Hydridocobalt Complexes in the Hydroformylation Reaction

The hydridocarbonylcobalt active species formed in the first step of the Heck and Breslow hydroformylation mechanism is the 16-electron HCo(CO)3 complex, obtained via dissociation of CO from HCo(CO)4, 1. The most recent theoretical studies indicated that the stable singlet state is a C2 -planar structure, which resulted from the loss of the equatorial CO from HCo(CO)4, 1. In an earlier study, Ziegler s group found that the Cs structure was the most stable.  

This HCo(CO)4 species is formed / s//u from the reaction between Co2(CO)g and H2 (Equation (1)). 

The rapid equilibrium between HCo(CO 4 and Co2(CO)g is one of the important key steps in hydroformylation reactions. A critical review of the existing data on this equilibrium has been made in parallel with new infrared spectroscopic measurements under semi-tfi-st/u conditions. Thermodynamic parameters obtained were A// = 4.054 0.175 kcal mol and AS = —3.067 0.488 cal mol Values have been obtained in the 50.7-117.0 °C temperature range in hexane as solvent.  

The study by Klinger and Rathke established the formation of radical Co(CO)4, which is involved in the equilibrium. Their observation was based on high-pressure NMR studies of hydrogen transfer and related dynamic process in 0x0 catalysis.They have determined the Co-Co and Co-H bond-dissociation enthalpies (19 and 59 kcal mol respectively), which have been previously obtained by Ziegler and Folga (148 and 283 kj mol , 

HCo(CO)4 is an unstable precursor, which decomposes rapidly below ca. 22 °C with a half-time of 10 min. Aylett ( / a/, studied the effect of the decomposition of DCo(CO 4 in porous silicon by chemical vapor infiltration and deposition. They have determined that this precursor was decomposed to afford mainly DCo3(CO 9, while above this temperature Co2(CO)8 was the main product.A detailed analysis of the infrared spectra of HCo(CO 4 and DCo(CO)4 has provided good agreement between experiment and theory.  

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