Dimethyl Sulfoxide - American Chemical Society

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CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 2 107-111

Articles Polymorphs of a 1:1 Cocrystal with Tunnel and Layer Structures: p,p′-Biphenol/Dimethyl Sulfoxide Shinbyoung Ahn, Benson M. Kariuki, and Kenneth D. M. Harris* School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Received September 10, 2000

ABSTRACT: Two polymorphs of the 1:1 cocrystal formed between p,p′-biphenol and dimethyl sulfoxide (DMSO) have been discovered. One polymorph has a tunnel type structure, which can be considered to comprise tunnels formed from p,p′-biphenol molecules accommodating the DMSO molecules as guests. The other polymorph has a layered structure, comprising layers of p,p′-biphenol molecules and layers of DMSO molecules. In both structures, there is direct hydrogen bonding between the p,p′-biphenol and DMSO components of the cocrystal. The general utility of the p,p′-biphenol molecule as a component in crystal engineering strategies is discussed. 1. Introduction The successful development of crystal engineering strategies relies first on developing an understanding and rationalization of intermolecular interactions in the context of crystal packing, followed by the subsequent utilization and exploitation of this understanding in the design of new solids with specific desired structural properties.1-7 Clearly much of the impetus for progress in this field is driven by the need to design crystalline solids with optimal structural properties for specific materials applications. In general, rationalization of the structural properties of organic molecular crystals is rendered difficult by the fact that crystal structures arise from the interplay of several different intermolecular interactions that are often of comparable importance. However, for cases in which one specific intermolecular interaction (or a small number of interactions) is clearly dominant, it may be possible to develop a reliable rationalization of the observed molecular packing arrangement in the crystal. In this regard, studies of hydrogen-bonded crystals have been particularly prominent in view of the fact that the hydrogen bond8-10 is generally stronger and more directional than the other types of intermolecular interactions that are typically present in organic molecular crystals, such that the formation of optimal hydrogen-bonding patterns can often be recognized as the main determinant of the observed crystal structures. Thus, the structures of hydrogen-bonded crystals can often be rationalized by identifying the preferred modes * To whom correspondence should be addressed. Fax: +44-121-4147473. E-mail: [email protected].

of interaction of the hydrogen bond donor and hydrogen bond acceptor groups present.11-24 The phenomenon of polymorphism25-27 (i.e., when a given type of molecule is able to form different crystal structures) is of considerable interest in the field of organic solid state chemistry, in part because comparison of the properties of different polymorphs provides an ideal basis for understanding relationships between solid-state properties and crystal structure. Furthermore, in the development of reliable strategies for the prediction, rationalization, and design of the crystal structure(s) formed by a given type of organic molecule, studies of polymorphic systems provide fertile grounds for exploring and testing our understanding of crystalpacking arrangements. In the case of molecular cocrystals (i.e., crystals containing more than one type of molecule), true polymorphism (in which the different polymorphs have precisely the same stoichiometry) has been relatively rarely reported (see refs 28-32 for examples). In contrast, pseudo-polymorphism (i.e., the case of cocrystals comprising the same molecules but with different stoichiometries and different crystal structures) has been much more widely reported. In this paper, we report a case of true polymorphism of a cocrystalsthe 1:1 cocrystal formed between p,p′biphenol (HOC6H4C6H4OH, abbreviated PBP; Figure 1) and dimethyl sulfoxide (OdS(CH3)2, DMSO)sin which the two polymorphs differ substantially in their structural properties. These structural differences may be understood on the basis of conformational differences in one component of the cocrystal promoting different geometric opportunities for intermolecular hydrogen bonding. In a broader context, the structural properties

10.1021/cg000010b CCC: $20.00 © 2001 American Chemical Society Published on Web 01/05/2001

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Ahn et al. Table 1. Crystal Data for Forms I and II of the PBP/ DMSO Cocrystal (C12H10O2‚C2H6SO)

Figure 1. Molecular structure of p,p′-biphenol (PBP).

of these polymorphs provide a basis for assessing the utilization of the PBP molecule as a component for inducing different packing arrangements within the context of crystal engineering strategies. 2. Experimental Section One polymorph (denoted form I) of the PBP/DMSO (1:1) cocrystal was prepared by slow evaporation of solvent from a solution of PBP in DMSO. The crystals have prismatic shape. As discussed below, this polymorph has a tunnel type structure in which tunnels formed from PBP molecules accommodate the DMSO molecules as guests. On obtaining this result, we next explored the possible formation of other cocrystals (inclusion compounds) of PBP containing other appropriate guest molecules in the same tunnel structure (i.e., to replace the DMSO component in this structure). These molecules typically either contained one or two hydrogen bond acceptor groups and/or were based on a long alkane chain (the latter feature chosen with the aim of assisting the templating of tunnel structures). However, most of the experiments involving crystallization of these potential guest molecules together with PBP in DMSO produced only crystals of pure PBP or the PBP/ DMSO cocrystal (form I) discussed above. When other solvents (DMF, ethyl acetate, acetone, and cyclopentanone) were used instead of DMSO, only crystals of the pure phase of PBP were produced. However, a second polymorph (denoted form II) of the 1:1 cocrystal of PBP and DMSO was obtained while exploring the cocrystallization of PBP and 1,10-dibromodecane from DMSO/ ethanol (4:1) solution at -4 °C. The crystals of form II were obtained together with crystals of form I but were readily distinguished on the basis of crystal morphology (form I, prismatic; form II, thin plates). Crystals of both forms I and II tend to be unstable with respect to the loss of DMSO over a period of about 1 day under ambient conditions, with form II probably less stable than form I. Crystals of forms I and II were studied by single-crystal X-ray diffraction, using graphite-monochromated Mo KR radiation (λ ) 0.710 69 Å) on a Rigaku R-Axis II single-crystal X-ray diffractometer equipped with an image plate detector and rotating anode X-ray source. Each data collection comprised a total crystal rotation of 180°, in 30 frames for form I and 60 frames for form II. The crystal-to-detector distance was 80 mm in both cases. In the course of data collection for form II, some extent of deterioration of the crystal was observed, yielding small amounts of polycrystalline material. This decomposition undoubtedly contributes to the comparatively poor quality of the diffraction data and, hence, of the refined structure for form II. The crystal structures of both forms I and II were solved by direct methods (SIR92 program33) and difference Fourier techniques and refined using full-matrix least-squares analysis (SHELXL program34). All calculations were performed using the TEXSAN crystallographic software package.35 For form I, the positions of all non-hydrogen atoms and the hydrogen atoms of the hydroxyl groups were refined freely. The hydrogen atoms of the aryl rings were placed in standard positions relative to the aryl rings. Anisotropic displacement parameters were refined freely for all non-hydrogen atoms, and isotropic displacement parameters were refined freely for all hydrogen atoms. For form II, the positions and anisotropic displacement parameters of all non-hydrogen atoms were refined, with the

PBP:DMSO cryst syst cryst size/mm3 temp/K a/Å b/Å c/Å space group cell vol/Å3 calcd density/g cm-3 Z data/params final R indices (I > 2σ(I)) R Rw

form I

form II

1:1 orthorhombic 0.17 × 0.3 × 0.1 248(2) 10.4405(11) 10.3493(8) 12.3209(14) P212121 1331.3(3) 1.319 4 2350/187

1:1 orthorhombic 0.15 × 0.12 × 0.1 293(2) 8.153(2) 24.106(5) 7.1794(7) P212121 1411.1(5) 1.244 4 1731/165

0.041 0.095

0.15 0.34

Table 2. Fractional Atomic Coordinates and Equivalent Isotropic Displacement Parameters for Form I of the PBP/DMSO Cocrystala S(1) O(1) O(2) O(3) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) H(2) H(3) H(5) H(6) H(8) H(9) H(11) H(12) H(13A) H(13B) H(13C) H(14A) H(14B) H(14C) H(15) H(16)

x

y

z

Ueq/Å2

-1.2831(1) -1.0584(2) -0.8707(2) -1.1608(2) -1.0303(2) -1.1002(2) -1.0748(2) -0.9793(2) -0.9121(3) -0.9377(2) -0.9495(2) -0.9595(2) -0.9330(3) -0.8957(2) -0.8846(2) -0.9107(2) -1.2467(3) -1.2910(4) -1.1646(2) -1.1225(2) -0.8482(3) -0.8925(2) -0.9852(2) -0.9403(3) -0.8592(2) -0.9022(2) -1.3172(10) -1.2335(20) -1.1695(12) -1.3637(15) -1.2129(11) -1.3009(25) -0.9861(34) -0.8548(38)

0.1576(1) -0.2915(2) -0.0523(2) 0.2314(2) -0.2521(3) -0.3065(3) -0.2707(2) -0.1803(2) -0.1255(2) -0.1599(3) -0.1464(2) -0.2389(2) -0.2106(2) -0.0867(2) 0.0069(2) -0.0224(2) 0.0615(3) 0.0328(3) -0.3673(3) -0.3078(2) -0.0639(2) -0.1208(3) -0.3231(2) -0.2750(2) 0.0910(2) 0.0421(2) 0.0029(15) 0.1169(4) 0.0121(16) -0.0226(15) -0.0178(16) 0.0705(3) -0.2704(39) -0.1237(40)

-0.4517(1) -0.1444(2) -0.8979(2) -0.4763(2) -0.2483(2) -0.3319(2) -0.4376(2) -0.4622(2) -0.3763(2) -0.2700(2) -0.5762(2) -0.6577(2) -0.7647(2) -0.7928(2) -0.7138(2) -0.6067(2) -0.3369(3) -0.5502(3) -0.3168(2) -0.4943(2) -0.3907(2) -0.2128(2) -0.6392(2) -0.8182(2) -0.7329(2) -0.5535(2) -0.3226(11) -0.2744(5) -0.3508(7) -0.5348(13) -0.5479(15) -0.6218(4) -0.1031(30) -0.9365(32)

0.046(1) 0.051(1) 0.046(1) 0.054(1) 0.038(1) 0.041(1) 0.041(1) 0.033(1) 0.041(1) 0.042(1) 0.033(1) 0.037(1) 0.038(1) 0.033(1) 0.035(1) 0.035(1) 0.061(1) 0.070(1) 0.052(8) 0.054(8) 0.049(8) 0.047(8) 0.038(7) 0.059(9) 0.035(7) 0.035(6) 0.086(12) 0.086(12) 0.074(11) 0.128(18) 0.098(14) 0.087(12) 0.079(12) 0.095(14)

a For atoms with anisotropic displacement parameters, U eq is defined as one-third of the trace of the orthogonalized Uij tensor. For atoms with isotropic displacement parameters, Ueq is equal to Uiso. Estimated standard deviations are given in parentheses.

geometry of each aryl ring of the PBP molecule constrained to be a regular hexagon. The positions of the hydrogen atoms of the hydroxyl groups (which were located by difference Fourier synthesis) were refined freely, with the isotropic displacement parameter of each of these hydrogen atoms taken to be 1.2 times the equivalent isotropic displacement parameter of the oxygen atom to which it is bonded. The hydrogen atoms of the aryl rings were placed in standard positions relative to the aryl rings, with the isotropic displacement parameter of each of these hydrogen atoms taken to be 1.2 times the equivalent isotropic displacement parameter of the carbon atom to which it is bonded. It became clear during the refinement that the DMSO molecule is disordered between two orientations, with the relative occupancies refined. As the

Polymorphs of a 1:1 Cocrystal

Crystal Growth & Design, Vol. 1, No. 2, 2001 109

Table 3. Fractional Atomic Coordinates and Equivalent Isotropic Displacement Parameters for Form II of the PBP/DMSO Cocrystala S(1) O(1) O(2) O(3) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(13A) C(14A) S(1A) H(2) H(3) H(5) H(6) H(8) H(9) H(11) H(12) H(13A) H(13B) H(13C) H(14A) H(14B) H(14C) H(13D) H(13E) H(13F) H(14D) H(14E) H(14F) H(15) H(16)

x

y

z

Ueq/Å2

occ

0.6684(12) 0.8124(10) 0.1638(11) 0.7504(4) 0.7198(4) 0.7662(4) 0.6715(4) 0.5302(4) 0.4838(4) 0.5785(4) 0.4292(4) 0.4651(4) 0.3764(4) 0.2520(4) 0.2162(4) 0.3048(4) 0.7956(35) 0.7393(43) 0.5966(34) 0.7719(69) 0.7749(17) 0.8607(4) 0.7025(4) 0.3893(4) 0.5474(4) 0.5483(4) 0.4004(4) 0.1329(4) 0.2808(4) 0.8505(168) 0.7308(43) 0.8755(136) 0.8570(43) 0.7009(176) 0.6999(170) 0.5967(169) 0.5881(176) 0.5050(40) 0.6739(177) 0.8661(190) 0.7740(364) 0.7479(86) 0.1841(159)

0.2426(4) -0.1630(4) 0.1834(5) 0.2404(2) -0.1137(2) -0.0709(2) -0.0231(2) -0.0181(2) -0.0609(2) -0.1087(2) 0.0365(2) 0.0761(2) 0.1253(2) 0.1350(2) 0.0954(2) 0.0462(2) 0.2903(12) 0.1868(10) 0.2659(17) 0.1670(7) 0.2365(6) -0.0742(2) 0.0056(2) -0.0576(2) -0.1374(2) 0.0696(2) 0.1518(2) 0.1019(2) 0.0197(2) 0.3128(49) 0.3137(48) 0.2709(12) 0.1869(38) 0.1885(38) 0.1534(10) 0.2667(117) 0.3030(45) 0.2444(70) 0.1516(18) 0.1505(16) 0.1594(8) -0.1836(18) 0.2072(72)

0.2796(8) 0.2230(13) 0.2770(14) 0.4684(5) 0.2262(5) 0.3446(5) 0.3559(5) 0.2488(5) 0.1303(5) 0.1190(5) 0.2620(5) 0.3969(5) 0.4035(5) 0.2752(5) 0.1404(5) 0.1338(5) 0.1675(30) 0.1600(28) 0.1772(37) 0.2196(42) 0.2546(11) 0.4163(5) 0.4352(5) 0.0586(5) 0.0398(5) 0.4827(5) 0.4937(5) 0.0545(5) 0.0435(5) 0.2588(32) 0.0879(191) 0.0942(189) 0.1611(197) 0.0337(68) 0.2175(141) 0.0435(38) 0.2244(343) 0.2202(355) 0.2730(350) 0.2777(339) 0.0884(42) 0.1407(150) 0.3774(232)

0.102(3) 0.089(3) 0.097(3) 0.090(3) 0.085(4) 0.091(4) 0.099(5) 0.079(4) 0.077(4) 0.082(4) 0.071(4) 0.081(4) 0.072(4) 0.071(3) 0.102(5) 0.106(6) 0.129(9) 0.118(7) 0.130(9) 0.129(11) 0.108(5) 0.109 0.119 0.092 0.098 0.097 0.086 0.122 0.127 0.155 0.155 0.155 0.142 0.142 0.142 0.156 0.156 0.156 0.154 0.154 0.154 0.107 0.116

0.63(2) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.63(2) 0.63(2) 0.37(2) 0.37(2) 0.37(2) 1 1 1 1 1 1 1 1 0.63(2) 0.63(2) 0.63(2) 0.63(2) 0.63(2) 0.63(2) 0.37(2) 0.37(2) 0.37(2) 0.37(2) 0.37(2) 0.37(2) 1 1

a For atoms with anisotropic displacement parameters, U eq is defined as one-third of the trace of the orthogonalized Uij tensor. For atoms with isotropic displacement parameters, Ueq is equal to Uiso. Estimated standard deviations are given in parentheses. Note that the DMSO molecule is disordered between two orientations, but with the same position of the oxygen atom in each orientation.

molecular geometry of the minor orientation tended to distort during refinement, the geometry of this component was restrained to standard geometry. Data concerning the crystal structures are reported in Table 1, and fractional atomic coordinates are given in Table 2 (form I) and Table 3 (form II).

3. Results and Discussion Form I of PBP/DMSO (1:1) has a helical tunnel structure along the crystallographic c axis, as shown in Figure 2. In projection on the plane perpendicular to the tunnel axis, four PBP molecules complete a given host tunnel around the DMSO guest molecules. The DMSO molecules in a given tunnel adopt two different orientations, which alternate along the tunnel, and the same orientation of both the PBP and DMSO molecules is repeated every 12.32 Å along the tunnel (the periodic repeat distance along the c axis). In the PBP molecule, the dihedral angle between the two aryl rings is 33.6° and the dihedral angles around the two C-O bonds are

Figure 2. (a, top) Crystal structure of form I of the PBP/ DMSO cocrystal viewed along the tunnel axis (c axis) and (b, bottom) a section of the crystal structure showing the hydrogenbonding arrangement.

such that the H-O‚‚‚O-H dihedral angle is 105.0°. The actual angle between the two O-H vectors is 107.3° (see section 4). The hydrogen atom of each hydroxyl group lies significantly out of the plane of the aryl ring to which it is bonded (H-O-C-C dihedral angles ca. 20 and 21°). As shown in Figure 2b, the crystal structure contains SdO‚‚‚H-O hydrogen bonds between the DMSO and PBP molecules, representing a helical hydrogen-bonded chain of alternating PBP and DMSO molecules. In these chains, a given DMSO molecule accepts hydrogen bonds from two different PBP molecules, and the graph set description36,37 is C12(13). For one of the hydrogen bonds formed to the DMSO molecule, the O‚‚‚H distance is 1.82 Å, the O‚‚‚O distance is 2.74 Å, and the O-H‚‚‚O angle is 167.0°. For the other hydrogen bond, the O‚‚‚H distance is 1.85 Å, the O‚‚‚O distance is 2.74 Å, and the O-H‚‚‚O angle is 173.7°. There is no direct hydrogen bonding between PBP molecules. Under ambient conditions, DMSO is lost from the cocrystal over a period of time and the tunnel structure collapses, producing the pure phase of PBP (as shown by powder X-ray diffraction). The crystal structure of form II of PBP/DMSO (1:1) is a layered structure, comprising alternating layers of PBP and DMSO molecules, as shown in Figure 3. The layers are perpendicular to the crystallographic b axis. The asymmetric unit comprises one PBP molecule and one DMSO molecule. In this structure, the DMSO molecule is disordered between two orientations (occupancies 0.63 and 0.37), with the same position of the oxygen atom in each case. The two orientations differ

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Crystal Growth & Design, Vol. 1, No. 2, 2001

Figure 3. (a, top) Crystal structure of form II of the PBP/ DMSO cocrystal viewed along the c axis and (b, bottom) a section of the crystal structure showing the hydrogen-bonding arrangement. Only the major orientation of the disordered DMSO molecule in this structure is shown.

in terms of reorientation about an axis almost parallel to the OdS bond; therefore, the disorder mainly involves different positions of the methyl groups. In the PBP molecule, the dihedral angle between the two aryl rings is 9.7° and the dihedral angles around the two C-O bonds are such that the H-O‚‚‚O-H dihedral angle is 161.8°. The actual angle between the two O-H vectors is 155.9° (see section 4). The hydrogen atoms of the hydroxyl groups lie virtually within the plane of the aryl rings (H-O-C-C dihedral angles ca. 1 and 7°). The conformation of the PBP molecule in form II is substantially closer to the conformation in the pure crystalline phase of PBP38,39 than is the conformation in form I (in the pure phase of PBP, the molecule is strictly centrosymmetric, with the two aryl rings coplanar and an angle of 180° between the two O-H vectors). As shown in Figure 3b, the DMSO molecule interacts through Sd O‚‚‚H-O hydrogen bonds with a PBP molecule in each of the two adjacent layers, forming essentially linear hydrogen-bonded chains that run along the b axis (i.e., perpendicular to the layers). The graph set description of these chains is C12(13). For one of these hydrogenbonds, the O‚‚‚H distance is 1.85 Å, the O‚‚‚O distance is 2.66 Å, and the O-H‚‚‚O angle is 144.5°. For the other hydrogen bond, the O‚‚‚H distance is 1.76 Å, the O‚‚‚O distance is 2.69 Å, and the O-H‚‚‚O angle is 167.5°. Thus, as in form I, the SdO group of the DMSO molecule acts as a bifurcated hydrogen bond acceptor. Recalling that the DMSO molecule is orientationally disordered in this structure, we note that the geometric features of the hydrogen-bonding arrangement discussed above are actually the same for the major and minor orientations of the molecule, as the position of the oxygen atom is the same in both orientations

Ahn et al.

(however, the orientation of specific orbitals of the DMSO molecule relative to the hydrogen bond donor (hydroxyl) groups of the PBP molecules clearly differs for the two orientations of the DMSO molecule). There is no direct hydrogen bonding between PBP molecules; however, within the layer of PBP molecules, the aryl rings of neighboring molecules interact through recognizable edge-to-face interactions. We now highlight some comparisons between the structures of forms I and II of the PBP/DMSO cocrystal. In form II, the conformation of the PBP molecule is such that the aryl rings are close to coplanar (dihedral angle between the aryl rings is 9.7°), whereas in form I the corresponding dihedral angle is substantially larger (33.6°). Importantly, in form I the angle between the O-H vectors of the molecule is 107.3°, whereas in form II the corresponding angle is 155.9°. This angle is critically important in determining the directionality of the hydrogen-bonding arrangement around a given PBP molecule, and it is clear that the different molecular conformations in forms I and II lead to different intermolecular hydrogen-bonding schemes. As discussed in more general terms in section 4, the large value (155.9°) of this angle in form II is completely consistent with the propagation of essentially linear hydrogenbonded chains of alternating PBP and DMSO molecules running perpendicular to the planes of the layers in this structure. In contrast, the substantially smaller angle of 107.3° in form I is consistent with the formation of an extended hydrogen-bonded array with significant curvature (rather than linear), as observed with the helical hydrogen-bonded chains of alternating PBP and DMSO molecules in this tunnel type structure. The calculated densities of the two polymorphs of PBP/DMSO are 1.32 g cm-3 for form I and 1.24 g cm-3 for form II. It is noteworthy that the polymorph (form I) that apparently has the greater stability is the polymorph of higher density. 4. Concluding Remarks The PBP molecule has considerable potential as a building block to construct a variety of hydrogen-bonded structures, as demonstrated in the crystal structures of the two polymorphs of the PBP/DMSO cocrystal reported in this paper and in the structures of other cocrystals involving PBP reported previously.40-43 Much of the potential for using PBP as a versatile component in the construction of hydrogen-bonded arrays in crystals originates from the compromise that it offers in terms of rigidity and flexibility, with regard to both molecular conformation and intermolecular hydrogenbonding capacity. Thus, while the biphenyl component of the molecule represents an essentially rigid linear unit, the molecule nevertheless has flexibility in terms of (i) the dihedral angles around the C-O bonds of the two hydroxyl groups and (ii) the dihedral angle around the C-C bond that links the two aryl rings. First, as discussed in section 3, the flexibility of the dihedral angles around the C-O bonds leads to a wide range of possible relative orientations of the two O-H bond vectors within the molecule, giving very wide scope for the PBP molecule to propagate extended hydrogenbonded arrays of differing geometries. In principle, the angle between the two O-H bond vectors may range

Polymorphs of a 1:1 Cocrystal

from ca. 39° (for the case with the hydroxyl groups cis; H-O‚‚‚O-H dihedral angle 0°) to 180° (for the case with the hydroxyl groups trans; H-O‚‚‚O-H dihedral angle 180°). Clearly form I of the PBP/DMSO cocrystal lies between these extremes, whereas form II of the PBP/ DMSO cocrystal is close to the upper limit of this range. The consequences in terms of the differing geometries of the extended hydrogen-bonded arrays formed in each case is readily apparent from the comparison of the structures of these polymorphs discussed in section 3, with form I exhibiting an extended hydrogen-bonded array (helical chain) of significantly greater curvature than the extended hydrogen-bonded array (essentially linear) in form II. Second, variation of the dihedral angle around the C-C bond that links the two aryl rings does not alter the geometric requirements of the PBP molecule for intermolecular hydrogen bonding but, rather, alters the steric nature of the molecule, which is clearly also an important factor influencing crystal packing. The provision of this flexibility in the steric character of the molecule is potentially advantageous within a component used for crystal engineering purposes, as it provides the molecule with some opportunity to find an optimal shape to facilitate the relative packing of extended hydrogen-bonded chains (formed by interactions of the hydroxyl groups). Furthermore, as illustrated in the structure of form II of the PBP/DMSO cocrystal, interactions between the aryl rings of neighboring PBP molecules may also influence the crystal packing arrangementsthis structure exhibits recognizable edge-to-face interactions between aryl rings, which may promote the observed aggregation of the hydrogenbonded chains into layers of PBP molecules and layers of DMSO molecules. In view of the features discussed above, it is not surprising that, in addition to the two polymorphs of the PBP/DMSO cocrystal reported in this paper, other crystal structures containing PBP exhibit a variety of different types of hydrogen-bonding networks, including one-dimensionalchains(forexample,PBP/1,4-diazabicyclo[2.2.2]octane (1:1)40 and PBP/hexamethylenetetramine (1:1)42), two-dimensional layers (for example, PBP/1,2diaminoethane/methanol (2:1:1)40), and three-dimensional diamondoid structures (for example, PBP/1,2diaminoethane (1:1),40 PBP/p-phenylenediamine (1:1),43 and PBP/benzidine (1:1)43). Acknowledgment. We are grateful to the EPSRC for general support and to the University of Birmingham for providing a studentship (to S.A.). Supporting Information Available: X-ray crystallographic information files (CIF) are available for forms I and II of the PBP/DMSO cocrystal. This material is available free of charge via the Internet at http://pubs.acs.org.

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