Second sphere coordination in binding large size anions: Synthesis, characterisation and X-ray structure determination of [trans-Co(en)2Cl2]C6NO2SO3H3Cl and [trans-Co(en)2Cl2]2S4O6

In an effort to explore [trans-Co(en)2Cl2]+ as anion receptor for large size anions, 2-chloro,5-nitrobenzenesulphonate and tetrathionate ion, green coloured single crystals of [trans-Co(en)2Cl2]C6NO2SO3H3Cl I and [trans-Co(en)2Cl2]2S4O6 II have been obtained by slowly mixing the separately dissolved trans-dichlorobis(ethylenediamine)cobalt(III) chloride with metal salts of 2-chloro,5-nitrobenzenesulphnic acid and tetrathionic acid in aqueous medium in 1:1 and 2:1 molar ratio, respectively. The newly synthesized complex salts were characterized on the basis of elemental analyses and spectroscopic techniques (IR, UV/visible, 1H and 13C NMR). Single crystal X-ray structure determinations revealed that I crystallizes in the monoclinic space group P21/c and II crystallizes in the triclinic space group P 1 ¯ . Supramolecular hydrogen bonding networks between ionic groups: oxygen atoms of 2-chloro,5-nitrobenzenesulphonate or tetrathionate and NH groups of coordinated ethylenediamine molecules, i.e. N–H⋯O− interactions by second sphere coordination besides electrostatic forces of attraction have been observed. This suggests that [trans-Co(en)2Cl2]+ is a promising anion receptor for the weakly coordinating sulphonate and tetrathionate ions. The solubility product measurements indicate that the affinity of cationic cobaltammine [trans-Co(en)2Cl2]+ is greater for 2-chloro,5-nitrobenzenesulphonate than tetrathionate ion.