Ligand Tuning Effects Upon the Multielectron Reduction and Single Electron

摘要

In this paper we report the effects of a more subtle perturbation due to changesin ligand substituents. The redox thermodynamics of a total of twenty cis and trans dioxo rhenium (V) complexes are surveyed; five of these have been subjected to more extensive pH dependent studies in order to elucidate how electron transfer is coupled to proton release and uptake. Also reported is a brief study of substituent effects upon electronic absorption spectra for the cis dioxo series. These measurements (along with resonance Raman and electrochemical studies) have been used to show that the cis spectra, in contrast to the trans, are dominated by charge-transfer transitions. Other significant findings are: (1) that the one-electron Re(VI) reduction potentials are influenced in a predictable fashion by substituent electronic effects, (2) that the two-electron Re(V) reduction potentials, surprisingly, are not affected appreciably by ligand-substituent based electron withdrawal or donation effects, and (3) that the two-electron kinetics, however, are influenced detectable by remote ligand substituents. The second and third observations are perhaps the most intriguing; they are tentatively interpreted in terms of proton demand and in terms of invisible oxidation state effects.