Dinuclear bis(bipyridine)ruthenium(II) complexes [(bpy)2 RuII{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)(2)Ru}(2)(mu-L)(2-)](2+), where L=L-1 (complex [1](2+)) and L-2 (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 (Delta(E)p=80) and 0.87 V (DeltaE(p)=90 mV) versus SCE due to successive Ru(II)/Ru(III) couples. The successive Ru(II)/ Ru(II) couples for [2](2+) appear at half wave potentials of 0.41 (DeltaE(p)=200) and 0.72 V (DeltaE(p)=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+) (DeltaE=700 mV) to [2](2+) (DeltaE=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 V-ab of 950 cm(-1) for [1](3+) and 900 cm(-1) for [2](3+). The complexes are weakly luminescent at 77 K.