Electrochemistry of metal phthalocyanines in organic solvents at variable pressure

High-pressure electrochemical investigations of representative metallophthalocyanines in solution are reported. The selected systems were ZnPc, CoPc, FePc, and CoTNPc (Pc = phthalocyanine, TNPc = tetraneopentoxyphthalocyanine) in several donor solvents and (for CoTNPc) dichlorobenzene, with [Bu(4)N][ClO(4)] as supporting electrolyte and a conventional Pt electrode referred to Ag(+)(CH(3)CN)/Ag. Electrode reaction volumes deltaV(cell) for CoTNPc and ZnPc show that consecutive ring reductions result in progressive increases in electrostriction of solvent in accordance with Drude-Nernst theory. Reductions of the metal center in CoTNPc and CoPc, however, result in much less negative values of deltaV(cell) than would be expected by analogy with ring reductions of the same charge type. This is attributable to loss of axial ligands following the insertion of antibonding 3d(z)2 electrons on going from Co(III) to low-spin Co(II) and then Co(I). In the same vein, rate constants for reduction of Co(III) centers to Co(II) were an order of magnitude slower than those for other metal center or phthalocyanine ring reductions because of Franck-Condon restrictions. The volumes of activation deltaV(el) were invariably positive for all the electrode reactions and in most cases were roughly equal to the volumes of activation for reactant diffusion deltaV(diff)(), indicating predominant rate control by solvent dynamics rather than by activation in the manner of transition-state theory for which negative deltaV(el) values are expected. For CoTNPc and CoPc in donor solvents, the deltaV(cell) and deltaV(el) data are consistent with the assignments of the successive reduction steps made for CoTNPc in DMF by Nevin et al. (Inorg. Chem. 1987, 26, 570).