Synthesis of [(DPPNCH₂CH₂)₃N]³⁻ Molybdenum Complexes (DPP = 3,5-(2,5-Diisopropylpyrrolyl)₂C₆H₃) and Studies Relevant to Catalytic Reduction of Dinitrogen

Molybdenum complexes that contain a new TREN-based ligand [(3,5-(2,5-diisopropyl-pyrrolyl)2C6H3NCH2CH2)3N]3− ([DPPN3N]3−) that are relevant to the catalytic reduction of dinitrogen have been prepared. They are [Bu4N]{[DPPN3N]MoN2}, [DPPN3N]MoN2, [DPPN3N]MoNNH, {[DPPN3N]MoNNH2}[BArf4], [DPPN3N]MoN, {[DPPN3N]MoNH}[BArf4], and {[DPPN3N]MoNH3}[BArf4]. NMR and IR data for [Bu4N]{[DPPN3N]MoN2} and [DPPN3N]MoN2 are close to those reported for the analogous [HIPTN3N]3− compounds (HIPT = hexaisopropylterphenyl), which suggests that the degree of reduction of dinitrogen is virtually identical in the two systems. However, X-ray studies and several exchange studies support the conclusion that the apical pocket is less protected in [DPPN3N]Mo complexes than in [HIPTN3N]Mo complexes. For example, 15N/14N exchange studies showed that exchange in [DPPN3N]MoN2 is relatively facile (t1/2 ≈ 1 h at 1 atm) and depends upon dinitrogen pressure, in contrast to the exchange in [HIPTN3N]MoN2. Several of the [DPPN3N]Mo complexes, e.g., the [DPPN3N]MoN2 and [DPPN3N]MoNH3 species, are also less stable in solution than the analogous “parent” [HIPTN3N]Mo complexes. Four attempted catalytic reductions of dinitrogen with [DPPN3N]MoN yielded 2.53 ± 0.35 equiv of total ammonia. These studies reveal more than any other just how sensitive a successful catalytic reduction is to small changes in the triamidoamine supporting ligand.