Syntheses, Characterization, and Application of Tridentate Phenoxyimino-Phenoxy Aluminum Complexes for the Coupling of Terminal Epoxide with CO2: From Binary System to Single Component Catalyst
Round 1
Reviewer 1 Report
The design of new bifunctional aluminum catalysts for the coupling of expoxides with carbon dioxide is presented. Although the manuscript is certainly interesting and fits well within the scope of Catalysts, publication in its present form is not recommended for the following reasons:
- The synthesis of ligand H2L1 and complex 1, along with its catalytic behavior in the coupling of propylene dioxide with CO2, has already been described by the same authors in a Chinese journal (Liaoming Huagong 2019, 48(5), 393-397). This is not indicated in the manuscript. It makes no sense to repeat the previous discussions about synthesis, characterizations and catalytic activity.
- Ligands H2L2 and H2L3 are new but their characterization is not complete (no elemental analysis or HRMS data provided). Inclusion in the Supplementary material of copies of the 13C NMR spectra of all new compounds is also needed.
- The discussion of the species found in the MS spectra of the new complexes 2 and 3 is speculative. A study of the reactivity of complexes 2 and 3 towards ethanol could help to support the structures given in Figure 1.
Author Response
- The synthesis of ligand H2L1 and complex 1, along with its catalytic behavior in the coupling of propylene dioxide with CO2, has already been described by the same authors in a Chinese journal (Liaoming Huagong 2019, 48(5), 393-397). This is not indicated in the manuscript. It makes no sense to repeat the previous discussions about synthesis, characterizations and catalytic activity.
Response
Thanks for your comments and suggestions.
As pointed out by the reviewer, the reaction of H2L1 with AlEt2Cl was first presented in Liaoning Huagong in 2019. We initially wanted to design an aluminum complex bearing a low coordination number ligand, which we thought would enhance the catalytic activity due to the increased acidity of the metal center. Therefore, with the preliminary analysis of the 1H NMR spectrum, we supposed that complex 1 was a mononuclear complex at that moment. The aluminum center is 4-coordinated by the phenoxyimino-phenoxy ligand and one chloride. However, when we discussed the structures of complexes 2 and 3 with Prof Gao at Jilin University later on, we realized that the structure of complex 1 published might be wrong, by considering the coordination saturation. Thus, we characterized complex 1 by HRMS conducted in DUT. After thorough analysis and discussion with Dr. Qinqin Du, we proposed the binuclear structure of complex 1 which might be generated via the chloride bridge. That is the reason HRMS was used to identify the structures of complexes 2 and 3, and the synthesis and characterization of complex 1 were also included in this manuscript. We also discussed with Dr. Meiheng Lv about the structures of complexes 2 and 3. By optimization of the structures by computer, the agostic coordination of the amine group to the Al center is impossible because of the long distance from N atom to Al center (see pictures below). This result, together with the analysis of 1H NMR spectra and MS, excluded the mononuclear structures of complexes 2 and 3. Thus, we proposed the binuclear structure of all three complexes.
- Ligands H2L2 and H2L3 are new but their characterization is not complete (no elemental analysis or HRMS data provided). Inclusion in the Supplementary material of copies of the 13C NMR spectra of all new compounds is also needed.
Response
The elemental analysis of the ligand precursors has been added to the supporting materials.
Because of the HRMS results of complexes 1-3, we did not do the elemental analysis for complexes 1-3. Usually, the 1H NMR spectra were included in the supporting materials and the data of 13C NMR spectra were listed in the experimental sections. That is the reason 13C NMR spectra were not attached.
- The discussion of the species found in the MS spectra of the new complexes 2 and 3 is speculative. A study of the reactivity of complexes 2 and 3 towards ethanol could help to support the structures given in Figure 1.
Response
We appreciate your suggestion.
Because the process of ionization of complexes in MS is so complicated that we almost could not predict what would happen to the complexes. And the addition of methanol or ethanol molecules to the complexes is not controlled by us. Therefore, we do not think the reaction of complexes 2 and 3 with methanol or ethanol will help to elucidate the structures of complexes. Surely the best direct resolution of the structures is by the X-ray diffraction of single crystals, but we still think that the characterization by NMR and MS is helpful to elucidate the structures of these complexes.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
aluminum complexes for the coupling of terminal epoxide with CO2: from binary system to single component catalyst” sent for review, the authors present the results of their research, in which the syntheses of a series of aluminum complexes supported by novel tridentate phenoxyimino-phenoxyligands and their catalytic behaviors for the coupling of terminal epoxides with CO2.
In my opinion, the work may be published in its present state. However, minor corrections should be taken into consideration:
-standardize the notation of the literature (e.g., journal names appear in short and full form, e.g. 52, 53),
-explain all abbreviations in the text at the first use, e.g., THF, H NMR, ESI +, etc.
Author Response
Reviewer 2
Comments and Suggestions for Authors
In my opinion, the work may be published in its present state. However, minor corrections should be taken into consideration:
-standardize the notation of the literature (e.g., journal names appear in short and full form, e.g. 52, 53),
Response
Thanks for your comments and suggestions.
We have checked the format of references 52 and 53 as pointed out by the reviewer. The abbreviation of that journal name has been corrected in the revised manuscript. Other abbreviations of the journal names such as the Journal of the Chemical Society, Perkin Transactions 2 (ref 72), Proceedings of the Royal Society of London (ref 73) and Journal of the Chemical Society, Transactions, (ref 74) have also been corrected.
-explain all abbreviations in the text at the first use, e.g., THF, H NMR, ESI +, etc.
Response
The abbreviations such as THF have been explained in the revised manuscript.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors describe in their manuscript "Syntheses, characterization and application of tridentate phenoxyimino-phenoxy aluminum complexes for the coupling of terminal epoxide with CO2: from binary system to single component catalyst" new homogeneous catalysts for the coupling with CO2, a promising and well-studied reaction using CO2. The study is interesting, the results are promising. The strong dependence on the CO2 pressure and temperature is surprising. The results should present in a way that the reader can clearly see the differences concerning CO2 pressure and temperature. I am missing any ideas to explain these observations. This should lead to some mechanistic insights and can really "shed light on the design of metal based catalyst of high activity" as the authors promised.
Author Response
Reviewer 3
The authors describe in their manuscript "Syntheses, characterization and application of tridentate phenoxyimino-phenoxy aluminum complexes for the coupling of terminal epoxide with CO2: from binary system to single component catalyst" new homogeneous catalysts for the coupling with CO2, a promising and well-studied reaction using CO2. The study is interesting, the results are promising. The strong dependence on the CO2 pressure and temperature is surprising. The results should present in a way that the reader can clearly see the differences concerning CO2 pressure and temperature. I am missing any ideas to explain these observations. This should lead to some mechanistic insights and can really "shed light on the design of metal based catalyst of high activity" as the authors promised.
Response
Thanks for your comments and suggestions.
To gain information about the mechanism of the coupling reaction, we tried the reaction of PO with complex 3, but no poly(propylene oxide) was isolated. This result may suggest that the ring opening of the epoxy ring was not achieved by complex 3. We also did the reactions of epoxide with water, by using complexes 2 and 3, but the analyses of the reaction mixtures did not provide too much useful information. Therefore, we speculated that the mechanism of the coupling reaction by complex 2 and 3 is through dual activation, that is, the activation of epoxide by its coordination to the metal center, and on the other hand, the amine group reacts with CO2 to form carbamate which will attack the activated epoxide to achieve the ring opening. However, we do not have solid evidence at this stage to confirm this mechanism. For the influence of temperature on the coupling, we thought that temperature may change the solubility of the bifunctional catalysts to some extent. On the other hand, the elevated temperature may induce the cleavage of the chloride bridge of the bifunctional catalysts. We are quite sure the solubility of the bifunctional catalyst will greatly influence the reactivity rate. A coupling of PO with CO2 by complex 3 conducted in cyclic propylene carbonate demonstrated that the reaction rate was doubled during the same period, in which the complex 3 was fully dissolved in propylene carbonate from the beginning without heating.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The rebuttal made by the authors to my previous criticisms are convincing. Acceptance of the manuscript is now recommended.
