OULU 1999

NEW HETERODONOR PHOSPHINE AND BIPYRIDINE LIGANDS

RIITTALAITINEN

Department of Chemistry

OULUN YLIOP ISTO, OULU 1999

NEW HETERODONOR PHOSPHINE AND BIPYRIDINE LIGANDS

RIITTA LAITINEN

Academic Dissertation to be presented with the assent of the Faculty of Science, University of Oulu, for public discussion in Raahensali (Auditorium L 10), on May 28th, 1999, at 12 noon.

Copyright © 1999Oulu University Library, 1999

OULU UNIVERSITY LIBRARYOULU 1999

ALSO AVAILABLE IN PRINTED FORMAT

Manuscript received 13.4.1999Accepted 16.4.1999

Communicated by Docent Erkki KolehmainenProfessor Markku Leskelä

ISBN 951-42-5229-2(URL: http://herkules.oulu.fi/isbn9514252292/)

ISBN 951-42-5228-4ISSN 0355-3191 (URL: http://herkules.oulu.fi/issn03553191/)

Laitinen, Riitt a H., New heterodonor phosphine and bipyri dine ligandsDepartment of Chemistry, University of Oulu, PO Box 3000, 90401 Oulu, Finland1999Oulu, Finland(Manuscript received 13.4.1999)

Abstract

Twenty-seven phosphine and six bipyridine ligands were synthesised and characterized.Additionally, a new route to the famil y of phosphine ligands via o-thioanisyldichlorophosphinewas found. The phosphine ligands contain thiomethylphenyl, methoxyphenyl, dimethylamino-phenyl, pyridyl, naphthyl and anthracenyl groups, and the bipyridine ligands thiomorpholineand piperidine groups. Metal complexes of 3-pyridyldiphenylphosphine, 6,6’-bis(methylthio-morpholine)-2,2’-bipyridine and 4,4’-dimethyl-6,6’-bis(methylthiomorpholine)-2,2’-bipyridinewere prepared.

Ligands and complexes were characterized by 1H-, 13C-, 31P- and two-dimensional HSQC-NMR spectroscopy, and crystal structures were determined for the ligands and two of the metalcomplexes. Tertiary phosphine ligands were prepared for catalytic purposes and tested inhydroformylation reaction at the Helsinki University of Technology and the University ofJoensuu. Bipyridine ligands were designed for bimetallic coordination.

The phosphine ligands cover a wide range of electronic and steric properties. Thespectroscopic parameters and crystal structures were studied with the purpose of charting trendsin the basicities and steric effects of the ligands.

Keywords: preparation, basicity,31P-NMR, steric effects

Acknowledgements

The present study was carried out in the Department of Chemistry at the University ofOulu, during the years 1994 – 1999.

I am most grateful to my supervisor Professor Jouni Pursiainen for his guidance,support and advice throughout the research. Especially, I am indepted for hisencouragement and help during the last, tense times preparing the papers and thethesis.

Likewise I wish to express my gratitude to the Head of Department, ProfessorRisto Laitinen for permission to use laboratory space and facilities at my disposal.

I am indepted to two referees, Professor Markku Leskelä and Docent ErkkiKolehmainen, for their careful reading of my manuscript, and Dr. Kathleen Ahonenfor revising the language of my thesis. I thank all my co-authors for theircontributions.

Sincere thanks go to my friends and colleagues in the Laboratory of PhysicalChemistry and the Laboratory of Inorganic Chemistry for the warm and supportingatmosphere. I wish also thank the always helpful staff in the Department ofChemistry.

Most importantly I wish to thank my parents Pirkko-Liisa and Pentti and thefamilies of my sister Anna-Maija and brother Markku for their support during mystudies. I am also grateful to Melli about filling my free time.

The study was financed by Neste Oy, TEKES and Neste Foundation. In addition,financial support from Alfred Kordelin and Finnish Konkordia foundations isgratefully acknowledged.

Oulu, April 1999 Riitta Laitinen

List of original papers

This thesis is based on the following papers, which are referred to in the text by theirRoman numerals:

I Laitinen RH, Csoban K, Pursiainen J, Huuskonen J & Rissanen K (1997)Synthesis and Characterization of Some New Bipyridyl-Based MultidentateLigands and Their CuI Complexes. Acta Chem Scand 51: 462-469.

II Laitinen RH, Losonczi J, Pursiainen J & Ahlgrén M (1997) X-Ray CrystalStructure of 1,2-Bis[bis(2-methylthiophenyl)phosphino]ethane. Acta ChemScand 51: 804-806.

III Laitinen RH, Soininen J, Suomalainen P, Pakkanen TA, Ahlgrén M &Pursiainen J (1999) Reactions of 3-Pyridyldiphenylphosphine withRh2(CO)4Cl2 and Co(CO)8. Crystallographic and Catalytic Studies ofRh(CO)Cl(3-PyPPh2)2. Acta Chem Scand, in press.

IV Laitinen RH, Riihimäki H, Haukka M, Jääskeläinen S, Pakkanen TA &Pursiainen J (1999) Syntheses and Characterization of New TertiaryPhosphine Ligands Prepared fromp-Anisyl- and p-Thioanisyldichloro-phosphines, submitted.

V Laitinen RH, Koskinen AMP & Pursiainen JT (1999)o-Thioanisyldichloro-phosphine: A New Route for the Preparation of Phosphine Ligands,submitted.

VI Laitinen RH, Heikkinen V, Haukka M, Koskinen AMP & Pursiainen J(1999) o-Thioanisyldichlorophosphine, New Starting Material for thePreparation of Multidentate Phosphine Ligands. Syntheses andCharacterization of Derivatives ofo-Anisyl- and o-Thioanisyldichloro-phosphines, submitted.

Contents

AbstractAcknowledgementsList of original papers1. Introduction…………………………………………………………………. 11

1.1. Phosphine ligands ……………………………………………………… 111.2. Bipyridine ligands ……………………………………………………… 12

2. Aims of the work …………………………………………………………… 143. Experimental ………………………………………………………………. 15

3.1. Reagents ………………………………………………………………... 153.2. Syntheses ………………………………………………………………. 153.3. Instrumentation ………………………………………………………… 16

4. Heterodonor phosphine ligands ……………………………………………... 174.1. Syntheses ………………………………………………………………. 17

4.1.1. Starting materials ……………………………………………. 174.1.2. Ligands derived from PPh2Cl, PPhCl2 and PCl3 ……………... 194.1.3. Ligand derived from Cl2PCH2CH2PCl2 ………………………. 214.1.4. Ligands derived fromp-substituted aryldichlorophosphines … 224.1.5. Ligands derived fromo-substituted aryldichlorophosphines … 234.1.6. Observations on the syntheses………………………………… 24

4.2. Characterization of the phosphine ligands ……………………………... 254.3. Properties and trends …………………………………………………… 30

4.3.1. Electronic effects …………………………………………….. 314.3.2. Steric effects ……………………………….………………….. 34

4.4. Rh(I) and Co(0) complexes of 3-pyridyldiphenylphosphine ………….... 355. Bipyridine based ligands ……………………………………………………. 36

5.1. Synthesis of starting materials ..………………………………………… 365.2. Synthesis of ligands …………………………………………………….. 365.3. Characterization ……………………………………………………….… 375.4. Cu(I) complexes of 6,6’-bis(methylthiomorpholine)-2,2’-bipyridine

and 4,4’-dimethyl-6,6’-bis(methylthiomorpholine)-2,2’bipyridine …….. 386. Conclusions ………………………………………………………………….. 40

References…………………………………………………………………….. 41

1. Introduction

In coordination chemistry, a ligand is generally understood as a complexing group thatcan either donate or accept electrons. The metal atoms in organometal complexesoften are in low-positive, zero or negative formal oxidation states, andπ-bondingligands typically stabilize these low oxidation states by delocalizing the high electrondensity of the metal atom onto the ligand.[1]

It is essential in coordination chemistry to understand how ligands are bonded tometal atoms. The interactions between ligands and metal atoms depend on theorientation of the orbitals with respect to each other. According to their interactions,ligands are classified into three types:σ-donor ligands,π-donor ligands andπ-acceptor ligands. Theσ-donor ligands have an electron pair capable of being donateddirectly an empty metal orbital.π-donor ligands may donate electrons, for example,from a filled p-orbital on a ligand. Inπ-acceptor ligands,σ donation is complementedby the ability of the ligand to accept electron density from the metal onto suitableacceptor orbitals.[2]

The design and development of new ligands for specific purposes is a growth areain organometallic and coordination chemistry.

1.1. Phosphine ligands

Work towards the synthesis of organophosphorus compounds began early in the 19thcentury. A major driving force was the recognition of their biochemical activity.

Tertiary phosphine ligands, PR3, behave as bothσ-donors andπ-acceptors. Thenature of the R groups determines the properties of the ligands, and the strength of thephosphine donor/acceptor properties can be modified through change in the Rgroups.[2] Steric and electronic properties provide the basis for the selection of aparticular ligand for catalytic purposes. Catalytic properties of phosphines dependamong other factors, on their basicity.

Organophosphine compounds [3] are usually prepared by one of three routes: 1) viaorganometallic reagents and appropriate halogenophosphines, 2) from metalphosphides or 3) by hydride reduction.

12

In the first route Grignard and organolithium reagents are the commonly usedorganometallic reagents. Occasionally also other organometallic compounds, such asorganozinc and organocadmium, have been used [4]. The first route is the most usefulfor the synthesis of tertiary phosphines of the type PR3, PR2R

1 and PRR1R2.[3]The second route can be used for the preparation of primary and secondary

phosphines. The required phosphide and organophosphide anions can be obtained bymetallating the phoshines with strong base, by halogen metal or metal metal exchangeor by reductive metallation.[3]

In the third route, two types of reduction are involved in the preparation ofphosphorus compounds: reduction that do not involve the formation of a new P-Hbond and those where new P-H bonds are formed. Typically, the first type is used forthe preparation of tertiary phosphines and the second for the preparation of secondaryand primary phosphines.

Phosphines are widely used as ligands for transition metals. They promote thesolubility of metal complexes in a wide range of organic media. The majority ofphosphines are insoluble in water, though water soluble phosphines are found amongthe sulfonated phenylphosphines and pyridylphosphines [5-8]. The property ofphosphines to stabilize low oxidation states of metal atoms provides compounds thatare useful in homogeneous catalysis. Asymmetric phosphines are designed forstereoselective catalytic reactions. Hydroformylation [7-9], hydrogenation [5] andhydrocyanation [10] are examples of catalytic reactions that use phosphine metalcomplexes as catalysts.

1.2. Bipyridine ligands

Pure 2,2’-bipyridine was synthesised for the first time in the late 19th century bydistillation of copper picolinate. A more convenient synthesis was eventually achievedwhen Raney-Nickel alloy was used as a catalyst. Since the preparation of the first2,2’-bipyridine complex in 1888, the derivatives of bipyridine have been widelyemployed in complexation experiments.

As described by Vögtle [11] 2,2’-bipyridine is both aσ-donor and aπ-acceptor.The lone electron pair of nitrogen can form aσ-bond with the central atom, while thearomatic system can take part in back-bonding. Bipyridine ligand stabilizes soft metalions, especially transition metal ions in low oxidation states. Ligands with two ormore donor atoms are able to form a chelate ring with metal atoms. 2,2’-Bipyridines,having two nitrogen donor atoms separated by two carbons, form five-memberedrings, which are the most stable structures. The diimine part of the bipyridinedelocalizes the electrons in the chelate ring.

Diimines like 2,2’-bipyridines are of special interest as complex ligands incoordination and supramolecular chemistry. Their long-lived, luminescent, charge-transfer state enables them to be used as sensitizers in photochemistry [12-13].Moreover, the ability of bipyridines to form ionic and neutral guest compounds makesthem useful precursors in supramolecular chemistry [11, 14-16]. Self-organization ofbipyridine structures and metal cations often leads to the formation of helicates, whichare supramolecular complexes formed between preorganized receptors and metal ions

13

[17]. Studies in this area focuse on design and control for the selective preparation ofcomplicated organized chemical structures. In addition to the areas mentioned above,bipyridine units are also significant in catalytic studies. Some examples of the widelystudied reactions are water-gas shift reaction [18-19], reduction of CO2 [20] andhydroformylation of olefins [21].

2. Aims of the work

The present study is part of a project targeted at developing new, selective catalystsfor hydroformylation reactions. The primary focus of the study was the preparationand characterization of new tertiary phosphine ligands. The ligands are now beingtested for their properties at the Helsinki University of Technology and the Universityof Joensuu.

The study of the phosphine ligands had two specific objectives: 1) to synthesise andcharacterize new heterodonor ligands for catalytic purposes and 2) to find trends intheir behaviour as ligands. Heterodonor phosphine ligands containing sulfur, oxygenor nitrogen donor atoms in addition to phosphorus were prepared with the purpose ofobtaining particular structural properties such as multidentate bonding. The structureswere modified systematically by binding thiomethylphenyl, methoxyphenyl,dimethylaminophenyl, pyridyl and large aromatic groups to phosphorus. Obtaining acomprehensive series of ligands required that some previously reported ligands wereprepared as well. The availability of a family of closely related tertiary phosphineligands allows comparative study of their electronic and steric effects, and predictionof the behaviour of the ligands in specific applications.

The promising results in catalytic tests [22] of a phosphine ligand containingphosphorus and sulfur donors encouraged the design of new starting materials. This,indeed, proved to be the most challenging part of the work, as the synthetic methodsreported in the literature failed when applied to the preparation of the desiredaryldichlorophosphine.

The preparation and characterization of several bipyridine based ligands constitutedanother part of the work. Bipyridine unit is a well-known building block insupramolecular chemistry and its rich coordination chemistry suggests that it, too,might find use in catalytic applications. The bipyridine ligands were specificallydesigned to combine two different metal centres. The bifunctionality of heterometallicstructures improves their range of properties.

3. Experimental

3.1. Reagents

Diethyl ether, tetrahydrofuran and toluene (Lab Scan) were distilled fromsodium/benzophenone ketyl under nitrogen before use. Chloroform (Lab Scan),dichloromethane (Kebo Lab), carbon tetrachloride (BDH Chemicals), acetone (Shell),petroleum ether (Lab Scan), methanol (Baker) and ethanol (Primalko) were usedwithout further purification. Sulfuric acid (Reachim) and hydrochloric acid (Baker)were degassed with argon. AlCl3 (Riedel-de Haen) and ZnCl2 (Aldrich) were fresh andanhydrous. RhCl3 (Johnson Matthey), N,N-dimethylaniline (EGA-Chem), NaOH (FF-Chemicals) and MgSO4 (Riedel-de Haen) were used without further purification. Allother reagents were obtained from Aldrich, Merck or Fluka and used without furtherpurification.

3.2. Syntheses

The tertiary phosphine ligands were prepared mainly by the following procedure. Thebrominated organic reagent was lithiated withn-butyl lithium in sodium-dried diethylether and the mixture was stirred until lithiation was complete. Halogenated aryl-phosphine was added in diethyl ether and the mixture was stirred until precipitationoccurred. The precipitate was filtered out, dried in vacuum and recrystallized fromethanol or ethanol toluene solution.

Pyridylphosphine ligands were extracted to aqueous phase with sulfuric acid,neutralized and extracted back to the organic phase. Purification was accomplished bycolumn chromatography.

Bipyridine ligands were prepared from appropriate starting materials under basicreaction conditions by mixing them in N,N-dimethylformamide solution. Theprecipitated products were purified by column chromatography.

All syntheses were performed in inert atmosphere by applying standard Schlenktechniques [23].

16

3.3. Instrumentation

Nuclear magnetic resonance spectroscopy.The characterization of the compoundswas based mainly on NMR techniques.1H- and13C-NMR spectra were recorded on aBruker AM200 spectrometer and13C-, 31P- and two-dimensional HSQC-NMR spectraon a Bruker DPX400 spectrometer.1H-, 13C- and HSQC-NMR spectra werereferenced to internal tetramethylsilane (TMS), and31P-NMR spectra to external 85%H3PO4. All samples were measured at room temperature with deuterated chloroform(99.8 atom % D, 0.03 % v/v TMS, Aldrich) supplying the solvent.

X-ray crystallography. X-ray diffraction measurements of the phosphine ligandsand one complex were performed on Siemens R3m and Enfrad-Nonius KappaCCDdiffractometers using graphite-monochromated Mo-Kα radiation. X-ray diffractionmeasurements of the bipyridine ligands and one complex were carried out on SyntexP21 diffractometer using graphite-monochromated Cu-Kα radiation and Enfrad-NoniusCAD4 diffractometer using graphite-monochromated Mo-Kα radiation. The phosphinestructures were solved by direct methods using the Siemens SHELXTL-Plus programpackage [24] or the SHELXS97 [25] or SIR97 [26] programs and subjected to full-matrix refinement using the SHELXL-93 or SHELXL97 programs [27-28]. Thebipyridine structures were solved by direct methods using the SHELXS [29] programsystem and subjected to full-matrix refinement using the CRYSTALS [30] program.X-ray diffraction measurements were done at the University of Joensuu and theUniversity of Jyväskylä.

FTIR spectroscopy. FTIR spectra were recorded on a Bruker IFS 66 spectrometer.Samples were dissolved in dichloromethane, which was also used as a referencesolvent, and measured in a 0.1 mm KBr cuvette at room temperature.

Other techniques. Mass spectra were recorded on a Kratos MS-80 spectrometer inEI or CI mode. Elemental analyses were carried out on a Perkin Elmer 2400 Series IICHNS/O Analyzer. Analytical thin layer chromatography (TLC) was conducted onKieselgel 60 F254 (Merck) silica gel precoated aluminium plates with fluorescentindicator UV254. Column chromatography was carried out on silica gel (particle size0.063-0.2 mm, 70-230 mesh, Aldrich).

4. Heterodonor phosphine ligands

4.1. Syntheses

The properties of organometallic metal centres largely depend on the ligands bound tothem. The steric and electronic effects of the ligands in most cases influence the rateand the selectivity of catalytic reactions, and change in the ligands allows fine-tuningof the selectivity and activity of catalysts.

In this work multidentate binding, systematically added steric stress and differentelectronic properties of the tertiary phosphine ligands were achieved by synthesisingheterodonor ligands containing sulfur, oxygen or nitrogen donor atoms in addition tophosphorus. Preparation of a family of closely related phosphines offered thepossibility to study their electronic and steric effects as well as to predict thebehaviour of the ligands in applications.

PPh3, a commercial phosphine ligand used in catalytic hydroformylation, was takenas the reference in varying the functionality of the ligands prepared in this work.Modifications were achieved by introducing thiomethylphenyl, methoxyphenyl,dimethylaminophenyl, pyridyl and two large aromatic groups as substituents atphosphorus atom. The position of the heteroatom was varied systematically.

4.1.1. Starting materialsIV, V

Halogenated arylphosphines are widely used as starting materials in the preparation ofphosphine ligands. Unfortunately, only a few such compounds are commerciallyavailable. Of these, PPh2Cl, PPhCl2, PCl3 and Cl2PCH2CH2PCl2 were used in thepresent work. To widen the choice of arylchlorophoshines, new methods were soughtfor their preparation and, indeed, this proved to be the most challenging part of thework.

One of the first reported syntheses of aryldichlorophosphines was published by A.Michaelis over a century ago [31]. In this method, which was later criticized for itsineffective reaction conditions [32],p-methoxyphenyldichlorophosphine was prepared

18

by aluminium trichloride catalysed Friedel-Crafts reaction. This method favours theformation of para isomers, whilemeta directing groups prevent the substitution.Syntheses modified from the original method [31] in reaction times, temperatures andcatalysts, have been used in most of the preparations of aryldichlorophosphinesreported in this century [33-37].

p-Methoxyphenyldichlorophosphine1 andp-thiomethylphenyldichlorophosphine2(Fig.1) were prepared from anisole and thioanisole, respectively, with AlCl3 andtrichlorophosphine, by the method of Davies and Mann [35] forp-methoxyphenyldi-chlorophosphine. In the preparation of compound2 the reaction time was prolonged to90 hours.

A method that relies on Grignard reagent for activating theortho or metapositionin the aromatic ring was developed by McEwenet al. [38] for the preparation oforthoand metasubstituted aryldichlorophosphines. In this method the Grignard reagent ofthe ortho or meta substituted aryl group is transmetallated with ZnCl2 to thecorresponding organozinc halide reagent, which is then allowed to react with PCl3 intetrahydrofuran under refluxing conditions.

Promising catalytic results for (o-thiomethylphenyl)diphenylphosphine [22]directed the design of new starting materials towardso-thiomethylphenyldichloro-phosphine4. In the preparation of this new material, examination was first made ofthe lithiation procedure in combination with excess of PCl3. The reaction resulted in atertiary phosphine ligand, tris(o-thiomethylphenyl)phosphine, rather than the targetedcompound. Stopping the reaction at the step where only one chloride was eliminatedproved difficult and the combination of organolithium and organozinc halide reagentsin diethyl ether was chosen for the preparation of a reagent that was then allowed toreact with PCl3.

o-Methoxyphenyldichlorophosphine3 ando-thiomethylphenyldichlorophosphine4(Fig. 1) were synthesised by a modified version of the method of McEwenet al. [38].The organozinc halide reagent of anisole or thioanisole was allowed to react withtrichlorophosphine in diethyl ether, producing the desired aryldichlorophosphine. Thesynthetic route is presented in Scheme 1.

Scheme 1. Synthetic route for the preparation ofo-substituted aryldichlorophosphines.

The improved method has two advantages over the original method of McEwenetal. [38]. In diethyl ether the activation of theortho position of the aryl ring is fasterwhen organolithium reagent is used in place of Grignard reagent. A side product, 4-chlorobutanol, is formed when the aryllithium compound is refluxed with ZnCl2 intetrahydrofuran as solvent [39], but the aryldichlorophosphine is obtained withoutsignificant formation of side products when diethyl ether provides the solvent.

XMe

Br

XMeLi

XMe

ZnYn-BuLi ZnCl 2 PCl3

XMe

PCl2

19

Fig. 1. Schematic structures ofp-methoxyphenyldichlorophosphine 1,p-thiomethylphenyl-dichlorophosphine 2, o-methoxyphenyldichlorophosphine 3 and o-thiomethylphenyl-dichlorophosphine 4.

4.1.2. Ligands derived from PPh2Cl, PPhCl2 and PCl3

The synthesis of tertiary phosphine ligands began with the use of commerciallyavailable arychlorophosphines. The replacement of the chlorine atoms inarylchlorophosphines by thiomethylphenyl, methoxyphenyl, dimethylaminophenyl orpyridyl groups provided a group of phosphine ligands in which multidentate binding,steric stress and various electronic properties were modified systematically.

The ligands treated in this section are reported in the literature. However, it wasconsidered important to prepare these compounds as well, in order to buildsystematically a family of substituted tertiary phosphine ligands. New spectroscopicand structural data were recorded from these structures. These ligands complement theseries of phosphine ligands synthesised here for the first time, providing a meaningfulset of ligands for the discussion of electronic and steric trends. Ligands5-12 weretested in catalytic experiments.

In general, the syntheses of phosphine ligands were carried out by lithiating thebrominated organic reagent in diethyl ether at 0ºC and adding the appropriatechlorophosphine. The synthetic route is presented in Scheme 2.

Scheme 2. Synthetic route for the preparation ofortho substituted phosphines derivedfrom PPh2Cl, PPhCl2 and PCl3.

XMe

Brn-BuLi

XMeLi

XMe

PPh2

P

XMe

MeX

P

XMe

MeX

XMe

PPh2Cl

PPhCl 2

PCl3

1 2 3 4

OMe

PC l2

SM e

PC l2

OMe

PC l2

SM e

PC l2

20

(o-Thiomethylphenyl)diphenylphosphine5, bis(o-thiomethylphenyl)phenylphos-phine6 and tris(o-thiomethylphenyl)phosphine7 (Fig. 2) were prepared from liquido-bromothioanisole and appropriate chlorophosphine according to the literature method[40]. The reaction mixtures were stirred two hours after lithiation and an other twohours after the addition of chlorophosphine. The ligands were recrystallized fromethanol and dried in vacuum.

(o-Methoxyphenyl)diphenylphosphine8, bis(o-methoxyphenyl)phenylphosphine9and tris(o-methoxyphenyl)phosphine10 (Fig. 2) were prepared basically by the samemethod as the thiomethylphenyl ligands, fromo-bromoanisole and chlorophosphines[40]. Both reaction times were shortened to one hour because the reactions proceededfaster than with thiomethylphenyl substituents, with fairly good yields. The ligandswere recrystallized from methanol or ethanol, and dried in vacuum.

Fig. 2. Schematic structures of (o-thiomethylphenyl)diphenylphosphine 5, bis(o-thio-methylphenyl)phenylphosphine 6, tris(o-thiomethylphenyl)phosphine 7, (o-methoxy-phenyl)diphenylphosphine 8, bis(o-methoxyphenyl)phenylphosphine 9 and tris(o-meth-oxyphenyl)phosphine 10.

For lack of theortho-brominated N,N’-dimethylaniline, the general method failedin the preparation of (o-N,N’-dimethylaminophenyl)diphenylphosphine11 (Fig. 3).Attempts to prepareo-bromo-N,N’-dimethylaniline by methylation from theo-bromoaniline [41] were unsuccessful: the reaction produced a mixture of partlymethylated amines, which could not be separated by treatment with acetic anhydride[42].

(o-N,N’-dimethylaminophenyl)diphenylphosphine11 was eventually prepared bythe literature method from N,N’-dimethylaniline,n-butyl lithium and diphenyl-chlorophosphine by refluxing the reaction mixture in hexane for ten hours [43]. Theproduct was crystallized in relatively low yield by cooling to –20°C.

P

SMe

P

SMe

MeS

P

SMeSMe

MeS

5 6 7

P

OMe

P

OMe

MeO

P

OMeOMe

MeO

8 9 10

21

Fig. 3. Schematic structure of (o-N,N’-dimethylaminophenyl)diphenylphosphine 11.

The formation of side products in the preparation of pyridylphosphines wasavoided by using low reaction temperatures. 3-Pyridyldiphenylphosphine12 (Fig. 4)was prepared according to the method of Kurtevet al. [44] by lithiating 3-bromopyridine withn-butyl lithium at -100°C and adding diphenylchlorophosphine.The amphiphilic property of pyridine was exploited to extract the unreacted materialbefore isolating the product. The raw product was extracted to the aqueous phase withsulfuric acid, neutralized with sodium hydroxide and extracted back to the organicphase. The final product was purified by column chromatography on silica gel using5% MeOH in CH2Cl2 as an eluent.

Fig. 4. Schematic structure of 3-pyridyldiphenylphosphine 12.

4.1.3. Ligand derived from Cl2PCH2CH2PCl2II

Diphosphine ligands are important compounds in catalytic reactions especiallybecause of their steric properties. Depending on the backbone that connects thephosphine atoms together, diphosphines can form either mono- or multimetalcomplexes. The nature of backbone also affects the natural bite angle, which is theligand-metal-ligand angle. The bite angle is an important variable, which can be usedto control the reaction pathway [45-46]. The bite angle of diphosphine ligands iseasily modified by varying the backbone between the phosphorus atoms.

The preparation of simple diphosphines, such as 1,2-bis(diphenylphosphino)ethane,can be carried out with use of the same general method as for tertiary phosphines. Thelength of the methylene chain between the phosphorus atoms can be varied to givedifferent separations between the donor atoms. The length of the backbonesignificantly affects the structure of the metal complexes. For example, Sanger [47]showed that bis(diphenylphosphino)ligands having one, three or four carbon atomsbetween the phosphorus atoms produce binuclear complexes with [Rh(CO)2Cl]2,whereas with two carbon atoms the complex is the chelated monomer.

P

NMe2

11

P

N

12

22

1,2-Bis[bis(o-thiomethylphenyl)phosphino]ethane13 (Fig. 5) was prepared fromliquid o-bromothioanisole and 1,2-bis(dichlorophosphino)ethane according to theliterature method for bis(o-methylthiophenyl)phenylphosphine [40]. The product wasdried in vacuum and recrystallized from dichloromethane. The ligand is a structuralanalogue of the diphenylphosphinoethane and thiomethylphenyl phosphine ligands5-7 prepared in this work. Both phosphorus and sulfur atoms have a strong tendency tocoordinate with late transition metal atoms.

Fig. 5. Schematic structure of 1,2-bis[bis(o-thiomethylphenyl)phosphino]ethane 13.

The diphosphine ligand prepared in this work had in addition to the diphosphinemoiety four sulfur donor atoms. Study of the metal complexes of the potentiallyhexadentate ligand13 turned out to be difficult because the reactions produced amixture of complexes with different coordination modes. Al-Dulayammiet al. [48]have, however, prepared molybdenum and tungsten complexes of13. In thesecomplexes 1,2-bis[bis(o-thiomethylphenyl)phosphino]ethane13 behaves as atridentate bridging ligand forming binuclear carbonyl complexes.

4.1.4. Ligands derived from p-substituted aryldichlorophosphinesIV

The six ligands discussed in this section were prepared with modified electronicproperties relative to the ligands described in section 4.1.2. Ligands14 and17, and15and18 are steric analogues of ligands9 and6, respectively, while ligands16 and19resemble PPh3. The electronic effects were modified by adding electron withdrawingsubstituents inpara position of the aromatic ring. Although ligands14 [49] and 16[50] have been mentioned in the patent literature, their preparation andcharacterization are not reported.

Ligands 14-19, which were derived from p-methoxyphenyl- and p-thiomethylphenyldichlorophosphines, each contain onepara substituted and twoorthosubstituted aryl groups. The synthesis of thepara substituted aryldichlorophosphinesused as starting materials has been described in section 4.1.1.

Syntheses of the ligands (Fig. 6) were carried out via the general method describedin section 3.2. The reaction times varied from one to two hours depending on thesubstituent group (methoxyphenyl and thiomethylphenyl, respectively). The reactiontemperature was kept at 0°C except for the pyridyl ligands, which were prepared at-100°C. The ligands were purified by recrystallization.

PP

SMe

MeS

SMe

MeS

13

23

Fig. 6. Schematic structures of (p-methoxyphenyl)bis(o-methoxyphenyl)phosphine 14, (p-methoxyphenyl)bis(o-thiomethylphenyl)phosphine 15, (p-methoxyphenyl)bis(2-pyridyl)-phosphine 16, (p-thiomethylphenyl)bis(o-methoxyphenyl)phosphine 17, (p-thiomethyl-phenyl)bis(o-thiomethylphenyl)phosphine 18 and (p-thiomethylphenyl)bis(2-pyridyl)-phosphine 19.

4.1.5. Ligands derived from o-substituted aryldichlorophosphinesVI

The ligands discussed in this section were prepared with both electronic and stericproperties modified relative to the ligands described above. Combining phosphorus,sulfur and oxygen donor atoms in the same structure served to modify the electronicproperties of the ligands, while addition of large aromatic groups made the ligandsmore bulky and altered their steric factors. The large aromatic groups also decreasedthe basicity of ligands.

The new ligands20-27 (Fig. 7), derived fromo-methoxyphenyl- ando-thiomethyl-phenyldichlorophosphines, were prepared by the general method described in section3.2. In all these ligands, large aromatic groups, in addition to methoxyphenyl andthiomethylphenyl groups, were introduced to modify the properties. The reactiontimes varied from one to two hours depending on the substituent group(methoxyphenyl and thiomethylphenyl, respectively). The reaction temperature waskept at 0°C in all reactions.

The synthesis of catalytically promising heterodonor ligands24-27 required thedevelopment of a synthetic route for the preparation of new starting material,o-thiomethylphenyldichlorophosphine4. This route has been described in section 4.1.1.Also o-methoxyphenyldichlorophosphine3 that was used for the preparation ofligands20-23 was prepared by this improved method.

161514

P

OMe

MeOMeO

P

SMe

MeSMeO

P

N

NMeO

P

OMe

MeOMeS

P

SMe

MeSMeS

P

N

NMeS

17 18 19

24

Fig. 7. Schematic structures of (o-methoxyphenyl)bis(o-thiomethylphenyl)phosphine 20,(o-methoxyphenyl)bis(p-thiomethylphenyl)phosphine 21, (o-methoxyphenyl)bis(1-naphthyl)phosphine 22, (o-methoxyphenyl)bis(9-anthracenyl)phosphine 23, (o-thiomethyl-phenyl)bis(o-methoxyphenyl)phosphine 24, (o-thiomethylphenyl)bis(p-thiomethylphenyl)-phosphine 25, (o-thiomethylphenyl)bis(1-naphthyl)phosphine 26 and (o-thiomethyl-phenyl)bis(9-anthracenyl)phosphine 27.

4.1.6. Observations on the syntheses

The general method in the preparation of tertiary phosphine ligands was quitestraigtforward. When appropriate chlorophosphine was available, syntheses proceededeasily at 0°C. The only exception was the syntheses of pyridylphosphines thatrequired low temperature, -100°C, in order to avoid the formation of side products.The pyridylphosphine ligands were also more difficult to handle because of their airand moisture sensitivity.

The synthetic experiments showed that obtaining reasonable yields of the ligandscontaining thiomethylphenyl groups demanded twice as long reaction times as for theligands containing only methoxyphenyl groups. The reason for this difference is notclear. Possibly, the lithiation reaction of brominated thiomethylphenyl group did notproceed to completion in the same time scale as the lithiation of brominated methoxy-phenyl group. In addition to longer reaction times the reaction might also havedemanded an excessn-butyl lithium.

The most challenging part of syntheses were the preparation of new arylchloro-phosphines. Every new chlorophosphine starting material opens a route to a series ofnew phosphines.

20 21 22

P

OMe

MeS

SMe

P

OMe

SMeMeS

P

OMe

P

OMe

23

P

SMe

MeO

OMe

P

SMe

SMeMeS

P

SMe

24 25 26

P

SMe

27

25

4.2. Characterization of the phosphine ligands

Characterization of the ligands was based mainly on NMR spectroscopy.1H-, 13C-,31P- and two-dimensional HSQC-NMR techniques were used routinely in theidentification of the prepared compounds. Although the combination of1H-, 13C- ,31P- and HSQC-NMR spectra clearly revealed the individual chemical structures ofthe ligands, the characterization was confirmed by X-ray diffraction studies, massspectroscopy and elemental analyses.31P-NMR data, boiling and melting points andyields of the syntheses are presented in Table 1.

Table 1. Analytical data of the tertiary phosphine ligands.

Ligand δ (31P), ppm Bb. or Mp., ºC Yield, %1 163.6 98-100 / 0.3 torr 11.02 165.1 125-126 / 0.15 torr 8.83 164.6 86-89 / 0.1 torr 37.44 150.5 100-104 / 0.1 torr 26.35 -18.1 104 62.76 -22.0 100-103 86.57 -30.2 169-171 94.88 -15.6 119-120 99.19 -26.7 144-147 87.1

10 -37.1 191-195 92.811 -12.5 115-118 12.412 -11.2 Oily 63.013 -30.1 214 69.614 -27.7 146-149 55.115 -23.1 155-157 59.216 -2.3 100-101 83.217 -25.9 151-152 62.018 -21.1 144 45.019 -1.6 122-123 38.720 -30.0 156-159 47.021 -15.3 103-106 71.622 -31.9 222-224 32.323 -40.4 232-233 17.124 -34.1 180-182 62.225 -13.9 113-115 40.426 -29.7 204-205 73.827 -38.4 231-232 64.3

The 31P-NMR spectra were characteristic for each of the ligands. The chemicalshifts varied from 0 to –41 ppm for the ligands having organic substituent groups, andfrom 150 to 165 ppm for the dichlorophosphines.

The 1H-NMR signals of substituent thiomethyl protons were observed as follows:o-thiomethyl protons gave a signal at about 2.4 ppm, while the correspondingp-thiomethyl protons gave a signal 0.1 ppm to lower field. With methoxy protons the

26

observed signals were at about 3.7 ppm and 3.8 ppm foro-methoxy andp-methoxyprotons, respectively.o-N,N-Dimethyamine had a typical methyl signal at 2.6 ppm.

The resonance signals of the aromatic protons varied from 6.7 to 7.3 ppm,excluding the pyridyl protons, which had signals up to 8.7 ppm. Partial overlapping ofthe signals in1H-NMR spectra complicated the interpretation and the couplinginformation could not be fully resolved. Therefore only the centre of the signal groupsis noted in the tables. Overlapped signals are given with 0.1 ppm accuracy, whileindependent signals are given with 0.01 ppm accuracy. Full interpretation of the1H-NMR spectra (and also13C-NMR spectra) required two-dimensional HSQC-NMRspectra and decoupling experiments.1H-NMR data of the main substituent groups isperformed in Tables 2-5.

Table 2.1H-NMR data of the o-thiomethylphenyl group.

Ligand H3 H4 H5 H6 H7

4 8.08 7.5 7.5 7.5 2.525 7.3 7.3 7.05 6.76 2.436 7.3 7.3 7.06 6.74 2.437 7.3 7.3 7.05 6.72 2.45

13 7.2 7.2 7.0 7.0 2.3415 7.3 7.3 7.05 6.74 2.4318 7.3 7.3 7.06 6.74 2.4420 7.3 7.3 7.05 6.76 2.4424 7.3 7.3 7.03 6.8 2.4325 7.3 7.3 7.06 6.78 2.4426 7.3 7.3 6.95 6.70 2.4627 7.3 7.3 6.99 6.86 2.31

Table 3.1H-NMR data of the o-methoxyphenyl group.

Ligand H3 H4 H5 H6 H7

3 6.94 7.51 7.12 7.93 3.938 6.9 7.3 6.9 6.66 3.759 6.9 7.3 6.9 6.66 3.74

10 6.89 7.32 6.89 6.69 3.7414 6.8 7.31 6.8 6.67 3.7317 6.9 7.33 6.8 6.68 3.7420 6.86 7.3 6.91 6.64 3.7521 6.9 7.34 6.9 6.69 3.7522 7.0 7.0 6.76 6.58 3.7823 6.7 7.3 6.86 6.7 3.3924 6.85 7.3 6.9 6.67 3.75

SMe

P1

2

34

5

67

OMe

P1

2

34

5

67

27

Table 4.1H-NMR data of the p-thiomethylphenyl group.

Ligand H2 H3 H5

2 7.78 7.32 2.5217 7.2 7.2 2.4818 7.2 7.2 2.4819 7.2 7.2 2.4821 7.2 7.2 2.4825 7.2 7.2 2.47

Table 5.1H-NMR data of the p-methoxyphenyl group.

Ligand H2 H3 H5

1 7.82 6.99 3.8414 7.22 6.8 3.8015 7.24 6.90 3.8116 7.3 6.99 3.82

As in the 1H-NMR spectra the methyl carbon signals in the13C-NMR spectraexhibited typical chemical shifts. The methyl carbon signals ofpara substitutedgroups appeared at higher field than those of the correspondingortho substitutedgroups. The carbon signals of theo-thiomethyl carbons were at about 17.2 ppm andthose of thep-thiomethyl carbons at about 15.2 ppm. Similarly the signals of theo-methoxy carbons andp-methoxy carbons appeared at about 55.7 and 55.1 ppm,respectively.o-N,N-Dimethyamine gave a characteristic methyl carbon signal at 45.5ppm. The signals of the aromatic carbons varied from 110 to 165 ppm. The13C-NMRdata of the main substituent groups are set out in Tables 6-9. The numbering followsthe schemes presented above.

Table 6.13C-NMR data of the o-thiomethylphenyl group.

Ligand C1 C2 C3 C4 C5 C6 C7

4 140.6 142.1 130.4 132.5 128.4 132.9 20.55 136.8 143.7 126.6 129.3 125.4 133.2 17.46 136.3 143.8 126.8 129.3 125.3 133.3 17.37 135.5 144.1 126.9 129.4 125.4 133.5 17.3

13 136.7 144.2 126.3 129.1 125.2 131.5 17.015 136.8 143.5 126.8 129.2 125.3 133.1 17.318 136.2 143.7 126.8 129.3 125.3 133.3 17.320 135.9 144.0 126.7 129.2 125.2 133.4 17.424 136.4 143.8 126.8 130.0 125.2 133.3 17.425 136.7 143.5 126.5 129.3 125.3 133.0 17.226 134.9 144.3 126.7 128.6 125.3 132.8 17.127 136.8 144.4 127.1 128.7 125.3 133.0 17.2

P

SMe

1

2

34

5

2

3

P

OMe

1

2

34

5

2

3

28

Table 7.13C-NMR data of the o-methoxyphenyl group.

Ligand C1 C2 C3 C4 C5 C6 C7

3 127.4 160.7 111.0 130.8 121.7 134.2 56.18 125.6 161.1 110.2 130.3 121.0 133.6 55.69 125.1 161.3 110.2 130.1 120.9 133.7 55.7

10 124.7 161.5 110.2 129.8 120.8 133.8 55.614 125.7 161.2 110.1 130.0 120.8 133.5 55.617 125.1 161.3 110.2 130.1 120.9 133.7 55.720 124.0 161.6 110.3 130.4 121.1 134.0 55.721 125.4 161.0 110.2 130.4 121.0 133.5 55.722 123.9 161.7 110.3 130.5 121.2 132.6 55.823 126.1 161.4 110.1 129.8 121.1 132.7 55.324 124.4 161.6 110.3 130.1 120.9 133.9 55.7

Table 8.13C-NMR data of the p-thiomethylphenyl group.

Ligand C1 C2 C3 C4 C5

2 131.9 127.0 132.2 148.3 15.117 132.4 125.8 134.4 139.1 15.218 131.3 126.1 134.7 140.0 15.119 132.3 126.5 136.6 142.0 14.821 132.6 125.9 134.2 139.5 15.225 132.1 126.0 134.3 139.9 15.2

Table 9.13C-NMR data of the p-methoxyphenyl group.

Ligand C1 C2 C3 C4 C5

1 131.9 114.4 132.6 162.3 55.414 126.8 113.9 135.6 160.1 55.115 125.9 114.4 136.1 160.5 55.116 127.0 115.2 137.7 162.0 55.5

The chemical shifts of the different proton and carbon atoms (Tables 2-9) weremore or less the same for the different phosphines. The one-dimensional1H- and13C-NMR spectra proved difficult to interpret, however, and two-dimensional HSQC-NMR spectra were required for full assignment of the signals. Assignment of the1H-and 13C-NMR signals was carried out step by step. The spectra of the ligands withonly thiomethylphenyl7 or methoxyphenyl10 substituent groups, were solved first,by considering the1H-, 13C- and HSQC-NMR data together. The interpretation ofthese substituent groups was then utilized along with the data for thetriphenylphosphine ligand in assigning the signals of other substituent groups.

Differences in the1H- and13C-NMR spectra, as well as in the31P-NMR spectra, aregreater for the metal complexes of the ligands than for the ligands themselves. The

29

magnitude of the shifts is dependent on both the binding behaviour of the ligands andthe nature of the coordinated metal atoms [43, 51-52].

X-ray crystal structures were systematically determined for all ligands. Singlecrystals of the ligands were obtained by slow evaporation of solvent or by vapourdiffusion method. Structures were determined at the University of Joensuu.

Fig. 8. Crystal structures of 1,2-bis[bis(o-thiomethylphenyl)phosphino]ethane 13, (p-thio-methylphenyl)bis(2-pyridyl)phosphine 19, (o-methoxyphenyl)bis(p-thiomethylphenyl)-phosphine 21 and (o-thiomethylphenyl)bis(9-anthracenyl)phosphine 27.

Molecular structures of the ligands were as expected: only minor differences werefound in the bond angles and distances. Differences in structures and bonding were morepronounced in transition metal complexes of the ligands. Multidentate as well asmonodentate behaviour was clearly observed [51].

S

S

S

S

P

P

13 19

21 27

30

4.3. Properties and trends

The ligands prepared in this work are potentially multidentate. Ligands that containo-methoxyphenyl,o-thiomethylphenyl ando-dimethylaminophenyl groups can coordinateto a metal atom by forming a bridge between the two donor atoms.

Experimental results show that the ligands containingo-thiomethylphenyl groupstypically behave as bidentate ligands in metal complexes [51]. Theo-methoxyphenylligands, in turn, mostly behave as monodentate ligands [53-54]. Diphosphine ligands,which have two CH2 groups as a backbone, typically behave as a bidentate, bridgingligands [47]. Although 2-pyridyl ligands are potentially multidentate, they have beenfound to form either monodentate complexes or to behave as bridging ligands inbimetallic coordination compounds [55-56]. Heterodonor atoms inpara position ofaromatic rings do not take part in the metal coordination. Their effect on the ligand ismainly electronic.

Nitrogen donors offer a way of modifying the solubilities of phosphines and theircomplexes. The solubility of pyridylphosphine ligands, in both aqueous and organicsolvents, is easily controlled simply by adjusting the acidity of the solution [6]. This maybe a useful property in homogeneous catalysis where the catalyst needs to be separatedfrom the products [7-8].

The basicity of the ligands reflects the electronic effects of the substituents. Electrondonating substituents such as alkyl groups increase the basicity of ligands, while electronwithdrawing substituents such as aromatic groups with electronegative substituentsdecrease it. The basicity of phosphine ligands can be varied systematically by altering thesubstituents on the phosphorus atom. [57-58]

Stability of phosphine ligands against air oxidation is dependent on their basicity. Theexperimental results show that the basic alkyl phosphines tend to be both air and moisturesensitive. The relatively good stability of the aromatic tertiary phosphines, in turn, is arecommending feature for the use of these ligands in catalysis.

Although the electronic and steric effects are intimately related, and in many systemsdifficult to distinguish, they are discussed separately in the next two sections for theclosely related tertiary phosphine ligands prepared in this work.

31

4.3.1. Electronic effects

The electron donor and acceptor properties of the phosphorus atom form the basis for thebehaviour of the phosphines as ligands.31P-NMR is a valuable tool for obtaininginformation about the electronic state of the phosphorus atom. The31P-NMR chemicalshifts have been shown to depend on the R groups of tertiary phosphine PR3 [59-60]. Thechemical shifts of tertiary phosphine ligands can be calculated with the empirical equation(1) [59]

�=

+−=3

1

62n

Pnσδ (1)

whereσP values are increments characteristic for each substituent group. The referencevalue represents PMe3 (δ -62 ppm, reference 85 % H3PO4).

The increments (σP) for the different substituent groups used in constructing the newtertiary phosphine ligands can be calculated from the31P-NMR chemical shifts reported inthe literature (Table 10). In the literature ligands all three substituent groups are the same.When these increments were used to calculate the31P-NMR chemical shifts for theligands of this work, relatively good agreement was obtained with the experimental values(Table 11).

Table 10.31P-NMR chemical shifts of the PR3 type phosphines and the correspondingcalculated increment values of the substituent groups.

Ligand 31P-NMR chemical shift (δ, ppm) σP increment (δ, ppm)PCl3 220.4 94.13PEt3 -20.0 14.0PPh3 -3.3 19.57(o-MeSC6H4)3P -30.2 [61] 10.60(p-MeSC6H4)3P -8.3 [62] 17.90(o-MeOC6H4)3P -37.1 [61] 8.30(p-MeOC6H4)3P -9.60 [63] 17.47(o-Me2NC6H4)3P -27.8 [64] 11.41(o-NC5H4)3P -1.3 [65] 20.23(m-NC5H4)3P -24.7 [65] 12.77P(naf)3 -32.6 [66] 9.80P(anthr)3 -44.0 [67] 6.00

Chemical shift values for the aryldichlorophosphines calculated by equation (1) aresystematically 15-30 ppm too low. Otherwise, the compatibility of the measured andcalculated chemical shifts is good, the largest difference (-18.1 ppm, -12.26 ppm) beingfor ligand5.

32

Table 11. Comparison of measured and calculated31P-NMR shifts of the ligandssynthesised in this work.

Ligand Measured31P-NMR shift (δ, ppm) Calculated31P-NMR shift (δ, ppm)

1 163.6 143.732 165.1 144.103 164.6 134.564 150.5 136.865 -18.1 -12.266 -22.0 -21.237 -30.2 -30.20 (see Table 10)8 -15.6 -14.569 -26.7 -25.83

10 -37.1 -37.10 (see Table 10)11 -12.5 -11.4512 -11.2 -10.0913 -30.1 -26.8014 -27.7 -27.9315 -23.1 -23.3316 -2.3 -4.0717 -25.9 -27.5018 -21.1 -22.9019 -1.6 -3.6420 -30.0 -32.5021 -15.3 -17.9022 -31.9 -34.1023 -40.4 -41.7024 -34.1 -34.8025 -13.9 -15.6026 -29.7 -31.8027 -38.4 -39.40

Tolman [68] has estimated the electron donating properties of tertiary phosphines fromthe IR frequencies of the carbonyl groups in Ni(CO)3(PR3). The frequencies of differentligands were compared with the frequency of tri-t-butylphosphine, which was the mostbasic ligand in the series. On the basis of the experimental results, Tolman calculated thesubstituent contributions (χi) for different substituent groups. The empirical equation (2)for the trend is

( ) 13

1

1.2056)( −

=�+= cmCOi

iχν (2)

where the frequency 2056.1 cm-1 is the CO stretching frequency of the nickel carbonylcomplex of tri-t-butylphosphine. The larger theχi value, the less basic is the ligand.

It has been postulated that there is no general correlation between basicity and31P-NMR chemical shifts of the phosphine ligands [58, 69]. However, the basicity ofphosphines has been shown to be directly related to the31P-NMR chemical shifts ofcorresponding phosphine oxides [70]. In that study, phosphine oxides were used to

33

minimize the effect of the lone pair of phosphorus on the chemical shift and thereby allowthe electronic effects to dominate.

Evidently within certain chemically related groups of phosphines, a relationship can befound between the31P-NMR shifts and basicity. For example, the values correlate fairlywell for trialkylphosphines: P(t-Bu)3 2056.1 cm-1 vs. 63 ppm, P(i-Pr)3 2059.2 cm-1 vs. 19.4ppm, PEt3 2061.7 cm-1 vs. –20.0 ppm and PMe3 2064.1 cm-1 vs. –62 ppm (Fig. 9).

Fig. 9. Correlation of νννν(CO) and δδδδ(31P) values of trialkylphosphines.

The same kind of correlation, although of opposite direction, is observed for the groupof anisyl substituted phosphine ligands.ν(CO) values of these ligands have beenexperimentally measured or they can be calculated by using the substituent contributionsof o-C6H4OCH3 (0.9 cm-1) and C6H5 (4.3 cm-1) reported by Tolman [68]. Comparison ofthe experimental31P-NMR shifts and calculatedν(CO) values reveals reasonably goodagreement: PPh3 2069.0 cm-1 vs. –3.3 ppm, PPh2(o-MeOC6H4) 2065.6 cm-1 vs. –15.6ppm, PPh(o-MeOC6H4)2 2062.2 cm-1 vs –26.7 ppm and P(o-MeOC6H4)3 2058.8 cm-1 vs.–37.1 ppm (Fig. 10).

Fig. 10. Correlation of Tolmans νννν(CO) values and experimental δδδδ(31P) values of anisylsubstituted phosphine ligands.

-80-60-40-20

020406080

2054 2056 2058 2060 2062 2064 2066

PMe3

PEt3

P(i-Pr)3P(t-Bu)3

δ(31P)

ν(CO)

-40

-30

-20

-10

0

2056 2058 2060 2062 2064 2066 2068 2070

ν(CO)

δ(31P)

PPh3

PPh2(o-MeOC6H4)

PPh (o-MeOC6H4) 2

P(o-MeOC6H4)3

34

The fairly good correlation between the31P-NMR shifts andν(CO) values in theseexamples suggests that the31P-NMR chemical shifts provide information about thebasicity of closely related phosphine ligands.

The behaviour of the phosphines as electron pair donors (Lewis basicity) has strongeffect on the coordination chemistry of these ligands. The experimental31P-NMR datasuggest that, of the ligands prepared in this work, the pyridyl ligands16 and 19, shouldhave the highest Lewis basicity, and the ligands with large aromatic groups23 and 27,should be less basic. These relationships are in good agreement with previous results [57].In the groups ofo-thiomethylphenyl5-7 ando-methoxyphenyl8-10 ligands, the basicityof the ligands seems to increase with the decreasing number of thiomethyl or methoxygroups in the ligand. Addition of the substituents topara position of the phenyl ringaffects only the electronic properties of the ligands, while substituents inortho positionsalso have steric effects.

4.3.2 Steric effects

The steric properties of the phosphine will affect the selectivity of organometalliccatalysts prepared from it. The steric effect is not merely a measure of a ligand’s size, butalso its spatial requirement in the coordination environment. Often the spatial requirementis expressed as the Tolman cone angleθ [69, 71]. For symmetrical phosphine ligands thecone angle,θ, is defined as the apex angle of a cylindrical cone with origin 2.28 Å fromthe centre of the P atom. For unsymmetrical ligands, the effective cone angle can bedefined by using a model to minimize the sum of half-angles [72].

As alternatives for cone angle determination, also some other methods such as solidangle measures, angular symmetric deformation coordinate and ligand repulsive energyparameters have been reported for estimating the steric properties [73].

In this work, a family of tertiary phosphine ligands was prepared with their stericproperties systematically modified. The reference ligand was PPh3, which has a coneangle of 145° [71]. The series of ligands5-7 and8-10, where steric stress relative to PPh3

was added systematically, were prepared witho-thiomethylphenyl ando-methoxyphenylsubstituent groups, respectively. In addition, large aromatic groups like naphthyl andanthracenyl were added modify the steric properties. In thepara substituted and pyridylligands that were prepared, the steric stress was kept more like that of the PPh3 ligand,allowing the electronic properties to be studied more independently. In the case ofbidentate coordination of the ligand steric effects can not be described with cone anglemeasurements.

Since the main burden of this work was the study of free ligands rather than ofcomplexes, detailed consideration of the steric effects is being deferred to future studies.

35

4.4. Rh(I) and Co(0) complexes of 3-pyridyldiphenylphosphineIII

Pyridylphosphines have shown clear potential in the modification of catalytically activemetal centres. The amphiphilic property of their complexes make them of considerableinterest for use in catalytic reactions. Preparation of metal complexes of the less known 3-pyridyldiphenylphosphine was considered important therefore. 3-Pyridyldiphenyl-phosphine is capable of bridging two metal centres to form bimetallic complexes, butmore importantly, its electronic properties as a ligand differ from those of 2- and 4-pyridyldiphenylphosphines. Sterically 3-pyridyldiphenylphosphine is similar to PPh3, andit also forms complexes with structures similar to complexes of PPh3.

The metal carbonyl reagents were dissolved in tetrahydrofuran, and 3-pyridyldiphenyl-phosphine in tetrahydrofuran was added dropwise. The mixture of Rh(I) complex wasstirred at room temperature for about four hours and the mixture of Co(0) complexovernight. The Rh(I) complex was precipitated with acetone, whereas the Co(0) complexprecipitated during the reaction. The products were filtered and dried in vacuum. Singlecrystals of (3-PyPPh2)2RhCOCl were obtained by slow evaporation of dichloromethanesolution. The crystallization of Co2(CO)6(3-PyPPh2)2 was not successful because thecomplex decomposed in solution. The crystal structure of (3-PyPPh2)2Rh(CO)Cl ispresented in Fig. 11.

Fig. 11. The X-ray crystal structure of (3-PyPPh2)2Rh(CO)Cl.

3-PyPPh2 preferentially binds through its phosphorus atom to give the mononuclearcomplex. The crystal structure of (3-PyPPh2)2RhCOCl is very similar to the structures ofits 2-PyPPh2 [55, 74] and PPh3 [75-76] analogues. Comparison of the spectroscopicparameters of (3-PyPPh2)2Rh(CO)Cl with those of the analogues [74] confirms thesimilarity in structure of (3-PyPPh2)2Rh(CO)Cl and the corresponding Rh(I) complexes of2-PyPPh2 and PPh3 also in solution.

The reaction of 3-pyridyldiphenylphosphine with Co2(CO)8 produced the bimetalliccompound Co2(CO)6(3-PyPPh2)2. In the solid state, the product was reasonably air-stable.In solution, however, it decomposed slowly. The IR spectra in the carbonyl regionindicated an unbridged structure [77]: no signals were found in the bridged carbonylregion around 1700 cm-1. Comparison of the IR parameters of Co2(CO)6(3-PyPPh2)2 withthose of the analogous complexes of 2-PyPPh2 [77] and PPh3 [78] suggests similarstructures.

5. Bipyridine based ligandsI

The bipyridine ligands prepared in this work were designed to coordinate with twodifferent metal centres. Piperidine ligands were prepared to study the square planarcoordination site, and thiomorpholine ligands to study the potential binding together oftwo different metal centres. The bifunctional nature of these heterometallic structuresbroadens the range of their properties. In catalysts containing two metal centres, one mayact to bind the substrate while the other acts to feed electrons to or remove them from thefirst site. Also, the different metal centres can preferentially bond to different substrates.

5.1. Synthesis of starting materials

6,6’-Bis(bromomethyl)-2,2’-bipyridine28 and 4,4’-dimethyl-6,6’-bis(bromomethyl)-2,2’-bipyridine 29 were prepared by a modification of the methods of Craig [79] and Rodeand Breitmaier [80]. The 2-amino-6-methylpyridine was brominated, coupled with Raney-Nickel catalyst [81] and the fresh 6,6’-dimethyl-2,2’-bipyridine obtained was brominatedwith N-bromosuccinimide.

Fig. 12. Schematic structures of 6,6’-bis(bromomethyl)-2,2’-bipyridine 28 and 4,4’-dimethyl-6,6’-bis(bromomethyl)-2,2’-bipyridine 29.

5.2. Synthesis of ligands

6,6’-Bis(methylthiomorpholine)-2,2’-bipyridine30, 4,4’-dimethyl-6,6’-bis(methylthio-morpholine)-2,2’-bipyridine31, 6,6’-bis(methylpiperidine)-2,2’-bipyridine32 and 4,4’-di-

N N

BrBr

28

N N

BrBr

H3C CH3

29

37

methyl-6,6’-bis(methylpiperidine)-2,2’-bipyridine33 were prepared by the followinggeneral method. An excess of thiomorpholine or piperidine was added dropwise to asolution of 6,6’-bis(bromomethyl)-2,2’-bipyridine or 4,4’-dimethyl-6,6’-bis(bromo-methyl)-2,2’-bipyridine and potassium carbonate in N,N-dimethylformamide at roomtemperature. The reaction mixture was stirred for one hour and filtered using a doubleended needle. The solution was evaporated and the solid product subjected to columnchromatography on silica gel (CH2Cl2 : EtOH : acetone, 2 : 2 : 1, v/v). The major yellowband was collected, the solvent removed and the light yellow ligand dried in vacuum. Thesynthetic route is presented in Scheme 3.

Scheme 3. Synthetic route for the preparation of bipyridine based ligands.

5.3. Characterization

The characterization of the bipyridine based ligands and their Cu(I) complexes was basedmainly on1H-NMR measurements. Bipyridine protons gave signals in the aromatic area,at 7.2-8.3 ppm. Thiomorpholine and piperidine units had characteristic signals at 2.7-2.9and 1.7-2.6 ppm, respectively. The –CH2 arm in the 6,6’-position of the pyridyl ringtypically exhibited a signal at about 4.0 ppm and the methyl group in 4,4’-position atabout 2.5 ppm. Changing the substituent from thiomorpholine to piperidine did notsignificantly affect the positions of the resonance signals of other groups. The analyticaldata of the bipyridine ligands is compiled in Table 12. and the1H-NMR data in Table 13.

Table 12. Analytical data of the bipyridine ligands.

Ligand Mp., ºC Yield, %30 153 74.831 168 67.132 194 65.033 252 69.1

K2CO3N N

R R

Br Br

S

N

N

H

H

N N

R R

S

N

S

N

N N

R R

N N

30 R=H31 R=Me

32 R=H33 R=Me

N N

RR

N N

X X

1

23

4

5 6

7

89

10

11

12

13

14

38

Table 13.1H-NMR data of the bipyridine ligands

Ligand H4 H5 H6 H7 H9, H13 H10, H12 H10, H11, H12 H14

30 7.4 7.8 8.3 3.8 2.9 2.7 - -31 7.3 - 8.1 3.8 2.9 2.8 - 2.432 7.5 7.8 8.3 3.8 2.6 - 1.7 -33 7.6 - 8.2 4.1 2.6 - 1.9 2.5

Structural analyses of ligands30 and31 were performed by X-ray crystallography. Thecrystal structure of 4,4’-dimethyl-6,6’-bis(methylthiomorpholine)-2,2’-bipyridine31 isshown in Fig. 13.

Fig. 13. Crystal structure of 4,4’-dimethyl-6,6’-bis(methylthiomorpholine)-2,2’-bipyridine 31.

5.4. Cu(I) complexes of 6,6’-bis(methylthiomorpholine)-2,2’-bipyridineand 4,4’-dimethyl-6,6’-bis(methylthiomorpholine)-2,2’-bipyridine

In the syntheses of copper complexes of bipyridine based thiomorpholine ligands the aimwas to obtain complexes with square planar coordination where all nitrogen donors werecoordinated to metal atom. This requires an octahedral or a square planar coordinationsphere around the metal atom. Attempts were made to prepare Cu(II) complexes, but theinsolubility of the products made interpretation difficult. Cu(I) formed soluble complexesand was used in complexation studies, although it has tetrahedral coordination geometryand can not coordinate all nitrogen atoms at the same time. Later, the Cu(II) complex wasprepared with the analogous piperazine ligand [82].

The Cu(I) complexes of ligands30 and 31 were prepared by the following method.6,6’-Bis(methylthiomorpholine)-2,2’-bipyridine or 4,4’-dimethyl-6,6’-bis(methylthio-morpholine)-2,2’-bipyridine was dissolved in ethanol water 4:1 (v/v) mixture. Solid CuClwas added to the solution in one portion. The reaction mixture changed colour from greento orange during one hour stirring. The excess of CuCl was filtered of with a double endedneedle and the solvent was evaporated. The precipitate was subjected to columnchromatography on silica gel with acetone used as eluent. The dark orange band wascollected, the solvent removed and the product dried in vacuum.

39

The 1H-NMR spectra of the Cu(I) complexes of ligands30 and31 showed only minordifferences, from the spectra of the free ligands. The signals were slightly broadened, butthe chemical shifts were the same as for the free ligands. Structural analysis of the Cu(I)complex of ligand30 was performed by X-ray crystallography (Fig. 14).

The Cu(I) complex of the ligand30 (Fig. 14) formed spontaneously pseudo-helicalstructure (Fig. 15), where the dimeric assemblies were bound together by intermolecularcontacts between sulfur and hydrogen atoms. This kind of self-organization is typical forthe Cu(I) complexes of bipyridine and polypyridine structures [83]. Similar helical entitieshave been constructed with a variety of transition metals, for example Zn(II) [84] andAg(I) [85].

Fig. 14. Crystal struture of the Cu(I) complex of 6,6’-bis(methylthiomorpholine)-2,2’-bipyridine 30.

Fig. 15. Side view of the helical packing of the Cu(I) complex of 6,6’-bis(methylthio-morpholine)-2,2’-bipyridine 30.

6. Conclusions

The research reported here was directed at the preparation and characterization ofheterodonor ligands. Most of the work pertained to the preparation and characterizationof tertiary phosphine ligands with systematically modified electronic and steric properties.Thiomethylphenyl, methoxyphenyl, dimethylaminophenyl, pyridyl and large aromaticsubstituent groups were introduced to the phosphorus environment.

Some of the prepared ligands,1-3 and 5-13, were known already, but newspectroscopic and structural data were collected from them. Two ligands mentioned in thepatent literature,14 and 16, were presented as new ligands because no syntheticinformation had been reported. One new dichlorophosphine4, which offers a newsynthetic route for the preparation of aryldichlorophosphines, and fourteen new ligands14-27 were prepared during the work. All ligands were characterized by NMRspectroscopic methods. In addition, X-ray crystal structures were systematically measuredfor all ligands.

The phosphine ligands were prepared for catalytic purposes and were taken forhomogeneous hydroformylation studies at the Helsinki University of Technology and theUniversity of Joensuu. In catalytic studies a family of ligands offers the best basis for thefuture development of new effective and selective catalysts, and for this reason somepreviously reported ligands were synthesised as well.

Ligands 7 and 13 were sent for testing in the disease-orientedin vitro anti-cancerscreening program of the National Cancer Institute, USA, and ligand13 has shown someinteresting activity.

Bipyridine based ligands were prepared at the beginning of the work with the objectiveof obtaining ligands capable of forming bimetallic complexes. Six ligands28-33 wereprepared and characterized by NMR spectroscopic and X-ray diffraction methods, andused in complexation experiments. Unfortunately, most of the complexes proved to be ofvery low solubility, and this could not be improved significantly by adding methyl groups.The studies on bipyridines were not continued therefore.

The synthesis of tertiary phosphine ligands will continue. In future work, tailoring ofthe ligands will be directed more strictly towards existing applications. The trends thatwere observed provide a solid basis for the development of other ligands for newapplications in the future.

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