Hybrid Network Formation via Halogen Bonding of the Neutral Bromo

Jun 20, 2012 - Published in print 1 August 2012 ... Slapping a 25% tariff on $200 billion worth of Chinese imports would raise the prices of countless...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/crystal

Hybrid Network Formation via Halogen Bonding of the Neutral Bromo-Substituted Organic Molecules with Anionic Metal−Bromide Complexes Sergiy V Rosokha* and Michael K. Vinakos Department of Biological, Chemical and Physical Sciences, Roosevelt University, Chicago, Illinois 60605, United States S Supporting Information *

ABSTRACT: X-ray measurements revealed that cocrystallization of various bromocarbons and tetraalkylammonium bromometallate salts lead to the formation of hybrid 3D-networks, which show short intermolecular C−Br···Br−M contacts resulting from halogen bonding between electrophilic organic species and anionic metal−bromide complexes. In particular, halogen bondings of carbon tetrabromide with [MBr4]2− complexes (M = Co, Zn, Cd) produce diamandoid networks in which nodes are occupied interchangeably by tetrahedral organic and inorganic counterparts. (NBu4)2[ZnBr4]·C3Br2F6 structure comprises diamandoid-like network in which nodes are occupied by the tetrahedral [ZnBr4]2− species connected via pairs of bromine substituents in C3Br2F6. Halogen bonding of planar [Pt2Br6]2− complexes with CBr4 and linear [CuBr2]− complexes with CHBr3 produces 3D-networks consisting of interconnected ladders. The structural characterization of this series of hybrid networks demonstrates that halogen bonding is strong enough to bring together disparate partners such as neutral aliphatic molecules and ionic salts and is able to accommodate a variety of geometries of the interacting species. Comparison of the intermolecular contact locations with the surface electrostatic potentials and molecular orbital shapes of the bromometallates suggests the importance of the covalent component in halogen bonding.

■

acceptors and their counterions as the halogen-bond donors.9,10 For example, structural studies of halometallates or cyanometallates, [MX4]2− (where M is a divalent metal ion, e.g., CoII, PtII, or PdII, and X is a halide or cyanide ligand), with halogensubstituted pyridinium-type counterions, revealed halogen bonding between coordinated halides (or cyanide) and pyridinium halogens.13 Similarly, halogen bonding between halogen-substituted cation-radicals and metal-containing anionic counterions has been identified in the tetrathiafulvalene-based systems.14 Another large group of hybrid systems includes trans-[MX2(4-X′py)2] complexes in which halogen atoms coordinated to metal ions form halogen bonds with the halogen substituent in the 4-X′py ligand of the neighboring complex.15,16 Short interatomic separations and the directionality of the contacts involving halogen atoms clearly point to the existence of halogen bonding in the above-mentioned systems. Furthermore, Zordan, Brammer, and Sherwood pointed out that this attractive interaction is facilitated by the complementary features of the electrophilic carbon-bonded halogen atoms and nucleophilic halide ligands.15 However, while halogen bonding plays a significant role in the solid-state architecture of

INTRODUCTION An intermolecular attraction between electrophilic halogen atoms and electron-rich centers, referred to as halogen bonding, has been recently recognized as a powerful method for crystal engineering involving halogenated molecules and ions.1,2 This interaction is commonly explained as electrostatic attraction between electron-rich centers and areas of positive electrostatic potential existing on the surfaces of covalently bonded halogen atoms (σ-hole concept).3−5 Experimental and computational data presented in several recent reports indicated the importance of weakly covalent interaction, charge-transfer, and other components in halogen bonding.6−8 While the majority of studies of halogen bonding were focused on the interactions between organic molecules,1 more recent publications point out its significant potential for preparation of hybrid metal−organic substances.9,10 In fact, the combination of redox, magnetic, and optical properties of metal complexes with the adjustable organic frameworks has great potential for material science.2 Also, halogen bonding provides an opportunity for cocrystallization of pharmaceutical substances (containing peripheral halogen substituents) with a variety of organic and inorganic counterparts, which represents a significant interest for medical applications.11,12 It is noticeable, however, that halogen bonding involving metal complexes was observed for the most part in the networks comprising anionic species as the halogen bond © 2012 American Chemical Society

Received: May 14, 2012 Revised: June 18, 2012 Published: June 20, 2012 4149

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156

Crystal Growth & Design

■

Article

RESULTS AND DISCUSSION Cocrystallization of Bromo-Substituted Organic Molecules with Metal−Bromide Complexes. Careful examination of the UV−vis spectra of dichloromethane solutions containing high concentrations of tetrabromozincate anion, [ZnBr4]2− (taken as the salt with bulky tetraalkylammonium cations) and carbon tetrabromide revealed an absorption in the 250−300 nm range, which is absent in the spectra of the separate solutions of these reactants (Figure 1). This absorption is

these materials, their chemical compositions are determined by the coordination or covalent bonds between counterparts and/ or the charge balance of the ionic components. In comparison, there are very few publications about hybrid systems formed by halogen bonding between two independent compounds, which exist as separate individual compounds at ambient conditions.12 In our earlier communication,17 we reported halogen-bond driven cocrystallization of carbon tetrabromide with tetra-npropylammonium salt of copper(I) bromide. In another example, attraction of iodine atoms of halogen bond donors to electron-rich centers (e.g., halide ligands or peripheral pyridine’s nitrogen) of neutral organometallic halogen bond acceptor resulted in formation of one-dimensional hybrid organic-organometallic supramolecular ribbons.18 It is necessary to mention in this respect the halogen bonding involving halide ligands from crowded neutral palladium complexes and the study of thermodynamics of halogen bonding between nickel-coordinated fluoride and iodopentafluorobenzene.19,20 Also, chemical literature contains examples of close contacts between covalently bound halogens and metal ions.21,22 Still, cocrystallization of organic and inorganic counterparts in these rather isolated examples might be related to the fortuitous structural fitting of the components or to the fact that halogen bonding involves atoms that are located on the peripheries of the extended ligands where the effects of metal ions are inconsequential. In the present study, we intend to demonstrate that halogen bonding may serve as a primary force that induces formation of hybrid networks incorporating compounds with different shapes and denticities and involves atoms in the immediate vicinity of metal ions. Following earlier works,8,17,23,24 we selected electrophilic tetrabromo-, tribromo-, and dibromosubstituted aliphatic molecules illustrated in Chart 1 as halogenbond donors.

Figure 1. Spectral changes attendant upon the addition of (Pr4N)2[ZnBr4] to 1.6 mM solution of CBr4 (in CH2Cl2, 22 °C). Concentration of (Pr4N)2[ZnBr4] (mM, solid lines from the bottom to the top): 0, 50, 100, 165, 200, 265, 330, 397, and 530. Dashed line represents the spectrum of the separate 133 mM solution of (Pr4N)2[ZnBr4]. Inset: UV−vis spectra of the CBr4/(Pr4N)2[ZnBr4] mixtures after subtraction of the components’ absorption.

relatively weak, and it is partially overshadowed by the tails of absorption bands of the individual components. Digital subtraction of the latter revealed the absorption band with maximum at λmax = 255 nm (note that the reported earlier24 complex of CBr4 with the separate Br− anion shows a band at λmax = 288 nm). Application of the Job’s method25 indicated that this band is related to the complex between CBr4 and [ZnBr4]2−, which is formed according to eq 1.

Chart 1

CBr4 + [ZnBr4]2 − ⇄ [CBr4, ZnBr4 2 −]

(1)

Slow room-temperature evaporation of the dichloromethane solutions containing tetra-n-butylammonium salt of tetrabromozincate together with slight excess of carbon tetrabromide resulted in the formation of uniform colorless crystals. FT-IR measurements indicated that these crystals contain both CBr4 and (Bu4N)2[ZnBr4] components (see Figure S1 in the Supporting Information). Notably, the C−Br vibration bands in these crystals are shifted about 10 cm−1 as compared to the spectrum of the separate carbon tetrabromide molecule. Most importantly, X-ray crystallographic analysis revealed that these cocrystals comprise 1:1 hybrid networks in which metal− bromide complexes interchange with carbon tetrabromide, and the cavities are occupied by the bulky tetra-n-butylammonium cations (vide infra). UV−vis measurements of the dichloromethane solutions of tetrabromozincate and dibromohexafluoropropane, followed by the digital subtraction of absorption of the individual components revealed only a tail of very weak additional absorption. Apparently, the absorption bands of the [C3Br2F6, ZnBr42−] associate, if any, are blue-shifted as compared to that of the [CBr4, ZnBr42−] complex, and they are overshadowed by the stronger absorption of the components. Such an

As their metal-containing halogen-bond acceptor, we used linear, tetrahedral, and planar metal−bromide anionic complexes, which are presented in Chart 2 (these anionic Chart 2

complexes were taken as salts with tetraalkylammonium counterions). The diverse stereochemistry of these counterparts makes it possible to establish the potential of halogen bonding in cocrystallization of a variety of organic substances and metal− ion complexes and to explore the effects of the metal−ion coordination of the halogen-bond acceptors, and the nature of halogen bonding in general. 4150

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156

Crystal Growth & Design

Article

observation is consistent with the fact that the absorption bands of the complexes of bromide anion with dibromohexafluoropropane are significantly blue-shifted as compared to the [CBr4,Br−] complexes.26 Most importantly, diffusion of hexane into dichloromethane solutions containing (Bu4N)2 [ZnBr4] salt and excess of C3Br2F6 at low temperature resulted in the formation of crystals that contain both organic molecules and zinc−bromide salt. Spectral measurements of the solutions of cadmium(II) tetrabromide with CBr4 also revealed an additional absorption band in the 250−270 nm range indicating formation of the supramolecular [CBr4, CdBr42−] complex. Slow evaporation of the dichloromethane solutions containing mixtures of CBr4 and (Bu4N)2[CdBr4] resulted in the formation of colorless crystals that show C−H vibrations bands of tetra-n-butylammonium and a (blue-shifted) C−Br vibrations band in the FT-IR spectrum (Figure S1, Supporting Information). X-ray crystallographic measurements revealed that these crystals comprise hybrid CBr4/[CdBr4]2− networks similar to that with the (Bu4N)2[ZnBr4] salt (vide infra). The UV−vis spectra of dichloromethane solutions containing carbon tetrabromide together with high concentrations of the bromocobaltate ion are dominated by the absorption bands of the transition metal complex itself. Slow evaporation of the dichloromethane solutions containing ∼1:1 mixtures of these components resulted in the formation of blue−violet crystals of (Bu4N)2 [CoBr4]·CBr4 (see IR spectra in the Supporting Information). Similarly, although our efforts to identify absorption bands of halogen bonded associates involving copper(I)− or platinum(II)−bromide were unsuccessful, slow evaporation of the dichloromethane solutions containing (Pr4N) [CuBr2]/CHBr3 and (Bu4N)2[Pt2Br6]/CBr4 mixtures resulted in the formation of single crystals comprising hybrid networks formed by organic molecules and halometallates. Xray crystallographic analysis of these complexes provided structural characteristics of such networks, as follows. Crystal Structures of Hybrid Halogen-Bonded Networks. (NBu4)2 [MBr4]·CBr4 (M = Co, Zn, Cd). X-ray structural analysis revealed that crystallization of the tetra-n-butylammonium salts of tetrabromometallate complexes with tetrabromomethane produced an isostructural series of crystals with 1:1 CBr4/[MBr4]2− stoichiometry in a space group of I4̅ symmetry. Their structures show three-dimensional diamandoid networks (somewhat similar to that formed by CBr4 with bromide anions24) in which tetrahedral CBr4 molecules and MBr42− dianions alternate in a regular manner at the nodes of the adamantane-like cages (Figure 2). The cavities of the network are occupied by the Bu4N+ cations (Figure S2 in the Supporting Information). In these networks, each tetrabromomethane molecule coordinates four tetrabromometallates, and vice versa, each MBr42− dianion coordinates four CBr4 molecules via four uniform CBr···BrM contacts of ∼3.4 Å (Table 1), which are markedly shorter than the sum of the bromine’s van der Waals radii (3.70 Å27). The C−Br···Br angles of ∼165° measured in these structures (Table 1) are typical for halogen bonded systems.28 These geometric features indicate that the formation of the (NBu4)2[MBr4]·CBr4 cocrystals results from the halogen bonding between electrophilic CBr4 molecules and electron rich MBr42− dianions. For example, the (NBu4)2 [ZnBr4]·CBr4 structure shows a short intermolecular Br···Br distance of 3.400 Å between bromine atoms of tetrabromomethane and a bromide anion of

Figure 2. Diamandoid network formed via halogen bonds between tetrahedral CBr4 and [ZnBr4]2− counterparts in (NBu4)2[ZnBr4]·CBr4 cocrystals. Colors as follows: C, gray; Br, brown; Zn, magenta; halogen bonds in this and following figures are shown as light blue lines (for clarity, tetra-n-butylammonium cations are not shown).

tetrabromozincate (Table 1). Such contraction (8%) of the interatomic separation relative to the sum of van der Waals radii is characteristic for the halogen-bonded bromine−bromide contacts.8 The C−Br···Br angle of 164.7° (Table 1) is also quite common for halogen bonded systems.28 In comparison, the Zn−Br···Br angle of 144.8° is somewhat higher than those previously measured with different halogen-bonded halometallates.13 Since C−Br···Br and M−Br···Br angles deviate from 180°, bromine atoms depart from the straight line connecting carbon and metal−ion nodes of the diamandoid network. This results in some distortion of the edges of the adamantane-like cage. Geometric characteristics of the (NBu4)2[CoBr4]·CBr4 network are essentially the same as those measured in the (NBu4)2[ZnBr4]·CBr4 structure. This is probably related to the practically identical size of the [ZnBr4]2− and [CoBr4]2− complexes, which are both characterized by the same M−Br distances of 2.418 Å. In comparison, the M−Br distance in the cadmium−bromide complex is higher (2.589 Å). Because of the larger size of the [CdBr4]2− dianions, geometric characteristics of their diamandoid networks with CBr4 are somewhat different from that of their ZnBr42− and CoBr42− analogues. However, while the Cd−Br bond is 0.17 Å longer than the Zn−Br or Co−Br analogues, the overall distance between the nodes of the diamandoid networks (i.e., M···C separations of 7.476 Å) in the cadmium-containing cocrystals is only ∼0.03 Å higher than that in their zinc- or cobalt-containing counterparts (7.442 Å). Apparently, the larger size of the CdBr42− complex is partially compensated by the shorter M−Br···Br−C separation. (NBu4)2 [ZnBr4]·2C3Br2F6. Single-crystal analysis revealed that this salt is characterized by a noncentrosymmetric space group I4̅2d. As in the previous systems, these cocrystals comprise 3D hybrid networks formed by halogen bonding between tetrabromozincate with 1,2-dibromohexafluoropropane. Their general architecture represents the (distorted) diamandoid network in which the nodes are occupied by the tetrabromozincate dianions linked by C3Br2F6 molecules (Figure 3 and Figure S3 4151

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156

Crystal Growth & Design

Article

Table 1. Geometric Characteristics of the C−Br···Br−M Halogen Bonds in the Hybrid Networks

network (NBu4)2 (NBu4)2 (NBu4)2 (NBu4)2 (NBu4)2

[ZnBr4]·CBr4 [CoBr4] · CBr4 [CdBr4] · CBr4 [ZnBr4] ·2C3F6Br2 [Pt2Br6] ·2CBr4

(NPr4)2 [CuBr2]·CHBr3

short contactsa

d (Å) (C−Br···Br)

α (deg) ∠(C−Br···Br)

β (deg) ∠(M−Br···Br)

4:4 4:4 4:4 2:4 3:6

3.4024(10) 3.4002(8) 3.3209(10) 3.246(1) 3.5106(13) 3.5561(13) 3.5817(13) 3.4228(9) 3.5101(9) 3.5277(9)

164.612(14) 164.65(3) 164.97(3) 171.9(7) 168.7(2) 170.7(2) 164.1(2) 165.23(15) 167.93(14) 164.54(15)

144.836(14) 144.87(2) 142.40(2) 153.5(5) 104.94(3) 85.96(3) 122.13(3) 88.48(2) 108.13(3) 76.20(3)

4:4b

a Number of short contacts per halogen bond donor and acceptor, respectively. bBromoform forms three halogen bonds and one hydrogen bond with [CuBr2]− anions.

Figure 3. Elementary cage of the halogen-bonded hybrid network in the (NBu4)2[ZnBr4]·2C3Br2F6 cocrystals (for clarity, the nodes of the network, zinc ions, are connected by the magenta lines, and tetra-nbutylammonium cations and fluorine atoms are not shown). Figure 4. Interconnected ladders formed by the [Pt2Br6]2− complexes linked by pairs of CBr4 molecules in halogen-bonded hybrid networks in the (NBu4)2[Pt2Br6]·2CBr4 cocrystals.

in the Supporting Information). Each [ZnBr4]2− complex forms halogen bonds via its four bromide ligands with bromine substituents in C3Br2F6 molecules, and each C3Br2F6 molecule forms halogen bonds with two tetrabromozincate dianions. The C−Br···Br−Zn separation of 3.246 Å and the C−Br···Br and Zn−Br···Br angles of 171.9 and 153.5°, respectively, are in the same ranges of the corresponding angles in the CBr4/MBr4 networks (see Table 1). (NBu4)2 [Pt2Br6]·2CBr4. X-ray measurements reveal that these cocrystals comprise an infinite 3D network formed via halogen bonding of [Pt2Br6]22− complexes and CBr4 molecules. Each tetrabromomethane forms three relatively long halogen bonds with bromide ligands. In turn, each [Pt2Br6]22− complex forms halogen bonds with six CBr4 molecules. As a result of such bonding, the 3D network shows inclined ladders in which pairs of planar [Pt2Br6]22− boards are linked by pairs of tetrabromomethane molecules (Figure 4 and Figure S3 in the Supporting Information). The third halogen bond formed by each CBr4 molecule provides a connection between neighboring ladders. Three distinct intermolecular Br···Br contacts in this network of 3.511, 3.556, and 3.582 Å are longer than those in the tetrabromomethane cocrystals with other tetrahalometallate or bromide anions. The C−Br···Br and Pt−Br···Br angles, which are rather close to 180 and 90°, respectively, are typical for the halogen bonds reported earlier for metal−halide complexes.15

(NPr4)[CuBr2]·CHBr3. This structure comprises columns consisting of [CuBr2]2− dianions propagating along the c axis. Neighboring (perpendicular) dibromocuprate within these columns are linked by halogen bonding with CHBr3 molecules, while adjacent columns are linked by a combination of halogen and hydrogen bonds (see Figure S3 in the Supporting Information). More detailed consideration of the structure reveals zigzag chains consisting of alternating (larger and smaller) rings propagating along the a axis (Figure 5). Along these chains, the CHBr3 and [CuBr2]2− counterparts are linked by halogen bonds, while hydrogen bonds serve as crossbars. Both types of rings in the chain are formed by pairs of CHBr3 molecules and pairs of [CuBr2]2− dianions linked by two halogen and two hydrogen bonds. The smaller rings include only one bromide from each [CuBr2]2− complex, which form a halogen bond (3.423 Å) with one CHBr3 molecule and a hydrogen bond with another bromoform. In the larger rings, each [CuBr2]2− forms a halogen bond of 3.51 Å to a bromoform molecule via one of its ligands and forms a hydrogen bond with another bromoform molecule via a second bromide ligand. Different chains in these crystals are linked via the longest halogen bond of 3.528 Å. 4152

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156

Crystal Growth & Design

Article

the C−Br axis and that major segments of their LUMOs are located along the same axis (see Figures S3 and S4 in the Supporting Information). Accordingly, the electrostatic attraction of the halogen bond donors with the electron-rich acceptor as well as their HOMO/LUMO overlap (necessary for covalent interaction) both attain their highest values when intermolecular contacts occur near the extension of the C−Br axes and the C−Br···Br angles are close to 180° (Figure S4 in the Supporting Information). In comparison, quantum mechanical computations indicated that the most negative areas on the surfaces of the bromometallates do not necessarily coincide with their HOMO localization (vide infra). Together with the diverse stereochemistries of the metal−ion complexes, these differences make it possible to examine the ability of different models to account for the locations of intermolecular contacts on the surfaces of the bromometallates, i.e., to explore the geometry of the Br···Br contacts as viewed from the halogen-bond acceptor side. On the whole, the anionic bromometallates are characterized by negative ESP values, and any point on their surfaces attracts areas of positive potential of the halogen-bond donors (Figure 6,

Figure 5. Interconnected chains formed via halogen and hydrogen bonding of [CuBr2]− anions and CHBr3 molecules in the (NPr4) [CuBr2]·CHBr3 (for clarity, tetra-n-propylammonium cations are not shown).

Geometric Features of Halogen Bonding in the Hybrid Networks: View from the Halogen-Bond Acceptor Side. The hybrid networks described in the previous sections are formed by cocrystallization of two distinct substances, i.e., bromocarbons and alkylammonium halometallate salts, which are linked together only by halogen bonding.29 The diverse molecular structures of the counterparts demonstrate the wide-ranging potential of this intermolecular interaction as a primary driving force of cocrystallization of neutral organic molecules and ionic metal−ion complexes. The data in Table 1 demonstrate significant variation of the C−Br···Br−M distances, which change from ∼3.25 Å in (NBu4)2[ZnBr4]·C3Br2F6 cocrystals to ∼3.55 Å in (NBu4)2[Pt2Br6]·CBr4 networks (note that the corresponding van der Waals contacts are 3.70 Å27). Intermolecular C−Br···Br− separations from 3.15 Å to 3.35 Å were also observed earlier in the cocrystals formed by CBr4 or C3Br2F6 molecules and alkylammonium salts of bromide anions.8,23,24 These variations of the Br···Br distances indicate that halogen bonding is sufficiently strong and adjustable to induce cocrystallization of distinct (aliphatic and ionic) substances with diverse crystal-packing requirements. The adaptability of halogen bonding is further illustrated by a structural comparison of the networks formed by carbon tetrabromide with zinc(II), cobalt(II), or cadmium(II) bromides. The geometries of the [ZnBr4]2− and [CoBr4]2− complexes are nearly the same, and the halogen bond lengths in the (Bu4N)2[ZnBr4]·CBr4 and (Bu4N)2[CoBr4]·CBr4 crystals are essentially identical. In comparison, Cd−Br bond length in the [CdBr4]2− complex is 7% higher than the corresponding M−Br distances in the tetrabromocobaltate or tetrabromozincate. Still, in the (Bu4N)2[CdBr4]·CBr4 crystals, the increase of the M−Br bond length is largely compensated by the shortening of the intermolecular Br···Br separation, and the distance between nodes of this network are close to those of the cobalt and zinc analogues (vide supra). The values of the C−Br···Br angles measured in the hybrid networks (Table 1) vary in a relatively narrow range from 164 to 171.9°, which is typical for halogen bonding.28 As was pointed out earlier,15 these nearly linear C−Br···Br arrangements can be alternatively explained using electrostatic and molecular orbital models of the halogen bond formation. Indeed, quantum mechanical evaluation of the bromosubstituted organic molecules in Chart 1 indicates that they show positive σ-holes on the surfaces of bromine atoms along

Figure 6. (Top row) ESP of the halogen bond donors and acceptors superimposed onto X-ray structures of their complexes: (A) CBr4/ [CuBr2]−; (B) CBr4/[ZnBr4]2−; (C) CBr4/[Pt2Br6]2−. Blue and red colors depict positive and negative potentials, respectively. (Bottom row) ESP of the halogen-bond acceptors shown within narrow limits from the most negative to the least negative value: (A) CBr4/ [CuBr2]−, from −100 (red) to −80 (blue); (B) CBr4/[ZnBr4]2−, from −170 (red) to −150 (blue); (C) CBr4/[Pt2Br6]2−, from −155 (red) to −125 (blue). Electrostatic potentials are calculated on the 0.001 electrons bohr−3 molecular surfaces; ESP values are in kcal/mol.

top row). However, scrutiny of the bromometallates’ ESP within narrower ranges revealed noticeable variation of their values (Figure 6, bottom row). On the basis of the electrostatic model, one might expect that the areas of positive ESP of the bromocarbons would be attracted preferably to the most negative areas on the surfaces of the bromometallates. Yet, superimposition of the ESPs onto the crystal structures of the complexes of carbon tetrabromide with different bromometallates revealed that halogen-bond donors approach the most negative area of the halogen-bond acceptor only in the [CuBr2]−/CBr4 dyad (Figure 6A). In the CBr4/[MBr4]2− associates, intermolecular Br···Br contacts are located close to the areas along the M−Br axes, which are characterized by the least negative potentials (Figure 6B). The most negative potentials on the surfaces of the tetrahedral bromometallates are observed in the valleys between bromide ligands, which are less accessible for interaction with the halogen-bond donor. However, steric hindrances do not play a major role in complexes of CBr4 with the planar [Pt2Br6]2− dianions (Figures 6C). Still, the intermolecular [Pt2Br6]2− /CBr4 contacts are observed on the surface of bromide ligands, while 4153

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156

Crystal Growth & Design the areas of the most negative ESP for the [Pt2Br6]2− dianions are located in the center of the triangle formed by the PtII and the two outer bromide anions. In general, while the locations of the most negative potential on the surface of the halogen-bond acceptor vary significantly with the bromometallate, the intermolecular halogen bonds are always directed toward their bromide ligands. It should be noted in this respect that HOMOs of bromometallates are located mostly on the bromide ligands. The superimpositions of the frontier orbitals of halogen bond donors and acceptors onto the crystal structures of their complexes demonstrate that all associates show significant HOMO/LUMO overlaps (Figure 7).

■

CONCLUSIONS

■

EXPERIMENTAL SECTION

Article

X-ray structural analyses of the hybrid 3D-networks formed via cocrystallization of bromo-substituted organic molecules with tetraalkylammonium bromometallates demonstrate that halogen bonding is sufficiently strong to bring together disparate partners such as neutral aliphatic substances and ionic salts.31,32 Such bonding is able to assemble species with a variety of geometric features, such as linear, planar, tetrahedral, and octahedral halogen-bond acceptors and di-, tri-, and tetrabromo-substituted halogen-bond donors. Furthermore, its length is adjustable in response to the crystal packing requirements. Analysis of the locations of intermolecular contacts in comparison with the surface electrostatic potentials and molecular orbital shapes of the bromometallates points out the importance of the covalent component in halogen bonding.

Starting Materials. Tetrabromomethane, 1,2-dibromohexafluoromethane, and bromoform were purified by sublimation or distillation in vacuo. The (NBu4)2[MBr4] (M = Zn, Co, Cd) salts were prepared by reaction of commercially available metal bromides and 2 equiv of tetra-n-butylammonium bromide.33 (NPr4)[CuBr2] salt was prepared from CuBr and 1 equiv of tetra-n-propylammonium bromide.34 (NBu4)2 [Pt2Br6] was prepared by ion-exchange of potassium salts of platinum(II)−bromide complexes with the tetra-n-butylammonium bromides.35 Crystallization of the Hybrid Networks. Halogen-bonded networks were crystallized by either evaporating solutions containing a 1:1 mixture of an organic acceptor and a metal complex in dichloromethane or by diffusion of hexane into such dichloromethane solutions at low temperatures (−35 to −70 °C). For example, 86 mg of the tetrabromocobaltate salt (0.10 mmol) was dissolved in 3 mL of dichloromethane together with 33 mg of tetrabromomethane (0.10 mmol).

Figure 7. (Top) MO shapes of CBr4 and halometallates, superimposed onto crystal structures of their complexes: (A) CBr4/ [CuBr2]− ; (B) CBr4/[CoBr4]2−; (C) CBr4/[Pt2Br6]2−.

This suggests that the location of the Br···Br contacts are related to the molecular orbital interaction between halogen-bond donors and acceptors, which points to the importance of the covalent component in halogen bonding.30

Table 2. Crystallographic Parameters and the Details of the Structure Refinements cmpd color M system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z temperature (K) F(000) μ (mm−1) ρ (g/cm3) refls indpndt obsd (I > 2σ) parameters R1 (I > 2σ) wR2 (I > 2σ) R1 (all) wR2 (all) GooF

(Bu4N)2[ZnBr4]·CBr4 (Bu4N)2[CoBr4]·CBr4 (Bu4N)2[CdBr4]·CBr4 (Bu4N)2[ZnBr4]·2C3Br2F6

(Bu4N)2[Pt2Br6]·2CBr4

(Bu4N) [CuBr2]·CHBr3

colorless 1201.58 tetragonal I4̅ 10.729(2) 10.729(2) 20.633(9) 90.00 90.00 90.00 2375.0(14) 2 173(2)

blue 1195.14 tetragonal I4̅ 10.7332(11) 10.7332(11) 20.613(4) 90.00 90.00 90.00 2374.6(6) 2 173(2)

colorless 1248.61 tetragonal I4̅ 10.8409(18) 10.8409(18) 20.596(7) 90.00 90.00 90.00 2420.6(10) 2 173(2)

colorless 1489.63 tetragonal I42̅ d 15.0828(3) 15.0828(3) 24.7518(6) 90.00 90.00 90.00 5630.8(2) 4 100(2)

yellow 1008.93 monoclinic P21/c 10.389(2) 15.535(3) 17.856(4) 90.00 101.447(5) 90.00 2824.4(11) 4 173(2)

colorless 662.46 monoclinic P21/c 10.466(2) 13.532(3) 16.404(4) 90.00 108.214(4) 90.00 2206.8(9) 4 173(2)

1188 7.269 1.680 7828 3462 3122 102 0.0235 0.0486 0.0282 0.0497 0.980

1182 7.115 1.671 7879 3453 2691 102 0.0420 0.0796 0.0625 0.0869 0.995

1224 7.076 1.713 7877 3462 2740 102 0.0402 0.0718 0.0539 0.0751 0.954

2928 6.180 1.757 43675 3963 2687 182 0.0507 0.1140 0.1018 0.1422 1.096

1872 14.876 2.373 33065 8408 5788 239 0.0482 0.1221 0.0819 0.1362 1.043

1272 10.036 1.994 16309 6519 4225 188 0.0485 0.1070 0.0845 0.1226 0.985

4154

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156

Crystal Growth & Design

■

Slow evaporation of this solution resulted in formation of uniform blue−violet crystals. FT-IR spectra of these crystals demonstrated a strong absorption band related to the C−Br vibrations at 669 cm−1 (shifted by 10 cm−1 as compared to the separate CBr4 molecule) and C−H vibrations of tetra-n-butylammonium cation at 2959, 1469, 882, and 735 cm−1 (see Figure S1 in the Supporting Information), and single-crystal X-ray measurements provided the structure of the (NBu4)2[CoBr4]·CBr4 hybrid material. Alternatively, an analogous dichloromethane solution containing tetrabromocobaltate salt and tetrabromomethane was placed in a 25 mL Schlenk tube and carefully layered with 1 mL of 1:1 dichloromethane/hexane mixture and then with ∼10 mL of hexane. This solution was placed in a refrigerator (−30 °C), and slow diffusion of hexane resulted in the formation of the same (NBu4)2 [CoBr4]·CBr4 crystalline material (as confirmed by FT-IR and unit cell measurements). Colorless crystals of (NBu4)2 [ZnBr4]·CBr4 and (NBu4)2 [CdBr4]·CBr4 were prepared both by evaporation and diffusion methods in the same way as (NBu4)2[CoBr4]·CBr4, (NPr4)[CuBr2]·CHBr3 and (NBu4)2[Pt2Br6]·2CBr4 were prepared by similar evaporation of dichloromethane solutions containing both components, although these hybrid networks crystallized in the mixture with the crystals of the separate bromometallate. Crystals of (NBu4)2[ZnBr4]·2C3Br2F6 were obtained by slow diffusion of hexane into dichloromethane solution containing both components at −75 °C. Spectral and Electrochemical Measurements. UV measurements were carried out on an Agilent 8453 spectrophotometer using a quartz (1 mm path length) spectroscopic cell. The IR spectra of neat crystalline materials were recorded on a Shimadzu spectrometer using single-reflection HATR (Smart Miracle). X-ray Crystallography. Single crystals suitable for X-ray diffraction studies were mounted using oil (Infineum V8512) on a glass fiber. Measurements were made on a Bruker APEX-II CCD using Mo Kα radiation (λ = 0.71073 Å). The data were collected in a cold nitrogen stream at 173(2) K or 100(2) K using an APEX2 V2.1-4 (Bruker, 2007) detector and processed using the Bruker SAINT software package,36 and a semiempirical absorption correction using multiple measured reflections was applied using the program SADABS.37 The structures were solved by direct methods and refined by full matrix least-squares treatment.38 The crystal parameters and information pertaining to the data collection and refinement of the crystals are summarized in Table 2. Computations. Frontier orbital shapes (at 0.01 isovalue) and energies and electrostatic potentials on the 0.001 electrons bohr−3 molecular surfaces were calculated using the Gaussian03 program at the B3LYP/6-311+G(dp) level.39

■

REFERENCES

(1) (a) Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Acc. Chem. Res. 2005, 38, 386−95. (b) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem., Int. Ed. 2008, 47, 6114−27. (2) Metrangolo, P., Resnati, G., Eds. Halogen Bonding: Fundamentals and Applications. In Structure and Bonding; Springer: Berlin, Germany, 2008; Vol. 126. (3) (a) Politzer, P.; Murray, J. S.; Clark, T. Phys. Chem. Chem. Phys. 2010, 12, 7748−7757. (b) Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. J. Mol. Model. 2007, 13, 291−296. (4) Awwadi, F. F.; Willett, R. D.; Peterson, K. A.; Twamley, B. Chem.Eur. J. 2006, 12, 8952−60. (5) Lommerse, J. P. M.; Stone, A. J.; Taylor, R.; Allen, F. H. J. Am. Chem. Soc. 1996, 118, 3108−16. (6) (a) Lu, Y. I.; Zhou, J. W.; Wang, Y. I.; Jiang, Q. C.; Yu, Q. S. J. Phys. Chem. A 2007, 111, 10781−88. (b) Sarwar, M. G.; Dragisic, B.; Salsberg, L. J.; Taylor, M. S. J. Am. Chem. Soc. 2010, 132, 1646−1653. (c) Palusiak, M. J. Mol. Struct. 2010, 945, 89−92. (7) (a) Nelyubina, Y. V.; Antipin, M. Y.; Dunin, D. S.; Kotov, V. Y.; Lyssenko, K. A. Chem. Commun. 2010, 5325−5327. (b) Martinez Amezaga, N. J.; Pamies, S. C.; Peruchena, N. M.; Sosa, G. L. J. Phys. Chem. A 2010, 114, 552−562. (8) (a) Rosokha, S. V.; Neretin, I. S.; Rosokha, T. Y.; Hecht, J.; Kochi, J. K. Heteroat. Chem. 2006, 17, 449−59. (b) Rosokha, S. V.; Kochi, J. K. Structure and Bonding. Halogen Bonding: Fundamentals and Applications 2008, 126, 137−160. (9) Brammer, L.; Espallargas, G. M.; Libri, S. CrystEngComm 2008, 10, 1712−1727. (10) Bertani, R.; Sgarbossa, P.; Venzo, A.; Lelj, F.; Amati, M.; Resnati, G.; Pilati, T.; Metrangolo, P.; Terraneo, G. Coord. Chem. Rev. 2010, 254, 677−695. (11) (a) Schultheiss, N.; Newman, A. Cryst. Growth Des. 2009, 9, 2950−2967. (b) Braga, D.; Grepioni, F.; Lampronti, G. I.; Maini, L.; Turrina, A. Cryst. Growth Des. 2011, 11, 5621−5627. (12) Cocrystals are defined as stable aggregates of two or more molecular components that form stable crystalline materials on their own at ambient conditions. At the same time, cocrystallization does not affect the chemical integrity of the components, which is particularly important for the active pharmaceutical ingredients.11 (13) (a) Espallargas, G. M.; Brammer, L.; Sherwood, P. Angew. Chem., Int. Ed. 2006, 45, 435−40. (b) Zordan, F.; Purver, S. L.; Adams, H.; Brammer, L. CrystEngComm 2005, 7, 350−354. (c) OrmondProut, J.; Smart, P.; Brammer, L. Cryst. Growth Des. 2012, 12, 205− 216. (14) Miyazaki, A.; Yamazaki, H.; Aimatsu, M.; Enoki, T.; Watanabe, R.; Ogura, E.; Kuwatani, Y.; Iyoda, M. Inorg. Chem. 2007, 46, 3353− 3366. (15) Zordan, F.; Brammer, L.; Sherwood, P. J. Am. Chem. Soc. 2005, 127, 5979−5989. (16) Awwadi, F.; Willett, R. D.; Twamley, B. Cryst. Growth Des. 2011, 11, 5316−5323. (17) Rosokha, S. V.; Lu, J. J.; Rosokha, T. Y.; Kochi, J. K. Chem. Commun. 2007, 3383−5. (18) Sgarbossa, P.; Bertani, R.; Di Noto, V.; Piga, M.; Giffin, G. A.; Terraneo, G.; Pilati, T.; Metrangolo, P.; Resnati, G. Cryst. Growth Des. 2012, 12, 297−305. (19) Johnson, M. T.; Džolić, Z.; Cetina, M.; Wendt, O. F.; Ö hrström, L.; Rissanen, K. Cryst. Growth Des. 2012, 12, 362−368. (20) Libri, S.; Jasim, N. A.; Perutz, R. N.; Brammer, L. J. Am. Chem. Soc. 2008, 130, 7842−4. (21) Ramasubbu, N.; Parthasarathy, R.; Murray-Rust, P. J. Am. Chem. Soc. 1986, 108, 4308−4314. (22) Makiura, R.; Nagasawa, I.; Kimura, N.; Ishimary, S.; Kitagawa, H.; Ikeda, R. Chem. Commun. 2001, 1642−1643. (23) Farina, A.; Meille, S. V.; Messina, M. T.; Metrangolo, P.; Resnati, G.; Vecchio, G. Angew. Chem., Int. Ed. 1999, 38, 2433−2436. (24) Lindeman, S. V.; Hecht, J.; Kochi, J. K. J. Am. Chem. Soc. 2003, 125, 11597−606.

ASSOCIATED CONTENT

S Supporting Information *

IR spectra of components and cocrystals; elementary cage of the network in the (NBu4)2[ZnBr4]·CBr4 cocrystals; halogenbonded hybrid networks; ESP and LUMO of halogen-bond donors; UV−vis spectra of the C3Br2F6/Br− solutions; CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Tel: 847-330-4519. Fax: 847-619-8555. E-mail: srosokha@ roosevelt.edu. Website: http://sites.roosevelt.edu/srosokha/. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Charlotte Stern for the X-ray measurements of (Bu4N)2[ZnBr4]·C3Br2F6, Jian Jiang Lu for the crystallographic assistance, and the National Science Foundation (CHE1112126 grant) for financial support of this work. 4155

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156

Crystal Growth & Design

Article

(25) Drago, R. S. Physical Methods in Chemistry; Saunders Co.: Philadelphia, PA, 1977. (26) [CBr3F6,Br−] complex is characterized by the absorption band at λmax = 245 nm; see Figure S6 in Supporting Information (note that λmax = 288 nm for the [CBr4,Br−] complex24). (27) Bondi, A. J. Phys. Chem. 1964, 68, 441−451. (28) Rissanen, K. CrystEngComm 2008, 10, 1107−1113. (29) Except for the [CuBr2]−/CHBr3 network, which is supported also by one hydrogen bond. (30) It should be noted that the crystal-packing (space-filling) requirements play a vital role in the mutual arrangements of the bromocarbons and bromometallates in their cocrystals. However, the consistent location of the intermolecular contacts on the surface of the bromide ligands implies the existence of a significant force responsible for this preferable direction of approach of halogen-bond donors to halogenbond acceptors (our analysis suggests that this force is related to the frontier orbital interaction). (31) Note that the current work presents halogen-bonded systems involving halometallate complexes. In comparison, previously reported examples of halogen-bond-driven cocrystallization of neutral organic molecules and ionic salts (e.g., dibromphexafluoropropane and bromide;23 bromoform and bromide;8 carbon tetrabromide and chloride, bromide, or iodide;24 diiodoperfluoroalkanes and iodide32) involve naked halide anions as halogen bond acceptors. (32) (a) Liantonio, R.; Metrangolo, P.; Pilati, T.; Resnati, G. Cryst. Growth Des. 2003, 3, 355−361. (b) Fox, D. B.; Liantonio, R.; Metrangolo, P.; Pilati, T. W.; Resnati, G. J. Fluorine Chem. 2004, 125, 271−281. (c) Liantonio, R.; Metrangolo, P.; Meyer, F.; Pilati, T.; Navarrini, W.; Resnati, G. Chem. Commun. 2006, 1819−1821. (d) Casnati, A.; Liantonio, R.; Metrangolo, P. W.; Resnati, G.; Ungaro, R.; Ugozzali, F. Angew. Chem., Int. Ed. 2006, 45, 1915−1918. (33) Sabatini, A.; Sacconi, L. J. Am. Chem. Soc. 1963, 86, 17−20. (34) Asplund, M.; Jagner, S.; Nilsson, M. Acta Chem. Scand., Sect. A 1983, A37, 57−62. (35) Sharutin, V. V.; Senchurin, V. S.; Sharutina, O. K.; Somov, N. V.; Gushchin, A. V. Russ. J. Coord. Chem. 2011, 37, 854−860. (36) SAINT, program for area detector absorption correction; Siemens Analytical X-Ray Instruments Inc.: Madison, WI, 1994. (37) Sheldrick, G. M. SADABS, program for siemens area detector absorption correction; University of Gottingen: Gottingen, Germany, 1996. (38) Sheldrick, G. M. SHELXS-97, a program for crystal structure solution; University of Gottingen: Gottingen, Germany, 1997. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision A.01; Gaussian, Inc.: Wallingford, CT, 2003.

4156

dx.doi.org/10.1021/cg300654e | Cryst. Growth Des. 2012, 12, 4149−4156