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

Abstract

Eight complexes LnIII(tfa)3(dmpy) (Ln = Gd 1, Dy 2, tfa = 1,1,1-trifluoro-2,4-pentanedione dmpy = 4,4′-dimethyl-2,2′-bipyridyl), LnIII(tfpa)3(dmpy) (Ln = Gd 3, Dy 4, tfpa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione), [LnIII(tfa)3]2(bpyd) (Ln = Gd 5, Dy 6, bpyd = 2,2′-bipyrimidine) and [LnIII(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 LnIII(tfa)3(dmpy) and LnIII(tfpa)3(dmpy) are mononuclear, while [LnIII(tfa)3]2(bpyd) and [LnIII(tfpa)3]2(bpyd) are binuclear complexes with bipyrimidine acting as bridging ligands. The LnIII ions in these complexes are octahedral structures. Direct current (dc) susceptibility data indicate that bipyrimidine can effectively transfer the magnetic exchange of two LnIII ions. The results of alternating current (AC) magnetization characterization reveal that the magnetic exchange of adjacent DyIII 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 Ueff values of 319.38 and 173.18 K, respectively. Theoretical calculations suggest that the weak DyIII-DyIII dipole-dipole interaction in complex 6 can effectively hinder the magnetic quantum tunneling in the lowest four Kramers doublets (KDs) of DyIII fragments. Consequently, the Ueff value of complex 6 is notably higher than that of complex 8. These findings could provide insights for achieving higher energy barriers in binuclear DyIII compounds.