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

Abstract

Three dinuclear complexes [{ mer -Ru(NH 3 ) 3 (bpy)} 2 (μ-L)][ClO 4 ] 4, where L = 2,5-dimethyl- (Me 2 dicyd 2 - ), 2,5-dichloro- (Cl 2 dicyd 2 - ), and unsubstituted 1,4-dicyanamidobenzene dianion (dicyd 2 - ), have been synthesized and characterized by magnetic resonance, electrochemical, and electronic absorption spectroscopic techniques. A crystal structure of [{ mer -Ru(NH 3 ) 3 (bpy)} 2 (μ-dicyd)][ClO 4 ] 4 ·3H 2 O showed dicyd 2 - to be approximately planar with the cyanamido groups in an anti configuration. Crystal structure data are space group = P 1̄, 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) Å 3, 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(NH 3 ) 3 (bpy)} 2 (μ-L)] 3+ where L = Me 2 dicyd 2 -, dicyd 2 -, and Cl 2 dicyd 2 - had decreasing comproportionation constants of 1.3 × 10 7, 9.3 × 10 6, and 3.5 × 10 5, 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.