What Limits Turnover Number in NH₃ Synthesis on a PNP Pincer Molecule?

Abstract

Abiological NH₃ synthesis under mild conditions on a PNP [=2,6-bis(di-tert-butyl-phosphinomethyl)pyridine] pincer chelating an Mo(N₂)₃ group has been reported and attributed to a catalytic cycle wherein only the equatorial N₂ is reduced. The present calculations suggest that reduction of axial N₂ ligands was the reason the cycle ended after production of 23 equiv. of NH₃. NH₃ detachment from axial NNH₃ initiates a terminal reaction pathway by leaving an axial nitrido N behind on the Mo. The transinfluence of the strong axial nitride bond causes the second axial N₂ to detach from the Mo, with N-Mo-N₂ remaining as a non-catalytic, pincer-chelated group.

Keywords:

• ammonia
• mild
• pincer
• synthesis
• turnover

ACKNOWLEDGMENTS

I am grateful to colleagues R. A. Kemp, D. R. Wheeler, D. Ingersoll, and S. L. Rempe for helpful discussions, and to G. Henkelman for assistance in implementing the dimer method.

Notes

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