Ferromagnetic heterotrinuclear Cu–Ni complexes of a compartmental chiral Schiff base

Abstract

Nickel(II) and copper(II) acetate react with the trinucleating compartmental Schiff base H₄L (H₄L = 6,6′-(E)-3,3′-(ethane-1,2-diyl)bis(1-(2-((E)-3-bromo-5-chloro-2-hydroxybenzylideneamino)ethyl)imidazolidine-3,2-diyl)bis(2-bromo-4-chlorophenol)) to produce the heterotrinuclear complexes [Ni₂CuL(OAc)₂]·0.25H₂O·2.5MeOH (1·0.25H₂O·2.5MeOH) and [NiCu₂L(OAc)₂]·3.25H₂O·0.5MeOH (2·3.25H₂O·0.5MeOH) as a function of the Ni(OAc)₂ : Cu(OAc)₂ molar ratio. The crystal structures of H₄L, 1·0.25H₂O·2.5MeOH and 2·3.25H₂O·0.5MeOH could be solved. The free ligand presents two stereogenic methine groups on the imidazolidine heterocycles. X-Ray diffraction studies on H₄L determined that the solved crystal structure corresponds to a racemate formed by the (2R,2′R) and (2S,2′S) enantiomers, without detecting the (2R,2′S) diastereoisomer. The crystal structures of both heterotrinuclear complexes reveal that Ni^II has a preference for the central ligand pocket, showing that this cavity discriminates between Ni^II and Cu^II when both species are present in the reaction medium. These results are validated by DFT calculations. As a consequence of the coordination, 1·0.25H₂O·2.5MeOH and 2·3.25H₂O·0.5MeOH are also chiral, but crystallise as racemates. In addition to their asymmetric methine groups, these complexes present four other stereogenic centres: the four coordinated imidazolidineN atoms. The luminescent properties of the ligand and both complexes were analysed, showing that the presence of the metals partially inhibits the emission of the ligand and apparently tunes the position of the secondary fluorescence emission band. The magnetic characterisation of 1·0.25H₂O·2.5MeOH and 2·3.25H₂O·0.5MeOH was also performed, showing the ferromagnetic behaviour of both complexes.