Synthesis, structure and spectral and redox properties of new mixed ligand monomeric and dimeric Ru(ii) complexes: predominant formation of the “cis-α” diastereoisomer and unusual 3MC emission by dimeric complexes

Abstract

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(μ-Cl)]₂(ClO₄)₂1 and [Ru(bbdo)(μ-Cl)]₂(ClO₄)₂2 respectively. The new di-μ-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2′-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO₄)₂3, [Ru(bbdo)(bpy)](ClO₄)₂4, [Ru(pdto)(dpa)](ClO₄)₂5 and [Ru(bbdo)(dpa)](ClO₄)₂6 and with the triimine ligand 2,2′:6,2″-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO₄)₂7 and [Ru(bbdo)(terpy)](ClO₄)₂8. In 1 the dimeric cation [Ru(pdto)(μ-Cl)]₂²⁺ is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a “cis-α” configuration [ΔΔ/ΛΛ (rac) diastereoisomer] supporting the possibility of some attractive π-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a “cis-α” isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The ¹H and ¹³C NMR spectral data of the complexes throw light on the nature of metal–ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-μ-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t_2g⁶ (¹A_1g) → t_2g⁵ e_g¹ (¹T_1g, ¹T_2g) d–d transition. They also display an intense Ru(II) dπ → py/bzim (π*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3–8 exhibit dπ → π* MLCT transitions in the range 340–450 nm. The binuclear complexes 1 and 2 exhibit a ligand field (³MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion (³MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru^IIRu^III state (K_c: 1, 3.97 × 10⁶; 2, 1.10 × 10⁶). The mononuclear complexes 3–7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated ‘soft’ thioether raises the redox potentials significantly by stabilising the ‘soft’ Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.