Comparison of Hole-Transfer Superexchange in Dinuclear Mixed-Valence Ruthenium Complexes

Three dinuclear complexes [{mer-Ru(NH3)3(bpy)}2(μ-L)][ClO4]4, where L = 2,5-dimethyl- (Me2dicyd2-), 2,5-dichloro- (Cl2dicyd2-), and unsubstituted 1,4-dicyanamidobenzene dianion (dicyd2-), have been synthesized and characterized by magnetic resonance, electrochemical, and electronic absorption spectroscopic techniques. A crystal structure of [{mer-Ru(NH3)3(bpy)}2(μ-dicyd)][ClO4]4·3H2O showed dicyd2- to be approximately planar with the cyanamido groups in an anti configuration. Crystal structure data are space group = P1̄, with a, b, and c equal to 12.5613(1), 12.8738(1), and 16.3267(2) Å, respectively, α, β, and γ equal to 76.756(1), 83.893(1), and 69.053(2)°, respectively, V = 2399.28(4) Å3, and Z = 2. The structure was refined using 6112 independent reflections with I > 2.5σ(I) to a final R factor of 0.0568. The strongly coupled mixed-valence complexes [{mer-Ru(NH3)3(bpy)}2(μ-L)]3+ where L = Me2dicyd2-, dicyd2-, and Cl2dicyd2- had decreasing comproportionation constants of 1.3 × 107, 9.3 × 106, and 3.5 × 105, respectively, which are consistent with a hole transfer superexchange mechanism for metal−metal coupling. The mixed-valence properties of these complexes together with analogous systems were compared in the context of a transformation from a localized to a delocalized mixed-valence state.