Electrochemistry of 2,2′‐Bipyridine Complexes of Cobalt in the Presence of Acrylonitrile

Abstract

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. ExperimentalApparatus. -Polarograms were obtained with a Princeton Applied Research (PAR) Model 174 polarographic analyzer and recorded with a Hewlett-Packard Model 7004 X-Y recorder. Cyclic voltammograms were obtained by means of a PAR Model 173 potentiostat driven by a conventional signal generator. The voltammograms were recorded with the X-Y recorder or by photographing the screen of a Tektronix 564 oscilloscope. Controlled potential coulometry was conducted with the PAR Model 173 potentiostat equipped with a Model 179 digital coulometer. Gas chromatographic analyses for acrylonitrile in acetonitrile were obtained with a Hewlett-Packard Model 5830 A gas chromatograph with an 18 ft column containihg Carbowax 20 M on Chromosorb W. Spectra were recorded with a Cary 11 spectrophotometer using cuvettes capped with rubber septums which were deoxygenated by purging with argon.Measurements were conducted in a three-compartment cell. The reference electrode was a silver wire immersed in 0.1M AgN0 3 in acetonitrile which was double isolated by frit...

Catalysis of the Electroreduction of Allyl Chloride by Cobalt 2,2′‐Bipyridine Complexes

“…3 show that substitution of Co(bipy)z+ for Co(bipy) 3 + significantly increases the rate at which reaction [1] proceeds from left to right (the same is true for the analogous reaction involving acrylonitrile). In the case of Co (bipy) 3 + the slow step is apparently the loss of one of the coordinated 2,2'-bipyridine ligands.…”

“…to exceed its value in the absence of allyl chloride ( Fig. 2) except at scan rates high enough to prevent reaction [1] (and the subsequent intramolec- for the allyl chloride complex.…”

“…Of the three characteristic voltammetric waves exhibited by Co(bipy)s 3 + in acetonitrile (1,2) the second is the first to be affected by the presence of allyl chloride. In addition, as was true for acrylonitrile ( 1), a new wave appears between the original second and…”

“…:<:= Co(bipy)2(CHzCHCH2Cl) + + bipy [1] However, in contrast with the cobalt (I) acrylonitrile system, the mixed complex with allyl chloride is unstable with respect to intramolecular reduction of the coordinated allyl chloride by the cobalt(!). This reaction, which results in the regeneration of cobalt (II) at the electrode surface, is the reason that the addition of allyl chloride causes the peak current of the wave corresponding to the reduction of cobalt (II) to cobalt(!)…”

“…Co(bipy)s(Cl0 4 )s, Co(bipy)s(Cl04) 2 (bipy = 2,2'-bipyridine), and Co(phen)s(Cl0 4 h (phen = 1,10-phenanthroline) were prepared as previously described (1). Solutions of Co(bipy)2(Cl04)2 were prepared by mixing Co (Cl0 4 )2 and 2,2'-bipyridine in a molar ratio of 1:2 in acetonitrile.…”