Internal carbon monoxide exchange and CO dissociation in cobalt carbonyl carbene complexes. A density functional study

Abstract

Structures, intramolecular CO-exchanges, and CO-dissociation of ethoxycarbonylcarbene-bridged dicobalt carbonyl complexes [μ 2-{ethoxycarbonyl(methylene)}-μ 2-(carbonyl)- bis(tricarbonyl-cobalt) (Co–Co)] Co 2(CO) 7(CHCO 2Et) ( 1) and [di-μ 2-{ethoxycarbonyl(methylene)}-bis(tricarbonyl-cobalt) (Co–Co)] Co 2(CO) 6(CHCO 2Et) 2 ( 2) were investigated by utilizing the density functional theory at the B3LYP/6-31G(d) level. In the lowest energy isomer of 1 the equatorial carbonyl group cis to the bridging ethoxycarbonylcarbene is prone to dissociate resulting in a coordinative unsaturated Co 2(CO) 6(CHCO 2Et) complex stabilized by an intramolecular cobalt–oxygen orbital interaction. Several mechanisms describing the fluxional behavior of 1 and 2 were found. It was found that the internal transformation designated as ‘tripodal rotation’ is responsible for the temperature-dependent broadening of the 13C NMR signals for compound 2. For 1 the tripodal rotation needs to be taken into account as well, however an even faster internal CO-exchange allows for the carbonyls to switch between the terminal and bridging positions. In the coordinative unsaturated complexes Co 2(CO) 6(CHCO 2Et) and Co 2(CO) 5(CHCO 2Et) 2 the CO ligands show also many variations of internal rearrangements. In complex 1 the effect of the rotation of the C carbene–C carbonyl bond on the energy of the rotamers was also examined.