Redox-Asymmetric Bisruthenium Complex Bridged by a Pyridin-4-yl Moiety: Synthesis, Characterization, and Electronic Coupling Studies

A polyazine ligand, 5,5′-di­(1-butyl-1,2,3-triazol-4-yl)-2,2′-bipyridine
(Bu-dtabpyH), has been prepared by a “click” reaction.
The treatment of Bu-dtabpyH with [Ru­(bpy)2Cl2] affords complex [Ru­(bpy)2(Bu-dtabpyH)]2+ (bpy
= 2,2′-bipyridine), which has been characterized by single-crystal
X-ray analysis. The reaction of [Ru­(bpy)2(Bu-dtabpyH)]2+ with another equivalent of [Ru­(bpy)2Cl2] in the presence of AgOTf gives a redox-asymmetric bisruthenium
complex, [Ru­(bpy)2(Bu-dtabpy)­Ru­(bpy)2]3+ (Bu-dtabpy is the 4-deprotonated form of Bu-dtabpyH), with one ruthenium
atom binding to the pyridin-4-yl bridge with a Ru–N bond and
the other with a Ru–C bond. This complex displays two Ru­(II/III)
redox couples at +0.79 and +1.28 V vs Ag/AgCl. After being transformed
into a mixed-valence species, [Ru­(bpy)2(Bu-dtabpy)­Ru­(bpy)2]4+, by one-electron oxidation, it exhibits a moderate
metal-to-metal charge-transfer band around 1300 nm. Density functional
theory and time-dependent density functional theory calculations have
been carried out for the model complex [Ru­(bpy)2(Me-dtabpy)­Ru­(bpy)2]3+ and the mixed-valence state [Ru­(bpy)2(Me-dtabpy)­Ru­(bpy)2]4+ to complement these
experimental results, where Me-dtabpy is the 4-deprotonated form of
5,5′-di­(1-methyl-1,2,3-triazol-4-yl)-2,2′-bipyridine.