The following molecules are related:

Transcription

1 Isolobal Analogy Inclusion of the ligand η-c 5 H 5 - which, as a donor of 3 π-electron pairs formally occupies 3 coordination sites, yields the analogies: The following molecules are related: 1

2 Isolobal Analogy Another analogy is that between cyclopropane and µ-alkylidene complexes: 2

3 Isolobal Analogy xxxx 3

4 Isolobal Analogy 4

5 Isolobal Analogy The realm of isolobal connections is considerably extended if the mutual replacement of σ-donor ligands and metal electron pairs is introduced: 5

6 Isolobal Analogy 6

7 Isolobal Analogy 7

8 Isolobal Analogy 8

9 Isolobal Analogy 9

10 Isolobal Analogy Examples of isolobal fragments: 10

11 Reactivity of Coordinated Ligands Cl 2- Cl Pd Cl Cl C 2 H 4 Cl Cl Pd Cl - H 2 O Cl H 2 O Pd Cl Pd (0) + H + + H 2 O + 2 Cl e (CuCl 2 CuCl) Cl H 2 O OH - Pd Cl OH - Cl H 2 O Pd Cl H CH 3 CHO Cl H 2 O Cl - Pd H O - "β-h elim" Cl H 2 O Pd OH ins Cl H 2 O Cl H 2 O - Cl - OH Pd β-h elim OH Pd H

12 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 12

13 Dissociative ligand substitution Example: L n M CO L n M + CO L' L n M L' 18 e 16 e 18 e Factors influencing ease of dissociation: 1 st row < 2 nd row > 3 rd row d 8 -ML 5 > d 10 -ML 4 > d 6 -ML 6 stable ligands (CO, olefins, Cl - ) dissociate easily (as opposed to e.g. CH 3, Cp). 13

14 Dissociative substitution in ML 6 16-e ML 5 complexes are usually fluxional; the reaction proceeds with partial inversion, partial retention of stereochemistry. or 18-e oct The 5-coordinate intermediates are normally too reactive to be observed unless one uses matrix isolation techniques. SP 16-e distorted TBP 14

15 Associative ligand substitution Example: L' L n M L n M L' - L L n-1 M L' 16 e 18 e Sometimes the solvent is involved. Reactivity of cis-platin: 16 e Br (NH 3 ) 2 PtCl Cl - slow H 2 O - Cl - (NH 3 ) 2 Pt(Cl)(Br) Br - fast - H 2 O (NH 3 ) 2 Pt(Cl)(H 2 O) + NucleoBase - H2 O fast - Cl - slow (NH 3 ) 2 Pt(Cl)(NB) + 15

16 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 O 3-e 1-e M CO R O M R 5-e M 3-e M (1+2)-e 1-e 16

17 Redox-induced ligand substitution Unlike 18-e complexes, 17-e and 19-e complexes are labile. Oxidation and reduction can induce rapid ligand substitution. L n M 18-e - e - + e - L' L n M + L n M L' + 17-e 19-e L n M - 19-e 17-e L n-1 M - + L Reduction promotes dissociative substitution. Oxidation promotes associative substitution. In favourable cases, the product oxidizes/reduces the starting material redox catalysis. 17

18 Redox-induced ligand substitution Fe(CO) 4 L Fe(CO) 5 CO Fe(CO) 4 Fe(CO) 5 Fe(CO) 4 L L Initiation by added reductant. Sometimes, radical abstraction produces a 17-e species (see C103). 18

19 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 σ hν d π Requirement: the complex must absorb, so it must have a colour! or use UV if the complex absorbs there 19

20 Photochemical ligand substitution Some ligands have a low-lying π* 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 σ π* hν HOMO d π LUMO M-M bonds dissociate easily (homolysis) on irradiation (n-1)-e associative substitution 20

21 Electrophilic and nucleophilic attack on activated ligands Electron-rich metal fragment: ligands activated for electrophilic attack N N N Rh N H + S N N N Rh N S H 2 O is acidic enough to protonate this coordinated ethene. Without the metal, protonating ethene requires H 2 SO 4 or similar. 21

22 Electrophilic and nucleophilic attack on activated ligands Electron-poor metal fragment: ligands activated for nucleophilic attack. BuLi Bu H - Li + OC OC Cr CO OC OC Cr CO BuLi does not add to free benzene, it would at best metallate it (and even that is hard to do). 22

23 Electrophilic attack on ligand Hapticity may increase or decrease. Formal oxidation state of metal may increase. + M I H + M I + M (0) H + M II 23

24 Electrophilic addition O OEt + Et 3 O + Fe(CO) 3 Fe(CO) 3 Is formally oxidation of Fe (0) to Fe II (the ligand becomes anionic). Ligand hapticity increases to compensate for loss of electron. 24

25 Electrophilic abstraction Electrophilic abstraction also by Ph 3 C +, H + Cp 2 Zr Me Me B(C 6 F 5 ) 3 Cp 2 Zr + Me + MeB(C 6 F 5 ) 3-16 e 14 e Alkyl exchange also starts with electrophilic attack: Me Zn Me Me Zn Me Me Me Zn Zn Me Me 25

26 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 H + Ni 26

27 Electrophilic attack at the metal Can be the start of oxidative addition I 2 (CO) 2 Rh Me I I 2 (CO) 2 RhMe + I - I 3 (CO) 2 RhMe (although this could also happen via concerted addition) CH 3 COOH HI CH 3 OH CH 3 COI H 2 O CH 3 I Key reaction in the Monsanto acetic acid process: MeOH + CO HI "Rh" MeCOOH MeCORh(CO) 2 I 3 - Rh(CO) 2 I 2 - MeRh(CO) 2 I 3 - CO MeCORh(CO)I 3-27