Comment on “Novel Macrocyclic Bidentate Schiff’s base Hg (II) Complexes, Hirshfeld surface analysis, NCI analysis, and Antimicrobial activity studies” by M. Singh et al. [Polyhedron 264 (2024) 117194]

Abstract

M. Singh et al. in their article “Novel Macrocyclic Bidentate Schiff’s base Hg (II) Complexes, Hirshfeld surface analysis, NCI analysis, and Antimicrobial activity studies” (Polyhedron 264 (2024) 117194) reported on the synthesis of a new macrocyclic Schiff base named 2,10,12,20-tetrahydroxy-3,6,9,13,16,19-hexaaza-1,11(1,3)-dibenzenacycloicosaphane-2,9,12,19-tetraene-1⁵,11⁵-dicarboxylic acid (L), which was obtained through a condensation reaction of two equivalents of trimesic acid and two equivalents of diethylenetriamine. The reported Schiff base L was involved in the complexation reaction with an equimolar amount of HgCl₂ and HgBr₂ in a mixture of ethanol and acetonitrile, yielding complexes [HgCl₂L] (1) and [HgBr₂L] (2). It should be mentioned, that the formation of both complexes led to the formation of a doubly deprotonated parent ligand L²⁻, with the elimination of both NH hydrogen atoms. This, in fact, means the formation of complex anions 1²⁻ and 2²⁻, while no cationic counterparts were provided. Although the authors reported some geometrical parameters of the obtained metallocomplexes, no crystal structures of neither the parent ligand L nor complexes 1 and 2 were provided. Furthermore, the corresponding request from the Cambridge Structural Database revealed no deposited crystal structures associated with this publication. However, the authors reported the crystal structure of one of the starting reagents, viz., trimesic acid, by providing a calculated powder X-ray diffraction pattern of its methanol solvate, and using the Hirshfeld surface analysis, RDG and NCI plots. Notably, methanol was not applied neither for the synthesis of L nor for the fabrication of 1 and 2, thus, the formation of the methanol solvate of trimesic acid is unclear. It is, in general, unclear the reason to discuss the latter compound in the light of the scope of this work. The authors also discussed the IR and ¹H NMR data for both metallocomplexes although only one IR and one ¹H NMR spectra were provided for one unspecified complex. To be said, the ¹H NMR is completely incorrectly discussed since it does not contain signals for the expected molecules, but only signals for the solvents. Some results of the molecular docking studies were also provided, however, for a doubly charged parent ligand, viz., H₂L²⁺, with two quaternary amine nitrogens. This is also unclear, since one would expect studies for L, 1 and 2. Finally, the whole manuscript is dubiously written with a plethora of grammatical and scientific concerns. In this work, I provide a step-by-step critical analysis of the discussed article.