Chiral tetranuclear copper(ii) complexes: synthesis, optical and magnetic properties

Abstract

Treating the enantiomerically pure Schiff base ligand (R-H₂L₁ or S-H₂L₁) with equimolar copper nitrate led to the isolation of two chiral tetranuclear Cu(II) cubane complexes with the general molecular formula [Cu₄(R-L₁)₄] (R-1) and [Cu₄(S-L₁)₄] (S-1), which are characterized by single-crystal X-ray diffraction. Both R-1 and S-1 are structurally analogous to each other, which is reflected from their unit cell parameters. The chiral ligands employed efficiently transfer the chirality to the metal complex, which is reflected from their flack parameters and the complex R-1 crystallizes in the chiral, polar point/space group (C2). The chirality is maintained not only in the solid-state but also in the solution state for both R/S-H₂L₁ and R/S-1. The direct current magnetic susceptibility measurements performed on a polycrystalline sample of a representative complex R-1 exhibits a dominant ferromagnetic interaction resulting in an S = 2 ground state, which is in contrast to the other alkoxide bridged [Cu₄O₄] complexes reported. The electronic structure and the observed ferromagnetic exchange coupling in R-1 are rationalized by detailed theoretical calculations.