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

Abstract 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 1.8 K. 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.