Issue Date: March 17, 2008
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.
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