Oxidation-Potential Tuning of Tungsten-Alkylidyne Complexes over a 2 V Range

The electrochemistry and electronic structures of over 30 tungsten-alkylidyne compounds of the form W(CR)LnL'X4-n (R = H, Bu-t, Ph, p-C6H4CCH, p-C6H4CCSiPr3i; X = F, Cl, Br, I, OTf, Bu-n, CN, OSiMe3, OPh; L/L' = PMe3, 1/2 dmpe, 1/2 depe, 1/2 dppe, 1/2 tmeda, P(OMe)(3), CO, CNBut, py), in which the alkylidyne R group and L and X ligands are systematically varied, have been investigated using cyclic voltammetry and density functional theory calculations in order to determine the extent to which the oxidation potential may be tuned and its dependence on the nature of the metal-ligand interactions. The first oxidation potentials are found to span a range of similar to 2 V. Symmetry considerations and the electronic-structure calculations indicate that the highest occupied molecular orbital (and redox orbital) is of principal d(xy) orbital parentage for most of the compounds in this series. The dependence of the oxidation potential on ligand is a strong function of the symmetry relationship between the substituent and the d(xy) orbital, being much more sensitive to the nature of the equatorial L ligands (pi symmetry, with respect to d(xy), Delta E-1/2 congruent to 0.5 V/L) than to the axial CR and X ligands (nonbonding with respect to d(xy), Delta E-1/2 < 0.3 V/L). The oxidation potential is linearly correlated with the calculated d(xy) orbital energy (slope congruent to 1, R-2 = 0.97), which thus provides a convenient computational descriptor for the potential. The strength of the correlation and slope of unity are proposed to be manifestations of the small inner-sphere reorganization energy associated with one-electron oxidation.