On the Selective Conversion of Methane to Methanol Facilitated by Coordinatively Unsaturated Transition Metal Complexes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper presents a rigorous and detailed study for methane to methanol reaction catalyzed by three transition metal complexes. The authors built this work on their prior work where they computationally discovered a Rh complex that can catalyze methane to methanol exhibiting desirable performance. In this work, they expanded their findings to the same type of complex where Rh was replaced by Co and Fe, which are abundant elements and much cheaper, making the work significant. The detailed catalytic mechanism of all these catalysts (Rh, Co, and Fe complexes) have been elucidated at an atomic level for their frontier orbitals, spin states, and energetics along the reaction coordinate, using state of the art theory level. The results are also well presented and the paper is well structured. The authors also show the chemical structures and atom coordinates used in their DFT computation, which are helpful for the readers to reproduce the results. I recommend publication of this paper, with a couple of comments for the authors to consider.

1. Methane to methanol reaction has been widely studied for heterogenous catalysis using oxygen (O2) as the oxidant. It is encouraged to comment briefly on the advantages of using metal complexes and ozone (O3) in this study.
2. A comparison of catalytic properties of the complexes of Rh, Co, and Fe would be interesting.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript entitled "On the selective conversion of methane to methanol facilitated by coordinatively unsaturated transition metal complexes" performed density functional theory calculations to examine the reactivity of (NH₃)₄RhO²⁺, (NH₃)₄CoO²⁺, and (NH₃)₄FeO⁺with methane and methanol, but there are still some issues that need to be modified before the publication on Chemistry.

 

Issue 1: There are font differences between lines 355-384 in the manuscript, which are neither consistent with the main text nor consistent with itself. Please review this section and confirm the required format. If there are any formatting issues, please make modifications here.

 

Issue 2: As a theoretical calculation paper, this article should include more introductions to the model structure. We noticed that the description of the model parameters in the main text was not accompanied by the actual structure diagram of the model, and only the energy of the relevant reaction steps were given in the subsequent calculations. It is recommended to add corresponding structure diagrams in the energy diagram as well.

 

Issue 3: Is it the result of this study that both kcal/mol and eV appear as energy units in the text? If so, please consider unifying these different units. If not, please provide reference and conversion relationships.

 

Issue 4: As a purely computational article, this article has limited computational content. Please consider adding more computational content to enrich the workload of this article and make up for the shortcomings in experimental aspects.

 

Issue 5: Please pay attention to the format specifications for corner markers in references [10] and [25].

 

Issue 6: Please consider enhancing the innovation of this study.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript describes a computational study of the catalytic activity of simple gas-phase ammonia complexes of three transition metal (Rh, Co, and Fe) oxide cations and their ability to act as selective catalysts for the conversion of methane to methanol at room temperature. The authors use density-functional theory (DFT) calculations, supplemented by selected CASSCF calculations, to examine the origin of the catalytic activity of the Rh compound, focusing specifically on the orbital origin of the electron acceptor properties of the Rh-oxide cation and on the role of unpaired electrons in promoting this process. The analysis of the electronic structure provides insight into why the low-spin state promotes a 2+2 activation of a methane molecule (as opposed to a radical proton coupled electron transfer path), which is a key requirement for high selectivity during the methane-to-methanol conversion. The authors then examine the corresponding ammonia complexes in which Rh is replaced by Co and Fe. Whereas Co is shown to favor a radical activation mechanism, Fe leads again to a non-radical path, providing a potential equally selective and cheaper alternative to Rh for oxidation reaction of hydrocarbon substrates.

The work is well organized and well described. The computational methods used are reliable and robust. The DFT calculations (geometry optimizations, orbital analysis, reaction free energy estimations) are carried out using the MN15 functional, whose accuracy has been validated by comparison to CASSCF calculations on the FeOCH3+ species in the presence of a methane molecule. The orbital analysis for the Rh system is insightful and convincing, and it provides a simple model of how spin and orbital energies determines the reactivity of the species examined in methane oxidation. On the whole, this is high-level and well presented computational work, and the paper is in my opinion publishable in Chemistry. I would like the authors however to address the following minor points before publication.

1) Lines 200-210 discuss the important issue of the ability of the Rh species to act as a selective catalyst for methane, as opposed to methanol, oxidation. The authors mention that their calculation on methanol oxidation yield free energy reaction barriers which are very close to those for [2+2] methane oxidation, and that larger rate constants for the latter substrate are due to methanol establishing hydrogen bonds with the coordinated ammonia molecules. So it seems, from this discussion, that the coordination environment play quite an important role in determining selectivity. Can the authors briefly discuss how important the structure of the metal coordination environment can be for selectivity? Are there experimental findings about the spectroscopic and electronic properties and the reactivity of MO+ species that can be used to support the link proposed in the paper of spin state, reactivity, and selectivity?

2) Concerning the activation of hydrocarbons (and other substrates) there is an extensive literature focussing on the role of the Fe-oxo species (Fe(IV)O2+), in both abiotic and biological conditions. One of the main finding of the work presented in the manuscript is that, in the case of the MO+ ion, low spin states favor reactivity in oxidation reactions, in contrast with what is observed in Fe-oxo species. Can the authors briefly summarize what is the origin of this difference and if/how MO+ species could provide alternative, more efficient, or more selective catalysts? 

3) The caption to Figure 1 seems to contain only a placeholder. A caption describing the two panels in the figure should be added.  

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf