Syntheses and Crystal Structures of New “Extended” Building Blocks

Syntheses and Crystal Structures of New “Extended” Building Blocks for Crystal Engineering: (Pyridylmethylene)aminoacetophenone Oxime Ligands Christer B. Aakero¨y,* Alicia M. Beatty, and Destin S. Leinen

CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 1 47-52

Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received August 7, 2000

ABSTRACT: The syntheses and crystal structures of four new ligands based on “extended” (pyridylmethylene)aminoacetophenone oxime derivatives are reported. The ligands are all prepared in good yields by allowing a suitably substituted pyridine to react with hydroxylamine, followed by Schiff-base condensation with the appropriate pyridinecarboxaldehyde. Their solid-state structures show that the dominating hydrogen bond, a head-to-tail O-H‚‚‚N interaction between the oxime O-H moiety and the pyridine nitrogen atom, is present in each of the four reported cases despite considerable differences in crystal packing. These structures also contain a multitude of weaker π-π interactions, but they do not display the same consistency and regularity as the O-H‚‚‚N hydrogen bond, which illustrates the importance of hydrogen bonding as the primary supramolecular synthetic tool. Introduction One of the most prolific areas of current research in crystal engineering1 is targeted on the predictable assembly of coordination complexes and molecular solids into extended open-framework architectures.2 Such structures have potential applications as versatile hostguest materials for catalysis or separation.3 An important synthetic strategy for such compounds utilizes ligands that can coordinate to a metal ion while at the same time participating in self-complementary intermolecular interactions, notably hydrogen bonds, thereby promoting the inherent geometry of the metal complex into infinite 1-D, 2-D, or 3-D networks. Pyridine derivatives with carboxylic acid or carboxamide substituents have proved very successful to date; the pyridine nitrogen atom interacts with the metal ion, whereas the substituent can form directional hydrogen bonds to neighboring ligands.4 The size and shape of the ligands are of key importance to the resulting structure, and it is highly desirable to have access to ligands that can modify the separation between metal centers, thereby providing a tool for controlling cavity size and channel diameters in the resulting extended framework. A functional group that has not been extensively explored in crystal engineering is the oxime moiety (Chart 1), which is similar to carboxylic acid in that it contains one hydrogen-bond donor and two acceptor atoms. Structurally characterized oxime moieties are much less common than carboxylic acids and amides (a recent survey of the Cambridge Structural Database (CSD)5 found 370 entries containing the oxime moiety6), but from a supramolecular perspective, this functionality does have some unique and desirable features. By changing the R substituent (Chart 1), it is possible to fine-tune the steric and electronic nature of this moiety in a systematic fashion. Such modifications may also provide better control over the way in which the oxime functionality participates in noncovalent interactions. A variety of oximes (R ) H, Me, Ph, NH2, NO2, etc.)

Chart 1. Oxime Functionality

can be prepared through well-established synthetic methods. Typically, a ketone or aldehyde is allowed to react with hydroxylamine to form the desired oxime.7 We have previously shown that oxime-substituted pyridines can be utilized as crystal engineering tools for designing lamellar silver(I) compounds8 and also in the construction of host-guest materials based on nickel(II) coordination compounds.4c We now present the syntheses and crystal structures of four new ligands where the intended metal-coordination site (the pyridine nitrogen atom) and the hydrogen-bond functionality (the oxime moiety) are separated by a larger distance. Such ligands may facilitate the construction of new metalcontaining host-guest structures and provide more flexibility in the supramolecular design of new opennetwork structures. Experimental Section All starting materials were obtained from Aldrich and used as received without further purification. Melting points were determined using a Fisher-Johns Mel-Temp melting point apparatus and are uncorrected. 1H NMR spectra were recorded on a Varian 400 MHz spectrometer. Synthesis of 3′-Aminoacetophenone Oxime. 3′-Aminoacetophenone (6.00 g, 4.44 × 10-2 mol) was dissolved in ethanol (100 mL) with stirring. NH2OH‚HCl (3.09 g, 4.44 × 10-2 mol) in 40 mL of water and Na2CO3 (2.35 g, 2.22 × 10-2 mol) in 40 mL of water were added to the ethanolic 3′aminoacetophenone solution. The reaction was monitored by TLC (developer: 1/1 hexanes/EtOAc). The reaction mixture was filtered through Celite, and upon subsequent concentration of the solution, a tan solid precipitated from the solution. The solid was collected via vacuum filtration and washed with cold water. Recrystallization from 1-propanol gave tan needles. Yield: 5.50 g (82%). Mp: 133-134.5 °C. 1H NMR (δH; 400 MHz, DMSO-d6): 2.05 (3H, s), 5.08 (2H, s), 6.52-6.54 (1H, d), 6.74-6.76 (1H, d), 6.85 (1H, s), 6.97-7.01 (1H, t), and 10.97 (1H, s).

10.1021/cg0055068 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2000

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Table 1. Crystal Data and Refinement for 1-4 empirical formula mol wt cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) χ (deg) V (Å3) Z Dcalcd (g cm-3) F(000) µ(Mo KR) (mm-1) temp (K) R1/wR2(obsd data) R1/wR2(all data) S

1

2

3

4

C14H13N3O 239.27 triclinic P1 h 10.966(3) 11.767(3) 11.931(3) 62.281(4) 68.493(4) 67.851(5) 1227.4(5) 4 1.295 504 0.085 173(2) 0.0564/0.1315 0.0937/0.1479 1.022

C14H13N3O 239.27 monoclinic P21/c 13.205(2) 11.263(1) 8.2638(9) 90 97.050(2) 90 1219.8(2) 4 1.303 504 0.085 173(2) 0.0510/0.1298 0.0824/0.1465 1.054

C14H13N3O 239.27 monoclinic P21 6.137(1) 7.289(2) 13.509(3) 90 93.927(4) 90 602.8(2) 2 1.318 252 0.086 173(2) 0.0503/0.1309 0.0572/0.1375 1.026

C14H13N3O 239.27 monoclinic Pn 7.296(2) 5.986(2) 27.196(8) 90 91.430(6) 90 1187.3(6) 4 1.339 504 0.088 173(2) 0.0610/0.1547 0.0817/0.1661 1.030

Synthesis of 4′-Aminoacetophenone Oxime. 4′-Aminoacetophenone (2.00 g, 1.48 × 10-2 mol) was dissolved in ethanol (55 mL) with stirring. NH2OH‚HCl (1.03 g, 1.48 × 10-2 mol) in 20 mL of water and Na2CO3 (0.78 g, 7.40 × 10-3 mol) in 20 mL of water were added to the ethanolic 4′-aminoacetophenone solution. The mixture was heated under reflux, and the reaction was monitored by TLC (developer: 1/1 hexanes/EtOAc). Upon concentration of the solution, yellow crystals precipitated from the solution. Yellow needles were collected via vacuum filtration and washed with cold water. Yield: 1.77 g (80%). Mp: 152-153 °C. 1H NMR (δH; 400 MHz, DMSO-d6): 2.01 (3H, s), 5.25 (2H, s), 6.49-6.51 (2H, d), 7.297.31 (2H, d), 10.58 (1H, s). General Procedure for the Synthesis of (Pyridylmethylene)aminoacetophenone Oximes. One equivalent of the appropriate aminoacetophenone oxime was combined with 100 mL of benzene and the mixture stirred. One equivalent of the appropriate pyridinecarboxaldehyde was added neat to the reaction mixture. The mixture was heated under reflux using a Dean-Stark apparatus, until water was no longer observed collecting in the sidearm of the apparatus. The mixture was cooled to room temperature and then placed in the freezer to facilitate precipitation. The precipitate was collected via vacuum filtration, washed with benzene, and dried with the aspirator. Synthesis of 3′-[N-(4-Pyridylmethylene)]aminoacetophenone Oxime (1). 3′-Aminoacetophenone oxime (4.98 g, 3.32 × 10-2 mol) was condensed with 4-pyridinecarboxaldehyde (3.55 g, 3.32 × 10-2 mol) as described above. Yield: 6.63 g (84%). Mp: 156-158 °C. 1H NMR (δH; 400 MHz, DMSO-d6): 2.18 (3H, s), 7.30-7.33 (1H, d), 7.42-7.46 (1H, t), 7.54 (1H, s), 7.57-7.59 (1H, d), 7.84-7.86 (2H, d), 8.71 (1H, s), 8.748.75 (2H, d), 11.27 (1H, s). Pale yellow plates suitable for X-ray diffraction were grown from a saturated solution of 1 in a 1/1 H2O/EtOH mixture. Synthesis of 3′-[(N-(3-Pyridylmethylene)]aminoacetophenone Oxime (2). 3′-Aminoacetophenone oxime (4.61 g, 3.07 × 10-2 mol) was condensed with 3-pyridinecarboxaldehyde (3.29 g, 3.07 × 10-2 mol) as described above. Yield: 6.92 g (94%). Mp: 137-139 °C. 1H NMR (δH; 400 MHz, DMSO-d6): 2.17 (3H, s), 7.26-7.28 (1H, d), 7.40-7.44 (1H, t), 7.51-7.55 (3H, m), 8.29-8.31 (1H, d), 8.68-8.69 (1H, d), 8.72 (1H, s), 9.05 (1H, s), 11.24 (1H, s). Pale yellow plates suitable for X-ray diffraction were grown from a saturated solution of 2 in dichloromethane. Synthesis of 4′-[N-(3-Pyridylmethylene)]aminoacetophenone Oxime (3). 4′-Aminoacetophenone oxime (1.75 g, 1.17 × 10-2 mol) was condensed with 3-pyridinecarboxaldehyde (1.25 g, 1.17 × 10-2 mol) as described above. Yield: 2.62 g (94%). Mp: 186-189 °C. 1H NMR (δH; 400 MHz, DMSO-d6): 2.15 (3H, s), 7.28-7.31 (2H, d), 7.50-7.54 (1H, m), 7.68-7.70 (2H, d), 8.28-8.30 (1H, d), 8.67-8.69 (1H, d), 8.71 (1H, s), 9.04

Table 2. Hydrogen Bonds for 1-4 D-H‚‚‚A

D-H

d, Å H‚‚‚A

D‚‚‚A

angle, deg DHA

O(27)-H(27)‚‚‚N(31) O(47)-H(47)‚‚‚N(1)#1 C(45)-H(45)‚‚‚N(7)#2

Compound 1a 0.86 1.90 1.04 1.78 0.98 2.63

2.727(3) 2.803(3) 3.489(3)

162.9 166.5 146.4

O(27)-H(27)‚‚‚N(1)#1 C(25)-H(25)‚‚‚N(7)#2

Compound 2b 0.82 1.96 0.93 2.61

2.750(2) 3.497(2)

161.6 158.0

O(27)-H(27)‚‚‚N(1)#1

Compound 3c 1.00 1.76

2.749(3)

171.0

Compound 4d O(27)-H(27)‚‚‚N(1)#1 1.10 1.75 O(47)-H(47)‚‚‚N(31)#2 0.92 1.89

2.772(6) 2.777(5)

151.7 162.2

a Symmetry transformations used to generate equivalent atoms: (#1) x + 1, y - 1, z - 2; (#2) x + 1, y - 1, z - 1. b Symmetry transformations used to generate equivalent atoms: (#1) -x + 1, -y + 1, -z; (#2) -x + 1, y + 1/2, -z + 1/2. c Symmetry transformations used to generate equivalent atoms: (#1) x, y, z + 1. d Symmetry transformations used to generate equivalent atoms: (#1) x + 1/2, -y + 1, z - 1/2; (#2) x - 1/2, -y + 2, z + 1/2.

(1H, s), 11.21 (1H, s). Pale yellow plates suitable for X-ray diffraction were grown from a saturated solution of 3 in acetone. Synthesis of 4′-[N-(4-Pyridylmethylene)]aminoacetophenone Oxime (4). 4′-Aminoacetophenone oxime (5.12 g, 3.41 × 10-2 mol) was condensed with 4-pyridinecarboxaldehyde (3.65 g, 3.41 × 10-2 mol) as described above. Yield: 7.78 g (95%). Mp: 199-203 °C. 1H NMR (δH; 400 MHz, DMSO-d6): 2.15 (3H, s), 7.32-7.34 (2H, d), 7.70-7.72 (2H, d), 7.83-7.84 (2H, d), 8.70 (1H, s), 8.72-8.74 (2H, d), 11.22 (1H, s). Pale yellow plates suitable for X-ray diffraction were grown from a saturated solution of 4 in ethyl acetate. X-ray Crystallography. Crystalline samples of 1-4 were placed in inert oil, mounted on a glass pin, and transferred to the cold gas stream of the diffractometer. Crystal data were collected and integrated using a Bruker SMART 1000 system, with graphite-monochromated Mo KR (λ ) 0.710 73 Å) radiation at 173 K. The structures were solved by direct methods using SHELXS-97 and refined using SHELXL-97.9 Nonhydrogen atoms were found by successive full-matrix leastsquares refinement on F2 and refined with anisotropic thermal parameters. Hydrogen atom positions were located from difference Fourier maps, and a riding model with fixed thermal parameters (uij ) 1.2[Uij(eq)] for the atom to which they are bonded) was used for subsequent refinements. The structural homogeneity for all four crystalline samples was verified by comparing experimental X-ray powder diffraction data with simulated powder patterns based on single-crystal data. Table

(Pyridylmethylene)aminoacetophenone Oxime Ligands

Crystal Growth & Design, Vol. 1, No. 1, 2001 49

Figure 1. Thermal ellipsoid plots (30%) of (a) 1, (b) 2, (c) 3, and (d) 4.

Figure 2. Infinite 2-D sheets in 1. 1 provides crystallographic details for 1-4, and Table 2 provides information about some of the hydrogen-bond geometries in 1-4. The molecular geometries, thermal ellipsoids (30%), and numbering schemes are shown in Figure 1.

Results The crystal structure of 1 contains two molecules in the asymmetric unit. The aromatic rings within each

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molecule are twisted with respect to each other by ca. 53 and 28°, respectively. Neighboring molecules form infinite 1-D chains held together by head-to-tail O-H‚‚‚N hydrogen bonds involving the oxime O-H moiety and the pyridine nitrogen atom (O27‚‚‚N31 ) 2.727(3) Å; O47‚‚‚N1 ) 2.803(3) Å) (Figure 2). The chains are positioned into infinite 2-D sheets via a C-H‚‚‚N hydrogen bond (C45‚‚‚N7 ) 3.489(3) Å) involving the imine nitrogen atom and a C-H donor of the phenyl ring. Chains within a sheet run parallel to each other, but adjacent sheets are arranged in an antiparallel manner. In the crystal structure of 2, the two rings within the molecule are twisted with respect to each other by ca. 16°. Neighboring molecules form dimers held together by two O-H‚‚‚N hydrogen bonds involving the oxime O-H moiety and the pyridine nitrogen atom (O27-N1 ) 2.750(2) Å) (Figure 3a). The dimers are arranged in a herringbone-like motif with each dimer hydrogenbonded to two other dimers via a C-H‚‚‚N hydrogen bond from the aromatic C-H donor to an imine nitrogen atom (C25‚‚‚N7 ) 3.497(2) Å) (Figure 3b). This interaction creates a buckled hydrogen-bonded layer of dimers, and there are no hydrogen bonds between layers. In the crystal structure of 3, the two rings within the molecule are twisted with respect to each other by ca. 19°. The molecules form infinite 1-D chains (similar to 1) held together by a head-to-tail O-H‚‚‚N hydrogen bond from the oxime O-H donor to the pyridine nitrogen atom (O27‚‚‚N1 ) 2.749(3) Å) (Figure 4). The chains run parallel to each other and are positioned in layers, although there are no hydrogen bonds between the chains. Subsequent layers run antiparallel to each other, with no hydrogen bonds between the layers. In the crystal structure of 4, there are two crystallographically unique molecules in the asymmetric unit. The two rings within each molecule are twisted with respect to each other by ca. 46 and 17°, respectively. The molecules form infinite 1-D chains held together by hydrogen bonds (Figure 5). Again, the head-to-tail O-H‚‚‚N hydrogen bond involving the oxime O-H moiety and the pyridine nitrogen atom (O27-N1 ) 2.772(6) Å; O47‚‚‚N31 ) 2.777(5) Å) lead to infinite chains. The 1-D chains are arranged into layers, albeit without any significant hydrogen-bonding interactions between the chains within the layer. The chains within each layer run parallel to each other, creating a polar 2-D assembly. However, neighboring layers have their polar axis arranged in an antiparallel fashion. The layer-layer separation is about 3.4 Å (π-π contacts), and there are no hydrogen bonds between neighboring layers. Discussion The oximes of 3′- and 4′-aminoacetophenone were synthesized in good yields by allowing the appropriate aminoacetophenone to react with hydroxylamine. Compounds 1-4 were synthesized in good to excellent yields by Schiff base condensation of 3′- or 4′-aminoacetophenone oxime with the appropriate pyridinecarboxaldehyde. The chemical shifts, splitting patterns, and integration in the 1H NMR spectra supported the identities of the desired compounds, and no trace of starting materials was observed in the spectra.

Figure 3. (a) Hydrogen-bonded dimer in 2. (b) Dimers arranged in a herringbone fashion in 2.

The orientation (torsion angle) of the two rings with respect to each other in this family of molecules fall in the range of 16-53°, and there is no preference for a specific orientation. This is not surprising, since the barrier of rotation around a single C-C bond is very small, which also makes it very difficult to unambiguously relate a specific intermolecular packing feature to a specific torsion angle.

(Pyridylmethylene)aminoacetophenone Oxime Ligands

Crystal Growth & Design, Vol. 1, No. 1, 2001 51

Figure 4. Infinite 1-D chain in 3.

Figure 5. Infinite 1-D chain in 4.

Chart 2. Three Possible Hydrogen Bond Modes for Oxime-Pyridines

Previous structural studies of molecules containing both oxime and carboxylic acid moieties have demonstrated that there is a pronounced preference for heteromeric (oxime‚‚‚acid) interactions over homomeric (oxime‚‚‚oxime, or acid‚‚‚acid) motifs in the absence of other strong hydrogen-bond donors or acceptors.10 For molecules containing oxime-pyridine functionalities, the expected hydrogen-bond modes are restricted to only three different modes of hydrogen bonding (in the absence of other strong hydrogen-bond donors/acceptors) (Chart 2).11 The molecule will contain a head-to-tail O-H(oxime)‚‚‚N(pyridine) interaction (resulting in an infinite 1-D chain), a catemeric O-H‚‚‚O interaction (leading to infinite chains or rings), or a self-complementary oxime-oxime motif (resulting in a discrete

dimer), which leaves the pyridine nitrogen atom free to accept another hydrogen-bond donor (e.g. C-H). A search of the CSD shows that seven out of nine compounds with an oxime functionality and an available pyridine nitrogen atom contain oxime‚‚‚pyridine hydrogen bonds and, although this is a small sample, there is a clear preference for head-to-tail interactions for this type of molecule. The same trend is also borne out, as expected, in the crystal structures of 1-4. In each case, the O-H‚‚‚N hydrogen bond persists, despite the fact that the overall packing arrangements and topologies in these four structures vary dramatically (from dimers to chains). In addition to the hydrogen-bond modes of oximesubstituted pyridines depicted in Chart 2, the (pyridyl-

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methylene)aminoacetophenone oximes have additional moieties that are capable of directly dictating the structural outcome. First, the syntheses of 1-4 introduce yet another hydrogen-bond acceptorsthe imine nitrogen atom. This imine nitrogen atom could potentially compete successfully with the pyridine nitrogen atom as a hydrogen-bond acceptor for the oxime O-H moiety. Second, these molecules have essentially twice the number of delocalized electrons compared to the oxime-substituted pyridines, thus increasing the relative contribution made by stabilizing π-π contacts to the lattice energies of these compounds. However, neither the weaker π-π interactions nor the imine nitrogen atom participate in intermolecular interactions that display identifiable motifs with the same consistency and regularity as the O-H‚‚‚N hydrogen bond, which serves to illustrate the importance of hydrogen bonding as the primary supramolecular synthetic tool. The appearance of a dimer in 2 is not easy to rationalize, but it is clear that such motifs are prevented in the other three structures due to their specific molecular geometries. From a thermodynamic standpoint, formation of discrete dimers or infinite chains would be expected to have similar enthalpies, since both would contain the same types and numbers of interactions; however, dimer formation would be entropically favored over formation of infinite chains. Since the pyridine nitrogen atom is the best hydrogen-bond acceptor in this system, it typically competes successfully for the best donor, the oxime moiety. Having established the preferred hydrogen-bonding modes in this family of compounds, we will continue to explore and exploit these molecules as supramolecular tools in the design of both organic molecular solids and in the assembly of inorganic-organic hybrid materials. As shown previously,8 these ligands are ideal for organizing coordination complexes into extended motifs. The pyridine nitrogen atom can coordinate to a metal ion, thereby ensuring that the best hydrogen-bond acceptor is blocked, which then enables the formation of a complementary head-to-head (oxime‚‚‚oxime) interaction. The latter interaction finally serves to propagate the geometry of the metal complex into 1-D, 2-D, or 3-D architectures. Acknowledgment. We are grateful for financial support from the NSF (Grant No. CHE-0078996) and Kansas State University.

Aakero¨y et al. Supporting Information Available: Crystallographic data, in CIF format, are available for all the crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) Aakero¨y, C. B. Acta Crystallogr., Sect. B 1997, 53, 569. (b) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids; Elsevier: Amsterdam, 1989. (c) Zaworotko, M. J. Nature 1997, 386, 220. (d) Crystal Engineering: From Molecules and Crystals to Materials; Braga, D., Grepioni, F., Orpen, A. G., Eds.; NATO Science Series 538; Kluwer: Dordrecht, The Netherlands, 1999. (2) (a) Swift, J. A.; Pivovar, A. M.; Reynolds, A. M.; Ward, M. D. J. Am. Chem. Soc. 1998, 120, 5887. (b) Eddaoudi, M.; Li, H.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 1391. (c) Batten, S. R.; Hoskins, B. F.; Moubaraki, B.; Murray, K. S.; Robson, R. Chem. Commun. 2000, 1095. (d) Brunet, P.; Simard, M.; Wuest, L. D. J. Am. Chem. Soc. 1997, 119, 2737. (e) Biradha, K.; Seward, C.; Zaworotko, M. J. Angew. Chem., Int. Ed. 1999, 38, 492. (f) Wang, Z.; Xioing, R. G.; Foxman, B. M.; Wilson, S. R.; Lin, W. Inorg. Chem. 1999, 38, 1523. (g) Blake, A. J.; Champness, N. R.; Cooke, P. A.; Nicolson, J. E. B. Chem. Commun. 2000, 665. (h) Batasanov, A. S.; Hubberstey, P.; Russel, C. E.; Walton, P. H. J. Chem. Soc., Dalton Trans. 1997, 2667. (i) Munakata, M.; Wu, L. P.; Yamamoto, M.; Kuroda-Sowa, T.; Maekawa, M. J. Am. Chem. Soc. 1996, 118, 3117. (3) (a) Aoyama, Y. Top. Curr. Chem. 1998, 198, 131. (b) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Acc. Chem. Res. 1998, 31, 474. (4) (a) Aakero¨y, C. B.; Beatty, A. M.; Helfrich, B. A. J. Chem. Soc., Dalton Trans. 1998, 1943. (b) Rivas, J. C. M.; Brammer, L. New J. Chem. 1998, 22, 1315. (c) Aakero¨y, C. B.; Beatty, A. M.; Leinen, D. S. Angew. Chem., Int. Ed. 1999, 38, 1815. (d) Aakero¨y, C. B.; Beatty, A. M. Chem. Commun. 1998, 1067. (e) Aakero¨y, C. B.; Beatty, A. M.; Leinen, D. S.; Lorimer, K. R. Chem. Commun. 2000, 935. (5) Cambridge Structural Database version 5.17 (April 1999): Allen, F. H.; Kennard, O.; Taylor, R. Acc. Chem. Res. 1983, 16, 46. (6) Maurin, J. K. Pol. J. Chem. 1998, 72, 786. (7) (a) Grammaticakis, P. Bull. Soc. Chim. Fr. 1953, 20, 93. (b) March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley: New York, 1992; p 906. (8) (a) Aakero¨y, C. B.; Beatty, A. M.; Leinen, D. S. J. Am. Chem. Soc. 1998, 120, 7383. (b) Aakero¨y, C. B.; Beatty, A. M.; Helfrich, B. A.; Leinen, D. S. Trans. Am. Crystallogr. Assoc. 1999, 33, 101. (9) Sheldrick, G. M. University of Go¨ttingen, Go¨ttingen, Germany. (10) Maurin, J. K. Acta Crystallogr., Sect. B 1998, 54, 866. (11) Note, however, that “motifs” other than those depicted here are possible.

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