Cation recognition by benzene sandwich compounds – a DFT perspective†

Cation–π interactions between alkali, alkaline earth and ammonium cations and sandwich compounds of benzene and the cyclopentadienyl (Cp) anion were studied using quantum chemical CCSD(T)/CBS and DFT (B3LYP/def2-TZVP) calculations. The results show significantly stronger interactions of sandwich compounds with respect to (uncoordinated) benzene. Moreover, very strong cation–π interactions of Cp sandwich compounds are furthermore surpassed by cation–π interactions of benzene sandwich compounds, which are capable of reaching a remarkable interaction energy value of −196.8 kcal mol⁻¹ (Mg²⁺/W(benzene)₂). While there are only small variations of interaction energy values for sandwich compounds of different transition metals (3d metals < 4d < 5d), cation–π interactions progressively become stronger in the following order: (uncoordinated) benzene < Cp sandwich < benzene sandwich. Aside from interaction energies, the cation–π interactions can be assessed by means of their influence on the geometries of sandwich compounds, which are found to strongly correlate with the strength of cation–π interactions. These results emphasize sandwich compounds, particularly those containing C₆ aromatic rings, as promising candidates for new receptors for common metal cations.