Dinuclear bis(bipyridine)ruthenium(ii) complexes [(bpy)2RuII{L}2–RuII(bpy)2]2+ incorporating thiouracil-based dianionic asymmetric bridging ligands: synthesis, structure, redox and spectroelectrochemical properties

The reactions of [Ru(bpy)2(EtOH)2]2+ with the asymmetric bridging ligands 2-thiouracil (H2L1) and 6-methyl-2-thiouracil (H2L2) in the presence of NEt3 result in dinuclear complexes of the type [{(bpy)2Ru}2(μ-L)2−]2+, where L = L1 (complex [1]2+) and L2 (complex [2]2+). In [1]2+ and [2]2+ the bridging functions act as a dinegative unit and bind with the two [Ru(bpy)2]2+ units through the terminal N,S and N,O donor sites. The crystal structure of the complex [2](ClO4)2 has been determined. The structural parameters of complex [2](ClO4)2 suggest a charge distribution with one negative charge associated with each binding site, which accordingly behave like a pyridonate and a thiopyridonate ligand. In acetonitrile, complex [1]2+ exhibits two reversible one-electron redox processes at half-wave potentials 0.17 (ΔEp = 80) and 0.87 V (ΔEp = 90 mV) versus SCE due to successive Ru(II)/Ru(III) couples. The successive Ru(II)/Ru(III) couples for [2]2+ appear at half wave potentials of 0.41 (ΔEp = 200) and 0.72 V (ΔEp = 100 mV) versus SCE. Both the complexes display a third oxidation process in the range 1.54–1.6 V versus SCE, which is ascribed to a process centred on the thiolato unit of the bridging ligand. The bipyridine based multiple reductions are observed in the range of −1.4 to −1.8 V versus SCE. The correlation between the structural parameters and the decrease in separation of the successive Ru(II)/Ru(III) couples while moving from [1]2+ (ΔE = 700 mV) to [2]2+ (ΔE = 310 mV) has been noted. A UV-Vis-NIR spectroelectrochemical study was carried out at 243 K on both the complexes. The mixed valence species [1]3+ and [2]3+ display broad intervalence charge-transfer transitions at 1170 nm and 1140 nm respectively, characteristic of class II complexes, which correspond to the electronic coupling constants Vab of 950 cm−1 for [1]3+ and 900 cm−1 for [2]3+. The complexes are weakly luminescent at 77 K.