Density functional theory study of redox pairs. 1. Dinuclear iron complexes that undergo multielectron redox reactions accompanied by a reversible structural change

The single-two-electron reduction for the Fe-Fe bonded dinuclear complexes Fe-2(CO)(6)(mu(2)-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 + A(2-) 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 salvation 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.