Synthesis and characterization of bis(di-2-pyridylmethanamine)ruthenium(II)

The complex Ru(dipa)(2)(2+) (dipa = di-2-pyridylmethanamine) has been prepared, yielding approximately a statistical ratio of the meso and rac isomers. The electronic spectra of both isomers show pyridyl pi --> pi transitions in the UV region and MLCT bands in the visible region. The solvent dependence of the spectra provides evidence of hydrogen bond formation between the solvent and the NH(2) site on the ligand. The electrochemical properties of the two isomers are identical; each undergoes a reversible one-electron oxidation in acetonitrile (E(1/2) = 0.933 V vs Ag/AgCl) and in aqueous solution below pH 3 (E(1/2) = 0.786 V vs Ag/AgCl). In aqueous solution above pH 3, one-electron oxidation of the ruthenium center is followed by deprotonation of the ligand NH(2) site yielding a reactive amidoruthenium(III) species. The ruthenium-bound dipa ligand possesses structural constraints that prevent the usual oxidative dehydrogenation reaction, which would yield exclusively the corresponding imine. Instead the amidoruthenium(III) intermediate finds alternative reaction routes leading to multiple products.