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van Langevelde P, Engbers S, Buda F, Hetterscheid DGH. Elucidation of the Electrocatalytic Nitrite Reduction Mechanism by Bio-Inspired Copper Complexes. ACS Catal 2023; 13:10094-10103. [PMID: 37560187 PMCID: PMC10407843 DOI: 10.1021/acscatal.3c01989] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2023] [Revised: 06/26/2023] [Indexed: 08/11/2023]
Abstract
Mononuclear copper complexes relevant to the active site of copper nitrite reductases (CuNiRs) are known to be catalytically active for the reduction of nitrite. Yet, their catalytic mechanism has thus far not been resolved. Here, we provide a complete description of the electrocatalytic nitrite reduction mechanism of a bio-inspired CuNiR catalyst Cu(tmpa) (tmpa = tris(2-pyridylmethyl)amine) in aqueous solution. Through a combination of electrochemical studies, reaction kinetics, and density functional theory (DFT) computations, we show that the protonation steps take place in a stepwise manner and are decoupled from electron transfer. The rate-determining step is a general acid-catalyzed protonation of a copper-ligated nitrous acid (HNO2) species. In view of the growing urge to convert nitrogen-containing compounds, this work provides principal reaction parameters for efficient electrochemical nitrite reduction. This contributes to the investigation and development of nitrite reduction catalysts, which is crucial to restore the biogeochemical nitrogen cycle.
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Affiliation(s)
| | - Silène Engbers
- Leiden Institute of Chemistry, Leiden University, 2300 RA Leiden, The Netherlands
| | - Francesco Buda
- Leiden Institute of Chemistry, Leiden University, 2300 RA Leiden, The Netherlands
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Cu(Proline) 2 Complex: A Model of Bio-Copper Structural Ambivalence. MOLECULES (BASEL, SWITZERLAND) 2022; 27:molecules27185846. [PMID: 36144582 PMCID: PMC9502899 DOI: 10.3390/molecules27185846] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/22/2022] [Revised: 08/17/2022] [Accepted: 09/05/2022] [Indexed: 11/17/2022]
Abstract
Complexes of Cu2+(d9) with proline may be considered a simple model to address the structural flexibility and electronic properties of copper metalloproteins. To discuss optical electronic spectra and infrared spectral responses, we use quantum chemistry applied to model systems prepared under different geometries and degree of hydration. A comparison of experimental data with calculations indicates that first explicit neighbor water clustering next to the Cu2+(d9) complex is critical for a correct description of the electronic properties of this system. We deduce that the moderately hydrated trans conformer is the main structural form of the complex in water. Further, we suggest that the antisymmetric stretching mode of the carbonyl moieties of the conformer is dominant in the spectrally broadened infrared resonance at 1605 cm−1, where inhomogeneity of the transition at the blue side can be ascribed to a continuum of less optimal interactions with the solvent. Extracted structural properties and hydration features provide information on the structural flexibility/plasticity specific to Cu2+(d9) systems in correlation with the electronic behavior upon photoexcitation. We discuss the role and the nature of the axial ligand in bio-copper structural ambivalence and reactivity.
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Stauffer M, Sakhaei Z, Greene C, Ghosh P, Bertke JA, Warren TH. Mechanism of O-Atom Transfer from Nitrite: Nitric Oxide Release at Copper(II). Inorg Chem 2021; 60:15968-15974. [PMID: 34184870 DOI: 10.1021/acs.inorgchem.1c00625] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Nitric oxide (NO) is a key signaling molecule in health and disease. While nitrite acts as a reservoir of NO activity, mechanisms for NO release require further understanding. A series of electronically varied β-diketiminatocopper(II) nitrite complexes [CuII](κ2-O2N) react with a range of electronically tuned triarylphosphines PArZ3 that release NO with the formation of O═PArZ3. Second-order rate constants are largest for electron-poor copper(II) nitrite and electron-rich phosphine pairs. Computational analysis reveals a transition-state structure energetically matched with experimentally determined activation barriers. The production of NO follows a pathway that involves nitrite isomerization at CuII from κ2-O2N to κ1-NO2 followed by O-atom transfer (OAT) to form O═PArZ3 and [CuI]-NO that releases NO upon PArZ3 binding at CuI to form [CuI]-PArZ3. These findings illustrate important mechanistic considerations involved in NO formation from nitrite via OAT.
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Affiliation(s)
- Molly Stauffer
- Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057, United States
| | - Zeinab Sakhaei
- Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057, United States
| | - Christine Greene
- Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057, United States
| | - Pokhraj Ghosh
- Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057, United States
| | - Jeffery A Bertke
- Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057, United States
| | - Timothy H Warren
- Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057, United States
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Dong Y, Wrobel AT, Porter GJ, Kim JJ, Essman JZ, Zheng SL, Betley TA. O-Heterocycle Synthesis via Intramolecular C-H Alkoxylation Catalyzed by Iron Acetylacetonate. J Am Chem Soc 2021; 143:7480-7489. [PMID: 33949855 DOI: 10.1021/jacs.1c02074] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
Intramolecular alkoxylation of C-H bonds can rapidly introduce structural and functional group complexities into seemingly simple or inert precursors. The transformation is particularly important due to the ubiquitous presence of tetrahydrofuran (THF) motifs as fundamental building blocks in a wide range of pharmaceuticals, agrochemicals, and natural products. Despite the various synthetic methodologies known for generating functionalized THFs, most show limited functional group tolerance and lack demonstration for the preparation of spiro or fused bi- and tricyclic ether units prevalent in molecules for pharmacological purposes. Herein we report an intramolecular C-H alkoxylation to furnish oxacycles from easily prepared α-diazo-β-ketoesters using commercially available iron acetylacetonate (Fe(acac)2) as a catalyst. The reaction is proposed to proceed through the formation of a vinylic carboradical arising from N2 extrusion, which mediates a proximal H-atom abstraction followed by a rapid C-O bond forming radical recombination step. The radical mechanism is probed using an isotopic labeling study (vinyl C-D incorporation), ring opening of a radical clock substrate, and Hammett analysis and is further corroborated by density functional theory (DFT) calculations. Heightened reactivity is observed for electron-rich C-H bonds (tertiary, ethereal), while greater catalyst loadings or elevated reaction temperatures are required to fully convert substrates with benzylic, secondary, and primary C-H bonds. The transformation is highly functional group tolerant and operates under mild reaction conditions to provide rapid access to complex structures such as spiro and fused bi-/tricyclic O-heterocycles from readily available precursors.
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Affiliation(s)
- Yuyang Dong
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Alexandra T Wrobel
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Gerard J Porter
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Jessica J Kim
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Jake Z Essman
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Shao-Liang Zheng
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | - Theodore A Betley
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
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