Near-perfect dipole parallel-alignment in the highly anisotropic crystal structure of 4-iodoacetophenone-(4-methoxyphenylethylidene) hydrazone

Article abstract of DOI:10.1023/A:1011311918880

The title compound crystallizes in the space group Pna2(1) with cell parameters a = 6.4606(3), b = 7.2155(3) and c = 33.5878(16) A. The azine shows a gauche conformation about the N - N bond and there is an angle of 58.1° between the benzene rings of each azine. This conformation allows for two intermolecular arene-arene T-contacts between pairs of benzene rings. The characteristic structural motif features T-contact formation between like-substituted arene rings and this architecture results in a highly dipole-parallel aligned lattice. All azines are perfectly colinear within each layer and the orientations of the azines in different layers are nearly the same. The surfaces of the layers exhibit a quadrilateral kite-shaped arrangement of I-atoms and of OCH₃-substituents. The layers pack such that the OCH₃ - carbon atoms are placed above the interstices between the I-atoms in the adjacent layer.

Full text of DOI:10.1023/A:1011311918880

Journal of Chemical Crystallography, Vol. 30, No. 7, 2000
Near-perfect dipole parallel-alignment in the highly
anisotropic crystal structure of
4-iodoacetophenone-(4-methoxyphenylethylidene)
hydrazone(1)
Michael Lewis,(2) Charles L. Barnes,(2) and Rainer Glaser(2)*
Received December 3, 2000
The title compound crystallizes in the space group Pna2(1) with cell parameters
˚
a = 6.4606(3), b = 7.2155(3) and c = 33.5878(16) A. The azine shows a gauche conformation
----
about the N N bond and there is an angle of 58.1 between the benzene rings of each azine.
This conformation allows for two intermolecular arene–arene T-contacts between pairs of
benzene rings. The characteristic structural motif features T-contact formation between like–
substituted arene rings and this architecture results in a highly dipole-parallel aligned lattice.
All azines are perfectly colinear within each layer and the orientations of the azines in different
layers are nearly the same. The surfaces of the layers exhibit a quadrilateral kite-shaped ar-
----
rangement of I-atoms and of OCH3-substituents. The layers pack such that the OCH3 carbon
atoms are placed above the interstices between the I-atoms in the adjacent layer.
KEYWORDS: Dipoleparallel-alignment;anisotropy;arene–arenecontacts;intermolecularinteractions;
nonlinear optics; azines.
Introduction
arene–arene double T-contact throughout the lattices
of the previously studied azines. A double T-contact
We have been investigating the stereochem-
occurs when the two spacer-connected arene rings of
one azine interact with the two arene rings of another
such molecule to form two intermolecular arene–
arene T-contacts. Scheme 2 illustrates four types of
double arene–arene contacts; I, II, and III are all dou-
ble T-contacts and IV is a double face-to-face con-
tact. Type I has a planar spacer and the two phenyl
rings in each molecule are coplanar. Types II and III
contain a twisted spacer and the two phenyl rings
4
6
10
istry,1 electronics,1,4 and crystal packing1,4,7
of symmetric and unsymmetric azines. An azine
is the condensation product between two carbonyl
molecules and hydrazine. The azines that we have
studied have the general structure shown in Scheme 1,
and they are called symmetric if X = Y and un-
symmetric, or mixed, if X = Y. The symmetric3 and
unsymmetric4,7,8 azines that we have investigated all
----
have an N N gauche conformation about the azine
----
bridge. The N N gauche conformation, in conjunc-
tion with a phenyl ring twist, causes the two phenyl
rings to be near orthogonal, and this allows for the
(1) Part 15 in the series “Stereochemistry and Stereoelectronics of
Azines.”
(2) Department of Chemistry, University of Missouri–Columbia,
Columbia, Missouri 65211. Telephone: (573) 882-0331, Facsimile:
(573) 882-2754, E-mail: GlaserR@missouri.edu.
* To whom correspondence should be addressed.
Scheme 1.
489
C
1074-1542/00/0700-0489$18.00/0 2000 Plenum Publishing Corporation

490
Lewis, Barnes, and Glaser
Scheme 2.
occurs when polar molecules aggregate in the solid
phase: the molecules align such that their molecular
dipole moments cancel and the dipole moment of the
crystal is zero.
in each molecule are twisted with respect to each
other. Note that the double T-contacts II and III are
interconverted by exchanging the top and bottom
molecules; these arrangements are structural isomers.
The double face-to-face contact, illustrated by IV, oc-
curs when the two phenyl rings of each molecule in-
teract in a face-to-face fashion. The double T-contacts
present in all but a few of the azines we have pre-
pared to date have the conformations shown in II
and III.
We are interested in mixed azines with the gen-
eral structure shown in Scheme 1 because of their
promise as nonlinear optical (NLO) materials.11,12 In
order for a crystal to exhibit a NLO response, there
are two necessary criterion. First, the crystals must be
noncentrosymmetric, or chiral, and this is a require-
ment for all types of polar effects of crystals (NLO
activity, piezo- and pyroelectric, ferroelectric, ferroe-
lastic, and many others). Noncentrosymmetry, how-
ever, is readily obtained for organic solids as chiral
space groups occur in approximately 20% of organic
crystals.13 The second criterion, and the central is-
sue, in the aggregation of crystals with macroscopic
polarizations is the parallel alignment of the dipole
moments of the individual chromophores, and this is
a challenge orders of magnitude more complicated
than the quest for non-centrosymmetry. We have ap-
proached the challenge of parallel dipole-alignement
in the absence of external fields in a systematic fashion
and with a rational design. Scheme 3 helps to illustrate
the difficulties in this proposal. Picture (a) in Scheme
1 depicts our goal, the alignment of dipole moments
in a crystal lattice. Picture (b) shows what most often
Scheme 3.

Near-perfect dipole parallel-alignment
491
Table 1. Crystal Data and Structure Refinement of 1
The design concept that we have employed to-
ward preparing dipole parallel-aligned crystals incor-
porates dipole moment minimization via the use of
the azine bridge. Previous studies5,14,15 have shown
that the azine bridge reduces the molecular dipole
moment by acting as a conjugation stopper. A small
dipole moment is obviously a key strategic fea-
ture in the preparation of dipole parallel-aligned
materials because the smaller the molecular dipole
moment, the less the electrostatic repulsion. Our
design concept also incorporates the use of arene–
arene interactions.16 Numerous theoretical stud-
ies have shown that arene–arene contacts are
attractive17,18 and the energy we gain from these in-
teractions can be used to overcome the electrostatic
repulsion of parallel-aligned dipole moments. Follow-
ing this design concept, we have been successful in
preparing dipole parallel-aligned crystals using chro-
mophores with the general structure in Scheme 1. In
this paper we report the crystal structure of the dipole
----
---- == ==
C 6 H 4 C(CH3) N N
Compound
I
== ----
----
C(CH3) C 6 H 4 OCH3
CCDC deposit no.
Color/shape
Chemical formula
Formula weight
Temperature, K
Crystal system
Space group
Unit cell dimensions
(8793 reflections in full
CCDC-1003/5909
Yellow/plates
C17H17IN2O
392.23
173(2)
Orthorhombic
Pna2(1)
˚
˚
a = 6.4606(3) A
b = 7.2155(3) A
˚
range)
Volume, A
c = 33.5878(16) A
3
˚
1565.74(12)
Z
4
Density (calculated), mg/mm3
Absorption coefficient, mm
1.664
2.046
1
Diffractometer/scan
Bruker SMART CCD area
detector
2.43 to 27.11
8793
range for data collection, deg
Reflections measured
Independent/observed reflections 3428 [R = 0.0235]/3428
int
Data/restraints/parameters
Extinction coefficient
3428/1/194
None used
==
==
parallel-aligned material where X OCH3 and Y
I
Absolute structure parameter
0.280(18)
1.135
(1). Wehavepreviouslyreportedthecrystalstructures
where X OCH3 and Y Br (2) or Cl (3). Com-
parisons will be drawn between the crystal structure
Goodness of fit on F2
4
9
==
==
Final R indices [I > 2s(I)]
R indices (all data)
R1 = 0.0223, wR2 = 0.0533
R1 = 0.0237, wR2 = 0.0538
of 1 and the crystal structure of the symmetric iodo-
10
== ==
Y I)
substituted crystal structure, 4 (Scheme 1, X
and the symmetric methoxy-substituted crystal struc-
3
== ==
ture, 5 (Scheme 1, X
Y OCH3) with attention to
Table 2. Atomic Coordinates ( 104) and Equivalent Isotropic
the packing between layers.
2
103) for 1
˚
Displacement Parameters (A
Atom
x
y
z
U(eq)a
Experimental
I(1)
1791(1)
4452(4)
1258(4)
2426(5)
2206(5)
1467(5)
2473(5)
1548(5)
361(5)
1390(5)
477(5)
4610(6)
989(5)
338(1)
166(3)
538(4)
6(4)
500(4)
56(4)
720(4)
645(4)
192(4)
946(5)
866(4)
662(6)
34(4)
0(1)
4219(1)
2008(1)
1720(1)
2382(1)
1314(1)
982(1)
606(1)
558(1)
887(1)
1260(1)
1769(1)
2673(1)
3076(1)
3395(1)
3772(1)
3838(1)
3525(1)
3148(1)
2614(1)
4301(1)
32(1)
29(1)
26(1)
22(1)
26(1)
23(1)
27(1)
30(1)
26(1)
26(1)
24(1)
40(1)
22(1)
22(1)
23(1)
26(1)
21(1)
23(1)
22(1)
35(1)
33(1)
Azine
1 was prepared by first reacting
O(1)
N(1)
C(1)
N(2)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
4-methoxyacetophenone with hydrazine hydrate to
give 4-methoxyacetophenone hydrazone. 4-Methoxy-
acetophenone hydrazone was condensed with
4-iodoacetophenone to yield 1. Crystals were ob-
tained from slow diffusion of hexane into a solution
of 1 in chloroform. The crystallographic data is given
in Table 1 and the fractional coordinates are shown
in Table 2.
1903(5)
926(5)
24(4)
Results and discussion
867(4)
894(5)
33(4)
881(4)
881(4)
538(6)
619(5)
1818(5)
3709(5)
4705(5)
3808(5)
1225(7)
6390(5)
The ORTEP II diagram of 1 is shown in Fig. 1 and
a view of the packing is shown in Figs. 2 and 3. The
crystallographic data information for 1 is presented
in Tables 1 and 2. The crystal structure of 1 has one
independent molecule and it is not planar. Figure 1 re-
---- == ---- == ----
bridge in 1 assumes a
aU(eq) is defined as one third of the trace of the orthogonalized
Ui j tensor.
veals that the
C
N
N
C
gauche conformation ( = 144.3 ). The angle for 1 is

492
Lewis, Barnes, and Glaser
Fig. 1. ORTEPII drawing of azine 1.
ω between the best planes through the benzene rings.
about 10 smaller than the respective angles for 2 ( =
135.8 ) and 3 ( = 134.7 ). The dihedral angles relat-
ing the twist of the two phenyl rings with respect to
the Cipso-imine bond planes in 1, 1 for the methoxy-
substitutedphenylringand 2 fortheiodo-substituted
phenyl ring, are 14.4 and 8.0 , respectively. Since the
Recognizing that 0 and all serve to increase ω, this
i
approximation gives ω 58.1 and leaves the ben-
zenes gauche. As was the case for the dihedral an-
gle , the ω angle for 1 is smaller than the respec-
tive angle for 2 (ω = 64.2 ) and 3 (ω = 64.0 ). The
fact that the dihedral and twist angles of the iodo-
substituted structure differ from the respective angles
of the chloro- and bromo-substituted analogs is not
surprising in light of an earlier analysis of the symmet-
ric 4-iodoacetophenone azine, 4. The and ω angles of
4 differ substantially from the same angles in the sym-
metric 4-chloro- and 4-bromoacetophenone azines.10
----
----
----
N1 N2, C1 C2 and0C9 C10 bonds in 1 are almost
0
parallel, the sum of (where = 180
),
and
1
provides an excellent approximation for the angle
2
----
Our previous investigations into the N
N
gauche conformation of mixed azines showed that
the azine bridge is essentially a conjugation stopper.
Crystallographic and ab initio studies5,9 showed that
the bond distances and populations of one phenyl
ring are virtually unaffected by the substitution pat-
tern on the other phenyl ring. Likewise, electrochem-
ical studies14,15 have also shown that the azine bridge
----
impedes conjugation. The N N gauche conformation
in 1 would suggest that the azine bridge is once again
acting as a conjugation stopper. If this is true then we
would expect little contribution from resonance form
1a (Scheme 4) in the ground state structure of 1.
Fig. 2. PLUTO drawing of the crystal packing of azine 1.
Scheme 4.

Near-perfect dipole parallel-alignment
493
Fig. 3. Chem3D representation of the crystal packing of azine 1, viewed down the diagonal of the ab-plane
with the c-axis on the horizontal.
----
==
The importance of resonance form 1a should be
manifest through a structural comparison of mixed
azine 1 with the symmetrical azines 4-iodoacet-
ophenone azine (4)10 and 4-methoxyacetophenone
azine (5).3 If there existed a substantial decrease in
and 4 and for 1 and 5. The I(1) C(5), C(1) N(1),
----
----
C(2) C(1) and N(1) N(2) bond lengths in 1 are
all approximately the same as those in azine 4. Simi-
larly, the C(13) O(1), C(9) N(2), C(9) C(10) and
N(1) N(2) bond lengths in 1 are all approximately
the same as those in azine 5. In all cases, deviations
----
==
----
----
----
----
the C(2) C(1) and N(1) N(2) bond lengths, and
˚
----
==
an increase in the I(1) C(5) and C(1) N(2) bond
lengths in 1 with respect to 4, then this would suggest
that conjugation plays a role in the ground state of 1.
of no greater than 0.02 A exist between the perti-
nent bond lengths in azines 1 and 4 and between
those in azines 1 and 5. Moreover, the directions of
these small changes are not consistent with contri-
bution from resonance form 1a to the ground state
of 1. Thus, the azine bridge in 1 is operating as a
conjugation stopper. The half of azine 1 with the
methoxy-substituted phenyl ring is impervious to the
substituent located on the phenyl ring in the other
half, and the same statement can be made about the
half of azine 1 with the iodo-substituted phenyl ring.
The B3LYP/6-31G* calculated dipole moment
of 1 is 3.3 Debye. We have previously studied the ef-
fects of azine conformation on the molecular dipole
moment.5 In this report we studied azines with the
general formula shown in Scheme 1 where the donor
substituent was OCH3 and the acceptors were F, Cl,
Br, CN and NO2. The ground state dipole moments
were calculated for the optimized gauche conforma-
tions and the planar structures. The dipole moments
all fell in the range 3–7 D and for each structure the
effect of conformation was less than 0.12 D. The same
would be expected for 1.
----
----
Likewise, a decrease in the C(13) O(1), C(10) C(9)
and N(1) N(2) bond lengths, and an increase in
the C(9) N(2) bond length in 1 with respect to 5
----
==
would also suggest that conjugation plays a role in
the ground state of 1. Table 3 shows the compar-
isons between the four pertinent bond lengths for 1
Table 3. Comparisons of Bond Distances in 1 and 4 and in 1 and 5
to Determine the Extent of Conjugation in 1a
˚
˚
˚
Bond
----
1 (A)
4 (A)
5 (A)
I(1) C(5)
2.09
1.29
1.50
1.40
1.37
1.30
1.48
1.40
2.10
1.29
1.49
1.40
—
—
—
—
—
—
—
==
C(1) N(1)
----
C(2) C(1)
----
N(1) N(2)
—
----
C(13) O(1)
1.36
1.28
1.48
1.41
==
C(9) N(2)
----
C(9) C(10)
----
N(1) N(2)
aThe esd values for molecules 1, 4, and 5 are all to the third decimal
place. Our comparisons all involve bond distances to the second
decimal place, and thus the esd values are not pertinent.
The crystal structure of azine 1 is highly
anisotropic with the chromophores ordered into

494
Lewis, Barnes, and Glaser
ers such that the arene–arene T-contacts always oc-
cur between two methoxy-substituted phenyl rings
or two iodo-substituted phenyl rings, but never be-
tween a methoxy- and an iodo-substituted ring. The
distance between the iodo-substituted arene rings
˚
in the type-II double T-contact is 3.8–4.2 A. The
methoxy-substituted arene rings in II are separated
˚
by 3.6–4.1 A. For the type-III double T-contacts, the
distance between the iodo-substituted arene rings is
˚
3.5–4.0 A and the distance between the methoxy-
˚
substituted arene rings is 3.6–4.1 A. It is important to
note that the double T-contacts always occur between
P- and M-conformational isomers.19 Figure 5 shows
the P- and M-isomers involved in the arene–arene
double T-contacts II and III. The designation P indi-
----
cates that the shortest rotation about the N N bond
required to eclipse one phenyl ring with the other is
in a clockwise direction. Conversely, the designation
M indicates a counterclockwise rotation.
Fig. 4. Layer of azine 1 viewed down the long molecular axis.
Since the arene–arene T-contacts always occur
between like substituted phenyl rings, each layer in
the crystal structure of 1 forms a two-dimensional
array of perfectly dipole parallel-aligned chro-
mophores. If one achieves dipole parallel-alignment
within a two-dimensional layer, then electrostatics
dictate that the third dimension must also be dipole
parallel-aligned. This is realized in the crystal struc-
ture of 1; however, the packing between the layers
repeating layers (Fig. 3). Rotation of these layers by
90 with respect to the plane of the page (Fig. 4) re-
veals that the predominant intermolecular interac-
tions within each layer are the arene–arene double
T-contacts (Fig. 5) of the types described by II and
III in Scheme 2. The molecules pack within the lay-
Fig. 5. P- and M-isomers in the two types of arene–arene double T-contacts in azine 1.

Near-perfect dipole parallel-alignment
495
is not quite the perfect scenario depicted in Scheme
3a. The packing between the layers deviates from per-
fect dipole parallel-alignment by an angle of approxi-
mately35 . Takingintoaccountthisanglebetweenthe
layers, the crystal structure of 1 is 95% dipole parallel-
aligned.
The azine layers in the crystal structure of 1
are terminated by surfaces containing iodine atoms
and methoxy substituents. The interface between the
layers is interesting with regards to how the iodine
atoms of one layer pack with the methoxy substituents
of the neighboring layer. The iodine surface can be
viewed as a plane of iodine atoms arranged in a kite-
shaped quadrilateral with two adjacent sides of iden-
tical length and three unique angles (Fig. 6, top). The
two unique iodine–iodine distances are a = 4.5 and
˚
b = 5.2 A. Angle is between the two iodine–iodine
contacts labeled a and b, angle is between the two
b-type contacts and angle is between the two a-type
contacts (Fig. 6, top), and these three angles measure
95.7 , 76.5 , and 92.0 . The iodine surface in 1 is very
similar to the iodine surface in 4.10 The a and b dis-
˚
tances in 4 are 4.5 and 5.1 A, respectively, and the
and angles are 97.1 , 76.9 , and 88.8 . Thus,
,
asymmetrization does not significantly affect the ar-
rangement of the iodine surface.
The methoxy surface can be viewed in a similar
fashion, where the carbon atoms are arranged in a
kite-shaped quadrilateral (Fig. 6, middle). The oxy-
gen atoms also form a kite-shaped quadrilateral, but
Fig. 6. The top shows the arrangement of iodine atoms at the surface of a layer and the
middle picture shows the arrangement of the methoxy substituents. The iodine atoms
and the methoxy substituents are in a quadrilateral kite-shaped motif. The drawing at the
bottomshowshowthetwosurfacespack, withthecarbonatomsofthemethoxysubstituents
lying in the interstices created by the iodine atoms.

496
Lewis, Barnes, and Glaser
it is the carbon atoms that pack in the interstices of the
iodine surface and thus we will focus on the carbon
atom pattern. The a and b distances for the quadri-
and the use of donors that avoid the occurrence of
any short intermolecular donor-acceptor interactions.
˚
lateral formed by the carbon atoms are 4.2 A and
Acknowledgments
˚
5.5 A and the angles are = 94.4 ,
= 71.3 ,
=
99.9 (Fig. 6, middle).
The University of Missouri Research Council
supported this work. M. L. thanks the Natural Sci-
ences and Engineering Research Council of Canada
for a Post-Graduate Scholarship, Type-B.
The interfacial packing of the iodine and
methoxy surfaces is described in the bottom picture of
Fig. 6. The carbon atoms of the methoxy groups lie in
the square interstices of the kite-shaped arrangement
of iodine atoms in the next layer. One of the methoxy
hydrogen atoms points straight into the center of the
interstice of the iodine atoms. The distances between
this hydrogen atom and the four iodine atoms are
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4336.
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˚
between 3.5 and 3.9 A and this is well outside the van
20
˚
der Waals distance of 3.3 A. The four distances re-
lating the methoxy carbon atoms of one layer with the
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˚
iodine atoms of an adjacent layer are c = 4.1 A, d =
˚
˚
˚
4.5 A, e = 4.3 A and f = 3.8 A (Fig. 6, bottom). This
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˚
atom. This oxygen-iodine distance is 3.1 A and it is
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˚
distance of 3.4 A for oxygen and iodine.
----
The short oxygen iodine packing between the
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molecules with the general structure in Scheme 1 with
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Highly polarizable substituents, such as bromine and
iodine, can better withstand being in close contact
with an oxygen atom than the highly electronegative,
relatively nonpolarizable fluorine and cyano groups.
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