The Effect of Steric Hindrance on the Association of Telluradiazoles

Sep 23, 2005 - Synopsis. DFT calculations have been successfully used to predict the degree of association of benzotelluradiazoles through secondary b...
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CRYSTAL GROWTH & DESIGN

The Effect of Steric Hindrance on the Association of Telluradiazoles through Te-N Secondary Bonding Interactions

2006 VOL. 6, NO. 1 181-186

Anthony F. Cozzolino, James F. Britten, and Ignacio Vargas-Baca* Department of Chemistry, McMaster UniVersity, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada ReceiVed June 10, 2005; ReVised Manuscript ReceiVed August 18, 2005

ABSTRACT: DFT calculations were used to compare the magnitude of steric repulsion to the strength of secondary bonding interactions (SBIs) in the (Te-N)2 supramolecular synthon to explain or predict the supramolecular structures assembled by two derivatives of the 1,2,5-telluradiazole ring: benzo-2,1,3-telluradiazole (4c) and 3,6-dibromobenzo-2,1,3-telluradiazole (5). The crystallographically determined structures were consistent with the computational predictions. In sharp contrast with the previously known structures of its sulfur and selenium analogues, 4c forms infinite ribbon chains in the solid state with 2.682(7)-2.720(7) Å Te-N SBIs. Steric hindrance in 5 restricted the supramolecular association to form discrete dimers with 2.697(8) Å Te-N SBIs. In addition to discrete dimers, the dibromo derivative crystallizes as solvated dimers in 5‚DMSO with 2.834(5) Å Te-O SBIs. Other weaker SBIs were identified in the crystal lattices and were assessed by the computational method. Introduction

Chart 1

There is increasing interest in the use of nontraditional supramolecular interactions in chemistry. This is well illustrated by two examples that recently have received much attention: the halogen bond1 and the self-assembly of tubular structures by chalcogen-chalcogen interactions.2-8 Both of these are cases of a more general phenomenon that is frequently observed in the chemistry of compounds that contain heavy main-group elements: secondary bonding interactions (SBIs). These attractive interactions are the product of a combination of electrostatic and orbital contributions. Despite the frequency with which SBIs are observed in the solid state, there are few cases in which they have been successfully used on purpose as an element of design in supramolecular architecture. It has been pointed out that this is due to a paucity of reliable supramolecular synthons rather than to an inherent weakness or lack of directionality of the SBIs.9 It has been proposed that stable and highly directional SBI supramolecular synthons would be formed by molecules linked to each other through simultaneous multiple points of attachment, analogous to the most successful hydrogen-bonded structures. One example is the (Te-N)2 supramolecular synthon (Scheme 1), which is observed in the crystal structure of TeScheme 1

(NMe2)2 (1),10 where the molecules are arranged in ribbon polymers held by 2.96 Å long Te-N SBIs. Tellurium diamides are, however, not very convenient supramolecular building blocks because they would likely yield puckered structures and, more importantly, changes of the substituents on nitrogen are known to disrupt the formation of the (Te-N)2 supramolecular synthon.11,12 A careful survey of crystallographic data showed that the same motif is ubiquitous in the structures of the 1,2,5chalcogenadiazoles (2) and their derivatives (Chart 1).13 The Te-N SBIs in the case of telluradiazoles are shorter, 2.77 Å * Corresponding author. Tel: 905 525 9140, ext 23497. Fax: 905 522 2509. E-mail: [email protected].

on average. A DFT analysis provided interaction energies comparable to those of hydrogen bonds and described the orbital contribution of these SBIs as a double donor-acceptor interaction between the lone pairs of nitrogen and the σ* orbitals centered on tellurium. Reasonable estimations for the stability of polymeric structures were obtained by building short chain models. In addition, the binding energies were compared to contributions from factors that could potentially compete with supramolecular association such as geometric distortions and steric repulsion. For example, it was found that the cost of puckering to accommodate the formation of infinite chains observed in the crystal structure of phenanthrotelluradiazole (3) is offset by the very strong binding energy of the (Te-N)2 supramolecular synthons. On the other hand, H-H steric repulsions would be strong enough to interfere with the formation of the (E-N)2 synthon in the cases of benzo-2,1,3-

10.1021/cg050260y CCC: $33.50 © 2006 American Chemical Society Published on Web 09/23/2005

182 Crystal Growth & Design, Vol. 6, No. 1, 2006

thia- (4a) and benzo-2,1,3-selenadiazole14 (4b); the isomorphic crystal structures of these compounds only display very long intermolecular E-N distances and no coplanar association. In contrast, the calculations forecasted the formation of ribbon chains for benzo-2,1,3-telluradiazole (4c);13 this appears to be in conflict with the nonassociated structure that has been claimed for perfluorobenzotelluradiazole, although actual crystallographic data is not published.15 We have investigated the crystal structure of benzotelluradiazole and hereby report that it indeed consists of infinite ribbon chains as predicted by DFT calculations. We have also challenged the computational method to predict the structure of a benzotelluradiazole substituted with bromine atoms, 5, in which the increase of steric repulsion might induce changes in the molecular organization. The use of DFT in the study of the association of molecules that contain heavy main-group elements is an approximation since dispersion forces are not properly accounted for, but these are not expected to contribute more than 5 kJ/mol.16 Experimental Section Materials and Methods. The manipulation of air-sensitive materials was performed under an atmosphere of dry argon or nitrogen with standard Schlenk and glovebox techniques. All solvents and reagents were dried and purified by standard procedures immediately before each experiment. Phenylenediamine was recrystallized before use; TeCl4 was prepared by direct combination of the elements;17 3,6-dibromo1,2-phenylenediamine was synthesized following a literature method.18 Elemental analyses were performed by Guelph Chemical Laboratories Inc. (Guelph, Ontario, Canada). Spectroscopic Instrumentation. IR spectra were recorded using a Bio-Rad FTS-40 FT-IR spectrometer. Each spectrum was acquired from a KBr pellet with a resolution of 8 cm-1, and the background, which was simultaneously subtracted, was recorded prior to a spectral acquisition. FT Raman spectra were recorded at ambient temperature in sealed Pyrex melting point capillaries using a Bruker RFS 100 spectrometer equipped with a quartz beam splitter and a liquid nitrogen cooled Ge diode detector. The 1064 nm line of an Nd:YAG laser (350 mW maximum output) was used for excitation of the sample with a spot of ca. 0.2 mm at the sample using 10-300 mW of power, and the backscattered radiation was sampled. The actual usable Stokes range was 100-3500 cm-1 with a spectral resolution of 2 cm-1. The Fourier transformations were carried out by using a Blackman-Harris fourterm apodization and a zero-filling factor of 4. The 1H, 13C{1H}, and 125 Te NMR spectra in solution were recorded on a Bruker AV200 (200.13 MHz) or DRX500 (500.13 MHz) spectrometer. Chemical shifts are reported in ppm and were referenced to the residual proton peak of d6-DMSO (δ 2.50, 1H NMR; δ 39.52, 13C{1H} NMR) or an external standard solution of Ph2Te2 in CH2Cl2 (δ 420.36; 125Te NMR) previously referenced to Me2Te (δ 0.0; 125Te NMR). The d6-DMSO solvent for NMR spectroscopic measurements was dried over freshly washed and activated 4-Å molecular sieves. For the 125Te NMR spectrum, 50 000 transients were recorded, and a line broadening of 50 Hz was used. The benzotelluradiazoles 4c and 5 were prepared using a modification of Zibarev’s procedure;15 the reaction of TeCl4 with the appropriate diamine in pyridine was followed by the addition of a strong base. In a typical reaction, TeCl4 (5.00 g, 18.5 mmol) was dissolved in 275 mL of pyridine. This was added dropwise with stirring to a solution of the diamine (92.8 mmol) in 175 mL of the same solvent. The mixture was stirred for 10 min and an excess of Et3N (15 mL) was added. Stirring was maintained for 1 h before removing the solvent under vacuum. The residue was recrystallized from DMSO and washed with toluene. The product obtained in this way can be further purified by successive steps of recrystallization, sublimation, or both. TeN2C6H4 (4c). Yield: crude 4.1 g (0.017 mol, 94%); after recrystallization 2.3 g (0.0098 mol, 53%). Anal. Calcd. for TeN2C6H4: C, 31.10; H, 1.74; N, 12.09. Found: C, 31.36; H, 1.54; N, 12.35. 1H NMR (200 MHz, d6-DMSO): δ 7.50, 7.48, 7.47, 7.45, 7.26, 7.24, 7.22, 7.20 (AA′BB′, 4H, aryl). 13C{1H} NMR (50 MHz, d6-DMSO): δ 166.58 (aryl, 4°), 128.68, 128.16 (aryl, CH). 125Te NMR (158 MHz, TeMe2): δ 2403. Raman (cm-1): 3061m, 3053m, 3045m, 3035m,

Cozzolino et al. 3001w, 2961w, 1511m, 1440vs, 1350m, 1343m, 1304vs, 1285w, 1203vw, 1153vw, 1148m, 1132m, 982m, 938vw, 799m, 702m, 691vs, 593vw, 545s, 472m, 299s. IR (cm-1): 3067m, 3053m, 3008m, 1508s, 1150m, 1132w, 902w, 778w, 731vs, 702m, 685vs. MS (EI, %): m/z 233.9437 (M+, 10), 129.9121 (M+ - C6H4N2, 10). TeN2C6H2Br2 (5). Yield: crude 1.1 g (0.0027 mol, 92%); after recrystallization 0.51 g (0.0012 mol, 43%). Orange crystals suitable for X-ray diffraction were grown from a DMSO solution. Anal. Calcd. for TeN2C6H2Br2: C, 18.50; H, 0.52; N, 7.19. Found: C, 18.77; H, 0.69; N, 6.88. 1H NMR (200 MHz, d6-DMSO): δ 7.54 (s, 2H, aryl). 13 C NMR: The sample was not soluble enough to give a solution NMR spectrum. 125Te NMR (158 MHz, TeMe2): δ 2368. Raman (cm-1): 3047w, 3021w, 1479s, 1371vs, 1350w, 1227m, 1077m, 830vw, 725w, 701m, 662s, 577w, 554m, 325m. IR (cm-1): 3063w, 3049w, 3020m, 1860w, 1656w, 1585w, 1476m, 1350w, 1329m, 1163s, 1076m, 921s, 890s, 831s, 725s, 702m, 577m, 549m, 431vs. MS (EI, %): m/z 389.7621 (M+, 10σ(I) after integration of all the frames using SAINT20 for each of 4c, 5, and 5‚ DMSO. The data were empirically corrected for absorption and other effects using TWINABS23 for 4 and SADABS24 for both 5 and 5‚DMSO. The structures were solved by direct methods and refined by full-matrix least squares on all F2 data using SHELXL25 as part of the WINGX package.26 The non-H atoms were refined anisotropically, while H atoms were constrained to idealized positions using appropriate riding models. Although the thermal refinement on the twinned samples did not behave ideally, no restraints were employed. Molecular graphics were produced using ORTEP-327 or Mercury 1.4.28,29 Computational Methods. The structures considered in this study were fully optimized using the ADF DFT package (SCM, version 2004.01).30-32 The adiabatic local density approximation (ALDA) was used for the exchange-correlation kernel,33,34 and the differentiated static LDA expression was used with the Vosko-Wilk-Nusair parametrization.35 The calculations of model geometries were gradientcorrected with the exchange and correlation functionals of the gradient correction proposed by Perdew and Wang,36,37 which usually better reproduce the geometries of heavy main-group systems38 than the Becke and Perdew corrections used in previous studies.13 Preliminary geometry optimizations were conducted using a double-ζ basis set with frozen cores corresponding to the configuration of the preceding noble gas and no polarization functions; the resulting structures were refined using a triple-ζ all-electron basis set with one polarization function and applying the zeroth-order relativistic approximation (ZORA)39-43 with specially adapted basis sets. Geometry constraints were used when point group symmetry was applicable. Association energies were calculated as the difference of total bonding energies of adducts and molecular fragments according to the scheme of Ziegler and Rauk.44-46 Additional visualization of the computational results was performed using Cerius2 (Accelrys) supplemented by the ADF SDK (SCM) interface.

Results and Discussion Orange crystals of 4c suitable for X-ray diffraction were grown from a pyridine solution. Although a clean rectangular plate was mounted in the diffractometer, analysis of the scattering data revealed that the sample consisted of multiple crystals. An initial structure was obtained using the data for one component; the remaining components where then inte-

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Crystal Growth & Design, Vol. 6, No. 1, 2006 183

Table 1. Crystallographic and Refinement Data for 4c, 5, and 5‚DMSO compound empirical formula crystal system space group a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] V [Å3] Z F(calcd) [g‚cm-3] T [K] µ [mm-1] θ range (deg) limiting indices

4c C6H4N2Te monoclinic C2/c (No. 15) 10.397(5) 11.403(6) 22.46(1) 90.00 101.399(9) 90.00 2610(2) 16 2.359 173(2) 4.457 2.68-28.33 -13 e h e 13 0 e k e 15 0 e l e 29 11312/6507 0.0552 165 0.0599/0.1672 0.0687/0.1738 1.084 1.199/-1.164

reflns collected/unique R(int) no. of params R1/wR2 (I > 2σ(I))a R1/wR2 for all dataa GOF on F2 largest difference peak/hole [e‚Å-3] a

5 C6H2Br2N2Te monoclinic P21/c (No. 14) 3.9476(9) 19.707(4) 10.501(2) 90.00 100.755(7) 90.00 802.6(3) 4 3.224 173(2) 13.591 1.97-28.55 -5 e h e 5 -25 e k e 25 -13 e l e 13 6913/1914 0.0639 102 0.0432/0.0913 0.0568/0.0954 1.040 2.080/-1.161

5‚DMSO C8H8Br2N2TeSO triclinic P1h (No. 2) 6.905(2) 8.431(2) 11.932(3) 100.118(7) 104.354(7) 109.822(7) 606.8(2) 2 2.559 173(2) 2.559 1.84-36.28 -9 e h e 11 -13 e k e 10 -19 e l e 17 11043/4994 0.0485 137 0.0506/0.1028 0.1134/0.1242 0.957 2.157/-2.560

R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo2 - Fc2)2]/∑w(Fo2)2}1/2.

Table 2. Selected Bond Distances (Å) and Angles (deg) for 4c, 5, and 5‚DMSO

Te(1)-N(1) Te(1)-N(2) Te(2)-N(3) Te(2)-N(4) N(1)-Te(1)-N(2) N(3)-Te(2)-N(4)

4c

5

5‚DMSO

2.015(7) 1.988(7) 2.007(7) 2.001(7) 83.7(3) 84.0(3)

1.994(8) 1.982(7)

2.002(5) 2.000(5)

85.5(3)

85.1(2)

grated separately to reach the final solution. An array of infinite ribbon chains, as had been predicted on the basis of the balance of steric repulsions and SBIs, was revealed. The crystallographic data and refinement parameters are summarized in Table 1, and selected bond lengths and angles are given in Table 2. The asymmetric unit (Figure 1) consists of two molecules of 4c associated through the (Te-N)2 supramolecular synthon and sits in general positions; the two molecules are almost coplanar (the intermolecular dihedral angle is 2.5(4)°) and are not related by symmetry, but their internal dimensions are essentially

Figure 1. ORTEP representation and numbering scheme for the asymmetric unit in the crystal structure of 4c. Displacement ellipsoids are shown at the 50% probability level.

identical. The Te-N bond distances (average 2.003 Å) are comparable to those observed in the telluradiazoles 2c (2.023(6) Å)47 and 3 (2.023(6) Å).48 The C-C bond distances within each aromatic ring are not equal: d(C2-C3), d(C4-C5), d(C8C9), and d(C10-C11) range from 1.32(1) to 1.37(1) Å, while the alternate bond lengths are between 1.40(1) and 1.48(1) Å. The C-N bond distances, 1.31(1)-1.33(1) Å, are typical of double bonds. These features suggest a strong degree of localization of the double bonds and have also been observed in the sulfur49 and selenium50 analogues. The (Te-N)2 supramolecular synthon is repeated at both ends of the asymmetric unit connecting the molecules generated by the combination of the C-centering operation and the translation of the unit cell; this builds the ribbon-polymer chains. Application of the inversion center and the inverted C-centering operation generates parallel ribbons that stack on top of each other at intervals of 3.670 and 3.710 Å. A c-glide generates another stack of ribbons rotated by 80.59(7)° and completes the crystal structure (Figure 2). Aside from the (Te-N)2 supramolecular synthon, all the distances from tellurium to its closest neighbors are comparable to or greater than the sum of van der Waals radii. The average

Figure 2. Two views of the crystal structure of 4c: (a) along (5, 6, -2); (b) along (1, 0, -6). Displacement ellipsoids are shown at the 50% and 25% probability levels, respectively.

184 Crystal Growth & Design, Vol. 6, No. 1, 2006

Cozzolino et al.

Figure 3. Optimized arrangement of a set of three molecules of 5.

d(Te-N, SBI) of 2.701(7) Å is comparable to that of 2c (2.764(6) Å)47 but shorter than the distances observed in 3 (2.825(8) and 2.842(8) Å);48 both telluradiazoles have ribbon-polymer structures. The SBI lengths observed in 3 can be explained by strong steric repulsions. The structure of the ribbon polymer of 4c from this crystallographic determination is in excellent agreement with the values that had been estimated from the DFT optimization of a model hexamer chain, both at the intramolecular and supramolecular levels. For example, within the central telluradiazole rings, the calculated Te-N distances were 2.054 and 2.055 Å, and the Te-N SBI distance calculated at the middle was 2.690 Å. Due to the reorganization of molecular dimensions that takes place upon association, only the dimensions at the center of the model should be compared. Because the DFT calculations had been successful at predicting the association of 4c molecules into a ribbon and provided excellent SBI distances, an attempt was made to test the ability of the method to deal with more intense steric demands. The very large tert-butyl substituents in 6 restrict supramolecular association to discrete dimers.51 Instead of resorting to such voluminous groups, smaller bromine atoms were considered in the target compound 5, raising the question: what supramolecular structure would be obtained? The first possibility is a ribbon polymer with longer SBI, just as in 5, the second is the coplanar dimer, and the third is an alternative helical chain based on the crystallographic structures of some tellurazoles.52,53 The computational method succeeded in optimizing a coplanar dimer. In the system with three molecules, the calculation predicted an associated dimer with the third molecule appearing to have the tellurium atom oriented toward the nitrogen atom but the bromine-bromine repulsion preventing the formation of the second supramolecular synthon (Figure 3). The target compound 5 was prepared and crystallized from DMSO. Two crystalline phases were readily identified, separated by hand, and analyzed by X-ray diffraction. Analysis of the data collected from the diffraction experiment for the first phase indicated that the sample was twinned. The diffraction data was processed by first classifying the reflections in two different sets and then applying an appropriate rotation matrix to one set to align it with the other. After the final structure was solved, the fraction of the minor component was determined to be 44.093%. Final crystallographic data and refinement parameters for this compound are summarized in Table 1; selected bond lengths and angles are displayed in Table 2. The asymmetric unit consists of one molecule of 5, which is connected through the (Te-N)2 supramolecular synthon (Figure 4) to the molecule generated by the inversion center and the

Figure 4. ORTEP representation and numbering scheme for the asymmetric unit plus the unit at -x, -y + 1, -z in the crystal structure of 5. Displacement ellipsoids are shown at the 50% probability level.

Figure 5. Two views of the unit cell of 5: (a) along a; (b) along (0, 1, 0). Displacement ellipsoids are shown at the 50% and 25% probability levels, respectively.

translation of a unit cell along b to form a discrete dimer as was predicted by the computational method. The dimers are arranged in parallel planes that stack separated by 3.518 Å along (1, 0, -2) (Figure 5). The intramolecular Te-N bond distances of 5, 1.994(8) and 1.982(7) Å, are statistically equal to each other and to those measured in 6, 2.006(4) and 2.002(3) Å,51 but slightly shorter than the values calculated for the model dimer 52, 2.024 and 2.041 Å. In this case, the bond distances also indicate a strong localization of double bonds. The Te-N SBI length, 2.697(8) Å, is shorter than that observed in 6, 2.764(6) Å,51 but longer than the 2.620 Å calculated for the 52 model. Interestingly, the dimers in the crystal of 5 appear to engage in SBIs with other molecules in the same plane (Figure 5). The Br1 atom is in close proximity to Br2 (3.506(2) Å) and Te1 (3.683(1) Å) of a dimer generated by a screw axis followed by a unit cell translation along a; the two contacts are shorter than the sum of the van der Waals radii (3.70 and 3.91 Å, respectively).54 All contacts between planes are comparable to the sum of van der Waals radii. A detailed analysis of these interactions was conducted by optimizing the tetramer model 5-52-5 as observed in the crystal structure (Scheme 2). The interaction energy was decomposed as described by eq 1 where

∆Etotal ) ∆Ereorganization + ∆Ebinding + ∆EBSSE

(1)

∆Ereorganization is the energetic cost of the geometrical changes that take place upon interaction, ∆Ebinding is the energy of formation the SBI between the building blocks, and ∆EBSSE is

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Crystal Growth & Design, Vol. 6, No. 1, 2006 185

Scheme 2

Table 3. Contributions to the Energy of Interaction (kJ/mol) for the Supramolecular Structures of 5 (Te-N)2 synthon 52 5-52-5 52DMSO2

∆Ebind

∆Ereorg

-90.91 -90.31 -89.76

13.22 13.88 16.28

other SBIs ∆Ebind -38.97 -92.86

∆Ereorg

∆EBSSE

∆Etotal

1.20 4.98

5.46 15.32 12.3

-72.23 -98.88 -149.06

Scheme 3

the basis set superposition error. The results are summarized in Table 3. A similar scheme was previously employed to describe these types of systems,13 but in this case the zero-point energy, which would be a small contribution, was not calculated. Instead the potentially more significant ∆EBSSE values are included; these are much smaller than the total interaction energies and are naturally dependent on the size of the model. Structural optimization of the extended model 5-52-5 did not have a discernible effect on intramolecular distances and angles but did elongate the Te-N SBI to 2.663 Å. There was a small energetic cost for this, but an overall stabilization by the additional SBIs was observed. The Br-Br contact appears to be a case of halogen bonding, one bromine acting as donor and the other as acceptor.1 Calculations performed on two molecules of 5 associated through the Br-Br interaction provided an optimized Br-Br distance of 3.500 Å and estimated a total association energy of only 0.88 kJ/mol. On the other hand, the Te-Br SBI was optimized at 3.633 Å, and the total association energy was calculated at 19.66 kJ/mol. The Te-Br SBI, however, appears to be supplemented by a weak hydrogen bond (C3-H2‚‚‚N1). By replacing Br with H in the model to give 5-7 (Scheme 3), the H-bond contribution was estimated to be 8.60 kJ/mol. Single-crystal X-ray diffraction of the second phase obtained by the crystallization of 5 from DMSO revealed the unexpected structure of a solvated dimer. Powder XRD showed peaks that corresponded only to the Miller indices of the unsolvated phase suggesting that the DMSO solvated phase is a very minor component. The crystallographic data and refinement parameters are summarized in Table 1 and the bond lengths and angles in Table 2. The asymmetric unit consisted of one molecule of 5 and one molecule of DMSO bound by a Te-O SBI. The inversion center completes the dimeric structure (Figure 6). A Te-DMSO SBI (2.82-2.91 Å long) has only been observed in the case of 2,2,7,7-tetraiodo-1,3,6,8-tetrahydro-

Figure 6. ORTEP representation and numbering scheme for the asymmetric unit plus the molecule at -x + 1, -y + 2, -z in the crystal structure of 5‚DMSO. Displacement ellipsoids are shown at the 50% probability level.

benzo[1,2-c;3,4-c′]ditellurophene.55 The structure for the solvated dimer was fully optimized by DFT and used to evaluate the strengths of the SBIs (Table 3). The calculations indicate that the two the solvent molecules are bonded to tellurium by 87.88 kJ/mol, more than the 73.48 kJ from the (Te-N)2 supramolecular synthon. There is some indication of packinginduced strain in the structure; in the optimized geometry, the oxygen atoms appear in the plane of the rings, but the crystal structure shows a deviation of 0.240(5) Å; the calculated d(TeO, SBI) is 2.737 Å, while the experimental value is 2.834(5) Å. The calculation indicates that solvation by DMSO results in lengthening of the Te-N SBI distance to 2.728 Å; this is consistent with the experimental observation, 2.744(4) Å. Similarly, the calculations predict small changes in the intramolecular bond lengths in going from the nonsolvated dimer to the DMSO solvate; the standard deviations of the present measurements do not permit the appreciation of any such changes. The crystallographic study has identified different structures in the solid state for the telluradiazoles 4c and 5; it is also of interest to probe the possibility of supramolecular association in solution. In this respect, the sensitivity of 125Te NMR to the environment, geometry, and electronegativity of substituents may be advantageous. The spectra in DMSO solution provided single resonances in both cases, at δ ) 2403 ppm for 4c and 2368 ppm for 5. The difference of just 35 ppm in the context of the 125Te NMR window is negligible and strongly suggests that these two compounds exist in solution with the same degree of association, either monomers or dimers that must be strongly solvated. Conclusions The calculations presented here and in the previous study13 have been successful at explaining and predicting the basic supramolecular structures derived from chalcogenadiazoles. Despite its intrinsic limitations, DFT was able to reproduce with remarkable accuracy the dimensions related to the SBIs. This is not the result of a fortuitous cancellation of errors; telluriumcentered SBIs have a strong degree of covalency, which minimizes the relevance of dispersion forces and base superposition errors. In the case of the benzo-derivatives, 4, the S-N and Se-N SBIs are not strong enough to overcome the repulsion between hydrogen atoms, and no coplanar association can happen. In contrast, the Te-N SBIs are so strong that the ribbon polymer is formed. The method was also able to predict the

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formation of dimers by 3,6-dibromo-1,4,7-benzotelluradiazole, 5, and provided a rational interpretation of the organization of the dimers in the crystal in terms of additional Te-Br secondary bonding interactions. In addition to the structure of the simple dimer, 5 also provided a hitherto not observed solvated telluradiazole dimer. The formation of SBIs between the tellurium atom and both DMSO and a bromine atom of a neighboring molecule highlights the electron acceptor ability of the σ* orbitals and strongly suggests that these molecules will be able to associate with other Lewis bases. Despite the difference in solid-state structures, the two benzotelluradiazoles studied here have a virtually identical 125Te NMR spectrum, which suggests that it might be possible to study the association of these supramolecular building blocks in solution. Acknowledgment. We thank McMaster University, the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, and the Ontario Innovation Trust for their financial support and McMaster’s Biomolecular Interactions Initiative for CPU time. Supporting Information Available: Crystallographic information files (CIF) for 4c, 5, and 5‚DMSO, additional crystallographic bond lengths and angles for 4c, 5, and 5‚DMSO, crystallographic and DFT calculated supramolecular bond distances and angles of 4c, 5, and 5‚ DMSO, and DFT-optimized molecular coordinates for 4c, 4c2, 4c6, 5, 52, 53, 5-5 (Br-Br SBI), 5-5 (Te-Br SBI), 5-52-5, 52‚DMSO2, DMSO, and 5-7. This material is available free of charge via the Internet at http://pubs.acs.org.

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