IRON(II) COMPLEXES WITH POLYDENTATE SCHIFF-BASE LIGANDS AS MODELS OF THE PHOTOSYNTHETIC MONONUCLEAR NONHEME FERROUS SITES - SYNTHESIS, CHARACTERIZATION, MOLECULAR-CRYSTAL STRUCTURE, EXAFS AND XANES STUDIES, MOSSBAUER-SPECTROSCOPY AND MAGNETIC-PROPERTIES

A series of four high-spin ferrous complexes of polydentate Schiff base ligands has been prepared and studied with IR, X-ray absorption and Mossbauer spectroscopy and variable-temperature magnetic susceptibility. The ligands include potentially tridentate and hexadentate Schiff bases with N2O and N4O2 donor sets, respectively, and result from the condensation of 5-nitrosalicylaldehyde with 2-(aminoethy)pyridine or tetramines. The results obtained provide evidence that the four complexes described herein are mononuclear high-spin iron(II) species at room temperature. The crystal and molecular structure of [Fe(5NO2-salaep)2] (1) has been determined. 1 crystallizes in the orthorhombic system, space group Pbcn with Z = 4 and a = 14.756(3), b = 9.682(3), c = 18.632(4) angstrom. The structure was solved by the heavy-atom method and refined to conventional agreement indices R = 0.042 and R(w) = 0.043 with 2845 unique reflections for which I > 3-sigma. The structure of 1 consists of [Fe(5NO2-salaep)2] complex molecules stacked through pi interactions involving the salicylaldimine rings of adjacent molecules to afford ribbons along the [001] direction. The central iron atom of each molecule is triply coordinated to two 5NO-2-salaep ligands, affording a distorted coordination octahedron. The Mossbauer spectroscopy is consistent with an increase of the distortion of the iron(II) ligand environment in the series from [Fe11(5NO2-salaep)2] (1) to Fe11[5NO2-sal-N(1,5,9,13)] (4). The variable temperature magnetic susceptibility evidences appreciable zero-field splitting of the iron(II) ground state in [Fe11(5NO2-salaep)2] (1) and Fe11[5NO2-sal-N(1,5,8,12)] (3). 1 exhibits several properties required to afford a modelling of the iron center of the 'ferroquinone complex' of photosystem 2. Fe11[5NO2-sal-N(1,4,7,10)] (2) exhibits a thermally induced 5T2g <--> 1A1g spin conversion with unprecedented features: iron(II) center in a N4O2 ligand environment, spin conversion of discontinuous nature occurring in two steps separated by a 30 K broad spin equilibrium domain in which c. 50% of high-spin and low-spin molecules coexist.