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Spectroscopic and Structural Characterization of the [Fe(imidazole)6]2+ Cation

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journal contribution
posted on 14.08.2003, 00:00 by Graham Carver, Philip L. W. Tregenna-Piggott, Anne-Laure Barra, Antonia Neels, John A. Stride
Spectroscopic, structural, and magnetic data are presented for Fe(C3H4N2)6(NO3)2, which facilitate a precise definition of the electronic and molecular structure of the [Fe(Im)6]2+ cation. The structure was determined at 120(1) K by X-ray diffraction methods. The salt crystallizes in the trigonal space group R3̄ with unit-cell parameters a = 12.4380(14) Å, c = 14.5511(18) Å, and Z = 3. All the imidazole ligands are equivalent with an Fe−N bond distance of 2.204(1) Å. Variable-temperature inelastic neutron scattering (INS) measurements identify a cold magnetic transition at 19.4(2) cm-1 and a hot transition at 75.7(6) cm-1. The data are interpreted using a ligand field Hamiltonian acting in the weak-field 5D basis, from which the diagonal trigonal field splitting of the 5T2g (Oh) term is estimated as ∼485 cm-1, with the 5Ag (S6) component lower lying. High-field multifrequency (HFMF) EPR data and measurements of the magnetic susceptibility are also reported and can be satisfactorily modeled using the energies and wave functions derived from analysis of the INS data. The electronic and molecular structures are related through angular overlap model calculations, treating the imidazole ligand as a weak π-donor.