Electroreduction of the Ruthenium Complex [(bpy)2Ru(tatpp)]Cl2 in Water: Insights on the Mechanism of Multielectron Reduction and Protonation of the Tatpp Acceptor Ligand as a Function of pH

Abstract

The mononuclear ruthenium complex [(bpy) 2Ru(tatpp)] (2+) ( 1 (2+); bpy is 2,2'-bipyridine and tatpp is 9,11,20,22-tetra-aza-tetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3''']-pentacene) undergoes up to four reversible tatpp ligand-based reductions as determined by electrochemistry in aqueous solution. Specific redox and protonation states of this complex were generated by stoichiometric chemical reduction with cobaltocene and protonation with trifluoroacetic acid in acetonitrile. These species exhibit unique UV-visible absorption spectra, which are used to determine the speciation in aqueous media as a function of the potential during the electrochemical reduction. A combination of cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemistry showed that the voltammetric reduction peaks are associated with two-electron/two-proton processes in which the details of stepwise electron transfer and protonation steps vary as a function of the pH. Spectroelectrochemistry performed during potential scans with and without a small superimposed sinusoidal potential waveform was used to examine the mechanistic details of this proton-coupled multielectron reduction process. Under basic conditions, the radical [(bpy) 2Ru(tatpp (*-))] (+)( 1 (*+)) is the first electrogenerated species that converts to doubly reduced, single-protonated [(bpy) 2Ru(Htatpp-)] (+) (H 1 (+)) and doubly protonated [(bpy) 2Ru(H 2tatpp)] (2+)(H 2 1 (2+)) by subsequent electron-transfer (ET) and proton-transfer (PT) reactions. Partial dimerization of radical 1 (*+) is also observed in basic media. Neutral or acidic conditions favor an initial ET-PT reaction leading to the protonated, radical species [(bpy) 2Ru(Htatpp (*))] (2+) (H 1 (*2+)), which rapidly disproportionates to give 1 (2+) and H 2 1 (2+). This intermediate, H 1 (*2+), is only observed when potential modulation is used in the spectroelectrochemical experiment. At more negative potentials, the doubly reduced complexes (e.g., H 2 1 (2+), H 1 (+)) undergo a two-electron/two-proton reductions to give the quadruply reduced and protonated species H 4 1 (2+) and/or H 3 1 (+) throughout the pH range investigated. These species are also only detectable when potential modulation is used in the spectroelectrochemical experiment, as they rapidly comproportionate with 1 (2+) in the bulk, leading to the regeneration of intermediate double-reduced species, H 2 1 (2+).