Spin-Parity Behavior in the Exchange-Coupled Lanthanoid-Nitroxide Molecular Magnets

摘要

To develop lanthanoid-based magnetic materials and relevant devices, reliable prescriptions for molecular/crystal design have long been desired. Ln~(3+)-ion dependence on the molecular magnetism was investigated in the isomorphous series [Ln(hfac)3(2pyNO)] (Ln = Tb, Dy, Ho, Er), where 2pyNO stands for tert-butyl 2-pyridyl nitroxide as a paramagnetic ligand, and hfac for 1,1,1,5,5,5-hexafluoropentane-2,4-dionate. The slow magnetization reversal was evaluated as an indication of single-molecule magnets (SMMs) by out-of-phase ac magnetic susceptibility χ Whereas the Tb~(3+) (4f~8) and Ho~(3+) (4f~(10)) derivatives exhibited frequency-dependent χ", practically null χ' was recorded for the Dy~(3+)(4f~9) and Er~(3+)(4f~(11)) derivatives. As for another series with Ln/radical = 1/2, [Ln(hfac)3(TEMPO)2] complexes were prepared (Ln = Tb, Dy, Ho, Er, Tm; TEMPO = 2,2,6,6-tetramethylpiperidin-1-oxyl). The Dy~(3+) and Er~(3+) derivatives showed appreciable χ", but the Tb~(3+), Ho~(3+), and Tm~(3+) derivatives did not. Thus, the S = 1/2 paramagnetic ligands play a role of a spin-parity switch to regulate whether the compound behaves as an SMM. In the strongly exchange-coupled regime owing to the direct radical coordination bond, the whole molecular electron counting may provide a useful criterion to predict Kramers molecules and accordingly to explore potential SMM candidates.