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<middle dot><middle dot><middle dot>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<middle dot><middle dot><middle dot>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 (sigma(p)) and the Cu<middle dot><middle dot><middle dot>Nu distance of regium bonding.