Strong ferromagnetic exchange interactions in hinge-like Dy(O₂Cu)₂ complexes involving double oxygen bridges

Abstract

Two trinuclear isomeric compounds, [{(Cu^II(salpn))(Me(CO)Me)}₂Dy^III(NO₃)₃] (1) and [{Cu^II(salpn)}₂Dy^III(H₂O)(NO₃)₃]•MeOH (2), along with one polymeric compound, {[{Cu^II(salpn)}₂Dy^III(NO₃)₃bpy]•MeOH•H₂O}_n (3), were synthesized using a metalloligand, [Cu^II(salpn)], where H₂salpn and bpy stand for N,N′-bis(salicylidene)-1,3-propanediamine and 4,4′-bipyridine, respectively. Compounds 1 and 2 were selectively prepared with two solvents: the less polar acetone led to the exclusive crystallization of 1 with a transoid trinuclear architecture, while more polar solvent methanol provided sole construction of 2 with a cisoid trinuclear architecture. Compound 3 was prepared from 1 or 2 after bpy was introduced as a bridge. The Dy and Cu ions are doubly bridged with oxygen atoms, and the core DyO₂Cu skeletons are characterized by different “butterfly angles” of 140.9(1)°, 147.1(19)° and 142.4(2)° for 1, 2, and 3, respectively. We have examined the molecular structures and magnetic properties of 1–3 using high-frequency electron paramagnetic resonance (HF-EPR), magnetization, and magnetic susceptibility techniques. These compounds showed slow magnetization reversal in the measurements of alternating current magnetic susceptibility. We analyzed EPR frequency-field diagrams using an effective spin-Hamiltonian including only one doublet of Dy sublevels and found that the exchange couplings are ferromagnetic in all compounds. The exchange coupling parameters J_Dy–Cu of 1, 2 and 3 were determined as 2.25 ± 0.05, 1.82 ± 0.04 and 1.79 ± 0.04 K, respectively. These values are larger than those found in previous research using EPR analysis on [Cu^II(L^A)(C₃H₆O)Dy^III(NO₃)₃] (H₂L^A = N,N′-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane) and [Dy^IIIB₂ (NO₃)₂{Cu^II(CH₃OH)} ₂](NO₃)(CH₃OH) (H₂L^B = 2,6-bis(acetylaceto)pyridine). The present result shows an advantage of doubly oxygen-bridged motifs to built strong ferromagnetic interactions between lanthanide and transition metal ions. We found that the exchange coupling strength is sensitive to the structural parameters such as bond angles, bond lengths, and butterfly angles. Precise determination of the exchange parameters would contribute to development of exchange-coupled 4f–3d heterometallic complexes.