Importance of an axial Ln(III)-F bond across the lanthanide series and single-molecule magnet behavior in the Ce and Nd analogues

The recently reported compound [Dy(III)LF](CF₃SO₃)2·H₂O (L = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane) displays a strong axial magnetic anisotropy, due to the short axial Dy-F bond, and single-mol. magnet (SMM) behavior. Following our earlier [Dy(III)LF]²⁺ work, herein we report the systematic structural and magnetic study of a family of [Ln(III)LF](CF₃SO₃)2·H₂O compounds (Ln(III) = 1-Ce, 2-Pr, 3-Nd, 4-Eu, 5-Tb, 6-Ho, 7-Er, 8-Tm, and 9-Yb). From this series, the Ce(III) and Nd(III) analogs show slow relaxation of the magnetization under an applied d.c. magnetic field, which is modeled using a Raman process. Complete active space SCF theor. calculations are employed to understand the relaxation pathways in 1-Ce and 3-Nd and also reveal a large tunnel splitting for 5-Tb. Addnl. computational studies on model compounds where we remove the axial F- ligand, or replace F- with I-, highlight the importance of the F- ligand in creating a strong axial crystal field for 1-Ce and 3-Nd and for promoting the SMM behavior. Importantly, this systematic study provides insight into the magnetic properties of these lighter lanthanide ions.