Efficient preparation of multimetallic ONO-based Schiff base complexes of nickel(ii) and copper(ii)

A series of symmetrical D–π–D type complexes based on ONO tridentate Schiff base nickel(II) and copper(II) building blocks connected by a bidentate conjugated spacer [(R-ONO)M(μ-4,4′-bipy)M(R-ONO)] (M = NiII, R = An = anisyl (1), R = Fc = ferrocenyl (2); M = CuII, R = An (3), R = Fc (4); 4,4′-bipy = 4,4′-bipyridine) have been prepared in 70–85% yield. The four new compounds were characterized by elemental analysis, FT-IR and UV-vis spectroscopy, and by 1H-NMR spectroscopy in the case of the diamagnetic NiII derivatives 1 and 2. The molecular structures of complexes 1–3 were further confirmed by single-crystal X-ray diffraction methods, along with that of the doubly phenoxide-bridged derivative [Cu2(μ-Fc-ONO)2] 5, formed upon dissociation of the 4,4′-bipy spacer from 4. In 1, 2 and 5, the two four-coordinated nickel and copper atoms adopt a square planar geometry, whereas in 3 that forms a coordination polymer through apical Cu–O interactions (dCu–O = 2.411(5) A) between dimers, the CuII metal ion is five-coordinated in a square pyramidal environment. The electrochemical behavior of compounds 1–4 has also been investigated by cyclic voltammetry. Variable temperature (2–300 K) magnetic susceptibility measurements reveal a weak antiferromagnetic coupling (J = −0.83 cm−1) in 3, and that there is no interaction between the two CuII spins carriers in 4.