Organometallic Chemistry

Kenneth D. Karlin Department of Chemistry, Johns Hopkins University

karlin@jhu.edu http://www.jhu.edu/~chem/karlin/

JHU Course 030.442 Prof. Kenneth D. Karlin

Spring, 2010

Organometallic Chemistry 030.442 Spring 2010 Prof. Kenneth D. Karlin karlin@jhu.edu

Class Meetings: TTh, 12:00 – 1:15 pm ++ Remsen Hall 347

Textbook – Organometallic Chemistry (2nd Edition) Gary O. Spessard, Gary L. Miessler Oxford University Press

Course Construction: Homeworks, Midterm Exam(s), Oral Presentations (Grads) TA: Craig Bettenhausen (bettenhausen@jhu.edu)

Course Information: http://www.jhu.edu/~chem/karlin/ Rough Syllabus Most or all of these topics • Introduction, History/Key advances • Transition Metals, d-electrons • Bonding, 18 e– Rule (EAN Rule) • Ligand Types / Complexes • Types of Compounds M-carbonyls, M-alkyls/hydrides M-olefins/arenes

M-carbenes (alkylidenes alkylidynes) Other

• Reaction Types Oxidative Addition

Reductive elimination Insertion – Elimination Nucleophilic/electrophilic Rxs.

• Catalysis – Processes Wacker oxidation Monsanto acetic acid synthesis

Hydroformylation Polymerization- Olefin metathesis

Water gas-shift reaction Fischer-Tropsch reaction

p. 1

p. 2

Reaction Examples p. 3

•  Oxidative Addition Reductive Elimination

• Carbonyl Migratory Insertion

• Reaction of Coordinated Ligands CH3Mn(CO)5 CH3CMn(CO)5

OCO

Vaskaʼs complex

(Iron pentacarbonyl) (CO)4Fe– + :OH– ––––> (CO)4Fe

––––––> (CO)4Fe–H + CO2

OO

HC O

Reaction Examples - continued p. 4

• Wacker Oxidation C2H4 (ethylene) + ½ O2 –––> CH3CH(O) (acetaldehyde) Pd catalyst, Cu (co-catalyst)

• Monsanto Acetic Acid Synthesis CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)

• Ziegler-Natta catalysts – Stereoregular polymerization of 1-alkenes (α-olefins) 1963 Nobel Prize Catalyst: Ti compounds and organometalllic Al compound (e.g., (C2H5)3Al )

• Olefin metathesis – variety of metal complexes

2005 Nobel Prize – Yves Chauvin, Robert H. Grubbs, Richard R. Schrock

n CH2=CHR –––> –[CH2-CHR]n–

Organo-transition Metal Chemistry History-Timeline p. 5

•  Main-group Organometallics 1760 - Cacodyl – tetramethyldiarsine,

from Co-mineral with arsenic 1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)

•  1827 – “Zeiseʼs salt” - K+ [(C2H4)PtCl3]–

n-Butyl-lithium

Synthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl Bonding- Dewar-Chatt-Duncanson model

Organo-transition Metal Chemistry History-Timeline (cont.) p. 6

1863 - 1st metal-carbonyl, [PtCl2(CO)2] 1890 – L. Mond, (impure) Ni + xs CO –––> Ni(CO)4 (highly toxic)

1900 – M catalysts; organic hydrogenation (---> food industry, margerine) 1930 – Lithium cuprates, Gilman regent, formally R2Cu–Li+

1951 – Ferrocene discovered. 1952 -- Sandwich structure proposed

Ferrocene was first prepared unintentionally. Pauson and Kealy, cyclopentadieny-MgBr and FeCl3 (goal was to prepare fulvalene) But, they obtained a light orange powder of "remarkable stability.”, later accorded to the aromatic character of Cp– groups. The sandwich compound structure was described later; this led to new metallocenes chemistry (1973 Nobel prize, Wilkinson & Fischer). The Fe atom is assigned to the +2 oxidation state (Mössbauer spectroscopy).

The bonding nature in (Cp)2Fe allows the Cp rings to freely rotate, as observed by NMR spectroscopy and Scanning Tunneling Microscopy. ----> Fluxional behavior. (Note: Fe-C bond distances are 2.04 Å).

(Cp)2Fe Cp = cyclopentadienyl anion)

(h5-C5H5)2Fe

(pentahapto) Solid-state structure

1955 - Cotton and Wilkinson (of the Text) discover organometallic-complex fluxional behavior (stereochemical non-rigidity)

The capability of a molecule to undergo fast and reversible intramolecular isomerization, the energy barrier to which is lower than that allowing for the preparative isolation of the individual isomers at room temperature. It is conventional to assign to the stereochemically non-rigid systems those compounds whose molecules rearrange rapidly enough to influence NMR line shapes at temperatures within the practical range (from –100 °C to +200 °C ) of experimentation. The energy barriers to thus defined rearrangements fall into the range of 5-20 kcal/mol (21-85 kJ/mol).

Organo-transition Metal Chemistry History-Timeline (cont.)

Aside: Oxidation State 18-electron Rule

p. 7

Fluxional behavior; stereochemical non-rigidity (cont.) Butadiene iron-tricarbonyl

Xray- 2 COʼs equiv, one diff., If retained in solution, expect, 2:1 for 13-C NMR. But, see only 1 peak at RT. Cooling causes a change to the 2:1 ratio expected. Two possible explanations: (1) Dissociation and re-association or (2) rotation of the Fe(CO)3 moiety so that COʼs become equiv.

Former seems not right, because for example addition of PPh3 does NOT result in substitution to give (diene)M(CO)2PPh3.

Note: You can substitute PPh3 for CO, but that requires either high T or hv. So, the equivalency of the CO groups is due to rotation without bond rupture, pseudorotation.

13C-NMR spectra CO region, only

p. 8

Pseudorotation: Ligands 2 and 3 move from axial to equatorial positions in the trigonal bipyramid whilst ligands 4 and 5 move from equatorial to axial positions. Ligand 1 does not move and acts as a pivot. At the midway point (transition state) ligands 2,3,4,5 are equivalent, forming the base of a square pyramid. The motion is equivalent to a 90° rotation about the M-L1 axis. Molecular examples could be PF5 or Fe(CO)5.

Berry Pseudorotation p. 9

The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation. It most commonly occurs in trigonal bipyramidal molecules, such as PF5, though it can also occur in molecules with a square pyramidal geometry. The process of pseudorotation occurs when the two axial ligands close like a pair of scissors pushing their way in between two of the equatorial groups which scissor out to accommodate them. This forms a square based pyramid where the base is the four interchanging ligands and the tip is the pivot ligand, which has not moved. The two originally equatorial ligands then open out until they are 180 degrees apart, becoming axial groups perpendicular to where the axial groups were before the pseudorotation.

p. 10

Organo-transition Metal Chemistry History-Timeline (cont.) p. 11

1961 – D. Hodgkin, X-ray structure – Coenzyme Vitamin B12 (see other page) Oldest organometallic complex (because biological) (see other page)

1963 - Ziegler/Natta Nobel Prize, polymerization catalysts 1964 - Fischer, 1st Metal-carbene complex

1965 – Cyclobutadieneiron tricarbonyl, (C4H4)Fe(CO)3 – theory before experiment (C4H4) is anti-aromatic (4 π-electrons) With -Fe(CO)3, C4H4 behaves as aromatic

1965 – Wilkinson hydrogenation catalyst, Rh(PPh3)3Cl 1971 – Monsanto Co. – Rh catalyzed acetic acid synthesis

C CHH

R HC C

HH

H RCatalysis of 1,2-shifts (mutases)

or Homocysteine

methylation

Methylmalonyl-CoA ––> Succinyl-CoA (CoA = coenzyme A)

(homocysteine) RSH RSCH3 (methionine)

[B12CoIII-CH3]+ [B12CoI]–

Vitamin B-12 is a water soluble vitamin, one of the eight B vitamins. It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production. Vitamin B-12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin. A common synthetic form of the vitamin, cyanocobalamin (R = CN), does not occur in nature, but is used in many pharmaceuticals, supplements and as food additive, due to its stability and lower cost. In the body it is converted to the physiological forms, methylcobalamin (R = CH3) and adenosylcobalamin, leaving behind the cyanide.

Vitamin B-12 Co-enzyme p.12

5-deoxyadenosyl group

Organo-transition Metal Chemistry History-Timeline (cont.) p. 13

1973 – Commercial synthesis of L-Dopa (Parkinsonʼs drug) asymmetric catalytic hydrogenation 2001 Nobel Prize – catalytic asymmetric synthesis, W. S. Knowles (Monsanto Co.) R. Noyori,, (Nagoya, Japan), K. B. Sharpless (Scripps, USA)

1982, 1983 – Saturated hydrocarbon oxidative addition, including methane 1983 – Agostic interactions (structures)

AGOSTIC INTERACTIONS:

Agostic – derived from Greek word for "to hold on to oneself” C-H bond on a ligand that undergoes an interaction with the metal complex resembles the transition state of an oxidative addition or reductive elimination reaction.

Detected by NMR spectroscopy, X-ray diffraction

Compound above: Mo–H = 2.1 angstroms, IR bands were observed at 2704 and 2664 cm–1 and the agostic proton was observed at –3.8 ppm. The two hydrogens on the agostic methylene are rapidly switching between terminal and agostic on the NMR time scale.

p. 14

Organometallic Chemistry Definition: Definition of an organometallic compound Anything with M–R bond R = C, H (hydride) Metal (of course) Periodic Table – down & left

electropositive element (easily loses electrons) NOT: • Complex which binds ligands via, N, O, S, other M-carboxylates, ethylenediamine, water

• M–X where complex has organometallic behavior, reactivity patterns e.g., low-valent

Oxidation State Charge left on central metal as the ligands are removed in their ʻusualʼ closed shell configuration (examples to follow).

d n for compounds of transition elements N d < (N+1) s or (N+1) p in compounds e.g., 3 d < 4 s or 4 p

M –NR'

R''

p. 15

d n computation – very important in transition metal chemistry d n zero oxidation state of M in M-complex has a

configuration d n where n is the group #. Examples: Mo(CO)6 Mo(0) d n = d 6 (CO, neutral) HCo(CO)4 H is hydride, H–, --> --> Co(I), d n = d 8

Group 5 Group 6 Group 7 V(CO)6

– Cr(CO)6 Mn(CO)6+

V(–1) Cr(0) Mn(+1) d 6 d 6 d 6

Isoelectronic and isostructural compounds (importance of d n)

Effective Atomic # Rule; 18-Electron Rule (Noble gas formalism) # of electrons in next inert gas =

# Metal valence electrons + σ (sigma) electrons from ligands

Rule: For diamagnetic (spin-paired) mononuclear complexes in organotransition metal compounds, one never exceeds the E.A.N.

p. 16

Cr(CO)6 Cr ---> d6 6 electrons (CO)6 e– - pairs from 6 ligands 12 electrons ––> to [Ar] configuration 18 electrons

(will see more in M.O. diagram)

Consequence of EAN Rule: leads to prediction of maximum in coordination #

Max coordination # = (18 – n) / 2 n is from d n . d n 10 8 6 4 2 0 Max Coord # 4 5 6 7 8 9

– Change in 2-electrons results in change of only one in Coord. # – Any Coord. # less than Max # ---> “coordinatively unsaturated”

Fe(CO)42– Fe(CO)5

18 e– 18 e– Fe(–2) Fe(0)

d 10 d 8 4-coord 5-coord

both Coord. Saturated

p. 17

–2e– +CO

2e– –CO

[ReH9]2– e.g., as Ba2+ salt

Re(VII), (Mn,Tc, Re triad)

d 0, 9 hydride ligands; CN = 9

Geometry: Face capped trigonal prism

p. 18

A compound not obeying an rules

Fe5(CO)15C Iron-carbonyl carbide

Eighteen-Electron Rule - Examples Co(NH3)6

3+ Cr(CO)6

Obey 18-electron rule for different reasons

Carbonyl Compounds in Metal-Metal Bonded Complexes less straightforward

Fe2(CO)9 [π-Cp)Cr(CO)3]2 Co2(CO)8 (2 isomers)

p. 19

Δo

Δt

eg

t2g

t2

e

M+

M+

M+

M+

Freeionspherical

Freeionspherical

sixpointchargessphericallydistributed

fourpointchargessphericallydistributed

octahedralligand9ield

tetrahedralligand9ield

d6 Octahedral maximum of 6 coordinate p. 20

spherical9ieldof6charges

dz2,dx2-y2(e2g)(destabilized)

dxy,dxz,dyz(t2g) (stabilized)

10DqorΔo

Oh

lowercaselettersfororbital

Picture of Octahedral Complex

Various representations

(ignore “s orbital”

p. 21

The five d-orbitals form a set of two bonding molecular orbitals (eg set with the dz2 and the dx2-y2), and a set of three non-bonding orbitals (t2g set with the dxy, dxz, and the dyz orbitals).

eg orbitals point at ligands (antibonding) appropriate symmetry for σ-bonds to ligands σ-bonds will be six d2sp3 hybrids ndz2, ndx2-y2, (n+1)s, (n+1)px,py,pz t2g orbital set left as non-bonding

p. 22

p. 23

Standard MO diagram for Octahedral ML6 complexes

with σ-donor ligands

e.g., [Co(NH3)6]3+ (18 e–) e.g., W(Me)6 (12 e–)

Case I Electron-configuration unrelated to 18–-Rule

1st Row-Complexes with “weak ligands” Δo small or relatively small, eg* only weakly antibonding No restriction on # of d-electrons –– 12 to 22 electrons

p. 24

p. 25

Case II Compounds which follow rule insofar as they never exceed the 18-e– rule

• Metal in high oxidation state Δo is large(r) (for a given ligand) radius is small –-> ligands approach closely ––> stronger bonding

• 2nd or 3rd Row Metal - 4d, 5d Δo is large(er) (for a given ligand); d-orbitals larger, more diffuse.

Complex d n Total e– Complex d n Total e– ZrF6

2– 0 12 OsCl62– 4 16 ZrF7

3– 0 14 W(CN)83– 1 17

Zr(C2O4)44– 0 16 W(CN)6

4– 2 18 WCl6 0 12 PtF6 4 16 WCl6– 1 13 PtF6

– 5 17 WCl62– 2 14 PtF6

2– 6 18 TcF6

2– 3 15 PtCl42– 8 16

Less than 18 e–, but rarely exceed 18 e–

p. 26

Similar Result if ligands are high in Spectrochemical Series e.g., CN– Δo is larger

V(CN)63– d2

Cr(CN)63– d3

Mn(CN)63– d4 Less than or equal to 6 d-electrons

Fe(CN)63– d5 eg* not occupied

Fe(CN)63– d6

Co(CN)63– d6

however Co(II) d7 ––> Co(CN)53–

Ni(II) d8 ––> Ni(CN)42– and

Ni(CN)53–

Can have less than maximum # of non-bonding (t2g) electrons, because they

are nonbonding. Addition or removal of e– has little effect on complex stability

p. 27

Δo can get (or is) very small with π-donor ligands F– example (could be Cl–, H2O, OH–, etc.)

a)   Filled p-orbitals are the only orbitals capable of π-interactions

i)  1 lone pair used in σ-bonding ii)  Other lone pairs π-bond

b)  The filled p-orbitals are lower in energy than the metal t2g set

c)   Bonding Interaction i.  3 new bonding MOʼs filled by Fluorine electrons

ii.  3 new antibonding MOʼs form t2g* set contain d-electrons

iii.  Δo is decreased (weak field)

d)  Ligand to metal (L M) π-bonding i.  Weak field, π-donors: F, Cl, H2O ii.  Favors high spin complexes

p. 28

T1g,T2g

T1u,T2u

Eg T2g

A1g

T1u

A1g

T1u Eg

eg(σ)

eg(σ*)

t2g(π)

t2g(π*)

3d

4s

4p

π‐orbitalspx,py

σ‐orbital pz

MetalOrbitals

Molecular Orbitals

Ligand Orbitals

focusonthispartonly

bothsetsofdorbitalsaredriven↑inenergyduetolowerlyingligandorbitals Δo

p. 29

Δo Δo Δo

eg(σ*)

eg(σ*) eg(σ*)

t2g(π) t2g(n.b.)

t2g(π*)

M‐Lbondingnon‐bonding

bothareantibonding

π‐acceptor

largestseparation betweensetsofd‐orbitals

σ-donor

intermediateseparation

π-donor

smallestseparation

antibonding

Have discussed σ-donor and π-donor – now π-acceptor p. 30

T2g Eg

T1g, T2g T1u, T2u

A1g T1u Eg

eg (σM-L)

eg (σ* M-L)

t2g (π)

t2g(π*)

4d Δo

π*orbitals on CO (6 x 2 each - orthogonal)

σorbitals on CO (6 x 1 each)

Mo(CO)6 Metal Orbitals (only consider the d orbitals – 4s and 4p orbitals not included

in the analysis)

Molecular Orbitals

Ligand Orbitals

CASE III

L high in spectrochemical series: CO, NO, CN–, PR3, CNR

π-acid ligands – π-acceptors Can form strong π-bonds

18 e– rule followed rigorously

Orbitals on M used in such π-bonding are just those which are non-bonding

Result: Increase in Δo Imperative to not

Have electrons in eg* orbitals

Want to maximize occupation of t2g because they are stabilizing

p. 31

p. 32

Implications of 18e– Rule for Complexes with π-accepting ligands

In octahedral geometry almost always have 6 d-electrons 12 electrons from ligands

Other cases: # d-electrons and coordination # complementary • Coordination # exactly determined by electron-configuration and vice-versa

(see previous notes) BrMn(CO)5 (d ?) I2Fe(CO)4 (d ?) Fe(CO)5 (d ?) Ni(PF3)4 (d ?)

All 18-electron

When M has odd electron ––––> metal-metal bond (often bridging COʼs) Mn2(CO)10 Co2(CO)8

Some 17 electron species known: V(CO)6 d 5 Mo(CO)2(diphos)2]+ d 5

See MO diagram: Want to fill stable MOʼsʼ there is a large gap to LUMO

p. 33

eg

t2g dxz dyz

dxy

dz2

dx2-y2

Δo

(degenerate ) ML6 ML4

Major Exception: d 8 square-planar complexes

As one goes across periodic table, d and p orbital energy Level splitting gets larger – hard to use p orbitals for σ-bonding

Common to have 4-coordinate SP complexes – dsp2 hybridization

Common for: Rh(I), Ir(I)

Pd(II), Pt(II)

Rationalize d-orbital splittings look at d-orbital pictures/axes

Which d-orbitals?

p. 34

p. 35

Again, examples of complexes: dn C.N. Coord. Geom. Example(s)

d10 4 Td Ni(CO)4, Cu(py)41+

d10 3 Trig.planar Pt(PPh3)3

d10 2 Linear (PPh3)AuX, Cu(py)2+

d8 5 TBP Fe(PF3)5

d8 4 (square) planar Rh(PPh3)2(CO)Cl (trans)

d4 7 capped octahedral Mo(CO)5X2

d2 8 sq. antiprism ReH5(PMePh2)3, Mo(CN)84–

d0 9 D3h symmetry [ReH9]2–

tricapped trig. prism

p. 36

LIGANDS in Organometallic Chemistry:

Ligands, charge, coordination # (i.e., denticity)

X SnCl3 H (hydride) CH3 (alkyl, perfluoroalkyl)

Ar RC(O) (acyl) R3E (E = P, As, Sb, N) R2P

CO RNC (isonitrile, isocyanide) R2C (cabenoid, carbene)

R2N N2 C2H4 (olefin, alkene) R2C2 (acetylene)

C4H4 (cyclobutadiene) C3H5– (π-allyl) CH=CH-CH2

– (σ-allyl)

benzene (arenes) π-C5H5 (π-Cp) π-C7H7 (tropylium)

π-C3H3 (cyclopropenium, +) O (O-atom; oxide) NO (nitrosyl)

ArN2+ (diazonium)

p. 37

Carbon Monoxide – exceedingly important ligand CO-derivatives known for all transition metals

Structurally interesting, important industrially, catalytic Rxs Source of pure metal: Ni (Mond); Fe contaminated with Cu, purify via Fe(CO)5

Fe & Ni only metals that directly react with CO Source of oxygen in organics: RC(O)H, RC(O)OH, esters Processes: hydroformylation, MeOH ––> acetic acid double insertion into olefins, hydroquinone synthesis (acetylene + CO; Ru catalyst), acrylic acid synthesis (acetylene, CO, Ni catalyst) Fischer Tropsch Rx: CO + H2 ––> ––> CnH2n+2 + H2O

Most of these involve CO “insertion”

p. 38

Metal-Carbonyl Synthesis: Reduction of available (in our O2-environment) metal salts,

e.g., MX2, MʼX3, other (e.g., carbonates) M-carbonyls generally in low-valent oxidation states

––––> “Reductive Carbonylation” Reductants: CO itself ( ––> CO2), H2, Na-dithionite

Some Reactions: WMe6 + xs CO –––> W(CO)6 + 3 Me2CO

NiO + H2 (400 °C) + CO ––> Ni(CO)4

Re2O7 + xs CO ––> (OC)5Re–Re(CO)5 + 7 CO2

RhCl3 + CO + pressure + (Cu, Ag, Cd, Zn) –––> Rh4(CO)12 or Rh6(CO)16

Structures Possible: X-ray diffraction, Infrared spectroscopy Ni(CO)4 Fe(CO)5 M(CO)6 Td D3h Oh 2058 cm–1 2013, 2034 cm–1 2000 cm–1

Cl– acceptor/reductant

p. 39

13C NMR spectroscopy of M-CO fragments: 180 – 250 ppm Useful to use 13C enriched carbon monoxide

Can be useful to observed “coupling” to other spin active nuclei, e.g., 103Rh or 13P

H3B–CO = 2164 cm–1

no backbonding possible

p. 40

Metal-Carbonyl Structures (cont.): Polynuclear Metal-Carbonyls

p. 41

p. 42

p. 43

p. 44

The backbonding between the metal and the CO ligand, where the metal donates electron density to the CO ligand forms a dynamic synergism between the metal and ligand, which gives unusual stability to these compounds.

Valence Bond formalism: M–C O: M=C=O– + ::

p. 45

C–O stretching frequencies, ν(C-O)

Put more electron density on metal – by charge – by ligands which cannot π-accept

Remaining COʼs have to take up the charge (e–-density) on the metal See effects on ν(C-O).

Ni(CO)4 [Co(CO)4]– Fe(CO)42–

2057 cm–1 1886 cm–1 1786 cm–1

–––––––> –––––––> more –ve charge

Mn(dien)(CO)3+ ~ 2020, 1900 cm–1

Cr(dien)(CO)3 ~1900, 1760 cm–1 (dien not π-acceptor)

p. 46

Reactions of Metal-Carbonyl Complexes Substitution of CO: PX3, PR3, P(OR)3, SR2, NR3, pyridine, OR2, RNC, RCN, olefins, NO

Examples: Fe(CO)5 + 2 L (or L2) ––heat or hv––> [Fe(CO)3L2] + 2 CO Mo(CO)6 + cycloheptatriene –heat or hv––> [Mo(cht)(CO)3] + 3 CO Cr(CO)6 + arene ––heat or hv––> [Cr(arene)(CO)3] + 3 CO

L = PPh3 (trans) Fe(CO)5 + 2 H–C=C–H ––heat or hv––> [Fe(CO)3(C5H4O)] + CO Oxidation – Carbonyl Halides Mn2(CO)10 + Br2 ––heat ––> Mn(CO)5Br [FeCp*(CO)2]2 + Br2 ––heat ––> [FeCp*(CO)2Br] 2 PtCl2 + 2 CO –––> [Pt(CO)(Cl2]2 (µ2-Cl)2

O

FeCO

CO

CO

p. 47

Reactions of Metal-Carbonyl Complexes (cont.) Nucleophilic Attack – Reactions with bases Previous – Fe(CO)5 + hydroxide

Use: liberate M(CO)3 groups; or oxidize M

Fe2(CO)9 + 4 OH– ––––––> Fe2(CO)82–

Cr(CO)6 + 3 KOH ––––––> KHCr(CO)5 + K2CO3 + H2O

Cr(CO)6 + BH4– ––––––> [(CO)5Cr–H–Cr(CO)5]–

Fischer type M-carbene Fe

OCCO

CO

+ PF6–

FeOC

CO

C FeOC

CO

+ Na+BH4–

THF

–80 °CO

H

O–

H

M(CO)n + R3N+O– ––––> (CO)n–1M– –––> M(CO)n–1 + R3N + CO2CO

ON+R3

p. 48

Reactions of Metal-Carbonyl Complexes (cont.) Alkyl Metal Carbonyls NaMn(CO)5 + CH3X ––––> Mn(CO)5CH3

NaMn(CO)5 + RC(O)Cl ––––> Mn(CO)5C(O)R NaCo(CO)4 + (C2F5C(O))2O (anhydride) –––> Co(CO)4C(O)C2F5 +

Metal-Olefin Complexes Zeiseʼs Salt, Pt-olefin complex Ag-(triflate) + C2H4 ––––> [(C2H4)Ag-OSO2CF3] [PtCl4]2– + C2H4 ––––> [PtCl3(C2H4)]– + Cl– FeCp(CO)2I + C2H4 + AgBF4 –––> [FeCp(CO)2(C2H4)]BF4 + AgI Fp-CH2-CH=CH2 + H+ –––> [Fp-CH2-CH=CH2]+ Fp-CHMe2 + Ph3C+BF4

– –––> [Fp-CH2-CH=CH2]+ Fp = (Cp)Fe(CO)2– [ (Cp)FeI(CO)2• or (Cp)FeII(CO)2

+ ]

p. 49

Dewar-Chatt-Duncanson model for M-Olefin Bonding

Not unlike M-CO bonding

π and π* alkene orbitals Proper symmetry; good overlap σ -Bond – πolefin + empty d-orbital π – bond – π*olefin + filled d-orbital Overall double-bond character

M-olefin bonding reduces C–C bond-strength

p. 50

(η2-C70)Ir(X)(CO)(PPh3)2

C60 - Buckminsterfullerene – Buckyballs (Soccer-ball 6- and 5-membered rings) -

Fullerenes Buckyferrocene

p. 51

Olefin σ-donation to M, AND π-donation to π* by metal leads to reduction in C–C bond strength.

Also, for longer C–C distances, olefin is no longer planar. can regard metal-olefin as a metallacyclopropane (sp3 carbons)

M–L + olefin <––– Keq –––> M-olefin + L

Studied with Pd(II), Ni(0), Rh(I) Keq (M-olefin bond-strength) smaller for sterically hindered olefins Keq increased by e– -withdrawing substituents (-CN, -carboxyl) Keq decreased by e– -donating substituents

Back donation from M into olefin π*-orbital; predominant in M-olefin bonding.

Ni(0) – d10 – olefin cannot donate to the metal Electronic effect less pronounced with metal with less d e– -density Ni(0) > Fe(0) > Rh(I) > Pt(II)

p. 52

Also follow by IR spectroscopy: Ethylene, 1623 cm–1; Zeiseʼs salt, 1516 cm–1

p. 53

Olefin coordination tendencies:

Tend to be perpendicular to plane of Square-Planar Complexes

In the plane, for trigonal or TBP compounds (Not relevant for octahedral complexes)

In solution, olefins are not in fixed orientations – olefins rotate

(Cp)Rh(C2H4)2 –20 °C Cp : inner : outer “inner” and “outer” Hʼs = 5 : 4 : 4

RT two C2H4 peaks strongly broadened non-equiv Hʼs exchange at rate intermediate on NMR time scale + 57 °C two C2H4 peaks coalesce to one Exchange fast. NMR cannot distinguish between non-equiv Hʼs Cp remains singlet throughout whole T range

(Cp)Rh

i o

i o

p. 54

Two modes for rotation consistent with NMR spectroscopic data

C

CM

C

CM Propeller like movement

Os

PPh3

H''H''

H'H'

PPh3ON

OC

Hʼ ʻs are equiv to each other; same with Hʼʼ ʻs but

Hʼ ʻs are different from Hʼʼ ʻs

• Rotation about C–C axis would not change situation • Propeller movement would exchange non-equiv hydrogens

•NMR spectroscopy shows two separate peaks at –90 °C • they coalesce at –65 °C

–––> Propeller like movement is operative Measured barrier to rotation ~ 50-60 kJ/mole for C2H4

No rotation for CF2=CF2 and (NC)2C=C(CN)2 Stronger π-bonding restricts rotation

p. 55

Olefin metal complexes have a considerable use in organic synthesis

Metal alters chemical behavior of olefins

Metal can activate, deactivate or protect double bond for electrophilic or nucleophilic attack

Resolve optical or geometric isomers direct stereospecific attack aromatic or de-aromatize appropriate systems

Can effect olefin metathesis reactions - polymerizations

Fe Fe

Cp

CO

OC

CO

Cp

OC (CO)2CpFeClor

(CO)2CpFe–+ Cp(CO)2Fe

O–OTHF

0°C to 25 °C

30 min

2 HBF4

Et2O[Cp(CO)2Fe– ]+ BF4–

90 %

NaI

acetoneolefin liberated

Use method to reduce epoxides stereospecifically to olefins with retention of configuration

Example

p. 56

Another Important Reaction of Olefins Wacker Process – Hoechst-Wacker Process

Ethylene Oxidation - German Invention

1st Homogeneous Catalytic Process with organometallic (R-Pd) compound used on an industrial scale (related to hydroformylation)

Net Reactions:

[PdCl4]2– + C2H4 + H2O –––> CH3CHO + Pd + 2HCl + 2Cl–

Pd + 2CuCl2 + 2Cl– –––> [PdCl4]2– + 2CuCl

2CuCl + ½ O2 + 2HCl –––> 2CuCl2 + H2O ==============================================

C2H4 + ½ O2 –––> CH3CHO

p. 57

p. 58

p. 59

Mechanism summary Several interesting key points:

(1) there is no H/D exchange seen in this reaction. Reaction runs with C2D4 in water generate CD3CDO, and runs with C2H4 in D2O generate CH3CHO. Thus, keto-enol tautomerization is not a possible mechanistic step.

(2) There is a negligible kinetic isotope effect with fully deuterated reactants (k H/k D=1.07). Hence, it is inferred that hydride transfer is not a rate-determining step.

(3) a significant competitive isotope effect with C2H2D2, (k H/k D= ~1.9), suggests that the rate determining step should be prior to oxidized product formation.

The bulk of mechanistic studies on the Wacker Process debated whether nucleophilic attack occurred via an external (anti-addition) pathway or via an internal (syn-addition) pathway.

In summary, it was determined that syn-addition occurs under low-chloride reaction concentrations (< 1 mol/L, industrial process conditions), while anti-addition occurs under high-chloride (> 3 mol/L) reaction concentrations. However, the exact pathway and the reason for this switching of pathways is still unknown.

p. 60

Another key step in the Wacker process is the migration of the hydrogen from oxygen to chlorine and formation of the C-O double bond. This step is generally regarded to proceed through a so-called β-hydride elimination with a four-membered cyclic transition state:

One in silico study[JACS,2006] argues that the transition state for this reaction step is unfavorable (activation energy 36.6 kcal/mol) and proposes an alternative reductive elimination reaction mechanism in which the proton directly attaches itself to chlorine with an activation energy of 18.8 kcal/mol. The proposed reaction step gets assistance from a water molecule acting as a catalyst.

Pd(0) Reoxidation

Must be complicated

p. 61

Shows only Syn mechanism - Nucleophilic attack by OH- ligand

This and the next 5 slides

are due to Darren Achey

and Byron Farnum

Febʼ09

2

p. 62

Indicates Syn and Anti mechanisms – dependence on Cl- concentration p. 63

Beyramabadi, S. A.; Eshtiagh-Hosseini, H.; Housaindokht, M. R.; Morsali, A.; Organometallics, 2008, 27, 72-79.

- Water-Chain mechanism� - Compared Syn vs. Anti mechanisms for rate determining step� - All DFT calculations� - Accounted for kinetic isotope effect data for O-D vs O-H bond breaking

Syn addition

Anti addition

p. 64

Syn

Transition State

Syn-Product

p. 65

Anti

Transition State

Anti-Product

Lower Activation Barrier

Concluded to be the mechanism of rate-determining step

p. 66

Kieth, J. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A.; Henry, P. M.; Organometallics, Feb. 2009

- Rebuked the article by Beyramabadi et. al.

- Emphasized the well established nature of the syn mechanism at low [Cl-] (Standard Conditions)

Mech depends highly on [Cl-] and [CuCl2]

LL – syn mechanism

HL – Isomerization

HH – anti mechanism w/ chlorohydrin products

p. 67

Allyl Ligand Organometallic Complexes

unidentate 2-e– anionic ligand alkyl + neutral alkene (2-e– )––> bidentate rarely observed form most common structure [CH2=CHCH2Co(CN)5]3– behaves as delocalized π-system [CH2=CHCH2Mn(CO)5] 3 (4) electrons now valence electrons C–C stretch ~ 1620 cm–1

[(η3-C3H5)PdCl]2 [(π-C3H5)PdCl]2

∠C-C-C ~ 120 ° (sp2). C…C(observed) = 1.40-1.43 Å; C–C = 1.54 Å; C=C = 1.34 Å

p. 68

Allyl Ligand-M Complexes (continued)

[(h3-C3H5)PdCl]2 approximately square-planar (allyl as bidentate) 16-electron system

Pd(II), d8, 2 Clʼs (4 e–), allyl is 2 e–) (can think about allyl as 3e–)

Always Ligand to M. To maximize bonding: PdCl2Pd plane cuts π-allyl skeleton~ 2/3 of

the distance (center of gravity) from the central C-atom towards the terminal Cʼs

Can be M-to-L (L-anion) or L-to-M (allyl-cation) Maximize bonding for allyl ψ2:

want terminal Cʼs in the PdCl2Pd plane. So Allyl plane tilts wrt PdCl2Pd plane from 90° to ~ 110 ° (central C bent away); moves terminal Cʼs closer to M

Metal interaction with allyl ψ3: Always M-to-allyl ligand

p. 69

Allyl Ligand-M Complexes (continued)

Tilted allyl group: Organometallics 1985, 4, 285: Neutron diffraction structure (able to locate Hʼs) of a Ni-allyl complex. Hmeso and Hsyn are bent towards M (7° and 13° from planar); Hanti are bent 31° away. (D.Astruc text – says opposite) Typical static 1H-NMR of trihapto allyl: Hanti at 1 - 3 ppm, Hsyn at 2 - 5 ppm and Hmeso at4 - 6.5 ppm. There is no syn-anti proton-proton coupling. In the 13C-NMR, terminal Cʼs at 80 - 90 ppm; Central C, 110-130 ppm.

Allyl ligands can be fluxional on the NMR time scale; see “exchange” of Hʼs.

p. 70

Allyl Ligand-M Complexes (continued) Syntheses of Allyl-Metal complexes

From alkene-complex, H-attack on the metal Overall 1,3-hydrogen shift

Nucleophilic (electrophilic) substitution using allylic substrate

Oxidative addition of allylic substrate to low-valent metal

Protonation or insertion of a 1,3-diene complex

p. 71

Allyl-M (π-to-σ; η3-to-η1) interconversions important to catalysis/synthesis, as a way to create a vacent coordination site and a way to exert fluxional

behavior

With excess of PPh3, can observe η1-intermediate by IR spectroscopy 1600-1650 cm–1

[Mn(CO)5]– + C3H5Cl –––> (η1-C3H5)Mn(CO)5

––––> (η3-C3H5)Mn(CO)4 + CO

18-electron species

Allyl Ligand-M Complexes (continued)

Δ or hν

p. 72

Allyl Ligand-M Complexes (continued)

Some reactions:

Nucleophilic attack: can be stereoselective maybe useful

Insertion reaction

Reductive elimination

p. 73

Uranocene – Bis(cyclooctatetraene-dianion)-uranium Considerations of Aromaticity

Digression – A Metallocene of a Different Kind p. 74

METAL HYDRIDE COMPLEXES Important class of compounds (relate to M-olefin, M-R compounds) Hydrogenation (stereospecific), hydrogen storage (H2 economy), catalysis

Unstable hydrides discovered in 1930ʼs H2Fe(CO)4, HCo(CO)4 not understood

1957 breakthrough, J. Chatt (UK) trans H-Pt(Cl)(PEt3)2 discovered by accident, found good prep later cis-PtCl2(PEt3)2 + N2H4 –––> trans H-Pt(Cl)(PEt3)2 + N2 could be sublimed strong, sharp Pt-H stretch in infrared spectrum

confirmed by replacement by D Later, an X-ray structure was obtained, ––> ʻnormalʼ Pt(II) complex

Using M-H stretch, trans H-Pt(Cl)(PEt3)2 used to measure “trans effect” confirmed later via studies on actual relative rates of substitution (will discuss)

p. 75

IR/Raman Spectroscopy Terminal M–H ν(M-H) 1900 ± 300 cm–1

more intense than ν(C-H), ~ 3000 cm–1 weaker than ν(C-O), ν(N-N), ν(N-C), RNC

Bridging M–H 1000 ± 300 cm–1

(broad; v1/2 ~ 100 cm–1)

Metal-Hydride Complexes (continued) p. 76

Metal-Hydride Complexes (continued)

Aspects of Bonding

p. 77

Problem: Given a complex formulated as Ru(H)(CO)(Cl)(PPh3)3 IR bands are observed at 2020 cm–1 and 1933 cm–1. How can you assign the bands either to the Ru–H or C-O stretching frequencies?

M–Hterminal and M–CO terminal IR stretches are in similar regions of spectrum. Thus, cannot assign directly. A solution would be to prepare Ru(D)(CO)(Cl)(PPh3)3 or Ru(H)(13CO)(Cl)(PPh3)3, because IR bands would undergo an isotope shift.

Heavy isotope substitution reduces frequency of corresponding vibration; reduced mass, µ, in Hookeʼs Law, increases {Force constant k doesnʼt change; bond strengths change little with isotope substitution.}

Hookeʼs Law:

Metal-Hydride Complexes (continued) p. 78

For vibration at 2000 cm–1: 2000 cm–1 x (0.71) = 1420 cm–1 For Ru(D)(CO)(Cl)(PPh3)3: Should observe Ru–D at ~1420 cm–1

νRu-D / νRu-H = µRu-H / µRu-H1/2

~ 0.71

ν(13C–16O) / ν(12C–16O) = 0.978 For a Metal-Carbonyl at: ν(12O–16O) = 2000 cm–1

Δ(13C–O) ~ 44 cm–1

ν(18O–18O) / ν(16O–16O) = 0.9 For a Metal-Peroxide: ν(16O–16O) ~ 800 cm–1

Δ(18O2) ~ 48 cm–1

Metal-Hydride Complexes (continued)

ν1 / ν2 = µ2 / µ11/2

p. 79

NMR spectroscopy. high fields, delta (δ) 15-30 ppm relative to TMS along with coupling constants (e.g., JP,H) e.g., distinguish cis vs. trans.

useful for sterochemical analysis. Other book: The chemical shift range for hydrides is approximately +25 to –60 ppm. The downfield shifts are most common in d0, d10 and early transition metal cases whereas those with other dn counts and late transition metals tend to be upfield of zero. Coupling to other spin active nuclei such as 31P often makes structural assignments unambiguous.

Xray diffraction: near other heavy atoms, position often inferred Other ligands bend towards position of ʻhydrogenʼ, because it is small Neutron diffraction; finding of an atom proportional to Z (X-ray, it is Z2)

HMn(CO)5 Neutron diffraction study) Mn–H = 1.6 Å, which equals sum of the covalent radii.

Metal-Hydride Complexes (continued) p. 80

TRANS – EFFECT (INFLUENCE) in INORGANIC CHEMISTRY

Trans effect (influence) - trans effect is the labilization of ligands trans to certain other ligands, which can thus be regarded as trans directing ligand. It is attributed to electronic effects and it is most notable in square planar complexes, In addition to this kinetic trans effect, trans ligands also have an influence on the ground state of the molecule, notably on bond lengths and stability

Some authors prefer the term trans influence to distinguish it from the kinetic effect, while others use more specific terms such as structural trans effect (i.e., elongated trans M-L distances) or thermodynamic trans effect.

The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence:

F−, H2O, OH− < NH3 < py < Cl− < Br− < I−, SCN−, NO2−, SC(NH2)2, Ph− <

SO32− < PR3, AsR3, SR2, CH3

− < H−, NO, CO, CN−, C2H4

Established by substitution kinetic measurements, M–H stretch (as mentioned above), or other observations

p. 81

PtIICl Cl

Cl Cl

2–

PtIICl NH3

Cl Cl

1–

PtIICl NH3

Cl NH3

PtIIH3N NH3

H3N NH3

2+

PtIIH3N Cl

H3N NH3

1+

PtIIH3N Cl

Cl NH3

+ NH3 + NH3

–NH3

+ Cl–

–NH3

+ Cl–

– Cl–– Cl–

Cis

Trans

Classic example of the trans effect: the synthesis of cis-platin. Starting from PtCl42−, the first NH3 ligand is added to any of the four equivalent positions at random, but the second NH3 is added cis to the first one, because Cl− has a larger trans effect than NH3. If, on the other hand, one starts from Pt(NH3)4

2+, the trans product is obtained instead.

p. 82

In hydrido carbonyls, get mixing of ν(M-CO) and ν(M-H) modes, especially when CO and H are trans. So deuterate to shift M-D to lower energy and separate out (and less mode mixing).

p. 83 Metal-Hydride Complexes (continued)

Metal-Hydride Complexes – More Syntheses by Protonation p. 84

MnL L

L

H H

OC

[Cp2Re]– + H+ –––> HCp2Re

H–Ir(CO)L3 + HCl –––> Cl

+ H+ –––>

Mn(CO)5– + H+ H–Mn(CO)5

Ni{P(OEt)3}4 + H+ Ni(H){P(OEt)3}4

+

Fe Fe H

+

Metal-Dihydrogen (H2) Complexes

Figure: ORTEP Drawing

Neutron diffraction study, 30 K W(CO)3(PnPr)3)2(η2-H2)

Intact H–H bond Elongated (by ~ 20 %)

to 0.82(1) Å (lower PR3 disordered)

p. 85

LnMH

HLnM

H

HLnM

H

Hdihydrogencomplex

dihydridecomplex

Metal-Dihydrogen (H2) Complexes – (continued)

Elongated H–H bond: H2 is not physisorbed

but chemisorbed

H2 bond “activated” toward breaking

“This initially enigmatic

Interaction lies at the heart of all

Interactions of sigma bonds

X–Y with metals” (G.Kubas)

p. 86

H2-binding is reversible (to this analogue with P(Cy)3) “relevant to new materials for hydrogen storage” ?

Metal-Dihydrogen (H2) Complexes – (continued) p. 87

Metal-Dihydrogen (H2) Complexes – (continued) p. 88

p. 89

Metal-Dihydrogen (H2) Complexes – (continued)

Metal-Dihydrogen (H2) Complexes – (Syntheses) p. 90

M-H2 complexes also called “non-classical” hydrides

Metal-Dihydrogen (H2) Complexes –> Hydrides p. 91

Solution 1H NMR spectra of η2-H2 ligands normally give broad uncoupled signals throughout a large range of chemical shifts (2.5 to –31 ppm) that can overlap with those for classical hydrides. NMR can be used to determine dHH in solution by two different techniques involving measurement of either JHD or relaxation time, T1. JHD for the HD isotopomer of an H2 complex is the premier diagnostic for H2 versus hydride coordination. The 1H-NMR signal for an HD complex becomes a 1:1:1 triplet (D has I = 1 : (2I + 1) with a much narrower line width and is direct proof of the existence of an H2 ligand, since classical hydrides do not show significant JHD because no residual H–D bond is present. JHD for HD gas is 43 Hz, the maximum value (dHD ) 0.74 Å), and lower values (20 – 34 Hz) represent proportionately longer (shorter) dHD. JHD determined in solution correlates well with dHH in the solid state, and both Morris and Heinekey developed empirical relationships:

dHH = 1.42 – 0.0167(JHD) Å (Morris) dHH = 1.44 – 0.0168(JHD) Å (Heinekey)

p. 92 Metal-Dihydrogen (H2) Complexes – (H–H bonding / NMR)

Metal-Dihydrogen (H2) Complexes – (continued)

Dynamics in M–H2 and M–H Complexes

p. 93

p. 94 Metal-Dihydrogen (H2) Complexes –dynamics (cont)

Hydrogenase Metalloenzymes

Redox enzymes: billions of years old – found in microorganisms Catalyze complete reversible interconversion of H2 & H+ / e–

H2 as energy source or dispose of excess electrons via H2 release

High turnover rates: 104 turnovers/s

H2 2 H+ + 2e–

True equilibrium; position (e)affected by H2 pressure

H2 + D2O HD + HDO (rx observed) pH dependent ––> infer that H2 is split heterolytically at metal(s)

p. 95

Understanding the mechanism of hydrogenase might help scientists design clean biological energy sources, such as algae, that produce hydrogen

Hydrogenase – (continued) p. 96

Fe – Fe Hydrogenase

Molecular “wire”

p. 97

p. 98 Hydrogenase – (continued) Active-site attached

only at one point

Iron-Iron bond: Site of

H2 heterolysis

CO and CN ligands

On low-spin

Fe(II)

Possible Mechanism for Hydrogenase: H2 2H+ + 2e– p. 99

Proposed H2ase Mechanism With dithiolate bridge Based on DFT Calculations Overall charges not shown

p. 100

Proposed H2ase Mechanism aza-dithiolate bridge DFT calculations Overall charges not shown

p. 101

p. 102

Hydride transfers to M, resulting olefin may or may not stay coordinated Requirements – (i) Vacant site. (ii) complex usually has less than 18e–, Otherwise a 20 electron complex results immediately

Beta-hydride Elimination Mechanism –––> Four-center transition state inferred

M

HCH2CH2

M

H

CC

H

H H

H MCH2

CH2

H

Olefin-insertion – microscopic reverse reaction critical step in olefin polymerization

β-hydride elimination is a termination step in olefin polymerization

Known structures (X-ray and/or Neutron diffraction), Supporting the proposed four-center transition state

Stable M-alkyls – No beta-hydride elimination

Ex: Stabilize M-alkyl-to β-hydride elimination: have a stable

complex where ligands do not come off to create vacant site,

that which is needed

MCMe3

MCMe2Ph

MSiMe3

MCMe3 M R

Mneopentyl neophyl "silyl-neopentyl"

norbornyl

benzyl alkynyl

Rh(III) d 6

low-spin

Alpha-hydride elimination is the transfer of a hydride (hydrogen atom) from the alpha-position on a ligand to the metal center. The process can be thought of as a type of oxidative addition reaction as the metal center is oxidized by two electrons (Eq 1). As the reaction involves a formal oxidation of the metal, alpha-elimination can not occur in a d0 or d1 metal complex. In these cases, a variant called alpha-abstraction can occur. Alpha-abstraction does not result in a change of oxidation state and the alpha-hydrogen is transferred directly to an adjacent ligand instead of the metal center (Eq 2):

α-hydride elimination

Delta and gamma eliminations also exist

INSERTION REACTIONS

U = an unsaturated ligand

Insertion Reaction Net Result: Decrease in coordination, formation of new U–X bond

Reverse reaction referred to as deinsertion When deinsertion group is an olefin –––> β-hydride elimination

Migratory Insertions Anionic and neutral couple too form a new anionic ligand

{makes the neutral ligand (e.g., CO) more electrophilic and Susceptible to nucleophilic attack by the anionic ligand.}

Mechanistic Considerations: CO Insertion or Me Migration

For a generic insertion of CO into a metal alkyl bond, one can envision two mechanistic extremes, one in which the methyl migrates to the carbonyl and a second in which the carbonyl moves and inserts into the metal-methyl bond. Either way, this generates an open coordination site denoted here as the small box:

Mechanistic Considerations: CO Insertion or Me Migration

Label on one CO (13CO) cis to an acyl group, can differentiate the two possible mechanisms. If the CO moves during the deinsertion, then it can only move to a cis position, displacing another CO in the process. As there are four cis CO's and only one of them is labeled with 13CO, then we would expect to remove the labeled carbonyl 25% of the time. If the methyl group moves, it can also displace the 13CO 25% of the time. However, if it moves into one of the other three cis positions, it can do so in a cis and trans fashion with respect to the 13CO, something that can be detected spectroscopically:

MnOC CH3

CO

COCO

OC

MnOC

CO

COCO

OC

MnOC CH3

CO

COCO

OC

MnOC CH3

CO

COCO

OC

MnOC CH3

CO

COCO

OC

O

CH3

CO – CO

25%

50%

25%

This subtle but important difference was studied by Calderazzo (see Ang. Chem. Int. Ed. Eng. 1977, 16, 299 for his classic IR spectroscopy study) who examined the reverse reaction, deinsertion of CO from a metal acyl complex. By the Principle of Microscopic Reversibility, the insertion and deinsertion must follow the same mechanistic route, only in different directions.

Optically active compound. CH3NO2, MeCN, Me-migration DMSO, DMF, proplyene carbonate, HMPA CO migration

Possible intervention of η2-acyl-intermediate could make the interpretation of which is the migrating group “less than definitive”– RBJordanp. 170

FeP

CH3C

O

Most systems undergo migration; both mechanisms can occur

82 % yield 95 % e.e.

Inversion on Fe Et migrated

Retention on Fe CO migrated

CpFe(CO)(PPh3)Me N.R. after 5 days (rate < 10-7s-1)1 atm CO

0° C, CH2Cl2

CO few % Cp2Fe+

(Ph3P)(CO)CpFeC

Me

Ocomplete in <2 min under similar conditions

rate enhancement ~ 107!!

Mechanism:

CpFe(CO)(PPh3)Me– e–

[CpFe(CO)(PPh3)Me]+

CO

(Ph3P)(CO)CpFeC

Me

O

(Ph3P)(CO)CpFeC

Me

O

18 e– 17 e–(oxidizes starting material)

Electron-transfer Induced Insertion Reaction Chemistry

Ferrocinium cation

Alkene (Olefin) Migratory Insertions

This is the basis for almost all transition metal-based polymerization catalysts. A polymerization rxn is just many, many migratory insertions of an alkene and alkyl (the growing polymer chain) interspaced with alkene ligand addition reactions.

Alkynes can also do migratory insertions to produce vinyl groups:

Oxidative Addition Reactions

Oxidative addition is formally the microscopic reverse of reductive elimination, and it is not surprising that a series of reactions involving an oxidative addition, a rearrangement and then a reductive elimination form the basis for a variety of industrially important catalytic cycles

Oxidative addition reactions are most facile when there is a good two-electron redox couple. In other words, both the starting and final oxidation states are relatively stable. For example, oxidative addition from Ir(I) to Ir(III) is common but an oxidative addition from Fe(III) to Fe(V), while possible, is generally unlikely

The more reduced a metal center is (also electron-rich because of ligands), the greater the reactivity towards oxidative addition.

The likelihood of oxidative addition of A–B to a metal, M, depends on the relative strengths of the A–B, M–A and M–B bonds. For example, oxidative addition of an alkane is much less common than oxidative addition of an alkyl halide. For the alkane case, the C-H bond is fairly strong compared to the M-H and M-R (R = alkyl) bonds

Vaska Complex Oxidative Addition Reactions

Reductive Elimination Reactions

Eliminating groups must be

cis

If we consider that the DH-H = 104 kcal/mol and that the DM-H is 50-60 kcal/mol we see that these are essentially balanced and there should be no thermodynamic preference for a dihydride versus a reduced metal center. But DR-H is typically 100 kcal/mol versus a metal alkyl bond strength of 30 to 40 kcal/mol. We see that the thermodynamic situation is again approximately balanced with a slight preference for the forward reaction. DR-R is typically around 90 kcal/mol, so for two alkyl substituents, there is a strong thermodynamic driving force for the reaction to go to the right. C-C bond activation is unusually rare, but more examples continue to be found.

But important; C–H “activation”

Carbenes: neutral divalent six-electron carbon atom species

Transition Metal Carbene Complexes

Ground-state, single or triplet, depends on R and Rʼ

Transition Metal Carbene Complexes – 2 M-carbon double bonds ––> Metal-carbene complexes – 2 types

Fischer carbene complexes (right) low oxidation state M; heteroatoms at carbene carbon atom E.O. Fischer (1st Carbene complex (1964, then Nobel Prize with Wilkinson, for metallocenes) Schrock carbene complexes: higher oxidation state; C or H substituents at carbene C-atom “alkylidene complex” Richard Schrock MIT, 2005 Nobel Prize for olefin metathesis (shared with Robert Grubbs (Cal Tech) and Y. Chauvin (France).

MO/AO perspective: one lone pair is donated from the singlet carbene to an empty d-orbital on the metal (red), and a lone pair is back-donated from a filled metal orbital into a vacant pz orbital on carbon (blue). There is competition for this vacant orbital by the lone pair(s) on the heteroatom, consistent with our second resonance structure. Overall, bonding resembles that of carbon monoxide. Therefore, carbene ligands are usually thought of as neutral species, unlike dianionic Schrock alkylidenes (which usually lack electrons for back-donation). However, electron counting is just a formalism!

Transition Metal Carbene Complexes – 3

Transition Metal Carbene Complexes – 4

• Nucleophilic attack on coordinated carbonyl, then alkylation

• Protonation (akylation) of neutral acyl-complex

Transition Metal Carbene Complexes – 5 • Addition of ROH to a coordinated isocyanide ligand

• Synthesis of a N-heterocyclic Carbene Complex. NHCʼs are useful co-ligands (ancillary ligands) in reactive transition-metal carbene complexes.

Transition Metal Carbene Complexes – 6 • The first “non-stabilized” (i.e., with C-heteroatom) metal carbene complex).

• Schrock Carbene Complexes: α-deprotonation of a metal alkyl

Transition Metal Carbene/Carbyne Complexes – 7 Metal-alkyl hydride abstraction:

Metal Carbyne Complexes – Metal-carbon triple bond

Made serendipitously

Transition Metal Carbyne Complexes – 8

Reactions of Fischer Carbenes • Heteratom substitution – reflects electrophilic nature of the ʻCʼ-atom

Methyl group C–H acidity is enhanced because the carbanion formed is stabilized by the carbene group:

M-carbenes generally not good carbene transfer agents, however:

A ʻfreeʼ carbene is not likely formed

Reactions of Fischer Carbenes: continued • Olefin Metathesis or Cyclopropanation – Olefin [2 + 2] addition to M-carbene gives a metallocyclobutane. Then, a retro [2 + 2] could give olefin metathesis. However, cyclopropane formation is more (energetically) likely:

Reaction stereospecific

First isolated metallocyclobutane

complex Grubbs, 1980

Reactions of Fischer or Schrock Carbenes Another example: if you replace CO by PPh3 ––> proceeds with enantiomeric excess

Schrock carbenes are more reactive than Fischer carbenes The alkylidene carbon is nucleophilic

Reactions of Schrock Carbenes

Reactions of Schrock Carbenes - continued

Tebbe chemistry – Tebbe reagent

Reactions of Schrock Carbenes - continued

Carbene Migratory Insertions:

If we electron-count the carbene as a dianionic ligand we are reacting a monoanionic ligand (X) with a dianionic ligand (carbene) to make a new monoanionic ligand. Now formally a reductive coupling reaction (since the metal is being reduced and we are coupling together two ligands). One Can/Should consider the carbene (or alkylidene) as a neutral ligand.

For X = H–, the reverse reaction is called an α-hydride abstraction or elimination.

Carbene Migratory Insertion:

Note: This is Schrock type M-carbene complex, where carbene carbon is not electrophilic (like CO is traditional migration reactions). The reaction is probably aided by the overall metal complex positive charge.

Alkene/Olefin Metathesis Reaction Types

Very useful in natural product syntheses

Very useful in polymer syntheses

Olefin Metathesis Catalysts (there are many others)

From, “Organometallic chemistry and catalysis”, Didier Astruc (Springer, 2007)

From, “Organometallic chemistry and catalysis”, Didier Astruc (Springer, 2007)

Chauvin Mechanism of Alkene Metathesis

D2C=CD2 is the product from the deuterated substrate, only. From 1:1 substrate mixture, of you get 1:2:1 mixture of ehtylenes

ROMP Examples Schrock asymmetric catalyst (based on BINOL)

First ROMP commercial catalyst

ROMP & RCM

ROMP incredibly useful/practical in materials/polymer syntheses Ring closing metathesis (RCM) incredibly useful in

natural product and pharmaceutical syntheses.

Hepatitis B protease inhibitor

TaVCH2

HCp*2 TaIII CH3Cp*2

H2OH2O H2O

OxidativeAddition

SiH4

CH4 +

TaIII OCp*2

H

TaVO

HCp*2

TaV

SiH3

CH3Cp*2

H

TaVCH2

OHCp*2H2 +

Migratory Insertion may be disfavored (equilibrium lies toto the left) due to higher energy of the insertion product(right side). BUT - this higher energy may favorreactivity with the insertion product

Alpha Elimination

R

R

R'

R'M

R

R

R'

R'M

M

R 'R

R R'

Pairwise Mechanism:

R R

M CH2M

R

H2C CHR

R' R'++

MR

R' R'

R' R

MR'

+

+ D2

D2

+

Distinguished by an experiment with 1:1 mixture of:

Metathesis products:from 100% protiosubstrateThese olefins do not back-react with catylst (i.e. are not metathezised) due to A: low concentration of ethylene in reaction mixture B: stabilization of phenanthene

Carbene "stepwise" mechanism:

H2C CH2+

or

D

D

D

D

D2C CD2+

M M

M CH2 H2C CH2

M

M CH2++

M CH2

D2C

CD2

H2C CD2

M+

M CD2+M CD2

+

This leads to the observed product distribution:1:2:1

H2C=CH2 : H2C=CD2 : D2C=CD2

By Pairwise Mechanism(after initiation)

Stepwise

Predicts 1:1 product distribution

Homogeneous Catalysis EXAMPLES:

Wacker Process oxidation C2H4 (ethylene) + ½ O2 –––> CH3C(O)H (acetaldehyde) (Pd, Cu) Monsanto acetic acid synthesis CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst) Alkene Hydrogenation CH3CH2=CH2 + H2 ––> CH3CH2CH3 (all gases) ΔGf° = – 20.6 kcal/mol Hydroformylation

Polymerization Ziegler-Natta (1963 Nobel Prize) n CH2=CHR ––> –[CH2-CHR]n–

Olefin metathesis (Chauvin, Schrock, Grubbs, 2005 Nobel Prize)

Water gas-shift reaction H2O + CO –––> H2 + CO2 (all gases) ΔGf° = – 6.9 kcal/mol

Fischer-Tropsch reaction

Coupling reactions

Catalyst: Increases overall rate of a reaction

Does not change equilibrium position Significantly lowers activation energy

Is not used up in the reaction Normally interacts with substrate –––> alternative reaction pathway

Homogeneous Catalysis

Products of un-catalyzed and catalyzed reactions may be different

A Catalyst Significantly Lowers the Reaction Activation Energy

• Rate of all of the steps of the reaction are the “same” for catalytic cycle But, of course, the slowest individual step (“weakest link”) dictates the Rx rate

i.e., “turnover limiting step”

• TON, turnover #; # of reactant molecules the catalyst converts to product • TOF, turnover frequency; TON per unit time

To achieve high TON, reactants and products cannot bind too tightly to M • A catalyst may influence initial product distribution, giving preferential formation

of a less thermodynamically stable product, i.e., “selectivity”

Kinetic Competence Catalysis is a kinetic phenomenon: Activity may rely on minor (even minuscule) component (of catalyst or intermediate) Danger in relying too much on spectroscopic studies of catalytic systems

where you “see” only major components Must demonstrate a given step is ʻkinetically competent” to carry out the reaction. •• proposed intermediates reacts sufficiently fast to account for product formation

One issue is (has been) catalyst decomposition to metal, M(0) --- > heterogeneous catalysis ʻhomogeneous catalyst is heterogeneous catalyst in disguiseʼ (Crabtree)

Examples; hydrogenation catalysts are Pt group metal (Ru, Os, Rh, Ir, Pd, Pt) halides. MX2 in polar solvent with H2 (g) --- > colloidal metal particles (heterog.) Test: Add Hg(l), which selectively poisons any heterog. Pt group M –Absorption to

ʻactive sitesʼ

The Monsanto acetic acid process is the major commercial production method for acetic acid. Methanol, which can be generated from synthesis gas ("syn gas", a CO/H2 mixture), is reacted with carbon monoxide/catalyst ––> acetic acid. In essence, you have the insertion of carbon monoxide into the C-O bond of methanol, i.e. the carbonylation of methanol.

This process operates at a pressure of 30–60 atm and a temperature of 150–200 °C and gives a selectivity greater than 99%.

Limitations/Drawbacks 1. Rhodium is an expensive starting material. 1 mole of RhCl3•3H2O costs ~ $30,000! 2. I2 is cheap (about $20 per mole), but is extremely corrosive. Other halogens or halogen substitutes do not work nearly as well.

The Monsanto process has largely been supplanted by the Cativa process, a similar iridium-based process developed by BP Chemicals Ltd which is more economical and environmentally friendly

“The Cativa process: A method for the production of acetic acid by carbonylation of MeOH. The technology, similar to the Monsanto process, was developed by BP Chemicals and is under license by BP Plc.[1] [2]The process is based on an iridium-containing catalyst, such as the complex [Ir(CO)2I2]−. The Cativa and Monsanto processes are similar; they can use the same chemical plant. Initial studies by Monsanto had shown that iridium to be less active than the rhodium for the carbonylation of methanol. Subsequent research, however, showed that the iridium catalyst could be promoted by ruthenium, and this combination leads to a catalyst that is superior to the rhodium-based systems. The switch from rhodium to iridium also allows the use of less water in the reaction mixture. This change reduces the number of drying columns necessary, decreases by-products formation, and suppresses the water gas shift reaction. Furthermore, the process allows a higher catalyst loading. Compared with the Monsanto process, the Cativa process generates less propionic acid by-product.”

Homogeneous Hydrogenation Catalysis

Wilkinson’s Catalyst (1966) Nobel Laureate (1973; on another subject)

Catalyst Synthesis:

RhCl3(H2O)3 + 4 PPh3 –––>

RhCl(PPh3)3 + O=PPh3 + 2 HCl + 2 H2O

Formally: double bond attacks H+

(R)-Limonene smells of oranges (S)-limonene smells of lemons.

Insects use chiral chemical messengers (pheromones) as sex attractants; one of the enantiomers of the insect pheromone, olean, attracts male fruit flies, while its mirror image operates on the female of the species.

Most drugs consist of chiral molecules. Since a drug must match the receptor in the cell, it is often only one of the enantiomers that is of interest (active).

In the 1960ʼs, the drug thalidomide was prescribed to alleviate morning sickness in pregnant woman. Tragically, the drug also caused deformities in the limbs of children born by these woman. May have been the “wrong” enantiomer (?).

In drug development, pharmaceutical companies are required to carefully purify and test both enantiomers.

Chiral Molecules – Enantiomers with different Biological Effects

Catalytic Asymmetric Syntheses Early success, 1968: Dr William S. Knowles, Monsanto Company, St Louis, USA

Following (i)  Wilkinson catalyst discovery, and (ii) synthetic methods for chiral phosphines, Knowlesʼ strategy: replace triphenylphosphine in Wilkinsonʼs catalyst with the enantiomer of a known chiral phosphine and hydrogenate a prochiral olefin.

• Knowlesʼs catalytic asymmetric hydrogenation of α-phenylacrylic acid using a rhodium catalyst containing (-)-methylpropylphenylphosphine (69% ee) gave (+)-hydratropic acid in 15% ee.

Industrial synthesis of the rare amino acid L-DOPA Proved useful in the treatment of Parkinsonʼs disease

Early BIG Success in Catalytic Asymmetric Syntheses – Knowles (Monsanto)

enamide

Monsanto Process - first commercialized catalytic asymmetric synthesis employing a chiral transition metal complex. In operation since 1974.

protected AA

rate limiting

Rh(III)-alkyl complex Stepwise addition of H

Mechanism (J. Halpern): Asymmetric Hydrogenation of Enamides

COD or other weak

ligand(s) dissociated

Noyoriʼs (R. Noyori, Nagoya U.) General Hydrogenation Catalysts

1. Developed Widely Useful BINAP Chelating Diphosphine

Noyori’s (R. Noyori, Nagoya U.) General Hydrogenation Catalysts

2. Application

Noyori’s catalyst

Noyori’s (R. Noyori, Nagoya U.) General Hydrogenation Catalysts 3.  Application – Expand synthetic organic utility:

With Ru, hydrogenation of carbonyl group (rather than olefin)

The (R)-BINAP-Ru-(II)-catalyzed hydrogenation of acetol to (R)-1,2- Propanediol: Used for the industrial synthesis of antibacterial levofloxacin.

2001 Nobel Prize in Chemistry Catalytic asymmetric synthesis Dr William S. Knowles, Monsanto Company, St Louis, USA; Professor Ryoji Noyori, Nagoya University, Nagoya, Japan: Professor K. Barry Sharpless, The Scripps Research Institute, La Jolla, CA USA.

Nobel Prize ”their development of catalytic asymmetric synthesis”.

Knowles and Noyori receive half the Prize for: “ their work on chirally catalysed hydrogenation reactions” and Sharpless, the other half of the Prize for: ”his work on chirally catalyzed oxidation reactions”.

Key: enantiopure dialkyltartrate ligands for Ti

Also, developed Osmium catalyzed asymmetric dihyroxylation of olefins.

Chemistry Nobel Prizes: Four (4) in Organometallic Chemistry: Ziegler-Natta, Wilkinson, 2001/Enantioselective organic rxs., 2005/ Olefin Metathesis

Thermal or Photochemical Water-Gas Shift Reaction

The water-gas shift reaction (WGS) is widely employed in industry to enrich the hydrogen content in water gas (synthesis gas; Syngas; H2(g)/CO(g) ) after the steam reforming of methane. The WGS reaction is typically performed at high temperatures over heterogeneous iron oxide or copper oxide catalysts. Interest in the WGS shift reaction under mild, homogeneous conditions has been long-standing. Many soluble transition metal carbonyl complexes show activity for thermal WGS catalysis, usually in basic media. WGS activity is promoted photocatalytically, where the photons are typically used to open coordination sites by the expulsion of CO or photoextrusion of H2 from the transition metal center.

Water gas-shift reaction H2O + CO <––

––> H2 + CO2 (all gases) ΔGf° = – 6.9 kcal/mol

Proposed mechanism for the thermal (or hv) WGS reaction catalyzed by homoleptic Group 6 carbonyls. Chem. Rev. 2007, 107, 4022. A. J. Esswein & D. G. Nocera

* hv 1 photon

Photochemical Water-Gas Shift Reaction

bipy; 2,2’-bipyridine