Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO2 Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH3)]2+ (6DHBP = 6,6′-(OH)2bpy)

Ruthenium complexes with proton-responsive ligands [Ru­(tpy)­(nDHBP)­(NCCH3)]­(CF3SO3)2 (tpy = 2,2′:6′,2″-terpyridine; nDHBP = n,n′-dihydroxy-2,2′-bipyridine, n = 4 or 6) were examined for reductive chemistry and as catalysts for CO2 reduction. Electrochemical reduction of [Ru­(tpy)­(nDHBP)­(NCCH3)]2+ generates deprotonated species through interligand electron transfer in which the initially formed tpy radical anion reacts with a proton source to produce singly and doubly deprotonated complexes that are identical to those obtained by base titration. A third reduction (i.e., reduction of [Ru­(tpy)­(nDHBP–2H+)]0) triggers catalysis of CO2 reduction; however, the catalytic efficiency is strikingly lower than that of unsubstituted [Ru­(tpy)­(bpy)­(NCCH3)]2+ (bpy = 2,2′-bipyridine). Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical infrared experiments suggest the reactivity of CO2 at both the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl formed by the intermediacy of a metallocarboxylic acid is stable against reduction, and mass spectrometry analysis of this product indicates the presence of two carbonates formed by the reaction of DHBP–2H+ with CO2.