Harnessing Nitrous Oxide for Sustainable Methane Activation: A Computational Exploration of CNC-Ligated Iron Catalysts
Abstract
:1. Introduction
- Electronic Flexibility: The CNC framework offers strong σ-donation and π-acceptance, stabilizing high-valent iron–oxo intermediates crucial for OAT and methane C–H activation. The absence of BR2 groups eliminates potential electronic interferences, allowing the study to focus on primary metal–ligand interactions [52].
- Steric Profile: Excluding bulky BR2 substituents ensures a compact coordination environment around the iron center. This steric simplicity reduces hindrance at the active site, facilitating efficient catalytic turnover [44].
- Catalyst Modularity: The simplified ligand design enables direct comparisons with previously studied systems that include BR2 groups, isolating the effects of the primary coordination sphere. This approach provides clearer mechanistic insights into the role of the Fe center [53].
- Suitability for High-Valent Oxo Complexes: The CNC ligand’s electronic properties support the stabilization of FeIV=O intermediates, which are critical for methane C–H activation. Excluding BR2 substituents avoids secondary interactions that might introduce competing pathways, simplifying mechanistic investigations [31,54,55,56,57,58].
Through Computational Modeling, This Research Aims to Achieve the Following
- Examine the impact of spin states on methane activation pathways.
- Assess the activation barriers for the OAT, HAA, and ORR steps.
- Explore the role of CNC ligands in stabilizing reactive intermediates and facilitating efficient catalysis.
2. Computational Methods
2.1. Level of Theory
2.2. Geometric Optimization and Frequency Analysis
2.3. Spin-State Considerations
2.4. Energetic Calculations
2.5. Computational Model
2.6. Reaction Pathways Investigated
- Oxygen Atom Transfer (OAT): Coordination of N2O to the Fe center and oxygen atom transfer to form the high-valent FeIV=O intermediate.
- Hydrogen Atom Abstraction (HAA): Interaction of the FeIV=O species with methane to cleave the C–H bond.
- Oxygen Radical Rebound (ORR): Rebound of the hydroxyl group with the methyl radical to yield methanol.
2.7. Software and Computational Resources
3. Results and Discussion
3.1. [(CNC)FeII]2+ Cation Complexes
3.2. Geometric and Spin-State Analysis
3.3. Electronic Structure and Reactivity Implications
3.4. Conclusion for [(CNC)FeII]2+ Cation Complexes
4. Coordination and Energetics of N2O to [(κ3-CNC)FeII]2+ Cation Complex
4.1. Geometric Features of the Quintet State
4.2. Conclusion for N2O Coordination
5. Oxygen Atom Transfer of N2O to Form Oxo FeIV Cation
5.1. Geometry and Transition State of OAT
- Fe–O Bond Length: 1.83 Å, reflecting the partial formation of the Fe–O bond.
- O–N Bond Length: 1.40 Å, showing significant elongation as the N–O bond undergoes cleavage.
- N–N Bond Length: 1.14 Å, consistent with the release of N2.
- Fe–O–N–N Bond Angle: 180.0°, indicative of a linear arrangement facilitating efficient oxygen transfer and nitrogen release.
5.2. Spin-State Dependence of OAT
5.3. Solvent Effects and Alternative Solvent Recommendations
5.4. Alternative Solvents
- 1.
- Polar Aprotic Solvents:
- Acetonitrile (CH3CN): A highly polar, non-coordinating solvent that minimizes interactions with the Fe center while providing the strong stabilization of polar intermediates and transition states. Previous studies have shown that acetonitrile improves reaction kinetics by facilitating charge separation and stabilizing high-valent metal–oxo species [87].
- Dimethylformamide (DMF): A high-polarity solvent that enhances the stability of charged species but may introduce undesired ligand competition with the Fe center. Studies indicate that DMF can reduce transition state energy barriers for oxidative transformations in Fe-based catalysis.
- 2.
- Dimethyl Sulfoxide (DMSO):
- DMSO effectively stabilizes charged intermediates due to its high dielectric constant. However, its coordinating nature may lead to direct interactions with Fe, potentially altering the catalytic pathways [87]. Some Fe-based systems show enhanced selectivity in DMSO, while others experience competitive ligand effects [56].
- 3.
- Moderate Polarity Solvents:
- Dichloromethane (DCM) and ethyl acetate (EtOAc): These solvents provide a balance between polarity and inertness, stabilizing key intermediates without significant coordination with the Fe center. Their ability to influence reaction rates and selectivity has been demonstrated in transition-metal-catalyzed oxidation reactions [58].
- 4.
- Nonpolar Solvents:
- Toluene and other nonpolar solvents may reduce solvation effects and lower transition state energies, particularly in systems where polar solvents over-stabilize reactants. Previous research suggests that nonpolar solvents improve Fe-catalyzed hydrocarbon oxidations by favoring reactive high-spin states [67].
- Co-solvent Strategies: Combining nonpolar solvents with small amounts of polar co-solvents could fine-tune the solvation effects, optimizing catalytic performance [69].
5.5. Conclusion of OAT Mechanism
6. Structure and Properties of the FeIV=O Complex
Geometric Characterization of Oxo/Oxyl Complex
- Fe=O Bond Length: 1.62 Å, indicative of a strong double bond with significant oxo character.
- CNC Ligand Coordination: The CNC ligand maintains a distorted square planar geometry around the Fe center, with the oxo ligand occupying an axial position.
- Spin-State Configurations: The geometric variations between the triplet (S = 1) and quintet (S = 2) states highlight the influence of the spin state on bond lengths and the coordination environment.
7. Spin-State Energetics
- Quintet (S = 2) State: The quintet spin state is the ground-state configuration for the FeIV=O species. The high spin density (3.16 e− on Fe and 0.87 e− on O) reflects the distribution of unpaired electrons, which enhances the oxyl radical character of the oxygen atom. This configuration is stabilized by the strong σ-donation from the oxo group and π-backbonding interactions with the CNC ligand.
- Triplet (S = 1) State: While the triplet state plays a critical role during the OAT step, it lies higher in terms of energy (by ~3–7 kcal/mol depending on the environment phase) compared to the quintet state for the FeIV=O species. This energy gap facilitates a spin flip to the quintet state after OAT, enabling the complex to adopt its thermodynamically preferred configuration.
7.1. Electronic Structure
- Iron Center (Fe): Spin density of 3.17 e− in the quintet state, consistent with its d4 configuration.
- Oxo Ligand (O): Partial spin density of 0.87 e−, indicative of an oxyl radical. This feature is critical for activating methane via HAA.
7.2. Reactivity Implications
- Oxo Radical Rebound: The oxyl radical character facilitates rapid rebound with the methyl radical during the oxygen radical rebound (ORR) step, ensuring the efficient formation of methanol.
- Hydrogen Atom Abstraction (HAA): The FeIV=O species initiates HAA by abstracting a hydrogen atom from methane, driven by its strong oxidizing potential and spin-state reactivity.
7.3. Conclusion Oxo/Oxyl Characterization
8. Methane C–H Activation by Oxo/Oxyl Cation Complex
8.1. Hydrogen Atom Abstraction (HAA)
8.2. Geometric Descriptions of Figure 5 and Figure 6
8.3. The Quintet and Triplet State Spin Densities
8.4. Energetic Barrier of Figure 5
9. Oxygen Radical Rebound (ORR)
9.1. Geometric Features of ORR TS Structure
9.2. Spin Densities of the ORR TS Structure
9.3. Energetic Barrier of Figure 6
9.4. Conclusions
10. Summary and Conclusions
- The CNC-ligated iron complexes effectively stabilize high-valent Fe intermediates, facilitating methane C–H activation under mild conditions.
- The triplet (S = 1) and quintet (S = 2) spin states play distinct but complementary roles in the catalytic cycle, with intersystem crossing (ISC) enhancing reactivity.
- The OAT step is the rate-determining step, with the energy barriers influenced by solvation effects and the electronic structure.
- The FeIV=O intermediate exhibits significant oxyl radical character, enabling efficient methane C–H activation via HAA.
- The ORR step proceeds with moderate energy barriers, ensuring methanol formation is feasible.
11. Prospectus
- Experimental Validation: The computationally derived mechanisms and spin-state energetics provide a foundation for experimental efforts to synthesize and characterize CNC-ligated iron complexes under conditions analogous to those modeled computationally.
- Ligand Optimization: Future work could explore alternative CNC ligand scaffolds or substitutions to enhance stability, selectivity, and catalytic turnover, especially for systems involving polar or nonpolar solvent environments.
- Solvent Engineering: The demonstrated impact of solvation on OAT and methane activation highlights the need for further exploration of mixed solvent systems or highly nonpolar environments to optimize solvation-induced energy barriers.
- Broader Substrate Scope: Beyond methane, this catalytic approach could be extended to other hydrocarbons or small molecules, broadening its applicability in chemical synthesis.
- Industrial Scale-Up: Developing scalable processes for methanol production using N2O as an oxidant could reduce the reliance on conventional, energy-intensive methods like steam methane reforming, aligning with global sustainability goals.
Supplementary Materials
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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Spin State | Fe–C (Å) | Fe–N (Å) | C–Fe–C (°) | Spin Density (Fe, e−) |
---|---|---|---|---|
Singlet (S = 0) | 1.97 | 1.86 | 163.5 | 0.00 |
Triplet (S = 1) | 1.97 | 1.87 | 163.0 | 2.17 |
Quintet (S = 2) | 2.06 | 2.13 | 152.3 | 4.00 |
Parameter | Gas Phase (Triplet) | Gas Phase (Quintet) | THF Solvent (Triplet) | THF Solvent (Quintet) |
---|---|---|---|---|
ΔG‡ HAA (kcal/mol) | 16.0 | 18.0 | 23.5 | 25.2 |
ΔG‡ ORR (kcal/mol) | 6.4 | 6.6 | 16.8 | 16.3 |
Fe–O (HAA, Å) | 1.67 | 1.72 | 1.67 | 1.72 |
O–H (HAA, Å) | 1.37 | 1.22 | 1.37 | 1.22 |
C–H (HAA, Å) | 1.20 | 1.33 | 1.20 | 1.33 |
Fe–O–H (HAA, °) | 114.0 | 94.1 | 114.0 | 94.1 |
Fe–O (ORR, Å) | 1.82 | 1.82 | 1.80 | 1.82 |
O–C (ORR, Å) | 2.08 | 2.17 | 2.08 | 2.17 |
Fe–O–C (ORR, °) | 74.2 | 71.4 | 74.2 | 71.4 |
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Prince, B.M. Harnessing Nitrous Oxide for Sustainable Methane Activation: A Computational Exploration of CNC-Ligated Iron Catalysts. Methane 2025, 4, 6. https://doi.org/10.3390/methane4010006
Prince BM. Harnessing Nitrous Oxide for Sustainable Methane Activation: A Computational Exploration of CNC-Ligated Iron Catalysts. Methane. 2025; 4(1):6. https://doi.org/10.3390/methane4010006
Chicago/Turabian StylePrince, Bruce M. 2025. "Harnessing Nitrous Oxide for Sustainable Methane Activation: A Computational Exploration of CNC-Ligated Iron Catalysts" Methane 4, no. 1: 6. https://doi.org/10.3390/methane4010006
APA StylePrince, B. M. (2025). Harnessing Nitrous Oxide for Sustainable Methane Activation: A Computational Exploration of CNC-Ligated Iron Catalysts. Methane, 4(1), 6. https://doi.org/10.3390/methane4010006