Assembly of chiral 3d–4f wheel-like cluster complexes with achiral ligands: single-molecule magnetic behavior and magnetocaloric effect

Abstract

Under solvothermal conditions, five chiral 3d–4f wheel-like heterometallic hexanuclear clusters with the Schiff base ligand [H₂L = N,N′-bis(3-methoxysalicylidene)-1,3-diaminopropane] were synthesized by the in situ ligand reaction of the 1,3-bis(2-pyridyl)-1,3-propanedione and generated the pyridine-2-carboxylate (PyCO₂⁻) that acted as a bridging ligand; a peroxide anion formed by the spontaneous reduction of dioxygen plays a key role in the assembly of these complexes. The five isomeric 3d–4f heterometallic complexes, with molecular formula [M₃Ln₃(O₂)L₃(PyCO₂)₃](OH)₂(ClO₄)₂·8H₂O (M = Zn, Cu; Ln = Dy, Tb, Gd) represent the first anion-orientated chiral heterometallic cluster complexes composed of achiral components. Except for complex 1, we randomly measured the crystal structure of one of the two chiral enantiomers of complexes 2–5. Magnetic measurements show that the Zn₃Gd₃ complex exhibits a moderate magnetocaloric effect. The Cu₃Tb₃ complex is a single-molecule magnet (SMM) with an energy barrier (U_eff/k) value of 31.7(0.3) K under zero dc field, which is a larger value for Cu–Tb SMMs; the other three compounds Zn₃Dy₃, Zn₃Tb₃, and Cu₃Dy₃ exhibit field-induced magnetic relaxation. The U_eff/k value of Zn₃Dy₃ [>100 K, H_dc = 1000 Oe] is obviously larger than those of equilateral triangular Dy₃ complexes (≤62 K).