Ligand Tuning Effects upon the Multielectron Reduction and Single-Electron Oxidation of (Bi)pyridyl Complexes of cis- and trans-Dioxorhenium(V): Redox Thermodynamics, Preliminary Electrochemical Kinetics, and Charge-Transfer Absorption Spectroscopy

The effects of ligand substituents upon the electrochemistry and charge-transfer absorption spectroscopy of complexes of the type rreni-(O)₂Re^V(py-X)₄⁺ (py-X is a substituted pyridine) and cis-(O)₂Re^V(bpy-Y₂)(py-X)₂⁺ (bpy-Y₂ is a doubly substituted 2,2′-bipyridine ligand) have been examined. Both series of complexes undergo a simple one-electron oxidation, but the Re(VI/V) potentials (E_f) are roughly 600 mV higher for the trans series compared with the cis. For both series, (bi)pyridyl substituents exert large effects: E_f(VI/V) increases by as much as several hundred millivolts upon replacement of electron-withdrawing substituents by electron-donating groups. Reduction of Re(V) is more complex. For the cis series, and a portion of the trans, it occurs by a two-electron process followed by a one-electron step. For some of the trans species, however, a three-electron reduction (to Re(II)) is seen. Furthermore the reductive reactions are pH dependent, indicating the uptake of protons (and oxo to hydroxo or aqua ligand conversion) upon Re(III) or Re(II) formation. A surprising finding in view of the Re(VI/V) results is that E_f(V/IIĬ) is essentially independent of ligand composition for both the trans and cis series. A careful consideration of substituent effects for cis-(OH)₂Re^III,(bpy-Y₂)(py-X)₂⁺ reduction, which displays both pH-dependent (low and intermediate pH’s) and pH-independent (high pH) behavior, suggests an explanation: electron-donating substituents evidently function simultaneously to decrease the affinity of the lower oxidation state for electrons (thereby making E_f more negative) while increasing the affinity for protons (thereby making E_f more positive), resulting in only a small net substituent effect. The electron/proton compensation effect appears also to be operative in the electrochemical kinetics of reduction of trans-(O)₂Re(py-X)₄⁺. Preliminary experiments show that, at E_f, the two-electron reduction is controlled by the rate of the Re(IV) to Re(III) step and that this step is preceded by a single protonation step. Qualitative rate comparisons, based on cyclic voltammetry peak separation measurements, reveal significant ligand substituent effects. There is no systematic dependence, however, of rate upon ligand electron-withdrawing or -donating character. The lack of correlation is interpreted in terms of compensating “electron demand” and “proton demand” effects in the overall two-electron, two-proton kinetic process. Studies of electronic absorption reveal complex correlations between metal-to-ligand charge transfer (MLCT) energies and E_f(VI/V) for both the cis and the trans series. For the cis series, the charge-transfer absorptions are assigned as Re(V)-to-pyridine (higher energy) and Re(V)-to-bipyridine (lower energy) on the basis of resonance Raman enhancement effects. The effects of solvent on MLCT energies and Re(VI/V) potentials are described in the Appendix. An empirical correlation between both quantities and the so-called solvent acceptor number is found. The correlations are tentatively interpreted in terms of the metal oxidation-state dependence of specific interactions between comparatively electron-rich oxo ligands and electron-deficient solvent functionalities.