Salts and Co-Crystalline Assemblies of Tetra(4-Pyridyl)Ethylene with Di-Carboxylic Acids

crystals

Article Salts and Co-Crystalline Assemblies of Tetra(4-Pyridyl)Ethylene with Di-Carboxylic Acids

Moustafa T. Gabr and F. Christopher Pigge * ID

Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-319-335-3805

Received: 22 December 2017; Accepted: 15 January 2018; Published: 17 January 2018

Abstract: Tetraarylethylene derivatives are emerging as an increasingly important family of supramolecular building blocks in both solution phase and the solid state. The utility of tetraarylethylenes stems from appealing structural features (rigidity and symmetry) and their propensity to exhibit aggregation induced emission (AIE). In an effort to investigate the luminescent sensing ability of heteroaromatic tetraarylethylenes, we previously prepared tetra(4-pyridyl)ethylene and characterized its solution phase AIE properties. We here report the successful incorporation of tetra(4-pyridyl)ethylene into three distinct salts and co-crystalline assemblies with three organic di-carboxylic acids (oxalic acid, malonic acid, and fumaric acid). Interactions between the tetra(pyridyl)ethylene and di-acid components were found to vary from conventional to charge-assisted hydrogen bonding according to the extent of proton transfer between the acid and pyridine groups. Notably, the formation of pyridinium-carboxylate adducts in the salts does not appear to be strongly correlated with acid pKa. Three distinct network topologies were observed, and all featured the bridging of two or three tetra(pyridyl)ethylene groups through di-acid linkers. Crystalline assemblies also retained the AIE activity of tetra(pyridyl)ethylene and were luminescent under UV light. As tetra(4-pyridyl)ethylene features four Lewis basic and potentially metal ligating pyridine rings in a relatively well-deﬁned geometry, this compound represents an attractive building block for the design of additional crystalline organic and metal–organic functional materials.

Keywords: supramolecular chemistry; crystal engineering; co-crystals; hydrogen bonded organic frameworks; aggregation induced emission

1. Introduction The design of functional crystalline materials has emerged as a principal objective of contemporary solid state supramolecular chemistry. It is envisioned that crystalline assemblies prepared from appropriate combinations of organic and/or metal–organic building blocks will afford functional materials that exhibit targeted bulk properties. Desirable bulk properties include inter alia porosity, catalysis, magnetism, conductivity, luminescence, and non-linear optical activity [1–12]. Of these targeted functions, the construction of porous crystalline materials has garnered considerable attention, in large part spurred by the desire to identify energy-related materials that can be potentially used for reversible gas storage [13–15]. A common strategy implemented in many crystal engineering approaches to functional materials aims to exploit well-deﬁned elements of molecular recognition or metal ligating ability that are arranged in pre-deﬁned patterns about rigid or semi-rigid organic building blocks to mediate supramolecular assembly. For example, aromatic polycarboxylic acids, such as terephthalic acid (1,4-benzene dicarboxylic acid), isophthalic acid (1,3-benzene dicarboxylic acid), and trimesic acid (1,3,5-benzene tricarboxylic acid), have provided a wealth of robust crystalline assemblies (both purely

Crystals 2018, 8, 41; doi:10.3390/cryst8010041 www.mdpi.com/journal/crystals Crystals 2018, 8, 41 2 of 12 organic as well as metal–organic) with diverse properties [5,6,10]. Similarly, N-functionalized Lewis basicCrystals building 2018, 8,blocks 41 (such as bipyridine and related polypyridines) have proven to be valuable2 crystalof 12 engineering tools [16]. Benzene polycarboxylic acids, polypyridine derivatives, and related congeners Lewis basic building blocks (such as bipyridine and related polypyridines) have proven to be offer predictable and geometrically deﬁned supramolecular or metal ligating interactions. Consonant valuable crystal engineering tools [16]. Benzene polycarboxylic acids, polypyridine derivatives, and with this strategy, new rigid or semi-rigid organic architectures that can be decorated with hydrogen related congeners offer predictable and geometrically defined supramolecular or metal ligating bonding and/or metal ligating moieties offer opportunities to expand the crystal engineering toolbox interactions. Consonant with this strategy, new rigid or semi-rigid organic architectures that can be anddecorated further facilitatewith hydrogen construction bonding of and/or additional metal functional ligating moieties materials. offer opportunities to expand the crystalIn this engineering context, tetraarylethylenes toolbox and further represent facilitate anconstruction attractive of platform additional for functional the construction materials. of novel crystallineIn this assemblies. context, tetraarylethylenes The parent hydrocarbon represent an tetraphenylethylene attractive platform for (1 ,the Figure construction1) and numerous of novel substitutedcrystalline derivatives assemblies. are The known parent to hydrocarbon exhibit the phenomenon tetraphenylethylene of aggregation-induced (1, Figure 1) and emission numerous (AIE) in whichsubstituted enhanced derivatives luminescence are known is observedto exhibit uponthe phenomenon the restriction of aggregation-induced of intramolecular bond emission rotations (AIE) due to aggregationin which enhanced of tetraarylethylene luminescence is molecules observed in upon solution the restriction (e.g., in the of presenceintramolecular of poor bond solvents) rotations or in thedue solid to aggregation state [17,18 of]. tetraarylethylene This general property molecules of tetraarylethylenesin solution (e.g., in the coupled presence with of poor a geometrically solvents) appealingor in the molecularsolid state [17,18]. structure This has general resulted property in studies of tetraarylethylenes examining the coupled utility with of tetraarylethylenea geometrically derivativesappealing inmolecular solution structure phase and has solid resulted state in supramolecular studies examining assembly the utility processes, of tetraarylethylene often with the aimderivatives of capitalizing in solution on enhanced phase and luminescence solid state supram to deliverolecular new assembly optical processes, or sensing often materials with the [19aim,20 ]. Indeed,of capitalizing several crystalline on enhanced luminescent luminescence metal to deliver organic new frameworks optical or (MOFs)sensing ma haveterials been [19,20]. prepared Indeed, using theseveral symmetrical crystalline tetraphenylethylene luminescent metal carboxylic organic fram acideworks (2) as (MOFs) well as have the extended been prepared biphenylethylene using the analoguesymmetrical (3) as tetraphenylethylene rigid metal ligating carboxylic components acid [21(2)– 35as]. well Additionally, as the extended we have biphenylethylene investigated the electroopticalanalogue (3) properties as rigid metal of crystalline ligating components organic assemblies [21–35]. constructedAdditionally,from we have phenoxyacetic investigated acid the (4) electrooptical properties of crystalline organic assemblies constructed from phenoxyacetic acid (4) and bis(pyridine) derivatives [36]. We have also characterized halogen bond-mediated self-assembly and bis(pyridine) derivatives [36]. We have also characterized halogen bond-mediated self-assembly and luminescence in crystals of tetraphenylethylene halophenyl esters (5)[37]. The tetrapyridine and luminescence in crystals of tetraphenylethylene halophenyl esters (5) [37]. The tetrapyridine derivative (6), featuring four pyridine units grafted onto the periphery of tetraphenylethylene [38], derivative (6), featuring four pyridine units grafted onto the periphery of tetraphenylethylene [38], has also been utilized as a metal ligating component in several MOFs, as well as a ligand in dynamic has also been utilized as a metal ligating component in several MOFs, as well as a ligand in dynamic 6 solutionsolution phase phase metal–organic metal–organic assemblies assemblies [39 [39–50].–50]. The The solid solid state state halogen halogen bondbond acceptingaccepting abilityability of 6 in combinationin combination with with diiodoarene diiodoarene halogen halogen bond bond donors donors has has also also been been examined examined [51 [51].].

Figure 1. Tetraphenylethylene (1) and tetraarylethylene derivatives (2–6) with proven utility as crystal Figure 1. Tetraphenylethylene (1) and tetraarylethylene derivatives (2–6) with proven utility as crystal engineering building blocks. Described below are the structures of three salts and co-crystals engineering building blocks. Described below are the structures of three salts and co-crystals assembled assembled from 7 and oxalic acid, malonic acid, and fumaric acid. from 7 and oxalic acid, malonic acid, and fumaric acid. Recently, we have initiated research aimed at developing small-molecule sensors for select analytes based on the metal chelating ability of poly(pyridyl) tetraarylethylenes in which two or more

Crystals 2018, 8, 41 3 of 12

Recently, we have initiated research aimed at developing small-molecule sensors for select analytes based on the metal chelating ability of poly(pyridyl) tetraarylethylenes in which two or more phenyl rings in tetraphenylethylene have been replaced with pyridine groups [52–54]. During the course of these studies, we also prepared tetra(4-pyridyl)ethylene (7) and characterized its solution phase AIE luminescent properties [54]. This tetra(pyridine) derivative resembles its larger analogue 6 in that the four pyridine rings are oriented in more or less ﬁxed positions about the central alkene linkage to provide geometrically well-deﬁned Lewis basic sites that are available for intermolecular interactions. We envision that synthetic accessibility and desirable molecular features will render 7 a useful building block in a range of crystal engineering applications. In preliminary efforts to explore the solid state supramolecular chemistry of 7, we report here the structures of three crystalline networks between 7 and oxalic, malonic, and fumaric acids.

2. Materials and Methods Tetra(pyridyl)ethylene (7) was synthesized as reported previously [54]. Co-crystals of 7 and the three dicarboxylic acids were obtained under similar conditions. A capped sample vial containing tetra(pyridine) 7 (33.6 mg, 0.1 mmol) and 0.2 mmol of the dicarboxylic acid in 6 mL of a 2:1 MeOH/acetone solution was heated in a 60 ◦C water bath for 1 h to obtain a homogeneous solution. The vial was then allowed to cool to room temperature. Slow evaporation of solvent under ambient conditions deposited X-ray quality single crystals. Diffraction data were collected on a Nonius Kappa CCD diffractometer (Bruker-Nonius, Delft, The Netherlands) equipped with Mo Kα radiation with λ = 0.71073 Å. Crystallographic data is shown in Table S1. Structures were solved by direct methods, and data were refined by full matrix least squares refinement of F2 against all reflections. Supplementary crystallographic data for this paper can be found in CCDC 1590576-1590578. This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

3. Results and Discussion Tetra(pyridyl)ethylene (7) was prepared via double Suzuki coupling between 1,1-dibromo-2,2- bis(4-pyridyl)ethylene and 4-pyridyl boronic acid as described previously [54]. Initial attempts to prepare co-crystals involved mixing 7 with terephthalic or isophthalic acid in different solvents followed by slow evaporation, but crystalline materials were not deposited under these conditions. We speculate that the combined rigidity of 7 and the benzene dicarboxylic acids may not be compatible with efficient crystal nucleation and growth. Consequently, we selected linear acyclic dicarboxylic acids as crystallization partners with the expectation that the smaller size and greater flexibility of these acids may be conducive to co-crystallization with 7. Gratifyingly, slow evaporation of methanol/acetone solutions containing 2:1 ratios of di-acid/7 yielded crystalline salts or co-crystalline assemblies. Of the simple di-acids screened in this study, X-ray quality single crystals were obtained from 7 and oxalic, malonic, and fumaric acids, while microcrystalline powders unsuitable for X-ray analysis were obtained from succinic, glutaric, and adipic acids. Crystals of 7 (oxalic acid)2 were formed with a 2:1 ratio of oxalic acid/7 along with seven H2O molecules of hydration. Crystallographic data is provided in Table S1. The asymmetric unit is comprised of one molecule of oxalic acid, 0.5 molecules of 7, and 3.5 H2O molecules. The C–O bond lengths in the oxalic acid component are all comparable and range from 1.288 to 1.219 Å, indicative of doubly deprotonated oxalate(2-) anions. Each pyridine group has been converted to a protonated pyridinium cation as evidenced by intra-annular N–C–N angles of ~121◦. These angles are significantly larger than comparable bond angles found in neutral pyridines (typically ~116◦)[55]. Oxalate and pyridinium groups are connected through a network of charge-assisted hydrogen bonds as shown in Figure2. One pyridinium group engages oxalate in a bifurcated H-bonding interaction, but the H4–O2 distance is much shorter than the H4–O1 distance (1.806 Å and 2.319 Å, respectively). The O1 oxygen of oxalate is also involved in a second strong H-bonding interaction with H10 of a neighboring molecule Crystals 2018, 8, 41 4 of 12 of 7 that serves to bridge two tetraarylethylenes (H10–O1 d = 1.474 Å). Individual molecules of 7 adoptCrystals a propeller-like 2018, 8, 41 conformation typical of tetraarylethylenes [17]. The oxalate ions are4 of further12 linkedCrystals through 2018, 8 an, 41 extensive network of H-bonds emanating from discrete T4(1) water4 tetramersof 12 (FigureH23OA) molecules [ 56]. Hydrogen range from bond 1.833 distances to 2.080 from Å. Two H2 Ovi toews oxalate of the andextended among packing H2O moleculesare illustrated range in from 1.833Figure toH2 2.080O molecules 3B,C. Å. TwoFigure range views 3B showsfrom of the1.833 the extended view to 2.080 down packingÅ. c Two in which vi areews offset illustrated of the stacked extended in columns Figure packing3 B,C.of alternating are Figure illustrated3B bridged shows in the viewtetra(pyridinium)ethylenes downFigure c3B,C.in which Figure offset3B shows stacked are the separated view columns down by c ofinoxal which alternatingate–water offset stackedclusters, bridged columns while tetra(pyridinium)ethylenes Figureof alternating 3C depicts bridged the are tetra(pyridinium)ethylenes are separated by oxalate–water clusters, while Figure 3C depicts the separatedpacking by down oxalate–water b. clusters, while Figure3C depicts the packing down b. packing down b.

Figure 2. Charge-assisted H-bonding interactions in oxalate-bridged tetra(pyridinium)ethylene co-crystals. FigureFigure 2. Charge-assisted 2. Charge-assisted H-bonding H-bonding interactions interactions in in oxalate-bridged oxalate-bridged tetra(pyridinium)ethylene tetra(pyridinium)ethylene co-crystals. co-crystals.

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(B)

Figure 3. Cont.

Crystals 2018, 8, 41 5 of 12 Crystals 2018, 8, 41 5 of 12 Crystals 2018, 8, 41 5 of 12

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Figure 3. (A) Tetrameric water clusters linking oxalate(C) ions in 7 (oxalic acid)2 7H2O. (B,C) Two views Figure 3. (A) Tetrameric water clusters linking oxalate ions in 7 (oxalic acid)2 7H2O; (B,C) Two views of the extended packing in 7 (oxalic acid)2 7H2O: (B) down c; (C) down b. of theFigure extended 3. (A) Tetrameric packing in water7 (oxalic clusters acid) linking2 7H2O: oxalate (B) down ions cin;( 7C (oxalic) down acid)b. 2 7H2O. (B,C) Two views of theThe extended structure packing of 7 (malonic in 7 (oxalic acid) acid)1.5 exhibits2 7H2O: significantly (B) down c; different(C) down features b. compared to the oxalic acid salt described above. First, solvate-free close-packed crystals were obtained in stoichiometry The structure of 7 (malonic acid)1.5 exhibits signiﬁcantly different features compared to the oxalic acid saltconsistingThe described structure of one above.of molecule 7 (malonic First, of 7acid)solvate-free and 1.51.5 exhibits molecules close-packed significantly of malonic different crystalsacid (crystallographic werefeatures obtained compared data in is stoichiometryprovidedto the oxalic acidin salt Table described S1). The malonicabove. First,acid groups solvate-free are present close-packed as neutral carboxyliccrystals were acids obtained (rather than in carboxylatestoichiometry consisting of one molecule of 7 and 1.5 molecules of malonic acid (crystallographic data is provided in consistinganions asof observedone molecule for oxalic of 7 acid). and 1.5This molecules is clearly evident of malonic by differences acid (crystallographic in the two C–O data bond is lengths provided Table S1). The malonic acid groups are present as neutral carboxylic acids (rather than carboxylate in Tablein each S1). carboxylic The malonic acid unitacid (1.202 groups + 1.309 are present Å; 1.203 as + 1.314neutral Å; 1.202carboxylic + 1.306 acids Å). Each (rather unique than carboxylic carboxylate anions as observed for oxalic acid). This is clearly evident by differences in the two C–O bond lengths anionsacid as residue observed engages for oxalic a pyridine acid). ring This of is 7 clearl in a conventionaly evident by (neutral)differences hydrogen in the twobonding C–O interactionbond lengths in each(Figure carboxylic 4, H–N acid distances unit (1.202 range + from 1.309 1.685 Å; 1.203 to 1.847 + 1.314 Å, O–N Å; 1.202 distances + 1.306 2.655–2.680 Å). Each Å). unique The fourth carboxylic in each carboxylic acid unit (1.202 + 1.309 Å; 1.203 + 1.314 Å; 1.202 + 1.306 Å). Each unique carboxylic acid residuepyridine engages ring of 7 a, however, pyridine does ring not of 7participatein a conventional in any H-bonding (neutral) interactions. hydrogen Evidence bonding that interaction the acid residue engages a pyridine ring of 7 in a conventional (neutral) hydrogen bonding interaction pyridines are acting as H-bond acceptors (and not as pyridinium H-bond donors) is further provided (Figure(Figure4, H–N4, H–N distances distances range range from from 1.685 1.685 to to 1.847 1.847 Å,Å, O–NO–N distancesdistances 2.655–2.680 Å). Å). The The fourth fourth pyridineby the ring intra-annular of 7, however, C–N–C does angles not (which participate range from in any 116.91 H-bonding to 117.69°) interactions.[55]. These angles Evidence are similar that the pyridineto the ringangle of found 7, however, in the remaining does not non-H-bonded participate in pyridineany H-bonding (115.93°) interactions.and are significantly Evidence smaller that the pyridinespyridines are are acting acting as as H-bond H-bond acceptors acceptors (and(and notnot as pyridinium H- H-bondbond donors) donors) is isfurther further provided provided than the analogous angles found in tetra(pyridinium)ethylene described◦ above. byby the the intra-annular intra-annular C–N–C C–N–C angles angles (which (which rangerange fromfrom 116.91 to 117.69°) 117.69 )[[55].55]. These These angles angles are are similar similar ◦ to theto the angle angle found found in in the the remaining remaining non-H-bonded non-H-bonded pyridine (115.93°) (115.93 )and and are are significantly signiﬁcantly smaller smaller thanthan the the analogous analogous angles angles found found in in tetra(pyridinium)ethylene tetra(pyridinium)ethylene described described above. above.

Figure 4. Hydrogen bonding interactions in 7 (malonic acid)1.5 co-crystals.

The structural topology observed in co-crystals of 7 (malonic acid)1.5 can be described as a (6,3) net [57]. Each molecule of 7 functions as a three-connected node bridged by malonic acid linkers. Six malonic acid molecules and six molecules of 7 comprise a cyclic motif that share common edges with Figure 4. Hydrogen bonding interactions in 7 (malonic acid)1.5 co-crystals. adjacent cyclicFigure assemblies, 4. Hydrogen affording bonding two-dimensional interactions in layers7 (malonic shown acid) in Figure1.5 co-crystals. 5. The large cavities evident in Figure 5 (cross-sectional dimensions of ~24 × 34 Å) are filled with two additional The structural topology observed in co-crystals of 7 (malonic acid)1.5 can be described as a (6,3) intertwined (6,3) nets as shown in Figure 6. Offset stacking of 2D layers then completes the structure. netThe [57]. structural Each molecule topology of 7 functions observed as in a co-crystals three-connected of 7 (malonic node bridged acid) 1.5by canmalonic be described acid linkers. as aSix (6,3) netmalonic [57]. Each acid moleculemolecules ofand7 sixfunctions molecules as aof three-connected7 comprise a cyclic node motif bridged that share by common malonic edges acid linkers.with Sixadjacent malonic cyclic acid moleculesassemblies, and affording six molecules two-dimensional of 7 comprise layers a shown cyclic motifin Figure that 5. share The commonlarge cavities edges withevident adjacent in cyclicFigure assemblies, 5 (cross-sectional affording dimensions two-dimensional of ~24 layers× 34 Å) shown are infilled Figure with5. Thetwo largeadditional cavities evidentintertwined in Figure (6,3)5 (cross-sectional nets as shown in dimensions Figure 6. Offset of ~24 stacking× 34 Å) of are 2D ﬁlledlayers with then two completes additional the intertwinedstructure. (6,3) nets as shown in Figure6. Offset stacking of 2D layers then completes the structure.

Crystals 2018, 8, 41 6 of 12 Crystals 2018, 8, 41 6 of 12 Crystals 2018, 8, 41 6 of 12

Figure 5. Spacefilling representation of a 2D (6,3) net formed from 3-connected tetra(pyridyl)ethylene Figure 5. SpacefillingSpaceﬁlling representation of a 2D (6,3) net fo formedrmed from 3-connected tetra(pyridyl)ethylene nodes and malonic acid linkers. nodes and malonic acid linkers.

Figure 6. Interdigitation of three (6,3) nets in 7 (malonic acid)1.5. Three identical independent (6,3) Figure 6. Interdigitation of three (6,3) nets in 7 (malonic acid)1.5. Three identical independent (6,3) Figurenetworks 6. Interdigitationare color coded ofred, three green, (6,3) and nets blue. in 7 (malonic acid)1.5. Three identical independent (6,3) networks are color coded red, green, and blue. networks are color coded red, green, and blue. Crystals of 7 and fumaric acid were also successfully grown. X-ray diffractometry revealed a Crystals of 7 and fumaric acid were also successfully grown. X-ray diffractometry revealed a close-packedCrystals structure of 7 and fumaricof stoichiometry acid were 7 (fumaric also successfully acid)4 (crystallographic grown. X-ray data diffractometry is shown in revealed Table S1). a close-packed structure of stoichiometry 7 (fumaric acid)4 (crystallographic data is shown in Table S1). The asymmetric unit consists of a 0.5 tetra(pyridyl)ethylene molecule that features one neutral close-packedThe asymmetric structure unit consists of stoichiometry of a 0.57 tetra(pyridyl)ethylene(fumaric acid)4 (crystallographic molecule datathat isfeatures shown one in Table neutral S1). pyridine (N–C–N angle 117.93°) and one protonated pyridinium cation (N–C–N angle 122.54°). Thepyridine asymmetric (N–C–N unit angle consists 117.93°) of a 0.5 and tetra(pyridyl)ethylene one protonated pyridinium molecule thatcation features (N–C–N one neutralangle pyridine122.54°). Additionally, two molecules◦ of fumaric acid are present, one of which exists as a◦ mono-anion as (N–C–NAdditionally, angle two 117.93 molecules) and one of protonatedfumaric acid pyridinium are present, cation one (N–C–Nof which angle exists 122.54 as a mono-anion). Additionally, as evidenced by similar C–O bond lengths at the anionic carboxylate terminus (1.240 and 1.262 Å) twoevidenced molecules by ofsimilar fumaric C–O acid bond are present,lengths oneat the of which anionic exists carboxylate as a mono-anion terminus as evidenced(1.240 and by 1.262 similar Å) compared to distinctly shorter and longer C–O bond lengths at the carboxylic acid terminus (1.212 C–Ocompared bond to lengths distinctly at the shorter anionic and carboxylate longer C–O terminus bond lengths (1.240 at and the1.262 carboxylic Å) compared acid terminus to distinctly (1.212 and 1.316 Å). The second fumaric acid molecule is present as a neutral dicarboxylic acid with C=O shorterand 1.316 and Å). longer The second C–O bond fumaric lengths acid at molecule the carboxylic is present acid terminusas a neutral (1.212 dicarboxylic and 1.316 Å).acid The with second C=O bond lengths of 1.202 and 1.211 Å and C–OH bond lengths of 1.322 and 1.312 Å. The protonated fumaricbond lengths acid moleculeof 1.202 and is present 1.211 asÅ aand neutral C–OH dicarboxylic bond lengths acid of with 1.322 C=O and bond 1.312 lengths Å. The of protonated 1.202 and pyridinium cations in each full molecule of 7 are in a trans-1,2-orientation about the central ethylene 1.211pyridinium Å and C–OHcations bondin each lengths full molecule of 1.322 andof 7 1.312are in Å. a trans The protonated-1,2-orientation pyridinium about the cations central in ethylene each full bond. Each pyridinum moiety is engaged with the carboxylate terminus of a fumarate mono-anion moleculebond. Each of pyridinum7 are in a trans moiety-1,2-orientation is engaged aboutwith the centralcarboxylate ethylene terminus bond. of Each a fumarate pyridinum mono-anion moiety is to form a cyclic hydrogen bonded motif featuring a pyridinium-carboxylate charge-assisted engagedto form witha cyclic the carboxylatehydrogen terminusbonded motif of a fumarate featuring mono-anion a pyridinium-carboxylate to form a cyclic hydrogen charge-assisted bonded hydrogen bond (NH+–O− distance 2.008 Å, N–H–O angle 147.05°, C–O–H angle 126.54°) and a C– hydrogen bond (NH+–O− distance 2.008 Å, N–H–O angle 147.05°, C–O–H angle 126.54°) and a C– H···O hydrogen bond (pyCH–O distance 2.400 Å) [58]. In complementary fashion, the neutral H···O hydrogen bond (pyCH–O distance 2.400 Å) [58]. In complementary fashion, the neutral

Crystals 2018, 8, 41 7 of 12 motif featuring a pyridinium-carboxylate charge-assisted hydrogen bond (NH+–O− distance 2.008 Å, N–H–O angle 147.05◦, C–O–H angle 126.54◦) and a C–H··· O hydrogen bond (pyCH–O distance Crystals 2018, 8, 41 7 of 12 2.400Crystals Å) 2018 [58, 8]., 41 In complementary fashion, the neutral pyridine rings in 7 each engage the neutral7 of 12 carboxylicpyridine rings acid in terminus 7 each engage of fumarate the neutral mono-anion carboxylic in a acid conventional terminus hydrogenof fumarate bonding mono-anion interaction in a ◦ (OH–Nconventionalpyridine distance rings hydrogen in 1.826 7 each Å, bonding engage O–H–N interactionthe angle neutral 169.26 (OH–N carboxylic). Thus, distance acid two terminus fumarate1.826 Å, O–H–Nof mono-anions fumarate angle mono-anion 169.26°). serve to Thus, bridge in a twotwoconventional bis(pyridyl)-bis(pyridinium)fumarate hydrogenmono-anions bonding serve interaction to ethylenes, bridge two(OH–N ultimately bis(pyridyl)-bis(pyridinium) distance giving 1.826 rise Å, to O–H–N two-dimensional ethylenes, angle 169.26°). ultimately hydrogen Thus, bondedgivingtwo fumarate rise chains, to two-dimens mono-anions as shown inional Figure serve hydrogen 7to. bridge bonded two chains,bis(pyridyl)-bis(pyridinium) as shown in Figure 7. ethylenes, ultimately giving rise to two-dimensional hydrogen bonded chains, as shown in Figure 7.

FigureFigure 7.7. Charge-assistedCharge-assisted parallelparallel H-bondedH-bonded chainschains ofof fumarate fumarate mono-anion mono-anion and and bis(pyridyl)-bis bis(pyridyl)- Figure 7. Charge-assisted parallel H-bonded chains of fumarate mono-anion and bis(pyridyl)- (pyridinium)ethylene.bis(pyridinium)ethylene. bis(pyridinium)ethylene. The neutral fumaric acid molecules are observed to support the charge-assisted carboxylate– The neutral fumaric acid molecules are observed to support the charge-assisted carboxylate–pyridinium pyridiniumThe neutral hydrogen fumaric bonding acid molecules interactions are as observ showned into Figuresupport 8. the A distortedcharge-assisted hydrogen carboxylate– bonded hydrogen bonding interactions as shown in Figure8. A distorted hydrogen bonded trimer involving trimerpyridinium involving hydrogen the carboxylate bonding terminusinteractions of fumara as shownte mono-anion, in Figure 8.a carboxylic A distorted acid hydrogen group of neutralbonded thefumarictrimer carboxylate involving acid, and terminus the a carboxylate pyridinium of fumarate terminus residue mono-anion, oforients fumara fumaricte a carboxylicmono-anion, acid acidgroups a carboxylic group perpendicular of acid neutral group fumaric to of the neutral acid,2D andhydrogenfumaric a pyridinium acid, bonded and residuechains a pyridinium described orients fumaricresidue above. acidTheorients rema groups fumaricining perpendicular carboxylic acid groups acids to perpendicular the of 2Dneutral hydrogen fumaric to the bonded acid 2D chainsgroupshydrogen described are bonded hydrogen above. chains bonded The described remaining to carboxylate above. carboxylic The ox remaygen acidsining atoms of neutral carboxylic of fumarate fumaric acids acid mono-anionsof neutral groups fumaric are found hydrogen acid in bondedadjacentgroups to areH-bonded carboxylate hydrogen chains. oxygenbonded Thus, atoms to the carboxylate fumaric of fumarate acid ox mono-anionsygencomponents atoms serveof found fumarate to inconnect adjacent mono-anions parallel H-bonded chains found chains. of in7 Thus,fumarateadjacent the fumaricH-bondedhydrogen acid bondedchains. components Thus,polymers the serve tofumaric form to connect cross-linkedacid co parallelmponents sheets. chains serve of to7 fumarateconnect parallel hydrogen chains bonded of 7 polymersfumarateto hydrogen form cross-linked bonded polymers sheets. to form cross-linked sheets.

Figure 8. 2D chains of 7 fumarate assemblies are connected in the third dimension by hydrogen FigurebondingFigure 8. 8. to 2D2D neutral chainschains fumaric ofof 77 fumaratefumarate acid components. assembliesassemblies are connected in in the third third dimension dimension by by hydrogen hydrogen bondingbonding to to neutral neutral fumaric fumaric acidacid components.components. Interdigitation of a second network of fumaric acid-linked 7 fumarate chains gives rise to an alternatingInterdigitation sheet-like of structurea second networkwith an ofABAB fumaric stacking acid-linked motif. 7 fumarateThe elliptical chains openings gives rise in to the an supramolecularalternating sheet-like macrocycles structure formed with through an ABAB fumara stackingte–pyridine/pyridinium motif. The elliptical H-bonding openings in one in sheet the (seesupramolecular Figure 7) possess macrocycles diagonal formed 2D cross-sections through fumara of ~1te–pyridine/pyridinium nm (i.e., distance from pyridineH-bonding to inpyridine one sheet N atoms(see Figure and pyridinium 7) possess diagonalto pyridinium 2D cross-sections N atoms). Th ofese ~1 openings nm (i.e., distanceare occupied from by pyridine two neutral to pyridine fumaric N atoms and pyridinium to pyridinium N atoms). These openings are occupied by two neutral fumaric

Crystals 2018, 8, 41 8 of 12

Interdigitation of a second network of fumaric acid-linked 7 fumarate chains gives rise to an alternating sheet-like structure with an ABAB stacking motif. The elliptical openings in the supramolecular macrocycles formed through fumarate–pyridine/pyridinium H-bonding in one sheet (see Figure7) possess diagonal 2D cross-sections of ~1 nm (i.e., distance from pyridine to pyridine Crystals 2018, 8, 41 8 of 12 N atoms and pyridinium to pyridinium N atoms). These openings are occupied by two neutral acidfumaric molecules acid molecules that support that support the thecharge-assisted charge-assisted carboxylate–pyridinium carboxylate–pyridinium hydrogen hydrogen bonding interactions connectingconnecting twotwo adjacentadjacent offset offset parallel parallel sheets. sheets. Two Two views views of thisof this packing packing arrangement arrangement are areshown shown in Figure in Figure9 with 9 with identical identical independent independent networks networks color color coded coded green green and red.and red.

(A)

(B)

Figure 9. Interpenetration of fumaric acid cross-linked 7 fumarate sheets. Identical independent Figure 9. Interpenetration of fumaric acid cross-linked 7 fumarate sheets. Identical independent networks are color coded green and red. (A) shows the view down the c axis (parallel to sheet axes). networks are color coded green and red. (A) shows the view down the c axis (parallel to sheet axes); (B) shows the view roughly perpendicular to the sheet axes with the ABA stacking motif represented (B) shows the view roughly perpendicular to the sheet axes with the ABA stacking motif represented by green–red–green networks. by green–red–green networks. Three distinct crystalline organic composites have been obtained from tetra(4-pyridyl)ethylene 7 andThree three distinctdifferent crystalline linear dicarboxylic organic composites acids. Intere havestingly, been the obtained assemblies from characterized tetra(4-pyridyl)ethylene in this study7 featureand three 7 in different three different linear dicarboxylic formal protonation acids. Interestingly, states: a tetra-protonated the assemblies tetracation characterized in combination in this study withfeature oxalic7 in acid, three a differentdi-protonated formal dication protonation in the presence states: a tetra-protonatedof fumaric acid, and tetracation a neutral intetra(pyridine) combination inwith the oxalic presence acid, of a di-protonatedmalonic acid. dicationThus, it inappears the presence that the of solid fumaric state acid, basicity and aof neutral 7 can tetra(pyridine)respond to an acidicin the environment presence of malonic in a manner acid. that Thus, facilitates it appears the formation that the solid of stable state crystalline basicity of networks.7 can respond Although to an oxalic acid is the strongest dicarboxylic acid among the three utilized in this study (pKa values of 1.27 and 4.28), both malonic and fumaric acid possess comparable pKa1 (2.85 and 3.02, respectively), and oxalic and fumaric acid possess comparable pKa2 (4.28 and 4.39, respectively). It appears, then, that consideration of pKa alone is insufficient to explain the conversion of 7 to poly(pyridindium) cations, and the tendency of 7 to experience protonation in the solid state may be strongly influenced by self- assembly dynamics along with the size and shape of acidic additives.

Crystals 2018, 8, 41 9 of 12 acidic environment in a manner that facilitates the formation of stable crystalline networks. Although oxalic acid is the strongest dicarboxylic acid among the three utilized in this study (pKa values of 1.27 and 4.28), both malonic and fumaric acid possess comparable pKa1 (2.85 and 3.02, respectively), and oxalic and fumaric acid possess comparable pKa2 (4.28 and 4.39, respectively). It appears, then, that consideration of pKa alone is insufﬁcient to explain the conversion of 7 to poly(pyridindium) cations, and the tendency of 7 to experience protonation in the solid state may be strongly inﬂuenced byCrystals self-assembly 2018, 8, 41 dynamics along with the size and shape of acidic additives. 9 of 12

4. Conclusions The results of of this this study study demonstrate demonstrate the the capabi capabilitylity of of Lewis Lewis basic basic tetra(pyridyl)ethylene tetra(pyridyl)ethylene (7) (to7) toparticipate participate in inco-crystal co-crystal and and salt salt formation formation with with organic organic acids. acids. The The distinct distinct topologies topologies of the crystalline assemblies characterized in this work provide a glimpse of the variedvaried architecturesarchitectures potentially accessible from small-molecule building blocks such as 7 and structurallystructurally related analogues. Additionally, Additionally, insertion insertion of ofmetal metal ions ions into into poly(pyridyl)ethylene–poly(acid) poly(pyridyl)ethylene–poly(acid) networks networks offer offeropportunities opportunities for construction for construction of novel of novel metal–orga metal–organicnic frameworks frameworks that may that mayexhibit exhibit topologies topologies and andproperties properties distinct distinct from from those those obtained obtained using using significantly signiﬁcantly larger larger tetraaryle tetraarylethylenethylene components components such suchas 6 (see as 6 Figure(see Figure 1). Research1). Research along along these these lines lines is currently is currently underway. underway. Finally, Finally, a defining a deﬁning feature feature of oftetraarylethylene tetraarylethylene assemblies assemblies is isthe the luminescent luminescent prop propertieserties that that often often emerge emerge upon upon restriction of intramolecular bondbond rotationrotation (i.e.,(i.e., AIEAIE effects). The AIE activity activity of of tetra(pyridyl)ethylene tetra(pyridyl)ethylene 7 previously established in solution phase studies is retained in the solid state assemblies prepared in this work. 7 For example, the luminescence of 7 (fumaric acid)4 isis visible visible to to the the naked naked eye eye as as shown shown in in Figure Figure 10.10. The other two crystallinecrystalline assemblies displayed similar luminescence, as shown in Figures S1 and S2 (see(see SupplementarySupplementary Materials).Materials).

Figure 10. Luminescence of 7 (fumaric acid)4 as revealed in photographs of microcrystalline samples Figure 10. Luminescence of 7 (fumaric acid) as revealed in photographs of microcrystalline samples taken under ambient light (left) and UV light4 (365 nm) (right). taken under ambient light (left) and UV light (365 nm) (right). Tetra(pyridyl)ethylene (7) is a promising supramolecular building block with appealing molecularTetra(pyridyl)ethylene and structural features. (7) is a promising Moreover, supramolecular 7 itself is easily building prepared block in with good appealing overall yield, molecular and andnumerous structural additional features. structural Moreover, analogues7 itself should is easily become prepared available in good via overall similar yield, synthetic and sequences. numerous additionalAdditional structuralapplications analogues of heteroarylethylenes should become in available both organic via similar and metal–organic synthetic sequences. crystal engineering Additional applicationsare under investigation, of heteroarylethylenes and it is envisioned in both organic that these and efforts metal–organic will offer crystal new approaches engineering to are a variety under investigation,of functional crystalline and it is envisioned materials. that these efforts will offer new approaches to a variety of functional crystalline materials. Supplementary Materials: The following are available online at www.mdpi.com/2073-4352/8/1/41/s1. Table S1: Supplementarycrystallographic Materials:data. FigureThe S1: following photograph are available illustrating online luminescent at www.mdpi.com/2073-4352/8/1/41/s1 properties of 7 (oxalic acid). Figure. Table S2: S1: crystallographic data. Figure S1: photograph illustrating luminescent properties of 7 (oxalic acid). Figure S2: photograph illustratingillustrating luminescentluminescent propertiesproperties ofof 77 (malonic(malonic acid).acid). Acknowledgments: We thank the Department of Chemistry, UniversityUniversity of Iowa, for partial support of this this work. work. We thankthank DaleDale C.C. SwensonSwenson (University(University ofof Iowa)Iowa) forfor assistanceassistance withwith X-rayX-ray crystallography.crystallography.

Author Contributions:Contributions: F. ChristopherChristopher PiggePigge conceived the work; F. ChristopherChristopher Pigge and Moustafa T. Gabr designed the experiments and analyzed the data; Moustafa T. Gabr performedperformed the experiments; F. Christopher Pigge wrote the paper. Pigge wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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