Importance of an Axial Ln^III–F Bond across the Lanthanide Series and Single-Molecule Magnet Behavior in the Ce and Nd Analogues

Abstract

The recently reported compound [Dy^IIILF](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-molecule magnet (SMM) behavior. Following our earlier [Dy^IIILF]²⁺ work, herein we report the systematic structural and magnetic study of a family of [Ln^IIILF](CF₃SO₃)₂·H₂O compounds (Ln(III) = <b>1</b>-Ce, <b>2</b>-Pr, <b>3</b>-Nd, <b>4</b>-Eu, <b>5</b>-Tb, <b>6</b>-Ho, <b>7</b>-Er, <b>8</b>-Tm, and <b>9</b>-Yb). From this series, the Ce(III) and Nd(III) analogues show slow relaxation of the magnetization under an applied direct current magnetic field, which is modeled using a Raman process. Complete active space self-consistent field theoretical calculations are employed to understand the relaxation pathways in <b>1</b>-Ce and <b>3</b>-Nd and also reveal a large tunnel splitting for <b>5</b>-Tb. Additional 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 <b>1</b>-Ce and <b>3</b>-Nd and for promoting the SMM behavior. Importantly, this systematic study provides insight into the magnetic properties of these lighter lanthanide ions.