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-pi 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-pi half-sandwich complexes. For cation-pi 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-pi sandwich E-bind values (E-bind,E-S) are approximately equal to twice the respective half-sandwich E-bind values (E-bind,E-HS), or if cation-pi sandwich E-bind,E-S values are less than double the respective half-sandwich E-bind,E-HS values. The work presented here shows that for cation-pi sandwich complexes involving substituted aromatics the E-bind,E-S values are less than twice the respective half-sandwich E-bind,E-HS values, and this is termed nonadditive. The extent to which the cation-pi 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.