Fluorobenzonitriles: Influence of the Substitution Pattern on Melting

Sep 15, 2010 - ence between ortho-, meta-, and para-substituted arenes is based on electronic arguments.1. But the substitution pattern also has an in...
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DOI: 10.1021/cg100794s

Fluorobenzonitriles: Influence of the Substitution Pattern on Melting and Crystallization Properties

2010, Vol. 10 4250–4255

Vera Vasylyeva and Klaus Merz* Chair of Inorganic Chemistry 1, Ruhr-University Bochum, Universit€ atsstrasse 150, D-44801 Bochum, Germany Received August 4, 2009; Revised Manuscript Received August 29, 2010

ABSTRACT: The influence of the substitution pattern on the crystal packing of mono-fluorine-substituted benzonitriles was investigated by DSC measurements and crystal structure determination. The low melting compounds ortho-fluorobenzonitrile and meta-fluorobenzonitrile were crystallized by in situ crystallization directly on the diffractometer. Introduction Substituted fluorinated arenes are well-known building blocks in preparative organic chemistry. In particular, due to the orthogonal reactivity of fluorine and hydrogen, this class is suitable for nucleophilic substitution reactions. Fundamental organic chemistry shows that the reactivity difference between ortho-, meta-, and para-substituted arenes is based on electronic arguments.1 But the substitution pattern also has an influence on the aggregation of the molecules and consequently an influence on macroscopically measurable physical properties such as boiling points or melting points of the substances. Besides hydrogen bonding, important roles in the structure stabilization in the solid state are weaker intermolecular interactions such as halogen 3 3 3 halogen interactions.2-5 In previous studies, we have shown that the substitution patterns on the benzene backbone of iodo-substituted benzonitriles and iodo-substituted nitrobenzenes have an influence on the formation of the crystal packing.6,7 The aggregation behavior of fluorinated arenes is therefore especially interesting. While the F--ion is an excellent hydrogen bond acceptor, the C-F-group does not show any significantly strong hydrogen bonds.8-11 In several studies, it was shown that C-F groups do not favor the formation of F 3 3 3 F contacts as do the C-Cl, C-Br, and C-I groups.12-15 A number of studies have been carried out in recent years which show that fluorine generates different types of packing motifs via C-H 3 3 3 F, C-F 3 3 3 F, and C-F 3 3 3 π interactions, especially in the absence of strong hydrogen bond donors and/or acceptors such as N-H, O-H, CdO, etc.16-20 The question of whether F 3 3 3 F contacts can really be called attractive interactions or if they are simply caused by the close packing in the crystal is controversial. The nature of F 3 3 3 F interactions is still not understood because fluorine is a complex element from the supramolecular perspective.21 According to Pauling’s22 principle, fluorine has a low polarizability, so that the attractive interatomic dispersion forces are rather low, and F 3 3 3 F interactions were rarely observed and, if present, they are very weak. Hulliger determined the crystal structures of some fluorinated benzophenones23 where all possible interactions of the organic fluorine C-F including F 3 3 3 F and F 3 3 3 H are involved in the crystal structures depending on the number of fluorine atoms. Schwarzer and Weber reported for 18 fluorinated *To whom correspondence should be addressed. Telephone: þ 49 234 32 24187. Fax: þ 49 234 32 14378. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 09/15/2010

Figure 1. Classification of supramolecular synthons based on CF 3 3 3 H interactions.

N-phenylmaleimides18 and corresponding phthalimides an increase of F 3 3 3 F contacts with increasing number of fluorine atoms in the molecule, while the C-H 3 3 3 O and C-H 3 3 3 F contacts decrease. Nevertheless, these contacts are supposed to be rather secondary and mostly determined by the crystal packing. However, the examples of F 3 3 3 F contacts in 2,3,6-trifluoropyridine,24 pentafluoropyridine,25 highly fluorinated benzophenones,23 and N-phenylmaleimides18 may suggest that a bottom limit of fluorine atoms seems to be necessary for successful competition with the hydrogen type of interactions. In contrast, the groups of Guru Row26 and Bagryankaya19 consider the existence of F 3 3 3 F noncovalent contacts to exhibit a stabilizing effect on the packing. It seems that fluorine would rather form C-H 3 3 3 F interactions than F 3 3 3 F contacts, whereas the heavier halogens seem to prefer the formation of halogen 3 3 3 halogen interactions.16 Boese et al. defined four possible types of C-H 3 3 3 F synthons (Figure 1) and has shown that low-melting fluorobenzenes have topologically similar molecular C-H 3 3 3 F recognition patterns to those for well-known C-H 3 3 3 O and C-H 3 3 3 N hydrogen bonds, that can be as important as C-H 3 3 3 N hydrogen bonds in stabilizing specific crystal structures.16 r 2010 American Chemical Society

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The comparison of 2,4,6-trifluoropyridine24 with 1,3,5trifluorobenzene and 1,3,5-triazine16 proves this statement. Interestingly, we have shown that a tendency for the coplanar arrangement of pyridine molecules is observed with increasing number of fluorine substituents.24 In contrast, a comparable coplanarization effect depending on the number of fluorine substituents is not visible in fluorinated benzenes. So, fluorinated arenes such as benzonitriles therefore make ideal objects for the study of crystal packing where dipole 3 3 3 dipole interactions play only an inferior role. Each o-,16 m-,27 and p-difluorobenzene16 has one polymorph, that crystallizes in a distorted edge-to-edge conformation. These three fluorinated benzenes show C-H 3 3 3 F contacts, forming different types of synthons. For instance, in ortho-compounds, the screw related molecules are connected by C-H 3 3 3 F interactions, giving catemeric chains along [010], whereas, in 1,4-difluorobenzene, all hydrogen atoms are involved in C-H 3 3 3 F interactions that generate dimer and catemer synthons. In the case of dicyanobenzenes,28-32 most crystal structures are determined by herringbone arrangements with different types of C-H 3 3 3 N hydrogen bonds. Only in the case of the triclinic polymorph of 1,4-dicyanobenzene does the crystal packing show a completely different crystal packing with parallel stacked layers.32 Because fluorobenzene16 and benzonitrile33 are isostructural and both substituent groups have a weak directing effect on the aggregation of the molecules in solid state, the question of whether additional cyano or fluorine substituents have an influence on the crystal packing becomes interesting. In continuation of our investigations on aggregation phenomena,6,7 we have studied the o-, m-, and p-fluorobenzonitriles. These molecular structures allow the study of packing modes, which specially involve interactions with fluorine and cyano groups. Halogen 3 3 3 cyano interactions have been utilized in supramolecular synthons for crystal engineering.34 Similar to the X 3 3 3 X interactions, only the heavier halogens (X = I, Br, Cl) in CN 3 3 3 X interactions are strongly structure-directing. The chemical reactivity of fluorobenzonitriles is well-known, but surprisingly, there is only incomplete and inaccurate information about melting and crystallization properties. p-Fluorobenzonitrile is described as a solid with a melting point of 37-40 °C,35-37 and the melting point of m-fluorobenzonitrile is mentioned: -16 °C.38 In the case of the liquid o-fluorobenzonitrile, no further information about the melting point is given. To analyze the substituent effect, the melting and crystallization properties of fluorobenzonitrile are completed, and the crystal structures of o-fluorobenzonitrile and m-fluorobenzonitrile are presented. The aggregation motifs of the crystal packing of the fluorobenzonitriles are compared with those of fluorobenzenes and benzonitrile in Scheme 1. Experimental Section X-ray Crystallography. Single-crystal X-ray diffraction measurements of o-fluorobenzonitrile and m-fluorobenzonitrile were carried out on a Bruker Smart 1000 CCD diffractometer using graphite-monochromated Mo KR radiation (λ = 0.71073). The compounds were purified by distillation in high vacuum and then transferred into a capillary, attached to the vacuum line. The capillary was sealed and then transferred to the diffractometer by means of a detachable cooling device. Single crystals suitable for X-ray diffraction were grown in situ using a computer-controlled device that applied a focused CO2 laser beam to be shifted along the capillary. Because of the geometry of the crystallization device, ω-scans (one cycle for each measurement) were collected.39 Structures were solved by direct methods, and all non-hydrogen

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Scheme 1

Table 1. Summary of Crystal Data, Data Collection, and Refinement Parameters for o-Fluorobenzonitrile and m-Fluorobenzonitrile formula formula weight temperature (K) crystal system space group crystal size (mm) a b c β V (A˚3) Z calc density (g cm-3) F (000) no. of rflns measured no. of unique rflns μ (mm-1) 2θmax (deg) parameters S (F2) R1 [I > 2σ(I)] wR2 (all rflns) max ΔF (e A˚-3)

o-fluorobenzonitrile

m-fluorobenzonitrile

C7H4FN 121.11 213 orthorhombic Pbca 0.3  0.3  0.1 11.094(7) 7.803(3) 14.174(9) 90 1227(1) 8 1.311 496 928 928 0.1 24.99 83 1.042 0.0699 0.1895 0.250, -0.169

C7H4FN 121.11 213 monoclinic C2/c 0.3  0.3  0.1 7.100(1) 13.201(3) 7.052(1) 115.11(3) 598.6(2) 4 1.333 244 762 413 0.103 24.99 49 1.266 0.0891 0.2456 0.325, -0.238

atoms were refined anisotropically on F2 (program SHELXTL-97, G. M. Sheldrick, University of G€ ottingen, G€ ottingen, Germany). Crystallographic crystal data and processing parameters are shown in Table 1. DSC Measurements. Differential scanning calorimetry (DSC) of o-fluorobenzonitrile, m-fluorobenzonitrile, and p-fluorobenzonitrile were carried out on a Netzsch DSC 250 Phoenix under nitrogen atmosphere at samples that were hermetically sealed in Al pans and cooled. The samples were reheated at a cooling and heating rate of 5 K min-1. The melting points (TM) were determined from the DSC thermograms during the programmed reheating steps, based on the onset temperatures.

Results and Discussion Earlier SCF-wave function calculations of disubstituted benzenes suggested that the total energies increase in the sequence m-, p-, o-compound. An explanation of the fact that o-fluorobenzonitrile was found to be least stable may be a repulsive interaction between the neighboring substituents.40 These calculations do not generally reflect the melting points of disubstituted benzenes. For example, in the case of iodo-substituted benzontrile, the mp increases in the sequence m-, o-, and p-iodobenzonitrile.6 The results of the DSC-measurements of fluorine-substituted benzonitriles are summarized and compared with values from the literature in Table 2. o-Fluorobenzonitrile shows a melting point of -13.7 °C. Additionally to the main peak, a further exothermic peak at -30 °C is observed, which is attributed to a glass transition. Surprisingly, in the case of the m-fluorobenzonitrile, our investigations

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Figure 2. Packing view of o-fluorobenzonitrile. Table 2. Melting Points of Fluorobenzonitriles from DSC Measurements

o-fluorobenzonitrile m-fluorobenzonitrile p-fluorobenzonitrile

melting point (Tm) (°C)

melting point (Tm) reported (°C)

- 13.7 12.5 38.5

-1638 37-4035-37

show a melting point of 12.5 °C. This value differs significantly from the reported value (-16 °C) in the literature.38 The determined melting point of 38.5 °C of p-fluorobenzonitrile is comparable to values from 37 and 40 °C.35-37 The melting points increase from o- to p-fluorobenzonitrile and are an indication of different intermolecular interactions in the crystal packing.41 To analyze the substituent effect of fluorine on the crystal structures of fluorobenzonitriles, the series was completed and the crystal structures of o-fluorobenzonitrile and m-fluorobenzonitrile were determined. o-Fluorobenzonitrile and m-fluorobenzonitrile were sucked into a capillary, and a polycrystalline mass was obtained by freezing at -70 °C. The sample was then recrystallized in the capillary using the laserassisted procedure of Boese & Nussbaumer.39 o-Fluorobenzonitrile crystallizes in the orthorhombic space group Pbca. The cyano substituent has an influence on the geometry of the molecular backbone. The angle at the ipso C atom of the phenyl group is enlarged (C-C1-C, 117.8(3)°). Simultaneously, the bond angle of the cyano group is almost linear (N-C-C1, 178.7(4)°). A similar situation was found for the crystal structure of o-iodobenzonitrile.6 m-Fluorobenzonitrile crystallizes in the monoclinic space group C2/c. In contrast to the cases of o-fluorobenzonitrile and p-fluorobenzonitrile, the C-F bond length in m-fluorobenzonitrile is expanded to 1.387(6) A˚. Furthermore, the C-N bond length of the cyano group is slightly reduced to 1.128(8) A˚, comparable to the C-N bond length determined in m-iodobenzonitrile.6 In the crystal packing of fluorobenzonitriles, the possible intermolecular contacts are hydrogen bonds of the type H 3 3 3 F and C-H 3 3 3 N, dipole 3 3 3 dipole interactions of the type F 3 3 3 F, F 3 3 3 CN, and CN 3 3 3 CN, as well as π 3 3 3 π interactions. As mentioned before, the monosubstituted benzenes fluorobenzene16 and benzonitrile33 crystallize isostructurally. In crystalline fluorobenzene, each molecule is linked to four neighbors via mutually perpendicular C-H...F-mediated helices. In benzonitrile, C-H...N hydrogen bonds substitute the C-H...F interactions.

Figure 3. Packing view of 2,6-difluorobenzonitrile.

The additional substitution of a fluorine atom in the ortho-postion of the CN group leads to a change of the helical aggregation. Now, the phenyl rings in ortho-fluorobenzonitrile are arranged in a herringbone fashion as in crystalline benzene (Figure 2). Herringbone interactions are identified by their characteristic T-shaped geometry, and their importance in the crystal structures of aromatic compounds has been discussed repeatedly in the past.42-44 The crystal structure of ortho-fluorobenzonitrile shows no significant F 3 3 3 F, CN 3 3 3 CN, and C-H 3 3 3 F intermolecular interactions. The crystal packing is determined by weak CH 3 3 3 N interactions (2.62 A˚) involving the proton in the position meta to the CN-group. These values are similar to the CH 3 3 3 N interactions in benzonitrile (2.66 A˚). A comparison with 2,6-difluorobenzonitrile45 (Figure 3) shows that the second substitution of a fluorine atom in the ortho position of the CN groups leads to the formation of a planar arrangement of the molecules. Similar to the case of ortho-fluorobenzonitrile is the crystal packing of 2,6-difluorobenzonitrile determined by weak CH 3 3 3 N interactions involving the proton in the position meta to the CN-group. Surprisingly, in the case of m-fluorobenzonitrile we observe a structural similarity in the aggregation to p-fluorobenzonitrile. This phenomenon is unusual for disubstituted arenes. For example the crystal packing of p-iodobenzonitrile,34 p-bromobenzonitrile,34 or p-iodonitrobenze46 shows a completely different structural motif to ortho- and meta-substitution, with a linear array of molecules strung by C-N 3 3 3 X and C-N 3 3 3 NO2 contacts. In the crystal packing of m-fluorobenzonitrile, the molecules are arranged antiparallel in planes (Figure 4). The layers are formed by C-H 3 3 3 F (2.57 A˚)

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Figure 4. Packing view of m-fluorobenzonitrile.

Figure 5. Structural similarity between p-dicyanobenzene (a), m-fluorobenzonitrile (b), and 3,5-difluorobenzonitrile (c).

Figure 6. Packing view of p-fluorobenzonitrile.

and C-H 3 3 3 N interactions (2.68 A˚) which involve the proton in the ortho position to the CN-group. Noteworthy is the structural similarity between m-fluorobenzonitrile, p-dicyanobenzene,47 and 3,5-difluorobenzonitrile48 (Figure 5). Nearly the same supramolecular synthon is found in the crystal packing of these compounds. The further substitution of a fluorine atom in the meta position of the CN group in 3,5-difluorobenzonitrile leads only to a small expansion of the distance between neighboring molecules. A rather weak directing influence of the fluorine substituent is found in p-fluorobenzonitrile. The crystal structure of p-fluorobenzonitrile is described in the literature (Figure 6).49 In contrast to the crystal packing of m-fluorobenzonitrile, the molecules in p-fluorobenzonitrile are parallel to each other and occur in layers that are parallel to (101). Comparable with the case of m-fluorobenzonitrile, short C-H 3 3 3 F (2.66 A˚) hydrogen bonds are observed also in p-fluorobenzonitrile. But in the case of p-fluorobenzonitrile, the fluorine substituent is

located over the fluorine atom of the molecule of an adjacent layer. The CN groups interact electrostatically. The negatively charged N atom of one group lies opposite to the positively charged C atoms of the CN group of an adjacent layer. Conclusion Based on the isostructural compounds fluorobenzene and benzonitrile with weak secondary CH 3 3 3 N and CH 3 3 3 F interactions, the substitution patterns of fluorobenzonitrile show a significant effect on the aggregation of the molecules in the solid state. Comparable with some highly fluorosubstituted benzenes, no short F 3 3 3 F contacts are observed. In contrast to the well-known halogen substituted benzonitriles and nitrobenzenes, p- and m-fluorobenzonitrile show almost similar supramolecular synthons which are comparable with motifs in the triclinic polymorph of p-dicyanobenzene and 3,5-difluorobenzonitrile. The crystal structures are stabilized

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Figure 7. Crystal packing of 2-fluorobenzonitrile (a), 3-fluorobenzonitrile (b), and 4-fluorobenzonitrile (c).

by CH 3 3 3 N and CH...F interactions. Similar to 2,4,6-trifluoropyridine and 1,3,5-trifluorobenzene, a synthon containing a CH 3 3 3 F interaction is observed for the para-substituted compound. The weak directing influence of the fluorine substituent on the crystal packing of fluorobenzonitrile is remarkable first of all in the increasing melting point of the fluorobenzonitriles in the sequence ortho-, meta-, para-compound. In contrast, a comparable effect on the densities of fluorobenzonitriles is not visible. Furthermore, fluorine substitution shows an enormous influence on the crystallization behavior of fluorobenzonitriles. With a migration of fluorine on the backbone of benzonitriles, the molecular aggregation changes from the T-shape arrangement to coplanar layers (Figure 7). Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft (FOR 618: “Molecular Aggregation”) for financial support. Supporting Information Available: DSC diagrams of the fluorosubstituted benzonitriles, and selected bond lengths and bond angles of o-fluorobenzonitrile and m-fluorbenzonitrile. CIF files giving X-ray data with details of refinement procedures for o-fluorobenzonitrile and m-fluorobenzonitrile: CCDC Nr. 741209 and 741210. This material is available free of charge via the Internet at http://pubs.acs.org.

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