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Ligand Substitution Reactivity of Coordinated Ligands 2- Cl Cl Pd Cl Cl C2H4 Cl Cl Pd H2O Cl Cl H2O Pd Cl OH(0) + Pd + H + H2O + 2 Cl- Cl H2O Pd - 2 e (CuCl2 CuCl) Cl H2O Pd OH Cl - Cl- H - Cl OH Cl H2O Pd CH3CHO -H elim OH ClCl H2O Pd O "-H elim" H Cl H2O Pd OH ins Cl H2O Pd H Peter H.M. Budzelaar Why care about substitution ? Basic premise about metal-catalyzed reactions: • Reactions happen in the coordination sphere of the metal • Reactants (substrates) come in, react, and leave again • Binding or dissociation of a ligand is often the slow, rate-determining step This premise is not always correct, but it applies in the vast majority of cases. Notable exceptions: • Electron-transfer reactions • Activation of a single substrate for external attack – peroxy-acids for olefin epoxidation – CO and olefins for nucleophilic attack 2 Ligand Substitution Dissociative ligand substitution Example: LnM CO 18 e LnM 16 e + CO L' LnM L' 18 e Factors influencing ease of dissociation: • 1st row < 2nd row > 3rd row • d8-ML5 > d10-ML4 > d6-ML6 • stable ligands (CO, olefins, Cl-) dissociate easily (as opposed to e.g. CH3, Cp). 3 Ligand Substitution Dissociative substitution in ML6 16-e ML5 complexes are usually fluxional; the reaction proceeds with partial inversion, partial retention of stereochemistry. or 16-e 18-e oct SP distorted TBP The 5-coordinate intermediates are normally too reactive to be observed unless one uses matrix isolation techniques. 4 Ligand Substitution Associative ligand substitution Example: LnM L' LnM L' 16 e -L 18 e Sometimes the solvent is involved. Reactivity of cis-platin: Ln-1M L' 16 e Br(NH3)2PtCl2 (NH3)2Pt(Cl)(Br) - Clslow Br- - H O H2O - Cl 2 fast (NH3)2Pt(Cl)(H2O)+ - Cl- NucleoBase - H O 2 fast slow (NH3)2Pt(Cl)(NB)+ 5 Ligand Substitution Ligand rearrangement Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactions of an apparently saturated complex: M N O M N 3-e 1-e O O CO M R M M M 5-e 3-e R (1+2)-e 6 1-e Ligand Substitution Redox-induced ligand substitution Unlike 18-e complexes, 17-e and 19-e complexes are labile. Oxidation and reduction can induce rapid ligand substitution. + LnM - e- 17-e L' LnM L' + 19-e LnM 18-e + e- LnM19-e Ln-1M- + L 17-e • Reduction promotes dissociative substitution. • Oxidation promotes associative substitution. • In favourable cases, the product oxidizes/reduces the starting material redox catalysis. 7 Ligand Substitution Redox-induced ligand substitution Fe(CO)4L CO Fe(CO)5 Fe(CO)4 Fe(CO)5 Fe(CO)4L L Initiation by added reductant. Sometimes, radical abstraction produces a 17-e species (see C103). 8 Ligand Substitution Photochemical ligand substitution Visible light can excite an electron from an M-L bonding orbital to an M-L antibonding orbital (Ligand Field transition, LF). This often results in fast ligand dissociation. M(CO)6 d h d Requirement: the complex must absorb, so it must have a colour! or use UV if the complex absorbs there 9 Ligand Substitution Photochemical ligand substitution Some ligands have a low-lying * orbital and undergo Metal-to-Ligand Charge Transfer (MLCT) excitation. This leads to easy associative substitution. – The excited state is formally (n-1)-e ! – Similar to oxidation-induced substitution M(CO)4(bipy) d HOMO * h d LUMO M-M bonds dissociate easily (homolysis) on irradiation (n-1)-e associative substitution 10 Ligand Substitution Electrophilic and nucleophilic attack on activated ligands Electron-rich metal fragment: ligands activated for electrophilic attack. + ++ N N Rh N N N H+ S N S Rh N N H2O is acidic enough to protonate this coordinated ethene. Without the metal, protonating ethene requires H2SO4 or similar. 11 Ligand Substitution Electrophilic and nucleophilic attack on activated ligands Electron-poor metal fragment: ligands activated for nucleophilic attack. Bu H BuLi Cr Cr OC OC Li+ CO OC OC CO BuLi does not add to free benzene, it would at best metallate it (and even that is hard to do). 12 Ligand Substitution Electrophilic attack on ligand Hapticity may increase or decrease. Formal oxidation state of metal may increase. + M I H+ MI + M(0) 13 H+ MII Ligand Substitution Electrophilic addition + O OEt Et3O+ Fe(CO)3 Fe(CO)3 • Is formally oxidation of Fe(0) to FeII (the ligand becomes anionic). • Ligand hapticity increases to compensate for loss of electron. 14 Ligand Substitution Electrophilic abstraction + Electrophilic abstraction also by Ph3C+, + H+ Me Cp2Zr B(C6F5)3 MeB(C6F5)3- Cp2Zr Me Me 16 e 14 e Alkyl exchange also starts with electrophilic attack: Me Zn Me 15 Zn Me Me Me Me Zn Zn Me Me Ligand Substitution Electrophilic attack at the metal If the metal has lone pairs, it may compete with the ligand for electrophilic attack Transfer of the electrophile to the ligand may then still occur in a separate subsequent step + Fe H+ Fe H + ?via? Ni 16 H+ Ni Ligand Substitution Electrophilic attack at the metal Can be the start of oxidative addition I2(CO)2Rh Me I I2(CO)2RhMe + I- (although this could also happen via concerted addition) I3(CO)2RhMe HI CH3COOH H2O CH3COI Key reaction in the Monsanto acetic acid process: HI MeOH + CO MeCOOH "Rh" CH3I Rh(CO)2I2- MeCORh(CO)2I3- CO 17 CH3OH MeRh(CO)2I3- MeCORh(CO)I3- Ligand Substitution Nucleophilic attack on ligand Nucleophile "substitutes" metal hapticity usually decreases Oxidation state mostly unchanged Competition: nucleophilic attack on metal usually leads to ligand substitution 18 Ligand Substitution Nucleophilic abstraction - Mostly ligand deprotonation Na+ NaH Cr Cr OC OC Cp2WH2 19 CO BuLi OC OC Cp2WH CO Li Ligand Substitution Nucleophilic addition OH (H2O)Cl2Pd + OH- (H2O)Cl2Pd Key reaction of the Wacker process: C2H4 + ½ O2 20 PdCl2, H2O CH3CHO CuCl2 Ligand Substitution The Wacker process 2- Cl Cl Pd Cl Cl C2H4 Cl Cl Pd H2O Cl Cl H2O Pd Cl OH(0) + Pd + H + H2O + 2 Cl- Cl H2O Pd - 2 e (CuCl2 CuCl) Cl H2O Pd OH Cl - Cl- H - Cl OH Cl H2O Pd CH3CHO -H elim OH ClCl H2O Pd 21 O "-H elim" H Cl H2O Pd OH ins Cl H2O Pd H Ligand Substitution The Wacker process Characteristics of the Wacker process: • The oxygen in the product derives from water, not directly from the oxygen used as oxidant • higher olefins yield ketones, not aldehydes • large amounts of halides required: corrosive • side products resulting from nucleophilic attack of halide on olefin No longer important for acetic acid synthesis Several variations (with more complicated nucleophiles) used in organic synthesis 22 Ligand Substitution Nucleophilic attack on the ligand How can you distinguish between internal and external attack of OH- ? Cl OHH2O Pd OH- - Cl H2O Pd Cl Cl- OH Cl ?? - Cl H2O Pd - ins OH Use trans-CHD=CHD and trap the intermediate OH Cl Pd-C-C-OH with CO: H2O Pd Cl - Cl H2O Pd H O CO - Cl- OH Pd CO ins HO O O 23 nucl attack Pd O Ligand Substitution Using isotopic labelling to study mechanisms HO D OH Pd D D Pd D - O CO O D D OH Pd D D HO Pd Pd D 24 HO D CO D D O D O D Ligand Substitution Using isotopic labelling to study mechanisms Could acetaldehyde be formed directly as vinyl alcohol ? OH OH diss Pd H H2O O CH3 H ins OH Pd "-H elim" O Pd diss H CH3 H Perform reaction in D2O: OD OD D2O diss Pd O H O CH2D H ins Pd OD "-H elim" Pd D 25 O diss H O CH3 Ligand Substitution