N–H Bond Activation\nof Ammonia by a Redox-Active\nCarboranyl Diphosphine

In this work, we report the room-temperature N–H\nbond activation\nof ammonia by the carboranyl diphosphine 1-P<i><sup>t</sup></i>Bu<sub>2</sub>-2-P<i><sup>i</sup></i>Pr<sub>2</sub>-<i>closo</i>-C<sub>2</sub>B<sub>10</sub>H<sub>10</sub> (<b>1</b>) resulting in the formation of zwitterionic 7-P(NH<sub>2</sub>)<i><sup>t</sup></i>Bu<sub>2</sub>-10-P(H)<i><sup>i</sup></i>Pr<sub>2</sub>-<i>nido</i>-C<sub>2</sub>B<sub>10</sub>H<sub>10</sub> (<b>2</b>). Unlike the other phosphorus-based ambiphiles\nthat require geometric constraints to enhance electrophilicity, the\nnew mode of bond activation in this main-group system is based on\nthe cooperation between electron-rich trigonal phosphine centers and\nthe electron-accepting carborane cluster. As an exception among many\nother metal-based and metal-free systems, the N–H bond activation\nof gaseous ammonia or aqueous ammonium hydroxide by carboranyl diphosphine <b>1</b> proceeds with tolerance of air and water. Mechanistic details\nof ammonia activation were explored computationally by DFT methods,\ndemonstrating an electrophilic activation of ammonia by the phosphine\ncenter. This process is driven by the reduction of the boron cluster\nfollowed by an ammonia-assisted deprotonation and proton transfer.\nA subsequent reaction of <b>2</b> and TEMPO results in the cleavage\nof all N–H and P–H bonds with the formation of a cyclic\nphosphazenium cation supported by an anionic cluster N(7-P<i><sup>t</sup></i>Bu<sub>2</sub>-8-P<i><sup>i</sup></i>Pr<sub>2</sub>)-<i>nido</i>-C<sub>2</sub>B<sub>9</sub>H<sub>10</sub> (<b>3</b>). Transformations reported herein represent\nthe first example of ammonia oxidation via triple hydrogen atom abstraction\nfacilitated by a metal-free system.