Synthesis, Structure and Characterisation of a New Trinuclear Di‐μ‐phenolato‐μ‐carboxylato MnIIIMnIIMnIII Complex with a Bulky Pentadentate Ligand: Chemical Access to Mononuclear MnIV–OH Entities

A new trinuclear MnIIIMnIIMnIII complex has been isolated and X‐ray characterised, namely [(py‐salpn)MnIII(μ‐OAc)MnII(μ‐OAc)MnIII(py‐salpn)]2+ (1), where H2py‐salpn is the new bulky [N3O2] ligand derived from the H2salpn Schiff base by the addition of one pyridine arm and the reduction of the imine function. The crystal structure reveals that the complex has a strictly 180° MnIII···MnII···MnIII angle, the MnII ion being located at an inversion centre. The complex is valence‐trapped, with the terminal MnIII ions showing a Jahn–Teller elongation along the pyridine–MnIII–acetate axis. The MnII···MnIII separation is 3.1224(13) Å. The EPR spectra recorded on solid and frozen solutions are consistent with an MnIIIMnIIMnIII species. The electrochemical response of complex 1 in acetonitrile solution exhibits two, one‐electron reduction waves at E1/2 = 0.140 and –0.075 V vs. SCE.Phenolato and acetato→MnIII ligand‐to‐metal charge‐transfer transitions are detected by UV/Visible spectroscopy at 359 and 587 nm, respectively. Chemical oxidation of an acetonitrile solution with tert‐butyl hydroperoxide leads to mononuclear MnIV–hydroxo species, as evidenced by UV/Visible and EPR spectroscopy as well as ESI mass spectrometry.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)