Synthesis and structural and magnetic characterisation of tetranuclear Cu(ii) complexes possessing novel [Cu4(μ4-PO4)2(μ2-CO3)] butterfly cores that exhibit supramolecular isomerism

Abstract

The reaction in water of Cu(OH)₂ with 2,2′-bipyridine (bipy) and (NH₄)₂HPO₄ (4 ∶ 4 ∶ 2) in air leads to the formation of two structurally related complexes following the fixation of CO₂: {[(H₂O)₂Cu₄(bipy)₄(μ₄-PO₄)₂(μ₂-CO₃)]·22.5H₂O}, 1, and {[(H₂O)₂Cu₄(bipy)₄(μ₄-PO₄)₂(μ₂-CO₃)]·15.5H₂O}, 1a, as determined by single crystal X-ray diffraction. Whilst each complex contains isostructural tetranuclear Cu(II) butterfly cores, they differ in the way they pack in the solid state: adjacent units in 1 align in parallel fashion, whereas those in 1a form H-bonded dimers in an antiparallel arrangement. 1 and 1a are therefore supramolecular isomers. 1 is encapsulated within an ‘ice-like’ 3D H-bonded water network whereas the solvent sheath about 1a forms a 2D H-bonded network. A similar product results when bipy is replaced with 1,10-phenanthroline (phen) giving {[(H₂O)₂Cu₄(phen)₄(μ₄-PO₄)₂(μ₂-CO₃)]·21.5H₂O}, 2, which possesses an isostructural butterfly core shrouded within a 3D H-bonded water network. 2 packs in the solid-state in a similar way to 1 although, in this instance, adjacent units align in an antiparallel fashion. Both 1 and 2 may be directly synthesised in higher yields when (NH₄)HCO₃ is included within the reaction medium. A magnetic susceptibility study performed upon 1 reveals net weak intramolecular antiferromagnetic coupling between the Cu(II) centres.