Effects of the Aromatic Substitution Pattern in Cation−π Sandwich Complexes

A computational study investigating the effects of the aromatic substitution pattern on the structure and binding energies of cation−π sandwich complexes is reported. The correlation between the binding energies (E bind) and Hammett substituent constants is approximately the same as what is observed for cation−π half-sandwich complexes. For cation−π sandwich complexes where both aromatics contain substituents the issue of relative conformation is a possible factor in the strength of the binding; however, the work presented here shows the E bind values are approximately the same regardless of the relative conformation of the two substituted aromatics. Finally, recent computational work has shown conflicting results on whether cation−π sandwich E bind values (E bind,S) are approximately equal to twice the respective half-sandwich E bind values (E bind,HS), or if cation−π sandwich E bind,S values are less than double the respective half-sandwich E bind,HS values. The work presented here shows that for cation−π sandwich complexes involving substituted aromatics the E bind,S values are less than twice the respective half-sandwich E bind,HS values, and this is termed nonadditive. The extent to which the cation−π sandwich complexes investigated here are nonadditive is greater for B3LYP calculated values than for MP2 calculated values and for sandwich complexes with electron-donating substituents than those with electron-withdrawing groups.