Six-coordinate lanthanide complexes based on bidentate phosphine oxide ligands: synthesis, structure and magnetic properties

Three six-coordinate mononuclear lanthanide complexes of the formula [Ln{(O = PPh₂)₂CH₂}₂Cl₂]Cl·THF based on the Kramers ions Dy³⁺ (1), Er³⁺ (2) and Yb³⁺ (3) have been synthesized and structurally characterized. Single-crystal X-ray diffraction reveals that these complexes adopt an octahedral geometry with <i>cis</i>-arranged chloride ligands and four oxygen atoms from two bidentate phosphine oxide ligands. The octahedral coordination sphere generates a crystal field environment conducive to field-induced slow magnetic relaxation for lanthanide ions with both oblate (Dy³⁺) and prolate (Er³⁺, Yb³⁺) electron density distributions. Under optimized direct-current fields, complexes 1Dy and 2Er show Orbach relaxation processes with effective energy barriers of 28.0(5) and 16.2(5) cm^-1, respectively. In contrast, complex 3Yb exhibits more complex relaxation dynamics involving a combination of quantum tunnelling, direct, and Raman processes. <i>Ab initio</i> calculations demonstrate that complexes 1Dy and 2Er undergo magnetic relaxation through their first excited Kramers doublets, while the higher-lying first excited state in complex 3Yb (177 cm^-1) prevents thermal relaxation <i>via</i> this pathway.