Coligand-driven modulation of magnetic properties in trigonal prismatic cobalt(ii) single-ion magnets

Abstract

Identifying the influencing factors and, consequently, manipulating the magnetic anisotropy are crucial for enhancing the performance of single-molecule magnets (SMMs). Using the rigid tetradentate macrocyclic host ligand 1,4,7,10-tetrabenzyl-1,4,7,10-tetraazacyclododecane (L), a series of mononuclear six-coordinate Co(II) complexes with trigonal prismatic geometry, including various bidentate coligands NO₃⁻, CH₃COO⁻, dhbq²⁻ (H₂dhbq = 2,5-dihydroxy-1,4-benzoquinone), and AHA⁻ (HAHA = acetohydroxamic acid), were synthesized. The target complexes [CoL(NO₃)](NO₃) (1), [CoL(CH₃COO)](PF₆) (2), [CoL(dhbq)]·2CH₃OH·H₂O (3) and [CoL(AHA)](BPh₄) (4) were characterized by X-ray diffraction, magnetic characterization, and first-principles calculation. Changing the coligands triggers significant alterations in the trigonal prismatic geometries, which allows systematic evaluation of the impacts of structural distortions. Direct current magnetic data revealed the large and negative axial zero-field splitting parameter, D, in all four complexes. Furthermore, theoretical calculations indicated that the trend in the absolute values of the D parameter follows the order of 1 > 2 > 3 > 4. Dynamic magnetic susceptibilities indicated that complexes 1–3 act as zero-field SMMs, while complex 4 shows a slow relaxation of magnetization under the external direct current field. These results indicated that the O–Co–O bite angle is the primary factor affecting magnetic anisotropy, while longer Co–O bond lengths lead to a weaker ligand field, thereby facilitating spin–orbital coupling to enhance the magnetic anisotropy.