Tuning of Metal–Metal Interactions in Mixed-Valence States of Cyclometalated Dinuclear Ruthenium and Osmium Complexes Bearing Tetrapyridylpyrazine or -benzene

New dinuclear ruthenium or osmium complexes with cyclometalated bonds in either tridentate bridging (BL) or ancillary ligands (L), [(L)­M­(BL)­M­(L)] (where M = Ru, Os; L = bis­(N-methylbenzimidazolyl)­pyridine, -benzene; BL= tetrapyridylpyrazine (tppz), -benzene (tpb)), were synthesized, and their mixed-valence-state characteristics were investigated. All of the complexes showed successive one-electron redox processes, each of which correspond to M­(II/III) (M = Ru, Os) or ligand reduction waves. In addition, an M­(III/IV) couple was observed in cyclometalated [M2(bis­(benzimidazolyl)­benzene)2(BL)] complexes (M = Ru, Os). Effects of the cyclometalated bonds on the redox behaviors and the accessibility to the mixed-valence M­(II)–M­(III) dinuclear complexes are discussed. Introduction of a cyclometalated bond induced a large negative potential shift in the redox potentials of dinuclear ruthenium and osmium complexes, depending on either bridging or ancillary sites of the cyclometalated bonds: the change falls within the range of −1.0 to −1.2 V for the bridging sites and −0.65 to −0.7 V for the ancillary ones. This large negative potential shift arises from the strong electron-donating property of the phenyl anion in a metal–C bond. Replacing the ruthenium by osmium in the dinuclear complexes with the same bridging ligand results in an increase of the potential separation (ΔE(1)) and the comproportionation constant (Kcom) of the mixed-valence complexes having the tppz bridging ligand (ΔE(1) and Kcom values: Os > Ru); however, complexes having the tpb bridging ligand showed the opposite trend (ΔE(1) and Kcom: Os * orbitals, which leads to either hole- or electron-transfer exchange mechanisms.