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, (R-F)(3)B-XR3 and (R-F)(3)Al-XR3 bond dissociation energies (Delta E-DA) are comparable for several R substituents. For R-F = CF3, both are predicted to exhibit M-X Delta E-DA values in the range 55-80 kcal mol(-1), exceptionally strong for dative bond energies. For R-F = C6F5, the Delta E-DA values are predicted to lie in the range 30-45 kcal mol(-1). (F5C6)(3)BP(t-Bu)(3), which does not contain a B-P bond, is predicted to display Delta E-DA = 19 kcal mol(-1). The Delta E-DA energies do not change smoothly as the steric bulk of the phosphine increases. However, intrinsic Delta E-DA energies Delta E-int 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 Delta E-DA. The data indicate that PPh3 is approximately equivalent to PMe3 as a donor in terms of Delta E-int.