Polymorphism, Halogen Bonding, and Chalcogen Bonding in the Diiodine Adducts of 1,3- and 1,4-Dithiane

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Article Polymorphism, Halogen Bonding, and Chalcogen Bonding in the Diiodine Adducts of 1,3- and 1,4-Dithiane

Andrew J. Peloquin 1, Srikar Alapati 2, Colin D. McMillen 1, Timothy W. Hanks 2 and William T. Pennington 1,*

1 Department of Chemistry, Clemson University, Clemson, SC 29634, USA; [email protected] (A.J.P.); [email protected] (C.D.M.) 2 Department of Chemistry, Furman University, Greenville, SC 29613, USA; [email protected] (S.A.); [email protected] (T.W.H.) * Correspondence: [email protected]

Abstract: Through variations in reaction solvent and stoichiometry, a series of S-diiodine adducts of 1,3- and 1,4-dithiane were isolated by direct reaction of the dithianes with molecular diiodine in solution. In the case of 1,3-dithiane, variations in reaction solvent yielded both the equatorial and the axial isomers of S-diiodo-1,3-dithiane, and their solution thermodynamics were further studied via

DFT. Additionally, S,S’-bis(diiodo)-1,3-dithiane was also isolated. The 1:1 cocrystal, (1,4-dithiane)·(I2) was further isolated, as well as a new polymorph of S,S’-bis(diiodo)-1,4-dithiane. Each structure showed signiﬁcant S···I halogen and chalcogen bonding interactions. Further, the product of the diiodine-promoted oxidative addition of acetone to 1,4-dithiane, as well as two new cocrystals of 1,4-dithiane-1,4-dioxide involving hydronium, bromide, and tribromide ions, was isolated. Keywords: crystal engineering; chalcogen bonding; halogen bonding; polymorphism; X-ray diffraction Citation: Peloquin, A.J.; Alapati, S.; McMillen, C.D.; Hanks, T.W.; Pennington, W.T. Polymorphism, Halogen Bonding, and Chalcogen Bonding in the Diiodine Adducts of 1. Introduction 1,3- and 1,4-Dithiane. Molecules 2021, Halogen bonding, and the related chalcogen bonding, interactions occur due to an inter- 26, 4985. https://doi.org/10.3390/ action between the electrophilic region on a halogen or chalcogen atom within a molecule and molecules26164985 a nucleophilic region on a different atom [1–4]. Due to their strength [5] and directionality [6], these interactions have emerged as useful tools in crystal engineering [7,8], pharmaceutical Academic Editors: Ryan Groeneman design [9–11], and in the understanding of synthetic transformations [12–14]. and Eric Reinheimer The reaction of diiodine, one of the simplest halogen bond donors, with thioethers allows the concomitant study of halogen and chalcogen bonding, as well as the competition Received: 26 July 2021 between the two. Several notable examples of reaction products from diiodine with cyclic Accepted: 12 August 2021 polythioethers exist in the literature, ranging from simple examples such as 1,4-dithaine [15] Published: 17 August 2021 and 1,3,5-trithiane [16] to larger macrocycles [17–19]. A common structural motif that emerges is the presence of an S···I halogen bond of varying strength, with a combination of Publisher’s Note: MDPI stays neutral halogen and chalcogen bonding to consolidate packing in the crystalline state. The charge with regard to jurisdictional claims in transfer component of these interactions has also been extensively characterized, with the published maps and institutional afﬁl- interaction primarily attributed to an n→σ type interaction that increases in strength in the iations. order of O < S < Se [20–22]. Herein, we report the isolation of a variety of crystalline polymorphs from the reaction of 1,3- and 1,4-dithiane with diiodine. In the case of 1,3-dithiane, two geometric isomers of S-diiodo-1,3-dithiane were isolated, with their solution behavior further studied via Copyright: © 2021 by the authors. DFT computations, as well as S,S’-bis(diiodo)-1,3-dithiane. For 1,4-dithiane, the previously Licensee MDPI, Basel, Switzerland. reported 1:1 cocrystal with I2 was isolated [15,23], as well as a new polymorph of S,S’- This article is an open access article bis(diiodo)-1,4-dithiane. Each of these structures reveals an interplay between S···I halogen distributed under the terms and and chalcogen bonding. In the course of our investigations, we found that diiodine conditions of the Creative Commons Attribution (CC BY) license (https:// promoted oxidative addition of acetone to 1,4-dithiane, and we isolated two structures creativecommons.org/licenses/by/ from the direct oxidation of the compound to dioxides, revealing alternative reaction 4.0/). pathways in these halogen-containing systems.

Molecules 2021, 26, 4985. https://doi.org/10.3390/molecules26164985 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 10

Molecules 2021, 26, 4985 2 of 10 oxidation of the compound to dioxides, revealing alternative reaction pathways in these halogen-containing systems. 2. Results and Discussion 2. Results and Discussion 2.1. Reaction of 1,3-Dithiane with I2 2.1. Reaction of 1,3-Dithiane with I2 While the product of the reaction of 1,4-dithiane and I2 was ﬁrst reported in the literatureWhile in the 1960 product [15,23 of], the reaction analogous of 1,4 reaction-dithiane of 1,3-dithianeand I2 was first has reported not been in reportedthe litera- inture the in intervening 1960 [15,23] 6, the decades. analogous Through reaction variations of 1,3-dithiane in stoichiometry has not been and reported solvent in utilized the in- fortervening the reaction, 6 decades. we obtainedThrough threevariation distincts in stoichiometry products: S-diiodo-1,3-dithiane and solvent utilized infor both the reac- the equatorialtion, we obtained (1) and three axial distinct (2) orientation products: of S the-diiodo diiodo-1,3- pendant,dithiane in as both well the as equatorial S,S’-bis(diiodo)- (1) and 1,3-dithianeaxial (2) orientation (3) (Figures of the1 and diiodo2 and pendant, Figures as S10–S12). well as WhenS,S’-bis(diiodo) the reaction-1,3- wasdithiane conducted (3) (Fig- atures a 1:11 and stoichiometry 2). When the inreaction methanol, was conducted crystalline at1 a was1:1 stoichiometry obtained, with in methanol, the S-I-I crystalline group in a1 pseudo-equatorialwas obtained, with the position, S-I-I group with in an a pseudo angle of-equatorial 52.29(3)◦ position,between with I-I andan angle the RMSof 52.29(3) ring° planebetween (Table I-I and1). Thethe RMS position ring ofplane this (Table substituent 1). The signiﬁcantly position of this deviates substit fromuent thesignificantly typical equatorialdeviates from position the typical (though equatorial not to the position degree (though of the axial not conformerto the degree2 described of the axial below). con- ◦ Forformer example, 2 described the analogous below). For angle example, for an equatorialthe analogous C-H angle in this for molecule an equatorial is only C 8.43(16)-H in this. Individualmolecule is moleculesonly 8.43(16) pack°. Individual in pairs in molecules the solid-state pack in via pa twoirs in C-H the ···solidS hydrogen-state via bondstwo C- (H···SdH···S hydrogen= 2.8840(17) bonds Å, R (d=H···S 0.93). = 2.8840(17) These crystallographicÅ, R = 0.93). These dimers crystallographic further interact dimers to further form sheetsinteract in to the formbc plane sheets by in anthe S bc··· Splane contact by an involving S···S contact an unsubstituted involving an unsubstituted sulfur atom on sulfur one ringatom and on one the ring diiodo-substituted and the diiodo-sub sulfurstituted atom sulfur on aatom neighboring on a neighboring ring (d Sring···S =(d 3.3571(6)S···S = 3.3571(6) Å, ◦ RÅ,= R 0.89 = ).0.89). Alternating Alternating dimers dimers are rotated are rotated by approximately by approximately 72 to 72 one° to another. one another. The sheets The stacksheets in stack the a indirection the a direction by a type by Ia halogentype I halogen interaction interaction between between terminal terminal iodine atoms iodine d ◦ (atomsI···I = ( 3.8019(3)dI···I = 3.8019(3) Å, θC-H Å,··· Sθ=C-H···S 128.64(15) = 128.64(15)). °).

FigureFigure 1. 1.The The solid-statesolid-state structurestructure ofof1 1( a(a),),2 2( b(b),), and and3 3( c().c). Carbon Carbon atomsatoms areare gray.gray. Sulfur Sulfur atoms atoms areare yellow. yellow. Iodine Iodine atoms atoms areare purple. purple. HydrogenHydrogen atomsatoms are are cyan. cyan. IntermolecularIntermolecularinteractions interactionsare areshown shown as as black black dotted dotted lines. lines. OnlyOnly hydrogenhydrogen atoms atoms involved in the indicated intermolecular interactions are shown for clarity. Atomic displacement ellipsoids are shown at involved in the indicated intermolecular interactions are shown for clarity. Atomic displacement ellipsoids are shown at the the 50% probability level. 50% probability level.

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FigureFigure 2. SingleSingle molecules molecules of of1 (1a),( a2), (b2), (andb), and3 (c),3 arranged(c), arranged to show to position show position of S-I-I relative of S-I-I to relative dithiane to ring dithiane plane. ring Hydrogen plane. Hydrogenatoms have atoms been haveomitted been for omitted clarity.for Atomic clarity. displacement Atomic displacement ellipsoids ellipsoidsare shown are at the shown 50% at probability the 50% probability level. level.

Table 1.TableS···I-I 1. interactionS···I-I interaction distances distances (Å) and (Å) angles and angles (◦). (°).

a Crystal dS-I dI-I θS-I-I θring-Ia-I Crystal dS-I dI-I θS-I-I θring-I-I 1 2.7124(4) 2.85193(18) 179.438(9) 52.29(3) 1 2.7124(4) 2.85193(18) 179.438(9) 52.29(3) 2 2 2.7465(6)2.7465(6) 2.8364(2) 2.8364(2) 176.579(12)176.579(12) 85.45(4)85.45(4) 2.8135(9)2.8135(9) 2.8072(4) 2.8072(4) 177.140(18)177.140(18) 40.70(6)40.70(6) 3 3 2.8329(9)2.8329(9) 2.7874(4) 2.7874(4) 176.229(18)176.229(18) 49.07(7)49.07(7) 4 4 3.0813(7)3.0813(7) 2.7827(4) 2.7827(4) 179.074(12)179.074(12) 38.25(5)38.25(5) 5 2.7891(6) 2.8117(3) 178.576(11) 42.78(5) 5 2.7891(6) 2.8117(3) 178.576(11) 42.78(5) 6 2.8036(7) 2.8031(3) 177.426(16) 47.40(5) a: angle calculated6 between2.8036(7) I-I bond and RMS plane2.8031(3) of dithiane ring. 177.426(16) 47.40(5) a: angle calculated between I-I bond and RMS plane of dithiane ring. When the 1:1 reaction of I2 and 1,3-dithiane was conducted in ethyl acetate, the axial isomerWhen of S-diiodo-1,3-dithiane the 1:1 reaction of I2 ( 2and) was 1,3 obtained.-dithiane Inwas this conducted case, the S-I-Iin ethyl group acetate, is in athe typical axial axialisomer position, of S-diiodo with- an1,3 angle-dithiane of 85.45(4) (2) was◦ obtained.between I-I In and this the case, RMS the ring S-I-I plane. group The is in geometric a typical changeaxial position, in the resulting with an angle molecule of 85.45(4) drastically° between affects I- theI and packing the RMS of thering molecules plane. The in geometric the solid- state.change Chains in the areresulting formed molecule by weak drastically C-H···S hydrogenaffects the bondspacking (d ofH··· theS = molecules 2.992(2) Å, inR the= 0.97solid),- propagatingstate. Chains in are the formedb direction. by weak Neighboring C-H···S hydrogen chains interact bonds ( withindH···S = 2.992(2) the ab plane Å, R by= 0.97), S···S prop- close contactsagating in (d Sthe···S b= direction. 3.6110(8) Neighboring Å, R = 0.96). chains The packing interact is wi furtherthin the reinforced ab plane inby theS···Sc closedirection con- bytacts abifurcated (dS···S = 3.6110(8) halogen Å, bonding R = 0.96). interaction, The packing involving is further both reinforced a weak I··· inI( thedI··· cI =direction 3.9808(2) by Å a, Rbifurcated= 0.98) and halogen I···S( dbondingI···S = 3.8928(5) interaction, Å, R involving= 0.99) contact. both a weak With I···I the (d primaryI···I = 3.9808(2) intermolecular Å, R = 0.98) ◦ interactionand I···S (dI···S being = 3.8928(5) of the Å, weak R = 0.99) C-H contact.···S type, With the the decomposition primary intermolecular temperature interaction of 2, 107 beingC, ◦ isof signiﬁcantlythe weak C-H···S lower type, than the in decomposition1, 148 C, where temperature stronger of S ···2, S107 interactions °C, is significantly are involved lower (Figuresthan in 1, S1 148 and °C S2)., where stronger S···S interactions are involved. TheThe product of the the I I22 additionaddition to to both both sulfur sulfur atoms, atoms, S,S’ S,S’-bis(diiodo)-1,3-dithiane-bis(diiodo)-1,3-dithiane (3), was (3), wasobtained obtained by reaction by reaction of 1,3- ofdithiane 1,3-dithiane with two with equivalents two equivalents of I2 in methanol. of I2 in methanol. In 3, both InS-I3,-I ◦ bothmoieties S-I-I are moieties in pseudo are- inequatorial pseudo-equatorial positions, with positions, angles withof 40.70(6) angles° and of 40.70(6) 49.07(7) betweenand 49.07(7) each betweenI-I and the each RMS I-I ring and plane. the RMSDouble ring-stranded plane. chains Double-stranded propagate in chainsthe a direction, propagate formed in the by aa direction, formed by a cooperative series of S···I chalcogen bonds (d = 3.8445(8) Å and cooperative series of S···I chalcogen bonds (dS···I = 3.8445(8) Å and 3.8712(8)S···I Å, R = 0.98 and 3.8712(8)0.99) Å, R = 0.98 and 0.99). 2.2. Computational Investigation of S-Diiodo-1,3-Dithiane Ring Flip 2.2. Computational Investigation of S-Diiodo-1,3-Dithiane Ring Flip The electrostatic potential map of 1,3-dithiane, generated by DFT calculations, shows The electrostatic potential map of 1,3-dithiane, generated by DFT calculations, shows dis- distinct regions of negative electrostatic potential around sulfur, as would be expected from a tinct regions of negative electrostatic potential around sulfur, as would be expected from a sp3 sp3 hybridized atom. These are localized both above and below the ring plane (Figure3a). hybridized atom. These are localized both above and below the ring plane (Figure 3a). Either Either region could potentially serve as a hydrogen or halogen bond acceptor. This is seen in region could potentially serve as a hydrogen or halogen bond acceptor. This is seen in struc- structures 1–3, with 1 having a pseudo equatorial halogen bond to diiodine, while 2 and 3 havetures axial1–3, with halogen 1 having bonds. a pseudo equatorial halogen bond to diiodine, while 2 and 3 have axial halogen bonds.

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FigureFigure 3.3. Electrostatic potential potential (ESP) (ESP) map map of of 1,3 1,3-dithiane-dithiane (a)( aand) and reaction reaction coordinate coordinate of the of ring the ringflip from flip from1 to 21 (bto). 2The(b ).electro- The electrostaticstatic potential potential is calculated is calculated at the at0.005 the 0.005au isodensity au isodensity surface surface and ranges and ranges from − from25 kcal/mol−25 kcal/mol (red) to (red) 38 kcal/mol to 38 kcal/mol (blue). (blue).

TheThe conformations conformations andand inversioninversion pathwayspathways ofof 1,3- 1,3- and and 1,4-dithiane 1,4-dithiane have have previously previously beenbeen examinedexamined byby gas-phase gas-phase DFT DFT calculations calculations [ 24[24––2626]].. In In general, general, the the energetics energetics of of these these processesprocesses werewere found to to be be similar similar to to those those of ofcyclohexane, cyclohexane, which which has hasbeen been thoroughly thoroughly stud- studiedied both both experimentally experimentally and andcomputationally. computationally. The Theinversion inversion of the of the1,3- 1,3-dithianedithiane ring ring was wasreported reported to proceed to proceed through through twisted twisted boat intermediates boat intermediates,, lying 4.72 lying kcal/mol 4.72 kcal/mol above the above chair theconfirmation chair conﬁrmation (slightly (slightlylower tha lowern the equivalent than the equivalent cyclohexane cyclohexane intermediates) intermediates).. The calculated The calculatedtransition transitionstates for this states process for this were process found were to be found 9.87 kcal/mol to be 9.87 above kcal/mol the chair above state, the chairwhile state,the boat while transitio the boatn state transition between state the betweentwo intermediate the two intermediate structures was structures only 0.80 was kcal/mol only 0.80higher kcal/mol in energy. higher The in interconversion energy. The interconversion of the equatorial of the and equatorial axial isomers and axialof 1,3 isomers-dithiane/I of 2 1,3-dithiane/Ican proceed through2 can proceed multiple through pathways multiple since pathways the addition since of I the2 at additiononly one ofsulfur I2 at breaks only one the sulfursymmetry breaks of thethe symmetrysystem. Gas of-phase the system. calculations Gas-phase of one calculations of the inversion of one pathways of the inversion of the S- pathwaysdiiodo substituted of the S-diiodo system substituted indicate a systemΔE‡ of indicate11.23 kcal/mol a ∆E‡ ofbetween 11.23 kcal/mol 1 and its between respective1 andhalf its-chair respective conformer. half-chair Conformer conformer. 2 lies Conformeronly slightly2 lieshigher only in slightlyenergy higherthan 1 in(ΔE energy = 1.24 thankcal/mol),1 (∆E =with 1.24 a kcal/mol), ΔE‡ of 9.02 with kcal/mol a ∆E‡ of between 9.02 kcal/mol 2 and betweenits respective2 and half its respective chair. The half S-I chair.binding The energy S-I binding remains energy relatively remains consistent relatively throughout consistent throughoutthe ring inversion, the ring at inversion, approxi- atmately approximately 11 kcal/mol. 11 kcal/mol. The calculated The calculated energy differences energy differences remain remain consistent consistent relative relative to the togas the-phase gas-phase when solvation when solvation in methanol in methanol or ethyl or acetate ethyl was acetate included was included via SMD. via These SMD. re- Thesesults indicate results indicate the preferential the preferential formation formation of 1 or 2 of is1 notor 2 drivenis not drivenby conformational by conformational equilib- equilibriumrium in solution. in solution.

2.3. Reaction of 1,4-Dithiane with I2 2.3. Reaction of 1,4-Dithiane with I2 Through variations in solvent and stoichiometry, three new structures resulting from Through variations in solvent and stoichiometry, three new structures resulting from the reaction of I2 and 1,4-dithiane were obtained. The first, (1,4-dithiane)(I2)(4), is a 1:1 the reaction of I2 and 1,4-dithiane were obtained. The first, (1,4-dithiane)(I2) (4), is a 1:1 co- cocrystal obtained when the reaction is conducted in perfluoropyridine, and it possesses acrystal significantly obtained longer when S··· theI distance reaction than is conducted any of the in other perfluoropyridine products observed, and init possesses this study a (Figuresignificantly4a and longer Supplementary S···I distance Figure than S13),any of such the thatother the products S ···I interactions observed in in this4 distinguish study (Fig- themselvesure 4a), such from that the the more S···I covalentlyinteractions bonded in 4 distinguish adducts of themselves the other structures from the (more1–3, 5 covalently, 6). Each bonded adducts of the other structures (1–3, 5, 6). Each iodine atom is involved in both a iodine atom is involved in both a strong halogen bond (dS···I = 3.0813(7) Å, R = 0.78) and strong halogen bond (dS···I = 3.0813(7) Å, R = 0.78) and much weaker chalcogen bond (dS···I = much weaker chalcogen bond (dS···I = 3.8728(7) Å, R = 0.99). The two types of interactions can3.8728(7) be easily Å, R distinguished = 0.99). The two by thetypes orientation of interactions of the can interaction be easily relativedistinguished to the by I-I the bond. orienta- For thetion halogen of the bond, interaction the S··· relI-Iative angle to of the 179.074(12) I-I bond.◦ indicates For the halogen involvement bond, of the the S···Iσ hole-I angle on the of iodine179.074(12) atom° [indicates27–29]. The involvement chalcogen of bond the showsσ hole aon distinctly the iodine different atom [27 orientation,–29]. The chalcogen with an Sbond···I-I angleshows of a 77.077(14)distinctly different◦, as well orientation, as a C-S···I with angle an of S···I 177.17(8)-I angle◦ .of These 77.077(14) two angles°, as well taken as a togetherC-S···I angle demonstrate of 177.17(8) an° interaction. These two between angles taken the band together of increased demonstrate electrostatic an interaction potential be- ontween the iodinethe band atom, of increased located normal electrostatic to the potential I-I bond, o asn the well iodine as the atom,σ hole located of the normal sulfur atom, to the locatedI-I bond, along as well the as elongation the σ hole ofof the sulfur C-S bond. atom, located along the elongation of the C-S bond. While increasing the iodine available for reaction to 2:1 diiodine:dithiane, variations in reaction solvent again produced polymorphic structures with 1,4-dithiane (Figure 4b,c); however, in this case, the polymorphic structures are of S,S’-bis(diiodo)-1,4-dithiane. When

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the reaction was conducted in methanol, structure 5 formed. This is isostructural to the IBr adduct, with an elongation of the S-I bond from 2.687(2) Å in the IBr compound to 2.7891(6) Å in 5 [30]. The packing is dominated by S···I chalcogen bonds (dS···I = 3.7591(6) Å, R = 0.96, θC–S···I = 176.81(9)°), which contribute to the formation of ribbons in the ab plane. Neighboring ribbons interact through a weak type I interaction between terminal iodine atoms. To provide the most accurate comparison to the McCollough (1,4-dithiane)(I2)2 polymorph, the reaction was also conducted in dichloromethane, yielding polymorph 6, with the same reduced unit cell parameters as those previously reported [15]. While the overall packing arrangement remains consistent, the reduction in data collection temperature causes a noticeable reduction in intermolecular contact distances. The S···I distance is 2.8036(7) in 6, reduced from 2.87 Å in the room temperature structure. A network of chalcogen bonds (dS···I = 3.6692(7) Å, R = 0.93, θC-S···I = 166.55(9)°) promotes the formation of sheets within the (1 0 1) plane. This distance is again reduced from the room temperature structure (3.78 Å). Neigh- boring sheets stack by weak S···I halogen bonds (dS···I = 3.9907(7) Å, R = 1.02). The arrangement of S-I-I moiety is pseudo in both 5 and 6, with an angle from the dithiane RMS ring plane of Molecules 2021, 26, 4985 47.22(5)° and 42.60(5)° respectively. Surprisingly, despite significant differences in the5 of pri- 10 mary halogen bonding interactions between 4, 5, and 6, the decomposition temperatures are within 7 °C of one another.

FigureFigure 4.4. TheThe solid-statesolid-state structurestructure ofof 44 ((aa),), 55( b(b),), andand6 6( c(c).). IntermolecularIntermolecular interactionsinteractions areare shownshown asas blackblack dotteddotted lines.lines. HydrogenHydrogen atomsatoms havehave beenbeen omitted omitted for for clarity. clarity. Atomic Atomic displacement displacement ellipsoids ellipsoids are are shown shown at at the the 50% 50% probability probability level. level.

2.4. OtherWhile Dihalogen increasing Promoted the iodine Reactions available of for1,4- reactionDithiane to 2:1 diiodine:dithiane, variations in reactionThroughout solvent again this investigation, produced polymorphic several other structures interesting with products 1,4-dithiane that (Figure involve4b,c the and for- Figuresmation S14of new and covalent S15) however, bonds in to this sulfur case, were the polymorphicobtained by the structures reaction are of of 1,4 S,S’-bis(diiodo)--dithiane with 1,4-dithiane.both I2 and Br When2. If the the 2:1reaction I2:1,4-dithiane was conducted reaction is in conducted methanol, in structure acetone, a5 sulfoniumformed. This cation is isostructuralis formed from to thethe IBraddition adduct, of withacetone an elongationto one of the of sulfur the S-I atoms bond of from the 2.687(2) dithiane Å ring in the (7) IBr(Figure compound 5a). A totriiodide 2.7891(6) anion Å in is5 also[30]. present The packing to balance is dominated the charge. by S A··· Ichalcogen chalcogen bond bonds is ◦ (presentdS···I = 3.7591(6) between Å the, R =disubstituted 0.96, θC–S···I =sulfur 176.81(9) atom), whichand the contribute central atom to the of formation the triiodide of ribbons anion in(dS···I the = ab3.8586(7)plane. Å, Neighboring R = 0.98, θC-S···I ribbons = 177.55(10)°). interact throughThe bromide a weak salt typeof this I interactioncompound betweenhas been terminalprepared iodine previously atoms. by Tothe provide reaction the of bromoacetone most accurate and comparison 1,4-dithiane to the in McColloughethanol [31]. (1,4- dithiane)(IFirst reported2)2 polymorph, in 1927, the the reaction reaction was of 1,4 also-dithiane conducted with inBr2 dichloromethane, provides 1,4-dithiane yielding-S,S’- polymorphdioxide.[32]6 The, with crystalline the same structure reduced unitof this cell molecule parameters in isolation as those has previously been further reported reported [15]. While the overall packing arrangement remains consistent, the reduction in data collection temperature causes a noticeable reduction in intermolecular contact distances. The S···I distance is 2.8036(7) in 6, reduced from 2.87 Å in the room temperature structure. A network ◦ of chalcogen bonds (dS···I = 3.6692(7) Å, R = 0.93, θC-S···I = 166.55(9) ) promotes the formation of sheets within the (1 0 1) plane. This distance is again reduced from the room temperature structure (3.78 Å). Neighboring sheets stack by weak S···I halogen bonds (dS···I = 3.9907(7) Å, R = 1.02). The arrangement of S-I-I moiety is pseudo in both 5 and 6, with an angle from the dithiane RMS ring plane of 47.22(5)◦ and 42.60(5)◦ respectively. Surprisingly, despite significant differences in the primary halogen bonding interactions between 4, 5, and 6, the decomposition temperatures are within 7 ◦C of one another (Figures S4–S6).

2.4. Other Dihalogen Promoted Reactions of 1,4-Dithiane Throughout this investigation, several other interesting products that involve the formation of new covalent bonds to sulfur were obtained by the reaction of 1,4-dithiane with both I2 and Br2. If the 2:1 I2:1,4-dithiane reaction is conducted in acetone, a sulfonium cation is formed from the addition of acetone to one of the sulfur atoms of the dithiane ring (7) (Figure5a and Figure S16). A triiodide anion is also present to balance the charge. A chalcogen bond is present between the disubstituted sulfur atom and the central atom ◦ of the triiodide anion (dS···I = 3.8586(7) Å, R = 0.98, θC-S···I = 177.55(10) ). The bromide Molecules 2021, 26, x FOR PEER REVIEW 6 of 10

twice in the literature.[33,34] While our attempts to investigate the halogen bonding of Br2 with 1,4-dithiane were unsuccessful, resulting in the dioxide due to the increased oxidizing power of Br2 relative to I2, two new structures containing 1,4-dithiane-S,S’-dioxide were obtained. The first, (O=S(CH2CH2)2S=O)(H3O)(Br) (8), was obtained by the reaction of 1,4-dithiane with Br2 in a 5:1 mixture of methanol:acetone (Figure 5b). Acetone is required to resolubilize the dioxide which immediately precipitates from the solution upon Br2 addition. The resulting crystalline material showcases chains formed by hydrogen bonding between oxygen atoms on two separate 1,4-dithiane-S,S’-dioxide molecules and two hydrogen atoms of a hydronium ion. The third hydronium hydrogen atom participates in a hydrogen bond Molecules 2021, 26, 4985 to a bromide anion. In contrast to 8, the reaction of 1,4-dithiane with Br2 in acetonitrile6 ofpro- 10 duced crystalline (O=S(CH2CH2)2S=O-H)(Br3) (9). Resulting from the single protonation of a S=O oxygen atom, likely from HBr formed from water contamination of Br2, the mixed sulfoxide/sulfoxonium cations form 1-D chains along the b axis (Figure 5c). The single −OH salt of this compound has been prepared previously by the reaction of bromoacetone and hydrogen atom is equally shared between two oxygen atoms. Four neighboring chains are 1,4-dithiane in ethanol [31]. consolidated through a series of C-H···Br hydrogen bonds to the tribromide anion.

FigureFigure 5.5. TheThe solid-statesolid-state structure structure of of7 7(a (),a),8 8(b (),b), and and9 (9c ,(dc). and Both d). the Both O··· theH··· O···H···OO hydrogen-bonded hydrogen-bonded chains chains (c) and ( connectionc) and con- ofnection neighboring of neighboring chains by chains C-H ···byBr C- hydrogenH···Br hydrogen bonding bonding (d) are ( shown.d) are shown. Bromine Bromine atoms atoms are brown. are brown. Oxygen Oxygen atoms atoms are red. are red. Intermolecular interactions are shown as black dotted lines. Atomic displacement ellipsoids are shown at the 50% Intermolecular interactions are shown as black dotted lines. Atomic displacement ellipsoids are shown at the 50% probability probability level. level. 3. Materials and Methods First reported in 1927, the reaction of 1,4-dithiane with Br2 provides 1,4-dithiane- S,S’-dioxide3.1. Materials [ 32]. The crystalline structure of this molecule in isolation has been further reported1,3-dithiane twice in (> the 97%, literature CAS registry [33,34 no.]. While505-23- our7) was attempts obtained to from investigate Oakwood the Chemical halogen bonding(Estill, SC of, USA Br2)with. 1,4-dithiane 1,4-dithiane (> 97%, were CAS unsuccessful, registry no. 505 resulting-29-3) was in obtained the dioxide from due Millipore to the increasedSigma (St. oxidizing Louis, MO power, USA of). Iodine Br2 relative (> 99.8%, to I2 ,resublimed, two new structures CAS registry containing number 1,4-dithiane- 7553-56-2), S,S’-dioxidebromine (> 99.8%, were CAS obtained. registry The no. ﬁrst,7726- (O=S(CH95-6), methanol2CH2) (100%,2S=O)(H CAS3O)(Br) registry (8), no. was 64- obtained56-1), and byethyl the acetate reaction (>of 99.5%, 1,4-dithiane CAS registry with Br no.2 in 141 a 5:1-78 mixture-6) were of obtained methanol:acetone from Fisher (Figure Scientific5b). Acetone(Waltham is, requiredMA, USA to). resolubilizePentafluoropyridine the dioxide (99%, which CAS immediatelyregistry no. 700 precipitates-16-3) was fromobtained the solutionfrom Synquest upon BrLaboratoires2 addition. (Alachua, The resulting FL, USA crystalline). All materials material were showcases used as received chains. formed by hydrogen bonding between oxygen atoms on two separate 1,4-dithiane-S,S’-dioxide molecules3.2. Thermal and Analysis two hydrogen atoms of a hydronium ion. The third hydronium hydrogen atom participates in a hydrogen bond to a bromide anion. In contrast to 8, the reaction of 1,4-dithiane with Br2 in acetonitrile produced crystalline (O=S(CH2CH2)2S=O-H)(Br3)(9). Resulting from the single protonation of a S=O oxygen atom, likely from HBr formed from water contamination of Br2, the mixed sulfoxide/sulfoxonium cations form 1-D chains along the b axis (Figure5c and Figure S18). The single −OH hydrogen atom is equally shared between two oxygen atoms. Four neighboring chains are consolidated through a series of C-H···Br hydrogen bonds to the tribromide anion.

3. Materials and Methods 3.1. Materials 1,3-dithiane (>97%, CAS registry no. 505-23-7) was obtained from Oakwood Chemical (Estill, SC, USA). 1,4-dithiane (>97%, CAS registry no. 505-29-3) was obtained from Millipore Sigma (St. Louis, MO, USA). Iodine (>99.8%, resublimed, CAS registry number 7553-56-2), bromine (>99.8%, CAS registry no. 7726-95-6), methanol (100%, CAS registry no. 64-56-1), Molecules 2021, 26, 4985 7 of 10

and ethyl acetate (>99.5%, CAS registry no. 141-78-6) were obtained from Fisher Scientific (Waltham, MA, USA). Pentafluoropyridine (99%, CAS registry no. 700-16-3) was obtained from Synquest Laboratoires (Alachua, FL, USA). All materials were used as received.

3.2. Thermal Analysis Simultaneous thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were made using a TA Instruments Q600 or Discovery 650 (New Castle, DE, USA). Samples were heated from room temperature to 500 ◦C in air (100 mL/min) at a rate of 10 ◦C/min.

3.3. Single-Crystal X-ray Analysis Single crystal X-ray diffraction data were collected using a Bruker D8 Venture diffrac- tometer (Madison, WI, USA) equipped with Mo Kα radiation (λ = 0.71073 Å) and a Photon 100 detector. Data were collected in 0.5◦ oscillations of phi and/or omega at a temperature of 100 K. Data were processed and corrected for absorption using the SAINT and SADABS routines in the Apex 3 software suite (v2017.3, Madison, WI, USA, 2017) [35]. The structures were solved by intrinsic phasing (SHELXT) [36] and subsequently refined within Olex2 [37,38]. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms attached to carbon atoms were refined in calculated positions using riding models. Hydrogen atoms attached to oxygen atoms in 8 and 9 were located in the electron difference map and freely refined. Appropriate DFIX constraints were used in 9. Crystallographic data are reported in Table S1.

3.4. Computational Study of 1 and 2 All DFT calculations were conducted using the Gaussian 09 package (B.01, Wallingford, CT, USA, 2010) [39]. Structures 1, 2, and their respective twisted boat conformations were optimized using the M06-2X functional and cc-pVTZ basis set with the LANL2DZ pseudopotential [40–43]. These optimized structures were used as starting points in the qst3 method to ﬁnd the transition states. Frequency calculations were performed using M06-2X/cc-pVTZ with LANL2DZ pseudopotential at 298.15 K and 1 atm of pressure. Each optimized transition state structure had one imaginary frequency and was used in intrinsic reaction coordinate (IRC) calculations to examine the reaction path of the transition state.

3.5. Synthesis of Cocrystals The synthesis of all cocrystals was scaled for a maximum yield of 100 to 200 mg of the desired product. Reagents were dissolved at the indicated ratios in a minimum amount of solvent and allowed to evaporate slowly at room temperature. Vials were sealed to halt evaporation as soon as crystals were observed to ensure sample purity. S-diiodo-1,3-dithiane (equatorial) (1). 1,3-dithiane (44 mg, 0.37 mmol) and diiodine (93 mg, 0.37 mmol) were dissolved in methanol (10 mL) and the solvent was allowed to ◦ slowly evaporate at room temperature. Tdecomp 148 C. Anal Calcd for C4H8I2S2 (374.0): C, 12.84; H, 2.16; S, 17.15; Found: C, 12.65; H, 2.13; S, 16.88. S-diiodo-1,3-dithiane (axial) (2). 1,3-dithiane (37 mg, 0.31 mmol) and diiodine (78 mg, 0.31 mmol) were dissolved in ethyl acetate (10 mL) and the solvent was allowed to slowly ◦ evaporate at room temperature. Tdecomp 107 C. Anal Calcd for C4H8I2S2 (374.0): C, 12.84; H, 2.16; S, 17.15; Found: C, 13.10; H, 2.36; S, 17.12. S,S0-bis(diiodo)-1,3-dithiane (3). 1,3-dithiane (26 mg, 0.22 mmol) and diiodine (110 mg, 0.44 mmol) were dissolved in methanol (10 mL) and the solvent was allowed to slowly ◦ evaporate at room temperature. Tdecomp 115 C. Anal Calcd for C4H8I4S2 (627.9): C, 7.65; H, 1.28; S, 10.21; Found: C, 7.80; H, 1.04; S, 10.40. 1,4-dithiane·I2 (4). 1,4-dithiane (34 mg, 0.28 mmol) and diiodine (72 mg, 0.28 mmol) were dissolved in perﬂuoropyridine (10 mL) and the solvent was allowed to slowly evap- ◦ orate at room temperature. Tdecomp 126 C. Anal Calcd for C4H8I2S2 (374.0): C, 12.84; H, 2.16; S, 17.15; Found: C, 12.65; H, 2.46; S, 16.81. Molecules 2021, 26, 4985 8 of 10

S,S0-bis(diiodo)-1,4-dithiane (5). 1,4-dithiane (28 mg, 0.23 mmol) and diiodine (118 mg, 0.27 mmol) were dissolved in methanol (10 mL) and the solvent was allowed to slowly ◦ evaporate at room temperature. Tdecomp 129 C. Anal Calcd for C4H8I4S2 (627.9): C, 7.65; H, 1.28; S, 10.21; Found: C, 7.71; H, 1.14; S, 10.46. S,S0-bis(diiodo)-1,4-dithiane (6). 1,4-dithiane (31 mg, 0.26 mmol) and diiodine (131 mg, 0.52 mmol) were dissolved in methanol (10 mL) and the solvent was allowed to slowly ◦ ◦ evaporate at room temperature. Tmelt 62 C. Tdecomp 122 C. Anal Calcd for C4H8I4S2 (627.9): C, 7.65; H, 1.28; S, 10.21; Found: C, 7.77; H, 1.29; S, 10.45. 1-(2-oxopropyl)-1,4-dithianium triiodide (7). 1,4-dithiane (34 mg, 0.28 mmol) and diiodine (144 mg, 0.57 mmol) were dissolved in acetone (10 mL) and the solvent was allowed to ◦ slowly evaporate at room temperature. Tdecomp 126 C. Anal Calcd for C7H13I3OS2 (558.0): C, 15.07; H, 2.35; S, 11.49; Found: C, 14.72; H, 2.05; S, 11.34. 1,4-dithiane-S,S0-dioxide hydronium bromide (8). 1,4-dithiane (112 mg, 0.93 mmol) was dissolved in methanol (10 mL) and bromine was added (0.25 mL, 4.6 mmol), resulting in immediate precipitation of a white solid. Acetone (5 mL) was added to resolubilize the precipitate and the resulting solvent mixture was allowed to slowly evaporate at room ◦ temperature. Tdecomp 96 C. Anal Calcd for C4H11BrO3S2 (251.2): C, 19.13; H, 4.41; S, 25.53; Found: C, 19.26; H, 4.40; S, 25.92. 1,4-dithiane-S-oxide-S0-oxonium tribromide (9). 1,4-dithiane (141 mg, 0. 17 mmol) was dissolved in acetonitrile (10 mL) and bromine was added (0.31 mL, 5.9 mmol). The solvent ◦ was then allowed to slowly evaporate at room temperature. Tdecomp 98 C. Anal Calcd for C4H9Br3O2S2 (393.0): C, 12.23; H, 2.31; S, 16.32; Found: C, 11.99; H, 1.98; S, 16.37.

Supplementary Materials: The following are available online. Figures S1–S9, Simultaneous differential scanning calorimetry and thermal gravimetric analysis (SDT) of 1–9, Figures S10–S18, Unit cell packing diagrams of 1–9, Table S1, Crystallographic data and selected data collection parameters. Author Contributions: A.J.P.: methodology, formal analysis, investigation, data curation, writing— original draft preparation; S.A.: methodology, formal analysis, investigation, writing—review and editing; C.D.M.: writing—review and editing; T.W.H.: formal analysis, writing—review and editing, funding acquisition; W.T.P.: writing—review and editing, project administration, supervision. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported in part by the National Science Foundation EPSCoR Program under NSF Award #OIA-1655740. Computational resources were provided by the Research Corpo- ration for Science Advancement, Award # 27446, and the NSF through the MERCURY consortium, CHE-122935. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect those of the National Science Foundation. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Crystal data for 1–9 were deposited with the Cambridge Crystal- lographic Data Center (CCDC) (https://www.ccdc.cam.ac.uk, accessed on 17 august 2021) with deposition numbers 2097610–2097618. Acknowledgments: AJP thanks the US Air Force Institute of Technology Civilian Institutions Pro- gram for fellowship support. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples of the compoundsare not available from the authors.

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