Inner Coordination Sphere Control of Metal−Metal Superexchange in Ruthenium Dimers

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Abstract
The dinuclear Ru(III) complexes trans-[{(NH3)4Ru(py)}2(μ-L)][PF6]4, where py represents pyridine and L represents 1,4-dicyanamidobenzene dianion (dicyd2-) derivatives dicyd2- (1), Me2dicyd2- (2), Cl2dicyd2- (3), and Cl4dicyd2- (4), have been prepared and characterized by electronic absorption spectroscopy and cyclic voltammetry. A crystal structure of the complex trans-[{(NH3)4Ru(py)}2(μ-dicyd)][PF6]4·1/2H2O showed the dicyd2- ligand to be approximately planar with the cyanamido groups in a syn configuration. Crystal structure data are space group P21, with a, b, and c = 7.826(3), 20.455(7), and 14.428(5) Å, respectively, β = 95.76 (3)°, V = 2296.7(14) Å3, and Z = 2. The structure was refined by using 3292 reflections with I > 2.5σ(I) to an R factor of 0.069. Solid state magnetic susceptibility measurements of the Ru(III)−Ru(III) dimers showed diamagnetic behavior at room temperature, and this is suggested to be due to strong antiferromagnetic superexchange via the HOMO of the dicyd2- ligand. The bridging ligand dependence of metal−metal coupling in the Ru(III)−Ru(II) complexes of 1, 2, 3, and 4 in acetonitrile solution was demonstrated by the trend in comproportionation constants, 1.5 × 106, 5.7 × 106, 1.4 × 104, and 1.1 × 103, respectively. In addition, comparison to the analogous pentaammineruthenium dimers showed that the magnitude of metal−metal superexchange could be controlled by the nature of the spectator ligand. Spectroelectrochemical methods were used to acquire the absorption spectra of the mixed-valence complexes, and the intervalence band properties were modeled with PKS theory. Metal−metal coupling in the Ru(III)−Ru(II) complexes of 1, 2, 3, and 4 was analyzed by using Hush and CNS theories.

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