Ammonia synthesis by photocatalytic hydrogenation of a N2-derived molybdenum nitride

Although metal complexes are known to split dinitrogen at ambient temperature and pressure, the synthesis of ammonia from these compounds with H2 as the terminal reductant is rarely achieved. Here we report a photocatalytic ammonia synthesis from a N2-derived terminal molybdenum nitride by using H2 as the terminal reductant. An iridium hydride photocatalyst mediates the reaction on irradiation with blue light. A molybdenum pentahydride was identified as the principal metal product to arise after ammonia release. Conversion of the molybdenum pentahydride back to the terminal molybdenum nitride was accomplished in three steps and completed a synthetic cycle for NH3 formation from N2 and H2. Mechanistic investigations support a pathway that involves photoexcitation of the iridium hydride and a subsequent energy transfer rather than electron transfer. Deuterium labelling confirmed H2 as the source of the N–H bonds. This photodriven, proton-coupled electron transfer allows the use of H2 as the terminal reductant for the catalytic formation of NH3 from N2 using metal catalysts. Although dinitrogen cleavage by metal complexes is known, the subsequent formation of N–H bonds using H2 is thermodynamically challenging. Now, ammonia synthesis using an Ir photocatalyst and H2 for the hydrogenation of a N2-derived molybdenum nitride is reported. The starting molybdenum nitride can be regenerated to complete a synthetic cycle for the preparation of ammonia from N2 and H2.