Ring-Slippage and Multielectron Redox Properties of Fe/Ru/Os-Bis(arene) Complexes: Does Hapticity Change Really Cause Potential Inversion?

Abstract

Bis(hexamethylbenzene) complexes of the group 8 metals (Fe, Ru, Os) show surprising diversity in their electron-transfer mechanisms and associated thermodynamics for the M(II) -> M(I) -> M(0) redox series. In electrochemical experiments, the Fe complex exhibits normally ordered potentials separated by similar to 1 V, the Ru system shows nearly overlapping one-electron redox events, and Os demonstrates a one-step, two-electron transfer with a peak potential separation suggestive of highly inverted potentials. It has been conjectured that the sequential one-electron transfers observed for Fe are due to the lack of an accessible eta(4):eta(6) Fe(0) state, destabilizing the fully reduced species. Using an established model chemistry based on DFT, we demonstrate that the hapticity change is a consequence of the bonding throughout this transition metal triad and that apparent multielectron behavior is controlled by the vertical electron attachment component of the M(II) -> M(I) redox event. Furthermore, the eta(6):eta(6) Fe(0) triplet state is more favorable than the hypothetical eta(4):eta(6) singlet state, emphasizing that the hapticity change is not sufficient for multielectron behavior. Despite both displaying two-electron redox responses, Ru and Os traverse fundamentally different mechanisms based on whether the first (Os) or second (Ru) electron transfer induces the hapticity change. While the electronic structure analysis is limited to the Fe triad here, the conceptual model that we developed provides a general understanding of the redox behavior exhibited by d(6) bis(arene) compounds.