N–H Bond Activation of Ammonia by a Redox-Active Carboranyl Diphosphine

In this work, we report the room-temperature N-H bond activation of ammonia by the carboranyl diphosphine 1-P<i>^t</i>Bu₂-2-P<i>ⁱ</i>Pr₂-<i>closo</i>-C₂B₁₀H₁₀ (<b>1</b>) resulting in the formation of zwitterionic 7-P(NH₂)<i>^t</i>Bu₂-10-P(H)<i>ⁱ</i>Pr₂-<i>nido</i>-C₂B₁₀H₁₀ (<b>2</b>). Unlike the other phosphorus-based ambiphiles that require geometric constraints to enhance electrophilicity, the new mode of bond activation in this main-group system is based on the cooperation between electron-rich trigonal phosphine centers and the electron-accepting carborane cluster. As an exception among many other metal-based and metal-free systems, the N-H bond activation of gaseous ammonia or aqueous ammonium hydroxide by carboranyl diphosphine <b>1</b> proceeds with tolerance of air and water. Mechanistic details of ammonia activation were explored computationally by DFT methods, demonstrating an electrophilic activation of ammonia by the phosphine center. This process is driven by the reduction of the boron cluster followed by an ammonia-assisted deprotonation and proton transfer. A subsequent reaction of <b>2</b> and TEMPO results in the cleavage of all N-H and P-H bonds with the formation of a cyclic phosphazenium cation supported by an anionic cluster N(7-P<i>^t</i>Bu₂-8-P<i>ⁱ</i>Pr₂)-<i>nido</i>-C₂B₉H₁₀ (<b>3</b>). Transformations reported herein represent the first example of ammonia oxidation via triple hydrogen atom abstraction facilitated by a metal-free system.