Penta- and decafluorinated dibenzalacetones: Synthesis, crystal structure, and cocrystallization experiments

Article abstract of DOI:10.1021/cg5000342

The nonfluorinated parent dibenzalacetone 1 as well as the corresponding penta- (2) and decafluorinated (3) derivative compounds were prepared, crystallized, and subjected to co-crystallization experiments. Only 3 yielded a 1:1 co-crystal with 1, while 2 did not form co-crystals with either 1 or 3. Powder X-ray diffraction patterns were determined to verify the co-crystallization experiments. The influence of the fluorine on the molecular geometry and crystal packing were studied and comparatively discussed. Conclusions with reference to the priority of Ar...Ar^F contact modes in the crystalline packing being in competition with other fluorine and non-fluorine involved supramolecular interactions were drawn.

Full text of DOI:10.1021/cg5000342

Article
pubs.acs.org/crystal
Penta- and Decafluorinated Dibenzalacetones: Synthesis, Crystal
Structure, and Cocrystallization Experiments
Anke Schwarzer*,† and Edwin Weber‡
‡
†Institut fur Anorganische Chemie, and Institut fur Organische Chemie, Technische Universitat Bergakademie Freiberg, Leipziger
̈
̈
̈
Strasse 29, D-09596 Freiberg, Sachsen, Germany
S
* Supporting Information
ABSTRACT: The nonfluorinated parent dibenzalacetone 1 as well as the corresponding penta-
(2) and decafluorinated (3) derivative compounds were prepared, crystallized, and subjected to
co-crystallization experiments. Only 3 yielded a 1:1 co-crystal with 1, while 2 did not form co-
crystals with either 1 or 3. Powder X-ray diffraction patterns were determined to verify the co-
crystallization experiments. The influence of the fluorine on the molecular geometry and crystal
packing were studied and comparatively discussed. Conclusions with reference to the priority of
Ar···ArF contact modes in the crystalline packing being in competition with other fluorine and
non-fluorine involved supramolecular interactions were drawn.
pounds38,39 are reported. Examples of rarely described
INTRODUCTION
■
compounds with strong hydrogen donor and acceptor groups
such as OH, COOH, and NH/NH2 are benzanilide,40
phenylureas,41 amines,42 and fluorinated anilines.43−46 With
reference to the Ar···ArF interaction, steric hindrance, large
torsion angles between adjacent phenyl rings, and bulky
substituents favor C−H···O and C−H···F contacts at the
expense of Ar···ArF stacking.33,47 Furthermore, it has also
turned out that a planar environment of the molecule is not an
absolute requirement for Ar···ArF interaction but preferable. In
short, conditions enabling a sure prediction of Ar···ArF
formation in a crystal packing being in competition with
other interaction modes are still rather shaky, requiring more
examination.
Here, we present the synthesis and crystal structure analyses
of the compounds 1−3 (Scheme 1) which could make a
contribution for investigating this question. They refer to
fluorinated dibenzalacetones 2−3 deriving advantage from (1)
an expected almost planar molecular geometry based on the π-
conjugation and promoting the formation of Ar···ArF stacks and
(2) the possibility to have one or two perfluorinated aryl rings.
In addition, we include the unfluorinated parent compound 1
in the study considering reasonable structural comparison and
also to probe feasibility of co-crystallization between fluorinated
and unfluorinated species. All this is subject to an interplay
between C−H···O, C−H···F, C−H···π, C−F···πF, F···F, and
Ar···Ar contacts competing with the Ar···ArF interaction under
discussion.
Noncovalent interactions of fluorinated compounds in the solid
state have been investigated and reviewed in the last few
decades.1−6 Regarding the fluorine-involved interactions, differ-
ent modes are discussed: (1) C−H···F, (2) C−F···F, (3) C−
F···C(O), (4) C−F···ArF, and (5) the most relevant Ar···ArF.
Nevertheless, all fluorine-containing interactions including Ar···
ArF are rather weak and still under controversial discussion.
As presently seen, the Ar···ArF interaction is a stabilizing
contact that occurs between a perfluorinated and a non-
fluorinated aromatic ring.2,3,5 The distances between adjacent
centers of the rings are in the range of 3.4−4.8 Å with ring-
plane angles up to 20°.2 This kind of interaction is not only
used for the generation of molecular stacks in crystal
engineering7,8 but is also a well-known supramolecular
synthon2,3,5,9 being profitable in the solid state [2 + 2]
photocycloaddition.10,11 Moreover, it offers advantages for the
optical and electronic properties of organic compounds12 and is
used in the design of liquid crystals13 and π-gels.14 Two models
are discussed in the recent literature to explain and quantify the
Ar···ArF interaction. The first is based on polarization of the π-
system,15,16 while the second is derived from substituent
effects.17,18 Considering this element of uncertainty, a multi-
tude of co-crystals generated by a perfluorinated and a non-
fluorinated aromatic ring have been described. Furthermore,
stacking interactions in isolated structures have been
observed.10,19−25
In addition, significance of the C−F···H−(O,N) type
hydrogen bonding is currently discussed rather intensely and
controversially as well.26−30 Previous studies regarding
investigations of C−H···F and C−F···F contacts have reported
mainly on molecular systems without strong hydrogen donors
and acceptor groups. Instead crystal structure investigations on
cyclic imides,31,32 benzophenones,33 pyridines,34 benzoni-
triles,35 benzylideneanilines36,37 and aromatic azo com-
Received: January 8, 2014
Revised: April 4, 2014
Published: April 25, 2014
© 2014 American Chemical Society
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Scheme 1. Preparation of Dibenzalacetones 1−3
Found: C, 49.25; H, 1.24; C17H4F10O requires C, 49.30; H, 0.97%.
νmax (KBr)/cm−1 3032 (w, CArH), 1680 (s, CO), 1618 (s, CC),
1523 (s, Ph), 1497 (s, Ph). δH (400 MHz; CDCl3; Me4Si) 7.71 (2H, d,
EXPERIMENTAL SECTION
■
General Remarks. Melting points (uncorrected) were determined
using a microscope heating stage PHMK Rapido (VEB Wagetechnik).
̈
3
3JHH = 16.4 Hz, HCCHPh), 7.31 (2H, d, JHH = 16.4 Hz, HC
1
IR spectra were measured on a FT-IR 510 Nicolet as KBr pellets. H,
CHPh); δC (100 MHz; CDCl3; Me4Si) 187.5 (s, CO), 144.7, 147.2
13C, and 19F NMR spectra were recorded in chloroform solution at
1
1
(d, JCF = −251.5 Hz, C1/C5/C13/C17), 140.7, 143.3 (d, JCF
=
room temperature on a Bruker Avance DPX 400 at 400, 100, and 376
MHz, respectively. The splitting of proton and carbon resonances is
defined as s = singlet, d = doublet, t = triplet, and m = multiplet.
Resonances are assigned according to the calculation of increments of
groups. The same numbering is used as for the crystallographically
determined molecular structure (Figure 2). Elemental analyses were
performed on a Heraeus CHN rapid analyzer. Mass spectra were
obtained using a Hewlett-Packard GC-MS 5890. Powder diffraction
data were recorded on a Siemens D5000 at room temperature.
Materials. Organic solvents were purified by standard procedures.
Starting compounds 2,3,4,5,6-pentafluorobenzaldehyde (98%) and
benzaldehyde (98%) were purchased from ABCR (Germany). 4-
Phenylbut-3-en-2-one48 and dibenzalacetone 149 were prepared
according to the literature procedures using NaOH in a water−
ethanol mixture.
1
−258.5, C3/C15), 136.6, 139.1 (d, JCF = −254.5 Hz, C2/C4/C14/
2
C16), 131.7 (s, CCPh), 127.8 (CCPh, s), 110.0 (t, JCF = 13.4
Hz, C6/C12); δF (376 MHz; CDCl3) −139.4 (m, 4F, F1/F5/F6/
F10), −150.8 (m, 2F, F3/F8), −161.9 (m, 4F, F2/F4/F7/F9); m/z:
414 [M]+, 395, 221, 193, 143.
Crystal Preparation and Crystal Structure Determination.
Single crystals of 2, 3, and 1·3 were obtained by isothermal
evaporation of solutions of ethanol (2) and chloroform (3, 1·3) at
room temperature. In an optically opaque sample tube 0.4 mmol of 2
(0.13 g, 0.4 mmol), 3 (0.16 g, 0.4 mmol), 1 (0.09 g, 0.4 mmol), and 3
(0.16 g, 0.4 mmol) were dissolved in ethanol (2) or chloroform (3, 1·
3). All crystals were measured on a Bruker Kappa Apex II using
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Software
for data collection: SMART Version 5.628,51 cell refinement: SMART
Version 5.62851 and data reduction: SAINT Version 6.45a.51
Preliminary structure models were derived by Direct Methods using
the SHELXTL package52,53 and were refined by full-matrix least-
squares calculation based on F2 for all reflections.52,54 All non-
hydrogen atoms were refined anisotropically. The hydrogen atoms
were included in the models in calculated positions and were refined as
constrained to bonding atoms. The crystal data and experimental
parameters are given in the References.55
Synthesis. Synthesis of 1-Phenyl-5-(2, 3, 4, 5, 6-
pentafluorophenyl)penta-1,4-dien-3-one, 2. A solution of sodium
hydroxide (1.24 g, 31.0 mmol) in 15 mL of water and 12 mL of
ethanol, 2,3,4,5,6-pentafluorobenzaldehyde (5.00 g, 25.0 mmol), and
4-phenyl-but-3-en-2-one (3.65 g, 25.0 mmol) was stirred for 30 min at
room temperature. The solid raw product was collected by suction
filtration and washed with water to remove the sodium hydroxide.
Recrystallization from n-hexane yielded 3.50 g (43%) of a yellow
crystalline powder. Mp 154−155 °C (from n-hexane). Found: C,
63.08; H, 2.96; C17H9F5O requires C, 62.97; H, 2.80%. vmax (KBr)/
cm−1 3063 (w, CHAr), 1674 (s, CO), 1612 (m, CC), 1594 (s,
Ph); δH (400 MHz; CDCl3; Me4Si) 7.78−7.38 (8H, m, Ph, HC
RESULTS AND DISCUSSION
■
Synthesis. Dibenzalacetones as representative of the
substance class of α,β-unsaturated carbonylic compounds are
normally accessible via Claisen−Schmidt reaction involving the
addition of a C−H acetic reagent to a carbonylic carbon atom
followed by dehydratization (Scheme 1).56,57 This reaction
commonly requires a basic catalyst, for instance, sodium
hydroxide dissolved in ethanol which is used for compounds 1
and 2. However, using NaOH as the base was found unsuitable
for the synthesis of the decafluorinated derivative 3 due to the
nucleophilic attack of the hydroxide at the aromatic ring.
Mikhalina and Fokin described the synthesis of 3 applying
gaseous hydrogen chloride for several days.50 In exchange for
this acidic medium, stirring of the reaction compounds in
concentrated sulfuric acid at room temperature for 4 days was
applied, which was a more agreeable procedure in handling.
Here, sulfuric acid acts both as proton donor and hygroscopic
medium.
3
CHPh), 7.02 (1H, d, JHH = 16.0 Hz, HCCHPh); δC (100 MHz;
1
CDCl3; Me4Si) 188.1 (s, CO), 144.9, 147.3 (d, JCF = −238.4 Hz,
1
C1/C5), 144.7 (s, CCPh), 140.4, 143.0 (d, JCF = −258.5 Hz, C3),
1
136.6, 139.2 (d, JCF = −257.5 Hz, C2/C4), 134.4 (s, C12), 131.8 (t,
3JCF = −6.9 Hz, CCPhF5), 130.9 (s, CCPh), 129.0 (s, C13/C17),
128.6 (s, C14/C16), 126.5 (s, CCPhF5), 125.6 (s, C15), 110.4 (t,
2JCF = 13.4 Hz, C6); δF (376 MHz; CDCl3) −139.8 (2F, m, F1/F5),
−151.8 (1F, m, F3), −162.3 (2F, m, F2/F4); m/z: 324 [M]+, 323,
193, 103.
Synthesis of 1,5-Bis(2,3,4,5,6-pentafluorophenyl)penta-1,4-dien-
3-one, 3. A solution of 2,3,4,5,6-pentafluorobenzaldehyde (5.0 g, 25.0
mmol) and acetone (0.73 g, 12.5 mmol) in 25 mL of sulfuric acid was
stirred for 4 days at room temperature. Sulfuric acid (5 mL) was added
and the mixture was stirred for 5 additional days. After the mixture was
carefully quenched on ice, the solid was separated via suction filtration
and recrystallized from n-hexane to yield 1.71 g (33%) of a yellow
crystalline solid. Mp 125 °C (from n-hexane; lit.50 127−129 °C).
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Crystal Preparation. Single crystals suitable for diffraction
measurements were achieved via isothermal evaporation of
solutions of ethanol (2) and chloroform (3). In order to study
the potential behavior of the different compounds to form co-
crystals that might be stabilized by Ar···ArF stacking
interactions, all possible 1:1 combinations of the dibenzalace-
tones 1−3 were dissolved in an equimolar ratio in chloroform
and crystallized from this solvent. The crystalline samples
obtained from each of the crystallization experiment were
subjected to unit cell measurements at 93 K using 15 single
crystals of each crystal batch. As a result of this analysis, it is
shown that only the combination of 1 and 3, i.e., the 1:1
mixture of the non-fluorinated parent compound 1 and the
decafluorinated derivative 3, gave rise to the formation of the
1:1 stoichiometric co-crystal 1·3, whereas the other two
combinations (1·2 and 2·3) failed yielding only a mixture of
crystals of the single components.
The signal-to-noise ratio of the XRD pattern of co-
crystallization phase is very low showing an amorphous
character of the bulk material. Though crystallinity is indicated,
a clear assignment to a specific phase cannot be drawn.
Apparently, different amorphous and crystalline phases are
more or less equal during the crystallization process applied in
this experiment. It shows that the 1:1 co-crystal does not
represent the main product phase and the influence of several
aspects on crystallization of a specific phase is significant.
Possible aspects are temperature, solvent, and concentration. A
similar behavior may be the reason for the failed isolation of co-
crystals of 1 with 2 and 2 with 3. This latter finding is
remarkable since 2, due to its unfluorinated and pentafluori-
nated aryl units, has a chance to form Ar···ArF stacking with
either 1 or 3. Finally, the co-crystal 1·3 was found to melt at
180 °C, in contrast to the single components showing a melting
point of 127−129 °C (1) and 111 (3). This thermal behavior
was observed earlier, e.g., the 1:1 mixture of benzene and
hexafluorobenzene melts at 23.7 °C.19
Crystal Structure Description. Compound 1 has been
described to show a photoinduced rearrangement depending
on UV/vis irradiation and irradiation time.58 All independent
molecules determined in the respective experiment exhibit a
molecular disorder. The compound occurs in two different
orientations not symmetry related. The isomerism of the
olefinic moiety with respect to the CO bond58 can be
specified as follows: (1) The C−O double bond is orientated
almost parallel to the olefinic double bond (∼180°); (2) the
angle between the CO group and the C−C double bond is
about 60°. A similar disorder was found in the crystal structure
of 1,5-bis(4-fluorophenyl)penta-1,4-dien-3-one in all three
independent molecules in the crystal structure.59 In the current
study of 2, 3, and 1·3, however, the non-, penta-, and
decafluoro substituted dibenzalacetones, respectively, do not
exhibit a disorder.55 Instead, they show only one isomer with an
angle of about 60° between the olefinic and carbonylic double
bonds in their crystal structures (Figure 2). In conformity with
the behavior of 2, 3, and 1·3, the 2,6,2′,6′-tetrafluoro
substituted dibenzalacetone was also reported to crystallize in
only one conformational isomer.60
Nevertheless, the 1:1 co-crystal 1·3 seems not to be a favored
crystal phase. Single crystals in this connection were either
identified as 1·3 or the isolated components 1 or 3. The XRD
pattern determined after the removal of the 15 single crystals
(the rest of the sample) is given in Figure 1b. Figure 1a shows
Figure 1. (a) Calculated XRD pattern based on the single crystal data
of 1·3; (b) experimental XRD pattern of the sample from the co-
crystallization experiment of 1 and 3; experimental XRD pattern of the
single components of (c) 3 and (d) 1.
The two positions in the disorder of the unfluorinated
compound 1 derive from conformational isomerism involving
rotation around the C−C bond neighboring the carbonyl group
[O(1)−C(9)−C(8)−C(7) and O(1)−C(9)−C(10)−C(11)].
The corresponding torsion angles in 2, 3 and 1·3 are in the
range of −2.3 to 5.7°, while 1 shows angles of 21 and −164°
(Table 1). Obviously, the fluorinated compounds having more
than one fluorine atom per phenyl ring tend to crystallize with
the calculated pattern based on the single crystal data of the co-
crystal 1·3, while Figure 1c,d gives the experimental XRD
pattern of the isolated components 1 and 3.
Figure 2. Perspective views of the asymmetric unit of 2, 3, and 1·3 in the respective crystals including atom numbering scheme. Thermal ellipsoids
are at the 50% probability level.
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from the least-squares plane of about 0.38 Å, the components
in the structures 2, 3 and 1·3 are almost planar with deviations
of only 0.06−0.10 Å. The mean planes of the phenyl rings in
compound 1 form dihedral angles of about 52° but show an
average angle of only 4.8° in 2, 3 and the co-crystal 1·3. The
angles between the mean plane of the phenyl moiety and the
penta-1,4-dien-3-one unit are 10.91° in average for all
unsubstituted phenyl rings and 4.56° in average for all
pentafluoro substituted rings regarding 2, 3, and 1·3. In
compound 1 the corresponding angle is about 30°. Moreover, it
has to be considered that the penta-1,4-dien-3-one unit is not
planar in 1, while in 2, 3, and 1·3 this moiety shows an average
of the r.m.s. of 0.0196. Hence, it is obvious that the fluorine
substituents exert a distinct influence on the molecular
geometry. In summary, the molecular structure of 1
crystallizing as single molecule cannot be described as planar
but is forced to adopt a planar geometry in the 1:1 co-crystal
with the decafluorinated compound 3. In addition, 2 and 3
exhibit an almost complete planar molecular geometry. This
indicates that the molecular arrangement and planarity in detail
is influenced distinctly by the number of fluorine atoms bonded
to the aromatic ring.
Table 1. Torsion Angles To Describe the Rotational Isomers
for Compounds 1−3 and 1·3
atoms involved in dihedral angle
angle in deg
158
2
O1a−C9a−C8a−C7a
O1b−C9b−C8b−C7b
C7−C8−C9−O1
O1−C9−C10−C11
C7−C8−C9−O1
O1−C9−C10−C11
C24−C25−C26−O2
O2−C26−C27−C28
C7−C8−C9−O1
O1−C9−C10−C11
C24−C25−C26−O2
O2−C26−C27−C28
21.06
−164.82
−2.3(4)
3.2(4)
1.6(4)
1.0(4)
5.7(4)
−3.5(4)
−1.8(6)
1.3(6)
3.2(6)
0.6(6)
3
1·3
only one isomer indicating an influence of the fluorine atoms
on the molecular geometry.
Both compound 158 and the para-fluoro disubstituted
derivative mentioned above59 are not planar with respect to
the whole molecule. Due to the close similarity between the
two compounds, we limit the discussion to 1. While the
molecular structure of 1 shows a maximum atomic deviation
a
Table 2. Geometrical Parameters for Intermolecular C−H···O-, C−H···F, and C−F···ArF Contacts in Compounds 1−3 and 1·3
interaction
symmetry
d(X···A) in Å
d(C···A) in Å
θ(C−X···A) in deg
b
158
2
C2A−H2A···Cg1
C11−H11···O1
C17−H17···O1
C14−H14···F2
C10−H10···F4
C13−H13···F4
C11−H11···O2
C28−H28···O1
C24−H24···F10
C7−H7···F20
x, −1 − y, −1/2 + z
1 − x, −1 − y, −z
1 − x, −1 − y, −z
x, 1/2 − y, −1/2 + z
−x, 1 − y, −z
−x, 1 − y, −z
−1 + x, y, z
1 + x, y, z
3.176
3.717
119
159
149
176
174
159
170
169
168
169
156
155
122
150
101
90
2.52
3.420(3)
3.531(3)
3.526(3)
3.422(3)
3.544(3)
3.566(3)
3.590(3)
3.454(3)
3.474(3)
3.553(3)
4.798(3)
4.486(3)
4.225(4)
3.710(3)
3.694(3)
3.662(3)
3.762(3)
3.698(3)
4.593(3)
3.768(3)
3.363(5)
3.428(5)
3.339(4)
3.399(5)
3.250(4)
3.573(4)
3.320(4)
3.479(4)
3.488(4)
3.420(4)
3.355(4)
3.462(4)
2.68
2.58
2.48
2.64
3
2.63
2.65
1 + x, y, z
2.52
−1 + x, y, z
2.54
C27−H27···F8
x, 1/2 − y, −1/2 + z
2 − x, −1/2 + y, 3/2 − z
2 − x, −1/2 + y, 3/2 − z
−1 + x, 1/2 − y, 1/2 + z
x, y, z
2.67
b
C2−F2···Cg4
3.546(2)
3.634(3)
3.015(3)
3.2198(19)
3.4333(19)
3.3179(19)
3.365(2)
3.2126(19)
3.371(2)
3.3401(19)
2.44
b
C32−F18···Cg4
c
C15−F8···π(C27−C28)
b
C1−F1···Cg5
C3−F3···Cg3
b
2 − x, 1 − y, 1 − z
2 − x, 1 − y, 1 − z
x, y, z
b
C13−F6···Cg2
94
b
C17−F10···Cg4
C18−F11···Cg3
C19−F12···Cg5
C34−F20···Cg2
96
b
x, y, z
100
151
98
b
−1 + x, 1/2 − y, 1/2 + z
x, y, z
b
1·3
C28−H28···O2
C11−H11···O1
C8−H8···F7
C24−H24···F10
C1−H1···F2
1 − x, 2 − y, 1 − z
2 − x, 2 − y, 1 − z
1 − x, 1 − y, 1 − z
1 − x, 2 − y, −z
2 − x, 2 − y, −z
1 + x, y, −1 + z
1 + x, y, −1 + z
1 − x,1 − y, 1 − z
1 − x, 1 − y, 1 − z
−x, 1 − y, 1 − z
−x,1 − y, 1 − z
1 − x, 1 − y, 1 − z
165
158
166
161
126
165
128
67
2.53
2.41
2.49
2.60
C3−H3···F8
C3−H3···F9
2.65
2.65
C18−F1···Cg2
C19−F2···Cg2
C21−F4···Cg2
C31−F7···Cg3
C34−F10···Cg3
3.771(3)
3.784(3)
3.640(3)
3.598(3)
3.638(3)
67
70
69
72
a
b
d(C−H) = 0.95 Å; X is the interacting atom, A is the acceptor. All angles are rounded off. Cg is defined as the centroid of the rings (center of
c
gravity): Cg1: C1A−C6A; Cg2: C1−C6, Cg3: C12−C17; Cg4: C18−C23; Cg5: C29−C34. Geometric parameter are given for C15−F8···C28.
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Table 3. Geometrical Parameters for Intermolecular Ar···ArF Interactions in Compounds 2 and 1·3
a
a
b
c
symmetry code
CgI
CgJ
CgI···CgJ/Å
CgI···P(J) /Å
CgJ···P(I) /Å
2
1 − x, −y, −z
2 − x, −y, −z
−x, 1 − y, 1 − z
1 − x, 1 − y, 1 − z
−x, 1 − y, 1 − z
1 − x, 1 − y, 1 − z
Cg2
Cg2
Cg4
Cg4
Cg5
Cg5
Cg3
Cg3
Cg2
Cg2
Cg3
Cg3
4.0255(16)
3.8088(16)
3.736(2)
3.693(2)
3.641(2)
3.791(2)
3.5444(11)
3.5029(11)
3.4681(16)
3.4928(16)
3.3879(15)
3.4471(15)
3.4208(12)
3.3874(12)
3.3939(16)
3.4054(16)
3.3459(16)
1·3
3.4075(16)
b
a
Cg is defined as the centroid of the rings (center of gravity): Cg2: C1−C6, Cg3: C12−C17; Cg4: C18−C23; Cg5: C29−C34. Perpendicular
distance of the centroid CgI on ring plane J. Perpendicular distance of the centroid CgJ on ring plane I.
c
Figure 3. Views of the crystalline packing of 2: (a) along the b axis showing the shortest distances (Å) between the centroids of adjacent stacks; (b)
along the a axis giving rise to the C−H···O and C−H···F contacts of one selected layer. Noncovalent contacts are represented as broken lines.
Figure 4. Crystalline packing of 3: (a) showing relevant intermolecular C−F···Ar contacts; (b) along the b axis giving rise to relevant C−H···O and
C−H···F interactions. Noncovalent contacts are represented as broken lines.
Apart from the molecular features, the crystalline state is
influenced by the molecular arrangement and the interaction
between adjacent units. In the current study, compound 1 is the
only one not bearing a fluorine atom potentially competing
with the carbonyl oxygen atom.58 Unlike the expectation, the
carbonylic oxygen atom is not involved in any interaction.
However, C−H···π contacts form zigzag chains along the
crystallographic c axis (Table 2).
On the other hand, compound 2 bearing a phenyl and a
pentafluorinated phenyl ring should be able to form
intermolecular strands of the Ar···ArF type.2,3,5 In fact, the
structure of 2 features stacks of alternately arranged phenyl and
pentafluorophenyl rings in an offset face-to-face orientation of
adjacent aromatic rings. The perpendicular distances of the
centroids of the interacting ring planes, as specified in Table 3
and given in Figure 3a, are 3.8 and 4.0 Å. While the latter
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a
Table 4. Geometrical Parameters for Intermolecular C−F···F Contacts in Compounds 2, 3, and 1·3
interaction
symmetry
C−F/Å
F···F/Å
C−F···F/deg
Type
2
3
C5−F5···F5−C5
C2−F2···F3−C3
−x, 1 − y, −z
−x, −1/2 + y, 1/2 − z
1.340(3)
1.347(3)
1.343(3)
1.338(3)
1.344(3)
1.337(3)
1.347(3)
1.343(3)
1.350(3)
1.339(3)
1.341(4)
1.337(4)
2.700(3)
2.881(2)
169
140
151
156
126
170
126
166
140
109
88
I
II
C5−F5···F5−C5
C2−F2···F14−C21
3 − x, −y, 2 − z
1 − x, −1/2 + y, 1.5 − z
2.531(3)
2.736(2)
I
II
C4−F4···F6−C13
1 − x, 1/2 + y, 1.5 − z
−1 + x, 1/2 + y, 1.5 − z
2.767(2)
2.775(2)
II
II
C20−F13···F17−C31
1≅3
C19−F2···F2−C19
C21−F4···F4−C21
1 − x, 2 − y, −z
1 − x, 1 − y, − z
2.866(4)
2.822(4)
I
I
126
a
All angles are rounded off.
Figure 5. Crystalline packing of 1·3: (a) along the b axis showing the shortest distances (Å) between the centroids of adjacent stacks; (b) along the a
axis giving rise to relevant C−H···O and C−H···F interactions. Noncovalent contacts are represented as broken lines.
distance is quite long indicating only a weak contact, the other
one is in a range suggesting a stabilizing Ar···ArF interaction. In
addition, layers are formed involving C−H···O61 bonded
dimers (2.5−2.6 Å) that interact via C−H···F and F···F
contacts resulting in a herringbone mode of supramolecular
architecture [Table 2, Figure 3b]. The C−H···F interactions are
in the range of 2.5−2.6 Å with angles of 159−176° indicating a
significant impact on the molecular arrangement. In contrast,
the distances between adjacent fluorine atoms are 2.700 Å
(angle at F = 169°) and 2.881 Å (angle at F = 140° and 151°).
However, their importance for the crystal packing seems
secondary. Instead, they support stabilizing the stacks of
molecules generated by the Ar···ArF interaction.
Thus, C−H···O and C−H···F interactions occur in 2 not
only as connecting element between the stacks but also show a
significant impact on the strength of the Ar···ArF synthon. The
longer distances between the adjacent stacks indicate a
weakened Ar···ArF interaction.
The crystal structure of the derivative 3, not containing any
aromatic bonded hydrogen atoms, does not feature stacking
interactions. The shortest distance between adjacent ring
centroids is 4.407 Å. Instead, several robust C−F···ArF
contacts31,33 are found with angles around and below 100°
and some angles between 120 and 155° (Table 2) leading to
domains of molecular stacks. In addition, there is a C−F··· π
contact involving F(8) and the olefinic double bond as π-
moiety62 with a distance of 3.015 Å and an angle of 149.5°.
Apart from the interactions including the aromatic units, several
other contacts are found, in particular of the C−H···O and C−
H···F type. As depicted in Figure 4 and Table 2, the C−H···O
contacts with a distance of about 2.6 Å give rise to dimer
formation within a layer. These layers are connected via C−H···
F interactions in the range of 2.5−2.6 Å (Table 2).
Furthermore, several fluorine atoms are located closer to each
other than their sum of the van der Waals radii of 1.47 Å.63
They show distances in the range of 2.5−2.8 Å with angles of
109−170° (Table 4). While in 2 types I and II of halogen−
halogen interaction3 occur, the decafluorinated derivative 3
seems to prefer type II. These contacts connect the domains of
stacks generated by the C−F···πF interaction.
Hence, the C−F···ArF contact seems to be the interaction
with the greatest impact on the crystal structure of compound
3, similar to the C−H···π interaction in compound 1. But in 3
additional C−H···O and C−H···F interactions are found. This
suggests a weaker influence of the C−F···ArF contacts on a
crystal structure than C−H···π in 1 since here the carbonyl
group is not involved in intermolecular interactions.
The co-crystals between compounds 1 and 3, 1·3, generated
via crystallization of the components in a 1:1 ratio from
chloroform crystallizes in the triclinic space group P1 with two
̅
independent molecules in the asymmetric unit. As mentioned
above, the molecular structure features planarity and shows no
disorder or isomerism. In keeping with the expectation, the
molecular arrangement in the crystal structure is mainly
determined by an Ar···ArF stacking motif (Table 3). The
resulting strands with distances between adjacent molecules of
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3.6−3.8 Å are illustrated in Figure 5a. The slippage of the
phenyl moieties indicates an offset face-to-face orientation.
Figure 5b shows one layer generated by the connection
between several stacks being realized by C−H···O and C−H···F
interactions in the range of 2.4−2.6 Å (Table 2). Furthermore,
the distances between fluorine and the center of adjacent
pentafluorophenyl rings ranging between 3.6 and 3.8 Å suggest
C−F···ArF interactions (Table 3). Nevertheless, the contact
angles are below 72° and therefore indicate no significance.
Table 4 gives the geometric parameters of located type I C−F···
F contacts with distances of about 2.8 Å and angles of 88° and
126°. These contacts assist in connecting the decafluorinated
molecules of adjacent stacks.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information file. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Anke.Schwarzer@chemie.tu-freiberg.de. Fax: +49
3731-39-4058.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
In the crystal structure of the co-crystal 1·3 a similar interplay
of the respective interactions is found as in 2. However, the
significance of the Ar···ArF interaction is more important in 1·3
indicated by the distances of the aromatic moieties.
A.S. thanks the TU Bergakademie Freiberg for the Postdoctoral
Research Fellowship “Mary Hegeler”.
DEDICATION
■
This paper is dedicated to Dr. Wilhelm Seichter on the
occasion of his 65th birthday.
CONCLUSION
■
Three different dibenzalacetones including the fluorine-free
parent compound and the pentafluoro- and the decafluoro
substituted derivatives have been prepared. They were studied
regarding their capacity to form a π···πF stacking motif in the
crystalline state competing with or supported by other types of
possible noncovalent interactions, allowing for the potential
generation of co-crystals among one another. Considering the
raised questions and based on the results, conclusions can be
drawn as follows.
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2342
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