Frustration Begets Dihydrogen Cleavage

Phosphines and boranes very much like to form Lewis acid-base adducts, but bulky substituent groups can sometimes get in the way and prevent close encounters. Such "frustrated" Lewis pairs, nonetheless, have been shown to facilitate chemical transformations, such as cleaving H₂ under mild conditions. A theoretical study led by Imre Pápai and Tibor Soós at the Chemical Research Center of the Hungarian Academy of Science, Budapest, now illustrates a possible mechanism for how such metal-free H₂ activation might take place (Angew. Chem. Int. Ed. 2008, 47, 2435). The researchers demonstrated that (tert-butyl)₃P and B(C₆F₅)₃ form a flexible, weakly bound complex through C–H···F hydrogen bonds and transient dipole interactions. An H₂ molecule may then enter into the phosphine-borane adduct as shown (P is magenta, B is aqua, H is white). As H₂ interacts with the phosphorus and boron centers, it becomes polarized such that the electron density shifts away from phosphorus and toward boron. This partial electron transfer weakens the H–H bond, which finally dissociates to form [(tert-butyl)₃PH][HB(C₆F₅)₃]. The frustrated phosphine-borane complex, although weakly bound, is a strained system that lowers the activation energy for H₂ cleavage, the authors conclude.