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

The complexes [(η5-C5R5)­Co­(iPr2Im)­(η2-C2H4)] (R = H 1; Me 2) were synthesized in good yields via reaction of one equivalent of the N-heterocyclic carbene iPr2Im (R2Im = 1,3-dialkylimidazolin-2-ylidene) and the bis­(ethylene) complexes [(η5-C5R5)­Co­(η2-C2H4)2]. These complexes serve as convenient starting materials for chemistry using the [(η5-C5R5)­Co­(iPr2Im)] complex fragment. The reaction with carbon monoxide leads to the carbonyl complexes [(η5-C5R5)­Co­(iPr2Im)­(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 [(η5-C5H5)­Co­(iPr2Im)­(CO)] (3) decomposes upon reaction with O2 to intractable products, [(η5-C5Me5)­Co­(iPr2Im)­(CO)] (4) yields the structurally characterized cobalt­(III) carbonato complex [(η5-C5Me5)­Co­(iPr2Im)­(κ2-CO3)] (5). This reaction represents the first example of O2 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 [(η5-C5Me5)­Co­(iPr2Im)­(κ2-C,O-C­{O}­OO)] (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 O2 to the carbonyl complexes 4 to give the peroxo acyl complex [(η5-C5Me5)­Co­(iPr2Im)­(κ2-C,O-C­{O}­OO)] (B), the other for the rearrangement of B to give the carbonato complex [(η5-C5Me5)­Co­(iPr2Im)­(η2-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.