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 = Ni^II, R = An = anisyl (1), R = Fc = ferrocenyl (2); M = Cu^II, 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 Ni^II 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 (d_Cu-O = 2.411(5) Å) between dimers, the Cu^II 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 Cu^II spins carriers in 4.