Density Functional Theory Study of Redox Pairs. 1. Dinuclear Iron Complexes That Undergo Multielectron Redox Reactions Accompanied by a Reversible Structural Change

Abstract

The single two-electron reduction for the Fe−Fe bonded dinuclear complexes Fe2(CO)6(μ2-PR2)2 (R = CH3, 1-CH3; R = CF3, 1-CF3) is studied by electronic structure calculations based on density functional theory (DFT) methods. Several theoretical models are evaluated, including gas-phase models and models that include solvation (COSMO model) and/or countercations. The experimentally observed cleavage of the Fe−Fe bond upon addition of electrons is reproduced in all calculations. The different theoretical models are evaluated by calculating the energy of the disproportionation reaction 2A- → A + A2- using the energies of the complexes [1-R]0, [1-R]-, and [1-R]2-. As expected, gas-phase calculations poorly model the experimental redox behavior, and the inclusion of solvation or countercations is necessary to correctly predict that the disproportionation reaction is energetically downhill. The distribution of the added electrons over the molecules and the charge distribution as a function of alkali metal countercation (Li+, Na+, K+) are evaluated using the Hirshfeld charge analysis scheme. A qualitative correlation is found between the HOMO/LUMO energies of the redox species and the calculated redox potentials.