Ruthenium(II/III)–bipyridine complexes with four-membered sulphur donor co-ligands: synthesis, metal valence preference, spectroscopic and electron-transfer properties

Abstract

A group of stable ruthenium(II) and (III) mixed-ligand tris-chelated complexes of the type [Ruⁿ⁺(bpy)(L)₂]^z+ (1–8, n=2, Z=0; 9, n=3, Z=1) have been synthesized and characterized (bpy=2,2'-bipyridine; L=anionic form of the ligands, ROC(S)SK, (R=Me, Et, n Pr, i Pr, n Bu, i Bu, –CH₂–Ph) or (EtO)₂P(S)SNH₄ or (Et)₂NC(S)SNa). The complexes 1–8 are diamagnetic and electrically neutral and the complex 9 is one-electron paramagnetic and behaves as 1:1 electrolyte in acetonitrile solvent. The complexes 1–8 and 9 display two MLCT transitions near 530, 370 nm and 663, 438 nm respectively. Intra-ligand bipyridine based π–π^∗ transition is observed near 300 nm. The complexes 1–8 exhibit room-temperature emission from the highest energy MLCT band (~370 nm). At room temperature the lifetime of the excited states for the complexes 2 and 8 are found to be 90 and 95 ns respectively. In acetonitrile solution the complexes 1–9 show a reversible ruthenium(III)–ruthenium(II) couple in the range -0.08 → 0.40 V and irreversible ruthenium(III)–ruthenium(IV) oxidation in the range 1.19–1.45 V vs Ag/AgCl. One reversible bipyridine reduction is observed for each complex in the range -1.70 → -1.85 V vs Ag/AgCl. The presence of trivalent ruthenium in the oxidized solution for one complex 1 is evidenced by the axial EPR spectrum at 77 K. The isolated trivalent complex 9 also exhibits an axial EPR spectrum at 77 K. The EPR spectra of the trivalent ruthenium complexes (1⁺ and 9) have been analyzed to furnish values of distortion parameters (Δ(cm^-1)→1⁺, 3689; 9, 3699) and energies of the two expected ligand field transitions (ν₁(cm^-1)→1⁺, 3489; 9, 3497 and ν₂(cm^-1)→1⁺, 4339; 9, 4348) within the t₂ shell. One of the ligand field transitions has been experimentally observed at 4673 cm^-1 for complex 9 and which close to the computed ν₂ value (4348 cm^-1).