Modulating axial anisotropy in tetrahedral Co(II) phosphonate single-ion magnets through N-donor ligand

Abstract

Mononuclear cobalt(II) phosphonate complexes [Co(HL)2(dmpz)2] (1), [Co(HL)2(imz)2] (2) and [Co(HL)2(bimz)2] (3) (where dmpz = 3,5-dimethylpyrazole; imz = imidazole and bimz = benzimidazole) have been synthesized by reacting cobalt acetate tetrahydrate and the α-hydroxyarylphosphonate [(2,6-iPr2C6H3O)P(O)(OH)(CMe2(OH)] (H2L) with chosen N-donor ligands. Complexes 1-3 have been characterized by various spectroscopic and analytical techniques. Single-crystal X-ray diffraction studies reveal that these complexes 1 and 2 exist in tetrahedral geometries with {CoN2O2} as the core coordination unit, while complex 3 shows a vacant trigonal bipyramidal geometry with one exceptionally long Co-O bond. Magnetic susceptibility measurements reveal the existence of high-spin cobalt(II) centres with zero-field splitting parameters (D) of -18.9 cm-1 (for 1), -13.7 cm-1 (for 2), and 22.3 cm-1 (for 3) with E/D values of 0.02, 0.11 and 0.25, respectively. Ab initio CASSCF/NEVPT2 calculations and X-band EPR spectroscopic analysis further confirm the observed trend, revealing a switch in the sign of the zero-field splitting parameter (D) from negative in complexes 1 and 2 to positive in complex 3. Investigation of the dynamic magnetic behaviour of these complexes shows field-induced slow relaxation of magnetization under an applied dc field with energy barriers for reversal of magnetization (Ueff) of 31.9 K (1), 32.5 K (2), and 26.2 K (3), indicating SIM behaviour. Ab initio CASSCF/NEVPT2 calculations performed on three structurally related Co(II) complexes reveal a switch in the sign of the axial zero-field splitting parameter (D), despite their comparable chemical environments. A detailed magneto-structural correlation study identifies the combined influence of conventional angular distortions from ideal tetrahedral geometry and the Co–O–P bond angle as key determinants, establishing a predictive framework for tuning magnetic anisotropy in cobalt phosph(on)ate based SIMs.