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. This is because the triple NN bond is strong (942 kJ/mol) and its cleavage requires a six-electron 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 surfaces promoted by alkali metals are used (Haber-Bosch process). In both cases, N2 reduction (“nitrogen fixation”) is proposed to take place on iron sites. However, mechanistic details of these reduction reactions remain speculative, mainly due to 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 flexible interactions between iron atoms and the central carbon atom, providing open sites for substrate binding. 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 team up with potassium to break the NN triple bond in N2  (Figure 4). This was the first example of an iron complex that cleaves molecular N2 to give nitrides. Reaction of acids with the nitrides quantitatively produces ammonia. Our current efforts concentrate on understanding the mechanism and scope of nitrogen reduction in this system.

Figure 2

Figure 3

Figure 4

In another important facet of our work, we are incorporating sulfur donors that mimic the sulfur-rich environment of the iron atoms in the FeMoco (Figure 1). 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). 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 6). 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

Relevant Publications

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, in press.  
Amy M. Speelman, Patrick L. Holland
Sulfur-Supported Iron Complexes for Understanding N2 Reduction  
Topics in Organometallic Chemistry: Nitrogen Fixation  2017, in press.  
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.  
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.  Reprint
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.