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Group 4 5 6 7 8 9 10 3d row Ti V Cr Mn Fe Co Ni 4d row 5d row Zr Hf Nb Ta Mo W Tc Re Ru Os Rh Ir Pd Pt Neutral stable compounds 0 I II III IV V ML7 ML6 MXL 6 MX2L6 MXL 5 MX2L5 MX3L4 (16e) MX4L4 (16e) ML5 ML4 MXL 3 (16e) MX2L4 MX3L4 MX4L3 (16e) MX2L2 (16e) MX3L3 MX4L3 MX5L2 (16e) Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count. MX4L2 Group 4 5 6 7 8 9 10 3d row Ti V Cr Mn Fe Co Ni 4d row 5d row Zr Hf Nb Ta Mo W Tc Re Ru Os Rh Ir Pd Pt Stable monocationic compounds 0 I II III IV V Now looking at compounds having a charge of +1 to obey 18 e rule. Elec count: 4 (M) +2 (NO) +12 (L6) = 18 Group 4 5 6 7 8 9 10 3d row Ti V Cr Mn Fe Co Ni 4d row Zr 5d row Hf Nb Ta Mo W Tc Re Ru Os Rh Ir Pd Pt Stable monocationic compounds [M(NO)L6]+ 0 [M(NO)L5]+ [ML6]+ (16e) I [MXL7]+ II IV [ML6]+ [MXL6]+ [MX2L5]+ (16e) III [M(NO)L4]+ [MX3L5,6]+ [ML4]+ (16e) [MXL5]+ [MX2L5]+ [MX3L4]+ (16e) MX2L2 (16e) [MX2L4]+ [MX3L4]+ [MX4L3]+ (16e) V NO+ is isoelectronic to CO X increases O N by 1 ML4 Elec Count: 4 (M) + 4 (L2) + 10 (L5) MX4L2 Actors and spectators Actor ligands are those that dissociate or undergo a chemical transformation (where the chemistry takes place!) Spectator ligands remain unchanged during chemical transformations They provide solubility, stability, electronic and steric influence (where ligand design is !) Organometallic Chemistry 1.2 Fundamental Reactions Fundamental reaction of organo-transition metal complexes Reaction D(FOS) D(CN) D(NVE) Association-Dissociation of Lewis acids 0 ±1 0 Association-Dissociation of Lewis bases 0 ±1 ±2 Oxidative addition-Reductive elimination ±2 ±2 ±2 0 0 0 Insertion-deinsertion FOS: Formal Oxidation State; CN: Coordination Number NVE: Number of valence electrons Association-Dissociation of Lewis acids D(FOS) = 0; D(CN) = ± 1; D(NVE) = 0 Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2 H H + BF3 W: H BF3 W H This shows that a metal complex may act as a Lewis base The resulting bonds are weak and these complexes are called adducts Association-Dissociation of Lewis bases D(FOS) = 0; D(CN) = ± 1; D(NVE) = ±2 A Lewis base is a neutral, 2e ligand “L” (CO, PR3, H2O, NH3, C2H4,…) in this case the metal is the Lewis acid HCo(CO) 4 HCo(CO) 3 + CO Crucial step in many ligand exchange reactions For 18-e complexes, only dissociation is possible For <18-e complexes both dissociation and association are possible but the more unsaturated a complex is, the less it will tend to dissociate a ligand Oxidative addition-reductive elimination D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2 Mn+ + X-Y M(n+2)+ X Y H Ph3P Cl I Ir CO PPh3 Vaska’s compound + H2 Ph3P IrIII Cl H PPh3 CO Very important in activation of hydrogen Oxidative addition-reductive elimination H becomes H- Concerted reaction H Ph3P CO IrI Cl + H2 Cl IrI PPh3 H H M PPh3 via H CO Ir: Group 9 SN2 displacement CO IrIII Cl PPh3 Vaska’s compound Ph3P Ph3P + CH3I cis addition CH3+ has become CH3+ CH3 Ph3P IrIII Cl CO PPh3 I- CH3 Ph3P IrIII Cl CO PPh3 I trans addition Also radical mechanisms possible Oxidative addition-reductive elimination Mn+ + X-Y M(n+2)+ X Y Not always reversible Mn+ + R-X Mn+ + R-H M(n+2)+ X R M(n+2)+ H R Insertion-deinsertion D(FOS) = 0; D(CN) = 0; D(NVE) = 0 M-X + L (CO)5Mn-CH3 + CO M-L-X O (CO)5Mn-C-CH 3 Mn: Group 7 Very important in catalytic C-C bond forming reactions (polymerization, hydroformylation) Also known as migratory insertion for mechanistic reasons Migratory Insertion CH3 OC CO + CO CO Mn OC OC O C CH3 Mn CO OC CO CO CO k1 k2 O OC + CO C CH3 Mn OC CO CO Also promoted by including bulky ligands in initial complex Insertion-deinsertion The special case of 1,2-addition/-H elimination R2C LnM CR'2 H LnM R2 C H C R'2 A key step in catalytic isomerization & hydrogenation of alkenes or in decomposition of metal-alkyls Also an initiation step in polymerization Attack on coordinated ligands Nu- Favored for electron-poor complexes (cationic, e-withdrawing ligands) M L E+ Favored for electron-rich complexes (anionic, low O.S., good donor ligands) Very important in catalytic applications and organic synthesis Some examples of attack on coordinated ligands Electrophilic addition Nucleophilic addition Cl Pt py py Cl Et Pt Cl py O O + N Cl Et3O+ + Fe(CO)3 Fe(CO)3 Electrophilic abstraction Nucleophilic abstraction Cp Cp + Ta CH3 CH3 Cp Me3PCH2 + Me4P+ Ta Cp Cp CH2 CH3 Fe OC OC OH- OH Cp Fe OC OC + OH2 -H2O Cp Fe OC OC Brooklyn College Chem 76/76.1/710G Advanced Inorganic Chemistry (Spring 2009) Unit 6 Organometallic Chemistry Part 2. Some physical and chemical properties of important classes of coordination and organometallic compounds Suggested reading: Miessler/Tarr Chapters 13 and 14 Metal Carbonyl Complexes M-CO CO as a ligand s donor, π-acceptor strong trans effect small steric effect CO is an inert molecule that becomes activated by complexation to metals “C-like MO’s” Frontier orbitals Larger homo lobe on C Mo(CO)6 anti bonding “metal character” non bonding “18 electrons” 6CO ligands x 2s e each 12 s bonding e “ligand character” Metal carbonyls may be mononuclear or polynuclear Synthesis of metal carbonyls Characterization of metal carbonyls IR spectroscopy M-C-O (C-O bond stretching modes) Effect of charge Lower frequency, weaker CO bond u(free CO) 2143 cm-1 Effect of other ligands PF3 weakest donor (strongest acceptor) PMe3 strongest donor (weaker acceptor) The number of active bands as determined by group theory 13C 13C NMR spectroscopy is a S = 1/2 nucleus of natural abundance 1.108% 1.6% as sensitive as 1H only For metal carbonyl complexes d 170-290 ppm (diagnostic signals) Very long T1 (use relaxation agents like Cr(acac)3 and/or enriched samples) Typical reactions of metal carbonyls Ligand substitution: Cr(CO)6 + CH3CN Cr(CO)5(CH3CN) + CO Always dissociative for 18-e complexes, may be associative for <18-e complexes Migratory insertion: CH3 OC Mn OC CO CO CO H3C CO H3C C CO O Mn OC CO CO CO C CO O Mn OC CO CO Metal complexes of phosphines M-PR 3 PR3 as a ligand Generally strong s donors, may be π-acceptor strong trans effect Electronic and steric properties may be controlled Huge number of phosphines available Metal complexes of phosphines M-PR 3 Basicity: PCy3 > PEt3 > PMe3 > PPh3 > P(OMe)3 > P(OPh)3 > PCl3 > PF3 Can be measured by IR using trans-M(CO)(PR3) complexes Steric properties: M R1 P R2 R2P M PR2 R3 The cone angle 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: HCo(CO) 4 + PBu3 HRh(CO)(PPh 3)3 + C2H4 HCo(CO) 3(PBu3) + CO HRh(CO)(PPh 3)2(C2H4) + PPh 3 presence of bulky ligands (large cone angles) can lead to more rapid ligand dissociation Very important in catalysis Mechanism depends on electron count Metal hydride and metal-dihydrogen complexes M M H H Terminal hydride (X ligand) M Bridging hydride (m-H ligand, 2e-3c) H Coordinated dihydrogen (h2-H2 ligand) M H Hydride ligand is a strong s donor and the smallest ligand available H2 as ligand involves s-donation and π-back donation Synthesis of metal hydride complexes IrCl(CO)(PPh 3)2 + H2 RuCl 2(PPh 3) 3 + H 2 Ir(H) 2Cl(CO)(PPh 3)2 Et3N RuHCl(PPh 3) 3 + Et 3N.HCl Co2(CO) 8 + H 2 2 HCo(CO)4 [Fe(CO) 4] 2- + H+ [HFe(CO) 4]- Cp2ZrCl2 + NaBH 4 Cp2ZrHCl Characterization of metal hydride complexes 1H NMR spectroscopy High field chemical shifts (d 0 to -25 ppm usual, up to -70 ppm possible) Coupling to metal nuclei (101Rh, 183W, 195Pt) J(M-H) = 35-1370 Hz Coupling between inequivalent hydrides J(H-H) = 1-10 Hz Coupling to 31P of phosphines J(H-P) = 10-40 Hz cis; 90-150 Hz trans IR spectroscopy n(M-H) = 1500-2000 cm-1 (terminal); 800-1600 cm-1 bridging n(M-H)/n(M-D) = √2 Weak bands, not very reliable Some typical reactions of metal hydride complexes Transfer of HCp2Zr(H)2 + 2CH2O Cp2Zr(OCH3)2 Transfer of H+ HCo(CO)4 H+ + [Co(CO)4]- A strong acid !! Insertion IrH(CO)(PPh3)3 + (C2H4) Ir(CH2CH3)(CO)(PPh3)3 A key step in catalytic hydrogenation and related reactions Bridging metal hydrides Anti-bonding Non-bonding 2-e ligand 4-e ligand bonding Metal dihydrogen complexes M Characterized by NMR (T1 measurements) H H i H M H OC OC P Pr3 CO H W H Very polarized d+, d- PiPr3 back-donation to s* orbitals of H2 the result is a weakening and lengthening of the H-H bond in comparison with free H2 If back-donation is strong, then the H-H bond is broken (oxidative addition) Metal-olefin complexes 2 extreme structures sp3 sp2 Zeise’s salt metallacyclopropane π-bonded only Net effect weakens and lengthens the C-C bond in the C2H4 ligand (IR, X-ray) Effects of coordination on the C=C bond Compound C-C (Å) M-C (Å) C2H4 1.337(2) C2(CN)4 1.34(2) C2F4 1.31(2) K[PtCl3(C2H4)] 1.354(2) 2.139(10) Pt(PPh3)2(C2H4) 1.43(1) 2.11(1) Pt(PPh3)2(C2(CN)4) 1.49(5) 2.11(3) Pt(PPh3)2(C2Cl4) 1.62(3) 2.04(3) Fe(CO)4(C2H4) 1.46(6) CpRh(PMe3)(C2H4) 1.408(16) 2.093(10) C=C bond is weakened (activated) by coordination Characterization of metal-olefin complexes IR n(C=C) ~ 1500 cm-1 (w) NMR 1H and 13C, d < free ligand X-rays C=C and M-C bond lengths indicate strength of bond Synthesis of metal-olefin complexes [PtCl4]2- + C2H4 [PtCl3(C2H4)]- + Cl- RhCl3.3H2O + C2H4 + EtOH [(C2H4)2Rh(m-Cl)2]2 Reactions of metal-olefin complexes Metal alkyl, carbene and carbyne complexes Metal-alkyl complexes Main group metal-alkyls known since old times (Et2Zn, Frankland 1857; R-Mg-X, Grignard, 1903)) Transition-metal alkyls mainly from the 1960’s onward W(CH3)6 Ti(CH3)6 Cp(CO)2Fe(CH2CH3)6 PtH(CCH)L2 [Cr(H2O)5(CH2CH3)6]2+ Why were they so elusive? Kinetically unstable (although thermodynamically stable) Reactions of transition-metal alkyls R LnM LnM + R-X X LnM R + H+ LnM+ + R-H Blocking kinetically favorable pathways allows isolation of stable alkyls Metal-carbene complexes pz pz sp2 sp2 : . R C R triplet carbene d : ds M L ligand Late metals Low oxidation states Electrophilic : C C R M . ds . . . C R Fischer carbene M R C R singlet carbene d . R Schrock carbene OR R M - M + C OR R R C R R X2 ligand Early metals High oxidation states Nucleophilic Fischer-carbenes Schrock-carbenes Synthesis t-Bu Cl Np 3Ta t-Bu 2LiNp -NpH Np 3Ta Np3 Ta H Cl t-Bu Typical reactions O X t-Bu Np 3Ta Np3Ta + X Y O H + Y + olefin metathesis (we will speak more about that) t-Bu Grubbs carbenes Excellent catalysts for olefin metathesis Metal cyclopentadienyl complexes M Metallocenes (“sandwich compounds”) M Bent metallocenes “2- or 3-legged piano stools” M M L L L L L Homogeneous catalysis: an important application of organometallic compounds M H M CO M H Very important fundamentally M PR3 Many synthetic and industrial applications M Cp Catalysis in a homogeneous liquid phase M Comparison of heterogeneous and homogeneous catalysts • 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 Difficulties in separation and catalyst regeneration have prevented a wider use of homogeneous catalysts in industry Fundamental reaction of organo-transition metal complexes Reaction D(FOS) D(CN) D(NVE) Association-Dissociation of Lewis acids 0 ±1 0 Association-Dissociation of Lewis bases 0 ±1 ±2 Oxidative addition-Reductive elimination ±2 ±2 ±2 0 0 0 Insertion-deinsertion Combining elementary reactions H H ML n H H -L + (oxidative addition) ML n ML n + H2 H H H ML x H ML x H C ML n C (ligand exchange) H (insertion) Completing catalytic cycles Olefin isomerization H H H ML x H C C H -H elimination no net reaction ML n H MLx H H H3C H C MLn CH3 CH3 C H3C H MLx H CH3 -H elimination resulting in C=C bond migration Completing catalytic cycles Olefin isomerization H H MLx H H3 C H2 H H MLx H MLx MLx CH3 H H3 C H C MLn CH3 C H CH3 Completing catalytic cycles Olefin hydrogenation H H ML x H C ML n C H C C H (insertion) ML n H ML n + H H C C (reductive elimination) Completing catalytic cycles Olefin hydrogenation H H H2 H2 C CH2 MLx H H MLx MLx H3 C CH3 H H H C MLn H C H H Wilkinson’s hydrogenation catalyst RhCl(PPh3)3 Very active at 25ºC and 1 atm H2 Very selective for C=C bonds in presence of other unsaturations AcO H2 RhCl(Ph3)3 Widely used in organic synthesis AcO Prof. G. Wilkinson won the Nobel Prize in 1973 H H The mechanism of olefin hydrogenation by Wilkinson’s catalyst Other hydrogenation catalysts [Rh(H)2(PR3)2(solv)2]+ With a large variety of phosphines including chiral ones for enantioselective hydrogenation RuII/(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 R + H2 + CO cat R R + H O O n-isomer Cat: i-isomer HCo(CO)4; HCo(CO)3(PnBu3) HRh(CO)(PPh3)3; HRh(CO)(TPPTS)3 6 million Ton /year of products worldwide Aldehydes are important intermediates towards plastifiers, detergents Olefin hydrogenation H H H ML x H C C C C H (insertion) ML n H ML n ML n + H H C C (reductive elimination) What else could happen if CO is present? O H OC C ML n C H H CO C O C ML n CO insertion C C H ML n + H reductive elimination C C H Olefin hydroformylation H H H2C H2 CH2 MLx H MLx MLx H3C H2 C H CHO O H CH3 C C H H H H C H H C H H MLn MLn CO 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 H2 C CH2 isotactic H2 C C H syndiotactic CH3 atactic Monomers Polymerization catalysts Polymers Catalytic synthesis of polyolefin H2C CH2 High density polyethylene (HDPE) is linear, d 0.96 “Ziegler catalysts”: TiCl3,4 + AlR3 Vacant site Ti Cl + R3Al + Ti R Electrophilic metal center Insoluble (heterogeneous) catalyst Coordinated alkyl Catalytic synthesis of polyolefin H2 C C H CH3 Isotactic polypropylene is crystalline “Natta catalysts”: TiCl3 + AlR3 Vacant site Ti Cl + R3Al + Ti R Coordinated alkyl Electrophilic metal center Insoluble (heterogeneous) catalyst, crystal structure determines tacticity Catalytic synthesis of polyolefin H2 C C H CH3 “Kaminsky catalysts” Vacant site + Zr X + Zr + MAO R X R Coordinated alkyl Electrophilic metal center Soluble (homogeneous) catalyst, structural rigidity determines tacticity Polymerization mechanism M X + "R-Al" initiation M R R' + M M R R R' M propagation M -H +H2 M H + P M H + P M X + P +HX termination The catalytic synthesis of acetaldehyde (Wacker process, oxidation of ethylene) C2H4 + PdCl 2 Pd(0) + 2CuCl2 2CuCl + 2HCl + 1/2O2 C2H4 + 1/2O2 CH3CHO + Pd(0) + 2 HCl PdCl2 + 2CuCl 2CuCl2 + H2 O CH3CHO The catalytic synthesis of acetaldehyde (Wacker process, oxidation of ethylene) C2H4 + PdCl2 CH3CHO + Pd(0) + 2 HCl H+/O2 H2O 2Cu+ 2Cu2+ OH2 PdII H+ Pd(0) CH3CHO H PdII H H H OH HO PdII H2 C CH2 OH PdII Nucleophilic attack Olefin metathesis The Nobel Prize 2005 (Chauvin, Schrock, Grubbs) 2RCH=CHR' RCH=CHR + R'CH=CHR' N N H R R N Cl Ru Mo Cl PCy3 Grubbs catalyst CMe 2Ph Ph H3 C(F2C)2CO O-C(CF3)2 CH3 Schrock catalyst ring-closing (RCM) (CH2)n (CH2)n + ring-opening (ROM) ADMET n ROMP n The metathesis mechanism (Chauvin, 1971)