The reaction of Co(CHCOO)·4HO with the Schiff base ligand LH derived from o-vanillin and tris(hydroxymethyl)aminomethane produces the dinuclear mixed-valence complex [CoCo(LH)(CHCOO)(HO)](HO) (1), which has been investigated using IR spectroscopy, X-ray crystallography, temperature-dependent magnetic susceptibility, magnetization, HFEPR spectroscopy, and ac susceptibility measurements at various frequencies, temperatures, and external magnetic fields. The structure of 1 consists of neutral molecules in which two cobalt ions with distorted octahedral geometries, CoO and CoNO, are bridged by two deprotonated -CHO groups of the two LH ligands. 1 completes a series with Cl, Br, NO, and NCS anions published before by different authors. Low-temperature HFEPR measurements reveal that the ground electronic state of the Co(II) center in 1 is a highly anisotropic Kramers doublet; the effective g values of 7.18, 2.97, and 1.96 are frequency-independent over the frequency ranges 200-630, 200-406, and 200-300 GHz for the highest, intermediate, and lowest g values, respectively. The two lower values were not seen at higher frequencies because the magnetic field was not high enough. Temperature-dependent magnetic susceptibility and field-dependent magnetization data confirm high magnetic anisotropy of the easy axis type. Complex 1 behaves as a single-ion magnet under a small applied external field and demonstrates two relaxation modes that strongly depend on the applied static dc field. The observation of multiple relaxation pathways clearly distinguishes 1 from the Cl and Br analogues.
The "direct synthesis" approach, namely one-pot reaction of metal powders and ammonium salt with a methanol solution of a polydentate Schiff base (HL) formed in situ from salicylaldehyde and ethanolamine, has been successfully used for the preparation of the new heterometallic compounds [CuMn(L)(CHOH)]I (1), [CuMn(L)(CHOH)(HO)]NCS·HO (2), [CuMn(L)(CHOH)(HO)]Br·0.45HO (3) and [CuMn(L)(HO)]BF·0.6HO (4). Crystallographic analysis revealed that 1-4 are based on the tetranuclear core {CuMn(μ-O)} where the metal centres are joined by the oxygen bridges of Schiff base ligands forming a cube-like arrangement. The novel heterometallic compound [CuMn(L)(CHOH)][Mn(NCS)]·2CHOH (5) has been obtained by the "building block" approach using the reaction of [Cu(HL)] with manganese acetate and NHNCS in methanol. The crystal structure of 5 revealed the {CuMn(μ-O)(μ-O)} metal core which can be viewed as a double open cube. In spite of a similar {CuMnO} atom set in the cores of 1-5, the complexes show rather different molecular structures and significantly differ by the number and combinations of coordinated CHOH/HO solvent molecules. Variable-temperature (2-300 K) magnetic susceptibility along with variable-field magnetization measurements of 1-5 showed a decrease of the effective magnetic moment value at low temperature, indicative of the antiferromagnetic coupling of medium size (-55 to -22 cm). For these systems resembling a compressed prism the coupling constant in walls J correlates with the averaged bonding angles in walls α: Jvs. α develops approximately according to a straight line.
The novel hexanuclear complexes [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4(tBuDea)4Cl] (2) were prepared through one-pot self-assembly reactions of copper powder and iron(ii) or manganese(ii) chloride with N-tert-butyldiethanolamine (H2tBuDea) and pivalic acid (HPiv) in acetonitrile. Crystallographic studies revealed the uncommon molecular core type M6(μ-X)7(μ3-X)2 in 1 and 2, which can be viewed as a combination of two trimetallic M3(μ-X)2(μ3-X) fragments joined by three bridging atoms. The analysis and classification of the hexanuclear complexes having a M3(μ-X)2(μ3-X) moiety as a core forming fragment using data from the Cambridge Structural Database (CSD) were performed. Variable-temperature (1.8-300 K) magnetic susceptibility measurements of 1 showed a decrease of the effective magnetic moment value at low temperature, indicative of antiferromagnetic coupling between the magnetic centres (JFe-Cu/hc = -6.9 cm-1, JCu-Cu/hc = -4.1 cm-1, JFe-Fe/hc = -24.2 cm-1). Complex 1 acts as a catalyst in the reaction of mild oxidation of cyclohexane with H2O2, showing the yields of products, cyclohexanol and cyclohexanone, up to 17% using pyrazinecarboxylic acid as a promoter. In the oxidation of cis-1,2-dimethylcyclohexane with m-chloroperoxybenzoic acid (m-CPBA), 70% of retention of stereoconfiguration was observed for tertiary alcohols. Compound 1 also catalyses the amidation of cyclohexane with benzamide. In all three catalytic reactions the by-products were investigated in detail and discussed.
Abstract:The magnetostructural correlations in Fe(III) complexes, originally outlined by Gorun and Lippard has been revised. Other correlation variants have been tested and the dataset enlarged for more recent entries possessing the Fe-O-Fe bridge including dinuclear, tetranuclear and hexanuclear Fe(III) complexes. The resulting relationships confirm that instead of the original suggestions, the correlation could stay as a linear relationship which covers the possibilities of positive values of exchange coupling constants.
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