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 [Mn-2(bpmp)(mul-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 CD3CN-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, similar to 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-mu-oxo bridged III, IV dimer [Mn-2(bpmp)(mu-O)(2)](2+) (m/z 335.8) can be generated at potentials below the III, III/II, III couple of the di-mu-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.