Substituent Effect on Regium Bonding in Copper(II) Complexes of 4‑Substituted Pyridines: Crystallographic and Computational Study

Regium bonding (also called coinage or noble bond) is an attractive weak interaction between the electrophilic region of the regium atom (group 11 element: Rg = Cu, Ag, or Au) and a nucleophilic (Nu) area of the same (intramolecular) or another (intermolecular) molecular entity. The strengths of regium bonding at Rg···Nu depend on the nature of both interacting patterns and electron-donating or -withdrawing character of substituents at Rg and Nu. Experimental and computational studies show that the strengths of Cu···Nu (Nu = Cl or Br) intermolecular regium bonds between the [Cu­(Nu)2(4-substituted pyridine)2] tectons can be regulated by the replacement of substituents (−OCH3, −CH3, −H, −Cl, −CONH2, −COOCH3, or −CN) at the para- position of the pyridine ligand. However, due to the involvement of these substituents in various sorts of weak interactions, there is no linear correlation between Hammett’s para-substituent constant (σp) and the Cu···Nu distance of regium bonding.