Metal carbonyls may be mononuclear or polynuclear

Characterization of metal carbonyls IR spectroscopy (C-O bond stretching modes)

Effect of charge Effect of other ligands PF 3 weakest donor (strongest acceptor) PMe 3 strongest donor (weaker acceptor) Lower frequency, weaker CO bond  (free CO) 2143 cm -1 Increasing elec donating ability of phosphines

The number of active bands as determined by group theory

13 C NMR spectroscopy 13 C is a S = 1/2 nucleus of natural abundance 1.108% 1.6% as sensitive as 1 H only For metal carbonyl complexes  ppm (diagnostic signals) Very long T 1 (use relaxation agents like Cr(acac) 3 and/or enriched samples)

Typical reactions of metal carbonyls Ligand substitution: Always dissociative for 18-e complexes, may be associative for <18-e complexes Migratory insertion:

Metal complexes of phosphines PR 3 as a ligand Generally strong  donors, may be π-acceptor strong trans effect Electronic and steric properties may be controlled Huge number of phosphines available

Metal complexes of phosphines Basicity: PCy 3 > PEt 3 > PMe 3 > PPh 3 > P(OMe) 3 > P(OPh) 3 > PCl 3 > PF 3 Can be measured by IR using trans-M(CO)(PR 3 ) complexes Steric properties: Rigid structures create chiral complexes apex angle of a cone that encompasses the van der Waals radii of the outermost atoms of the ligand

Tolman’s electronic and steric parameters of phosphines

Typical reactions of metal-phosphine complexes Ligand substitution: Very important in catalysis Mechanism depends on electron count presence of bulky ligands (large cone angles) can lead to more rapid ligand dissociation

Metal hydride and metal-dihydrogen complexes Terminal hydride (X ligand) Bridging hydride (  -H ligand, 2e-3c) Coordinated dihydrogen (  2 -H 2 ligand) Hydride ligand is a strong  donor and the smallest ligand available H 2 as ligand involves  -donation and π-back donation

Synthesis of metal hydride complexes Characterize these kinds of reactions.

Characterization of metal hydride complexes 1 H NMR spectroscopy High field chemical shifts (  0 to -25 ppm usual, up to -70 ppm possible) Coupling to metal nuclei ( 101 Rh, 183 W, 195 Pt) J(M-H) = Hz Coupling between inequivalent hydrides J(H-H) = 1-10 Hz Coupling to 31 P of phosphines J(H-P) = Hz cis; Hz trans IR spectroscopy (M-H) = cm -1 (terminal); cm -1 bridging (M-H)/ (M-D) = √2 Weak bands, not very reliable

Some typical reactions of metal hydride complexes Transfer of H - Transfer of H + A strong acid !! Insertion A key step in catalytic hydrogenation and related reactions

Bridging metal hydrides 2-e ligand 4-e ligand bonding Non-bonding Anti-bonding

Metal dihydrogen complexes If back-donation is strong, then the H-H bond is broken (oxidative addition) Very polarized  +,  - Characterized by NMR (T 1 measurements) back-donation to  * orbitals of H 2 the result is a weakening and lengthening of the H-H bond in comparison with free H 2

Metal-olefin complexes 2 extreme structures metallacyclopropane π-bonded only sp 3 sp 2 Zeise’s salt Net effect weakens and lengthens the C-C bond in the C 2 H 4 ligand (IR, X-ray)

Effects of coordination on the C=C bond Compound C-C ( Å )M-C ( Å ) C2H4C2H (2) C 2 (CN) (2) C2F4C2F4 1.31(2) K[PtCl 3 (C 2 H 4 )]1.354(2)2.139(10) Pt(PPh 3 ) 2 (C 2 H 4 )1.43(1)2.11(1) Pt(PPh 3 ) 2 (C 2 (CN) 4 )1.49(5)2.11(3) Pt(PPh 3 ) 2 (C 2 Cl 4 )1.62(3)2.04(3) Fe(CO) 4 (C 2 H 4 )1.46(6) CpRh(PMe 3 )(C 2 H 4 )1.408(16)2.093(10) C=C bond is weakened (activated) by coordination

Characterization of metal-olefin complexes NMR 1 H and 13 C,  < free ligand X-rays C=C and M-C bond lengths indicate strength of bond IR (C=C) ~ 1500 cm -1 (w)

[PtCl 4 ] C 2 H 4  [PtCl 3 (C 2 H 4 )] - + Cl - Synthesis of metal-olefin complexes RhCl 3.3H 2 O + C 2 H 4 + EtOH  [(C 2 H 4 ) 2 Rh(  -Cl) 2 ] 2

Reactions of metal-olefin complexes

Metal alkyl, carbene and carbyne complexes

Main group metal-alkyls known since old times (Et 2 Zn, Frankland 1857; R-Mg-X, Grignard, 1903)) Transition-metal alkyls mainly from the 1960’s onward W(CH 3 ) 6 Ti(CH 3 ) 6 PtH(C  CH)L 2 Cp(CO) 2 Fe(CH 2 CH 3 ) 6 [Cr(H 2 O) 5 (CH 2 CH 3 ) 6 ] 2+ Why were they so elusive? Kinetically unstable (although thermodynamically stable) Metal-alkyl complexes

Reactions of transition-metal alkyls Blocking kinetically favorable pathways allows isolation of stable alkyls

Metal-carbene complexes L ligand Late metals Low oxidation states Electrophilic X 2 ligand Early metals High oxidation states Nucleophilic

Fischer-carbenes

Schrock-carbenes Synthesis Typical reactions + olefin metathesis (we will speak more about that) Compare to Wittig

Grubbs carbenes Excellent catalysts for olefin metathesis

Metal cyclopentadienyl complexes Metallocenes (“sandwich compounds”) Bent metallocenes “2- or 3-legged piano stools”

Homogeneous catalysis: an important application of organometallic compounds Catalysis in a homogeneous liquid phase Very important fundamentally Many synthetic and industrial applications

Usually distinct solid phase Readily separated Readily regenerated and recycled Rates not usually as fast as homogeneous May be difussion limited Quite selective to poisons Lower selectivity Long service life Often high-energy process Poor mechanistic understnding Same phase as reaction medium Often difficult to separate Expensive/difficult to recycle Often very high rates Not diffusion controlled Usually robust to poisons High selectivity Short service life Often takes place under mild conditions Often mechanism well understood Comparison of heterogeneous and homogeneous catalysts Difficulties in separation and catalyst regeneration have prevented a wider use of homogeneous catalysts in industry

Reaction  (FOS)  (CN)  (NVE) Association-Dissociation of Lewis acids 0±10 Association-Dissociation of Lewis bases 0±1±2 Oxidative addition-Reductive elimination ±2 Insertion-deinsertion 000 Fundamental reaction of organo-transition metal complexes

Combining elementary reactions

Completing catalytic cycles (reductive elimination) Olefin hydrogenation

Completing catalytic cycles  -H elimination no net reaction  -H elimination resulting in C=C bond migration Olefin isomerization

Completing catalytic cycles Olefin isomerization

Completing catalytic cycles Olefin hydrogenation

Wilkinson’s hydrogenation catalyst RhCl(PPh 3 ) 3 Very active at 25ºC and 1 atm H 2 Very selective for C=C bonds in presence of other unsaturations Widely used in organic synthesis Prof. G. Wilkinson won the Nobel Prize in 1973

The mechanism of olefin hydrogenation by Wilkinson’s catalyst

Other hydrogenation catalysts [Rh(H) 2 (PR 3 ) 2 (solv) 2 ] + With a large variety of phosphines including chiral ones for enantioselective hydrogenation Ru II /(chiral diphosphine)/diamine Extremely efficient catalysts for the enantioselective hydrogenation of C=C and C=O bonds Profs. Noyori, Sharpless and Knowles won the Nobel Prize in 2001

Olefin hydroformylation Cat:HCo(CO) 4 ; HCo(CO) 3 (PnBu 3 ) HRh(CO)(PPh 3 ) 3 ; HRh(CO)(TPPTS) 3  6 million Ton /year of products worldwide  Aldehydes are important intermediates towards plastifiers, detergents

(reductive elimination) Olefin hydrogenation What else could happen if CO is present? CO insertion reductive elimination R behaves as H did

Olefin hydroformylation

Catalysts for polyolefin synthesis Polyolefins are the most important products of organometallic catalysis (> 60 million Tons per year) Polyethylene (low, medium, high, ultrahigh density) used in packaging, containers, toys, house ware items, wire insulators, bags, pipes. Polypropylene (food and beverage containers, medical tubing, bumpers, foot ware, thermal insulation, mats)

Catalytic synthesis of polyolefin

High density polyethylene (HDPE) is linear, d  0.96 “Ziegler catalysts”: TiCl 3,4 + AlR 3 Electrophilic metal center Vacant site Coordinated alkyl Insoluble (heterogeneous) catalyst

Catalytic synthesis of polyolefin Isotactic polypropylene is crystalline “Natta catalysts”: TiCl 3 + AlR 3 Electrophilic metal center Vacant site Coordinated alkyl Insoluble (heterogeneous) catalyst, crystal structure determines tacticity

Catalytic synthesis of polyolefin “Kaminsky catalysts” Electrophilic metal center Vacant site Coordinated alkyl Soluble (homogeneous) catalyst, structural rigidity determines tacticity

Polymerization mechanism

The catalytic synthesis of acetaldehyde (Wacker process, oxidation of ethylene) Pd(0) + 2CuCl 2 PdCl 2 + 2CuCl 2CuCl + 2HCl + 1/2O 2 2CuCl 2 + H 2 O

The catalytic synthesis of acetaldehyde (Wacker process, oxidation of ethylene) C 2 H 4 + PdCl 2 CH 3 CHO + Pd(0) + 2 HCl Nucleophilic attack

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

The metathesis mechanism (Chauvin, 1971)