Apparently improved energy barrier of Dy(iii) SMMs by adopting the bridging bipyrimidine ligand

Abstract

Eight complexes Ln^III(tfa)₃(dmpy) (Ln = Gd 1, Dy 2, tfa = 1,1,1-trifluoro-2,4-pentanedione dmpy = 4,4′-dimethyl-2,2′-bipyridyl), Ln^III(tfpa)₃(dmpy) (Ln = Gd 3, Dy 4, tfpa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione), [Ln^III(tfa)₃]₂(bpyd) (Ln = Gd 5, Dy 6, bpyd = 2,2′-bipyrimidine) and [Ln^III(tfpa)₃]₂(bpyd) (Ln = Gd 7, Dy 8) were designed to investigate the influence of magnetic coupling transfer and changes in molecular geometry on the performance of single-molecule magnets (SMMs). The single-crystal X-ray diffraction results show that Ln^III(tfa)₃(dmpy) and Ln^III(tfpa)₃(dmpy) are mononuclear, while [Ln^III(tfa)₃]₂(bpyd) and [Ln^III(tfpa)₃]₂(bpyd) are binuclear complexes with bipyrimidine acting as bridging ligands. The Ln^III ions in these complexes are octahedral structures. Direct current (dc) susceptibility data indicate that bipyrimidine can effectively transfer the magnetic exchange of two Ln^III ions. The results of alternating current (AC) magnetization characterization reveal that the magnetic exchange of adjacent Dy^III ions can obviously suppress the quantum tunneling effect. Therefore, complexes 2 and 4 display weak magnetic relaxation with the absence of peaks. Comparatively, complexes 6 and 8 exhibit distinct slow magnetic relaxation behaviors under the same conditions, with U_eff values of 319.38 and 173.18 K, respectively. Theoretical calculations suggest that the weak Dy^III–Dy^III dipole–dipole interaction in complex 6 can effectively hinder the magnetic quantum tunneling in the lowest four Kramers doublets (KDs) of Dy^III fragments. Consequently, the U_eff value of complex 6 is notably higher than that of complex 8. These findings could provide insights for achieving higher energy barriers in binuclear Dy^III compounds.