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.