Mononuclear Co(II) phosphoric triamide complexes: synthesis, crystal structures and field-induced single-ion magnet behavior.

Precise tuning of magnetic anisotropy in tetrahedral Co(II) complexes is challenging as structural distortions often influence the ligand field simultaneously. In this report, we introduce a series of mononuclear cobalt(II) complexes of the general formula [CoX{OP(NHiPr)3}3]X (X = Cl (1), Br (2), I (3)) and the anion exchanged analogue of 1 with a BF4- anion [CoCl{OP(NHiPr)3}3]BF4 (1') that enables systematic tuning of anisotropy without altering the tetrahedral coordination framework. Single-crystal X-ray diffraction studies reveal a distorted tetrahedral coordination geometry around the Co(II) centers, with subtle variations in bond angles and Co-X distances across the halide series. Static magnetic measurements reveal large axial zero-field splitting (ZFS) parameters of D = -9.4 cm-1 for 1, -6.1 cm-1 for 2, 10.9 cm-1 for 3 and -9.5 cm-1 for 1', respectively. The corresponding E/D values of 0.14, 0.22, 0.25 and 0.05, respectively, are indicative of strong magnetic anisotropy. Ab initio CASSCF/NEVPT2 calculations reveal that the change observed in the sign of D does not arise from the halide identity alone, but from a cooperative interplay between equatorial torsional distortion and metal-halide bond elongation that reorganizes low-lying excited states and switches the anisotropy from easy-axis to easy-plane. Dynamic magnetic measurements reveal that under optimal applied external fields of 450 to 1000 Oe, all complexes exhibit slow relaxation of magnetisation with energy barriers (Ueff) of 14.7 K (1), 31.5 K (2), 35.1 K (3) and 24.8 K (1'). These results demonstrate that subtle changes in the ligand environment and counterions modulate the dynamic magnetic behavior, thus highlighting the potential of mono-halide, phosphoric triamide based Co(II) systems as field-induced single-ion magnets (SIMs).