SUPEREXCHANGE METAL METAL COUPLING IN DINUCLEAR PENTAAMMINERUTHENIUM COMPLEXES INCORPORATING A 1,4-DICYANAMIDOBENZENE DIANION BRIDGING LIGAND

Four dinuclear complexes, {mu-Dicyd-[(NH3)5Ru]2}[ClO4]4 (1), {mu-Me2Dicyd-[(NH3)5Ru]2}[ClO4 4 (2), {mu-Cl2Dicyd-[(NH3)5Ru]2}[Cl]4 (3), and {mu-Cl4Dicyd-[(NH3)5Ru]2}[Cl]4 (4) where Dicyd2- = 1,4-dicyanamidobenzene dianion, Me2Dicyd2- = 1,4-dicyanamido-2,5-dimethylbenzene dianion, Cl2Dicyd2- = 1,4-dicyanamido-2,5-dichlorobenzene dianion, and Cl4Dicyd2- = 1,4-dicyanamido-2,3,5,6-tetrachlorobenzene dianion, have been synthesized. A crystal structure of the tosylate salt of 1 was determined. Two conformations of the dinuclear complex were revealed in the unit cell. For conformer A, both Ru(III)-cyanamide bonds are essentially linear, having a Ru(1)-N(6)-C(1) bond angle of 175-degrees. For conformer B, both Ru(III)-cyanamide bonds are bent, having a corresponding bond angle of 150-degrees. In both conformers, the Dicyd2- bridging ligand is planar with the cyanamide groups in an anti configuration. Crystal structure data for the complex are space group P2(1)/a, with a, b, and c = 7.5861 (6), 23.0450 (19), and 32.078 (3) angstrom, respectively, beta = 95.771 (7)-degrees, V = 5579.5 angstrom3, and Z = 4. The structure was refined using 3947 significant Cu (1.54056 angstrom) reflections to an R factor of 0.063. The dinuclear complexes were characterized by cyclic voltammetry, UV-vis NIR spectroscopy, and magnetic susceptibility. The mixed-valence complexes [Ru(III), Ru(II)] of 1-4 were shown to be weak coupling cases with the order of coupling inferred from com-proportionation constants at 1 < 4 < 3 < 2. Temperature dependent magnetic susceptibility measurements of the oxidized complexes [Ru(III), Ru(III)] of 1-4 showed antiferromagnetic behavior. Isotropic spin only models of the data for 3 and 4 and the tosylate salt of 1 derived magnetic exchange coupling constants of J = -95.9, -61.9, and -100 cm-1, respectively (where H = -2JS(a)-->.S(b)-->). For the perchlorate salt of 1, a slight rise in magnetic susceptibility as temperature approached 300 K allowed an estimate of J greater-than-or-equal-to 400 cm-1. For 2, antiferromagnetic coupling was of sufficient magnitude to render the complex diamagnetic at room temperature. Antiferromagnetic coupling of this magnitude at an estimated through space separation of 13.2 angstrom is unprecedented. Coupling in both mixed-valence and oxidized complexes is suggested to be dominated by a superexchange mechanism involving the Ru(III) d-pi-orbitals and the r HOMO of the Dicyd2- bridging ligand. Extended Huckel calculations have been performed using the crystal structure data of the free Dicyd2- ligands to illustrate that a continuous pi-interaction between metal ions is possible.