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.