Two bifunctional RuII/ReI photocatalysts for CO2 reduction: A spectroscopic, photocatalytic, and computational study

Two Ru-II/Re-I dimers, [Ru(dmb)(2)(L1)Re(CO)(3)Cl] (1) and [Ru(dmb)(2)(L2)Re(CO)(3)Cl] (2), were synthesized and characterized, and their electrochemical and spectroscopic properties together with their photocatalytic CO2 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 (-CH2-CH2- 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 CO2 pressure have been studied by means of cyclic voltammetric measurements. When the cyclic voltammetry was performed in the presence of CO2, 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 CO2 reduction are observed for 1 than that for 2. Although complex 2 exhibited longer wavelength absorption, its photocatalytic activation for the CO2 reduction was lower than that of 1, due to the effect of conjugated linkage on the reduction potential and low emission quantum yield. (c) 2011 Elsevier Ltd. All rights reserved.