A pentaammineruthenium(III) dimer with the novel bridging ligand 4,4′-dicyanamidobiphenyl dianion

Abstract

The novel ligand 4,4′-dicyanamidobiphenyl dianion (bp2−) has been synthesized and characterized by 13C NMR spectroscopy, cyclic voltammetry, and crystallography. The crystal structure of [Ph4As]2[bp]•H2O showed that bp2− is approximately planar with a dihedral angle of 8.2° between phenyl ring planes and the cyanamide groups in an anti conformation. The water of crystallization is asymmetrically hydrogen bonded between cyanamide groups of adjacent bp2− ions. The crystal data for C62H48N4As2 + H2O are monoclinic crystal system and space group P21/c with a = 12.998(5) Å, b = 13.465(4) Å, c = 28.703(13) Å, β = 98.94(3)°, V = 4963(3) Å3 and Z = 4. The structure was refined by using 4555 reflections with I > 2.5σ(I) to an R factor of 0.058. The complex, [{(NH3)5Ru)2(μ-bp)][X]4, where X = tosylate or PF6− ions, was also synthesized and characterized by 1H NMR spectroscopy, cyclic voltammetry, spectroelectrochemistry, and temperature-dependent magnetic susceptibility measurements. From cyclic voltammetry measurements, the comproportionation constants to form the mixed-valence complex [{(NH3)5Ru)2(μ-bp)]3+ were estimated to be 4.1, 16, and 22 in water, acetonitrile, and nitromethane, respectively. The trend and magnitude of Kc suggests solvent valence trapping of a weakly coupled Class II ion. The MMCT band of the mixed-valence complex had to be deconvoluted from the low-energy LMCT band and had the following properties in acetonitrile, νmax = 8400 cm−1, εmax = 3300 M−1 cm−1, and Δν1/2 = 3300 cm−1. The weak superexchange mediating properties of bp2− compared to 1,4-dicyanamidobenzene dianion were suggested to arise from the larger barrier to the formation of the radical anion bp−. Key words: cyanamido, mixed valence, superexchange, ruthenium.