Catalytic conversion of nitrogen to ammonia by an iron model complex

TLDR: Results indicate that a single iron site may be capable of stabilizing the various NxHy intermediates generated during catalytic NH3 formation, and propose that the interstitial carbon atom recently assigned in the nitrogenase cofactor may have a similar role, perhaps by enabling a singleIron site to mediate the enzymatic catalysis through a flexible iron–carbon interaction.

Abstract: Threefold symmetric Fe phosphine complexes have been used to model the structural and functional aspects of biological N2 fixation by nitrogenases. Low-valent bridging Fe-S-Fe complexes in the formal oxidation states Fe(II)Fe(II), Fe(II)/Fe(I), and Fe(I)/Fe(I) have been synthesized which display rich spectroscopic and magnetic behavior. A series of cationic tris-phosphine borane (TPB) ligated Fe complexes have been synthesized and been shown to bind a variety of nitrogenous ligands including N2H4, NH3, and NH2 - . These complexes are all high spin S = 3/2 and display EPR and magnetic characteristics typical of this spin state. Furthermore, a sequential protonation and reduction sequence of a terminal amide results in loss of NH3 and uptake of N2. These stoichiometric transformations represent the final steps in potential N2 fixation schemes. Treatment of an anionic FeN2 complex with excess acid also results in the formation of some NH3, suggesting the possibility of a catalytic cycle for the conversion of N2 to NH3 mediated by Fe. Indeed, use of excess acid and reductant results in the formation of seven equivalents of NH3 per Fe center, demonstrating Fe mediated catalytic N2 fixation with acids and protons for the first time. Numerous control experiments indicate that this catalysis is likely being mediated by a molecular species. A number of other phosphine ligated Fe complexes have also been tested for catalysis and suggest that a hemi-labile Fe-B interaction may be critical for catalysis. Additionally, various conditions for the catalysis have been investigated. These studies further support the assignment of a molecular species and delineate some of the conditions required for catalysis. Finally, combined spectroscopic studies have been performed on a putative intermediate for catalysis. These studies converge on an assignment of this new species as a hydrazido(2-) complex. Such species have been known on group 6 metals for some time, but this represents the first characterization of this ligand on Fe. Further spectroscopic studies suggest that this species is present in catalytic mixtures, which suggests that the first steps of a distal mechanism for N2 fixation are feasible in this system.

Figures

Figure 3.6. Low-temperature X-Band EPR spectra for complexes 3.1-3.6. Conditions: 3.1: Toluene, 8 K; 3.2: 2:1 Toluene:Et2O, 10 K 3.3: 2-MeTHF, 10 K; 3.4: 2-MeTHF, 10 K; 3.5: 2-MeTHF, 10 K; 3.6: Toluene, 10 K.

Figure 6.3. X-Band EPR spectra of (A) 6.1, (B) [(TPB)Fe≡N(C6H4OMe)] +, and (C) [(TPB)Fe≡NAd][BArF4]. Conditions for all: 77 K, 2-MeTHF, (A) 9.38 GHz, (B) 9.40 GHz, (C) 9.42 GHz.

Figure 2.4. (A) Near-IR spectrum of complex 2.2. (B) 4 K X-band EPR spectrum of 2.2. Conditions for (A): THF, 0.013 M. Note that the asterisk denotes a large feature due to

Figure 2.2. XRD structures of complexes 2.1, 2.2, and 2.3 with ellipsoids at 50% and hydrogens omitted for clarity. Fe atoms are shown in orange, S in yellow, P in purple, Na in blue, O in red, and C in white.

Figure 4.1. UV-Visible spectrum of the indophenol dye generated from NH3. This particular absorbance spectrum corresponds to 7.5 equivalents of NH3 generated per Fe center.

Figure 3.4. Reaction kinetics of the thermolysis of 3.3 to 3.4 at 60 °C in a 6:1 mixture of C6D6:THF-d8.

Full text

CATALYTIC CONVERSION OF NITROGEN TO
AMMONIA BY AN IRON MODEL COMPLEX
Thesis by
John S. Anderson
In Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy
California Institute of Technology
Pasadena, CA
2014
(Defended September 3, 2013)

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© 2013
John S. Anderson
All Rights Reserved

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Acknowledgements
Throughout the course of my doctoral studies, I have been blessed to be
surrounded by some of the most intelligent, helpful, and enjoyable people in the world.
Any success that I may have had during my Ph.D. reflects more on the capabilities of the
people around me than on any trait of my own. A large number of people have
contributed to this work either directly in the form of co-workers and collaborators, or
indirectly in the form of people who did not work on the science herein, but without
whom this work would not have been possible. I will do my best to thank all of them.
Firstly, I have to thank Professor Jonas Peters. As many will tell you, Jonas is a
fantastic advisor and I could not have asked for a better mentor for the past five years.
Jonas is far more intelligent than he gives himself credit for and has a unique gift in the
design and implementation of research projects. His approach to scientific problems and
his thought process in designing experiments is something I hope I will take with me as I
leave my doctoral studies. I also have to thank him for the patience he has shown in
developing me as a scientist. I am far from a finished product, but any progress that I
have made is largely a result of his efforts. Aside from his scientific prowess, Jonas has
been a thoroughly helpful and thoughtful person to work with and, in general, an
exceptional human being. Any professor can attest that time comes as a premium, but
Jonas has consistently managed this research lab in a remarkable manner and has been a
pleasure to work with. Although it is difficult to be friends with your advisor during the
course of one’s Ph.D., I have enjoyed my time working for him immensely and look
forward to our future interactions.

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During the move from M.I.T. to CalTech, I spent a brief “sabbatical” in the
laboratories of Professor Greg Fu. Although the joint project I was working on during
that time was an unmitigated disaster, I enjoyed the members of the Fu lab that I met
during that time, some of whom I still talk to today. I also have to thank Greg for letting
me work in his lab, and for our interactions during that time and since. The Fu lab has
since moved to CalTech as well and having them across the hall has been great.
Four graduate students started in the same year as I did, and then there was one. I
find it unlikely that I was the only person during my year that graduated with a Ph.D.
from the Peters lab, considering the exceptionally talented group of students that I came
in with. I would like to thank Laura Gerber, Kenny Lotito, and Alicia Chang for the
social and scientific interactions that we had during our time together. I hope our paths
will cross again in the future.
Outside of my classmates, the Peters lab has been filled with a an outstanding
group of people. I shared a glovebox with Caroline Saouma during my early time in the
lab and I have to thank her for getting me started in the laboratory. Although the time
that we spent together was relatively short, Professor Neal Mankad is an impressive
scientist and continues to host our yearly fantasy football league. Professor Yunho Lee
brought an incredibly cheerful demeanor to the lab every day and I thank him for it. I
would also like to thank Professor Nate Szymczak for getting me started on
electrochemistry and for, despite being a vegan, being an overall good labmate. I did not
interact with Professor Louise Berben very much during my time in the Peters Lab, but I
still have the koala she brought back from Australia for everyone in the lab. Samantha
Macmillan was also a part-time glovebox mate as well as perhaps the best person in the

v
lab to talk to. Charlene Tsay was also a glovebox partner for some time and I have to
thank her for putting up with my mess frequently.
Daniel Suess has been a brilliant, eccentric, and hysterical co-worker throughout
my Ph.D. I have learned a great deal about chemistry through my discussions with Dan
and I have to thank him for all of the help he has given me over the past five years.
Similarly, Professor Hill Harman has brought a unique, and sometimes belligerent,
outlook on chemical problems. In many ways his insights have made me re-evaluate the
way I think about molecules. Dr. Ayumi Takaoka was a good friend before he fled this
country for Japan. Early on in my Ph.D. Ayumi frequently referred to me as being “too
much of a cowboy.” Although I didn’t realize it at the time, I had a lot to learn from
Ayumi about scientific rigor. Fortunately, he had a lot to learn from me about fantasy
football, and the lessons are ongoing. I frequently enjoyed talking to Professor Chris
Uyeda, and his knowledge of synthesis has been a great resource during my doctoral
studies.
Dr. Marc-Etienne Moret was a co-author with me, and is in general more
enthusiastic about chemistry than any other person I know. He thinks deeply about
chemical problems and one can not help but be enlightened by discussions with him. Dr.
Charles McCrory has put up with many hours of my questions about electrochemistry
and other physical methods which are frequently beyond the scope of my knowledge.
Although he is often grumpy, I personally enjoy Charles’ dour moods, and must sincerely
thank him for all of his help. More recently, I have had the pleasure of working with
Dr.’s Yichen Tan and Tzu-Pin Lin, who I have to thank for discussions about chemistry

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Catalytic conversion of nitrogen to ammonia by an iron model complex thesis by john s. anderson in partial fulfillment of the requirements for the degree of doctor of philosophy" ?

In this paper, a number of low-valent bridging Fe-S-Fe complexes with high spin S = 3/2 have been synthesized for N2 reduction. 

Q2. What future works have the authors mentioned in the paper "Catalytic conversion of nitrogen to ammonia by an iron model complex thesis by john s. anderson in partial fulfillment of the requirements for the degree of doctor of philosophy" ?

Although reasonable arguments can be made for both distal and alternating mechanisms, another intriguing possibility is a disproportionation pathway from 6. 1 to generate diazene, hydrazine, and finally NH3. Nevertheless, the identification of further species en route to NH3 formation will be of great interest in future studies and will be required to further elucidate the mechanism of NH3 formation. The disproportionation chemistry shown in Chapter 3 offers further support for the feasibility of such a mechanism. 

Q3. How many equivalents of NH3 are formed per Fe center?

use of excess acid and reductant results in the formation of seven equivalents of NH3 per Fe center, demonstrating Fe mediated catalytic N2 fixation with acids and protons for the first time. 

Q4. How does the second reduction couple behave?

The second reduction couple at -2.09 V versus Fc/Fc+ displays a typical cathodic increase in current upon scanning negatively, but upon reversal of the scan polarity two re-oxidation peaks are observed. 

Q5. What is the steric profile of the (PhBP3) framework?

The steric profile of the (PhBP3) framework allows for substantially bent Fe-X-Fe geometries7c and so the linearity in complexes 2.1-2.3 is also likely indicative of an electronic preference for this geometry such as multiple bonding across the Fe-S-Fe unit. 

Q6. What is the key feature of the tris-phosphine borane set?

In order to combine the N2 binding capabilities and functionalization capabilitiesof the trigonal bipyramidal system with the stability of the pseudo-tetrahedral systems towards π-basic ligands, a new tris-phosphine borane (TPB) ligand set was utilized. 

Q7. What is the main feature of the PhBP3 complexes?

these dimers display interesting reactivity towards small molecule substrates broadly relevant to N2 fixation such as CO, H +, and N2H4.9As was mentioned, a key feature of (PhBP3)Fe complexes was the stabilization ofstrong π-donors in species such as Fe nitrides or imides that would be found along a distal mechanism. 

Q8. What is the description of the two nitrogenous ligands?

The two nitrogenous ligands are best described as a hydrazine and a hydrazido(2-) as indicated by long N-N distances of 1.449(5) and 1.451(4) respectively. 

Q9. What is the average Fe-P distance in the CSD?

While complex 2.1 possesses Fe-P bond distances consistent with other24examples of high-spin Fe(II) from their laboratory,7a a contraction of 0.22 Å in the average Fe-P bond lengths is apparent upon reduction from 2.1 to 2.3, resulting in an exceptionally short average Fe-P bond distance of 2.17 Å in 2.3 (The average Fe-P distance in the CSD is 2.24 Å).8 

Q10. What is the main reason for the lack of information on how N2 reduction is mediated?

Despite the realization of catalysis in the (TPB)Fe system, there remains little tono mechanistic information on how N2 reduction is mediated. 

Q11. What is the way to probe the coupling between the two metal centers?

in the case of mixed-valence complexes like 2.2, one way to probe this coupling has been to examine the line shape of the inter-valence charge transfer (IVCT) band via near-IR spectroscopy. 

Q12. What was the stoichiometric conversion of FeN2 into NH3?

Prior to the studies described herein, even stoichiometric conversion of FeN2 into NH3 was limited to yields of ~ 0.1 equivalents of NH3 per Fe center. 

Q13. What are the parameters of the Fe(I) complexes on PhBP3?

Previously reported Fe(I) complexes on PhBP3 are high-spin, and their Mössbauer parameters do not show good agreement with the parameters found in 2.3, suggesting that this site is not well modeled as a high-spin Fe(I) site, leaving a low-spin Fe(I) site as the most plausible alternative, especially when considering the magnetic and structural data already presented.