Synthesis and Characterization of Mononuclear and Dinuclear Ruthenium Complexes with Tris(2-pyridylmethyl)amine and Tris(5-methyl-2-pyridylmethyl)amine

Novel Ru(II) and Ru(III) complexes having TPA (tris(2-pyridylmethyl)amine, L1) and 5-Me3-TPA (tris(5-methyl-2-pyridylmethyl)amine, L2) were prepared to establish their synthetic routes and to elucidate coordination geometry and interactions between tightly bound tripodal tetradentate ligands and RuII/RuIII centers. They include mononuclear Ru(II) complexes [RuCl(DMSO)(L)]ClO4 (1 (L1), 2 (L2)), dinuclear bis-μ-chloro Ru(II) complexes [RuCl(L)]2(ClO4)2 (3 (L1), 4 (L2)), and mononuclear Ru(III) complexes [RuCl2(L)]ClO4 (5 (L1), 6 (L2)). They were characterized by X-ray crystallography (for 2, 3, and 5), 1H NMR spectroscopy, and cyclic voltammetry. For compound 2, the crystal structure was determined to possess S-bound DMSO ligand which was trans to pyridine and Cl- trans to the tertiary amino group of L2, and this isomer was obtained exclusively. Complex 1 was also isolated as a single isomer. Complex 3 was revealed to be a dinuclear bis-μ-chloro Ru(II) species with the center of symmetry midway between two Cl-. 1H NMR spectra of Ru(II) complexes 1−4, the molecular structure of 2, and comparison of the molecular structure of 3 with 5 suggest that the interaction between the Ru(II) center, pyridyl moieties, and the DMSO ligand is strengthened by π-back-bonding from the Ru(II) center to the ligands in addition to σ-bonding of the tertiary amino group. Electrochemical measurements on 1−6 in CH3CN revealed that the methyl groups on pyridyl rings exert electron-donating effects onto the Ru centers to lower each redox process and such effect strengthens the Ru−S bonding in 2 compared with that in 1, accommodating π-back-bonding from Ru(II) center to other π-acceptors such as DMSO in 2 enough to prevent isomerization of DMSO binding mode. The dinuclear complexes 3 and 4 showed relatively large comproportionation constants, which suggest mixed-valent RuIIRuIII states would be stabilized.