Second sphere coordination in anion binding: Synthesis, characterization and X-ray structure of [cis-diazidobis(ethylenediamine)cobalt(III)][trans-diamminetetranitrocobaltate(III)]

Dark maroon coloured single crystals of [cis-Co(en)2(N3)2][trans-Co(NH3)2(NO2)4] were obtained by slowly mixing the separately dissolved [cis-diazidobis(ethylenediamine)cobalt(III)] nitrate with potassium [trans-diamminetetranitrocobaltate(III)] in aqueous medium in 1:1 molar ratio. The complex salt was characterized by elemental analyses, spectroscopic studies (IR, UV/visible, 1H and 13C NMR) and solubility product measurements. The title complex salt crystallises in triclinic space group P1¯ having cell dimensions, a = 7.4210(2), b = 11.0618(3), c = 12.3763(3) Å, α = 95.5850(10), β = 91.4400(10), γ = 108.9070(10)°, V = 954.853 Å3, Z = 2 and R-factor = 5.16%. Single crystal X-ray structure determination revealed an ionic structure consisting of [cis-Co(en)2(N3)2]+ and [trans-Co(NH3)2(NO2)4]−. Supramolecular hydrogen bonding networks involving second sphere coordination like [NHen+⋯Xanion-] besides electrostatic forces of attraction have been observed to stabilize the crystal lattice. This is the first crystal structure of a salt containing [trans-diamminetetranitrocobaltate(III)] anion and [cis-diazidobis(ethylenediamine)cobalt(III)] cation. The formation of complex salt of definite composition and solubility product measurements show that the cationic cobaltammine [cis-Co(en)2(N3)2]+ is a potential anion receptor for complex anion [trans-Co(NH3)2(NO2)4]− in aqueous medium.