Inner Coordination Sphere Control of Metal-Metal Superexchange in Ruthenium Dimers.

The dinuclear Ru(III) complexes trans-[{(NH(3))(4)Ru(py)}(2)(&mgr;-L)][PF(6)](4), where py represents pyridine and L represents 1,4-dicyanamidobenzene dianion (dicyd(2)(-)) derivatives dicyd(2)(-) (1), Me(2)dicyd(2)(-) (2), Cl(2)dicyd(2)(-) (3), and Cl(4)dicyd(2)(-) (4), have been prepared and characterized by electronic absorption spectroscopy and cyclic voltammetry. A crystal structure of the complex trans-[{(NH(3))(4)Ru(py)}(2)(&mgr;-dicyd)][PF(6)](4).(1)/(2)H(2)O showed the dicyd(2)(-) ligand to be approximately planar with the cyanamido groups in a syn configuration. Crystal structure data are space group P2(1), with a, b, and c = 7.826(3), 20.455(7), and 14.428(5) Å, respectively, beta = 95.76 (3) degrees, V = 2296.7(14) Å(3), and Z = 2. The structure was refined by using 3292 reflections with I > 2.5sigma(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 dicyd(2)(-) 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 x 10(6), 5.7 x 10(6), 1.4 x 10(4), and 1.1 x 10(3), 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.