M.C. White. Chem 153 Mechanism -43- Week of September 24th, 2002 Ligand Exchange mechanisms Associative ligand substition: is often called square planar substition becausel6 e-, d8 square planar complexes generally undergo ligand substitution via an associative mechanism( the M-Nu bond is formed before the M-X bond breaks ). The intermediate is 18e-and therefore provides a lower energy route to the product than a 14e -intermediate formed via dissociative substitution(the M-X bond is fully broken before the M-Nu bond begins to form). Analogous in many ways to SN2 reactions empty, non-bonding p, orbital can act as an acceptor orbital for the e- density of the incoming nucleophile L X 16e 1 8 e- intermediate 16 M= Ni(D), Pd(il) Pt(D), Rh(), Ir Dissociative ligand substitution is most favored in coordinatively saturated xes(e.g. d tetrahedral, octahedral ) In the dissociative mechanism, the M-X bond is fully Nu broken before the M-nu bond form thereby avoiding an energetically unfavorable 20 L L reactions Note that in all ligand M= Ru(i), Co(lr) processes, there Is no Rh(im), Ir(lm) state change at the met
M.C. White, Chem 153 Mechanism -43- Week of September 24th, 2002 Ligand Exchange Mechanisms Note that in all ligand substition processes, there is no oxidation state change at the metal center. Nu M = Ni(II), Pd(II), Pt(II), Rh(I), Ir(I). M L L X L Nu L M X Nu L M L L Nu L X L M L L X empty, non-bonding pz orbital can act as an acceptor orbital for the e- density of the incoming nucleophile 18 e- intermediates L 16 eL M L L Nu 16 eAssociative ligand substition: is often called square planar substition because16 e-, d8 square planar complexes generally undergo ligand substitution via an associative mechanism (the M-Nu bond is formed before the M-X bond breaks). The intermediate is 18e- and therefore provides a lower energy route to the product than a 14e- intermediate formed via dissociative substitution (the M-X bond is fully broken before the M-Nu bond begins to form). Analogous in many ways to SN2 reactions. Dissociative ligand substitution is most favored in coordinatively saturated 18e- complexes (e.g. d10 tetrahedral, d6 octahedral). In the dissociative mechanism, the M-X bond is fully broken before the M-Nu bond forms thereby avoiding an energetically unfavorable 20e- intermediate. Analogous in many ways to SN1 reactions. L M L L X L L M = Ru(II), Co(III), Rh(III), Ir(III) L M L L L L 18eNu: X- 16eL M L L Nu L L 18e-
M C. White. Chem 153 Structure bonding-17- Week of september 17, 2002 MO Description of o bonding in ML square planar Metal valenceorbitals Linear Combinations of The metry is favored by (), Pd (ID), Pt(D), Ir (D), rh(i) a stable electronic configuration is achieved at LUMO When combining orbitals, the resulting repelled by magnetic fields) and may be MOs must be symmetrically dispersed readily characterized by nmr between bonding and antibonding Thus, combining 3 orbitals(i.e. algs) requires one of the orbitals to be non- bonding HOMO eg In a square planar ligand field the degenerate d orbitals split into 9 symmetries.The degenerate p orbitals split into orbitals of eu and dxy
MO Description of σ bonding in ML4 square planar M.C. White, Chem 153 Structure & Bonding -17- Week of September 17, 2002 L M L L L L L L L y x dz2 dx2-y2 dxy dxz dyz pz px py Linear Combinations of Ligand σ Donor Orbitals Metal ValenceOrbitals LUMO 16 e - Rule: The square planar geometry is favored by d8 metals (e.g. Ni (II), Pd (II), Pt(II), Ir (I), Rh(I)). A stable electronic configuration is achieved at 16 e-, where all bonding and non- bonding orbitals are filled. Spin-paired compounds display diamagnetic behavoir (i.e. weakly repelled by magnetic fields) and may be readily characterized by NMR. s b2g eg b1g a1g a1g eu a2u a1g eu b1g a2u eu a1g a1g b1g eg b2g In a square planar ligand field the degenerate d orbitals split into orbitals of a1g, b1g, eg, and b2g symmetries. The degenerate p orbitals split into orbitals of eu and a2u symmetries. When combining orbitals, the resulting MO's must be symmetrically dispersed between bonding and antibonding. Thus, combining 3 orbitals (i.e. a1g's) requires one of the orbitals to be nonbonding. eg b2g a2u σ* HOMO n n σ
M.C. White. Chem 153 Mechanism -44- Week of september 24th, 2002 Associative substitution: the nucleophile Rate=-d[PtCl]=k/[PtCl,]+k,[Nul[PtChl dt Clv Meoh rt k: first order rate constant that arises from substition of leaving group by solvent k,: second-order rate constant for bi-molecular 16e attack of Nu on metal complex relative rate NI relative rate Meoh CHiCO Basicity of the incoming ligand F <158 (nucleophile) plays only role in its reactivity for soft 2754 metal centers. In general, the softest (i.e. most polarizable) nucleophiles react fastest with Br- soft metals like Pt(Il)via CHO 15.000 associative substitution. Steric 1175 29X10 ChUc (i.e. picoline vs pyridine)can retard the rate of substition 1349 (CH3O)3P Ph3P 85x10 1096 Et P 9.8x108 NH 1175
M.C. White, Chem 153 Mechanism -44- Week of September 24th, 2002 Nu relative rate 1 <100 <100 <158 <250 1175 MeOH CH3CO2- CO F- CH3O- (Et)3N N N H ClNH3 Nu relative rate N BrIC6H11CN (CH3O)3P PhSPh3P Et3P 158 1349 1096 1175 1549 N N H 2754 15,000 2.9 x 105 2.2 x 106 1.7 x 107 1.5 x 107 8.5 x 108 9.8 x 108 PtII Cl N Cl N Nu 16 ePtII Cl N Nu N 16 e- + Cl MeOH, rt - Associative Substitution:the nucleophile Basicity of the incoming ligand (nucleophile) plays only a minor role in its reactivity for soft metal centers. In general, the softest (i.e. most polarizable) nucleophiles react fastest with soft metals like Pt(II) via associative substitution. Steric hinderance at the nucleophile (i.e. picoline vs pyridine) can retard the rate of substition. Rate = -d [PtCl2] = k1[PtCl2] + k2[Nu][PtCl2] dt k1: first order rate constant that arises from substition of leaving group by solvent. k2: second-order rate constant for bi-molecular attack of Nu on metal complex
M.C. White. Chem 153 Mechanism -45- Week of september 24th, 2002 Associative substitution sterics Sterically shielding the positions above and below the plane of the square planar complex can lead to significant decreases in the rates of associative substition Et3 PIr, rt Eta k EtaP Py 100.000M-I sec EtaN Eta EtP EtaP EtaP IM-Isec Pearson Chem Soc. 1961 220
M.C. White, Chem 153 Mechanism -45- Week of September 24th, 2002 Associative Substitution: Sterics Pearson J. Chem. Soc. 1961 2207. k2 = 100,000 M-1 sec-1 + ClPtII Et3P Et3P Cl k2 = 200 M-1 sec-1 + ClPt Et3P II Et Cl 3P k2 = 1 M-1 sec-1 , rt + Cl- Pt Et3P II Et3P Py N N , rt Pt Et3 II P Et3P Py N , rt PtII Et3P Et3P Cl PtII Et3P Et3P Py Sterically shielding the positions above and below the plane of the square planar complex can lead to significant decreases in the rates of associative substition
M. C. White. Chem 153 Mechanism -46- Week of September 24, 2002 Associative substitution sterics as the steric bulk of the im ine backbone increases the associative second order rate constants fo aryl groups become more rigidly locked perpendicula ethylene exchange were examined by HNMR to the square plane making their ortho substituents more effective at blocking the axial sites above and in cdch at -85C (BAr4) (BAr4 k= too fast to measure k=8100 L/mol/sec k=45 L/mol/sec even at-1000C Brookhart JACS 1995(117)6414 (BAr4) CH.R Ruffo om1998(17)2646
M.C. White, Chem 153 Mechanism -46- Week of September 24, 2002 = Brookhart JACS 1995 (117) 6414. N N Pt CH3 R + (BAr'4)- N N Pd CH3 N N Pd CH3 N N Pd CH3 k = too fast to measure even at -100oC. + (BAr'4)- + (BAr'4)- k = 8100 L/mol/sec + (BAr'4)- k = 45 L/mol/sec as the steric bulk of the imine backbone increases, the aryl groups become more rigidly locked perpendicular to the square plane making their ortho substituents more effective at blocking the axial sites above and below the plane. associative second order rate constants for ethylene exchange were examined by 1HNMR in CDCl2 at -85oC Associative Substitution: Sterics Ruffo OM 1998 (17) 2646
M.C. White. Chem 153 Mechanism -47 Week of Septem ber 24th, 2002 Brookhart polymerization catalysts (BAr'4) CH Polymer Mw=110,000 Polymer Mw=390,000 (BAr‘4) BAr‘4 CH3 CH Insertion elimination (BAr‘4) (BAr'4 人m三心 CHa displacement Low Mw Brookhart JACS 1995(117)6414
M.C. White, Chem 153 Mechanism -47- Week of September 24th, 2002 N N R R Pd CH3 N N R R Pd CH3 H N N R R Pd CH3 N N R R Pd CH3 H N N R R Pd H N N R R Pd H N N R R Pd CH3 H + (BAr'4)- + (BAr'4)- + (BAr'4)- polymer propagation + (BAr'4)- β-hydride elimination insertion High Mw polymers associative displacement + (BAr'4)- + (BAr'4)- Low Mw polymers + (BAr'4)- N N Pd CH3 N N Pd CH3 + (BAr'4) - Polymer Mw = 110,000 + (BAr'4) - Polymer Mw = 390,000 Brookhart JACS 1995 (117) 6414. Brookhart Polymerization Catalysts
M.C. White/QChen Chem 153 Mechanism -48- Week of Septem ber 24th, 2002 n to nl-Cp via Slipped n'-Cp Intermediate lipped n@CpI P(CH3)3 P(H3)3 Oc Re OC—ReP(CH3) P(CH3)3 apped n cp 2(18e-) 18e Based on the observation that the rate of reaction of 1 with P(CH3)3 /C1 to form 2 depends on both the concentration of 1 and P(CH3),an associative mechanism was proposed. To account for associative substition at a formally coordinatively and electronically saturated center, the authors propose an m"slipped"Cp intermediate that forms concurrently with phosphine attack. c10 Casey oM1983(2)535
M.C. White/Q.Chen Chem 153 Mechanism -48- Week of September 24th, 2002 Based on the observation that the rate of reaction of 1 with P(CH3)3 to form 2 depends on both the concentration of 1 and P(CH3)3, an associative mechanism was proposed. To account for associative substition at a formally coordinatively and electronically saturated center, the authors propose an η3 "slipped" Cp intermediate that forms concurrently with phosphine attack. Casey OM 1983 (2) 535. η5 to η1-Cp via Slipped η3-Cp Intermediate Re OC OC CO OC Re P(CH3)3 OC CO proposed "slipped" η3 Cp intermediate 1 (18 e-) P(CH3)3 OC Re OC P(CH3)3 P(CH3)3 CO H 2 (18 e-) 18 e- η1 η Cp 5 Cp slipped η3 Cp 64oC 2
M C. White/M. w. Kanan Chem 153 Mechanism -49- Week ofSeptember 24th, 2002 n5 to n3 ring Folding co 18 18 18 e-complexes with cyclopentadienyl ryl, indenyl ligands under associative"substitution avoiding an energetically unfavorable 20 e intermediate via ligand rearrangement from n5 to n3 (cyclopentadienyl and indenyl) Haptotropic rearrangement may take the form of ring"slippage"where the ring is 2.98A centrally bonded to the metal and its aromaticity is disrupt a non-bonding distance "bending"where the conjugation of th 228A π system is broker Huttner J Organometallic Chem. 1978(145)329
M.C. White/M.W. Kanan Chem 153 Mechanism -49- Week of September 24th, 2002 η5 to η3 Ring Folding 2.28Å WII CO 18 eCO WII CO CO 18 e- 2.98Å: a non-bonding distance Huttner J. Organometallic Chem. 1978 (145) 329. 18 e- complexes with cyclopentadienyl, aryl, indenyl ligands may undergo "associative" substitution avoiding an energetically unfavorable 20 eintermediate via ligand rearrangement from η5 to η3 or even η1 (cyclopentadienyl and indenyl). Haptotropic rearrangement may take the form of ring "slippage" where the ring is acentrally bonded to the metal and its aromaticity is disrupted or ring "bending" where the conjugation of the π system is broken
M C. White. Chem 153 Mechanism -50 Week of september 24th, 2002 Ligand exchange: Dissociative Mechanism The rate-determining step in a dissociative ligand substition pathway is breaking the M-L bond. Because of the late, product-like transition state for forming the coordinatively unsaturated intermediate in such a process, the M-L BDE is a good approximation of the activation energy(EA) 18e rate of ethylene exchange via rate of ethylene exchange via associative displacement at dissociative displacement at 25i~4x10-0sec4 e Q BDE =31 kcal/mol rXn coordinate PPh3 (mmol) kx 10 sec-I 0.20 PPh RhPPh3 1.23 1.73 a 6-fold increase in the concentration of nucleophile does 18e- 16e 18e affect the rxn rate. Results are consistent with a mechanism where the rate determining step is ethylene rate =d [rh(cH4h= k, [rh(chahl di d is not affected by the concentration of the Cramer JACS 1972(94)5681 dt nucleophile
M.C. White, Chem 153 Mechanism -50- Week of September 24th, 2002 Ligand Exchange: Dissociative Mechanism O RhI O 16 erate of ethylene exchange via associative displacement at 25oC is ~ 104 sec-1 RhI 18 erate of ethylene exchange via dissociative displacement at 25oC is ~ 4 x 10-10 sec-1 BDE = 31 kcal/mol The rate-determining step in a dissociative ligand substition pathway is breaking the M-L bond. Because of the late, product-like transition state for forming the coordinatively unsaturated intermediate in such a process, the M-L BDE is a good approximation of the activation energy(EA). 1014 slower at 25oC RhI RhI EA BDE E rxn coordinate RhI 18 eRhI 16 e- PPh3 RhI PPh3 18 erate = -d [LRh(C2H4)2] dt = k1 [LRh(C2H4)2] PPh3 (mmol) 0.20 1.65 1.23 1.73 k x 104 sec -1 a 6-fold increase in the concentration of nucleophile does not significantly affect the rxn rate. Results are consistent with a mechanism where the ratedetermining step is ethylene dissociation and is not affected by the concentration of the nucleophile. 118oC Cramer JACS 1972 (94) 5681
M C. White. Chem 153 Mechanism -51- Week of september 24th. 2002 igand dissociation: sterics t-Bu t-Bu t-B Pd Pd catalyst"resting state Bi-Bi ineffective catalyst +L‖ dissociative-L The steric bulk of the bidentate phosphine ligand is thought to weaken the Pd-p bond, thereby favoring ligand dissociation requ to form the cata lytically active species Pdo catalytically active HiN NaOt-Bu also aryl Br, I, OTs also aniline, piperidine Hartwig JACS 1998(120)7369
M.C. White, Chem 153 Mechanism -51- Week of September 24th, 2002 Ligand dissociation: sterics P P Pd0 P Fe P Fe "ineffective catalyst" Hartwig JACS 1998 (120) 7369. The steric bulk of the bidentate phosphine ligand is thought to weaken the Pd-P bond, thereby favoring ligand dissociation required to form the catalytically active species. P P Pd0 P t-Bu t-Bu t-But-Bu Fe t-Bu t-Bu P t-Bu t-Bu Fe Cl also aryl Br, I, OTs + H2N also aniline, piperidine N H catalyst "resting state" dissociative P P Pd0 t-Bu t-Bu t-But-Bu Fe catalytically active species NaOt-Bu +L -L