Temperature-controlled polymorphism of chiral CuII–LnIII dinuclear complexes exhibiting slow magnetic relaxation

Abstract

A new family of 3d–4f dinuclear complexes derived from a chiral Schiff-base ligand, (R,R)-N,N′-bis(3-methoxysalicylidene)cyclohexane-1,2-diamine (H₂L), has been synthesized and structurally characterized, namely, [Cu(L)Ln(NO₃)₃(H₂O)] (Ln = Ce (1) and Nd (2)), [Cu(L)Sm(NO₃)₃]·2CH₃CN (3) and [Cu(L)Ln(NO₃)₃] (Ln = Eu (4), Gd (5 and 5′), Tb (6 and 6′), Dy (7 and 7′), Ho (8), Er (9) and Yb (10)). Structural determination revealed that these complexes are composed of two diphenoxo-bridged Cu^II–Ln^III dinuclear clusters with slight structural differences. Complexes 1, 2 and 4–7 crystallize in the chiral space group P1, and the space group of 3 is P2₁, while the other six complexes (5′–7′ and 8–10) are isomorphous and each of them contains two slightly different Cu^II–Ln^III dinuclear clusters in the asymmetric unit with the chiral space group P2₁. Magnetic investigations showed that ferromagnetic couplings between the Cu^II and Ln^III ions exist in 5–7 and 5′–7′. Moreover, the alternating current (ac) magnetic susceptibilities of 6, 6′, 7 and 7′ showed that both the in-phase (χ′) and out-of-phase (χ′′) are frequency- and temperature-dependent with a series of frequency-dependent peaks for the χ′′, which being typical features of field-induced slow magnetic relaxation phenomena. For 8, a frequency dependent χ′ with peaks but χ′′ without peaks appeared; however, the compound displays field-induced slow magnetic relaxation behavior. Furthermore, no obvious frequency-dependent ac signal was observed in 9 owing to the absence of the easy-axis anisotropy. More significantly, we observed the temperature-controlled reversible conversion from one chiral single-crystal (5–7) to another chiral single-crystal (5′–7′) exhibiting slow magnetic relaxation.