Proton to Hydride Umpolung at a Phosphonium Center Via Electron Relay: A New Strategy for Main-Group Based Water Reduction

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry are well known for water activation by heterolytic H-OH bond cleavage, to the best of our knowledge, there is only one case showing water reduction by metal-free FLP systems, in which a silylene (SiII) was used as the Lewis base. This work reports a moleular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shaffling a pair of electrons from the hydroxide (OH–) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new revenues for this main-group chemistry in their growing roles in chemical transformations.