International Journal of Molecular Sciences Article Uracil Derivatives for Halogen-Bonded Cocrystals Mónica Benito 1, * , Yannick Roselló 2,† , Miquel Barceló-Oliver 2 , Antonio Frontera 2, * and Elies Molins 1, * 1 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain 2 Departament de Química, Universitat de les Illes Balears, Ctra. Valldemosa km 7.5, 07122 Palma de Mallorca, Spain; ynkrm11@gmail.com (Y.R.); miquel.barcelo@uib.es (M.B.-O.) * Correspondence: mbenito@icmab.es (M.B.); toni.frontera@uib.es (A.F.); elies.molins@icmab.es (E.M.) † Present address: Department de Química Física i Inorgànica, Universitat Rovira i Virgili, 43007 Tarragona, Spain; yannick.rosello@urv.cat. Abstract: Among non-covalent interactions, halogen bonding is emerging as a new powerful tool for supramolecular self-assembly. Here, along with a green and effective method, we report three new halogen-bonded cocrystals containing uracil derivatives and 1,2,4,5-tetrafluoro-3,6-diiodobenzene as X-bond donor coformer. These multicomponent solids were prepared both by solvent-drop grinding and solution methods and further characterized by powder and single-crystal X-ray diffraction, Fourier-transformed infrared spectroscopy, and thermal methods (TGA-DSC). In order to study the relative importance of hydrogen versus halogen bonds in the crystal packing, computational methods were applied. Keywords: nucleobases; cocrystal; crystal engineering; mechanochemistry; noncovalent interactions; DFT   Citation: Benito, M.; Roselló, Y.; 1. Introduction Barceló-Oliver, M.; Frontera, A.; In 2013, after several years working on the rationalization and unification of interac- Molins, E. Uracil Derivatives for tions involving halogens, the IUPAC defined the halogen bond as a non-covalent interaction Halogen-Bonded Cocrystals. Int. J. when there is evidence of a net attractive interaction between an electrophilic region associ- Mol. Sci. 2021, 22, 10663. https:// ated with a halogen atom in a molecular entity and a nucleophilic region in another, or the doi.org/10.3390/ijms221910663 same, molecular entity [1]. Academic Editor: Woo-Sik Kim Over the last decades, the use of this non-covalent interaction has grown rapidly as shown by several reviews [2–5]. A plethora of opportunities in areas such as organic Received: 7 September 2021 chemistry [4], supramolecular chemistry [5], crystal engineering including pharmaceu- Accepted: 28 September 2021 tical cocrystals [6–11], polymer science [12,13], medicinal chemistry, and chemical biol- Published: 1 October 2021 ogy [14–18] have grown, taking advantage of its high linear directionality, tunable bonding strength, hydrophobicity, or the size of the halogen atom. Furthermore, in biological sys- Publisher’s Note: MDPI stays neutral tems such as proteins or nucleic acids, halogen bonding contribution has been found in with regard to jurisdictional claims in many protein-ligand complexes corroborated by several publications [19,20]. Particularly, published maps and institutional affil- in the case of nucleobases, some reported studies are theoretical [21,22], while others were iations. carried out with halogenated nucleobases due to the use of these compounds as intermedi- ate scaffolds for the synthesis of active pharmaceutical ingredients (APIs) [23,24]. However, alkylated nucleobases also represent important models for explaining the contribution of DNA methylation in gene expression through its regulation or inhibition [22]. For this Copyright: © 2021 by the authors. reason, an increased effort from experimental and theoretical points of view is necessary for Licensee MDPI, Basel, Switzerland. understanding this type of non-covalent interaction, in many cases with a similar strength This article is an open access article to hydrogen bonds. distributed under the terms and The renaissance of mechanochemistry has encompassed a whole range of possibili- conditions of the Creative Commons ties not only for crystal engineering and polymorphism but also for synthetic processes, Attribution (CC BY) license (https:// especially when products different from the ones obtained via solution methods are ac- creativecommons.org/licenses/by/ complished. From a green chemistry perspective, this synthetic method as well as other 4.0/). Int. J. Mol. Sci. 2021, 22, 10663. https://doi.org/10.3390/ijms221910663 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 10663 2 of 15 Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 solvent-free andsolvent-free more sustainable and moreapproaches sustainableare highly demanded approaches [25]. The are highly required[25]. demanded acti-The requ vation energy isactivation obtainedenergy by mechanical forces is obtained in solid-state by mechanical grinding forces methods. in solid-state Moreover, grinding methods. M although far away over,from solvent although far techniques, the addition away from solvent of catalytic techniques, amounts the addition of solvent, of catalytic amounts of known as liquid-assisted grinding (LAG), has even permitted a great advance. vent, known as liquid-assisted grinding (LAG), has even permitted a great advance. In previous work, we reported In previous thewe work, preparation reported using the mechanochemical the preparation method of a using the mechanochemical met halogen bondedofcocrystal based on 9-ethyladenine and 1,2,4,5-tetrafluoro-3,6-diiodobenzene a halogen bonded cocrystal based on 9-ethyladenine and 1,2,4,5-tetrafluoro-3,6-diio (TFDIB) [26]. The present benzene contribution (TFDIB) is devoted [26]. The presentto the ability of contribution is three devotedderivatives of uracil to the ability of three der for halogen bonding, tives of uracil for halogen bonding, taking advantage of this strategy.are taking advantage of this strategy. The selected compounds Thetheselected c canonical nucleobase poundsthymine (THY), thenucleobase are the canonical aliphatic derivative 1-ethyluracil thymine (THY), (1ETURA), the aliphatic and 1-ethylu derivative the antineoplastic and well-known (1ETURA), API 5-fluorouracil and the antineoplastic (5FU). All of and well-known API them have been(5FU). 5-fluorouracil used All of th as building blocks havefor preparing been halogen-bonded used as building blocks forcocrystals preparing with the classicalcocrystals halogen-bonded iodofluo-with the c robenzene compound (see Scheme 1) through sical iodofluorobenzene the mechanochemical compound (see Scheme 1) method. through Furthermore, the mechanochem solution crystallizations to afford suitable method. Furthermore, single solution crystals for their crystallizations structural to afford resolution suitable as single crystals for t well as the physico-chemical characterization of the solids were performed. Finally, structural resolution as well as the physico-chemical characterization of the solids w DFT calculations of performed. the three new cocrystals Finally, containingof DFT calculations these uracilnew the three derivative cocrystalsmolecules containingas these u X-bond acceptors and TFDIB as X-bond donor were conducted. derivative molecules as X-bond acceptors and TFDIB as X-bond donor were conducte Scheme 1. Scheme 1. Uracil derivatives Uracil derivatives and the halogenated and the halogenated coformer coformer used used in this in this work. work. 2. Results and 2. Discussion Results and Discussion 2.1. Spectroscopic Characterization 2.1. Spectroscopic Characterization As stated before, Asfrom stated thebefore, supramolecular perspective, all perspective, from the supramolecular the nucleobases show all the poten- show nucleobases tial groups for forming non-covalent interactions such as hydrogen bonds or tential groups for forming non-covalent interactions such as hydrogen bonds or π–π stacking interactions. However, stacking observation interactions.ofHowever, other weak interactions observation of such otherhalogen bonds in thesuch halo weak interactions structural motifs could also be very interesting to know how nucleobase-based bonds in the structural motifs could also be very interesting to know APIs inter- how nucleob act. For this reason, an initial cocrystal screening using the mechanochemical based APIs interact. For this reason, an initial cocrystal screening using the method was mechanoch carried out with canonical nucleobases as well as for two more derivatives: 1-ethyluracil ical method was carried out with canonical nucleobases as well as for two more der and the active pharmaceutical tives: 1-ethyluracil ingredient and the 5-fluorouracil as targetingredient active pharmaceutical molecules.5-fluorouracil The coformeras target m used was the ditopic and well-known 1,2,4,5-tetrafluoro-3,6-diiodobenzene. In our multi- ecules. The coformer used was the ditopic and well-known 1,2,4,5-tetrafluoro-3,6-diio component screening, different stoichiometric compositions (nucleobase: coformer benzene. In our multicomponent screening, different stoichiometric compositions ratio: (nu 1:1, 1:2, 2:1, andobase: 4:1) and different solvents were explored. coformer ratio: 1:1, 1:2, 2:1, and 4:1) and different solvents were explored. At first, the analysis At first,bythe powder X-ray analysis diffraction by powder X-raymeans of the means diffraction solids after of thegrinding solids after grind confirmed the presence of new phases for the systems: thymine-TFDIB and confirmed the presence of new phases for the systems: thymine-TFDIB and 1ETU1ETURA-TFDIB in a nucleobase: coformer TFDIB in a 2:1 molar ratio nucleobase: (Figure2:1 coformer 1a,b), molarindicating that only ratio (Figure 1a,b),the aliphaticthat only indicating substituted nucleobases, thymine or 1-ethyluracil, aliphatic substituted nucleobases, underwent thymine orcocrystal formation 1-ethyluracil, resultingcocrystal underwent in compoundsmation 1 and resulting 2, respectively. The rest of the canonical nucleobases (adenine, in compounds 1 and 2, respectively. The rest of the canonical nucleob guanine, uracil, and cytosine) (adenine, guanine, remained uracil, andunreacted cytosine)as remained their powder patterns unreacted werepowder as their the patt sum of the corresponding were the sum of the corresponding former substances even when differentor former substances even when different stoichiometric ratios stoichiome solvents were usedratiosduring the grinding or solvents screening were used during(Figures S1–S4). the grinding Nucleobases screening (Figuresshow poor Nucleob S1–S4). aqueous solubilities show poor aqueous solubilities and have high melting points (see Table of and have high melting points (see Table S1). These are indicative S1). These the strength ofindicative their homosynthon motifs of the strength and homosynthon of their therefore theirmotifslow affinity for rendering and therefore their low affinity other multicomponent renderingcrystals. A CSD surveycrystals. other multicomponent (version 2020.2.0, A CSD surveyupdate August (version 2020) 2020.2.0, update Au revealed that multicomponent solids of pure uracil or guanine nucleobases are scarce Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 3 of 15 Int. J. Mol. Sci. 2021, 22, 10663 3 of 15 2020) revealed that multicomponent solids of pure uracil or guanine nucleobases are scarce since one cocrystal was found for each one (VIFKUR [27] and QQQFGM [28]). In since one cocrystal was found for each one (VIFKUR [27] and QQQFGM [28]). In contrast, contrast, several multicomponent solids are well reported for adenine, cytosine, or thy- several multicomponent solids are well reported for adenine, cytosine, or thymine. Most mine. Most of the solid forms described were salts for the first two and cocrystals for the of the solid forms described were salts for the first two and cocrystals for the last one. It last one. It is important to remark that thymine is the lowest melting point nucleobase as is important to remark that thymine is the lowest melting point nucleobase as shown in shown Table in S1.Table S1. Figure Figure 1. Comparison 1. Comparison of of calculatedand calculated andexperimental experimentalPXRD PXRD patterns patterns for for the the cocrystals cocrystals 11(a), (a),22(b), (b),and and3 (c) and 3 (c) their and their respective respective precursors. precursors. Surprisingly,for Surprisingly, for5-fluorouracil, 5-fluorouracil, aa different differentpowder powderpattern patternwas wasobtained obtained when when using using a mixture of precursors in 4:1 molar ratio. Figure 1c shows the resulting powder pattern a mixture of precursors in 4:1 molar ratio. Figure 1c shows the resulting powder pattern which confirms the successful formation of the cocrystal by the excellent agreement with which confirms the successful formation of the cocrystal by the excellent agreement with the simulated PXRD from the single crystal structure. A search in the CSD (version 2020.2.0, the simulated PXRD from the single crystal structure. A search in the CSD (version update August 2020) showed that this is the first halogen-bonded cocrystal with such a 2020.2.0, update August high stoichiometric ratio2020) showed and the halogen that bondthisinteraction is the firstishalogen-bonded between a carbonyl cocrystal group with as such a high stoichiometric an acceptor and this ditopicratio and the halogen bond interaction is between a carbonyl coformer. group Refcode as an acceptor ZIJHAD and this ditopic reports the onlycoformer. example found for a halogen-bonded cocrystal in Refcode ZIJHAD reports 4:1 ratio with a racemic sulfonamide the only and example 1,4-DITFB found for a halogen-bonded containing a S=O···I interaction cocrystal [29]. in 4:1 ratio with athe Finally, racemic measured sulfonamide PXRD patterns and 1,4-DITFB of the threecontaining a S=O···I by cocrystals obtained interaction grinding [29]. are Finally, in good the measured agreement PXRD with those patternsfrom calculated of the three single cocrystals crystal obtained data, thus by grinding confirming that allare insolids good were obtained agreement withas those pure (Figure calculated 1). from single crystal data, thus confirming that all solids were obtained as pure (Figure 1). the halogen bonding formation provokes small Previous studies have proven that shifts in the vibrational Previous studies have modes either proven thatfor the the donor halogen or the acceptor bonding X-bond bands. formation provokesIn this small sense, shifts in athe careful examination vibrational modes ofeither the FTIR for spectra the donor of the or new synthesized the acceptor X-bond compounds bands. Inandthis their parent compounds allowed sense, a careful examination of the us to check those differences. Typical FTIR spectra of the new synthesized compounds and bands for TFDIB appear at 1455, 937, and 755 cm−1 and correspond tentatively to the νC-C stretch ring, their parent compounds allowed us to check those differences. Typical bands for TFDIB νC-F stretch, and νC-I asymmetrical stretch, respectively [30]. In the case of the compound appear at 1455, 937, and 755 cm−1 and correspond tentatively to the νC-C stretch ring, νC- 1 (THY-TFDIB), almost no changes were observed for the characteristic modes of the F stretch, and νC-I asymmetrical stretch, respectively [30]. In the case of the compound 1 coformer, although differences in the carbonyl or the N-H and C-H regions were observed (THY-TFDIB), almost (Figure respective to thymine no changes S5 andwere Tableobserved S2). For the fornewthecocrystal characteristic modes of the 2 (1ETURA-TFDIB), coformer, althoughindifferences a slight increase the νC-I band in the wascarbonyl observed or the (758N-Hcm−and C-Hinregions 1 ) and the region werefor observed C=O respective to thymine (Figure S5 and Table S2). For the new cocrystal stretchings, some shifts were also observed (now at 1704 and 1651 cm ) or again in the 2 − 1 (1ETURA-TFDIB), a N-H slightandincrease in the C-H region νC-I band (3200–2800 −1 ) (Figure cmwas observed S6). (758 cmthe Finally, −1) and in the region for C=O C=O and N-H stretchings stretchings, some shifts in the 5-fluorouracil were also compound observed exhibited at (now at 1704 1720/1648 and and31151651cm− cm1 , −1 ) or again inAfter respectively. the N- Hformation and C-H region (3200–2800 of cocrystal cm )band 3, the C=O −1 (Figure S6). Finally, appeared at 1672the cmC=O− 1 andand theN-H N-Hstretchings stretching in band the at 3150/3088 5-fluorouracil cm−1 . Moreover, compound exhibited the atthree characteristic 1720/1648 and 3115 bands cmof−1, the coformer suffered respectively. After for- from a slightly increase and appeared at 1463, 943, and 758 cm −1 (see Figure S7). All these mation of cocrystal 3, the C=O band appeared at 1672 cm and the N-H stretching band −1 atchanges 3150/3088 confirm cm−1. the formation Moreover, theofthree new interactions characteristic in bands the synthesized compounds. of the coformer suffered from Int. J. Mol. Sci. 2021, 22, 10663 4 of 15 2.2. Thermal Analysis TGA-DSC measurements were carried out to determine the formation of new crys- talline solids. Moreover, by thermal analysis, the three compounds were confirmed to be anhydrous as no weight losses were observed before their melting (with decomposi- tion). The TG (black curve) and DSC (orange curve) are displayed in the Supplementary Info. For compound 1 (THY- TFDIB), the melting was at approximately 139.7 ◦ C (Tpeak), which is a temperature in between the melting of the precursors (see Table 1). During melting, decomposition of this cocrystal happened as indicated by a loss on drying (lod) of 60.3%, in agreement with the theoretical value for the release of a 1,2,4,5-tetrafluoro- 3,6-diiodobenzene molecule (calc. 61%) (Figure S8) and also with the stoichiometric ratio confirmed by single-crystal resolution (see next section). In a similar manner, the TGA of compound 2 showed a lod of 59% (calc. 59% for one TFDIB molecule) during its melting process (Tp : 154.7 ◦ C) and this happened at a higher temperature than the corresponding ones for the precursors (1ETURA, Tp = 144.9 ◦ C), showing a higher thermal stability (see Figure S9). Finally, in the TGA of compound 3, the mass loss observed (42.7%) can also be attributed to the release of the coformer (calc. 43.6%). In the same range in the DSC trace, a single peak occurred at 161.8 ◦ C, the melting of this cocrystal. A second peak at around 282.0 ◦ C matches with melting of the 5-fluorouracil (Figure S10). This value was the highest for the new cocrystals and was in between the melting points of the former compounds (5-fluorouracil Mp , 282–286 ◦ C). Table 1. Physical data of the starting compounds and the new cocrystals. νC=O νC=O ∆E Compound Mp (◦ C) * Cocrystal Mp (◦ C) dO···I (Å) (cm−1 ) (cm−1 ) (kcal/mol) 1,4-TFDIB 108–110 - - - - - - THY 320 1720/1667 1 139.7 1717 3.15 −10.1 1ETURA 144.9 1707/1645 2 154.7 1704 2.849; 2.910 −14.0 5FU 282–286 1720/1648 3 161.8 1710; 1672 3.097; 3.575 −4.3 * From literature. 2.3. Crystal Structure Analysis 2.3.1. Thymine–TFDIB (2:1) Cocrystal Suitable single crystals of a thymine–TFDIB system for X-ray structural determination were grown from a mixture of methanol-acetone. Cocrystal 1 crystallizes in the triclinic space group P-1 containing two molecules of thymine and a molecule of the halogenated coformer in the asymmetric unit. No solvent molecules were found in agreement with the TGA trace. A representative ORTEP with atom labeling is shown in Figure 2a. For this compound, each thymine molecule established four strong hydrogen bonds through N(1)-H···O(2) and N(3)-H···O(2) interactions by two distinct R22 (8) dimeric mo- tifs and resulting in ribbons in a similar fashion as observed for polymorphic forms of thymine [31]. These ribbons were connected among them through symmetrically equiva- lent halogen bonds with the halogenated coformer through O(4)···I interactions (distance: 3.148 Å; angle: 172.26◦ ) (Figure 2b). The resulting 2D ladders, in which the self-assembled nucleobases represent the steps of the ladder, were pilled forming the tridimensional struc- ture shown in Figure 2c. Other short contacts were observed, such as π–π stacking among the nucleobases rings, through C(5M)-H···C(6) interactions or among the TFDIB rings by F(2)···C(3) contacts. 2.3.2. 1-Ethyluracil–TFDIB (2:1) Cocrystal This compound crystallized in the triclinic space group P-1 containing two molecules of 1-ethyluracil and one of 1,2,4,5-tetrafluoro-3,6-diiodobenzene in the asymmetric unit. No solvent molecules were present as confirmed by the thermogravimetric analysis. A representative ORTEP with atom labeling is shown in Figure 3a. Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 5 of 1 Int. J. Mol. Sci. 2021, 22, 10663 5 of 15 Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 6 of 15 Figure 2. (a) ORTEP view of cocrystal 1 showing atom labeling. (b) 2D ladders formed by thymine– Figuremolecules TFDIB 2. (a) ORTEP view through of cocrystal H-bond 1 showing and X-bond atom (c) interactions. labeling. (b) 2Dinladders 3D structure cocrystalformed 1. by thymine– TFDIB molecules through H-bond and X-bond interactions. (c) 3D structure in cocrystal 1. For this compound, each thymine molecule established four strong hydrogen bond through N(1)-H···O(2) and N(3)-H···O(2) interactions by two distinct 𝑅 (8) dimeric motif and resulting in ribbons in a similar fashion as observed for polymorphic forms of thy mine [31]. These ribbons were connected among them through symmetrically equivalen halogen bonds with the halogenated coformer through O(4)···I interactions (distance 3.148 Å; angle: 172.26°) (Figure 2b). The resulting 2D ladders, in which the self-assembled nucleobases represent the steps of the ladder, were pilled forming the tridimensiona structure shown in Figure 2c. Other short contacts were observed, such as π–π stacking among the nucleobases rings, through C(5M)-H···C(6) interactions or among the TFDIB rings by F(2)···C(3) contacts. 2.3.2. 1-Ethyluracil–TFDIB (2:1) Cocrystal This compound crystallized in the triclinic space group P-1 containing two molecules of 1-ethyluracil and one of 1,2,4,5-tetrafluoro-3,6-diiodobenzene in the asymmetric unit No solvent molecules were present as confirmed by the thermogravimetric analysis. A representative ORTEP with atom labeling is shown in Figure 3a. Figure Figure3. 3. (a)(a) ORTEP ORTEPview viewofofcocrystal cocrystal2 2showing showingatom atomlabeling labelingininthe theasymmetric asymmetricunit. unit.(b) (b)Planes Planes formed formedbyby1-ethyluracil–TFDIB 1-ethyluracil–TFDIBmolecules moleculesthrough through H-bond and X-bond H-bond and X-bondinteractions. interactions.(c) (c)3D 3Dstructure struc- ture in cocrystal in cocrystal 2. 2. Unlike compound 1, this structure was characterized by only one type of self-assem- bly between two molecules of 1-ethyluracil through a H-bond interaction among N(3)- H···O(2) and N(3′)-H···O(2) giving to 𝑅 (8) motifs. The coformer served again as a bridge Int. J. Mol. Sci. 2021, 22, 10663 6 of 15 Unlike compound 1, this structure was characterized by only one type of self-assembly between two molecules of 1-ethyluracil through a H-bond interaction among N(3)-H···O(2) and N(30 )-H···O(2) giving to R22 (8) motifs. The coformer served again as a bridge be- tween two uracil dimers forming halogen bonding interactions through the O(40 )···I(1) or O(4)···I(2) (distances, 2.849 and 2.910 Å; angle: 175.25◦ ) generating zig-zag chains (Figure 3b). Moreover, short contacts were formed between 1-ethyluracil molecules from different chains through C(7) or C(70 )-H from the ethyl group and O(4) or O(40 ), C(60 )- H···F(1) and also through F···F contacts along the TFDIB molecules forming a 2D sheet. The final 3D structure was obtained by the stacking of these sheets in which the ethyl tails were faced every two sheets while the TFDIB or nucleobase rings were stacked with the next sheet, as displayed in Figure 3c. 2.3.3. 5-Fluorouracil–TFDIB (4:1) Cocrystal Compound 3 crystallized in the monoclinic space group P21/n containing two molecules Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 7 of 15 of 5-fluorouracil and a half of 1,2,4,5-tetrafluoro-3,6-diiodobenzene in the asymmetric unit. No solvent molecules were present as confirmed by the thermogravimetric analysis. A representative ORTEP with atom labeling is shown in Figure 4a. Figure 4. (a) ORTEP representation showing the cocrystal 3 with atom labeling. (b) Belts formed Figure 4. (a) ORTEP representation showing the cocrystal 3 with atom labeling. (b) Belts formed by by 5-fluorouracil–TFDIB molecules through H-bond and X-bond interactions. (c) 3D structure in 5-fluorouracil–TFDIB molecules through H-bond and X-bond interactions. (c) 3D structure in co- cocrystal 3. crystal 3. Self-assembly among the nucleobases was formed through bifurcated hydrogen bond Self-assembly interactions between among the N(11) O(2) and nucleobases or N(13)wasandformed through in a similar bifurcated fashion, hydrogen O(12) established bond interactions between O(2) and N(11) or N(13) and in a similar bifurcated H-bonds with N(1) and N(3) forming zig-zag ribbons (see Figure 4b). fashion, O(12) estab- As lished bifurcated H-bonds with N(1) and N(3) forming zig-zag ribbons (see observed in the two previous structures, these ribbons were interconnected through the Figure 4b). As observed halogenatedin the two previous coformer structures, which acted these as a bridge ribbons forming were interconnected bifurcated halogen bondthrough the interactions halogenated coformer which acted as a bridge forming bifurcated halogen bond through O(4)···I(1) and F(5)···I(1) contacts, leading to belts in which the coformer was tilted interac- tions from through the planeO(4)···I(1) formed byand theF(5)···I(1) contacts, nucleobases leading (dihedral to between angle belts in which theTFDIB rings of coformer was and 5FU: tilted from ◦ the plane formed by the nucleobases (dihedral angle between rings 61.4(6) ) and all the TFDIB rings were π–π stacked. The resulting belts were accommodated of TFDIB and 5FU: 61.4(6)°) establishing and further all the short TFDIBgiving contacts rings the werefinal π–πtridimensional stacked. The resulting structurebelts shownwere in accommodated Figure 4c. establishing further short contacts giving the final tridimensional struc- ture shown in Figure 4c. Interestingly, the presence of a methyl or a fluoro atom in C(5) in these uracil deriv- atives altered the hydrogen and halogen bond interactions observed in cocrystals 1 and 3. This effect has also been observed previously when using hydroquinone as coformer with these two molecules (refcodes: OGIYOU and UDOVIV) [32,33]. Int. J. Mol. Sci. 2021, 22, 10663 7 of 15 Interestingly, the presence of a methyl or a fluoro atom in C(5) in these uracil deriva- tives altered the hydrogen and halogen bond interactions observed in cocrystals 1 and 3. This effect has also been observed previously when using hydroquinone as coformer with these two molecules (refcodes: OGIYOU and UDOVIV) [32,33]. The halogen bond interaction is typically colinear with an angle between the donor and acceptor components near to 175◦ . From the three cocrystals, 1 and 2 had angles of about 172.26 and 175.25◦ , respectively, in agreement with the described results in the literature. However, compound 3 was the one that had the lowest O···I-C angle, with a value of 160.03◦ as a result of a bifurcated X-bond also with the fluorine atom from the 5-fluorouracil (see for comparison Figure S11). In our previous cocrystal with 9-ethyladenine and this coformer, N···I halogen bonds were obtained. It is interesting to remark that herein, the halogen bonds in the new cocrys- tals were formed through O···I similar to the multicomponent solids formed between the active pharmaceutical ingredient pentoxifylline with diiodotetrafluorobenzene or dibro- motetrafluorobenzene [11]. However, while the ratio in the pentoxifylline–TFDIB cocrystal was 1:1 and halogen bond interactions among N(secondary nitrogen)···I or O(xanthyl fragment)···I were observed, for compounds 1 and 2, the observed ratio was 2:1 and for cocrystal 3, 4:1, and the halogen bonds were exclusively formed through O···I interactions. This behavior is closer to the cocrystal of pentoxifylline with the weaker halogen bond donor dibromotetrafluorobenzene. For multicomponent solids 1–3, the available nitrogen atoms were involved in self-assembly between nucleobases. Finally, the excellent agreement between the simulated powder patterns obtained from the crystal structures and the experimental from the bulk solids confirmed the results and the purity of new compounds (Figure 1). 2.4. Theoretical Study The theoretical study was focused on the analysis of the energetic features of several supramolecular assemblies observed in the solid state of the cocrystals focusing on some recurrent motifs and comparing the energies H-bonding and halogen bonding interactions. This study is helpful to evaluate the relative importance of the interactions that govern the crystal packing. Figure 5 shows the molecular electrostatic potential surfaces of all molecules used in this work. It can be observed that for the halogen bond acceptors (1ETURA, THY, and 5FU), the most negative atom was the O4-atom (ranging from −35 to −40 kcal/mol) followed by the O2 (ranging from −32 to −35 kcal/mol). The most positive values were located at the N1–H group (+55 kcal/mol in THY and +56 kcal/mol in 5FU). In 1ETURA, the presence of the ethyl group bonded to N1 caused changes in the charge distribution. In this case, the most positive region was located at the C–H group adjacent to N1 (+38 kcal/mol) followed by the N3–H group, which was slightly smaller (+34 kcal/mol). The MEP was more positive at the C–H group due to the contribution of the adjacent H-atoms belonging to the methylene group of the ethyl substituent. In THY and 5FU, the MEP at the N3–H group were +34 and +40, respectively, in line with the electron withdrawing effect of the fluorine substituent. For TFDIB coformer, the maximum MEP was located at the σ-hole of iodine (+33 kcal/mol) and the minimum at the F-atom (−8 kcal/mol). This MEP analysis anticipates that the uracil derivatives are strong H-bond donor and good H-bond and X-bond acceptors and that the coformer is a good X-bond donor. group were +34 and +40, respectively, in line with the electron withdrawing effect o fluorine substituent. For TFDIB coformer, the maximum MEP was located at the σ-ho iodine (+33 kcal/mol) and the minimum at the F-atom (−8 kcal/mol). This MEP ana anticipates that the uracil derivatives are strong H-bond donor and good8 H-bond Int. J. Mol. Sci. 2021, 22, 10663 of 15 an bond acceptors and that the coformer is a good X-bond donor. Figure 5. MEP surfaces of 1ETURA (a), THY (b), 5FU (c), and TFDIB (d) at the PBE0-D3/def2-TZVP level of theory. Figure 5. MEP surfaces of 1ETURA (a), THY (b), 5FU (c), and TFDIB (d) at the PBE0-D3/def2-TZVP level of theory. Isosur- Isosurface 0.001 a.u. The values at selected points of the surface are given in kcal/mol. face 0.001 a.u. The values at selected points of the surface are given in kcal/mol. Figure 6a shows the two R22 (8) motifs observed in the solid state of THY–TFDIB with the participation Figure 6a shows of both the two 𝑅 (8)asmotifs NH groups H-bond observed donors and the O2 atom in the solidas state H-bond ofacceptor. THY–TFDIB The interaction energy of the tetrameric fragment used as model of the infinite 1D tape the participation of both NH groups as H-bond donors and the O2 atom as H-bond a described above was ∆E1 = −35.2 kcal/mol, thus revealing the strong nature of the H-bonds tor. The interaction (around energy −5.9 kcal/mol eachof the tetrameric H-bond). fragment The strong nature used aswas of the H-bonds model of the confirmed by infinit tape described the NCIplotabove was ΔE index analysis 1 = −35.2 which kcal/mol, revealed dark bluethus revealing isosurfaces thethe between strong nature of th interacting atoms, coincident with the location of the bond critical points (CP, represented as a red bonds (around −5.9 kcal/mol each H-bond). The strong nature of the H-bonds was sphere) that characterize the H-bonds. The formation of the self-assembled tetramer left the firmedO4by the available atoms NCIplotasindex halogenanalysis whichInrevealed bond acceptors. dark fact, the two blue isosurfaces symmetrically equivalentbetwee interacting atoms, and highly coincident directional with X-bonds that the location are shown of the in Figure 6bbond critical points were evaluated (CP, represe by calculating the sphere) as a red hexamericthatassembly, using the tetramer characterize as starting the H-bonds. The material. The formation formation energy of the self-assemble was ∆E2 = −10.1 kcal/mol, thus revealing that the X-bonds are slightly weaker than the H-bonds (around 5.0 kcal/mol). Each X-bond was characterized by a bond CP connecting the I to the O-atom and a blue isosurface located between the I and O-atoms (see Figure 6b). The color of the NCIplot isosurface was bluish, thus confirming that the X-bond is weaker than the H-bond, in agreement with the MEP analysis (the MEP value at the N1-H was significantly more positive than that at the σ-hole and the MEP value at N3–H was very similar). The NCIplot and QTAIM analysis also revealed the existence of a weak (green isosurface) contact between the H-atom of the thymine’s methyl group and the I-atom, characterized by a bond CP and bond path interconnecting both atoms. Figure 7a shows the R22 (8) motif observed in the solid state of 1ETURA–TFDIB. The H-bond acceptor was O2 thus leaving the most nucleophilic O4 atom available as halo- gen bond acceptor. The dimerization energy was ∆E3 = −9.4 kcal/mol, thus each HB was −4.7 kcal/mol, slightly smaller than the contribution of the H-bonds in the tetramer (formed by two different R22 (8) dimers) in compound THY-TFDIB in agreement with the MEP analysis (see Figure 5). Interestingly, the R22 (8) dimer established two symmetri- cally equivalent and highly directional X-bonds that were stronger than the H-bonds (∆E4 = −14.0 kcal/mol, see Figure 7b) and also the X-bonds computed for the THY-TFDIB cocrystal. Each X-bond was characterized by a bond CP connecting the I to the O-atom and a blue isosurface located between the I and H-atoms (see Figure 7b). the X-bond is weaker than the H-bond, in agreement with the MEP analysis (the MEP value at the N1-H was significantly more positive than that at the σ-hole and the MEP value at N3–H was very similar). The NCIplot and QTAIM analysis also revealed the ex- istence of a weak (green isosurface) contact between the H-atom of the thymine’s methyl Int. J. Mol. Sci. 2021, 22, 10663 group and the I-atom, characterized by a bond CP and bond path interconnecting9bothof 15 atoms. the R 2 Figure Figure 6.6. (a) (a) QTAIM QTAIM distribution distribution of of bond bond and and ring ring CPs CPs (red (red and and yellow yellow spheres, respectively) and spheres, respectively) and NCIplot NCIplot of of the 𝑅2 (8) (8) tetramer. tetramer. Int. J. Mol. Sci. (b) 2021, 22,(b) QTAIM QTAIM x FOR distribution REVIEW of of distribution PEER bond bond andand ringring CPs CPs (redyellow (red and and yellow spheres,spheres, respectively) respectively) and NCIplot and NCIplot of the of the hexamer 10 of 15 hexamer of compoundof compound THY–TFDIB. THY–TFDIB. The formation The formation energies energies (∆E) computed (ΔE) computed at the atPBE0-D3/def2-TZVP the PBE0-D3/def2-TZVP level level of theory of theory are are also indicated. also indicated. Figure 7a shows the 𝑅 (8) motif observed in the solid state of 1ETURA–TFDIB. The H-bond acceptor was O2 thus leaving the most nucleophilic O4 atom available as halogen bond acceptor. The dimerization energy was ΔE3 = −9.4 kcal/mol, thus each HB was −4.7 kcal/mol, slightly smaller than the contribution of the H-bonds in the tetramer (formed by two different 𝑅 (8) dimers) in compound THY-TFDIB in agreement with the MEP analy- sis (see Figure 5). Interestingly, the 𝑅 (8) dimer established two symmetrically equivalent and highly directional X-bonds that were stronger than the H-bonds (ΔE4 = −14.0 kcal/mol, see Figure 7b) and also the X-bonds computed for the THY-TFDIB cocrystal. Each X-bond was characterized by a bond CP connecting the I to the O-atom and a blue isosurface lo- cated between the I and H-atoms (see Figure 7b). 2 FigureFigure (a) QTAIM 7. (a)7.QTAIM distributionofofbond distribution bond and and ring ringCPs CPs(red andand (red yellow spheres, yellow respectively) spheres, and NCIplot respectively) and of NCIplot the 𝑅 (8) the R2 (8)ofdimer of compound 1ETURA-TFDIB. (b) QTAIM distribution of bond and ring CPs (red and yellow spheres, dimer of compound 1ETURA-TFDIB. (b) QTAIM distribution of bond and ring CPs (red and yellow spheres, respectively) respectively) and NCIplot of the tetramer of compound 1ETURA–TFDIB. The formation energies (∆E) computed at and NCIplot of the tetramer of compound 1ETURA–TFDIB. The formation energies (ΔE) computed at the PBE0-D3/def2-the PBE0-D3/def2-TZVP TZVPlevel levelofoftheory theoryareare also indicated. also indicated. Figure 8a shows the R2 (8) and R3 (9) motifs observed in the tetrameric assembly Figure 8a shows the 𝑅 (8) 2 and 𝑅 3(9) motifs observed in the tetrameric assembly ex- extracted from the solid state of 5FU-TFDIB showing the participation of both NH groups tracted from donors as H-bond the solidandstate ofatom the O2 5FU-TFDIB as H-bond showing acceptor.the participation Moreover, of both NH the O4 established groups an H- as H-bond bond with donors the C–H and the O2 group atom of 5FU, as as H-bondby evidenced acceptor. the bondMoreover, CP and bond the O4connecting path established an H-bond with the C–H group of 5FU, as evidenced by the bond CP and bond the CH to the O4-atom (see Figure 8a). In the solid state, both lone pairs of O2 participated path connect- inginthe CH to strong N–H the···O4-atom O H-bonds(see andFigure 8a). one lone Inof pair the O4solid in thestate, C–H···both lone pairs O H-bond, thusof O2 partic- leaving one in ipated lone pair of strong the O4 H-bonds N–H···O atom available and oneas halogen lone pair bond acceptor of O4 in the(see FigureH-bond, C–H···O 8b). The thus formation leaving energy one lone (∆Eof pair −42.7 5 =the O4 kcal/mol) was larger atom available (in absolute as halogen bond value) than the acceptor (seetetramer Figure 8b). of compound 1 THY–TFDIB, likely due to the additional CH ··· The formation energy 3(ΔE5 = −42.7 kcal/mol) was larger (in absolute value) thanO interactions. In this case, the te- one 5FU unit of the R3 (9) synthon established a bifurcated X-bond (BXB) with one TFDIB tramer of compound 1 THY–TFDIB, likely due to the additional CH···O interactions. In molecule where both the O4 and F5 acted as electron donors. The interaction energy was thisquite case,modest one 5FU unit of the 𝑅 (9) synthon established a bifurcated X-bond (BXB) with (∆E6 = −4.3 kcal/mol, see Figure 8b), thus suggesting that the bifurcated onebinding TFDIBmode molecule where is weaker thanboth the the O4 andXBs directional F5 observed acted as electron in THY–TFDIBdonors. andThe interaction 1ETURA– TFDIB co-crystals. The BXB was characterized by two bond CPs connecting the I to thethat energy was quite modest (ΔE 6 = −4.3 kcal/mol, see Figure 8b), thus suggesting O the bifurcated binding mode is weaker than the directional XBs observed in THY–TFDIB and 1ETURA–TFDIB co-crystals. The BXB was characterized by two bond CPs connecting the I to the O and F atoms of 5FU. The interaction was further characterized by a ring CP (yellow sphere) due to the formation of a supramolecular ring (see Figure 8b). The formation energy (ΔE5 = −42.7 kcal/mol) was larger (in absolute value) than the te- tramer of compound 1 THY–TFDIB, likely due to the additional CH···O interactions. In this case, one 5FU unit of the 𝑅 (9) synthon established a bifurcated X-bond (BXB) with one TFDIB molecule where both the O4 and F5 acted as electron donors. The interaction Int. J. Mol. Sci. 2021, 22, 10663 energy was quite modest (ΔE6 = −4.3 kcal/mol, see Figure 8b), thus suggesting that 10 of 15 the bifurcated binding mode is weaker than the directional XBs observed in THY–TFDIB and 1ETURA–TFDIB co-crystals. The BXB was characterized by two bond CPs connecting the Iand to the O and F atoms F atoms of 5FU. The of 5FU. Thewas interaction interaction was further by further characterized characterized by a ring a ring CP (yellow CP sphere) (yellow sphere) due to the formation of a supramolecular ring due to the formation of a supramolecular ring (see Figure 8b). (see Figure 8b). Figure 8. (a) QTAIM distribution of bond and ring CPs (red and yellow spheres, respectively) and NCIplot of the tetrameric assembly of compound 5FU-TFDIB. (b) QTAIM distribution of bond and ring CPs (red and yellow spheres, respectively) and NCIplot of the pentamer of compound 5FU-TFDIB. The formation energies (∆E) computed at the PBE0-D3/def2-TZVP level of theory are also indicated. 3. Materials and Methods 3.1. Materials Thymine and 5-fluorouracil were obtained from commercial suppliers and used without further purification. 1-Ethyluracil was prepared following a modified synthesis reported previously [34]. TFDIB was purchased from Cymit Química S.L. (Barcelona, Spain). Methanol, acetone, acetonitrile, and ethyl acetate used for the synthesis and crystallization experiments were p.a. grade from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany). 3.2. Methods 3.2.1. Synthesis of the Cocrystal thymine–1,2,4,5-tetrafluoro-3,6-diiodobenzene (2:1) A mixture containing thymine (100.08 mg, 0.79 mmol) and 1,2,4,5-tetrafluoro-3,6- diiodobenzene (159.49 mg, 0.40 mmol) in a 2:1 stoichiometric ratio was ground using a Retsch MM400 ball mixer (Retsch, Haan, Germany) in 10 mL agate grinding jars with two 5 mm agate balls at 30 Hz during 30 min. Single crystals were obtained by slow evaporation of a solution containing a mixture of acetone and methanol. 3.2.2. Synthesis of the Cocrystal 1-Ethyluracil–1,2,4,5-tetrafluoro-3,6-diiodobenzene (2:1) A mixture containing 1-ethyluracil (75.19 mg, 0.54 mmol) and 1,2,4,5-tetrafluoro-3,6- diiodobenzene (107.74 mg, 0.27 mmol) in a 2:1 ratio was ground using a Retsch MM400 ball mixer (Retsch, Haan, Germany) in 10 mL agate grinding jars with two 5 mm agate balls at 30 Hz during 30 min. Int. J. Mol. Sci. 2021, 22, 10663 11 of 15 Suitable single crystals were obtained by slow evaporation of methanol-ethyl ac- etate mixture. 3.2.3. Synthesis of the Cocrystal 5-Fluorouracil–1,2,4,5-tetrafluoro-3,6-diiodobenzene (4:1) A mixture containing 5-fluorouracil (75.0 mg, 0.58 mmol) and 1,2,4,5-tetrafluoro-3,6- diiodobenzene (57.9 mg, 0.144 mmol) in a 4:1 ratio was ground using a Retsch MM400 ball mixer (Retsch, Haan, Germany) in 10 mL agate grinding jars with two 5 mm agate balls at 25 Hz during 30 min. Suitable single crystals were obtained by slow evaporation of a methanolic solution. 3.3. Characterization Powder X-ray Diffraction (PXRD). PXRD data were collected using a Siemens D5000 powder X-ray diffractometer with Cu-Kα radiation (λ = 1.54056 Å), with 39 kV and 40 mA voltage and current applied. An amount of powder was gently pressed onto a glass slide to afford a flat surface and then analyzed. The samples were scanned from 2θ (◦ ): 2–50 using a step size of 0.02◦ and a scan rate of 1 ◦ /s. Single Crystal X-ray Diffraction (SC-XRD). Suitable crystals of compounds 1–3 were selected for single crystal X-ray diffraction experiments and mounted at the tip of a nylon CryoLoop on a BRUKER APEX-II CCD diffractometer (Bruker, Karlsruhe, Germany) using graphite monochromated MoKα radiation (λ = 0.71073 Å). Crystallographic data were collected at 294(2) K. Data reduction was performed using SAINT V6.45A and SORTAV in the diffractometer package [35]. Data were corrected for Lorentz and polarization effects and for absorption by SADABS [36]. The structural resolution procedure was made using SHELXT [37]. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced in calculated positions and refined riding on their parent atoms. Calculated X-ray powder patterns were obtained from single crystal structure data using Mercury 4.3.1 software (CCDC, Cambridge, UK) [38]. In Table 2, details about general and crystallographic data for compounds 1–3 are summarized. Hydrogen-bond parameters can be found in Supplementary Information (Table S3). Thermogravimetric analysis-Differential scanning calorimetry (TGA-DSC). Thermal analyses were carried out on a simultaneous thermogravimetric analysis (TGA)-differential scanning calorimetry/differential thermal analysis (heat flow DSC/DTA) system NET- ZSCH -STA 449 F1 Jupiter. Samples (3–8 mg) were placed in an alumina pan and measured at a scan speed of 10 ◦ C min−1 from ambient temperature to 250 or 350 ◦ C under N2 atmosphere as protective and purge gas (their respective flow velocities were 20 and 40 mL/min). Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR-FT-IR). Infrared spectra were recorded with a Jasco 4700LE spectrophotometer (JASCO, Tokyo, Japan) with attenuated total reflectance accessory. The scanning range was 4000 to 400 cm−1 and at a resolution of 4.0 cm−1 . 3.4. Computational Methods The theoretical study reported herein was carried out using the Gaussian-16 [39] program package (Gaussian, Wallingford, CT, USA) and the PBE0-D3/def2-TZVP level of theory [40]. Since we intend to estimate the interactions in the solid state, the crystallo- graphic coordinates were used. This level of theory and methodology has been previously used to evaluate similar interactions [41]. In this work, the binding energy and interaction energy terms were used indistinctly. They were computed as the difference between the energies of the isolated monomers and their assembly and were corrected for the basis set superposition error (BSSE) by using the Boys–Bernardi counterpoise method [42]. The Bader’s quantum theory of “Atoms in molecules” (QTAIM) and the noncovalent interaction index (NCIPlot) were used to characterize the noncovalent interactions using the AIMall program [43]. The molecular electrostatic potential (MEP) surfaces were generated using Int. J. Mol. Sci. 2021, 22, 10663 12 of 15 the Gaussian-16 software (Gaussian, Wallingford, CT, USA) and the PBE0-D3/def2-TZVP wave-functions. The 0.001 a.u. isosurface was used as the best estimate of the van der Waals surface. Table 2. Crystallographic data and refinement for cocrystals 1–3. 1 2 3 Empirical Formula C16 H12 F4 I2 N4 O4 C18 H16 F4 I2 N4 C22 H12 F8 I2 N8 O8 Mr 654.11 682.15 922.20 Crystal system Triclinic Triclinic Monoclinic Space group P-1 P-1 P 21 /n a/Å 4.6812(15) 8.2094(7) 6.837(3) b/Å 5.7924(19) 8.4322(7) 15.423(6) c/Å 18.787(6) 16.4377(13) 13.611(5) α/◦ 86.472(6) 87.684(2) 90 β/◦ 86.409(6) 87.889(2) 92.662(8) γ/◦ 84.697(5) 83.752(2) 90 V/Å3 505.4(3) 1129.59(16) 1433.7(9) Z 1 2 2 Radiation type Mo Kα Mo Kα Mo Kα µ/mm−1 3.179 2.849 2.309 Temperature/K 294(2) 294(2) 294(2) Crystal size/mm 0.27 × 0.26 × 0.15 0.330 × 0.130 × 0.100 0.340 × 0.080 × 0.080 Dcalc /g·cm−3 2.149 2.006 2.136 Reflections collected 10,285 33,259 31,225 Independent Reflections 2601 [R(int) = 0.0448] 5642 [R(int) = 0.0235] 3621 [R(int) = 0.0450] Completeness to θ = 25.242◦ 99.8% 100.0% 99.7% F(000) 310 652 884 Data/restraints/parameters 2601/0/137 5642/0/291 3621/0/241 Goodness-of-fit 1.277 1.039 1.065 R1 = 0.0607, R1 = 0.0240, R1 = 0.0434, Final R indices [I < 2d(I)] wR2 = 0.1384 wR2 = 0.0573 wR2 = 0.0917 R1 = 0.0649, R1 = 0.0280, R1 = 0.0563, R indices (all data) wR2 = 0.14 wR2 = 0.0599 wR2 = 0.0981 Largest diff. peak and hole/eÅ−3 1.505 and −1.394 0.847 and −0.551 1.468 and −1.280 CCDC n◦ 2,101,095 2,101,094 2,101,096 4. Conclusions In conclusion, we prepared and characterized three new halogen-bonded cocrystals comprising the canonical nucleobase thymine and two derivatives, 1-ethyluracil and 5- fluorouracil, with the strong halogen-bond donor 1,2,4,5-tetrafluoro-1,3-diiodobenzene. All compounds form self-assembled H-bonded R22 (8) dimers in the solid state, which is an important and recurrent binding motif. The interactions were studied through DFT calculations and characterized by a combination of NCIPlot/QTAIM methods. In general, the behavior of the cocrystals reported herein agrees well with the MEP surface analysis. The energetic results reveal that the H-bonds are stronger than the halogen bonds apart from the 1ETURA–TFDIB cocrystal, where the X-bonds are stronger, likely due to the presence of the ethyl substituent in N1, which is the best H-bond donor group in THY and 5FU compounds. In view of the present results, our future perspective is to continue exploring the ability and importance for hydrogen and halogen bonds in other nucleobase- derivate APIs, as well as to determine the modification of their physico-chemical properties (solubility, stability, hygroscopicity, pharmacological activity). This type of study is scarcely found in the literature. Consequently, further investigation related to the formation of cocrystals is needed, particularly considering that many APIs contain halogen atoms in their structure and that cocrystals are a real alternative to salts and polymorphs in drug formulation. Int. J. Mol. Sci. 2021, 22, 10663 13 of 15 Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/ijms221910663/s1, Figure S1. Comparison of the powder patterns of adenine (ADE), 1,2,4,5- tetrafluoro-1,3-diiodobenzene (TFDIB) and the mixtures prepared in methanol (MT) or water (WA) and the different ratios used. Figure S2. Comparison of the powder patterns of guanine (GUA), 1,2,4,5-tetrafluoro-1,3-diiodobenzene (TFDIB) and the mixtures prepared in methanol (MT) or water (WA) and the different ratios used. Figure S3. Comparison of the powder patterns of uracil (URA), 1,2,4,5-tetrafluoro-1,3-diiodobenzene (TFDIB) and the mixtures prepared in methanol (MT) or water (WA) and the different ratios used. Figure S4. Comparison of the powder patterns of cytosine (CYT), 1,2,4,5-tetrafluoro-1,3-diiodobenzene (TFDIB) and the mixtures prepared in methanol (MT), water (WA) or nitromethane (NM) and the different ratios used. Table S1. Melting points and solubilities in water of canonical nucleobases. Figure S5. Comparison of ATR-FT-IR spectra of thymine, 1,2,4,5- tetrafluoro-1,3-diiodobenzene (TFDIB) and cocrystal 1. Figure S6. Comparison of ATR-FT-IR spectra of 1-ethyluracil, 1,2,4,5-tetrafluoro-1,3-diiodobenzene (TFDIB) and cocrystal 2. Figure S7. Comparison of ATR-FT-IR spectra of 5-fluorouracil, 1,2,4,5-tetrafluoro-1,3-diiodobenzene (TFDIB) and cocrystal 3. Table S2. Some characteristic frequencies for the modified uracil compounds, the coformer and the new cocrystals. Figure S8. TGA-DSC of the cocrystal 1. Figure S9. TGA-DSC of the cocrystal 2. Figure S10. TGA-DSC of the cocrystal 3. Figure S11. X-bonds angles in cocrystals 1-3. Table S3. Hydrogen bonds for cocrystals 1 (THY-TFDIB), 2 (1ETURA-TFDIB) and 2 (5FU-TFDIB) [Å and ◦ ]. Author Contributions: Y.R. and M.B.-O. prepared 1-ethyluracil. M.B. designed, synthesized, charac- terized, and obtained the single crystals of the cocrystals. E.M. performed the data collection and the resolution of the crystal structures. A.F. performed the DFT study. M.B., A.F. and E.M. designed the concept. All wrote and revised the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: M.B. and E.M. are grateful to the Severo Ochoa FunFuture project (MICINN, CEX2019-917S) and Generalitat de Catalunya (2017SGR1687). A.F. thanks the MICIU/AEI from Spain for financial support (Projects CTQ2017-85821-R and PID2020-115637GB-I00, Feder funds). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: We thank R. Frontera from the Centre de Tecnologies de la Informació (CTI) at the UIB for computational facilities. The authors also thank the powder diffraction and thermal analysis services from the ICMAB. Conflicts of Interest: The authors declare no conflict of interest. References 1. Desiraju, G.R.; Ho, P.S.; Kloo, L.; Legon, A.C.; Marquardt, R.; Metrangolo, P.; Politzer, P.; Resnati, G.; Rissanen, K. Definition of the halogen bond (IUPAC Recommendations 2013). Pure Appl. Chem. 2013, 85, 1711–1713. [CrossRef] 2. Cavallo, G.; Metrangolo, P.; Milani, R.; Pilati, T.; Priimagi, A.; Resnati, G.; Terraneo, G. The Halogen Bond. Chem. Rev. 2016, 116, 2478–2601. [CrossRef] [PubMed] 3. Costa, P.J. The halogen bond: Nature and applications. Phys. Sci. Rev. 2017, 2, 20170136. [CrossRef] 4. Bulfield, D.; Huber, S.M. Halogen Bonding in Organic Synthesis and Organocatalysis. Chem. Eur. J. 2016, 22, 1–18. [CrossRef] [PubMed] 5. Gilday, L.C.; Robinson, S.; Barendt, T.; Langton, M.J.; Mullaney, B.R.; Beer, P.D. Halogen bonding in supramolecular chemistry. Chem. Rev. 2015, 115, 7118–7195. [CrossRef] 6. Christopherson, J.-C.; Topic, F.; Barret, C.J.; Friscic, T. Halogen-bonded cocrystals as optical materials: Next-generation control over light–matter interactions. Cryst. Growth Des. 2018, 18, 1245–1259. [CrossRef] 7. Baldrighi, M.; Cavallo, G.; Chierotti, M.R.; Gobetto, R.; Metrangolo, P.; Pilati, T.; Resnati, G.; Terraneo, G. Halogen bonding and pharmaceutical cocrystals: The case of a widely used preservative. Mol. Pharm. 2013, 10, 1760–1772. [CrossRef] 8. Baldrighi, M.; Bartesaghi, D.; Cavallo, G.; Chierotti, M.R.; Gobetto, R.; Metrangolo, P.; Pilati, T.; Resnati, G.; Terraneo, G. Polymorphs and co-crystals of haloprogin: An antifungal agent. CrystEngComm 2014, 16, 5897–5904. [CrossRef] 9. Aakeröy, C.B.; Welideniya, D.; Desper, J.; Moore, C. Halogen-bond driven co-crystallization of potential anti-cancer compounds: A structural study. CrystEngComm 2014, 16, 10203–10209. [CrossRef] 10. Miroslaw, B.; Plech, T.; Wujec, M. Halogen bonding in the antibacterial 1,2,4-triazole-3-thione derivative—Spectroscopic properties, crystal structure. J. Mol. Struct. 2015, 1083, 187–193. [CrossRef] Int. J. Mol. Sci. 2021, 22, 10663 14 of 15 11. Choquesillo-Lazarte, D.; Nemec, V.; Cinčić, D. Halogen bonded cocrystals of active pharmaceutical ingredients: Pyrazinamide, lidocaine and pentoxifylline in combination with haloperfluorinated compounds. CrystEngComm 2017, 19, 5293–5299. [CrossRef] 12. Berger, G.; Soubhye, J.; Meyer, F. Halogen bonding in polymer science: From crystal engineering to functional supramolecular polymers and materials. Polym. Chem. 2015, 6, 3559–3580. [CrossRef] 13. Berger, G.; Frangville, P.; Meyer, F. Halogen bonding for molecular recognition: New developments in materials and biological sciences. Chem. Commun. 2020, 56, 4970–4981. [CrossRef] 14. Auffinger, P.; Hays, F.A.; Westhof, E.; Ho, P. Halogen bonds in biological molecules. Proc. Natl. Acad. Sci. USA 2004, 101, 16789–16794. [CrossRef] 15. Persch, D.E.; Dumele, O.; Diederich, F. Molecular recognition in chemical and biological systems. Angew. Chem. Int. Ed. 2015, 54, 3290–3327. [CrossRef] 16. Wilcken, R.; Zimmermann, M.O.; Lange, A.; Joerger, A.C.; Boeckler, F.M. Principles and applications of halogen bonding in medicinal chemistry and chemical biology. J. Med. Chem. 2013, 56, 1363–1388. [CrossRef] 17. Lu, Y.; Wang, Y.; Zhu, W. Nonbonding interactions of organic halogens in biological systems: Implications for drug discovery and biomolecular design. Phys. Chem. Chem. Phys. 2010, 12, 4543–4551. [CrossRef] 18. Rowe, R.K.; Ho, S. Relationships between hydrogen bonds and halogen bonds in biological systems. Acta Cryst. 2017, B73, 255–264. [CrossRef] [PubMed] 19. Kolář, M.H.; Tabarrini, O. Halogen bonding in nucleic acid complexes. J. Med. Chem. 2017, 60, 8681–8690. [CrossRef] [PubMed] 20. Hardegger, L.A.; Kuhn, B.; Spinnler, B.; Anselm, L.; Ecabert, R.; Stihle, M.; Gsell, B.; Thoma, R.; Diez, J.; Benz, J.; et al. Systematic Investigation of Halogen Bonding in Protein–Ligand Interactions. Angew. Chem. Int. Ed. 2011, 50, 314–318. [CrossRef] [PubMed] 21. Marín-Luna, M.; Alkorta, I.; Elguero, J. The influence of intermolecular halogen bonds on the tautomerism of nucleobases. I. Guanine. Tetrahedron 2015, 71, 5260–5266. [CrossRef] 22. Marín-Luna, M.; Alkorta, I.; Elguero, J. The effect of cytosine methylation on its halogen-bonding properties. Comput. Theor. Chem. 2016, 1076, 101–108. [CrossRef] 23. Valkonen, A.; Chukhlieb, M.; Moilanen, J.O.; Tuononen, H.M.; Rissanen, K. Halogen and Hydrogen bonded complexes of 5-iodouracil. Cryst. Growth Des. 2013, 13, 4769–4775. [CrossRef] 24. Gerhardt, V.; Egert, E. Cocrystals of 6-chlorouracil and 6-chloro-3-methyluracil: Exploring their hydrogen-bond-based synthon motifs with several triazine and pyrimidine derivatives. Acta Crystallogr. Sect. B Struct. Sci. Cryst. Eng. Mater. 2015, 71, 209–220. [CrossRef] 25. Howard, J.L.; Cao, Q.; Browne, D.L. Mechanochemistry as an emerging tool for molecular synthesis: What can it offer? Chem. Sci. 2018, 9, 3080–3094. [CrossRef] 26. Roselló, Y.; Benito, M.; Molins, E.; Barceló-Oliver, M.; Frontera, A. Adenine as a halogen bond acceptor: A combined experimental and DFT study. Crystals 2019, 9, 224. [CrossRef] 27. Thomas, R.; Kulkarni, G. A hydrogen-bonded channel structure formed by a complex of uracil and melamine. Beilstein, J. Org. Chem. 2007, 3, 17. [CrossRef] 28. Madden, J.J. The unit cell of a mixed crystal of guanine and 8-azaguanine. Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 1973, 29, 914–915. [CrossRef] 29. Eccles, K.S.; Morrison, R.E.; Daly, C.A.; O’Mahony, G.E.; Maguire, A.; Lawrence, S. Co-crystallisation through halogen bonding with racemic or enantiopure sulfinamides. CrystEngComm 2013, 15, 7571–7575. [CrossRef] 30. Wang, H.; Jin, W.J. Cocrystal assembled by 1,4-diiodotetrafluorobenzene and phenothiazine based on C-I···p/N/S halogen bond and other assisted interactions. Acta Cryst. Sect. B 2017, B73, 210–216. [CrossRef] [PubMed] 31. Braun, D.E.; Gelbrich, T.; Wurst, K.; Griesser, U.J. Computational and Experimental Characterization of Five Crystal Forms of Thymine: Packing Polymorphism, Polytypism/Disorder, and Stoichiometric 0.8-Hydrate. Cryst. Growth Des. 2016, 16, 3480–3496. [CrossRef] [PubMed] 32. Sridhar, B.; Babu Nanubolu, J.; Ravikumar, K. Four cocrystals of thymine with phenolic coformers: Influence of the conformer on hydrogen bonding. Acta Cryst. 2015, C71, 602–609. [CrossRef] [PubMed] 33. Kumar, S.S.; Athimoolam, S.; Sridhar, B. Structural, spectral, theoretical and anticancer studies on new co-crystal of the drug 5-fluorouracil. J. Mol. Struct. 2018, 1173, 951–958. [CrossRef] 34. Roselló, Y.; Benito, M.; Barceló-Oliver, M.; Frontera, A.; Molins, E. 1-Ethyluracil, a New Scaffold for Preparing Multicomponent Forms: Synthesis, Characterization, and Computational Studies. Crystallogr. Growth Des. 2021, 21, 4857–4870. [CrossRef] 35. Blessing, R.H. An empirical correction for absorption anisotropy. Acta Crystallogr. Sect. A Found. Crystallogr. 1995, 51, 33–38. [CrossRef] 36. Bruker, A.P.E.X.; Saint, A.X.S. Inc., Madison, WI, 2004 Search PubMed; (b) GM Sheldrick. 2004. Acta Crystallogr. Sect. A Fundam. Crystallogr. 2008, 64, 112. 37. Sheldrick, G.M. SHELXT—Integrated space-group and crystal-structure determination. Acta Crystallogr. Sect. A Found. Adv. 2015, 71, 3–8. [CrossRef] 38. Macrae, C.F.; Sovago, I.; Cottrell, S.J.; Galek, P.T.A.; McCabe, P.; Pidcock, E.; Platings, M.; Shields, G.P.; Stevens, J.S.; Towler, M.; et al. Mercury 4.0: From visualization to analysis, design and prediction. J. Appl. Crystallogr. 2020, 53, 226–235. [CrossRef] Int. J. Mol. Sci. 2021, 22, 10663 15 of 15 39. Frisch, M.J.E.A.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.; et al. Gaussian 09, Revision d. 01, Gaussian; Gaussian, Inc.: Wallingford, CT, USA, 2016. 40. Adamo, C.; Barone, V. Toward reliable density functional methods without adjustable parameters: The PBE0 model. J. Chem. Phys. 1999, 110, 6158–6169. [CrossRef] 41. Rozhkov, A.; Eliseeva, A.A.; Baykov, S.V.; Galmés, B.; Frontera, A.; Kukushkin, V.Y. One-Pot Route to X-perfluoroarenes (X = Br, I) Based on FeIII-Assisted C–F Functionalization and utilization of these arenes as building blocks for crystal engineering involving halogen bonding. Cryst. Growth Des. 2020, 20, 5908–5921. [CrossRef] 42. Boys, S.F.; Bernardi, F. The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors. Mol. Phys. 1970, 19, 553–566. [CrossRef] 43. Keith, T.A. AIMAll (Version 13.05.06); TK Gristmill Software: Overland Park, KS, USA, 2013.