Dinuclear bis(bipyridine)ruthenium(II) complexes [(bpy)_2Ru~(II){L}~2-Ru~(II)(bpy)_2]~(2+) incorporating thiouracil-based dianionic asymmetric bridging ligands: synthesis, structure, redox and spectroelectrochemical properties

摘要

The reactions of [Ru(bpy)_2(EtOH)_2]~(2+) with the asymmetric bridging ligands 2-thiouracil (H_2L~1) and 6-methyl-2-thiouracil (H_2L~2) in the presence of NEt_3 result in dinuclear complexes of the type [{(bpy)_2Ru}_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](ClO_4)_2 has been determined. The structural parameters of complex [2](ClO_4)_2 suggest a charge distribution with one negative charge associated with each binding site, which accordingly behve 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 (DELTAE_p = 80) and 0.87V (DELAE_p = 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 (DELTAE_p = 200) and 0.72V (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.