Electron transfer reactions of copper(II)/(I) couples with bidentate and quadridentate polypyridine ligands: is gated behavior common among outer-sphere electron transfer reactions?

Abstract

We examined redox behaviors of bis(2,9-diphenyl-1,10-phenanthroline)copper(II)/(I) couple and the oxidation reaction of structurally more constrained bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper(I) in acetnitrile: the electron transfer reactions of the former couple took place in an ordinary concerted manner with the self-exchange rate constant k ex 298=9.2×10 4 kg mol −1 s −1, while a dicopper(II) intermediate with a tetrahedral coordination geometry was spectrophotometrically detected during the course of the oxidation reaction of the latter compound. It was also found that the reduction of (1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)copper(II), the oxidized form of the corresponding dicopper(I) species, proceeded through the dimerization process. The oxidation reaction of bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper(I) by [Ni(1,4,7-triazacyclononane) 2] 3+ in acetonitrile was a bi-phasic reaction involving fast electron transfer (ET) with k ex 298=(4.8±0.1)×10 4 kg mol −1 s −1 followed by the very slow monomerization of the intermediate [ k 298=(3.46±0.03)×10 −4 s −1]. These observations indicate that the electron transfer reactions involving (1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)copper(II)/bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper(I) couple are gated through the conformational change (Cu(II) monomer–dimer). We re-examined the gated ET reactions of Cu(II)/(I) couples so far reported from the view point of the time scale of internal rearrangement and of the merit brought about by the geometric changes around either Cu(I) or Cu(II). We have reached a tentative conclusion that gated ET is observed only when the following conditions are fulfilled: (1) direct ET for either oxidation or reduction direction is non-adiabatic; and (2) a low energy CT perturbation is expected by the uneven structural change (change in the coordination geometry takes place only in the species which exhibits no low-lying CT band in its ground state). In addition to the above two conditions; (3) time scale of the structural change is slow compared with the lifetime of the encounter complex may be added.