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

Abstract

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 (ΔE_p = 80) and 0.87 V (ΔE_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 (ΔE_p = 200) and 0.72 V (ΔE_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⁺ (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 V_ab of 950 cm⁻¹ for [1]³⁺ and 900 cm⁻¹ for [2]³⁺. The complexes are weakly luminescent at 77 K.