Synthesis, structure and spectral, and electrochemical properties of new mononuclear ruthenium(III) complexes of tris[(benzimidazol-2-yl)methyl]amine: role of steric hindrance in tuning the catalytic oxidation activity

Abstract

The new mononuclear Ru^III complexes [Ru(ntb)Cl₂]Cl (1), [Ru(ntb)Cl₂]ClO₄ (1a), and [Ru(mntb)Cl₂]Cl (2) (ntb = tris(benzimidazol-2-ylmethyl)amine, mntb = tris(N-methylbenzimidazol-2-ylmethyl)amine) have been synthesized and characterized. The X-ray crystal structure of 1 reveals that the coordination geometry around the Ru^III center is distorted octahedral in which four sites are occupied by the tetradentate ligand ntb and the remaining cis positions by two chloride ions. The stronger Ru-N_bzim bonds elongate the Ru-Cl bonds thereby labilizing the coordinated chloride ions. In the electronic absorption spectra the Ru^III complexes show two bands corresponding to π(bzim) → t_2g(Ru) and pπ(Cl^-) → t_2g(Ru) ligand-to-metal charge transfer (LMCT) transitions along with intraligand transitions in the UV region. Complex 1 shows rhombic EPR spectral features (g₁, 2.238; g₂, 2.071; g₃, 1.790). The Ru^III complexes display both Ru^III → RuII reduction and Ru^III → Ru^IV oxidation processes. Complexes 1 and 2 catalyze the allylic oxidation of cyclohexene, selective and higher epoxidation of cyclooctene, and hydroxylation of alkanes in the presence of the peroxide TBHP and the peracid m-CPBA as cooxidants illustrating that the electronic and steric effects of tripodal 4N ligands can be tuned to catalyze the effective oxidative transformation of organic compounds. ESI-MS studies reveal the formation of ruthenium peroxido species in these catalytic reactions.