Slow electron self-exchange in spite of a small inner-sphere reorganisation energy - The electron-transfer properties of a copper complex with a tetradentate bispidine ligand

The electron self-exchange rate of [Cu(L)(OH2)](2+/1+), k(11)(exp)(298.13 K) = 15 +/- 11 m(-1)s(-1) {L = dimethyl 3,7-dimethyl-9-oxo-2,4-bis(2-pyridyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-di- carboxylate}, was determined by a cross reaction. The analysis, based on classical Marcus theory, indicates that this relatively slow rate is to a large extent due to enthalpic terms (DeltaG(11)(double dagger,exp) = 62.8 +/- 3.5 kJ.mol(-1), DeltaH(11)(double dagger,exp) = 36.0 2.7 kJ-mol(-1) and DeltaS(11)(double dagger,exp) = -92 +/- 10 J.mol(-1)K(-1)). The activation entropy is significant but not unusually large and the calculated outersphere reorganization energy, DeltaG(out)(.,calc) = 20.5 kJ.mol(-1), is at least of the same order of magnitude as the calculated innersphere reorganisation energy DeltaG(in)(.,calc) = 18.6 kJ.mol(-1), i.e. the deformation of the solvent sheath is a major reason for the slow electron transfer rate. This is believed to be due to the highly elastic coordination geometry which leads to little strain upon distortion enforced by the electron transfer but to comparably large structural changes and, hence, to a large outer-sphere reorganisation term. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).