Catalysis of the Electroreduction of Allyl Chloride by Cobalt 2,2'-Bipyridine Complexes

The electrochemical reduction of allyl chloride is strongly catalyzed in the presence of cobalt complexes of 2,2'-bipyridine. A prominent reaction product of the catalyzed reduction is 1,5-hexadiene. Voltammetry, coulometry, and gas chromatographic data are presented and analyzed and a mechanistic scheme proposed to. account for the catalytic action of the cobalt-2,2'-bipyridine complexes. Recently we reported studies (1) which examined a previous claim (2) that the electroreduction of acrylonitrile in acetonitrile was catalyzed by a complex of cobalt with 2,2'-bipyridine. These studies demonstrated the absence of such catalysis for acrylonitrile and a number of other vinyl monomers but one substrate, allyl chloride, did exhibit a very large enhancement in its reduction rate in the presence of cobalt 2,2'-bipyridine complexes. This report summarizes our experimental observations with this system and presents a possible reaction scheme to account for the observed catalysis. Exp·erimental Reagents.-The solvent was prepared from spectroscopic grade acetonitrile which was stirred over CaH2 for 24 hr, distilled near room temperature under reduced pressure, and stored under argon. Polarographic grade tetraethylammonium perchlorate (TEAP) (Southwestern Analytical Company) was vacuum dried and used as the supporting electrolyte without additional purification. Co(bipy)s(Cl04)s, Co(bipy)s(Cl04) 2 (bipy = 2,2'bipyridine), and Co(phen)s(Cl04h (phen = 1,10phenanthroline) were prepared as previously described (1). Solutions of Co(bipy)2(Cl04)2 were prepared by mixing Co (Cl04)2 and 2,2'-bipyridine in a molar ratio of 1:2 in acetonitrile. Allyl chloride (Eastman) and allyl alcohol (Aldrich) were distilled just prior to their use. Apparatus.-Cyclic voltammograms were obtained with a Princeton Applied Research (PAR) Model 173 potentiostat driven by a PAR Model 175 programmer. Current-voltage curves were recorded with a Tektronix Model 564 storage oscilloscope and photographed with a Tetronix Model C-12 camera. Controlled potential coulometry was conducted with the same potentiostat equipped with a Model 179 digital coulometer. A three-compartment electrochemical cell was employed with double isolation of the Ag/0.1M Ag+ reference electrode against which all potentials are quoted. Solutions were deoxygenated with prepurified argon which was passed successively through aqueous chromous chloride solution, a calcium chloride drying tower, and purified acetonitrile before entering the test solution. Concentrations of allyl chloride and 1,5hexadiene were monitored gas chromatographically with a Hewlett-Packard Model5830 chromatograph using a 12 ft column packed with Carbowax 20M on Chromosorb W. Results Of the three characteristic voltammetric waves exhibited by Co(bipy)s3 + 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 • Electrochemical Society Active Member.