Synthesis of DiamidoPyrrolyl Molybdenum Complexes Relevant to Reduction of Dinitrogen to Ammonia

摘要

A potentially useful trianionic ligand for the reduction of dinitrogen catalytically by molybdenum complexes is one in which one of the arms in a [(RNCH2CH2)3N]³⁻ ligand is replaced by a 2-mesitylpyrrolyl-α-methyl arm, i.e., [(RNCH2CH2)2NCH2(2-MesitylPyrrolyl)]³⁻ (R = C6F5, 3,5-Me2C6H3, or 3,5-t-Bu2C6H3). Compounds have been prepared that contain the ligand in which R = C6F₅ ([C₆F₅N)₂Pyr]³⁻); they include [(C₆F₅N)₂Pyr]Mo(NMe₂), [(C₆F₅N)₂Pyr]MoCl, [(C₆F₅N)₂Pyr]MoOTf, and [(C₆F₅N)₂Pyr]MoN. Compounds that contain the ligand in which R = 3,5-t-Bu₂C₆H₃ ([Ar^t-BuN)₂Pyr]³⁻) include {[(Ar^t-BuN)₂Pyr]Mo(N₂)}Na(15-crown-5), {[(Ar^t-Bu N)₂Pyr]Mo(N₂)}[NBu₄], [(Ar^t-Bu N)₂Pyr]Mo(N₂) (ν_NN = 2012 cm⁻¹ in C₆D₆), {[(Ar^t-Bu N)₂Pyr]Mo(NH₃)}BPh₄, and [(Ar^t-Bu N)₂Pyr]Mo(CO). X-ray studies are reported for [(C₆F₅N)₂Pyr]Mo(NMe₂), [(C₆F₅N)₂Pyr]MoCl, and [(Ar^t-BuN)₂Pyr]MoN. The [(Ar^t-BuN)₂Pyr]Mo(N₂)^0/− reversible couple is found at −1.96 V (in PhF versus Cp₂Fe^+/0), but the [(Ar^t-BuN)₂Pyr]Mo(N₂)^+/0 couple is irreversible. Reduction of {[(Ar^t-BuN)₂Pyr]Mo(NH₃)}BPh₄ under Ar at approximately −1.68 V at a scan rate of 900 mV/sec is not reversible. Ammonia in [(Ar^t-BuN)₂Pyr]Mo(NH₃) can be substituted for dinitrogen in about 2 hours if 10 equivalents of BPh₃ are present to trap the ammonia that is released. [(Ar^t-BuN)₂Pyr]Mo-N=NH is a key intermediate in the proposed catalytic reduction of dinitrogen that could not be prepared. Dinitrogen exchange studies in [(Ar^t-BuN)₂Pyr]Mo(N₂) suggest that steric hindrance by the ligand may be insufficient to protect decomposition of [(Ar^t-BuN)₂Pyr]Mo-N=NH through a variety of pathways. Three attempts to reduce dinitrogen catalytically with [(Ar^t-BuN)₂Pyr]Mo(N) as a “catalyst” yielded an average of 1.02 ± 0.12 equivalents of NH₃.