Catalytic Performances of Platinum Containing PLLA Macrocomplex in the Hydrogenation of α,β-Unsaturated Carbonyl Compounds
and Marco Frediani 1,*Round 1
Reviewer 1 Report
The paper by Rosi, Frediani and co-workers describes the preparation of a platinum complex with a bipyridine ligand linked to a poly(lactide) polymer and its use as catalyst in the hydrogenation of α,β-unsaturated carbonyl compounds. The catalyst preparation and characterization is reported together with preliminary catalytic tests. Although this is an interesting piece of work, the results as presented are not mature enough to be published in its present form. Thus I recommend reconsidering the paper after major revision following the points listed below:
- The first key point is the characterization of the PLLA-Bipy and Pt(PLLA-Bipy)Cl2: the 1H NMR characterization provided does not match with the formula provided in Figure 2 (especially regarding the number of lactic acid units). The number of lactic units reported is different in the ligand and in the complex. In fact, the authors had a note in Italian also questioning this point in the description of the 1H NMR (lines 78 and 91). The coupling constants reported for the 5 and 6 pyridine protons in the 1H NMR are not reciprocal.
- The second key point regards the scope of the process: only two substrates are being tested which give different selectivity. The data presented is not enough to draw any conclusion on the activity and selectivity of the catalyst prepared. More cases need to be studied.
- The third key point is the deactivation of the catalyst: The deactivation mechanism presented is not convincing. The authors mentioned the formation of Pt(PLLA-Bipy)2Cl2 and argued that no platinum is leached because no free Bipy is observed on the NMR spectra. First Pt has strong preference for square planar coordination, but more importantly, if starting with a catalyst system in which all the bipyridine rings are coordinated in the form Pt(PLLA-Bipy)Cl2 and at the end all the bipyridine rings are coordinated in the form Pt(PLLA-Bipy)2Cl2, half of the metal loaded has been leached from the catalytic material. Therefore, it is important that the leaching is better studied (by carrying out hot-tests or analyzing the content of platinum in the reaction mixture) to propose a more plausible and consistent deactivation mechanism.
- The following formal points should also be considered when redrafting the manuscript:
o Table 1 is very difficult to follow. The structure of the substrates should be given in or near the table.
o Affiliations are not correctly stated.
o α,β-unsaturated needs to be written consistently throughout the manuscript
o Line 41-44: which macrocyclic ligands are the authors referring to?
o Line 140: secondary absorption at 480 is not observed but an absorption below 400nm. Revise data and text accordingly.
Author Response
Dear Editor,
we would like first to thank the referees for the time dedicated to reviewing this article and their critical comments aiming to improve the quality of the paper.
REFEREE 1
Comment
The paper by Rosi, Frediani, and co-workers describes the preparation of a platinum complex with a bipyridine ligand linked to a poly(lactide) polymer and its use as catalyst in the hydrogenation of α,β-unsaturated carbonyl compounds. The catalyst preparation and characterization are reported together with preliminary catalytic tests. Although this is an interesting piece of work, the results as presented are not mature enough to be published in its present form. Thus I reccomend reconsidering the paper after major revision following the points listed below:
- The first key point is the characterization of the PLLA-Bipy and Pt(PLLA-Bipy)Cl2: the 1H NMR characterization provided does not match with the formula provided in Figure 2 (especially regarding the number of lactic acid units). The number of lactic units reported is different in the ligand and in the complex. In fact, the authors had a note in Italian also questioning this point in the description of the 1H NMR (lines 78 and 91). The coupling constants reported for the 5 and 6 pyridine protons in the 1H NMR are not reciprocal.
Answer
It is obvious that the formation of the Pt complex does not affect the number of lactic acid unities in the ligand because the reaction conditions are quite soft. The difference between hydrogen numbers is imputable to several issues. The first and the main relevant among them is the integral processing of the signals that were affected by noise and baselines correction. A % error in integral evaluations is introduced in experimental. Considering these points, the data of macro complex and macroligand are in good agreement among them. For a more precise molecular weight and fragment numbers please consider the value obtained from GPC analysis reported in the NMR characterization of the complex.
Comment
- The second key point regards the scope of the process: only two substrates are being tested which give different selectivity. The data presented is not enough to draw any conclusion on the activity and selectivity of the catalyst prepared. More cases need to be studied.
Answer
Thanks for the comment. Of course, the number of substrates is limited but this is a preliminary work presented as a short communication. These results are important to show that the system works as a catalyst. Further substrates will be tested in following and more comprehensive studies.
Comment
- The third key point is the deactivation of the catalyst: The deactivation mechanism presented is not convincing. The authors mentioned the formation of Pt(PLLA-Bipy)2Cl2 and argued that no platinum is leached because no free Bipy is observed on the NMR spectra. First Pt has strong preference for square planar coordination, but more importantly, if starting with a catalyst system in which all the bipyridine rings are coordinated in the form Pt(PLLA-Bipy)Cl2 and at the end all the bipyridine rings are coordinated in the form Pt(PLLA-Bipy)2Cl2, half of the metal loaded has been leached from the catalytic material. Therefore, it is important that the leaching is better studied (by carrying out hot-tests or analyzing the content of platinum in the reaction mixture) to propose a more plausible and consistent deactivation mechanism.
Answer
Probably we were not clear to expose our point of view, taking into account also a typographical error present in the previous text. Now it has been corrected. We suppose the formation of a Pt dimer with the bipyridine ligand placed in a bridge position between two Pt atoms. We added a reference to support our hypothesis with a theoretical study on the formation of binuclear bipyridyl-platinum based complex. We will deeply study the deactivation mechanism in a next and more exhaustive paper.
Comment
- The following formal points should also be considered when redrafting the manuscript:
- Table 1 is very difficult to follow. The structure of the substrates should be given in or near the table.
Answer Done
Comment - Affiliations are not correctly stated.
Answer Done
Comment - α,β-unsaturated needs to be written consistently throughout the manuscript
Answer Done
Comment -Line 41-44: which macrocyclic ligands are the authors referring to?
Answer Done
Comment -Line 140: secondary absorption at 480 is not observed but an absorption below 400nm. Revise data and text accordingly.
Answer Done
Reviewer 2 Report
The work by Rosi, Frediani and co-workers reports hydrogenation of model a,b-unsaturated carbonyl substrates, namely, 2-cyclohexenone and but-2-enal catalyzed by platinum(II) complex with 2,2’-bipy-appended PLLA as a unique macromolecular ligand.
I would recommend the acceptance provided that a significant improvement of the manuscript will be done:
1.The discussion of data presented in Figure 3 should be improved. Specifically, it is unclear why the spectra in Fig. 3b-c for the same complex Pt[PLLA-Bipy]Cl2 are different.
2. It should be clearly specified in the text which stereoisomer of but-2-enal was used (according to reaction scheme in Figure 1, isomer trans was employed).
3.The discussion of orbital (not “orbitalic”!) interactions should be improved as it is not informative at all. What type of interactions are relevant for the observed spectroscopic features of the catalyst? This should be clearly specified as it seems that the authors are not familiar with the electronic structure of studied platinum complexes.
4. The presentation of UV-Vis spectra in Figure 4 should be improved: the solvent(s) and compound concentration(s) should be specified in the caption.
5. The description of 1H NMR data for PLLA-Bipy and Pt[PLLA-Bipy]Cl2 in Section 2.2. should be corrected as it contains phrases in Italian. In addition, I am wondering, whether the NMR spectra showed satellite signals due to 195Pt-1H NMR and 195Pt-13C couplings. Such an information in the text would be valuable.
6. Last but not least, I am not an expert in English but I noticed many language deficiencies throughout the manuscript including grammar errors, typos, incorrect words or phrases, etc. I expect that the revised manuscript will be carefully checked by a person having good knowledge of scientific English.
Author Response
Dear Editor,
we would like first to thank the referees for the time dedicated to reviewing this article and their critical comments aiming to improve the quality of the paper.
REFEREE 2
Comment
The work by Rosi, Frediani and co-workers reports hydrogenation of model a,b-unsaturated carbonyl substrates, namely, 2-cyclohexenone and but-2-enal catalyzed by platinum(II) complex with 2,2’-bipyappended PLLA as a unique macromolecular ligand.
I would recommend the acceptance provided that a significant improvement of the manuscript will be done:
1. The discussion of data presented in Figure 3 should be improved. Specifically, it is unclear why the spectra in Fig. 3b-c for the same complex Pt[PLLA-Bipy]Cl2 are different.
Answer
We have hypothesized that the differences are due to the transformation of the starting complex Pt(PLLA-Bipy)Cl2 into a dimeric specie [Pt(PLLA-Bipy)Cl2]2 where the ligands are placed between the two Pt atoms. In the previous work, a typographical error was present, now it is corrected. Considering the preliminary data reported, we will be in-depth to this topic in a future more comprehensive study.
Comment It should be clearly specified in the text which stereoisomer of but-2-enal was used (according to the reaction scheme in Figure 1, isomer trans was employed).
Answer Done
Comment The discussion of orbital (not “orbitalic”!) interactions should be improved as it is not informative at all. What type of interactions are relevant for the observed spectroscopic features of the catalyst?
Answer As reported in the paper and in the reference ( 27-28) the orbital interaction is mainly due to the d systems of platinum and the free doublet of the nitrogen involving the p-system of bipyridine ligand (line 135).
Comment This should be clearly specified as it seems that the authors are not familiar with the electronic structure of studied platinum complexes.
Answer The section was fixed. Appropriate references about theoretical studies were added.
Comment The presentation of UV-Vis spectra in Figure 4 should be improved: the solvent(s) and compound concentration(s) should be specified in the caption.
Answer Done.
Comment The description of 1H NMR data for PLLA-Bipy and Pt[PLLA-Bipy]Cl2 in Section 2.2. should be corrected as it contains phrases in Italian. In addition, I am wondering, whether the NMR spectra showed satellite signals due to 195Pt-1H NMR and 195Pt-13C couplings. Such information in the text would be valuable.
Answer The section was corrected. The satellite signals due to 195Pt-1H NMR and 195Pt-13C couplings were introduced.
Comment Last but not least, I am not an expert in English but I noticed many language deficiencies throughout the manuscript including grammar errors, typos, incorrect words or phrases, etc. I expect that the revised manuscript will be carefully checked by a person having a good knowledge of scientific English.
Answer The overall manuscript was edited improving the quality of the English language
Round 2
Reviewer 1 Report
The key points raised in the review report have not been addressed in the new version of the paper resubmitted:
- characterization: the information provided in the response letter needs to be provided in the manuscript to clarify the characterization of the products synthesized.
- scope: even if this is a short communication, only two cases is not enough to draw any conclusion on the activity of the catalyst. Publication in a subsequent paper leads to an unnecessary fragmentation of the work.
- deactivation study: the concerns raised in the initial revision have not been solved.
Thus, I can only recommend again reconsideration after a major revision.
Author Response
Referee 1:
The key points raised in the review report have not been addressed in the new version of the paper resubmitted:
- characterization: the information provided in the response letter needs to be provided in the manuscript to clarify the characterization of the products synthesized.
Information is introduced in the text
- scope: even if this is a short communication, only two cases is not enough to draw any conclusion on the activity of the catalyst. Publication in a subsequent paper leads to unnecessary fragmentation of the work.
As before stated this is only a preliminary communication and two substrates have been presented to show the possibility of this catalyst to perform the catalytic hydrogenation. In a subsequent full paper the work will be extended. It is actually not possible to perform the tests required in a week, as requested by the editor.
- deactivation study: the concerns raised in the initial revision have not been solved.
A figure explaining the possible structure of the dimeric specie is reported as requested by referee 2
Thus, I can only recommend again reconsideration after a major revision.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors improved the manuscript, especially with respect to language correctness.
Unfortunately, the description of NMR data is still not satisfactory, e.g., the coupling constants are not given at all including those resulting from 195Pt-1H NMR and 195Pt-13C NMR couplings. In fact, it is intriguing as in their response, the authors stated:
'The section was corrected. The satellite signals due to 195Pt-1H NMR and 195Pt-13C couplings were introduced.'
In addition, the authors proposed that partial deactivation of their catalytic system was due to formation of dimeric Pt complex [Pt(PLLA-‐‑Bipy)Cl2]2 (Bipy part of its 1H NMR is shown in Figure 3c). However, in line 187 it is formulated as [Pt(PLLA-‐‑Bipy)2Cl2]2. This needs revision. The spatial arrangement of Pt atoms and ligands (Bipy and Cl) in the dimer should be presented to shed more light on its structural specificity.
Author Response
Referee 2:
The authors improved the manuscript, especially with respect to language correctness.
Unfortunately, the description of NMR data is still not satisfactory, e.g., the coupling constants are not given at all including those resulting from 195Pt-1H NMR and 195Pt-13C NMR couplings. In fact, it is intriguing as in their response, the authors stated:
'The section was corrected. The satellite signals due to 195Pt-1H NMR and 195Pt-13C couplings were introduced.'
We are sorry for our erroneous reply. We referred to the H-H coupling constants that were introduced. Probably due to the low concentration of the complex in the solution (caused by the high molecular weight of the complex itself) we were not able to detect the 195Pt-1H NMR and 195Pt-13C NMR couplings. It would be taken into account that these couplings must be J3 Pt-H and J2 Pt-C through an intermediate nitrogen atom. However, the coordination of Pt to the ligand is confirmed by the resonance shift of the ligand in the complex if compared to the pure ligand itself
In addition, the authors proposed that partial deactivation of their catalytic system was due to formation of dimeric Pt complex [Pt(PLLA- ‐‑Bipy)Cl2]2 (Bipy part of its 1H NMR is shown in Figure 3c). However, in line 187 it is formulated as [Pt(PLLA- ‐‑Bipy)2Cl2]2. This needs revision. The spatial arrangement of Pt atoms and ligands (Bipy and Cl) in the dimer should be presented to shed more light on its structural specificity.
The error remained on line 187 has been corrected and a figure reporting the possible structure of the dimeric specie has been introduced
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
With the exception of the scope study, the other points have now been addressed in the present version of the manuscript, thus I recommend acceptance of the manuscript.
