Single molecule magnet behaviors of Zn4Ln2 (Ln = DyIII, TbIII) complexes with multidentate organic ligands formed by absorption of CO2 in air through in situ reactions

Abstract

In this work, we report the syntheses, crystal structures and magnetic properties of three novel Zn–Ln mixed metal complexes, namely [Zn₄Dy₂(L₁)₂(L₂)₂(N₃)₂]Cl₂·2H₂O (1), [Zn₄Tb₂(L₁)₂(L₂)₂(Cl)₂][ZnN₃Cl₃]·2H₂O (2), and [Zn₄Gd₂(L₁)₂(L₂)₂(Cl)₂][ZnN₃Cl₃]·2H₂O (3), in which L₁²⁻ and L₂³⁻ were formed from the ligand L [L = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine] through in situ reactions. Interestingly, carbon dioxide in air was absorbed in the process of forming carbamate ligand L₂³⁻; this can be ascribed to the insertion of CO₂ into M–N amide bonds. Moreover, 1 and 2 represent the first series of 3d–4f SMMs containing carbamate ligands by fixation of CO₂ in air. Single-crystal X-ray diffraction analyses reveal that the crystal structures of 1 and 2 are anion-dependent, i.e., the apical positions of the two Zn^II ions in 1 and 2 are occupied by an N atom of N₃⁻ and by Cl⁻, respectively. However, the topologies of 2 and 3 are similar. Two Zn^II ions and one Ln^III (Ln = Dy (1), Tb (2) and Gd (3)) form nearly linear trinuclear [Zn₂Ln] units which are double-bridged by two L₂³⁻ ligands. Magnetic studies reveal that two complexes show single molecule magnet behavior under a direct current (dc) field, with effective energy barriers (U_eff) of 30.66(5) K for 1 and 8.87(3) K for 2. Ab initio calculations reveal that the Dy^III ions in 1 and the Tb^III ions in 2 are axial in nature; however, a difference in the tunnel splitting of 1 and 2 leads to variation in the magnetization blockades of the two complexes. Theoretical calculations also indicate that the directions of the main magnetic axes severely deviate from the coordination atoms of the first spheres of Dy^III and Tb^III in 1 and 2; thus further results in poor SMM behavior of the two complexes.