Novel Heterometallic Schiff Base Complexes Featuring Unusual Tetranuclear {CoIII2FeIII2(μ-O)6} and Octanuclear {CoIII4FeIII4(μ-O)14} Cores: Direct Synthesis, Crystal Structures, and Magnetic Properties

A one-pot reactions of cobalt powder with iron­(II) chloride in dimethylformamide (DMF; 1) or dimethyl sulfoxide (DMSO; 2) solutions of polydentate salicylaldimine Schiff base ligands (H2L1, 1; H4L2, 2) based on 2-aminobenzyl alcohol (1) or tris­(hydroxymethyl)­aminomethane (2), formed in situ, yielded two novel heterometallic complexes, [CoIII2FeIII2(L1)6]·4DMF (1) and [CoIII4FeIII4(HL2)8(DMSO)2]·18DMSO (2). Crystallographic investigations revealed that the molecular structure of 1 is based on a tetranuclear core, {CoIII2FeIII2(μ-O)6}, with a chainlike metal arrangement, while the structure of 2 represents the first example of a heterometallic octanuclear core, {CoIII4FeIII4(μ-O)14}, with a quite rare manner of metal organization, formed by two pairs of {CoFe­(HL2)2} and {CoFe­(HL2)2(DMSO)} moieties, which are joined by O bridges of the Schiff base ligands. Variable-temperature (1.8–300 K) magnetic susceptibility measurements showed a decrease of the μB value at low temperature, indicative of antiferromagnetic coupling (J/hc = −32 cm–1 in 1; J/hc = −20 cm–1 in 2) between the FeIII magnetic centers in both compounds. For 2, three J constants between FeIII centers were assumed to be identical. High-frequency electron paramagnetic resonance spectra allowed one to find spin Hamiltonian parameters in the coupled-spin triplet and quintet states of 1 and estimate them in 2. The “outer” and “inner” Fe atoms in 2 appeared separately in the Mössbauer spectra.