Syntheses, structures, and magnetic properties of a family of end-on azido-bridged CuII–LnIII complexes

Abstract

The reaction of Cu(valdmpn) (H₂valdmpn = N,N′-bis-(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane), LnCl₃·nH₂O (Ln = La(1), Ce (2), Pr(3), Nd(4), Sm(5), Eu(6), Gd(7), Tb(8), Dy(9), Ho(10), Er(11), Tm(12), Yb(13), Lu(14)) and NaN₃ in methanol led to a whole series of heterometallic Cu^II–Ln^III complexes. The basic structural components of these complexes are the [Cu^II(valdmpn)Ln^III] units with a compartmental Schiff-base ligand. These [Cu^IILn^III] units were then further connected by the end-on (EO) azides between the Ln³⁺ centers to form tetranuclear {[CuLn](μ-N₃)_n[LnCu]} motifs, where n is 3 for the complexes of larger radii lanthanides (1_CuLa to 5_CuSm) and 2 for the complexes of smaller radii lanthanides (6_CuEu to 14_CuLu). While compounds 6_CuEu to 14_CuLu remain isolated tetranuclear clusters, compounds 1_CuLa to 5_CuSm are one-dimensional chains where tetranuclear motifs are further connected by end-to-end (EE) azides between Cu^II and Ln^III centers. Direct current (dc) magnetic susceptibilities have been measured in the range of 1.8–300 K for all complexes, revealing the different nature of magnetic interactions between the spin centers. Among them, dominant ferromagnetic interactions were found in compounds 7_CuGd, 8_CuTb and 9_CuDy. Alternating current (ac) magnetic measurements performed on 8_CuTb, 9_CuDy, and 10_CuHo demonstrated the presence of slow magnetic relaxation under zero applied dc field. The spin relaxation barrier U_eff was determined to be 19.54 cm⁻¹ (28.12 K) and 10.34 cm⁻¹ (14.87 K) for compounds 8_CuTb and 9_CuDy. The SMM behavior of 8_CuTb was further confirmed by the presence of magnetic hysteresis loops; while no hysteresis loop was observed for 9_CuDy and 10_CuHo. These results demonstrated that azido-bridged copper(II)–lanthanide(III) complexes could be synthesized successfully despite the generally weak azido–lanthanide interaction.