Trapping of a pseudotetrahedral CoIIO4 core in mixed-valence mixed-geometry [Co5] coordination aggregates: synthetic marvel, structures, and magnetism

A systematic one-step one-pot multicomponent reaction of Co(ClO4)2·6H2O, H3L (2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol), and readily available carboxylate salts (RCO2Na; R = CH3, C2H5) resulted in the two structurally novel coordination aggregates [CoIICoIII4L2(μ1,3-O2CCH3)2(μ-OH)2](ClO4)4·4H2O (1) and [CoIICoIII4L2(μ1,3-O2CC2H5)2(μ-OH)(μ-OMe)](ClO4)4·5H2O (2). At room temperature, reactions of H3L in MeOH with cobalt(II) perchlorate salts led to coassembly of initially formed ligand-bound {CoII2} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L(μ1,3-O2CCH3)(μ-OH/OMe)Co2}+ fragments were utilized to trap a central CoII ion during the formation of [Co5] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo-Td CoII ion present. The structural distortion leads to an easy-axis magnetic anisotropy (D = −31.31 cm–1 for 1 and −21.88 cm–1 for 2) and a small but non-negligible transverse component (E/D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O–Co–O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co2(μ-L)} hosts controls the distortion of the central {CoO4} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.