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.
Created by Tarun Narayan (Tarun Narayan <tnarayan@hmc.edu>), Kristine Fong and Eric Nacsa and posted on VIPEr on December 2009. Copyright Tarun Narayan 2009. This work is licensed under the Creative Commons Attribution Noncommercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
A Holy Grail
Low value
High value
• 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
Created by Tarun Narayan (Tarun Narayan <tnarayan@hmc.edu>), Kristine Fong and Eric Nacsa and posted on VIPEr on December 2009. Copyright Tarun Narayan 2009. This work is licensed under the Creative Commons Attribution Noncommercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
Early N2 Complexes
First N2 complex
• Allen & Senoff, 1965
First side-on N2 complex
• Ozin and Vander Voet, 1973
• Matrix-isolated CoN2
• IR: one 15N–14N stretch
First side-on N2 crystal structure
• Crissey, 1982
• N2 binds η1- and (μ2,η2)- to two Ti2 units
Created by Tarun Narayan (Tarun Narayan <tnarayan@hmc.edu>), Kristine Fong and Eric Nacsa and posted on VIPEr on December 2009. Copyright Tarun Narayan 2009. This work is licensed under the Creative Commons Attribution Noncommercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
“Side-On” Bonding
• Most N2 complexes bond end-on, but side-on geometry
gives other ligands access to both N2 π* MOs
• N2 complex characterization
– X-ray crystallography: molecular structure, N–N bond length
reflects activation
– IR/Raman: N–N stretching frequency reflects activation
Created by Tarun Narayan (Tarun Narayan <tnarayan@hmc.edu>), Kristine Fong and Eric Nacsa and posted on VIPEr on December 2009. Copyright Tarun Narayan 2009. This work is licensed under the Creative Commons Attribution Noncommercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
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
Created by Tarun Narayan (Tarun Narayan <tnarayan@hmc.edu>), Kristine Fong and Eric Nacsa and posted on VIPEr on December 2009. Copyright Tarun Narayan 2009. This work is licensed under the Creative Commons Attribution Noncommercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
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)
Created by Tarun Narayan (Tarun Narayan <tnarayan@hmc.edu>), Kristine Fong and Eric Nacsa and posted on VIPEr on December 2009. Copyright Tarun Narayan 2009. This work is licensed under the Creative Commons Attribution Noncommercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
Functionalization Beyond H
• Carboxylation and alkylation
• Organic adduct to nitrogen with alkynes
• Full N–N cleavage
Created by Tarun Narayan (Tarun Narayan <tnarayan@hmc.edu>), Kristine Fong and Eric Nacsa and posted on VIPEr on December 2009. Copyright Tarun Narayan 2009. This work is licensed under the Creative Commons Attribution Noncommercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.