A Rational Approach to the Preparation of Polydipyridyldiacetylenes

A Rational Approach to the Preparation of Polydipyridyldiacetylenes: An Exercise in Crystal Design Sean M. Curtis, Nam Le, Frank W. Fowler,* and Joseph W. Lauher*

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 6 2313-2321

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794 Received May 8, 2005;

Revised Manuscript Received August 11, 2005

ABSTRACT: Crystals have been designed for the purpose of carrying out the topochemical polymerization of two isomeric dipyridyldiacetylenes, 1,4-di(pyrdin-3-yl)buta-1,3-diyne (3-pda) and 1,4-di(pyrdin-4-yl)buta-1,3-diyne (4pda). A successful polymerization depends on achieving a monomer spacing, dm, of 4.90 Å with a close contact R1,4, between the reacting atoms, C1-C4′, of neighboring diacetylenes. A series of five oxalamide dicarboxylic acid host molecules were chosen for the study since they were known to form hydrogen-bond networks with spacings near the required 4.90 Å. Five host molecules and two diacetylene guests should have given 10 cocrystals, but only eight were successfully obtained. The best parameters were found for the cocrystal of the 4-pyridyl isomer, 4-pda, with the oxalamide of glycine, 1-ox. The repeat distance, dm, was 4.93 Å with an R1,4 contact of 3.62 Å. The 4-pda:1-ox cocrystals were stable to heat, but ultraviolet irradiation polymerized the compound with an isolated yield of 61%. Introduction Crystal Growth & Design is a somewhat provocative title that suggests that we know what we are doing. Is this an accurate assumption? Can we design a crystal, synthesize the necessary compound or compounds, and grow the designed crystal? We would like to think that we can. Here is the story of one attempt. Topochemical diacetylene polymerizations were first reported over 30 years ago by Wegner.1 Like all topochemical reactions, a polymerization requires a precise prealignment of the reactant molecules within a crystal.2 For a successful polymerization,3,4 the most important parameter is the monomer repeat distance, dm (Figure 1). The corresponding polymer will possess a particular polymer repeat distance, dp. If the polymer is to be formed by a topochemical reaction then these two spacings must be roughly the same; otherwise, there will be too much molecular motion and the crystal will be disrupted before the polymerization is completed. Experience has shown that the average repeat distance, dp, for a polydiacetylene is 4.90(2) Å. The reaction is also favored by a close approach of the reacting atoms. If the monomeric diacetylene molecules are spaced at the required 4.90 Å and if they are aligned at an angle, φ, of 45° with respect to the crystallographic axis then the neighboring C1-C4 distance, R1,4, will be 3.5 Å, which corresponds to a van der Waals contact between the reacting molecules. If this required supramolecular structure can be achieved then the diacetylene molecules may undergo the polymerization reaction upon the application of heat or radiation. In truly ideal cases the polymerization can go crystal-to-crystal yielding single crystals of the resulting conjugated polymers. These single crystals have the polymers in perfect alignment, and unique optical and spectroscopic properties have been demonstrated. Unfortunately, there is a big problem. Most diacetylenes do not crystallize in accordance to these precise structural requirements. A given molecule will in gen-

Figure 1. The ideal structural parameters for a diacetylene polymerization. A diacetylene polymer will have a molecular repeat distance, dp, of 4.9 Å. For a topochemical polymerization to take place, the monomers must be aligned along some axis with a crystallographic repeat distance, dm, of similar length. The reacting atoms of neighboring molecules, C1 and C4, must also be in close contact. If the molecules are oriented at an angle, φ, of 45°, then this R1,4 distance will be 3.5 Å.

eral have one crystalline form, perhaps two or even three if polymorphism occurs. But for a given preexisting molecule, there is nothing one can do to change the crystallography. Either the molecule crystallizes properly or it does not. This limitation is a serious one. One has to come up with some method to alter the crystallographic environment of the diacetylene monomers to impose upon them a more suitable supramolecular structure. The answer is simple in concept. Use two molecules, not just the one. Design a second molecule, a host molecule that will reliably form crystals with a crystallographic repeat distance corresponding to the targeted repeat distance, dm. Make sure that the host molecule also has the functionality to form a reliable intermolecular bond to the original monomer, now serving the role of guest molecule. In the design, pay attention to

10.1021/cg050204x CCC: $30.25 © 2005 American Chemical Society Published on Web 10/08/2005

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allowable angles and distances of close approach. Do your best to grow a cocrystal between the host and guest; relative solubilities do not always allow it. Once you have your host-guest cocrystal, determine the structure. If the parameters look good, subject the crystal to the necessary polymerization conditions. We have used this technique to form a variety of new polydiacetylenes,2 and with suitably altered parameters we have designed and carried out the only known 1-6triacetylene polymerization.5 It seems to be a general synthetic method. However, like any new synthetic method one needs to test the method to find both its limitations and its extensions. This brings us to the subject of the present paper. We needed a challenging goal.

The two isomeric dipyridyldiacetylenes, 3-pda and 4-pda, are ideal polymerization candidates. Both compounds are known,6,7 and their crystal structures have been determined7,8 (Figure 2). Each compound forms an appropriate stack along a crystallographic axis, but the crystallographic repeat distances, dm, are much too short. Neither compound shows any sign of polymerization. There are no reported polymorphs for either molecule. A polymer of either 3-pda or 4-pda would be of high interest. The pyridine rings would function to extend the conjugation of the polymer π-system. This might be an important factor in enhancing nonlinear optical properties of conjugated polymers. Also, the pyridine ring is an effective functionality for bonding to metal atoms. This could be important for the establishment of an electrical connection between the conjugated polymer and a metal surface.

Figure 2. The crystal structures of 3-pda7 and 4-pda.8 In both cases the diacetylenes stack appropriately along a crystallographic axis, but the structural parameters are far from the ideal values shown in Figure 1.

The syntheses of aryl-substituted polydiacetylenes have been attempted before. In the most thorough study, Sarkar and Talwar9 examined seven different bisaryl-diacetylenes. Only one compound, bis-3-quinolyldiacetylene (qda), crystallized with structural parameters even close to the necessary range. The critical distances in the qda structure are 4.81 Å for dm and 3.61 Å for R1,4. Samples of qda undergo polymerization, but only to an extent of about 25%. Curtis and coworkers10 studied the polymerization of a bithazolyldiacetylene, btda. The critical distances in the btda structure are 5.19 Å for dm and a close 3.48 for R1,4.

Despite exhaustive attempts, btda failed to polymerize in the solid state, although it underwent some sort of polymerization in the melt. Curtis suggested that the rigidity of the aryl group might be the factor hindering the polymerization. These studies and others showed us in advance the difficulties that we might anticipate with our attempts to polymerize 3-pda and 4-pda. Having identified our diacetylene targets, we needed to choose a host system. In previous work we have shown that the oxalamides,11 diamides of oxalic acid, are ideal for this purpose.2,12 A family of oxalamides, from 1-ox to 5-ox, derived from R, ω-amino acids were synthesized and their crystal structures were determined, Chart 1. These centrosymmetric oxalamides are molecules that we designed to form two-dimensional β-networks.11 The molecules are self-complementary in two dimensions. The primary one-dimensional R-network is formed by the diamide functionality, while a second R-network is formed by the two terminal carboxylic acid groups. Together the two nonparallel R-networks generate a β-network, Chart 1. The β-network of 4-ox is shown as

Preparation of Polydipyridyldiacetylenes

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Chart 1. Oxalamides Derived from Simple r,ω-Amino Acidsa

a Four of the five compounds crystallize with like-to-like acid-acid and amide-amide hydrogen bonds as illustrated by the 4-ox structure. The 3-ox structure is an exception with the molecules forming like-to-unlike amide-to-acid hydrogen bonds.

an example; three of the other four structures are quite similar. Significantly, the repeat distance of the primary R-network is fixed at about 5.0 Å by presence of the two amide hydrogen bonds. The crystallographic symmetry of the layers is a natural consequence of the chemical functionalities. The molecules are centrosymmetric, Ci point group symmetry. The primary R-network is held together by diamide hydrogen bonds that form about inversion centers yielding an R-network of p1 h rod symmetry.13,14 The carboxylic acid dimers also form about inversion centers, meaning that the second R-network also has p1 h rod symmetry. The resulting β-network has p1 h layer symmetry. The layers pack via simple translation, meaning that the final crystals have P1 h space group symmetry. The design, from molecular structure to hydrogen-bond network to the crystal, is thus all very rational and easy to understand, and it works for four of the five compounds. The molecule 3-ox is the lone exception; it does not form the designed β-network. For reasons that we do not fully understand, the molecules of 3-ox form a complex three-dimensional γ-network via like-to-unlike acid-to-amide hydrogen bonds. We will come back to this exception later in this paper. Taken alone, the oxalamides form interesting p1 h layered structures, but they can also be used as host molecules to organize a wide variety of guest molecules at the 5.0 Å amide repeat distance. The terminal acid functionalities can bind a variety of bases or the terminal acid functionalities can be replaced by a basic pyridine that can bind a variety of acidic guest molecules. We have used the oxalamide hydrogen-bonding

motif to organize diacids and dibases.2,12 Using pyridyloxalamides, we have organized metal atoms15 and in a recent study, diiododiacetylene,16 via a Lewis acid-base interaction. The five oxalamides are thus perfect host candidates for the polymerization of 3-pda and 4-pda. We anticipated β-networks such as the one shown for 1-ox:4-pda in Chart 2 with the diamides again imposing the desired 5.0 Å spacing. Furthermore, since both diacetylenes have inversion centers we expected that β-network layer symmetry would again be p1 h , strongly favoring the space group P1 h for the final cocrystals. Having a family of host molecules gives us a combinatorial advantage as we look to the final polymerization step. If any one of the host molecules gives us our desired chemistry, we can consider the experiment to be a success. With two diacetylenes, 3-pda and 4-pda, and five hosts, we anticipated 10 cocrystals. Experimental Section The diacetylenes6,7 and oxalamides11 were all prepared according to literature methods. The cocrystals were grown by slow evaporation of methanol solutions of the two components. Typically, 0.1 mmol of host and guest were dissolved in 10 mL of hot methanol, and the solution was allowed to evaporate yielding the desired crystals. Cocrystal 1-ox:3-pda. Colorless needles. mp 260-264°. Cocrystal 1-ox:4-pda. Colorless needles which darken to purple. mp 235-240°. Cocrystal 2-ox:3-pda. Colorless plates. mp 187-189°. Cocrystal 2-ox:4-pda. Brown crystals. mp > 300°. Cocrystal 4-ox:3-pda. Colorless rhombs. mp 197-199°. Cocrystal 4-ox:4-pda. Blue needles. mp 210-211°.

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Chart 2. Predicted β-Network Structure for the 1-ox:4-pda Cocrytstal Showing the Multitude of Intramolecular and Intermolecular Inversion Centers that Lead One to Predict a P1 h Layer Structure

Cocrystal 5-ox:3-pda. Colorless plates. mp 192-195°. Cocrystal 5-ox:4-pda. Brown crystals. mp 189-193°. The X-ray crystallographic data was collected on a Bruker AXS Smart 1000 diffractometer using graphite-monochromated Mo radiation. Structures were solved and refined using the SHELX17 crystallographic program package. The γ-ray source was 60Co with a dose rate of 0.8 Mrad/h at Brookhaven National Laboratory. The Raman spectra were collected on a Kaiser optical system Raman microscope equipped with a 633 nm laser.

Results All seven compounds involved in this study are known. However, the designed cocrystals were unknown. The growth and X-ray structure determinations of the 10 anticipated crystals were the focus of our initial studies. Crystal preparation proved to be easy for eight cases. X-ray quality 3-pda and 4-pda cocrystals of 1-ox, 2-ox, 4-ox, and 5-ox were grown by slow evaporation of methanol solutions of stoichiometric mixtures of the two components. In contrast, cocrystals of 3-ox could not be prepared. What is wrong with 3-pda or 4-pda cocrystals of 3-ox? It is always difficult to explain a negative result. However, we can say that two different chemists working separately at different times worked very hard to prepare the missing cocrystals. A large variety of different solvents and conditions were used. Despite repeated attempts the two components always crystallized separately. Ground powders were studied using FTIR, but we could see no evidence of a cocrystalline phase. As we will shortly see all the eight of the observed cocrystals have structures in accordance with our design shown in Chart 2. There seems to be no particular reason similar cocrystals with 3-ox would not be stable. There must be some inherent kinetic or thermodynamic problem with 3-ox. Looking back at the structures of our five host molecules we are reminded of the fact the 3-ox structure was the exception in our original study of the oxalamide hosts. The other four compounds formed designed β-networks with like-to-like acid-to-acid and amide-to-amide hydrogen bonds, Chart 1. The 3-ox structure was different; a three-dimensional γ-network featuring like-to-unlike acid-to-amide hydrogen bonds is formed. This odd circumstance perplexed us then as it does now. The best explanation seems to be an unanticipated thermodynamic stability for the observed 3-ox structure. Although we lack quantitative data, qualitative observations indicate that the solubil-

ity of 3-ox in common solvents is less than the solubilities of the other oxalamides in the series. The Layers. Each of the eight cocrystals examined contain layers that have the supramolecular β-network structure we anticipated in our initial design, Chart 2. The β-networks each have two intersecting R-networks, one based upon the oxalamide diamide hydrogen bonds, the other based upon the carboxylic acid to pyridine hydrogen bonds. Each layer has p1 h layer symmetry with each molecule sitting on a crystallographic inversion center. The 3-pda structures are shown in Figure 3, while the 4-pda structures are shown in Figure 4. Table 1 gives a summary of the crystallographic results including a listing of the important intramolecular parameters defined in Figure 1. Although the molecular and layer symmetry is the same for all eight structures, molecular structure and conformations do influence the details. Qualitatively, one can see an “odd-even” structure effect with respect to the number of methylenes in the side chains.18 This is most significant in the 3-pda structures. The “even” structures, 2-ox:3-pda and 4-ox:3-pda, have the 3-pda molecule pointing downward in Figure 3, while in the odd structure, 3-ox:3pda, it points upward. The 1-ox: 3-pda cannot really be classified so easily since it is the only structure of the eight in which the carboxylic acid group is not coplanar with its hydrogen-bonded pyridine. The dihedral angle between the carboxylic acid group and the pyridine is 113° in 1-ox:3-pda. It is near zero in all the other structures. In the 4-pda structures, the “odd-even” effect can also be seen for the three heavier structures, but it is less obvious since the dicarboxylic acid-dipyridine R-networks adopt a more linear conformation. Indeed, the 4-ox:4-pda and 5-ox:4-pda structures are remarkably similar despite the difference of one methylene. It is reasonable to assume that the R-networks pack better when they adopt a nearly linear conformation. Stacking the Layers. From a crystal engineering perspective, it is important to understand the stackings of these eight p1 h layered structures to form threedimensional crystals. To do this, we must consider the various symmetry operations that could be invoked to relate the structure of one layer in a stack of layers to its neighbors. There are five common symmetry elements that one commonly encounters in organic structures: translation axes, inversion centers, 2-fold axes, 2-fold screw axes, and glide planes. Mirror planes are never found between molecules since they require



Figure 4. The four 4-pda structures. All four structures form in accordance with the basic design shown in Chart 1.

Figure 3. The four 3-pda structures. All four structures form in accordance with the basic design shown in Chart 1.

unfavorable like-to-like intramolecular interactions,19 and higher order axes are rare for small organics unless the molecule itself has high molecular symmetry. A p1 h layer has a multiplicity of two, an original molecule (or half molecule) and its inversion partner.

This means that there will of necessity be two paired symmetry operators relating one layer to the next one in the stack (Figure 5). A p1 h layer is in general oblique, with its lone independent angle (γ) not equal to 90° (Scheme 1). Seven of the eight layers in this study are oblique as expected. As we will shortly see, the eighth structure is not so simple. Oblique layers are not orthogonal, so when they stack they must either form a triclinic cell or they may lie parallel to the basal plane of a monoclinic cell. There are only two possible space group symmetries. In the simplest case, neighboring layers are related by paired translation and inversion operators, yielding a triclinic P1 h cell (Figure 5a). Alternatively, neighboring layers can be related by paired 2-fold screw and glide operations yielding a monoclinic P21/c cell. The axis must be

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Table 1. Selected Crystallographic Data a (Å) b (Å) c (Å) R (°) β (°) γ (°) volume (Å)3 space group Z density (g/cm3) dm, (Å) C1-C4′ (Å) φ (°)

1-ox:3-pda

1-ox:4-pda

2-ox:3-pda

2-ox:4-pda

4-ox:3-pda

4-ox:4-pda

5-ox:3-pda

5-ox:4-pda

17.576(3) 4.895(1) 22.989(4) 90 97.32(4) 90 1984.8(6) C2/c 4 1.380 4.90 4.17 55.1

4.928(2) 10.098(4) 19.901(8) 90 95.03(9) 90 986.6(7) P21/n 2 1.375 4.93 3.62 47.7

5.0624(6) 10.196(4) 10.958(64) 105.163(1) 100.651(6) 97.373(6) 527.1(3) P1 h 1 1.375 5.04 3.84 48.8

5.2321(8) 7.7730(12) 14.009(2) 88.809(3) 79.727(3) 72.899(3) 535.51(14) P1 h 1 1.353 5.23 5.66 76.0

5.1373(7) 5.1989(8) 23.223(3) 86.042(3) 88.088(3) 23.223(3) 617.31(15) P1 h 1 1.325 5.20 3.70 45.4

5.1801(10) 5.2099(10) 23.409(5) 88.219(6) 85.486(4) 88.992(5) 629.4(2) P1 h 1 1.299 5.21 5.53 74.2

5.215(2) 6.860(3) 18.858(7) 84.790(7) 89.247(7) 80.962(7) 663.5(4) P1 h 1 1.303 5.22 3.96 49.1

5.2263(10) 6.9022(13) 19.145(4) 94.246(4) 90.234(4) 100.533(4) 677.0(2) P1 h 1 1.277 5.23 4.91 64.0

perpendicular to the layer and the glide plane must be parallel to the layer (Figure 5b). Six of the observed structures in this study are triclinic, P1 h . One structure, 1-ox:4-pda, follows the monoclinic P21/c path. Symmetry does not require a p1 h layer to be anything other than oblique, but symmetry does not prohibit orthogonality. An otherwise oblique layer may have orthogonality imposed upon it if it is found as a substructure of a higher symmetry cell.20 A parallel 2-fold screw axis paired with a perpendicular glide can generate a second class of P21/c structures, but only if the original p1 h layer adopts an orthogonal lattice structure Figure 5c. In a similar manner a 2-fold axis can pair with a perpendicular glide to generate a P2/c cell (Figure 5d). Finally there is one more complex possibility. If the original p1 h layer adopts a centered orthogonal structure its multiplicity becomes four. Four new symmetry operators are needed to generate the next layer. These four operators can be a 2-fold, a 2-fold

screw, and a pair of glide planes (one will be a n-glide.) This combination of symmetry operators generates the centered monoclinic cell C2/c (Figure 5d). Surprisingly, one of the eight structures in this study, 1-ox:3-pda, adopts this structure. This means that the original layer as shown in Figure 3 is not oblique. It has a centered orthogonal lattice as shown in Figure 6. Six of the eight these cocrystals crystallize in P1 h as did the four original oxalamide hosts molecules when crystallized alone. A search of the Cambridge Crystallographic Data Base21 has been carried out for single molecules (no cocrystals) that form hydrogen-bonded β-networks with p1 h layer symmetry.22 A total of 123 structures were found, including our four original oxalamides. Of these 123 structures, 89 were triclinic P1 h, path a in Figure 5. There were 27 P21/c structures, including 12 structures with oblique p1 h layers packing as in Figure 5b and 15 structures with orthogonal p1 h layers packing as in Figure 5c. There were six C2/c

Figure 5. This diagram shows the five simple ways p1 h layers can be brought together to give a three-dimensional cell (original and translationally related layers are red; other symmetry generated layers are green). A p1 h layer has a multiplicity of two (the original molecule and its inversion partner), so at least two operations are needed to go from a p1 h layer to a space group. In (a) simple translation and inversion generate the triclinic cell, space group P1 h . The space group P21/c can be generated two ways. In (b), a perpendicular screw axis and a parallel glide plane combine to give a cell made up of two oblique p1 h layers. In (c), the original p1 h layer must have imposed orthogonality. The screw axis is now parallel, and the glide plane is perpendicular to the original layer. Substituting a parallel 2-fold axis for the screw axis generates the space group P2/c as shown in (d). It also contains p1 h layers with imposed orthogonality. Finally, you can add both a screw axis and a 2-fold along with two different glide planes (the second one is an n glide.) This generates the space group C2/c as shown in (e). C2/c with a multiplicity of eight contains four orthogonal p1 h layers. To generate the C2/c structure, the original p1 h layer must adopt a centered structure with imposed orthogonality. In this study, all eight structures contain p1 h layers. Six of them stack via path (a) to give a triclinic cell, P1 h . One structure, 1-ox:4-pda, follows path (b) to give a P21/c cell. The most complicated path (e) is followed by 1-ox:3-pda (Figure 6).


Crystal Growth & Design, Vol. 5, No. 6, 2005 2319 Scheme 1

structures (Figure 5e) and a lone example of a P2/c structure (Figure 5d). Thus, one can say that p1 h β-networks are much more likely to crystallize in the P1 h space group than in any other space group. Most of the remaining cases will be P21/c. C2/c will be unusual and P2/c is rare. The Targeted Parameters. Although we have seen that our basic structural goals have been met with each cocrystal forming out designed β-networks, we have to see if we have achieved the precise structural parameters necessary for a topochemical reaction. These numbers can be found in last rows of Table 1. The single most important parameter is dm, the repeat distance between the diacetylene monomers (Figure 1). For a polymerization to take place, this distance must match as close as possible the expected polymer repeat distance, dp, of 4.90 Å. The reaction is also facilitated if the C1-C4′ distance is a short as possible. The shortest possible value would be 3.4-3.5 Å corresponding to a van der Waals contact. Combining these two distances with the fixed length of the diacetylene functional group defines a triangle, which in turns mandates that the tilt angle, φ, be 45°. A look at the values in Table 1 reveals the 1-ox and 2-ox cocrystals of 3-pda and the 1-ox cocrystal of 4-pda have acceptable values of dm. The C1-C4′ distance is

Figure 6. The 1-ox:3-pda structure consists of layers of p1 h symmetry. These layers adopt a centered lattice with imposed orthogonality to meet the symmetry requirements of the C2/c space group (Figure 5e).

the shortest for the 1-ox:4-pda and the 2-ox:3pda structures. The same distance is rather long for the 1-ox:3-pda structure. Overall, the best and a near ideal parameter set are found for 1-ox:4-pda. The 4.93 Å dm value, the 3.62 Å C1-C4′ contact, and the 47.7° φ angle are all close to the ideal values. The other 4-pda structures have parameters far from the ideal, with dm values greater than 5.2 Å and long C1-C4′ contacts. The second best candidate is the 2-ox:3pda structure with a 5.23 Å dm value, a 3.84 Å C1-C4′ contact, and a φ angle 48.8°. As mentioned in the introduction, the most important parameter is the monomer repeat distance dm. If this distance is not a close match for the expected polymer repeat distance dp of 4.9 Å, then experience has taught us that polymerization is unlikely. An examination of the parameters shown in Table 1 shows that the cocrystals of the longer oxalamides, 4-ox and 5-ox, all have dm values in excess of 5.2 Å. There is a general trend for the distance to increase as the number of methylene units increases. The repeat distance of an R-network of any amide will vary depending upon the side-to-side displacement of one amide relative to the next.

If the translation brings a N-H group directly over the neighboring N-H group, the distance will be short, in the 4.5-7 Å range as is found in a urea structure. If the N-H sits directly on top of the neighboring carbonyl the distance will lengthen to the more desirable 4.9-

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Figure 7. In (a), the structure the 5-ox R-network as it appears in the 5-ox:4-pda cocrystal is shown, and the experimental repeat distance is 5.23 Å. In (b), the top molecule has been arbitrarily moved to the right into a position where it assumes a shorter repeat distance of 4.9 Å equal to the targeted distance shown in Figure 1. One can see that this movement brings the methylene groups of the upper molecule directly over those in the lower molecule. The unfavorable oxygen-oxygen contact distance also becomes more of a problem. This perhaps explains why the longer oxalamides, those with more methylene groups, show longer repeat distances.

Figure 8. The Raman spectrum of a 12 M HCl solution of 4-pda and its polymer. The 2210 and 2120 cm-1 bands are assigned to the carbon-carbon triple bond stretch. The 1638 and 1517 cm-1 are pyridine bands. The new peak at 1460 cm-1 is assigned to the new carbon-carbon double bond in the polydiacetylene polymer backbone.

5.0 Å range. If the N-H moves to the opposite side of the carbonyl the distance will lengthen further to the undesirable 5.2-4 Å range. The longer oxalamides may be forced into this longer repeat range by an increase in direct methylene-methylene repulsion as shown in Figure 7. Polymerization Studies. The X-ray data indicated that cocrystals 1-ox:4-pda and 2-ox:3-pda were the

Curtis et al.

best polymerization candidates. Interestingly, the 1-ox: 4-pda crystals were colorless when first prepared, but immediately turned purple. Such color changes are common when polymerization to a conjugated polymer occurs. Thermal annealing caused the crystals to turn black, but heating for long periods of time up to the melting point of 235-240° C brought about no visible changes in the single-crystal diffraction parameters and no change in the melting point. Crystals of 2-ox:3-pda were colorless and remained unchanged upon heating to their melting point of 260-264° C. Having had no success with thermal polymerization we tried UV irradiation. Irradiation of the 2-ox:3-pda crystals with a Hanovia 550 W medium-pressure mercury lamp for 72 h resulted in no change to the crystals. In contrast the 1-ox:4-pda cocrystals were quite sensitive to UV irradiation. The purple crystals turned purple and crumbled as the irradiation continued. After 120 h of irradiation the material was washed with hot methanol to remove the host oxalamide and the unreacted diacetylene. After extraction, a red brown amorphous polymer remained. Mass balance indicated a conversion yield of 61%. The polymer did not melt up at a temperature up to 300 °C. Since the polymer is a poly-pyridine it dissolves in 12 M HCl. The Raman spectrum of the resulting HCl solution was obtained and compared to a similar solution of the monomer. The monomer shows two prominent bands that we assign to the diacetylene triple bonds and to the pyridine. These peaks shift in the polymer and a strong new band appears at 1460 cm-1 that can be assigned to the new carbon-carbon double bonds of the polydiacetylene backbone. Diacetylene polymerizations that do not occur thermally can sometimes be induced by the use of gamma rays. The technique may not be successful since it depends on the competition between polymerization and gamma-ray decomposition. Nevertheless, it is known to work in difficult cases. Crystals of 1-ox:4-pda and 2-ox: 3-pda were both subjected to 85 Mrads of 60Co γ-ray. Both compounds turned dark, and both were examined using single-crystal X-ray diffraction. The crystallographic parameters for the 2-ox:3-pda crystals showed no change from the monomeric crystals. There was also no change in the melting point. The 1-ox:4-pda crystals did show a change. The crystals, now having a deep crimson color, no longer melted up to a temperature of 300° C. Unfortunately, the diffraction spots were now broad and diffuse, and no structure could be obtained despite repeated attempts. A unit cell could be determined from the X-ray limited data, and it showed that a significant change in cell parameters had taken place.23 Crystals of the polymerized sample were extracted in a manner similar to that used for the UV experiment, and a polymer yield of 41% was obtained. Conclusions. When we initiated this project, we knew that it would be difficult to induce a polymerization of a diacetylene with a direct aryl group attached. We hoped that if we could find a cocrystal with a nearly perfect set of structural parameters, we could induce the polymerization. Did we succeed? Like our reported polymerizations, our success has not been 100 percent,


but overall we have demonstrated several important points and we have made one of our two desired polymers. The most important point for this work is that the crystal design worked. Our host-guest cocrystal system gave us a supramolecular structure in accordance with our design in eight out of 10 cases. The two 3-ox cocrystals simply could not be grown. The structural parameters of the eight structures varied a bit, but for both 3-pda and 4-pda, one crystal had structural parameters near the ideal. All attempts to polymerize the 3-pda samples failed as did the thermal polymerization of 4-pda. UV and γ-ray irradiation of the 1-ox:4-pda cocrystals led to polymerization, and soluble polymers samples could be isolated. The preparation is simple enough that significant quantities of the 4-pda polymer can now be prepared by others. Future physical and chemical studies of the 4-pda polymer may prove to be most interesting. We agree with Curtis and co-workers10 that the fundamental problem with polymerizing an aryl diacetylene is the rigidity of the directly attached aryl group. In the monomer, the angle of the sp hybridized acetylenic carbon atom is 180°; upon polymerization these atoms take on sp2 hybridization with an ideal angle of 120°. In the known crystal-to-crystal diacetylene polymerizations,12 one can see that the diacetylene group itself rotates 32-35°, accommodating about over half of this angle difference. This leaves 25-28° of movement for the substituent. If the substituent is a methylene group that can rotate, there is no big problem, but it is very difficult to rotate a large aryl group, such as a pyridine, by such a large amount without destroying the crystal in the process.24 Final Conclusion. The name of the journal is not overly optimistic. We can design a crystal, synthesize the necessary compounds, and grow the crystals to yield a desired supramolecular structure with a function in accordance with our design. The preparation of designed functional crystals is a realistic endeavor. Acknowledgment. We wish to join our colleagues from around the world in celebrating Professor Mike McBride’s many accomplishments. We remember well Professor McBride’s visit to Stony Brook fifteen years ago. His colloquium inspired us to think the organic solid state in new ways, some of them odd, and some of them even. We thank Dr. Sergie Lymar of Brookhaven National Laboratories for assistance with the γ irradiations, Drs. Alasdair F. Bell and Peter J. Tonge for assistance with the Raman spectroscopy, and the National Science Foundation for financial support, CHE 0300008. Supporting Information Available: CIF files for the eight structures discussed in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.


References (1) Kiji, J.; Kaiser, J.; Wegner, G.; Shulz, R. C. Polymer 1973, 14, 433-439. (2) Fowler, F. W.; Lauher, J. W. J. Phys. Org. Chem. 2000, 13, 850-857. (3) Baughman, R. H.; Yee, K. C. J. Polym. Sci., Part D: Macromol. Rev. 1978, 13, 219-239. (4) Enkelmann, V. Adv. Polym. Sci. 1984, 63, 91-136. (5) Xiao, J.; Yang, M.; Lauher, J. W.; Fowler, F. W. Angew. Chem., Int. Ed. 2000, 39, 2132-2135. (6) Della Ciana, L.; Haim, A. J. Heterocycl. Chem. 1984, 21, 607-608. (7) Rodriques, J. G.; Marin-Vallamil, R.; Cano, F. H.; Fonseca, I. J. Chem. Soc., Perkin Trans. 1 1997, 709-714. (8) Allan, J. R.; Barrow, M. J. Beaumont, P. C.; Macindoe, L. A.; Milburn, G. H. W.; Werninck, A. R. Inorg. Chim. Acta 1988, 148, 85. (9) Sarkar, A.; Talwar, S. S. J. Chem. Soc., Perkin Trans. 1 1998, 4141-4146. (10) Lee, J.-H.; Curtis, M. D.; Kampf, J. W. Macromolecules 2000, 33, 2136-2144. (11) Coe, S.; Kane, J. J.; Nguyen, T. L.; Toledo, L. M.; Wininger, E.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 1997, 119, 86-93. (12) Xi, O. Y.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 2003, 125, 12400-12401. (13) Rod groups and layer groups are subperiodic groups. Recently, these groups have been defined in a manner similar to the more familiar space groups. Kopsky, V.; Litvin, D. B. International Tables for Crystallography, Volume E Subperiodic Groups; Kluwer Academic Publishers: Dordecht, 2002. (14) The use of rod and layer groups as a design tool has been discussed. Lauher, J. W. Trans. Am. Crystallogr. Assoc. 2004, 39, published online at http://www.hwi.buffalo.edu/ ACA/Publication_39/Lauher.pdf. (15) Schauer, C. L.; Matwey, E.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 1997, 119, 10245-10246. (16) Goroff, N. S.; Curtis, S. M.; Webb, J. A.; Fowler, F. W.; Lauher, J. W. Org. Lett. 2005, 7, 1891-1893. (17) Sheldrick, G. M. In SHELX-97: Program for Crystal Structure Refinement; University of Go¨ttingen, Germany, 1997. (18) McBride, J. M.; Bertman, S. B.; Cioffi, D. Z.; Segmuller, B. E.; Weber, B. A. Mol. Cryst. Liq. Cryst. 1988, 161, 1-24. (19) Pratt, C. P.; Dunitz, J. D. Chem. Mater. 1994, 6, 118-1127. (20) An oblique lattice with imposed orthogonality is not the same as a rectangular lattice. Rectangular lattices are defined by the presence of certain higher symmetry elements. As an analogy, imagine a P21/c crystal lattice that just happens to have an experimental β angle of 90°. The 90° β angle would not make the P21/c structure orthorhombic; only the presence of additional symmetry operators would make the structure orthorhombic. (21) Allen, F. H. Acta Crystallogr. B58, 2002, 380-388. (22) The study was limited to organic structures of single molecules with located hydrogen atoms. The molecule was required to have at least one O-H, N-H, or S-H hydrogenbond donor. This generated about 27 000 candidate structures. Hydrogen bond networks were generated for each structure, and the symmetry cards used to generate the networks were examined to determine the network symmetries. (23) The cell constants for the partially polymerized crystals were a ) 4.878(7) Å, b ) 10.00(1) Å, c ) 20.17(1) Å, b ) 97.75(2)°, volume ) 986.1(1) Å3. (24) The role of local stress is solid-state radical reactions has been reviewed. McBride, J. M. Acc. Chem. Res. 1983, 16, 304-312.

CG050204X

A Rational Approach to the Preparation of Polydipyridyldiacetylenes

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