Comment on “Comment on “Crystallographic and theoretical study of the atypical distorted octahedral geometry of the metal chromophore of zinc(II) bis((1R,2R)-1,2-diaminocyclohexane) dinitrate””

Abstract

We defend argumentation that entitled compound (1) (CCDC 1576255) crystallizes into centrosymmetric space-group type P21/n as reported, previously [J. Mol. Struct. 1248 (2022) 131488]. Recent paper [J. Mol. Struct. 1284 (2023) 135362] supports negation of this statement, claiming that (1) is assigned to non-centrosymmetric P21 structure (CCDC 2226587). Inter-laboratory crystallographic experiments support opposite statements, regarding, space-group type. We must be able to argue what fails through experimentation or chemometrics, assessing validity of structural solutions. Single crystal 3D X-ray diffraction analysis is carried out via statistical tests, assessing whether measurands are used reliably. Results from work [J. Mol. Struct. 1284 (2023) 135362] fail to realize robust statistical tests of CheckCIF and PLATON, testing most serious problems of crystallographically determined structure. Major con-arguments against structural solution of CCDC 1576255 are based on our chemical synthesis, using presumably enantiopure analytes. Such argument fails to a significant extend, however. Many reports show that centrosymmetric crystals could contain either non- or centrosymmetric molecules. Moreover, study [J. Mol. Struct. 1284 (2023) 135362] does not provide neither spectroscopic nor chiral chromatographic or mass spectrometric experiments, evidencing chirality and purity of starting ligand and synthetic product (1), in addition to, a lack of data on elemental analysis and synthetic yield. The structural solution is argued only assessing Flack parameter of space-group type P21 (x(u)=0.078(17)) and R1=0.0316 factor. However, the x(u)-value is relatively high. When x(u)≈0 and its sd(yEr±)≈0, then absolute structure is likely considered to be correct one. There is a lack of comments on cases of chiral racemization, typical for ligands such as 1,2-diaminocyclohexane, when x(u)=0.5; and it cannot provide properly in a large number of cases unambiguous justification for correct space group assignment. Small R1-quantity is incapable of justifying unambiguously an absolute structure, as well. Our arguments against structure CCDC 2226587 focus on effect of temperature and energy of X-ray irradiation, in addition to chiral racemization processes. The study uses Mo Kα irradiation. Structural solution CCDC 2226587 does not meet robust test, considering symmetry elements as mentioned above. Conversely, structure CCDC 1576255 satisfies this criterion. The same is true for independent structural solutions of crystals of our sample (CCDC 2256980) examined in paper [J. Mol. Struct. 1248 (2022) 131488], which are presented, as well. Therefore, structure CCDC 1576255 is accurate and precise. It is obtained under repeatability and reproducibility conditions. This is not valid to work [J. Mol. Struct. 1284 (2023) 135362]. The robust capability of tests, considering symmetry elements of distinguishing among space-group types is illustrated via our new data on replicates of non-centrosymmetric P21 structure of (1) (CCDC 2256806) (a = 6.7580(14), b = 7.1970(14), and c = 21.385(4)Å, β=96.69(3)o; Z = 2, V = 1033.0(4) Å3; and R1=0.0770). Its space-group type is proven by the latter test. Comparative analysis with our resolving data on CCDC 2226587 in P21/n structure is also carried out. The latter structural resolving satisfies the test, considering symmetry elements. Chemometrics and high-resolution single crystal X-ray diffraction measurands are used.