Steric Effects of Alkyl Substituents at N-Donor Bidentate Amines Direct the Nuclearity, Bonding and Bridging Modes in Isothiocyanato-Copper(II) Coordination Compounds

Round  1

Reviewer 1 Report

In this manuscript, Salah S. Massoud and their co-workers described the results of crystal structures of Thiocyanato-Copper(II) complexes with bidentate amine ligands. This manuscript is well-written, and the contents and results of this manuscript are also well-matched to an object of Crystal journal. However, I reviewed that the original manuscript is necessary to improve as follows.

1. There are several typos in original manuscript. Authors should recheck the English.

In column 38-39, … factors which include which include electronic … → … factors which include electronic ….

In column 40-41, co-ligand(s) and coligand(s).

In column 64, Density Functional theory → Density Functional Theory.

2. Authors measured the absorption spectra of complexes 1-4 in CH₃CN. However, as you known, these measurements are not suitable for the coordination polymers (complexes 1-3) because the dissolve of coordination polymers in CH₃CN indicate the decomposition or fragmentation of coordination polymers. So, in general, the absorption spectra of coordination polymers are measured at the solid-state. Although authors mentioned that “The geometrical finding around the Cu(II) ion in acetonitrile solution was retained in the solid state as it supported with the X-ray structural data”, this is the speculation and many readers have doubts about that. Thus, I strongly recommend authors to measure the solid-state diffuse reflectance spectra of complexes 1-5 and should add their spectra as the figure in the revised manuscript.

3. In the DFT calculations, authors performed the geometry optimizations for the fragment geometries of complexes 4 and 5 at three different oxidation states to estimate the relative energies of their complexes. However, the methods for calculations of relative energies are not described in details.

i). Did you check the optimized geometries of complexes 4 and 5 by the frequency analyses?

ii). Did you use the zero-point energies or free energies of optimized geometries of 4 and 5, but not total energies of them?

iii) How is compared the relative energies between Cu(I), Cu(II), and Cu(III) complexes? Did you correct the energies of deficiency and excess electrons? 

Author Response

In this manuscript, Salah S. Massoud and their co-workers described the results of crystal structures of Thiocyanato-Copper(II) complexes with bidentate amine ligands. This manuscript is well-written, and the contents and results of this manuscript are also well-matched to an object of Crystal journal. However, I reviewed that the original manuscript is necessary to improve as follows.

My deep gratitude to the referee for his positive comments and time to review the Ms

1. There are several typos in original manuscript. Authors should recheck the English.

In column 38-39, … factors which include which include electronic … → … factors which include electronic ….

In column 40-41, co-ligand(s) and coligand(s).

In column 64, Density Functional theory → Density Functional Theory.

All of these points were fixed and corrected.

2. Authors measured the absorption spectra of complexes 1-4 in CH₃CN. However, as you known, these measurements are not suitable for the coordination polymers (complexes 1-3) because the dissolve of coordination polymers in CH₃CN indicate the decomposition or fragmentation of coordination polymers. So, in general, the absorption spectra of coordination polymers are measured at the solid-state. Although authors mentioned that “The geometrical finding around the Cu(II) ion in acetonitrile solution was retained in the solid state as it supported with the X-ray structural data”, this is the speculation and many readers have doubts about that. Thus, I strongly recommend authors to measure the solid-state diffuse reflectance spectra of complexes 1-5 and should add their spectra as the figure in the revised manuscript.

Thank you again for raising this issue as he is absolutely right.  The solid-state diffuse spectra for the polymeric and dimeric complexes 1-4 were measured and are given in Figures S1-S4. Please see the 2^nd paragraph on p-9.

3. In the DFT calculations, authors performed the geometry optimizations for the fragment geometries of complexes 4 and 5 at three different oxidation states to estimate the relative energies of their complexes. However, the methods for calculations of relative energies are not described in details.

The figure 6 caption contains the sentence “The Dimer geometries are relative to the energy of the Cu(II) + Cu(II) monomer asymptote at infinite separation”. This was in fact a typo and should read “Cu(I) + Cu(I)” instead. We have revised this typo in the manuscript. This is an obvious choice since the Cu(I) + Cu(I) monomer is the lowest energy oxidation state. The energies of all other structures and oxidation states are relative to the Cu(I) + Cu(I) monomer asymptote. We therefore do not believe further clarification is necessary.

i). Did you check the optimized geometries of complexes 4 and 5 by the frequency analyses?

The computations were already challenging with the available resources. Computing the normal mode wavenumbers would add to the already prohibitive expense of the computations. We have included added a sentence on page 16 advocating this limitation. We note that the dimeric units were especially hard to converge. This can be understood by considering the relatively weak interaction between pairs of complexes (cf. the intrinsic dative bonds of the monomer). The optimization steps were difficult to converge, indicating that the change in potential energy as a function of displacements across the 3n-6 (n = number of atoms) degrees-of-freedom is very shallow around the global minima.

ii). Did you use the zero-point energies or free energies of optimized geometries of 4 and 5, but not total energies of them?

Since normal mode wavenumbers were not computed we did not zero-point correct the total energies, nor derive free-energies. However, when comparing relative energies between oligomers, the zero-point corrections to the total energy are negligible since the number of degrees-of-freedom are retained in all cases. Comparing total energies are therefore adequate.

iii) How is compared the relative energies between Cu(I), Cu(II), and Cu(III) complexes? Did you correct the energies of deficiency and excess electrons? 

There are no spurious corrections to the total energy of the various oxidation states. For such approximate energies, it is unreliable to further correct states with excess or deficient electrons. It is well known that DFT is sufficient at computing both ionization energies and electron affinities. In addition, the Cu(I) and Cu(III) oxidation states represent stable states in which the associated compounds clearly attach an electron. Therefore, there is no need to use arbitrary non-Hermitian quantum mechanics to stabilize such states

Also, there was a typo error in the original Figure 6 concerning the oxidation states for Cu and this was corrected and we apologize for this.

Author Response File: Author Response.pdf

Reviewer 2 Report

Review

The submitted report describes the synthesis, structure and properties of five coordination compounds. The presented results show novelty and can be interesting for readers of Crystals, but, due to small errors and omissions (described below), the reviewer cannot recommend publication of the manuscript in the current form and suggests revision.

Comments:

In general the language of the manuscript is understandable, but it contains some inaccuracies, mainly related to chemical terminology. For example the authors use extremely non-recommended (by IUPAC) and obsolete term “complex” instead of  term “coordination compound”,  the phrase “absorption band” (used in description of UV-Vis spectra)  is incorrect and should be changed to “absorption maxima”. These and similar argot phrases should be corrected. For guidance please see the IUPAC Compendium of Chemical Terminology (the Gold Book) available online, free of charge.

The determination of the electrolyte type on the basis of point measurement, however relatively common, is generally incorrect. The authors should determine the Kohlrausch curves to study the electrochemical character of synthesised compounds solutions. Additionally the authors should prove the constancy of the compounds composition in the solution (by any suitable analytical technique), because dissolution may lead to partial or total dissociation of the coordination compounds studied in the solid state. Alternatively authors may delete the sentence describing the conductivity (lines 77 and 78).

The shifts of the position of absorption maxima observed in the solution spectra might originate from the partial dissociation of the studied compounds. Authors should prove the constancy of the compounds composition in the solution (by any suitable analytical technique), or register the solid state UV-Vis spectra and compare them with solution spectra. Alternatively, the possibility of dissociation (and the fact that the registered spectra may reflect at least partially dissociate compound) should be mentioned in the manuscript.

The Figures 1, 2 and 3 should be replaced by three new figures (one per compound), and each one must contain the asymmetric unit atoms placed in bonded positions (represented as anisotropic displacement ellipsoids) and the directly neighbouring the symmetry-generated atoms (only atoms bonded directly to the atoms of the asymmetric unit) represented by dashed ellipsoids (or hollow ellipsoids or other kind of ellipsoids different from asymmetric unit ellipsoids). The bonds linking asymmetric unit atoms with the symmetry generated atoms also must be represented by dashed lines (or hollow lines or other kind of lines different from asymmetric unit bonds).

Beside this the figure representing the simplified polymeric chains should be added to the manuscript.

The Figures 4 should contain the asymmetric unit atoms placed in bonded positions (represented as anisotropic displacement ellipsoids) and the symmetry-generated atoms represented by dashed ellipsoids (or hollow ellipsoids or other kind of ellipsoids different from asymmetric unit ellipsoids). The bonds linking asymmetric unit atoms with the symmetry generated atoms (and the symmetry generated atoms with symmetry generated atoms)  also must be represented by dashed lines (or hollow lines or other kind of lines different from asymmetric unit bonds).

The hydrogen bonds existing in the studies compounds should be shortly and correctly described in terms of graph sets (the unitary graph set must be given and most characteristic motifs of the higher graph set levels should be listed). The respect description of the graph sets idea is given e.g. in Angew. Chem. Int. Ed. Engl. 34, 1555-1573.

The geometrical parameters of all intermolecular interactions should be tabularised, instead of reporting them in text.

The bond lengths and angles multiply repeated in text and in tables must be deleted from the manuscript text and replaced by reference to the respect table.

The paragraph 2.4 contains description of some virtual models of hypothetical compounds (copper(III) compounds, copper(I) compounds and compound with too small number ligands in coordination sphere). Consequently discussion is based only on theoretical arrangements, most probably non-synthesisable in real experiments, and is only slightly relevant to the manuscript merit. Reviewer suggests deletion of the whole paragraph 2.4 and 3.4, as they do not provide important insights to the studied topic.

The used restrains and twinning model should be described in the experimental section. If the atoms with restrained U(i,j) tended to fall out of metric space, they should be refined with the XNPD command (incorporated in the latest SHELXL software), otherwise they should be refined as non- restrained.

Author Response

Review

The submitted report describes the synthesis, structure and properties of five coordination compounds. The presented results show novelty and can be interesting for readers of Crystals, but, due to small errors and omissions (described below), the reviewer cannot recommend publication of the manuscript in the current form and suggests revision.

Thank you for careful revision.

Comments:

In general the language of the manuscript is understandable, but it contains some inaccuracies, mainly related to chemical terminology. For example the authors use extremely non-recommended (by IUPAC) and obsolete term “complex” instead of  term “coordination compound”,  the phrase “absorption band” (used in description of UV-Vis spectra)  is incorrect and should be changed to “absorption maxima”. These and similar argot phrases should be corrected. For guidance please see the IUPAC Compendium of Chemical Terminology (the Gold Book) available online, free of charge.

Thank you for these crucial points.  Both terms were fixed (p-4) and are highlighted all over the Ms.

The determination of the electrolyte type on the basis of point measurement, however relatively common, is generally incorrect. The authors should determine the Kohlrausch curves to study the electrochemical character of synthesised compounds solutions. Additionally the authors should prove the constancy of the compounds composition in the solution (by any suitable analytical technique), because dissolution may lead to partial or total dissociation of the coordination compounds studied in the solid state. Alternatively authors may delete the sentence describing the conductivity (lines 77 and 78).

With our full respect to the reviewer’s opinion, we do not agree with him in this point. The molar conductivity was not an essential part of our study.  It was only used to confirm the electrolytic nature of the compounds as non-electrolyte, 1:1 or 1:2 electrolytes.  As the compounds were shown to be non-electrolyte (L_M measured in CH₃CN = 7-12 W^-1cm²mol^-1;   (L_M = 130-160 W^-1cm²mol^-1 for 1:1 electrolyte and ~240 W^-1cm²mol^-1 for 1:2 electrolytes).  Therefore, the measured L_M of 7-12 W^-1cm²mol^-1, not only supporting the non-electrolytic nature of the compounds but also presents an evidence for the non-dissociation/decomposition of the polymeric species, otherwise the measured L_M values should be much higher than that! Therefore, we did not delete the sentence describing the conductivity. 

The shifts of the position of absorption maxima observed in the solution spectra might originate from the partial dissociation of the studied compounds. Authors should prove the constancy of the compounds composition in the solution (by any suitable analytical technique), or register the solid state UV-Vis spectra and compare them with solution spectra. Alternatively, the possibility of dissociation (and the fact that the registered spectra may reflect at least partially dissociate compound) should be mentioned in the manuscript.

Solid spectra was measure and the measured L_M in CH₃CN = 7-12 W^-1cm²mol^-1 clearly reveals the non-electrolytic nature; no decomposition or dissociation which means that compounds retain un-dissociated in solution

The Figures 1, 2 and 3 should be replaced by three new figures (one per compound), and each one must contain the asymmetric unit atoms placed in bonded positions (represented as anisotropic displacement ellipsoids) and the directly neighbouring the symmetry-generated atoms (only atoms bonded directly to the atoms of the asymmetric unit) represented by dashed ellipsoids (or hollow ellipsoids or other kind of ellipsoids different from asymmetric unit ellipsoids). The bonds linking asymmetric unit atoms with the symmetry generated atoms also must be represented by dashed lines (or hollow lines or other kind of lines different from asymmetric unit bonds).

Beside this the figure representing the simplified polymeric chains should be added to the manuscript.

New Figures 1a, 2a, 3a, for compounds 1-3, respectively,  have been prepared with atoms of asymmetric unit and symmetry related atoms connected by different bond types. Also, new Figures 1b, 2b, 3b showing simplified polymeric chains for 1-3, respectively, are added.

The Figures 4 should contain the asymmetric unit atoms placed in bonded positions (represented as anisotropic displacement ellipsoids) and the symmetry-generated atoms represented by dashed ellipsoids (or hollow ellipsoids or other kind of ellipsoids different from asymmetric unit ellipsoids). The bonds linking asymmetric unit atoms with the symmetry generated atoms (and the symmetry generated atoms with symmetry generated atoms)  also must be represented by dashed lines (or hollow lines or other kind of lines different from asymmetric unit bonds).

FIXED ! New modified Figure 4 is given!

The hydrogen bonds existing in the studies compounds should be shortly and correctly described in terms of graph sets (the unitary graph set must be given and most characteristic motifs of the higher graph set levels should be listed). The respect description of the graph sets idea is given e.g. in Angew. Chem. Int. Ed. Engl. 34, 1555-1573.

The geometrical parameters of all intermolecular interactions should be tabularised, instead of reporting them in text.

The intermolecular interactions are listed in a new Table 3  “Possible hydrogen bonds of 1 -5.”

In the review article of Joel Bernstein et al. Angew. Chem. Int. Ed. Engl. 34,1555-1573 graph set analysis for hydrogen bonds is described for “discrete” (organic) molecules, however compounds 1-3 in the present manuscript are 1D coordination polymers dominated by coordinative bonds.

The bond lengths and angles multiply repeated in text and in tables must be deleted from the manuscript text and replaced by reference to the respect table.

FIXED !

The paragraph 2.4 contains description of some virtual models of hypothetical compounds (copper(III) compounds, copper(I) compounds and compound with too small number ligands in coordination sphere). Consequently discussion is based only on theoretical arrangements, most probably non-synthesis able in real experiments, and is only slightly relevant to the manuscript merit. Reviewer suggests deletion of the whole paragraph 2.4 and 3.4, as they do not provide important insights to the studied topic.

Paragraph 2.4 contains important discussions on the geometric reasons for why 4 and 5 form, respectively, monomeric and dimers and not subsequent oligomers. It is clear that this type of computation is important in complementing the observed experimental results and clarifying the interpretation

The used restrains and twinning model should be described in the experimental section. If the atoms with restrained U(i,j) tended to fall out of metric space, they should be refined with the XNPD command (incorporated in the latest SHELXL software), otherwise they should be refined as non- restrained.

FIXED !

A detailed description of the refinement procedure and twinning model for 1 is included in the experimental section. The number of restraints is also given now in Tables 4 and 5, i.e. 13 restraints in case of 1 and 0 for all other structure refinements of 2-4. Therefore new crystal structure refinements were performed for 3, 4, and 5, excluding all restraints. Updated data are given in text, Figures and Tables and revised CIF-files deposited at CCDC.

Author Response File: Author Response.pdf

Reviewer 3 Report

In my opinion, the Experimental Details for the present manuscript are incomplete. In the case of IR, the sample preparation is not given (KBr pellet, Nujol mulls, etc.). In the case of X-ray crystal structure determinations I am missing at lot. If restraints were used: how many and for what purpose? If disorder has been detected: how was it treated and how can it be chemically explained? If inversion twinning was present, how did the authors deal with it? How were hydrogen atoms treated in the structure determination? (The latter is important because hydrogen bonds are discussed in the text.)
In the attachment of this review I show a histogram of the scale factor k=F_obs^2/F_calc^2 for complex 3. This factor should be constantly 1 over all intensity bins. There seems to be a significant problem, here.
Four of the five discussed crystal structures are described as pentacoordinated. (Because of the used disorder model this is dubious for complex 1). The authors give a percentage of the way between trigonal bipyramid and square pyramid. This is based on the concept of the Berry pseudorotation. The model compound for Berry pseudorotation is the (non-transition-metal) molecule PF5. In the whole pathway, a twofold rotation symmetry is preserved. In the case of transition metals, this is often not the case, and descriptors for the Berry pseudorotation cannot be applied. Possible reasons are the electron configuration of the central metal and/or the presence of chelating ligands which break the twofold symmetry. I encourage the authors to analyze the molecular symmetry before they apply descriptors for the Berry pseudorotation (i.e. tau-parameter).
The fifth described complex is tetrahedral four-coordinated. Such copper(II) complexes are rare in synthetic chemistry (and somewhat more common in proteins). It would be interesting to give a comparison to other such complexes from the literature.
When reading the discussion of the crystal structures, I was wondering about the oxidation state of the copper ion. No answer is given. Only in section (4) "Conclusions" the oxidation state of compound 5 is given as Cu(II). For complexes 1-4 this analysis is left to the reader. Interestingly, the DFT calculations involve different oxidation states for copper. Why is this?
I am not an expert in inorganic nomenclature but I was surprised to see the NCS ion consequently be described as "thiocyanate". I thought that this should be "isothiocyanate".
Two minor comments:
line 6: author affiliation as superscipts
line 38: "which include which include"  --> "which include"

Author Response

Comments and Suggestions for Authors

In my opinion, the Experimental Details for the present manuscript are incomplete. In the case of IR, the sample preparation is not given (KBr pellet, Nujol mulls, etc.). In the case of X-ray crystal structure determinations I am missing at lot. If restraints were used: how many and for what purpose? If disorder has been detected: how was it treated and how can it be chemically explained? If inversion twinning was present, how did the authors deal with it? How were hydrogen atoms treated in the structure determination? (The latter is important because hydrogen bonds are discussed in the text.)

FIXED ! See comments given above for response to reviewer 2 !
In the attachment of this review I show a histogram of the scale factor k=F_obs^2/F_calc^2 for complex 3. This factor should be constantly 1 over all intensity bins. There seems to be a significant problem, here.

Thanks for this valuable hint ! We have repeated the integration of the intensities with more proper weighting scheme and got now better refinement results for 3 ! The revised structural data are updated in text, Figures and Tables. Revised CIF-files 3 (and also for 4 and 5) are deposited at CCDC!

Four of the five discussed crystal structures are described as pentacoordinated. (Because of the used disorder model this is dubious for complex 1). The authors give a percentage of the way between trigonal bipyramid and square pyramid. This is based on the concept of the Berry pseudorotation. The model compound for Berry pseudorotation is the (non-transition-metal) molecule PF5. In the whole pathway, a twofold rotation symmetry is preserved. In the case of transition metals, this is often not the case, and descriptors for the Berry pseudorotation cannot be applied. Possible reasons are the electron configuration of the central metal and/or the presence of chelating ligands which break the twofold symmetry. I encourage the authors to analyze the molecular symmetry before they apply descriptors for the Berry pseudorotation (i.e. tau-parameter).
We have omitted the tau-values in the description of the crystal structures. We inserted also a comment on the partially disordered compound 1.

The fifth described complex is tetrahedral four-coordinated. Such copper(II) complexes are rare in synthetic chemistry (and somewhat more common in proteins). It would be interesting to give a comparison to other such complexes from the literature.

In the Conclusion we have discussed this topic of tetrahedrally four-coordinated copper(II) compounds and given additional references .
When reading the discussion of the crystal structures, I was wondering about the oxidation state of the copper ion. No answer is given. Only in section (4) "Conclusions" the oxidation state of compound 5 is given as Cu(II). For complexes 1-4 this analysis is left to the reader. Interestingly, the DFT calculations involve different oxidation states for copper. Why is this?

SORRY ! Missing oxidation states of Cu(II) are now given for all five compounds.

Interestingly, the DFT calculations involve different oxidation states for copper. Why is this?

As we have highlighted in the manuscript, we explored different oxidation states of copper in order to determine the lowest energy oxidation state. We were also interested in whether electron transfer within an oligomer affects the structure.
I am not an expert in inorganic nomenclature but I was surprised to see the NCS ion consequently be described as "thiocyanate". I thought that this should be "isothiocyanate".

FIXED !
Two minor comments:
line 6: author affiliation as superscipts
line 38: "which include which include"  --> "which include"

FIXED !
peer-review-3485943.v1.pdf

Submission Date

01 December 2018

Date of this review 09 Dec 2018 12:58:54

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Round  2

Reviewer 3 Report

For the ORTEP plots I recommend to provide the probability level in the figure captions.

line 7: please give author affiliations in superscript

Figure 1a: please indicate that only the major disorder form is shown

Figure 1b: this figure is missing in my pdf-file

line 238: sample preparation for IR is not explained (KBr pellets, Nujol mulls, etc.)

line 297: please indicate the SHELXL treatment of H atoms that are involved in hydrogen bonds

line 304: "and refined with BASF of 0.33" --> "resulting in BASF = 0.33"

Author Response

For the ORTEP plots I recommend to provide the probability level in the figure captions.

Fixed in all figures

line 7: please give author affiliations in superscript

Fixed

Figure 1a: please indicate that only the major disorder form is shown

Fixed

Figure 1b: this figure is missing in my pdf-file

Fixed

line 238: sample preparation for IR is not explained (KBr pellets, Nujol mulls, etc.)

Fixed: In the ATR-IR: solid samples are measured DIRECTLY without need to make KBr pellets or nujol mulls sample.

line 297: please indicate the SHELXL treatment of H atoms that are involved in hydrogen bonds

Fixed

line 304: "and refined with BASF of 0.33" --> "resulting in BASF = 0.33"

Fixed

Author Response File: Author Response.pdf