Electrochemistry of 2 , 2 ' - Bipyridine Complexes of Cobalt in the Presence of Acrylonitrile

The previously claimed (1) catalysis of the electroreduction of acrylonitrile by means of a complex of Co(l) and 2,2'-bipyridine is shown to be erroneous. The "catalytic currents" result instead from the two-electron reduction of a mixed complex of Co (I) acrylonitrile and 2,2' -bipyridine. The equilibrium and forward rate constants for the formation of the mixed complex have been estimated and its spectrum is given. The behavior of a number of other vinyl monomers, which mimic acrylonitrile, is described. Some time ago Tanaka and Sato described experiments (1) in which they claimed to have shown that a low valent complex of cobalt with 2,2'-bipyridine, generated electrochemically in acetonitrile as solvent, was an effective catalyst for the electrochemical reduction of acrylonitrile. We were intrigued by this report for three reasons: (i) The claimed catalysis was very notable CE112 for the wave attributed to acrylonitrile reduction is shifted from -2.3 to -1.3V vs. SCE); (ii) the reduction of acrylonitrile is a reaction of considerable industrial importance; and (iii) the catalytic mechanism was speculated to involve the adsorption of a low valent complex on the surface of the mercury electrode. We have therefore examined this system in more detail at both mercury and platinum electrodes by means of controlled potential electrolysis, cyclic voltammetry, and polarography. Contrary to the previous report (1) we find that the cobalt-2,2'bipyridine complexes do not function as catalysts for the reduction of acrylonitrile. The increase in current that results when both reactants are present arises instead from the reduction of a complex containing bipyridine, acrylonitrile, and cobalt(!) to a new complex containing cobalt in the (-I) oxidation state. Several other vinyl monomers which form complexes with cobalt (I), e.g., 2,2'-bipyridine, also facilitate the reduction to cobalt( -I). This report summarizes the experimental evidence which has led us to these conclusions.