Theoretical investigation on the nature of substituted benzene⋯AuX interactions: covalent or noncovalent?

Abstract

The nature of coinage–metal bonds between AuX (X = F, Cl, Br, CN, NO₂, CH₃) and aromatic moieties with different electronic properties (benzene, 1,3,5-trifluorobenzene and hexafluorobenzene) has been characterized within the framework of energy decomposition analysis, natural bond orbital analysis, quantum theory of atoms in molecules, and the electron localization function method. The strength and covalency of π⋯AuX interactions depend on the nature of the aromatic moiety and coin metal substituents. The binding energy varies from −6.6 to −34.8 kcal mol⁻¹, and becomes more negative in the order of π = C₆F₆ < C₆H₃F₃ < C₆H₆ and X = CH₃ < NO₂ < CN < Br < Cl < F. Most of the coinage–metal bonds have partially covalent characters, and the degree of covalency is larger in electron-deficient π-systems than in electron-rich π-systems. For the same aromatic moiety, the coinage–metal bond is more covalent when the substituent is a halogen, and the degree of covalency increases with the sequence of CH₃ < CN < NO₂ < Br < Cl < F. Appreciable tuning of the coinage–metal bond would be possible by varying the substituents on the coinage–metal and aromatic ring. The polarization effect plays an important role in the π⋯AuX interactions, and there is significant charge transfer from the occupied π(C–C) orbital to the empty Au orbital or antibonding Au–X orbital in the formation of coinage–metal bonds.