Redox Reactions: Oxidative-Addition (OA) and Reductive-Elimination (RE)
Oxidative-Addition LnM + X-Y → LnM(X)(Y)
oxidation state increases by 2 units common for electron counts of 16 e- or less
Vaska’s complex (shown below) is a classic example of 16 e-d8 compound that undergoes oxidative addition with a wide range of substrates and does so by a variety of mechanisms. Some of these mechanisms are dealt with in the following sections. a) Three-centre concerted additions
H-H, C-H and C-C additions are usually of this type
H2 addition
H P Cl P H Ir + H2 Ir OC P OC P
Cl thermodynamics
o ∆H is roughly 2 D(Ir-H) – D(H-H) = 2(-60) – (104) = -16 kcal/mol (-67 kJ/mol) ∆So is negative and large: -30 eu (-125 J mol-1 K-1) therefore ∆Go = -16 – (298)(30)/1000 = -7 kcal/mol (-29 kJ/mol)
kinetics rate = kobs[Complex][H2]
consistent with a concerted process
from: Organometallic Chemistry by Spessard and Miessler M.O. picture for this reaction is straightforward:
(i) σ-donation from filled H2 σ orbital
(ii) π-back donation from metal orbital to H2 σ*
It is interaction (ii) that causes the H-H bond to break.
• metals with positive charges, higher oxidation states or competing π-acceptors cannot back donate well, so stable η2-
H2 complexes are obtained:
Compare:
H H HH R P PR R P CO 3 Mo 3 3 Mo R P PR OC PR 3 3 3 PR OC 3 7-coordinate 6-coordinate 18 electron 18 electron dihydride dihydrogen
HH + H H H + Ir IrH5(PCy3)2 H + Cy3P H
PCy3 6-coordinate 18 electron dihydrogen C-H addition
‘C-H activation’ is one of the most sought after reactions in organometallic chemistry: potential economic impact is huge
• intermolecular C-H activation is still rare but intramolecular cases (cyclometallation) are common (entropy effect)
hν R-H Ir - H Ir (solvent) Ir H 2 H Me3P Me3P Me3P H R Ir(III) 18 e- Ir(I) 16 e- Ir(III) 18 e- R = Me, Ph, Cy
- PEt Et P 3 3 Pt Et P Pt 3 Et3P Pt Et3P H H
+ PEt3 - CMe4
Et P 3 Pt Et3P b) Nucleophilic oxidative addition of RX
filled dz2 R L L slow L L M + RX M X- L L r.d.s. L L +
rate = k [M][RX] fast obs
R L L M L L
X
‡ ∆S is large and negative suggesting an SN2-like transition state: L L H δ− δ+ δ− M C X L L H H
- - - - - • rate depends on leaving group: CF3SO3 > I > OTs , Br > Cl • inversion observed at C
Ph Ph L - 2 L XC + PdL4 C Pd X
H R R H L
• polar solvents increase rate (polarized transition state) • electron-releasing ligands increase rate (metal more nucleophilic)
Reductive-elimination
X L Y L L M M + XY L L L L
L Mn+2 Mn
• very important as the C-C bond forming step in catalytic cycles (where X and Y are organic groups) • cis orientation of X and Y is required for concerted elimination • proceeds with retention of stereochemistry at C:
nucleophilic O.A. PR PR Ph 3 (inversion) 3 Br Br + Me Pd PR3 Me Pd PR3 H D Me Ph Me H D concerted R.E. (retention)
Ph PR3 Me + Me Pd PR3 D H Br
• favoured by:
bulky ligands (relief of crowding) high oxidation state ancillary ligands that can stabilize the lower
oxidation state (CO, alkenes, PR3 etc.)
• intermolecular reductive elimination processes can be ruled out by conducting the crossover experiment:
PR3 PR3 heat D C Pd PR H C Pd PR 3 3 + 3 3 D3CCD3 + H3C CH3
CD3 CH3 but NO H3C CD3