Increased photocatalytic activity in Ru(II),Rh(III) supramolecular bimetallic complexes with terminal ligand substitution

Abstract

Three new Ru(II),Rh(III) supramolecular bimetallic complexes of the design [(Ph2phen)2Ru(dpp)RhCl2(R2-bpy)](PF6)3 (R=CH3 (Ru-Rh(Me2bpy)), H (Ru-Rh(bpy)), or COOCH3 (Ru-Rh(dmeb)); Ph2phen=4,7-diphenyl-1,10-phenanthroline; dpp=2,3-bis(2-pyridyl)pyrazine; dmeb=4,4′-dimethyl ester-2,2′-bipyridine; bpy=2,2′-bipyridine; Me2bpy=4,4′-dimethyl-2,2′-bipyridine) have been synthesized and analyzed to determine the impact that the polypyridyl terminal ligand (TL) coordinated to the cis-dihalide rhodium(III) metal center has on the photocatalytic activity for water reduction. The bimetallic complexes demonstrate that a correlation exists between the σ-donating ability of the substituted bipyridine ligand, the rate of chloride dissociation upon electrochemical reduction and the activity towards photocatalytic hydrogen production. The weaker σ-donating –COOCH3 substituent in Ru-Rh(dmeb) increases the rate constant for Cl− dissociation (k−Cl=0.7s−1) and the amount of H2 produced photocatalytically (37±4μmol H2, 63±7 turnovers after 20h; turnovers=mol H2/mol photocatalyst) when compared to –H and –CH3 substituted complexes Ru-Rh(bpy) (k−Cl=0.2s−1, 21±2μmol H2, 35±3 turnovers) and Ru-Rh(Me2bpy) (k−Cl=0.2s−1, 18±2μmol H2, 30±4 turnovers), respectively. Varied catalytic activity with respect to the σ-donor capacity of the Rh-TL is attributed to the relative ease of ligand dissociation and the ability to afford rapid electron collection at the Rh metal center.