Enantiospecific inclusion of chiral 1,2-dichloroethane rotamers in the crystal lattice of chiral square-pyramidal Cu(II) complexes with perfectly polar alignment of guest and host molecules

Article abstract of DOI:10.1039/b501642h

The inclusion compounds, [CuL¹₂(H₂O)] ·(P)-C₂H₄Cl₂ and [CuL² ₂(H₂O)]·(M)-C₂H₄Cl ₂ (HL¹ = N-(2-hydroxy-5-nitroben

Full text of DOI:10.1039/b501642h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Enantiospecific inclusion of chiral 1,2-dichloroethane rotamers in the
crystal lattice of chiral square-pyramidal Cu(II) complexes with
perfectly polar alignment of guest and host molecules
Vamsee Krishna Muppidi, Panthapally S. Zacharias{* and Samudranil Pal*
Received (in Cambridge, UK) 3rd February 2005, Accepted 18th March 2005
First published as an Advance Article on the web 4th April 2005
DOI: 10.1039/b501642h
produce the complexes, [CuL1 (H2O)] (1) and [CuL2 (H2O)] (2) in
The inclusion compounds, [CuL1 (H2O)]?(P)-C2H4Cl2 and
2
2
2
[CuL2 (H2O)]?(M)-C2H4Cl2 (HL1
5 N-(2-hydroxy-5-nitro-
good yields.{ X-Ray quality single crystals were grown by slow
evaporation of 1,2-dichloroethane solutions of the complexes.
2
benzyl)-(R)-a-methylbenzylamine and HL2 5 N-(2-hydroxy-5-
nitrobenzyl)-(S)-a-methylbenzylamine), crystallise in the
non-centrosymmetric space group C2; intermolecular hydrogen
bonding leads to a perfectly polar alignment of both host and
guest molecules with enantioselectivity.
Both complexes crystallise as [CuLn (H2O)]?C2H4Cl2 in the non-
2
centrosymmetric space group C2 with essentially identical unit cell
parameters.§ The molecular structures of 1 and 2 are mirror
images of each other (Fig. 1). In each complex molecule, the metal
centre is in a distorted square-pyramidal N2O3 coordination
sphere. Two bidentate chelating ligands form the N2O2 basal plane
and the water oxygen occupies the apical site. The bidentate
ligands are related by a crystallographically imposed two-fold axis
passing through the Cu and the apical water O. Both atoms are at
special positions on the b-axis. The dihedral angles between the
plane defined by Cu, N1 and O1 and that defined by Cu, N19 and
O19 are 18.41(4)u and 18.97(5)u for 1 and 2, respectively. These
values indicate a tetrahedral distortion of the N2O2 basal plane.
Polar and non-centrosymmetric organization of molecules in the
crystalline state is a prerequisite for molecular materials having
technologically important physical properties such as ferro-, pyro-
or piezoelectricity and nonlinear optical effects.1 Inclusion of
chirality in the molecule is a common approach to ensure non-
centrosymmetric organization in the crystals.2 On the other hand,
perfectly polar assembly in the crystal lattice is rare and requires
special design strategies.3 Synthesis of inclusion compounds having
polar host frameworks that can confine and align guest molecules
in a polar order is one of the successful strategies.4 In this regard, it
may well be noted that the synthesis of host frameworks with
chiral cavities is a noteworthy challenge as these solids also have
potential applications in the separation of enantiomers and in
asymmetric catalysis.5 In this work, we report the enantioselective
isolation of the right handed (P) and the left-handed (M) gauche
form of 1,2-dichloroethane in the host lattices formed by the chiral
square-pyramidal Cu(II) complexes having the general formula
[CuLn (H2O)] (HL1 5 N-(2-hydroxy-5-nitrobenzyl)-(R)-a-methyl-
2
benzylamine and HL2
5
N-(2-hydroxy-5-nitrobenzyl)-(S)-
a-methylbenzylamine,
H
represents the dissociable phenolic
proton) with perfectly polar alignment of both guest and host
molecules.
HL1 and HL2 were prepared by condensation reactions of one
mole equivalent of 5-nitrosalicylaldehyde with one mole equivalent
of the corresponding enantiomerically pure a-methylbenzylamine
in methanol followed by reduction with NaBH4. Reactions of
CuCl2?2H2O, HLn and LiOH (1:2:2 mole ratio) in methanol
Fig. 1 Molecular structures of (a) [CuL1 (H2O)] (1) and (b)
2
[CuL2 (H2O)] (2) with the atom-labelling schemes. All non-hydrogen
2
atoms are represented by their 35 and 50% probability thermal ellipsoids
{ Present address: Vice Chancellor, Goa University, Goa 403 206, India.
*spsc@uohyd.ernet.in (Samudranil Pal)
for 1 and 2, respectively.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2515–2517 | 2515

View Article Online
The bond parameters associated with the Cu(II) centres in 1 and 2
are unexceptional.6
Three rotamers are possible for 1,2-dichloroethane. These are
the trans or anti, the cis or eclipsed and the gauche forms. The
eclipsed form is energetically most unfavorable. Between the trans
and the gauche forms the former is more stable (by y1 kcal
mol21).7 The gauche form is chiral and also polar. One of the
enantiomers has the right-handed (P) form and the other one has
the left-handed (M) form. In the structures of 1?C2H4Cl2 and
2?C2H4Cl2, the asymmetric unit contains one-half of the 1,2-
dichloroethane molecule. The other half can be generated by
rotation about a two-fold axis passing through the mid-point of
the C–C bond. The molecule is in gauche form in each structure
(Fig. 2). The torsion angles involving Cl–C–C–Cl are 63(1)u and
64.6(9)u in 1?C2H4Cl2 and 2?C2H4Cl2, respectively. The molecule is
trapped in the P-form in 1?C2H4Cl2 (Fig. 2a). On the other hand,
in the case of 2?C2H4Cl2 only the M-form of it exists (Fig. 2b).
Thus the chiral host complexes 1 and 2 produce the appropriate
frameworks in the crystal lattice for exclusive accommodation of
the P- and M-forms of 1,2-dichloroethane, respectively.
Reports of inclusion crystals of optically active and polar
rotamers of 1,2-dichloroethane with a chiral host compound are
very rare.8 There are no reports on inclusion crystals of a chiral
rotamer of 1,2-dichloroethane with polar alignment of the guest
molecules in the host framework. The crystals of 1?C2H4Cl2 and
2?C2H4Cl2 provide examples, not only of enantioselective isolation
of the P- and M-forms of the chiral rotamers, but also of the
unprecedented, perfectly polar alignment of both host and guest
molecules in the crystal lattice. In each case, the polar alignments
of host and guest molecules are along the b-axis. It may be noted
that for 1 and 2, and also for (P)-C2H4Cl2 and (M)-C2H4Cl2, the
molecular dipole axis is along the b-axis or parallel to the b-axis.
The polar alignments are in opposite directions due to the
enantiomeric relationship between the two host molecules and that
between the two guest molecules in 1?(P)-C2H4Cl2 and
2?(M)-C2H4Cl2 (Fig. 3). In the crystal lattice of each structure,
the apical water molecule of the complex is involved in hydrogen
bonding interactions with the nitro groups of the neighboring
Fig. 3 Polar alignments of (a) 1?(P)-C2H4Cl2 and (b) 2?(M)-C2H4Cl2
viewed along the a-axis; H-atoms of the ligands and 1,2-dichloroethane are
omitted for clarity.
interaction involving one of the methylene hydrogens.9 The
…
C7 O2 distance and the C7–H O2 angle are 3.388(4) A and
…
˚
˚
146u and 3.336(4) A and 147u for 1 and 2, respectively. These
…
…
O–H O and C–H O interactions connect the complex mole-
cules and a polar two-dimensional layered structure is formed
(Fig. 4). In the channels formed by the parallel layers of the
complex molecules, the 1,2-dichloroethane molecules are trapped
and oriented in a polar order. The methylene groups of the 1,2-
…
dichloroethane are involved in intermolecular C–H O interac-
tions with the other O-atom of the nitro group (Fig. 5). The
…
…
˚
C16 O3 distance and the C16–H O3 angle are 3.285(14) A and
˚
150u and 3.195(11)
A and 149u for 1?(P)-C2H4Cl2 and
…
molecules. Most likely these O–H O interactions are primarily
responsible for the alignment of the host molecules in a head-to-
…
tail manner along the b-axis. For 1 the O4–H, H O2 and
…
…
O4 O2 distances and O4–H O2 angle are 0.86(6), 2.03(7) and
˚
2.846(3) A and 160(9)u, respectively. For 2 the values of these
˚
parameters are 0.89(5), 1.98(6) and 2.816(3) A and 156(7)u,
respectively. In both cases, the same O-atom of the nitro group
…
participates in an additional weak intermolecular C–H
O
Fig. 4 View of the two-dimensional layered structures of (a) 1 and (b) 2
along the c-axis; only the H2O and the CH2 group H-atoms involved in the
non-covalent interactions (dashed lines) are shown.
Fig. 2 (a) The right-handed (P) and (b) the left-handed (M) forms of 1,2-
dichloroethane in 1?C2H4Cl2 and 2?C2H4Cl2, respectively.
2516 | Chem. Commun., 2005, 2515–2517
This journal is ß The Royal Society of Chemistry 2005

View Article Online
suspended in water was extracted with (2 6 30 ml) CH2Cl2. The CH2Cl2
extracts were dried over anhydrous Na2SO4 and then evaporated to
dryness. The compound (HL1) was obtained in the form of light yellow
solid (1.96 g, yield 72%, m.p. 91–93 uC). HL2 was prepared in similar yield
by following the same procedure as described above using (S)-
a-methylbenzylamine instead of (R)-a-methylbenzylamine. The elemental
analysis data and the 1H NMR spectra (in CDCl3) of the compounds are
consistent with the expected molecular formula and structures.
Synthesis of 1: a methanol solution (10 ml) of CuCl2?2H2O (89 mg,
0.5 mmol) was added to a methanol solution (20 ml) of HL1 (272 mg,
1 mmol) and LiOH?H2O (42 mg, 1 mmol). The mixture was stirred at room
temperature for 1 h. The green solid separated was collected by filtration,
washed with methanol and dried in vacuum. Yield, 230 mg (74%).
Complex 2 was synthesized in similar yield and isolated as a green solid by
following the same procedure as described for 1 using HL2 instead of HL1.
§ Crystal data: for 1?C2H4Cl2: CuC32H36N4O7Cl2, M 5 723.09, mono-
Fig. 5 Projections of the crystal lattices of (a) 1?(P)-C2H4Cl2 and (b)
2?(M)-C2H4Cl2 onto the ac-plane.
˚
clinic, space group C2, a 5 20.2834(12), b 5 8.1769(5), c 5 12.8799(7) A,
˚
3
b 5 128.6480(10)u, V 5 1667.94(17) A , T 5 293(2) K, Z 5 2, 3842
independent reflections, 3572 reflections with I . 2s(I), 215 parameters,
…
2?(M)-C2H4Cl2, respectively. These C–H O interactions act as
bridges between the parallel layers of the complex molecules and a
three-dimensional network, with the polar alignment of both host
and guest molecules, is formed.
R1 5 0.0411 and wR2 5 0.1035 [I . 2s(I)], R1 5 0.0443 and wR2 5 0.1056
(all data), goodness-of-fit on F2 5 1.068, Flack parameter 5 20.01(1),
3
largest peak 0.647 e A . For 2?C2H4Cl2: CuC32H36N4O7Cl2, M 5 723.09,
˚
monoclinic, space group C2,
a 5 20.1329(17), b 5 8.0328(6),
3
Self-assembly via intermolecular non-covalent interactions is one
of the powerful tools for designing and synthesising polar crystals
as well as enantioselective chiral host frameworks. The spatial
disposition of the functional groups participating in the inter-
molecular non-covalent interactions is expected to control the
alignment of the host and guest molecules as well as spontaneous
resolution of chiral guest compounds. The structures of
1?(P)-C2H4Cl2 and 2?(M)-C2H4Cl2 reveal this functional group
directed alignment of molecules and selective trapping of guest
molecules. The self-assembly of the host molecules (1 and 2) occurs
˚
˚
c 5 12.7987(13) A, b 5 128.290(2)u, V 5 1624.6(2) A , T 5 293(2) K,
Z 5 2, 3815 independent reflections, 3661 reflections with I . 2s(I), 215
parameters, R1 5 0.0433 and wR2 5 0.1063 [I . 2s(I)], R1 5 0.0451 and
wR2 5 0.1076 (all data), goodness-of-fit on F2 5 1.053, Flack
3
parameter 5 20.01(1), largest peak 1.187 e A . Data were collected on a
˚
Bruker-Nonius SMART APEX CCD single crystal diffractometer
˚
using Mo Ka radiation (l 5 0.71073 A) and v-scans. An absorption
correction was applied to each data set with the help of the SADABS10
program. The structures were solved by direct methods and refined by
least-squares methods on F2 with all reflections (SHELX-9711). CCDC
263054–263055. See http://www.rsc.org/suppdata/cc/b5/b501642h/ for
crystallographic data in CIF or other electronic format.
…
through the O–H O interactions and the guest molecules (1,2-
…
dichloroethane) are held via the C–H O interactions. In both
1 Molecular Nonlinear Optics: Materials, Physics and Devices, ed. J. Zyss,
Academic Press, London, 1994; Handbook of Laser Science and
Technology, ed. M. J. Webber, CRC Press, Boca Raton, FL, 1995.
2 J. Zyss, J.-F. Nicoud and M. Coquillay, J. Chem. Phys., 1984, 81, 4160;
D. E. Eaton, Science, 1991, 253, 281.
3 S. R. Marder, J. W. Perry and W. P. Scaefer, Science, 1989, 245, 626;
S. R. Marder, J. W. Perry and C. P. Yakymyshyn, Chem. Mater., 1994,
6, 1137; S. P. Anthony and T. P. Radhakrishnan, Chem. Commun.,
2001, 931; P. K. Thallapally, G. R. Desiraju, M. Bagieu-Beucher,
R. Masse, C. Bourgogne and J.-F. Nicoud, Chem. Commun., 2002,
1052; A. Jouaiti, M. W. Hosseini and N. Kyritsakas, Chem. Commun.,
2002, 1898.
4 W. Tam, D. F. Eaton, J. C. Calabrese, I. D. Williams, Y. Wang and
A. G. Anderson, Chem. Mater., 1989, 1, 128; O. Ko¨nig, H.-B. Bu¨rgi,
T. Armbruster, J. Hullinger and T. Weber, J. Am. Chem. Soc., 1997,
119, 10632; A. Quintel, J. Hullinger and M. Wu¨bbenhorst, J. Phys.
Chem. B, 1998, 102, 4277; K. T. Holman, A. M. Pivovar and
M. D. Ward, Science, 2001, 294, 1907.
5 X. X. Zhang, J. S. Bradshaw and R. M. Izatt, Chem. Rev., 1997, 97,
3313; E. Brunet, Chirality, 2002, 14, 135; L. Pe´rez-Garc´ıa and
D. B. Amabilino, Chem. Soc. Rev., 2002, 31, 342; B. Kesanli and
W. Lin, Coord. Chem. Rev., 2003, 246, 305; L.-X. Dai, Angew. Chem.
Int. Ed., 2004, 43, 5726; M. A. Mateos-Timoneda, M. Crego-Calama
and D. N. Reinhoudt, Chem. Soc. Rev., 2004, 33, 363.
intermolecular interactions, ligand nitro group O-atoms act as the
acceptors. The positions and orientations of the nitro groups on
the enantiomeric ligands in 1 and 2 are befitting for a perfectly
polar alignment of the host molecules and for enantioselective
confinement of the chiral rotamers of 1,2-dichloroethane in a polar
order.
Financial support for this work was provided by the Council of
Scientific and Industrial Research (CSIR), New Delhi [Grant No.
01(1880)/03/EMR-II to S. P.]. V. K. M. thanks the CSIR for a
senior research fellowship. X-Ray crystallographic studies were
performed at the National Single Crystal Diffractometer Facility,
School of Chemistry, University of Hyderabad (funded by the
Department of Science and Technology, New Delhi). We thank
the University Grants Commission, New Delhi for the facilities
provided under the Universities with Potential for Excellence
program.
Vamsee Krishna Muppidi, Panthapally S. Zacharias{* and
Samudranil Pal*
School of Chemistry, University of Hyderabad, Hyderabad 500 046,
India. E-mail: spsc@uohyd.ernet.in; Fax: +91 40 2301 2460;
Tel: +91 40 2313 4756
6 L. L. Koh, J. O. Ranford, W. T. Robinson, J. O. Svensson, A. L. C. Tan
and D. Wu, Inorg. Chem., 1996, 35, 6466; C. T. Yang, H. Moubaraki,
K. S. Murray and J. J. Vittal, Dalton Trans., 2003, 880.
7 E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds,
Wiley, New York, 1994, p. 606.
8 F. Toda, K. Tanaka and R. Kuroda, Chem. Commun., 1997, 1227.
9 G. R. Desiraju, Acc. Chem. Res., 1996, 29, 441.
10 G. M. Sheldrick, SADABS, Program for area detector adsorption
correction, Institute for Inorganic Chemistry, University of Go¨ttingen,
Germany, 1996.
11 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Go¨ttingen, Germany, 1997; G. M. Sheldrick,
SHELXS-97, Program for solution of crystal structures, University of
Go¨ttingen, Germany, 1997.
Notes and references
{ Synthesis of HL1: a methanol solution (30 ml) of 5-nitrosalicylaldehyde
(1.67 g, 10 mmol) was added to a methanol solution (30 ml) of (R)-
a-methylbenzylamine (1.21 g, 10 mmol) and stirred at room temperature
for K h. To the resulting yellow solution, 0.74 g (20 mmol) of NaBH4 was
added and the mixture was stirred for another K h until a colourless
solution was obtained. The reaction mixture was evaporated to dryness on
a rotary evaporator, followed by addition of 100 ml of water. The solid
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2515–2517 | 2517