Assessing the stability of the Co(I) species of two mononuclear dichlorocobalt compounds bearing 2,2′-bipyridine and trans-2-(2′-quiolyl)methylene-3-quinuclidione via59Co NMR spectroscopy, electrochemical, and catalyzed proton electroreduction studies

Abstract

The Co(I) species resulting from the reduction of two mononuclear species [CoLnCl2] (where L=2,2′-bipyridine (bpy) and n=2, or L=trans-2-(2′-quiolyl)methylene-3-quinuclidione (quin) and n=1), was studied by 59Co NMR spectroscopy, UV–visible-NIR spectroscopy, and spectroelectrochemical techniques. A quasi-stable Co(I) species produced by the reduction of [Co(bpy)2Cl2] was studied and gave a 59Co NMR spectroscopic chemical shift of 2934ppm in CD3CN-D2O (4:1, v/v) and a broad peak of λmax=600nm in the UV–visible region of the spectrum. The Co(I) species generated from [Co(quin)Cl2) (when reduced by electrochemical methods) was found to be unstable and produced a transient of λmax=530nm. Both [CoLnCl2] species were examined as electrocatalysts for the reduction of protons in an acetonitrile–water solvent mixture in the presence of p-cyanoanilinium tetrafluoroborate. The electrochemical properties of both species showed a dependence on the supporting electrolyte which also affected the electrocatalytic behavior. The simulation of the cyclic voltammograms in CH3CN-H2O (4:1, v/v) allowed for the extraction of kinetic data and suggested a homogenous reaction following the reduction to a Co(I) metal centre with rate constants k=0.01s−1 and 2Ms−1 for the Co(I) species with the ligands, bpy and quin, respectively. Analysis of the head space of controlled potential electrolysis experiments for an hour, in the presence of p-cyanoanilinium tetrafluoroborate, confirmed the production of hydrogen, and also showed that the supporting electrolyte affected the production of hydrogen at a glassy carbon electrode in CH3CN–H2O (either 1:1 or 4:1, v/v). A mechanism was postulated which involved a CoIII–H species as the most likely candidate and appeared to involve both a homolytic and a heterolytic pathway towards the production of hydrogen.