Syntheses, structures, and magnetic properties of a family of end-on azido-bridged Cu-Ln complexes.

The reaction of Cu(valdmpn) (Hvaldmpn = N,N'-bis-(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane), LnCl·nHO (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 NaN in methanol led to a whole series of heterometallic Cu-Ln complexes. The basic structural components of these complexes are the [Cu(valdmpn)Ln] units with a compartmental Schiff-base ligand. These [CuLn] units were then further connected by the end-on (EO) azides between the Ln centers to form tetranuclear {CuLn[LnCu]} motifs, where n is 3 for the complexes of larger radii lanthanides (1 to 5) and 2 for the complexes of smaller radii lanthanides (6 to 14). While compounds 6 to 14 remain isolated tetranuclear clusters, compounds 1 to 5 are one-dimensional chains where tetranuclear motifs are further connected by end-to-end (EE) azides between Cu and Ln 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, 8 and 9. Alternating current (ac) magnetic measurements performed on 8, 9, and 10 demonstrated the presence of slow magnetic relaxation under zero applied dc field. The spin relaxation barrier U was determined to be 19.54 cm (28.12 K) and 10.34 cm (14.87 K) for compounds 8 and 9. The SMM behavior of 8 was further confirmed by the presence of magnetic hysteresis loops; while no hysteresis loop was observed for 9 and 10. These results demonstrated that azido-bridged copper(ii)-lanthanide(iii) complexes could be synthesized successfully despite the generally weak azido-lanthanide interaction.