Optimal N–Co–N bite angle for enhancing the magnetic anisotropy of zero-field Co(II) single-ion magnets in tetrahedral [N4] coordination environment

Highlights: • A tetrahedral Co(II) complex with CoN₄ coordination environment was successfully synthesized. • The complex displays zero-field slow magnetic relaxation and a hysteresis loop at 1.8 K. • Magnetic and computational studies reveal that the N-Co-N bite angle seems to govern the overall magnetic anisotropy. • The D value of tetrahedral Co(II) complexes can be maximized by reducing the N-Co-N bite angle to a certain extent. By modulating the substitution on the oxamide ligand, a mononuclear tetrahedral Co(II) complex (HNEt₃)₂[CoL₂] (1) (H₂L = N,N′-bis(methanesulfonyl)oxamide) with subtle variation in the CoN₄ coordination environment was successfully synthesized, which displays zero-field slow magnetic relaxation and a hysteresis loop at 1.8K. The complex adopts distorted tetrahedral coordination geometries with acute N–Co–N bite angle imposed by two mutually perpendicular oxamide ligands. Magnetic studies and computational results reveal that the acute N–Co–N bite angle seems to govern the overall magnetic anisotropy of tetrahedral Co(II) complexes with two bidentate chelated ligands containing N₄-donor set. Upon reducing the bite angle, the D parameter increases gradually and reaches the maximum at about 81°, and then show a significant decrease as the angle continuing to decline. This research demonstrates that the easy-axial magnetic anisotropy can be maximized by reducing the bite angle to a certain extent, which is also feasible for tetrahedral cases with other donor sets.