Polymorphs of Octaphenylcyclotetrasiloxane - American Chemical

Chapter 3

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date (Web): November 9, 2010 | doi: 10.1021/bk-2010-1051.ch003

Polymorphs of Octaphenylcyclotetrasiloxane Abhilasha M. Baruah, Anirban Karmakar, and Jubaraj B. Baruah* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039 Assam, India *email: [email protected]

Three polymorphs of octaphenylcyclotetrasiloxane are described in order to show an uncommon type of polymorphism in which the number of symmetry independent molecules are sequentially increased. The C-H··π interactions play a crucial role in the stability of the polymorphs and in these polymorphs two different types of end-to-face hydrogen bonding interactions play a crucial role in the packing pattern.

Introduction In order to understand the polymorphism occurring from variation of the number of symmetry independent molecules per unit cell (1) it is essential to identify suitable systems that exhibit polymorphism (2). It has been already shown that hydrogen bonding interactions are responsible for holding multiple numbers of symmetry nonequivalent units (3) in the unit cell. However, there are systems in which weak interactions such as C-H•••π interactions (4) becomes prominent which may also lead to polymorphs. It is already reported that polymorphism in siloxanes plays a major role in understanding their physical properties (5). In this study we have chosen a phenyl substituted cyclic siloxane, namely octaphenylcyclotetrasiloxane, (A) for understanding the role of C-H•••π interactions in polymorphs as this molecule has several phenyl groups oriented in different directions with a relatively simple structure and it can be easily prepared by different methods (6).

© 2010 American Chemical Society In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Experimental The crystallographic details and synthetic procedure for polymorph I and II of octaphenylcyclotetrasiloxane are also available in reference (6).


The Synthesis of Polymorph III of Octaphenylcyclotetrasiloxane A solution of diphenylsilane (0.18 gm, 1mmol) in ethanol/ allyl alcohol (3ml, 1:1mixture) were stirred with catalytic amount of tetramethylammoinumborohydride (0.088gm, 0.1mmol) 80°C for five minutes. The solution was cooled and kept standing for three days; white crystals of octaphenyl-cyclotetrasiloxane (30%). Crystal data for polymorph III: CCDC number 634266: Colorless blocks (0.48 x 0.32 x 0.20mm3), C48H40O4Si4, 793.16, Triclinic (P-1), a=10.7494(9)Å, b=20.969(2)Å, c=21.3035(19)Å, α = 66.788(5)°, β = 75.973(5)°, γ = 89.359(5)°, Z = 4,V = 4262.4(7)Å (3), ρcalcd = 1.236 Mgm-3, µ (MoKα) = 0.101mm-1, Goof = 0.972, Final R1 = 0.0879 for 20205 reflections of I > 2σ(I), R1 = 0.1693, wR2 = 0.2196 for all data.

Results A base catalyzed hydrolytic reaction of diphenylsilane leads to octaphenylcyclotetrasiloxane (A) (see Equation 1). The formation of different polymorphs (I-III as illustrated in Figure 1) of A is governed by the solvent used in the crystallization as well as reaction medium. For example the hydrolytic reaction of diphenylsilane in aqueous methanol gives polymorph II. The polymorph II is formed when such hydrolytic reaction and crystallization is carried out from mixture of allyl alcohol and ethanol. Similar reaction in aqueous acetonitrile (5%) gives the polymorph I.

20 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. The ORTEP of polymorphs of octaphenylcyclotetrasiloxane: polymorph (a) I (b) II and (c) III each are drawn with a 20% thermal ellipsoid. (Red = oxygen atoms; Pink = silicon atoms). Two polymorphs of compound A were reported earlier (7), but these polymorphs were not being discussed in detail. We have observed that by variation of the reaction medium and crystallization from different solvents, three polymorphs of octaphenylcyclotetrasiloxane can be obtained. The two already known polymorphs of octaphenylcyclotetrasiloxane crystallizes in a monoclinic, P2(1)/c (polymorph I) or a triclinic, P-1 (polymorph II) space group (7). Whereas the third polymorph reported here is also triclinic, P-1 (polymorph III) but has a different crystal dimension. The polymorph I, II and III have one, two and three symmetry independent molecules respectively, in their unit cells as illustrated in Figure 1. However, each of the polymorphs has characteristic features, for example the siloxane ring in the polymorph I has one type of rings that have two distinct types of Si-O-Si bond angles of 167.2° and 152.8° making the ring slightly distorted. In the polymorph II, the two rings of the two molecules are not equivalent. One of the rings is relatively symmetric with Si-O-Si bond angles 158.9° and 157.5°, whereas the other ring is distorted having two sets of Si-O-Si bond angles 165.4° and 152.5° respectively. The polymorph III has three symmetry independent cyclic units per unit cell; of which two rings resemble the rings of polymorph II; each ring having Si-O-Si bond angles 158.1°, 157.5° and 21 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


160.9°, 152.9° respectively; whereas the third ring has a set of bond angles 159.3° and 153.6°. Thus, it may be imagined that the polymorph III is arising from a combination of polymorph II and III. In all the three cases the cyclic siloxane molecules are held together by weak C-H··π interactions, but the orientation of the siloxane rings in the lattice makes the C-H groups of phenyl groups to orient in different manner in each lattices; making the packing pattern distinguishable. Among these the polymorph II is the most stable and upon recrystallisation from different solvents of I and III leads to formation of II. The crystal packing in each case is basically governed by end-face C-H··π interactions and two different types of such interactions are observed as illustrated in Figure 2. Linear chain like structures in the case of polymorph I are held by end-face interactions of the type 1 as illustrated in Figure 2. By such interactions a helical assembly is formed as shown in Figure 3a. In the case of polymorph II these interactions leads to three dimensional structure; whereas in the case of the polymorph III the molecules are held in multiple number of chains which incidentally crosses at certain point in a cross-wise fashion and thereby provide stability to the lattice. The weak interactions involved in the lattice are illustrated in Figure 3c. In the case of polymorph II the three dimensional network is formed by both the types of end-face C-H··π interactions shown in Figure 2. The three dimensional growth makes the system thermodynamically more favorable and is readily formed during crystallization and this polymorph is commonly observed during crystallisation. The polymorph III is governed by end-face C-H··π interactions of type 1 as shown in figure 2 and these interactions results in two dimensional structure. Since rings in some system are disordered we prefer to discuss it qualitatively. The IR spectra of the three polymorphs are indistingushable and is illustrated in Figure 4(i). The X-ray powder pattern of polymorphs of A formed from different crystallisation from different solvent such as aqeous methanol, ethanol, ethanol/allylalcohol mixture and also from acetonitrile are shown in Figure 4(ii)-(v). It is clear that the solvent has a significant role in the polymoph formation. In the case of methanol as solvent only polymorph I is formed, whereas the mixed solvent of ethanol and allyl alcohol leads to the formation of polymorph III as the major product, crystallisation from acetonitrile leads to I as major polymorph, whereas from ethanol a mixture of II and III are formed.

Figure 2. Two types of end-face C-H··π interactions



Figure 3. Packing in the part of the lattice of the polymorphs (a) I (b) II (c) III showing the weak interactions (black dashed lines) associated in packing. (see color insert)



Figure 4. (i) IR spectra of polymorphs (a) I, (b) II, (c) III in KBr (cm-1). The X-ray powder diffraction pattern from the octaphenylcyclotetrasiloxane obtained from (ii) methanol (iii) from ethanol (iii) mixed solvent of allylalcohol and ethanol (1:1) and (v) acetonitrile.


Conclusions In conclusion we have described here three polymorphs of octaphenylcyclotetrasiloxane arising from packing differences in the numbers of symmetry independent molecules in a unit cell having slight conformational difference in the rings.


Acknowledgments We thank the Department of Science and Technology, New Delhi, India for the X-ray diffraction facility used in this research.

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Polymorphs of Octaphenylcyclotetrasiloxane - American Chemical

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