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 < Ru). In addition to the results
of EPR and DFT calculation, it was found that the orbital energy levels
of the central metal ion (namely, either Ru or Os) in the mixed-valence
complex determines the degree of orbital mixing between metal dπ
orbitals and bridging-ligand π or π* orbitals,
which leads to either hole- or electron-transfer exchange mechanisms.