A New Polymorph of 4-Pyridinethione Containing a Helical Assembly Based on N-H‚‚‚S Hydrogen Bonds Sebastian Muthu and Jagadese J. Vittal* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Received March 28, 2004;
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1181-1184
Revised Manuscript Received July 5, 2004
ABSTRACT: Tautomerization of 4-mercaptopyridine in absolute ethanol at two different temperatures leads to the formation of two polymorphs. In the polymorph 1, obtained by refluxing the ethanolic solution of 4-mercaptopyridine and layering with ether, the molecules are arranged in a helical conformation through N-H‚‚‚S hydrogen bonds. On the other hand, the previously reported polymorph 2 is an N-H‚‚‚S bonded zigzag chain and can be prepared by layering an ethanolic solution of 4-pyridinethione with ether at 5 °C. Apart from the different structural motifs, N-H‚‚‚S, C-H‚‚‚S hydrogen bonds, π‚‚‚π and C-H‚‚‚π interactions make a significant difference between the two structures. Both polymorphs exhibit solid-state photoluminescence due to π-π* transition. Introduction Polymorphism can be defined as the existence of a particular compound in more than one crystal structure.1 Apart from the structural differences, physical properties of the polymorphs may differ remarkably.2 Controlling the formation of a specific polymorph of desired properties is of special interest in pharmaceutical industries.3 Frequent occurrence of polymorphism in organic compounds appears to be very common.4 Further, the small energy difference between polymorphs (generally less than 12 kJ/mol) renders it more challenging to rationalize and predict the occurrence of polymorphism.5 However, the influence of solvent, temperature, and crystallization conditions is evident in some cases.2,6 Solvent inclusion in crystals may lead to one or more solvated forms of the same compound, known as pseudopolymorphs.7 Conformationally flexible molecules have often been found to give rise to polymorphism.8 Trans-1,4-diethynylcyclohexane-1,4-diol, for example, exhibits conformational polymorphism due to the flexibility of the cyclohexane backbone.8a Similarly, molecules with multiple functional groups capable of forming different hydrogen bonding motifs has been used for deliberate synthesis of polymorphism.9 On the other hand, combination of weak intermolecular forces, such as hydrogen bonds and π‚‚‚π interactions may also contribute to the formation of different crystalline phases. In general, packing and overall structural motifs of the resulting polymorphs depend on a delicate balance of different types of supramolecular interactions.10 Of these, the reports pertaining to N-H‚‚‚S and C-H‚‚‚S bonds are relatively fewer than that of N-H‚‚‚O and C-H‚‚‚O bonds,11 which may be attributed to the fact that such interactions have gained recognition only recently. Further, the importance of N-H‚‚‚S, C-H‚‚‚S hydrogen bonds in biological systems and material science has attracted special interest in this field of study.12 Previously, Etter and co-workers have reported a 1D zigzag N-H‚‚‚S hydrogen-bonded poly* To whom correspondence should be addressed. Tel: + (65) 68742975; e-mail:
[email protected].
meric structure of 4-pyridinethione.13 Here, we report on a polymorph that has an interesting helical conformation based on N-H‚‚‚S interactions. To the best of our knowledge, this appears to be the first report on helical structure built on N-H‚‚‚S hydrogen bonds by a single component system. Results and Discussion Thione-thiol tautomerism of 4-mercaptopyridine is a well-known phenomenon.14 It was shown that polar solvents shift tautomeric equilibrium significantly toward the thione form.14a Etter and co-workers demonstrated that only the thione form rather than the thiol form exists in the solid state.13 It was shown that slow evaporation of toluene solution of 4-mercaptopyridine gave rise to a crystal structure containing hydrogenbonded zigzag chains.13 However, refluxing an ethanolic solution of 4-mercaptopyridine under nitrogen atmosphere followed by layering ether over the mother solution gave crystals of a new polymorph 1. Singlecrystal X-ray diffraction studies show that 1 has a hydrogen-bonded helix comprised of both right- and lefthanded helices. To investigate the influence of solvent and temperature in controlling polymorphic structure, we have crystallized 2 by diffusing ether into an ethanolic solution of 4-mercaptopyridine at 5 °C. Singlecrystal X-ray diffraction study shows that unit cell parameters and supramolecular structure of 2 are the same one reported by Etter and co-workers.13 DSC experiments and powder X-ray diffraction pattern of the samples at different temperatures (room temperature and at 100 °C) show that there is no phase transition between the two polymorphs. It appears that these two polymorphs may not be converted to one another in the solid state by the influence of temperature. Unfortunately, the IR spectrum and the melting point cannot distinguish these forms. Both compounds melt unusually in the temperature range 145-173 °C. Both the forms 1 and 2 crystallize in the monoclinic system (with cell choices P21/n and P21/c, respectively, as given in Table 1). The main difference is that there are two independent 4-pyridinethione molecules in the
10.1021/cg0498812 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/19/2004
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Muthu and Vittal Table 3. Hydrogen Bond Parameters in 1 D-H‚‚‚A
D-H/Å D‚‚‚A/Å H‚‚‚A/Å D-H‚‚‚A/°
N(1)-H(1)‚‚‚S(2) 0.83(2) 2.36(3) 3.183(1) N(2)-H(2)‚‚‚S(1) 0.93(2) 2.28(2) 3.198(1)
Figure 1. Asymmetric unit of 1. Table 1. Crystal Data for 1 and 2 compound formula formula weight T/K crystal system space group a/Å b/Å c/Å β/° V/Å3 Z µ(Mo KR)/mm-1 density g/cm3 θ range/° reflections collected independent reflections reflections observed R(int) R1 [I > 2σ(I)] wR2 [I > 2σ(I)] final diff. Fmax (e Å -3) GOF
1 C5H5NS 111.16 223(2) monoclinic P21/n 6.1082(4) 19.6249(14) 8.9246(6) 90.184(2) 1069.8(1) 8 0.458 1.380 2.08-30.05 8619 3023 2545 0.0198 0.0393 0.1021 0.377, -0.221 1.052
2 C5H5NS 111.16 223(2) monoclinic P21/c 7.2281(8) 6.1499(7) 11.6316(13) 90.449(2) 517.0(1) 4 0.474 1.428 2.88-30.0 3284 1335 1159 0.0223 0.0625 0.1441 0.091, -0.381 1.111
Table 2. Selected Bond Lengths(Å) and Angles(°) in 1 S(1)-C(3) S(2)-C(8) N(1)-C(1) N(1)-C(5) C(1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) N(2)-C(10) N(2)-C(6) C(6)-C(7) C(7)-C(8) C(8)-C(9) C(9)-C(10)
1.698(1) 1.703(1) 1.338(2) 1.338(2) 1.361(2) 1.142(2) 1.142(2) 1.362(2) 1.343(2) 1.346(2) 1.360(2) 1.417(2) 1.413(2) 1.362(2)
C(1)-N(1)-C(5) N(1)-C(1)-C(2) C(1)-C(2)-C(3) C(4)-C(3)-C(2) C(4)-C(3)-S(1) C(2)-C(3)-S(1) C(5)-C(4)-C(3) N(1)-C(5)-C(4) C(10)-N(2)-C(6) N(2)-C(6)-C(7) C(6)-C(7)-C(8) C(9)-C(8)-C(7) C(9)-C(8)-S(2) C(7)-C(8)-S(2) C(10)-C(9)-C(8) N(2)-C(10)-C(9)
120.6(1) 121.1(1) 120.9(1) 115.3(1) 121.4(1) 123.3(1) 121.1(1) 120.9(1) 121.0(1) 120.6(1) 121.1(1) 115.6(1) 122.6(1) 121.8(1) 121.1(1) 120.7(1)
asymmetric unit of 1 but only one present in the asymmetric unit of 2. The C-S and C-N bond lengths in 1 are similar to that of 2 and 2-pyridinethione, which shows the characteristic CdS and C-N bond lengths of the tautomeric thione form (Table 2).13,15 The N-H hydrogen atom of the pyridone ring is oriented toward the terminal sulfur atom of the adjacent ring as shown in Figure 1. Thus, both the molecules in the asymmetric unit are linked by N-H‚‚‚S hydrogen bonds. Dihedral angles between the two 4-pyridinethione rings are 67.6° and 8.8°.
169(2) 171(2)
symmetry operators 1/2 - x, 1/2 + y, 1/2 - z
As shown in Figure 2, 4-pyridinethione molecules are aligned around the spine of the helix via complimentary N-H‚‚‚S bonds along the b-axis, and the relevant hydrogen bond parameters are displayed in Table 3. It is noted that the helical motifs arising from hydrogen bonds alone, without the influence of metal atom, are rare.16,17 Solid-state structures of such hydrogen-bonded helical motifs self-assembled via N-H‚‚‚O bonds are common,17 but similar structures arising from relatively weak N-H‚‚‚S bonds are not known, at least to the best of our knowledge.11 Although the role of N-H‚‚‚S bonds in helical conformation of cysteine-containing proteins are known,18 self-assembly of a helical motif via single component N-H‚‚‚S bonds remains unexplored. A recent CSD survey by Allen and co-workers and our own survey reveal that N-H‚‚‚S hydrogen bonds fall in the region 2.2 to 2.5 Å.11,19 It is noted that although the graph set notation C(6) and N-H‚‚‚S bond parameters are the same in both the polymorphs (hydrogen bond parameters in 2: N‚‚‚S, 3.219(3) Å, H‚‚‚S, 2.42(4) Å, N-H‚‚‚S, 175(4)°),13 the overall structure of the polymorphs are different. In addition to the C(6) motif, neighboring helical strands in 1 are hydrogen-bonded via C-H‚‚‚S bonds, forming a 2D sheet approximately in the (103) plane as shown in the Figure 3. The interactions may be represented by the graphic notation R42(10).20 Hydrogen bond parameters (C-H‚‚‚S: H‚‚‚S, 2.89 Å, C‚‚‚S, 3.730(3) Å, C-H‚‚‚S, 160°) found in 1 are normal.11 A CSD search11 for C-H‚‚‚S bond lengths within the range of C-H‚‚‚S angles, 130-180° shows 124 hits below 2.9 Å. Further analysis of the structure reveals that the 4-pyridinethione rings from the adjacent helices are held together by π‚‚‚π interaction (Figure 4). 4-Pyridinethione rings in the adjacent helical strands are stacked in an offset arrangement with an angle of 22.8° between the ring normal of the 4-pyridinethione plane and centroid vector. The distance between the centers of the two adjacent 4-pyridinethione rings is 3.61 Å, which is well below the maximum distance of 3.8 Å for such interactions.21 However, both C-H‚‚‚S and π‚‚‚π interactions were not observed in the polymorph 2. A weak C-H‚‚‚π interaction (H‚‚‚centroid distance, 2.89(4) Å and θ, 131°) between the adjacent zigzag chains are noted in 2. The C-H‚‚‚π bond length is at the higher values of the accepted distance range (2.5-2.9 Å).22,23 It is obvious that better packing efficiency is achieved in the zigzag
Figure 2. (a, b) Two different views of a portion of the helical strand formed by N-H‚‚‚S bonds in 1.
New Polymorph of 4-Pyridinethione
Crystal Growth & Design, Vol. 4, No. 6, 2004 1183 Scheme 1. Formation of Two Different Polymorphs of 4-Pyridinethione
Figure 3. Diagram showing C-H‚‚‚S bonds between the adjacent helical strands in 1.
a helical conformation based on N-H‚‚‚S hydrogen bonds, while 2 has N-H‚‚‚S hydrogen bonded zigzag chains. The C-H‚‚‚S hydrogen bonds, R42(10) motif, between the neighboring chains and π-π interactions between the 4-pyridinethione rings of adjacent left- and right-handed helix further stabilize the structure of 1. Difference in the π-π interactions between the polymorphs is not reflected in solid-state emission spectra. Both the compounds exhibit intense emission bands at 390 nm due to π-π* transition. Experimental Section
Figure 4. Diagram showing π‚‚‚π interactions between the helical strands in 1.
Figure 5. Solid-state emission spectrum of (a) 1 and (b) 2.
arrangement of the molecule in 2 over the helical packing as indicated by the density differences (Table 1). Photophysical Properties of the Polymorph. Absorption spectrum of the 4-pyridinethione exhibits an absorption maximum at 340 nm in ethanol solution.14a,b Solid-state emission spectra of both compounds exhibit intense emission bands at 390 nm when excited at 330 nm (Figure 5). The emission bands are assigned to π-π* transition state. The difference in the emission spectra of 1 and 2 is not significant enough to attribute to the difference in the packing arrangements or π-π interactions observed in the polymorphs. Summary In summary, we have obtained a new polymorphic form of 4-pyridinethione by a different crystallization method as depicted in Scheme 1. The polymorph 1 has
4-Mercaptopyridine was purchased from Sigma-Aldrich. For crystallization, AR grade solvents were used as received without further purification. IR spectra (KBr pellet) of the compounds were recorded using Bio-Rad FT-IR spectrometer. DSC was performed under a flow of inert N2 gas (flow rate 90 mL/min) and heating rate 10 °C/min using a DSC-2920 thermal analyzer. X-ray powder diffraction patterns were recorded using a D5005 Bruker AXS X-ray diffractometer at 25 °C. UV-Vis absorption and emission spectra were recorded on a Hewlett-Packard HP8452A diode array spectrometer and a Perkin-Elmer LS-55 fluorescence spectrometer, respectively. Melting points of the compounds were recorded using a Hoover capillary melting point apparatus and were found to melt over the temperature range 145-173 °C. X-ray Crystallography. The diffraction experiments were carried out on a Bruker SMART APEX diffractometer equipped with a Mo-KR sealed tube and a CCD detector at -50 °C. The program SMART24 was used for collecting frames of date, indexing reflection and determination of lattice parameters, SAINT24 for integration of the intensity of reflections and scaling. SADABS25 was used for absorption corrections and SHELXTL26 for space group and structure determination and least-squares refinements on F2. The space groups were determined from the systematic absences, and their correctness was confined by successful solution and refinement of structures. Anisotropic thermal parameters were refined for all the non-hydrogen atoms. All the hydrogen atoms were located in the Fourier difference routine. Individual isotropic thermal parameters and positional parameters were refined for all the hydrogen atoms in 1 and 2.
Acknowledgment. We would like to thank The National University of Singapore for financial support (Grant. No. R-143-000-153-112). Supporting Information Available: Histogram of C-H‚‚‚S interactions, IR spectra of 1 and 2, a diagram showing the C-H‚‚‚π interaction in 2, and X-ray crystallographic information files (CIF) for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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