Mn-carbonyl molecular catalysts containing a redox-active phenanthroline-5,6-dione for selective electro- and photoreduction of CO2 to CO or HCOOH

Abstract

One advantage of using homogenous molecular catalysts versus their heterogeneous counterparts for CO2 reduction is the greater degree of control one usually retains over the selectivity of the catalyst. In this context, a pair of new manganese(I) tricarbonyl species, based on the N,N’-coordinated 1,10-phenanthroline-​5,​6-​dione (phen-dione) ligand, have been synthesized and characterized by spectroscopic methods and X-ray diffraction. They exhibit excellent selectivity for homogeneous catalytic CO2 reduction to CO or HCOOH through electrochemical or photochemical activation, respectively. Under the latter particularly, switching the solvent from dimethylformamide to acetonitrile remarkably enhances the selectivity of the reduction reaction towards formic acid. To obtain deeper mechanistic insight, the evolution of the catalysts and products in solution during the catalytic processes was investigated by combined electrochemical, chromatographic and spectroscopic methods It has been shown that the presence of the π-accepting, redox-active phen-dione ligand leads to different reduced intermediates compared with standard 2,2′-bipyridine and 1,10-phenanthroline complexes, with the two first redox processes remaining localized on the ligand.