Second sphere coordination in oxoanion binding: Synthesis, spectroscopic characterisation and crystal structures of trans-[bis(ethylenediamine)dinitrocobalt(III)] diclofenac and chlorate

In the exploration of cationic cobaltammine [trans-Co(en)2(NO2)2]+ as an anion receptor, binding with oxoanions diclofenac and chlorate ions has been investigated. Yellow crystals of [trans-Co(en)2(NO2)2]C14H10Cl2NO2. 2H2O I, and [trans-Co(en)2(NO2)2]ClO3 II, have been obtained from a mixture of trans-[bis(ethylenediamine)dinitrocobalt(III)] nitrate solution with sodium diclofenac and sodium chlorate, respectively, in aqueous medium. The products were characterised by elemental analyses, IR, UV/vis, 1H and 13C NMR spectroscopy. Single crystal X-ray structure determinations revealed that electrostatic forces of attraction besides second sphere hydrogen bonding interactions stabilize the crystal lattice. Oxygen atoms of the halate and carboxylate group in diclofenac ions act as hydrogen bond acceptors thereby forming NHen⋯O bonds. The results show that [trans-Co(en)2(NO2)2]+ is a promising anion receptor for the weakly coordinating halate and diclofenac ions in aqueous medium. Solubility measurements indicate that the affinity of cationic cobaltammine [trans-Co(en)2(NO2)2]+ is greater for diclofenac than for the chlorate ion.