Photochemical Two-Electron Reduction of a Dinuclear Ruthenium

SHREEYUKTA SINGH; NORMA R. DE TACCONI; N.R.G. DIAZ; REYNALDO O. LEZNA; MUÑOZ ZUÑIGA, JACKELINE; KENNETH ABAYAN; FREDERICK M. MACDONNELL

INORGANIC CHEMISTRY

AMER CHEMICAL SOC

Lugar: WASHINGTON DC; Año: 2011 vol. 50 p. 9318 - 9328

0020-1669

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]pentacene (tatppALFA) and its dinuclear ruthenium complex, (phen)2Ru(tatppALFA)Ru(phen)2][PF6]4 (14+), are described as potential photocatalysts for CO2 reduction and/or generation of solar hydrogen. The tatppALFA ligand is structurally very similar to 9,10,20,33-tetraazatetrapyrido[3,2-a:2´,3´-c:3´´2´´-m:2´´´,3´´´-n]pentacene (tatppBETA), except that, instead of a linear tetraazapentacene  backbone, tatppALFA has an ortho (or ALFA) substitution pattern about the central benzene ring, leading to a 120° bend. Complex 14+ shows tatppALFA-based reductions at -0.73 and -1.14 V vs Ag/AgCl/saturated KCl and has an absorption spectrum showing the typical RuIId-->phen-like p* metal to-ligand charge-transfer transition centered at »­450 nm. In acetonitrile, visible-light irradiation of 14+ 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 tatppALFA ligand. These assignments were made by comparison of the spectral changes observed in 14+ upon stoichiometric chemical reduction with cobaltocene and by spectroelectrochemical analysis. Significantly, DFT calculations are very predictive of the optical and reductive behavior of the tatppALFA  complex relative to the tatppBETA 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 tatppALFA ligand to species such as SIGMA-{1}2 6+, which is only slowly reversible upon exposure to air and may limit the complexe´s  14+ utility for driving photochemical H2 production.