Gold(III) derivatives as the noncovalent interaction donors: theoretical study of the π-hole regium bonds

Except for the well-known σ-hole regium bonds formed by metal nanoparticles and M(I) (M = Cu, Ag, and Au) derivatives, the existence of π-hole regions located above and below the Au atom in gold(III) derivatives suggests that gold(III) also functions as an efficient electrophilic site. In this study, a comprehensive analysis was conducted on the electrophilicity of trichloro-(p-toluonitrilo-N)-gold(III) derivatives AuL₃(NCC₆H₄X) (L = Cl, Br, CN; X = NH₂, CH₃, CF₃, NC, and CN) and the nature of π-hole regium bonds in the AuL₃(NCC₆H₄X)⋯LB (LB = NH₃, N(NH₃)₃, CH₂O, C₂H₂, C₂H₄, C₆H₆) and (AuCl₃(NCC₆H₄Y)) (Y = Cl, CN, NC, NO₂; n = 2, 3)) complexes. The characteristics of the π-hole regium bonds were studied with respect to the influence of ligands and substituents, the strength of intermolecular interactions between Au(III) derivatives and Lewis bases, and those in the polymers. In the case of the AuL₃(NCC₆H₄X)⋯NH₃ complexes, the strength of the regium bonds increases gradually in the order of L = Cl < Br < CN and X = NH₂ < CH₃ < CF₃ ≈ NC < CN. The ligands (L) attached to the Au atom exert a significant effect on the strength of the π-hole regium bonds in comparison to the substituents (X) on the benzene ring. The regium bonds are primarily dominated by electrostatic interaction, accompanied by moderate contribution from polarization. Linear relationships were identified between the electrostatic energies and the local most positive potentials over the Au atom, as well as between the polarization energies and the amount of charge transfer. Most of the π-hole regium bonds in the AuL₃(NCC₆H₄X)⋯LB complexes exhibit the characters of closed shell noncovalent interactions. In the polymers (AuCl₃(NCC₆H₄Y)), weak face-to-face π-π stacking interactions are also present, in addition to regium bonds. The trimers displayed a slightly negative cooperativity in comparison to the dimers.