Two bifunctional Ru II/Re I photocatalysts for CO 2 reduction: A spectroscopic, photocatalytic, and computational study

Abstract

Two Ru II/Re I dimers, [Ru(dmb) 2( L1)Re(CO) 3Cl] ( 1) and [Ru(dmb) 2( L2)Re(CO) 3Cl] ( 2), were synthesized and characterized, and their electrochemical and spectroscopic properties together with their photocatalytic CO 2 reduction activities were evaluated (dmb = 4,4′-dimethyl-2,2′-bipyridine; L1 = 1,2-bis(4′-methyl-2,2′-bipyridyl-4-yl)ethane; L2 = 1,2-bis(4′-methyl-2,2′-bipyridyl-4-yl)ethene). The structures of 1 and 2 are identical except for the difference in the conjugation content of bridging ligands (–CH 2–CH 2– for 1 and –CH CH– for 2). Density functional theory (DFT) methods were employed to model the ground-state electronic transition and electrochemical properties of both catalysts. Electronic transitions were identified using UV–Vis spectroscopic techniques, aided by time-dependent density functional theory (TD-DFT) methods. The redox properties of two complexes under N 2 and CO 2 pressure have been studied by means of cyclic voltammetric measurements. When the cyclic voltammetry was performed in the presence of CO 2, a substantial current enhancement was observed for the reduction wave of 1 and 2. Additionally, significant higher turnover numbers of CO formation in the photocatalytic CO 2 reduction are observed for 1 than that for 2. Although complex 2 exhibited longer wavelength absorption, its photocatalytic activation for the CO 2 reduction was lower than that of 1, due to the effect of conjugated linkage on the reduction potential and low emission quantum yield.