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Dinitrogen Activation with “Side-On” N 2 Complexes

Dinitrogen Activation with “Side-On” N 2 Complexes. Kristine Fong, Tarun Narayan, Eric Nacsa.

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Dinitrogen Activation with “Side-On” N 2 Complexes

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  1. Dinitrogen Activation with “Side-On” N2 Complexes Kristine Fong, Tarun Narayan, Eric Nacsa This powerpoint presentation was created as part of the requirements for Chemistry 165 "Organometallics" at Harvey Mudd College during the fall semester 2009. Student groups were asked to collaboratively research and prepare an extensive literature review on an organometallic topic. Some of these topics were introduced in class. If so, they were expected to extend beyond the specific metal or system discussed in class. Each presentation went through two rounds of peer review.

  2. A Holy Grail • Need for functionalized nitrogen • Essential component of proteins • Food: 80% of ammonia produced globally is for fertilizers • NH3 production represents 1% of global energy use • Difficult goal • N2 very inert: strong bond, no dipole moment, poor electron donor and acceptor • Drawbacks of current methods • Ammonia from Haber process requires 250 atm, 500º C • Ammonia from nitrogenase requires specific bacteria, lots of energy (16 ATP : 1 N2) and thus food Low value High value

  3. Early N2 Complexes • First side-on N2 complex • Ozin and Vander Voet, 1973 • Matrix-isolated CoN2 • IR: one15N–14N stretch • First N2 complex • Allen & Senoff, 1965 • First side-on N2 crystal structure • Crissey, 1982 • N2 binds η1- and (μ2,η2)- to two Ti2 units

  4. “Side-On” Bonding • Most N2 complexes bond end-on, but side-on geometry gives other ligands access to both N2π* MOs • N2 complex characterization • X-ray crystallography: molecular structure, N–N bond length reflects activation • IR/Raman: N–N stretching frequency reflects activation

  5. Side-On Complexes • More electrophilic early metals often needed • Lower-row metals are larger, have enough space for N2 • Ligands must be finely tuned • Spacing and electronics around N2 binding site strongly affect bonding mode and degree of activation • Metallocenes have received much attention (happen to have correct steric and electronic factors) • Strategies to bind dinitrogen to coordination complexes • KC8, Na(Hg), reductive elimination of alkanes

  6. Functionalization with H • Stoichiometric hydrogenation of dinitrogen to ammonia achieved • Zr and Hf octamethyl metallocenes • Zirconocene performs first hydrogenation at ambient conditions • Need higher temperatures to hydrogenate fully • Hafnocene performs first step faster, but needs higher pressure • Both processes require alkali amalgams (Hg)

  7. Functionalization Beyond H • Carboxylation and alkylation • Organic adduct to nitrogen with alkynes • Full N–N cleavage

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