Synthesis, Characterization, and Anticancer Activity of 3-Chlorothiophene-2-carboxylic Acid Transition Metal Complexes
, Xingzhi Yang 4, Yanting Yang 1 and Mei Luo 1,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsReferee report on the manuscript “Synthesis, Characterization and Anticancer Activity of 3-Chlorothiophene-2-Carboxylic acid Transition Metal Complexes” by Baiquan Hu et al. to be published in Inorganics /mdpi (manuscript ID: 3550147).
The manuscript presents results on the preparation of copper, nickel and cobalt complexes of 3-chlorothiophene-2-carboxylic acid. Although the aforementioned new complex molecules may be of interest, there are several potential problems with the MS.
- Metal complexes based on 3-chloro-thiophene-2-carboxylic acid ligands may even be interesting as anti-cancer compounds, but this would be more of an accidental finding, which has no significant history. The claim that compounds containing carboxylic acid ligands would be particularly preferred in cancer research needs some proof and explanation in the introduction.
- At least ESI-MS data are missing for the characterization of the complexes.
- The characterization of physicochemical properties of the newly synthesized compounds did not take place, it is not clear that biological experiments could have been performed at all. Authors should have basic information about the solubility, stability, coordination chemistry, electrochemical properties before performing biological experiments.
- In vitro characterization of compounds requires that at most one compound be present at a time, but complexes 3 and 4 contain at least two different complexes plus the uncoordinated ligand. In this case, proving stability in the physiological system would be particularly important.
- In vitro data should be given in IC50 units, the method in the manuscript does not show any meaningful results, 100 µM is too high concentration to draw any conclusions.
I shell suggest a major revision of the manuscript for the above reasons.
Author Response
Thank you very much for your professional review of our article. As you noted, there are several issues that need to be addressed. Based on your valuable suggestions, we have revised the previous manuscript. The detailed revisions are as follows:
1.Metal complexes based on 3-chloro-thiophene-2-carboxylic acid ligands may even be interesting as anti-cancer compounds, but this would be more of an accidental finding, which has no significant history. The claim that compounds containing carboxylic acid ligands would be particularly preferred in cancer research needs some proof and explanation in the introduction.
A: Thank you for your valuable comments. We have rewritten the introduction part of the manuscript and highlighted it in red in the manuscript.
2.At least ESI-MS data are missing for the characterization of the complexes.
A: Thank you for your valuable comments. We have added the ESI-MS data to the manuscript and highlighted it in red.
3.The characterization of physicochemical properties of the newly synthesized compounds did not take place, it is not clear that biological experiments could have been performed at all. Authors should have basic information about the solubility, stability, coordination chemistry, electrochemical properties before performing biological experiments.
A: Thank you for your valuable comments. We have added the corresponding electrochemical data and stability data to the manuscript and highlighted them in red.
4.In vitro characterization of compounds requires that at most one compound be present at a time, but complexes 3 and 4 contain at least two different complexes plus the uncoordinated ligand. In this case, proving stability in the physiological system would be particularly important.
A: Thank you for your valuable comments. We have added the corresponding thermogravimetric data to the manuscript and highlighted them in red.
5.In vitro data should be given in IC50 units, the method in the manuscript does not show any meaningful results, 100 µM is too high concentration to draw any conclusions.
A: Thank you for your valuable comments. Since the inhibitory effect at 100 µM concentration was not satisfactory, no subsequent IC50 test was performed.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsPlease see attached PDF document which contains detailed comments on this manuscript.
Comments for author File: Comments.pdf
Author Response
Thank you very much for your professional review of our article. As you noted, there are several issues that need to be addressed. Based on your valuable suggestions, we have revised the previous manuscript. The detailed revisions are as follows:
1.Physical Techniques of Characterization
The structural and spectroscopic properties of the copper(II), nickel(II) and cobalt(II) complexes depend on the ground-state electron configurations (d9, d8 and d7, respectively), yet nowhere in this manuscript have the authors mentioned the dn configurations. Moreover, the authors did not carry out magnetic susceptibility measurements even though these complexes are paramagnetic. Room-temperature magnetic data [μeff = (8χMT)½] would have confirmed the oxidation states inferred from the crystallographic analyses; furthermore, in the cases of the nickel(II) (d8) and cobalt(II) (d7) the spin states would have been established. The copper(II) complex could have also been characterized by electron spin resonance (ESR) spectroscopy.
A: Thank you for your valuable comments. We have added the corresponding content to the manuscript and highlighted it in red in the manuscript.
2.Representation and Designations of the Complexes
In the abstract and elsewhere in the manuscript, the authors referred to the complexes as C20H18Cl2CuN2O6S2 (1), C20H18Cl2CoN2O6S2 (2), C30H48Cl6Ni3O30S6 (3) and C30H48Cl6Ni3O30S6 (4).
- Unfortunately, these chemical formulae do not represent the structural constituents of these complexes properly. Complex 1 (C20H18Cl2CuN2O6S2) could be better represented as [Cu(C5H2ClO2S)2py2(OH2)2], where (C5H2ClO2S)=3-chlorothiophene-2-carboxylate, py=pyridine. Likewise, complex 2 (C20H18Cl2CoN2O6S2) could be represented as [Co(C5H2ClO2S)2py2(OH2)2]. The chemical formulae of complexes 3 and 4 (C30H48Cl6Ni3O30S6 and C30H48Cl6Co3O30S6, respectively), are not consistent with the schematic representations shown in Scheme 1 and the X-ray structures depicted in Figure 1. In Scheme 1 and Figure 1, the structures are presented as follows: [{Ni(C5H2ClO2S)2(OH2)4}{Ni(C5H2ClO2S)(OH2)5}]C5H2ClO2S·2H2O and [{Co(C5H2ClO2S)2(OH2)4}{Co(C5H2ClO2S)(OH2)5}]C5H2ClO2S·2H2O. The authors did not provide an explanation for the discrepancy between the chemical formulae C30H48Cl6Ni3O30S6 and C30H48Cl6Co3O30S6 and the structures presented in Scheme 1 and Figure. Unfortunately, the authors did not provide the CIFs (crystallographic information files) for these X-ray structures. When I tried to access the structures of complexes 3 and 4 from the CCDC (Cambridge Crystallographic Data Centre) the data were not available. The data available were those of complexes 1 and 2. The authors also did not provide the checkCIFs for the X-ray structures as supporting/supplementary information. • The designations of the complexes 1, 2, 3 and 4 should be in bold type throughout the manuscript. That is, 1, 2, 3 and 4.
A: Thank you for your valuable comments. We have modified the corresponding content in the manuscript and highlighted it in red in the manuscript.
3.Chemical Formulae of the Hydrated Acetate Salts
The chemical formulae of copper(II) acetate monohydrate and cobalt(II)acetate tetrahydrate are written incorrectly as Cu(CH3COO)2·H2O and Co(CH3COO)2·4H2O, respectively. The conventional chemical formulae of these two salts are (CH3COO)2Cu·H2O and (CH3COO)2Co·4H2O, respectively.
A: We are very sorry for our negligence and thank you for your reminder. We have revised the content in the manuscript. The revised parts have been marked in red in the manuscript.
4.Single-Crystal X-ray Analyses
- Table 1 The monoclinic space group is written wrongly. It should be italicized as follows: P21/c The triclinic space groups is written wrongly. It should be as follows: P-1 • Bond distances and bond angles missing Surprisingly, the authors decided omitted the table of bond distances and abngles from the text and relegated it to supplementary/supporting information as Table S1. The authors must move Table S1 to the text and renumber it as Table 2. They must mention and discuss the bond distances and angles, especially those in the coordination sphere. The ligand bond angles C=O, C–O (carboxylate) and C–S (thiophene ring) must be incuded because they are being discussed in the section on IR spectroscopy. The current Table 2 (hydrogen bond lengths and angles) must be renumbered as Table S1 and moved to supporting/supplementary information. • Descriptions of X-ray structures Very poor! Complex 1: [Cu(C5H2ClO2S)2py2(OH2)2] The authors failed to recognize the tetragonal distortion (Jahn-Teller elongation along the z-axis). The axial Cu–O water distances are much longer than those in the equatorial plane as a consequence of the Jahn-Teller effect consistent with the d9 configuration of the central metal ion. Such tetragonally elongated Cu–O water bonds have distances approximately 2.4 Å. In contrast, normal Cu–O water bonds in the equatorial plane have distances around 2.0 Å. Please refer to the following journal articles about Jahn-Teller elongated and normal CuII–O water distances: Molecules 2020, 25, 1898; doi:10.3390/molecules25081898 Acta Cryst. 2015, C71, 386–393 Polyhedron 26 (2007) 4525–4532 J Incl Phenom Macrocycl Chem 2011, 71, 557–566 J Mol Struct 2009, 933, 126–131 Inorg Chim Acta 2012, 384, 47–53 J Coord Chem, 2017, 70, 3660–3676.Figure 1 Complex 1. The water molecules must be attached to the copper(II) ion as aqua ligands even though Cu–O bonds are long. Currently, the structure shows a square planar coordination geometry (which is wrong!). It should be a tetragonally elongated octahedral coordination geometry. Description of Complex 2. Surprisingly, the authors wrote the following statement: “The structure of complex 2 is similar to that of complex 1, with only the metal center changed to from Cu2+ to Co2+ …” This is NOT true! The six-coordinate geometry of complex 2 (the CoII complex, high-spin d7) is not subject to the Jahn-Teller effect, thus the axial CoII–Owater bonds are normal, i.e. not tetragonally elongated. Moreover, the ionic radius of high-spin CoII is different from that of CuII and so the coordinate M L distances in complexes 1 and 2 are different. It is noteworthy that the CoII–L distances in complex 2 are consistent with the high-spin state of this complex. The coordination geometry and spin state of complex 2 are also supported by the purple colour of the complex, typical of six coordinate cobalt(II) compounds. Description of all four X-ray structures (complexes 1–4) The authors should mention the monodentate coordination of the carboxylate group and point out the differences in the C=O and C–O distances. They should also mention the two thiophene C–S single bonds and comment on their lengths. Descriptions of complexes 3 and 4 • Comment on the chemical formulae which show three (3) metal centres, yet Scheme 1 and Figure 1 show only two (2) metal centres • Write the triclinic space group correctly as P1 • Describe the coordination geometries at the NiII and CoII centres and mention deviations of the angles from the idealized octahedral angles and comment on the distortion.
A: Thank you for your valuable comments. We have revised the crystal structure analysis part of the manuscript and highlighted it in red in the manuscript.
5.IR Spectroscopy
Mention that the samples were compressed as KBr disks. The difference between the carboxylate νas(COO–) and νs(COO–) is indicative of the monodentate coordination of this group.
A：Thank you for your valuable comments. We have revised the infrared spectroscopy analysis part of the manuscript and highlighted it in red in the manuscript.
6.UV-visible Spectroscopy
In what solvent were the complexes dissolved? What were the concentrations of the solutions of the complexes? Figure 3: Why is the absorbance scale beginning from –0.2? If a blank was used, the scale should begin from 0.0. What were the molar absorptivity values (epsilon) for the absorption bands? UV-visible spectra of the complexes 1–4: There is no such electronic transition as d–d* transition. These ligand transitions are also described as d–d transitions, NOT d–d* transitions. On the other hand, the intra-ligand π–π* transition is correct. Molar absorptivities are required to assign the electronic transitions (d–d and π–π*) correctly.
A：Thank you for your valuable comments. We have revised the UV-vis diffuse reflectance spectroscopy analysis part of the manuscript and highlighted it in red in the manuscript.
7.Cytotoxicity
Evaluation Explain briefly the principle of the MTS cell viability assay. Were the measurements performed in triplicate? Are there any graphs of the results for the four complexes? How were the inhibitory rates of cell proliferation determined?
A: Thank you for your valuable comments. We have rewritten the method of MTS test cell activity in the manuscript and highlighted it in red in the manuscript.
8.Conclusions
Very poorly written. Improvement is required.
A: Thank you for your valuable comments. We have revised the conclusion and highlighted it in red in the manuscript.
9.References
The references are listed inconsistently. Some of the names of the journals are written in full while others are abbreviated. They should all be abbreviated.
A: We are very sorry for our negligence and thank you for your reminder. We have revised and checked the format of the references and highlighted them in red in the manuscript.
10.General comment
Some of the statements are inaccurate. Examples are as follows: Conclusion: “… including crystal structure analysis via single-crystal X-ray diffraction and absorption spectrum evaluation using infrared spectroscopy, alongside an assessment of their anticancer activities …” (very strange sentence and misleading)! Introduction: “… this study characterized their structural features using techniques such as elemental analyses (EA), UV-visible spectroscopy (UV-vis), …” Elemental analysis and UV-visible spectroscopy are NOT structural techniques!
A: Thank you for your valuable comments. We have modified the corresponding parts in the manuscript and highlighted them in red in the manuscript.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe quality of the English text can be improved
Author Response
Thank you for your valuable comments.
Reviewer 2 Report
Comments and Suggestions for AuthorsDetailed comments and suggested corrections are given in the attached PDF document.
Comments for author File: Comments.pdf
Author Response
Comments 1: Abstract
Lines 17–25: All four compounds were identified by elemental analysis and ESI mass spectrometry, and subsequently characterized by IR spectroscopy, UV-visible diffuse reflectance spectroscopy, electron paramagnetic resonance spectroscopy, thermogravimetric analysis, single-crystal X-ray crystallography and cyclic voltammetry. X-ray analyses revealed that complexes 1 and 2 exhibit a centrosymmetric pseudo-octahedral coordination geometry; the copper(II) and cobalt(II) metal ions, respectively, are located at the crystallographic center of inversion. The coordination sphere of the copper(II) complex is axially elongated in accordance with the Jahn-Teller effect. Intriguingly, for charge-neutrality, compounds 3 and 4 crystallized as three independent mononuclear octahedrally-coordinated metal centers, which are two [ML2(OH2)4] complex molecules and one [ML(OH2)5]+ complex cation (M = NiII and CoII, respectively), with the ligand anion L– serving as the counter ion.
Response 1: Thank you for your valuable comments. We have revised the abstract of the manuscript and highlighted it in red in the manuscript (lines 17-28).
Comments 2: Keywords
“crystal structure” should be replaced by “X-ray crystal structures” The authors can add “spectroscopic techniques” to the list of keywords.
Response 2: Thank you for your valuable comments. We have modified the keywords in the manuscript and highlighted them in red in the manuscript (lines 33-34).
Comments 3 : Results and Discussion
Line 110: “… water molecules (O3, O3#i), …” Change to “… aqua ligands (O3, O3#i), …”.
Response 3: Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 113).
Line 111: “… forming an octahedral coordination geometry.” Change to “… forming a distorted octahedral coordination geometry.”
Response 4 : Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 114).
Line 112: The distance of the Cu–N (Cu1–N1/N1#i) is given as 2.4473(14) Å which is wrong. The correct distance is 2.0039(15) Å.
Response 5: Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 115).
Line 125: The statement “The central Co2+ ion in complex 2 does not exhibit a significant Jahn-Teller effect due to its d7 electron configuration …” is WRONG! Complex 2 does NOT exhibit a Jahn-Teller distortion.
The statement should be: “The copper(II) (d9 configuration) and cobalt(II) (d7 configuration) complexes 1 and 2, respectively, are isostructural in that they are both centrosymmetric (Figure 1), but differ in that the six-coordinate geometry of the latter is not tetragonally distorted since this complex is singly degenerate. The equatorial CoII–Ocarboxylate [Co1–O1/O1#i = 2.0744(9) Å] and CoII–Npy [Co1–N1/N1#i = 2.1341(11) Å], and the axial CoII–Oaqua [Co1–O3/O3#i = 2.1419(9) Å] are normal.
Response 6：Thank you for your valuable comments. We have revised the manuscript and highlighted them in red in the manuscript (lines 128-133).
Line 136: The triclinic space group is P¯1 , not P1.
Response 7：Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 141).
Line 157: Figure 1. crystal structure of complexes 1–4. Figure 1. Crystal structures of complexes 1–4.
Response 8: Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 161).
Line 160: Table 1. Data of main bond lengths (Å) … Table 1. Selected bond lengths (Å) …
Response 9 : Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 164).
Line 163: 2.2 IR Analysis of Complexes 1–4 2.2 Infrared Spectra of Complexes 1–4
Response 10 : Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 167).
Line 164: Figure 2 displays the infrared spectral data of four complexes.
Infrared spectra of complexes 1–4 are displayed in Figure 2. The broad absorption bands between 3200 and 3600 cm–1 are attributable to the ν(O–H) for the aqua ligands. In the IR spectra of complexes 3 and 4, these absorptions are conspicuously more intense due to the presence of the solvent molecules of crystallization.
Response 11: Thank you for your valuable comments. We have revised the manuscript and highlighted them in red in the manuscript (lines 168-171).
Line 225: 2.4 Electron paramagnetic resonance spectrum of complex 1
Response 12: Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 232).
Lines 230 and 231: Additionally, the sequence ge < gê“• < g‖ indicates …
Additionally, the magnitude of the hyperfine parameters g‖ > gê“• > 2.0 indicates …
Response 13 : Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 238).
Line 232: “… with a low-spin state (S = 1/2).” For the d9 electron configuration, there is no high-spin or low-spin.
Response 14: Thank you for your valuable comments. We have deleted the corresponding part in the manuscript.
Figure 4. X-band EPR spectra spectrum for complex 1 (powder crystals polycrystalline sample) at 298 K. What was the microwave frequency (9.xx GHz)?
Response 15：Thank you for your valuable comments. We have revised the manuscript and added the microwave frequency (9.42 GHz) of the electron paramagnetic resonance spectrum, which is highlighted in red in the manuscript (lines 242-243).
Lines 271 & 272: The reduction potentials are referenced to what electrode? That is, versus NHE? versus Ag+/Ag? versus Fc+/Fc?
Response 16：Thank you for your valuable comments. We have added the corresponding part to the manuscript and highlighted it in red (lines 280-281).
Line 289: 3.1 General Methods 3.1 Materials and Methods of Characterization
Response 17: Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 298).
Line 311 &312: At what microwave frequency was the EPR spectrometer operating?
Response 18 : Thank you for your valuable comments. We have added the microwave frequency (9.42 GHz) of the electron paramagnetic resonance spectroscopy to the manuscript and highlighted it in red (lines 319-321).
Lines 312–317: What was the internal reference standard? That is, the reduction potentials are referenced to what redox couple?
Response 19 : Thank you for your valuable comments. We have revised the manuscript and highlighted it in red (lines 326-327).
Line 318: 3.2 X-Ray Structure 3.2 Single-Crystal X-ray Structure Determinations
Response 20：Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 328).
Line 320: Ga kα Ga-Kα
Response 21: Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 330).
Line 328: Table 3. crystallographic data … Crystallographic data …
Response 22 : Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript (line 338).
Triclinic space group: P1 P¯1
Response 23 : Thank you for your valuable comments. We have revised the manuscript and highlighted it in red in the manuscript.
Author Response File: Author Response.pdf
