A hexaureachromium(III) complex as a molecular receptor for oxoanions

The hourglass shaped molecular receptor, [Cr(CO(NH2)2)6]3+ has been recognized and its binding properties with oxoanions ( Cr 2 O 7 2 - and CrO 4 2 - ) were studied. Binding constants with different potassium salts of oxoanions were determined by standard UV–vis titration techniques in aqueous medium. According to the titration data, receptor containing six urea molecules exhibits high affinity (log K = 3.7–3.8) for oxoanions in the presence of Cl−. The UV–vis spectroscopic titration data were found to fit well to a 2:3 binding model for the interactions between the receptor and Cr 2 O 7 2 - and 1:2 for receptor and CrO 4 2 - . The newly synthesized receptor-anion complexes were also characterized by elemental analyses and IR spectroscopy. Single crystal X-ray study of [Cr(CO(NH2)2)6]2(Cr2O7)3 reveals the structure with a honeycomb-like pattern, consisting of columns of coordination cations, running parallel to c axis, being surrounded by a “cylinder” of oxoanions. The structure is stabilized by strong charge assisted (HNH⋯OCr) intermolecular hydrogen bonds (CAHB) between cations and anions and intramolecular (CO⋯HNH) hydrogen bonds within cations.