Trapping of a Pseudotetrahedral Co^IIO₄ Core in Mixed-Valence Mixed-Geometry [Co₅] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism

Abstract

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 [Co^IICo^III₄L₂(μ_1,3-O₂CCH₃)₂(μ-OH)₂](ClO₄)₄·4H₂O (1) and [Co^IICo^III₄L₂(μ_1,3-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 {Co^II₂} 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(μ_1,3-O₂CCH₃)(μ-OH/OMe)Co₂}⁺ fragments were utilized to trap a central Co^II 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- T _d Co^II 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.