Synthesis, Structure and Spectral, and Electrochemical Properties of NewMononuclear Ruthenium(III) Complexes of Tris[(benzimidazol-2-yl)-methyllamine: Role of Steric Hindrance in Tuning the Catalytic OxidationActivity

摘要

The new mononuclear Rum complexes [Ru(ntb)Cl_2]Cl (1),[Ru(ntb)Cl_2]ClO_4(1a), and [Ru(mntb)Cl_2]Cl(2)(ntb = tris-(benzimidazol-2-ylmethyl)amine, mntb = tris(N-methyl-benzimidazol-2-ylmethyl)amine) have been synthesized andcharacterized. The X-ray crystal structure of 1 reveals thatthe coordination geometry around the Rum center is distortedoctahedral in which four sites are occupied by the tetraden-tate ligand ntb and the remaining cis positions by two chlo-ride ions. The stronger Ru-Nb,,,, bonds elongate the Ru-Clbonds thereby labilizing the coordinated chloride ions. In theelectronic absorption spectra the Rum complexes show twobands corresponding to n(bzim) t2g(Ru) and pn(C1-) —>t2g(Ru) ligand-to-metal charge transfer (LMCT) transitionsalong with intraligand transitions in the UV region. Complex1 shows rhombic EPR spectral features (g1, 2.238; g2, 2.071;g3, 1.790). The Rum complexes display both Rum Run re-duction and Rum —> Rulv oxidation processes. Complexes 1and 2 catalyze the allylic oxidation of cyclohexene, selectiveand higher epoxidation of cyclooctene, and hydroxylation ofalkanes in the presence of the peroxide TBHP and the per-acid m-CPBA as cooxidants illustrating that the electronicand steric effects of tripodal 4N ligands can be tuned to cata-lyze the effective oxidative transformation of organic com-pounds. ESI-MS studies reveal the formation of rutheniumperoxido species in these catalytic reactions.