Synthesis, characterization, and electrocatalytic oxidation of benzyl alcohol by a pair of geometric isomers of [Ru(trpy)(4,4′-Me 2dppi)(OH 2)] 2+ where 4,4′-dppi is 3,6-di-(4-methylpyrid-2-yl)pyridazine

Abstract

The geometric isomers, in-[Ru(trpy)(4,4′-Me 2dppi)Cl] + ( 1a), in-[Ru(trpy)(4,4′-Me 2dppi)OH 2] 2+ ( 1b), and in-[Ru(trpy)(4,4′-Me 2dppi)(NCCH 3)] 2+ ( 1c), out-[Ru(trpy)(4,4′-Me 2dppi)Cl] + ( 2a), out-[Ru(trpy)(4,4′-Me 2dppi)OH 2] 2+ ( 2b), and out-[Ru(trpy)(4,4′-Me 2dppi)(NCCH 3)] 2+ ( 2c), where trpy is 2,2′,2″-terpyridine and 4,4′-Me 2dppi is 3,6-di-(4-methylpyrid-2-yl)pyridazine, were synthesized and characterized by UV–Vis and 1H NMR spectroscopy. The in and out notation refers to the orientation of the non-imine group which is directed towards the center of the complexes in the in complexes and away from the center in the out complexes. Compounds 1a and 2c were additionally characterized by X-ray crystallography. Reaction of 1a or 2a with aqueous silver ion produces the corresponding aqua complexes, 1b and 2b, which after dissolution in acetonitrile produces the analogous acetonitrile complexes, 1c and 2c. The redox properties of 1a, 1b, 2a, and 2b were examined using cyclic voltammetry. In acetonitrile, compounds 1a and 2a display reversible 1e − waves assigned to the Ru(III)/Ru(II) couple, while in aqueous solutions 1b and 2b show pH-dependent 2e − waves corresponding to the formation of Ru(IV)O complexes. Second-order rate constants, k cat, for oxidation of benzyl alcohol derivatives by the Ru(IV)O complex 2b were determined electrochemically. Complex 1b severely passivated the working electrode upon oxidation, and reliable catalytic rate constants could not be measured. This passivation may be related to self oxidation of the Me 2dppi ligand by the electrochemically generated Ru(IV)O complex.