Lithium and sodium cation binding of cyclopentadienyl anions: Electronic effects of cyclopentadienyl substitution

摘要

The Li+ and Na+ binding of substituted cyclopentadienyl (Cp) anions were investigated using computational techniques. The ring centroid-metal distances and the binding energies of the Cp-metal complexes correlate very well with the Sigma sigma(m) of the substituted Cp ring. These properties also correlate well with the Cp Theta(zz) values. The trend in the correlations is the more electron-rich the Cp (negative Sigma sigma(m) and Theta(zz) values values), the shorter the Cp-metal bond and the stronger the binding energy. The NBO metal charges correlate, though not very well in either case, with the Cp Theta(zz) and Sigma sigma(m) values. However, there is a substantial increase in correlation when the sum of the absolute value of the Hammett sigma(m) (Sigma vertical bar sigma(m)vertical bar) is employed. The significantly improved correlation when the Sigma vertical bar sigma(m)vertical bar values are employed leads us to propose a model for substituted Cp charge transfer upon Li+ or Na+ complexation, and it also informs us that the Hammett substituent constant sigma(m) contains information about substituent polarizabilities, at least in the case of Li+- and Na+-substituted Cp anions.