The Role of Monomers and Dimers in the Reduction of Ruthenium (II) Complexes of

N.R. DE TACCONI; R. CHITAKUNYE; F.M. MACDONNELL; R. O. LEZNA

JOURNAL OF PHYSICAL CHEMISTRY A

AMERICAN CHEMICAL SOCIETY

Lugar: Washington, DC, USA; Año: 2008 vol. 112 p. 497 - 507

1089-5639

A combination of electrochemistry, spectroelectrochemistry, and 1H NMR has been used to study the reduction and solution speciation in acetonitrile of two mononuclear Ru complexes containing the redox-active 9,11,-20,22-tetraazatetrapyrido [3,2-a:2¢,3¢-c:3¢¢,2¢¢-l:2¢¢¢,3¢¢¢-n]pentacene (tatpp) ligand. These complexes, [(bpy)2Ru-(tatpp)][PF6]2 (1[PF6]2), and [(phen)2Ru(tatpp)][PF6]2 (2[PF6]2) (where bpy is 2,2¢-bipyridine and phen is 1,10-phenanthroline), form p-p stacked dimers (eg., p-{1}24+ and p-{2}24+) in solution as determined by 1H NMR studies in an extended concentration range (90 - 5000 µM) as well as via simulation of the electrochemical data. The dimerization constant for 12+ in acetonitrile is 2 x 104 M-1 as determined from the NMR data. Slightly higher dimerization constants (8 x 104 M-1 ) were obtained via simulation of the electrochemical data and are attributed to the presence of the supporting eletrolyte. Electrochemical and spectroelectrochemical data show that the p-p stacked dimers are electroreduced in two consecutive steps at -0.31 and -0.47 V vs Ag/AgCl, which is assigned to the uptake of one electron by each tatpp ligand in p-{1}24+ to give first p-{1}23+ and then p-{1}22+. At potentials negative of -0.6 V, the electrochemical data reveal two different reaction pathways depending on the complex concentration in solution. At low concentrations (£20 µM), the next electroreduction occurs on a monomeric species (e.g., [(bpy)2Ru(tatpp)]+/0) showing that the doubly reduced p-p dimer (p-{1}2 2+ and p-{2}2 2+) dissociates into monomers. At high concentrations (³100 µM), reduction of p-{1}2 2+ or p-{2}2 2+ induces another dimerization reaction, which we attribute to the formation of a s-bond between the radical tatpp ligands and is accompanied by the appearance of a new peak in the absorption spectrum at 535 nm. This new s-dimer can undergo one additional tatpp based reduction to form s-{1}20 or s-{2}20, in which the tatpp-bridged assembly is the site of all four reductions. Finally, potentials negative of -1.2 V result in the electroreduction of the bpy or phen ligands for complexes 12+ or 22+, respectively. For the latter complex 22+, this process is accompanied by the formation of an electrode adsorbed species.