4.8 Article

Formation of the N=N Triple Bond from Reductive Coupling of a Paramagnetic Diruthenium Nitrido Compound

Journal

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 144, Issue 7, Pages 3259-3268

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/jacs.1c13396

Keywords

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Funding

  1. U.S. Department of Energy, Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science [DE-SC0021021]
  2. National Science Foundation [DGE-0718123]
  3. NIH [S10 OD012245]
  4. U.S. Department of Energy (DOE) [DE-SC0021021] Funding Source: U.S. Department of Energy (DOE)

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Construction of nitrogen-nitrogen triple bonds via homocoupling of metal nitrides is a significant reaction relevant to a potential Nitrogen Economy. Room temperature photolysis of Ru-2(chp)(4)N-3 (chp(-) = 2-chloro-6-hydroxypyridinate) in CH2Cl2 produces N-2 via reductive coupling of Ru-2(chp)(4)N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane zigzag geometry instead of the anticipated planar zigzag TS.
Construction of nitrogen-nitrogen triple bonds via homocoupling of metal nitrides is an important fundamental reaction relevant to a potential Nitrogen Economy. Here, we report that room temperature photolysis of Ru-2(chp)(4)N-3 (chp(-) = 2-chloro-6-hydroxypyridinate) in CH2Cl2 produces N-2 via reductive coupling of Ru-2(chp)(4)N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane zigzag geometry instead of the anticipated planar zigzag TS. However, with intentional exclusion of dispersion correction, the planar zigzag TS geometry can also be found. Both the out-of-plane and planar zigzag TS geometries feature two important types of orbital interactions: (1) donor-acceptor interactions involving intermolecular donation of a nitride lone pair into an empty Ru-N pi* orbital and (2) Ru-N p to Ru-N pi* interactions derived from coupling of nitridyl radicals. The relative importance of these two interactions is quantified both at and after the TS. Our analysis shows that both interactions are important for the formation of the N-N sigma bond, while radical coupling interactions dominate the formation of N-N pi bonds. Comparison is made to isoelectronic Ru-2-oxo compounds. Formation of an O-O bond via bimolecular oxo coupling is not observed experimentally and is calculated to have a much higher TS energy. The major difference between the nitrido and oxo systems stems from an extremely large driving force, similar to-500 kJ/mol, for N-N coupling vs a more modest driving force for O-O coupling, -40 to -140 kJ/mol.

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