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Fraker A, Linn BN, McSkimming A. Low-Coordinate Iron Hydride Chemistry at an N,N,C-Heteroscorpionate Platform. Inorg Chem 2024; 63:14449-14458. [PMID: 39037731 PMCID: PMC11304392 DOI: 10.1021/acs.inorgchem.4c01596] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2024] [Revised: 06/21/2024] [Accepted: 06/30/2024] [Indexed: 07/23/2024]
Abstract
Locally high-spin iron hydrides are proposed to play a critical role as intermediates in iron-molybdenum cofactor (FeMoco)-catalyzed N2 fixation. Inspired by these biological systems, we report herein our initial investigations into low-coordinate iron hydride chemistry supported by our N,N,C-heteroscorpionate ligands. Those ligands with smaller steric profiles are unable to completely suppress the formation of a binuclear [Fe(μ2-H)]2 complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin (S = 2) Fe2+ hydride. Fourier transform infrared spectroscopy suggests an unusually weak Fe-H bond in the latter molecule. Mössbauer spectroscopies, coupled with density functional theory calculations, highlights the substantial influence of the terminal hydride ligand on 57Fe isomer shift.
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Affiliation(s)
- Addison Fraker
- Department
of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
| | - Brittany N. Linn
- Department
of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139, United States
| | - Alex McSkimming
- Department
of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
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2
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Drena A, Fraker A, Thompson NB, Doan PE, Hoffman BM, McSkimming A. Terminal Hydride Complex of High-Spin Mn. J Am Chem Soc 2024; 146:18370-18378. [PMID: 38940813 PMCID: PMC11240256 DOI: 10.1021/jacs.4c03310] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2024] [Revised: 06/10/2024] [Accepted: 06/12/2024] [Indexed: 06/29/2024]
Abstract
The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N2 via Fe hydride intermediates. Our understanding of these species has relied heavily on the characterization of well-defined 3d metal hydride complexes, which serve as putative spectroscopic models. Although the Fe ions in FeMoco, a weak-field cluster, are expected to adopt locally high-spin Fe2+/3+ configurations, synthetically accessible hydride complexes featuring d5 or d6 electron counts are almost exclusively low-spin. We report herein the isolation of a terminal hydride complex of four-coordinate, high-spin (d5; S = 5/2) Mn2+. Electron paramagnetic resonance and electron-nuclear double resonance studies reveal an unusually large degree of spin density on the hydrido ligand. In light of the isoelectronic relationship between Mn2+ and Fe3+, our results are expected to inform our understanding of the valence electronic structures of reactive hydride intermediates derived from FeMoco.
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Affiliation(s)
- Alex Drena
- Department
of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
| | - Addison Fraker
- Department
of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
| | - Niklas B. Thompson
- Chemical
Sciences and Engineering Division, Argonne
National Laboratory, Lemont, Illinois 60439, United States
| | - Peter E. Doan
- Department
of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
| | - Brian M. Hoffman
- Department
of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
| | - Alex McSkimming
- Department
of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
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Cruz TFC, Loupy V, Veiros LF. Zinc-Catalyzed Hydroboration of Carbon Dioxide Amplified by Borane-Tethered Heteroscorpionate Bis(Pyrazolyl)methane Ligands. Inorg Chem 2024; 63:8244-8256. [PMID: 38656156 PMCID: PMC11080050 DOI: 10.1021/acs.inorgchem.4c00500] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2024] [Revised: 04/09/2024] [Accepted: 04/11/2024] [Indexed: 04/26/2024]
Abstract
The borane-functionalized (BR2) bis(3,5-dimethylpyrazolyl)methane (LH) ligands 1a (BR2: 9-borabicyclo[3.3.1]nonane or 9-BBN), 1b (BR2: BCy2), and 1c (BR2: B(C6F5)2) were synthesized by the allylation-hydroboration of LH. Metalation of 1a,b with ZnCl2 yielded the heteroscorpionate dichloride complexes [(1a,b)ZnCl2] 3a,b. The reaction of 1a with ZnEt2 led to the formation of the zwitterionic complex [Et(1a)ZnEt(THF)] 5. The reaction of complex 3a with two equivalents of KHBEt3 under a carbon dioxide (CO2) atmosphere gave rise to the formation of the dimeric bis(formate) complex [(1a)Zn(OCHO)2]2 8, in which its borane moieties intermolecularly stabilize the formate ligands of opposite metal centers. The allylated precursor Lallyl and its zinc dichloride, diethyl and bis(formate) complexes [(Lallyl)ZnCl2] 2, [(Lallyl)ZnEt2] 4, and [(Lallyl)Zn(OCHO)2] 7 were also isolated. The catalyst systems composed of 1 mol % of 3a or 3b and two equivalents of KHBEt3 hydroborated CO2 at 1 bar with pinacolborane (HBPin) to the methanol-level product H3COBPin (and PinBOBPin) in yields of 42 or 86%, respectively. The catalyst systems using the unfunctionalized complex [(LH)ZnCl2] 6 and KHBEt3 or KHBEt3/nOctBR2 (BR2: 9-BBN or BCy2) hydroborated CO2 to H3COBPin but in 2.5- to 6-fold lower activities than those exhibited by 3a,b/KHBEt3. The hydroboration of CO2 using 8 as a catalyst led to yields of 39-43%, comparable to those obtained with 3a/KHBEt3. The results confirmed that the catalytic intermediates benefit from the incorporated boranes' intra- or intermolecular stabilizations.
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Affiliation(s)
- Tiago F. C. Cruz
- Centro de Química
Estrutural, Institute of Molecular Sciences, Departamento de Engenharia
Química, Instituto Superior Técnico,
Universidade de Lisboa, Av. Rovisco Pais, 1049 001 Lisboa, Portugal
| | - Valentin Loupy
- Centro de Química
Estrutural, Institute of Molecular Sciences, Departamento de Engenharia
Química, Instituto Superior Técnico,
Universidade de Lisboa, Av. Rovisco Pais, 1049 001 Lisboa, Portugal
| | - Luís F. Veiros
- Centro de Química
Estrutural, Institute of Molecular Sciences, Departamento de Engenharia
Química, Instituto Superior Técnico,
Universidade de Lisboa, Av. Rovisco Pais, 1049 001 Lisboa, Portugal
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Gu L, Fraker A, Thompson NB, McSkimming A. Four-Coordinate Co(III) Imide with an Unusually Tilted Terminal Imido Ligand. Organometallics 2024; 43:341-348. [PMID: 38362485 PMCID: PMC10865434 DOI: 10.1021/acs.organomet.3c00473] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2023] [Revised: 12/20/2023] [Accepted: 12/21/2023] [Indexed: 02/17/2024]
Abstract
We report herein the synthesis and characterization of a terminal Co(III) imido complex supported by an intermediate field N,N,C heteroscorpionate. This chemistry is enabled through the development of an additional member of this ligand type featuring Ph2(CH3)C- substituents, one of which weakly binds and stabilizes Co in the corresponding Co(I) precursor. The Co(III) imide is low-spin with no evidence for thermal population of open-shell excited states. Unusually, the imido ligand in this molecule tilts markedly toward the Calkyl donor. DFT calculations suggest this structural feature to be largely a result of strong Co-C covalency, underscoring the importance of M-C bonding in determining the (electronic) structure of metal centers supported by this class of ligand.
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Affiliation(s)
- Li Gu
- Department
of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
| | - Addison Fraker
- Department
of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
| | - Niklas B. Thompson
- Chemical
Sciences and Engineering Division, Argonne
National Laboratory, Lemont, Illinois 60439, United States
| | - Alex McSkimming
- Department
of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
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Barchenko M, O’Malley PJ, de Visser SP. Mechanism of Nitrogen Reduction to Ammonia in a Diiron Model of Nitrogenase. Inorg Chem 2023; 62:14715-14726. [PMID: 37650683 PMCID: PMC10498488 DOI: 10.1021/acs.inorgchem.3c02089] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2023] [Indexed: 09/01/2023]
Abstract
Nitrogenase is a fascinating enzyme in biology that reduces dinitrogen from air to ammonia through stepwise reduction and protonation. Despite it being studied in detail by experimental and computational groups, there are still many unknown factors in the catalytic cycle of nitrogenase, especially related to the addition of protons and electrons and their order. A recent biomimetic study characterized a potential dinitrogen-bridged diiron cluster as a synthetic model of nitrogenase. Using strong acid and reductants, the dinitrogen was converted into ammonia molecules, but details of the mechanism remains unknown. In particular, it was unclear from the experimental studies whether the proton and electron transfer steps are sequential or alternating. Moreover, the work failed to establish what the function of the diiron core is and whether it split into mononuclear iron fragments during the reaction. To understand the structure and reactivity of the biomimetic dinitrogen-bridged diiron complex [(P2P'PhFeH)2(μ-N2)] with triphenylphosphine ligands, we performed a density functional theory study. Our computational methods were validated against experimental crystal structure coordinates, Mössbauer parameters, and vibrational frequencies and show excellent agreement. Subsequently, we investigated the alternating and consecutive addition of electrons and protons to the system. The calculations identify a number of possible reaction channels, namely, same-site protonation, alternating protonation, and complex dissociation into mononuclear iron centers. The calculations show that the overall mechanism is not a pure sequential set of electron and proton transfers but a mixture of alternating and consecutive steps. In particular, the first reaction steps will start with double proton transfer followed by an electron transfer, while thereafter, there is another proton transfer and a second electron transfer to give a complex whereby ammonia can split off with a low energetic barrier. The second channel starts with alternating protonation of the two nitrogen atoms, whereafter the initial double proton transfer, electrons and protons are added sequentially to form a hydrazine-bound complex. The latter split off ammonia spontaneously after further protonation. The various reaction channels are analyzed with valence bond and orbital diagrams. We anticipate the nitrogenase enzyme to operate with mixed alternating and consecutive protonation and electron transfer steps.
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Affiliation(s)
- Maxim Barchenko
- Manchester
Institute of Biotechnology, The University
of Manchester, 131 Princess Street, Manchester M1 7DN, U.K.
- Department
of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
| | - Patrick J. O’Malley
- Department
of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
| | - Sam P. de Visser
- Manchester
Institute of Biotechnology, The University
of Manchester, 131 Princess Street, Manchester M1 7DN, U.K.
- Department
of Chemical Engineering, The University
of Manchester, Oxford
Road, Manchester M13 9PL, U.K.
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