Stable and metastable crystal phases of 4-methylbenzophenone

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 374 (1996) 129-135

Stable and metastable crystal phases of 4-methylbenzophenone H. Kutzke*, M. AI-Mansour, H. Klapper Mineralogisch-Petrologisches Institut, Universitiit Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany

Received 17 February 1995;accepted in final form 28 April 1995

Abstract

4-Methylbenzophenone crystallizes in a monoclinic stable form (mp 59°C) with a = 5.70,4,, b = 13.89 .~, c = 14.08 A, = 95.18° and space group P21/c, and a trigonal metastable form (mp 55°C) with a = 9.12 ,~, c = 11.28,4, and space groups P31 or P32. The metastable phase is obtained only in highly supercooled melts at -25 ° to -35°C. It can undergo a monotropic phase transition into the stable form. Large and optically homogeneous crystals of both phases were grown from melts slightly supercooled below their melting temperatures. The structures of both crystals were determined by X-ray diffraction methods. The metastable phase contains molecules of only one chirality, packed into columns along three-fold screw axes. The centrosymmetric stable structure is racemic. Both structures and their different nucleation properties in supercooled melts are discussed.

1. Introduction

With increasing interest in the growth and application of organic crystals, the investigation of polymorphism of these materials has increased in importance. Different modifications of one compound exhibit quite different physical properties. They are also of interest for the study of the relation between structure and properties of crystals and for chemical applications (e.g. for the separation of enantiomers). An introduction into this subject with a comprehensive list of references is given by Bernstein [1,2]. Besides benzophenone, 4-methylbenzophenone was one of the first organic compounds for which polymorphism was observed. It crystallizes in two modifications, the stable monoclinic a-form (mp 59°C) and a metastable trigonal * Corresponding author.

~-form (mp 55°C) [3]. Crystals of both forms were obtained and studied as early as the 1870s [3-6]. The monotropic character of the phase transformation /3 ~ a was also recognized. This transformation may be initiated by contact with a crystal of the stable phase or by mechanical stress, or it may occur spontaneously. In 1876, Bodewig [7] described the morphology and optical properties of both phases in the first investigation of an organic metastable crystal. Bodewig also mentioned the significant pyroelectricity of the metastable form and the high optical dispersion of the stable form. The nucleation of the various phases of benzophenone and 4-methylbenzophenone was studied by Schaum [8,9], who heated the melt for a few hours up to 250°C and cooled it down to about - I 0 ° to -30°C. After a period of a few minutes to a few hours, crystals of the metastable phase nucleated frequently and grew in this highly supercooled, viscous melt. Based on numerous observations of

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H. Kutzke et al./Journal o f Molecular Structure 374 (1996) 129-135

60°C Nucleation of the stable phase only 0*C Nucleation of the stable phase favoured -25oc Nucleation of the metastable phase favoured .45oc , No nucleation -70oc Vitreous state Fig. 1. Nucleationbehaviour of 4-methylbenzophenone.Note that the temperaturerangesare not sharplydefined. the nucleation after various pre-treatments of the melt, Schaum concluded that during the heating cycle the molecules change their shape from an a-form into a E-form. During cooling into the supercooled state, the/3-form was presumed to be preserved and to induce the nucleation of the metastable phase. Since Schaum did not know the nature of the transformation, he called the phenomenon "cryptochemical polymorphism". Later investigations of the nucleation of stable and metastable benzophenone by Melnik et al. [10] showed that the pre-treatment of the melt has no influence on the nucleation of both phases and that metastable nucleation requires a degree of supercooling. Unfortunately, these authors gave no details about crystal growth, quality or size. The aim of the present study is to determine the crystal structures of both phases of 4-methylbenzophenone and to clarify their molecular and structural relationship. We also describe the growth of large crystals of both modifications. The structure of the stable a-phase, first determined in 1987 by Ito et al. [11], was redeterminated and refined in this work to a high accuracy for reliable comparison of the two structures.

the stable a-form, suitable for structure determination, were easily obtained by precipitation from ethanolic solution or from the supercooled melt. Crystals of a few millimetres diameter, to be used as seed crystals for the growth of large single crystals, were obtained in the same way. For the preparation of the metastable form, the material was melted in a sealed glass tube heated to I00-150°C. After a few hours, the melt was supercooled down to a temperature between -25°C and -35°C. Within a few minutes spontaneous nucleation of the metastable E-form occurred. At higher temperatures, between -20°C and +59°C, nucleation of the stable a-form was favoured. At lower temperatures, below about -45°C, nucleation or growth of crystals was not observed (see Fig. 1). The influence of pre-heating the melt on the nucleation of the metastable phase, as stated by Schaum [8,9], could not be confirmed by our studies. For metastable nucleation, it is crucial to meet the favoured temperature range. In this respect 4-methylbenzophenone behaves similarly to benzophenone [10]. The fine polycrystalline material obtained by the above nucleation procedure was placed in the melt supercooled by 5 to 10°C below the melting point (55°C) of the metastable E-form. The small grains grew to needles of the metastable phase, fractions of which were used for structure determination. Larger crystals of 4-6 mm diameter, appropriate for seed crystals, were obtained after longer periods of growth. Both phases were characterized by powder diffraction patterns [12]. Large single crystals of both forms, suitable for the investigation of physical properties, were grown from slightly supercooled melts at 0.1 to 0.5°C below the melting point. This was simply done by suspending the seed crystal with a thin thread into the supercooled temperature-controlled melt. Large and optically homogeneous single crystals of several centimetres diameter were obtained within a growth period of 2-7 days.

2. Experimental

2.2. Structure determination 2.1. Nucleation and crystal growth 4-Methylbenzophenone (Merck) was purified by recrystaUization from ethanol. Small crystals of

The structure determinations of both modifications were carried out on a RIGAKU AFC6R four-circle diffractometer at room temperature,

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H. Kutzke et al./Journal of Molecular Structure 374 (1996) 129-135

(010)

(ool)

/

(011)

H15

~

(100)

HI0

HI6~ Cla CI0£ H I ~ ~

H9

~_____C . 4 CS

a)

02)

Fig. 3. Molecule. of 4-methylbenzophenone with labelling scheme.

structures were solved by direct methods. Fullmatrix least-squares refinement on F for all data was performed for the non-hydrogen atoms. The positions of the hydrogen atoms were refined isotropically. All calculations were done using the program package TEXSAN [13]. Figures of molecular structure and packings were drawn with ORTEe[14].

b) Fig. 2. Typical morphologies of 4-methylbenzophenone crystals, grown from supercooled melt: (a) stable a-form; (b) metastable E-form.

using Mo K a radiation with a graphite monochromator. Intensity data were measured by w/20 scanning mode. Data reduction was carried out using Lorentz and polarization corrections but neglecting absorption and extinction effects. The

3. Results and discussion

3.1. Crystal growth Following the procedures described in section 2, large single crystals of both forms were grown from

Table 1 Crystal data and some details of the measurement

Molecular formula Molecular weight (kDa) Crystal system Space group a b c a /3 7 Unit-cell volume Volume per molecule Dc Z No. of independent reflections 20ma x

Rint

R/Rw

Stable form

Metastable form

CI4HI20 196.2 monoclinic P21/c 5.701 (1) 4 13.890 (2) A 14.077 (2) A 90.00° 95.18 (2)° 90.00° 1110.25 (30) A) 277.56 ~3 1.173 4 3357

C14HI20 196.2 trigonal P31 or P32 9.217 (3) .~ 9.217 (4) tk 11.294 (2) 90.00° 90.00° 120.00 ° 829. I 1 (54) ,~3 276.37 ,A,3 1.179 3 1049 43° 2.8% 2.9/2.1%

60 ° 2.0% 4.2/3.3%

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H. Kutzke et al./Journal of Molecular Structure 374 (1996) 129-135

Table 2 Selected bond lengths and angles (e.s.ds in parentheses) Stable form 0(1)..-C(I) C(1)... C(2) C(1)... C(8) C(I 1)-.-C(14)

Metastable form

1.219 (2)/~. 1.488 (3) ,~. 1.475 (3) ,~ 1.501 (4) ,~

Distances betweenthe ring carbon atoms C-..H O(1) ... C(1)... C(2) O(1)... C(1)... C(8) C(2)... C(7)..- C(6) C(10)...C(ll)-..C(14)

1.3-1.4 ,~ about 1.0 ,~ 119.2° (2) 120.5° (2) 120.6° (3) 121.7° (3)

Tilt angle betweenthe phenyl rings

1.223 (2) ,~, 1.482 (3) ,/k 1.480 (3) ,~, 1.496 (4) ,~

63°

119.8° (2) 119.0° (2) 121.1° (3) 121.4° (3) 58°

Table 3 Final atomic coordinates and equivalent isotropic displacement parameters B of stable a-4-methylbenzophenone(B = 8/37r2 ~-~qE j Vii a~. a; ai aj) Atom

x/a

y/b

z/c

beq (A.2)

O(1) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(I1) C(12) C(13) C(14) H(3) H(4) H(5) H(6) H(7) H(9) H(10) H(12) H(13) H(14) H(15) H(16)

0.2762(3) 0.2781(4) 0.1444(4) 0.2305(5) 0.1096(7) -0.0992(6) -0.1880(6) -0.0672(5) 0.4093(4) 0.3336(4) 0.4500(5) 0.6488(4) 0.7243(4) 0.6061(4) 0.788(1) 0.378(3) 0.185(4) -0.180(4) -0.336(4) -0.127(4) 0.197(3) 0.387(3) 0.859(3) 0.655(3) 0.794(6) 0.711(6) 0.902(5)

0.1126(1) 0.1072(2) 0.0287(2) -0.0186(2) -0.0964(2) -0.1240(2) -0.0771(2) -0.0020(2) 0.1782(2) 0.2069(2) 0.2785(2) 0.3215(2) 0.2916(2) 0.2225(2) 0.3994(3) 0.001(1) -0.124(2) -0.179(2) -0.097(2) 0.033(2) 0.176(1) 0.299(1) 0.319(1) 0.205(1) 0.457(3) 0.415(2) 0.373(2)

0.6669(I) 8.5(1) 0.7534(2) 5.2(1) 0.7960(2) 4.3(1) 0.8778(2) 5.5(1) 0.9113(2) 6.8(2) 0.8641(3) 7.2(2) 0.7845(3) 6.6(2) 0.7495(2) 5.3(1) 0.8153(2) 4.2(1) 0.9013(2) 4.9(I) 0.9544(2) 5.3(1) 0.9247(2) 5.0(1) 0.8391(2) 5.3(1) 0.7846(2) 5.0(1) 0.9819(3) 8.4(2) 0.907(1) 5.0(6) 0.965(2) 7.8(8) 0.887(2) 8.4(8) 0.751(2) 8.2(8) 0.689(2) 7.4(7) 0.923(1) 5.7(6) 1.013(1) 6.2(6) 0.816(1) 5.9(6) 0.724(1) 6.2(6) 0.948(2) 16(1) 1.035(2) 12(1) 1.091(2) 11(1)

supercooled melts within periods of 2 to 7 days. Optically perfect crystals with lengths up to 7 cm were obtained; Fig. 2(a) shows the typical morphology of the monoclinic stable form. It is dominated by prisms {011}, followed (with decreasing morphological significance) by the pinacoids {010}, the prisms {110}, {111}, {120} and the pinacoids {001} and {100}. The metastable form crystallizes in the trigonal system. The typical shape of crystals grown from supercooled melts, shown in Fig. 2(b), is dominated by the trigonal prism {0116} and the trigonal pyramids { 10i 1} and {01 ii}, followed by the prism {1010} and the minor pyramids {1102} and {10il}. This morphology suggests the ditrigonal pyramidal class 3m, as reported by the early authors [3,7]. X-ray investigations, however, prove the existence of a three-fold right or left-handed screw axis. Thus the trigonal pyramidal class 3 is correct. The metastable fl-form is rather stable and almost indefinitely preserved if it is not subjected to severe mechanical load or brought into contact with the stable form. If it converts into the stable form, the crystal becomes milky white and non-transparent due to micro-cracking. There are no conditions known under which the/3-form is more stable than the a-form. 3.2 C r y s t a l structures

The crystallographic data of both phases are

H. Kutzke et al./Journal of Molecular Structure 374 (1996) 129-135

133

collected in Table 1. The densities of the two forms are nearly equal, with the density of the metastable phase 0.5% higher than that of the stable form.

Table 4 Final atomic coordinates and equivalent isotropic displacement parameters B o[ metastable B-4-methylbenzophenone

Molecular shape

Atom

x/a

y/b

z/c

0(1) c(I) c(2) c(3) c(4) c(5) c(6) c(7) c(8) c(9) C(lO) c(11) c(12) c(13) c(14) H(3) H(4) H(5) H(6) H(7) H(9) H(10) H(12) H(13) H(15) H(14) H(16)

0.3358(2) 0.2175(3) 0.2329(2) 0.1532(3) 0.1803(3) 0.2843(4) 0.3627(4) 0.3392(3) 0.0607(2) -0.0961(3) -0.2392(3) -0.2339(3) -0.0774(3) 0.0665(3) -0.3908(5) 0.071(3) 0.125(3) 0.306(2) 0.444(3) 0.394(3) -0.105(2) -0.344(2) -0.072(2) 0.181(3) -0.461(4) -0.437(5) -0.353(6)

0.3526(2) 0.2123(3) 0.0629(3) -0.0813(3) -0.2141(3) -0.2053(5) -0.0653(5) 0.0682(4) o. 1954(2) 0.0604(3) 0.0544(3) o. 1778(3) 0.3102(3) 0.3210(3) 0.1705(6) -0.090(3) -0.323(3) -0.299(3) -0.051(3) 0.166(3) -0.031(2) -0.043(3) 0.393(2) 0.414(3) 0.082(5) 0.252(5) 0.240(6)

1.0507 7.37(6) 1.0315(2) 4.98(8) 1.0572(2) 4.79(8) 0.9896(2) 5.5(1) 1.0085(3) 6.8(1) 1.0993(3) 7.5(1) 1.1684(3) 7.6(1) 1.1475(3) 6.5(1) 0.9824(2) 4.29(8) 1.0095(2) 4.89(9) 0.9680(2) 5.2(1) 0.8951(3) 5.37(9) 0.8668(3) 5.6(1) 0.9105(2) 5.3(1) 0.8517(4) 8.4(2) 0.937(2) 5.8(5) 0.953(2) 8.9(6) 1.114(2) 6.4(5) 1.235(2) 9.0(6) 1.192(2) 6.1(5) 1.061(1) 4.3(4) 0.990(2) 5.4(5) 0.811(2) 5.9(5) 0.895(1) 6.1(5) 0.850(3) 12(1) 0.913(3) 18(1) 0.810(5) 16(2)

In both modifications the molecules have the same shape, with bond distances and bond angles close to those of other, comparable compounds (Table 2). The apparent hydrogen positions of the methyl group are not reasonable, probably because of a statistical disorder of this group. The crucial parameter determining the shape of the molecule is the torsion of the two phenyl groups. The tilt angle between the planar phenyl groups is 63 ° for the stable and 58 ° for the metastable modification. This asymmetry results in the existence of two enantiornorphous molecules related by a symmetry centre or a reflection plane. The labelled molecule is shown in Fig. 3.

Structure ofthe stable phase The positional parameters of atoms as provided by the structure refinement are given in Table 3. Fig. 4 shows the packing of the molecules in the structure projected along the a-axis. In this structure (space group P21/c), both enantiomorphic molecules, related by the symmetry centre (e.g. in the comers

G

•

"°

-

(B = 8/3 7r2Ei ~'~4Uija~ a~ aiai) Beq (,~2)

of the unit cell) and by the glide plane, are combined and arranged in columns along the a axis. Neighbouring columns are connected by symmetry centres or by two-fold screw axes.

Structure of the metastable phase

O"q"&o ~..0 0 o

cFg

The positional parameters of atoms are presented in Table 4. The packing of the molecules is shown in Fig. 5. In contrast to the stable phase, molecules of only one enantiomer are involved, forming a right- or left-handed screw arrangement corresponding to the space groups P31 or P32, respectively.

3.3. Discussion Fig. 4. Structure of stable 4-methylbenzophenone(space group P21/c) projectedalong the a-axis. The monoclinicaxis b is horizontal. Dark atoms: oxygen.Hydrogen atoms are omitted.

It is well known that compounds with enantiomeric molecules can form chiral as well as

134

H. Kutzke et al./Journal of Molecular Structure 374 (1996) 129-135

a)

Fig. 5. Structure of metastable 4-methylbenzophenone (space group P31 ) projected parallel (a), and normal (b), to the trigonal screw axis. Dark atoms: oxygen. Hydrogen atoms are omitted.

centrosymmetric racemic crystal structures. Kress et al., for example, studied the two forms of binaphthyl [15]; this exists in a lower melting (mp 145°) racemic modification (sp#ce group C2/c) and in a higher melting (mp 159°) chiral one (space group P41212 or P43212). In contrast to 4-methylbenzophenone, here the chiral form is the stable one. Brock et al. [16] report a list of 129 pairs of chiral and racemic crystals and compare their density and stability. The relative density differences range from 0.1 to 5% with an average of 1%. In our case, the density change of 0.5% is close to the lower limit of this range. Moreover, 4-methylbenzophenone does not obey the rule of Wallach (1895) [17], which postulates that racemic crystals tend to be denser than their chiral counterparts. This is frequently observed for enantiomers that racemize rapidly, forming a mixture containing both enantiomers in equal parts; 4-methylbenzophenone belongs to this group of compounds. During the rapidly proceeding monotropic solid-state transformation at room temperature from the metastable to the stable phase, 50% of the molecules switch to the other enantiomorphic state. From this it can be concluded that the energy barrier of switching is low and is thermally activated at normal and higher temperatures. At lower temperatures (e.g. -30°C), the switching of the molecules is presumed to be frozen in. This may be the essential cause for the quite different nucleation behaviour of the stable and metastable phases of 4-methylbenzophenone in supercooled melt, as shown in Fig. 1. A theoretical treatment applying

b) Fig. 5.

molecular modelling and energetic considerations is in progress.

Acknowledgements The authors would like to thank A. Katrusiak, Poznafi, and Mrs. K. Marthi, Copenhagen, for helpful advice about the literature. We thank also B. Barbier, Bonn, for his help in powder diffractometry.

References [1] J. Bernstein, J. Phys. D, 26 (1993) B66. [2] J. Bernstein, J.B. Garbarczyk and D.W. Jones (Eds.), Organic Crystal Chemistry, IUCr/Oxford University Press, Oxford, 1991, pp. 6-26. [3] P. Groth, Chemische Krystallographie, Band V, W. Engelmann, Leipzig, 1919, p. 124. [4] M. Kollarits and V. Merz. Ber. Dtsch. Chem. Ges., 6 (1873) 536. [5] Plascuda and Th. Zincke, Ber. Dtsch. Chem. Ges., 6 (1873) 908. [6] A. Behr and W.A. van Dorp, Ber. Dtsch. Chem. Ges., 6 (1873) 754. [7] C. Bodewig, Ann. Phys., 158 (1876) 235. [8] K. Schaum, Chem.-Z., 23 (1914) 257. [9] K. Schaum, Ann. Chem., 411 (1915) 161. [10] V.I. Melnik, K:I. Nelipovich, M.T. Shpak, Izv. Akad. Nauk. SSSR, Ser. Fiz., 44(4) (1980) 827. [11] Y. Ito, T. Matsuura, K. Tabata, M. Ji-Ben, K. Fukuyama, M. Sasaki and S. Okada, Tetrahedron, 43 (1987) 1307. [12] H. Rose, E. Lilly, IUPAC Data f-de 33-1754. A proposal for a data file of the metastable phase is in preparation.

H. Kutzke et al./Journal of Molecular Structure 374 (1996) 129-135

[13] Molecular Structure Corporation, TEXSAN:Single Crystal Structure Analysis Software, Version 5.0, Molecular Structure Corporation, The Woodlands, TX 77 381, 1989. [14] C.K. Johnson, ORTEP-n,Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976.

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[15] R.B. Kress, E.N. Duesler, M.C. Etter, I.C. Paul and D.Y. Curtin, J. Am. Chem. Soc., 102 (1980) 7709. [16] C.P. Brock, W.B. Schweizer and J.D. Dunitz, J. Am. Chem. Sot., 113 (1991) 9811. [17] O. Wallach, Justus Liebigs Ann. Chem., 286 (1895) 90.