The B-H•••H-P Dihydrogen Bonding in Ion Pair Complexes [(CF3)3BH-][HPH3-n(Me)n+] (n=0-3) and Its Implication in H2 Elimination and Activation Reactions

The B-H delta-center dot center dot center dot H delta+-P dihydrogen bonding (DHB) in ion pair complexes [(CF3)(3)BH-][HPH3-n(Me)(n)(+)] (n = 0-3) and its role in the combination of proton and hydride with the release of H-2 or, reversibly, the heterolytic activation of H-2 by Lewis pairs (CF3)(3)BPH3-n(Me)(n) have been theoretically investigated at the MP2 and DFT levels. It is found that the B-H center dot center dot center dot H-P bonds behave similarly to those in neutral pairs and ion-molecule complexes in most respects, such as the linearity of the H center dot center dot center dot H-P moiety, the characteristics of the electron transfer and rearrangement, and the topological properties of the DHB critical point, except that in certain cases, a blue-shifting of the H-bond vibrational frequency is observed. In [(CF3)(3)BH-][HPH3-n(Me)(n)(+)], the proton shifting within the complexes leads to the formation of the dihydrogen complex B(CF3)(3)(eta(2)-H-2) which is followed by a subsequent H-2 release. The stability of B(CF3)(3)(eta(2)-H-2) (D-e/D-0 = 10.8/6.0 kcal/mol) makes the proton-hydride combination proceed in a fashion similar to the protonation reactions in transition-metal hydrides rather than those in group 13 hydrides EH4- (E = B, Al, Ga). As for the H-2-splitting reaction R3BPR'(3) + H-2 -> [R3BH-][HPR3+], classical Lewis pair (CLP) (CF3)(3)BPH3 exhibits a high barrier and results in an unstable ion pair product [(CF3)(3)BH-][HPH3+] compared with the "frustrated Lewis pair" (FLP) (C6F5)(3)BP(tBu)(3). A detailed analysis of the mechanistic aspects of H-2 activation by (CF3)(3)BPH3 and (C6F5)(3)BP(tBu)(3), supported by another CLP (CF3)(3)BP(tBu)(3) which has a binding energy comparable to (CF3)(3)BPH3 but a reaction exothermicity comparable to (C6F5)(3)BP(tBu)(3), allows us to suggest that the low stability of FLP (C6F5)(3)BP(tBu)(3) is the determining factor for the low reaction barrier. The relative stability and other properties of the ion pair products [R3BH-][HPR'(+)(3)] have also been analyzed. Results strongly Support the view proposed by Rokob et al. [Rokob, T. A.; Hamza, A.; Stirling, A.; Soos, T.; Papai, I. Angew. Chem., Int. Ed. 2008, 47, 2435] that the frustration energy lowers the energy barrier and increases the exothermicity of the reaction.