Aerobic CO Oxidation of a Metal-Bound Carbonyl in a NHC-Stabilized Cobalt Half-Sandwich Complex

The complexes [(eta(5)-C5R5)Co(lPr(2)Im)(eta(2)-C2H4)] (R = H 1; Me 2) were synthesized in good yields via reaction of one equivalent of the N-heterocyclic carbene (i)Pr(2)Im (R(2)Im = 1,3-dialkylimidazolin-2-ylidene) and the bis(ethylene) complexes [(eta(5)-C5R5)Co(eta(2)-C2H4)(2)]. These complexes serve as convenient starting materials for chemistry using the [(eta(5)-C5R5)Co((i)Pr(2)Im)] complex fragment. The reaction with carbon monoxide leads to the carbonyl complexes [(eta(5)-C5R5)Co((i)Pr(2)Im)(CO)] (R = H 3; Me 4) in good to excellent yields. The carbonyl complexes 3 and 4 are very air sensitive and react readily with oxygen in the solid state and in solution. Whereas the cyclopentadienyl-substituted complex [(eta(5)-C5H5)Co((i)Pr(2)Im)(CO)] (3) decomposes upon reaction with O-2 to intractable products, [(eta(5)-C5Me5)Co((i)Pr(2)Im)(CO)] (4) yields the structurally characterized cobalt(III) carbonato complex [(eta(5)-O5Me5)Co((i)Pr(2)Im)(kappa(2)-CO3)] (5). This reaction represents the first example of O-2 oxidation of a metal-bound carbonyl for a 3d transition metal complex. The oxidation is too fast to be monitored by NMR spectroscopy, and application of low-temperature time-resolved UV/vis spectroscopy combined with stopped-flow techniques led to the detection of a possible intermediate. On the basis of these experiments and computational investigations using density functional theory (DFT) the peroxo acyl complex [eta(5)-C5Me5)Co((i)Pr(2)Im)(kappa(t)-C,O-(C) under barC{O}O (O) under bar)] (B) is assumed to be the key intermediate detected. The DFT calculations further reveal that this reaction is strongly exothermic with two kinetic barriers, one for the exothermic addition of O-2 to the carbonyl complexes 4 to give the peroxo acyl complex [(eta(5)-C5Me5)Co((i)Pr(2)Im)(kappa(2)-C,O-(C) under barC{O}O (O) under bar)] (B), the other for the rearrangement of B to give the carbonato complex [(eta(5)-C5Me5)Co((i)Pr(2)Im)(eta(2)-CO3)] (5). The key step for the rearrangement is the formation of CO2 in the coordination sphere of cobalt and the attack of metal-bound oxygen at the carbon atom of CO2.