Photodriven Multi-electron Storage in Disubstituted RuII Dppz Analogues

Four derivatives of the laminate acceptor ligand dipyrido-[3,2-a:2',3'-c[phenazine (dppz) and their corresponding ruthenium complexes, [Ru(phen)(2)(dPPzX(2))](2+), were prepared and characterized by NMR spectroscopy, ESI-MS, and elemental analysis. The new ligands, generically denoted dppzX(2), were symmetrically disubstituted on the distal benzene ring to give 10,13-dibromodppz (dppz-p-Br), 11,12-di-bromodppz (dppz-o-Br), 10,13-dicyanodppz (dppz-p-CN), 11,12-dicyanodppz (dppz-o-CN). Solvated ground state MO calculations of the ruthenium complexes reveal that these electron withdrawing substituents not only lower the LUMO of the dppz ligand (dppz(CN)(2) < dppzBr(2) < dppz), but that the para disubstitution results in a lower LUMO than the ortho disubstitution (dppz-p-CN < (dppz-o-CN), and dppz-p-Br < dppz-o-Br). The validity of the calculations was confirmed experimentally using cyclic voltammetry. Of the complexes evaluated in this study, only the dicyanodppz complexes showed multiple dppz-based reductions prior to reduction of the phen ligands. The capacity to form singly and doubly reduced dppz-based anions at modest reduction potentials was confirmed using a combination of spectroelectrochemical and chemical titration methods. When subjected to photolysis with visible light in the presence of a sacrificial donor, such as triethylamine, both cyano complexes showed multi-electron reduction. The other complexes only show a single reduction.