Ferrocene/ferrocenium, cobaltocene/ cobaltocenium and nickelocene/nickelocenium: from gas phase ionization energy to one-electron reduction potential in solvated medium

We propose a theoretical procedure for accurate determination of reduction potentials for three metallocene couples, Cp2M(+)/Cp2M, where M = Fe, Co and Ni. This procedure first computes the gas phase ionization energy (IE) using the explicitly correlated CCSD(T)-F12 method and includes the zero-point energy correction, core-valence electronic correlation, and relativistic and spin-orbit coupling effects. By means of Born-Haber thermochemical cycle, the one-electron reduction potential is obtained as the sum of the gas phase IE and the corresponding Gibbs free energies of solvation (Delta G(solv)) for both the neutral and cationic species. Among the three solvent models (PCM, SMD and uESE) investigated here, it turns out that only the SMD model (computed at the DFT level) gives the best estimation of the value for ''Delta G(solv)(cation)- Delta G(solv)(neutral)'' and thus, combining with the accurate IE values, the theoretical protocol is capable of yielding reliable values (in V) for E-circle (abs) (Cp(2)Fethorn=Cp2FeTHORN 1/4 4:616 owaterTHORN and 4:644oacetonitrileTHORN, E abs oCp2Cothorn=Cp2CoTHORN 1/4 3:347owaterTHORN and 3:369oacetonitrileTHORN and E absoCp2Nithorn=Cp2NiTHORN 1/4 4:289owaterTHORN and 4:321oacetonitrileTHORN. These predictions compare favorably with the available experimental data (in V): E abs oCp2Fethorn=Cp2FeTHORN 1/4 4:682 owaterTHORN and 4:763 oacetonitrileTHORN, E abs oCp2Cothorn=Cp2CoTHORN 1/4 3:321owaterTHORN and 3:369 V, and E absoCp2Nithorn=Cp2NiTHORN 1/4 4:291 V oacetonitrileTHORN. We show that our theoretical procedure is reliable for accurate reduction potential predictions of Cp2Fe+/Cp2Fe, Cp2Co+/Cp2Co and Cp2Ni+/Cp2Ni redox couples in aqueous and non-aqueous media; the maximum absolute deviation is as small as E120 mV, which outperforms those of the existing theoretical methods. [GRAPHICS]

Automated summary

This article proposes a theoretical procedure for accurately determining the reduction potentials of three metallocene couples. By calculating the gas phase ionization energy and the solvation free energy, the one-electron reduction potential is obtained. The SMD model yields the best estimation for the solvation free energy and provides reliable values for the reduction potentials.