Ligand exchange upon oxidation of a dinuclear Mn complex-detection of structural changes by FT-IR spectroscopy and ESI-MS

The structural rearrangements triggered by oxidation of the dinuclear Mn complex [Mn₂(bpmp)(μ-OAc)₂]⁺ (bpmp = 2,6-bis[bis(2-pyridylmethyl)amino]methyl-4-methylphenol anion) in the presence of water have been studied by combinations of electrochemistry with IR spectroscopy and with electrospray ionization mass spectrometry (ESI-MS). The exchange of acetate bridges for water (D₂O) derived ligands in different oxidation states could be monitored by mid-IR spectroscopy in CD ₃CN-D₂O mixtures following the v_as(c-o) bands of bound acetate at 1594.4 cm^-1 (II,II), 1592.0 cm^-1 (II,III) and 1586.5 cm^-1 (III,III). Substantial loss of bound acetate occurs at much lower water content (< 0.5% v/v) in the III,III state than in the II,II and II,III states (≥10%). The ligand-exchange reactions do not initially reduce the overall charge of the complex but facilitate further oxidation by proton-coupled electron transfer as the water-derived ligands are increasingly deprotonated in higher oxidation states. In the IR spectra deprotonation could be followed by the formation of acetic acid (DOAc, ∼1725 cm^-1, v_(c-o)) from the released acetate (1573.6 cm ^-1, v_as(c-o)). By the on-line combination of an electrochemical flow cell with ESI-MS several product complexes could be identified. A di-μ-oxo bridged III,IV dimer [Mn₂(bpmp)(μ-O) ₂]²⁺ (m/z 335.8) can be generated at potentials below the III,III/II,III couple of the di-μ-acetato complex (0.61 V vs. ferrocene). The ligand-exchange reactions allow for three metal-centered oxidation steps to occur from II,II to III,IV in a potential range of only 0.5 V, explaining the formation of a spin-coupled III,IV dimer by photo-oxidation with [Ru[bpy) ₃]³⁺ in previous EPR studies.