Crystal structure and vibrational spectra of salts of 1H-pyrazole-1-carboxamidine and its protonation route
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ORIGINAL RESEARCH
Crystal structure and vibrational spectra of salts of 1H-pyrazole-1-carboxamidine and its protonation route Piotr Rejnhardt 1 & Marek Daszkiewicz 1 Received: 2 October 2020 / Accepted: 27 October 2020 # The Author(s) 2020
Abstract Crystal structures of five salts of 1H-pyrazole-1-carboxamidine, PyCA, with various inorganic acids were determined, (HPyCA)Cl, (HPyCA)Cl·H2O, (HPyCA)Br, (HPyCA)2(I)I3, and (HPyCA)HSO4. Theoretical calculations of the protonation route of PyCA showed that the cationic form present in the studied crystals is energetically privileged. Tautomeric equilibrium constants indicated two isomers as the most stable neutral forms. Calculations for two other tautomers failed resulting in pyrazole and carbodiimid tautomer of cyanamide. Such decomposition is important in a view of guanylation reaction. Hydrogen bonding patterns were studied by means of the graph-set approach. Similarities of the patterns in different crystal structures were demonstrated by the algebraic relations between descriptors of the patterns. The strength of hydrogen bonding network in the crystals was assessed analyzing vibrational spectra. The bands were assigned on the basis of theoretical calculations for the complex [(HPyCA)2Cl4]2– ion and potential energy distribution analysis. The strength of hydrogen bonds was set in the following ascending series (HPyCA)2(I)I3 (4) < (HPyCA)Br (3) < (HPyCA)Cl (1) < (HPyCA)Cl·H2O (2) < (HPyCA)HSO4 (5). Keywords Guanylation . Protonation route . Hydrogen bonding . Elementary graph-set descriptor . Vibrational spectroscopy . PED
Introduction The 1H-pyrazole-1-carboxamidine, hereafter PyCA, has been a next-generation substrate for guanylation of amines after Smethylisothiouronium sulfate, cyanamide or 3,5-dimethyl-1guanylpyrazole nitrate [1]. For instance, it was successfully used in the introduction of guanidinium group as the last step of synthetic procedure of zanamivir, an influenza A and B drug [2], or Boc protected PyCA was a guanylation agent in synthesis of dengue and West Nile virus protease inhibitors [3]. The 1H-pyrazole-1-carboxamidine is commercially available as a hydrochloride and is widely used in organic synthesis for years. It can be synthesized by mixing equimolar amount of pyrazole and cyanamide, and further crystallization from Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s11224-02001671-0. * Marek Daszkiewicz [email protected] 1
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna str. 2, 50-422 Wrocław, Poland
the reaction mixture [1]. This very simple one-step procedure and very simple isolation method of pure compound make the PyCA a low-cost reagent in organic synthesis. The (HPyCA)Cl has very good solubility in water or DMF and is stable even at basic conditions, i.e., 1 M water solution of 1 M Na2CO3 [1]. So, it was suggested that the guanylation mechanism proceeds with cationic form of 1H-pyrazole-1carboxamidine and deprotonated for
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