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Interplay of hydrogen bonding and π–π interactions in the molecular complex of 2,6-lutidine N-oxide and water
Journal of Molecular Structure 787 (2006) 121–126 www.elsevier.com/locate/molstruc
Interplay of hydrogen bonding and p–p interactions in the molecular complex of 2,6-lutidine N-oxide and water Jose´ Giner Planas a, Gehad G. Mohamed a, Reijo Sillanpa¨a¨ b, Raikko Kiveka¨s c, Francesc Teixidor a, Clara Vin˜as a,* a
Inorganic Materials and Catalysis Laboratory, Institut de Cie`ncia de Materials de Barcelona, Campus UAB, 08193 Bellaterra, Spain b Department of Chemistry, University of Jyva¨skyla¨, FIN-40531, Finland c Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Finland Received 28 September 2005; received in revised form 31 October 2005; accepted 1 November 2005 Available online 20 December 2005
It is a pleasure for the authors to dedicate this paper to Prof. Hursthouse on the occasion of his 65th birthday in recognition of his great contribution to the field of X-Ray Crystallography
Abstract The crystal and molecular structure of 2,6-lutidine N-oxide monohydrate (1) has been determined by X-ray diffraction analysis. Each water molecule is acting as bridging ligand between the N/O moieties of two 2,6-lutidine N-oxide molecules through moderate strong intermolecular ˚ ) giving rise to a one-dimensional (1D) polymeric helical chain. A twohydrogen bonding (O–H/O, O/O distances are 2.787(2) and 2.832 (2) A dimensional (2D) layered network is then formed by self-assembly of 1D helical chains via strong p–p interactions of the aromatic rings ˚ ). The molecular structure of 1 is compared with that for the already reported molecular structures of 2-acetylamino(interplanar distances 3.385 A 6-methylpyridine N-oxide monohydrate and pyridine trihydrate. Finally, on the basis of the present studies a possible explanation for the formation of the molecular complexes is proposed and discussed. q 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen bond; p–p interactions; Aromatic N-oxides
1. Introduction The large amount of organic structures containing solvent of crystallization in the Cambridge Structural Database (CSD) [1] have been rationalised by assuming that crystallisation begins with solute–solvent aggregates that contain solute–solute, solute– solvent and solvent–solvent interactions [2]. If solute–solvent interactions are unusually important, say because of multipoint recognition, the entropic advantage associated with solvent expulsion into the bulk may be overriden by these additional enthalphic factors resulting in retention of some solvent in the crystal. The latter is often encountered when using organic solvents capable of strong hydrogen bonding such as water [3]. The inclusion of water within organic crystals is a matter of both fundamental and practical importance [3] with increasing interest * Corresponding author. E-mail addresses: [email protected] (J.G. Planas), [email protected] (C. Vin˜as).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.11.005
in the new area of crystal engineering [4]. Water molecules have two hydrogen atoms and two lone pairs enabling them to participate, in general, in four hydrogen bonds in a tetrahedral arrangement [3,4]. Water is incorporated into organic crystals far more frequently than other common solvents [3]. One family of compounds that easily crystallizes in the form of hydrates are that of amine N-oxides. These oxygen bases are characterized by the ability to form hydrogen-bonded complexes of various compositions by interaction with one or two proton donors from donor molecules through one or two electron lone-pairs of the oxygen, respectively (Scheme 1, I-II) [5]. Without exception, crystal structures for hydrated aliphatic N-oxides are consolidated by strong hydrogen bonding between the water molecules, as well as between the N-oxide and water molecules, giving rise to puckered layers or three-dimensional networks composed of edge-sharing rings of different sizes [6,7]. As an example, the crystal structure of trimethylamine N-oxide dihydrate (WEMFEY) [7] shows an eight-membered ring composed of edge-sharing water and N-oxide molecules (Scheme 1). As part of a synthetic work on NS2 aza-thio podand or macrocyclic ligands [8], we obtained crystals for the 2,6-lutidine
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˚ ). Scheme 1. Amine N-oxide complexes (O/O distances in III are 2.69-2.83 A
N-oxide monohydrate (1). This represents a rare example of a N-oxide monohydrate molecular complex that does not contain any strong hydrogen bonding between the water molecules. A CSD search for aromatic N-oxides disclosed at least six examples of hydrated aromatic N-oxides, not including strong hydrogen bonding between the water molecules in their molecular structures [9]. The present paper provides an analysis of the structure for the molecular complex 1 and explores the possible reasons for the absence of strong hydrogen bonding between the water molecules in these compounds. 2. Experimental
2.2. Synthesis of the compound
2.1. General remarks 2,6-Lutidine N-oxide was prepared as previously reported [10]. NMR spectra were acquired on Bruker ARX 300 MHz spectrometer and referenced to the solvent (residual CHCl3) Table 1 Crystal data and experimental details of 1 Formula Mr Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) U (A Z m (Mo Ka) (cmK1) Unique reflections Parameters Sa R1b (IO2s(I)) wR2c (IO2s(I)) a b c
SZ ½SðwðF02 KFc2 Þ2 Þ=ðnKpÞ1=2 . R1Z SjjF0 jKjFc jj=SjF0 j. wR2Z fS½wðF02 KFc2 Þ2 =S½wðF02 Þ2 g1=2 .
[11]. Chemical shifts are reported in ppm. Multiplets nomenclature is as follows: s, singlet; d, doublet; t, triplet. The thermogravimetric analysis (TGA) was carried out in dynamic nitrogen atmosphere (20 ml minK1) with a heating rate of 10 8C minK1 using a Perkin–Elmer 13A TGA7 thermal analyzer. Cambridge Structural Database searches for nonbonded contacts were carried out using the program ConQuest (version 1.7) [12]. The search criteria (CSD dated February 2005) were error and disorder free only for organic structures with R factors less than 0.05. Structures were retrieved from the Cambridge Structural Database as mol2 files and analysed using Mercury (version 1.4) [12].
C7H11NO2 141.17 Monoclinic C2/c 14.174(2) 8.2706(18) 13.877(2) 108.469(13) 1543.0(5) 8 0.89 1393 124 1.034 0.0470 0.1201
White crystals of 2,6-lutidine N-oxide monohydrate (1) were obtained from a chloroform or carbon tetrachloride solution of 2,6-lutidine N-oxide and a few drops of water. The compound was characterized spectroscopically and by single crystal X-ray difraction. 2,6-lutidine N-oxide monohydrate (1): 1H NMR (CDCl3): 7.18 (d, 3J(H,H)Z8.5 Hz, 2H, m-pyridine), 7.09 (t, 3J(H,H)Z 6.9 Hz, 1H, p-pyridine), 2.55 (s, 6H, CH3). 13C{1H} NMR (CDCl3): 149.0, 124.7, 124.0 (s, pyridine), 18.4 (s, CH3). 2,6-lutidine N-oxide: 1H NMR (CDCl3): 7.01 (d, 3J(H,H)Z 8.3 Hz, 2H, m-pyridine), 7.05 (t, 3J(H,H)Z7.0 Hz, 1H, p-pyridine), 2.48 (s, 6H, CH3). 13C{1H} NMR (CDCl3): 148.8, 124.5, 123.8 (s, pyridine), 18.1 (s, CH3). 2.3. X-ray crystallography and data collection Single-crystal data collection for 1 was performed at K80 C on a Rigaku AFC7S diffractometer using graphite mono˚ ). The unit cell chromatized Mo Ka radiation (lZ0.71073 A parameters were determined by least-squares refinement of 25 carefully centered reflections. The structure was solved by direct methods and refined on F2 by the SHELXL97 program [13]. Non-hydrogen atoms were refined with anisotropic
J.G. Planas et al. / Journal of Molecular Structure 787 (2006) 121–126
displacement parameters but for hydrogen atoms, positional parameters were refined with fixed isotropic displacement parameters. Crystal data are cathered in Table 1. 3. Results and discussion Thermogravimetric analysis of 1 under nitrogen atmosphere shows the following clear and well-separated weight loss steps. A total weight loss of 12.88% occurred in the range 30–70 8C, corresponding to the removal of guest water molecules (12.77% calculated), followed by a weight loss of 87.12% in the range 70–130 8C consistent with the decomposition of the 2,6-lutidine N-oxide molecule (87.23% calculated). 3.1. Crystal structure of 2,6-lutidine N-oxide monohydrate (1) Asymmetric unit of the structure consists of one 2,6-lutidine N-oxide and one crystal water molecule connected by hydrogen bond (Fig. 1). Approximate symmetry of the planar 2,6-lutidine N-oxide molecule is C2v but crystallographic symmetry of the molecule is C1. Bond lengths and angles of 1 (Table 2) agree well with those of comparable compounds [9] and do not reserve any comment. Instead, main interest of the structure is focused on the molecular packing with hydrogen bonds and p–p stacking. These aspects are discussed more in the next section. 3.2. Molecular packing Fig. 2 shows a projection of the structure for the 2,6-lutidine N-oxide monohydrate complex (1). It can be seen that the molecules are held by a network of hydrogen bonds. Table 3 lists the intermolecular distances and angles of interest. The water molecules link 2,6-lutidine N-oxide molecules by hydrogen bonds in the direction of the b-axis (Fig. 2). The result is a one-dimensional chiral polymeric helical chain. Surprisingly, the moderate strong hydrogen bonding between the water molecules observed in organic hydrated N-oxides [6,7] is not found in this simple compound. In contrast, both H atoms from each water molecule in the structure of 1 behave as proton donors to two oxygen atoms from two different 2,6lutidine N-oxide molecules (O–H/O, interaction (a) and (b) in
Fig. 1. A view of asymmetric unit of 1 with one extra molecule 2,6-lutidine molecule showing labelling system utilized and hydrogen bonds of the water molecule. Thermal ellipsoids are drawn with 30% probability level. Superscipt b on O1 refers equivalent position 1/2Kx, 1/2Cy, 1/2Kz.
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Table 2 ˚ ) and angles (8) for 1 Selected bond lengths (A Bond lengths N–O1 N–C1 N–C5 C1–C2 C1–C6 C2–C3 C3–C4 C4–C5 C5–C7
Bond angles 1.3314(19) 1.362(2) 1.361(2) 1.379(3) 1.484(3) 1.373(3) 1.372(3) 1.379(3) 1.487(3)
O1–N–C1 O1–N–C5 C1–N–C5 N–C1–C2 N–C1–C6 C2–C1–C6 N–C5–C7 C4–C5–C7
118.91(14) 118.74(15) 122.34(15) 118.34(17) 117.06(17) 124.60(19) 116.85(17) 124.63(19)
Table 3) giving rise to the one-dimensional helical structure shown in Fig. 2. These intermolecular interactions lead to the formation of infinite helical chains consisting of two zones with their hydrophobic heads (non-polar side of the lutidine rings) exposed to the exterior and the polar ends of the lutidine rings embedded in the interior. The two dimensional structure of 1 is then constructed by interactions in the hydrophobic zone (exterior of helices) through strong p–p stacking interactions. The shortest C/C ˚ , for distances between neighbouring molecules are 3.382(3) A C(3)/C(5). The average interplanar distance between inter˚ (centroid–centroid distance acting 2,6-lutidine rings is 3.385 A ˚ 3.647 A). The formation of the 2D structure can be described by two helices (in the crystallographic bc-plane) that approach each other along the crystallographic c-axis (Fig. 2). This indicates an association between the hydrophobic moieties of the helical structures. As mentioned above, the most intriguing feature of the molecular complex 1—and other aromatic N-oxide hydrates— is that unlike all other reported hydrated aliphatic N-oxides [6,7], they do not contain any strong hydrogen bonding between the water molecules. Now the question is why this unusual structure is adopted at all. A possible rationale is
Fig. 2. The molecular packing of 2,6-lutidine molecules in one of the layers showing hydrogen bonds in the direction of b-axis (1D structure) and the stacking of 2,6-lutidine rings. The CH hydrogens are omitted for clarity.
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Table 3 ˚ , 8), involved in the Geometrical parameters of hydrogen bonds (A supramolecular construction in 1 D–H/A
D(D–H)
d(H/A)
d(D/A)
!(DHA)
(a) O(2)–H(2A)/O(1) (b) O(2)–H(2B)/O(1)a (c) C(3)–H(3)/O(2)b (d) C(7)–H(7B)/O(2)c
0.84(3) 0.86(3) 0.92(2) 0.97(3)
1.95(3) 1.97(3) 2.68(2) 2.65(3)
2.787(2) 2.832(2) 3.459(3) 3.569(3)
174(3) 173(3) 142.7(17) 159(2)
D,donor; A, aceptor. a Symmetry codes: 1/2Kx, 1/2Cy, 1/2Kz. b 1/2Kx, 1/2Ky,Kz. c x,Ky, K1/2Cz.
obtained by considering the packing of aromatic rings in the structure of 2,6-lutidine N-oxide monohydrate. Fig. 3 shows that planar aromatic rings in the structure for 1 are arranged in a sandwich-herringbone fashion, typical of aromatic rings in crystalline pyrene and perylene [14]. In this type of structures both herringbone and stacking geometries of molecular dimers are observed. Each sandwich (dimer) in 2,6-lutidine N-oxide monohydrate corresponds to those aromatic moieties with short ˚ ) including strong p–p interinterplanar distances (3.385 A actions (Fig. 2). Water molecules are then enclosed by molecular dimers with herringbone geometries as shown in Fig. 4. Besides the strong O–H/O hydrogen bonds described above (interactions (a) and (b) in Table 3), there are other pair of weak C–H/O hydrogen bonds, involving phenyl ring or methyl group hydrogens and the water oxygen (interactions (c) and (d) in Fig. 4 and Table 3). It is important to note that tetrahedral hydrogen bond coordination of each water is fullfilled in this way, so that no water–water interactions are needed. The absence of strong hydrogen bonding between the water molecules can then be understood as a result of the need for the aromatic rings of the 2,6-lutidine N-oxide molecules to establish a sandwich-herringbone arrangement. The optimization of the p–p interactions between aromatic rings rather than the formation of O–H/O hydrogen bonds between water
Fig. 3. Perspective for an X-ray diagram for 1 (10% probability level) along the diagonal between the a and b-axes, showing the sandwich herringbone interactions. Note that the arrangement of aromatic rings is similar to that in the structure of pyrene. H atoms and water molecules are omitted for clarity.
Fig. 4. Cameron view of 1 at the 20% probability level showing tetracoordination of a water molecule (bold dashed lines). O–H/O (a, b), C–H/O (c,d), and p–p interactions (wide dashed lines).
molecules seems to be the primary structural effect in 1. It is known that electron withdrawing substituents or heteroatoms decrease the p-electron density in the arene rings and lead to stronger p–p interactions [15]. Moreover, if molecules have a permanent dipole moment there is a tendency, especially in flat aromatic species, for them to stack in an anti-parallel fashion ˚ ) so that dipole-dipole and with short axis (7.5–7.8 A interactions are optimised [14b,c]. This is the case of the pyridine N-oxide derivatives since the N/O moiety both withdraw electron density and impose a permanent dipole to the molecule. However, it is important to emphasize that the only examples of N-oxides hydrate molecular complexes, apart from 1, not having water–water interactions that we have found in a CSD search correspond to aromatic (pyridine) N-oxide derivatives [9]. Analysis of the crystal packing for these molecular complexes show that all of them contain other functionalities capable of acting as strong hydrogen bond donors and acceptors, leading to unnecessary and avoidable complications that further complicate the system. Nevertheless, a somewhat related compound, namely 2-acetylamino-6-methylpyridine N-oxide monohydrate (HIQKIB) [9], give a similar molecular structure to that of 1. A water molecule is acting as bridging ligand between the N/O moiety of one molecule and the carbonyl oxygen from another. But more importantly is that well defined p–p interactions are ˚ ; centroid–centroid also found (interplanar distance, 3.473(2) A ˚ ) as well as weak C–H/O hydrogen bonds distance 3.65 A (Fig. 5). As can be seen in Fig. 5, tetrahedral coordination of
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expected to play a significant role in tailoring the topology and architecture of solid materials and their hydrates. These observations also serve in the understanding of the basic principles of supramolecular chemistry and crystal engineering. 5. Supplementary material
Fig. 5. Cameron view of 2-acetylamino-6-methylpyridine N-oxide monohydrate at the 15% probability level showing tetracoordination of a water molecule (thin dashed lines) and p–p interactions (wide dashed lines).
the water molecule is fulfilled without the need of any strong water–water hydrogen bond, just like in case of 1. With these considerations in hand, one might expect that aromatic compounds with weaker p–p interactions than that of pyridine N-oxides will yield molecular complexes with water, including strong hydrogen bonding between them. One simple related structure available is that of pyridine trihydrate which does form both water O–H/O hydrogen bonds and aromatic ring p–p interactions [16]. But in this case, p–p interactions are merely an overlayering at the edges of the rings (centroid– ˚ ) and pyridine rings are not stacked in centroid distance 3.75 A anti-parallel fashion [17], as expected for molecules having a permanent dipole moment [14b,c]. Uneffective p–p interactions of pyridine trihydrate are probably due to the small permanent dipole moment compared with that of 1 or 2-acetylamino-6-methylpyridine N-oxide monohydrate. Thus, unlike the latter two, the optimization of the water O–H/O hydrogen bonds rather than the formation of aromatic ring p–p interactions seems to be the primary structural effect in the pyridine case.
4. Conclusions We have reported the crystal structure of the 2,6-lutidine N-oxide monohydrate complex (1), which shows the existance of moderate strong hydrogen bonding between 2,6-lutidine N-oxide and water molecules (not including water–water interactions) giving rise to a 1D helical structure and its selfassembly through p–p interactions to give a 2D network. In addtion, a comparison of the structure for 1 with those for 2-acetylamino-6-methylpyridine N-oxide monohydrate and pyridine trihydrate complexes indicate that correspondence between molecular and crystal structure are the result of effective interference between different sets of significant intermolecular interactions during the crystallization process. In the present case, these sets correspond to solute–water hydrogen bonding interactions and solute–solute p–p interactions. The latter are favored in aromatic rings having the strongly polarized N/O group. Thus, it is important to note that the introduction of this group in crystal engineering is
Crystallographic data (excluding struture factors) for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 276992. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: C44 1223 336033; e-mail: [email protected] or www:http://www.ccdc.cam.ac.uk Acknowledgements We thank CICYT (Project MAT2004-01108), Generalitat de Catalunya (2001/SGR/00337), CSIC (I3P contract to J.G.P.). References [1] A. Nangia, G.R. Desiraju, Chem. Commun. (1999) 605. [2] V.S.S. Kumar, S.S. Kuduva, G.R. Desiraju, J. Chem. Soc., Perkin Trans. 2 (1999) 1069. [3] G.R. Desiraju, J. Chem. Soc., Chem. Commun. (1991) 426. [4] L. Infantes, S. Motherwell, CrystEngComm 4 (2002) 454. [5] Z. Dega-Szafran, Z. Kosturkiewicz, E. Tykarska, M. Szafran, D. Leman´ski, B. Nogaj, J. Mol. Struct. 404 (1997) 25, and references there in. [6] C.-K. Kwok, T.C.W. Mak, Can. J. Chem. 66 (1987) 1836. [7] (a) W.-J. Lu, L.-P. Zhang, H.-S. Chan, T.-L. Chan, T.C.W. Mak, Polyhedron 23 (2004) 1089; (b) E. Maia, S. Perez, Acta Crystallogr. B 38 (1982) 849; (c) E. Maia, E. Peguy, S. Perez, Acta Crystallogr. B 37 (1981) 1858; (d) M. Jaskolski, Acta Crystallogr. C 43 (1987) 2391; (e) V. Kettmann, I. Masterova, J. Tomko, Acta Crystallogr. C 46 (1990) 1334; (f) P.K. Hon, T.C.W. Mak, J. Crystallogr. Spectrosc. Res. 17 (1987) 419; (g) H.G. Aurich, M. Soeberdt, K. Harms, Eur. J. Org. Chem. (1999) 1249; (h) O. Ritzeler, S. Parel, B. Therrien, N. Bensel, J.-L. Reymond, K. Schenk, Eur. J. Org. Chem. (2000) 1365; (i) M. Jaskolski, Z. Kosturkiewicz, D. Mickiewicz-Wichlacz, M. Wiewiorowski, J. Mol. Struct. 52 (1979) 77; (j) M. Rospenk, A. Koll, T. Glowiak, L. Sobczyk, J. Mol. Struct. 195 (1989) 33; (k) P. Dapporto, A. Guerri, P. Paoli, P. Rossi, M. Altamura, F.R. Calabri, A. Guidi, J. Mol. Struct. 617 (2002) 189; (l) T.C.W. Mak, J. Mol. Struct. 178 (1988) 169; M. Enders, O. Fritz, H. Pritzkow, Private Communication, 2002. (m) J.D. Sauer, H.Y. Elnagar, F.R. Fronczek, Acta Crystallogr. C 59 (2003) o62. [8] (a) F. Teixidor, G. Sa´nchez-Castello´, N. Lucena, L. Escriche, R. Kiveka¨s, M. Sundberg, J. Casabo´, Inorg. Chem. 30 (1991) 4931; (b) F. Teixidor, G. Sa´nchez, N. Lucena, L. Escriche, R. Kiveka¨s, J. Casabo´, J. Chem. Soc., Chem. Commun. 163 (1992); (c) C. Vin˜as, P. Angle´s, G. Sanchez, N. Lucena, F. Teixidor, Ll. Escriche, J. Casabo´, J.F. Piniella, A. Alvarez-Larena, R. Kiveka¨s, R. Sillanpa¨a¨, Inorg. Chem. 37 (1998) 701. [9] (a) X. Mei, C. Wolf, Chem. Commun. (2004) 2078; (b) X.-M. Gan, R.T. Paine, E.N. Duesler, H. Noth, Dalton Trans. (2003) 153;
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(c) B.-G. Zhang, Y.-J. Liu, S.-H. Gou, C.-Y. Duan, X.-Z. You, Acta Crystallogr. C55 (1999) 1929; (d) E. Bermejo, R. Carballo, A. Castineiras, R. Dominguez, C. MaichleMossmer, J. Strahle, D.X. West, Polyhedron 18 (1999) 3695; (e) S. Goswami, K. Ghosh, A.K. Mahapatra, G.D. Nigam, K. Chinnakali, H.-K. Fun, Acta Crystallogr. C 55 (1999) 579; (f) A.S. Batsanov, Acta Crystallogr. C 56 (2000) e269. [10] Z. Talik, A. Puszko, Rocz. Chem. 50 (1976) 2209. [11] H.E. Gottlieb, V. Kotlyar, A. Nudelman, J. Org. Chem. 62 (1997) 7512. [12] I.J. Bruno, J.C. Cole, P.R. Edgington, M. Kessler, C.F. Macrae, P. McCabe, J. Pearson, R. Taylor, Acta Crystallogr. B 58 (2002) 389.
[13] G.M. Sheldrick, SHELXK97, Program for Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997. [14] (a) J.M. Robertson, Proc. R. Soc. Lond., A207101; (b) G.R. Desiraju, Crystal Engineering: The Design of Organic Solids, Elsevier, Amsterdam, 1989, and references there in (c) A. Dey, G.R. Desiraju, Chem. Commun.2486. [15] C. Janiak, J. Chem. Soc., Dalton Trans. (2000) 3885, and references there in. [16] D. Mootz, H.-G. Wussow, J. Chem. Phys. 75 (1981) 1517. [17] Pyridine rings do not stack in an anti-parallel fashion: torsion angle N(1)-centroid-centroid-N(1) for a pair of stacked molecules is K94.568.
Interplay of hydrogen bonding and p–p interactions in the molecular complex of 2,6-lutidine N-oxide and water Jose´ Giner Planas a, Gehad G. Mohamed a, Reijo Sillanpa¨a¨ b, Raikko Kiveka¨s c, Francesc Teixidor a, Clara Vin˜as a,* a
Inorganic Materials and Catalysis Laboratory, Institut de Cie`ncia de Materials de Barcelona, Campus UAB, 08193 Bellaterra, Spain b Department of Chemistry, University of Jyva¨skyla¨, FIN-40531, Finland c Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Finland Received 28 September 2005; received in revised form 31 October 2005; accepted 1 November 2005 Available online 20 December 2005
It is a pleasure for the authors to dedicate this paper to Prof. Hursthouse on the occasion of his 65th birthday in recognition of his great contribution to the field of X-Ray Crystallography
Abstract The crystal and molecular structure of 2,6-lutidine N-oxide monohydrate (1) has been determined by X-ray diffraction analysis. Each water molecule is acting as bridging ligand between the N/O moieties of two 2,6-lutidine N-oxide molecules through moderate strong intermolecular ˚ ) giving rise to a one-dimensional (1D) polymeric helical chain. A twohydrogen bonding (O–H/O, O/O distances are 2.787(2) and 2.832 (2) A dimensional (2D) layered network is then formed by self-assembly of 1D helical chains via strong p–p interactions of the aromatic rings ˚ ). The molecular structure of 1 is compared with that for the already reported molecular structures of 2-acetylamino(interplanar distances 3.385 A 6-methylpyridine N-oxide monohydrate and pyridine trihydrate. Finally, on the basis of the present studies a possible explanation for the formation of the molecular complexes is proposed and discussed. q 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen bond; p–p interactions; Aromatic N-oxides
1. Introduction The large amount of organic structures containing solvent of crystallization in the Cambridge Structural Database (CSD) [1] have been rationalised by assuming that crystallisation begins with solute–solvent aggregates that contain solute–solute, solute– solvent and solvent–solvent interactions [2]. If solute–solvent interactions are unusually important, say because of multipoint recognition, the entropic advantage associated with solvent expulsion into the bulk may be overriden by these additional enthalphic factors resulting in retention of some solvent in the crystal. The latter is often encountered when using organic solvents capable of strong hydrogen bonding such as water [3]. The inclusion of water within organic crystals is a matter of both fundamental and practical importance [3] with increasing interest * Corresponding author. E-mail addresses: [email protected] (J.G. Planas), [email protected] (C. Vin˜as).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.11.005
in the new area of crystal engineering [4]. Water molecules have two hydrogen atoms and two lone pairs enabling them to participate, in general, in four hydrogen bonds in a tetrahedral arrangement [3,4]. Water is incorporated into organic crystals far more frequently than other common solvents [3]. One family of compounds that easily crystallizes in the form of hydrates are that of amine N-oxides. These oxygen bases are characterized by the ability to form hydrogen-bonded complexes of various compositions by interaction with one or two proton donors from donor molecules through one or two electron lone-pairs of the oxygen, respectively (Scheme 1, I-II) [5]. Without exception, crystal structures for hydrated aliphatic N-oxides are consolidated by strong hydrogen bonding between the water molecules, as well as between the N-oxide and water molecules, giving rise to puckered layers or three-dimensional networks composed of edge-sharing rings of different sizes [6,7]. As an example, the crystal structure of trimethylamine N-oxide dihydrate (WEMFEY) [7] shows an eight-membered ring composed of edge-sharing water and N-oxide molecules (Scheme 1). As part of a synthetic work on NS2 aza-thio podand or macrocyclic ligands [8], we obtained crystals for the 2,6-lutidine
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˚ ). Scheme 1. Amine N-oxide complexes (O/O distances in III are 2.69-2.83 A
N-oxide monohydrate (1). This represents a rare example of a N-oxide monohydrate molecular complex that does not contain any strong hydrogen bonding between the water molecules. A CSD search for aromatic N-oxides disclosed at least six examples of hydrated aromatic N-oxides, not including strong hydrogen bonding between the water molecules in their molecular structures [9]. The present paper provides an analysis of the structure for the molecular complex 1 and explores the possible reasons for the absence of strong hydrogen bonding between the water molecules in these compounds. 2. Experimental
2.2. Synthesis of the compound
2.1. General remarks 2,6-Lutidine N-oxide was prepared as previously reported [10]. NMR spectra were acquired on Bruker ARX 300 MHz spectrometer and referenced to the solvent (residual CHCl3) Table 1 Crystal data and experimental details of 1 Formula Mr Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) U (A Z m (Mo Ka) (cmK1) Unique reflections Parameters Sa R1b (IO2s(I)) wR2c (IO2s(I)) a b c
SZ ½SðwðF02 KFc2 Þ2 Þ=ðnKpÞ1=2 . R1Z SjjF0 jKjFc jj=SjF0 j. wR2Z fS½wðF02 KFc2 Þ2 =S½wðF02 Þ2 g1=2 .
[11]. Chemical shifts are reported in ppm. Multiplets nomenclature is as follows: s, singlet; d, doublet; t, triplet. The thermogravimetric analysis (TGA) was carried out in dynamic nitrogen atmosphere (20 ml minK1) with a heating rate of 10 8C minK1 using a Perkin–Elmer 13A TGA7 thermal analyzer. Cambridge Structural Database searches for nonbonded contacts were carried out using the program ConQuest (version 1.7) [12]. The search criteria (CSD dated February 2005) were error and disorder free only for organic structures with R factors less than 0.05. Structures were retrieved from the Cambridge Structural Database as mol2 files and analysed using Mercury (version 1.4) [12].
C7H11NO2 141.17 Monoclinic C2/c 14.174(2) 8.2706(18) 13.877(2) 108.469(13) 1543.0(5) 8 0.89 1393 124 1.034 0.0470 0.1201
White crystals of 2,6-lutidine N-oxide monohydrate (1) were obtained from a chloroform or carbon tetrachloride solution of 2,6-lutidine N-oxide and a few drops of water. The compound was characterized spectroscopically and by single crystal X-ray difraction. 2,6-lutidine N-oxide monohydrate (1): 1H NMR (CDCl3): 7.18 (d, 3J(H,H)Z8.5 Hz, 2H, m-pyridine), 7.09 (t, 3J(H,H)Z 6.9 Hz, 1H, p-pyridine), 2.55 (s, 6H, CH3). 13C{1H} NMR (CDCl3): 149.0, 124.7, 124.0 (s, pyridine), 18.4 (s, CH3). 2,6-lutidine N-oxide: 1H NMR (CDCl3): 7.01 (d, 3J(H,H)Z 8.3 Hz, 2H, m-pyridine), 7.05 (t, 3J(H,H)Z7.0 Hz, 1H, p-pyridine), 2.48 (s, 6H, CH3). 13C{1H} NMR (CDCl3): 148.8, 124.5, 123.8 (s, pyridine), 18.1 (s, CH3). 2.3. X-ray crystallography and data collection Single-crystal data collection for 1 was performed at K80 C on a Rigaku AFC7S diffractometer using graphite mono˚ ). The unit cell chromatized Mo Ka radiation (lZ0.71073 A parameters were determined by least-squares refinement of 25 carefully centered reflections. The structure was solved by direct methods and refined on F2 by the SHELXL97 program [13]. Non-hydrogen atoms were refined with anisotropic
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displacement parameters but for hydrogen atoms, positional parameters were refined with fixed isotropic displacement parameters. Crystal data are cathered in Table 1. 3. Results and discussion Thermogravimetric analysis of 1 under nitrogen atmosphere shows the following clear and well-separated weight loss steps. A total weight loss of 12.88% occurred in the range 30–70 8C, corresponding to the removal of guest water molecules (12.77% calculated), followed by a weight loss of 87.12% in the range 70–130 8C consistent with the decomposition of the 2,6-lutidine N-oxide molecule (87.23% calculated). 3.1. Crystal structure of 2,6-lutidine N-oxide monohydrate (1) Asymmetric unit of the structure consists of one 2,6-lutidine N-oxide and one crystal water molecule connected by hydrogen bond (Fig. 1). Approximate symmetry of the planar 2,6-lutidine N-oxide molecule is C2v but crystallographic symmetry of the molecule is C1. Bond lengths and angles of 1 (Table 2) agree well with those of comparable compounds [9] and do not reserve any comment. Instead, main interest of the structure is focused on the molecular packing with hydrogen bonds and p–p stacking. These aspects are discussed more in the next section. 3.2. Molecular packing Fig. 2 shows a projection of the structure for the 2,6-lutidine N-oxide monohydrate complex (1). It can be seen that the molecules are held by a network of hydrogen bonds. Table 3 lists the intermolecular distances and angles of interest. The water molecules link 2,6-lutidine N-oxide molecules by hydrogen bonds in the direction of the b-axis (Fig. 2). The result is a one-dimensional chiral polymeric helical chain. Surprisingly, the moderate strong hydrogen bonding between the water molecules observed in organic hydrated N-oxides [6,7] is not found in this simple compound. In contrast, both H atoms from each water molecule in the structure of 1 behave as proton donors to two oxygen atoms from two different 2,6lutidine N-oxide molecules (O–H/O, interaction (a) and (b) in
Fig. 1. A view of asymmetric unit of 1 with one extra molecule 2,6-lutidine molecule showing labelling system utilized and hydrogen bonds of the water molecule. Thermal ellipsoids are drawn with 30% probability level. Superscipt b on O1 refers equivalent position 1/2Kx, 1/2Cy, 1/2Kz.
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Table 2 ˚ ) and angles (8) for 1 Selected bond lengths (A Bond lengths N–O1 N–C1 N–C5 C1–C2 C1–C6 C2–C3 C3–C4 C4–C5 C5–C7
Bond angles 1.3314(19) 1.362(2) 1.361(2) 1.379(3) 1.484(3) 1.373(3) 1.372(3) 1.379(3) 1.487(3)
O1–N–C1 O1–N–C5 C1–N–C5 N–C1–C2 N–C1–C6 C2–C1–C6 N–C5–C7 C4–C5–C7
118.91(14) 118.74(15) 122.34(15) 118.34(17) 117.06(17) 124.60(19) 116.85(17) 124.63(19)
Table 3) giving rise to the one-dimensional helical structure shown in Fig. 2. These intermolecular interactions lead to the formation of infinite helical chains consisting of two zones with their hydrophobic heads (non-polar side of the lutidine rings) exposed to the exterior and the polar ends of the lutidine rings embedded in the interior. The two dimensional structure of 1 is then constructed by interactions in the hydrophobic zone (exterior of helices) through strong p–p stacking interactions. The shortest C/C ˚ , for distances between neighbouring molecules are 3.382(3) A C(3)/C(5). The average interplanar distance between inter˚ (centroid–centroid distance acting 2,6-lutidine rings is 3.385 A ˚ 3.647 A). The formation of the 2D structure can be described by two helices (in the crystallographic bc-plane) that approach each other along the crystallographic c-axis (Fig. 2). This indicates an association between the hydrophobic moieties of the helical structures. As mentioned above, the most intriguing feature of the molecular complex 1—and other aromatic N-oxide hydrates— is that unlike all other reported hydrated aliphatic N-oxides [6,7], they do not contain any strong hydrogen bonding between the water molecules. Now the question is why this unusual structure is adopted at all. A possible rationale is
Fig. 2. The molecular packing of 2,6-lutidine molecules in one of the layers showing hydrogen bonds in the direction of b-axis (1D structure) and the stacking of 2,6-lutidine rings. The CH hydrogens are omitted for clarity.
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Table 3 ˚ , 8), involved in the Geometrical parameters of hydrogen bonds (A supramolecular construction in 1 D–H/A
D(D–H)
d(H/A)
d(D/A)
!(DHA)
(a) O(2)–H(2A)/O(1) (b) O(2)–H(2B)/O(1)a (c) C(3)–H(3)/O(2)b (d) C(7)–H(7B)/O(2)c
0.84(3) 0.86(3) 0.92(2) 0.97(3)
1.95(3) 1.97(3) 2.68(2) 2.65(3)
2.787(2) 2.832(2) 3.459(3) 3.569(3)
174(3) 173(3) 142.7(17) 159(2)
D,donor; A, aceptor. a Symmetry codes: 1/2Kx, 1/2Cy, 1/2Kz. b 1/2Kx, 1/2Ky,Kz. c x,Ky, K1/2Cz.
obtained by considering the packing of aromatic rings in the structure of 2,6-lutidine N-oxide monohydrate. Fig. 3 shows that planar aromatic rings in the structure for 1 are arranged in a sandwich-herringbone fashion, typical of aromatic rings in crystalline pyrene and perylene [14]. In this type of structures both herringbone and stacking geometries of molecular dimers are observed. Each sandwich (dimer) in 2,6-lutidine N-oxide monohydrate corresponds to those aromatic moieties with short ˚ ) including strong p–p interinterplanar distances (3.385 A actions (Fig. 2). Water molecules are then enclosed by molecular dimers with herringbone geometries as shown in Fig. 4. Besides the strong O–H/O hydrogen bonds described above (interactions (a) and (b) in Table 3), there are other pair of weak C–H/O hydrogen bonds, involving phenyl ring or methyl group hydrogens and the water oxygen (interactions (c) and (d) in Fig. 4 and Table 3). It is important to note that tetrahedral hydrogen bond coordination of each water is fullfilled in this way, so that no water–water interactions are needed. The absence of strong hydrogen bonding between the water molecules can then be understood as a result of the need for the aromatic rings of the 2,6-lutidine N-oxide molecules to establish a sandwich-herringbone arrangement. The optimization of the p–p interactions between aromatic rings rather than the formation of O–H/O hydrogen bonds between water
Fig. 3. Perspective for an X-ray diagram for 1 (10% probability level) along the diagonal between the a and b-axes, showing the sandwich herringbone interactions. Note that the arrangement of aromatic rings is similar to that in the structure of pyrene. H atoms and water molecules are omitted for clarity.
Fig. 4. Cameron view of 1 at the 20% probability level showing tetracoordination of a water molecule (bold dashed lines). O–H/O (a, b), C–H/O (c,d), and p–p interactions (wide dashed lines).
molecules seems to be the primary structural effect in 1. It is known that electron withdrawing substituents or heteroatoms decrease the p-electron density in the arene rings and lead to stronger p–p interactions [15]. Moreover, if molecules have a permanent dipole moment there is a tendency, especially in flat aromatic species, for them to stack in an anti-parallel fashion ˚ ) so that dipole-dipole and with short axis (7.5–7.8 A interactions are optimised [14b,c]. This is the case of the pyridine N-oxide derivatives since the N/O moiety both withdraw electron density and impose a permanent dipole to the molecule. However, it is important to emphasize that the only examples of N-oxides hydrate molecular complexes, apart from 1, not having water–water interactions that we have found in a CSD search correspond to aromatic (pyridine) N-oxide derivatives [9]. Analysis of the crystal packing for these molecular complexes show that all of them contain other functionalities capable of acting as strong hydrogen bond donors and acceptors, leading to unnecessary and avoidable complications that further complicate the system. Nevertheless, a somewhat related compound, namely 2-acetylamino-6-methylpyridine N-oxide monohydrate (HIQKIB) [9], give a similar molecular structure to that of 1. A water molecule is acting as bridging ligand between the N/O moiety of one molecule and the carbonyl oxygen from another. But more importantly is that well defined p–p interactions are ˚ ; centroid–centroid also found (interplanar distance, 3.473(2) A ˚ ) as well as weak C–H/O hydrogen bonds distance 3.65 A (Fig. 5). As can be seen in Fig. 5, tetrahedral coordination of
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expected to play a significant role in tailoring the topology and architecture of solid materials and their hydrates. These observations also serve in the understanding of the basic principles of supramolecular chemistry and crystal engineering. 5. Supplementary material
Fig. 5. Cameron view of 2-acetylamino-6-methylpyridine N-oxide monohydrate at the 15% probability level showing tetracoordination of a water molecule (thin dashed lines) and p–p interactions (wide dashed lines).
the water molecule is fulfilled without the need of any strong water–water hydrogen bond, just like in case of 1. With these considerations in hand, one might expect that aromatic compounds with weaker p–p interactions than that of pyridine N-oxides will yield molecular complexes with water, including strong hydrogen bonding between them. One simple related structure available is that of pyridine trihydrate which does form both water O–H/O hydrogen bonds and aromatic ring p–p interactions [16]. But in this case, p–p interactions are merely an overlayering at the edges of the rings (centroid– ˚ ) and pyridine rings are not stacked in centroid distance 3.75 A anti-parallel fashion [17], as expected for molecules having a permanent dipole moment [14b,c]. Uneffective p–p interactions of pyridine trihydrate are probably due to the small permanent dipole moment compared with that of 1 or 2-acetylamino-6-methylpyridine N-oxide monohydrate. Thus, unlike the latter two, the optimization of the water O–H/O hydrogen bonds rather than the formation of aromatic ring p–p interactions seems to be the primary structural effect in the pyridine case.
4. Conclusions We have reported the crystal structure of the 2,6-lutidine N-oxide monohydrate complex (1), which shows the existance of moderate strong hydrogen bonding between 2,6-lutidine N-oxide and water molecules (not including water–water interactions) giving rise to a 1D helical structure and its selfassembly through p–p interactions to give a 2D network. In addtion, a comparison of the structure for 1 with those for 2-acetylamino-6-methylpyridine N-oxide monohydrate and pyridine trihydrate complexes indicate that correspondence between molecular and crystal structure are the result of effective interference between different sets of significant intermolecular interactions during the crystallization process. In the present case, these sets correspond to solute–water hydrogen bonding interactions and solute–solute p–p interactions. The latter are favored in aromatic rings having the strongly polarized N/O group. Thus, it is important to note that the introduction of this group in crystal engineering is
Crystallographic data (excluding struture factors) for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 276992. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: C44 1223 336033; e-mail: [email protected] or www:http://www.ccdc.cam.ac.uk Acknowledgements We thank CICYT (Project MAT2004-01108), Generalitat de Catalunya (2001/SGR/00337), CSIC (I3P contract to J.G.P.). References [1] A. Nangia, G.R. Desiraju, Chem. Commun. (1999) 605. [2] V.S.S. Kumar, S.S. Kuduva, G.R. Desiraju, J. Chem. Soc., Perkin Trans. 2 (1999) 1069. [3] G.R. Desiraju, J. Chem. Soc., Chem. Commun. (1991) 426. [4] L. Infantes, S. Motherwell, CrystEngComm 4 (2002) 454. [5] Z. Dega-Szafran, Z. Kosturkiewicz, E. Tykarska, M. Szafran, D. Leman´ski, B. Nogaj, J. Mol. Struct. 404 (1997) 25, and references there in. [6] C.-K. Kwok, T.C.W. Mak, Can. J. Chem. 66 (1987) 1836. [7] (a) W.-J. Lu, L.-P. Zhang, H.-S. Chan, T.-L. Chan, T.C.W. Mak, Polyhedron 23 (2004) 1089; (b) E. Maia, S. Perez, Acta Crystallogr. B 38 (1982) 849; (c) E. Maia, E. Peguy, S. Perez, Acta Crystallogr. B 37 (1981) 1858; (d) M. Jaskolski, Acta Crystallogr. C 43 (1987) 2391; (e) V. Kettmann, I. Masterova, J. Tomko, Acta Crystallogr. C 46 (1990) 1334; (f) P.K. Hon, T.C.W. Mak, J. Crystallogr. Spectrosc. Res. 17 (1987) 419; (g) H.G. Aurich, M. Soeberdt, K. Harms, Eur. J. Org. Chem. (1999) 1249; (h) O. Ritzeler, S. Parel, B. Therrien, N. Bensel, J.-L. Reymond, K. Schenk, Eur. J. Org. Chem. (2000) 1365; (i) M. Jaskolski, Z. Kosturkiewicz, D. Mickiewicz-Wichlacz, M. Wiewiorowski, J. Mol. Struct. 52 (1979) 77; (j) M. Rospenk, A. Koll, T. Glowiak, L. Sobczyk, J. Mol. Struct. 195 (1989) 33; (k) P. Dapporto, A. Guerri, P. Paoli, P. Rossi, M. Altamura, F.R. Calabri, A. Guidi, J. Mol. Struct. 617 (2002) 189; (l) T.C.W. Mak, J. Mol. Struct. 178 (1988) 169; M. Enders, O. Fritz, H. Pritzkow, Private Communication, 2002. (m) J.D. Sauer, H.Y. Elnagar, F.R. Fronczek, Acta Crystallogr. C 59 (2003) o62. [8] (a) F. Teixidor, G. Sa´nchez-Castello´, N. Lucena, L. Escriche, R. Kiveka¨s, M. Sundberg, J. Casabo´, Inorg. Chem. 30 (1991) 4931; (b) F. Teixidor, G. Sa´nchez, N. Lucena, L. Escriche, R. Kiveka¨s, J. Casabo´, J. Chem. Soc., Chem. Commun. 163 (1992); (c) C. Vin˜as, P. Angle´s, G. Sanchez, N. Lucena, F. Teixidor, Ll. Escriche, J. Casabo´, J.F. Piniella, A. Alvarez-Larena, R. Kiveka¨s, R. Sillanpa¨a¨, Inorg. Chem. 37 (1998) 701. [9] (a) X. Mei, C. Wolf, Chem. Commun. (2004) 2078; (b) X.-M. Gan, R.T. Paine, E.N. Duesler, H. Noth, Dalton Trans. (2003) 153;
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