Isomeric and Isostructural OligothienylsilanesâStructurally Similar

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Isomeric and Isostructural Oligothienylsilanes−Structurally Similar, Physicochemically Different: The Effect of Interplay between C− H···C(π), S···C(π), and Chalcogen S···S Interactions Krzysztof Durka,*,† Krzysztof Gontarczyk,† Sergiusz Luliński,† Janusz Serwatowski,† and Krzysztof Woźniak‡ †

Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664, Warszawa, Poland ‡ Chemistry Department, Biological and Chemical Research Centre, University of Warsaw, Ż wirki i Wigury 101, 02-089 Warszawa, Poland S Supporting Information *

ABSTRACT: The solid state and solution properties of tris(2-thienyl)methylsilane, I, tetrakis(2-thienyl)silane, III, and their positional isomers bearing 3-thienyl groups (II and IV) were investigated and compared. The tris(thienyl)silanes (I, II) crystallize in different space groups, but their respective structural motifs are very comparable. In turn, the tetrathienyl isomers are isostructural. Furthermore, in all studied systems the same set of C−H···C(π), S···C(π), S···S, C−H···S interactions are engaged in supramolecular structure formation. These interactions are interchangeable as thienyl rings (excluding structure II) are affected by 2-fold positional disorder. Despite the high level of structural similarity, the studied thienylsilanes show very different physicochemical behavior: (1) much higher melting points and larger enthalpies of fusion for II (mp = 71.3 °C, ΔH = 20.9 kJ mol−1) and IV (mp = 221.2 °C, ΔH = 29.1 kJ mol−1) with respect to their isomeric counterparts I (mp = 28.6 °C, ΔH = 16.0 kJ mol−1) and III (mp = 131.5 °C, ΔH = 27.0 kJ mol−1), (2) different temperaturedependence unit-cell evolution, and (3) much lower solubility of IV compared to III. The computations show that the strength of interactions decreases in the series C(α)−H···C(π) > C(β)−H···C(π) > S···C(π) ≫ S···S. In a combination with crystal symmetry, this leads to a different distribution of energy within the corresponding crystal structures, and as a consequence, results in their different macroscopic behaviors. In addition, the solid−liquid equilibrium studies suggest that the specific S···S chalcogen bonding between molecules of IV is responsible for decreased solubilities of this compound. To characterize the specific interactions involving sulfur atoms (S···S and S···C(π)), the quantum theory of atoms in molecules has been successfully applied.

1. INTRODUCTION Crystal engineering aspires to understand the influence of noncovalent interactions on the macroscopic properties of organic crystals (density, solubility, thermal and mechanical behavior, electronic and optical properties), which would help to design materials possessing desired parameters. Consequently, the studies on polymorphic and isostructural chemical compounds have attracted a lot of attention nowadays, because they provide an opportunity to directly address the fundamental questions about the effect of intermolecular forces on the crystal structure of compound. While polymorphism implies the ability of the compound to self-assemble into multiple crystal structures, the isostructurality refers to the similarity of the molecular arrangement in the crystal of different compounds. Much research in these areas focuses on systems that involve strong interactions such as metal−ligand coordination bonds and hydrogen-bond interactions;1−9 © XXXX American Chemical Society

however, in the past two decades a significant body of work was undertaken to study more subtle effects resulting from weak solid-state interactions such as π···π stacking and weak hydrogen bonds.10−17 A lot of attention also has been paid to halogen interactions.18−23 For instance, structurally equivalent halogen-bonded iodine and bromine atoms have been explored as a tool to construct isostructural co-crystals.24−26 The formation of halogen bonds has been rationalized using a σhole concept consistent with the anisotropical charge density distribution within the halogen atomic basin.27−30 As a consequence, electrophilic and nucleophilic regions can simultaneously be involved in highly directional halogen bond interactions. More recently, this concept has been expanded to Received: March 4, 2016 Revised: June 20, 2016

A

DOI: 10.1021/acs.cgd.6b00358 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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groups, and their crystal lattice assembly results from the molecular symmetry; (b) they are relatively simple as only several types of intermolecular interactions are formed including C−H···C(π), S···S, S···C(π), C−H···S, while the C(π)···C(π) stacking interactions are avoided, which reduces the number of possible crystal motifs, (c) and finally, the molecules are very similar one to another, which would facilitate the rationalization of their differences in macroscopic properties (solubility, thermal behavior) on the basis of conformation−structure−energy relationships. The proper description of the studied systems is not simple as the thienyl groups are affected by a 2-fold disorder. Thus, the additional aim of this work is to show the origin of this process and its influence on the crystal stability.

the Main Group VI elements in order to rationalize frequently occurring chalcogen Chal1···Chal2 interactions.31−58 Their role in crystal engineering,46−51 molecular recognition,52,53 biological systems,54−57 and materials science58 is the subject of increasing interest. It is supposed that they may significantly influence the solid-state properties of organic systems. For instance, the analyses of crystal structures of oligothiophenes revealed many short S···S contacts, along with the C−H···π, π···π, and C−H···S interactions, suggesting their key role in the molecular packing.58−66 It is also supposed that they are responsible for the charge transport properties of thiophenebased conducting and semiconducting materials.67,68 The presented considerations have prompted us to study the substantial relationships between molecular conformation, structural organization, energetic features, and macroscopic properties of some model oligothiophene systems. For these studies we have selected pairs of isomeric tris(thienyl)methyland tetrakis(thienyl)silanes (Chart 1). It was expected that they

2. RESULTS AND DISCUSSION 2.1. Crystal Structure Analysis from the Perspective of Molecular Packing and Interaction Energies. 2.1.1. Molecular Conformation and Disorder. Both tetrathienyl isomers III and IV are isostructural and belong to the tetragonal crystal system (P4̅21c space group) reflecting the high symmetry of the molecule. The cell axis and volume metric parameters are almost identical (III: V = 805.07(3) Å3; V = IV: 807.68(3) Å3). They can be included in a family of isomorphous compounds of a general chemical formula Ar4X (X = C, Si, Ge, Pb, Sn, Os, Mo; Ar = Ph, 2-MePh, 3-MePh) and tetragonal P4̅21c symmetry.69−74 On the other hand, the crystal setting of tetrakis(2-thienyl)methane (CTh4)75a carbon analogue of IIIis monoclinic P21/n. Furthermore, tris(2-thienyl)silanes I and II crystallize in the orthorhombic Pccn and monoclinic Cc space groups of symmetry, respectively, and their structures differ from the phenyl analogues (CMePh3, SiMePh3).76,77 However, the close inspection of intermolecular arrangement reveals that many crystal motifs are preserved along these two series. This confirms the general principle that systems with very similar molecular features exhibit the tendency to form identical or similar packing motifs. The molecular structures are presented in Figure 1.

Chart 1. Studied Thienylsilanes: Tris(2-thienyl)methylsilane (I), Tris(3-thienyl)methylsilane (II), Tetrakis(2thienyl)silane (III), and Tetrakis(3-thienyl)silane (IV)

would exhibit a number of useful features: (a) they belong to the highly symmetric C3 (I and II) and S4 (III and IV) point

Figure 1. Labeling of atoms and the atomic thermal motion estimation as ADPs (50% probability level) for I−IV together with corresponding crystal snapshots. Disorder within thienyl rings is shown separately for clarity. B



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balance of intermolecular interactions formed when the molecule joins the growing crystal from solution. As is usually observed for disordered structures, this process was not affected by the choice of a solvent (we have tested acetone, toluene, and thiophene) or crystallization rates. Importantly, the single-molecule calculations show that 2thienylsilanes (I and III) are by ca. 20 kJ·mol−1 more stable than the 3-thienyl isomers (II and IV, respectively). On the other hand, it is supposed that the crystal structures of 3thienylsilanes will be more stable than structures of 2thienylsilanes, as the former contain two C(α)−H groups and one C(β)−H group per each thienyl ring, while for the latter ones this proportion is reversed (Chart 2). C(α)-bonded hydrogen atoms are more acidic, and therefore it is expected that they would be involved in stronger intermolecular interactions.

As is usually observed in the structures containing thienyl moieties,78 the studied compounds are affected by rotational disorder of thienyl groups. For I, III, and IV, the refinement shows that thienyl moieties can occupy two positions being rotated by 180° around the Si−C bond. For the purpose of further considerations thienyl components with higher occupancy factors will be labeled as S1, S2, S3, and S4 (according to the sulfur atom labeling), whereas the second components will be stared S1*, S2*, S3*, and S4*. For I, there is one molecule in the asymmetric part of the unit cell, and two of three thienyl groups (S2, S3) were found to be disordered with the occupancy ratios of 76:24 (S2:S2*) and 80:20 (S3:S3*). This gives four possible combinations of molecular conformations in the structure, I_S1S2S3, I_S1S2*S3, I_S1S2S3*, and I_S1S2*S3*, with a diminishing probability of occurrence (P, calculated from experimentally obtained occupancy factors) of 60.8%, 19.2%, 15.2%, and 4.8%, respectively. In the case of II, the difference Fourier synthesis (Figure S5, SI) shows that all three thienyl rings are affected by disorder; however, since the contribution of second fraction is low (∼5%), it was not possible to propose any reasonable model including disorder within thienyl rings in this structure. In III and IV, thienyl groups were found to be disordered with the occupancy ratios of 60:40 and 86:14, respectively. Because of the location of the silicon atom at the 4-fold inversion axis, the four thienyl rings of the molecules III and IV are symmetrically equivalent. This limits possible conformations for this crystal setting to III/IV_S1S2S3S4 and III/ IV_S1*S2*S3*S4*. Therefore, for the purpose of further analyses, we have transformed the studied system to the lower symmetry P21 space group preserving all metric parameters. This operation allowed us to generate other possible conformations: III/IV_S1*S2S3S4, III/IV_S1*S2*S3S4, and III/IV_S1*S2*S3*S4. The probability for the occurrence (P) of a particular conformation decreases as follows: I V _ S 1 S 2 S 3 S 4 ( 5 4 . 7 % ) , I V _ S 1 * S 2 S 3 S 4 ( 35 .6 %) , IV_S1*S2*S3S4 (8.7%), IV_S1*S2*S3*S4 (0.94%), and IV_S1*S2*S3*S4* (0.04%). A quite different situation occurs in the case of III, where the most probable conformations are III_S1*S2S3S4 and III_S1*S2*S3S4 (34.6%), then III_S1*S2*S3*S4 (15.4%), III_S1S2S3S4 (12.9%), and III_S1*S2*S3*S4* (2.5%). This clearly demonstrates that all conformations and resulting crystal motifs should be taken into account for the proper crystal structure description. It is expected that all conformers are randomly distributed in the crystal structure leading to an enormous number of possible combinations. However, the analysis of structures built solely by one particular conformer allows one to study all possible close-distance intermolecular arrangements, and therefore we have limited our considerations only to these several cases. According to the theoretical calculations performed with Gaussian0979 for the isolated molecules of I−IV at the M062X80/6-311+G**81 level of theory, all conformers are characterized by comparable stabilities (the energy differences are less than 1 kJ mol−1). Furthermore, the molecules show a low barrier for the rotation of thienyl group along the Si−C bond (about 15 kJ mol−1). This is also consistent with the NMR studies,82−84 which show that rotations of thienyl groups are not restricted in solution. In turn, in a crystal the positions of thienyl rings are constrained. The repeated measurements of single crystals of III and IV at 300 K show that the occupancy factors are preserved. Therefore, it can be concluded that the observed disorder is static in nature and results from the

Chart 2

2.1.2. Supramolecular Structure and Energetic Landscape of I−IV. The observed static disorder results in the formation of many different crystal motifs randomly distributed in the crystal structure. To simplify the consideration, we have analyzed structures built solely by one particular molecular conformer. This was restricted to conformations, which persist in the experimentally obtained structures (see Discussion in the previous paragraph). Overall, four structures of I, one of II, five of III, and five of IV have been generated based on the experimentally obtained atomic coordinates. Then, the constrained geometry optimization using periodic density-functional approach (B3LYP85−88/TZVP89) implemented in the CRYSTAL0990,91 program was performed (details in Experimental Section). This procedure allowed us to obtain more reliable positions of hydrogen atoms and geometry of thienyl rings (Figure S6, SI). As is already shown in the literature,92−103 such an approach is crucial for the proper description of intermolecular interactions, and thus, it has a significant impact on the estimation of cohesive energy. In the next step, the optimized geometries were used for the PIXEL104−106 calculations (M06-2X80/6-311+G**81), which allowed for the estimation of cohesive energies EcohPIX for each system along with the interaction energies between pairs of molecules (ED). In general, the obtained EcohPIX energies stay in agreement with CRYSTAL09 EcohCRYST values (Tables 2 and 3). While the effect of the conformation on the stability of a particular compound is rather clear based on these calculations, the direct comparison between structures I−IV is not straightforward. Therefore, we have estimated the averaged energy values (E̅ D, E̅coh) for structures I, III, and IV (in the case of II only one conformation was analyzed, thus Ecoh = E̅coh). They were obtained from the Ecoh and ED values calculated for each structure multiplied by the probability of occurrence of a corresponding conformation in the structure (P): E D̅ =

∑ EDi P i ; i

C

Ecoh ̅ =

i Pi ∑ Ecoh i DOI: 10.1021/acs.cgd.6b00358 Cryst. Growth Des. XXXX, XXX, XXX−XXX


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Figure 2. Packing diagram of I (a, b) and II (c, d) in the (001) plane (a, c) and along the molecular columns (b, d) showing the basic structural motifs. Hydrogen atoms are omitted for clarity. Blue and yellow arrows show the directions for the double-layer motif propagation; red arrows indicate molecular columns constructed from dimers D1.

Figure 3. Packing diagram of IV showing intermolecular relations in the (001) plane (a) and along the [110] and [010] directions (b, c). Hydrogen atoms are omitted for clarity.

In all studied compounds the C−H···C(π), C−H···S, and S··· C(π) interactions play an essential role in crystal packing. On the other hand, the π−π stacking interactions are not formed. The first coordination sphere of the basic molecule contains 12 (I and II) or 14 (III and IV) neighboring molecules. As a result several types of dimeric molecular motifs can be distinguished. They have been further abbreviated as dimers D1−D6 (dimer description refers to the unique (in terms of symmetry) pair of interacting molecules) and are shown in corresponding

molecular packing diagrams in Figure 2 (I and II) and Figure 3 (III and IV), and separately for each structure in Figures 4, 5, and 6. The full list of intermolecular interactions for all studied systems is deposited in Table S4 (SI). Dimers D1 link molecules into columns involving C−H··· C(π) and/or S···C(π) interactions. They represent the most strongly bonded dimeric molecular motifs with the distance between central silicon atoms dSi···Si ≈ 6.62 Å for tris(thienyl)methylsilanes and dSi···Si ≈ 6.35 Å for tetrakis(thienyl)silanes; D



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with Gaussian09 (M06-2X/6-311+G**) using pairs of thiophene molecules as model systems with the geometry taken directly from the CRYSTAL09 optimized structures. Estimated values (Table 1) confirm the observed order of intermolecular interactions: C(α)−H···C(π) > C(β)−H···C(π) > S···C(π) ≫ S···S ≈ C−H···S > C−H···H−C. It is noticeable that the energy of particular interaction also depends on a mutual orientation of thiophene molecules that may vary for different compounds and dimeric molecular motifs. In the case of tris(2-thienyl)methylsilane (I), the dimeric molecular motif D1 involves the herringbone arrangement between the S1 and S3 thienyl rings of one molecule and S2 thienyl ring of the neighbored molecule. The propagation of this dimer along the [001] crystallographic direction leads to the formation of a molecular 1D column. When molecules in the structure adopt the I_S1S2S3 conformation, the C−H··· C(π) and S···C(π) interactions are involved, and the interaction energy between molecules is −30.3 kJ mol−1 (Table 1, Figure 4a). A similar value was obtained when both S2 and S3 rings adopt S2* and S3* conformations. This is rationalized by the fact that the reorientation of both rings simply reverses interaction pattern leading to a very similar crystal motif (Figure 4d). However, when only one ring (S2 or S3) changes its conformation, the interaction energy increases (I_S1S2*S3, Figure 4b) or decreases (I_S1S2S3*, Figure 4c) by ca. 5 kJ mol−1 as two C−H···C(π) or two S···C(π) interactions are involved. In the dimeric molecular motif D2, the C−H···C(π) interactions occur between the thiophene rings S1 and spin two neighbored [001] columns in a herringbone fashion (Figure 5a). The dimer interaction energy is ca. −20 kJ mol−1, being the smallest figure among the dimers D1−D5. On the other hand, the geometry of C−H···C(π) interaction seems to be very favorable in this case. The S1 thienyl groups are aligned perpendicular to each other in such a way that the C−H hydrogen atom resides on the center of the neighbored thienyl group. The distances between H atom and all five atoms from neighbored thienyl ring are closer than the sum of the corresponding van der Waals radii (dH···X: 2.638−2.913 Å; dX···H: 3.633−3.820 Å; θC−H···X: 135.9−165.5°; X = C, S; Figure 5a, Table S4, SI). This results from the higher acidity of the C(α)-bonded H atoms. Considering the inversion of the S1 ring to the S1* position, the less acidic (i.e., carrying less positive charge) C(β)-bonded H atoms would be involved, which would reduce the dimer stability. Furthermore, the rotation of the S1 ring would not significantly change the intermolecular relations in other dimers. This probably explains why the S1 ring is exceptionally not affected by the disorder.

Figure 4. (a−d) Effect of the thienyl ring conformation on the intermolecular interaction pattern in dimer D1 of structure I. Distances closer than sum of van der Waals atomic radii are shown as red dashed lines. The full list of intermolecular interactions is provided in Table S4 (SI).

interaction energies calculated with the PIXEL program vary in the range of 25−35 kJ mol−1 (I and II) and 35−70 kJ mol−1 (III and IV). Other important structural motifs arise from the dimers D2−D4, which spin adjacent columns by means of C− H···C(π) and/or S···C(π) interactions and are responsible for the formation of molecular layers. The Si···Si distances between the molecules oscillate in the range of 7.6−8.6 Å, and the interaction energies are in the range of 20−30 kJ mol−1. For structures I and II, dimers D5 can be additionally distinguished. They arrange thiophene moieties and/or CH3 groups from the neighbored molecules (dSi···Si = 6.940(1) Å for I, dSi···Si = 7.342(1) Å for II) and are responsible for the interactions between layers. Finally, dimers D6 link molecules across the layers (dSi···Si ≈ 12.7 Å for I and II, dSi···Si ≈ 11.2 Å for III and IV) involving chalcogen S···S, hydrogen C−H···S, or dihydrogen C−H···H−C interactions. The interaction energies are in the range of 4−7 kJ mol−1. From the comparison between conformers, we have found that the replacement of C−H···C(π) interaction by the S···C(π) one in a particular dimeric molecular motif (when thiophene group rearrange between S and S*) decreases its stability by about 4 kJ mol−1. Furthermore, the dimers containing C(α)−H···C(π) interaction are by about 1−3 kJ mol−1 more stable than corresponding dimers stabilized by C(β)−H···C(π) interactions. As the dimer interaction energies cannot be simply deconvoluted to contributions from the separated interaction types, we have performed additional single-point computations

Figure 5. Dimeric molecular motifs D2−D5 (a−d) in the structure I. Distances closer than sum of van der Waals atomic radii are shown as red dashed lines. Blue arrows indicate the possibility for reorientation of thienyl groups. E



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Figure 6. Dimeric molecular motifs D1 (a) and D2 (b) in the structures III and IV. Distances closer than the sum of van der Waals atomic radii are shown as red dashed lines. Blue arrows indicate the possibility for reorientation of thienyl groups.

The propagation of dimers D1−D4 results in the formation of a double-deck layered molecular motif (Figure 2a, blue and orange arrows). The additional C−H···H−C homo-hydrogen or hetero-hydrogen107 contacts appear across the layer producing a dimeric molecular motif D6. The neighbored layers are connected due to the interactions formed between the methyl groups and S3 thienyl rings (D5, Figure 5d). The comprehensive analysis of all dimeric molecular motifs in I shows that the reorientations of thienyl rings between S and S* conformations change the interaction pattern, but only slightly affect the overall crystal stability. This is because each thienyl group is usually involved in the formation of two or three different dimeric molecular motifs. The formation of very favorable intermolecular interactions (such C(α)−H···C(π)) in a chosen dimer is usually counterbalanced by the appearance of less advantageous interactions (e.g., S···C(π)) in other dimers. Hence, all structures derived from different conformers of I are characterized by very similar cohesive energy values (Table 2). This rationalizes the occurrence of a conformational disorder of thienyl rings S2 and S3. On the other hand the S1 ring adopts only one conformation owing to the formation of very favorable C−H···C(π) interactions involving acidic C(α)bonded H atoms. It is also noticeable that the structure I_S1S2S3 is the most stable of the series (EcohCRYST = −141.1 kJ mol−1, EcohPIX = −146.7 kJ mol−1), which is in agreement with the highest probability of occurrence of this conformation in the structure. However, three remaining structures do not follow this relation. The crystal structure of II resembles many structural motifs of I, although the molecules are more effectively packed and interact more strongly than in I. The molecular columns defined by the dimers D1 are essentially the same. The molecules also aggregate in a double-layer pattern. The main difference between two isomers lies in the mutual orientation of neighbored molecules in layers (Figure 2c). Furthermore, the neighbored layers are shifted with respect to each otherthe dimers D5 describing the relations between layers are now based on C−H···C(π) interactions between S1 thienyl groups leaving the methyl group out of the intermolecular arrangement. Importantly, the homohydrogen C−H···H−C contacts are replaced by the chalcogen S···S interactions (dimer D6). In most cases, the corresponding dimer interaction energies in II are higher compared to I (Table 2). The structure II is also ca. 5 kJ mol−1 more stable (II: EcohCRYST = −145.8 kJ mol−1, EcohPIX

Table 1. Comparison of Interaction Energies in Model Thiophene Dimeric Systemsa

dimer

interaction

D1

C(α)−H···C(π) C(β)−H··· C(π) S···C(π) C(α)−H···C(π) C(β)−H··· C(π) S···C(π) C(α)−H···C(π) C(β)−H··· C(π) C(α)−H···C(π) C(β)−H··· C(π) CH3... C(π) C(α)−H···C(π) S···S C−H···S C−H···H− C

D2

D3 D4 D5 D6

I

II

III

−13.5 −10.0 −7.4 −12.9

−10.6

−11.0 −8.0 −10.8 −9.6

IV −12.1 −10.3 −13.2 −7.1

−10.8 −8.8 −8.9 −7.2 −6.2

−11.6

−10.3 −3.9 −1.0

−1.3

−3.8 −4.3 −0.5

a

The calculations were performed at the M06-2X/6-311+G** level of theory. The results are grouped by the interaction type. Note that the occurrence of a particular interaction type in dimer depends on the conformation of thienyl ring (S or S*), and thus the calculations were performed separately for each conformer. All values are given in kJ mol−1.

The formation of dimeric molecular motifs D3 and D4 arises from the C−H···C(π) interactions between the S2 or S3 thienyl rings, respectively (Figure 5b,c). The interaction pattern is similar to that found in D2, but the interactions seem to be less advantageous considering geometrical parameters (Table S4, SI). On the other hand, the computations show that the interaction energies between the molecules are higher in the former case and, depending on the thienyl ring conformation, they vary in the range −23 to −31 kJ mol−1 (Table 2). This additional stabilization comes from the contribution of auxiliary interactions of different types (C−H···S, S···S, van der Waals) formed with neighboring S1 (D3) or S2 (D4) thienyl groups. F



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Table 2. Comparison of Cohesive Energies (Ecoh) and Dimer Interaction Energies (ED) in the Structures I (Four Possible Conformers) and IIb I EcohCRYST S1S2S3 S1S2*S3 S1S2S3* S1S2*S3* E̅a

−141.1 −136.2 −139.2 −139.3 −140.2

EcohPIX −136.7 −132.8 −133.5 −134.6 −135.6 N: dSi···Si/Å

ED1

ED2

ED3

ED4

ED5

ED6

P (%)

−30.3 −34.5 −25.3 −29.9 −29.7 2 6.615(1)

−19.0 −19.0 −19.1 −19.2 −19.2 2 8.144(1)

−25.5 −28.7 −25.9 −23.6 −25.3 2 8.660(1)

−25.9 −23.3 −31.1 −23.4 −26.1 2 8.393(1)

−29.9 −24.3 −25.4 −30.1 −29.0 2 6.940(1)

−3.4 −3.1 −3.3 −3.7 −3.4 2 11.116(1)

60.8 19.2 15.2 4.8

II EcohCRYST S1S2S3

−145.8

EcohPIX −140.8 N: dSi···Si/Å

ED1

ED2

ED3

−41.7 2 6.577(1)

−28.1 4 7.662(1)

−22.2 2 8.663(1)

ED4

ED5

ED6

P (%)

−21.4 2 7.342(1)

−4.2 2 11.902(1)

100

a

Average energy values. bN stands for the number of symmetrically equivalent dimers that are formed by each molecule, dSi···Si indicates the distance between silicon centers, and P is the probability of occurence of a given conformation in the structure. Cohesive energy values were calculated using both CRYSTAL (EcohCRYST) and PIXEL (EcohPIX) approaches, whereas dimer interactions energies were calculated using PIXEL. All energy values are given in kJ mol−1.

Figure 7. Relative contributions to Hirshfeld surface areas for various intermolecular contacts in III (a) and IV (b).

= −140.8 kJ mol−1 vs I: E̅ cohCRYST = −140.2 kJ mol−1, E̅ cohPIX = −136.6 kJ mol−1). This is because all thienyl groups are arranged in the C(α)−H···C(π) interactions, while less favored S···C(π) and C(β)−H···C(π) interactions are absent. Similar to I and II, the molecules of III and IV are packed in columns (dimer D1, Figure 3b,c). However, in the latter case this motif involves an intermolecular mutual arrangement of four perpendicular thienyl ringstwo from each molecule. As a result, four edge-to-face C−H···C(π) or S···C(π) interactions (or combinations of these two interaction types) are formed (Figure 6a). The second characteristic motif arises from dimers D2 which link molecules from neighbored columns in the faceto-edge fashion involving C−H···C(π) or S···C(π) interactions (Figure 6b). Each molecule is involved in the formation of eight such dimerstwo per each thienyl ring. The interactions between molecules generated by [100] and [010] translational vectors provide the third type of dimer. It arranges molecules in a similar fashion as in the dimeric molecular motif D6 formed in I and II, and therefore has the same descriptor. Depending on the orientation of the thienyl ring, the dimer D6 involves S···S, C−H···S, or C−H···H−C contacts. The propagation of dimers D2 and D6 leads to the formation of the (001) waved layer (Figure 3a), and thus, the

supramolecular structure of III and IV can also be interpreted as a stack of such molecular layers along the c direction. The reorientation of thienyl rings between S and S* positions changes the intermolecular interaction scheme, but the origin of this process is different for III and IV. This can be visualized using the Hirshfeld surface approach.108 In the case of III, the relative contribution of the Hirshfeld surface area for S···C(π) contacts systematically decreases with a gradual reorientation of thienyl rings to S* positions (Figure 7). This is associated with the increase of the contribution of C−H··· C(π) and C−H···S interactions. The situation is reversed in IVthe reorientation of thienyl rings to S* positions leads to an increased S···C(π) contact area, while the contributions from C−H···C(π) interactions are reduced. It is interesting to note that S conformations of thienyl rings usually favor the formation of S···S contacts. In III_S1S2S3S4, a net attraction calculated for the dimer D1 is −47.0 kJ mol−1 and gradually increases when the thienyl rings reorient to the S* position to reach −63.0 kJ mol−1 in III_S1*S2*S3*S4*. This is a consequence of changing the intermolecular interaction schemea set of four S···C(π) interactions are systematically replaced by more favorable C(β)−H···C(π) interactions, and each of them increases the G



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Table 3. Comparison of Cohesive Energy (Ecoh) and Dimer Interaction Energy Values (ED, ΣED - Sum of All ED of One Type Per One Molecule) in the Structures III and IVa III EcohCRYST

EcohPIX

ED1c

ED2d −27.3 −23.6, − −28.2, − −23.5, − −26.2, − −24.3, − −24.2, − −24.5 −25.4 8 8.572(1) IV

S1S2S3S4 S1*S2S3S4

−171.6 −169.7

−168.7 −163.6

−47.0 −49.0

S1*S2*S3S4

−170.3

−167.6

−56.5

S1*S2*S3*S4

−170.7

−167.9

−59.8

S1*S2*S3*S4* E̅ b

−171.2 −170.3

−168.1 −166.4 N dSi···Si/Å

−63.0 −53.4 2 6.349(1)

EcohCRYST

EcohPIX

ED1c

25.8 25.2 26.9 23.6 21.8 26.1

ΣED2

ED6e

ΣED6

P (%)

−109.2 −102.8

−6.7 −6.7 −6.1 −6.4 −6.0 −6.2 −5.3 −5.4 −6.2 4 11.261(1)

−13.4 −12.8

13.0 34.5

−12.4

34.5

−11.4

15.4

−10.8 −12.4

2.6

−100.0 −96.4 −94.0 −101.5

ED2d

ΣED2 −108.4 −98.3

S1S2S3S4 S1*S2S3S4

−190.8 −180.9

−184.8 −176.4

−60.3 −60.5

−27.1 −25.4, − 27.9 −25.5, − 19.5

S1*S2*S3S4

−178.3

−172.6

−63.8

S1*S2*S3*S4

−175.5

−166.8

−65.9

S1*S2*S3*S4* E̅b

−177.0 −186.0

−167.7 −180.6 N dSi···Si/Å

−71.0 −60.7 2 6.302(1)

−25.7, − −16.8, − −27.0, − −16.5, − −20.1 −26.3 8 8.603(1)

27.1 18.8 21.0 18.8

−88.4 −83.3 −80.4 −106.2

ED6e −4.4 −4.5 −6.4 −6.5 −6.6 −4.7 −6.4 −5.0 −5.0 4 11.321(1)

ΣED6

P (%)

−8.8 −10.9

54.7 35.6

−13.0

8.7

−11.0

0.94

−10.0 −10.0

0.04

a

N stands for the number of dimeric molecular motifs of the same type formed by each molecule, dSi···Si indicates the distance between silicon centers, and P is probability of occurence of a given conformation in the structure. Cohesive energy were calculated using both CRYSTAL (EcohCRYST) and PIXEL (EcohPIX) approaches, whereas the dimer interactions energies were calculated using PIXEL. All energy values are given in kJ mol−1. b Average energy values. cAs N = 2, ED1 = ΣED1. dThere can be four symmetrically nonequivalent dimers of this type. eThere can be two symmetrically nonequivalent dimers of this type.

dimer stabilization by 4.0 kJ·mol−1. Quite a different situation occurs in molecular columns formed in the structure IV. Irrespective of orientations of the thienyl ring, a set of four C− H···C(π) interactions contribute to the dimer D1. Differences in interaction energies arise from the different acidities of H atoms. When all thienyl rings adopt S conformations, the C− H···C(π) interactions involve less acidic C(β)-bonded H atoms (ED ≈ − 60 kJ mol−1). In turn, the S* conformations favor the occurrence of C(α)−H···C(π), which enhances the interactions between molecules. As a result, the interaction energy reaches −71 kJ mol−1 in IV_S1*S2*S3*S4*. The calculations clearly show that the dimers D1 in III and IV gain additional stability when thienyl rings adopt the less favored S* conformation. On the other hand, the stability of D2 dimers decreases. In both structures, the S conformations favor the formation of the C− H···C(π) interactions with the acidic C(α)-bonded H atoms, thus giving a comparable dimer interaction energy values of ca. −27 kJ mol−1. However, when the thienyl rings reorient to S* positions, the interaction energy decreases. In the case of III, the less acidic C(β)-bonded H atoms are involved, while for IV, C−H···C(π) interactions are replaced by significantly less advantageous S···C(π) interactions. As a consequence, the stability of the dimer D2 reaches −24.5 kJ mol−1 in III_S1*S2*S3*S4* and −20.1 kJ mol−1 in IV_S1*S2*S3*S4*. These energy differences are rather small, but they become relevant when we sum up the contributions from all dimers of this type. Furthermore, there are four dimers D2 per one dimer

D1. Table 3 and Figure 8 contain the comparison of such overall energies calculated with respect to one molecule of

Figure 8. Comparison of cohesive and dimer interaction energies calculated as a sum of contributions from all dimers of the same type (values are given per one molecule).

thienylsilane. It clearly emphasizes differences in energetic features between the crystal lattices of III and IV. In III the reorientation of thienyl groups between S and S* positions increase the ED1 and decrease ΣED2 energies to a similar extent, thus leaving the cohesive energy at a similar level (EcohPIX ≈ H



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Figure 9. Results of crystallization of equal amounts of III and IV from THF solution (a), overlap of disordered molecular structures in co-crystal (III+IVa) (b), melting points observation of pure III (left) and IV (right), and their co-crystal (middle) (c).

Figure 10. Laplacian map in the thiophene ring plane and in the respective perpendicular plane for IV (a). Molecular graph showing the formation of S···S (b) and S···C(π) (c) interactions together with the disposition of critical points of electron density (BCP(3, −1) - red, RCP(3, +1) - bright yellow, CCP(3, +3) - white) and its Laplacian (CC(3,−3) - blue, CD(3,−1) - gray, CD(3,+1) - green). Contours are in logarithmic scale (e·Å−5) (a), blue solid lines denote positive values and red dashed lines to the negative ones. Theory level: M06-2X/6-311+G**.

−167 kJ mol−1; EcohCRYST ≈ −170 kJ mol−1). In turn, in IV, the reorientation of all the thienyl rings to the S* positions lowers the stability of all D2 dimers by the total value of ca. 28 kJ mol−1, while the ED1 increases only by ca. 11 kJ mol−1. As a result, the stability of IV gradually decreases from −184.8 kJ mol−1 (IV_S1S2S3S4) to −167.7 kJ mol−1 (IV_S1*S2*S3*S4*). The presented theoretical results are in line with the experimentally obtained values of the S:S* occupation factors for the both structures −60:40 for III and 86:14 for IV. Furthermore, the comparison of the average cohesive energies shows that the crystal structure IV is noticeably more stable than III (III: E̅ cohCRYST = −170.3 kJ mol−1, E̅cohPIX = −166.4 kJ mol−1 vs IV: E̅cohCRYST = −185.5 kJ mol−1, E̅cohPIX = −180.3 kJ mol−1). 2.1.3. Co-Crystallization of III and IV. The structural similarity of III and IV should facilitate the formation of cocrystals. According to some literature examples, the isostructural compounds should form solid solutions in all proportions.109−111 Thus, we have dissolved equal amounts of III and IV in THF, and the solution was left until the solvent slowly evaporated to dryness. As III and IV show very different solubility behavior (IV is significantly less soluble in THF than III), we have observed the formation of small crystallites of pure IV together with much bigger co-crystals of III+IV

(Figure 9a). This was confirmed by the 1H NMR analyses, which showed that the content of IV in co-crystals usually varies in the range of 5−35%. Overall, we have measured crystal structures of three different co-crystals (III+IVa − III/IV = 70:30, III+IVb − III/IV = 82:18, III+IVc − III/IV = 94:6), and then we have confirmed their composition by 1H NMR analysis. The averaged structures of co-crystals showed superimposed positions of 2- and 3-thienyl groups, which is due to their random distribution when going from one unit cell to another. Furthermore, each thienyl ring is disordered, and thus four different positions of thienyl groups overlap in the structure (Figure 9b). It is noticeable that the relative occupancy factors of each disordered fraction within III or IV components approximately resemble those obtained for pure structures III and IV. A detailed analysis of intermolecular interaction pattern is rather complex in this case, but it is expected that all types of interactions observed in the structures of III and IV will occur. It is intriguing that, despite a smaller contribution of IV, the co-crystal starts to melt at the temperature which is closer to the melting point of IV (Figure 9c). 2.1.4. Basic Features of S···S and S···C(π) Interactions in the Studied Systems. Studies on organic conducting materials (such as tetrathiafulvene) show that the S···S chalcogen I



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Å−5) are comparable with experimental and theoretical values obtained for S···S interactions reported previously.59−61 According to the Espinosa−Lecomte estimation,116,117 the corresponding interaction energy is −5.3 kJ mol−1, which is close to the value obtained from the PIXEL (−4.4 kJ mol−1) and Gaussian (−3.8 kJ mol−1) calculations. The sulfur atom is also involved in the S···C(π) interaction. The Gaussian calculations on the thiophene dimeric motifs (Table 1) give an approximate value of S···C(π) interaction energy of −8.0 kJ mol−1 (III, D1 dimer) or −7.1 kJ mol−1 (IV, D2 dimer). This is comparable to the value obtained by Meng et al. (−12.7 kJ mol−1) for OCS···C6H6 complex.118 In general, S···C(π) interactions are classified as lone pair-π (lp-π), where the aromatic ring acts as an acceptor of an electron lone pair from the sulfur atom.119−122 The analysis of electron density shows the formation of three bond critical points between the sulfur atom and the thienyl ring along with the three ring critical points (RCP, (3,+1)) and one cage critical point (CCP, (3,+3)), which is a common feature of lp-π interaction. On the other hand, the inspection of Laplacian function revealed that the nature of this interaction is more complex as both types of sulfur critical points (CC1 and CD1) are present in the interaction region (Figure 10c). Thus, besides the expected donor character of the sulfur lone pair represented by the CC1 point, the sulfur atom may also act as an acceptor of electron density from the aromatic ring. At this point it is also worth mentioning that thiophene ring is considered electron rich. Overall, this may suggests a complementary arrangement of electron deficient and electron concentration sites on the sulfur atom and the thiophene ring rather than a simple lp-π bond character. This is consistent with the anisotropic distribution of electron density around the sulfur atom. 2.2. Physicochemical Properties. The differences in structural properties of I−IV have a strong impact on their physicochemical behavior. In a subsequent paragraph, we compare the properties such as melting points and enthalpy of fusion of all studied systems including co-crystal III+IV. Then we focus our particular attention on the isostructural tetrathienyl isomers, as they exhibit very distinctive macroscopic properties. 2.2.1. Thermal Behavior of I−IV. Thermal behavior of all studied compounds was monitored by DSC (Figure 11 and Table 5). The tetrakis(thienyl)silanes melt at higher temperatures than tris(thienyl)methylsilanes. Furthermore, 3-thienylsilanes (II, IV) melt at significantly higher temperatures than the respective 2-thienyl isomers (I, III). In the case of the pair

interactions can direct the molecular assembly and are responsible for many functions of these systems.59,41 The S··· S intermolecular interactions formed between two thienyl moieties in the solid state have been recently quantified using theoretical charge density analysis.112 It was shown that the S··· S interaction geometry and electron density features are in agreement with the σ-hole interaction concept, where the positive potential on one sulfur atom (σ-hole) interacts with the negative one on the surface of the second sulfur atom (the lone electron pair). This enables the electrostatic attraction between two sulfur atoms. Depending on the geometry and sulfur environment character (aliphatic, aromatic, thione), the energy of this interaction varies in the range of 2−15 kJ mol−1.48,113,114 The topological analysis of electron density and its Laplacian provided the regions of locally concentrated (L(r) > 0) or depleted (L(r) > 0) distribution of charge density. According to the QTAIM,114,115 the (3, −3) critical points of L(r) function correspond to the maxima of electron density (charge concentrations, CCs), while remaining critical points correspond to local minima of ρ(r) function along with one (3, −1), two (3, +1), or three (3, +3) directions (charge depletions, CDs). Figure 10a shows the Laplacian map plotted over the thiophene ring plane and in the plane bisecting the C−S−C angle (IV as a model compound) with the dispositions of CC and CD sites. The topological features of electron density around the sulfur atom in our systems are similar to those observed in thiophthalic anhydrides studied by Espinosa and co-workers.48 According to expectations two charge concentration points (CC1, CC2) appear in the plain bisecting the C− S−C angle. They can be directly associated with sulfur electron lone pairs. Within the thiophene ring plane, the couple of (3, +1) CDs (CD2, CD3) can be found approximately along the C−S bond directions, and one CD1 (3,+1) is located in the bisecting direction. Selected numerical values of ρ(r) and L(r) are given in Table 4 (full information is deposited in the SI, Table S8). Table 4. Topological Characteristics of Selected Critical Points IV_S1S2S3S4 D6 dimer S···S interaction III_S1S2S3S4 D1 dimer S···C(π) interaction

CP type/function

ρ(rCP)/e·Å−3

L(rCP)/e·Å−5

CC1 (3, −3)/L(rCP) CC2 (3, −3)/L(rCP) CD1 (3, +1)/L(rCP) BCP1(S···S)/ρ(rCP) BCP2(S···C(π))/ρ(rCP) BCP3(S···C(π))/ρ(rCP) BCP4(S···C(π))/ρ(rCP) CCP1(S···C(π))/ρ(rCP)

1.25 1.24 0.35 0.053 0.041 0.042 0.044 0.035

11.45 11.42 −1.69 −0.55 −0.49 −0.47 −0.54 −0.55

Depending on the molecular environment, each CC and CD point can be arranged in an intermolecular interaction. However, the approach to CD2 and CD3 sites is hampered due to close proximity of the C−H group or the silicon atom. Therefore, the formation of S···S contact with expected n(S) → σ*(C−S) geometry is not observed in studied systems. Instead, the thienyl rings are oriented in such a way that the CC1 site of one molecule is directed toward CD1 of another one and vice versa (Figure 10b). The distance between sulfur atoms in this contact is 3.60 Å, and the QTAIM calculations revealed the formation of (3, −1) bond critical point (BCP) of electron density between them. The values of electron density and Laplacian at this point (ρ(r) = 0.053 e·Å−3, L(r) = −0.55 e·

Figure 11. DSC patterns for studied thienylsilanes. J



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on the corresponding dSi···Si = f(T) curve of III and a plateau for IV (Figure 12b,c). Such discontinuities are usually explained by molecular flipping, group rotation, or phase transitions. However, the full single-crystal measurements and structure refinements at 200 K (III), 220 K (IV), and 300 K (III and IV) do not support these theses; i.e., the intermolecular distances change, but the molecular geometry and supramolecular arrangements are essentially preserved. The expansion within columns proceeds in a quite similar manner for both tetrakis(thienyl)silanes, but in the case of IV, the molecules are closer to each other by about 0.08 Å. This is because the C(α)−H···C(π) interactions play a vital role in IV, while weaker S···C(π) contacts contribute to the dimers D1 of III. Quite a different thermal expansion occurs in the molecular layers. First of all, the distance relation is reversed; i.e., the molecules within layers are slightly better packed in III. However, when the temperature exceeds ca. 250 K, these differences become less pronounced. 2.2.3. Solid−Liquid Equilibria for III and IV. To shed more light on the properties of crystal lattices of III and IV, solid− liquid equilibria (SLE) for the binary systems with 1,4-dioxane, thiophene, or toluene have been determined by a dynamic method in the temperature range of 293−363 K. The respective solubility curves are presented in Figure 13. For comparison,

Table 5. Melting points (Tm) and Enthalpies of Fusion (ΔH) Derived from DSC Measurements sample mass/mg

Tm/°C

ΔH/kJ mol−1

4.27 3.38 2.44 4.46 0.94

28.6 71.3 131.5 221.2 180−210

16.0 20.9 27.0 29.1 28.0

I II III IV III+IVa a

One single crystal.

(I, II), this difference is ca. 40 °C, while for (III, IV), it is significantly higher and equals ca. 90 °C. The enthalpy of fusion is also notably higher for the 3-thienylsilanes. Furthermore, the DSC analysis performed for a co-crystal III+IV shows that it melts over a very wide temperature range (180−210 °C), and the enthalpy of fusion is in between values obtained for III and IV. 2.2.2. Temperature-Dependent Evolution of III and IV Crystal Structures. To analyze the strength of competitive intermolecular interactions, we performed variable temperature single crystal X-ray diffraction analysis of III and IV by heating single crystals of these compounds from 100 to 320 K with an interval of 10 K. The thermal expansion of a material strongly depends on its crystal packing;123−129 therefore it may indicate some differences in intermolecular interactions between the molecules III and IV. A full set of crystal data for all temperatures is not necessary, as the Si···Si distances between the tetrathienylsilane molecules in the dimeric molecular motifs D1, D2, and D6 are simply related to the unit-cell parameters (dSi···Si (D1) = c, dSi···Si (D2) =

c2 4

+

a2 2

, dSi···Si (D6) = a). It is

well-known that the structural deformations caused by changes of temperature are more pronounced in the direction in which the intermolecular interactions are weaker and vice versa. Indeed, in the both cases, the distances between molecules increase more rapidly in the [001] columns (the D1 dimer) than in the (001) layers (D2 and D6 dimers), which indicates that the intermolecular interactions between the molecules in columns are not as strong as the interactions in layers. This is in a good agreement with theoretical calculations (ED1 vs ΣED2). In the range of 180−200 K for III and 200−220 K for IV, this expansion is even more pronounced (Figure 12a), and it is accompanied by the contraction of the structure in the perpendicular directions which gives a well-defined minimum

Figure 13. Solubilities of III and IV in toluene, thiophene, and 1,4dioxane. Ideal solubilities are represented by solid lines. x stands for a mole fraction of solute in saturated solution, and T is absolute temperature. Numerical data are deposited in the SI (Table S2).

Figure 12. Thermal expansion of Si···Si distances in dimers D1 (a), D2 (b), and D6 (c) of III and IV. The size of the points approximate measurement deviations. Numerical values are deposited in SI (Table S3). Note that the dSi···Si-axis scale is different for a, b, c. K



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Such structural and energetic features determine the macroscopic properties of the studied compounds. 3Thienylsilanes present much higher melting points and larger enthalpy of fusion with respect to their 2-thienyl isomers (for example, IV melts at about 220 °C, while III melts at only 130 °C). The studied systems show also very different solubility behaviors in toluene, 1,4-dioxane, and thiophene, indicating the involvement of specific solute−solvent interactions (probably chalcogen bonds such as S···S and O···S). In the case of IV, the strong positive deviation from ideality determined for all studied binary systems is due to the strong self-association of this compound in solution. The differences in energy distribution within crystal structures of III and IV are also manifested by different crystal thermal expansion behaviors monitored by multitemperature unit-cell X-ray diffraction measurements. On the other hand, the structural similarity of III and IV facilitates the formation of co-crystals; however, because of the different solubility behaviors, they do not form solid solutions in any proportion. Finally, we have also analyzed the topological features of intermolecular S···S and S···C(π) interactions. The character of these interactions is determined by the anisotropic distribution of electron density around the sulfur atom and the occurrence of electron deficient and electron concentration sites. Consequently, the S···S and S···C(π) interactions involve a mutual arrangement of CC1 (associated with sulfur lp) and CD1 (located on C−S−C bisecting direction) atomic sites. It is noticeable that the topological feature of the S···S interaction is similar to the ones observed in other chalcogen-bonded systems and confirms the highly directional character of this interaction type. We suggest that the presented observations regarding the effect of positional isomerism on the structural and selected physicochemical features of oligothienylsilanes may have a more general character. For instance, even low-molecular weight thiophene derivatives such as 2,2′- and 3,3′bithiophenes have very different melting points (31 and 133 °C, respectively),130−132 which is in line with our results indicating stronger stabilization of the crystal lattice in 3-thienyl isomers relative to their 2-thienyl analogues. In practical terms, this is especially interesting and important how the structural diversity of oligothiophenes may be reflected by their properties related to widespread use in optoelectronics (e.g., charge transport ability). We believe that a better understanding of possible intermolecular interactions and structural phenomena in this class of compounds will facilitate the rational design of novel materials based on a thiophene core.

ideal solubilities have been calculated based on the melting point values and molar enthalpies of fusion obtained from DSC measurements. It is clear from these experiments that III is much better soluble than IV in all tested solvents. For both isomeric silanes, the solubilities increase in the order: toluene ≪1,4-dioxane < thiophene. This indicates that specific solute− solvent interactions involving heteroatoms in 1,4-dioxane and thiophene (such as S···S and S···O chalcogen bonds) may be responsible for the improved solubilities with respect to toluene. For III, a comparison with the ideal solubility shows that a strong positive deviation from ideality occurs only for the system III+toluene. A different situation occurs for IV as a strong positive deviation from ideality was determined for all studied binary systems. This points to a stronger tendency to self-association of IV in solution. This is also in agreement with our structural and computational analyses, which shows that IV is better stabilized in its crystal form than III.

3. SUMMARY AND CONCLUSIONS In summary, we have presented detailed investigations on the structure−energy−property relationships for the series of model oligothienylsilanes, tris(thienyl)silanes (I, II) and tetrakis(thienyl)silanes (III, IV), using a combination of experimental and computational procedures. Despite the fact that all studied systems show a high level of structural similarity (III and IV are isostructural), they revealed very different physicochemical behaviors. We found that all structures are dominated by C−H···C(π), S···C(π), C−H···S, and S···S interactions. The computations show that the strength of these interactions decreases in the series C(α)−H···C(π) (∼10−14 kJ mol−1) > C(β)−H···C(π) (∼7−11 kJ mol−1) > S···C(π) (∼7−8 kJ mol−1) ≫ C−H···S (∼4 kJ mol−1) ≈ S···S (∼3 kJ mol−1) > C−H···H−C (∼1 kJ mol−1). In the combination with crystal symmetry, this leads to a different distribution of energy within the crystal structures. For instance, in III, the 1D columnar motif (dimer D1) involves the S···C(π) interactions, which are gradually replaced by more favorable C(β)−H···C(π) interactions when thienyl rings reorient to the S* position. This additional stability is counterbalanced at the expense of a decreased stability of the dimer D2 (C(α)−H··· C(π) interactions are replaced by the C(β)−H···C(π) ones). However, these effects are comparable in magnitude, and so the cohesive energy of III is at a similar level irrespective of the orientation of the thienyl groups. The different situation occurs for IVthe stabilization of the dimer D1 slightly increases when C(β)−H···C(π) interactions are replaced by more favorable C(α)−H···C(π) ones. Simultaneously, the dimers D2 become significantly less advantageous as C(α)−H···C(π) interactions are interchanged by S···C(π) ones. As there are four dimers D2 per one dimer D1, the conformation I_S1S2S3S4 is the most stable. This rationalizes the observed disorder magnitude in structures III (60:40) and IV (86:14). Similar conclusions can be derived from the comparison of I and II. In the case of I, two of the three thienyl rings are affected by disorder, which is because the reorientation of thienyl groups only changes the pattern of the interaction leaving the cohesive energy at the same level. In contrast, the crystal structure of II is based mainly on C(α)−H···C(π) interactions which are beneficial for the crystal stability. We have estimated the average cohesive energy values concluding that the 3-thienylsilanes are evidently more stable than their 2thienyl counterparts (I vs II: −140.2 kJ mol−1 vs −145.8 kJ mol−1; III vs IV: −166.4 kJ mol−1 vs −180.3 kJ mol−1).

4. EXPERIMENTAL SECTION 4.1. Materials and Crystallization. All studied thienylsilanes were synthesized in accordance with the procedure already published in the literature.82−84 Crystallizations of II−IV were achieved by partial evaporation of corresponding acetone solutions, whereas I was crystallized by cooling its acetone solution slowly to −20 °C. All compounds form crystals with well-defined faces. The crystal structure of III has been known since 1974, but it was then measured at room temperature.133 Therefore, to get data of better quality and to compare it with remaining structures, the structure of III was determined again by the measurement performed at 100 K. The co-crystallization of equal amounts of III and IV provided co-crystals III+IV. We performed X-ray diffraction experiments for three different single crystals obtaining structures differing in their compositions (III+IVa − III/IV = 70:30), (III+IVb − III/IV = 82:18), (III+IVc − III/IV = 94:6). Their composition was confirmed by the 1H NMR spectroscopy L



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Table 6. Selected Crystal Data, Data Collection and Refinement Parameters for Measured Thienylsilanes I formula molecular mass, Mr/a.u. temperature, T/K crystal system space group a/Å b/Å c/Å α/° β/° γ/° volume, V/Å3 dcalc/g·cm−3 F(000) radiation type absorption coefficient, μ/mm−1 no. of measured, independent, observed [I > 2σ(I)] reflections Rint (%) GOF R[F]/wR[F2] (I > 2σ(I)) max. and min residual density/e·Å−3

C13H12S3Si 292.50 100(1) orthorhombic Pccn 15.157(1) 27.989(1) 6.615(1) 90 90 90 2806.1(5) 1.385 1216 Cu Kα 5.43 31956, 2961, 2905 2.9 1.051 2.64%/6.92% +0.41/−0.33

II C13H12S3Si 292.50 100(1) monoclinic Cc 9.281(1) 28.557(1) 6.577(1) 90 125.38(1) 90 1421.14(4) 1.367 608 Cu Kα 5.36 9260, 2687, 2686 2.2 1.095 4.08%/11.21% +1.11/−0.77

III

IV

C16H12S4Si 360.59 100(1) tetragonal P42̅ 1c 11.261(1) 11.261(1) 6.349(1) 90 90 90 805.07(3) 1.487 372 Cu Kα 6.04 5960, 845, 843

C16H12S4Si 360.59 100(1) tetragonal P42̅ 1c 11.321(1) 11.321(1) 6.302(1) 90 90 90 807.68(3) 1.483 372 Cu Kα 6.02 5330, 841, 834

3.3 1.225 2.24%/5.54% +0.25/−0.20

2.7 1.069 1.84%/4.72% +0.19/−0.17

performed using the same crystals after the diffraction experiment performance (Figures S7−S9, SI). 4.2. Crystal Structure Determination. X-ray diffraction data sets for single crystals of I−IV were collected at 100 K on a SuperNova diffractometer equipped with an Atlas detector (Cu-Kα radiation, λ = 1.5418 Å). The structure of co-crystal III+IVa was determined on SuperNova diffractometer equipped with an EOS measurement device type (Mo-Kα radiation, λ = 0.7107 Å). Data reduction and analysis were carried out with the CrysAlisPro program.134 All structures were solved by direct methods using SHELXS-97135 and refined using SHELXL-2014.136 All non-hydrogen atoms were refined anisotropically. Crystallographic information files (CIFs) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications no. 1456382−1456388. Relevant crystallographic data are provided in Table 6. 4.3. Disorder Modeling. In almost all studied thienylsilanes, the thienyl rings were disordered over two positions. The only exception is the S1 ring of Ithe difference-Fourier map shows no particular regions of positive or negative electron density (Figure S5, SI). In the case of II, the difference-Fourier is not flat as there are regions of negative electron density found around the sulfur atom, and positive electron density around the neighbored carbon atom, which clearly indicates the disorder of the thienyl ring. However, since the contribution of the second fraction is low (∼5%), we were unable to propose any reasonable model including the disorder. In I, III, and IV the occupancies were refined freely. In the co-crystal III+IVa, the thienyl groups were observed to be disordered over four positions, which is due to the superpositions of 2thienyl and 3-thienyl isomers and disorder of the thienyl rings between two positions within each component. The positions of carbon atoms of the minor component were restrained using the EXYZ and EADP commands. In the case of the crystal structures III+IVb and III+IVc, the contribution of IV was too low to model the atomic positions of the second fraction of this component. Therefore, in the final refinement, the thienyl groups were disordered over three positions. 4.4. Thermal Characterization. Differential scanning calorimetric analysis was conducted by differential scanning calorimetry (DSC) (model DSC 1, Mettler-Toledo) under the flow of nitrogen atmosphere. The calibration of the instrument was performed using the phase-transition temperature and phase-transition enthalpy of indium as a reference material. The samples were prepared in covered 40 μL aluminum pans with a hole in the lid to allow venting. An empty

III+IVa

III+IVb

C16H12S4Si 360.59 100(1) tetragonal P42̅ 1c 11.335(1) 11.335(1) 6.293(1) 90 90 90 808.51(3) 1.483 372 Mo Kα 0.65 13533, 2619, 2442 3.3 1.189 5.75%/13.25% +0.40/−0.54

C16H12S4Si 360.59 100(1) tetragonal P42̅ 1c 11.281(1) 11.281(1) 6.340(2) 90 90 90 806.93(4) 1.484 372 Cu Kα 6.02 5795, 843, 841 3.4 1.226 2.53%/6.46% 0.24/−0.26

III+IVc C16H12S4Si 360.59 100(1) tetragonal P42̅ 1c 11.274 (1) 11.274 (1) 6.349 (1) 90 90 90 806.98 (2) 1.484 372 Mo Kα 0.65 24955, 1696, 1636 3.5 1.080 1.90%/5.01% 0.28/−0.12

pan was used as a reference. The samples were reheated after cooling with the heating rate of 5 K/min from −30 to 80 °C (I), 25 to 100 °C (II), 25 to 150 °C (III), and 25 to 240 °C (IV, III+IV). The melting points (Tm) were determined from the DSC thermograms during the programmed reheating steps, based on the onset temperatures. The enthalpy of fusion was determined as the area under the curve. 4.5. Solid−Liquid Equilibria. The dynamic method was used to study the solid−liquid equilibria (SLE) in binary systems III/IV and 1,4-dioxane/thiophene/toluene in the temperature range of 293−363 K. A sealed glass flask containing a weighted amount of solute, and solvent was stirred and slowly heated at a constant rate until the last portion of solid disappeared at temperature T. This experiment was repeated several times for each binary system changing the mole fraction of the corresponding solutes (x). The obtained results were compared with ideal solubilities of compounds. According to the Schröder approximation,137,138 the enthalpy of fusion (ΔH) is temperature constant and equals enthalpy of fusion at the melting point of pure substance (Tm). This leads to the following relation of ideal solubility (xideal) with temperature (T):

ln x ideal = −

ΔH ⎛ 1 1 ⎞ ⎜ − ⎟ R ⎝T Tm ⎠

The ΔH and Tm values were obtained from the DSC experiments.

5. COMPUTATIONAL METHODS 5.1. General Comment. To provide more reliable geometries for further computational investigations, atomic coordinates of all studied structures were subjected to constrained optimization in the CRYSTAL09 program.90,91 The starting geometries were taken directly from the corresponding X-ray structures. In the case of III and IV the P4̅21c space group of symmetry was transferred to P21 and then five different structures resulting from different conformation of molecules were generated. All X−H bonds were elongated to standard neutron distances prior optimization.139 As our intention was to stick relatively close to the experimental crystal geometries, we have only optimized the positions of atoms within thienyl rings including sulfur, carbon, and hydrogen atoms, while the positions of silicon and all C−Si carbon atoms as well as dihedral angles between thienyl moieties were constrained. The unit-cell parameters were also constrained. This allowed us to obtain more reliable intramolecular bond angles and distances, which was especially important for the structures generated M



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from S* conformers as the atomic positions in these cases were poorly described from the experiment. In the case of structures derived from the S conformations, the obtained intermolecular distances and angles match very well to the experimental ones. The overlay of the molecular geometries before (X-ray experimental structures with C−H normalized distances) and after optimization procedure is provided in Figure S6 (SI). The atomic coordinates obtained after periodic optimizations (Tables S5−S7, SI) were used for further computational investigations in PIXEL and Gaussian09. According to studies by Alkorta and co-workers, the hybrid B3LYP and M06-2X functionals give good results for neutral complexes featuring chalcogen interactions (CCSD(T)/aug-cc-pVTZ as reference),140 and thus the Gaussian09 calculations were performed using the M06-2X functional with the 6-311+G** basis set. 5.2. Gaussian09 Calculations. The single-molecule optimizations were performed for all conformers of I−IV at the M06-2X/6311+G** level of theory. The minima were confirmed by the absence of imaginary frequencies. During the calculations no symmetry constraints were applied. To estimate the values of C−H···C(π), S··· S, S···C(π), C−H···S interaction energies, we have performed singlepoint calculations on model thiophene dimeric systems (M06-2X/6311+G**). The initial geometries were extracted from the CRYSTAL09 optimized structures, and then silicon atom together with remaining thienyl groups were replaced by the hydrogen atom with the C−H distance of 1.083 Å (average neutron value). The obtained values were corrected for the basis-set superposition error using the counterpoise procedure. 5.3. CRYSTAL09 Calculations. All energy computations within the CRYSTAL09 program package were performed at the DFT(B3LYP) level of theory with POB triple-ζ valence + polarization basis set (TZVP).89 Both Grimme dispersion correction and correction for the basis set superposition error were applied.141,142 Ghost atoms were selected up to 15 Å distance from the studied molecule in a crystal lattice and were used for the basis set superposition error estimation. The evaluation of Coulomb and exchange series was controlled by five thresholds, set arbitrary to values of 10−7, 10−7, 10−7, 10−7, 10−25. The condition for the SCF convergence was set to 10−7 on the energy difference between two subsequent cycles. Shrinking factor was equal to 4, which refers to 30−36 k-points (depending on space group symmetry) in the irreducible Brillouin zone in the case of the studied systems, and ensures the full convergence of the total energy. The cohesive energy (Ecoh) was calculated as described below: Ecoh =

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00358. Experimental details regarding crystallographic data, details on thermal analysis, theoretical calculations, 1H NMR spectra and supplementary results (PDF) Accession Codes

CCDC 1456382−1456388 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Warsaw University of Technology. The X-ray measurements were carried out at the Biological and Chemical Research Centre, University of Warsaw, established within the project cofinanced by European Union from the European Regional Development Fund under the Operational Programme Innovative Economy 2007-2013. The authors thank the Interdisciplinary Centre for Mathematical and Computational Modelling in Warsaw (G33-14) and the Wrocław Centre for Networking and Supercomputing (Grant No. 285) for providing computational facilities. We gratefully acknowledge the Aldrich Chemical Co., Milwaukee, WI, USA for a long-term collaboration.

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REFERENCES

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1 E bulk − Emol Z

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