Interaction of Benzene with Transition Metal Cations: Theoretical Study of Structures, Energies, and IR Spectra.

The cation-π interactions have been intensively studied. Nevertheless, the interactions of π systems with heavy transition metals and their accurate conformations are not well understood. Here, we theoretically investigate the structures and binding characteristics of transition metal (TM) cations including novel metal cations (TM(n+) = Cu(+), Ag(+), Au(+), Pd(2+), Pt(2+), and Hg(2+)) interacting with benzene (Bz). For comparison, the alkali metal complex of Na(+)-Bz is also included. We employ density functional theory (DFT) and high levels of ab initio theory including Møller-Plesset second-order perturbation (MP2) theory, quadratic CI method with single and double substitutions (QCISD), and the coupled cluster theory with single, double, and perturbative triple excitations (CCSD(T)). Each of the transition metal complexes of benzene exhibits intriguing binding characteristics, different from the typical cation-π interactions between alkali metal cations and aromatic rings. The complexes of Na(+), Cu(+), and Ag(+) favor the conformation of C6v symmetry with the cation above the benzene centroid (πcen). The formation of these complexes is attributed to the electrostatic interaction, while the magnitude of charge transfer has little correlation with the total interaction energy. Because of the TM(n+)←π donation, cations Au(+), Pd(2+), Pt(2+), and Hg(2+) prefer the off-center π conformation (πoff) or the π coordination to a C atom of the benzene. Although the electrostatic interaction is still important, the TM←π donation effect is responsible for the binding site. The TM(n+)-Bz complexes give some characteristic IR peaks. The complexes of Na(+), Cu(+), and Ag(+) give two IR active modes between 800 and 1000 cm(-1),which are inactive in the pure benzene. The complexes of Au(+), Pd(2+), Pt(2+), and Hg(2+) give characteristic peaks for the ring distortion, C-C stretching, and C-H stretching modes as well as significant red-shifts in the CH out-of-plane bending.