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The Balance Between Hydrogen Bonds, Halogen Bonds, and Chalcogen Bonds in the Crystal Structures of a Series of 1,3,4-Chalcogenadiazoles

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The Balance Between Hydrogen Bonds, Halogen Bonds, and Chalcogen Bonds in the Crystal Structures of a Series of 1,3,4-Chalcogenadiazoles molecules Article The Balance between Hydrogen Bonds, Halogen Bonds, and Chalcogen Bonds in the Crystal Structures of a Series of 1,3,4-Chalcogenadiazoles Viraj De Silva, Boris B. Averkiev, Abhijeet S. Sinha and Christer B. Aakeröy * Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA; [email protected] (V.D.S.); [email protected] (B.B.A.); [email protected] (A.S.S.) * Correspondence: [email protected]; Tel.: +1-785-532-6096 Abstract: In order to explore how specific atom-to-atom replacements change the electrostatic potentials on 1,3,4-chalcogenadiazole derivatives, and to deliberately alter the balance between inter- molecular interactions, four target molecules were synthesized and characterized. DFT calculations indicated that the atom-to-atom substitution of Br with I, and S with Se enhanced the σ-hole potentials, thus increasing the structure directing ability of halogen bonds and chalcogen bonds as compared to intermolecular hydrogen bonding. The delicate balance between these intermolecular forces was further underlined by the formation of two polymorphs of 5-(4-iodophenyl)-1,3,4-thiadiazol-2-amine; Form I displayed all three interactions while Form II only showed hydrogen and chalcogen bonding. The results emphasize that the deliberate alterations of the electrostatic potential on polarizable atoms can cause specific and deliberate changes to the main synthons and subsequent assemblies in the structures of this family of compounds. Citation: De Silva, V.; Averkiev, B.B.; Keywords: Sinha, A.S.; Aakeröy, C.B. The chalcogen bond; halogen bond; hydrogen bond; intermolecular interactions; polymor- Balance between Hydrogen Bonds, phism; σ-hole Halogen Bonds, and Chalcogen Bonds in the Crystal Structures of a Series of 1,3,4-Chalcogenadiazoles. Molecules 2021, 26, 4125. https:// 1. Introduction doi.org/10.3390/molecules26144125 One of the main goals of crystal engineering is to employ intermolecular interactions in the assembly of crystalline materials with desired physiochemical properties [1–3]. To Academic Editor: Borys Osmialowski realize this objective, and to effectively manipulate these synthetic vectors, we need to further improve our understanding of the nature of these interactions. Received: 22 May 2021 The most influential and well-understood intermolecular force is, undoubtedly, the Accepted: 5 July 2021 hydrogen bond [4], and, consequently, the active use of this supramolecular tool has Published: 7 July 2021 been extensively mapped out [5–7]. Hydrogen bonding as a structure directing force has found many practical applications in pharmaceutical co-crystallizations [8,9], new Publisher’s Note: MDPI stays neutral agricultural formulations [10], as well as in the design of new energetic materials [11]. with regard to jurisdictional claims in However, over the past few decades, the recognition and understanding of potentially published maps and institutional affil- iations. competing intermolecular interactions, such as halogen and chalcogen bonding, have opened alternative ways for bottom-up design of new crystalline materials [12–14]. Hydrogen bonding is a noncovalent interaction involving the positive electrostatic potential on the hydrogen atom and a negative electrostatic potential of the acceptor atom (Scheme1). Electrostatic models can predict the directionality of the hydrogen bond rea- Copyright: © 2021 by the authors. sonably well; however, consideration of other components, such as charge transfer and Licensee MDPI, Basel, Switzerland. dispersion, is needed to properly understand the directionality [15,16]. Like hydrogen This article is an open access article bonds, halogen bonds share some of the same characteristics, whereby a positive electro- distributed under the terms and conditions of the Creative Commons static potential on a halogen atom and the negative electrostatic potential on an acceptor Attribution (CC BY) license (https:// atom can produce a ‘halogen bond’. In fact, this interaction is even more directional than creativecommons.org/licenses/by/ the hydrogen bond; however, it is worth keeping in mind that halogen bonds are not purely 4.0/). electrostatic. Using high-level theoretical models, such as symmetry-adapted perturbation Molecules 2021, 26, 4125. https://doi.org/10.3390/molecules26144125 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 13 Molecules 2021, 26, 4125 2 of 13 purely electrostatic. Using high-level theoretical models, such as symmetry-adapted per- turbationtheory (SAPT), theory it is(SAPT), possible it tois obtainpossible the to breakdown obtain the of breakdown the intermolecular of the forcesintermolecular into indi- forcesvidual into components individual of components electrostatic, of polarization, electrostatic, repulsion, polarization, and repulsion, dispersion and for adispersion complete forunderstanding a complete understanding of the forces at of play, the whichforces at shows play, electrostatics which shows alone electrostatics cannot completely alone can- notdescribe completely and predict describe the and structure predict directing the structure ability directing of non-covalent ability of interactionsnon-covalent [17 inter-,18]. actionsSAPT calculations [17,18]. SAPT and calculations other periodic and other DFT calculationsperiodic DFT can calculations be computationally can be computa- taxing, tionallyhence one taxing, of the hence simplest one waysof the to simplest estimate ways the directionality to estimate the of directionality such systems withof such accept- sys- temsable accuracywith acceptable is through accuracy the use is through of molecular the use electrostatic of molecular potential electrostatic maps potential (MEPs). maps This (MEPs).approach This provides approach a simple provides method a simple for mapping method outfor mapping the distribution out the ofdistribution electron densities of elec- tronin molecular densities systemsin molecular (in vacuo), systems whereby (in vacuo), electron-efficient whereby electron-efficient and -deficient and areas -deficient can be areasidentified can be [19 identified,20]. The most[19,20]. prominent The mostσ -holesprominent (electron-deficient σ-holes (electron-deficient areas) of a molecule areas) of can a moleculebe identified can bybe theidentified positive by electrostatic the positive potential. electrostatic The σ potential.-hole on halogen The σ-hole atoms on provideshalogen atomsthe main provides electrostatic the main driver electrostatic for halogen driver bonding for halogen interactions bonding [21, 22interactions]. [21,22]. Scheme 1. A simple view of the electrostatic component of hydrogen, halogen, and chalcogen bonds. Scheme 1. A simple view of the electrostatic component of hydrogen, halogen, and chalcogen bonds.Chalcogen bonds, a relatively recent ‘discovery’, also share the same aspects of the halogen bond, including the high directionality. However, chalcogen atoms can form Chalcogen bonds, a relatively recent ‘discovery’, also share the same aspects of the two highly directional chalcogen bonding interactions as a result of the presence of two halogenσ-holes directlybond, including opposed the to high R-Ch directionali bonds (Schemety. However,1)[ 23–26 chalcogen]. Importantly, atoms can by changingform two highlythe electrostatic directional potentials chalcogen on bo thesending donor interactions atoms, theseas a result noncovalent of the presence interactions of two can beσ- holesfine-tuned directly to beopposed strong structureto R-Ch bonds directing (Scheme tools that1) [23–26]. can be utilizedImportantly, in crystal by changing engineering the electrostaticfor creating competitionpotentials on between these donor hydrogen, atoms, halogen, these noncovalent and chalcogen interactions bonding can [27, 28be]. fine- tunedMany to bedrug strong molecules structure contain directing multiple tools th functionalat can be groups utilized comprising in crystal engineering hydrogen, halo- for creatinggen, and competition chalcogen atoms, between and hydrogen, the presence halogen, of these and can chalcogen lead to increasing bonding [27,28]. complexity as to howMany molecules drug molecules interact notcontain only multiple in drug formulations,functional groups such comprising as co-crystals, hydrogen, but also hal- at ogen,an active and sitechalcogen of specific atoms, receptors and the [8 ,presence29]. Each of approved these can drug lead costs to increasing billions of complexity dollars in asresearch to how and molecules development interact and not many only potentially in drug formulations, useful candidates such fail as dueco-crystals, to low solubility but also ator an poor active stability, site of and specific such receptors problems [8,29]. may be Each addressed approved with drug a more costs complete billions understand-of dollars in researching of how and the development balance between and many intermolecular potentially forces useful leads candidates to a specific fail due structure to low solu- with bilityunique or bulk poor properties. stability, and Furthermore, such problems 1,3,4-chalcogenadiazole may be addressed moieties with a more are known complete to show un- derstandinganti-inflammatory of how andthe balance anticancer between properties intermol [30ecular,31], and forces broadly leads to speaking, a specific chalcogen structure withatoms unique and their bulk structural properties. influence Furthermore, can also 1,3,4-chalcogenadiazole positively impact synthetic moieties transformations are known to
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