From Noncovalent ChalcogenâChalcogen Interactions to

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Cite This: Chem. Rev. 2018, 118, 2010−2041

From Noncovalent Chalcogen−Chalcogen Interactions to Supramolecular Aggregates: Experiments and Calculations Rolf Gleiter,*,† Gebhard Haberhauer,*,‡ Daniel B. Werz,§ Frank Rominger,† and Christian Bleiholder∥ †

Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany Institut für Organische Chemie, Universität Duisburg-Essen, Universitätsstraße 7, D-45117 Essen, Germany § Institut für Organische Chemie, Technische Universität Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany ∥ Department of Chemistry and Biochemistry & Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306-4390, United States ‡

ABSTRACT: This review considers noncovalent bonds between divalent chalcogen centers. In the first part we present X-ray data taken from the solid state structures of dimethyl- and diphenyl-dichalcogenides as well as oligoalkynes kept by alkyl-sulfur, -selenium, and -tellurium groups. Furthermore, we analyzed the solid state structures of medium sized (12−24 ring size) selenium coronands and medium to large rings with alkyne and alkene units between two chalcogen centers. The crystal structures of the cyclic structures revealed columnar stacks with close contacts between neighboring rings via noncovalent interactions between the chalcogen centers. To get larger space within the cavities, rings with diyne units between the chalcogen centers were used. These molecules showed channel-like structures in the solid state. The flexibility of the rings permits inclusion of guest molecules such as five-membered heterocycles and aromatic six-membered rings. In the second part we discuss the results of quantum chemical calculations. To treat properly the noncovalent bonding between chalcogens, we use diffuse augmented split valence basis sets in combination with electron correlation methods. Our model substances were 16 dimers consisting of two Me-X-Me (X = O, S, Se, Te) pairs and dimers of Me-X-Me/Me-X-CN (X = O, S, Se, Te) pairs. The calculations show the anticipated increase of the interaction energy from (Me-O-Me)2 (−2.15 kcal/mol) to (Me-O-Me/Me-TeCN) (−6.59 kcal/mol). An analysis by the NBO method reveals that in the case of the chalcogen centers O and S the hydrogen bridges between the molecules dominate. However, in the case of Se and Te the major bonding between the pairs originates from dispersion forces between the chalcogen centers. It varies between −1.7 and −4.0 kcal/mol.

CONTENTS 1. Introduction 2. Noncovalent Chalcogen−Chalcogen Interactions: Experimental Results 2.1. Linear Systems with Chalcogen Centers 2.1.1. Di- and Trichalcogenides 2.1.2. Carbon Rods between Methylchalcogen Units 2.2. Cyclic Systems with Chalcogen Centers 2.2.1. Ring Systems with Alkane Units between Chalcogen Centers 2.2.2. Ring Systems with Alkene and Alkyne Units between Two Chalcogen Centers 2.2.3. Ring Systems with Both Alkane and Alkyne Units between Two Chalcogen Centers 2.2.4. Columnar Structures with Guest Molecules: Elastic Cycles 2.2.5. Chalcogens as Part of Aromatic Systems 2.2.6. Intramolecular Chalcogen−Chalcogen Interactions as Stabilizing Factor

© 2018 American Chemical Society

3. Theoretical Investigations on Noncovalent Chalcogen−Chalcogen Interactions 3.1. Computational Methods 3.1.1. Definition of Interaction Energy 3.1.2. Selection of Methods and Basis Sets 3.2. Homonuclear Model Systems 3.2.1. Model Systems 3.2.2. Results from NBO Analyses 3.3. Homo- and Heteronuclear Models 3.3.1. Model Systems 3.3.2. Optimized Geometries and Supramolecular Interaction Energies 3.3.3. Results from NBO Analyses 3.3.4. Results from SAPT Calculations 3.3.5. Sigma Hole Bonding 4. Conclusion Author Information Corresponding Authors ORCID

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Received: July 27, 2017 Published: February 8, 2018 2010

DOI: 10.1021/acs.chemrev.7b00449 Chem. Rev. 2018, 118, 2010−2041

Chemical Reviews Notes Biographies Acknowledgments References

Review

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1. INTRODUCTION The S−S bond lengths of 1,2-dithiolane (1)1 and the radical cation of 1,5-dithiacyclooctane (2)2 vary considerably as shown in Figure 1. This large variation finds its counterpart

Figure 3. Packing of C16S8 (6) (“Sulflower”) in the solid state.

restricted to chalcogens and play a pivotal role to describe the interaction between and within molecules, we depicted in Figure 4 just four examples concerning the noncovalent

Figure 1. Lengths and S−S bond energies of a (2c-2e) and a (2c-3e) bond in 1,2-dithiolane (1) and in 1,5-dithiacyclooctyl cation (2).

in the bond energies of two sulfur centers3,4 also listed in this figure. The longer distance and smaller energy in the case of the radical cation are due to a (2c-3e) bond which is considerably weaker than the (2c-2e) bond in 1,2-dithiolane.1 In the cases of noncovalent (van der Waals) bonding between the chalcogen centers of two dimethyl chalcogenides (3−5), the bond lengths are considerably longer and the interaction energies are smaller than those shown in Figure 1. In Figure 2 we show the optimized geometries of the three

Figure 2. Calculated geometries, chalcogen−chalcogen distances, and interaction energies of two dimethylsulfides (3), dimethylselenides (4), and dimethyltellurides (5).

Figure 4. Four solid state structures of molecules belonging to groups III (7), IV (8), V (9), and VII (10). Distances are given in angstroms.

dimers and list their chalcogen−chalcogen distances together with the interaction energies.5,6 A comparison between the three nonbonding species in Figure 2 reveals bond lengths between 3.7 Å (S) and 4.0 Å (Te) which are equal (S) or smaller than the van der Waals distances (Se···Se 4.0 Å, Te··· Te 4.4 Å).3,7 The interaction energies of the three dimers vary between 2.79 (S), 2.82 (Se), and 3.4 kcal/mol (Te). All three values are small and in the order of weak hydrogen bonds, such as CH···O and OH···π.8−11 Nevertheless, these weak bonds are important for the formation and structures of many types of condensed matter: As examples for molecules with noncovalent interactions we mention in this chapter the supramolecular assemblies to yield macrocycles, helices, and container molecules to name only a few.12−16 On behalf of these supermolecules and their supramolecular assemblies, we have depicted in Figure 3 C16S8 (6) and its association in the solid state by noncovalent intermolecular S···S contacts between 3.2 and 3.5 Å.17 Further examples of crown ethers with S, Se, and Te as heteroatoms and their association to further species with and without inclusion of molecules will be discussed in section 2. To show that noncovalent interactions leading to long chalcogen−chalcogen bonds (Figures 2 and 3) are not

bonding between two Tl-, two Sb-, and two iodine centers as well as the effect of the dispersion attraction between bulky substituents. The first example in Figure 4 presents a molecule with long bonds of Tl cation centers complexed by bis(8-quinolinyl)amine rings. The crystal structure of 7 reveals a (Tl)n zigzag chain with a Tl···Tl distance of 3.53 Å.18,19 This length is longer than the sum of the ionic radii of Tl cations (3.28 Å)20 but shorter than that of the van der Waals distance of two Tl centers (3.92 Å). The structure of 7 is probably supported by the π stacking of the aromatic ligands19 together with a noncovalent Tl···Tl bonding. The second example, 2-(1diamantyl)[121]tetramantane (8) shows a very long carbon− carbon single bond (1.71 Å) between the bulky groups.21 This unusual bond length and the high stability of the hydrocarbon 8 and related structures21 were ascribed to “numerous attractive dispersion forces between inward H-terminated surfaces surrounding the long bonds”.21 In 9 we present, as a member of group V, 2,2′,5,5′-tetramethyldistiboyl.22 The purple blue crystals of 9 show long Sb···Sb contacts of 3.63 Å, which are considerably longer than the Sb−Sb single bonds (2.83 Å). Similar intra- and intermolecular distances were 2011


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Table 1. Typical Distances between Directly Connected Homonuclear Chalcogen−Chalcogen Centers (E−E, E = S, Se, Te), Not Embedded in a Cyclea Element

Compound

S

CH3-S-S-CH3 C6H5-S-S-C6H5 CH3-Se-Se-CH3 C6H5-Se-Se-C6H5 CH3-Te-Te-CH3 C6H5-Te-Te-C6H5

Se Te

11 12 13 14 15 16

E−E

E−C

E···E

ref

avg value

2.03 2.03 2.31 2.31 2.71 2.71

1.80 1.79 1.94 1.93 2.15 2.15

3.78 3.65 3.55 3.65 3.74 4.18

26 27, 28 26 28, 29 26 28, 30

3.72 3.60 3.96

a

All values are given in angstroms.

reported for the solid state of tetramethyldistibane.23 The long Sb···Sb contacts remind strongly of the cases shown in Figure 2. In 10 we represent the crystal structure of iodine at 110 K.24 The I−I bond length (2.72 Å) is significantly longer than that in the gaseous state (2.66 Å).25 The short distance between the I−I dumbbells of 3.50 Å is considerably smaller than the van der Waals distance of 4.30 Å and is due to a strong noncovalent bonding between the molecules. All the “long bonds” just discussed in compounds with either heavier elements or with bulky groups are mainly due to dispersion forces. In the following section we present a series of experiments which show the synthesis of organic rings and rods containing chalcogen centers and their tubular aggregates. These results are underpinned by high level quantum chemical calculations to understand what “noncovalent bonding” really means.

2. NONCOVALENT CHALCOGEN−CHALCOGEN INTERACTIONS: EXPERIMENTAL RESULTS 2.1. Linear Systems with Chalcogen Centers

2.1.1. Di- and Trichalcogenides. In this section we consider intermolecular interactions between divalent chalcogen species substituted by simple organic substituents. First we consider dichalcogenides of the type R-E-E-R with E = S, Se, or Te and R = alkyl or aryl substituents. We limit our considerations mostly to intermolecular interactions because in the cases of intramolecular interactions the close proximity of chalcogen centers might be caused by steric effects. We also restrict R to carbon-substituents such as alkyl-, alkenyl-, alkynyl- and aryl in order to avoid further complications by substituents such as halogens, strong electron acceptors, or electron donors. In the first section we discuss as prime examples the intermolecular interactions of the dimethyldichalcogenides of sulfur, selenium, and tellurium,26 as well as the diphenyldichalcogenides of sulfur,27,28 selenium,28,29 and tellurium.28,30 The most pertinent bond lengths within the molecules (E−E) and between them (E···E) are listed in Table 1. In Figure 5 we show parts of the solid state structures of dimethyldiselenide (13), 26 dimethylditelluride (15), 26 diphenyldiselenide (14),28,29 and diphenylditelluride (16).28,30 As anticipated, the average values obtained for the nonbonded contacts (Table 1) between Se···Se (3.6 Å) and Te···Te (4.0 Å) are shorter than the sum of the van der Waals radii for Se (4.0 Å) and Te (4.4 Å),3,7 whereas the average S···S contacts (3.7 Å) are close to the sum of the van der Waals radii of two sulfur centers (3.7 Å).3,7 In Figure 6a we show the results of X-ray investigations on the dimers of [Se2Me3]TeF5 (17)31 and [S3Me3]AsF6 (18a).32 In Figure 6b a second modification of [S3Me3]AsF6 (18b)33 is

Figure 5. Structures (X-ray) of dimethyldiselenide (13) (a), dimethylditelluride (15) (b), diphenyldiselenide (14) (c), and diphenylditelluride (16) (d). The homonuclear contacts between the chalcogen centers are indicated by broken lines. All hydrogen atoms are omitted for the sake of clarity.

shown, in which four [S3Me3]+ units form a chain with short S···S contacts (see Table 2). In the deep red colored salt [Se3Me3]SbCl6 (19)34 the Se3Me3+ units form a chain with short intermolecular Se···Se distances. The intermolecular chalcogen−chalcogen distances are summarized in Table 2. The values reported for the intermolecular Se···Se distances in 17 and 19 are either slightly above or slightly below the sum of the van der Waals radii for Se (4.0 Å).3,7 In the case of the two [S3Me3]+ cations of 18a and 18b, the short S···S distances vary between 3.79 and 3.93 Å (Table 2). These values are slightly above the sum of the van der Waals radii of two S centers. These results parallel those reported for 11−14 (see Table 1). Noncovalent chalcogen−chalcogen interactions between organo-ditellurides can also be observed in solution by measuring 125Te NMR chemical shifts in dependence on the concentration.35 The resonance of 125Te of two ditellurides RTe-Te-R (R = 4-CH3C6H4 and 2-(CH3)2NCH2C6H4) under2012


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Scheme 1. Preparation of the Polyalkynes Capped by Methylsulfide [22(n = 2-4)], Methylselenide [25(n = 2-4)], and Methyltelluride [26(n = 2-4)]a

Figure 6. (a) Dimers of [Se2Me3]+TeF6− (17) and [S3Me3]+AsF6− (18a). (b) Chains of [S3Me3]+AsF6− (18b) and [Se3Me3]+SbCl6− (19).

Reaction conditions: (i) MeLi, LiBr, −78 to 30 °C. (ii) MeSCN, THF, −30 °C. (iii) Se, THF, rt. (iv) MeI, THF, 0 °C. (v) Te, THF, rt. a

Table 2. Intermolecular Bond Lengths between the Chalcogen Centers of 17−19 (cf. Figure 6) Compound [Se2Me3]TeF5 [S3Me3]AsF6 [S3Me3]AsF6 [Se3Me3]SbCl6

17 18a 18b 19

distance [Å], a

distance [Å], b

4.02 3.93 3.79 3.65

3.85 3.81

distance [Å], c

3.94

discussed below. The three ditellura species, 26(2,3,4), were isolated in 8% (26(2)), 14% (26(3)) and 20% (26(4)) yield as brown solids.37 The He(I) and He(II) photoelectron spectra of 22(1),41 22(2),36 25(1),41 25(2), 25(3),36 and 26(1)36 as well as the UV/vis spectra of 22(1−4),36 25(1−4),36 and 26(1−4)37 reveal significant interactions between the chalcogen units within one molecule. The molecular structures of 25(4) and 26(1−4) show C2 symmetry in the solid state.36,37 For 25(4) and 26(2−4), X-ray studies on single crystals showed relatively short contacts between the neighboring chalcogen centers. For 25(4) as well as for 26(2) to 26(4), the chalcogen−chalcogen contacts between the stacks (3.67 and 3.73 Å for 25(4), 3.74 and 3.82 Å for 26(2), 3.69 and 3.84 Å for 26(3), 3.74 and 3.73 Å for 26(4)) are shorter than the van der Waals distances between two Se atoms (4.0 Å)3,7 or two Te centers (4.4 Å).3,7 However, the distances between chains within the stacks are relatively large (4.3 Å for 25(4) and 4.5 Å for 26(2) to 26(4)).36,37 In Figure 7 we show a side view (left) and a top view (right side) of the solid state structure of 26(2). Due to the C2 symmetry of the diyne rods of this molecule, it forms a triple helix in the solid state connected to neighboring helices by short Te···Te contacts indicated by broken lines in the right picture. Inside the channels of Figure 7, one can detect residual electron density due to n-hexane molecules from recrystallization. In the case of 26(3) the tellurium centers of four piles of molecules form a rhombohedric arrangement (Figure 8, top),37 whereas four piles of 25(4)36 form a rectangle with almost equal distances (Figure 8, bottom). The piles of 25(4) are connected by short Se···Se bonds (3.73 and 3.67 Å). A very similar but not isomorphous structure was reported for the corresponding tellurium congener 26(4).37 The short Te···Te bonds connecting the piles were reported to be 3.74 and 3.73 Å. In the solid states of 25(4), 26(3), and 26(4), the spaces left by the methyl groups of one molecule are filled by those of neighboring piles to avoid any empty space. The reaction of 26(2) with CH3I gave in 42% yield methylbis(methyltelluro)acetylene iodide (27) as a colorless salt (Scheme 2).42 In contrast to the sulfonium and selenium salts

ref 31 32 33 34

goes a low-field shift with increasing concentration of the ditellurides which can be explained by chalcogen-centered supramolecular interactions. In sterically hindered ditellurides, such interactions are prevented and no positive concentration coefficients are observed. However, the measured equilibrium constants for the autoassociation of the molecules through tellurium-centered supramolecular interactions in solution are rather small.35 2.1.2. Carbon Rods between Methylchalcogen Units. The bis(methylchalcogena)oligoalkynes are appropriate model systems to study close contacts between divalent chalcogen centers related to dimethyl dichalcogenides such as 11, 13, and 15. In Scheme 1 we show ways to form carbon rods, existing of conjugated triple bonds capped by methylchalcogenide units.36,37 The syntheses of these model systems commence by preparing the corresponding alkyne rods, capped on both sides by trimethylsilyl groups.38,39 In a further step the bis(lithium)salts 21n were prepared, following procedures developed by Brandsma.40 The bis(methylsulfides) 22n were obtained by reacting 21n in situ with methylrhodanide at −30 °C.36 To achieve the syntheses of the Se and Te congeners of 22n, 25n, and 26n, the second intermediates 23n and 24n had to be generated by reacting 21n in situ with the corresponding chalcogens. These intermediates, 23n and 24n, were trapped with methyliodide (step (iv)) to afford 25n and 26n, respectively.36,37 The disulfides 22(2) to 22(4) were isolated as yellow to brown colored oils in 20% to 40% yield.36 The first two Me-Secapped rods 25(2) and 25(3) were isolated as brown colored oils in 44% and 33% yield, respectively.36 The 2,11diselenadeca-3,5,7,9-tetrayne (25(4)) was obtained as a deep yellow solid in 24% yield. Its structural properties will be 2013


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Figure 9. Part of the layer of 27 in the solid state. The Te atoms are depicted in red and iodide anions in blue. All hydrogen atoms are omitted for the sake of clarity. Adapted with permission from ref 42. Copyright 2004 Elsevier.

chalcogen contacts slightly shorter than the sum of the corresponding van der Waals radii. There are several tubular species based on noncovalent forces in nature, such as helices built of α-D-glucose or from peptides, to name only the two most important representatives. A rather flexible way for building tubular constructs is the stacking of cyclic units. This concept can be varied in different ways: the rings can be connected with each other by strong forces along the stacking direction only (Figure 10a). This motif is found in tubes obtained from cyclic peptides,43,44 N,Nlinked oligoureas,45 or cyclodextrins.46 A strong in plane connectivity but weak intrastack connection is found for shapepersistent macrocycles47−49 containing aromatic building units (Figure 10b). Due to the π−π repulsion of the aromatic units, the planes of the rings are shifted sidewise and, hence, the resulting tubular holes are reduced. Common to these structures discussed are hydrogen bonds or π−π interactions as noncovalent forces. If the connectivity occurs in a zigzag fashion as shown in Figure 10c, the tubuli are stacked on top of each other. However, the distances between the rings within one stack are further apart than in cases a and b, whereas the distances between the rings of neighboring stacks are relatively short. There are many examples of tubular structures assembled according to the situations depicted in Figure 10a and 10b; however, only recently has the zigzag connection (Figure 10c) emerged as an alternative concept.50,51 2.2.1. Ring Systems with Alkane Units between Chalcogen Centers. To begin with, we consider the structure of 1-thiacyclododeca-3,10-diyne (28).52 This molecule provides a rather flat ring due to the pentamethylene bridge on one end of the alkyne units and a CH2-S-CH2 bridge on the other end. This connectivity guarantees a rather flat arrangement of all five CH2 groups (Figure 11a). In the solid state the rings of 28 associate in such a way (Figure 11b) that the sulfur center of one ring of one stack keeps close contact with two sulfur centers of two neighboring rings of another stack as described in Figure 10c.53 The close contacts, a and b, between the centers E in Figure 12 and the distance c within one stack are compiled in Table 3. In some cases the binding motif is changed to a ladder-type arrangement as shown on the right of Figure 12. In such a case, the distance b is considerably longer than a. In the case of 28, the distances a and b are equal, 3.53 Å, which is shorter than the sum of the radii of two sulfur atoms (3.7 Å).3,7 Similar distances were reported for 11, 12 (Table 1) and 18a, 18b (Table 2). The formation of these zigzag and ladder connections (Figure 10c and 11b) can be explained by two simple models. In the first case, the attractive interaction between the occupied p-type orbital of one chalcogen center E and the empty E-C σ*orbital of another chalcogen unit, as shown in Figure 13a, results in a highly directed orientation of the

Figure 7. Side view (left) and top view (right) of the solid state structure of 26(2). The top view shows residual electron density in the channels due to disordered n-hexane molecules. Reproduced from ref 37. Copyright 2003 American Chemical Society.

Figure 8. Top views of the solid state structures of 25(4) (bottom) and 26(3) (top). All hydrogen atoms are omitted for the sake of clarity. Reproduced from ref 37. Copyright 2003 American Chemical Society.

Scheme 2. Reaction of 26(2) with CH3I Yielded Methylbis(methyltelluro)acetylene Iodide (27)

17 to 19 given in Figure 6, where the molecules are tied up in dimers or chains via the divalent chalcogen centers, in the case of 27, close contacts are found between the trivalent and one divalent tellurium center of 27. In the solid state, 27 crystallizes in layers consisting of Me2Te+-CC-TeMe molecules with short contacts (3.77 Å) between one divalent and one trivalent Te center and iodide anions. In Figure 9 a part of such a layer is shown. 2.2. Cyclic Systems with Chalcogen Centers

In this section we consider the structural properties of cyclic systems containing divalent chalcogen centers connected with chains containing as building units alkanes, alkenes, or/and alkynes. As will be shown, these cycles are prone to selforganization, yielding columnar structures with chalcogen− 2014


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Figure 10. Three possible arrangements to interconnect rings to tubes in the solid state.

outer surface are located along the extensions of the E-C covalent bond. The relative orientations of two Me2S units having the lowest energy are such that the region with positive electrostatic potential on the outer surface of one chalcogen atom is approaching a negative region on the lateral side of the other chalcogen atom (Figure 13b).59,60 In section 3 we show that these effects describe only a part of the chalcogen− chalcogen interactions and dispersion energy dominates these types of noncovalent interactions. However, these models remain useful tools to predict the spatial orientation of the chalcogen-containing units. Another source of columnar structures which follows the interaction patterns shown in Figures 10 and 12 is selenium coronands, which are presented schematically in Figure 14.

Figure 11. (a) Molecular structure of 28. (b) Top view of stacks of 28 in the solid state. The hydrogen atoms are omitted for the sake of clarity. Reproduced from ref 53. Copyright 2002 American Chemical Society.

Figure 12. Schematic drawing of a zigzag (left) and ladder (right) interaction of chalcogen−chalcogen interactions in the solid state (side view). Definition of distances a, b, and c.

Figure 14. Schematic drawing of the selenium coronands 29−33.

Their syntheses could be achieved in a straightforward way, by reaction of the bis-sodium salts of propene-1,3-bis(selenolate) or ethane-1,2-bis(selenolate) with α,ω-dibromalkanes.54,55 These reactions yield the selenium crown ethers 29−33 depicted in Figure 14. The geometrical parameters of all five of the selenium coronands could be elucidated by X-ray investigations of single crystals. A detailed analysis yielded not only the bond angles and lengths but also intermolecular close Se···Se contacts between the rings.54,55 The threedimensional networks of these species were analyzed,56 and also columnar structures were found with zigzag interactions between the Se centers and neighboring stacks, as discussed in the case of 28. The data for the distances a, b, and c as defined in Figure 12 are compiled in Table 3. In the case of 29 to 32, the values for a and b are below the van der Waals radii for Se (4.0 Å).3,7 In Figure 15 we show a plot of the columnar arrangements of 30 and 32 in the solid state. The nonbonding interactions between the Se centers of neighboring columns are indicated by broken lines. The 24-membered ring of 33 adopts a deck chair shape in the solid state (Figure 16a).54 The stacks formed by these molecules show no significant Se···Se interactions. However, a look at the stacks in Figure 16b reveals weak Se···H interactions between the columns of the rings, as indicated by broken lines. The Se···H distances vary between 3.1 and 3.5 Å; most of them are slightly longer than the sum of the van der Waals radii of Se and H (3.2 Å). Two examples in which sulfides form stacks, despite very bulky groups, are the bis(hexacarbonyldicobalt) complexes of N-isopropyl-6-aza-1-thia-cyclodeca-3,8-diyne (34) and 1-thiacyclotetradeca-4,11-diyne (35). Both species were prepared

Table 3. Distances between Chalcogen Centers of 28−32 within a Zigzag Arrangement of Two Stacks (a, b) and Distances within One Stack (c) (cf. Figure 12) element S Se Se Se Se

28 29 30 31 32

distance [Å], a

distance [Å], b

distance [Å], c

(a + b)/2

ref

3.53 3.77 3.69 3.41 3.63

3.53 3.77 3.69 3.94 3.81

4.74 5.63 5.53 5.43 5.43

3.53 3.77 3.69 3.68 3.72

53 54−56 54−56 54−56 54−56

Figure 13. Explanation of the relative orientation of two chalcogencontaining units: (a) Interaction of an occupied p orbital at center E and the empty E-C σ* orbital. (b) Energetic most preferred orientation of two Me2S units due to their molecular electrostatic potential on a defined outer surface.

chalcogen centers.57,58 The second model relies on the electrostatic interactions. As an example, the molecular electrostatic potential of two Me2S units is shown in Figure 13b. The regions with positive electrostatic potential on the 2015


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Figure 17. Tubular structure of bis(hexacarbonyldicobalt) complexes of N-isopropyl-6-aza-1-thia-cyclodeca-3,8-diyne (34) (a) and 1thiacyclotetradeca-4,11-diyne (35) (b). In both complexes the CO groups and hydrogen atoms are omitted for the sake of clarity. The sulfur centers are marked in yellow, the nitrogen atoms in speckled blue, and the Co centers in blue.

5.91 Å and γ = 67.5° (Figure 17b).61 A closer look at these structures reveals stacks with short S···S contacts of 4.03 Å in the case of 34 and 3.88 Å in the case of 35 (see Figure 17). It is quite remarkable that the bulky Co2(CO)6 units cannot prevent the intermolecular interaction between the sulfur centers. 2.2.2. Ring Systems with Alkene and Alkyne Units between Two Chalcogen Centers. To get more rigidity into the larger ring systems, some CH2 groups were replaced by C,C-double- and C,C-triple bonds. To achieve such a goal, an efficient synthesis of heterocyclic diynes and dienes had to be developed. As an example, we show in Scheme 3 the

Figure 15. Plot of the columnar arrangement of 30 (a) and 32 (b) in the solid state. Broken lines indicate the Se···Se distances less than 4 Å. Hydrogen atoms are omitted for the sake of clarity. Reproduced with permission from ref 56. Copyright 2002 Elsevier.

Scheme 3. Synthesis of Cyclic Tetrathiadiynes [44(m,n)], Tetraselenadiynes [45(m,n)], as Well as Cyclic Tetrathiadienes [46(m,n)] and Tetraselenadienes [47(m,n)] for m,n = 2−5

Figure 16. (a) Framework of 33; the hydrogen atoms are omitted for the sake of clarity, and the selenium centers are indicated by full orange colored circles. (b) Top view of stacks of 33. The Se···H interactions between 3.10 and 3.45 Å are indicated by broken lines and the selenium centers by full orange colored circles.

from the corresponding heterocyclic rings and dicobalt octacarbonyl in light petroleum (30/40) or methylene chloride at 0 °C.61 The X-ray structural investigations of single crystals of both species reveal for [(CO)6Co2]234 a distance of d = 3.44 Å between the C2[Co2(CO)6] units and a torsional angle γ = 55.6° between the complexed triple bonds (see Figure 17a). For the larger ring 35, these parameters amount to d =

synthesis of cyclic tetrathiadiynes.62 The synthesis starts with trimethylsilyl acetylene (TMSA) (36), which was transformed to its lithium salt 37. The reaction of the latter with various α,ω-dithiocyanoalkanes 38(m) afforded in good yields the α,ω-bis(trimethylsilylalkynethio)alkanes 40(m), which could be converted to the diynes 42(m) by treatment with NaOH. 2016


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In Figure 19b we show the columnar structure of 44(5,5).53 Common to all these rings is a rectangular geometry. The short sides of the rectangles are spanned by rigid S-CC-S units, whereas the long sides are formed by alkane chains, adopting a zigzag conformation when the number of CH2 groups is uneven. The chalcogen atoms are located at the four corners of the rather flat rectangle; hence, they are ideally placed to interact with other chalcogen centers of a molecule of a neighboring stack. As just mentioned, the sulfur atoms of 44 and 46 keep close contacts with pairs of chalcogen centers of neighboring stacks, according to the zigzag arrangement depicted in Figure 12. The S···S distances of 44(3,3), 44(5,5), 46(3,3), and 46(4,4) for the parameters a and b are listed in Table 4. The shortest S···S distances (a) vary between 3.47 and 4.05 Å. The first value is below and the second one slightly above the sum of the van der Waals radii of S (3.7 Å).3,7 As shown at the end of Scheme 3, the diynes 44(m,n) can be reduced with diisobutyl-aluminum hydride (DIBAH) to give the tetrathiadienes 46(m,n) in good yields.63 In Figure 20 we show the molecular structures of (2Z,8Z)-1,4,7,10-tetrathiacyclododeca-2,8-diene [46(2,2)] and (2Z,10Z)-1,4,9,12-tetrathiacyclohexadeca-2,10-diene [46(4,4)]. The two structures presented reveal cis configurations at the double bonds. In both cases one observes almost planar S-CHCH-S units. Common to 46(2,2) and 46(4,4) is that both double bonds adopt an anti conformation with respect to each other. The hydrogen centers of the C2H4 bridges of 46(2,2) and the C4H8 bridges of 46(4,4) adopt a staggered conformation. In the latter case the bridges show a zigzag arrangement. The packing of 46(4,4) in the solid state is given in Figure 19b, showing columnar structures with close contacts between the sulfur atoms (see also Table 4). Similar columnar structures are found for 46(3,3) (cf. Table 4), 46(5,5), and 46(6,6).53,63 The values for the closest intertubular S···S distances (a) of neighboring rings vary between 3.4 and 4.1 Å. The distance between the rings of one stack (c) varies between 4.2 and 5.2 Å.63 The stepwise approach to synthesize the cyclic tetrathiadiynes 44(m,n) could be extended to prepare the tetraselenadiynes 45(m,n) with m,n = 2 to 5.64 In Figure 21 we show as examples for cyclic tetraselenadiynes the molecular structures of 45(2,2) and 45(3,3). The torsion angle β (cf. Figure 18) between the triple bonds of 45 was found to be 0° in those cases where Se-CC-Se units were connected by chains with an uneven number of methylene groups such as 45(3,3), 45(3,5), and 45(5,5).64 Consequently, chair conformations of the rings are adopted, as seen in the case of 45(3,3) in Figure 21a. A comparison among the torsion angles γ1 and γ2 between the CH2−Se σ bonds reveals values close to 60° for the rings with β = 0°. For 45(2,2) the values of γ1 = γ2 were 25° and β = 11°. For the other species, the values for γ vary between 65° and 89°.64 In the cases of the selenium rings 45(3,3) and 45(5,5), one finds isomorphous structures in the sulfur congeners 44(3,3) and 44(5,5), respectively. In Figure 21b, the crystal packing of 45(3,3) is shown. In common, the rings 45(3,3) and 45(5,5) are rectangular structures. The short sides of the rectangles are formed by the rigid Se−CC−Se units, whereas the long sides of the rings are composed of the alkane chains, adopting a zigzag conformation. The selenium atoms are located at the corners of the rather flat rectangle. As a result, the flat rings are excellent building blocks for making columnar stacks. As seen

In the ring closing step, one treats the dilithium salt of 42(m) with α,ω-dithiocyanoalkanes 38(n) under high dilution conditions to obtain 44(m,n). The yields reported by this protocol depend on the ring size.62 In an analogous sequence the tetraselenadiynes 45(m,n) can be prepared (Scheme 3). To describe the conformations of the ring systems 44(m,n) and 45(m,n), the torsional angles β and γ, depicted in Figure 18, were introduced. The dihedral angle β between the triple

Figure 18. Definition of the torsion angles β and γ in 44(m,n) and 45(m,n).

bonds in these ring systems is found to be usually 0° when the alkane chains(m,n) have an odd number of methylene brigdes. As a result, a chair conformation is adopted, as exemplified by 44(3,3) in Figure 19a. When one or both bridges show an even

Figure 19. (a) Molecular structures of 44(3,3) (left) and 44(4,4) (right). (b) Columnar structures of 44(5,5) (left) and 46(4,4) (right). The hydrogen atoms in part b are omitted for the sake of clarity. Reproduced from ref 53. Copyright 2002 American Chemical Society.

number of methylene groups, β is usually >0. Such a case is shown in 44(4,4), which adopts a twist conformation with β = 58.7°, as shown in Figure 19a.62 The comparison between the dihedral angles γ in 44(m,n) shows that the molecular geometries of these rings are determined by both steric and electronic factors. If both alkane bridges have the same odd numbers of CH2 units, γ is close to 63°, as expected for a chair conformation. In all other cases, γ was found considerably larger, in the range between 75° and 110°.62 These values are based on the fact that the 3p lone pairs at sulfur or 4p lone pairs at selenium centers interact through the π system of the triple bond and hence prefer a perpendicular position. This view was supported by B3LYP/6-31G* calculations.62 2017


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Table 4. Distances between Chalcogen Centers of 44, 45, 46, 25, and 26 within a Zigzag Arrangement of Two Stacks (a, b) and Distances within One Stack (c) (cf. Figure 12) element

compound

distance [Å], a

distance [Å], b

distance [Å], c

(a + b)/2

ref

S S S S Se Se Se Se Se Se Se Se Te Te Te

44(3,3) 44(5,5) 46(4,4) 46(3,3) 45(2,2) 45(3,3) 45(4,4) 45(5,5) 45(4,2) 45(5,2) 45(5,3) 25(4) 26(2) 26(3) 26(4)

3.75 3.63 3.47 4.05 3.70 3.75 3.63 3.68 3.87 3.58 3.90 3.67 3.74 3.69 3.73

3.89 3.86 4.13 4.08 4.12 3.80 3.82 3.84 3.90 3.78 3.81 3.73 3.82 3.84 3.73

4.55 4.61 5.2 5.2 4.63 4.56 4.63 4.63 4.63 4.63 4.63 4.3 4.5 5.5 4.45

3.92 3.75 3.80 4.06 3.91 3.78 3.72 3.76 3.90 3.58 3.86 3.70 3.78 3.77 3.73

53 53 53,63 53,63 64 64 64 64 64 64 64 36 37 37 37

chalcogen centers occurs (cf. Figures 10c and 12). In Table 4 we list the intermolecular Se···Se distances of seven cyclic tetraselenadiynes 45(m,n) with m,n = 2−5. The shortest distance (a) between the neighboring Se···Se centers varies between 3.63 Å for 45(4,4) and 3.90 Å for 45(5,3). All distances a and b are below the sum of the van der Waals radii for Se (4.0 Å).3,7 The reduction of 45(2,3) and 45(4,4) with DIBAH affords in 30% and 60% yield, respectively, the corresponding dienes 47(2,3) and 47(4,4), with close contacts between the Se centers: 47(2,3): Se···Se 3.71 and 3.78 Å; 47 (4,4): Se···Se 3.60, 3.81, and 3.85 Å.65 The chains between the rigid E-CC-E units in the cycles can be varied by including other firm units. We mention here solid state structures of isomeric [6.6]cyclophanes with 2,5diselenahex-3-yne bridges.66 In Scheme 4 we summarize

Figure 20. Molecular structures of (2Z,8Z)-1,4,7,10-tetrathiacyclododeca-2,8-diene [46(2,2)] and (2Z,10Z)-1,4,9,12-tetrathiacyclohexadeca-2,10-diene [46(4,4)].

Scheme 4. Synthesis of the o-, m-, and p-Isomeric [6.6]Cyclophanes with 2,5-Diselenahex-3-yne Bridges 51a− c

preparations of the o-, m-, and p-[6.6]cyclophanes 51a to 51c. The synthesis commences with 1,n-bis(selenocyanatomethyl)-benzenes 48a to 48c. The reaction of these species with lithiated trimethylsilylacetylene (cf. Scheme 3) yielded the bis(diselenoalkadiynes) as yellow oils (49a, 49b) or crystals (49c). The removal of the protecting groups afforded the alkynes 50a−c. The alkynes were transformed into bis-lithium salts and subsequently reacted with 1 equiv of the selenocyanates 48a−c. The yields of purified cyclophanes were low and varied between 7% (51b) and 2% (51c). From all three products, single crystals could be obtained.66 In Figure 22 we show a section of 51a. It is found that the rings are stacked on top of each other. A remarkable feature of this

Figure 21. (a) Molecular structures of 1,4,7,10-tetraselenacyclodeca2,8-diyne [45(2,2)] (left) and 1,4,8,11-tetraselenacyclotetradeca-2,9diyne [45(3,3)] (right). (b) Columnar structure of 45(3,3). The hydrogen atoms in part b are omitted for the sake of clarity. Reproduced from ref 64. Copyright 2002 American Chemical Society.

from Figure 21b, the selenium atoms in 45(3,3) keep close contacts with pairs of chalcogen atoms of neighboring stacks. As a result the often mentioned zigzag interaction between the 2018


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Figure 23. Molecular structure of 57(3). The close Se···Se distances are indicated by broken lines. All hydrogen atoms are omitted for the sake of clarity. Reproduced from ref 67. Copyright 2008 American Chemical Society.

Figure 22. Threaded arrangement in the solid state of 51a. Dotted lines in the center represent strong Se···Se interactions (Se1···Se1′ = 3.48 Å, Se2···Se2′ = 3.52 Å). The Se···Se distances with the outer units (Se4···Se4′ = 3.94 Å) are weaker. Hydrogen atoms are omitted for the sake of clarity. Adapted with permission from ref 66. Copyright 2009 John Wiley and Sons.

Se···Se distances of 3.6 and 3.7 Å.67 The cyclophane 56(3) forms pairs with close contacts (3.28 Å) between the parallel phenyl rings and close S···S contacts of 3.3 and 3.9 Å.67 In the metacyclophane 51b the rings adopt a chair conformation and are piled on top of each other. The resulting molecular channels are connected with each other by intermolecular Se···Se contacts. The latter are rather long (>4.36 Å); therefore, presumably weak Se···H and C−H···π interactions are responsible for the stacking, rather than Se···Se interactions.66 The paracyclophane 51c also adopts a chair conformation. In the solid state the molecules are interconnected by Se···Se bridges. Each Se center is involved in three short contacts (3.79 Å, 3.80 Å, and 3.81 Å) to selenium atoms of three different neighboring molecules. This gives rise to a structure with two-dimensional Se-regions showing multiply linked selenium networks.66 2.2.3. Ring Systems with Both Alkane and Alkyne Units between Two Chalcogen Centers. In connection with the discussion of the solid state structure of the 24membered rings of 33 (see Figure 16), we noticed that there is no considerable Se···Se interaction. However, there are several C−H interactions between the Se atoms of one ring and the C−H bonds of four neighbors in such a way that stacks are

structure is that only one Se−CC−Se unit of each molecule is involved in strong intermolecular interactions, as indicated by dotted lines in Figure 22. This leads to a one-dimensional thread of ···Se-CC-Se··· chains in one direction with the cyclophanes alternating on both sides as shown in Figure 22. The Se···Se contacts along the thread amount to only 3.48 and 3.52 Å, respectively. Parallel strings are connected by weaker Se···Se interactions (3.93 Å) in the other direction.66 Further examples of cyclophanes with intermolecular nonbonding contacts between chalcogen centers are 52(n), shown in Scheme 5a, as well as 56(n) and 57(n), depicted in Scheme 5b. As an example, for their syntheses we summarize the syntheses of 56(n) and 57(n) in Scheme 5b. The starting points were 1,4-di(ethynyl)benzene in the case of 52(n) and 1,4-di(ethynyl)-2,5-di(n-propyl)benzene (53) for the synthesis of 56(n) and 57(n). The diyne 53 was reacted in a threecomponent reaction with a strong base (BuLi or LiHMDS) followed by an α,ω-dithio- 54(n) or α,ω-diselenocyanatoalkane 55(n), as shown in Scheme 5b. This sequence afforded 56(3) in 17% yield and 57(3) in 4% yield. In Figure 23 we show the molecular structure of 57(3) with intermolecular

Scheme 5. (a) Cyclophane 52(n). (b) Synthesis of Cyclophanes 56(n) and 57(n)

2019


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Scheme 6. Syntheses of 60(m,n), 61(m,n), and 62(m,n) from α,ω-Diynes 58(m)

formed according to those shown in Figure 10c. A second reason why C−H forces might be important in thio-crown ethers is the fact that especially so-called short S···S bonds are slightly longer than the van der Waals distance (3.7 Å).3,7 This can be seen for the cyclic thioethers listed in the first section of Table 4. In contrast to these results, for Se···Se and Te···Te bonds the average values are clearly shorter than the sum of the corresponding van der Waals radii. This can be seen from the examples listed in the second and third section of Table 4. These results suggest that S···S interactions might be weaker than other weak forces such as CH−π interactions.8−11,68,69 Therefore, it is of interest to take a look at cyclic diynes with two CH2-CC-E instead of two E-CC-E (E = S, Se, Te) units, as discussed before. In Scheme 6 the syntheses of the cyclic systems 60(m,n), 61(m,n), and 62(m,n) are presented. These cycles are model systems which provide 50% less chalcogen centers for direct E···E interaction than 44(m,n) or 45(m,n).70 The syntheses follow strategies as described in Schemes 1 and 3 to obtain the corresponding ring systems 44(m,n) and 45(m,n) as well as the bis(methylchalcogena)alkynes 22(n), 25(n), and 26(n): the bis(lithium)salts of the terminal alkynes 58(m) (see Scheme 6) were reacted with either thiocyanates 38(n) or selenocyanates 39(n) to afford the cyclic systems 60(m,n) and 61(m,n), respectively. The yields of the products vary between 20% and 50%. To obtain the related ditellura rings 62(m,n) the bis(lithium) salts of 58(m) were treated with freshly ground tellurium metal, followed by α,ω-diiodoalkanes 59(n). The yields of the pale yellow solids 62(m,n) were about 50%.70 In Table 5 we have compiled the closest intermolecular distances between the chalcogen centers (E···E), the closest intermolecular distances between hydrogen atoms and chalcogen atoms (C−H···E), and between hydrogen atoms and the π systems (C−H···π) for all cycles 60(m,n), 61(m,n) and 62(m,n) which were investigated in the solid state.70 For five systems 60(5,2), 60(5,4), 60(7,3), 61(5,2), and 62(5,3) one encounters E···E distances (see Table 5) which are on the order of van der Waals distances (S···S = 3.7 Å, Se··· Se = 4.0 Å, Te···Te = 4.4 Å).3,7 As examples we show in Figure 24 the columnar structures in the solid state of 61(5,2) and 62(5,3). For 60(5,2), 61(5,2), and 62(5,3) the rings are stacked on top of each other to yield tubular structures. For 60(5,2) and 61(5,2) the rings in one plane are grouped in blocks of four in which the chalcogen atoms of neighboring rings form a distorted tetrahedron as shown in Figure 24a. The distances between the Se centers are 3.64 and 4.08 Å.70 In the

Table 5. Comparison of Short Intermolecular Distances [Å] E···E, C···E, and CH···π of 60(m,n) to 62(m,n) compound

distance [Å,] E···E

distance [Å], CH···E

distance [Å], CH···π

molecular structure

columnar stacking

60(4,3) 60(5,2) 60(5,3) 60(5,4) 60(5,5) 60(7,3) 60(7,5) 61(5,2) 62(5,3)

4.77 3.76 4.18 3.65 4.29 3.79 5.00 3.64 3.81

3.04 2.97 2.93 2.93 3.01 3.02 2.89 3.10 2.83

2.87 2.89 2.89 2.83 2.88 2.87 2.85 2.82 3.35

dist. chaira dist. chaira chair chair chair boat chair plane chair

− + + − − − + + +

a

Distorted chair.

Figure 24. (a) Columnar structure of 61(5,2) in the solid state. The shortest distances in the central distorted tetrahedron (3.64 Å) are indicated by broken lines. The diagonal Se···Se distances in 61(5,2) amount to 4.21 Å. (b) Columnar structure of 62(5,3) in the solid state. In both cases all hydrogen atoms are omitted for the sake of clarity. Adapted with permission from ref 70. Copyright 2003 Royal Society of Chemistry.

case of 62(5,3), the rings are arranged in such a way that the Te centers oppose each other, which leads to the formation of pairs of stacks, which allows all Te centers to interact with those of neighboring rings. The corresponding structure is depicted in Figure 24b. For 60(4,3), 60(5,3), 60(5,5), and 60(7,5) one finds relatively short distances for the C−H···π and C−H···E contacts, whereas the contacts among the sulfur centers vary between 4.0 and 5.0 Å (see Table 5). Nevertheless, it was reported that 60(5,3) and 60(7,5) show stacking of the rings as shown in Figure 25 for 60(7,5). The C−H···π distances recorded for these two ring systems (2.85 and 2.89 Å)70 compare well with other weak hydrogen 2020


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products of the synthesis of the central system. When 63(4) was recrystallized from n-hexane, one obtained single crystals which showed a tubular stacking of the rings by including nhexane in a disordered fashion.50,73 The sulfur atoms form close contacts with those in neighboring rings stacked on top of each other with S···S distances of 3.52 Å. The diameter of the tube built by this stacking system measures approximately 6 Å. Increasing the ring size by elongating the alkane chains between the rigid SCC-S units led to 63(6).51 Now the potential cavity created by the ring system collapsed (Figure 26), yielding columns of

Figure 25. Top view (right) and side view (left) of the columnar structure of 60(7,5). All hydrogen atoms at the right are omitted for the sake of clarity. In the side view the C−H···π interactions are indicated. Adapted with permission from ref 70. Copyright 2003 Royal Society of Chemistry.

bonds.8−11,68,69 The comparison of the data listed in Table 5 reveals that the contributions of the E···E interactions increase in the sequence S-Se-Te, whereas the other weak interactions decrease from S to Se and Te. A comparison of the C−H···S and S···S bond lengths in Table 5, respectively, reveals very similar distances as reported for the thiabowls trithia-[3]peristylane: C−H···S = 3.05 and 3.09 Å and tetrathia-[4]peristylane: C−H···S = 2.93 Å and S···S = 3.61 Å.71,72 2.2.4. Columnar Structures with Guest Molecules: Elastic Cycles. The diameters of the tubes provided by the columnar structures discussed so far proved to be too small for the inclusion of guest molecules. To provide larger space within the cavities, the previously used rings containing 1,4chalcogen alkyne units which are depicted in the center of Scheme 7 had to be enlarged. One way to achieve this goal, A, is the further repetition of the rigid donor unit to afford a ring with three alkyne units, 63(n) and 64(n).

Figure 26. Top view of the columnar structure of 63(6). The short S···S distances are indicated by dotted lines. The hydrogen atoms are omitted for the sake of clarity. Adapted with permission from ref 51. Copyright 2005 Chemical Society of Japan.

T-shaped molecules that are stacked in AAA fashion, favored by short S···S (3.53 and 4.13 Å) intertubular interactions. This observation shows that rigid units are crucial to guarantee a large cavity. In Scheme 8a we summarize the results of dimerization reactions of the dithiaalkadiynes 42(3) and 42(4) by applying Glaser conditions in methanol to yield the coupling products 65(3) and 65(4). A trimerization of the starting materials was not observed. As a second approach (Scheme 8b), a four component condensation of dilithiumdiacetylene 21(2) and dichalcogencyanides 38(n) and 39(n) is described. This protocol was previously applied to prepare systems such as 44(n) and 45(n)62 in Scheme 3 and polyalkynes capped by sulfur- and seleniumalkynes (cf. Scheme 1).36,37 In these experiments the rather stable bis(trimethylsilyl)butadiyne 20(2) was deprotected in THF at −78 °C (Scheme 8b). The bis-lithium salt 21(2) was reacted in situ with dithia- and diselenocyanatoalkanes 38(n) and 39(n), respectively, applying higher dilution conditions. In this approach not only the cyclic bis(diynes) 65(n) and 66(n) are generated but also the six-component products, the cyclohexaynes 67(n) and 68(n), could be isolated. The yields of this reaction varied between 3%, for 66(2), 68(4), 68(5), and 18%, for 65(3), 65(4), 66(4).74 All tetraynes show a center of inversion, i.e. adopt a chair conformation in the solid state.74 Channel-like structures were obtained in the cases of 65(5) and 66(5). Due to the butadiyne units, the rectangular cavity is enlarged and the chalcogen atoms are located at the four edges of the rather flat rectangle as depicted in Figure 27 for 65(5). As shown before, the cycles associate in such a way that the chalcogen centers of one cycle keep close contact with chalcogen centers of neighboring rings (cf. Figures 10c and

Scheme 7. Different Ways A, B, and C to Enlarge the Cavity of Ring Systems with Two 1,4-Chalcogena Units to Yield the Ring Systems 63(n) to 68(n)

Another alternative is to replace the two triple bonds in the central system of Scheme 7 by 1,3-butadiyne units as depicted in concept B to give 65(n) and 66(n). The third possibility is a combination of A and B to C with three 1,3-butadiyne units to yield 67(n) and 68(n). Concept A needed no further synthetic efforts, because ring systems with three 1,4-dichalcogenalkyne units were side 2021


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Scheme 8. (a) Preparation of 65(n) by Coupling Two Units of 42(n) under Glaser Conditions. (b) Four and Six Component Condensation of Dilithiumdiacetylide with Dithio- or Diselenocyanatoalkanes 38 or 39

in Table 6. The list comprises aliphatic, aromatic, as well as electron rich and electron poor molecules. The study reveals that all structures have very similar primitive unit cells, some of them leading to a higher symmetric R-centered lattice. Each modification of 68(5) is described by a greek letter (from α to μ). In some cases the guest molecules could not be localized at well-defined positions, as shown in Figure 28 top. In modifications (η to μ) when the guests were found on welldefined positions, a triclinic space group resulted (P1̅). The fact that many different molecules are incorporated in an ordered fashion was ascribed to weak C−H···π interactions68,69 between the alkane chains of the host and the π system of the guest. In the fourth column of Table 6 the solvent accessible volume is listed calculated with the PLATON program.75 It is seen that the diameter of the tubes and hence the solventaccessible volume in the solid state is increasing from top to bottom. The larger the guest, the more the tubes are widened. In the last column the angle ϕ between the medium plane of the macrocycle and the Se−CC−Se units is listed. It is seen that the smaller ϕ, the larger the solvent-accessible volume was found. This correlation can be rationalized by the fact that only 3.2 kcal/mol (13.5 kJ/mol) is predicted (B3LYP/6-311G**)64 to be necessary to reduce the torsion angle between the CH2Se groups from 60° to 0°. This movement makes the ϕ smaller and the diameter of the rings larger. The comparison between the solid state structures of 68(5) including toluene and mesitylene, respectively, in Figure 29 illustrates the above-mentioned correlation. The angle ϕ between the medium plane of the macrocycle and the SeCC-Se units is 35.3° when toluene was included and 30.6° when the larger mesitylene was the guest. More striking is the solvent accessible volume.75 It is 20.8% of the volume of the unit cell when toluene is included and 25.6% when the more voluminous guest mesitylene is included. The two additional methyl groups afford an increase in the channel diameter. This requirement can be met by a decrease of the axis in stacking direction by 2.3%, whereas the two longer axes of the unit cell increase by 3.2%. When the channel is occupied by n-hexane

Figure 27. Columnar structures (left) and side view (right) of 65(5) including toluene as guest. The hydrogen atoms are omitted for the sake of clarity. Reproduced from ref 74. Copyright 2004 American Chemical Society.

12). In the case of 65(5), one molecule of toluene is included per two cyclooctatetrayne units in a regular manner. In Figure 28 we compare two modifications of 68(4). Both are shown in a top view (right) and side view (left). In the upper modification disordered n-hexane molecules are incorporated and AAA stacking of the molecules is observed (modification α), with only a few interactions between the stacks. In the second case (modification β), an ABAB stacking is found, including toluene molecules. The side view of modification β reveals that one guest molecule per host is contained. There are also more interactions between the stacks via Se···Se contacts than in the α modification. The two modifications show rather short intermolecular Se···Se contacts between neighboring stacks (α: 3.60 and 3.78 Å, β: 3.66 and 3.68 Å).74 These contacts are considerably shorter than the sum of the van der Waals radii.3,7 Calculations of the solventaccessible volume75 reveal values of 19.2% for the α and 24% for the β modification. Choosing 68(5) as an example a systematic study was carried out on the solid state behavior with respect to the inclusion of guest molecules.74 These results are summarized 2022


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Figure 28. Tubular structures of two modifications of 68(4): top view at right and side view at left. The upper modification was obtained by recrystallization from n-hexane, and the lower modification from toluene. Reproduced from ref 62. Copyright 2000 American Chemical Society.

macrocycles.47−49 It was suggested to name such species elastic cycles.74 2.2.5. Chalcogens as Part of Aromatic Systems. Aromatic systems tend to form staggered structures in solid state. The main driving force for the formation of this stacking are π−π interactions between the aromatic units.76 However, in some cases it could be shown that the self-organized structures of chalcogen-containing aromatic systems are formed by chalcogen−chalcogen interactions. One example is the fused oligothiophene 69 (R = C12H25; Figure 30a).77 The aromatic core of 69 consists of nine nonlinearly connected fused oligothiophene units. DFT calculations show that the aromatic core of 69 is not planar but adopts a C3-symmetric propeller shape due to steric repulsion between proximate sulfur atoms. This deviation of the aromatic core from planarity should allow the formation of well-organized intermolecular S···S contacts in the columnar assembly of 69. Accordingly, 69 self-assembles into a hexagonal columnar liquid crystalline (LC) mesophase over a wide temperature range from −12 to 90 °C. X-ray diffraction analysis confirms that the columnar assembly of 69 adopts a helical geometry along the axis.77 The intermolecular S···S contacts are developed triple-helically along the columnar axis: Considering a stacked dimer of 69, three sets of intermolecular S···S interactions occur simultaneously (see Figure 30b). The stacking distance amounts to 3.64 Å and 11.1 molecules of 69 being involved in a single pitch, leading to a helical pitch of 40.40 Å (see Figure 30c).77

Table 6. Systematic Study of the Solid State Behavior of 68(5) Depending on the Included Guest Molecules structure

guest molecule

crystal system

volume [%]a

angle ϕ [deg]b

68(5)α 68(5)β 68(5)γ 68(5)δ 68(5)ε 68(5)ζ 68(5)η 68(5)θ 68(5)ι 68(5)κ 68(5)λ 68(5)μ

furane n-hexane thiophene benzene aniline chlorobenzene nitrobenzene toluene anisole p-xylene 4-bromoanisole mesitylene

trigonal trigonal trigonal trigonal trigonal trigonal triclinic triclinic triclinic triclinic triclinic triclinic

18.9 19.2 20.0 20.3 20.4 20.7 20.8 20.8 21.0 23.3 23.5 25.6

37.8 37.6 36.6 36.1 36.0 35.9 35.5 35.3 35.1 34.1 33.6 30.6

a

Solvent-accessible volume of the unit cell, calculated with PLATON.75 bAngle ϕ between the medium plane of the macrocycle and the Se-CC-Se units.

molecules, which are totally flexible, the solvent-accessible volume is decreased to 19.2%. It should be mentioned that in the case of 67(4) similar experiments have been carried out with n-hexane and toluene, respectively, showing quasiisomorphous structures.74 The experiments presented in this section reveal rather flexible rings which are in contrast to the shape-persistent 2023


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Figure 29. Side view (left) and top view (right) of 68(5) including toluene (top) and mesitylene (bottom), showing larger cavities in the case of mesitylene. Short Se···Se contacts are indicated by broken lines. The hydrogen atoms are omitted for the sake of clarity. Reproduced from ref 74. Copyright 2004 American Chemical Society.

Figure 30. (a) Chemical structure of the fused oligothiophene 69 (R = C12H25). (b) Schematic illustration of a stacked dimer of 69 (R = C12H25). (c) Schematic representation of a columnar assembly of the core structure of 69 (R = H) via multiple S···S contacts adopting a triple-helical geometry. Reproduced from ref 77. Copyright 2013 American Chemical Society.

hexane/CH2Cl2 mixture. In benzene as solvent, crystals containing infinite spiral chains of 70b are isolated. In all these aggregates, the aromatic units are connected via Te···O contacts whereby the oxygen atom of one molecule is bound to the tellurium atom of another one (Figure 31). The Te···O distances found in these structures range from 2.171 to 2.242 Å.79 The supramolecular tetramers and hexamers of 70 are so stable that they are even persistent in solution, which could be proven by multinuclear NMR spectroscopic experiments. The enthalpy ΔH per Te···O interaction was calculated on the DFT level to amount to −16.5 kcal/mol for (70b)4 and −18.2 kcal/

A fascinating and promising supramolecular self-assembly driven by chalcogen−chalcogen interactions was described for isotellurazole oxides 70 (Figure 31). These compounds assemble in a variety of supramolecular oligo- and polymers that includes the cyclic tetramers (70)4 and the hexamers (70)6 as well as a helical polymer.78,79 In solid state, the type of the supramolecular aggregate depends on the solvent used for crystallization. If dichloromethane is used as solvent, the macrocyclic tetramer (70b)4 as in a solvent-free polymorph was obtained. The hexamer (70b)6 is formed as the cocrystal 12(70b) × (CH2Cl2) by crystallization from THF and a 2024


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Figure 31. Isotellurazole oxides 70, their macrocyclic aggregates (70)4 and (70)6, as well as benzo-1,2-chalcogenazole 2-oxides 71.

Figure 32. Examples of 1,4-type intramolecular chalcogen−chalcogen interactions as stabilizing factor.

the furanose oxygen. In each case the distance between the chalcogen and oxygen atoms (2.958 Å in 72a and 3.012 Å in 72b) is significantly less than the sum of their van der Waals radii (3.30 Å for 72a and 3.40 Å for 72b). It has been proposed that these chalcogen−chalcogen interactions play an important role in the mechanism of several biological effects.82 1,4-Type intramolecular chalcogen−chalcogen interactions were found to be a driving force for the stereoselectivity of the synthesis of the lactones 73 (Figure 32).84 A series of syn,syn-βhydroxy-α-sulfenyl-γ-butyrolactones (syn,syn-73) having different R and Ar substituents were prepared and subsequently isomerized. The isomerization of syn,syn-73 or syn,anti-73 (or any mixture of both), which occurred easily under basic conditions, yields the syn,anti/syn,syn lactones in a ratio of ca. 6/4.84 The other two possible isomeric lactones (anti,syn-73 or anti,anti-73) were never observed. As the isomerization process proceeds via a retroaldol−aldol sequence, the two concomitant, attractive 1,4-intramolecular interactions of the divalent sulfur with both carbonyl and hydroxyl oxygens, which are only present in syn,syn-73 or syn,anti-73, are made responsible for this stereochemical preference.84 X-ray studies of the lactones syn,syn-73 demonstrate that the distances between the hydroxyl oxygen and sulfur (2.85−3.10 Å) and between the carbonyl oxygen and sulfur (3.05−3.21 Å) are less than the sum of S and O van der Waals radii (3.30 Å) in all compounds. Moreover, the three groups (OH, SAr, and C = O) are positioned in a way allowing an effective orbital interaction both between the oxygen lone pair and the σ* orbital of the S− C bond and between the sulfur lone pair and the π* of the C = O bond.84

mol for (70b)6 in the gas phase. A study of the concentration dependency of the 1H NMR spectrum of 70a at low temperature shows that tetramer and hexamer exist in equilibrium in solution. Furthermore, the self-assembled tetrameric structures form coordination complexes with transition-metal ions and act as fullerene receptors.79 Investigations of the benzo-1,2-chalcogenazole 2-oxides 71 reveal a similar behavior in solid state and in solution.80 However, for the selenium compound 71a (cf. Figure 31) the aggregation in solution occurs only at low temperatures. A similar aggregation pattern, which leads to infinite tapes in solid state, is found for 1,2,5-thiadiazole 2-oxides.81 The observed S···O contacts having an interatomic distance in the range of 2.7 Å are significantly shorter than the sum of the van der Waals radii of oxygen and sulfur (3.25 Å).81 2.2.6. Intramolecular Chalcogen−Chalcogen Interactions as Stabilizing Factor. In molecules displaying two chalcogen atoms in 1,4- or 1,5-position, the chalcogen− chalcogen interactions can control the molecular structure and/or the chemical reactivity. Such controlling interactions are mainly found when the chalcogen atoms are bound at a ring and/or are part of the ring system. Here, the entropy loss by the conformation stabilizing interactions is lower and the control of the chalcogen−chalcogen interaction is more pronounced. A 1,4-type S···O interaction is found in the thiazole nucleoside tiazofurin (72a; X = S) and its selenium analogue selenazofurin (72b; X = Se; Figure 32). Both nucleosides show biological effects, including antitumor activity. The crystal structures of tiazofurin82 and selenazofurin83 show close contacts between the thiazole or selenazole heteroatom and 2025


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Figure 33. Examples of 1,5-type intramolecular chalcogen−chalcogen interactions as stabilizing factor.

center in 77. This indicates an increased electron density at Se(1) due to van der Waals interactions with the second Se center. An even more sensitive probe for the Se···Se interactions are the coupling constants between the interacting Se atoms. The measured 4JSe−Se coupling constants amount to 37.8 (77a; R = Me), 40.9 (77b, R = CCH), and 58.8 Hz (77c, R = CN). The increase of the coupling constants 4JSe−Se with increasing electron-withdrawing group at the Se(1) center confirms a p−σ* interaction between the two Se centers.87 1,5-Type intramolecular chalcogen−chalcogen interactions are not limited to divalent chalcogen centers. An investigation of a series of new arylfluorinated compounds both with improved reactivity as deoxofluorinating agents and with high thermal stability shows that the most promising candidates contained one or more ether groups bound to the phenyl, in positions adjacent to the SF3 unit; one example is molecule 78 in Figure 33.88 By means of DFT methods, this phenomenon was explained by a 1,5-type intramolecular S···O chalcogen bond estimated in the range between 3 and 6 kcal/mol.89 Using statistical database analysis the presence of a multitude of intramolecular chalcogen−chalcogen interactions in proteins has been demonstrated.90 The analyzed data suggested that S···O and S···S interactions are important factors controlling not only the three-dimensional structure of proteins but also their functions to some extent.90

Very strong 1,5-type intramolecular S···O interactions were recognized in the (acylimino)thiadiazoline derivatives 74,85 being angiotensin II receptor antagonists and in thioindirubin86 (75; Figure 33). For the latter compound X-ray diffraction data revealed a S···O distance of only 2.70 Å.86 Even smaller S···O distances ranging from 2.54 to 2.60 Å were found for three (acylimino)thiadiazoline derivatives 74 by X-ray crystallographic analyses.85 However, a closer look at the electronic structures of 74 and 75 reveals that this type of 1,5type intramolecular S···O interactions is not nonbonded chalcogen−chalcogen interactions but long chalcogen−chalcogen bonds.1 A qualitative interaction diagram for 74 and 75 shows that the interaction between the lone pair of the divalent sulfur and the lone pair of the carbonyl group results in the formation of a bonding σ and an antibonding σ* orbital (Figure 34). The occupation of both orbitals (σ and σ*) would

3. THEORETICAL INVESTIGATIONS ON NONCOVALENT CHALCOGEN−CHALCOGEN INTERACTIONS

Figure 34. Qualitative interaction diagram for the bonding chalcogen−chalcogen interactions in 74 and 75. The electrons, which are considered in the MO diagrams, are shown in red color in the formulas.

3.1. Computational Methods

Strong attractions are not expected between closed shell species of zero charge or of the same charge. However, there are numerous observations that so-called weak noncovalent or van der Waals interactions play a role in the binding between different molecules.91,92 The development of these qualitative ideas from 1961 on has been summarized in the literature.91 The interaction between divalent chalcogen centers was initially rationalized by p−σ* interactions as shown in Figure 13a. However, this qualitative rationalization is incomplete because it is based on a single electron configuration. Research in the last two decades has shown that the inclusion of electron long-range electron correlation (dispersion interaction) is crucial for the analysis of noncovalent interactions between closed shell species such as divalent chalcogen centers.91−96 In this section we summarize recent work which is based on high

not lead to a bond formation. However, the interaction between the R2S unit and the carbonyl group in 74 and 75 results in the formation of an empty bonding aromatic π orbital. The distribution of electrons leads to the 10π aromatic compound 74 and to the 14π aromatic compound 75 (Figure 33). This resembles the binding situation found in trithiapentalene and its derivatives.1 The selenium compounds 77 were synthesized to study 1,5type intramolecular nonbonded Se···Se interactions by NMR spectroscopy in solution (Figure 33).87 They were chosen as the 77Se nucleus has a spin of I = 1/2 and the 77Se NMR chemical shift is very sensitive to the electronic environment of the selenium atom. A comparison of the NMR chemical shifts of the Se(1) center with the shifts of the Se(1) center in the reference compounds 76 reveals a high-field shift of the Se(1) 2026


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level quantum chemical calculations on divalent sulfur, selenium, and tellurium species with two methyl groups or one methyl and one ethynyl or one CN group, respectively.5,6 3.1.1. Definition of Interaction Energy. The interaction energy between two closed shell molecules A and B, denoted as Eint, can be defined according to eq 1. Eint( r ⃗ , ζ ⃗ , Q A , Q B) = EAB( r ⃗ , ζ ⃗ , Q A , Q B) − EA (Q A ) − E B(Q B)

(1)

In this equation (Eint) defines the interaction energy as the difference between the energy of a molecule EAB and those of the separated monomers EA and EB. For both the supermolecule AB and the monomers A and B, the same internal coordinates QA and QB are valid. The relative orientation of the monomers is described by the intermolecular vector r⃗ and the orientational angles ζ⃗ (see Figure 35).5,6 All quantities shown

Figure 36. Potential energy curves of H2Te···Te(H)CN as derived by different methods. Basis sets: aug-cc-pVTZ for C, N, and H and augcc-pVTZ-PP for Te.

distance was too long. The B3LYP method leads to quite good geometries but is incapable of recovering much of the interaction energy. The reason for this failure is the fact that B3LYP does not account for long-range electron correlation (dispersion interaction). However, if DFT methods with dispersion corrections (B3LYP-D3 and M06-2X) are used, the calculated values for the interaction energy deviate only slightly from the MP2, B2PLYPD, and CCSD(T) data (Figure 36). As dispersion-corrected DFT methods134 are much less timeconsuming than MP2 and CCSD(T), they are particularly suitable for the calculation of chalcogen−chalcogen interactions in larger systems. A comparison between the potential energy curves of B3LYP and B3LYP-D3 shows that the dispersion dominates the chalcogen−chalcogen interaction. For the model systems shown in Figure 37 (and in Figure 41, see below), the geometrical parameters were optimized

Figure 35. Definition of the three most important parameters used in eq 1. The distance r(E1E2) and the orientational angles ω(y , E1E 2) and ω(z , E 2Z), which were used to characterize the optimized geometries of the molecules R-E1-R and R-E2-Z, are shown.

below are corrected for basis set superposition error (BSSE) using the counterpoise (CP) procedure,97 if not otherwise noted. Please keep in mind that the interaction energy which is defined according to eq 1 cannot take into consideration zeropoint corrections. 3.1.2. Selection of Methods and Basis Sets. During the calculations, the choice of the basis set proved to be delicate.5,6 Quite a number of studies manifest that a polarization or diffuse enlarged split-valence triple-ζ basis set in association with electron-correlation methods was needed to obtain trustworthy results for van der Waals-type interactions.98 For heavier atoms, such as Te-containing compounds, a powerful effective core potential (ECP) was necessary. Therefore, Dunning’s correlation-consistent basis sets (correlation-consistent polarized valence triple-ζ, cc-pVTZ, cc-pVTZ-PP, SDBcc-pVTZ) were employed.99−107 Benchmark computations were performed by using the basis sets given above, combined with Pople’s 6-311G family108−111 for the lighter atoms (C, N, H) with and without polarization and diffuse functions in combination with a variety of electronic structure methods (HF,112−116 MPn, 117−119 B2PLYPD,120,121 CCSD(T),122−127 B3LYP,128−130 B3LYPD3,131,132 and M06-2X133). In this study one varied the distance r between two Te centers of TeH2 and HTeCN, leaving all other geometrical parameters fixed.5,6 In Figure 36 the potential energy curves derived by the above-mentioned methods using aug-cc-pVTZ for C, N, and H and aug-ccpVTZ-PP for Te are shown. These model studies revealed that the HF level of theory turned out to be insufficient for describing the intermolecular distance, as the resulting Te−Te

Figure 37. Dimeric homonuclear model systems 79−82 with one alkyne unit as acceptor group in the upper part.

with Gaussian03135 using the counterpoise protocol to obtain BSSE-corrected supramolecular geometries.136−138 For each molecular pair the minimum has been characterized by a frequency calculation.5,6 Particular notice was paid to the flatness of a van der Waals potential energy surface. This caused rather stringent convergence criteria during geometry optimizations to reach the minima. Furthermore, force constants were recalculated every five to ten steps. Perturbation theoretical interaction energy corrections were computed using SAPT 2002.139 For these calculations Atmol1024140 was used as the necessary SCF front end. In order to include the tellurium-containing model systems, summarized in Figure 37 and 41, in the SAPT calculations the DGDZVP141,142 basis set was chosen. The SAPT/DGDZVP calculations were performed on the dimer geometries optimized at the MP2/DGDZVP level of theory.6 To assess the relative amount of chalcogen−chalcogen bonding as compared to hydrogen bonding, NBO analyses143 were applied. The optimized geometries of the dimers were derived by using the HF/aug-cc-pVTZ-ECP level of theory. 2027


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Table 7. Calculated Interaction Energies Ecc‑pVTZ,ECP [kcal/ int,MP2 mol], Intermolecular Equilibrium Distance r(E1E2) [Å], and Orientational Angles ω(y , E1E2 ) and ω(z , E2Z) [deg] of 79−82

Each NBO interaction term between the distinct monomers was interpreted in terms of hydrogen bonding or chalcogen− chalcogen interaction. Charge transfer between the two molecular units was also obtained from NBO analyses.5,6 3.2. Homonuclear Model Systems

3.2.1. Model Systems. In Figure 37 we list four model systems, 79−82, which are closely related to those molecules discussed in section 2 in which at least one chalcogen center is directly connected to an alkyne unit. In our case we assume that the molecule with the alkyne unit acts as acceptor, whereas the other with two methyl groups stands for the donor. We suppose that the interaction of both units can be qualitatively described by the p−σ* interaction sketched in Figure 13a. However, experimental evidence suggests that this assumption is not always justified. Gas phase studies on the dimer of dimethyl ether (83) by molecular beam Fourier transform microwave and free jet millimeter wave absorption spectroscopies144 revealed a Cs symmetrical geometry with the two monomers bound by three weak C−H···O hydrogen bonds as shown in Figure 38. As a result, a similar geometry as

system

Ecc‑pVTZ,ECP int,MP2

r(E1E2)b

ω(y , E1E2)b

ω(z , E2Z)b

79 80 81 82

−2.58 −3.23 −3.66 −4.64

3.52 3.63 3.63 3.76

103.1 102.0 100.0 100.5

21.2 16.8 15.3 13.7

a

Corrected for BSSE. bFor the definition of the parameters, see Figure 35.

interaction [ω(z , E 2Z) ≈ 0°, ω(y , E1E 2) ≈ 90°] can be ascribed to steric effects. The interaction energies shown in Table 7 reveal an increase from 79 to 82. In Figure 39 the potential energy profiles at the

Figure 39. Interaction energies at the MP2/cc-pVTZ-ECP level of theory of the alkynyl-substituted dimers 79−82 as a function of r(E1E2). Reproduced from ref 5. Copyright 2006 American Chemical Society. Figure 38. Optimized geometries of two dimethyl ethers (83) (left) and two dimethyltellurides (right). Reproduced from ref 5. Copyright 2006 American Chemical Society.

MP2/cc-pVTZ-ECP level of theory for 79−82 are plotted. It can be seen that the energy minima increase only slightly between 79, 80, and 81 but considerably for 82. The trends from this discussion, documented in Figure 39 and Table 7, agree in a qualitative sense with the interaction model shown in Figure 13a. This qualitative model predicts an increase of interaction with decreasing energy difference between the p donor orbital and the σ* (E2-C) acceptor orbital.76 The energy of the donor orbital increases in the series O → S → Se → Te, as exemplified by the first ionization energies of Me2E (E = O, S, Se, Te).145,146 The energy of the acceptor σ* orbital of the E-CCH σ* bond should decrease. 3.2.2. Results from NBO Analyses. The NBO analysis143 provides an alternative way to characterize a noncovalent interaction in terms of the functional groups involved in a supermolecule. In the case of the models 79−82, this leads to hydrogen bonding between C−H groups and chalcogen atoms and to chalcogen−chalcogen interactions of two chalcogen centers E1 and E2 as depicted in Figure 35. The NBO analyses for 79 to 82 were performed by interpreting the sums of the second-order interaction terms of the NBO program in terms of hydrogen and chalcogen−chalcogen bonding.5 The results of this study are summarized in Table 8. It is interesting to note that the absolute values of hydrogen bonding vary only slightly from 79 to 82, ranging from 1.8 to 2.8 kcal/mol

anticipated from a p−σ*-type interaction emerges. For comparison, we show in this figure also the structure of the dimer of dimethyltellurium, which will be discussed later. High level ab initio calculations show that there is no C−H···Te interaction possible for geometrical reasons. 5,6 The C s symmetrical structure of the dimer of dimethyltellurium is due to a noncovalent interaction between the Te centers and can be described by a p−σ* interaction of the 5p lone pair of the lower Te center with the σ* orbital of the Te−Me bond.5,6 In Table 7 the calculated interaction energies Ecc‑pVTZ,ECP , int,MP2 the equilibrium distances r(E1E2), and the orientation angles ω are given. As anticipated, one finds for 79 that the calculated bond distance between the oxygen centers (3.52 Å) is substantially longer than the van der Waals distance of two oxygen centers (2.8 Å).3,7 For 80 to 82 the value for r(E1E2) is smaller than the corresponding sum of the van der Waals radii of S (3.7 Å), Se (4.0 Å), and Te (4.4 Å).3,7 The values calculated for the orientation angles ω(y , E1E 2) and ω(z , E 2Z) are close to those expected for a p−σ* interaction. The deviation from an ideal geometrical arrangement for a p−σ* 2028


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Several discussions on interactions between different divalent chalcogen centers in the literature inspired these studies.147−156,87,157−159 Although we restricted our discussions to examples of intermolecular interactions between divalent chalcogen units, we list here briefly also some examples of heteroatomic intramolecular chalcogen−chalcogen interactions. Further examples for experimental work are S···E interactions with (E = O, N, S, Se, Te),147−151 (O, N···S, Se, Te), and (N···S, Se, Te) interactions.152−156 Experiments with intramolecular S···Se′ interactions closely related to 83 and 79 revealed short Se···Se and Se···H contacts.87 Most results just summarized are based on X-ray and spectroscopic data. Further support for the interpretation was provided by model calculations.157−159 3.3.2. Optimized Geometries and Supramolecular Interaction Energies. In Figure 41 we list 32 dimeric model systems for which the geometrical parameters and electronic properties were calculated by using the same basis sets and methods as used for 79−82. The models with tellurium could be included in the SAPT calculations by using the DGDZVP141,142 basis set. In Figure 42 the difference Δr between r(E1E2) and the sum of the van der Waals radii for the chalcogen elements involved (O···O (2.80 Å), O···S (3.25 Å), O···Se (3.40 Å), O···Te (3.60 Å), S···S (3.7 Å), S···Se (3.85 Å), Se···Se (4.0 Å), S···Te (4.05 Å), Se···Te (4.20 Å), and Te···Te (4.40 Å)) is plotted for all 32 models. For the sake of clarity we have subdivided our figures into eight parts according to the eight families listed in Figure 41. The first four families in Figure 42 (83−98) are characterized by two positive values for Δr for those systems having oxygen or sulfur in the accepting fragment. A negative value for Δr is found for those model systems with Se or Te in the acceptor units, which means a smaller intramolecular distance r(E1E2) than the sum of the van der Waals radii. The second group is characterized by one electronwithdrawing CN group. In the four families, one finds that only the first family member (99, 103, 107, 111) reveals positive Δr values. For all other members, one finds Δr values between −0.17 and −0.79 Å (Figure 42). Looking closer at the trends within each group, it is seen that Δr becomes smaller when the element in the accepting unit is held constant. This is seen by looking at the series 83, 87, 91, 95, or 85, 89, 93, 97, etc. In the case of the two sulfurcontaining series (84, 88, 92, 96 and 100, 104, 108, 112), strong exceptions are noticed. In the first series a maximum of Δr is encountered for the homonuclear system 88, whereas in the related homonuclear system, 104 shows the smallest value for Δr. Looking at Figure 42 one sees that Δr is greater than zero for Z = O in the accepting subunit or smaller than zero for Z = Se, Te in the accepting unit. An exception is found for Z = S. For 84, 88, 92, and 96 Δr is positive, whereas for 100, 104, 108, and 112 a negative value is found. These observations support our earlier findings that there is an inherent difference between the noncovalent interactions of oxygen containing species and those with heavier chalcogen centers (Se, Te). The sulfur species behave as a “hybrid” between these extremes.6 According to Figure 13 one expects for the angle ω(y , E1E 2) a value of 90° if a p−σ* interaction is dominant. The calculations reveal a variation of ω between 44° and 137°.6 To survey these results, we compare as before the calculated values between the eight families defined in Figure 41. The highest values within each family are found for 83, 87, 91, 95, 99, 103,

Table 8. Partition of Interaction Terms of Model Systems 79 to 82 as Derived by a NBO Second-Order Perturbation Analysis into Chalcogen−Chalcogen Interactions (EEE) and Hydrogen Bonding (EH‑bond), the Largest Matrix Element of the Perturbation Analysis (Emax), and Charge Transfer (CT) from Donating Units Me-E1-Me to Accepting Units Me-E2CCH, given in 10−3 electronsa system

EEE

EH‑bond′

Emax

CT

79 80 81 82

0.00 1.82 4.42 10.85

2.81 1.93 1.75 1.78

0.63 1.04 2.13 7.30

+1.80 +5.41 +14.02 +37.81

a

The energies are given in kcal/mol.

compared to the values for the chalcogen−chalcogen interactions, ranging from 1.8 to 10.9 kcal/mol. With the exception of 79 and 80, the hydrogen bonds are weaker than the chalcogen−chalcogen interactions. For 80 to 82 the major single intermolecular NBO interaction term is of p−σ*-type. The relative contributions of the hydrogen and chalcogen− chalcogen bonding as derived by NBO analysis are visualized in Figure 40, in which the two competing interactions

Figure 40. Relative contributions of the hydrogen bonding and chalcogen−chalcogen bonding of 79−82 from NBO analysis. Reproduced from ref 5. Copyright 2006 American Chemical Society.

(summed up to 100%) are plotted. We note hydrogen bonds predominate for the oxygen species 79. The extent of chalcogen−chalcogen interaction increases significantly from 80 (50%) to 82 (85%). The results just discussed are corroborated by the values for the charge transfer (CT) from the donating units Me-E1-Me to the accepting units MeE2-CCH given in the last column of Table 8. 3.3. Homo- and Heteronuclear Models

3.3.1. Model Systems. The preliminary studies on 79 to 82 were the prelude for a more extended study concerning the nature of noncovalent interactions taking place between two heteronuclear chalcogen model species. To restrict the possible numbers of pairs with different atoms E1, E2 and substituent Z to a reasonable number, we selected in Figure 41 two families of dimers. In the first family of models, 16 dimers with only methyl groups as substituents were chosen. In the second family, the dimers consist of one Me-E1-Me molecule as donor and one Me-E2-CN molecule as acceptor. 2029


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Figure 41. (a) Dimeric model systems 83−98 with homonuclear and heteronuclear chalcogen−chalcogen interactions and methyl groups on both partners. (b) Dimeric model systems 99−114 with homonuclear and heteronuclear chalcogen−chalcogen interactions and one CN acceptor substituent on the upper partner.

Figure 42. Difference between the calculated distances r(E1,E2) and the sum of the van der Waals radii rvdW(E1) and rvdW(E2) of 83 to 114.

107, and 111. These species bear oxygen in the acceptor unit.6 The lowest values for ω are found for those pairs with Te in the accepting unit. Especially low values are encountered for 86 (62.3°) and 102 (43.6°). In these cases the donating unit (X1) is oxygen and the accepting unit (X2) is tellurium. Other low values are encountered for 90, 94, 98, 106, 110, and 114. These pairs contain also tellurium in the accepting unit. Regarding the donor part, the angle ω(z , E 2Z) is decreased in the molecules with Se or Te, with the tellurium-containing acceptor fragment showing the smallest values. The mean value for ω in these groups was found to be in the range between 22° and 36° for the CH3 unit, and between 18° and 28° for the CN moiety. It is noteworthy that the largest deviation from

geometries obtained from the simple p-σ* model is found for the heteronuclear systems 87, 91, 95, 103, 107, and 111, with the accepting unit containing an oxygen atom. The interaction energies, Ecc‑pVTZ,ECP , are larger for the int,MP2 systems with an electron-withdrawing substituent Z = CN (the second group 99 to 114) on the accepting subunit than they are for the acceptor bearing dimethyl ether or its sulfur, selenium, and tellurium analogs (the first group 83 to 98) (Figure 43). The interaction energy rises within each group when going from E2 = O to E2 = Te for the model systems having a cyano moiety in the accepting fragment. The largest stabilization for the heteronuclear pairs is reported for 102 (−6.59 kcal/mol), 106 (−6.42 kcal/mol), and 110 (−6.44 2030


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agrees with the above-mentioned results that these model systems also show the largest interaction energies Ecc‑pVTZ,ECP int,MP2 within each group and, thus, are expected to show the strongest influence on the E2-C vibrational mode. 3.3.3. Results from NBO Analyses. We discuss in this section the noncovalent interactions in the model systems 83− 114 in terms of hydrogen bonding and chalcogen−chalcogen interactions. To get the first insights into the various contributions, we rely on the results of NBO analysis.143 This was accomplished for 83−114 by interpreting the sums of the second-order interaction terms of the Fock operator in the NBO basis in terms of hydrogen bonding or chalcogen− chalcogen interactions. It is noteworthy that such an approach is based on the HF-SCF level of theory and that only bonding interactions are taken into account. Please note that these charge-transfer (CT) interactions make up only a fraction of the total interaction energy: In symmetry-adapted perturbation theory (SAPT), the charge-transfer energy is normally absorbed into the induction energy.160 The latter amounts to only ca. 10−35% of the sum of all attractive terms (see next section). Insofar, these charge-transfer interactions should not be used for a quantitative description of these noncovalent interactions. However, this model allows prediction of the spatial orientation of the interaction units and the change in the bond distances in a simple way. In Figure 44 the relative contributions of the hydrogen− chalcogen bonding and chalcogen−chalcogen bonding, for the model compounds listed in Figure 41, are visualized. It is seen that for each family of both groups the hydrogen−chalcogen bonding dominates (EH‑bond − EEE) for the first members (83, 87, 91, 95, 99, 103, 107, and 111). Hydrogen−chalcogen bonding also prevails in those model systems in which the accepting subunit is dimethylsulfide (84, 88−92, 96) although to a lesser extent. For the respective model compounds of the second group (100, 104, 108, 112) this is not the case. In the second group only those pairs in which the accepting subgroup contains an oxygen (99, 103, 107, 111) show a preference of hydrogen bonding. For the remaining 12 species chalcogen−chalcogen bonding prevails.

Figure 43. Calculated interaction energies Ecc‑pVTZ,ECP [kcal/mol] for int,MP2 the model systems 83−114.

kcal/mol). All these compounds show a CH3-Te-CN as acceptor unit. The values do not differ much from the homonuclear pair 114 (−6.18 kcal/mol). The values for the interaction energies decrease slightly in the second group (99 to 114) when changing E1 from O to E1 = Te (E2 = O: 0.2 kcal/mol; E2 = S: 0.5 kcal/mol; E2 = Se: 0.6 kcal/mol; E2 = Te: 0.4 kcal/mol). In contrast to these results are the data belonging to the first group with Z = CH3. The interaction energy, Ecc‑pVTZ,ECP shows a slight increase for E2 = O (83, 87, int,MP2 91, 95), E2 = S (84, 88, 92, 96), E2 = Se (85, 89, 93, 97), and E2 = Te (86, 90, 94, 98).6 A significant change for Δν̃sym for the symmetric stretching mode of the E2−C bond is found for the heteronuclear models.6 The changes observed for Δν̃sym are relatively small within the first group. By contrast, within the second group (99−114) the changes are relatively large (|Δν̃sym| ≤ 29 cm−1) and do not show any scattering. One observes within each of the families 99−102, 103−106, 107−110, and 111−114 for the last member (E2 = Te) the largest decrease in Δν̃sym. This

Figure 44. Relative contributions of the hydrogen− and chalcogen−chalcogen bonding as derived from NBO calculations. Reproduced from ref 6. Copyright 2007 American Chemical Society. 2031


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correlated methods are necessary to derive accurate values for the energies and geometries of aggregates with noncovalent bonding, such as 83−114. For this purpose the terminology derived from the symmetry-adapted perturbation theoretical (SAPT)94−96,139 treatment is suitable. The particularly intriguing aspect of the SAPT approach is that it reveals the mechanistic origin of a noncovalent interaction in addition to providing an accurate interaction energy. In this approach the interaction energy Eint is computed as an (infinite) expansion consisting of four principal components termed electrostatic (Eelst), induction (Eind), dispersion (Edisp), and exchange (Eexch) energies (eq 2 and 3).

In Figure 45 we display the optimized structures of 111, an example for hydrogen bonding, and of 112, a representative for

∞

Eint,SAPT =

∞

∞

(n) (nk) = ∑ ∑ ESAPT ∑ ESAPT n=1

n=1 k=0

(2)

For practical applications, each expansion coefficient E(n) SAPT is approximated using a perturbation expansion from the Hartree−Fock wave functions. In reality, this amounts to a double perturbation approach for the total interaction energy Eint,SAPT. In practice, these infinite expansions are truncated after a finite number of terms, as in the presently available implementation (SAPT 2002).56 Eint,SAPT is calculated as

Figure 45. Minimum energy conformation of 111 and 112 along with the bonding linear combination of the relevant p and σ* orbitals. The hydrogen bonds between the chalcogen atoms and the two methyl groups are not shown for the sake of clarity. American Chemical Society. Reproduced from ref 6. Copyright 2007 American Chemical Society.

(10) (20) (20) (10) Eint,SAPT = Epol + Eexch + Eind,resp + Eexch ‐ ind,resp + δ HF (1) (20) (2) + ϵ(1) pol (3) + ϵexch (2) + Edisp + ϵdisp(2) (20) + Eexch ‐ disp

(3)

To study the relative influence of the four principal forces, eqs 4−7 are used to sum up several expansion coefficients, resulting in a partition of Eint into Eelst, Eind, Edisp, and Eexch.

chalcogen−chalcogen bonding. Drawn are the bonding linear combinations of the corresponding p and σ* orbitals of 111 and 112 at their optimized geometries. It is seen that the bonding between both subunits in 111 is because of three hydrogen bonds between the CH3 group at the oxygen atom and the 5p orbital at the tellurium center. In contrast to this result is the van der Waals interaction in 112 between both chalcogen centers. For the calculated charge transfer (CT) of 83 to 114 from the donating units (CH3)2E1 to the accepting units CH3E2-Z, one observes a continuous increase in each family (except of 95 and 96).6 For model system 96 (1.42 × 10−3 electrons), a slightly smaller charge transfer was computed than for 85 (2.15 × 10−3 electrons). Aside from these small differences, we notice large differences between the two groups 83−98 and 99−114. This difference is due to the substituent Z, which is methyl in the first group and CN in the second. The strong electron-withdrawing nature of CN causes CT values from 5.01 (99) to 67.22 × 10−3 electrons (114), whereas the CH3 group of the first group causes a CT value of 0.63 (84) to 16.17 × 10−3 electrons (90). The calculated NBO second-order interaction terms between the two subunits of 83 to 114, for which chalcogen−chalcogen interactions dominate, range up to 12.3 kcal/mol.6 In all cases p−σ* interactions play a significant role. However, it should be taken into account that the p−σ* model can explain the chalcogen−chalcogen interactions only qualitatively. Because of their dominating dispersive character, the MO model is not able to describe chalcogen−chalcogen interactions in a full manner. Instead, a method which includes a high degree of electron correlation is required to quantitatively describe the nature of noncovalent bonding. 3.3.4. Results from SAPT Calculations. From the model calculations depicted in Figure 36, it is evident that highly

(10) (10) Eelst = Epol + Eexch + ϵ(1) pol (3)

(4)

(20) (20) Eind = Eind,resp + Eexch ‐ ind,resp

(5)

(20) (20) Edisp = Edisp + ϵ(2) disp(2) + Eexch ‐ disp

(6)

Eexch = ϵ(1) exch (2)

(7)

The results of the SAPT studies are condensed in Table 9 and are displayed in Figures 46−48. A first look at Table 9 and Figures 46−48 reveals considerable differences between both groups. In the first group (Figures 46 and 48, Z = Me, 83-98) the dispersion forces contribute most to the bonding (68−89% of the attractive terms) and the exchange forces to the antibonding energies. In the second group (Figures 47 and 48, Z = CN, 99−114) the bonding contributions of electrostatic (99−106) and inductive forces (107−114) contribute considerably to the bonding. However, the dispersion forces dominate also in this part (49−82% of the attractive terms). On a closer examination one sees that the electrostatic force is responsible for bonding in all dimers having at least one oxygen-containing subunit (83−86, 87, 91, 95, 99−102, 103, 107, 111). With the exception of 99−102 the size of Eelst stays relatively constant compared to that of other forces, ranging from −0.30 (85) to −0.68 kcal/mol (103). Only in the cases of 99 to 102 do the corresponding contributions exceed this range considerably: from 99 (E2 = O, −1.22 kcal/mol) to 102 (E2 = Te, −2.09 kcal/mol) an increase is observed. 2032


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strongly antibonding. Intermediate behavior is observed for the systems with S or Se in the accepting subunit. The induction force is bonding for all 32 families; however, there are large differences between the two groups. In the first group (Z = CH3), the bonding contribution varies between −0.24 (88) and −0.55 kcal/mol (86). In the second group (Z = CN), a clear dependence on the accepting element E2 is found. This shows up in the large difference within each family such as −0.39 to −1.56 (99 to 102), −0.26 to −1.71 (103 to 106), −0.28 to −2.24 (107 to 110), and −0.25 to −2.34 kcal/ mol (111 to 114). Whenever Z is either oxygen or sulfur, the size of induction is similar in both groups. The main difference is due to the higher polarizability of Se and Te as compared to O and S. The highest contribution to the bonding in all 32 model systems comes from the dispersion force, and it contributes most for the last member in each family. For those dimers in which the accepting unit contains either oxygen (83, 87, 91, 95, 99, 103, 107, 111) or sulfur (84, 88, 92, 96, 100, 104, 108, 112), the values for the dispersion remain rather constant, similar to induction. In these compounds the dispersion energies range from −1.70 (103) to −2.17 (95) and from −1.68 (88) to −2.22 kcal/mol (112). These values are in line with the poor polarizabilities of oxygen and sulfur. For the dimers containing either Se or Te in the acceptor unit, the contribution increases when going from Z = CH3 to Z = CN. This is due to the higher polarizabilities of Se and Te with respect to those of O and S. It is noteworthy that within each group, the contributions remain rather constant when E2 is kept constant. The exchange energies are repulsive for all 32 model systems. For the first group, the repulsive contribution is very low in each family for the dimers which contain a sulfur in the accepting unit (84, 88, 92, 96). In the second group the exchange repulsion increases with the atomic weight of the accepting chalcogen. The repulsion is highest for E2 = Te: 2.64 (102), 1.58 (106), 1.92 (110), and 1.86 kcal/mol (114). 3.3.5. Sigma Hole Bonding. The extent of the chalcogen−chalcogen interaction and especially its directionality in the dimeric model systems 83 to 114 can also be explained by the concept of the σ-hole bonding.161,162 Let us consider in a first step the electrostatic potential V(r). In atomic units for a set of atomic nuclei and electrons, the electrostatic potential V(r) at spatial position r is given by

Table 9. Percentage Contributions of the Attractive Terms (induction (Eind), dispersion (Edisp), and electrostatic (Eest)) to the Total Attraction and the Interaction Energy EDGDZVP int,SAPT on MP2/DGDZVP-Optimized Geometries, as Obtained by the SAPT2002 Program and Summed According to Eqs 3−7 percentage contributions system

E1

E2

Z

Eelst

Eind

Edisp

83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114

O O O O S S S S Se Se Se Se Te Te Te Te O O O O S S S S Se Se Se Se Te Te Te Te

O S Se Te O S Se Te O S Se Te O S Se Te O S Se Te O S Se Te O S Se Te O S Se Te

Me Me Me Me Me Me Me Me Me Me Me Me Me Me Me Me CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN

16 16 10 11 18 9 0 0 17 5 0 0 17 2 0 0 36 36 34 29 26 17 8 0 22 9 0 0 18 3 0 0

14 12 13 15 14 11 11 14 14 12 11 15 14 11 11 15 12 14 17 22 10 14 21 35 10 15 22 37 9 15 22 37

70 72 77 74 68 80 89 86 69 83 89 85 69 86 89 85 52 50 49 49 64 69 72 65 68 76 78 63 72 82 78 63

a EDGDZVP int,SAPT

b

−2.04 −2.03 −2.21 −2.56 −2.37 −1.97 −1.99 −2.11 −2.53 −2.08 −2.08 −2.17 −2.62 −2.11 −2.11 −2.15 −2.83 −3.67 −4.39 −5.30 −2.43 −2.68 −3.10 −3.88 −2.50 −2.68 −3.09 −3.96 −2.41 −2.53 −2.89 −3.72

a

All values are given in kcal/mol. This column collects the sum of the four contributions plus δHF.

A different behavior is observed for all systems not containing oxygen atoms. In these cases the bonding contribution of the electrostatic interaction decreases with an increased atomic weight of E2, whereas a dependence of the chalcogen element E1 as well as the substituent Z is seen. The remaining systems in the first group (Z = CH3) reveal a bonding electrostatic force whenever the accepting subunit contains sulfur, with a decrease in bonding from E1 = S to E1 = Te. Antibonding contributions are found for E2 = Se, Te; also, in these examples an increase in antibonding character is found when changing E1 from S to E1 = Te. A substitution of Z = CH3 in Z = CN (Figure 41b) leads to another situation in those model systems which contain an oxygen atom in the donating fragment (83 to 86 and 99 to 102). They reveal bonding electrostatic interactions irrespective of the chalcogen element in the accepting subunit. For all other families, for the first member (E2 = O) the electrostatic interaction is bonding and for the last member (E2 = Te) it is

V (r ) =

∑ A

ZA − |RA − r |

∫

ρ(re) dre |re − r |

(8)

in which ZA is the charge on nucleus A located at position RA, and ρ is the electron density at position re. The first positive term represents the electrostatic potential generated by the atomic nuclei, while the latter negative term accounts for the electrostatic potential of the electron cloud. Please note that V(r) is a physical observable which can be obtained either experimentally163,164 or by calculation using theoretical models.162 For molecules, it is common practice to calculate the surface with an electron density of 0.001 au and project the electrostatic potentials onto it.161,162 In Figure 49 the molecular electrostatic potentials of Me2O, Me2S, Me2Se, Me2Te, MeO-CN, MeS-CN, MeSe-CN, and MeTe-CN are depicted. The electrostatic potential was computed at the density functional B3LYP level. As basis set cc-pVTZ was used 2033


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Figure 46. Contribution of the electrostatic (Eest), induction (Eind), dispersion (Edisp), and exchange (Eexch) energies to the interaction energy (EDGDZVP int,SAPT ) as derived by the SAPT program for the model systems 83−98 (Figure 41a).

for C, H, N, S, and Se, whereas cc-pVTZ-PP was employed for Te. In Me2O, the surface of the chalcogen atom is completely negative (Figure 49a). In the case of the heavier chalcogens S, Se,and Te, the chalcogen atoms show a positive outer region with two local maxima. These regions with positive electrostatic potentials on the outer surface are located along the extensions of the E−C covalent bond and are named σholes.59,60 The σ-holes are most pronounced for Se and Te. This trend can easily be explained by the concept of atomic orbitals. As the hybridization for higher chalcogens is not significant, the contributions of the higher chalcogens to the E−C bonds contain almost pure half-filled p orbitals which results in a lack of electron density in the outer (noninvolved) lobe. Therefore, the regions of positive electrostatic potential are found along the extensions of the E−C bonds. Furthermore, the σ-holes increase with electron-withdrawing substituents bonded to the chalcogen atoms (see molecular electrostatic potential of MeE-CN in Figure 49b), especially when the chalcogen atoms become less electronegative and more polarizable (from S to Se to Te). The highly directed

chalcogen−chalcogen interactions in the dimeric model systems 83 to 114 can be explained as the electrostatic attraction between a σ-hole of one chalcogen atom and the lone pair of the other chalcogen atom. The resulting relative orientations of the two molecules are such that the σ-hole of the chalcogen atom with the higher positive outer region (MeE2-CN) is approaching a negative region on the lateral side of the other chalcogen atom (Me2E1). This corresponds to the concept of the interaction of an occupied p orbital at center E1 and the empty C-E2 σ* orbital (Figure 13a).

4. CONCLUSION The investigation of systems having divalent chalcogen centers in solid state shows that chalcogen−chalcogen interactions are driving forces for the formation of superior structures in crystals. This property can be used for crystal engineering. For example, cyclic systems containing divalent chalcogen centers connected with chains containing as building units alkanes, alkenes, or/and alkynes are prone to self-organization, yielding columnar structures with chalcogen−chalcogen contacts, 2034


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Figure 47. Contribution of the electrostatic (Eest), induction (Eind), dispersion (Edisp), and exchange (Eexch) energies to the interaction energy (EDGDZVP int,SAPT ) as derived by the SAPT program for the model systems 99−114 (Figure 41b).

chemical reactivity of the molecules. Even in solution the formations of supramolecular aggregates by chalcogen− chalcogen interactions were proved. A particularly impressive example is the self-assembly of isotellurazole oxides by stabilizing Te···O contacts. For the theoretical investigation of the noncovalent chalcogen−chalcogen interactions, the application of a method which also includes the long-range electron correlation (dispersion interaction) is of utmost importance. The values obtained by MP2 and dispersion-corrected DFT methods are in good agreement with the CCSD(T) data, which are considered to be the golden standard. The dispersioncorrected DFT methods have the advantage to be much less time-consuming than MP2 and can therefore be used for larger systems.

slightly shorter than the sum of the corresponding van der Waals radii. The usage of rigid units (e.g., two or three alkyne or 1,3-butadiyne units) between the chalcogen centers leads to columnar structures, enabling the inclusion of guest molecules within the cavity of the cycles. This flexibility of the cycles allows the change in the solvent accessive volume of the tubes and makes it possible to include guests with different shapes and volumes in the same tube. A closer look at the chalcogen− chalcogen interactions in solid state reveals that the interacting centers always show an exactly definite relative spatial orientation to each other. Chalcogen−chalcogen interactions as structure defining forces are not limited to solid state: Intramolecular chalcogen−chalcogen interactions stabilize certain conformations which is manifested in the molecular structure and/or the 2035


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Figure 48. Percentage contributions of the attractive terms (induction (Eind), dispersion (Edisp), and electrostatic (Eest)) to the total attraction as derived by the SAPT program for the model systems 83−114 (Figure 41a).

Figure 49. (a) Calculated (B3LYP/cc-pVTZ(-PP)) molecular electrostatic potentials on the 0.001 au electron density isosurface of Me2O, Me2S, Me2Se, and Me2Te. Color scheme spanning between −0.03 (red) and +0.03 (blue). (b) Calculated (B3LYP/cc-pVTZ(-PP)) molecular electrostatic potentials on the 0.001 au electron density isosurface of MeO-CN, MeS-CN, MeSe-CN, and MeTe-CN. Color scheme spanning between −0.04 (red) and +0.04 (blue).

acceptor unit only in a range smaller than 0.7 kcal/mol. The strongest noncovalent chalcogen−chalcogen interaction within both groups is found for the dimer Me2O···Te(CN)Me. These energies can be compared with those of the more familiar hydrogen bonds.8,11 This comparison leads to hydrogen bonds of moderate strength such as water molecules or HCl/H2O (ca. 5 kcal/mol) and weak hydrogen bonds found between acetylene and water (2.2 kcal/mol). The major bonding contribution originates from dispersion forces. In the first group (Me2E···EMe2), the dispersion energy amounts to 70−90% of the sum of all attractive terms. In the second group (Me2E···E(CN)Me), it is lower but still in the range between 49 and 82%. The electrostatic force can either be repulsive or attractive, whereby the following pattern was evident: The electrostatic forces are always repulsive if selenium and/or tellurium are present in both the donor and the acceptor unit. On the other hand, the electrostatic forces are always attractive if oxygen is part of either the donor or the acceptor unit. The highest percentage contributions of the electrostatic term to the total

For the comparison of the interaction energy, the studied model system can be divided into two groups: In the first group a dimethylchalcogenide as donor interacts with another dimethylchalcogenide as acceptor (Me2E···EMe2). In the second group the dimers consist of one Me-E-Me molecule as donor and one Me-E-CN molecule as acceptor (Me2E··· E(CN)Me). All interaction energies (EMP2) within the first group (Me2E···EMe2) lie between −2.0 and −3.5 kcal/mol. The highest absolute values are found for those dimers in which tellurium is present in the acceptor unit. The interaction energies (EMP2) of the second group (Me2E···E(CN)Me), having an electron-withdrawing group in the acceptor units, span a wider area, ranging from −2.7 to −6.6 kcal/mol. Decisive for the strength of the chalcogen−chalcogen interactions in this group is the type of chalcogen in the acceptor unit (E(CN)Me). The interaction energies (EMP2) increase going from O (−2.7 to −3.0 kcal/mol) via S (−3.6 to −4.2 kcal/mol) and Se (−4.4 to −5.1 kcal/mol) to Te (−6.2 to −6.6 kcal/mol). The type of chalcogen in the donor unit is less significant and influences the interaction energy for a given 2036


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attraction (up to 36%) are found for the dimers Me2O··· E(CN)Me. For the induction force, a large difference between the two groups is observed. In the first group (Me2E···EMe2), the percentage contributions of the induction term to the total attraction amount to only 10 to 15%. In the second group (Me2E···E(CN)Me), the induction reaches up to 37%. The highest values are found for the dimers Me2E···Te(CN)Me. Hydrogen bonds dominate only in those dimers which contain oxygen or sulfur atoms in the accepting fragment, but never when selenium or tellurium atoms are in the accepting unit. The p-σ* model, which is based on a one particle consideration, describes only the charge-transfer interaction and hence only a small part of the interaction. Hence, it should not be used for a quantitative explanation of the interaction energy between two chalcogen moieties. However, this nonperfect model has been appropriate enough to predict and reproduce trends and details of structures that exhibit this type of interaction. Both calculations and experiments show that within the chalcogen−chalcogen interactions the tellurium−chalcogen interactions are the strongest and accordingly the most promising ones for further applications. Particularly high values are found if an electron-withdrawing group is attached to the tellurium center. As tellurium−chalcogen interactions exhibit a distinct spatial orientation and as their magnitude is on the order of medium hydrogen bonds, we expect that they could replace in the future hydrogen bridges as driving forces in the formation of complex supramolecular structures (e.g., self-assembled containers, capsules, etc.) in solution.

Rebek, Jr. at The Scripps Research Institute in La Jolla. After working one year at BASF in Ludwigshafen, he returned to the University of Heidelberg, where he did his habilitation from 2001 to 2005. In 2005, he was appointed Professor for Organic Chemistry at the University of Duisburg-Essen. His current research interests include chirality induction, rotation in excited states, and the synthesis of molecular switches and motors. Daniel B. Werz is Professor for Organic Chemistry at TU Braunschweig. He studied chemistry at the Universities of Heidelberg and Bristol and received a Ph.D. from the University of Heidelberg under the supervision of Rolf Gleiter. From 2004 to 2006 he performed postdoctoral studies at ETH Zurich in the group of Peter H. Seeberger. In 2006 he started his independent career at the University of Göttingen as Emmy Noether Fellow of the German Science Foundation. After his habilitation in 2011 he was appointed in 2013 as Professor for Organic Chemistry at TU Braunschweig. His main areas of research include the design of novel synthetic methods based on cyclopropane and Pd chemistry, but they deal also with carbohydrates and fluorescent dyes. Frank Rominger is a staff crystallographer at Heidelberg University. He studied chemistry at the University of Ulm and received his Ph.D. from the same university under the supervision of Prof. U. Thewalt. He started his work at the Institute of Organic Chemistry at Heidelberg University in 1996 in the group of Prof. P. Hofmann and is meanwhile responsible for the crystallographic division and the structure analyses of the entire institute. Christian Bleiholder is an Assistant Professor of Chemistry at Florida State University. He received his M.Sc. from the University of Heidelberg under the supervision of Prof. R. Gleiter (2004) and pursued his Ph.D. under the supervision of Profs. R. Gleiter and S. Suhai (University of Heidelberg and German Cancer Research Center). After graduating in 2007, he conducted his postdoctoral research from 2008 to 2013 in the laboratory of Prof. M. T. Bowers (University of California, Santa Barbara). His current research interests focus on elucidation of structure and reactivity of biological molecules with ion mobility spectrometry/mass spectrometry methods.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rolf Gleiter: 0000-0001-5853-381X Gebhard Haberhauer: 0000-0002-5427-7510 Daniel B. Werz: 0000-0002-3973-2212

ACKNOWLEDGMENTS We would like to thank Petra Krämer for helpful support. Special thanks goes to Professor Dr. Georg Jansen (Essen) for numerous helpful discussions.

Notes

The authors declare no competing financial interest. Biographies

REFERENCES

Rolf Gleiter is professor emeritus and senior professor at the University of Heidelberg. He studied chemistry at TU Stuttgart where he gained his doctorate in organic synthesis in 1964 (F. Effenberger). After that he stayed for one year as postdoc at Princeton University (P.v.R. Schleyer) and two years at Cornell University (R. Hoffmann). Returning to Europe he qualified as professor in 1972 at the University of Basel in E. Heilbronner’s labratory. One year later he moved as full professor to TU Darmstadt and 1979 to the University of Heidelberg. His main areas of research concern PE- and UVspectroscopy, quantum chemical calculations combined with syntheses of hydrocarbons (alkynes, cyclophanes, cyclacenes) and organometallics. He has received many national and international awards for his scientific contributions.

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Gebhard Haberhauer is Professor for Organic Chemistry at the University of Duisburg-Essen. He studied chemistry at the Universities of Vienna and Heidelberg and received his Ph.D. from the University of Heidelberg under the supervision of Prof. R. Gleiter. From 1999 to 2000 he was a postdoctoral fellow in the laboratory of J. 2037


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