Binding of an acetonitrile molecule inside the ethereal cavity of a hexaarylbenzene-based receptor via a synergy of C-H...O/C-H...π interactions

Article abstract of DOI:10.1039/b707237f

A pair of hexaarylbenzene-based receptors, which contain a circular, as well as a partially-broken, ethereal fence around the central benzene ring, bind acetonitrile molecules via a synergy of C-H...O and C-H...π interactions, as probed by X-ray crystallo

Full text of DOI:10.1039/b707237f

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Binding of an acetonitrile molecule inside the ethereal cavity of a
… …
hexaarylbenzene-based receptor via a synergy of C–H O/C–H p
interactions{
Ruchi Shukla, Sergey V. Lindeman and Rajendra Rathore*
Received (in Austin, TX, USA) 15th May 2007, Accepted 22nd June 2007
First published as an Advance Article on the web 6th July 2007
DOI: 10.1039/b707237f
…
central benzene ring, prompted us to investigate their C–H
1–3
O
A pair of hexaarylbenzene-based receptors, which contain a
circular, as well as a partially-broken, ethereal fence around the
central benzene ring, bind acetonitrile molecules via a synergy
…
and C–H p interactions
guests.5
towards the binding of different
…
…
Accordingly, herein, we report the binding of acetonitrile to
hexaarylbenzene (HAB) derivatives 2 and 3, and delineate that the
binding of acetonitrile to these HAB derivatives occurs from a
of C–H O and C–H p interactions, as probed by X-ray
crystallography.
…
…
synergistic interplay of both C–H O and C–H p forces, as
established by X-ray crystallography.
Weak molecular forces play a vital role both in supramolecular
chemistry and biological sciences.1 Scores of studies have demon-
…
…
The hexaarylbenzene rotamers 2 and 3 were synthesized by a
Co2(CO)8-catalysed trimerization of the corresponding bridged
ortho-substituted diarylacetylene. The characterization data of the
various compounds are included in the ESI.{ Excellent single
crystals of 2 (or 3) were obtained by a slow evaporation of its
solution in a 9 : 1 mixture of dichloromethane–acetonitrile during
the course of 2 d at 22 uC. The crystal structure of the symmetrical
rotamer 2 showed that it contained two symmetrically-independent
but (almost) identical molecules of a [2?CH3CN] complex.{ Fig. 1
shows that the CH3 group of the guest acetonitrile nestles deep
inside the bowl-shaped cavity of 2, which is comprised of an
aromatic bottom (i.e. central benzene ring) and an ethereal rim
formed by six oxygens from the peripheral aryl groups. The depth
of the cavity, i.e. the distance between the aromatic bottom and the
ethereal rim, is y2.1 s. The radius of the cavity in 2 is y3.15 s,
which is significantly larger than the cavity found in 18-crown-6
(i.e. y2.85 s),7 and thus allows a CH3 group to reside inside it.
The acetonitrile molecule approaches the central benzene ring of
2 in a highly symmetrical fashion, with the 3-fold axis of both
molecules coinciding within 7u and with a distance as close as
3.24 s. Although acetonitrile usually forms directed (linear)
strated that C–H O and C–H p interactions are the two most
notable weak molecular forces, besides hydrogen bonding, that
control the crystal packing, as well as the stabilization, of both
natural and unnatural supramolecular assemblies.2 It has also been
suggested that these weak molecular forces play an important role
in controlling the selectivity/reactivity of a variety of chemical
transformations.3
We have recently uncovered that
a Co2(CO)8-catalysed
trimerization of (bridged) ortho-substituted diarylacetylenes4
allows the selective formation of only two easily-separable
rotamers of hexaarylbenzenes (Scheme 1).5 Note that simple
ortho-substituted diarylacetylenes produce (multiple) inseparable
mixtures of rotamers!6 The ready availability of well-defined
rotamers and especially symmetrical rotamers 2, in which the six
ethereal oxygens are arranged in a circular array on one face of the
…
C–H O bonds (vide infra), in this complex, however, it forms
Scheme 1 Synthesis of a pair of hexaarylbenzene rotamers.
Department of Chemistry, Marquette University, PO Box 1181,
Milwaukee, Wisconsin 53201-1881, USA.
E-mail: Rajendra.Rathore@marquette.edu; Fax: +1 414-288-7066;
Tel: +1 414-288-2076
{ Electronic supplementary information (ESI) available: Synthetic details
and spectral data of various compounds in Scheme 1. See DOI: 10.1039/
b707237f
Fig. 1 A partial space-filling representation of the [2?CH3CN] complex
showing the symmetrical juxtaposition of the CH3 group inside the bowl-
shaped cavity of 2.
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8
…
symmetrically-bifurcated C–H O bonds, where each H atom of
…
C–H O angle of 156u, while the remaining H atom lies at an
…
…
the methyl group is equally shared between two neighbouring
…
average H C distance of 2.67 s and a C–H C angle of 151u. On
the opposite side of the benzene ring, having just two ethereal
…
ethereal oxygens. The average H O distance of 2.48 s is con-
siderably shorter than the equilibrium van der Waals separation of
oxygens, the observed H O distance is longer, 2.60 s, with a
…
…
CAr interactions across
C–H O angle of 157u, whereas the C–H
2.72 s, and is also shorter than the value of y2.6 s expected
9
…
the ring are similar, i.e. 2.66 s and 157u, respectively.
It should be noted that the observed geometrical parameters are
from the average C–H O angle of y137u (Fig. 1).
As such, the staggered orientation of the CH3 group of the
acetonitrile molecule with respect to six ethereal oxygens in 2
results in its H atoms being positioned within the largest openings
between the pairs of ethereal oxygens from the rim of the cavity.
Moreover, such an arrangement of the methyl H atoms (Fig. 1)
allows optimum access to the central benzene ring surface. Indeed,
…
optimal for effective (individualized) C–H O and C–H
…
p
interactions in the [3?2CH3CN] complex, as opposed to the
geometrical parameters observed for the interaction of acetonitrile
with symmetrical rotamer 2 (Fig. 1). It is believed that the
symmetrical juxtaposition of the acetonitrile molecule in rotamer 2,
containing all the ethereal oxygens on one face, arises due to an
all the H atoms of the methyl group simultaneously approach the
…
…
…
effective synergy between C–H O and C–H p forces, despite
carbon atoms of the central benzene ring at an average H
C
…
less than optimal geometric conditions for the discrete C–H
…
O
separation of 2.92 s, which is comparable with the standard value
…
and C–H p interactions.§
of 2.90 s.§ Also note that the C–H CAr bond angle in the
These findings suggest that the synergy of various weak
molecular forces can play an important role in designing molecular
receptors for the effective binding of different guests for practical
applications. The construction of new receptors based on the
molecular motifs presented herein is being actively pursued.
We thank the National Science Foundation (CAREER Award)
for financial support and Dr I. A. Guzei (University of Wisconsin,
Madison) for help with X-ray crystallography.
[2?CH3CN] complex is 117u, which is a deviation from the optimal
value of y150u—a statistically preferred angle for effective
…
C–H p interactions.§
The observed mutual orientation of the acetonitrile within the
bowl-shaped cavity of 2 in Fig. 1 leads to a rather stable complex"
despite the absence of optimal geometric conditions for the discrete
…
…
C–H O and C–H p interactions. This most likely arises due to
the mutual cooperativity of two weak molecular forces.
Interestingly, the unsymmetrical rotamer 3, with two faces of
the central benzene ring containing 4 and 2 ethereal oxygens,
Notes and references
…
…
respectively, shows more localized C–H O and C–H p interac-
tions with a pair of complexed acetonitrile molecules. For example,
the crystal structure of the [3?2CH3CN] complex consists of three
symmetrically-independent but structurally similar molecular
complexes. As shown in Fig. 2, unsymmetrical complex 3 interacts
with two acetonitrile molecules above and below the plane of the
central benzene ring to produce a 2 : 1 molecular complex.
The mutual orientation of the two acetonitrile molecules relative
to the central benzene ring is surprisingly invariable in all three
independent molecules—with one H–C–H plane singularly
perpendicular (within just 2u) to the plane of the central benzene
ring. Such an orientation allows the H atoms to effectively interact
{ Crystal structure data for [2?CH3CN]. A suitable crystal (0.49 6 0.37 6
0.21 mm) of [2?CH3CN] was obtained from a mixture of dichloromethane–
acetonitrile solution at 22 uC. MW = 1108.97, C75H88.50N1.50O6.13, triclinic,
space group P-1, a = 14.1227(6), b = 216227(10), c = 23.5476(10) s, a =
107.8180(10), b = 92.4520(10), c = 102.2070(10)u, Dc = 1.108 Mg m23, V =
6646.3(5) s3, Z = 4. The total number of reflections measured were 73492,
of which 27031 reflections were symmetrically non-equivalent (R(int) =
0.0259). Final residuals were R1 = 0.0584 and wR2 = 0.1561 for 27031
reflections with I . 2s(I). Also note that the two isopropyl groups in one of
the molecules are rotationally disordered over 2 and 3 positions,
respectively, and were refined with appropriate geometrical restrains.
CCDC 647363.
Crystal structure data for [3?2CH3CN]. A suitable crystal (0.33 6
0.14 6 0.09 mm3) of [3?2CH3CN] was obtained from a mixture of
dichloromethane–acetonitrile solution at 22 uC. MW = 1127.72,
C75.76H89.68Cl0.16N1.84O6, monoclinic, space group P21/n, a = 11.1865(6),
b = 40.7401(19), c = 42.484(2) s, a = 90, b = 95.9670(10), c = 90u, Dc =
1.167 Mg m23, V = 19256.5(17) s3, Z = 12. The total number of reflections
measured were 89746, of which 27724 reflections were symmetrically non-
equivalent (R(int) = 0.0741). Final residuals were R1 = 0.0635 and wR2 =
0.1415 for 27724 reflections with I . 2s(I). Also note that in the second and
third symmetrically-independent units of the molecular complex, aceto-
nitrile molecules are partially substituted with dichloromethane molecules.
They were refined with a geometry restrained to be the same as that of the
unaffected acetonitrile molecules from the first unit. CCDC 647364. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b707237f
…
with the central benzene ring via C–H p bonding, whereas the
…
remaining H atoms are chiefly involved in C–H O interactions.
The acetonitrile molecule on the face of the benzene ring
containing four ethereal oxygens shows that the H atoms facing
…
the benzene ring have an average H O distance of 2.45 s and a
…
§ Statistical analysis of C–H O intermolecular interactions between
acetonitrile molecules and ethereal oxygens in various reported crystal
structures (based on 83 structures from the Cambridge Structural
…
Database) has revealed that 224 prospective H O contacts are shorter
than 3.0 s, 116 contacts are shorter than the van der Waals separation
(2.7 s), and there are 30 contacts shorter than 2.4 s. These contacts show
strong correlation with the corresponding C–H O angles, i.e. the shorter
…
…
the C–H O distance the more linear (close to 180u) is the angle. For
example, the shortest contacts only have angle variations between 140 and
…
180u. There are larger geometrical variations in C–H p intermolecular
interactions between acetonitrile molecules and benzene rings in various
…
crystal structures. 2328 prospective H C contacts shorter than 3.2 s were
found in 418 structures. Furthermore, 859 of these contacts are shorter
than the van der Waals separation (i.e. 2.9 s), whereas 72 of them are
Fig. 2 A partial space-filling representation of the [3?2CH3CN] complex
showing the binding of two acetonitrile molecules to 3.
3718 | Chem. Commun., 2007, 3717–3719
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shorter than 2.6 s. These contacts exhibit a strong correlation with the
corresponding C–H C angle, similar to that observed for C–H
interactions. For the purposes of a uniform comparison, all C–H bonds in
acetonitrile molecules were normalized throughout the search to a standard
neutronographic value of 1.083 s
" It is noteworthy that an exposure of a solution of 2 in CDCl3 to y10–
20 equiv. acetonitrile at 22 uC showed considerable shifts of the aromatic
1H NMR signals. However, it could not be ruled out that these changes in
chemical shift occurred due to the change in solvent polarity. The
crystallization of the molecular complexes of 1 and 2 is being pursued with
a variety of other guests such as nitromethane, toluene, methanol,
ammonium cation, etc., and will be reported in due course.
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O
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C
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Chem. Commun., 2007, 3717–3719 | 3719