Structural Variability in the Monofluoroethynylbenzenes Mediated

ARTICLE pubs.acs.org/crystal

Structural Variability in the Monofluoroethynylbenzenes Mediated through Interactions Involving “Organic” Fluorine Amol G. Dikundwar, Ranganathan Sathishkumar, Tayur N. Guru Row,* and Gautam R. Desiraju* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560 012, India

bS Supporting Information ABSTRACT: Competition among weak intermolecular interactions can lead to polymorphism, the appearance of various crystalline forms of a substance with comparable cohesive energies. The crystal structures of 2-fluorophenylacetylene (2FPA) and 3-fluorophenylacetylene (3FPA), both of which are liquids at ambient conditions, have been determined by in situ cryocrystallization. Both compounds exhibit dimorphs, with one of the forms observed in common, P21, Z = 2 and the other form being Pna21, Z = 4 for 2FPA and P21/c, Z = 12 for 3FPA. Variations in the crystal structures of the dimorphs of each of these compounds arise from subtle differences in the way in which weak intermolecular interactions such as C
H 3 3 3 π and C
H 3 3 3 F are manifested. The interactions involving “organic” fluorine, are entirely different from those in the known structure of 4-fluorophenylacetylene (4FPA), space group P21/c, Z = 4. The commonalities and differences in these polymorphs of 2FPA and 3FPA have been analyzed in terms of supramolecular synthons and extended long-range synthon aufbau module (LSAM) patterns. These structures are compared with the three polymorphs of phenylacetylene, in terms of the T-shaped C
H 3 3 3 π interaction, a feature common to all these structures.

’ INTRODUCTION Phenylacetylene (PA, ethynylbenzene, melting point
48
C), a simple aromatic compound with weak hydrogen bond donors (Csp2
H and Csp
H) and acceptors (aromatic, CtC) invokes interest because of its three known polymorphs, namely, the Rform reported by Weiss et al.,1 the β-form reported by Katrusiak and co-workers,2 and the γ-form reported recently by us.3 These trimorphs are obtained under different experimental conditions, the R with normal cooling of the liquid in a capillary, the β by application of pressure on the liquid in a diamond anvil cell, and the γ by quenching the compressed liquid with liquid N2. The R- and β-forms are “nearly isostructural” with the same space group P1, and they share a common packing feature, namely, the CtC
H 3 3 3 π (ethynyl/phenyl) cyclic tetramer (Scheme 2b,c). The γ-form takes the monoclinic space group C2/c with Z0 = 6. All three PA polymorphs are characterized by a number of T-shaped CtC
H 3 3 3 π(ethynyl) and CtC
H 3 3 3 π(aryl) contacts which are particularly common in this group of compounds. These weak interactions develop supramolecular synthons,4 for example, the cyclic tetramer found in the R- and β-forms or the C
H 3 3 3 π(ethynyl) zigzag chain found in the γ-form. Hydrogen-bonded complexes of phenylacetylene with a variety of solvent molecules, such as argon, water, ammonia, alcohols, amines, and N-heterocyclic aromatic molecules, have also been investigated using electronic and vibrational spectroscopic techniques in combination with high-level ab initio and density functional theory calculations.5 In contrast to other r 2011 American Chemical Society

common substituted benzenes such as phenol and aniline, the hydrogen-bonded complexes of phenylacetylene are numerous and span a variety of intermolecular patterns. This variety stems from a subtle balance of intermolecular interactions in the various possible structures. Owing to its switching character in offering a particular hydrogen bonding site by sensing the potential of the approaching reagent, PA has been termed a “hydrogen bonding chameleon” in a recent review.6 Fluorination of organic molecules, in general, affects their physicochemical properties such as chemical reactivity and the biological activity without much change in molecular size (isosteric substitution).7
10 A monofluorinated analogue is geometrically very similar to its parent molecule and hence satisfies isosteric requirements.11 However, there are considerable differences in the stereoelectronic properties of the C
H and C
F bonds because of the highly polar nature of the latter.12a Recent charge density studies provide evidence for the low but still effective polarizability associated with a fluorine atom bonded to carbon (the so-called “organic” fluorine).12b The reversal of the bond dipole moment in going from C
H to C
F is an important factor leading to different physical properties of fluorinated molecules when compared to their nonfluorinated precursors.13 The interactions offered by fluorine in an organic environment have been well characterized by both experiment Received: April 29, 2011 Revised: July 6, 2011 Published: July 07, 2011 3954

dx.doi.org/10.1021/cg2005453 | Cryst. Growth Des. 2011, 11, 3954–3963

Crystal Growth & Design

ARTICLE

Scheme 1. Chemical Structures of Phenylacetylene (PA) and Fluorophenylacetylenes (FPAs)

and computation.12,14 The role of these weak yet directional interactions has also been established in the generation of various polymorphic modifications15 and unusual molecular conformations.16 Fluoro substitution on the benzene ring of phenylacetylene increases the number of possible hydrogen bonding sites. The monofluorophenylacetylenes (2-fluorophenylacetylene, 2FPA; 3-fluorophenylacetylene, 3FPA; and 4-fluorophenylacetylene, 4FPA, Scheme 1) provide three H-bond acceptor sites, namely, the benzene ring and the acetylene fragment, acting as πacceptors and the fluorine atom which can act as an σ-acceptor. Gas phase IR studies on water complexes of monofluorinated phenylacetylenes6 show that organic fluorine is a weak hydrogen bond acceptor and that it increases the acidity of the ethynyl proton and the ring protons (at o-, m-, p- positions) catering to a variety of interactions in the resulting structures. The crystalline forms of 2FPA and 3FPA (Scheme 1) were thus probed in order to evaluate the propensity of formation of various synthons when one of the phenyl ring hydrogens is replaced by a fluorine atom. The ability of fluorophenylacetylenes to prioritize a particular mode of interaction in the crystal structure is an indicator of subtle intermolecular potentials and thus provides an excellent and unique opportunity to investigate competitive hydrogen bonding. 4FPA has been investigated previously crystallographically, but we repeated these experiments for the sake of completeness and also to search for polymorphs (which were not found). The technique of cryocrystallography brings several new compounds, which are liquids at room temperature, into the scope of studies pertaining to intermolecular interactions. Additionally, the experimental methods employed are conducive to the formation of polymorphs, especially in compounds whose crystal structures are constituted with very weak interactions. Our comparative study of the isomeric fluoroethynylbenzenes and the ethynylbenzene (PA) polymorphs therefore offers many new insights.

’ EXPERIMENTAL SECTION The fluorophenylacetylenes (2FPA, 3FPA, and 4FPA) were procured from Sigma-Aldrich and used directly. An initial differential scanning calorimetry (DSC) run was made to verify the melting and crystallization points (Figure 1). The DSC plots give a clear indication of the possibility of various crystalline forms for ortho- and meta-fluorophenylacetylenes, whereas a sharp peak in case of the para isomer suggests a stable monomorphic form.17 A Lindemann glass capillary of 0.3 mm diameter was filled with a liquid and flame-sealed at both ends. The capillary was aligned on a Bruker AXS Smart Apex diffractometer, and the temperature was maintained at 90 K using an OXFORD cryosystems N2 open flow cryostat. Data of 180 frames were collected on the resulting solid sample, generally consisting of more than one

Figure 1. DSC patterns for 2FPA (black), 3FPA (blue), and 4FPA (red). Notice the sharp crystallization exotherm for 4FPA. crystalline domain, with 2θ fixed at
28
and an ω scan-width of
1
. The obtained frames were processed using SMART (Bruker 2004), and the reflections were analyzed using RLATT (Bruker 2004, RLATT (version 30)) wherein the major crystalline domain was identified which was used for unit cell determination and further processing of the data. Full data sets were collected in which omega scans with 0.3
width were performed at a fixed chi angle (54.7
) of the SMART spindle on four sets of 606 frames with 2θ =
28
and with ϕ values of 0
, 90
, 180
, and 270
, respectively, using Mo KR radiation (λ = 0.71073 Å). All structures were solved by direct methods using SHELXS-97 and refined against F2 using SHELXL-97.18 H-atoms were fixed geometrically and refined isotropically. The WinGX package19 was used for refinement and production of data tables, and ORTEP-320 was used for structure visualization. Analysis of the H-bonded and π-interactions was carried out using PLATON21 for all the structures. Packing diagrams were generated with MERCURY.22 Crystallization Conditions. Form I, 2FPA and Form I, 3FPA: Crystallization was carried out by sudden quenching of the liquid (taken in a capillary kept in a hot water bath at ∼70
C), in liquid N2. The crystalline solid formed was taken to the N2 cryostream maintained at 90 K and was further refined by creating a temperature gradient within the capillary so as to obtain a single crystalline domain. Form II, 2FPA: The liquid taken in a capillary was crystallized at 200 K with normal cooling using the turbo cooling option (∼1000 K/h). The same crystalline domain was taken down to 90 K at which temperature the data were collected. Form II, 3FPA: Form I solid was melted completely and recrystallized under the N2 cryostream at 90 K. Data were collected at 90 K. 4FPA: The liquid in a capillary was crystallized at 90 K with normal cooling using the turbo cooling option (∼1000 K/h) and the data were collected at the same temperature.

’ RESULTS As mentioned earlier, the indication of polymorphism by the DSC patterns of both 2FPA and 3FPA prompted us to carry out the crystallization of these liquids in situ on a single crystal diffractometer by employing various cooling conditions. Also, the generation of a third polymorphic modification of phenylacetylene proves the utility of the method of sudden quenching to arrive at new structural forms of a given liquid compound. This can be easily envisaged for compounds with weak hydrogen bond donors and acceptors because the energies involved in the process of crystallization are much smaller than that in the case of compounds with stronger H-bond partners. Both 2FPA and 3955

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Crystal Growth & Design

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Table 1. Crystallographic Details of the Polymorphs Form I and Form II of Compounds 2FPA and 3FPA and a Monomorph of 4FPA at 90 K 2FPA data

3FPA

Form I

Form II

Form I

Form II

4FPA

CCDC number

823474

823475

823476

823477

823478

formula formula weight

C8H5F1 120.1

C8H5F1 120.1

C8H5F1 120.1

C8H5F1 120.1

C8H5F1 120.1

color

pale yellow

pale yellow

colorless

colorless

colorless

crystal morphology

cylindrical

cylindrical

cylindrical

cylindrical

cylindrical

crystal size (mm)

0.3 0.3 0.4

0.3 0.3 0.4

0.3 0.3 0.4

0.3 0.3 0.4

0.3 0.3 0.4

temperature/K

90

90

90

90

90 Mo KR

radiation

Mo KR

Mo KR

Mo KR

Mo KR

wavelength/Å

0.71073

0.71073

0.71073

0.71073

0.71073

crystal system space group

orthorhombic Pna21

monoclinic P21

monoclinic P21/n

monoclinic P21

monoclinic P21/c

a (Å)

7.584(4)

7.0506(15)

6.0163(11)

3.868(3)

7.101(2)

b (Å)

13.088(7)

5.9320(13)

13.997(3)

5.941(4)

6.5984(18)

c (Å)

6.198(3)

7.4848(16)

22.244(4)

13.486(9)

13.273(4)

R (
)

90

90

90

90

90

β (
)

90

103.460(3)

90.304(4)

98.089(12)

99.882(4)

γ (
)

90

90

90

90

90

volume (Å3) Z

615.2(6) 4

304.45(11) 2

1873.1(6) 12

306.8(4) 2

612.7(3) 4

density (g/mL)

1.30

1.31

1.28

1.30

1.30

μ (1/mm)

0.095

0.096

0.094

0.096

0.096

F (000)

248.0

124.0

744.0

124.0

248.0

θ (min, max)

3.1, 26.0

2.8, 25.0

1.7, 25.0

3.0, 25.0

2.9, 25

no. unique reflns

1188

1069

3285

1020

1084

no. of parameters

86

82

244

82

82

hmin,max kmin,max


8, 9
16, 9


8, 8
7, 7


7, 7
16, 16


4, 4
7, 7


8, 8
7, 7

lmin,max


7, 7


8, 8


26, 26


15, 15


15, 15

R_all, wR2_all

0.091, 0.144

0.039, 0.106

0.114, 0.150

0.094, 0.124

0.044, 0.104

R_obs, wR2_obs

0.064, 0.134

0.036, 0.104

0.060, 0.133

0.058, 0.113

0.042, 0.103

ΔFmin, ΔFmax (e Å
3)


0.225, 0.272


0.191, 0.264


0.222, 0.288


0.180, 0.242


0.174, 0.189

GOF

1.024

1.088

1.009

0.958

1.151

3FPA indeed exhibit polymorphism. The crystallographic details of the crystals we obtained are given in Table 1, which also contains details of the structure of 4FPA at 90 K. The structure of 4FPA, already reported by Weiss et al. at 125 K,23 was redetermined at 90 K for a better comparison of C
H 3 3 3 F and C
H 3 3 3 π hydrogen bonds in the structures of these isomeric compounds (2FPA, 3FPA, and 4FPA). The experimental conditions under which our crystals were obtained are provided in the Experimental Section of the manuscript. Two Polymorphic Forms of 2FPA. Form I, 2FPA. Analysis of the cylindrical crystal formed inside the capillary revealed that the structure belongs to the orthorhombic noncentrosymmetric space group Pna21 with Z0 = 1. Interestingly, the fluorine atom was found to be disordered over the two ortho positions of the ethynyl group with partial occupancies of 72 and 28% for the major and minor parts, respectively (Figure 2a). The packing arrangement of the molecules results in zigzag chains along the 21 screw axis (c-axis) formed via CtC
H 3 3 3 π(ethynyl) and C
H 3 3 3 F hydrogen bonds. There are two types of C
H 3 3 3 F

hydrogen bonds in the structure, one with an H-atom from an ethynyl group [CtC
H 3 3 3 F1a, 2.61 Å, 117
] and another with a phenyl ring hydrogen meta to the ethynyl group [(ph, m)C
H 3 3 3 F1b, 2.67 Å, 136
], as shown in Figure 3. The aryl and ethynyl protons appear to be the competing for an acceptor fluorine atom, F1 causing orientational disorder in the molecule. As can be seen from Table 2, the CtC
H 3 3 3 F1a hydrogen bond is shorter than the (ph, m)C
H 3 3 3 F1b H-bond because of the comparatively higher acidity associated with the ethynyl proton when compared to the aryl ones leading to the uneven occupancies of F1. Effectively, the ethynyl proton forms a donor bifurcated hydrogen bonded system with the F1a acceptor (occupancy 0.72) and a π-electron cloud of the ethynyl group (Figure 3). In addition, π 3 3 3 π stacking contacts with an interaction distance of 4.07 Å were noticed which further substantiate the interlayer packing. Form II, 2FPA. The crystal which resulted from normal cooling of liquid in the capillary takes the monoclinic space group P21 with Z = 2. The molecules are perfectly ordered and align 3956

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Crystal Growth & Design

Figure 2. Molecular structures (ORTEP diagrams drawn at 50% probability ellipsoids): (a) Form I, Pna21 of 2FPA showing the disordered F atom with the corresponding partial occupancies. (b) Form II, P21 of 2FPA. (c) Form I, P21/n of 3FPA showing three symmetry independent molecules with different orientations. (d) Form II, P21 of 3FPA.

Figure 3. Intermolecular interactions in 2FPA, form I showing the bifurcated tC
H 3 3 3 π(CtC) and tC
H 3 3 3 F1a hydrogen bonds and (ph, m)C
H 3 3 3 F1b hydrogen bonds.

themselves along the 21 screw axis (b-axis). Figure 2b shows an ORTEP view of the molecule drawn at 50% probability ellipsoids. This structure is characterized by a chain of cooperative tC
H 3 3 3 π(ethynyl) interactions, as seen previously

ARTICLE

in 1,4-diethynylbenzene1 and in the third polymorph of phenylacetylene.3 These molecular chains are further linked through (ph, m)C
H 3 3 3 F hydrogen bonds running along the same 21 screw axes as shown in Figure 5a. The form II structure can be viewed in either of two ways: the molecular chains formed via tC
H 3 3 3 π(ethynyl) interactions are interconnected with C(aromatic)
H 3 3 3 F contacts or the chains formed by C(aromatic)
H 3 3 3 F hydrogen bonds along the 21 screw are further interlocked with the co-operative tC
H 3 3 3 π(ethynyl) interactions. In form I, in contrast, there is an apparent competition between the fluorine atom and π(CtC) acceptors for the ethynyl hydrogen atom, and the overall crystal structure is a compromise between these two relatively weaker hydrogen bonds resulting in a structure with orientationally disordered molecules. Two Polymorphic Forms of 3FPA. Form I, 3FPA. The crystalline solid formed inside a capillary by sudden quenching of the liquid was indexed in the monoclinic system with the centrosymmetric space group P21/n and three independent molecules in the asymmetric unit (Z = 12). The random orientation of molecules in the asymmetric unit (Figure 2c) is a noteworthy aspect of this structure furnishing a variety of intermolecular interactions with all possible hydrogen bonding sites of a molecule. Figure 4 shows the packing diagram of the form I structure of 3FPA. The detailed description of the packing features of this structure and its correlation with form II of 3FPA and also with the β-form of PA is provided later in the discussion part of the manuscript. Form II, 3FPA. As in the form II structure of 2FPA, the crystal takes the monoclinic space group, P21 with Z = 2. Figure 2d shows an ORTEP view of the asymmetric unit drawn with 50% probability ellipsoids. The packing features adopted in this structure are similar to those observed for the form II crystal of 2FPA; i.e., the molecular chains formed via tC
H 3 3 3 π(ethynyl) interactions are interconnected by (ph, p)C
H 3 3 3 F hydrogen bonds (involving the ring hydrogen para to the ethynyl group) and vice versa. Figure 5b shows the herringbone arrangement of molecules formed through tC
H 3 3 3 π(ethynyl) interactions that are further connected via (ph, p)C
H 3 3 3 F interactions along the crystallographic b-axis. The molecular layers formed in this way are stacked with π 3 3 3 π interactions (Cg 3 3 3 Cg; 3.86 Å). Of the above-mentioned two forms of 3FPA, the packing of form II is mainly constituted with well-defined tC
H 3 3 3 π(CtC) and (ph, p)C
H 3 3 3 F hydrogen bonds, whereas different types of packing motifs involving tC
H 3 3 3 π(CtC), (ph)C
H 3 3 3 π(CtC) and CtC
H 3 3 3 F and (ph)C
H 3 3 3 F are observed in the crystal structure of form I.

’ DISCUSSION The crystal packing in all three phenylacetylene polymorphs is characterized by a number of T-shaped tC
H 3 3 3 π(ethynyl) and tC
H 3 3 3 π(aryl) contacts which are commonly seen in this group of terminal acetylenes. These weak tC
H 3 3 3 π interactions form supramolecular synthons, for example, the cyclic tetramer (Scheme 2b,c) found in the R- and β-forms or the tC
H 3 3 3 π(ethynyl) zigzag chain found in the γ-form (Scheme 2a) of phenylacetylene. An assessment of the competition between the aryl and ethynyl donor/acceptor functionalities in determining the overall crystal packing was addressed in our recent communication with a systematic Cambridge Structural 3957

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Table 2. Intermolecular Interactions in the Crystal Structures of Forms I and II of Compounds 2FPA and 3FPA and in a Monomorphic Structure of 4FPA at 90 Ka D
H 3 3 3 A

d(D
H)/Å

d(D
A)/Å

d(H 3 3 3 A)/Å

— D
H 3 3 3 A/


symmetry

2FPA Form I (Pna21, Z = 4) C8
H8 3 3 3 π (C7tC8) C8
H8 3 3 3 F1A C5
H5 3 3 3 F1B

0.93 0.93

3.870(6) 3.149(5)

0.93

3.406(7)

Cg(1) 3 3 3 Cg(1) Form II (P21, Z = 2) C8
H8 3 3 3 π (C7tC8) C3
H3 3 3 3 F1 C5
H5 3 3 3 F1 C4
H4 3 3 3 F1

2.94 2.617(3)

175.9 116.8(2)

2.671(6)

136.4(3)


x,
y, z + 1/2
x,
y, z
1/2
x,
y, z + 1/2 1/2 + x, 1/2
y, z

4.079(3) 0.93

3.588(2)

2.88

132.9


x, y
1/2,
z + 2

0.93

3.464(2)

2.594(1)

155.8(1)


x, y + 1/2,
z

0.93

3.371(3)

2.714(1)

128.3(1)

x + 1, y, z

0.93

3.494(3)

2.956(1)

118.3(1)

x + 1, y, z

3FPA Form I (P21/n, Z = 12) C8
H8 3 3 3 π (C23tC24) tetramer I: d1 C21
H21 3 3 3 π (C7tC8) tetramer I: d2 and LSAM: d2

0.93

3.603(4)

2.81

143.7 (θ1)


x,
y,
z

0.93

3.616(3)

2.72

160.0 (θ2)

x, y, z

C24
H24 3 3 C16
H16 3 3

3 π (C7tC8) LSAM: d1

0.93

3.656(3)

2.78

155.6


x,
y,
z

3 F3 tetramer II: d1

0.93

3.276(4)

2.393(2)

158.5(2) (θ1)

x + 1/2,
y + 1/2, z
1/2

C13
H13 3 3 3 F3 tetramer II: d2 C4
H4 3 3 3 F2

0.93

3.494(3)

2.586(2)

165.3(2) (θ2)


x
1/2, y
1/2,
z + 1/2

0.93

3.191(3)

2.512(2)

130.0(2)


x + 1/2, y + 1/2,
z + 1/2

0.93 0.93

3.223(3) 3.217(4)

2.525(2) 2.709(2)

132.0(2) 115.2(2)

x
1/2,
y + 1/2, z + 1/2 x + 1/2,
y + 1/2, z + 1/2

0.93

3.409(3)

2.929(2)

113.5(2)


x + 1/2, y + 1/2,
z + 1/2

0.93

3.449(4)

2.938(2)

116.0(2)

x, y, z

C18
H18 3 3 3 F1 C24
H24 3 3 3 F1 C5
H5 3 3 3 F2 C16
H16 3 3 3 F1 Cg(1) 3 3 3 Cg(2) Cg(2) 3 3 3 Cg(3) Form II (P21, Z = 2) C8
H8 3 3 3 π (C7tC8) C4
H4 3 3 3 F1 C2
H2 3 3 3 F1

3.742(2)

1/2
x,
1/2 + y, 1/2
z

3.964(2)


1 + x, y, z

0.93

3.667(3)

2.74

173.1


x, y
1/2,
z

0.93 0.93

3.370(5) 3.748(5)

2.507(3) 2.999(3)

154.4(3) 138.6(2)


x, y + 1/2,
z
x, +y
1/2,
z

x + 1, +y + 1, +z

4FPA P21/c, Z = 4 C8
H8 3 3 C3
H3 3 3

a

3 F1 3 F1

C8
H8 3 3 3 π (C7tC8) Cg(1) 3 3 3 Cg(1)

0.93

3.183(2)

2.384(1)

143.8(1)

0.93

3.424(2)

2.677(1)

137.7(1)


x,
y,
z + 2

0.93

3.867(1)

3.19

130.6


x, y + 1/2,
z + 1/2

3.796(1)

1
x, 1
y,
z

Cg represents a centroid of the phenyl ring.

Database (CSD) analysis.24 The patterns in Scheme 2a
c are the most common. This analysis indicates a full exploitation of the hydrogen bond donor and acceptor capabilities in terminal ethynyl aromatics with an equal propensity for both aryl and ethynyl donors/acceptors. A slight preference toward the tC
H 3 3 3 π(aryl) interactions was seen over the tC
H 3 3 3 π(ethynyl) and C(aryl)
H 3 3 3 π(aryl) interactions. An introduction of fluorine at one of the peripheral positions of PA molecule provides an additional H-bond acceptor site without altering the molecular geometry and spatial requirements significantly. The anticipated supramolecular synthon patterns in these structures are thus not only restricted to the π-acceptor property of the aryl and ethynyl donor/acceptor functionalities but are also augmented with the involvement of σ-acceptor F-atom. It is noteworthy that the commonly observed chains formed via tC
H 3 3 3 π(CtC) interactions in nonfluoro molecules

such as 1,4-diethynylbenzene and phenylacetylene are interconnected with many ph(π) 3 3 3 (π)ph, tC
H 3 3 3 π(Ph), (ph)C
H 3 3 3 π(ph), and (ph)C
H 3 3 3 π(CtC) contacts. This feature persists in the fluorophenylacetylenes, but there is also the further implication of F-atom contacts. For example, the form II structures of 2FPA and 3FPA are characterized by molecular chains formed via relatively robust T-shaped tC
H 3 3 3 π(CtC) synthons, but the chains are further interconnected through F-atom interactions, mainly C
H 3 3 3 F
C hydrogen bonds. However, in the form I structures of 2FPA and 3FPA, interactions involving F-atom orchestrate the overall crystal structure, demonstrating the role of fluorine in the generation of these closely related polymorphic structures. The isomorphous nature of the two form II structures of 2FPA and 3FPA is further substantiated by their nearly equal packing densities (1.31 and 1.30 g/cm3, respectively) and the 3958

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Figure 4. Packing diagram of 3FPA, form I (P21/n) showing two types of tetramers I and II. The symmetry independent molecules are color coded. Compare this crystal structure with the β-polymorph of phenylacetylene.2

Figure 5. Packing diagrams of (a) 2FPA, form II (P21) and (b) 3FPA, form II (P21) showing the tC
H 3 3 3 π(CtC) and tC
H 3 3 3 F hydrogen bonded molecular chains propagating along the 21-screw axis. Notice the similarity between these two crystal structures.

corresponding packing fractions (Kitaigorodskii packing index, 70.3 and 69.8%, respectively). Comparison of Form I of 3FPA (Z0 = 3) and β-Form of PA (Z0 = 3). A close inspection of the structure of form I of 3FPA reveals modes of interactions as shown in Figure 4. Among the five structures of the fluorophenylacetylenes (2FPA I and II, 3FPA I and II and 4FPA), this is the only structure with Z0 > 1. Importantly, in this structure, 3FPA molecules ascertain the formation of cyclic tetramer synthons (Figure 4) which are not seen in any of the other fluorinated phenylacetylenes structures discussed here. There are two types of tetramers noticed, tetramer I formed via C
H 3 3 3 π(ethynyl) H-bonds and tetramer II built with C
H 3 3 3 F interactions, wherein the F-atom acts exclusively as an H-bond acceptor (Figure 6). The striking difference between tetramer I in form I of 3FPA and the one noted in all three polymorphs (an incipient tetramer

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in the γ-form) of the parent molecule PA is that tetramer I involves two H-bond donors from each of the ethynyl groups and two from C(aromatic)
H groups meta to the ethynyl group, whereas in the tetramer in all the PA structures all participating H-bond donors and acceptors are from ethynyl groups. All tetramers in PA and the FPAs (Scheme 2b
f) occur across centers of inversion and are generated from two symmetry independent molecules in the asymmetric unit. They can be characterized by two distances and two angles, namely, d1, d2 and θ1, θ2 (marked in Table 2). Stacking of similar tetramers results in columnar structures interconnected with various C
H 3 3 3 π interactions. Surprisingly, the apparent structural unit (Scheme 2f) interlinking the columnar tetrameric structures is of similar pattern in PA and 3FPA and hence can be termed a LSAM25 connecting the similar tetrameric synthons in a diagonal fashion (Figures S1
3 in Supporting Information). Relationship of Form II, P21 (Z0 = 1) and Form I, P21/n 0 (Z = 3) of 3FPA. The high Z0 structures have been considered to be fossil relics of their lower Z0 (preferably Z0 = 1) counterparts.26 The very existence of robust synthons in the lower Z0 forms has been well associated with synthons in their evolutionary stage in the polymorphic structures of higher Z0 .27 For example, in the phenylacetylene structures, it was shown that the γ-form (Z0 = 6) actually represents an intermediate stage between the comparatively stable R- and β-forms and the unstructured liquid.3 A higher order common synthon (LSAM) proposed for all the forms of phenylacetylene structure was shown to evolve from the liquid to the R- and β-forms via the metastable γ-form. This has been termed as a long-range synthon aufbau module (LSAM) by Ganguly and Desiraju.25 Mechanistically, it is important to trace out similarities and differences among the form I and form II structures of 3FPA because the form II structure was obtained by annealing the domains of form I crystal inside the capillary (as explained in the Experimental Section). Although there is no direct resemblance between the crystal packing in form I and form II of 3FPA, it may be noted that the basic packing pattern of the molecules through the formation of molecular chains via tC
H 3 3 3 π(CtC) and (ph, p)C
H 3 3 3 F hydrogen bonds appears to be retained in both the forms. Figure 7 panels a and b show the molecular zigzag chains formed by tC
H 3 3 3 π(ethynyl) and (ph, p)C
H 3 3 3 F hydrogen bonds, respectively. All the three symmetry-independent molecules (red, green and blue) are used in this combined chain structure which replicates itself by the symmetry elements (21-screw and an n-glide) resulting in the formation of tetramers I and II generating a three-dimensional structure of form I. Comparison of FPAs with Simple Disubstituted Benzenes. Monofluorophenylacetylenes (2FPA, 3FPA, and 4FPA) fall under the general category of disubstituted benzenes. A CSD28 (Cambridge Structural Database, v1.13, update Feb 2011) analysis was carried out in order to obtain an overall estimate of centrosymmetric and non-centrosymmetric structures adopted by this class of compounds. The search was restricted to ortho, meta, and para disubstituted benzenes with simple substituents such as halogens,
CN,
OH,
NH2,
NO2,
CHO,
COOH,
COCH3, and
COOCH3. It was assured that the included structures do not contain any chiral centers so as to evaluate the propensity of adoption of non-centrosymmetric packing arrangements by achiral disubstituted benzenes, especially 1,2 and 1,3 disubstituted derivatives. 1,4 Disubstituted benzenes with same substituents (X1 = X2), in general, adopt centrosymmetric structures with the molecular inversion center 3959

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Scheme 2. C
H 3 3 3 π Interaction Patterns Observed in Phenylacetylene and Fluorophenylacetylenesa

a (a) Parallel arrangement of tC
H 3 3 3 π(ethynyl) zigzag chains (observed in the γ-form of PA and form II of 2FPA and 3FPA); (b) cyclic tetramer formed by tC
H 3 3 3 π(ethynyl) interactions in PA (R- and β-forms); (c) tetramer formed by tC
H 3 3 3 π(aryl) contacts in PA (R- and β-forms); (d) tetramer I formed through mixed (ethynyl)C
H 3 3 3 π(ethynyl) and (aryl)C
H 3 3 3 π(ethynyl) contacts in form I of 3FPA; (e) tetramer II formed through mixed (ethynyl)C
H 3 3 3 F and (aryl)C
H 3 3 3 F contacts in form I of 3FPA; (f) long range synthon aufbau module (LSAM) (ref 25) connecting the similar tetrameric columns in PA and form I of 3FPA.

Figure 6. Tetramers I and II in the structure of 3FPA, form I (P21/n). Tetramer I is constituted with tC
H 3 3 3 π(CtC) and (ph, m)C
H 3 3 3 π(CtC) hydrogen bonds; tetramer II is constituted with tC
H 3 3 3 F and (ph, m)C
H 3 3 3 F hydrogen bonds. Notice the participation of red-colored molecules in both the tetramers.

coinciding with the crystallographic center of inversion (Z0 = 0.5 or 0.25). Hence, the figures presented here are for the benzenes with two different substituents (X1 6¼ X2) at 1,4 positions. Although the general trend for adoption of centrosymmetric structures for homo 1,4 disubstituted benzenes (X1 = X2) is followed, there are a few structures with Z0 g 1; some of these are also non-centrosymmetric. Details are provided in the Supporting Information. Table 3 gives the distribution of centrosymmetric and non-centrosymmetric packing arrangements in all three cases. The percentage of non-centrosymmetric structures (column 5) must be compared with the overall figure of total number of structures in each case (column 2). The figures for 1,2 and 1,3 disubstituted benzenes show some interesting trends. There is a higher tendency for non-centrosymmetric space group adoption

Figure 7. Supramolecular fragments in form I (P21/n, Z0 = 3) structure of 3FPA. Panels (a) and (b) represent the common modes of interactions in form I and form II of 3FPA viewed in different orientations. Notice that green-colored molecules act as common H-bond donors in both the tC
H 3 3 3 π(CtC) and (ph, p)C
H 3 3 3 F interactions. Compare these diagrams with the packing diagram of form II structure of 3FPA in Figure 5b.

in 1,3 disubstituted benzenes (27.4%) compared to 1,2 disubstituted benzenes (18.6%). The trends for 1,2 and 1,3 disubstituted 3960

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Table 3. Distribution of Centrosymmetric and Non-Centrosymmetric Space Groups for Simple Disustituted Benzenes (Does Not Include the 2FPA, 3FPA, and 4FPA Structures) CSa

substitution type

a

total no. of hits structures

non-CS

% of non-CS

no. of hits with

no. of comps exhibiting

no. of polymorphic

structures

structures

Z0 > 1

polymorphism

structures

1,2 1,3

59 62

48 45

11 17

18.6 27.4

11 11

5 4

11 8

1,4

179

124

55

30.7

34

22

48

CS = centrosymmetric.

benzenes are in good agreement with a similar analysis reported several years ago29 for a restricted set of compounds in which at least one nitro group is substituted on an isocyclic or heterocyclic six-membered aromatic ring which may or may not be part of a fused system. However, 1,4 disubstituted benzenes do not follow the previously reported trend. Rather, they show a counterintuitive result in that they have the highest percentage of noncentrosymmetric crystal structures (30.7%) among disubstituted benzenes. Also, the number of structures added over the years in the CSD is the highest for 1,4 disubstituted benzenes. This raises the question as to whether or not the earlier study had a statistically significant number of structures. In the present case, the search pertains to disubstituted benzenes with simple substituents in order to compare these structures with a close resemblance to that of fluorophenylacetylenes (2FPA, 3FPA, and 4FPA). Both the polymorphs of 2FPA add to the minimally occupied set of 1,2 disubstituted benzenes with non-centrosymmetric packing (Table 3). While one of the structures of 3FPA is centrosymmetric, the other belongs to a non-centrosymmetric space group. Compound 4FPA crystallizes in the centrosymmetric space group P21/n as do its -Br and -I analogues as reported by Weiss et al.23 The propensity of adoption of higher Z0 structures (Z0 > 1) appears to be almost similar for 1,2, 1,3, and 1,4 disubstituted benzenes (Table 3, column 6). A comparison of fluorophenylacetylenes with the corresponding fluorohalobenzenes, fluoroanilines, and fluorophenols is interesting owing to their comparable molecular sizes and volumes. In particular, the structures of fluorophenols and fluoroanilines are of direct relevance because of the dual role (as H-bond donor and acceptor) offered by
OH,
NH2, and
CtC
H groups in the respective structures. More interestingly, dimorphs are known for both o- and p- fluorophenols.30a,b The structures of o-fluoroaniline and p-fluoroaniline are known,31 but the structure of m-fluoroaniline is not reported in the literature. Notably, 2-fluorophenol (phase I, obtained at 150 K) and 2-fluoroaniline are isostructural, exhibiting the same space group C2/c with Z0 = 1.5. A complete molecule and a half molecule of 2-fluoroaniline (2FA) shows disorder with respect to the fluorine position with partial occupancies of 0.73:0.27 and 0.50:0.50, respectively, whereas 2-fluorophenol (2FP) (phase I) structure has an ordered molecule and a disordered half molecule (partial occupancies of F-atom, 0.73:0.27) in the asymmetric unit. Phase II of 2FP which was obtained at 0.36 GPa and 403 K takes the non-centrosymmetric space group P212121. The structure is ordered and Z0 = 1. It is noteworthy that in all the abovediscussed structures, the putative interactions involving fluorine, namely, O
H 3 3 3 F and N
H 3 3 3 F are observed along the 2-fold axis (or 21-screw) interconnecting the molecular chains formed by the strong O
H 3 3 3 O and N
H 3 3 3 N hydrogen bonds in 2FP and 2FA, respectively. The formation

Figure 8. Packing diagram of 4FPA showing CtC
H 3 3 3 F and π 3 3 3 π interactions.

of comparatively weaker C
H 3 3 3 F interactions is also confirmed in both cases. 3-Fluorophenol crystallizes in the space group P21 with a single ordered molecule in the asymmetric unit. Once again, the molecules pack with 3 3 3 O
H 3 3 3 O
H 3 3 3 hydrogen bonds in a catemeric fashion to form chains along the crystallographic 21-screw axis which are further held together via supportive C
H 3 3 3 F interactions. Hence, the average packing characteristics of 2-fluorophenol, 2-fluoroaniline, and 3-fluorophenol resemble what we have observed in the structures of the corresponding fluorophenylacetylenes. This one-to-one correspondence in the modes of interactions dictated by the
OH,
NH2, and
CtC
H groups in the respective fluoro compounds proves the H-bond acceptor capability and consistency of fluorine in the context of crystal engineering. The nonspecific trend observed within para-disubstituted benzenes still continues for the structures of 4-fluorophenol (4FP), 4-fluoroaniline (4FA), and 4-fluorophenylacetylene (4FPA) showing the diversified modes of interactions in these structures. 4FA crystallizes in the non-centrosymmetric space group Pna21 (Z0 = 1), whereas 4FP phase I and phase II in space groups R3 (hexagonal setting, Z0 = 1) and P21/n (Z0 = 0.5), respectively. In both compounds, the intermolecular packing is mainly achieved through C
H 3 3 3 N and O
H 3 3 3 O hydrogen bonds, and there are no N
H 3 3 3 F and O
H 3 3 3 F interactions observed. The distinction between phase I and phase II of 4FP is reflected in the packing motifs involving O
H 3 3 3 O hydrogen bonds. In 4FP phase I, six molecules interact via O
H 3 3 3 O H-bonds forming an R66(12) ring motifs, whereas in 4FP phase II, molecular chains are formed through O
H 3 3 3 O H-bonds which are further interconnected through F 3 3 3 F contacts. The surrogate C
H 3 3 3 F interactions were noticed in all the structures. On the contrary, the main driving force in intermolecular packing of 4FPA is the formation of unusually short CtC
H 3 3 3 F hydrogen bonds (C8
H8 3 3 3 F1, 2.38 Å, 144
, Table 2) coupled with the π 3 3 3 π stacking interactions with a stacking 3961

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Crystal Growth & Design distance of 3.79 Å (Cg 3 3 3 Cg) providing additional stability (Figure 8). It must be noted that 4FPA does not favor an ideal supramolecular geometry for CtC
H 3 3 3 π(CtC) contacts (C
H 3 3 3 π; 3.19 Å and 130
, Table 2) which is generally seen in all the similar structures with ethynyl groups (4ClPA, 4BrPA, and 4IPA).23

’ CONCLUSIONS To summarize, the monofluorophenylacetylenes 2FPA, 3FPA, and 4FPA form multiple structural patterns owing to a variety of hydrogen bonded and van der Waals interactions. Variations in the solid forms of these isomers provide an opportunity to witness the mutual competition among weak intermolecular interactions. In situ cryocrystallization of liquids enables one to access various crystalline forms of a given substance under different sets of temperature and pressure conditions. These forms arise as a consequence of compromise between kinetic and thermodynamic factors governing the process of nucleation and eventually crystallization. In our experiments, the nuclei that are formed under a specific set of physical conditions were nurtured and were finally grown in the form of a single crystal inside the capillary by utilizing the thermal gradient created because of the movement of a cryojet over the capillary. The dimorphs of 2FPA and 3FPA and the monomorph of 4FPA demonstrate the directing nature of fluorine in determining packing motifs and thereby the overall crystal structure. Cryocrystallization using various cooling protocols is a powerful tool for tuning the balance between weak interactions, for example, hydrogen bonds and van der Waals contacts. The perceivable structural features of monofluorophenylacetylenes are associated with subtly different modes of interactions involving weak H-bond partners. The crystal structures of the dimorphs of 2FPA and 3FPA are related and interactions involving fluorine, mainly C
H 3 3 3 F hydrogen bonds, are attributed to be a principal cause for the onset of polymorphism in these compounds. One of the dimorphs each of 2FPA and 3FPA are isostructural (2FPA, form II, P21 and 3FPA, form II, P21) in terms of the helical arrangements formed through CtC
H 3 3 3 π and C
H 3 3 3 F interactions. The high Z0 polymorph of 3FPA (P21/n, Z0 = 3) has two different tetrameric synthons and appears to share some interaction modes with its phenylacetylene counterpart (β-form, Z0 = 3). Meta and para disubstituted benzenes seem to have a slightly higher tendency to adopt non-centrosymmetric crystal packing compared to ortho disubstituted benzenes. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figures; tables of CSD analysis; crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*(T.N.G.R.) Fax: +91-80-23601310. Tel: +91-80-22932796. E-mail: [email protected]. (G.R.D.) Fax: +91-80-23602306. Tel: +91-80-22933311. E-mail: [email protected].

’ ACKNOWLEDGMENT A.G.D. thanks CSIR, New Delhi, for a fellowship. T.N.G. and G.R.D. thank the DST for the award of a J. C. Bose fellowship.

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