Photochemical Two-Electron Reduction of a Dinuclear Ruthenium Complex Containing a Bent Tetraazatetrapyridopentacene Bridging Ligand: Pushing Up the LUMO for Storing More Energy

The synthesis and characterization of a ditopic bridging ligand, 9,12,21,22-tetraazatetrapyrido [3,2-a : 2',3'-c:3 '' 2 ''-m:2''',3'''-o]pentaphene (tatpp alpha) and its dinuclear ruthenium complex, [(phen)(2)Ru(tatpp alpha)Ru(phen)(2)] [PF6](4) (I4+), are described. The tatppa ligand is structurally very similar to 9,10,20,33-tetraazatetrapyrido[3,2-a:2',3'-c:3 '',2 ''-l:2''',3'''-n]pentacene (tatpp beta), except that, instead of a linear tetraazapentacene backbone, tatppa has an ortho (or alpha) substitution pattern about the central benzene ring, leading to a 120 degrees bend. Complex I4+ shows tatpp alpha-based reductions at -0.73 and -1.14 V vs Ag/AgCl/saturated KCl and has an absorption spectrum showing the typical Ru-II d pi -> phen-like pi* metal-to-ligand charge-transfer transition centered at similar to 450 nm. In acetonitrile, visible-light irradiation of I4+ in the presence of triethylamine leads to two sequential changes in the absorption spectra, which are assigned to the formation of the one- and two-electron-reduced species, with the electrons stored on the tatpp alpha ligand. These assignments were made by comparison of the spectral changes observed in I4+ upon stoichiometric chemical reduction with cobaltocene and by spectroelectrochemical analysis. Significantly, DFT calculations are very predictive of the optical and reductive behavior of the tatpp alpha complex relative to the tatpp beta complexes and show that modeling is a useful tool for ligand design. The chemical reactivity and differential reflectance spectroelectrochemical data reveal that the reductions are accompanied by radical dimerization of the tatpp alpha ligand to species such as sigma-{1}(2)(6+), which is only slowly reversible upon exposure to air and may limit the complexe's I4+ utility for driving photochemical H-2 production.