Ruthenium(II)/(III) bipyridine heterochelates incorporating phenolato imine functionalities. Synthesis, crystal structure, spectroscopic and electron-transfer properties and solution reactivities

Abstract

A new class of ruthenium–bipyridine complexes of the type [Ru(bpy)₂(L′)]ClO₄ [bpy = 2,2′-bipyridine; L′ = ⁻OC₆H₃(R)C(R′)NH; R = R′ = H 1a; R = H, R′ = CH₃1b; R = NO₂, R′ = H 1c] have been synthesized via cleavage of N–N and N–C (aliphatic and aromatic) bonds of binucleating imine functionalities. The formation of 1 has been authenticated by the single crystal structure determination of 1a. The complexes exhibit strong MLCT transitions in the visible region and intraligand transitions in the UV region and display moderately strong emission near 700 nm. The quantum yield (φ) of the emission process at 77 K (EtOH–MeOH, 4∶1 glass) varies in the range 2.2 × 10⁻²–6.5 × 10⁻³ depending on L′ in 1. The complexes show ruthenium(III)–ruthenium(II) and ruthenium(IV)–ruthenium(III) oxidations in the ranges 0.52–0.77 and 1.71–1.97 V versus SCE respectively. For 1a and 1b, one ligand centred oxidation near 2 V and for all the complexes two bipyridine based reductions have been detected in the ranges −1.51 to −1.56 V and −1.72 to −1.79 V versus SCE. Coulometric oxidation of 1 initially generates the unstable trivalent one-electron paramagnetic [Ru^III(bpy)₂(L′)]²⁺ congener, 1⁺. The complexes 1⁺ display one broad and moderately strong LMCT band near 750 nm and intraligand transitions in the UV region. The oxidised complexes 1⁺ exhibit rhombic EPR spectra at 77 K which have been analysed to furnish values of distortion parameters (Δ = 4543–5923 cm⁻¹, V = 3251–5127 cm⁻¹) and energy of the expected ligand field transitions (ν₁ 3086–3508, ν₂ 6409–8669 cm⁻¹) within the t₂ shell. One of the ligand field transitions has been experimentally observed (ν₂ 7092–7812 cm⁻¹). The oxidised species 1⁺ slowly changes to diamagnetic dimeric species of the type [(bpy)₂Ru^III{⁻OC₆H₃(R)C(R′)NNC(R′)(R)C₆H₃O⁻}Ru^III(bpy)₂]⁴⁺2 where the ruthenium(III) centres are antiferromagnetically coupled. In the presence of water 1⁺ as well as 2 return to the parent bivalent species 1. The second order rate constants (k) of the conversion process 1⁺ → 2 in dry acetonitrile and first order rate constants (k′) of 1⁺ → 1 in ordinary acetonitrile and the thermodynamic parameters (ΔH^‡ and ΔS^‡) of both have been determined spectrophotometrically in the temperature range 303–333 K.