Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

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-tetramethylguanidine, 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, U-eff = 263(28) cm(-1) in the tert-butoxide ligated version, 5-Dy-CYBu, to 817(22) cm(-1) in the hexafluoroisopropoxide-ligated complex, 5-Dy-F-6. 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-F-6, and 5-Dy-F-12 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-CYBu 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). [GRAPHICS] .