Hydrosilylation of a Molecular Molybdenum Nitride Provides Mechanistic Insights into Photodriven Ammonia Synthesis from N 2 and H 2

Addition of Ph2SiH2 to (depe)2Mo(N)BArF4 (depe = 1,2-bis(diethylphosphino)ethane, BArF4 = B(3,5-(CF3)2C6H3)4) at 60 °C generated the silyl imido molybdenum hydride complex, trans-(depe)2Mo(NSiHPh2)HBArF4, a surrogate for a proposed intermediate complex in the photodriven hydrogenation to free ammonia. Irradiation of a THF solution of trans-(depe)2Mo(NSiHPh2)HBArF4 with blue light under H2 produced free amine along with (depe)2MoH5BArF4 in 76% yield. This transformation occurred in the absence of a precious metal photocatalyst, suggesting that it was needed only for the initial addition of H2 to the molybdenum nitride during the first N-H bond-forming step in the photodriven hydrogenation. Deuterium labeling and crossover studies support concerted Si-H bond addition across the Mo≡N bond, enabled by the nucleophilicity of the nitride. Subsequent hydrogenation involves an intramolecular H migration from Mo to the imido ligand, as supported by electronic absorption spectroscopy, transient absorption spectroscopy, initial rate measurements, and deuterium kinetic isotope effect measurements. These findings provide insights into the photodriven hydrogenation of (depe)2Mo(N)BArF4 to ammonia and the role of the photocatalyst in this transformation.