Disordered crystal structure and magnetic behavior of [Cu ₂ (3-bromopropionato) ₄ (caffeine) ₂ ]·(H ₂ O)

The crystal structure of compound [Cu2(3-bromopropionato)4(caffeine)2]·(H2O) has been studied using single-crystal X-ray crystallography. The compound crystallizes in the Cc space group. Its structure consists of asymmetric binuclear complexes linked by stacking interactions between caffeine ligands. Bidentate 3-bromopropionato ligands are disordered across eight crystal sites. The carboxylic groups connect the copper atoms into the binuclear units. The oxygen atoms nearly form a plane and coordinate the copper atoms, which are 2.66(3) Å apart. The caffeine ligands are stacked along the edge of the unit cell. The relationship between the displacement of copper atoms from the mean plane of basal oxygen atoms and the Cu-N bond length at the apex has been modeled for binuclear oxo-bridged copper carboxylates. Magnetic susceptibility measurements between 77 and 290 K, along with EPR analysis of the polycrystalline sample, support the binuclear structure of the copper(II) units. The gas-phase quantum–chemical calculations of a single molecule indicated energetic stability of both the paramagnetic spin state and the diamagnetic spin state with broken symmetry.