Nitrogenase and Nitrogen Activation

All living beings need nitrogen for key molecules like DNA, RNA, and proteins, and this nitrogen originally comes from N2. Even though N2 constitutes the majority of the atmosphere, our bodies are unable to use it directly, and we depend on special routes for nitrogen reduction. In nature, nitrogen reduction to ammonia (NH3) is catalyzed by the iron-molybdenum cofactor (FeMoco: Figure 1) of nitrogenase enzymes in microorganisms. In industry, iron/alkali metal surfaces are used (Haber-Bosch process). In both cases, N2 reduction is proposed to take place on iron sites. However, mechanistic details remain speculative because of the complexity of the catalytic systems.

Figure 1

Our group uses synthetic molecules for which the structures are known in detail, in order to provide answers to fundamental questions about the nitrogen fixation process. For example, how important are the different components of the FeMoco like sulfides, carbide, hydrides, nearby amino acid residues, and metal-metal interactions? What is the mechanism through which N2 is reduced?

A key hypothesis that guides our work is that the FeMoco has reactive iron atoms. To evaluate this idea, we synthesized soluble complexes of low-coordinate iron and cobalt that reduce the NN bond order of N2 from three to two (Figure 2 and Figure 3), and showed the relationship between the metal’s properties and the NN bond weakening. We also discovered that three or more iron atoms can cooperate with potassium to break the NN triple bond in N2  (Figure 4). This was the first example of an iron complex that breaks the bond of molecular N2 to give nitrides. Reaction of acids with the system produces ammonia. 

Figure 2

Figure 3

Figure 4

In another important facet of our work, we are testing the idea that the reactivity of the FeMoco iron atoms is a result of the S and C atoms that surround them. Therefore, we are incorporating S and C donors that mimic the environment of the iron atoms in the FeMoco (Figure 1 above). For example, we have prepared synthetic complexes in which two iron atoms are bridged by sulfides, and they break the NN bond in certain hydrazines (Figure 5). More recently, we have discovered that complexes with carbon-based bridges can reduce N2 to ammonia (Figure 6). We have also designed new supporting ligands that coordinate through only sulfur and carbon, providing N2 complexes that more closely mimic Fe-N2 species on FeMoco (Figure 7). Our continuing synthetic efforts are focusing on preparation of new complexes that test the influence of multimetallic interactions, carbides, hydrides, and sulfur donors.

Figure 5

Figure 6

Figure 7

Current funding: National Institutes of Health, R01 GM065313, “Low-Coordinate Synthetic Models for Nitrogenase Activity.”

Relevant Publications

Gannon P. Connor, Daniel Delony, Jeremy E. Weber, Brandon Q. Mercado, Julia B. Curley, Sven Schneider, James M. Mayer, Patrick L. Holland
Facile conversion of ammonia to a nitride in a rhenium system that cleaves dinitrogen
Chem. Sci.  202213, 4010-4018.  
Kazimer L. Skubi, Reagan X. Hooper, Brandon Q. Mercado, Melissa M. Bollmeyer, Samantha N. MacMillan, Kyle M. Lancaster, Patrick L. Holland
Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure
Inorg. Chem.  202261, 1644-1658.  
Jeremy E. Weber, Samuel M. Bhutto, Alexandre T.-Y. Genoux, Patrick L. Holland
Dinitrogen Binding and Functionalization  
Comprehensive Organometallic Chemistry IV   , in press.  
Lynn S. Yamout, Mohamad Ataya, Faraj Hasanayn, Patrick L. Holland, Alexander J. M. Miller, Alan S. Goldman
Understanding Terminal versus Bridging End-on N2 Coordination in Transition Metal Complexes    
J. Am. Chem. Soc.  2021143, 9744–9757.  
Daniel W. N. Wilson, Patrick L. Holland
Nitrogenases and model complexes in bioorganometallic chemistry      
Comprehensive Organometallic Chemistry IV  , in press.  
Jeremy E. Weber, Faraj Hasanayn, Majed S. Fataftah, Brandon Q. Mercado, Robert H. Crabtree, Patrick L. Holland
Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System      
Inorg. Chem.  202160, 6115-6124.  
John W. Peters, Oliver Einsle, Dennis R. Dean, Serena DeBeer, Brian M. Hoffman, Patrick L. Holland, Lance C. Seefeldt
Comment on “Structural evidence for a dynamic metallocofactor during N2 reduction by Mo-nitrogenase”
Science  2021371, eabe5481.  
Amy L. Speelman, Kazimer L. Skubi, Brandon Q. Mercado, Patrick L. Holland
Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand
Inorg. Chem.  202160, 1965-1974.  
Quinton J. Bruch, Gannon P. Connor, Noah D. McMillion, Alan S. Goldman, Faraj Hasanayn, Patrick L. Holland, Alexander J. M. Miller
Considering Electrocatalytic Ammonia Synthesis via Bimetallic Dinitrogen Cleavage
ACS Catal.  202010, 10826–10846.  
Alexandra L. Nagelski, Majed S. Fataftah, Melissa M. Bollmeyer, Sean F. McWilliams, Samantha N. MacMillan, Brandon Q. Mercado, Kyle M. Lancaster, Patrick L. Holland
The Influences of Carbon Donor Ligands on Biomimetic Multi-Iron Complexes for N2 Reduction
Chem. Sci.  202011, 12710-12720.  Tenth anniversary issue
Patrick L. Holland
Introduction: Reactivity of Nitrogen from the Ground to the Atmosphere
Chem. Rev.  2020120, 4919-4920.  Guest Editor for Thematic Issue of Chemical Reviews
Gal Hochman, Alan S. Goldman, Frank A. Felder, James M. Mayer, Alexander J. M. Miller, Patrick L. Holland, Leo A. Goldman, Patricia Manocha, Ze Song, Saketh Aleti
The Potential Economic Feasibility of Direct Electrochemical Nitrogen Reduction as a Route to Ammonia
ACS Sustainable Chem. Eng.  20208, 8938-8948.  
Quinton J. Bruch, Gannon P. Connor, Chun-Hsing Chen, Patrick L. Holland, James M. Mayer, Faraj Hasanayn, Alexander J. M. Miller
Dinitrogen Reduction to Ammonium at Rhenium Utilizing Light and Proton-Coupled Electron Transfer
J. Am. Chem. Soc.  2019141, 20198-20208.  (Spotlight)
Amy L. Speelman, Ilija Čorić, Casey Van Stappen, Serena DeBeer, Brandon Q. Mercado, Patrick L. Holland
Nitrogenase-Relevant Reactivity of a Synthetic Iron-Sulfur-Carbon Site
J. Am. Chem. Soc.  2019141, 13148-13157.  
Daniel E. DeRosha, Vijay G. Chilkuri, Casey Van Stappen, Eckhard Bill, Brandon Q. Mercado, Serena DeBeer, Frank Neese, Patrick L. Holland
Planar three-coordinate iron sulfide in a synthetic [4Fe-3S] cluster with biomimetic reactivity  
Nature Chem.  201911, 1019-1025.  
Daniel E. DeRosha, Nicholas A. Arnet, Brandon Q. Mercado, and Patrick L. Holland
A [2Fe-1S] Complex that Affords Access to Bimetallic and Higher Nuclearity Iron-Sulfur Clusters
Inorg. Chem.  201958, 8829-8834.  
Samuel M. Bhutto, Patrick L. Holland
Dinitrogen Activation and Functionalization Using Beta-Diketiminate Iron Complexes
Eur. J. Inorg. Chem.  2019, 1861-1869.  
Sean F. McWilliams, Philip C. Bunting, Venkatesan Kathiresan, Brandon Q. Mercado, Brian M. Hoffman, Jeffrey R. Long, Patrick L. Holland
Isolation and Characterization of a High-Spin Mixed-Valent Iron Dinitrogen Complex
Chem. Commun.  201854, 13339-13342.  
Sean F. McWilliams, Eckhard Bill, Gudrun Lukat-Rodgers, Kenton R. Rodgers, Brandon Q. Mercado, Patrick L. Holland
Effects of NBinding Mode on Iron-Based Functionalization of Dinitrogen to Form an Iron(III) Hydrazido Complex  
J. Am. Chem. Soc.  2018140, 8586–8598.  
Kazimer L. Skubi, Patrick L. Holland
So Close, Yet Sulfur Away: Opening the Nitrogenase Cofactor Structure Creates a Binding Site
Biochemistry  201857, 3540-3541.  
Vladimir Pelmenschikov, Leland B. Gee, Hongxin Wang, K. Cory MacLeod, Sean F. McWilliams, Kazimer L. Skubi, Stephen P. Cramer, Patrick L. Holland
High-Frequency Fe-H Vibrations in a Bridging Hydride Complex Characterized by NRVS and DFT  
Angew. Chem. Int. Ed.  201857, 9367-9371.  
J. G. Chen, R. M. Crooks, L. C. Seefeldt, K. L. Bren, R. M. Bullock, M. Y. Darensbourg, P. L. Holland, M. J. Janik, A. K. Jones, M. G. Kanatzidis, P. King, K. M. Lancaster, S. V. Lymar, P. Pfromm, W. F. Schneider, R. R. Schrock
Beyond fossil-fuel-driven nitrogen transformations  
Science  2018360, 873.  
Daniel E. DeRosha, Patrick L. Holland
Incorporating light atoms into synthetic analogues of FeMoco  
Proc. Natl. Acad. Sci. USA  2018115, 5054-5056.  
Daniël L. J. Broere, Patrick L. Holland
Boron compounds tackle dinitrogen
Science  2018359, 871.  (reprint)
Gannon P. Connor, Nicholas Lease, Andrea Casuras, Alan S. Goldman, Patrick L. Holland, James M. Mayer
Protonation and electrochemical reduction of rhodium- and iridium-dinitrogen complexes in organic solution
Dalton Trans.  201746, 14325-14330.  
Nicholas A. Arnet, Sean F. McWilliams, Daniel E. DeRosha, Brandon Q. Mercado, Patrick L. Holland
Synthesis and Mechanism of Formation of Hydride-Sulfide Complexes of Iron  
Inorg. Chem.  201756, 9185-9193.  
Masaki Horitani, Katarzyna Grubel, Sean F. McWilliams, Bryan D. Stubbert, Brandon Q. Mercado, Ying Yu, Prabhuodeyara M. Gurubasavaraj, Nicholas S. Lees, Patrick L. Holland, Brian M. Hoffman
ENDOR Characterization of an Iron-Alkene Complex Provides Insight into a Corresponding Organometallic Intermediate of Nitrogenase
Chem. Sci.  20178, 5941-5948.  
Amy M. Speelman, Patrick L. Holland
Sulfur-Supported Iron Complexes for Understanding N2 Reduction  
In: Nishibayashi Y. (eds) Nitrogen Fixation.  2017Topics in Organometallic Chemistry, vol 60. Springer, New York, 197-213.  
Gannon P. Connor, Patrick L. Holland
Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction
Catalysis Today  2017286, 21-40.  
Sarina M. Bellows, Nicholas A. Arnet, Prabhuodeyara M. Gurubasavaraj, William W. Brennessel, Eckhard Bill, Thomas R. Cundari, Patrick L. Holland
The Mechanism of N-N Double Bond Cleavage by an Iron(II)-Hydride Complex
J. Am. Chem. Soc.  2016138, 12112–12123.  
K. Cory MacLeod, Fabian S. Menges, Sean F. McWilliams, Stephanie M. Craig, Brandon Q. Mercado, Mark A. Johnson, Patrick L. Holland
Alkali-Controlled C-H Cleavage or N-C Bond Formation by N2-Derived Iron Nitrides and Imides
J. Am. Chem. Soc.  2016138, 11185-11191.  
K. Cory MacLeod, Sean F. McWilliams, Brandon Q. Mercado, Patrick L. Holland
Stepwise N-H Bond Formation From N2-Derived Iron Nitride, Imide and Amide Intermediates to Ammonia
Chem. Sci.  20167, 5736-5746.  
Ilija Čorić, Patrick L. Holland
Insight into the FeMoco of nitrogenase from synthetic iron complexes with sulfur, carbon, and hydride ligands
J. Am. Chem. Soc.  2016138, 7200-7211.  
Sean F. McWilliams, Kenton R. Rodgers, Gudrun Lukat-Rodgers, Brandon Q. Mercado, Katarzyna Grubel, Patrick L. Holland
Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging Diiron Dinitrogen Complexes
Inorg. Chem.  201655, 2960-2968.  
Nicholas A. Arnet, Thomas R. Dugan, Fabian S. Menges, Brandon Q. Mercado, William W. Brennessel, Eckhard Bill, Mark A. Johnson, Patrick L. Holland
Synthesis, Characterization, and Nitrogenase-Relevant Reactions of an Iron Sulfide Complex with a Bridging Hydride  
J. Am. Chem. Soc.  2015137, 13220-13223.  (Highlight in Nature Chemical Biology)
Ilija Čorić, Brandon Q. Mercado, Eckhard Bill, David J. Vinyard, Patrick L. Holland
Binding of dinitrogen to an iron–sulfur–carbon site
Nature  2015526, 96-99.  
Sean F. McWilliams, Patrick L. Holland
Dinitrogen Binding and Cleavage by Multinuclear Iron Complexes
Acc. Chem. Res.  201548, 2059–2065.  
Megan E. Reesbeck, Meghan M. Rodriguez, William W. Brennessel, Brandon Q. Mercado, David J. Vinyard, Patrick L. Holland
Oxidized and Reduced [2Fe-2S] Clusters from an Iron(I) Synthon
J. Biol. Inorg. Chem.  201520, 875-883.  
Katarzyna Grubel, William W. Brennessel, Brandon Q. Mercado, Patrick L. Holland
Alkali Metal Control over N−N Cleavage in Iron Complexes
J. Am. Chem. Soc.  2014136, 16807-16816.  
K. Cory MacLeod, David J. Vinyard, Patrick L. Holland
A Multi-iron System Capable of Rapid N2 Formation and N2 Cleavage
J. Am. Chem. Soc.  2014136, 10226-10229.  
Thomas R. Dugan, Eckhard Bill, K. Cory MacLeod, William W. Brennessel, Patrick L. Holland
Synthesis, Spectroscopy and Hydrogen/Deuterium Exchange in High-Spin Iron(II) Hydride Complexes
Inorg. Chem.  201453, 2370-2380.  
Karen P. Chiang, Sarina M. Bellows, William W. Brennessel, Patrick L. Holland
Multimetallic cooperativity in activation of dinitrogen at iron-potassium sites
Chem. Sci.  20145, 267-274.  
Wenwen Yao, Prabhuodeyara M. Gurubasavaraj, Patrick L. Holland
All-Ferrous Iron-Sulfur Clusters
Struct. Bonding  2014160, 1-37.  
Christopher J. Pollock, Katarzyna Grubel, Patrick L. Holland, Serena DeBeer
Experimentally Quantifying Small Molecule Bond Activation Using Valence-to-Core X-ray Emission Spectroscopy
J. Am. Chem. Soc.  2013135, 11803-11808.  
Thomas R. Dugan, K. Cory MacLeod, William W. Brennessel, Patrick L. Holland
Cobalt-Magnesium and Iron-Magnesium Complexes with Weakened Dinitrogen Bridges
Eur. J. Inorg. Chem.  2013, 3891-3897.  
Bryan D. Stubbert, Javier Vela, William W. Brennessel, Patrick L. Holland
A Sulfide-Bridged Diiron(II) Complex with a cis-N2H4 Ligand  
Z. Anorg. Allg. Chem.  2013639, 1351-1355.  
K. Cory MacLeod, Patrick L. Holland
Recent Developments in Homogeneous Dinitrogen Reduction by Molybdenum and Iron
Nature Chem.   20135, 559-565.  
Meghan M. Rodriguez, Bryan D. Stubbert, Christopher C. Scarborough, William W. Brennessel, Eckhard Bill, Patrick L. Holland
Isolation and Characterization of Stable Iron(I)-Sulfide Complexes
Angew. Chem. Int. Ed.   201251, 8246-8520.  
Travis M. Figg, Patrick L. Holland, Thomas R. Cundari
Cooperativity Between Low-Valent Iron and Potassium Promoters in Dinitrogen Fixation
Inorg. Chem.   201251, 7546-7550.  
Katarzyna Grubel, Patrick L. Holland
New Iron-Sulfur Clusters Help Hydrogenases Tolerate Oxygen
Angew. Chem. Int. Ed.   201251, 3308-3310.  
Meghan M. Rodriguez, Eckhard Bill, William W. Brennessel, Patrick L. Holland
N2 Reduction and Hydrogenation to Ammonia by a Molecular Iron-Potassium Complex
Science  2011334, 780-783.  
Mario U. Delgado-Jaime, Benjamin R. Dible, Karen P. Chiang, William W. Brennessel, Patrick L. Holland, Uwe Bergmann, Serena DeBeer
Identification of Light Atoms within Multinuclear Metal Clusters using Valence-to-Core X-Ray Emission Spectroscopy  
Inorg. Chem.  201150, 10709-10717.  
Patrick L. Holland
Techniques Used in Functional and Structural Modeling of Nitrogenase
Methods in Molecular Biology - Nitrogen Fixation   2011(M. Ribbe, ed.), Springer: New York, 249-263.  
Patrick L. Holland
Metal-Dioxygen and Metal-Dinitrogen Complexes: Where Are The Electrons?
Dalton Trans.  201039, 5415-5425.  
Keying Ding, William W. Brennessel, and Patrick L. Holland
Three-Coordinate and Four-Coordinate Cobalt Hydride Complexes That React with Dinitrogen
J. Am. Chem. Soc.  2009131, 10804-10805.  Correction
Keying Ding, Aaron W. Pierpont, William W. Brennessel, Gudrun Lukat-Rodgers, Kenton R. Rodgers, Thomas R. Cundari, Eckhard Bill and Patrick L. Holland
Cobalt-Dinitrogen Complexes with Weakened N-N Bonds
J. Am. Chem. Soc.   2009131, 9471-9472.  
Thomas R. Dugan, Patrick L. Holland
New Routes to Low-Coordinate Iron Hydride Complexes: The Binuclear Oxidative Addition of H2
J. Organomet. Chem.   2009694, 2825-2830.  
Patrick L. Holland
Nitrogen Fixation
McGraw-Hill Yearbook of Science & Technology  2009, 255-256.  
Ying Yu, Azwana R. Sadique, Jeremy M. Smith, Thomas R. Dugan, Ryan E. Cowley, William W. Brennessel, Christine J. Flaschenriem, Eckhard Bill, Thomas R. Cundari, Patrick L. Holland
The Reactivity Patterns of Low-Coordinate Iron Hydride Complexes
J. Am. Chem. Soc.   2008130, 6624-6638.  
Nicholas S. Lees, Rebecca L. McNaughton, Wilda Vargas Gregory, Patrick L. Holland, and Brian M. Hoffman
ENDOR Characterization of a Synthetic Diiron Hydrazido Complex as a Model for Nitrogenase Intermediates
J. Am. Chem. Soc.   2008130, 546-555.  
Azwana R. Sadique, Elizabeth A. Gregory, William W. Brennessel, and Patrick L. Holland
Mechanistic Insight into N=N Cleavage by a Low-Coordinate Iron(II) Hydride Complex
J. Am. Chem. Soc.   2007129, 8112-8121.  Editor's Choice in Science
Ying Yu, William W. Brennessel, and Patrick L. Holland
Borane B-C Bond Cleavage by a Low-Coordinate Iron Hydride Complex and N-N Bond Cleavage by the Hydridoborate Product
Organometallics  200726, 3217-3226.  
Sebastian A. Stoian, Javier Vela, Jeremy M. Smith, Azwana R. Sadique, Patrick L. Holland, Eckard Münck, and Emile L. Bominaar
Mössbauer and Computational Study of an N2-Bridged Diiron Diketiminate Complex: Parallel Alignment of the Iron Spins by Direct Antiferromagnetic Exchange with Activated Dinitrogen
J. Am. Chem. Soc.   2006128, 10181-10192.  
Ying Yu, Jeremy M. Smith, Christine J. Flaschenriem, and Patrick L. Holland
Binding Affinity of Alkynes and Alkenes to Low-Coordinate Iron  
Inorg. Chem.  200645, 5742-5751.  cover of Inorg. Chem.
Jeremy M. Smith, Azwana R. Sadique, Thomas R. Cundari, Kenton R. Rodgers, Gudrun Lukat-Rodgers, Rene J. Lachicotte, Christine J. Flaschenriem, Javier Vela, and Patrick L. Holland
Studies of Low-Coordinate Iron Dinitrogen Complexes
J. Am. Chem. Soc.   2006128, 756-769.  
Patrick L. Holland
Low-Coordinate Iron Complexes as Synthetic Models of Nitrogenase
Can. J. Chem.   200583, 296-301.  
Javier Vela, Sebastian Stoian, Christine Flaschenriem, Eckard Münck, Patrick L. Holland
A Sulfido-Bridged Diiron(II) Compound and Its Reactions with Nitrogenase-Relevant Substrates
J. Am. Chem. Soc.   2004126, 4522-4523.  Correction
Patrick L. Holland
“Nitrogen Fixation,” In Comprehensive Coordination Chemistry 2 (McCleverty, J., Meyer, T. J., Eds.)
  2004Vol. 8; Elsevier: Oxford, 569-599.  On-line access
Jeremy M. Smith, Rene J. Lachicotte, Patrick L. Holland
N=N Bond Cleavage by a Low-Coordinate Iron(II) Hydride Complex
J. Am. Chem. Soc.   2003125, 15752-15753.  
Jeremy M. Smith, Rene J. Lachicotte, Karl A. Pittard, Thomas R. Cundari, Gudrun Lukat-Rodgers, Kenton R. Rodgers, Patrick L. Holland
Stepwise Reduction of N-N Bond Order by a Low-Coordinate Iron Complex
J. Am. Chem. Soc.   2001123, 9222-9223.