Halogen-bonded liquid crystals of 4-alkoxystilbazoles with molecular iodine: A very short halogen bond and unusual mesophase stability

Article abstract of DOI:10.1039/c3cc41227j

Complexes of molecular iodine with alkoxystilbazoles are liquid crystals with unusually high mesophase stability, predicated on an intermolecular I...I contact. Attempts to prepare analogous complexes with bromine led to an unexpected electrophilic substitution product.

Full text of DOI:10.1039/c3cc41227j

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Halogen-bonded liquid crystals of 4-alkoxystilbazoles
with molecular iodine: a very short halogen bond and
unusual mesophase stability†
Cite this: Chem. Commun., 2013,
49, 3946
Received 15th February 2013,
Accepted 20th March 2013
Linda J. McAllister,a Carsten Prasang,a Joanna P.-W. Wong,a Robert J. Thatcher,a
¨
Adrian C. Whitwood,a Bertrand Donnio,b Peter O’Brien,a Peter B. Karadakova and
Duncan W. Bruce*a
DOI: 10.1039/c3cc41227j
www.rsc.org/chemcomm
Complexes of molecular iodine with alkoxystilbazoles are liquid
crystals with unusually high mesophase stability, predicated on
an intermolecular IÁ Á ÁI contact. Attempts to prepare analogous
complexes with bromine led to an unexpected electrophilic sub-
stitution product.
Fig. 1 Molecular iodine complexes of alkoxystilbazoles.
yellow/orange complexes 1-n (Fig. 1) which, from elemental
analysis, showed a 1 : 1 stilbazole : I2 stoichiometry.
Halogen bonding is an established tool in supramolecular
chemistry.1 Some time ago, we reported the first examples of
liquid crystals formed using halogen bonding, namely complexes
between an alkoxystilbazole and pentafluoroiodobenzene.2
Since then, we have obtained 2 : 1 complexes of stilbazoles with
a, o-diiodoperfluoroalkanes,3 with 1,4-4 and 1,3-diiodotetra-
fluoro-benzenes5 (the latter showing chiral mesophases in
the absence of molecular chirality) and 2,3,5,6-tetrafluoro-
1-iodophenol,6 as well as performing an extensive study of
more than ninety 1 : 1 complexes.7 Keen to extend the range
of materials containing halogen bonds, we set out to pre-
pare complexes of stilbazoles with molecular iodine (I2) and
bromine (Br2).
In the case of octyloxystilbazole, single crystals were
obtained and the molecular structure is shown in Fig. 2. Thus,
there is a NÁ Á ÁI halogen bond at 2.353(5) Å, which at 67% of the
sum of the van der Waals radii of iodine and nitrogen, is almost
the shortest NÁ Á ÁI separation recorded in complexes of I2.
The I–I bond in the complex is at 2.8298(7) Å, compared to
2.715(4) Å in I2, while the NÁ Á ÁI–I angle at nitrogen (measured
using the bound iodine, the nitrogen and the ipso carbon of the
pyridine ring) is close to linear at 176.4(2)1. In complexes
with strongly basic donors, iodine can be polarised strongly
giving rise to so-called amphoteric adducts,8a characterised
by a right-angled ‘Type 2’ interaction. The structure here
features an intermolecular interaction that is topologically
Type I, with an iodine–iodine separation of 3.8891(7) Å which,
given a van der Waals’ radius for iodine of 1.98 Å, is at 98%
of twice this radius. This would not normally be held to be
significant (intermolecular IÁ Á ÁI distances in molecular iodine
are 3.496(4) Å).
Iodine is known to form strong halogen bonds,8 and indeed,
while incorrectly formulated, Guthrie’s paper of 1863 described
the iodine adduct of ammonia. The classic work of Hassel then
demonstrated an example in a co-crystal with 4-picoline9 in
which the IÁ Á ÁN separation was 2.31 Å – 65% of the sum of the
van der Waals’ radii. As our halogen bonding work described
above demonstrates, extending 4-alkoxystilbazoles via function-
alisation of the pyridine nitrogen is a good way to induce
mesomorphism,10 which we have demonstrated also using
metal complexation11 and hydrogen bonding.12 Thus, crystal-
lising solutions containing I2 and alkoxystilbazole led to the
The liquid crystal properties of 1-n (Table 1) were deter-
mined using polarised optical microscopy, which showed that
the longest-chain homologues, 1-10 and 1-12, gave straight-
forward behaviour. Thus, 1-10 melted directly to a SmC phase
at 108.5 1C giving way to a SmA at 117 1C. Both phases showed
characteristic textures and DSC showed the transition to be
a Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.
E-mail: duncan.bruce@york.ac.uk
b
´
CNRS-Universite de Strasbourg (UMR 7504), Institut de Physique et Chimie des
´
Materiaux de Strasbourg, 23 rue du Loess BP 43, 67034 STRASBOURG Cedex 2,
France
† Electronic supplementary information (ESI) available: X-ray data; cif files; NMR
spectra; experimental details. CCDC 905968–905970. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc41227j
Fig. 2 Molecular structure of the 1 : 1 complex between octyloxystilbazole and I2.
c
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Table 1 Thermal data for iodine complexes 1-n
consistent with the observation of a SmC phase. Determination
of the molecular areas13 reveals a tilt angle of ca. 301 in the
mesophase for each complex. Despite having the tilt angle
and being able to compute the molecular length of the 1 : 1
adduct, it is not then straightforward to determine the
parameter of real interest, namely the intermolecular iodine
separation as there is too much geometric flexibility that
cannot be pinned down. Nonetheless, the X-ray data show
unequivocally that the SmC phase arises from a 2 : 2 arrange-
ment, which must be stabilised by the development of the
intermolecular IÁ Á ÁI contact. The implied strength of this
contact in the mesophase is then remarkable given the tem-
peratures at which it is found.
Complex
Transition
T/1C
1-4
Cr–Cr + Iso
Cr + Iso–Iso
(Iso–SmA)
Cr–Cr + Iso
Cr + Iso–Iso
(Iso–SmA)
Cr–SmA
145
217
(214)
155
1-6
213
(204.5)
146.7
207
108.5
117.0
230
112.0
139.5
208
1-8
SmA–Iso
Cr–SmC
1-10
SmC–SmA
SmA–Iso
Cr–SmC
1-12
SmC–SmA
SmA–Iso
Having found iodine complexes, attempts were then made
to prepare a bromine analogue by allowing an alkoxystilbazole
second order. Clearing was found at 230 1C and was evidently and molecular bromine to crystallise together from THF. How-
accompanied by some decomposition as cooling the sample ever, rather than forming a halogen-bonded adduct, the crystals
did not reproduce the phase sequence. The behaviour of 1-12 that were obtained were of the stilbazolium bromide (Fig. 3) in
was the same at the temperatures included in Table 1. It is which one of the ethylenic hydrogens had been replaced by
testament to the stability of the stilbazoleÁ Á Áiodine halogen bromine (vinyl bromide trans-2 – Scheme 1).14
bond that the only real evidence of decomposition is found at
This observation was at first sight surprising and a search of
elevated temperatures or on prolonged heating, as previously the literature found only one example of such a stilbazole
we provided evidence of the lability of halogen bonds between derivative (Katritzky archive,15 no experimental data). Efforts
stilbazoles and iodobenzenes.4,5,7
to isolate appreciable quantities of vinyl bromide, 2, gave a
Complex 1-8 was a little different as on heating to 146.7 1C, solid that was shown by 1H NMR spectroscopy (Fig. S2, ESI†) to
the crystalline solid melted to form a SmA phase. If slow be a 10 : 1 mixture of trans-2 and cis-2 (Scheme 1). The minor
heating continued, the mesophase crystallised at 157 1C before component was confirmed as cis-2 by NOE experiments: trans-2
re-melting to a SmA at 177.5 1C and then clearing at 207 1C with showed a NOE between the vinyl proton and both aromatic
decomposition. However, if the sample was heated rapidly rings whereas cis-2 showed a NOE only to the alkoxyphenyl ring
above 146.5 1C, crystallisation and re-melting were not observed (no NOE to the pyridyl ring was observed). A mechanistic
and heating led straight to the isotropic state. Thus, the crystal- rationale for the reaction is depicted in Scheme 1. Thus,
line phase obtained on solution crystallisation appears to be cleavage of the C–Br bond adjacent to the alkoxyphenyl ring
metastable. No liquid crystal behaviour was obtained, however, could occur either from a bromonium ion or dibromo inter-
for 1-4 and 1-6. In each case, the complex melted to give a mediate, generating a carbocation stabilised by conjugation
mixture of an isotropic liquid and a crystalline solid, which from the lone pair on the alkoxy oxygen. Two conformations for
persisted until the new solid also melted. The first melting
was accompanied by a darkening attributed to extrusion of
molecular iodine (molten above 114 1C), which was confirmed
gravimetrically. Stoichiometry would suggest the formation of a
2 : 1 complex, although such structures have previously been
felt unlikely where strong donors are concerned.8a
However, it is surprising that a SmC phase is observed for
these complexes, as the weak intermolecular I2Á Á ÁI2 interaction
would suggest that in the mesophase the complex exists as Fig. 3 Molecular structure of 2.
shown in Fig. 1, i.e. a 1 : 1 complex. There is little or no
precedent for such simple, dipolar mesogens forming a SmC
phase. Therefore, complexes 1-10 and 1-12 were investigated by
X-ray diffraction in the SmC phase just above the Cr–SmC
transition, as the complexes were unstable over time at elevated
temperature and in the X-ray beam in the mesophase. In each
case, a single, small-angle reflection – d(001) – was observed,
consistent with a lamellar phase (Fig. S1, ESI†); the measured
d-spacings were 49.9 Å (1-12 at 120 1C) and 50.1 Å (1-10 at
110 1C). These distances are clearly much greater than the
length (26.8 Å) of the monomeric stilbazoleÁ Á ÁI2 unit and show
conclusively (and surprisingly) that the species giving rise to the
mesophase is in fact the 2 : 2 dimer found in the crystal structure, Scheme 1 Proposed mechanism for the formation of compound 2.
c
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intermolecular IÁ Á ÁI interaction found in the crystal structure
clearly changes its nature and strength in the mesophase and
appears crucial in promoting the formation of the SmC phase,
which proposal is supported by the results on X-ray diffraction
in the mesophase. Attempts to prepare analogous complexes of
bromine lead to electrophilic substitution in the ethylenic
fragment of the alkoxystilbazole.
EPSRC (LJM and CP), the University of York (RJT) and the
University of York Wild Fund (JPKW) are thanked for support.
Fig. 4 Single crystal X-ray structure of compound 3. Further details are found in
the ESI.†
Notes and references
HBr elimination (with the C–H bond approximately parallel to
the carbon p-orbital) of the carbocation A and B are shown.
Elimination from A (to give trans-2, the major product) is
presumably favoured as it minimises steric interactions
between the two aryl groups.
1 P. Metrangolo, H. Neukirch, T. Pilati and G. Resnati, Acc. Chem. Res.,
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¨
3 P. Metrangolo, C. Prasang, G. Resnati, R. Liantonio, A. C. Whitwood
If this mechanistic conjecture is correct, then using a
4-alkylstilbazole should inhibit elimination of bromide and
disfavour formation of vinyl bromide 2. Therefore, 4-butyl-
stilbazole and Br2 were mixed in THF solution under a range
and D. W. Bruce, Chem. Commun., 2006, 3290–3292.
4 D. W. Bruce, P. Metrangolo, F. Meyer, C. Prasang, G. Resnati and
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¨
¨
5 C. Prasang, A. C. Whitwood and D. W. Bruce, Chem. Commun., 2008,
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1
of conditions. Evidence from H NMR spectroscopy and from
¨
6 C. Prasang, H. L. Nguyen, P. N. Horton, A. C. Whitwood and
single crystals obtained showed (see also ESI†) only the for-
mation of the 1,2-dibromo adduct (3, Fig. 4). Thus, the 1H NMR
spectrum showed an AB spin system centered on d = 5.37 with
JAB = 11 Hz, as expected for anti stereochemistry about the
carbon–carbon bond. That this was the 1,2-dibromo adduct
was verified by its independent preparation by reacting alkyl-
or alkoxy-stilbazole with a mixture of Bu4NBr and Bu4NBr3
according to Bellucci et al.16
In the case of the alkyloxystilbazoles, all attempts to obtain
solid samples of the 1,2-dibromo adduct using the conditions
employed for the alkylstilbazole were unsuccessful and indeed
in contact with EtOH, the dibromo adduct was solvolyzed, with
the bromine attached to the ethylenic carbon next to the
alkoxyphenyl ring being replaced by EtO (4).
D. W. Bruce, Chem. Commun., 2008, 6164–6166.
7 D. W. Bruce, P. Metrangolo, F. Meyer, T. Pilati, C. Prasang,
G. Resnati, G. Terraneo, S. G. Wainwright and A. C. Whitwood,
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8 (a) W. T. Pennington, T. W. Hanks and H. D. Arman, in Halogen
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G. Resnati, Springer, Berlin, 2008, vol. 126, pp. 65–104;
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T. W. Hanks and W. T. Pennington, Cryst. Growth Des., 2001, 1,
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¨
9 O. Hassel, C. H. R. Romming and T. Tufte, Acta Chem. Scand., 1961,
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10 D. W. Bruce, Adv. Inorg. Chem., 2001, 52, 151–204; D. W. Bruce,
D. A. Dunmur, M. A. Esteruelas, S. E. Hunt, R. Le Lagadec,
P. M. Maitlis, J. R. Marsden, E. Sola and J. M. Stacey, J. Mater.
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D. W. Bruce, J. Mater. Chem., 2001, 11, 2852–2863; D. W. Bruce, Acc.
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12 See e.g.: J. P.-W. Wong, A. C. Whitwood and D. W. Bruce,
Chem.–Eur. J., 2012, 18, 16073–16089; D. J. Price, K. Willis,
T. Richardson, G. Ungar and D. W. Bruce, J. Mater. Chem., 1997,
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13 Method in: Z. T. Nagy, B. Heinrich, D. Guillon, J. Tomczyk, J. Stumpe
and B. Donnio, J. Mater. Chem., 2012, 22, 18614–18622.
14 When applied to the stilbazole derivatives, trans and cis refer to the
mutual disposition of the two six-membered rings.
15 http://www.ark.chem.ufl.edu/ (CAS 300396-11-6) and A. R. Katritzky,
private communication 2011.
Interestingly, this NMR spectrum (Fig. S3, ESI†) showed that
a single diastereoisomer is produced on addition of bromine,
consistent with the predicted lability of C–Br bonds expressed
in Scheme 1. The stereochemistry is presumed to be anti and
the value for JAB of 5.3 Hz is consistent with related methanol
adducts of anti-stereochemistry obtained from methanolysis of
1,2-dibromo adducts of stilbenes.17
The complexes between the alkoxystilbazole and I2 show a very
short halogen bond and are also found to be liquid crystalline; the
mesophases are unusually stable compared to other systems we
have investigated. More remarkably, the weak-to-non-existent,
3
16 G. Bellucci, R. Bianchini, C. Chiappe, R. S. Brown and H. Slebocka-
Tilk, J. Am. Chem. Soc., 1991, 113, 8012–8016.
17 M. F. Ruasse, G. Lo Moro, B. Galland, R. Bianchini, C. Chiappe and
G. Bullucci, J. Am. Chem. Soc., 1997, 119, 12492–12502.
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3948 Chem. Commun., 2013, 49, 3946--3948
This journal is The Royal Society of Chemistry 2013