Hexanuclear copper(ii) cage with {Cu3O⋯H⋯OCu3} core supported by a dicompartmental oxime ligand with m-xylyl spacer: synthesis, molecular structure and magnetic studies

Abstract

A new dicompartmental dioxime ligand (H₂L) with m-xylyl spacer between the donor sites has been synthesised by Schiff-base condensation of α,α′-diamino-m-xylene and diacetyl monooxime. The ligand reacts with copper(II) salts giving rise to hexanuclear tricationic copper(II) cage complexes [Cu^II₆(μ₃-O⋯H⋯O-μ₃)L₃(H₂O)₆]X₃ (X = BF₄, 1a; X = ClO₄, 1b). The complexes have been characterised by different analytical and spectroscopic techniques and confirmed the hexanuclear structure even in solution. Single crystal X-ray diffraction studies of both the complexes revealed a very similar core structure with three dicompartmental ligands supporting two triangular Cu₃O cores that share a proton, located on their common threefold axis and involved in a strong hydrogen bond interaction (O⋯O distance of 2.517(2) Å). Two Cu₃O units do not superimpose but are staggered and disposed with the formation of a helicate structure. However both the enantiomers are present in the centrosymmetric space group. The facing Cu₃-planes in 1a are separated at a distance of 3.476 Å. The temperature dependence of the magnetic behaviour of the hexanuclear complex 1a clearly indicates an overall antiferromagnetic exchange interaction between the spin carriers in the cage having two Cu₃O subunits and leaves a single unpaired electron in each triangular unit. The unpaired electrons in the two Cu₃O units interact antiferromagnetically through hydrogen bonding giving rise to an overall singlet-spin ground state.