A non-linear (2,2,2) alkoxo-bridging in a FeIII4O6 core potentially relevant to iron-tunicates

The tridentate unsymmetrical ligand N-(2-hydroxymethylphenyl)salicylideneimine H2L, derived from salicylaldehyde and 2-aminobenzylalcohol, with [ONO] donor atoms yields [L2FeIII2Cl2] (1) and [L6FeIII4] (2) complexes containing alkoxide bridges, which have been structurally characterized by X-ray diffraction. In complex 1, each ferric ion is five-coordinated with a distorted square-pyramidal geometry, the basal planes of which are symmetrically bridged by two alkoxide oxygen atoms. Analysis of the susceptibility data reveals antiferromagnetic interactions with an exchange parameter J = −15.8 cm−1 between the high-spin d5 ferric centers. The structure of 2 can be considered as “linear (2,2,2)” to specify the number of enolate oxygen atoms between four iron atoms. Variable-temperature magnetic susceptibility data are fitted to a “three-J” model, yielding pairwise antiferromagnetic exchange interactions, J12 = J34 = −13.4 cm−1, J13 = J24 = −7.1 cm−1, J23 = −14.9 cm−1, between the neighboring ferric centers; J14 is assumed to be negligible. Complex 2 has a complicated low-lying magnetic structure with a non-diamagnetic ground state. In addition, the Fe–O–Fe angles at the bridging ligands seem to be determinant for the strength of the antiferromagnetic interactions.