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

2004; Elsevier BV; Volume: 357; Issue: 3 Linguagem: Inglês

10.1016/j.ica.2003.09.037

1873-3255

Marciela Scarpellini, José Carlos Toledo, Ademir Neves, Javier Ellena, E.E. Castellano, Douglas Wagner Franco,

Oxidative Organic Chemistry Reactions

The present paper describes a new tripodal ligand containing imidazole and pyridine arms and its first cis-[Ru I I I (L)(Cl) 2 ]ClO 4 complex (1). The crystal structure of 1 shows Ru I I I 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 Ru I I I /Ru I I couple in the cis-[Ru I I I (L)(Cl) 2 ] + , cis-[Ru I I (L)(H 2 O)(Cl)] + and cis-[Ru I I (L)(H 2 O) 2 ] 2 + , respectively. After chemical reduction (Zn(Hg) or Eu I I ) only the cis-[Ru I I (L)(H 2 O) 2 ] 2 + species is observed in the cyclic voltammetry. Complex 1 absorbs at 470 nm (e = 1.4 x 10 3 mol - 1 L cm - 1 ), 335 nm (e = 7.9 x 10 3 mol - 1 L cm - 1 ), 301 nm (e = 6.7 x 10 3 mol - 1 L cm - 1 ) and 264 nm (e = 9.9 x 10 3 mol - 1 L cm - 1 ), in water solution (CF 3 COOH, 0.01 mol L - 1 , μ = 0.1 mol L - 1 with CF 3 COONa). Spectroelectrochemical experiments show a decrease of the bands at 335 and 301 nm, which are attributed to LMCT transitions from the chloride to the Ru I I I center and the appearance of a broad band at 402 nm ascribed to MLCT transition from the Ru I I center to the pyridine ligand. The lability of the water ligands in the cis-[Ru I I (L)(H 2 O) 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-[Ru I I (L)(H 2 O)(pz)] 2 + , which exhibits a reversible redox process at 493 mV versus SCE and absorbs at 438 nm (e = 5.1 x 10 3 mol - 1 L cm - 1 ) and 394 nm (e = 4.2 x 10 3 mol - 1 L cm - 1 ) Experiments performed with a large excess of pyrazine gave a specific rate constant k 1 = (2.8 ′ 0.5) x 10 - 2 M - 1 s - 1 , at 25 °C, in CF 3 COOH, 0.01 mol L - 1 , μ = 0.1 mol L - 1 (with CF 3 COONa).