Conformational Isomorphism - American Chemical Society

Feb 1, 2010 - †Department of Chemistry, University of Coimbra, Portugal, ‡Faculty of ..... (14) Barriau, E.; Cormack, P. A. G.; Daly, J. H.; Ligga...
6 downloads 0 Views 5MB Size
Download PDF
DOI: 10.1021/cg901160v

Polymorphism of trans-1,4-Cyclohexanediol: Conformational Isomorphism

2010, Vol. 10 1194–1200

Teresa M. R. Maria,*,† Ricardo A. E. Castro,‡ Suse S. Bebiano,† M. Ramos Silva,§ A. Matos Beja,§ Jo~ ao Canotilho,‡ and M. Ermelinda S. Eusebio*,† †

Department of Chemistry, University of Coimbra, Portugal, ‡Faculty of Pharmacy, University of Coimbra, Portugal, and §CEMDRX, Department of Physics, University of Coimbra, Portugal

Received September 22, 2009; Revised Manuscript Received November 27, 2009 w This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. n

ABSTRACT: In this work, an investigation on the polymorphism of trans-1,4-cyclohexanediol is performed. Polymorph screening is carried out by crystallization from solutions, by sublimation, and by cooling the melt, and a combined approach using differential scanning calorimetry, polarized light thermomicroscopy, and X-ray diffraction is employed in the results interpretation. Three solid forms, I, II, and III (Steiner et al. J. Chem. Soc., Perkin Trans. 2 1998, 371-377), are identified, I and II, for the first time, and their relative stability established. The crystal structure of polymorph II is resolved by single crystal X-ray diffraction: a monoclinic P21/a space group structure. This polymorph, as polymorph III (Steiner et al. J. Chem. Soc., Perkin Trans. 2 1998, 371-377), exhibits unusual conformational isomorphism, that is, the coexistence of the biequatorial and the biaxial conformers in the same crystal structure.

1. Introduction Dihydroxyl cyclohexane derivatives make up a class of compounds which bear structural features that make them interesting candidates to perform research on the correlations between molecular conformation and crystal structure. In fact, despite being simple molecular species, they may adopt various conformations, although the cyclohexane chair form is by far the most stable and the equatorial substituent orientation is the most favorable.2-4 For trans-1,2, cis-1,3, and trans-1,4-diols, the biequatorial chair forms are expected to be preferred relative to the biaxial ones. For the cis-1,2, trans-1,3-, and cis-1,4- isomers, one of the OH groups is always in the axial position.2-4 Some work exists on the solid-state characterization of the vicinal cyclohexanediols,5-11 and interesting differences were found between the isomers. For the pure enantiomeric forms of trans-1,2-cyclohexanediol, a crystalline structure belonging to the hexagonal system, made up of biequatorial molecules,6 and a metastable phase were identified.8 An orthorhombic crystal is described for the cis-1,2-isomer,5 and a cubic plastic crystal phase, enantiotropically related to the crystalline one, was also identified.7,8 For 1,4-cyclohexanediols, a 2:1 cis:trans cocrystal was recently described in which the trans isomer is present as the biequatorial conformer.12 In this work, following our interest in cyclohexane derivatives, a search for trans-1,4-cyclohexanediol polymorphic forms was carried out taking the pure compound as the starting material, and its thermal behavior was investigated from room temperature to fusion. One interesting crystalline structure was reported by Steiner and Saenger,1 where biequatorial and biaxial conformers coexist. Single crystals of this solid form were obtained by slow evaporation of an ethanolic solution of a mixture of the cis-1,4 and trans-1,4-isomers. *To whom correspondence should be addressed. E-mail: [email protected] (J.C.); [email protected] (J.C.). pubs.acs.org/crystal

Published on Web 02/01/2010

Besides the importance that dihydroxyl cyclohexane derivatives may have in a fundamental research perspective, they also find numerous applications in the chemical and in the pharmaceutical industries. trans-1,4-Cylohexanediol, for instance, is used as an active component in the treatment of rosacea, seborrheic dermatitis, or facial dermatitis,13 in the synthesis of biodegradable cycloaliphatic polyesters for medical purposes,14,15 and as an intermediary in the synthesis of pharmaceuticals with activity as anticancer agents.16 2. Experimental Procedures Materials. trans-1,4-Cyclohexanediol was acquired from YickVic Chemicals & Pharmaceuticals (H. K.) Ltd., China, purity greater than 98% and purified by sublimation at T = 90 °C and p = 103 Pa, using the coldfinger technique with cold water as the freezing fluid. Polymorph Search Methods. The search for polymorphs was carried out by crystallization from solution using apolar (1,2-dichloroethane, dichloromethane), polar aprotic (tetrahydrofurane, ethylacetate), and polar protic solvents (ethanol, methanol). Sublimation experiments were also performed, in the conditions referred to in the previous section and, in addition, from a cylindrical stainless steel box (1 cm diameter, 1 cm height) whose metallic cover has an orifice (3 mm diameter), covered with a glass slide or with a thermomicroscopy glass cell, at T = 75 °C and ambient pressure. Cooling the molten compound in a differential scanning calorimeter at different cooling rates was the other methodology used in the polymorph search assays. Differential Scanning Calorimetry (DSC). DSC experiments were performed using a Perkin-Elmer DSC7 calorimeter, with an intracooler cooling unit at -10 °C (ethylenglycol-water 1:1 v/v cooling mixture). The samples were hermetically sealed in aluminum pans, and an empty pan was used as reference. A 20 mL/min nitrogen purge was employed. Temperature calibration17,18 was performed with high grade standards, namely, biphenyl (CRM LGC 2610, Tfus = 68.93 ( 0.03 °C) and indium (Perkin-Elmer, x = 99.99%, Tfus = 156.60 °C). Enthalpy calibration was performed with indium (ΔfusH= 3286 ( 13 J/mol).17 DSC curves were analyzed with Pyris software version 3.5. Polarized Light Thermal Microscopy (PLTM). A DSC600 hot stage Linkam system, with a Leica DMRB microscope and a Sony r 2010 American Chemical Society

Article

Crystal Growth & Design, Vol. 10, No. 3, 2010

CCD-IRIS/RGB video camera were used. A 7 mm diameter glass cell was used. The images were obtained by combined use of polarized light and wave compensators, using a 200 magnification. Real Time Video Measurement System software by Linkam was used for image analysis.19 Single-Crystal X-ray Diffraction (SXD). A Bruker-Nonius Kappa Apex II CCD diffractometer using graphite monochromated Mo K R radiation (λ = 0.71073 A˚) was employed. Direct methods and conventional Fourier syntheses (SHELXS-97) were used to solve the structures, and the refinement was carried out by full matrix leastsquares on F2 (SHELXL-97).20 All non-H-atoms were refined anisotropically. The H atoms positions were initially placed at idealized calculated positions and refined with isotropic thermal factors while allowed to ride on the attached parent atoms using SHELXL-97 defaults. The fractional atomic coordinates, displacement parameters, and other supplementary data have been deposited at the Cambridge Crystallographic Data Centre (CCDC No. 747527 for trans-1,4cyclohexanediol). X-ray Powder Diffraction (XRPD). A glass capillary was filled with the powdered specimen. The samples were mounted on an ENRAF-NONIUS powder diffractometer (equipped with a CPS120 detector by INEL) and data were collected for 5 h using Debye-Scherrer geometry. Monochromatized Cu K R1 radiation was used (λ = 1.540598 A˚). Potassium aluminum sulfate dodecahydrate was chosen as an external calibrant.

3. Results and Discussion Crystallization experiments from solutions gave rise in all cases, as confirmed by DSC experiments, to the already published trans-1,4-cyclohexanediol crystalline structure,1 from now on called polymorph III. In the following sections,

1195

the results obtained in the sublimation experiments and by cooling the melt are discussed. Table 1. Crystal Data and Structure Refinement Parameters for trans-1,4-Cyclohexanediol Polymorph II crystal data empirical formula formula weight temperature/K wavelength/A˚ crystal system space group a/A˚ b/A˚ c/A˚ β/° volume/A˚3 Z calculated density/g cm-3 absorption coefficient/mm-1 FOOO crystal form, color crystal size/mm θ range for data collection/° index ranges reflections collected/unique completeness to θmax refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I )] R indices (all data) largest diff peak and hole /e A˚-3

C6H12O2 116.16 293(2) 0.71073 monoclinic P21/a 7.7980(8) 8.9221(8) 9.3291(9) 96.633(7) 644.72(11) 4 1.197 0.088 256 prism, colorless 0.24  0.13  0.07 2.20-28.07 -10 < h < 10, -11 < k < 11, -12 < l < 12 1548/1135 [R(int) = 0.0631] 99.1% full-matrix least-squares on F2 1548/0/75 1.090 R1 = 0.0631 wR2 = 0.1250 R1 = 0.0934 wR2 = 0.1402 -0.179/0.216

Figure 1. ORTEPII diagrams for trans-1,4-cyclohexanodiol: (a) polymorph II; (b) polymorph III.1 The displacement ellipsoids are drawn at the 50% level.

Figure 2. Packing diagrams of trans-1,4-cyclohexanodiol: (a) polymorph II; (b) polymorph III.1

1196

Crystal Growth & Design, Vol. 10, No. 3, 2010

Maria et al.

Table 2. Torsion Angles (°) of the Molecules of Polymorphs II and IIIa polymorph II axial molecule

equatorial molecule

polymorph III O1-C1-C2-C3i C3-C1-C2-C3i O1-C1-C3-C2i C2-C1-C3-C2i O2-C4-C5-C6ii C6-C4-C5-C6ii O2-C4-C6-C5ii C5-C4-C6-C5ii

66.9(2) -53.5(2) -69.2(2) 53.8(2) -175.8(2) -55.5(3) 178.3(2) 55.7(3)

equatorial molecule

axial molecule

a

O1-C1-C2-C3 C6-C1-C2-C3 O1-C1-C6-C5 C2-C1-C6-C5 C1-C2-C3-C4 C2-C3-C4-O2 C2-C3-C4-C5 O2-C4-C5-C6 C3-C4-C5-C6 C4-C5-C6-C1 O3-C7-C8-C9iii C9-C7-C8-C9iii O3-C7-C9-C8iii C8-C7-C9-C8iii C7-C8-C9iii-C7iii

177.4(2) 56.7(2) -178.5(2) -56.3(2) -56.9(2) 177.7(2) 56.9(2) -179.1(2) -56.2(2) 55.7(2) 66.7(2) -54.6(2) -64.8(2) 54.5(2) 55.4(2)

Symmetry codes: (i) -x, -y þ 1, -z þ 1; (ii) -x, -y, -z þ 2; (iii) -x, -y, -z.

Figure 3. Hydrogen bond details in (a) polymorph II; (b) polymorph III.1

Table 3. Hydrogen Bond Details for trans-1,4-Cyclohexanediol Polymorphs II and III H-bond polymorph II III

D-H 3 3 3 A/A˚ O1-H1 3 3 3 O2i O2-H2 3 3 3 O1ii O2-H2 3 3 3 O1iii O1-H1 3 3 3 O3iv O3-H3 3 3 3 O2v

D 3 3 3 A/A˚ 2.732(2) 2.714(2) 2.729(2) 2.748(2) 2.714(2)

Figure 4. DSC heating run and PLTM images of trans-1,4-cyclohexanediol polymorph III; m = 1.90 mg; β = 10 °C/min.

H 3 3 3 A/A˚ 1.91 1.91 1.83 1.84 1.78

D-H 3 3 3 A/° 177 168 176 176 173

symmetry code i: 1/2 - x, 1/2 þ y, 1 - z ii: -x, 1 - y, 1 - z iii: x, y, 1 þ z iv: x, y, -1 þ z v: 1 þ x, y, z

Figure 5. DSC heating/cooling cycle (a, b) of a trans-1,4-cyclohexanediol sample, illustrating the reversible character of polymorph III f polymorph I transition; (c) DSC heating curve obtained after the cooling run (b); m = 2.07 mg; β = 10 °C/min.

Article

Crystal Growth & Design, Vol. 10, No. 3, 2010

1197

Figure 6. PLTM images of a heating run of a mixture of trans-1,4-cyclohexanediol III and II polymorphs. Magnification 200; β=10 °C/min.

Figure 7. PLTM images of a heating run of a mixture of trans-1,4-cyclohexanediol III and II polymorphs, showing transition of form II to form I at T ∼ 119 °C. Magnification 200; β = 10 °C/min.

3.1. Polymorphs Obtained by Sublimation. The solid material obtained by sublimation at 90 °C and 103 Pa was identified by powder X-ray diffraction as polymorph III. Sublimation carried at T = 75 °C and ambient pressure gave rise to a new polymorphic form, II, which may be identified by its globular-like crystalline habit, most often concomitantly with polymorph III, whose crystalline habit is tabular. Structural Characterization. Good quality single crystals of both form II and form III were obtained by sublimation in the stainless steel box. The crystalline structure of II was determined. Crystallographic data are presented in Table 1, and the ORTEPII21 and the packing diagrams for this new polymorph II and for polymorph III are shown in Figures 1 and 2, respectively.

Polymorph II as well as polymorph III exhibits conformational isomorphism:22-24 both the biequatorial and the biaxial conformers are part of the crystal structure. In a search of the Cambridge Structural Database, September 2009, for trans-1,4-disubstituted cyclohexanes no other structures comprising both conformers could be found. trans-1,4-Cyclohexanediol seems indeed to be, as stated by Steiner,1 “a very exceptional case”. trans-1,4-Cyclohexanediol form III contains one and a half molecules in the asymmetric unit, one of them being the biequatorial conformer and the other, centrosymmetric, the biaxial one (proportion 2:1); the new polymorph II has two half molecules in the asymmetric unit and again both axial and equatorial conformations are present (proportion 1:1).

1198

Crystal Growth & Design, Vol. 10, No. 3, 2010

In both polymorphs, the six-membered rings exhibit chair conformations. The C-C-C-C and C-C-C-O torsion angles are given in Table 2. The differences in packing of the molecules in both polymorphs arise from the different H-bonds networks. Hydrogen bond details are given for both polymorphs in Figure 3 and in Table 3. In the polymorph II, there is a threedimensional O-H 3 3 3 O hydrogen bond network with each biequatorial molecule linked only to biaxial molecules and vice versa. The dihedral angle between the least-squares planes of the carbon atoms of both type of molecules (C1, C2, C3, C1i, C2i, C3i, i: -x, -y þ 1, -z þ 1) and (C4, C5, C6, C4ii, C5ii, C6ii, ii: -x, -y, -z þ 2) is 77.9(1)°. The dihedral angle between nearest axial-axial molecular planes (C1, C2, C3, C1i, C2i, C3i, i: -x, -y þ 1, -z þ 1) and (C1iii, C2iii, C3iii, C1iv, C2iv, C3iv, iii: x - 1/2, -y - 1/2, z, iv: -x þ 1/2, y - 1/2, 1 - z) is 86.4(1)°. In form III,1 the biequatorial molecules are joined in chains by O-H 3 3 3 O hydrogen bonds. Those chains are interconnected by H-bonds involving the biaxial conformer, joining the molecules in layers parallel to the ac plane. The angle between the least-squares planes of the carbon atoms of both type of molecules (C1, C2, C3, C4, C5, C6) and (C7, C8, C9, C7v, C8v, C9v, v: -x, -y, -z) is 37.6(1)°. In this polymorphic form, all the axial molecules have their mean planes parallel and so do all the equatorial molecules. Thermal Behavior. A typical DSC heating curve of form III, at a scanning rate β = 10 °C/min, is shown in Figure 4. Two endothermic transitions are observed; the first one, a solid-solid transition, as confirmed by PLTM (Figure 4), takes place at Ttrs = (78.7 ( 0.6) °C with transition enthalpy 4trsH = (1.5 ( 0.1) kJ/mol: on heating, polymorph III transforms into a new solid form, I, which melts at Tfus = (141.3 ( 0.2) °C with enthalpy of fusion, 4fusH=(21.5 ( 0.2) kJ/mol. The numeric values presented are the mean of six independent experiments, and the uncertainty is expressed by one standard deviation. After the III f I transition, polymorph I reverts to form III, on cooling, with an undercooling of about 25 °C, at β = 10 °C/min, as illustrated in Figure 5. From these results, and according to Burger and Ramberger heat of transition rule,25,26 it is concluded that polymorphs III and I are enantiotropically related with transition temperature TIIIfI = 78.6 °C. It was not possible to generate an appropriate amount of pure polymorph II in order to perform DSC experiments. Nevertheless, PLTM experiments could be carried out on mixtures of forms III and II. PLTM results obtained on the heating process of such a mixture are shown in Figure 6. For polymorph II fusion is the single phase transition observed at T = 138 °C. In this experiment, the sublimation of polymorph II at temperatures above 100 °C and concomitant growth of the other solid form are obvious. All transformations are clearly evident in the video movie recorded during this experiment and included as Video 1 available in the HTML version of the paper. The monotropic relationship between phases II and I may be evidenced from PLTM observations carried out in another mixture and shown in Figure 7. In this figure, the growth of crystal I is clearly visible from deposition of material sublimed from crystal II. At about 118 °C, crystal I touches crystal II which is converted to polymorph I. All crystals melt at the same temperature, T ∼ 141 °C. Video 2 recorded in this experiment is available in the HTML version of the paper.

Maria et al.

Figure 8. Schematic molar Gibbs energy versus temperature diagram, at atmospheric pressure, of trans-1,4-cyclohexanediol polymorphs III, II, and I.

Figure 9. DSC cooling curves of molten trans-1,4-cyclohexanediol samples: (a) m = 1.90 mg; (b) m = 2.53 mg; (c) m = 1.92 mg; (d, e) m = 2.07 mg; (f) m = 1.88 mg; cooling rate 10 °C/min.

Figure 10. DSC traces of trans-1,4-cyclohexanediol samples obtained in the heating runs following the corresponding cooling scans shown in Figure 9; (a) m=1.90 mg; (b) m=2.53 mg; (c) m=1.92 mg; (d, e) m = 2.07 mg; (f) m = 1.88 mg; heating rate 10 °C/min.

Article

Crystal Growth & Design, Vol. 10, No. 3, 2010

1199

Figure 11. Images observed by polarized light thermomicroscopy in a heating run of a trans-1,4-cyclohexanediol sample, crystallized by cooling the melt to 25 °C; magnification 200; heating rate 10 °C/min.

The thermal behavior study of the outcomes of the sublimation processes allowed us to identify three trans-1,4cyclohexanediol polymorphic forms and to establish their relative stability, as summarized in the molar Gibbs energy versus temperature diagram shown in Figure 8. 3.2. Polymorphs Generated by Cooling the Melt. Cooling molten trans-1,4-cyclohexanediol was carried out at different scanning rates ranging from 1 °C/min to nominal 50 °C/min. A particular behavior could not be ascribed to a specific thermal program. For instance, as illustrated in Figure 9, for a cooling rate of 10 °C/min, the thermogram may be more or less complex with crystallization peaks at temperatures that may range from 120 to 85 °C. Occasionally, a small peak at 55 °C exists, that is ascribed to the I f III transition. DSC traces recorded in the heating runs following the cooling steps shown in Figure 9 are presented in Figure 10. From the thermograms obtained, it is evident that crystallization of molten trans-1,4-cyclohexanediol often gives rise to mixtures of polymorphic forms - also illustrated in PLTM images shown in Figure 11. Moreover, from repeated experiments on the same sample, it could be concluded that the results are more dependent on the particular sample, most likely due to the presence of heteronuclei,27 than on the cooling rate of the preceding run. The presence of polymorph III is obvious, for instance, in the b, c, d, and f samples (Figure 10); also the melting peaks at ∼139 °C or ∼141 °C in samples b-f can be ascribed to forms II and I, respectively. A more complex behavior is shown in samples a-c. From the complexity of these thermograms, the existence of other solid forms may not be discharged. 4. Conclusions The results obtained in this work show that trans-1,4-cyclohexanediol presents polymorphism. Two new polymorphic

forms, I and II, were unequivocally identified, with close melting temperatures, Tfus, I = 141 °C and Tfus, II = 138 °C, being form II monotropic relatively to form I. The crystalline structure of polymorph II was resolved by single crystal X-ray diffraction. In its crystalline structure biaxial and biequatorial conformers coexist, as it was also found in the structure of the trans-1,4-cyclohexanediol polymorph III.1 These are interesting examples of conformational isomorphism. Form III was straightforwardly obtained in the present work either by sublimation at reduced pressure and T ∼ 90 °C, or by crystallization from solutions. From the thermal behavior study performed on form III, an enantiotropic relationship to form I is recognized, Ttrs = 79 °C. The results of the crystallization from the melt experiments indicate that other polymorphic forms are likely to exist. Acknowledgment. We are grateful to FEDER/POCI 2010 for financial support. Supporting Information Available: X-ray crystallographic information files (CIF) are available free of charge via the Internet at http://pubs.acs.org.

References (1) Steiner, T.; Saenger, W. J. Chem. Soc., Perkin Trans. 2 1998, 371– 377. (2) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry Structure and Function; W.H. Freeman and Company: New York, 2002. (3) Morris, D. G. Stereochemistry; The Royal Society of Chemistry: Cambridge, UK, 2001. (4) Juarist, E. Conformational Behavior of Six-Membered Rings, Methods in Stereochemical Analysis; VCH Publishers: New York, 1995. (5) Sillanp€a€a, R.; Leskel€a, M.; Hiltunen, L. Acta Chem. Scand. 1984, 38B, 249–254. (6) Hanessian, S.; Gomtsyan, A.; Simard, M.; Roelens, S. J. Am. Chem. Soc. 1994, 116, 4495–4496.

1200

Crystal Growth & Design, Vol. 10, No. 3, 2010

(7) Maria, T. M. R.; Costa, F. S.; Leit~ao, M. L. P.; Redinha, J. S. Thermochim. Acta 1995, 269, 405–413. (8) Leit~ ao, M. L. P.; Castro, R. A. E.; Costa, F. S.; Redinha, J. S. Thermochim. Acta 2001, 378, 117–124. (9) Maria, T. M. R.; Eusebio, M. E. S. J. Chem. Eng. Data 2008, 53, 1316–1320. (10) Leit~ ao, M. L. P.; Eusebio, M. E. S.; Maria, T. M. R.; Redinha, J. S. J. Chem. Thermodyn. 2002, 34, 557–568. (11) Lloyd, M. A.; Patterson, G. E.; Simpson, G. H.; Duncan, L. L.; King, D. P.; Fu, Y.; Patrick, B. O.; Parkin, S.; Brock, C. P. Acta Crystallogr., Sect. B: Struct. Sci. 2007, 63, 433–447. (12) Loehlin, J. H.; Lee, M.; Woo, C. M. Acta Crystallogr., Sect. B: Struct. Sci. 2008, 64, 583–588. (13) Silny, W.; Chomczy nski, P.; Czarnecka-Operacz, M.; Da nczakPazdrowska, A.; Silny, P. Post. Dermatol. Alergol. 2005, 22, 271– 277. (14) Barriau, E.; Cormack, P. A. G.; Daly, J. H.; Liggat, J. J.; Quincy, A. Eur. Cell. Mater. 2002, 4, 100. (15) Giavaresi, G.; Tschon, M.; Daly, J. H.; Liggat, J. J.; Fini, M.; Torricelli, P.; Giardino, R. Int. J. Artif. Organs 2004, 27, 796– 805. (16) ElSohly, M. A.; Ross, S. A.; Galal, A. M. Dihydroartemisinin and dihydroartemisitene dimers as anti-cancer and anti-infective agents.

Maria et al.

(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27)

United States Patent US7098242-B2, US 2004/0266860-A1, 30 Dec 2004. Sabbah, R.; An, X. W.; Chickos, J. S.; Leit~ao, M. L. P.; Roux, M. V.; Torres, L. A. Thermochim. Acta 1999, 331, 93–204. Della Gatta, G.; Richardson, M. J.; Sarge, S. M.; Stolen, S. Pure Appl. Chem. 2006, 78, 1455–1476. Linksys, version 2.35; Linkam Scientific Instruments Ltd.: Surrey, UK, 2000. Sheldrick, G. M. SHELXS97 & SHELXL97; Instit€ut f€urAnorganische Chemie der Universit€at: Gottingen, Germany, 1997. Johnson, C. K. ORTEPII; Oak Ridge National Laboratory: Tennessee, USA, 1976. Bernstein, J. Polymorphism in Molecular Crystals; Clarendon Press: Oxford, 2002. Bilton, C.; Howard, J. A. K.; Madhavi, N. N. L.; Nangia, A.; Desiraju, G. R.; Allen, F. H.; Wilson, C. C. Chem. Commun. 1999, 1675–1676. de Castro, R. A. E.; Canotilho, J.; Barbosa, R. M.; Silva, M. R.; Beja, A. M.; Paix~ao, J. A.; Redinha, J. S. Cryst. Growth Des. 2007, 7, 496–500. Burger, A.; Ramberger, R. Microchim. Acta 1979, 2, 259–271. Burger, A.; Ramberger, R. Microchim. Acta 1979, 2, 273–316. Mullin, J. W. Crystallization; Butterworth-Heinemann: Oxford, 2001.