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GAS PHASE STRUCTURE AND REACTIVITY OF ZIRCONOCENE CATIONS
By
ALEXANDER A. AKSENOV
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2005
This work is dedicated to my loving wife Karina
iii
ACKNOWLEDGMENTS
Author expresses his deepest gratitude to Dr. John R. Eyler for his knowledge, help
and concern and Dr. David E. Richardson for his sin qua non expertise and fruitful
discussion.
iv
TABLE OF CONTENTS page
ACKNOWLEDGMENTS ................................................................................................. iii
LIST OF TABLES............................................................................................................ vii
LIST OF FIGURES ......................................................................................................... viii
ABSTRACT.........................................................................................................................x
CHAPTER
1 CHEMISTRY OF ZIRCONOCENES AND ZIRCONOCENE CATIONS IN THE CONDENSED PHASE ................................................................................................1
1.1 Introduction and Historical Background ................................................................1 1.2 Structure of Zirconocenes.......................................................................................2 1.3 Reactivity of Zirconocene Derivatives ...................................................................4
1.3.1 Formation of Zirconium-Carbon Bonds.......................................................6 1.3.1 Cleavage of Zirconium-Carbon Bonds.........................................................7 1.3.2 Other Organozirconium Derivatives ............................................................7
1.4 Cationic Zirconocene Species.................................................................................9 1.4.1 Structure and Reactivity of Zirconocene Cations.........................................9 1.4.2 Nucleophilic Addition Reactions ...............................................................10 1.4.3 Cationic Zirconocenes as Lewis Acid Catalysts ........................................11
1.5 Single-Center Metal Polymerization ....................................................................12 1.5.1 Ziegler-Natta Polymerization .....................................................................12 1.5.2 Metallocene-Catalyzed Olefin Polymerization ..........................................13
2 FTICR MS: METHOD OVERVIEW.........................................................................15
2.1. Introduction and Historical Background .............................................................15 2.2 Theory and Instrumentation..................................................................................16
2.2.1 Ion Cyclotron Motion .................................................................................18 2.2.2 Acquisition of Mass Spectra.......................................................................20
2.3 Ionization Methods ...............................................................................................23 2.3.1 Electron Ionization .....................................................................................24 2.3.2 Chemical Ionization....................................................................................25 2.3.3 Electrospray Ionization...............................................................................25 2.3.4 Matrix Assisted Laser Desorption/Ionization.............................................26
v
2.3.5 Atmospheric Pressure Chemical Ionization ...............................................27 2.3.6 Cationization...............................................................................................28
3 GAS-PHASE CHEMISTRY OF METAL IONS AND IONIC COMPLEXES ........29
3.1 Gas-Phase vs. Solution Chemistry........................................................................29 3.2 Gas-Phase Reactivity of Bare and Oxo- Metal Ions.............................................30 3.3 Gas-phase Reactivity of Ligated Metal Ions ........................................................32 3.4 Reactions of the Bis(η5-cyclopentadienyl)methylzirconium Cation in the Gas
Phase .......................................................................................................................34 3.4.1 Reactions with Alkenes ..............................................................................34 3.4.2 Allyl Complex Formation: Mechanistic Differences of Reaction
Patterns in the Gas and Liquid Phases .............................................................38
4 GAS-PHASE REACTIONS OF BIS(η5-CYCLOPENTADIENYL) METHYLZIRCONIUM CATIONS WITH KETONES AND ALDEHYDES .........41
4.1 Rationale ...............................................................................................................41 4.2 Experimental Procedures ......................................................................................43 4.3 Reaction with Ketones..........................................................................................51
4.3.1 Reactions of Cp2ZrCH3+/Cp2ZrCD3
+ with Acetone/d6-Acetone ...............51 4.3.2. The Reactions of Cp2ZrCH3
+/Cp2ZrCD3+ with 2-Butanone, Methyl
Isobutyl Ketone and Cyclohexanone. ..............................................................61 4.3.3 Reactions with Other Ketones ....................................................................70
4.4 Reaction with Aldehydes......................................................................................72 4.5 Comments on Suggested Reaction Mechanisms ..................................................85
5 GAS-PHASE REACTIONS OF BIS(η5-CYCLOPENTADIENYL) METHYLZIRCONIUM CATIONS WITH IMINES ................................................92
5.1 Synthesis and Reactivity of Imines.......................................................................92 5.2 Gas-phase Reactions of the Bis(η5-Cyclopentadienyl)methylzirconium Cation
with Imines..............................................................................................................93 5.2.1 Reactions of Cp2ZrCH3
+/Cp2ZrCD3+ with 2,2,4,4-Tetramethyl-3-
Pentanone Imine...............................................................................................94 5.2.2 Reactions of Cp2ZrCH3
+/Cp2ZrCD3+ with Aryl Substituted Imines ..........98
6 CONCLUSIONS AND FUTURE WORK...............................................................102
6.1 Conclusions.........................................................................................................102 6.2 Future Work........................................................................................................103
6.2.1 IRMPD Spectroscopy...............................................................................103 6.2.2 Other gas-phase studies of organometallic compounds ...........................108
vi
LIST OF REFERENCES.................................................................................................111
BIOGRAPHICAL SKETCH ...........................................................................................132
vii
LIST OF TABLES
Table page 3.1. Reactions of Cp2ZrCH3
+ with alkenes. ......................................................................34
4.1. Isotopes of zirconium.................................................................................................45
4.2. Major reaction products observed in the reactions of 1 and 2 with 2-butanone, and methyl isobutyl ketone at various reaction times (1 to 5 seconds)....................63
4.3 Major reaction products observed in the reactions of 1 and 2 with diethyl carbonate at various reaction times (1 to 5 seconds)................................................71
4.4. Major reaction products observed in the reactions of 1 and 2 with acetaldehyde, benzaldehyde, propanal and n-hexanal at various reaction times (1 to 5 seconds)..73
4.5. The absolute electronic energy (kcal/mol) of the most stable structure is given, and the relative energy difference is given for the remaining structures (in kcal/mol). Lettering corresponds to the structures pictured in Figure 4.16, a) through f)..................................................................................................................91
5.1. Imines used in this work. ...........................................................................................94
viii
LIST OF FIGURES
Figure page 1.1. Structural types of Cp2Zr(IV) compounds (adapted from ref. [23]); examples of
each structural type are listed in the right column. ....................................................2
1.2. Dewar-Chatt-Duncanson model of alkene bonding by a metal complex (adapted from ref. [28]).............................................................................................................4
1.3. Two-electron reactions of zirconocene complexes......................................................6
1.4. Reactions of Cp2ZrC(O)RCl (adapted from Ref. [23])................................................8
2.1. Schematic representation of a cylindrical FTICR analyzer cell (adapted from Ref. [104]). .......................................................................................................................18
2.2. Schematic representation of a cross section of an FT-ICR analyzer cell, where ions are being excited by the RF potential applied to the excitation electrodes (adapted from Ref. [104]).........................................................................................21
2.3. Stages of data in the FTICR experiment....................................................................22
3.1. A reaction profile for hydrocarbon oxidation by a transition metal oxo-ion. ............31
3.2. Reactions of Cp2ZrCH3+ with alkenes (adapted from Ref. [147]).............................35
3.3. Comparison of qualitative potential energy surfaces for the reaction of Cp2ZrCH3
+ with ethylene in solution and the gas phase (adapted from Ref. [147]). .......................................................................................................................36
3.4. Mass spectrum for the reaction of Cp2ZrCH3+ with cyclopentane. ...........................38
3.5. Proposed structure of the complex with m/z 287.......................................................38
4.1. Schematic diagram of the mass spectrometer used in this work. ..............................44
4.2. Schematic diagram of the electron ionization source. ...............................................44
4.3. EI mass spectrum of Cp2Zr(CH3)2: Cp2ZrCH3+ (m/z 235-242), Cp2Zr+(m/z 220-
227) and Cp2ZrOH+ (m/z 237-244)..........................................................................46
4.4. Example of a SWIFT experiment. .............................................................................48
ix
4.5. Binuclear complexes Cp4Zr2(CH3)n+ (n = 0 - 3) ........................................................48
4.6. EI mass spectrum of Cp2Zr(CD3)2 (* Noise peaks)...................................................50
4.7. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone. ..................52
4.8. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone ...................54
4.9. The a) η3-enolate and b) η3-allyl complexes..............................................................56
4.10. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with methyl isobutyl ketone. ......................................................................................................................62
4.11. Ions produced in the reaction of Cp2ZrMe+ with benzoquinone: products of coordination of quinone molecule by the binuclear complexes Cp4Zr2(CH3)n
+ (n = 0 - 3) ......................................................................................................................71
4.12. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetaldehyde, 1 second reaction delay. ..............................................................................................75
4.13. Mass spectra of ions with m/z 279 and 280 formed in the reaction of Cp2ZrCD3+
with acetaldehyde, at a) 1 second and b) 5 seconds reaction delays. .......................76
4.14. Ions produced in the reaction of Cp2ZrMe+ with benzaldehyde at 1 second reaction delay. ..........................................................................................................76
4.15. Qualitative potential surface for the reaction of 1 with acetaldehyde/generic aldehyde in the gas phase and in solution. The lettering corresponds to that in Scheme 4.21. Gas-phase estimates of energetic effects are based on data from Ref. [173]. ................................................................................................................80
4.16. The most stable theoretically calculated structures for the Cp2ZrC3H5O+ cation. Canonical structures shown were calculated with the MPW1PW91/6-311+G(d,p) functional/basis set. The calculated absolute electronic energies of structures from a) through f) are listed in Table 4.5.................................................90
5.1. EI mass spectrum of benzophenone imine (molecular weight 181 amu). .................99
6.1. IRMPD spectrum of Cr+(Et2O)2 (adapted from Ref. [204]). ..................................104
6.2. Theoretical IR absorption spectra for the key structures discussed in the present work........................................................................................................................106
6.3. Depletion spectrum of ion with m/z 282 (postulated as [Cp2Zr(CH3CHO)(H2O)]+). ....................................................................................108
x
Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
GAS PHASE STRUCTURE AND REACTIVITY OF ZIRCONOCENE CATIONS
By
Alexander A. Aksenov
December 2005
Chair: John R. Eyler Major Department: Chemistry
The reactions of bis(η5-cyclopentadienyl)methylzirconium cation with aldehydes,
ketones and imines in the gas phase have been studied by Fourier transform ion cyclotron
resonance mass spectrometry. Reactions of bis(η5-cyclopentadienyl)methylzirconium
cation with a majority of the ketones studied resulted in consecutive addition of one and
two substrate molecules and/or elimination of neutral. The key ionic products of the
elimination reactions were identified as η3-enolate complexes. Similar product ion
structures are also postulated for the reactions with aldehydes, where these complexes are
either the only or the major reaction product.
Deuterium-labeled substrates and methylzirconocene were used to investigate
mechanistic details. Results indicate a multiple-step mechanism, with migratory insertion
of an aldehyde molecule into the methyl zirconocene cation, followed by β-H elimination
and, via a 6-membered cyclic transition state, formation of the resulting enolate complex.
When a β-H elimination pathway is not available for ketones, the reaction is proposed to
xi
proceed instead via direct nucleophilic attack of the metal-bound alkyl preceded by a fast
migratory insertion equilibrium.
In reactions with the majority of imines, the reactivity toward bis(η5-
cyclopentadienyl)methylzirconium cation is governed by the availability of a labile
hydrogen. This leads to reaction products different from those which might be expected
in analogy with reaction products observed for ketones and aldehydes, i.e., the
azomethyne/benzylidene species instead of the enamines. Also, due to lack of lability of
the aryl groups, the migratory insertion equilibrium was not possible for reactions of aryl-
substituted imines. Consequently, only methane elimination was observed. However, a
general reaction mechanism resembles that proposed for ketones. The proposed
mechanisms account very well for all of the observed reaction features.
1
CHAPTER 1 CHEMISTRY OF ZIRCONOCENES AND ZIRCONOCENE CATIONS IN THE
CONDENSED PHASE
1.1 Introduction and Historical Background
Among other transition metals, Zr and its complexes, especially cyclopentadienyl
derivatives, have special significance in view of their potential to serve as catalysts or
reagents for multitudes of synthetically interesting reactions. Even though zirconium
occurs to the extent of 0.022% in the lithosphere (more abundant than carbon but less
abundant than Ti, 0.63%) and is one of the least expensive metals, its application in
organic syntheses of any kind was virtually unknown until the 1970s (apart from use as a
rather unsuccessful substitute for Ti salts in the Ziegler-Natta polymerization) [1].
However, over the last twenty years organozirconium compounds (almost exclusively
zirconocenes) have become some of the most widely used organometallics, along with
other metals such as Ni, Cr, Rh, and Fe. The explosive growth of this area is documented
in multiple reviews (a few of the most recent are given [2-15]) and the chemistry of
organozirconium and other group 4 transition metals has been extensively discussed in
some monographs [16, 17]. Most importantly, the d0 group 4 metallocenes are of great
interest as precursors of alkene polymerization catalysts [18, 19], and can be tuned to
make polymers of very specific tacticities. Single-center metal-catalyzed polymerization
has attracted ample attention and evolved from an area of purely academic interest into
commercially important technology [20]. The remainder of this chapter reviews some of
the most important aspects of the chemistry of organozirconium compounds.
2
1.2 Structure of Zirconocenes
The majority of known Zr compounds are derivatives of the so-called
“Cp2Zr unit” (Cp = cyclopentadienyl or related ligand) with Zr in +4 or, less often, +2
oxidation states. Zr (+3) compounds are also known, but are generally considered as
unwanted by-products [21].
Figure 1.1. Structural types of Cp2Zr(IV) compounds (adapted from ref. [23]); examples of each structural type are listed in the right column.
3
Structural types of Cp2Zr(IV) compounds are shown in Fig. 1.1. In addition to the
structures shown in Fig. 1.1, it is well known for the coordinatively unsaturated 14 and 16
electron zirconocene species to form dimers, oligomeric species, or polymeric
aggregates. It is common for the “Cp2Zr unit” to remain intact in the majority of
chemical transformations. However, under certain conditions, for example, treatment
with an excess of very strong nucleophiles such as alkyl lithium compounds, the Cp ring
(or both) can be displaced [22].
Zirconocene (IV) derivatives represented by the general formula Cp2ZrR1R2 (both
R1 and R2 being X ligands), such as zirconocene dichloride, Cp2ZrCl2, the Schwartz
reagent Cp2ZrHCl [24, 25], Cp2ZrMe2, etc., are d0, 16-electron compounds, with an
empty valence-shell orbital, but with no filled orbitals available for interaction. This
makes these complexes inherently electrophilic (Lewis acidic), but not nucleophilic. The
empty 2d orbital of zirconium is able to interact with σ- and π- bonding electrons, as well
as nonbonding electron pairs, and this interaction is usually a first step in the reaction of
Zr derivatives. Formation of the π-complexes of transition metals is described by the
Dewar-Chatt-Duncanson model [26, 27] (Figure 1.2). In this model, a σ-type donation
from the C=C π orbital with concomitant π-backbonding into an empty π* orbital of the
alkene results in synergistic bonding: the greater the σ-donation to the metal, the greater
the π-backbonding. Also, the greater the electron density back-donated into the π* orbital
of the alkene, the greater the reduction in the C=C bond order, which can ultimately lead
to formation of η2 alkene insertion product. This, in turn, implies that only metals with at
least one filled nonbonding orbital can form such complexes. For the d0 zirconocenes,
only donation of electrons is possible from alkenes, so these complexes act solely as
4
Lewis acids, thus forming only unstable (usually transient) species. This interaction is
more efficient for the cationic zirconocenes.
Figure 1.2. Dewar-Chatt-Duncanson model of alkene bonding by a metal complex (adapted from ref. [28])
1.3 Reactivity of Zirconocene Derivatives
The reactivity of various zirconocene derivatives can be generalized into several
distinct types of reactions, as shown on Figure 1.3. The most important and relevant
reactions are discussed below.
5
a) σ-Complexation and σ-dissociation
b) π-Complexation and π-dissociation
c) Hydrozirconation and dehydrozirconation
d) Carbozirconation and decarbozirconation
e) Oxidative addition and reductive elimination
f) σ-Bond metathesis
6
g) Migratory insertion and migratory deinsertion
Figure 1.3. Two-electron reactions of zirconocene complexes.
1.3.1 Formation of Zirconium-Carbon Bonds
Apart from transmetallation, the most synthetically important reaction resulting in
zirconium-carbon bond formation is hydrozirconation [25, 29] (Fig. 1.2c), which
provides an easy and convenient way of converting alkenes and alkynes into alkyl- and
vinyl- zirconium derivatives, respectively [30]. The commonly used “Cp2Zr” precursor is
the Schwartz reagent, Cp2ZrHCl, in conjunction with an aluminium hydride, such as
LiAlH4 [24]. The formation of the insertion product occurs with good regio- and
stereoselectivity (anti-Markovnikov, syn addition, with the formation of the E-alkenyl
complex in reactions with alkynes) [31]. The availability of β-hydrogens in the insertion
product can possibly lead to intramolecular rearrangement.
Another synthetically important means of formation of organozirconocene
derivatives is oxidative addition (Fig. 1.2e). It should be noted, however, that since the
Cp2Zr(IV) complexes have d0 configuration, the oxidative addition process is not
possible, since it requires that both empty and filled orbitals be available for interaction.
7
It has been shown that the actual reaction is oxidative coupling-elimination [32]. The
oxidative addition reaction is valuable for converting allyl ethers into allylzirconocenes
[33], as well as for conversion of alkenyl chlorides into alkenylzirconocene chlorides.
1.3.1 Cleavage of Zirconium-Carbon Bonds
The zirconocene alkyl/alkenyl insertion product can be converted by
protonolysis/deuterolysis and halogenolysis into the corresponding (halogenated)
alkane/alkene. Cleavage of the Zr-C bond occurs in relatively mild conditions:
protonolysis requires treatment with diluted acid or even water, while halogenolysis
occurs under treatment with bromine or iodine, or, when an alkene group is present,
succinimides (NBS or NCS) at room temeperature. The configuration of alkene is usually
preserved [34]; however, some examples of concomitant skeletal rearrangement are
known [25, 29]. Another path for Zr-C bond cleavage is treatment with peroxides, which
leads to formation of alcohols [35].
The inherent low intrinsic nucleophilicity of alkyl/alkenyl zirconocenes renders
them unable to react with a majority of common electrophilic substrates, with the only
exception known to be the reaction with aldehydes [36]. On the other hand,
dienezirconocenes can react with a variety of substrates, such as alkenes, alkynes,
aldehydes/ketones and even nitriles [37].
1.3.2 Other Organozirconium Derivatives
An important class of compounds containing a Zr-C bond is the acylzirconocenes,
which can be easily and straightforwardly obtained via treatment of alkyl/alkenyl
derivatives with carbon monoxide at atmospheric pressure, resulting in a generally clean
reaction of CO inserting into the Zr-C bond [34, 38]. The stability of the
acylzirconocenes depends strongly on the ligand of the given derivative, with the
8
acylzirconocene chlorides being most stable (not losing CO even at pressures
significantly lower than atmospheric). Acylzirconocenes can be further involved in a
variety of other reactions shown in Figure 1.4. All of those reactions proceed via
migratory insertion [34, 39] (Fig. 1.2g).
Figure 1.4. Reactions of Cp2ZrC(O)RCl (adapted from Ref. [23]).
Another very important class of organozirconium compounds are the π-
complexes, which can be considered as zirconocyclopropanes. The π-complexation (or,
rather, oxidative π-complexation) can be considered as a two-electron red-ox process
with the alkene being reduced by zirconium. The required Zr(II) species can be generated
by a reducing agent (alkali metal amalgam) prior to reaction, or in situ [39-41]. The
resultant zirconacycle, called the “Negishi reagent” is a valuable reagent, enabling
synthesis of various zirconocycles and serving as a very convenient source of the
“Cp2Zr” unit. The sequence of reactions involving formation of zirconium π-complexes is
called the “Negishi protocol,” and was shown to be a concerted non-dissociative process (
the Cp2Zr species is not generated at any stage [42]). Along with the more recently
developed Erker-Buchwald protocol [43-47], these two protocols, often used
complementarily to each other, provide synthetic routes to tri-member zirconocycles and
a variety of compounds obtained from them.
9
1.4 Cationic Zirconocene Species
Zirconocene cations are not found as isolated species in the solution phase. The
high reactivity of such species usually leads to their immediate conversion/reaction.
Furthermore, if such cations are indeed produced, they always exist as an ionic pair with
varying degree of coordination by a counterion. In general, it is impossible to define a
discrete cation as part of a salt: in most cases, no indication of a cationic nature is found
either (by the means of X-ray) in the solid state, or in solution, and the only indication of
the zirconocene cation’s presence is the characteristic chemistry taking place.
Normally, the ionic species are formed from a suitable precursor with appropriate
in situ activation. The most widely used methods in organic synthesis include halide
abstraction from Cp2ZrCl2 with Ag salts, such as AgClO4, AgAsF6, AgSbF6, etc. (the
counterion must be non-nucleophilic) and protonation of methyl zirconocenes with acids
containing the BARF (tetrakis(3,5-bis(trifluoremethyl)phenyl)borate) [48] anion. The
variety of methods used for the generation of bis(η5-cyclopentadienyl)methylzirconium
and related cations employed in polymerization catalysis is considered below in Chapter
1.4. A vast number of studies have been carried out [49].
1.4.1 Structure and Reactivity of Zirconocene Cations
The zirconocene cations generated in solution always exist in their solvated form
and/or ion pairs with a counterion. Depending on the nucleophilicity of the anion, the
degree of coordination can vary and the free cation is never observed. Weakly
coordinating counterions such as BARF- provide the only way to utilize the reactivity of
the zirconocene cation, while common nucleophilic anions such as Cl- can completely
shut down the reactivity in most cases. Moreover, these cationic species exhibit a
tendency to abstract chlorine ion from chlorinated solvents, and show even greater
10
tendency to abstract and bind fluoride (usually forming Cp2ZrX2[50]) (so counterions like
BF4- cannot be used[51]). This feature is used for synthetic purposes for C-F bond
activation in glycosyl fluorides [52, 53].
The observed rate for reactions involving zirconocene cationic species
Cp2ZrX(L)+ depends on the rate of dissociation of the coordinated ligand to give the 14-
electron species. The latter complexes are the most reactive ones, since their
electrophilicity is unimpeded by electron donation from the base. Of course, such species
tend to coordinate solvent molecule(s), thus establishing a solvent association-
dissociation equilibrium which determines the cation’s reactivity.
1.4.2 Nucleophilic Addition Reactions
The Cp2ZrRCl complex is not electrophilic enough to react with the most
substrates efficiently, i.e., cleanly and with a sufficient rate; thus, the synthetically
interesting addition reactions may involve zirconocene cations as intermediates. Alkyl
and alkenyl zirconocene cations can be produced from the hydrozirconation products of
alkenes and alkynes by the Schwartz reagent via chloride abstraction by a catalytic
amount of (most commonly) AgClO4 [54].
In aldehyde addition reactions of Cp2ZrRCl complexes, a mechanism involving
generation of alkyl/alkenyl zirconocene was proposed [54]. The resultant reaction
products are alcohols formed through condensation of the aldehyde and R ligand of
zirconocene. Another application of silver-mediated addition is synthesis of homologues
of unsaturated aldehydes by hydrozirconation of alkene/alkyne ethers with the Schwartz
reagent and addition of aldehyde followed by acidic hydrolysis. This method allows 2
and 4 carbon homologization with very good to excellent yields [55]. Also, utilizing
11
addition of aldehydes by a bimetallic 3-methylsilyl-1-propenylzirconocene chloride
allows synthesis of such valuable organic reagents as 1,3 dienes [56, 57].
Other reactions of practical importance are the ring opening of oxiranes producing
alcohols [58] and carbometallation of alkenes and alkynes. The interesting feature of the
latter is that only single addition is achieved through the metal-alkyl addition, rather than
polymerization. Among possible carbometallations, carboalumination is the most
common; this method is highly stereo- and regioselective and can produce various
products depending on the starting alkene/alkyne and the subsequent Al-C bond cleavage
method [59-61].
1.4.3 Cationic Zirconocenes as Lewis Acid Catalysts
As in nucleophilic addition reactions, cationic zirconocene species are produced
by abstraction of an anion from a neutral complex, usually zirconocene dichloride or
triflate, by AgClO4. The main application of such species is catalysis of Diels-Alder
reactions of α,β-unsaturated aldehydes or ketones and common dienes [62]. Zirconocene
catalysts are advantageous over the usual AlCl3 Lewis acid catalyst because of their
lower susceptibility to water and oxygen, thus eliminating the need for extensive drying
of reaction solvents, and generally leading to significantly lower amounts of catalyst
required (<1 mol%). Albeit efficient (up to 106 reaction acceleration), the catalysts
provide rather poor reaction product regio- and stereoselectivities [63]. The major
drawback, however, is the tendency to catalyze polymerization reactions instead,
resulting in the competing side reaction of a diene polymerization, thus limiting possible
application of such catalysts to only very reactive diene/dienophile pairs. Other reactions
catalyzed by cationic zirconocene species include various rearrangements/isomerization
12
reactions, such as, for instance, rearrangement of epoxides into corresponding aldehydes
[58], which proceeds with generally good yields.
1.5 Single-Center Metal Polymerization
The main practical application of zirconocenes is undoubtedly in polymerization
catalysis. The active species is generally accepted to be a coordinatively unsaturated
cation such as LL’MCH3+ (M=Ti, Zr; L= η5-cyclopentadienyl or related ligand). The
extensive interest in this class of compounds was triggered not only by very high catalytic
activities (comparable to those of enzymes!), but also by the ability to exert stereocontrol
of the resultant polymer by modification of the “Cp” ligands, thus creating polymers
with a wide array of properties unattainable by other means [64]. Single-center metal
polymerization has stemmed from conventional Ziegler-Natta polymerization, which is
reviewed briefly below.
1.5.1 Ziegler-Natta Polymerization
In classical heterogeneous Ziegler-Natta polymerization, the process takes place
at the structural defects (dislocations and edges) of TiCl3 crystals [65]. The proposed
mechanism for such reactions postulates polymer chain growth by cis-insertion of the
olefin into a Ti-C bond on the surface [66, 67]. The non-uniformity of the active sites in
heterogeneous catalysts is partially responsible for the polymer’s high polydispersity.
The more advanced version of the Ziegler-Natta catalyst, developed on the basis
of group 4 metallocenes, was comprised of mixtures of Cp2TiCl2 and AlR2Cl and
exhibited moderate activity [68]. The proposed mechanism of its action involves olefin
coordination and insertion into the Ti-C bond of electron-deficient Cp2Tiδ+R species. The
latter are formed via ligand exchange between aluminum and titanium and formation of
Cp2TiRCl, followed by coordination of the Lewis acidic aluminum to the titanium’s
13
chloride and polarization of the Tiδ+-Clδ- bond. The Ti-C bond “prepared” in this way
readily inserts ethylene [69]. This mechanism is similar to that postulated for
heterogeneous Ziegler-Natta catalysis: in both cases the Lewis acidity of the metal center
plays a key role, and cis insertion of the olefin into the M-R bond is observed. Originally
the nature of the active center was not fully established; however there were early
indications that Cp2TiR+ was an actual catalyst [70]. Unambiguous proof of this premise
was obtained when the ionic species [Cp2ZrMe(L)]+[BPh4]- was found to be a very active
ethylene polymerization catalyst [71]. It was shown that this compound generates
cationic metallocene species that are capable of olefin polymerization without the
aluminum alkyl activators. Further studies confirmed the crucial role of metallocene
cations as catalytically active species in homogeneous polymerization catalysis [72-83].
1.5.2 Metallocene-Catalyzed Olefin Polymerization
The major drawback of homogeneous polymerization catalysts, which impeded
their industrial application, was unacceptably low efficiency and an inability to
polymerize alkenes other than ethylene. A breakthrough was achieved with the
application of methylaluminoxane (MAO) [84, 85], which, at the present time, is used
almost exclusively in catalytic olefin polymerization. Unlike in Ziegler-Natta catalysis,
treatment of the metallocene/Al alkyl catalyst with miniscule, but exact, amounts of water
can immensely augment the catalyst’s activity as well as providing the ability to
polymerize alkenes more complex than ethylene. Various otherwise inactive systems
were shown to achieve exceptional activity upon such treatment [84, 85].
The currently accepted mechanism of MAO formation assumes partial hydrolysis
of the solid AlMe3 resulting in formation of extended networks of (AlMeO)n.
Introduction of catalytic amounts of Cp2MR2 (Cp = cyclopentadienyl or related ligand; M
14
= Ti, Zr; R = Cl, Me) into an excess of MAO generates Cp2MMe+ species via initial
methylation of metal-halide bonds and further abstraction of a methyl anion from the
Cp2MMe2. The MAO serves as a very low-coordinating counterion, thus forming a
[Cp2MMe]+[MAO-Me]- “salt”. Very low coordination of the anion enables high activity
of the catalyst.
The importance of the zirconocene cation being “free” in order to exhibit its
activity, also discussed in Chapter 1.3, has led to development of various ways for
preparation of weakly coordinating anions. Such systems are called ‘single site’
metallocene catalysts, implying that polymerization occurs solely at the metal center.
Large anions such as [B(C6H4R)]- and carboranes have been used as counterions, but they
exhibit rather strong interactions with the cation [86]. Less than two decades ago, a very
weakly coordinating perfluorinated tetraphenylborate anion was introduced as a
counterion [79] which afforded exceptionally high activity of the single-site zirconocene
catalyst for alkene polymerization, exceeding that of the catalysts with MAO as a
counterion (co-catalyst). Various co-catalysts for metal-catalyzed olefin polymerization
have been reviewed [87].
15
CHAPTER 2 FTICR MS: METHOD OVERVIEW
2.1. Introduction and Historical Background
Valuable insights into reaction mechanisms, including those of polymerization
reactions, can be obtained using Fourier transform ion cyclotron resonance mass
spectrometry (FTICR MS) [88-92] for studies of the intrinsic gas-phase reactivity of
transition metal complexes. The spatial separation of reacting species under high-
vacuum conditions has certain advantages, most importantly the absence of complicating
factors prevailing in solution such as association by ion pairing, interactions with solvent,
and intra- and intermolecular processes leading to destruction/modification of the active
species. This is especially relevant in catalytic studies, since active species are often
present in trace amounts and very short-lived. On the other hand, reaction mechanisms in
the gas phase may be different from those observed in the condensed phase. However,
information about such processes can provide a valuable description of reactions
occurring at extreme conditions and assist in explanation of possible reaction
intermediates/side products which might occur in traditional chemistry in the liquid
phase. The key issues that can be addressed by the gas-phase studies are: the effects of
solvent and intrinsic reactivity, as well as the effects of ancillary ligands and impact of
aggregation and ion-pairing on reactivity of ionic species; the thermochemical
determinations of various gas-phase reaction energies; and, importantly, direct
observation of key reactive intermediates, as well as identification of solution species in
16
general (speciation). It is possible to refine our understanding of the condensed-phase
chemistry through the study of analogous gas-phase chemistry.
FTICR has become very important technique due to the method’s ability to
provide mass resolution and accuracy unmatched by any other type of mass spectrometer.
Another important factor is the ability to combine FTICR mass spectrometry with various
ionization techniques. Rapid growth of the number of FTICR mass spectrometers
installed in laboratories around the world has occurred after the discovery of electrospray
ionization (ESI), which triggered extensive application of FTICR mass spectrometry in
studies of biomolecules [93, 94]. Another advantage of this method is the capability of
trapping and storing ions inside the spectrometer, which enables investigation of the ion
chemistry.
The principles of Fourier transform mass spectrometry are based on the cyclotron
resonance theory, developed in the 1930s [95]. The first attempt at employing the
principle of ion cyclotron resonance in a mass spectrometer was undertaken in 1950 [96],
while the first FTICR mass spectrometer was built in 1974 by Comisarow and Marshall
[97] At the present time, FTICR mass spectrometry is used widely for studies and
characterization of a broad range of systems. A large number of reviews have been
written on this topic [92, 98-104]. The principles of the FTICR MS technique are briefly
reviewed below.
2.2 Theory and Instrumentation
All FTICR mass spectrometers are comprised of certain components. Since the
ion cyclotron motion is caused by a magnetic field, a magnet is required. The magnets
can be permanent, electromagnetic or superconducting. The first of these three have very
low field strengths, unacceptable in many cases for the purposes of FTICR. The best of
17
electromagnets are capable of producing fields in the 1-2 Tesla range, which is suitable
for ions of relatively low mass-to-charge ratio. Since the performance of the FTICR MS
correlates with the magnetic field strength, superconducting magnets capable of
producing fields ranging from 2 to 9.4 Tesla are commonly used. The highest magnetic
field used in any of the currently operational mass spectrometers is 14.5 T at the National
High Magnetic Field Laboratory in Tallahassee, Florida [105].
Another main component of any FTICR mass spectrometer is the analyzer cell.
All of the manipulations with ions (storage, ion/molecule reactions etc.) and ion signal
detection are carried out inside it. Various cell designs have been used, with cubic and
cylindrical cells being the most common ones. The cubic cell is composed of six plates:
two plates are perpendicular to the magnetic field and used to confine the ions inside the
cell, hence the “trapping” plates name. Normally the trapping plates of cubic cells have
openings for the electrons or ions to be introduced into the cell. The other four plates are
the ion excitation and detection plates. The function of those is discussed below. Other
types of cells, though different in design, are comprised of plates or cylinders which
perform the same functions as in the cubic cell. In the cylindrical cell, excitation and
detection plates form a cylinder, with the trapping plates remaining parallel to each other
(Fig 2.1), while in the open cylindrical cell trapping plates are replaced by two open
cylinders at either side of the cell. The advantage of such designs is better suitability for
positioning in the bore of a superconducting magnet. Various other cell designs have
been developed [92].
18
Figure 2.1. Schematic representation of a cylindrical FTICR analyzer cell (adapted from Ref. [104]).
2.2.1 Ion Cyclotron Motion
The magnetic field exerts force on a charged particle moving in it, if a component
of the particle’s velocity is perpendicular to the magnetic field axis. This force is called
the “Lorentz force”. For a charged particle its velocity, magnetic field and the exerted
Lorentz force are mutually perpendicular. The value of the Lorentz force acting on a
charged particle in the magnetic field is:
F = qvBsinθ
where q is the charge on the particle, v is the velocity of the particle, B is the magnetic
field strength (in Tesla) and θ is the angle between the velocity of the charged particle
and the direction of the magnetic field. In case when θ=π/4 (particle moves
perpendicularly to the magnetic field), this expression simplifies to
F = qvB
The total charge on an ion is equal to:
q = ze
19
where e is the charge of electron and z is the total number of electronic charges on the
ion. The resultant trajectory of an ion will be circular, so it will be precessing in the plane
perpendicular to the magnetic field in a fashion called “cyclotron motion”. The frequency
of such motion depends on the mass-to-charge ratio of the ion and strength of the
magnetic field:
or
Here m is the mass of the ion and ω is the angular frequency, f is the cyclotron frequency.
Therefore, ions of lower m/z have higher cyclotron frequencies than ions of higher m/z.
As mentioned above, the FTICR analyzer cell (Fig. 2.1) consists of two excitation
and two detection plates along with two trapping plates in order to confine the ions inside
the cell. Potentials applied to the trapping plates are usually low (~1 volt), but they leads
to the side effect of a non-zero potential at the center of the FT-ICR analyzer cell due to
electrostatic fields from the trapping plates. This can offset the center of the cyclotron
motion and result in the “magnetron motion” [92, 103] in addition to the orbiting motion.
The magnetron frequency can be calculated as follows:
Here fm is the magnetron frequency (in Hz), Vtrap is the potential applied to the trapping
plates, a is the distance between the trapping plates and α is a geometry factor of the
20
FTICR analyzer cell [92, 106]. Magnetron frequencies are usually of an order of
magnitude of a few hundred Hz (unlike cyclotron frequencies, which are typically within
the kHz to MHz range). The magnetron frequency depends upon cell dimensions, design,
applied trapping potentials, and magnetic field strength. The resultant trajectory of an ion
in the cell is a combination of three components: the cyclotron motion, the magnetron
motion, and the oscillation between the trapping plates along the magnetic field axis. The
magnetron motion [92, 106] and the overall complex ion motion [102, 107-109] are
discussed in detail in the literature.
2.2.2 Acquisition of Mass Spectra
When the packets of ions in cyclotron motion repeatedly move past two detection
plates, charge moves within the detection circuit to counteract the proximity of the
passing ions. The voltage change between the detection plates can be measured as a
function of time. The resultant data are called “transient”, “time-domain” data, or “free
induction decay” (Fig 2.3a). Prior to detection, however, it is necessary to perform ion
“excitation” - application of a radio frequency voltage to the two excitation plates at the
frequency resonant with the cyclotron frequency of individual ions. The excitation is
necessary due to two factors: when the ions are first formed in, or enter into the FTICR
cell, the radii of their cyclotron orbits are too small to be detectable and the ions therefore
must be excited to detectable radii (Fig 2.2). Also, prior to the excitation, initial cyclotron
motion of ions is incoherent, and the effect of ions passing opposite detection plates
cancels out so no transient can be observed. Application of the RF voltage drives the ions
to larger orbits along with collection of ions into “ion packets”, which results in coherent
motion, i.e. the cyclotron motion of an ion cloud, rather than individual ions. Such an ion
21
packet will induce a potential in the detection plates which alternates with the cyclotron
frequency of ions with various m/z.
Figure 2.2. Schematic representation of a cross section of an FT-ICR analyzer cell, where ions are being excited by the RF potential applied to the excitation electrodes (adapted from Ref. [104]).
The magnitude of the signal induced on detection plates is proportional to the total
charge and the proximity of the ions to the detection plates (orbital radius), and is
independent of the magnetic field strength. Such a waveform is comprised from different
cyclotron frequencies of all of the ions in the cell. The next step is, therefore, extraction
of data about the different ion packets. This is achieved through application of the
“Fourier transform” operation (FT)[110] where frequency information is obtained from
time-domain data (Fig 2.3b).
22
a) Time-domain waveform.
b) Frequency domain data.
c) Resultant spectrum calibrated in terms of m/z.
Figure 2.3. Stages of data in the FTICR experiment.
23
A resultant spectrum is a function of signal magnitude from cyclotron frequencies
of the ions present. As shown above, the cyclotron frequency depends on the m/z ratio, so
it is feasible to plot signal magnitude as a function of m/z (Fig 2.3c).
An excitation waveform consisting of a linear increase or sweep through the
resonant frequency range of interest at constant amplitude is known as a “chirp”. Ions of
different m/z values are excited to orbits of the same radius, even though their cyclotron
frequencies differ. Such application of a complex frequency signal to the excitation
plates, which causes the trapped ions to absorb energy at their specific resonance
frequency, can also be used for selective ejection of ions from the cell. If a broad range of
frequencies is applied to cover the mass range of interest with amplitude and duration
adjusted to drive all ions to cyclotron orbits still contained within the cell dimensions, the
optimal image current can be obtained. However, such excitation also allows ejection of
all of the ions from the cell if their orbits reach the electrodes. It is possible, therefore, to
construct a complex signal that misses one or more frequency windows of a
predetermined width. Such an approach is called “stored waveforms inverse Fourier
transform” or SWIFT [111]. Applying such a signal to the trapped ions causes ejection of
all ions with exception of a selected cluster of ions with given m/z.
2.3 Ionization Methods
Formation of ions can be achieved by multiple means: ions can be formed either
by removal/addition of an electron leading to ions M+/M-, by protonation/deprotonation
leading to protonated/deprotonated molecules MH+/(M-H)-, or by addition of another
small ion leading to cationized molecules such as MNa+. Ions can be formed in an MS
ion source from a neutral precursor, or pre-exist in a solution phase. Also, ionization can
be occurring within the homogeneous high magnetic field region (internal ionization
24
sources) or externally to the magnetic field. In the latter case, transfer of ions into the MS
cell is required. Despite sometimes limited understanding of the exact means of ion
formation [112] the use of various ionization methods enables mass spectroscopic
analysis of a wide array of species from small molecules to large proteins. A brief review
of some of the ionization methods is given below.
2.3.1 Electron Ionization
The process of electron ionization (EI) involves interaction of gas phase
molecules with electrons accelerated to about 10-100 eV. As a primary product, EI forms
an excited odd electron ion-radical M+.. These ions are additionally destabilized by
having an unpaired electron and, usually, are capable of high-energy, sometimes quite
complex, reactions. Although the stability of ions may be increased by lowering the
energy of the ionizing electrons, most of the time ion fragmentation is observed. In
general, the presence of molecular ions in spectra is an exception rather than a rule. Some
structural characteristics, such as those favoring charge localization, may have a profound
effect on fragmentation and may increase the relative stability of parent ions or channel
fragmentation processes. For example, for a series of normal hydrocarbons, the
intramolecular vibrational and rotational super-cooling of molecules in a supersonic
molecular beam (SMB) lead to a significant increase in the stability of molecular ions
formed by EI [113]. With conventional EI normal hydrocarbons show only barely
detectable M+., while such ions dominated in the spectra obtained with SMB.
In a variation on the EI method, metastable atom bombardment (MAB), gas phase
chemi-ionization (not chemical ionization!) can be induced by metastable atoms of noble
gases. Employing one of the five of them (with excitation energy decreasing from 20 to 8
25
eV from helium to xenon, it is possible to control the energy transferred to the molecules
being ionized.
2.3.2 Chemical Ionization
Chemical ionization (CI) is a gas phase process involving ion-molecule reactions
with species derived from a reagent gas (or gas mixture) that is first ionized by EI at high
electron energy. Typically, the even electron protonated molecule MH+ is formed. An
efficient CI process, in the case of ion sources producing a constant beam of ions,
requires relatively high pressure of the reagent gas (~1 Torr), but in instruments that trap
ions for an extended time, ion sources may work at much lower pressure. Due to
collisional cooling of the nascent ions by molecules of the reagent gas the CI process is
relatively soft. The ionization efficiency and type of ions formed depend on the
specificity of ion-molecule reactions. For example, a high electron affinity can provide
very efficient ionization leading to M- ions. Other types of CI ion formation include
association with species derived from the reagent gas or from “true” chemical reactions
[114].
2.3.3 Electrospray Ionization
Electrospray ionization (ESI) [115-118] is the ionization technique most
frequently used in conjunction with FTICR mass spectrometry. ESI has the advantage of
being a soft ionization technique, resulting in the vaporization and ionization of a
sample, at the same time minimizing fragmentation. The latter is particularly important
for labile molecules such as biological molecules, and for the study of complex mixtures
where fragments must not interfere with the mass spectrum so the original constituents
can be determined. ESI frequently leads to formation of multiply charged ions,
particularly in the case of large biomolecules. Since mass spectrometry is based upon the
26
determination of m/z, not mass alone, it is frequently the case that a mass spectrum will
contain the same molecules but in a variety of charge states, and therefore at several
different m/z values. As ions become more highly charged, the m/z becomes lower and
the spacing between the “isotopomers” becomes narrower. As a result, it becomes more
difficult to resolve the signals and the resolution of the mass analyzer becomes more
important. The charge state of an ion can be determined by examining the spacing
between the isotopomer signals. The increasing importance of the electrospray ionization
technique is promoting the growth of the FTICR mass spectrometry, due to the ultra-high
resolution of FTICR mass spectrometers. For these reasons, the ESI FTICR mass
spectrometry has become increasingly important for study of biological molecules in
recent years.
2.3.4 Matrix Assisted Laser Desorption/Ionization
An alternative mean of ion formation is desorption of ionic species from the solid
phase with desorption energy supplied by a laser. Direct laser desorption of analyte can
only be effective if the λmax of the analyzed compound matches the laser output. Matrix
assisted laser desorption/ionization (MALDI), on the other hand, does not pose this
restriction and provides good ionization efficiency at high mass ranges [119]. In MALDI,
the light energy is first absorbed by an excess (usually >1000) of a crystalline matrix with
the absorption maximum close to the laser output. A short burst of ions formed by a
single laser pulse is followed by ion analysis. In a simplified model, matrix evaporation
forms excited and ionized species that in turn ionize other components. As in the case of
other soft ionization methods, neutral species which are present in the plume (before its
dissipation in vacuum) provide a cooling effect, thus contributing to the stability of ions
27
formed. Many compounds have been tested as matrices for MALDI, but only some work
well. For different classes of compounds, matrices show striking differences in the
ionization efficiency and the difference in their suitability for different method of ion
analysis. For instance, matrices working well with MALDI TOF may not be suitable for
MALDI FTICR. MALDI may show lack of reproducibility resulting from the matrix
selection, matrix-to-compound ratio, form of matrix crystallization, and possible presence
of impurities including those formed by a degrading matrix. MALDI and other soft
ionization methods are highly selective and subject to strong suppression effects.
Although this selectivity may facilitate detection of compounds with well recognized
ionization characteristics, at the same time it may create an uncertainty in characterizing
novel compounds as well as in evaluating purity. Suppression effects may completely
prevent some or all components from producing ions and may lead to a situation in which
ions observed represent only the most easily ionized components and not those that are
quantitatively important. As a result, the removal of salts, organic buffers, and detergents
is important in avoiding sample unrelated ions and suppression effects [120].
2.3.5 Atmospheric Pressure Chemical Ionization
Atmospheric pressure chemical ionization (APCI) is most commonly used in
combination with liquid chromatography. This ionization technique involves high
temperature nebulization of solutions in the presence of a corona discharge or β-emitter
while solvents or solvent additives (such as, for example, ammonium acetate) play the
role of a reagent gas. Ions formed at the atmospheric pressure are transferred to the high
vacuum part of a mass spectrometer via several stages of pumping and mass analyzed
[121].
28
2.3.6 Cationization
The formation of cationized ions has been recognized as a very useful in the
analysis of compounds that resist protonation [122]. Even traces of alkali metals,
especially the ubiquitous sodium ion, may lead to full or partial cationization, but
dissolving salts or hydroxides in a matrix allows much better control of this process.
Elements with well recognizable isotopic patterns, for example silver or thallium, both
with two naturally occurring isotopes (107Ag 51.3% and 109Ag 48.7%; 203Tl 29.5% and
205Tl 70.5%), produce easily discernable attachment ions [123]. Analogously, mixtures of
alkali metal salts can produce well recognizable patterns. It is also possible to facilitate
cationization by using special derivatives incorporating groups with a high affinity for
alkali ions (such as, for example, crown ethers) [124].
29
CHAPTER 3 GAS-PHASE CHEMISTRY OF METAL IONS AND IONIC COMPLEXES
3.1 Gas-Phase vs. Solution Chemistry
It is quite apparent that ions studied under the conditions inside an FTICR mass
spectrometer will exhibit higher reactivity than those in the solution due to a combination
of two factors. The isolated charge on the ions due to the absence of counterions and
solvent, coupled with high-energy collisions, result in the accumulation of significant
energy that cannot be discarded into a solvent bath. Furthermore, the possible formation
of excited states during high energy ionization events, such as collisions with “hot”
electrons (exceeding the appearance energy (AE) by numerous electron volts) during EI,
can lead to a distribution of excited states. Often the ground states species comprise less
than a half of the total ion population [125-130]. Application of special techniques, like
surface ionization (SI) or high-pressure ionization sources, is required to overcome this
complication. The value of such gas-phase studies, therefore, is not only in the possibility
of determining thermodynamic and kinetic data (which often cannot be directly applied to
the processes in solution). It is also in the possibility of obtaining a precise and thorough
description of the impact of factors which would be very hard to account for in the actual
reaction in the solution phase due to interaction of species of interest with solvent,
clustering, nonhomogeneity and so on. Examples of such factors are the effects of
ligands, electronic configuration and periodic trends, as well as the detection of possible
elusive reaction intermediates, etc. A brief review of some aspects of the gas phase
chemistry of metal ions and simple organometallics is given below.
30
3.2 Gas-Phase Reactivity of Bare and Oxo- Metal Ions
Significant interest in the chemistry of metal ions in the gas phase was triggered
by a discovery of their ability to perform specific activation of carbon-hydrogen and
carbon-carbon bonds [131]. Both activation of unstrained carbon-carbon bonds by bare
transition metals [132, 133] as well as by oxide cations, MO+ [134] have been discussed
in number of reviews. A recent review provides an update of the chemistry of alkane
activation in the gas phase with a focus on thermochemical data for various metal-carbon
bond energies [135].
An alkane activation reaction is promoted when there is an empty orbital on the
metal which accepts electrons from the carbon-carbon bond being cleaved. If such an
orbital is filled, the repulsive interaction makes the reaction inefficient [130, 136].
Simultaneously, the electrons on the metal’s orbital with π-symmetry are back-donated
into the σ* orbital of the breaking bond, thus leading to its dissociation if the reactants are
capable of getting sufficiently close. For efficient activation both of the interactions
described above are required.
Interest in the chemistry of metal oxo-ions stems from their potential role as
reaction intermediates in various oxidation processes, in particular the oxidation of
hydrocarbons. Since alkane oxidation by a transition metal oxo-cation is generally
exothermic (Fig. 3.1), a catalytic cycle can potentially be devised if the metal cation can
be reoxidized in situ by a suitable reagent (such as oxygen, hydrogen peroxide, nitrogen
oxides, etc.).
31
Figure 3.1. A reaction profile for hydrocarbon oxidation by a transition metal oxo-ion.
An example of methane activation by FeO+, a most widely studied species, is
shown in Scheme 3.1 [134].
Scheme 3.1
An example of a true gas-phase catalytic cycle for the oxidation of ethylene by
nitrogen dioxide with CeO2+ as a catalyst is shown in Scheme 3.2 [137]. However, the
turnover rate was limited to only 8 cycles per CeO2+ ion due to reaction of the latter with
the hydrocarbon contaminants present in the NO2 gas, resulting in generation of CeO2H+
species, which were unreactive toward ethylene.
32
Scheme 3.2
3.3 Gas-phase Reactivity of Ligated Metal Ions
Even though studies of ligated metal ions in the gas phase are not as
comprehensive as those of bare ions, there is a significant number of publications on this
subject, especially for the complexes of group 8-10 metals, with the main focus on Fe and
Co.
The simplest complexes, metal hydrides, can react with alkanes and alkenes in the
gas phase to form either metal alkyl or allyl complexes by the loss of one or two
molecules of dihydrogen (or methane) [138]. Co(allyl)+ is generally more reactive than
Fe(allyl)+. Both complexes react rapidly with alkanes (excluding methane) by a C-H
bond activation mechanism resulting in the formation of one or two dihydrogen
molecules [139].
Among MCH3+ and LMCH3
+ ions, FeCH3+ and CoCH3
+ ions are also the most
widely studied. The latter is capable of reacting with alkanes larger than ethane by an
initial insertion into a C-H bond. Subsequent methane loss is followed by
dehydrogenation or alkane elimination resulting in cobalt allyl complexes [140]. Both
FeCH3+ and CoCH3
+ react with cyclic alkanes.
33
The reaction of MCH3+ ions with alkenes is particularly important with regard to
Ziegler-Natta polymerization. The Cp2ZrCH3+ complex exhibits distinct reactivity with
unsaturated hydrocarbons in the gas phase. Reactions of Cp2ZrCH3+ are reviewed in the
following sub-chapter. FeCH3+ is unreactive with ethylene, while CoCH3
+ forms an
insertion product which decomposes via loss of dihydrogen to form a stable allyl
complex. Such M(allyl)+ complexes are also formed by FeCH3+ and CoCH3
+ ions from
higher alkenes as a result of methane loss and possible elimination of dihydrogen or
alkene [139].
Alkene complexes Me(alkene)+ of Fe+[141], Co+ and Ni+ [142] primarily exhibit
ligand displacement and condensation reactions. However, bis(allyl) complexes resulting
from dehydrogenation by double allylic C-H activation could also be formed. These
complexes were not observed for the bare metal ions. Also, some ligand coupling, similar
to metal-assisted Diels-Alder reactions, was reported for Co(alkene)+ and to a small
extent for Fe(alkene)+ complexes.
Studies of cyclopentadienyl complexes, in particular CpCo+, in reactions with
aliphatic alkanes larger than methane revealed formation of mainly dehydrogenation
products along with a small amount of products resulting from C-C bond cleavages [143].
In the reaction of CpNi+ with a variety of aldehydes, formation of decarbonylation
products was reported [144].
It was shown that Cp2Zr+ is capable of abstracting a chlorine atom from chloro-
substituted methanes, indicating that the dissociation energy of the Zr-Cl bond is at least
81 kcal/mol [145]. In addition, the cyclopentadienyl complexes of iron and nickel were
34
shown to undergo a metal-switching reaction with a number of metal ions (Ti, Rh, Nb)
[146].
3.4 Reactions of the Bis(η5-cyclopentadienyl)methylzirconium Cation in the Gas Phase
The following sub-chapter reviews application of FTICR MS as a primary tool for
studying species involved in the process of metallocene-catalyzed olefin polymerization.
The physico-chemical properties of zirconocene ions, their intrinsic reactivity, and their
reaction patterns have been studied. The effect of the solvent on reactivity can be
estimated by comparison of gas-phase results with data for condensed phase single-site
polymerization catalysis.
3.4.1 Reactions with Alkenes
The first study of reactivity of Cp2ZrCH3+ with unsaturated hydrocarbon in the
gas phase [147] indicated that no polymerization took place at all, but rather that the
reaction proceeded exclusively via elimination of dihydrogen or alkene and formation of
an η3-allyl complex (Table 3.1) [147].
Table 3.1. Reactions of Cp2ZrCH3+ with alkenes (adapted from Ref. [147]).
35
The suggested reaction mechanisms involve insertion/dehydrogenation
accompanied by statistical H/D scrambling with deuterated ethylene or a σ-bond
metathesis (Fig. 3.2).
a) Insertion/dehydrogenation reaction mechanism
b) σ-bond metathesis reaction mechanism
Figure 3.2. Reactions of Cp2ZrCH3+ with alkenes (adapted from Ref. [147]).
The observed reaction pathways were rationalized by the insufficient
thermalization of “hot” ions produced the during electron ionization event (Fig. 3.3)
[147-151]. The observed loss of a neutral molecule (H2, CH4) in the gas-phase reactions
can be suppressed. Three ways can be suggested so the reaction pathway observed in the
liquid phase would be approached: first, increasing the pressure by, for example, pulsing
an inert gas, to quench excess internal energy by collisions; second, by coordination of a
weakly bound molecule (for example, a solvent) by the coordinatively unsaturated
36
metallocene cation. The loss of a solvent molecule will result in excess energy being
taken away as translational energy of a departing moiety; or third, by increasing the size
and complexity of coordinated ligands, creating an “energy sink” where the excess
energy can be efficiently redistributed among a multitude of available vibrational states,
thus decreasing the probability of allyl complex formation.
Figure 3.3. Comparison of qualitative potential energy surfaces for the reaction of Cp2ZrCH3
+ with ethylene in solution and the gas phase (adapted from Ref. [147]).
37
The first approach was implemented by Plattner et al. [152]. Stable solutions of
the tetrakis(pentafluorophenyl)borate salt of the methylzirconocene cation, Cp2ZrCH3+, in
a mixture of acetonitrile and CH2Cl2 were electrosprayed with a "high" pressure, 10
mTorr, of thermalization gas (Ar). This led to insertion of up to four molecules of alkene
before deactivation. The observed rate constant of propylene polymerization under these
conditions was estimated to be about 6 orders of magnitude higher than that in solution
[152, 153], which is an excellent illustration of intrinsic high reactivity of an isolated
cation in the absence of a coordinating counterion or coordination by solvent molecules.
The loss of hydrogen was more difficult to suppress –reaction with ethylene still led to
allyl complex formation.
Besides reactions of metallocenes with unsaturated hydrocarbons, previous gas-
phase studies have involved investigation of the reactivity patterns of bis(η5-
cyclopentadienyl)methylzirconium cations with nitriles [154] and effects of ancillary
ligands on LL’ZrCH3+ cation reactivity with dihydrogen and ethylene [150] and other
alkenes [151]. Also, the intrinsic reactivity of hydroxyzirconocenes was studied as a
model for metal-hydroxide complex ion reactions with a range of compounds [155].
Our experimental data (Figure 3.4) indicate the ability of C-H bond activation by
the Cp2ZrCH3+ complex in cyclic alkanes with consecutive formation of a complex with
cyclic allyl structure. In the reaction with cyclopentane, a complex with m/z 287 was
observed. The experimental procedure and appearance of mass spectra of zirconocene
complexes are discussed in detail in Chapter 4. The ion with m/z 287 was presumably
formed via loss of dihydrogen and methane (Scheme 3.3). The proposed structure of the
complex is shown on Figure 3.5.
38
Figure 3.4. Mass spectrum for the reaction of Cp2ZrCH3+ with cyclopentane.
Cp2ZrCH3+ + C5H10 ⎯→ Cp2ZrC5H7
+ + CH4 + H2 Scheme 3.3
Formation of such a complex assumes initial activation of a C-H bond by a metal center.
Figure 3.5. Proposed structure of the complex with m/z 287.
3.4.2 Allyl Complex Formation: Mechanistic Differences of Reaction Patterns in the Gas and Liquid Phases
One of the main drawbacks of metallocene catalysts is a gradual decrease in the
rate of monomer uptake with time, which occurs on the timescale of several hours, or, in
some cases, even minutes [156, 157]. The exact nature of this phenomenon is still
debatable; to a certain degree, the deactivation results from factors such as presence of
impurities and high-temperature decomposition. Recently, it has been postulated that a
m /z290280270260250240230220
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90
80
70
60
50
40
30
20
10
39
possible cause may be accumulation of a less active or inactive zirconium-allyl complex
[72, 151, 152, 158-162]. The formation of this complex was observed or inferred on
several occasions in both (as discussed above) gas-phase experiments [148, 154] and in
the solution phase [72, 151, 152, 158-161, 163-165]. Formation of H2 gas during the
polymerization reactions catalyzed by zirconocenes is a possible indication of the allyl
complex formation [166].
Various theoretical studies of the bis(η5-cyclopentadienyl)methylzirconium cation
reactivity toward dihydrogen and alkenes [167-171] or even alkanes [172, 173] have been
reported. However, the subsequent transformation of the allyl complex after its formation
in the reaction mixture is not completely clear. Initially, it was presumed that this
complex is catalytically inactive [72]. As shown by Ziegler and co-workers [174, 175],
the allyl complex can be reactivated by alkene molecule insertion; their theoretical
studies of zirconocene and titanocene cations indicate that the limiting step in the reaction
of the allyl complex formation reaction is a β-hydrogen elimination and that it has an
activation barrier of ~ 11 kcal/mol for the zirconium and ~ 13 kcal/mol for the titanium
complex. The liquid-phase studies by Britzinger and co-workers of the
Cp2ZrMe(methallyl) complex [162] showed that its perfluorotriphenylborane adduct, the
contact ion pair [Cp2Zr(methallyl)]+[MeB(C6F5)3]-, reacts with propene with rates
substantially lower than those of cationic Zr-alkyl species. The propene molecule inserts
between Zr and one of the allylic termini. Their DFT calculations have indicated that
insertion of propene directly into an η3-coordinated Zr-allyl unit occurs with lower
activation energy than insertion into an η1-bound Zr-allyl species and that the lowest-
40
energy pathway for the reactivation of a cationic zirconocene allyl species is its
reconversion to the corresponding Zr-alkyl species by H2.
Nevertheless, experimental proof of the allyl complex reactivation has yet to be
demonstrated. It is expected that formation of the transition state for alkene insertion by
the allyl complex and its consecutive transformation into a vinyl-terminated product can
be controlled by various means. It also appears to be necessary to test the intrinsic
reactivity of the Cp2Zr(allyl)+ complex by studying its reactivity under the gas phase low-
pressure conditions of an FTICR MS spectrometer, thus eliminating various perplexing
factors such as effects of impurities, electric permeability of the solvent, ligands
exchange, etc.
41
CHAPTER 4 GAS-PHASE REACTIONS OF BIS(η5-CYCLOPENTADIENYL)
METHYLZIRCONIUM CATIONS WITH KETONES AND ALDEHYDES
4.1 Rationale
Despite rapid growth in the gas-phase chemistry of organometallic complexes
and, especially, metal ions [134, 135, 176, 177], and the major importance of metallocene
catalysis, gas-phase studies of metallocenes are still rather scarce. The purpose of the
work reported in this chapter was to gain a deeper understanding of this chemistry, with a
focus on reaction mechanisms of the gas-phase chemistry of metallocenes, and to attempt
to draw general conclusions about these processes. For example, based on the work
reported in Chapter 3, it may be postulated that the formation of an η3-allyl complex is an
instance of a general process which takes place under high-vacuum, non-equilibrium
conditions.
Scheme 4.1 shows for the reaction mechanism suggested in earlier studies [151]
for the reactions of methyl zirconocenes with terminal olefins, Y = CH2. For reactions
with olefins such as isobutene or styrene, this scheme can be applied for the Y=C(R1)R2
substrate (R1 = H or R). In the case of 1,1 disubstituted alkenes, elimination of methane
was observed [147]. At the same time, in the liquid phase, the η2 alkene insertion
products of zirconocenes are generally stable and have no tendency to eliminate a neutral
molecule.
42
Scheme 4.1
In the case of aldehydes and ketones, the group Y is the carbonyl oxygen.
Compared to alkenes, the carbonyls are expected to be coordinated more strongly to
Zr(IV) by donation of an electron lone pair from oxygen (as opposed to π-bonding
electrons in the alkenes) to an empty d-orbital of the metal. Therefore, initial bonding of
the substrate with the metal is expected to occur via the substrate’s carbonyl oxygen. In
the gas phase, under high-vacuum, high-energy collision conditions, the reactivity
43
observed might possibly also be described by the general Scheme 4.1. This would be
quite different from the known chemistry in the solution phase, as discussed in the last
chapter. For example, for silver-mediated (i.e. chloride abstraction from Schwarz reagent
by a catalytic amount of AgClO4 [54]) aldehyde addition reactions of Cp2ZrRCl
complexes, a mechanism involving generation of alkyl/alkenyl zirconocene was proposed
[54]. The resultant reaction products are alcohols formed through condensation of the
aldehyde and R ligand of zirconocene.
The chemistry of zirconocene carbonyl, acyl and related species presents interest
due to, for example, the potential of complexes like [LnM{CHClCH2C(=O)R]+ to carry
out insertion polymerization/copolymerization of vinyl chloride, thus enabling better
control of PVC polymer structure and properties than is possible by conventional radical
methods [178].
The work presented in this chapter represents a study of the reactivity pathways of
bis(η5-cyclopentadienyl)methylzirconium (1), chosen as a model compound, with a series
of simple ketones and aldehydes in the gas phase.
4.2 Experimental Procedures
Ion-molecule reactions were studied using Fourier transform ion cyclotron
resonance mass spectrometry. The primary mass spectrometer used in the experiments
was based on a 2.0 Tesla magnet with an internal electron ionization (EI) ion generation
source (Fig. 4.1).
A common experiment sequence was used:
1) Quench: application of +/- 9.5 V to trapping plates in order to eliminate any
ions present prior to the experiment.
44
Figure 4.1. Schematic diagram of the mass spectrometer used in this work.
2) Electron beam: irradiation of the neutral substrates with the flux of electrons
produced by the filament of the EI (Electron Ionization) source (Fig. 4.2). The number of
electrons was controlled by the filament current, typically in the range of 1-2 A. The
electron energy distribution, controlled by the voltages on the repeller plate and the grid,
was varied from 15 to 50 eV in order to produce an optimal number of ions.
Figure 4.2. Schematic diagram of the electron ionization source.
3) Ion ejection (optional): if necessary, the ejection of ions with selected m/z from
the MS cell by applying SWIFT waveform [179] could be carried out.
45
4) Reaction delay: time delay varying from 0 to10 s allowing unejected ions to
react with the neutral(s) present in the cell.
5) Excitation: excitation of the reaction product ions.
6) Detection: detection of the image current with subsequent Fourier transform of
the transient to obtain the mass spectrum.
The above sequence of events (scan) was repeated for each experiment 20 to 40 or
more times in order to achieve a higher signal to noise ratio by a signal averaging.
Table 4.1. Isotopes of zirconium. Isotope Natural Abundance, %
90Zr 51.45
91Zr 11.22
92Zr 17.15
93Zr Radioactive
94Zr 17.38
95Zr Radioactive
96Zr 2.80
Zirconium has five stable isotopes (Table 4.1), which in superposition with 13C/12C
isotopes result in a unique pattern of seven peaks in mass spectra for all of the
organozirconium compounds studied in this work, enabling unambiguous identification
of the presence of Zr. Electron ionization of Cp2Zr(CH3)2 resulted in the formation of two
ions: Cp2ZrCH3+ and Cp2Zr+. The typically obtained mass spectra of Cp2Zr(CH3)2 are
shown in Fig. 4.3
46
m/z240235230225220
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90
80
70
60
50
40
30
20
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a) No reaction delay
m/z240235230225220
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90
80
70
60
50
40
30
20
10
b) Reaction delay 0.5 s.
Figure 4.3. EI mass spectrum of Cp2Zr(CH3)2: Cp2ZrCH3+ (m/z 235-242), Cp2Zr+(m/z
220-227) and Cp2ZrOH+ (m/z 237-244)
Since the Cp2Zr+ ion and its reaction products complicate the spectrum, in order to
distinguish products from these two possible parents, additional studies using selective
SWIFT ejection were implemented when necessary. An example of a SWIFT experiment
is shown in Figure 3.4. When one possible parent ion Cp2ZrMe+ (m/z 235-242) is ejected,
47
the reaction products’ peaks (m/z 277-284) disappear. On the other hand, when another
possible parent Cp2Zr+ (m/z 220-227) is ejected, no change is observed. This indicates
that the product is formed from the Cp2ZrMe+ ion. An incomplete removal of either
complex by SWIFT, especially of the Cp2ZrMe+ ion, stems from a partial re-formation
during a reaction delay event via collisions of neutral Cp2Zr(CH3)2, which is present in
the cell at all times, with Cp2Zr+ ions.
a) No ions ejected
b) Cp2Zr+ (m/z 220-227) is ejected m /z
290280270260250240230220210
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50
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30
20
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m /z29 02 8027 02 602502 4023022 021020 0
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9 0
8 0
7 0
6 0
5 0
4 0
3 0
2 0
1 0
48
m /z3 0 02 9 028 02 7 02 6 025 02 4 02 3 022 02 1 0
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1 0 0
9 0
8 0
7 0
6 0
5 0
4 0
3 0
2 0
1 0
c) Cp2ZrMe+ (m/z 235-242) is ejected Figure 4.4. Example of a SWIFT experiment.
The typical background pressure on the 2.0 T instrument was ~ 2·10-9 Torr;
pressure of zirconocene was ~ 10-7 Torr, decreasing throughout experiment at the rate of
approximately 10-8 Torr/hour due to the sample evaporation. A set of experiments at
different substrate pressures of 0.5·10-7, 1·10-7, 2·10-7 and 4·10-7 Torr and common
reaction times of ~ 0, 0.5, 1, 2, 5 s and longer were carried out for each reaction.
The formation of binuclear metal ions Cp4Zr2(CH3)n+ (n = 0 - 3) in reactions of
both parent ions with the neutral complicated the spectrum, especially at longer reaction
times (Fig. 4.5), which impeded the ability to study reactions with rate coefficients below
~ 109 1/M·s.
m/z500490480470460450440
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4
3
2
1
Figure 4.5. Binuclear complexes Cp4Zr2(CH3)n+ (n = 0 - 3)
49
The presence of water also decreased the amount of the ion of interest as it reacted very
rapidly to form a hydroxyl complex (m/z 237-244) (see Fig. 4.3):
Cp2ZrCH3+ + H2O → Cp2ZrOH+ + CH4
Therefore, precautions had to be taken to minimize the extent of hydrolysis. At
prolonged reaction times the Cp2ZrOH+ ion and its reaction products dominated the
spectrum. In some cases hydrolysis interfered with the identification of reaction products
even at typical reaction times of 1-5 seconds.
Cp2Zr(CH3)2 and all of the aldehydes, ketones, and imines used in these studies
were purchased from commercial sources. The liquids were purified by repeated freeze-
pump-thaw cycles. Cp2Zr(CD3)2 was synthesized from commercially purchased
precursors in accordance with a preparation procedure described in the literature [180]
via the following reaction:
Cp2ZrCl2 + 2CD3MgI → Cp2Zr(CD3)2 + 2MgICl
A Schlenk line was used at all times due to the extreme air and moisture sensitivity of
both the reactants and the product. The reaction was carried out in diethyl ether solvent
at room temperature under a flow of dry nitrogen in thoroughly desiccated glassware. The
reaction product’s precipitate was dissolved in ether/pentane mixture for the final product
extraction followed by a recrystallization from benzene at -20 °C. The mass spectra of the
Cp2Zr(CD3)2 obtained from this procedure are shown in Fig. 4.6. As expected, the
Cp2ZrCH3+ (m/z 235) complex is replaced with the Cp2ZrCD3
+ (m/z 238) complex, while
the hydrolysis product, Cp2ZrOH+ (m/z 237), is unaltered.
50
m/z230225220
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90
80
70
60
50
40
30
20
10
a) No reaction delay
m/z230225220
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90
80
70
60
50
40
30
20
10
b) Reaction delay 0.5 s.
Figure 4.6. EI mass spectrum of Cp2Zr(CD3)2 (* N
Ab initio (Hartree-Fock) as well as Density
were carried out by Cesar S. Contreras for the zirco
most stable structures.
*245240235
245240235
oise peaks).
Functional Theory (DFT) calculations
nocene complexes to establish the
51
4.3 Reaction with Ketones
For the sake of simplicity, all of the seven Zr-containing isotopic ions are further
reported as a single mass corresponding to the most abundant (51.45%) 90Zr isotope. In
cases where there is an overlap of several series of peaks corresponding to different
products, a detailed analysis of the contributions from individual ions has been carried
out. Since the isotopic abundances of the constituent elements are known, and at least one
peak containing the lightest 90Zr isotope and thus of lowest m/z does not overlap with
other peaks, the heights of individual series of peaks can be estimated.
In previous studies under similar conditions it has been demonstrated that the
cyclopentadienyls are solely spectator ligands – as no Cp “switching” reactions and HD
elimination for the complexes with fully deuterated ligands but non-deuterated Cp rings
have been observed [147]. Therefore, the Cp2Zr unit was presumed to remain intact
throughout all of the experiments. Where questions exist about the structures of the
product ions, their formulae are reported in the form Cp2ZrCxHyOz+ with the
corresponding m/z ratio. Since all of the reported ions were singly charged the term
“empirical formula” is used throughout the text. Detailed structural considerations are
presented later in this chapter.
4.3.1 Reactions of Cp2ZrCH3+/Cp2ZrCD3
+ with Acetone/d6-Acetone
The reaction of 1 (m/z 235) with acetone (nominal molecular weight 58 amu)
proceeds with formation of several products (Fig. 4.7). Two principal ones with m/z 293
and 351 correspond to the consecutive addition of one and two substrate molecules, as
shown in Scheme 4.2. Addition of the second acetone molecule becomes pronounced
only after 4 seconds of reaction time (at a total pressure of approximately 2·10-7 torr). A
variety of structures can be postulated for the observed adducts, e.g. η1 vs. η2 complexes,
52
etc., (only one of the likely structures is shown), and, therefore, theoretical geometry
optimizations and/or spectroscopic data are needed for more definitive “proof” of
structure.
a) 1 second reaction time
b) 5 seconds reaction time Figure 4.7. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone.
An ion with m/z 277 (Fig.4.7; the peak with m/z 278 corresponds to the product
depicted in Scheme 4.2), corresponding to the empirical formula Cp2ZrC3H5O+,
apparently resulting from elimination of methane from the ion with m/z 293, is formed to
a lesser extent than the latter, and its peak height decreases at longer reaction times. It is
coordinatively unsaturated and adds an additional acetone molecule, forming an ion with
m/z 335 (Scheme 4.3, Figure 4.8).
m/z350300250200
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80
70
60
50
40
30
20
10
m/z 277
m/z350300250200
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100908070605040302010
m/z 293
m/z 351
53
Scheme 4.2
Scheme 4.3
54
Figure 4.8. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone
The hydrolysis product of 1 (m/z 235), bis(cyclopentadienyl)zirconium hydroxyl
cation Cp2ZrOH+ (m/z 237, Fig. 4.3), formed by reaction with trace amounts of water
present in the mass spectrometer, reacts with acetone to give an addition product with
m/z 295, which becomes predominant at very long reaction times (5 seconds or more).
This indicates a slower reaction rate and, for these species reacting with acetone, is
consistent with the demonstrated intrinsic lower reactivity of the hydroxide derivative
compared to the methylzirconium cation itself [155]. The formation of binuclear metal
ions Cp4Zr2(CH3)n+ (n = 0 - 3) and their derivatives complicated the spectrum, especially
at longer reaction times and had to be taken into account as well.
Reaction of 1 with acetone proceeds differently for different stages of
thermalization of the reaction mixture. The consecutive addition of two acetone
molecules by the Cp2ZrCH3+ ion results in a coordinatively saturated 18 electron
complex. The loss of methane and the subsequent generation of the complex with m/z
277 results from the reaction of 1 ions with sufficient internal energy (Scheme 4.4):
m/z 280
15
10
5
m/z345340335
2
1
55
Cp2ZrCH3+ + O=C(CH3)2 → Cp2ZrC4H9O+ → Cp2ZrC3H5O+ + CH4
m/z 235 m/z 58 m/z 293 m/z 277 Scheme 4.4
Consideration of the gas-phase chemistry of bis(η5-cyclopentadienyl)
methylzirconium cations described in the literature suggests that the species with m/z 277
have structures similar to those postulated for the gas-phase reaction of 1 with
unsaturated hydrocarbons, i.e. η3-allyl complexes [147, 154]. The possibility of η3-allyl
complex formation is well documented [72, 151, 152, 158-161, 163, 164]. Density
functional theory studies also confirm the plausibility of allyl intermediate formation as a
possible side reaction in homogeneous single-site olefin polymerization [174, 175].
However, there is no direct evidence supporting η3-allyl structures postulated for the
reaction of 1 with acetone. Also, it is difficult to suggest tenable reaction mechanisms
which would lead to the formation of such complex(es).
Our DFT calculations, carried out by Cesar S. Contreras, suggest that the most
stable structure is not an η3-allyl complex, but rather that of the complex shown in Fig.
4.9, which is an η3-enolate complex. The higher stability of this complex can be
understood: the η3-enolate complex is isoelectronic with the η3-allyl complex and both
the allyl and enolate complexes in either resonance form participate in both σ and dative
bonding of the ligand to a metal center, thus contributing to high stability of the complex.
The ability of the oxygen atom in the η3-enolate complex (even though it is weaker σ-
donor compared to the methylene group) in the oxygen σ-bonded resonance form to
provide additional bonding by partially transferring its available lone pair into empty Zr
2d orbital results in increased total bond strength. In another resonance form, dative
bonding from the oxygen atom is substantially more efficient than π-complexation of the
56
C=C bond by a metal center. According to the Dewar-Chatt-Duncanson model discussed
above [26, 27], an efficient π-bonding occurs when a σ-type donation from the C=C π
orbital is synergistic with concomitant π-backbonding into an empty π* orbital of the
alkene which in turn implies that only metals with at least one filled nonbonding orbital
can efficiently form such complexes. For the d0 zirconocene, only donation of electrons is
possible from the C=C bond, making the π-complexation rather inefficient.
a) b)
Figure 4.9. The a) η3-enolate and b) η3-allyl complexes.
In the reactions of Cp2ZrMe+ with d6-acetone, normal addition products of d6-
acetone (nominal molecular weight 64 amu) to 1 are expected to have m/z 299 and m/z
363, 6 and 12 mass units, respectively, higher than for the products seen with the
nondeuterated substrate. Both of these ions were observed, and their intensities obeyed
the same trend as in case of nondeuterated acetone: only one molecule was coordinated
initially and the product with two acetone molecules became the major peak after few
57
seconds reaction time. In addition, formation of a product ion with m/z 302 was
observed, which became predominant after 5 s. The 3 mass unit gain can be assigned to
the following product (Scheme 4.5):
Scheme 4.5
This ion results, apparently, from CH3/CD3 scrambling between the metal center and
carbonyl substrate.
Three ions with m/z 279, m/z 280 (major product), and m/z 282, corresponding to
the empirical formula Cp2ZrC3H5-nDnO+, n = 2, 3 and 5 (approximate peak height ratios ~
2:5:1), replace Cp2ZrC3H5O+ (m/z 277), the ion reported above in reactions of 1 with
acetone (postulated to be an η3-enolate complex).
The observed CH3/CD3 scrambling and lack of H/D scrambling suggest reversible
transfer of the methyl group between the Zr and a coordinated acetone molecule as a unit,
rather than transfer of individual H atoms. This in turn implies that in the complex with
m/z 280 the CD3 group remains intact (Scheme 4.6):
Scheme 4.6
58
The transfer of the deuterated methyl group introduces only a negligible
secondary isotope effect, so the migratory insertion-deinsertion equilibrium remains
unaffected for all of the studied combinations of zirconocene/acetone: in the
approximation of a ∆E arising only from a change in zero-point energy of the stretching
vibration of the Zr-C bond the predicted isotope effect is:
kCD3/kCH3 = e-λ, where ⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧−= µµνλ
3
3
~
12
CD
CH
kThc
[181]
Assuming ~ν (Zr-C) ~ 500 cm-1, the calculated value for λ is ~ 1⋅10-3, thus kCD3/kCH3 ≈ 1.
Cp2ZrCD3+, (2), reacted with acetone similarly to 1, with a corresponding shift of
all the peaks by 3 mass units. However, two ions (m/z 277 and m/z 280, corresponding to
the empirical formulae Cp2ZrC3H5O+ and Cp2ZrC3H2D3O+)) with comparable peak
heights, and a small amount of an ion with m/z 279 were also formed.
Similarly, the reaction of 2 with d6-acetone resulted in formation of ions with m/z
302 and 366 (addition of one and two molecules of d6-acetone respectively) and an ion
with m/z 282, corresponding to the empirical formula Cp2ZrC3D5O+ (Scheme 4.7).
Cp2ZrCD3+ + O=C(CD3)2 → Cp2ZrC4D9O+ → Cp2ZrC3D5O+ + CD4
m/z 238 m/z 64 m/z 302 m/z 282
Scheme 4.7
This complex is analogous to the product with m/z 277 (η3-enolate complex), but
fully deuterated (except for the Cp rings). Formation of the complex with m/z 282
proceeded with a deuterium kinetic isotope effect as the apparent rate of formation of the
corresponding complex (m/z 277) in the reaction of 1 and acetone was approximately
twice as high. The primary isotope effect indicates C-D bond cleavage in the rate-
59
determining step in the reaction sequence leading to the formation of the enolate
complex, since, in this case, there are no C-H bonds available in the reacting ligands. All
other observed reactivity trends were similar to those described above for the reactions of
1: increasing reaction time led to increasing addition of a second acetone molecule to the
zirconocene complex, disappearance of the m/z 282 complex along with concomitant
hydration and hydrolysis reactions due to inevitable traces of water, prevalent at very
long reaction times (5 seconds and above).
The proposed reaction mechanism for the reaction of 1 with d6-acetone as an
example is shown in Schemes 4.8 a) and b). The second step, the migratory insertion of a
coordinated ketone molecule, is reversible and leads to CH3/CD3 scrambling. This
equilibrium is fast and proceeds via the intermediate insertion product where all three
substituents at the oxygen-bound carbon are equivalent, thus leading to a statistical
distribution of methyl and methyl-d3 groups between the metal and coordinated ketone
(Scheme 4.8a).
When a ketone molecule is coordinated by the metal center, the Brønsted-Lowry
acid-base reaction can take place. The metal-bound alkyl group is a good Brønsted base,
the α-proton of a ketone is slightly acidic (and significantly more acidic in the ketone
molecule coordinated by a metal) and the nascent enolate is expected to be a reasonably
good leaving group. The combination of these factors enables nucleophilic attack of the
alkyl on the α-proton of a ketone. This reaction is assumed to be the slowest step and to
proceed via the least strained 6-membered cyclic transition state. If two different groups
are available at the carbonyl group, two different products can be formed, which, in
combination with products formed in the reaction of either the CH3- or CD3- nucleophile,
60
will comprise the observed set of products (Scheme 4.8b). The products resulting from
reaction involving nucleophilic attack on a deuteron are expected to be less abundant due
to the primary isotope effect resulting from C-D bond cleavage.
Scheme 4.8a
The expected product abundance ratios are completely consistent with those
observed in the experiment.
61
Scheme 4.8b
4.3.2. The Reactions of Cp2ZrCH3+/Cp2ZrCD3
+ with 2-Butanone, Methyl Isobutyl Ketone and Cyclohexanone.
The product distributions in reactions of 1 and 2 with ketones other than acetone
are also entirely consistent with the proposed reaction mechanism. Both 1 and 2
sequentially add two substrate molecules (Scheme 4.9 illustrates the case for methyl
isobutyl ketones as a substrate, examples of spectra are shown in Fig. 4.10).
62
a) 1 second reaction time
b) 5 seconds reaction time
Figure 4.10. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with methyl isobutyl ketone.
m /z400350300250
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50
40
30
20
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m/z 335 m/z 277
m/z 319
m/z 435
m/z400350300250200
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5
m/z 277
m/z 335
m/z 319
m/z 235
m/z 237m/z 220
63
Scheme 4.9
The major products observed are summarized in Table 4.2
Table 4.2. Major reaction products observed in the reactions of 1 and 2 with 2-butanone, and methyl isobutyl ketone at various reaction times (1 to 5 seconds).
Observed Reaction Products Reaction
m/z Suggested Empirical Formula Neutral
eliminated
307 [Cp2ZrCH3 · O=C(CH3)C2H5]+
379 [Cp2ZrCH3 · 2O=C(CH3)C2H5]+
277 Cp2ZrC3H5O+ C2H6
1 (m/z 235) + 2-
butanone (nominal
molecular weight 72
amu) 291 Cp2ZrC4H7O+ CH4
64
Table 4.2 Continued Observed Reaction Products Reaction
m/z Suggested Empirical Formula Neutral
eliminated
310 [Cp2ZrCD3 · O=C(CH3)C2H5]+
382 [Cp2ZrCD3 · 2O=C(CH3)C2H5]+
279 Cp2ZrC3H3D2O+ C2H5D
280 Cp2ZrC3H2D3O+ C2H6
291 Cp2ZrC4H7O+ CHD3
293 Cp2ZrC4H5D2O+ CH3D
2 (m/z 238) + 2-
butanone (nominal
molecular weight 72
amu)
294 Cp2ZrC4H4D3O+ CH4
335 [Cp2ZrCH3 · O=C(CH3)i-C4H9]+
435 [Cp2ZrCH3 · 2O=C(CH3)i-C4H9]+
277 Cp2ZrC3H5O+ C4H10
319 Cp2ZrC6H11O+ CH4
1 (m/z 235) + methyl
isobutyl ketone
(nominal molecular
weight 100 amu)
377 [Cp2ZrC3H5O+ · O=C(CH3)i-C4H9]+
338 [Cp2ZrCD3 · O=C(CH3)i-C4H9]+
438 [Cp2ZrCD3 · 2O=C(CH3)i-C4H9]+
279 Cp2ZrC3H3D2O+ C4H9D
2 (m/z 238) + methyl
isobutyl ketone
(nominal molecular
weight 100 amu) 280 Cp2ZrC3H2D3O+ C4H10
65
Table 4.2 Continued Observed Reaction Products Reaction
m/z Suggested Empirical Formula Neutral
eliminated
319 Cp2ZrC6H11O+ CHD3
321 Cp2ZrC6H9D2O+ CH3D
2 (m/z 238) + methyl
isobutyl ketone
(nominal molecular
weight 100 amu)
322 Cp2ZrC6H8D3O+ CH4
333 [Cp2ZrCH3 · O=C(CH2)5]+
431 [Cp2ZrCH3 · 2O=C(CH2)5]+
1 (m/z 235) +
cyclohexanone
(nominal molecular
weight 98 amu)
317 Cp2ZrC6H9O+ CH4
336 [Cp2ZrCD3 · O=C(CH2)5]+
434 [Cp2ZrCD3 · 2O=C(CH2)5]+
2 (m/z 235) +
cyclohexanone
(nominal molecular
weight 98 amu)
317 Cp2ZrC6H9O+ CHD3
In the reaction of 1 with 2-butanone, a complex with m/z 277, (most likely
resulting from the loss of ethane from the single adduct ion with m/z 307), was produced
to a lesser extent than in the reactions with acetone. An ion with m/z 291 (most likely
produced by methane loss from the m/z 307 ion) was nearly as abundant as the m/z 277
ion. The intensities of both peaks appeared to change with time in a similar fashion.
Substantial fragmentation of the coordinated ligands resulted in multiple product peaks,
increasing with reaction time. Scheme 4.10 shows as an example possible origination of
the ion with m/z 293 in the reaction of 1 with 2-butanone:
66
Scheme 4.10
The reaction of 1 with methyl isobutyl ketone also produced a m/z 277 complex
(presumably resulting from C4H10 loss by the m/z 335 ion), which was the major reaction
product for reaction times less than one second. The complex with m/z 277 was nearly
three times more abundant than the complex with m/z 319 (presumably resulting from the
loss of methane by the m/z 335 ion). For both ketones, variations of peak heights with
reaction time followed the same trends as for complexes with m/z 277, 291 and 319.
As in the reaction with 2-butanone, apparent ligand fragmentation was also
observed. The loss of a zirconocene methyl group (and probable elimination of a neutral
alkene), possibly in a multiple-step process, was commonly observed (Scheme 4.11).
Scheme 4.11
The coordinatively unsaturated complex with m/z 293 reacted further by addition
of a methyl isobutyl ketone molecule, resulting in a product with m/z 399. The reactions
of 2 with 2-butanone and methyl isobutyl ketone produced fragmentation patterns that
were remarkably similar to those produced in reaction of 1. A series of peaks with same
67
m/z were observed, due to the common loss of the deuterated/nondeuterated methyl
group of the metallocene along with the fragmentation.
Scheme 4.12
The complex with m/z 277 is presumed to be the same reaction product for both of
these ketones and acetone, i.e. the η3-enolate complex. The complexes with m/z 291 and
m/z 319 were formed in comparable amounts with m/z 277, and, according to the
reaction mechanism proposed above, differ from the η3-enolate complex with m/z 277
Cp2ZrCD3+ + O=C(CH3)C2H5
m/z 238 m/z 72 m/z 310
[Cp2ZrCH3 • O=C(CD3)C2H5]+
Cp2ZrOC(C2H5)=CD2+ + CH3D
[Cp2ZrCD3 • O=C(CH3)C2H5 ]+
[Cp2ZrC2H5 • O=C(CH3)CD3] +
[Cp2ZrCH3 • O=C(CD3)C2H5]+
Cp2ZrC5H8D3O+
Cp2ZrOC(CD3)=CHCH3+ + CH4
m/z 293
m/z 294
Cp2ZrOC(C2H5)=CH2+ + CHD3
[Cp2ZrCD3 • O=C(CH3)C2H5 ]+
Cp2ZrOC(CH3)=CHCH3+ + CHD3
m/z 291
m/z 291
Cp2ZrOC(CD3)=CH2+ + C2H6
[Cp2ZrC2H5 • O=C(CH3)CD3] +
Cp2ZrOC(CH3)=CD2+ + C2H6
m/z 280
m/z 279
68
(carrying the methyl group) only by substitution of the corresponding alkyl (Et, iBu,) at
the central carbon, or both carbons (Me, Me and Me, iPr) of the enolate ligand. Thus, the
ketones other than acetone, R1-C(=O)-R2 (R1 = Me, R2 = Et, iBu) react with
Cp2ZrCH3+/Cp2ZrCD3
+ in a similar manner, but two different substituents at the carbonyl
carbon enable additional possible reaction pathway(s), with the loss of one of the alkyls
as the corresponding alkane, as shown in Scheme 4.12 for the reaction of 2 with 2-
butanone.
The expected product abundances are quite consistent with the experimental
values, except for higher abundance of the complex with m/z 277 in the reactions of 1
and 2 with methyl isobutyl ketone, which was almost three times more abundant than
would be expected from a purely statistical distribution of alkyl groups and an assumed
isotope effect kH/kD ~ 2. This possibly results from a combination of two factors: higher
nucleophilicity of the isopropyl as compared to the methyl group and release of steric
tension when the relatively bulky iBu group is removed from the metal coordination
sphere.
Scheme 4.13
69
Scheme 4.14
Cyclohexanone (nominal molecular weight 98 amu) reacted with 1 forming
simple addition products of m/z 333 and 431 only, with no other complexes and just two
products: the loss of methane leads to formation of complexes with m/z 317 and 415
respectively. The same enolate product (m/z 317) is formed in the reaction of 2 with
cyclohexanone. This is additional evidence for the proposed reaction mechanism, as two
70
groups at the ketone’s carbonyl are locked into the ring and unable to migrate so that CD3
group scrambling is impossible, and the deuterated group is eliminated as methane
resulting in the same complex as for the reaction of 1 (Scheme 4.13).
The fragmentation reactions which do not conform to the stoichiometry of the
expected products of reactions fitting the pattern of Scheme 4.8 for the reactions of both 1
and 2 with all of the ketones studied result in minor amounts of products and were
pronounced only for the more branched methyl isobutyl ketone and very long reaction
times. The observed fragmentation product peaks most likely result from ions with
different isomeric structures with the same m/z. One of the possible pathways is
shown in Scheme 4.14 for the reaction of 1 with 2-butanone. The formation of a
carbocation via C-H bond scission enables further rearrangements[182] and at some point
is followed by elimination of neutral(s) leading to the variety of fragmentation products.
4.3.3 Reactions with Other Ketones
In the reactions of Cp2ZrCH3+ with 1,4-benzoquinone and 1,4-naphthoquinone, no
interesting reactivity was observed (except for very small amounts of the complex with
m/z 277 formed in reaction with 1,4-benzoquinone). However, both substrates exhibited
an unusual tendency to coordinate readily with the binuclear metal complexes (Fig. 4.11)
In the reaction of Cp2ZrCH3+ with diethyl carbonate, the peaks corresponding to
the complex with m/z 471 (addition of two substrate molecules) overlap with the region
of the binuclear metal ions Cp4Zr2(CH3)n+ (n = 0-3), which severely impairs quantitative
analysis. The observed peaks are summarized in the Table 4.3
71
Figure 4.11. Ions produced in the reaction of Cp2ZrMe+ with benzoquinone: products of coordination of quinone molecule by the binuclear complexes Cp4Zr2(CH3)n
+ (n = 0 - 3)
Table 4.3 Major reaction products observed in the reactions of 1 and 2 with diethyl carbonate at various reaction times (1 to 5 seconds).
Reaction m/z Empirical Formula
353 [Cp2ZrCH3 · O=C(OC2H5)2]+
471 [Cp2ZrCH3 · 2O=C(OC2H5)2]+
1 + diethyl carbonate
(nominal molecular weight
118 amu) 277 Cp2ZrC3H5O+
356 [Cp2ZrCD3 · O=C(OC2H5)2]+
474 [Cp2ZrCD3 · 2O=C(OC2H5)2]+
2 + diethyl carbonate
(nominal molecular weight
118 amu) 279/280 [Cp2ZrC3H3D2O+/ Cp2ZrC3H2D3O]+
The complex with m/z 277 was formed in smaller amounts than in the reaction
with acetone. This complex was still abundant enough for the addition product (m/z 395)
to be observed. Several ligand fragmentation products, always accompanied by the loss
of a methyl (or methyl-d3 in the reactions of 2) group were observed; a few of the
possible resulting structures of some of the observed ions are shown in Scheme 4.15:
m/z550500
Abu
ndan
ce
15
10
5
72
Scheme 4.15
In the case of the reaction of 2 with diethyl carbonate, instead of the unsaturated
complex with m/z 277, two series of peaks, m/z 279 and 280, with relative intensities 1:3
were observed. This intensity ratio was independent of time. The total intensity obeyed
the same trends observed before: a maximum intensity is reached at 2 seconds of reaction
time with gradual disappearance at longer reaction times.
4.4 Reaction with Aldehydes
Reaction patterns for 1 and 2 with all of the studied aldehydes were remarkably
different from those observed in the reactions with various ketones: often no simple
substrate molecule addition products were formed at all. Instead, at short reaction times a
single (or major) product, resulting from dihydrogen loss from a postulated short-lived
carbonyl molecule addition product, was obtained in all of the cases (Table 4.4).
In reaction of 1 with acetaldehyde (Fig. 4.12), increasing the reaction time led to
the addition of another substrate molecule by the single reaction product, the postulated
η3-enolate complex (m/z 277), forming an ion with an m/z 321. The combined abundance
of the ions with m/z 277 and 321 decreased significantly at longer reaction times due to
water contamination of the acetaldehyde sample, and formation of the hydration product
of the ion with m/z 277 (m/z 295). The bis(η5-cyclopentadienyl)zirconium hydroxide
cation, Cp2ZrOH+ was also formed in significant amounts, resulting in an eventual
73
intensity shift into peaks belonging to the hydroxyl cation coordinating one substrate
molecule (m/z 281), Scheme 4.16.
Table 4.4. Major reaction products observed in the reactions of 1 and 2 with acetaldehyde, benzaldehyde, propanal and n-hexanal at various reaction times (1 to 5 seconds).
Observed Reaction Products Reaction
m/z Suggested Empirical Formula
277 [Cp2ZrC3H5O+] 1 (m/z 235) + acetaldehyde
(nominal molecular weight
44 amu)
321 [Cp2ZrC3H5O+ · CH3CHO]+
279/280 (ion
abundance ratio
~1/3)
[Cp2ZrC3H3D2O]+/[Cp2ZrC3H2D3O]+ 2 (m/z 238) + acetaldehyde
(nominal molecular weight
72 amu)
323/324 (ion
abundance ratio
~1/3)
[Cp2ZrC3H3D2O · CH3CHO]+/
[Cp2ZrC3H2D3O · CH3CHO] +
1 (m/z 235)+ benzaldehyde
(nominal molecular weight
106 amu)
339 [Cp2ZrC2H2(C6H5)O]+
2 (m/z 238) + benzaldehyde
(nominal molecular weight
106 amu)
341 [Cp2ZrC2D2(C6H5)O]+
74
Table 4.4 Continued Observed Reaction Products Reaction
m/z Suggested Empirical Formula
291 [Cp2ZrC4H7O]+
293 [Cp2ZrCH3 · C2H5CHO]+
1 (m/z 235) + propanal
(nominal molecular weight
58 amu) 349 [Cp2ZrC4H7O · C2H5CHO]+
293/294 (ion
abundance ratio
~1/1)
[Cp2ZrC4H5D2O]+/ [Cp2ZrC4H4D3O]+
295 [Cp2ZrCD3 · C2H5CHO]+
2 (m/z 238) + propanal
(nominal molecular weight
58 amu)
351/352 (ion
abundance ratio
~1/1)
[Cp2ZrC4H5D2O · C2H5CHO]+/
[Cp2ZrC4H4D3O · C2H5CHO]+
333 [Cp2ZrC7H13O]+
335 [Cp2ZrCH3 · C5H11CHO]+
1 (m/z 235) + n-hexanal
(nominal molecular weight
100 amu) 433 [Cp2ZrC7H13O · C5H11CHO]+
335/336 (ion
abundance ratio
~3/2)
[Cp2ZrC7H11D2O]+/ [Cp2ZrC7H10D3O]+
338 [Cp2ZrCD3 · C5H11CHO]+
2 (m/z 238) + n-hexanal
(nominal molecular weight
100 amu)
435/436 (ion
abundance ratio
~3/2)
[Cp2ZrC7H11D2O · C5H11CHO]+/
[Cp2ZrC7H10D3O · C5H11CHO]+
75
Figure 4.12. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetaldehyde, 1 second reaction delay.
Scheme 4.16
The intensity distribution of the two products (m/z 279 and 280) formed in
reaction of Cp2ZrCD3+ with acetaldehyde, was approximately 1:3, nearly independent of
reaction time (Fig. 4.13). Increasing the reaction time resulted in addition of another
acetaldehyde molecule by these complexes (m/z 323/324), with the intensity distribution
remaining unaffected. Interestingly, the same intensity ratios were observed for the water
molecule addition products, (m/z 297/298). This implies that, since these ions have
similar reactivity, both reaction products of 2 with acetaldehyde have similar structures,
m/z400350300250200
Abu
ndan
ce
100
90
80
70
60
50
40
30
20
10
m/z 277
76
i.e. both are η3-enolate complexes, and they differ only by the number of deuterium
atoms (Cp2ZrC3H3D2O+ and Cp2ZrC3H2D3O+ respectively).
a) b)
Figure 4.13. Mass spectra of ions with m/z 279 and 280 formed in the reaction of Cp2ZrCD3
+ with acetaldehyde, at a) 1 second and b) 5 seconds reaction delays.
In the reaction pattern of 1 with benzaldehyde (Fig.4.14), the complex with m/z
339 (Cp2ZrC2H2(C6H5)O+) is coordinatively unsaturated, analogous to the complex with
m/z 277.
Figure 4.14. Ions produced in the reaction of Cp2ZrMe+ with benzaldehyde at 1 second reaction delay.
m/z350300250
Abu
ndan
ce
100
90
80
70
60
50
40
30
20
10
m/z290285280275
Abu
ndan
ce
30
25
20
15
10
5
m/z285280275
Abu
ndan
ce
60
50
40
30
20
10
77
The reaction of 2 with benzaldehyde resulted in formation of the same set of
products as in reaction of 1, with a two mass unit shift corresponding to the two
deuterium atoms in the products, including the hydration product (m/z 359,
Cp2ZrC2H2D2(C6H5)O2+). The product of benzaldehyde molecule addition to Cp2ZrOH+
ion (m/z 343) dominated in the spectrum at long reaction times (>5 s.). In the reaction of
1 with propanal, a few minor products were observed at longer reaction times (unlike in
reaction with acetaldehyde), but the peak with m/z 291 was always the major peak in the
mass spectrum. In the reaction of 1 with n-hexanal, at reaction times 5 seconds and
longer, the direct addition of aldehyde molecule(s) to 1 (m/z 335 and 435) was
noticeable. Increasing the reaction time led to extensive ligand fragmentation resulting in
a multitude of peaks corresponding to the loss of dihydrogen or a variety of alkenes by
various ions. Analogous to the reaction patterns with other aldehydes, the ion with m/z
333 also coordinated a water molecule, forming a hydration product with m/z 351, which
was predominant at reaction times of 5 seconds and longer. The reaction of 2 with n-
hexanal showed the same trend of extensive ligand fragmentation at increasing reaction
times as in reaction of 1 with n-hexanal.
In summary, all three aldehydes reacted with 1 in the same fashion (Scheme 4.17):
Cp2ZrCH3+ + RCHO → Cp2ZrC2H2RO+ + H2
Scheme 4.17
where R = Me, Et, Ph, C5H11. These complexes are presumed to be the postulated η3-
enolate complexes with the corresponding R- substituent, and all of their isomers which
are possible according to the reaction mechanism. Two products were formed in the
78
reaction of 2 with all aldehydes (except for benzaldehyde), apparently differing only by
the number of deuterium atoms in the structure.
The slowest step in the reaction mechanism, as in the case of reactions with
ketones, is presumed to be a nucleophilic attack on the α-hydrogen (deuterium) of the
coordinated carbonyl. The key feature of aldehydes is the presence of a hydrogen atom at
the carbonyl carbon instead of an alkyl group. This leads to the possibility of β-H
elimination provided the reaction proceeds through the same initial equilibrium as that
with ketones, i.e., coordination of a substrate molecule and migratory insertion. This
hydrogen further acts as nucleophile. Since the attack on the hydrogen in the phenyl ring
in the reactions of 1 and 2 with benzaldehyde will lead to the disruption of aromaticity,
only deuteriums of the CD3 bond are available for the reactions, hence only one product
is observed. The proposed reaction mechanism, which demonstrates this for the example
of reaction of 2 with acetaldehyde, is shown in Scheme 4.18. The β-H elimination is
known to be a very facile process [183]. Thus, when in the rate limiting step of
nucleophilic attack on an α-H of the coordinated ketone an alkyl is replaced by hydrogen,
a better nucleophile, while the leaving group remains the same, the reduction of the
activation barrier is significant enough to make this reaction pathway favorable over any
other available for a given system. The 6-member transition state preceding dihydrogen
elimination is similar to that postulated for the reaction with ketones, and is assumed to
be the least strained. The observed kinetic isotope effect kD/kH ≈ 0.3 is consistent with
that typically observed for C-D vs. C-H bond cleavage for relatively linear transition
states [184, 185]. The noticeable effect of reduction in the amount of the complex
carrying 3 deuterium atoms in the series acetaldehyde – propanal – hexanal (resulting
79
from the attack on hydrogen of -CH3, -C2H5 and -C5H9 groups correspondingly) suggests
certain geometrical constrains in the reactions with bulkier radicals – the lower steric
availability of the secondary hydrogen atoms and/or removal from the reactive center of
more accessible primary ones.
A postulated reaction potential surface associated with this mechanism, for the
reaction of 1 with acetaldehyde, is shown in Fig. 4.15a.
a) Gas-phase
80
b) Solution
Figure 4.15. Qualitative potential surface for the reaction of 1 with acetaldehyde/generic aldehyde in the gas phase and in solution. The lettering corresponds to that in Scheme 4.21. Gas-phase estimates of energetic effects are based on data from Ref. [173].
Formation of ions with m/z 357 and 359 in the reactions of 1 and 2 with
benzaldehyde, and similar ions for other aldehydes, is most likely due to inevitable traces
of water in the instrument, leading to hydrolysis of the η3-enolate complex according to
the following proposed scheme (Scheme 4.19). This hydrolysis is the first directly
observed gas-phase reaction of the η3-enolate complex. The reactivity of the isoelectronic
η3-allyl complex has been studied on several occasions in the solution [162] and in the
gas phase [175, 186]. However, no studies have been carried out so far on the gas-phase
reactivity of the η3-enolate complex, leaving this topic open for future investigation.
81
Scheme 4.18a
The liquid-phase studies [187-193] indicate potential significance of such species
in polymerization catalysis: for example, in the investigation of the polymerization of the
methylmethacrylate (MMA) with the neutral enolate Cp2ZrMe[OC(OMe)=CMe2] in the
absence and in the presence of zirconocene cations Cp2ZrMe+ to bypass the rate-limited
initiation of the system, it was shown that by itself the enolate is not active in
polymerization, but as soon as the reaction system contains the cation, MMA is
quantatively converted into syndiotactic-rich PMMA with high molecular weight (Mn >
100,000 g/mol) and very narrow molecular weight distribution (Mw/Mn < 1.05) [187].
82
Scheme 4.18b
The gas-phase studies presented in this work augment our understanding of
related processes taking place in the condensed phase. Individual reactions stages can be
more easily discerned. The reaction mechanism postulated in the original study of the
83
silver-mediated addition of alkenyl- and alkylzirconocene chloride to aldehyde [54] is
shown in Scheme 4.20.
Scheme 4.19
The precursor alkyl/alkenyl zirconocene chloride complex is activated via chloride
abstraction by AgClO4 and resultant cation reacts with the aldehyde molecule. The
process leading from complex (IIIA) to complex (IVA) and regeneration of the active
species (IIA) (transfer of the R’ group) can be either inter- or intramolecular in nature.
Since only the structures of final products (RR’COH alcohols, formed after treatment of
the reaction mixture with NaHCO3 aqueous solution) were established, the detailed
reaction mechanism is not completely clear.
84
Scheme 4.20
The reaction mechanism which is consistent with the gas-phase studies data
presented earlier in this chapter is shown in Scheme 4.21. According to the reaction
mechanism postulated for the gas-phase reactions of 1 with aldehydes (Scheme 4.18), the
coordinated aldehyde molecule undergoes migratory insertion followed by β-H
elimination. This suggests formation of a side product (VI). Regeneration of the active
species (IIB) occurs via chloride abstraction by cationic complex (IVB) from neutral
precursor (I). Even though this process is endothermic (cation IVB is more stable than
IIB), the equilibrium can be shifted toward species (V) and (IIB), as the concentration of
(V) is much lower than that of (I) at all times. Hydrogen elimination and formation of η3-
enolate complex do not take place in the liquid phase due to efficient energy dissipation
in the presence of solvent bath (Figure 4.15b).
85
Scheme 4.21
As can be seen, the gas-phase studies can provide some comprehensive insight
into the reaction mechanism by direct monitoring of possible reaction intermediates and
eliminating perplexing factors associated with the solution chemistry.
4.5 Comments on Suggested Reaction Mechanisms
Several inter-related factors need to be considered in understanding the reactivity
of the bis(η5-cyclopentadienyl)methylzirconium cation. Having a d0 configuration, the Zr
metal center cannot be involved in further oxidation; therefore the number of possible
reaction pathways is reduced. Traces of background water invariably lead to hydrolysis
products which must be accounted for, especially in experiments with deuterium-labeled
compounds. The hydrolysis reaction leads to substitution of the species of interest, 1 or 2,
86
by the Cp2ZrOH+ cation, thus decreasing total ion abundances of these cations and their
reaction products. However, the significantly lower reactivity of the hydroxyl complex
[155] compared to 1 and 2 reduces possible competition for the substrate and does not
hinder reaction product identification for times less than 5 seconds at the pressures used
in these studies.
Extended reaction times lead to eventual thermalization of reactant ions through
collisions. Ignoring long range interactions, at short reaction times, the number of
molecular collisions under the high-vacuum conditions found in the FTICR cell is
insignificant: sincekT
pcz rel= , whereπµkTcrel
8= and
ba
ba
mmmm+
=µ , when assuming
collisional crossection for zirconocene cation and carbonyl molecule 2dπσ = ,
)(21
ba ddd += being approximately 1 nm2, for typical masses of Cp2ZrL+ ~ 235 amu
and higher and of carbonyl ~ 70 amu, µ ~ 9·10-26 kg, and =relc 337 m/s; thus, for T =
295 K and p = 1·10-7 torr (1.33·10-5 Pa), z ≈ 1.1. However, this number is actually
significantly higher (approximately an order of magnitude or more) for charged species
due to a long-range interaction of the positive charge of the cation with electron cloud of
the neutral. The ion-induced dipole (“Langevin”) potential and the ion-neutral collision
rate constant can be calculated. The simplest case of the Langevin theory considers
spiraling elastic collisions for a point-charge induced dipole force, with the attraction
potential, 4
2
2)(
rerV α−
= . Here, α is the average polarizability of the neutral molecule and
r is ion-molecule internuclear separation. The ion mobility, associated with this potential
can be converted into the momentum transfer collision rate constant:
87
mM
K rαα 101097.9)( −×= . Here Mr is the reduced mass of the system and m is the ion
mass in amu [194]. Better agreement with the experimental values, especially in the case
of anisotropic nonpolar molecules can be achieved if the average polarizability of the
neutral is replaced by the maximum component of the diagonalized polarizability tensor
α’ (this component is the same for spherical molecules and 5-20% larger for most
anisotropic molecules [195]. This substitution gives a modified expression for the rate
constant: mM
K r'1097.9)( 10 α
α −×= . The collision rate constant estimated for an ion of
100 amu colliding with a nitrogen molecule, k ~ 6.611*10-10 cm3 s-1 [92]. By multiplying
this number by the concentration of neutral, the number of collisions per second per ion
can be obtained.
The limited number of collisions will lead to non-equilibrium conditions, which
will result in kinetically controlled reaction products. At long reaction times, when
equilibrium conditions are more closely approached, the observed products will result
from thermodynamically controlled reactions. Here, the term “kinetically controlled
reaction products” does not refer to irreversible reaction(s), but rather to those resulting
from processes far from equilibrium.
Caution must be used when comparing the gas-phase reactivity of metallocenes to
the known chemistry of their condensed-phase analogs. Very few zirconocene complexes
are known in the liquid phase without triphenylphosphine or similar stabilizing ligands
and such complexes react with some substrates in an unusual manner. The reactivity of
direct liquid-phase analogs of the system of interest, i.e. the bis(η5-
cyclopentadienyl)methylzirconium cation, as discussed above, is greatly affected by the
88
nature of the counterion. These catalytic systems were quite thoroughly studied with
respect to their function in alkene polymerization reactions, but the reactivity of such
systems with various substrates has not been studied systematically. Thus, perhaps the
best approach for examining the gas-phase reactivity of Cp2ZrMe+ cations with carbonyls
would be to compare the reactivity of this complex with alkenes in both the gaseous and
liquid phases.
The reaction of olefin insertion into an M-X bond (X = H, R) proceeds via
equilibrium of the alkene coordination complex and the alkene insertion product, which
is exothermic by 25 kcal/mol. Both complexes are expected to be more stable for the
coordination and insertion of the carbonyl group. Even though there is no π-backbonding
possible for the d0 Zr4+ metal center, the lone electron pair of the carbonyl oxygen is
more accessible than π electrons of the alkene, making it a stronger Lewis base. Insertion
of the carbonyl into the M-X bond results in formation of the metal-oxygen σ bond,
which is typically significantly stronger than an M-C σ bond. This compensates for the
disruption of the C=C π bond, which is usually stronger than that of C=O by ~ 30
kcal/mol. Even though oxygen is a weaker σ-donor than the methylene group, it provides
additional bonding by partially transferring its available lone pair into the empty Zr d2-
orbital, thus increasing total bond strength. Theoretical calculations of ground state
energies clearly indicate higher stability of the enolate complex compared to other
possible isomers (Fig. 4.16, Table 4.5)
89
a) b)
c) d)
90
e) f)
Figure 4.16. The most stable theoretically calculated structures for the Cp2ZrC3H5O+ cation. Canonical structures shown were calculated with the MPW1PW91/6-311+G(d,p) functional/basis set. The calculated absolute electronic energies of structures from a) through f) are listed in Table 4.5.
The unsaturated nature of the bis(η5-cyclopentadienyl)methylzirconium cation
results in a propensity to coordinate at least two carbonyl molecules in order to increase
its electron count from 14 to a closed-shell 18 electrons. The extent of the insertion
reaction equilibrium depends on various factors such as the nature of the substituents at
the metal center, and for the reaction of olefin insertion into an M-X bond, Keq varies by
several orders of magnitude. On the other hand, the reaction of carbonyl insertion,
compared to the reaction with alkenes, can be expected to be shifted in the direction of
the substrate coordinated by the dative bond rather than the M-O-CR1R2R3 insertion
product, due to the higher stability of the carbonyl adduct. This assumption is supported
by the experiments reported here – no addition of a third molecule of carbonyl was
91
observed in any reaction of the carbonyls investigated, i.e. the C=O group acts as an L-
ligand, rather than an X-ligand in the insertion product, thus limiting the number of
coordinated entities to two.
Table 4.5. The absolute electronic energy (kcal/mol) of the most stable structure is given, and the relative energy difference is given for the remaining structures (in kcal/mol). Lettering corresponds to the structures pictured in Figure 4.16, a) through f).
Method Basis Sets Ea ∆Εb ∆Εc ∆Εd ∆Εe ∆Εf
MPW1PW91 sd(97), -393187 9.6 18.4 24.4 26.1 27.7
cep-121g, -393112 18.4
crenbl, -393103
lanl2dz -392738 33.5
B3LYP sd(97), -393269 9.9 20.6 30.1 31.0
lanl2dz, -392866 8.6 25.6 32.0 34.9 34.0
lanl2dz -392806 11.8 23.2 21.7 --- 34.3
RHF lanl2dz -390076 11.4 22.0 13.4 32.4 35.2
92
CHAPTER 5 GAS-PHASE REACTIONS OF BIS(η5-CYCLOPENTADIENYL)
METHYLZIRCONIUM CATIONS WITH IMINES
5.1 Synthesis and Reactivity of Imines
The general reaction mechanism suggested in chapter 4 for the reactions of the
methyl zirconocenes with R1R2C=X compounds was shown to encompass both the
reactions with alkenes and aldehydes, and, with the appropriate modifications, ketones. A
further advance toward generality would suggest extension of this mechanism to
reactions with the next logical class of compounds containing C=N bonds, i.e. imines.
However, the chemistry of imines is significantly different from that of both alkenes and
carbonyls. These compounds are usually observed as transitional species and very few
imines can be isolated and stored. Therefore, before consideration of the reactivity of
imines with organometallic species, traditional liquid phase chemistry of this class of
compounds needs to be considered and discussed.
Nucleophilic addition of ammonia/amines to aldehydes or ketones is the primary
means of imine synthesis; however, only a few of these reactions are synthetically useful.
For example, the addition of ammonia to aldehydes or ketones leads to two initial
products, hemiaminals (“aldehyde ammonias”) and imines:
R1R2C=O + NH3 → R1R2C(NH2)OH + R1R2C=NH
Both of these compounds are unstable. A majority of imines with a hydrogen on the
nitriogen atom polymerize spontaneously[196] (the simplest homologue, methanimine
CH2=NH is stable in solution for several hours at -95°C, but rapidly decomposes at -80°C
93
[196, 197]). The final reaction products are the result of various condensation reactions of
combinations of hemiaminals and imines, and/or of imine with ammonia or carbonyl
compounds. The most important example of such products is hexamethylenetetramine,
prepared from ammonia and formaldehyde. Aromatic aldehydes give hydrobenzamides
ArCH(N=CHAr).
The addition of primary, secondary and tertiary amines to aldehydes and ketones
results in various of products [198], but only the reaction of primary amines gives imines:
R1R2C=O + R3NH2 → R1R2C=NR3
The imines with a group at the nitrogen atom are, in general, more stable and sometimes
can be isolated. However, some imines, especially those with a simple R group, tend to
rapidly decompose or polymerize, unless there is at least one aryl group on the nitrogen
and/or the carbon. Such compounds are called Schiff bases. The formation of the Schiff
bases occurs via initial formation of hemiaminals followed by loss of water.
5.2 Gas-phase Reactions of the Bis(η5-Cyclopentadienyl)methylzirconium Cation with Imines
Given the instability of the imines, only a few are available commercially, thus
limiting our choice of substrates. The reactions with four compounds were investigated:
2,2,4,4-tetramethyl-3-pentanone imine, benzophenone imine, N-benzylidene aniline and
N-benzylidenebenzyl amine (Table 5.1). All four compounds contain either aryl, or bulky
alkyl groups, with or without the group on the nitrogen.
94
Table 5.1. Imines used in this work. Imine Structure Molecular
weight (amu)
2,2,4,4-tetramethyl-3-pentanone imine
141.26
Benzophenone imine
181.24
N-benzylidene aniline
181.24
N-benzylidenebenzyl amine
195.26
5.2.1 Reactions of Cp2ZrCH3+/Cp2ZrCD3
+ with 2,2,4,4-Tetramethyl-3-Pentanone Imine
2,2,4,4-tetramethyl-3-pentanone imine is the only stable aliphatic imine. The
experimental setup and conditions were similar to those described in the previous chapter
for the reaction with ketones and aldehydes: the pressure of bis(η5-
cyclopentadienyl)dimethylzirconium was in the 5*10-8 – 1*10-7 Torr range, and the
substrate was leaked in to ~ 1*10-7 Torr pressure. The reaction time was varied in the 0 –
5 second range and SWIFT ejection was selectively applied to different possible parent
ions in order to determine the origin of various reaction products.
95
Scheme 5.1a
In the reaction of 1, three major products were formed: m/z 376, m/z 360 and m/z
318. The first one is the product of addition of one imine molecule to zirconocene with
no neutral eliminated. The latter two result from elimination of methane (m/z 360) and
(most probably) isobutane (m/z 318). At the same time, reaction of 2 leads to the
products with m/z 379, m/z 360 and m/z 321, i.e. the deuterated methyl group is either
96
lost as methane (formation of m/z 360) or retained and isobutene is eliminated (formation
of m/z 321).
Scheme 5.1b
The course of these reactions appears to be quite similar to that of the reactions
with ketones (Scheme 4.8). Elimination of either methane or isobutene strongly suggests,
as shown in Scheme 4.8a, fast migratory insertion equilibrium preceding the elimination
(Scheme 5.1a). The next step in the reaction mechanism (Scheme 4.8) is nucleophilic
attack of the Zr-bound group on the α-hydrogen of a coordinated substrate. This,
however, is not the most efficient pathway in the case of imines.
For coordinated 2,2,4,4-tetramethyl-3-pentanone imine, the least strongly bound
hydrogen is expected to be that of the N-H bond. Even though the N-H bond is normally
97
6 kcal/mol weaker than the C-H bond, due to coordination of the imine by the metal
center, donation of a nitrogen atom’s lone pair results in an induced δ+ charge which
depletes N-H bond’s electron density and reduces the bond order. Therefore, nucleophilic
attack in this case is expected to target the nitrogen-bound hydrogen of imine (Scheme
5.1b).
Such attack leads to final product(s) different from those suggested by Schemes
4.1 and 4.8, as the enamine (analogously with the enolate) complex can no longer be
formed. The resultant azomethine structures have been postulated in earlier gas-phase
studies of reactions of bis(η5-cyclopentadienyl)methylzirconium cations with nitriles (as
opposed to nitriles coordinated by the methyl zirconocene cation) [154], as well as in
liquid phase studies of cationic Zr and Ti complexes [81, 199, 200]. The enamine
complex can be formed via facile H shift from the methyl carbon to the nitrogen (Scheme
5.2). The calculated absolute electronic energies (as described in previous section; used
functional/basis set: MPW1PW91/6-311+G(d,p)) of both structures are almost equal (in
kcal/mol): for the azomethine complex, Scheme 5.2 (left), Etot=-380330.7; for the
enamine complex, Scheme 5.2 (right), Etot=-380333.0. This suggests that these structures
are in equilibrium with each other and both complexes contribute to the observed peak
with m/z 318. In order for the enamine structure to be formed from the azomethine
complex with m/z 360, the methyl group transfer needs to take place. This is far less
likely than the hydrogen transfer, so the formation of the enamine structure in this case
cannot be expected.
98
Scheme 5.2
It also possible that the alternative isomeric complexes shown in Scheme 5.3
(coordinated nitriles) also partially account for the observed peaks. It has been discussed
that such complexes are not in equilibrium with azomethine species, but interconversion
between them is possible under CID conditions [154]. The nitrile coordination product,
however, is the most plausible complex in reactions of the concomitant parent ion,
Cp2Zr+ (m/z 220) as shown in Scheme 5.4.
Scheme 5.3
5.2.2 Reactions of Cp2ZrCH3+/Cp2ZrCD3
+ with Aryl Substituted Imines
The interesting chemistry exhibited in the reactions of 1 and 2 with 2,2,4,4-
tetramethyl-3-pentanone imine was not observed for the other imines studied.
99
Scheme 5.4
It was found that under EI conditions used in these experiments, the
benzophenone imine has an apparent tendency to decompose, with the biphenyl
(molecular weight 154 amu) being the most abundant product of decomposition (Figure
5.1).
m/z185180175170165160155150
Abu
ndan
ce
555045403530252015105
Figure 5.1. EI mass spectrum of benzophenone imine (molecular weight 181 amu).
The resultant spectra of reactions of 1 and 2 with the benzophenone imine appear
to be quite complex, partially due to benzophenone imine fragments which retained
100
positive charge forming complexes with the neutral Cp2Zr(CH3)2 (or Cp2Zr(CD3)2).
However, only the ion with m/z 400 was produced from the parent ions Cp2ZrCH3+ (m/z
235)/Cp2ZrCD3+ (m/z 238) and, most probably, resulted from elimination of methane by
the complex with m/z 416 (not observed or observed to a very small extent) (Scheme
5.5).
Scheme 5.5
An identical pattern is observed in reactions of 1 and 2 with N-benzylidene
aniline: the only product originating from Cp2ZrCH3+ (m/z 235)/Cp2ZrCD3
+ (m/z 238)
had m/z 400, most probably also resulting from the methane elimination by the complex
with m/z 416 (observed in very low abundance), i.e. the elimination reaction proceeds
almost to completion. Unlike the case of the benzophenone imine, there is no hydrogen
atom bound to nitrogen in the N-benzylidene aniline. Thus the carbon-bound hydrogen of
the imine group is the only labile electrophilic hydrogen available. Nucleophilic attack in
the reaction with N-benzylidene aniline, therefore, will be directed toward this hydrogen,
and, due to the lack of lability of the phenyl group, lead to a different reaction product
(Scheme 5.6).
101
Scheme 5.6
N-benzylidenebenzyl amine exhibits an almost complete absence of reactivity
with the methyl zirconocene cation as well as absence of coordination by the metal
center.
It is apparent that the very low lability of the aryl groups precludes the migratory
insertion equilibrium, which locks imine groups at the nitrogen. Consequently, only
methane elimination can be observed. Furthermore, this also makes improbable any
rearrangement leading to isomerization of the product structures in the Schemes 5.5 and
5.6 into one another. In summary, it is clear that the reactivity of imines with bis(η5-
cyclopentadienyl)methylzirconium cations is governed by the availability of a labile
hydrogen in the respective imine. This leads to reaction products different from expected
(Scheme 4.1), i.e. the azomethyne/benzylidene species instead of the enamines. However,
the reaction mechanism in general resembles that proposed for ketones (Scheme 4.8).
102
CHAPTER 6 CONCLUSIONS AND FUTURE WORK
6.1 Conclusions
The reactions and proposed reaction mechanisms reported here illustrate the
reactivity of bis(η5-cyclopentadienyl)methylzirconium cations toward ketones, aldehydes
and imines under gas-phase, low-pressure conditions in the FTICR analyzer cell. These
reactions exhibit distinct patterns for all of the compounds studied. In reactions with
ketones, addition of up to two molecules of the various carbonyls (resulting in formation
of 18e- complexes for Cp2ZrMe+) was observed, along with formation of the postulated
η3- enolate complex whenever possible. In the reactions with aldehydes, due to the
possibility of β-H elimination, the Zr-bound hydrogen acts as nucleophile, and the
reaction sequence leading to the formation of the proposed η3- enolate complex is very
efficient, so in all the cases this complex was the only or major product observed. Even
though the reaction with ketones proceeds by a similar mechanism, since β - H
elimination is not possible the metal-bound alkyl group acts as nucleophile instead, which
results in a major decrease in the amount of η3- enolate complex(es) and multiple
competing reaction pathways leading to several enolate products, as well as products of
ligand fragmentation, especially pronounced in reactions with long and branched
substrates. In reactions with the majority of imines, the reactivity toward bis(η5-
cyclopentadienyl)methylzirconium cation is governed by the availability of a labile
hydrogen. This leads to reaction products different from those which might be expected
in analogy with reaction products observed for ketones and aldehydes, i.e.
103
azomethyne/benzylidene species instead of the enamines. Also, due to lack of lability of
the aryl groups, the migratory insertion equilibrium was not possible for reactions of aryl-
substituted imines. Consequently, only methane elimination was observed. However, a
general reaction mechanism resembles that proposed for ketones. The proposed
mechanisms account very well for all the observed reaction features.
6.2 Future Work
Our data, presented earlier in Chapter 4, indicate formation of an η3-enolate
complex isoelectronic to an allyl in reactions of Cp2ZrMe+ with aldehydes and ketones.
Neither the structure nor the reactivity of the η3-enolate complex has ever been studied in
the gas phase. Even though the DFT calculations indicate that the η3-enolate structure is
the most stable among likely structures, it is still necessary to demonstrate
unambiguously that such structures are, indeed, formed. Furthermore, the majority of the
observed gas-phase species reported in the literature are postulated based on observed
reactivity trends, data from MS/MS studies or other indirect means. There is very little
definitive, spectroscopic evidence of structure for gas-phase ionic species. For example,
even though the allyl complex was one of the most commonly postulated structures
formed in reactions of various metal complexes with a wide array of organic substrates,
there is no spectroscopic evidence confirming that conjecture.
6.2.1 IRMPD Spectroscopy
Infrared multiple photon dissociation (IRMPD) spectroscopy provides a very
convenient way of obtaining infrared absorption spectra of gas phase ions. One of the
currently operational facilities is built around a continuously tunable Free Electron Laser
(FEL) which is located at the FOM Institute "Rijnhuizen" in Nieuwegein, the Netherlands
104
[201]. The principles of operation and experimental setup are discussed in details in
reference [202].
Figure 6.1. IRMPD spectrum of Cr+(Et2O)2 (adapted from Ref. [204]).
The possibility of confirming postulated ionic structures by obtaining infrared
spectra of the ions in question at the new FELIX-FTICR facility is quite exciting. The
current ionization capabilities for the FTICR mass spectrometer located at FELIX [203]
include electron ionization (EI), multiphoton ionization (MPI), matrix assisted laser
desorption ionization (MALDI) and electrospray ionization (ESI) with an octopole ion
guide and a pumping station for external ion sources. An example of a spectrum obtained
at the facility for the complex of chromium and diethyl ether is shown in Figure 6.1. On
the spectrum, the inset shows mass spectra taken with the FEL off and on the resonance
near 1010 cm-1. Observed dissociation channels at m/z=126 and m/z=52 correspond to
detachment of one and two ether ligands, respectively. Zooming in on the m/z = 126
105
channel reveals the pattern due to the naturally occurring Cr isotopes, showing the level
of detail typically obtainable with FT-ICR mass spectrometry.
Significant success has been achieved in the application of IRMPD spectroscopy
[205] for the studies of ions in the gas phase by using the FELIX free electron laser
interfaced to a FTICR spectrometer. For example, in the recent investigation of gas-phase
Cr+ complexes for both the monomer complex [Cr(aniline)]+ and the dimer complex
[Cr(aniline)2]+, the spectra showed features indicating binding of the metal ion to the
aromatic π cloud, as opposed to the N atom, thus resolving an ambiguity in computational
predictions of the preferred binding site [204]. As was discussed in Chapter 3, the
Cp2ZrC3H5+ ion which has been seen during the studies of the reactions of bis(η5-
cyclopentadienyl)methylzirconium(l+) with dihydrogen, ethylene, and propylene [147],
was postulated to contain an allyl moiety. Even though this has been strongly suggested
by earlier work [154, 166], there is no experimental (spectroscopic) evidence.
Comparison of experimentally observed vibrational frequencies to those known for the
allyl species and/or predicted by theoretical calculations for the Zr complex ion should
give definitive proof to the postulated structure. For example, when an allyl group is π-
bonded to a metal, three bands of medium to strong intensity are seen in the 1375 - 1510
cm-1 region, and three more, corresponding to metal-ligand stretching vibrations, are seen
in the 280 - 570 cm-1 region [206]. These are substantially shifted (and increased in
number) from the carbon-carbon double and triple bond stretching frequencies of 1550 -
1610 cm-1 and 2000 - 2070 cm-1, respectively, and metal-ligand frequencies closer to 500
cm-1 which should be seen for sigma metal-allyl bonds [206].
106
Figure 6.2. Theoretical IR absorption spectra for the key structures discussed in the present work.
A similar approach could be used for postulated product ions seen in other studies
as well. Since the ions of interest, such as zirconocene η3-allyl or η3-enolate can be (and
Zirconocene-methyl ZrC11H13+
0
50
100
150
200
250
300
0 500 1000 1500 2000
frequency (cm-1)
Inte
nsity
(km
/mol
)
Zirconocene-allyl ZrC13H15+
0
50
100
150
200
250
300
350
0 500 1000 1500 2000
frequency (cm-1)
Inte
nsity
(km
/mo
Zirconocene-enolate ZrC13H15O
0
50
100
150
200
250
300
350
0 500 1000 1500 2000
frequency (cm-1)
Inte
nsity
(km
/mo
107
were) formed by EI, they should be amenable to study at the FELIX-FTICR facility. A
probable complication is the strong coordination of the ligands by a metal center. The
binding energy of the η3-allyl ligand in the Cp2Zr(η3-allyl)+ moiety is estimated to be 22
kcal/mol above that of the corresponding alkene in the cation Cp2Zr(H)(η2-alkene)+
[162], thus requiring at least ~ 55-60 kcal/mol or greater to be provided in order for the
dissociation of the η3-allyl ligand to occur. This is close to the upper limit for bond
energy for bonds successfully dissociated at the FELIX facility. In cases like that, instead
of monitoring the lowest energy dissociation pathway, the lowest energy reaction
pathway with a suitable reagent could be followed. For example, a possible choice of
such a reagent for the zirconocene η3-allyl complex could be H2, which is known to react
in the gas phase with the zirconocene η3-allyl forming a single reaction product
(zirconocene alkyl) with an activation barrier of ~ 11 kcal/mol, well within the reach of
the laser power at the FELIX facility. The calculated spectra for the most important
species involved in the work presented in this thesis are shown on Figure 6.2.
Our preliminary data indicate that such spectra can possibly be obtained
experimentally. As can be seen in Figure 6.3, the most prominent feature for zirconocene
spectra, in-phase rocking motion of Cp ring hydrogen atoms at ~ 12.5 µm is reproduced
for the Zr(III) complex. The quality and reproducibility of the spectra can be dramatically
enhanced if the spectra acquisition were carried out in the dissociation (monitoring of
fragments’ intensities) rather than in the depletion (monitoring of parent ion’s
disappearance) mode.
108
Figure 6.3. Depletion spectrum of ion with m/z 282 (postulated as [Cp2Zr(CH3CHO)(H2O)]+).
6.2.2 Other gas-phase studies of organometallic compounds
As was mentioned above, several liquid-phase studies indicate that zirconocene
enolate complexes have potential importance as polymerization catalysts [187-193]. This
calls for more thorough investigation of such complexes both in liquid and gas phases.
By applying the FTICR MS approach, with all the advantages discussed above, it is
possible to enrich our understanding of the reactivity of the η3-enolate complex, which
has never been studied in the gas phase. Varying structural complexity of the aldehydes
reacting with the bis(η5-cyclopentadienyl)methylzirconium cation enabled us, as
described above, to control the substituent at the central carbon of the enolate ligand. It is
109
feasible to carry out series of experiments on the reactivity of the η3-enolate complex
toward alkenes and other molecules, such as water, alcohols, dihydrogen, etc. This will
provide valuable insight into the reactivity of such complexes and will extend our
knowledge of gas-phase metallocene chemistry. The exciting possibility of imitating of
the liquid-phase reactions of methylmethacrylate polymerization in the low-pressure
environment of FTICR mass spectrometer should help clarify the reaction mechanism.
However, the potential of FTICR MS studies could also be applied to solve other
unanswered questions. It is possible in general to investigate the effect of weakly bound
ligands (such as solvent molecules) on the metallocene catalysts LL*MCH3+ (L, L* = Cp
or related ligands; M = Ti, Zr) reactivity. An electrospray ionization (ESI) ion source
could be used. The suitable precursor (such as, for example,
[Cp2ZrMe]+[C6H5N(CH3)2B(C6F5)4]-) [79] in the solvent of interest can be introduced
directly, or the oxidation of the corresponding neutral in ESI source in order to produce
Cp2ZrMeSn+ (n = 1, 2) cation can be carried out. The choice of solvents is somewhat
limited by their suitability for electrospray, as well as, of course, the possibility of
reaction with the metallocene. Study of the reactivity of metallocene species with
alkenes, apart from the aspects discussed above, will also help to discern the effect of 16-
electron or closed 18-electron shells vs. the 14-electron shell of coordinatively
unsaturated Cp2MMe+. These experiments would somewhat mimic the effect of the
solvent shell in solution-phase catalysis. Therefore, it can be anticipated that the reaction
may lead to inserting of a second alkene molecule rather than formation of the allyl
complex. The presence of the solvent molecules in the ICR cell may help to resolvate the
metallocene complex cation. The probability of this process is negligible at typical ICR
110
cell operating pressures, but can increase to a noticeable value at elevated pressures
available in ion traps. If the metallocene cation has already undergone alkene insertion,
resolvating will lead to a consecutive insertion, ultimately resulting in oligomerization.
Experimental proof of this hypothesis is a very interesting challenge. In order to discern
the factors most affecting the extent of allyl complex formation vs.
dimerization/oligomerization it is essential to investigate a range of metallocenes with a
variety of solvent ligands. It is also possible to use different precursors LL*M(CH3)2 for
EI or [LL*MCH3]+X- (L, L* = Cp or related ligand; X- = [C6H5N(CH3)2B(C6F5)4]- [207-
224] or other suitable counterion; M = Ti, Zr) for ESI.
Further advances toward the understanding liquid-phase processes could
potentially be obtained from on-line ESI-FTICR MS studies of actual reaction mixtures
and real-time monitoring of cationic species. Change in concentrations of (potential)
active species as well as monitoring of growing polymer chains would provide important
clues and mechanistic details about catalyzed homogeneous polymerization processes.
Coupling of ESI-FTICR MS studies of actual reaction mixtures (not necessarily of
polymerization reactions) with IRMPD spectroscopy both at FELIX and, potentially, with
simpler CO2 laser based systems would create an immense opportunity for studying
structure and functions of various elusive species which bear a charge. Due to the unique
sensitivity of the FTICR MS technique it may be possible to detect and identify transient
species which cannot be characterized by other means. This will tremendously enhance
our understanding of various catalytic processes, as well as the intricacies of reaction
mechanisms.
111
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BIOGRAPHICAL SKETCH
I was born in 1977 in Molodechno, Belorussia - former USSR, now the
independent republic of Belarus. My family moved to Russia in 1988. In 1994, I
graduated from my high school and was accepted into the Chemistry Department of the
Moscow State University in Moscow, Russia. I graduated in August 1999 with B.S. in
Chemistry with the Honor of Excellence Diploma. Part of my diploma project research
was carried out in the School of Chemistry of the University of Nottingham, in
Nottingham, United Kingdom. In the fall of 2001 I started my PhD program at the
Chemistry Department of the University of Florida.
