Experimental and theoretical insights into magnetic exchange and anisotropy in multinuclear cobalt complexes displaying slow relaxation

Abstract

We report a combined experimental and theoretical magneto-structural study of three multinuclear cobalt complexes: [Co^II₇(L)₆](NO₃)₂ (1), [Co^II₄(L)₄(H₂O)₄]·1.5H₂O (2) and [Co^III₂Co^II₂(L)₄(NCS)₂(H₂O/MeOH)₂] (3), synthesized using the Schiff base 2-[(2-hydroxymethylphenyl)iminomethyl]-6-methoxy-4-methylphenol (H₂L) in the presence of different counter anions. X-ray crystallography reveals diverse architectures, including a disc-shaped heptanuclear cluster (1), a cubane-type tetranuclear core (2), and a mixed-valence defective dicubane motif (3). In addition to the topological variations induced by counterions and the diverse chelating and bridging behavior of the Schiff base, significant differences in coordination geometry are observed around the cobalt centers. The cobalt ions in complexes 2 and 3, along with the central metal ion in complex 1, adopt distorted octahedral geometry. In contrast, the peripheral metal centers in complex 1 exhibit significantly distorted trigonal prismatic geometries. Magnetic measurements, ab initio calculations, and broken-symmetry DFT analyses reveal competing ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions in complexes 1 and 2, resulting in an overall FM response. In complex 3, the magnetic interaction is exclusively AFM. These exchange interactions correlate with the phenoxo and alkoxo bridging angles, where larger angles promote AFM interactions, and smaller angles favor FM coupling. The Co^II ions in complex 3 exhibit moderate easy-plane anisotropy, whereas the Co^II centers in complex 1 show a predominant axial anisotropy. A mixed anisotropic behavior is observed in complex 2, in which the orthogonal arrangement of dominant single-ion easy axes results in a net negative axial anisotropy. Furthermore, all three complexes exhibit slow relaxation of magnetization in zero external magnetic field, primarily in the high-frequency region, with complex 2 showing the most well-resolved temperature-dependent AC signals. Overall, this study emphasizes the role of counter anions in tuning the structural and geometrical features, magnetic exchange pathways, and anisotropy in multinuclear cobalt complexes exhibiting slow magnetic relaxation.