Second sphere coordination complexes via hydrogen bonding: Synthesis, spectroscopic characterisation of [trans-Co(en)2Cl2]CdX4 (X=Br or I) and single crystal X-ray structure determination of [trans-Co(en)2Cl2]CdBr4

In an effort to explore [trans-Co(en)2Cl2]+ as anion receptor for tetrabromocadmate and tetraiodocadmate ion, green coloured single crystals of [trans-Co(en)2Cl2]CdBr4 I and [trans-Co(en)2Cl2]CdI4 II have been obtained by slowly mixing the separately dissolved trans-dichlorobis(ethylenediamine)cobalt(III) chloride with potassium tetrabromocadmate and tetraiodocadmate in aqueous medium in 2:1 molar ratio. The newly synthesized complex salt was characterized on the basis of elemental analysis and spectroscopic techniques (IR, UV/visible, 1H and 13C NMR). Single crystal X-ray structure determination of [trans-Co(en)2Cl2]2CdBr4 revealed that it crystallizes in the orthorhombic space group Pbca with a=18.6201(10), b=12.0872(6), c=24.0877(12) Å, V=5421.3(5) Å3, Z=8, R=0.0727. Supramolecular hydrogen bonding networks between ionic groups: bromide ions of tetrabromocadmate group and NH groups of coordinated ethylenediamine molecules, i.e. N–H⋯Br− 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 tetrabromocadmate.