Temperature-controlled polymorphism of chiral Cu(II)-Ln(III) dinuclear complexes exhibiting slow magnetic relaxation

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.