Structural, spectroscopic and redox studies of a new ruthenium(III) complex with an imidazole-rich tripodal ligand

The present paper describes a new tripodal ligand containing imidazole and pyridine arms and its first cis-[RuIII(L)(Cl)2]ClO4 complex (1). The crystal structure of 1 shows RuIII in a distorted octahedral geometry, in which two chloride ions, cis-positioned to each other, are coordinated besides the four nitrogen atoms from the tetradentate ligand L. The cyclic voltammogram of 1 exhibits three redox processes at −67, +73 and +200 mV versus SCE, which are attributed to the RuIII/RuII couple in the cis-[RuIII(L)(Cl)2]+, cis-[RuII(L)(H2O)(Cl)]+ and cis-[RuII(L)(H2O)2]2+, respectively. After chemical reduction (Zn(Hg) or EuII) only the cis-[RuII(L)(H2O)2]2+ species is observed in the cyclic voltammetry. Complex 1 absorbs at 470 nm (ε=1.4×103 mol−1 L cm−1), 335 nm (ε=7.9×103 mol−1 L cm−1), 301 nm (ε=6.7×103 mol−1 L cm−1) and 264 nm (ε=9.9×103 mol−1 L cm−1), in water solution (CF3COOH, 0.01 mol L−1, μ=0.1 mol L−1 with CF3COONa). Spectroelectrochemical experiments show a decrease of the bands at 335 and 301 nm, which are attributed to LMCT transitions from the chloride to the RuIII center and the appearance of a broad band at 402 nm ascribed to MLCT transition from the RuII center to the pyridine ligand. The lability of the water ligands in the cis-[RuII(L)(H2O)2]2+ species has been investigated using the auxiliary ligand pyrazine. Reactions in the presence of stoichiometric and excess of pyrazine yield the same species, cis-[RuII(L)(H2O)(pz)]2+, which exhibits a reversible redox process at 493 mV versus SCE and absorbs at 438 nm (ε=5.1×103 mol−1 L cm−1) and 394 nm (ε=4.2×103 mol−1 L cm−1). Experiments performed with a large excess of pyrazine gave a specific rate constant k1=(2.8±0.5)×10−2 M−1 s−1, at 25 °C, in CF3COOH, 0.01 mol L−1, μ=0.1 mol L−1 (with CF3COONa).