CRYSTAL GROWTH & DESIGN
Stabilization of a Metastable Polymorph of 4-Methyl-2-nitroacetanilide by Isomorphic Additives He,*,†,‡
Stowell,*,†
Xiaorong Joseph G. Kenneth R. Hui Li,† G. Patrick Stahly,§ and Stephen R. Byrn†
Morris,†
Ralph R.
Pfeiffer,†
2001 VOL. 1, NO. 4 305-312
Department of Industrial & Physical Pharmacy, Purdue University, West Lafayette, Indiana 47907-1336, and Solid State Chemical/Pharmaceutical Information, Inc., West Lafayette, Indiana 47906-1076 Received October 28, 2000
ABSTRACT: 4-Methyl-2-nitroacetanilide (1) crystallizes in white (1W), amber (1A), and yellow (1Y) modifications. The isomorphic molecules 4-chloro-2-nitroacetanilide (2) and 2-nitro-4-trifluoromethylacetanilide (3) were synthesized, and their effects on the crystallization of 1 were studied. The percentages of an additive incorporated into the 1W, 1A, and 1Y crystal lattices were determined by HPLC. Compound 2 is incorporated as a solid solution in 1A up to levels of 30% (w/w), whereas the incorporation efficiency of 3 is much lower at the same doping level. From these results it can be assumed that 2 causes less disruption to the host lattices than does 3. At the same doping level of an additive, 1A incorporates the additive at a greater level than 1W or 1Y. As the incorporation level of an additive increases, both the solution and solid-state transformation rate from 1A to either 1W or 1Y decreases. Structural comparison of the 1W, 1A, and 1Y crystal lattices indicates that the additives may be least disruptive to the 1A lattice, therefore explaining the greater incorporation efficiency of an additive in 1A. Introduction Crystallization is the main method of purification in the pharmaceutical industry. However, these substances frequently crystallize in more than one packing arrangement; the resulting crystal forms are referred to as polymorphs.1 Polymorphs can differ in solubility, dissolution rate, stability, and mechanical properties and may exhibit different bioavailability behaviors.2 In cases where polymorphism may have an impact on bioavailability, the Food and Drug Administration (FDA) requires pharmaceutical manufacturers to control the crystallization of drugs so that the desired polymorph is produced consistently.3 Crystallization, however, is a complex process, and the ability to control the crystallization of polymorphic systems may be limited. Many factors can influence the crystallization of polymorphs.4 Such factors are related to the thermodynamic stability or to the nucleation and growth kinetics of the polymorphic forms. In many cases, kinetic factors control the outcome of a crystallization process. For the past 20 years, control of crystallization kinetics by tailor-made additives has been the subject of extensive research.5,6 Additives that are structurally similar to host molecules may be recognized by and incorporated to various extents into the host crystal lattice. Once incorporated, an additive may interact with incoming molecules, impeding the incorporation of these molecules into the crystal. Additive-host interactions may be exploited to carry out specific tasks, such as morphology control7 and kinetic resolution of chiral compounds.8,9 Davey et al. designed additives that * To whom correspondence should be addressed. Tel: X.H., (616) 833-6999; J.G.S., (765) 494-1460. Fax: X.H., (616) 833-7290; J.G.S., (765) 494-6545. E-mail: X.H.,
[email protected]; J.G.S.,
[email protected]. † Purdue University. ‡ Current address: Pharmacia, Inc., 7000 Portage Rd., Kalamazoo, MI 49001-0199. § Solid State Chemical/Pharmaceutical Information, Inc.
Figure 1. Chemical structures of the substances studied.
induced crystallization of a metastable form of R-glutamic acid.10 Compared to other applications, the use of additives to control crystallization of polymorphs is a recent development. The lack of understanding in this area motivated the present work to search for additives that may stabilize a metastable form. Herein is described the stabilization of a metastable crystalline form of 4-methyl-2-nitroacetanilide (1) by the isomorphic additives 4-chloro-2-nitroacetanilide (2) and 2-nitro-4-trifluoromethylacetanilide (3) (see Figure 1). Experimental Section Materials. 4-Methyl-2-nitroacetanilide (1) was prepared by heating a mixture of 400 g (2.63 mol) of 4-methyl-2-nitroanailine (Aldrich) and 800 mL (7.26 mol) of acetic anhydride (Fisher) at 70 °C for 30 min. The solution was poured into 4 L of water; the yellow solids that precipitated were recovered by filtration and recrystallized from 10% aqueous ethanol to give 276 g (54% yield) of 1 as fine yellow needles (mp 91.5 °C, which is the 1Y polymorph). Similar procedures afforded a 48% yield of 4-chloro-2-nitroacetanilide (2) as gold needles (mp 98.5 °C, lit.11 mp 99-100 °C) and 22% yield of 2-nitro-4-trifluoromethylacetanilide (3) as pale yellow needles (mp 112.0 °C, lit.12 mp 112-113 °C). Single-Crystal X-ray Structure Determinations. Data were obtained using Mo ΚR radiation (λ ) 0.71073 Å) on a Nonius KappaCCD instrument at ambient temperature. Structures were solved by direct methods using SIR-9713 and were refined using SHELX-97.14 Packing diagrams were generated
10.1021/cg0055225 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/21/2001
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He et al.
Table 1. Crystallographic Data compd (polymorph) color cryst syst space group Z a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) density (103 kg/m3) cell vol (Å3) free vol (Å3) packing efficiency (%)a a
1W
1A
1Y
2
3
white monoclinic P21/c 4 10.421 9.980 9.568 90.000 99.51 90.000 1.273 981.41 317.61 67.7
golden yellow monoclinic P21/c 4 10.158 11.635 8.041 90.000 94.55 90.000 1.319 947.36 241.91 74.5
yellow triclinic P1 h 4 17.956 12.908 4.039 93.13 83.71 90.77 1.345 929.09 229.36 75.3
dark golden yellow triclinic P1 h 2 6.9787 7.2242 9.2504 82.1947 88.9712 84.6954 1.549 460.06 119.4 74.0
pale yellow monoclinic P21/c 4 10.7687 4.9642 19.7385 90.000 92.106 90.000 1.563 1054.47 284.77 73.0
Calculated as (1 - (free volume)/(cell volume)) × 100%.
using WebLab ViewerLite (version 3.7) from Molecular Simulations, Inc.15 X-ray Powder Diffraction. Analyses were carried out on a Shimadzu XRD-6000 X-ray powder diffractometer equipped with a fine-focus X-ray tube using Cu ΚR radiation (1.5406 Å). The tube voltage and amperage were set at 30 kV and 30 mA, respectively. The divergence and scattering slits were set at 1°, and the receiving slit was set at 0.15 mm. Diffracted radiation was detected by a NaI scintillation detector. θ-2θ continuous scans at 2°/min (with a step size of 0.02°) from 4 to 35° in 2θ were used. The instrument was calibrated using silicon. Differential Scanning Calorimetry. Data were obtained on a TA 2920 instrument utilizing a 25 mL/min flow of helium as the purge gas. A 1.5 ( 0.05 mg amount of sample was heated in a sealed aluminum pan. Either intact crystals or ground samples were heated at a rate of either 5 or 20 °C/ min. The instrument was calibrated using mercury and indium. Hot-Stage Microscopy. Data were obtained on a Mettler FP52 stage equipped with a FP5 temperature controller mounted on a Zeiss polarized microscope. Equilibrium melting points were determined by oscillating the heating and cooling temperatures slightly within a narrow range such that solid and melt coexisted. Thus, the material slowly solidified at the lower temperature or slowly liquefied at the higher temperature, but not to completion at either limit. Solubility of 1W, 2, and 3. The solubility of 1W in 50% aqueous ethanol was measured at 15.5, 20.5, 25.5, and 30.0 °C, and the solubilities of 2 and 3 were measured at 25.5 °C. The temperature was controlled within 0.2 °C by a programmable circulating bath (Model 9510, Fischer Scientific, Pittsburgh, PA). Solvent was equilibrated at the test temperature, and samples were filtered through 0.45 µm membrane filters (Varian, Nylon-66, Walnut Creek, CA). The concentration of solution was monitored with a Beckmann DU 640 UV/vis spectrophotometer at 234 nm (Beckmann, Fullerton, CA) until a plateau was reached. Crystallization of 1 with Additives. Additives 2 and 3 were mixed with 1 at various ratios. Crystallizations of these mixtures from 50% aqueous ethanol were studied under two sets of conditions. In case 1, 40 mg/mL solutions of 1 were prepared at elevated temperature and cooled to ambient temperature in an uncontrolled fashion. In case 2, 20 mg/mL solutions of 1 were prepared at elevated temperature, filtered through a 0.45 µm Nylon-66 membrane (Varian, Walnut Creek, CA), heated at 70 °C to remove/or reduce nuclei, and cooled to ambient temperature in an uncontrolled fashion. In both cases, crystals were collected on a Bu¨chner funnel and air-dried. The crystal form was checked by XRPD and DSC; the incorporation level of additives in 1 was measured by HPLC. In case 2, the effects of seeding with selected forms (0.5% w/w) were also studied. High-Performance Liquid Chromatography. Data were obtained on a Rainnen system consisting of two HPXL pumps, a Rheodyne injector, and a Dynamax UV detector interfaced
to a Macintosh SE computer. A reverse-phase C18 column (5 µm, length 250 mm, i.d. 4.6 mm, Altech, Inc., Deerfield, IL) was used to separate 1, 2, and 3. The mobile phase consisted of 50:50 acetonitrile/water (v/v), the injection volume was 10 µL, the flow rate was 1 mL/min, and the detector was set at 234 nm. A good baseline resolution was obtained with 1, 2, and 3, with the three compounds eluting at 6.10, 7.43, and 9.45 min, respectively. HPLC standard curves were constructed for 1, 2, and 3. The percentage of additives in 1 is reported as the percentage of 2 or 3 in the total weight of the material. Measurement of Additive Homogeneity in 1. To measure the homogeneity of additive incorporated in 1, crystals were placed on a 2 mL fritted funnel (Pyrex) and serially washed with 0.5 mL of aqueous acetonitrile; the material from each wash was collected and analyzed individually. The ratio of acetonitrile in water varied from 5% to 20%, such that the resulting concentration of dissolved material was within the linear range of the calibration curves developed for the HPLC analysis. About four to five washes were needed to dissolve all of the material; each wash was isolated and analyzed by HPLC. Thus, the impurity distribution from the surface to the core of the crystal could be analyzed. Structural Analysis. Electrostatic charges of the atoms in the crystal lattices of 1W, 1A, 1Y, 2, and 3 were determined using Cerius2 version 4.0.16 Utilizing the atomic coordinates from the single-crystal structures, the atom positions of a given molecule were standardized while the conformation was constrained. The resulting molecule was assigned the appropriate Dreiding 2.21 force-field type and then charged using the charge-equilibrium option. Cerius2 version 4.0 also provided geometry measurements and basic crystal property information (e.g., free volume, packing efficiency, etc.) of the various compounds and crystal forms studied.
Results and Discussion 4-Methyl-2-nitroacetanilide (1) was chosen as a model compound in part because it was found to exist in three color polymorphs: white (W), amber (A), and yellow (Y).17,18 Furthermore, the crystal structures of each of these polymorphs are known.19,20 Some of the crystallographic data obtained are reproduced in Table 1, and packing diagrams are shown in Figure 2. The isomorphous compounds 2 and 3 were selected as additives having the potential to influence the crystallization of 1 because of their structural and size similarity to 1. The aromatic substituents para to the acetanilide group in these three compounds are of similar size but different electronegativities. Approximate van der Waals radii are CH3 (2.0 Å), Cl (1.80 Å), and CF3 (2.15 Å);21 approximate electronegativities are CH3 (2.3 eV), Cl (3.03 eV), and CF3 (3.35 eV).22 Both 2
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Figure 2. Stereoview packing diagrams for 1W, 1A, 1Y, 2, and 3.
and 3, which are known compounds,11,12 were synthesized by acylation of the appropriately substituted aniline. Approximately 20-100 mg amounts of crystalline samples of 2 and 3 were produced by evaporation and cooling of a series of solutions using a wide variety of solvents (e.g., methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, isobutyl alcohol, 1-pentanol, isoamyl alcohol, acetone, methyl ethyl ketone, ethyl acetate, N,N-dimethylformamide, dimethyl sulfoxide, methyl tert-butyl ether, tetrahydrofuran, 1,4-dioxane, cyclopentane, cyclohexane, hexanes, heptane, benzene, o-xylene, dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, nitrobenzene, triethylamine, and pyridine). X-ray powder diffraction (XRPD) analyses of the samples indicated that each crystallization of 2 produced the same crystalline form; the same observation was true for each crystallization of 3. The crystal structures of 2 and 3 were solved using crystals grown by slow evapo-
ration of 50% aqueous ethanol solutions. Representative crystallographic data are shown in Table 1, and packing diagrams are shown in Figure 2. Observed XRPD patterns of 2 and 3 as well as those of the polymorphs of 1 are shown in Figure 3. The melting points of 1W, 1Y, 2, and 3 (see Table 2) were determined by the equilibrium technique described in the Experimental Section. Because 1A rapidly transforms to 1W or 1Y upon heating, the melting point and enthalpy of fusion (∆Hf) of 1A could not be accurately determined. Although it was reported in the literature that 1A has the lowest melting point and ∆Hf value among these three polymorphs,23 the results should be taken with caution, since different heating rates were used in the literature references for the different polymorphs. From our crystallization studies, it is possible to extrapolate the ∆Hf of pure 1A from the crystals containing additives. Since incorporation of 2 into 1A seems to have little impact on the ∆Hf value of 1A (Table
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He et al. Table 3. Selected Torsion Angles (deg)
Figure 3. X-ray powder diffraction patterns of 1W, 1A, 1Y, 2, and 3. Table 2. Thermal and Solubility Data polymorph (°C)a
mp mp (°C)b ∆Hf (J/g)c ∆Hf (J/g)b S at 25.5 °Ce order of stability
1W
1A
1Y
2
3
93.0 96.3 124.9 (0.51) 127.7 16.50 (0.17)
81-86 92.1 106-110d 110.4
91.0 93.9 115.2 (3.6) 115.4
98.5
112.0
8.52
4.98 (0.12)
most
least
intermed
a Equilibrium melting point determined by hot-stage microscopy in this study. b From ref 23. c ∆Hf or enthalpy of fusion was determined by DSC measurements at heating rates of 20 °C/min. Numbers in parentheses are standard deviations. d Extrapolated from ∆Hf of 1A crystals containing various amounts of 2 (see Table 5). e Solubility of 1W fits the linear van’t Hoff equation ln CS ) 17.7 - 4.44T-1 (T is in Kelvin, CS in mg/mL).
2), ∆Hf for pure 1A is estimated to be 106-110 J/g. This is consistent with the literature23 in that the stability order of the 1 polymorphs is 1W (most stable) > 1Y (intermediate) > 1A (least stable) at all temperatures. The system is monotropic on the basis of the heat of fusion rule.24 Note that, in most cases, a more stable form will have a greater density,24 but this is not the case for 1 (see Table 1). Despite their structural similarity to 1, the crystal structures of 2 and 3 are distinctly different from each of the 1 polymorphs. Polymorphs 1W and 1A and compound 3 belong to the monoclinic space group P21/ c. There are four molecules in each unit cell, each having the same conformation (one molecule per asymmetric unit). Form 1Y and compound 2 belong to the triclinic space group P1 h . Form 1Y has four molecules in the unit cell, but in two different conformations (two molecules per asymmetric unit), designated 1Y1 and 1Y2. Compound 2 has two molecules in the unit cell with one molecule per asymmetric unit. Form 1W has an intermolecular hydrogen bond (C-O‚‚‚H-N, 1.947 Å) between molecules related by c-glide planes, forming chains along the c direction. Forms 1A and 1Y, and compounds 2 and 3, have intramolecular hydrogen bonds (N-H‚‚‚O-N-O) ranging from 1.949 to 1.977 Å in length. Molecules in the form 1A unit cell are nearly parallel to one another, forming layers, which are held together by dipole-dipole interactions between closely spaced antiparallel carbon-
bonds
1W
1A
1Y1
1Y2
2
3
C9-C8-N1-C1 O1-C8-N1-C1 H7-N1-C1-C2 O2-N2-C2-C1
172.1 8.3 152.3 42.5
176.1 2.9 0.1 8.7
178.9 1.6 15.5 12.6
175.6 4.4 13.5 18.5
172.4 7.9 5.9 1.4
173.4 6.3 7.1 20.7
yl groups or between closely spaced antiparallel nitro groups. Like form 1A, compound 2 molecules pack in antiparallel sheets, with molecules rotated 180° in alternating layers. Because 2 and 3 contain halogen atoms, their densities are larger than any of the 1 forms. Forms 1A and 1Y, and compounds 2 and 3, exhibit similar packing efficiencies ranging from 73.0% for 3 to 75.3% for 1Y. The packing efficiency of 1W (67.7%) is significantly less than for the other forms (see Table 1) probably because the nitro group in 1W, being twisted out of the plane of the phenyl ring, provides a steric barrier to the close packing of adjacent molecules. The packing efficiency is defined as25
packing efficiency )
ZVm × 100% Vc
where Z is the number of molecules in the unit cell, Vm is the volume of one molecule, and Vc is the volume of the unit cell. According to Kitaigorodskii’s close-packing principle,25 a polymorph that is packed more tightly (efficiently) is more stable. Compound 1 is an exception to the closepacking principle, since form 1W is the most stable form but has the lowest packing efficiency (see Table 1). The presence of hydrogen bonding in the crystal lattice can and will cause deviations from the close-packing principle, as is the case for 1. Each crystalline form of 1, 2, and 3 studied differs in molecular conformation, specifically in the coplanarity of the acetanilido and nitro groups. Relevant torsional angles are shown in Table 3. The acetanilido groups are nearly coplanar with the aromatic rings in 1A, 1Y, 2, and 3, in that the out-of-plane twist of the C1-N1 bonds and the N1-C8 bonds are less than 18° in each case. In these structures, the amide hydrogen atom (H7) is intramolecularly hydrogen bonded to the nearest oxygen atom of the nitro group. In 1W, the amide hydrogen atom points away from the nitro group and is intermolecularly hydrogen bonded to the carbonyl oxygen atom of an adjacent molecule. Furthermore, the acetanilido group is approximately 28° out of coplanarity with the aromatic ring. The coplanarity of the nitro group and the aromatic ring varies in the order 2 (1.4°), 1A (8.7°), 1Y (12.6 and 18.5°), 3 (20.7°), and 1W (42.5°). The fact that 1W exists with both the acetanilido and nitro
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Crystal Growth & Design, Vol. 1, No. 4, 2001 309
Table 4. Crystallization of 1 from 50% Aqueous Ethanol at a σ Value of 2.5a additive
doping level (%)
none
0
2
3
form
incorporation level (%)
stability of 1Aa
3.4 in 1Y 7.5 in 1A; 5.0 in 1W 10 20 30
10 h to 5 days 1-5 days 2-4 weeks 2-4 weeks
1Y
2 5 7.5 10 20 30 50 75 80 90
1Y or 1W 1Y or 1W 1A or 1W 1A 1A 1A 1A and 2 2 2 2
1 2 5 10 20
1Y 1Y 1Y 1A 1A and 3
90 (or 9.7% of 1) 94 (or 6% of 1) 97 (or 2.8% of 1) undetectable
