“Caging” Anions through Crystal Engineering to Avoid Polymerization: Structural, Conformational and Theoretical Investigations of New Halocadmate [Cd2X7]3– Anions (X = Cl/Br)

Two new discrete anionic species, [Cd2Cl7]3– and [Cd2Br7]3–, have been isolated in the form of cobalt(III) complex salts, [Co(phen)3][Cd2Cl7]·3H2O (1) and [Co(phen)3][Cd2Br7]·2H2O (2). The complex salts have been characterized by spectroscopic techniques and other investigations like TGA, solubility product measurements, conductance measurements, luminiscent study, DFT calculations and single crystal structure determinations. The new discrete [Cd2X7]3– anions reside in cavities that are formed by complex cations and are stabilized by different noncovalent interactions (hydrogen bonding, π–π stacking, C–H···π and anion···π). With the help of DFT calculations, an in depth investigation has been conducted into what happens to the conformation of these anions in crystal, solution and gas phase, and the nature of the Cd–halogen bond has been elucidated.