The role of monomers and dimers in the reduction of ruthenium(II) complexes of redox-active tetraazatetrapyridopentacene ligand.

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 pi-pi stacked dimers (e.g., pi-{1}24+ and pi-{2}24+) in solution as determined by 1H NMR studies in an extended concentration range (90 - 5000 microM) as well as via simulation of the electrochemical data. The dimerization constant for 12+ in acetonitrile is 2 x 10(4) M(-1) as determined from the NMR data. Slightly higher dimerization constants (8 x 10(4) 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 pi-pi 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 pi-{1}24+ to give first pi-{1}23+and then pi-{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 (< or =20 microM), the next electroreduction occurs on a monomeric species (e.g., [(bpy)2Ru(tatpp)]+/0) showing that the doubly reduced pi-pi dimer (pi-{1}22+ and pi-{2}22+) dissociates into monomers. At high concentrations (> or =100 microM), reduction of pi-{1}22+ or pi-{2}22+ induces another dimerization reaction, which we attribute to the formation of a sigma-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 sigma-dimer can undergo one additional tatpp based reduction to form sigma-{1}20 or sigma-{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.