Reactions of cobalt carbonyls in methanol under high pressure of carbon monoxide: a reexamination of the hydroesterification mechanism

Abstract

High pressure IR and UV spectroscopy was used to study the disproportionation of Co 2(CO) 8 1 to [Co(MeOH) 6] [Co(Co) 4] 2 2 in methanol under the conditions of carbonylation reactions. The disproportionation is first order in [ 1] and inverse first order in [CO]. It is reversible under high pressure of CO. In the equilibrium the formation of 2 is favoured by addition of pyridine and KI, that of 1 by olefins and CO. The rate-determining step for the disproportionation is an associative ligand substitution of 1. The comproportionation of 2 under CO is first order in [ 2] and in [CO]. This is in agreement with a mechanism in which the resolvation of the cation or the electron transfer from the anion to the cation is rate-determining. The activation energies for the forward and backward reactions are 57 ± 12 kJ mol −1 and 60 kJ mol −1, respectively. The thermodynamic parameters for the equilibrium are Δ H 0 = −2.35 ± 0.3 kJ mol −1 and Δ S 0 ~ −4 J mol −1 K −1. The spectroscopic investigations concerning the hydroesterification (HE) of 1-octene are in line with a mechanism that starts with the ionic addition of H + and Co(CO) 4 −. The H + concentration is the rate-limiting factor and methanol is probably the source of the protons. Addition of pyridine leads to a higher concentration of Co(CO) 4 −, but it also solvates the H + more strongly. Thus a large excess of pyridine increases the activation energy of the HE. In the HE of butadiene, activation of the π-allyl complex η 3-C 4H 7-Co(CO) 3 is the slowest step.