A series of mononuclear lanthanide Zn–Dy–Zn type single-molecule magnets (SMMs) were synthesized and magnetically characterized. The four molecules ([Zn2(L1)2DyCl3]·2H2O (1), [Zn2(L1)2Dy(MeOH)Br3]·3H2O (2), [Zn2(L1)2Dy(H2O)Br2]·[ZnBr4]0.5 (3) and [Zn2(L2)2DyCl3]·2H2O (4)) all display remarkable magnetic relaxation behavior with a relatively high energy barrier and hysteresis temperature, despite possessing a low local geometry symmetry of the center Dy(III) ions. Ab initio studies revealed that the symmetry of the charge distribution around the Dy(III) ion is the key factor to determine the relaxation of the SMMs. The four complexes orient their magnetic easy axes along the negative charge-dense direction of the first coordination sphere. The entire molecular magnetic anisotropy was therefore controlled by a single substituent atom in the hard plane which consists of five coordination atoms (perpendicular to the easy axis), and the lower charge distribution on this hard plane in combination with the nearly coplanarity of the five coordination atoms ultimately lead to the prominent magnetic slow relaxation. This offers an efficient and rational method to improve the dynamic magnetic relaxation of the mononuclear lanthanide SMMs that usually possess a low local geometry symmetry around the lanthanide(III) center.

A series of mononuclear lanthanide Zn–Dy–Zn type single-molecule magnets (SMMs) were synthesized and magnetically characterized. The four molecules ([Zn2(L1)2DyCl3]·2H2O (1), [Zn2(L1)2Dy(MeOH)Br3]·3H2O (2), [Zn2(L1)2Dy(H2O)Br2]·[ZnBr4]0.5 (3) and [Zn2(L2)2DyCl3]·2H2O (4)) all display remarkable magnetic relaxation behavior with a relatively high energy barrier and hysteresis temperature, despite possessing a low local geometry symmetry of the center Dy(III) ions. Ab initio studies revealed that the symmetry of the charge distribution around the Dy(III) ion is the key factor to determine the relaxation of the SMMs. The four complexes orient their magnetic easy axes along the negative charge-dense direction of the first coordination sphere. The entire molecular magnetic anisotropy was therefore controlled by a single substituent atom in the hard plane which consists of five coordination atoms (perpendicular to the easy axis), and the lower charge distribution on this hard plane in combination with the nearly coplanarity of the five coordination atoms ultimately lead to the prominent magnetic slow relaxation. This offers an efficient and rational method to improve the dynamic magnetic relaxation of the mononuclear lanthanide SMMs that usually possess a low local geometry symmetry around the lanthanide(III) center.

Four Zn-Ln salen complexes are formed with the Schiff base ligand bis(3-methoxysalicylidene)ethylene-1,2-phenylenediamine (H2L). The complexes are trinuclear [LnZn2L2(OAc)2]·CF3SO3·Et2O (Ln = Eu (1) and Tb (2)), and dinuclear [EuZnL(OAc)(NO3)2MeOH] (3) and [TbZnL(OAc)2(NO3)] (4). The structures of 1–4 were determined by single crystal X-ray crystallographic studies and the respective luminescence properties in MeCN solution were determined.Graphical abstractFour Zn–Ln salen complexes are formed with the Schiff base ligand bis(3-methoxysalicylidene)ethylene-1,2-phenylenediamine (H2L). The complexes are trinuclear [LnZn2L2(OAc)2]·CF3SO3·Et2O (Ln = Eu (1) and Tb (2)), and dinuclear [EuZnL(OAc)(NO3)2MeOH] (3) and [TbZnL(OAc)2(NO3)] (4). The structures of 1–4 were determined by single crystal X-ray crystallographic studies and the respective luminescence properties in MeCN solution were determined.Highlights► Synthesis of Zn–Ln (Ln = Eu and Tb) salen complexes. ► Crystal structures of Zn–Ln salen complexes. ► Luminescence properties of Zn–Ln salen complexes.

Four Zn-Ln salen complexes are formed with the Schiff base ligand bis(3-methoxysalicylidene)ethylene-1,2-phenylenediamine (H2L). The complexes are trinuclear [LnZn2L2(OAc)2]·CF3SO3·Et2O (Ln = Eu (1) and Tb (2)), and dinuclear [EuZnL(OAc)(NO3)2MeOH] (3) and [TbZnL(OAc)2(NO3)] (4). The structures of 1–4 were determined by single crystal X-ray crystallographic studies and the respective luminescence properties in MeCN solution were determined.Graphical abstractFour Zn–Ln salen complexes are formed with the Schiff base ligand bis(3-methoxysalicylidene)ethylene-1,2-phenylenediamine (H2L). The complexes are trinuclear [LnZn2L2(OAc)2]·CF3SO3·Et2O (Ln = Eu (1) and Tb (2)), and dinuclear [EuZnL(OAc)(NO3)2MeOH] (3) and [TbZnL(OAc)2(NO3)] (4). The structures of 1–4 were determined by single crystal X-ray crystallographic studies and the respective luminescence properties in MeCN solution were determined.Highlights► Synthesis of Zn–Ln (Ln = Eu and Tb) salen complexes. ► Crystal structures of Zn–Ln salen complexes. ► Luminescence properties of Zn–Ln salen complexes.