Comparison of isomeric: Meta - And para -diiodotetrafluorobenzene as halogen bond donors in crystal engineering

Article abstract of DOI:10.1039/c8nj01368c

We provide a systematic investigation of the structures and composition of halogen-bonded cocrystals involving the bent (meta) halogen bond donor 1,3-diiodotetrafluorobenzene (1,3-tfib), in comparison to analogous systems based on its linear (para) 1,4-isomer. In particular, whereas 1,4-tfib has now been established as an archetypal, ubiquitous example of a halogen bond donor, its meta-isomer has remained almost completely unexplored. This study investigates the structures of 14 new cocrystals of 1,3-tfib and 9 new, analogous cocrystals of 1,4-tfib, with 9 monotopic and 7 ditopic nitrogen-based aliphatic and aromatic halogen bond acceptors. Combined with previously reported structures of cocrystals of 1,4-tfib, the results of our study indicate that whereas halogen bond lengths and the thermal stability of the investigated cocrystals are similar, the change in molecular shape between the two halogen bond donors brings about important consequences in supramolecular architectures and preferred stoichiometric compositions of otherwise analogous cocrystals. These preliminary results suggest that the principal factor responsible for such differences might be the different abilities of the cocrystals based on 1,3-tfib and 1,4-tfib to form close-packed structures.

Full text of DOI:10.1039/c8nj01368c

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Comparison of isomeric meta- and para-diiodotetrafluorobenzene
as halogen bond donors in crystal engineering
Nikola Bedeković1, Vladimir Stilinović*1, Tomislav Friščić2, and Dominik Cinčić*1
Received 00th January 20xx,
Accepted 00th January 20xx
We provide a systematic investigation of the structures and composition of halogen-bonded cocrystals involving the bent
(meta) halogen bond donor 1,3-diiodotetrafluorobenzene (1,3-tfib), in comparison to analogous systems based on its
linear (para) 1,4-isomer. In particular, whereas 1,4-tfib has now been established as an archetypal, ubiqitous example of a
halogen bond donor, its meta-isomer has remained almost completely unexplored. This study investigates the structures
of 14 new cocrystals of 1,3-tfib and 9 new, analogous cocrystals of 1,4-tfib, with 9 monotopic and 7 ditopic nitrogen-based
aliphatic and aromatic halogen bond acceptors. Combined with previosuly reported structures of cocrystals of 1,4-tfib the
results of our study indicate that, whereas halogen bond lengths and thermal stability of the investigated cocrystals are
similar, the change in molecular shape between the two halogen bond donors brings about important consequences in
supramolecular architectures and preferred stoichiometric compositions of otherwise analogous cocrystals. These
preliminary results suggest that the principal factor responsible for such differences might be different abilities of
cocrystals based on 1,3-tfib and 1,4-tfib to form close-packed structures.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
Although an iodine-ammonia adduct, today considered an
which made its 'supramolecular debut' in 1999 when it was
used by Dehnicke group for complexation of halides11 and,
soon afterwards, for cocrystallisation with neutral organic
molecules containing Lewis base heteroatoms nitrogen,12
oxygen13,14 and sulphur.15 Over three hundred (301) crystal
structures comprising 1,4-tfib have been deposited with the
Cambridge Structural Database16 to date, and 1,4-tfib can be
considered an archetype of a halogen bond donor.
archetypal example of halogen-bonded system, was
a
identified by J. J. Collin as far as 1814,1 halogen bond gained
widespread interest as an intermolecular interaction of
supramolecular chemistry only during the 1990s.2,3 This
ongoing renaissance of the halogen bond was pioneered by
the seminal studies of Legon in the gas phase,4 theoretical
work of Politzer and Murray,5 and pioneering and extensive
solid-state research by Resnati and Metrangolo.6 The latter
group revolutionised the approach to halogen-bonded
supramolecular chemistry by introducing perfluorinated iodo-
alkanes and arenes as promiscuous halogen donor building
blocks based on iodine donor atoms.7 Perfluorinated iodo- and
bromo-alkanes and arenes are today considered as classical
halogen bond donors, and remain among the most commonly
used building blocks in halogen-bonded crystal engineering. 8-10
Probably the most extensively used halogen bond donor to
Scheme. 1 Two isomers of diiodofluorobenzene used in this study
.
In principle, other isomeric of diiodofluorobenzenes should
provide access to halogen-bonded structures with identical
topologies as their 1,4-tfib equivalents, but with different
geometries. A particularly interesting, but poorly investigated
target for such studies is 1,3-diiodotetrafluorobenzene (1,3-
tfib), exhibiting a “bent” arrangement of the two donor atoms.
Compared to the other, more popular and also sterically
hindered “bent” isomer 1,2-diiodotetrafluorobenzene (1,2-
tfib), the arrangement of donor atoms in 1,3-tfib is close to an
angle of 120°, avoiding steric issues or secondary interactions
upon cocrystal formation. A potential advantage of 1,3-tfib in
crystal engineering is that it is a liquid at ambient conditions,
date is 1,4-diioditetrafluorobenzene
(1,4-tfib, Scheme 1),
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therefore circumventing problems of solubility of different formation, and a comparison with 1,4-tfib as the cocrystal
cocrystal components. former in cocrystal formation. Our screen explored cocrystal
Nevertheless, the properties of 1,3-tfib as a cocrystal former formation of 1,3-tfib with 16 nitrogen-based halogen bond
and similarities or differences to isomeric 1,4- and 1,2-tfib acceptors (compounds I-XVI Scheme 2). The obtained
,
remain largely unexplored. Indeed, a survey of the Crystal cocrystals were structurally characterized and compared to
Structure Database (CSD) reveals only 18 crystal structures so analogous cocrystals of 1,4-tfib that have been either reported
far reported using 1,3-tfib as the halogen bond donor.
in the literature or also prepared in this work (Table 1).
We now provide the first extensive, systematic screen of the
ability of 1,3-tfib to act as a halogen bond donor in cocrystal
Scheme.
2.
Halogen
bond
acceptors
used
in
this
study.
in melting point ranges is remarkable, considering that the two
halogen bond donors exhibit a significant difference in melting
points, with 1,3-tfib melting at ~20 °C and 1,4-tfib at ~108 °C.
Indeed, the highest melting point for a cocrystal in this study was
for a cocrystal of phenazine (VIII) with 1,3-tfib. An overview of I∙∙∙
N halogen bond lengths in the herein synthesized cocrystals also
does not reveal major differences between 1,3-tfib and 1,4-tfib as
the halogen bond donor (Table 1). For 1,3-tfib cocrystals they
halogen bond lengths all lie in the range of 2.74 Å to 3.0 Å , while
for 1,4-tfib cocrystals the corresponding range is 2.67 Å to 3.0 Å.
While 1,3-tfib cocrystals do sometimes exhibit shorter halogen
bond lengths, d(XB), compared to their 1,4-tfib analogues, the
differences are small compared to the general halogen bond length
dispersal within each series.
Results and discussion
An early comparison between 1,4-tfib and 1,3-tfib as halogen bond
donors17 indicated that the halogen bonding ability of the two
might be similar. Specifically, the study suggested there should be
no significant difference between the (positive) electrostatic
potential on iodine atoms in free 1,4-tfib and 1,3-tfib molecules, as
both formed cocrystals of 2:1 acceptor:donor stoichiometry with 4-
(N,N-dimethylamino)-pyridine (XVI). However, to this day XVI has
remained the only simple monotopic acceptor for which cocrystals
with both 1,4-tfib and 1,3-tfib have been prepared. Additionally,
only one ditopic acceptor (4,4’-bipyridine, VIII)13,18 has so far been
utilised in cocrystal formation with both halogen bond donors.
Consequently, the herein presented set of 14 cocrystals presents a
significant addition to knowledge of halogen-bonding ability of 1,3-
tfib , permitting a systematic exploration of halogen bond topology
and geometry in comparison to isomeric 1,4-tfib.
While the analyses of cocrystal melting points and halogen
bond lengths would suggest similarity between 1,3-tfib and 1,4-tfib
as halogen bond donors, the cocrystal structures and even the
stoichiometric compositions of nominally analogous cocrystals are
markedly different. Generally, ditopic acceptors (VI-XII) form the
anticipated 1:1 solid-state complexes with both donors, the only
exception being phenazine (VI). Monotopic acceptors (I-V, XIII-XVI),
however, do not form the expected 2:1 acceptor:donor cocrystals
with the same predictability. While 1,4-tfib has yielded such solid-
Overall results
All of the herein used acceptors formed cocrystals with both 1,3-
tfib and 1,4-tfib. The cocrystals with 1,3-tfib exhibit melting points
in the range of ca. 40 °C -170 °C, and those with 1,4-tfib melt
between 50 °C and 155 °C (see ESI, Tables S3 and S4). The similarity
2 | J. Name., 2012, 00, 1-3
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state complexes with almost all herein explored acceptors except II,
for 1,3-tfib only the cocrystals with III and XVI exhibited the
anticipated 2:1 stoichiometric ratio of acceptor and donor
components. The monotopic acceptors I, II, IV and
V form
cocrystals with 1,3-tfib in 1:1 stoichiometric ratio. Notably, the
cases in which monotopic acceptors yielded the cocrystals of
anticipated 2:1 stoichiometry with both 1,3-tfib and 1,4-tfib involve
the highly basic XVI and a moderate base quinoline (III) (Figure 1).
The cocrystals of XVI consist of discrete three-component
complexes, with each iodine atom of the halogen bond donor
involved in a halogen bond to a pyridine nitrogen atom of a
different acceptor molecule. Analogous cocrystals of quinoline (III)
also exhibit the formation of topologically identical discrete
complexes in the solid state, although with considerably longer
halogen bonds, consistent with the less basic nature of the
quinoline acceptor. The two I‧‧‧N halogen bonds in each complex
of (III)2(1,3-tfib) are of different lengths (d(I‧‧‧N) = 2.896(3) Å vs.
d(I‧‧‧N) = 2.911(2) Å), possibly due to steric effect caused by a
Fig. 1. Discrete molecular complexes in crystal structures of a) (III)2(1,3-tfib) and
b) (III)2(1,4-tfib).
The most significant difference between the two structures is in
the torsion angle ∠torsion(CNC(I)C) between quinoline and donor
molecules – in (III)2(1,4-tfib) the quinoline molecules are twisted
C−H‧‧‧F contacts between 1,3-tfib and quinoline molecules. In out of the plane of the donor molecule by 71.9(3)°, while in
(III)2(1,3-tfib) this is only 2.2(2)°, rendering the halogen-bonded
cocrystal (III)2(1,4-tfib) the halogen bonds are related by symmetry
assembly almost completely planar.
and therefore of the same length.
Table 1. Cocrystals covered by this analysis: p
1,3-tfib and 1,4-tfib, along with the length and relative shortening (R.S.) of corresponding halogen bonds. In compounds where the same donor participates in two
halogen bonds of different lengths and herein these are designated with XBI and XBII. Two independent donor molecules were found in cocrystals of 1,3-tfib with IX
Ka values (in aqueous solution, at room temperature) of the acceptors, stoichiometric compositions of cocrystals with
,
XI and XVI and these structures have two sets of data for corresponding halogen bond lengths. For previously reported compounds, the CSD codes are given in
parentheses.
d(XB)1,3 / Å; R.S.a / %
XBII
d(XB)1,4 / Å; R.S.a / %
XBI XBII
base
pKa
1,3-tfib cocrystal
XBI
1,4-tfib cocrystal
I
6.24
7.48
4.85
5.14
5.58
1.23
1.10
(I)(1,3-tfib)
(II)(1,3-tfib)
2.847(5); 19.3%
2.963(8); 16.7%
(I)2(1,4-tfib)
(II)(1,4-tfib)
2.803(3); 20.6%
3.008(1); 14.8%
2.868(2); 18.8%
2.834(6); 19.7%
2.971(2); 15.8%
2.970(3); 15.8%
2.934(5); 16.9%
II
III
IV
V
(III)2(1,3-tfib)
(IV)(1,3-tfib)
(V)(1,3-tfib)
2.888(6); 18.2%
2.933(6); 16.9%
(III)2(1,4-tfib)
2.842(6); 19.5%
(IV)2(1,4-tfib)
2.992(3); 15.2%
3.146(3); 10.9%
(V)2(1,4-tfib); (VOMHIP)
(VI)2(1,4-tfib); (VOMHAH)
(VII)(1,4-tfib)
VI
VII
(VI)2(VI)(1,3-tfib)
(VII)(1,3-tfib)
2.891(1); 18.1%
3.034(1): 14.1%
(VIII)(1,3-tfib);
(SIMHAY)
VIII
4.80
2.901(4); 17.8%
(VIII)(1,4-tfib); (QIHBEO)
2.851(4); 19.2%
2.768(3); 21.6%
2.795(3); 20.8%
2.908(2);
17.6%
2.889(2);
18.2%
IX
X
8.36
9.80
3.00
(IX)(1,3-tfib)
(X)(1,3-tfib)
(XI)(1,3-tfib)
(IX)(1,4-tfib); (DIVCOB)
(X)(1,4-tfib); (DIVCUH)
(XI)(1,4-tfib); (ISIHUN01)
2.874(6); 18.6%
2.830(6); 19.8%
2.792(2); 20.9%
2.790(3); 21.0%
2.842(3); 19.5%
2.798(2); 20.7%
2.745(2); 22.2%
2.764(4);
21.7%
2.774(4);
21.4%
XI
2.954(3);
16.3%
2.958(4);
16.2%
XII
0.60
(XII)(1,3-tfib)
2.911(1); 17.5%
2.953(1); 16.3%
(XII)(1,4-tfib)
XIII
XIV
XV
5.97
5.68
6.02
(XIII)(1,3-tfib)
(XIV)(1,3-tfib)
(XV)(1,3-tfib)
-
-
-
-
-
-
(XIII)2(1,4-tfib)
(XIV)2(1,4-tfib)
(XV)2(1,4-tfib)
2.871(4); 18.7%
2.781(3); 21.2%
2.808(5); 20.5%
(XVI)2(1,3-tfib);
XVI
9.50
2.783(4); 21.2%
2.800(4); 20.7%
(XVI)2(1,4-tfib) (RUYHID)
2.667(2); 24.4%
(RUYHOJ)
aCalculated as R.S. = 1 –
d(X···A)/[rvdW(X) + rvdw(A)]
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In contrast to III and XVI, cocrystallization of 3,5-lutidine (I),
isoquinoline (IV) and acridine (V) with 1,3-tfib yields solid-state
complexes with a 1:1 stoichiometric ratio of the halogen bond
donor and acceptor. In these complexes, only one iodine atom of
the halogen bond donor is involved in I···N halogen bind formation
(Figure 2), while the second iodine atom forms a close contact with
a fluorine atom of a neighbouring complex, forming chains through
I‧‧‧F contacts. For cocrystals of (I)(1,3-tfib) and (IV)(1,3-tfib), all
molecules within such chains are identically oriented and adopt an
approximately coplanar arrangement. Such a geometry cannot be
achieved with the bulky acridine molecule in (V)(1,3-tfib), resulting
in a different geometry of halogen-bonded chains, in which pairs of
halogen-bonded 1,3-tfib and acridine molecules adopt a coplanar
arrangement (dihedral angle of 6.3°), and the resulting halogen-
bonded planar units further assemble in mutually almost
perpendicular arrangement (dihedral angle of 88.8°).
In contrast to 1,3-tfib, cocrystals of 1,4-tfib with I, IV and V all
exhibit the anticipated 2:1 stoichiometric ratio of halogen bond
acceptors and donors, with both iodine atoms in each 1,4-tfib
molecule forming halogen bonds with the nitrogen atoms of the
respective bases. In the cocrystals (I)2(1,4-tfib) and (IV)2(1,4-tfib)
the resulting three-component halogen-bonded assemblies are
further interconnected via C−H‧‧‧F contacts, while the structure of
the cocrystal (V)2(1,4-tfib) did not reveal any contacts shorter than
the sum of the van der Waals radii.
Fig. 2. Halogen bonding contacts in crystal structures of a) (
tfib); c) IV)(1,3-tfib), d) IV)(1,4-tfib e) )(1,3-tfib
(VOMHIP).
I
)(1,3-tfib), b) (
V
I)(1,4-
and f) )(1,4-tfib)
(
(
)
(
V
)
(
Monotopic halogen bond acceptors
The explored picoline-based acceptors XIII-XV exhibit pKa values in
the range between 5.1 and 6.2 and, consequently, should readily
form cocrystals with both 1,3-tfib as well as 1,4-tfib, as observed for
the more, as well as less basic pyridines.9b Indeed, cocrystallisation
of the isomeric picoline halogen bond acceptors with 1,4-tfib
resulted in three cocrystals exhibiting the expected
2 : 1
stoichiometry of the halogen bond acceptors and donors (Figure 3).
The halogen bond donor 1,3-tfib, in contrast, appears to yield
cocrystals composed of halogen bond donors and acceptors in a 1:1
stoichiometric ratio. Unfortunately, single crystals of these could
not be obtained in order to compare their structures with the above
described ones. The cocrystals of 1,3-tfib and picolines were
obtained by mixing the reactants (both liquid at ambient
conditions) in a 1:1 stoichiometric ratio, which resulted in an almost
immediate solidification of the mixture. In the presence of a small
amount of a liquid additive or if excess picoline was used, the
reaction mixture remained liquid indefinitely. The solid-state
reaction mixture was also found to rapidly liquify in air, presumably
due to adsorption of moisture and/or loss of picoline. Moreover,
the material was also found to liquify in contact with cryoprotectant
oil which has made it impossible to conduct an X-ray single crystal
diffraction study with any of the three cocrystals.
Fig. 3. Discrete molecular complexes in crystal structures of a) (XIII)(1,4-tfib), b)
XIV)(1,4-tfib); c) (XV)(1,4-tfib).
(
The only monotopic acceptor which yielded cocrystals with a 1:1
donor:acceptor composition with both 1,3-tfib and 1,4-tfib was
2,4,6-colidine (II) (Figure 4). However, the two cocrystals exhibit
very different structural motifs. In the structure of the (II)(1,3-tfib)
cocrystal, one iodine atom of each halogen bond donor molecule
forms an I∙∙∙N halogen bond to a molecule of II, while the second
iodine atom forms a type I interhalogen contact19 with an iodine
atom from
a neighbouring 1,3-tfib molecule. This results in
centrosymmetric four-component assemblies (Figure 4a) of
composition (II)2(1,3-tfib)2. The assemblies are further
interconnected through C−H∙∙∙F contacts into a layered three-
dimensional (3D) structure. On the other hand, the crystal structure
of (II)(1,4-tfib) (Figure 4b) reveals chains of alternating donor and
acceptor molecules parallel to the crystallographic two-fold axis,
with one iodine atom of each 1,4-tfib molecule forming an I‧‧‧N
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halogen bond with a neighbouring molecule of II. The second iodine and hydrogen-bonded tapes consisting of alternating molecules of
1,4-tfib and pairs of VI (Figure 5a). However, in cocrystal with 1,3-
tfib, the steric demand of phenazine molecules precludes such a
coplanar arrangement, as was already observed for the cocrystal
(V)(1,3-tfib) involving the geometrically similar donor acridine
(Figure 2e). As a result, the I···N halogen bonds are directed towards
the π-system of phenazine molecules, each at an angle of 127o to
the plane of the corresponding halogen bond acceptor. This results
in bent halogen-bonded assemblies, with an angle of 86.2° between
the mean planes of two molecules of VI halogen-bonded to a
central molecule of 1,3-tfib. Each of the phenazine molecules is also
involved in a pair of C−H‧‧‧N hydrogen-bonded R22(8) motifs,
forming chains. The combination of I···N and C-H···N interactions
results in the formation of supramolecular ribbons with a V-shaped
cross-section, propagating along the crystallographic a-direction.
The spaces between consecutive 1,3-tfib molecules in each ribbon
are occupied by additional molecules of VI, resulting in an unusual
overall cocrystal composition (VI)3(1,3-tfib) (Figure 5). The
additional molecules of phenazine are not involved in any
significant halogen bonding, but form π···π stacks with 1,3-tfib
molecules, characterized by interplanar distances of ca. 3.34 Å and
distances between ring centroids of 3.45 Å and 3.56 Å.
atom of the 1,4-tfib molecule forms a short and linear I‧‧‧C
contact (I···C separation of 3.643(2) Å) involving a methyl group of a
neighboring II molecule (Figure 4c). The calculated electrostatic
surface potential of this halogen atom is positive which, considering
the generally positive nature of the -CH3 functionality, indicative of
a repulsive interaction. A survey of the CSD reveals that such linear
contacts, described by ∠(C–I‧‧‧CH3) and ∠(I‧‧‧CH3–C) angles of
180°, are rare: the database contains a total of 5 reported
structures exhibiting I···C(methyl) contacts with each of the two
angles higher than 170°. Moreover, the median values of ∠(C–
I‧‧‧CH3) and ∠(I‧‧‧CH3–C) angles for C–H‧‧‧I contacts found in
the CSD are ca. 110° and 150°, respectively, and are usually
associated with interactions of a positively charged hydrogen atom
of a methyl group and the region of the iodine atom with negative
electrostatic potential. Consequently, the short and linear I···C
contacts in (II)(1,4-tfib) cannot be explained through C-H···I
interactions or any other well-documented motif of molecular
packing, and we propose that the inability of one iodine atom in
each 1,4-tfib molecule to form a halogen bond is due to particularly
efficient close packing (Kitaigorodskii packing index, CK of 68.8%) of
the (II)(1,4-tfib) crystal structure.
Fig. 5. a) Halogen and C-H…N hydrogen bonded chains in
(VI)2(1,4-tfib)
(VOMHAH) b) Halogen bonded units in (VI)2(1,3-tfib); c) Linking of molecular
complexes in (VI)2(1,3-tfib) into a V-shaped ribbon through hydrogen bonded
phenazine-phenazine homosynthon, with additional phenazine molecules in
voids.
Other ditopic acceptors covered by this study (VII-XII) form
cocrystals of 1:1 stoichiometry with both donors. Specifically, all
ditopic cocrystal formers based exclusively on nitrogen-based
acceptor atoms (VII, VIII, X-XII) were found to form halogen-bonded
chains composed of alternating donor and acceptor molecules.
While the connectivity of the halogen bonding motif in such
structures is not affected by the choice of halogen bond donor,
switching between 1,4-tfib and 1,3-tfib leads to a pronounced
effect on the halogen-bonding geometry. As all the bases used have
the acceptor atoms at opposite ends of the molecules (closing an
angle of 180°), it was anticipated that the halogen-bonded chains in
Fig. 4. Discrete molecular complexes in crystal structures of a) (II)(1,3-tfib) and b)
(II)(1,4-tfib); c) Hirshfeld surface20 of a 1,4-tfib molecule in (II)(1,4-tfib) with
mapped electrostatic potential (colour scheme: red = -0.030 a.u.; blue = 0.120
a.u.), calculated at the HF 3-21G level of theory.
Ditopic halogen bond acceptors
With ditopic acceptors, both 1,3-tfib and 1,4-tfib donors were
found to yield cocrystals of 1:1 composition. The only exception is
phenazine (VI), which was found to act as a monotopic acceptor
towards both halogen bond donors, with one nitrogen atom
involved in I···N halogen bonding and the other involved in an
intermolecular C−H‧‧‧N hydrogen bond between neighbouring
molecules of VI (Figure 5). In the previously reported structure of
the (VI)2(1,4-tfib) cocrystal21 this leads to the formation of halogen-
cocrystals of 1,4-tfib will exhibit
a straight geometry, while
cocrystals of 1,3-tfib should be comprised of zig-zag-shaped chains.
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These expectations are indeed observed (Figure 6) in all cases a halogen bond acceptor. This led to a cocrystal of composition
except for cocrystals of quinoxaline as the acceptor (XII). The (IX)(1,3-tfib), with a very different halogen bonding topology
halogen bonds in the crystal structure of (XII)(1,4-tfib) are not compared to (IX)(1,4-tfib). Specifically, in this case only one iodine
coplanar with the quinoxaline ring, but are positioned on along the atom of each 1,3-tfib molecule participates in halogen bon
same face of the acceptor, each at an angle of ca 20° to the mean
formation, while the other one is involved in I‧‧‧F contacts.
Furthermore, only the nitrogen atoms of morpholine molecules are
involved in halogen bonding, with the oxygen atoms acting as
acceptors of N−H‧‧‧O hydrogen bonds formed between
neighbouring molecules of IX. As a result, each nitrogen atom in IX
is simultaneously an acceptor of an I···N halogen bond, and a
participant in the formation of hydrogen-bonded chains.
Consequently, the structure of (IX)(1,3-tfib) can be describes as
consisting of N-H···O hydrogen-bonded chains of morpholine
molecules, decorated through 1,3-tfib molecules acting as
monotopic halogen bond donors. The hydrogen-bonded chains
plane of the ring. In addition, the halogen bonds in (XII)(1,4-tfib) are
not perfectly linear, exhibiting a halogen bonding angle ∠(C–I‧‧‧N)
of 172.1° and 173.0°. The overall result is that each halogen-bonded
chain in the structure of (XII)(1,4-tfib) bends by ca. 60° at each XII
molecule, leading to a zig-zag shape. In the structure of the
(XII)(1,3-tfib) cocrystal the halogen bonding motifs around the
quinoxaline ring are identical to those observed in (XII)(1,4-tfib),
with approximately linear halogen bonds (halogen bonding angles
of 168.9° and 170.0°) deviating from the plane of the quinoxaline
ring by ca 20°. However, as the donor atoms on the 1,3-tfib
molecules are arranged at a 120° angle, this compensates for the
ca. 60° bend on the ring of XII, rendering the chains straight (Figures
6g,h).
adopt
a kinked structure, due to the alternation of two
conformationally different morpholine molecules, with the N-H
group occupying an equatorial position in one case, and axial in the
other. Conversely, the two types of morpholine molecules form
halogen bonds in axial and equatorial arrangements around the six-
membered ring, respectively (Fig 7a). Such considerable differences
between the crystal structures of (X)(1,3-tfib) and (IX)(1,3-tfib) are
surprising, considering that the analogous pair of 1,4-tfib cocrystals
are isostructural (Fig. 7b). We speculate that the difference in
behaviour of 1,3-tfib and 1,4-tfib towards the acceptor pair IX and
X
It is noteworthy that both 1,3-tfib and 1,4-tfib form solid-state
halogen-bonded chains with piperazine (X), with each donor and
each acceptor molecule involved in two halogen bonds. For 1,4-tfib
it has been shown that the cocrystal with piperazine is isostructural
with a series of cocrystals involving 1,4-diheteroatomic cyclohexane
ring derivatives, including morpholine (IX).9e In order to test
whether the same holds for 1,3-tfib, we explored morpholine (IX) as
Fig. 6. Halogen bonded chains in crystal structures of the a) (VII)(1,3-tfib), b) (VII)(1,4-tfib), c)
)
(
X
)(1,3-tfib), d) (
and
X
)(1,4-tfib) (DIVCUH), e) (XI)(1,3-tfib), f) (XI)(1,4-tfib
h) (XII)(1,4-tfib
)
)
(ISIHUN01),
g)
(
XII)(1,3-tfib
might be related to
a
combination of effects related to
a
differences in crystal packing efficiency of differently shaped
competition of different supramolecular forces, particularly the halogen-bonded structures based on a linear (1,4-tfib) and a bent
ability of IX to engage in either I···O or N-H···O interaction, and (1,3-tfib) halogen bond donor. The poorer packing efficiency of
6 | J. Name., 2012, 00, 1-3
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(IX)(1,3-tfib)
compared
to
were solved by direct methods and refined using the SHELXS
and SHELXL programs, respectively.23 The structural
(IX)(1,4-tfib) is evident from a comparison of the respective
crystallographic volumes per formula unit. The crystallographic unit refinement was performed on F2 using all data. The hydrogen
cell volume of (IX)(1,3-tfib) is 1384 Å3 for 4 formula units (i.e. 346 Å3
atoms were placed in calculated positions and treated as riding
on their parent atoms [C–H = 0.93 Å and Uiso(H) = 1.2 Ueq(C);
C–H = 0.97 Å and Uiso(H) = 1.2 Ueq(C)]. All calculations were
performed using the WinGX crystallographic suite of
per formula unit), whereas the structure of (IX)(1,4-tfib) exhibits a
volume of 326 Å3 per formula unit. Overall, (IX)(1,3-tfib) exhibits a
6.2% larger volume per formula unit compared to (IX)(1,4-tfib).
Similar difficulty of achieving a dense molecular packing is observed
for cocrystals of X, where the cocrystal (X)(1,3-tfib) exhibited a 5.2%
larger volume per formula unit compared to (X)(1,4-tfib). These
values stand out, as all other structurally characterised systems in
this study exhibited variations in formula unit volume of ±2.3% or
less upon replacing 1,4-tfib with 1,3-tfib. (see ESI, Table S2).
programs.24
A
summary of data pertinent to X-ray
crystallographic experiments is provided in Table 2. The
hydrogen- and halogen bond geometries were calculated using
Parst25 and Platon26. Further details are available from the
Cambridge Crystallographic Centre.15 Molecular structures of
compounds are presented by ORTEP-327 and their packing
diagrams were prepared using Mercury.28 Hirshfeld surfaces
and electrostatic potentials were calculated using
CrystalExplorer.20
Powder X-ray diffraction (PXRD) experiments on the
samples were performed on
a PHILIPS PW 1840 X-ray
diffractometer with CuKα1 (1.54056 Å) radiation at 40 mA and
40 kV. The scattered intensities were measured with a
scintillation counter. The angular range was from 5 to 45° (2θ )
with a continuous step size of 0.02°, and measuring time of 0.5
s per step. Data collection and analysis were performed using
the program package Philips X'Pert.29
Thermal analysis
Differential scanning calorimetry (DSC) measurements were
performed on a Mettler-Toledo DSC823e module in sealed
aluminium pans (40 μL) with three pinholes in the lid, heated
in a stream of nitrogen (150 mL min−1) at a heating rate of 10
°C min−1. Data collection and analysis were performed using
the program package STARe Software 14.00.30
Fig. 7. a) Halogen- and hydrogen-bonded chains (IX)(1,3-tfib); b) halogen bonded
chains in (IX)(1,4-tfib).
Conclusions
We have provided an extended structural study of halogen-
Experimental
bonded cocrystals involving 1,3-tfib,
a so far hardly
investigated isomer of the highly popular 1,4-tfib and 1,2-tfib
halogen bond donors. Whereas previous studies have
suggested that cocrystallization properties of 1,3-tfib should
be similar to those of the linear 1,4-tfib, this study has noted
significant differences. In particular, whereas our comparative
Synthesis
Halogen bond donors (1,3-tfib and 1,4-tfib) were purchased from
Manchester Organics Company, while other used chemicals and
solvents were purchased form Sigma-Aldrich Company and used as
received. Cocrystals were obtained by dissolving 1,3-tfib or 1,4-tfib study of 1,3-tfib and 1,4-tfib cocrystals with aromatic,
aliphatic, mono- and ditopic halogen bond acceptors reveals a
similarity in I···N halogen bond lengths and distribution of
melting points of explored cocrystals, it also reveals
differences in stoichiometric compositions of cocrystals and
the structures of underlying halogen-bonded motifs. For
example, both halogen bond donors form topologically
equivalent chains when combined with ditopic, nitrogen-based
halogen bond acceptors With monotopic acceptors, however,
1,4-tfib preferably (8 of 9 cases, 89%) forms cocrystals of 2:1
stoichiometry, while 1,3-tfib forms predominantly (7 of 9
cases, 78%) 1:1 complexes. Tentatively, the herein observed
differences in halogen-bonded motifs and cocrystal
composition are explained through changes in the efficiency of
crystal packing upon switching from a linear halogen bond
and the corresponding acceptor in hot acetone in 1:2 stoichiometric
ratio, whereupon solutions were left to cool and evaporate
(crystallization experiments involving I, II, III, IV, VIII, IX, XIII, XIV
and XV were performed in a refrigerator). Crystals suitable for
single crystal X-ray diffraction experiment appeared in three to five
days.
X-Ray Diffraction Measurements
Single crystal X-ray diffraction experiments were
performed using an Oxford Diffraction Xcalibur Kappa CCD X-
ray diffractometer with graphite-monochromated MoKα
0.71073 Å) radiation.22 The data sets were collected using the
-scan mode over the 2 -range up to 54°. The structures
(λ =
ω
θ
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Journal Name
Wainwright and A. C. Whitwood, Chem. Eur. J., 2010, 16
,
donor to the bent 1,3-tfib. This implies that, at least in some
cases, close packing, rather than halogen bonding strength
may be the force that determines the supramolecular
architecture and stoichiometric composition of the final the
product. Importantly, the herein reported comparative study
does not represent an exhaustive screen and it is likely that
some of the investigated systems could also form alternative
polymorphs stoichiometric variation. In that context, this
systematic investigation should be considered as a preliminary
report, and will be followed by further work.
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This research was supported by the Croatian Science
Foundation under the project IP-2014-09-7367.
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Cinčić, Cryst. Growth Des., 2016, 16, 6381; d) V. Nemec, L.
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Conflicts of interest
There are no conflicts to declare.
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The ability of meta- and para-diiodotetrafluorobenzene to act as halogen bond donors in crystal
engineering have been compared by synthesis and crystal structure analysis of a family of 20 novel
halogen-bonded cocrystals with simple monotopic and ditopic nitrogen-based acceptors.