Hexanuclear {Zn II4Fe III2} and {Zn II4Cr III2} complexes from the use of potentially tetradentate NOO′O′′ Schiff-base ligands

Abstract

The use of N-(2-carboxyphenyl)salicylideneimine (saphHCOOH) and N-(4-chloro-carboxyphenyl)salicylideneimine (4ClsaphHCOOH) for the synthesis of hexanuclear {Zn^II₄M^III₂} (M = Cr, Fe) complexes is described. [Zn₄Fe₂(saphCOO)₆(NO₃)₂(EtOH)₂] (1), [Zn₄Cr₂(saphCOO)₆(NO₃)₂(H₂O)₂] (2) and [Zn₄Fe₂(4ClsaphCOO)₆(NO₃)₂(EtOH)₂] (3), as 4CH₂Cl₂·2EtOH (1, 3) and 4MeCN·2EtOH (2) solvates, have been isolated and their structures have been determined by single-crystal X-ray crystallography. The three complexes are centrosymmetric and almost isostructural. The metal topology can be described as two isosceles triangles which are linked through phenolate oxygen atoms of two 3.2111 (Harris notation) saphCOO²⁻/4ClsaphCOO²⁻ ligands, each of which bridges exclusively three Zn^II atoms. Four 2.1111 ligands are also incorporated in the molecules, each bridging one M^III center and one Zn^II ion. The former is chelated with the O_phenolate, N_imine and one O_carboxylate being the donor atoms (thus forming two 6-membered chelating rings with a common M^III–N edge), while the latter is bound to the other carboxylate oxygen. The coordination spheres are of the types {FeO₄N₂}/{ZnO₄N}/{ZnO₅} for 1 and 3, and {CrO₄N₂}/{ZnO₄N}/{ZnO₄} for 2. Complexes 1–3 are the first heterometallic complexes (with any metals) based on saphHCOOH and 4ClsaphHCOOH, thus illustrating their ability for the preparation of mixed-metal molecular species. Interesting H-bonding patterns are present in the crystal structures. The IR and Raman spectra of the solid complexes are discussed in terms of the coordination modes of the ligands involved, while molar conductivity data and UV/VIS spectra in CH₂Cl₂ are also reported and interpreted. The δ and ΔE_Q⁵⁷Fe-Mössbauer parameters of 1 and 3 at 300 and 80 K indicate isolated high-spin Fe^III centers. Variable-temperature (1.8–300 K) magnetic susceptibility data for 1–3 suggest a very weak exchange interaction between the M^III ions in agreement with their long distances (∼9 Å) in the molecules.