Charge-Transfer Adducts of N-Methylthiazolidine-2-thione with IBr and I2: An Example of Polymorphism Featuring Interpenetrating Three-Dimensional Subcomponent Assemblies and Halogen‚‚‚π‚‚‚Ηalogen Weak Interactions
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 7 1284-1290
M. Carla Aragoni,‡ Massimiliano Arca,‡ Francesco Demartin,† Francesco A. Devillanova,‡ Thomas Gelbrich,§ Alessandra Garau,‡ Michael B. Hursthouse,*,§ Francesco Isaia,‡ and Vito Lippolis*,‡ Dipartimento di Chimica Inorganica ed Analitica, UniVersita` degli Studi di Cagliari, S.S. 554 biVio per Sestu, 09042 Monserrato (CA), Italy, Dipartimento di Chimica Strutturale e Stereochimica Inorganica, UniVersita` di Milano, Via G. Venezian 21, 20133 Milano, Italy, and School of Chemistry, UniVersity of Southampton, Highfield, Southampton SO17 1BJ, UK ReceiVed September 28, 2006; ReVised Manuscript ReceiVed December 21, 2006
ABSTRACT: Two polymorphic forms of the 1:1 charge-transfer adduct between N-methylthiazolidine-2-thione and IBr have been isolated: their structural analyses by a recently reported procedure for comparison of multiple crystal forms of the same molecular structure reveal some interesting geometrical 3D similarity relationships that can be interpreted in terms of different sets of halogen ‚‚‚π‚‚‚halogen weak interactions. The 1:1 charge-transfer adduct with I2 has also been isolated and structurally characterized and proven to be isostructural with one of the two polymorphic forms of the IBr adduct. Introduction Multiple crystal forms (polymorphs, cocrystals, and solvates) are of widespread current interest due to their recognized importance in a wide variety of research areas such as drug development in the pharmaceutical industry and in optoelectronic and pigment materials.1-18 In recent years, research in this field has been particularly well supported by improved methods of structure determination using X-ray diffraction techniques and other sophisticated analytical tools (microscopy, IR, Raman, and solid-state NMR spectroscopies, and thermal analysis). Furthermore, a better and more comprehensive access to the chemical literature has made the comparison of crystal and molecular structural features among compounds easier and has enhanced the understanding of physical-chemical characteristics.19-23 However, despite the development of automated highthroughput methods for the preparation and monitoring of large numbers of simultaneous or sequential crystallization experiments,1,24 the rational design of new molecular solid-state assemblies, by applying “crystal engineering” principles based on hydrogen- or coordination-bonding patterns,8,9,25-27 and the cultivation of new crystal forms or the control of the production of old ones, are still challenges. In all cases, the battle is against the general natural tendency for a given compound to crystallize in a particular preferred way. On the other hand, polymorphic forms are often discovered accidentally during the course of crystallization of new compounds for routine structural characterization. In this paper, we report the discovery of a rare example of polymorphism among charge-transfer adducts of chalcogen donors with di-halogens/inter-halogens. We also describe the use of a recently developed procedure for comparison of multiple crystal forms of the same molecular structure that has * To whom correspondence should be addressed. E-mail:
[email protected],
[email protected]. ‡ Universita ` degli Studi di Cagliari. † Universita ` di Milano. § University of Southampton.
enabled the identification of some intricate geometrical threedimensional (3D) relationships among the structures. Subsequent, detailed investigation of these features indicated that they can be interpreted in terms of different sets of halogen‚‚‚π ‚‚‚halogen weak interactions in the crystal structures. Results and Discussion Syntheses and X-ray Diffraction Analyses. After many attempts to crystallize the 1:1 charge-transfer adduct between N-methylthiazolidine-2-thione (1) and IBr, using different solvent mixtures and crystallization techniques (mainly slow evaporation or diffusion at different temperatures of vapor of Et2O into a solution containing 1 and IBr in a 1:1 molar ratio), we finally obtained, by slow evaporation at room temperature from a CH2Cl2/petroleum ether (60-80 °C) mixture, two kinds of crystals of different color and morphology in the same crop (Supporting Information, Figure S1). A careful examination convinced us that the different colors observed for the two kinds of crystals were determined by a different internal electronic structure rather than by a different thickness or morphology. Furthermore, although on the basis of microanalytical data both kinds of crystals corresponded to the formulation 1‚IBr, they showed somewhat different FT-Raman spectra (Supporting Information, Figure S2). Taking into consideration the chemistry of chalcogen donor molecules with di-halogens and interhalogens, it seemed therefore more likely that the two kinds of crystals isolated corresponded to two different compounds rather than to two crystal forms of the same molecular adduct.
Indeed, it is well-known that the reactions of di-halogens (I2, Br2) and inter-halogens (IBr, ICl) with organic molecules
10.1021/cg0606545 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007
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Figure 1. ORTEP view of the independent adduct molecule 1‚IBr in the asymmetric unit of form I with the numbering scheme adopted. Displacement ellipsoids are drawn at the 50% probability level. The same numbering scheme is adopted for the independent adduct molecules in the structures of 1‚IBr (form II) and 1‚I2.
containing Group 16 donor atoms (LE; L ) organic framework, E ) S, Se, Te) can follow a variety of pathways depending on both the acid/base nature of the reactants and the experimental conditions used (solvents and reaction molar ratios).28-32 X-ray diffraction analysis on the two kinds of crystals (Figure S1, Supporting Information) revealed the formation of two polymorphic forms of the same 1:1 charge-transfer “spoke” adduct 1‚IBr. Both structures have the same space group Cc but differ in the number of molecules per asymmetric unit (form I, yellow-orange blocks: Z ) 4; form II, red-brown blocks: Z ) 12). This is a rare example of polymorphism among neutral charge-transfer “spoke” adducts of chalcogen donor ligands with di-halogens and inter-halogens.33 Interestingly, the 1:1 I2 adduct of 1, 1‚I2, obtained from a CH2Cl2/CCl4 mixture (1:1 v/v) by slow evaporation, proved to be isostructural to form I of 1‚IBr. Adduct formation between 1 and I-Y (Y ) I, Br) involves a charge-transfer process from the lone pairs on the exocyclic sulfur atom of the donor molecule to the σ/u LUMO of the dihalogen/inter-halogen molecule lying along its main axis, with a consequent lengthening of the I-Y bond distances. In 1‚IBr (both polymorphic forms) and 1‚I2, the independent adduct molecules in the asymmetric units feature an almost linear S-I-Y system (Y ) I, Br) and an almost coplanar arrangement of the di-halogen/inter-halogen molecule with respect to the penta-atomic ring of the thiazolidine moiety, thus avoiding steric hindrance with the N-Me fragment (Figure 1 and Table 1). Following the discovery of the two polymorphs, we set out to compare and understand the packing in the crystal structures, with the aim of establishing possible explanations for their formation.
Structural Analyses Using the Computer Program XPac. In general, polymorphs and related solid forms of one particular compound, and often, the different crystal structures of compounds with similar molecular structures, may contain similar component assemblies such as identical isolated units, differently packed, identical chains or sheets, differently bundled or stacked, respectively.34-36 An objective and rigorous assessment of such structural features can be very difficult since it should take into account not only intermolecular bonding interactions between contributing functional groups (e.g., H-bonding, π-π interactions, coordination bonding) but also less directional intermolecular interactions, such as diffuse Coulombic or van der Waals forces, which cannot be localized on structurally recognizable molecular sites and therefore are more difficult to “pinpoint” structurally. In this respect, the development of new methodologies able to search for and quantify, in a general way, geometrical similarities in multiple crystal forms of the same molecular structure, independently of connectivity similarities, is an important goal. Very recently, we have described such a procedure and developed a versatile computer program, XPac,34 that achieves this objective. In the present study, the comparison of the two polymorphic forms of the adduct 1‚IBr (forms I and II) was greatly aided by the use of this program that searches for, and quantifies, similarities in relative geometrical arrangements in the packing of molecules in different crystal forms. Any similar arrangements occurring in more than one structure are identified as “supramolecular constructs”, or SCs. These recurring arrangements are considered to merit further attention as possible subassembly constituents, whether or not they are built using specific “supramolecular synthons” via specific intermolecular bonding interactions. It is important to keep in mind the difference between the concept of an SC and that of “supramolecular synthon” employed in crystal engineering. An SC exists only if a common spatial arrangement of molecules is present in two (or more than two) crystal structures, regardless of the nature of intermolecular interactions within these fragments of crystal structures. As currently used by the crystal engineering community, the term “supramolecular synthon” focuses instead explicitly on how molecules are linked through definable interatomic interactions. Thus, while a particular synthon could be present in an SC, the same synthon can exist in fundamentally different spatial arrangements, and SCs may assemble without involvement of identifiable synthons. Full technical details of the way in which the program works are described in ref 34, but it is useful here to summarize some general aspects of the XPac procedure to see the way in which a geometrical similarity is identified. In a crystal structure, the arrangements formed by each central independent molecule (kernel) and its closest neighbors (shell) constitute a cluster (a crystal structure with Z′ independent molecules has Z′ crystallographically independent clusters that can always be generated once the symmetry operations, unit cell parameters, and atomic coordinates are known). From another point of view, any crystal structure can be portioned, in various ways, into individual subassemblies of molecules (dimers, chains, sheets, 3D nets). As for the entire structure, a representative cluster consisting
Table 1. Selected Bond Distances (Å) and Angles (°) for 1‚IBr (Form I), 1‚IBr (Form II), and 1‚I2 1‚IBr (form II) S(1)-I I-Y S(1)-I-Y C(1)-S(1)-I-Y
1‚IBr (form I)
unit A
unit B
unit C
1‚I2
2.589(2) 2.7487(9) 177.03(5) -167.2(11)
2.617(5) 2.790(2) 177.26(10) 160.2(26)
2.582(5) 2.797(2) 175.26(13) -132.3(17)
2.546(5) 2.791(3) 177.26(12) 162.7(28)
2.658(2) 2.8947(7) 178.17(5) -167.0(17)
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Figure 2. Structure of form I (Z ) 4, space group Cc) of 1‚IBr viewed along c (each single adduct molecule in the diagram represents a column of molecules related by translation along the c-axis). The deconstruction of the structure into two 3D “supramolecular constructs” (SCs), X and Y, and the further division of Y into two occurrences of X is shown. All molecules are crystallographically equivalent. The relationship between green occurrence of X on the one hand and the two purple occurrences of X on the other is a crystallographic translation. The SCs X and Y are colored differently to aid the interpretation of the full structure in the top diagram.
of a kernel and one or more shell molecules can be assigned for any such subassembly. This is the “seed” from which this assembly may grow. As this seed must be contained in the cluster(s) of the crystal structure, it follows that if representative clusters of the compared structures will have a certain subunit in common, this will be the seed of an SC. The program XPac identifies SCs simply by comparing the representative clusters of crystal structures with the aim to identify similar cluster subunits. The characterization of an SC (dimensionality and corresponding lattice vectors) is straightforward using the crystallographic information contained in the seed. The first step of the procedure is to define a “corresponding ordered set of points” (COSP, generally a suitable selection of atoms), one set for each independent molecule, that describes a common or very similar geometrical features of any molecule in the compared structures. The cluster of each independent molecule in the structures is generated by application of the appropriate symmetry operations to the COSP, and the similarity of their subunits is established by comparing representative lists of intermolecular, dihedral, and torsion angles, to gradually assemble the seed of the SC. For the study described here, the atoms of the penta-atomic ring in the N-methylthiazolidine-2-thione moiety of the adduct were used to define the COSP; these atoms, in fact, describe the basic geometrical features of any adduct molecule in the two polymorphic forms. In this way, XPac revealed that the structures of form I (Z′ ) 1) and form II (Z′ ) 3) are based on different arrangements of two interpenetrating 3D supramolecular constructs (SCs), X and Y (Figures 2 and 3). Form I (Figure 2) is characterized by only one independent adduct molecule in the asymmetric unit packed in columns and, according to the XPac analysis, consists of three single occurrences of the SC X (see Figure 2, bottom), which are related by a crystallographic translation. Each occurrence consists of one-third of all adduct molecules of the crystal
structure. Any two of these occurrences of X may be combined to give a composite SC Y (Figure 2, middle), and the overall structure also can be depicted as one occurrence of X (green) interpenetrating one occurrence of Y (purple, see top section of Figure 2). Form II (Figure 3) is characterized by three independent adduct molecules in the asymmetric unit, again packed in columns. Together, two independent adduct molecule types (A and B) form two occurrences of a composite (AB) SC X (colored purple, Figure 3, bottom), which can be merged to give a composite SC Y (Figure 3, middle). There is a third occurrence of SC X (colored yellow, Figure 3, bottom) that originates only from the third independent adduct molecule type C. Hence, the overall structure of form II, like that of form I, can be deconstructed into one occurrence of the SC X and one occurrence of the SC Y. The relationship between the yellow occurrence of X on the one hand and each of the two purple occurrences of X on the other may be described as a pseudoinversion (Figure 3, top).37 Because the SC Y is present in both structures, it represents the largest component of the similarity between the two structures, and it is important to identify the similarity at the highest level. Essentially, the structures each contain one occurrence of Y (a doubly interpenetrating network structure, comprised of two congruent X’s) and a single, separate X (a single network), but the relationships between the common Y’s and the X’s are different. One X is congruent with the two X subcomponents in Y, and the other is inverted. Polymorphic modifications that consist of the same one- or two-dimensional structural fragments are not uncommon, and cases of 3D similarities between different forms have also been described in the past.35 However, the packing arrangement of all molecules remains essentially the same in these cases. The 3D structural relationship between forms I and II of 1‚IBr, identified by XPac, is not described by one single 3D SC
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Figure 3. Structure of form II (Z ) 12, space group Cc) of 1‚IBr viewed along c (each single adduct molecule in the diagram represents a column of molecules related by translation along the c-axis). The deconstruction of the structure into two 3D “supramolecular constructs” (SCs), X and Y, and the further division of Y into two occurrences of X is shown. The SCs X and Y are colored differently to aid the interpretation of the full structure in the top diagram. The relationship between the yellow occurrence of X on the one hand and the two purple occurrences of X on the other may be described as a pseudo-inversion. There are three independent molecules, A, B (together forming Y), and C (forming X).
comprising the entirety of the crystal structure but by two unequal 3D fragments, the SCs X and Y, which are mutually interpenetrating, and their reciprocal orientation is different in the two forms. An alternative view along the [101] direction of the forms I and II of 1‚IBr is available as Supporting Information (Figure S3). These results from the XPac calculations not only allow the investigator to perceive immediately and easily the geometrical relationships/differences between the structures of the two polymorphic forms but also invite further analysis to see if it is possible to identify the origins of these relationships, in terms of particular complementary connectivity patterns. Thus, we set out to investigate whether the SCs identified by the XPac calculations might point to particular patterns of specific intermolecular interactions. In fact, a more detailed inspection of the crystal packing reveals that in form I of 1‚IBr, each adduct molecule is sandwiched between the bromine atom and the iodine atom of two adjacent adduct molecules; the distances from the mean plane of the penta-atomic ring of the thiazolidine fragment are 3.58 and 3.87 Å (3.67 and 4.04 Å from the centroid), respectively (Figure 4, top). These possible very weak I‚‚‚π‚‚‚Br interactions (at the limit of the sum of van der Waals radii) mainly involving the π-system delocalised over the endo-cyclic S-C-N fragment of the penta-atomic ring define (0 0 1)oriented infinite 2D layers in the crystal lattice. If form I is decomposed, according to Figure 1, into an SC X and an SC Y, then these layers of weakly linked molecules consist of alternating single and double rows of molecules belonging to X and Y, respectively. An identical pattern of I‚‚‚π‚‚‚I intermolecular contacts is recognizable in the crystal structure of the adduct 1‚I2 which is isostructural to form I of 1‚IBr. Here, the distances of the iodine atoms from the mean plane of the penta-atomic ring of the thiazolidine fragment are 3.93 and 3.73 Å (4.05 and 3.86 Å to the centroid).
In form II of 1‚IBr, three different types of halogen‚‚‚π‚‚‚halogen weak interactions are observed (see Figure 4, bottom). Two bromine atoms lie on either side of the thiazolidine framework of each A-type molecule. The distances between Br and the mean plane of the penta-atomic ring are 3.72 (BrC) and 3.57 Å (BrB) (3.82 and 3.64 Å to the centroid, respectively). Similarly, each B-type molecule interacts with two iodine atom. Their distance to the mean plane of the penta-atomic ring is 3.85 (IA) and 3.77 Å (IC) (3.94 and 3.82 Å to the centroid, respectively). In contrast, each molecule of type C is sandwiched between a bromine and a iodine atom. The distances between the mean plane of the penta-atomic ring of its thiazolidine fragment and these atoms are 3.65 (BrA) and 3.71 Å (IB) (3.80 and 3.76 Å to the centroid, respectively). Figure 4 (bottom) shows the (0 0 1)-oriented infinite 2D layers of connected molecules that result when these halogen‚‚‚π‚‚‚halogen contacts are considered. There is a close relationship between this 2D assembly and its counterpart in form I (Figure 4, top). Within each layer, the sequence of rows assigned to the SCs X and Y is the same as in form I. Rows belonging exclusively to X and Y propagate in the [1 3h 0] direction in form I, and they are parallel to [1h 1 0] in form II. While the orientation of all Y-double rows in a given layer is similar in the two modification of 1‚IBr, each X-row is related to adjacent rows by translation and pseudoinversion in form I and form II, respectively (the lengths of the translation vector between adjacent adduct molecules is almost identical (11.72 Å (I) and 11.80 Å (II)). The three independent molecules in form II are arranged in such a way that A and B belong to Y and C belongs to X. This requires the existence of three different halogen‚‚‚π‚‚‚halogen weak interactions in form II (I‚‚‚π‚‚‚I, I‚‚‚π‚‚‚Br, Br‚‚‚π‚‚‚Br) rather than only one (I‚‚‚π‚‚‚Br) as in form I. Neighboring halogen‚‚‚π‚‚‚halogen bonded layers of both forms of 1‚IBr are weakly joined by C-H‚‚‚X (X ) S, Br, I) H-bonding interactions (Figures S4 and S5, Supporting Informa-
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Figure 4. 2D nets of molecules linked by weak halogen‚‚‚π‚‚‚halogen interactions. (a) Form I - layer parallel to (0 0 1), composed of molecules linked by I‚‚‚π‚‚‚Br interactions (
