Catalytic Reduction of Dinitrogen via Hydroboration.

Abstract

The conversion of dinitrogen (N₂) into ammonia (NH₃) or other nitrogenous compounds under mild conditions remains highly desirable, given the extremely harsh conditions required by the typically employed Haber-Bosch process, which reacts N₂ with dihydrogen (H₂) to produce NH₃. However, the catalytic conversion of N₂ using H₂ or related compounds containing H-E bonds (where E represents a general element) as reductants under mild conditions has remained elusive. Herein, we report the catalytic reduction of N₂ via hydroboration, wherein N₂ (1 atm) reacts with catecholborane in the presence of molybdenum-nitride complexes bearing PCP-type pincer ligands as catalysts at 60 °C, yielding up to 32 equiv of borylamines based on the molybdenum atom. The catalytic reaction proceeds through three successive 1,2-additions of hydroborane to the molybdenum-nitrogen bond, disproportionation of the molybdenum-hydride complex, and regeneration of the molybdenum-nitride complex via cleavage of the nitrogen-nitrogen triple bond. The catalytic reduction of N₂ with a "H₂ equivalent" under mild reaction conditions represents a notable advance toward the development of ideal nitrogen fixation systems.