Effect of variation of substitution groups attached on benzoxazole ligands on relaxation dynamics of a family of nona-coordinate mononuclear dysprosium zero-field single-molecule magnets

Taking advantage of modified substitution group strategy, the magneto-structural correlation of four nonacoordinate mononuclear dysprosium zero-field single-molecule magnets derived from benzoxazole ligands is established. Mixing of the corresponding benzoxazole ligands and dysprosium trifluoromethanesulfonate in the presence of methanol solvent yielded four nona-coordinate mononuclear dysprosium complexes 1-4 with the core molecular formula Dy(Lx-R)3 (R = F, 1, x = 1; CN, 2, x = 2; CF3, 3, x = 3 and C(CH3)3, 4, x = 4). The coordination geometries of the dysprosium metal site with N6O3 donor atoms in 1-4 are all belong to spherical capped square antiprism (C4v) and the detailed corresponding parameters are 1.509, 1.173, 1.412 and 1.377, respectively. The static and dynamic magnetic susceptibilities measurements were performed on 1-4. They all display frequencies-dependent ac out-of-phase component signal peaks at different temperatures and zero external field, characteristic of single-molecule magnets behaviour. Variable-field magnetization measurements performed at 2 K confirm the magnetization blocking for 1-4 and reveal the presence of butterfly shape magnetic hysteresis. The experimental resulted effective energy barriers for magnetization reversal for 1-4 are 48.6, 102.2, 113.5, and 120.6 K, respectively. To understand the origin of variation observed, we have performed ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations on complexes 1-4. The calculations reveal axial g-tensors within the ground KD state, aligning with experimental results, and contributing to the observed trend in barrier heights. The computed crystal field parameters show an increasing trend from 1 to 4, indicating strengthening axial bonds, while non-axial CF parameters decrease. Steric hindrance from bulky ligands correlates with weakened equatorial bond strength, further contributing to higher barrier heights. Additionally, the LoProp charge on axial atoms strengthens the bonds in the order 1 -* 2 -* 3 -* 4, reaffirming the importance of strengthen axial bond and weakening equatorial bonds towards better performant SIMs. Our work demonstrates that ligand substituent group strategy applied on benzoxazoles can utilize to tune the magnetic anisotropy of nona-coordinate mononuclear dysprosium complexes.