Transition Metal Complexes of Schiff Base Ligands Prepared from Reaction of Aminobenzothiazole with Benzaldehydes

Round 1

Reviewer 1 Report

The Authors have greatly improved the Manuscript. However, some of the points need further revision.

 

1. Page 4, line 149 “However, the magnetic moment values of Cu, Cr and Fe complexes could not exclude the possibility of tetrahedral geometry…” Please discuss also the possibility of trigonal bipyramidal or square pyramidal geometry. Such geometry is also typical for Cu(II) and Co(II) compounds.
2. Page 6, line 200 “Thus, the free HB ligand exhibited broad bands in the range from 2500 to 2800 cm-1 that can be ascribed to intramolecular H- bonding”. Could the Authors provide some proofs? In ref. 30, it is stated “No bands corresponding to either O-H or N-H stretching were observed as a consequence of a strong intramolecular hydrogen bond” for the aldehydes. In ref. 29, it is stated “H2L exhibited a broad band characteristic of the OH group at 3300–3500 cm−1.”
3. Page 6, line 201 “These bands disappeared upon complexation with metal ions due to deprotonation and a chelate O,N coordination of HB to metals.” This contradicts with the proposed formulae [M(HB)₂Cl(OH)] in the Experimental Section. Please revise the discussion and/or the formulae.
4. The Cr(III) and Fe(III) compounds are formulated as ML₂Cl(OH)·xH₂O or ML₂Cl₂H₂O. However, these formulae are not charge-balanced. Please revise them (there should be three negative charges per M).
5. Page 8, line 224 “The broad hydroxyl peaks of HB at 10.69 and 12.01 ppm exhibited decrease in intensity and disappearance in the case of Cd…”. Figure 13S reveals the presence more than one set of peaks. Can this be a consequence of partial dissociation of Cd-HB? If so, the hydroxyl peaks and some other peaks could attribute to the free HB.

Author Response

Reviewer #1

The Authors have greatly improved the Manuscript. However, some of the points need further revision.

Page 4, line 149 “However, the magnetic moment values of Cu, Cr and Fe complexes could not exclude the possibility of tetrahedral geometry…” Please discuss also the possibility of trigonal bipyramidal or square pyramidal geometry. Such geometry is also typical for Cu(II) and Co(II) compounds.
Response: The possibility of trigonal bipyramidal or square pyramidal geometry for all the complexes was discussed. Line 141-202.

“The measured values of magnetic moment (µeff, B.M) are given in Table 1. These values were compared with those of previous reported ranges and those expected from calculating spin-only magnetic moment: (µ s.o. = √(4S(S+1)) B.M.); where: S: spin quantum number of unpaired electrons. The comparison with µ s.o. allows predicting orbital contribution to magnetic moment which is dependent on electronic configuration of complexes. The observed magnetic moments of Cu-NB, Cu-CB and Cu-HB complexes were 1.88, 1.88 and 1.84 B.M, respectively, which agreed with the literature experimental range for Cu(II) octahedral complexes which is 1.7-2.2 B.M.[22, 23] and µ s.o. (d9, S=1/2) = √(4(1/2) (1/2+1)) = 1.73 B.M. Similarly, the observed values of Cr-NB, Cr-CB and Cr-HB complexes (3.77, 3.73 and 3.87 B.M., respectively) were within the reported range for Cr(III) octahedral complexes which is 3.7-3.9 [22] and µ s.o. (d3, S=3/2) = √(4(3/2) (3/2+1)) = 3.87. Furthermore, the observed values of Fe-NB, Fe-CB and Fe-HB complexes (6.01, 5.85 and 5.15 B.M., respectively) were within the reported range for high spin Fe(III) octahedral complexes which is 5.7-6.0 [22] and µ s.o. (d5, S=5/2) = √(4(5/2) (5/2+1)) = 5.92. Since the measured magnetic moments of Cu(II), Cr(III) and Fe(III) complexes agree with µ s.o., orbital contribution to magnetic moment is minimal. This can be ascribed to the symmetrical occupation of electronic configuration of t2g in octahedral geometries [Housecroft and Sharpe 2005, Dalal 2019]. 

 

The unsymmetrical t2 occupation of electrons in tetrahedra Cu(II) and Cr(III) (e4 t25 and e2 t21 , respectively) results in orbital contribution to magnetic moment [Dalal 2019], thus, tetrahedral geometry for Cu(II) and Cr(III) complexes may be not plausible.  On the other hand, symmetrically occupied e2 t23 in tetrahedral Fe(III) configuration does not have orbital magnetism and thus tetrahedral geometry for Fe(III) complexes is possible. Using the same approach, symmetrical electron occupation of (dxz,dyz) (dx2-y2, dxy) (dz2) in trigonal bipyramidal  and (dxz,dyz) (dxy) (dz2) (dx2-y2) in square pyramidal Cu(II) and Fe(III) complexes implies absence of orbital magnetizmm and thus these geometries may be possible. Furthermore, the unsymmetrical electron occupation (dxz,dyz)2 (dx2-y2, dxy)1  in trigonal bipyramidal  and symmetric electron occupation (dxz,dyz)2 (dxy)1 in square pyramidal Cr(III) complexes indicates that trigonal bipyramidal geometry is not  possible while square pyramidal geometry may be plausible .

 

The observed magnetic moments of Ni-CB and Ni-HB complexes (3.37 and 2.90 B.M., respectively) were in agreement with that reported for Ni(II) octahedral complexes (2.8-3.5) [22] and µ s.o. (d8, S=1) = √(4(1) (1+1)) = 2.83 B.M. The literature range for Ni(II) tetrahedral complexes, which is 4.2-4.8 [22], is far from our values, supporting the octahedral geometry for Ni(II). The higher magnetic moment for Ni(II) tetrahedral complexes is due to the unsymmetrical electron occupation in tetrahedral e4 t24 Ni(II) configuration which has orbital contribution to magnetic moment. Square planar geometry for Ni(II) complexes is excluded since it is diamagnetic [25, Willis and Mellor 1947]. The unsymmetrical electron occupation (dxz,dyz)4 (dx2-y2, dxy)3 (dz2)1 in trigonal bipyramidal  and symmetrical occupation (dxz,dyz)4 (dxy)2 (dz2)1  (dx2-y2)1 in square pyramidal Ni(II) complexes [Tripathi et al. 2018] reflect orbital magnetizm in the former. Since no orbital contribution to magnetic moment was observed in Ni(II) complexes, trigonal bipyramidal is not possible while square pyramidal may be plausible.

 

The observed magnetic moments of Co-NB, Co-CB and Co-HB complexes (4.84, 5.28 and 5.73 B.M., respectively) were in agreement with that reported for Co(II) high spin octahedral complexes (4.3-5.2) [22, 24], but higher than the µ s.o. (d7, S=3/2) = √(4(3/2) (3/2+1)) = 3.87 B.M. Thus, orbital contribution to the magnetic moment is expected due to the unsymmetrical electron occupation t2g5 eg2 in high spin octahedral Co(II) configuration. Symmetrical electron occupation in tetrahedral e4 t23 Co(II) configuration has no orbital magnetism [Housecroft and Sharpe 2005] and consequently tetrahedral geomtry could be excluded. The literature range for Co(II) tetrahedral complexes 4.2-4.8 [22] is far from our values, supporting the octahedral geometry for Co(II) complexes. Square planar geometry for Co(II) is very rare [Tripathi et al. 2018]. The symmetrical electron occupation (dxz,dyz)4 (dx2-y2, dxy)2 (dz2)1 in trigonal bipyramidal  and (dxz,dyz)4 (dxy)1 (dz2)1  (dx2-y2)1 in square pyramidal Co(II) complexes [Tripathi et al. 2018] implies no orbital magnetism. Since orbital contribution to magnetic moment was observed in Co(II) complexes, these geometries are not plausible. As expected, the Cd(II) complexes in Table 1 were diamagnetic [26].”

 

C. E. Housecroft and A. G. Sharpe (2005) Inorganic chemistry, Pearson Education Limited England, pp. 583.

 

Tripathi, S., Dey, A., Shanmugam, M., Narayanan, R.S., Chandrasekhar, V. (2018). Cobalt(II) Complexes as Single-Ion Magnets. In: Chandrasekhar, V., Pointillart, F. (eds) Organometallic Magnets . Topics in Organometallic Chemistry, vol 64. Springer, Cham. https://doi.org/10.1007/3418_2018_8

 

Dalal M. (2019) A textbook of inorganic chemistry volume 1, chapter 9, Dalal institute.

 

J. B. Willis and D. P. Mellor, The Magnetic Susceptibility of Some Nickel Complexes in Solution. J. Am. Chem. Soc. 1947, 69, 6, 1237–1240. https://doi.org/10.1021/ja01198a001

 

 

Page 6, line 200 “Thus, the free HB ligand exhibited broad bands in the range from 2500 to 2800 cm-1 that can be ascribed to intramolecular H- bonding”. Could the Authors provide some proofs? In ref. 30, it is stated “No bands corresponding to either O-H or N-H stretching were observed as a consequence of a strong intramolecular hydrogen bond” for the aldehydes. In ref. 29, it is stated “H2L exhibited a broad band characteristic of the OH group at 3300–3500 cm−1.” 
Response: Line 263: “In their study of intramolecular hydrogen bonding in N-salicylideneaniline, Moosavi-Tekyeh and Dastani attributed the weak band between 2700 and 3100 cm−1, which was sensitive to deuteration, to the OH group involved in strong intramolecular OH···N hydrogen bonding (in CCl4 solvent). The higher frequency broad bands at 3200-3500 cm-1  in HB can be attributed to the second phenolic OH group.”

 

Z. Moosavi-Tekyeh, N. Dastani,(2015) Intramolecular hydrogen bonding in N salicylideneaniline: FT-IR spectrum and quantum chemical calculations, Journal of Molecular Structure, Volume 1102, 2015, Pages 314-322,   https://doi.org/10.1016/j.molstruc.2015.09.001.

 

 

Page 6, line 201 “These bands disappeared upon complexation with metal ions due to deprotonation and a chelate O,N coordination of HB to metals.” This contradicts with the proposed formulae [M(HB)2Cl(OH)] in the Experimental Section. Please revise the discussion and/or the formulae.
Response: The deprotonation idea was omitted since no basic conditions are used. Line 261: “These bands disappeared upon complexation with metal ions due to involvement of O and N of HB in coordination to metals.” 

 

The Cr(III) and Fe(III) compounds are formulated as ML2Cl(OH)·xH2O or ML2Cl2H2O. However, these formulae are not charge-balanced. Please revise them (there should be three negative charges per M).
Response: The formulae were corrected. 

Cr-NB: Yield: 66%, color: dark green, m.p.: 70-72 °C, analysis: C 48.67, H 3.30, N 11.52, S 8.78%, calculated for CrL2(OH)3.0.5H2O: C 48.78, H 3.55, N 11.38, S 8.68%. 

Fe-NB: Yield: 57%, color: brown, m.p.: 134-136 °C, analysis: C 47.98, H 3.11, N 11.85, S 8.89%, calculated for FeL2Cl(OH)2: C 47.92, H 3.22, N 11.18, S 8.53%. 

 

 

Cr-CB: Yield: 62%, color: blue, m.p.: 278-280 °C (dec.), analysis: C 49.85, H 3.54, N 7.75, S 8.78%, calculated for CrL2Cl(OH)2: C 49.56, H 3.33, N 7.71, S 8.82%.

Fe-CB: Yield: 57%, color: black, m.p.: 88-90 °C, analysis: C 49.87, H 3.60, N 7.96, S 8.97%, calculated for FeL2(OH)3.0.5H2O: C 49.95, H 3.63, N 7.77, S 8.89%.

 

Cr-HB: Yield: 66%, color: reddish-brown, m.p.: 92-94 °C, analysis: C 49.75, H 3.87, N 7.51, S 9.12%, calculated for CrL2Cl(OH)2: C 49.90, H 3.63, N 7.76, S 8.88%.

Fe-HB: Yield: 67%, color: black, m.p.: 95-97 °C, analysis: C 49.87, H 3.64, N 7.88, S 8.91%, calculated for FeL2Cl(OH)2: C 49.63, H 3.61, N 7.72, S 8.83%.

 

Page 8, line 224 “The broad hydroxyl peaks of HB at 10.69 and 12.01 ppm exhibited decrease in intensity and disappearance in the case of Cd…”. Figure 13S reveals the presence more than one set of peaks. Can this be a consequence of partial dissociation of Cd-HB? If so, the hydroxyl peaks and some other peaks could attribute to the free HB.
Response: We agree with the reviewer that “the Cd-HB complex was partially dissociated in DMSO since the 1H-NMR of HB ligand (Figure 11S) and Cd-HB (Figure 13S) are almost identical with with more splitting of peaks in the case of complex .” Line 298.   

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper 'Transition metal complexes of Schiff base ligands prepared from reaction of aminobenzothiazole with benzaldehydes' is devoted to the synthesis of complexes of several d-metal ions with three Schiff bases and their characterization by means of physical and chemical methods. The manuscript has been improved significantly since the first submission; and the most commentaries were addressed. However, there are still some questions and comments remain:

1. Do the Schiff bases actually melt or decompose? (line 86)
2. The assignment of peaks in NMR spectra should be done on some basis, e.g. 2D NMR experiments or predictions made in any available software. The location of azomethine proton resonanse in the range of 9-9.5 ppm seems too downfield. The same refers to carbon resonances assignment. For example, in hydrazones, azomethyne protone signal is observed at 7.5-8.0 ppm while carbon signal can be found at 135-140 ppm.
3. If the complexes contain solvent molecules, it should affect the elemental analysis results. Results for no complex show the presence of ethanol molecule.
4. The spectrum shown in Fig. 8S is more similar to that of low-spin complex of Co(II). Moreover, if this solution is allowed to stay for a coubple of weeks, its gonna change due to Co(II) oxidation to Co(III). I recommend perform such an experiement. If two weeks is too much to wait, couple drops of 3% H₂O₂ would do the same trick immediately. (see the paper 10.1080/00958972.2018.1512708)
5. The quality of Figures as Schemes should be improved as they are blur.

Author Response

Reviewer #2

The paper 'Transition metal complexes of Schiff base ligands prepared from reaction of aminobenzothiazole with benzaldehydes' is devoted to the synthesis of complexes of several d-metal ions with three Schiff bases and their characterization by means of physical and chemical methods. The manuscript has been improved significantly since the first submission; and the most commentaries were addressed. However, there are still some questions and comments remain:

1. Do the Schiff bases actually melt or decompose? (line 86).

Response: All Schiff bases ligands were melted without decomposition except HB as indicated in the experimental part.

 

1. The assignment of peaks in NMR spectra should be done on some basis, e.g. 2D NMR experiments or predictions made in any available software. The location of azomethine proton resonanse in the range of 9-9.5 ppm seems too downfield. The same refers to carbon resonances assignment. For example, in hydrazones, azomethyne protone signal is observed at 7.5-8.0 ppm while carbon signal can be found at 135-140 ppm.

 

Response: Line 363: “The assignment of all ¹H and ¹³C-NMR chemical shifts to their corresponding protons and carbons of NB ligand was accomplished by assistance of 2D-NMR which include COSY, HMQC and HMBC experiments (Table 1S and Figures 15S-17S)”.

 

Line 392: “The assignment of all ¹H and ¹³C-NMR chemical shifts was verified using 2D-NMR which include COSY, HMQC and HMBC experiments (Table 1S and Figures 18S-20S)”.

 

 

The previously reported chemical shifts for azomethine proton are 9.2 (CDCl₃) for HB ligand [Ha et al. 2010], 9.06-9.09 for HB analogues [Yeap et al. 2012] and 9.13-9.42 ppm for Schiff bases derived from 3-amino-1,2,4-triazole [Issa et 2009].

 

G-Y Yeap, B-T Heng, N. Faradiana, R. Zulkifly, M. M. Ito, M. Tanabe, D. Takeuchi “Synthesis, molecular structures and phase transition studies on benzothiazole-cored Schiff bases with their Cu(II) and Pd(II) complexes: Crystal structure of (E)-6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneamino)benzothiazole” Journal of Molecular Structure, 1012, 2012, 1-11, https://doi.org/10.1016/j.molstruc.2011.12.048. 457

 

S-T Ha, T-M Koh, G-Y Yeap, H-C Lin, S-L Lee, Y-F Win & S-T Ong (2010) Synthesis and Mesomorphic Properties of 6-458 Methoxy- and 6-Ethoxy-2-(2-Hydroxy-4 Alkanoyloxybenzylidenamino)Benzothiazoles, Molecular Crystals and Liquid 459 Crystals, 528:1, 10-22, DOI: 10.1080/15421406.2010.504510

 

Y.M. Issa, H.B. Hassib, H.E. Abdelaal, (2009) ¹H NMR, ¹³C NMR and mass spectral studies of some Schiff bases derived from 3-amino-1,2,4-triazole,

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 74, 902-910. https://doi.org/10.1016/j.saa.2009.08.042.

 

 

1. If the complexes contain solvent molecules, it should affect the elemental analysis results. Results for no complex show the presence of ethanol molecule.

Response: The complexes contain water from water of hydration of metal salts used in synthesis of complexes (line 402). The complexes were dried in vacuum oven at 60 °C which remove ethanol solvent from the complexes.

The sentence: Line 135 “. Such low melting points may be due to either the hydroxyl or water molecules in the complexes (section 3.3.1)”

 

1. The spectrum shown in Fig. 8S is more similar to that of low-spin complex of Co(II). Moreover, if this solution is allowed to stay for a couple of weeks, its gonna change due to Co(II) oxidation to Co(III). I recommend perform such an experiment. If two weeks is too much to wait, couple drops of 3% H₂O₂ would do the same trick immediately. (see the paper 10.1080/00958972.2018.1512708).

Response: We agree with the reviewer that the Co (II) complex is oxidized with air to Co(III) ,  it is fixed this in line 303. “Consequently, so we believe that the Co-NB (Figure 3S) and Co-CB (Figure 8S) complexes are oxidized in air to Co(III ) complexes  which is the reason for the noisy spectrum in the range 0 to 11 ppm”

 

1. The quality of Figures as Schemes should be improved as they are blur.

Response: The quality of all Figures in the supplementary material was improved, as well as the schemes in the manuscript.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The Authors have responded to all the points and revised the Manuscript accordingly. I believe that now the Manuscript can be accepted for publication.

Author Response

The authors express their great thanks for the reviewer for his comments and suggestions. 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The Manuscript reports the synthesis and characterization of three Schiff bases (NB, CB, and HB) and their Co(II), Ni(II), 16 Cu(II), Cd(II), Cr(III), and Fe(III) complexes. The compounds were characterized by IR, NMR spectroscopy, elemental analysis, magnetic susceptibility, and molar conductivity analysis to reveal the composition and the structure of the complexes. The analysis of the data is questionable; I would recommend rejecting the Manuscript in order to make significant revisions and then resubmit the Manuscript. The points that should be revised are as follows:

 

1. The Authors claim that the three Schiff bases (NB, CB, and HB) ligands were prepared for the first time. However, all of them have been reported previously, for instance, in the following works: Weihua Li, et al, Chirality, Volume 24, Pages 223-231 https://doi.org/10.1002/chir.21986. Weihua Li, Volume 2018, Issue39, 2018, Pages 5422-5426 https://doi.org/10.1002/ejoc.201801013; Mohammad H.El-Dakdouki et al, Tetrahedron Volume 73, Issue 39, 2017, Pages 5769-5777 https://doi.org/10.1016/j.tet.2017.08.027; Guan-YeowYeap et al, Journal of Molecular Structure Volume 1012, 2012, Pages 1-11 https://doi.org/10.1016/j.molstruc.2011.12.048; Sie-Tiong Ha et al, Molecular Crystals and Liquid Crystals Volume 528, Issue 1, 2010 Pages 10-22 https://doi.org/10.1080/15421406.2010.504510. Please revise the corresponding phrases.
2. The synthesis of the complexes seems to be the main topic of the work. However, it was not discussed at all. Please discuss the synthesis of the complexes (what metal salts were used, what solvents were used, what concentrations and/or molar ratios of the Schiff base and metal salts were used, etc.). Section 3.3 can be moved from Experimental Section to the Results and Discussion Section, while appropriate experimental details for the synthesis of the complexes (loadings in mmol and mL and yields) should be included in the Experimental Section.
3. Line 127 “Remarkably, the melting points of HB complexes were much lower than that of free HB ligand” Such low melting points likely indicate that the compounds comprise solvent molecules. Please discuss this in the Manuscript.
4. “The results indicated that Cu(II), Ni(II), and Co(II) complexes have octahedral geometry. The observed magnetic moment of Cu(II) complexes ranges from 1.84-1.88 B.M which is in agreement with the reported values (1.70-2.20) [19] and μo. (d9, S=1/2) = √(4(1/2) (1/2+1)) = 1.73 B.M. Similarly, the observed magnetic moment of Ni (II) complexes ranges from 2.90-3.37 B.M, which is in agreement with the reported values (2.8-3.5) [20]…” and related sentences. Works [18-21] do not contain any data proving that Cu(II), Ni(II) and Co(II) showing such magnetic moments have octahedral geometry. Please provide correct references clearly distinguishing between octahedral and other (square planar, tetrahedral, square pyramidal, trigonal bipyramidal, etc.) coordination environments based on the magnetic moment. The work [23] also does not allow one to distinguish between Fe(II) and Fe(III). As far as I understand, the calculated magnetic moment for Fe(III) ion is ca. 5.9 MB, which agrees with the experimental data (Table 2). Please carefully analyze the literature data and revise the magnetic properties Section.
5. Abstract and Conclusion: “tetrahedral [ML2] for M = Cd” I did not find any discussion of the tetrahedral coordination environment of Cd; furthermore, it contradicts with Scheme 2. In my point of view, large Cd ion (largest in the series) would rather show an octahedral environment, but this, of course, needs some proof.
6. “Cr(III) and Fe(III) are assumed to undergo reduction to Cr(II) and Fe(II) in the complexes.” This is quite a strong conclusion that needs a deep study. Fe(III) in such complexes is hard to reduce; Cr(III) is reduced to Cr(II) even harder only with strong reducing agents, such as H₂. In the reaction conditions, no strong reducing agents were added; Schiff bases also do not act as a reducing agent, as represented by numerous examples of crystal structures of Fe(III) and Cr(III) complexes with Schiff bases. Some of them taken randomly are: CCDC Refcode ACOREQ, DOI: 10.1246/bcsj.79.442; CCDC Refcode AETSFE, DOI: 10.1039/dt9750001344; CCDC Refcode NALVOM, DOI: 10.1016/S0020-1693(03)00311-6; CCDC Refcode JAFKUX, DOI: 10.1002/ejic.200200575; CCDC Refcode AESCRI, DOI: 10.1107/S0567740871004291; CCDC Refcode AFUBUZ, DOI: 10.1021/ja800302c. Thus, please avoid claiming reduction to Fe(II) and Cr(II) in the Manuscript.
7. “Cr(III) … are assumed to undergo reduction to Cr(II) …” This phrase contradicts with the phrase below: “The observed magnetic moment of Cr(III) complexes ranges from 3.73-3.90 B.M, which is in agreement with the reported values (3.70-3.90) B.M [18] and μo. (d3, S=3/2)” Please revise the phrase(s)
8. Line 185: “The azomethine ν(-HC=N-) and benzothiazole imine ν (C=N) were shifted to higher frequency upon complexation with metal ions.” The sentence contradicts Table 2; for instance, -C=N- band for Co-NB is shifted to lower frequencies. Anyway, IR spectroscopy does not allow one to unequivocally define the coordination mode of the ligands using only IR spectroscopy. Please revise the IR Section.
9. “the following proposed structures: [CuL2Cl2], [CoL2Cl2], [NiL2Cl2], [CrL2]Cl2, [FeL2]Cl2 and [CdL2]” In the formulae, inner- and/or outer-sphere ethanol and water molecules are ignored, although they can present in the compounds according to the low melting temperature in some cases. In addition, deprotonation of water and/or HB can proceed in the complexation reaction. In fact, tens of variants of formulae can agree with the experimental elemental analysis. For instance, for Cr-NB, one can imagine the following formulae: CrL₃Cl₃(H₂O) (C 48.4, N 11.3 H 3.2, S 8.6%); Cr₂L₃ClO₂ (C 48.6, N 11.3 H 3.0, S 8.6%), Cr₃L₅Cl₃(H₂O) (C 48.8, N 11.4 H 3.1, S 8.7%). Furthermore, a mixture of complexes of different compositions can form as well as free Schiff bases precipitate, which also was not considered at all. Thus, I would recommend further studying the possible compounds formed. For instance, thermogravimetry analysis could help to determine the number of solvate molecules.
10. “These bands disappeared upon complexation with metal ions due to involvement of nitrogen of azomethine with bonding with metal ion.” These bands can also disappear due to deprotonation and a chelate O,N coordination of HB to metals as frequently observed in Schiff-based complexes even without adding additional bases.
11. Table 2. what is “-----“ in this case? Does this mean that the measurement was not carried out? If yes, why? This should be clarified in the Manuscript.
12. Molar conductivity of metal complexes Section. “The relatively small values of molar conductivity values of Cu(II), Co(II), Ni(II) and Cd(II) complexes” and “relatively higher molar conductivity values [17], [18] of Fe(III) and Cr(III) complexes” According to Table 2, the value for Co-CB is relatively high, while the value for Cr-CB is relatively low. Please, revise the sentences.
13. Section 7. (1H-NMR study of complexes). The complexes reveal different behavior showing upfield or downfield shifts of the signals compared with free Schiff bases. Then why do the Authors assume a similar coordination mode for all the complexes?
14. Scheme 2 for M = Fe and Cr contradicts the conclusion from conductivity analysis about outer sphere chloride ions. Please revise the Scheme and/or revise the Conductivity Section.
15. Line 84. “The colors of ligands were yellow … and orange … due to the high conjugation in these ligands” The color of the compounds does not depend on the degree of conjugation, but rather on the presence of a charge transfer. In fact, the benzothiazole moiety rather than the other groups is likely responsible for the color of the compounds, so their color should be compared with the benzothiazole precursor.
16. Figure 5. Please provide the NMR spectra for the other complexes in the Supporting Information.
17. Conclusions and Abstract are questionable and contradict the Results and Discussion Section. Please carefully revise the Sections.

Reviewer 2 Report

The paper "Schiff base ligands from the reaction of aminobenzothiazole with benzaldehydes: synthesis and their transition metal complexes" is devoted to the synthesis of three ligands and several complexes of different d-metal ions with these ligands. All the synthesized compounds are thoroughly characterized by modern physical and chemical methods of analysis, and the reasonable suggestion on the composition and structure of the compounds are deduced from the spectral data.

The paper is suitable for publication in Inorganics; however, some minor comments require attention:

 

1. The quality of all the figures should be improved as now they (except Figs. 3, 4) look like screenshots. NMR spectra can also be exported in ASCII form and re-drawn in any plotter.
2.  What is the chemical shift of the formyl proton of the starting aldehydes? It would be great to demonstrate the disappearance of the CHO signal and the appearance of methine resonance instead in ¹H NMR spectra.
3. Why the molar conductivity of metal complexes is measured in a non-polar solvent such as ethanol? What is the concentration of the metal complex? Is it the same in all the measurements?
4. The finding that Co(II) complex is in a high-spin configuration confirmed by the magnetic susceptibility data is in somewhat contradiction with the ¹H NMR spectrum of this complex. High-spin Co(II) complex should have a huge paramagnetic shift (up to 100 ppm) and narrow spectral bands. Please, clarify. I also recommend adding the 1H NMR spectra for all the complexes to the supplementary material file as now only spectrum for Cu(II) is shown. If necessary, these spectra should be re-registered with the broadened spectral width to catch the possible paramagnetic shifted lines.
5. If Cr(III) and Fe(III) ions are assumed to undergo reduction to Cr(II) and Fe(II) in the complexes, what undergoes oxidation? What donates the electrons to metal ions?