Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands
Zhenhua Zhu, Yi‐Quan Zhang, Xiaolei Li, Mei Guo, Jingjing Lu, Shuting Liu, Richard A. Layfield, Jinkui Tang
Abstract
Eight square-pyramidal dysprosium complexes of the type [Dy(X)(DBP)2{TMG(H)}2] were synthesized, where TMG(H) denotes 1,1,3,3-tetramethylgua-nidine, X denotes an alkoxide ligand or anionic guanidinate in the apical position, and DBP denotes 2,6-di-tert-butylphenoxide. These complexes, expressed as 5-Dy-X, are single-molecule magnets (SMMs) with properties dependent on the nature of the apical X ligand. The series shows remarkable variation in the effective energy barrier to reversal of the magnetization from, for example, Ueff = 263(28) cm−1 in the tert-butoxide ligated version, 5-Dy-OtBu, to 817(22) cm−1 in the hexafluoroisopropoxide-ligated complex, 5-Dy-F6. Analysis of the electronic structure using ab initio calculations reveals that the apical X ligand plays a key role in tuning the energy barrier by allowing modifications to the dominant crystal-field interaction. The strong magnetic axiality in 5-Dy-TFE (TFE = 2,2,2-trifluoroethoxide), 5-Dy-F6, and 5-Dy-F12 results in thermal relaxation via the third-excited Kramers doublet (KD), whereas stronger interactions of the apical TMG ligand in 5-Dy-TMG and bending of the ODBP-Dy-ODBP connectivity in 5-Dy-OtBu lead to relaxation only via the first-excited KD. By deconstructing the complex 5-Dy-TFE in silico, we identify the cationic two-coordinate complex [Dy(DBP)2]+, which is predicted to have an extremely high barrier of 2286 cm−1