Stereoselective formation of a meso-diruthenium(ii,ii) complex and tuning the properties of its monoruthenium analogues

Abstract

A novel bis(bidentate) ligand dgpm (dgpm = diguanidylpyrimidine) was synthesized by a catalyst-free C–N bond forming reaction in high yield (90%) by microwave-assisted heating. The ligand was coordinated to two [Ru(bpy)₂]²⁺ cores to afford a meso-di-Ru(II,II) complex (1-meso) with high diastereoselectivity over its homochiral form. Three mononuclear ether-functionalized Ru(II) complexes (2: ethoxyether; 3: butoxyether; 4: 2-hydroxy-1-ethoxyether) were also isolated. The ligand and complexes were fully characterized by a variety of techniques including X-ray crystallography. In cyclic voltammetric studies, the complexes exhibit a Ru^III/II couple, which is ∼500 mV less positive than the Ru^III/II couple in Ru(bpy)₃²⁺. The ¹MLCT absorption maxima of all the complexes (510–550 nm) are considerably red-shifted as compared to that of Ru(bpy)₃²⁺ (450 nm). The ³MLCT emission maxima of complexes 1-meso and 3 are also red-shifted by about 120 nm compared to that of Ru(bpy)₃²⁺ (620 nm), whereas the corresponding maxima for complexes 2 and 4 are shifted by 75 nm and 25 nm, respectively. These relative trends in redox potentials and ¹MLCT maxima are in good agreement with DFT and TD-DFT calculations, performed for all complexes. Complexes 1-meso and 3 display emission from a Ru^II-to-bpy ³MLCT state, which is rarely the emitting state at λ > 700 nm in [Ru(bpy)₂(N-N)]²⁺ complexes when the ancillary ligand is neutral.