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Ferromagnetic Coupling between Low- and High-Spin Iron(III) Ions in the Tetranuclear Complex fac-{[FeIII{HB(pz)3}(CN)2(μ-CN)]3FeIII(H2O)3}· 6H2O ([HB(pz)3]- = Hydrotris(1-pyrazolyl)borate)

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journal contribution
posted on 19.10.2002, 00:00 by Rodrigue Lescouëzec, Jacqueline Vaissermann, Francesc Lloret, Miguel Julve, Michel Verdaguer
The novel mononuclear PPh4-fac-[FeIII{HB(pz)3}(CN)3]·H2O (1) [PPh4+= tetraphenylphosphonium cation; (HB(pz)3)- = hydrotris(1-pyrazolyl)borate] and tetranuclear fac-{[FeIII{HB(pz)3}(CN)2(μ-CN)]3FeIII(H2O)3}·6H2O (2) have been prepared and characterized by X-ray diffraction analysis. Crystal data for compound 1:  monoclinic, space group P21/c, a = 9.575(3) Å, b = 21.984(4) Å, c = 16.863(3) Å, β = 100.34(2)°, U = 3486(1) Å3, Z = 4. Crystal data for compound 2:  orthorhombic, space group Pnam, a = 14.084(3) Å, b = 14.799(4) Å, c = 25.725(5) Å, U = 5362(2) Å3, Z = 4. Compound 1 is a low-spin iron(III) compound with three cyanide ligands in fac arrangement and a tridentate pyrazolylborate ligand building a distorted octahedral environment around the iron atom. Compound 2 is the first example of a molecular species containing three peripheral low-spin iron(III) ions linked to a central high-spin iron(III) cation by single cyanide bridges, the anion of 1 acting as a monodentate ligand in 2. Variable-temperature magnetic susceptibility measurements of 2 reveal the occurrence of a significant ferromagnetic coupling between the three peripheral low-spin iron(III) centers and the central high-spin iron(III) ion cations leading to a low-lying nonet spin state.