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 via Co-59 NMR spectroscopy, electrochemical, and catalyzed proton electroreduction studies

摘要

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 Co-59 NMR spectroscopy, UV-visible-NIR spectroscopy, and spectroelectrochemical techniques. A quasi-stable Co(I) species produced by the reduction of [Co(bpy)(2)Cl-2] was studied and gave a Co-59 NMR spectroscopic chemical shift of 2934 ppm in CD3CN-D2O (4:1, v/v) and a broad peak of lambda(max) = 600 nm in the UV-visible region of the spectrum. The Co(I) species generated from [Co(bpy)(2)Cl-2) (when reduced by electrochemical methods) was found to be unstable and produced a transient of lambda(max) = 530 nm. 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 = 0.01 s(-1) and 2 M s(-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 Co-III-H species as the most likely candidate and appeared to involve both a homolytic and a heterolytic pathway towards the production of hydrogen. (C) 2015 Elsevier B.V. All rights reserved.