Donor-Acceptor Dissociation Energies of Group 13-15 Donor-Acceptor Complexes Containing Fluorinated Substituents: Approximate Lewis Acidities of (F3C)3M vs (F5C6)3M and the Effects of Phosphine Steric Bulk.

To study donor-acceptor complexes containing fluoroalkyl and -aryl substituents on the acceptors, ONIOM methods for optimizing large complexes and determining single point energies were tested. A two-layer ONIOM optimization procedure utilizing the MPW1K model followed by single point calculations using the composite three-layer ONIOM G2R3 method proved acceptable. The optimization model predicts M-X bond distances well when compared to experiment and shows that the distances increase discontinuously with the bulk of the phosphine. Unexpectedly, (RF)3B-XR3 and (RF)3Al-XR3 bond dissociation energies (ΔEDA) are comparable for several R substituents. For RF = CF3, both are predicted to exhibit M-X ΔEDA values in the range 55-80 kcal mol(-1), exceptionally strong for dative bond energies. For RF = C6F5, the ΔEDA values are predicted to lie in the range 30-45 kcal mol(-1). (F5C6)3BP(t-Bu)3, which does not contain a B-P bond, is predicted to display ΔEDA = 19 kcal mol(-1). The ΔEDA energies do not change smoothly as the steric bulk of the phosphine increases. However, intrinsic ΔEDA energies ΔEint show a regular increase as the donor ability of the phosphine increases, confirming that the reorganization energy of the individual moieties contributes sizably to the overall ΔEDA. The data indicate that PPh3 is approximately equivalent to PMe3 as a donor in terms of ΔEint.