Can electrochemical measurements be used to predict X-ray photoelectron spectroscopic data? the case of Ferrocenyl-β-Diketonato complexes of Manganese(III)

In order to better understand intramolecular communication between molecular fragments, a series of ferrocene-functionalized β-diketonato manganese(III) complexes, [Mn(FcCOCHCOR)₃] with R = CF₃, 1, CH₃, 2, Ph = C₆H₅, 3, and Fc = Fe^II(η⁵-C₅H₄)(η⁵-C₅H₅), 4, the mixed ligand β-diketonato complex [Mn(FcCOCHCOFc)₂(FcCOCHCOCH₃)], 5, as well as the acac complex [Mn(CH₃COCHCOCH₃)₃], 6, were subjected to an electrochemical and X-ray photoelectron spectroscopy (XPS) study. The ferrocenyl (Fe^II) and Mn^III redox potentials, E°′, and photoelectron lines were sufficiently resolved in each complex to demonstrate a linear correlation between E°′ and group electronegativities of ligand R groups, _R, or ς_R, as well as with binding energies of both the Fe 2p_3/2 and Mn 2p_3/2 photoelectron lines. These relationships are consistent with effective communication between molecular fragments of 1-5. From these relationships, prediction of Mn and Fe core electron binding energies in [Mn(R¹COCHCOR²)₃] complexes from known manganese and/or ferrocenyl redox potentials are, therefore, now possible. Ligand infrared carbonyl stretching frequencies were successfully related to binding energy as a measure of the energy required for inner-sphere reorganization. In particular it became possible to explain why, upon electrochemical oxidation or photoionization, the ferrocenyl Fe^II inner-shell of 1-5 needs more energy in complexes with ligands bearing electron-withdrawing (CF₃) groups than in ligands bearing electron-donating groups such as ferrocenyl. The XPS determined entity I_ratio (the ratio between the intensities of the satellite and main metal 2p_3/2 photoelectron lines) is an indication not only of the amount of charge transferred, but also of the degree of inner-sphere reorganization. Just as for binding energy, the quantity I_ratio was also found to be related to the energy requirements for the inner-sphere reorganization depicted by the vibrational frequency, v_co.