Role of second-sphere coordination in anion binding: Synthesis, characterization and X-ray structure of hexaamminecobalt(III) chloride hydrogen phthalate trihydrate and sodium hexaamminecobalt(III) benzoate monohydrate

In an effort to utilize [Co(NH3)6]3+cation as a new host for carboxylate ions, orange coloured crystalline solids of composition [Co(NH3)6]Cl(C8H5O4)2·3H2O (1) and Na[Co(NH3)6](C7H5O2)4·H2O (2) were obtained by reacting hot aqueous solutions of hexaamminecobalt(III) chloride with potassium hydrogen phthalate and sodium benzoate in 1:3 molar ratio, respectively. The title complex salts were characterized by elemental analyses and spectroscopic studies (IR, UV/Visible and NMR). Single crystal X-ray structure determinations revealed the formation of second-sphere coordination complexes based on hydrogen bond interactions. In complex salt 1 only two out of three ionisable chloride ions present in [Co(NH3)6]Cl3 were replaced by two C8H5O4− ions whereas in complex salt 2 all the three ionisable chloride ions present in [Co(NH3)6]Cl3 were replaced and the final product was an adduct with another mole of sodium benzoate in solid state. The crystal lattice is stabilized by electrostatic forces of attraction and predominantly N-H⋯O interactions.