Investigating the Competitive Factors of Metal–Anion Coordination, Coordination by Water, and Hydrogen Bonding in Complexes of Cyanuric Acid with the Alkaline Earth Metals

Round 1

Reviewer 1 Report

Thank you for the opportunity to see the interesting work presented by the authors. Please, see some points that should be adressed to improve the quality of the article.

Please, add formulas (if appropriate, general formulas) to the Abstract section.

Please indicate the purity grade of the reagents (metal salts) and where they were purchased.

Why IR spectra or X-ray powder diffraction (or other methods) were not used to verify that one crystal corresponds to the whole sample?

Author Response

Thank you for your positive assessment of this manuscript.  We have made small changes in response to the points raised.

Chemical formulae for the four products have been added to the Abstract.

The suppliers and purity grade of the reagents have been given in the Experimental section.

We had no convenient access to X-ray powder diffraction facilities at the time of these experiments for checking phase purity.  This is a minor drawback but does not affect the validity of our results.  We have noted in the Experimental section that the same structures have been found by others using similar conditions, with no suggestion of other polymorphs except in hydrothermal synthesis of two forms of the Sr complex having the same chemical formula.

Reviewer 2 Report

This manuscript is interesting for crystal engineering and supramolecular chemistry community and could be accepted for publication. The topic is up to date and actual. A survey of the trends in metal-ligand coordination and hydrogen bonding has been carried out on complexes of singly deprotonated cyanuric acid CY− and alkaline earth metals, synthesised from aqueous solution and characterised by X-ray crystallography. Involvement of cyanurate in the metal coordination increases from Mg through Ca and Sr to Ba relative to coordination by water, in parallel with the increasing ionic radius and coordination number. The incidence of bridging of metal centres by both water and cyanurate also increases in this series. The Mg complex contains hexaaqua-coordinated cations, uncoordinated CY− anions, and additional water molecules. Ca and Sr complexes feature polymeric chains of coordinated metal ions, the Ca complex with uncoordinated anions and the Sr complex with all potential ligands coordinated. The Ba complex has a two-dimensional coordination network. Extensive hydrogen bonding plays a key role in generating a three-dimensional network in all four structures. The competition between cyanurate and water for coordination to the alkaline earth metal cations, and that between coordination and hydrogen bonding propensities of the cyanurate anion, appear to be finely balanced, with the structural outcomes depending on the relative sizes and degrees of hardness of the four cations of the group. The subject addressed in this article is worthy of investigation. The information presented is new. The methodology of research is appropriate. The conclusions supported by the data. The manuscript is good illustrated and interesting to read. I have only two suggestions for minor revision. First, it would be a good idea to cite following relevant reference in introduction: ACS omega 2021 6 (27), 17267-17275. Second, some more detailed perspectives about the future research could be formulated in conclusions.

Author Response

Thank you for your positive assessment of this manuscript.  There are two suggestions for minor revision and both have been adopted.  The cited ACS Omega reference has been added in the Introduction where cocrystals of cyanuric acid and melamine are mentioned; and a short paragraph has been added to the Conclusions describing possible future extensions of this work.

Reviewer 3 Report

I have only one minor comment: the chemical structure of the cyanuric acid, including the triol tautomer and the keto form, should be inserted in the Introduction section.

Author Response

Thank you for the positive assessment of this manuscript.  The chemical structures of the two tautomers of cyanuric acid have been added to the Introduction section as recommended.