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 (MnMnMnIII)-Mn-III-Mn-II complex has been isolated and X-ray characterised, namely [(py-salpn)Mn-III(mu-OAc)-Mn-II( mu-OAc)Mn-III(py-salpn)](2+) (1), where H2PY-salpn is the new bulky [N3O2] ligand derived from the H(2)salpn 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 degrees Mn-III... Mn-II... Mn-III, angle, the Mn-II ion being located at an inversion centre. The complex is valence-trapped, with the terminal Mn-III ions showing a JahnTeller elongation along the pyridine-Mn-III-acetate axis. The Mn-II---Mn-III separation is 3.1224(13) angstrom. The EPR spectra recorded on solid and frozen solutions are consistent with an (MnMnMnIII)-Mn-III-Mn-II. species. The electrochemical response of complex 1 in acetonitrile solution exhibits two, one-electron reduction waves at E-1/2 = 0.140 and -0.075 V vs. SCE. Phenolato and acetato -> Mn-III 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 Mn-IV-hydroxo species, as evidenced by UV/Visible and EPR spectroscopy as well as ESI mass spectrometry. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).