When CuCl₄ ^2- and CuBr₄ ^2- Form Anion···Anion Networks Assembled via Cu···Cl/Br Regium Bonds

This study presents the design, preparation, and single-crystal X-ray characterization of CuCl₄ ^2- and CuBr₄ ^2- salts, wherein dianions self-assemble into supramolecular (4,4) networks via short Cu···Cl/Br contacts (regium bonds). In the described salts, the organic cations play a pivotal role in promoting the ability of Cu atoms to act as electrophiles. The used cations are the protonated forms of primary amines and form a tight network of N⁺H···Cl/Br hydrogen bonds that promote the regium bond donor ability of Cu by dissipating the anion negative charge. Calculations, namely determination of the molecular electrostatic potential, quantum theory of atoms in molecules and natural bond orbital analyses and other approaches, afford a comprehensive understanding of the Cu···Cl/Br short contacts identified via crystallography and confirm the attractive character of the charge transfer from an occupied lone pair on the halogen of one anion to an empty σ* orbital on copper of another anion.