Combinatorial chemistry approach to new materials for non-linear optics. I. Five Schiff bases

Article abstract of DOI:10.1107/S0108270100002845

A combinatorial chemistry approach has been used to synthesize an array of Schiff bases, five of which, namely N-[(E,2E)-3-(4-methoxyphenyl)-2-propenylidene]-3-nitroaniline, C₁₆H₁₄N₂O₃, (1a), N-[(E,2E)-3-(4-meth

Full text of DOI:10.1107/S0108270100002845

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
to obtain suitable single crystals. To our knowledge, this study
represents the ®rst systematic application of chemical crys-
tallography to the investigation of an array of combinatorial
products. For the targeted library, we used amines (1±4) and
aldehydes (a, b) as the building blocks and we assumed that
we would be able to synthesize eight polar Schiff bases by
condensation reactions (scheme).
ISSN 0108-2701
A combinatorial chemistry approach
to new materials for non-linear optics.
I. Five Schiff bases
Vladimir N. Nesterov,a* Tatiana V. Timofeeva,a Oleg Ya.
Borbulevych,a Mikhail Yu. Antipina,b and Ronald D. Clarka
aDepartment of Physical Sciences, New Mexico Highlands University, Las Vegas,
NM 87701, USA, and bInstitute of Organoelement Compounds of the Russian
Academy of Sciences, 28 Vavilov St., Moscow, Russian Federation
Correspondence e-mail: vlad@kremlin.nmhu.edu
Received 25 November 1999
Accepted 21 February 2000
A combinatorial chemistry approach has been used to
synthesize an array of Schiff bases, ®ve of which, namely N-
[(E,2E)-3-(4-methoxyphenyl)-2-propenylidene]-3-nitro-
aniline, C16H14N2O3, (1a), N-[(E,2E)-3-(4-methoxyphenyl)-2-
propenylidene]-4-nitroaniline, C16H14N2O3, (2a), N-{(E,2E)-3-
[4-(dimethylamino)phenyl]-2-propenylidene}-3-nitroaniline,
C17H17N3O2, (1b), N-{(E,2E)-3-[4-(dimethylamino)phenyl]-2-
propenylidene}-4-nitroaniline, C17H17N3O2, (2b), and N-
{(E,2E)-3-[4-(dimethylamino)phenyl]-2-propenylidene}-2-
methyl-4-nitroaniline, C18H19N3O2, (3b), have been structu-
rally characterized. A stack structure is observed for (1a) and
(1b) in the crystal phase. Experimental and calculated
molecular structures are discussed for these compounds which
belong to a chemical class having potential applications as
non-linear optical materials.
Because of the structural characteristics of the Schiff base
products (i.e. electron donor and acceptor groups connected
to a ꢁ-conjugated chain), they will have potential as NLO or
electro-optical materials. Quantum chemical calculations of
average molecular hyperpolarizabilities (ꢀ) support this
conclusion. It can be seen from Table 2 that the ꢀ values
increase in the series (1) ! (4) and (a) ! (b), in agreement
with the activity of donor and acceptor groups in these
compounds.
In each case, the color of the product in comparison with
that of the starting material indicated that the expected
chemical reaction had taken place. In all cases, we also
observed precipitation of crystalline products. We were able to
obtain suitable single crystals for all products except for (3a),
where the crystals were too small. The results of structure
investigations indicated that the conditions we used for the
condensation reactions were not favorable for obtaining the
Schiff base for adducts (4a) and (4b). From the ®rst of these
reactions, a molecular complex (4-a) was observed while, in
the second, one of the starting materials (b) was identi®ed.
The structures of these two compounds are described in a
following paper (Nesterov et al., 2000).
Bond-length alternation is observed in the central
ꢁ-conjugated chain of all ®ve molecules (1a), (2a), (1b), (2b)
and (3b) (see Fig. 1, respectively, and Table 1). Similar alter-
nations were found previously in Schiff bases with different
substituents (Cl, Br or Me) on one of the phenyl rings (Karaev
& Furmanova, 1984; Childs et al., 1989; Ercan et al., 1996). The
C1ÐN1, N1 C7 and C8ÐC9 bond lengths in the chains are
similar both in the ®ve molecules studied here (Table 1) and in
Comment
Combinatorial chemistry is a new ®eld of synthetic chemistry
which leads to fast synthesis of a wide variety (libraries) of
products for particular applications. The recent success of
combinatorial methods in the ®eld of drug discovery (Tarby et
al., 1996) has triggered exploration in the ®eld of materials
such as inorganic luminescent materials and catalysts
(Danielson et al., 1997; Shimizu et al., 1997).
There are two approaches in combinatorial synthesis which
lead to either diverse or focused combinatorial libraries.
Recently, we started an investigation of the combinatorial
synthesis procedure for the rapid formation of large libraries
of polar compounds (a focused library) which can be used for
preparation of non-linear optical (NLO) materials (Zyss et al.,
1994). We performed quantum chemical evaluations of the
®rst-order molecular hyperpolarizabilities (ꢀ) of the
compounds in question before undertaking the synthesis of a
small combinatorial library. Since the crystal structure is a very
important characteristic of such materials, we performed
X-ray analyses of the products in all cases where we were able
ꢀ
Acta Cryst. (2000). C56, 971±975
# 2000 International Union of Crystallography
Printed in Great Britain ± all rights reserved 971

organic compounds
four analogous molecules (Karaev & Furmanova, 1984; Childs
et al., 1989; Ercan et al., 1996). In contrast, the C7ÐC8 and
C9ÐC10 distances in (1b) and (3b) are considerably shorter
than those observed in all other compounds. It should be
noted that phenyl rings with a dimethylamino substituent, as
seen in (1b) and (3b), exhibit noticeable quinoid character
Figure 1
Views of the ®ve title Schiff bases with the
non-H atoms shown with displacement
ellipsoids drawn at the 50% probability
level. H atoms are drawn as circles of
arbitrary small radii for clarity.
ꢀ
972 Vladimir N. Nesterov et al. Five Schiff bases
Acta Cryst. (2000). C56, 971±975

organic compounds
1±3 min. The precipitates were isolated and recrystallized from
ethanol (30 ml) in the presence of a small amount of acetic acid.
Yields: (1a) 78, (1b) 74, (2a) 85, (2b) 82, (3a) 78 and (3b) 70%.
Crystals were obtained by isothermal evaporation from CH3CN
solutions. UV/vis (ꢂmax/nm, acetone solution): 381 (1a), 413 (1b), 365
(2a), 444 (2b), 355 (3a) and 431 (3b).
demonstrated by the shortening of the C11ÐC12 and C14Ð
. . .
C15 bond lengths (Table 1) compared to the standard C
Ê
C
distance of 1.398 A (Allen et al., 1987). The shortening of
these bonds is less pronounced in (1a), (2a) and (2b). Other
. . .
phenyl-ring bond lengths lie close to that of the standard C
C
bond. The quinoid character of the phenyl ring is actually
rather typical for rings bearing electron donor and acceptor
substituents in para positions (Domenicano, 1992), and this
feature is considered important in potential NLO compounds
(Zyss & Chemla, 1987).
Compound (1a)
Crystal data
3
Molecules of (2a), (2b) and (3b) (Fig. 1) are non-planar due
to rotation of both phenyl rings with respect to the central
planar molecular fragment, but the rotation of the nitrophenyl
ring is consistently greater than that of the other aromatic ring:
AM1 calculations (Dewar et al., 1985) reproduce the features
of the non-planarity in these molecules very well (Table 1). A
parallel observation was made for related compounds where
rotations of the aniline rings were about 40ꢁ (Karaev &
Furmanova, 1984; Childs et al., 1989; Ercan et al., 1996).
In contrast, molecules of (1b) (Fig. 1) are almost planar in
the solid state (Table 1). Since AM1 calculations predict non-
planarity of this molecule (Table 1), the in¯uence of crystal-
packing forces might be invoked to explain the discrepancy
between experimental and theoretical results. Indeed, mol-
ecules are stacked along [100] with a separation of only
C16H14N2O3
Mr = 282.29
Monoclinic, P21=n
Ê
a = 15.7883 (8) A
Dx = 1.391 Mg m
Mo Kꢃ radiation
Cell parameters from 5200
re¯ections
ꢄ = 2±24ꢁ
ꢅ = 0.098 mm
T = 100 (2) K
Ê
b = 3.9239 (2) A
1
Ê
c = 22.6063 (11) A
ꢀ = 105.7670 (10)ꢁ
3
Ê
V = 1347.80 (12) A
Z = 4
Square prism, yellow
0.3 Â 0.1 Â 0.1 mm
Data collection
Siemens SMART CCD area-
detector diffractometer
! scans
14 296 measured re¯ections
3938 independent re¯ections
2603 re¯ections with I > 2ꢆ(I)
Rint = 0.063
ꢄmax = 30.03ꢁ
h = 22 ! 20
k = 5 ! 5
l = 31 ! 31
Intensity decay: none
Re®nement
Re®nement on F2
R[F2 > 2ꢆ(F2)] = 0.054
wR(F2) = 0.126
S = 1.024
3925 re¯ections
246 parameters
All H-atom parameters re®ned
w = 1/[ꢆ2(Fo2) + (0.0889P)2]
where P = (Fo2 + 2Fc2)/3
(Á/ꢆ)max = 0.001
Ê
3.31 (1) A between the least-squares mean planes of neigh-
boring molecules. In addition, similar packing but in the
direction [010] is observed in the crystal of (1a). Consequently,
molecule (1a) is closer to planarity than (2a), (2b) and (3b)
(Table 1). Signi®cantly, such stacking is absent in the structures
of (2a), (2b) and (3b).
3
Ê
Áꢇmax = 0.50 e A
3
Ê
0.22 e A
Áꢇmin
=
Our results show the potential use of combinatorial synth-
esis for obtaining targeted libraries in materials chemistry.
Although, in this case, six Schiff base compounds from a small
array of eight possible products were synthesized, all ®ve
which were the subject of successful structure determination
were found to crystallize in centrosymmetric space groups.
Since in centrosymmetric crystals the superposition of
inverted tensors of the third rank which describe molecular
hyperpolarizability (ꢀ) is equal to zero, such crystals do not
generate second harmonics (Zyss & Chemla, 1987). This
precludes the application of bulk crystals or crystalline ®lms of
the compounds studied as NLO materials. On the other hand,
the centrosymmetric patterns might be easily destroyed by
introducing these compounds into polymer matrices doped by
an active NLO chromophore (Agullo-Lopez et al., 1994).
Thus, the title compounds might be utilized as active dopants
in polymer-based second harmonic generating, electrooptic or
photorefractive materials.
Compound (2a)
Crystal data
C16H14N2O3
Mr = 282.29
Orthorhombic, Pbca
Ê
a = 10.413 (2) A
Ê
b = 7.5990 (15) A
Mo Kꢃ radiation
Cell parameters from 24
re¯ections
ꢄ = 10±11ꢁ
1
ꢅ = 0.092 mm
T = 298 (2) K
Ê
c = 36.010 (7) A
Ê
V = 2849.4 (10) A
3
Plate, yellow
0.60 Â 0.40 Â 0.30 mm
Z = 8
Dx = 1.316 Mg m
3
Data collection
Enraf±Nonius CAD-4 diffract-
ometer
ꢄ/2ꢄ scans
3579 measured re¯ections
3400 independent re¯ections
1593 re¯ections with I > 2ꢆ(I)
Rint = 0.044
ꢄmax = 27.98ꢁ
h = 13 ! 0
k = 10 ! 0
l = 0 ! 47
2 standard re¯ections
every 98 re¯ections
intensity decay: 5%
Re®nement
Re®nement on F2
R[F2 > 2ꢆ(F2)] = 0.050
wR(F2) = 0.106
S = 1.159
3346 re¯ections
w = 1/[ꢆ2(Fo2) + (0.0427P)2
+ 1.1439P]
where P = (Fo2 + 2Fc2)/3
(Á/ꢆ)max = 0.001
Experimental
3
Ê
Áꢇmax = 0.13 e A
All Schiff bases were obtained by the reaction of 4-methoxy-
cinnamaldehyde, (a), or 4-dimethylaminocinnamaldehyde, (b)
(0.005 mol), with amines (1)±(4) (0.005 mol) in the presence of a
catalytic amount of acetic acid in ethanol (20 ml) under re¯ux for
3
Ê
0.15 e A
247 parameters
All H-atom parameters re®ned
Áꢇmin
=
Extinction correction: SHELXTL-
Plus (Sheldrick, 1994)
Extinction coef®cient: 0.0137 (14)
ꢀ
Acta Cryst. (2000). C56, 971±975
Vladimir N. Nesterov et al. Five Schiff bases 973

organic compounds
Compound (1b)
Data collection
Enraf±Nonius CAD-4 diffract-
ometer
ꢄ/2ꢄ scans
4620 measured re¯ections
4007 independent re¯ections
1534 re¯ections with I > 2ꢆ(I)
Rint = 0.031
ꢄmax = 28.96ꢁ
Crystal data
h = 0 ! 8
C17H17N3O2
Mr = 295.34
Triclinic, P1
Z = 2
Dx = 1.299 Mg m
Mo Kꢃ radiation
k = 9 ! 9
3
l = 23 ! 23
2 standard re¯ections
every 98 re¯ections
intensity decay: 5%
Ê
a = 6.7910 (14) A
Ê
Cell parameters from 24
re¯ections
b = 10.204 (2) A
Ê
c = 12.185 (2) A
ꢄ = 10±11ꢁ
ꢅ = 0.087 mm
T = 295 (2) K
1
ꢃ = 113.20 (3)ꢁ
ꢀ = 98.97 (3)ꢁ
ꢈ = 95.53 (3)ꢁ
Re®nement
Re®nement on F2
R[F2 > 2ꢆ(F2)] = 0.062
wR(F2) = 0.107
S = 1.224
3955 re¯ections
267 parameters
All H-atom parameters re®ned
w = 1/[ꢆ2(Fo2) + (0.0454P)2]
where P = (Fo2 + 2Fc2)/3
(Á/ꢆ)max = 0.001
Plate, yellow
0.60 Â 0.30 Â 0.30 mm
3
Ê
V = 754.9 (3) A
Data collection
3
Ê
Áꢇmax = 0.16 e A
3
Enraf±Nonius CAD-4 diffract-
ometer
ꢄ/2ꢄ scans
2899 measured re¯ections
2650 independent re¯ections
1255 re¯ections with I > 2ꢆ(I)
Rint = 0.047
ꢄ
max = 24.98ꢁ
Ê
0.17 e A
Áꢇmin
=
h = 0 ! 8
k = 12 ! 12
l = 14 ! 14
Compound (3b)
2 standard re¯ections
every 98 re¯ections
intensity decay: 5%
Crystal data
C18H19N3O2
Mr = 309.36
Triclinic, P1
a = 8.438 (2) A
Ê
b = 9.609 (2) A
Z = 2
Dx = 1.260 Mg m
Mo Kꢃ radiation
3
Re®nement
Re®nement on F2
R[F2 > 2ꢆ(F2)] = 0.040
wR(F2) = 0.094
S = 1.135
2595 re¯ections
201 parameters
H-atom parameters constrained
w = 1/[ꢆ2(Fo2) + (0.0578P)2]
where P = (Fo2 + 2Fc2)/3
(Á/ꢆ)max = 0.001
Ê
Cell parameters from 24
re¯ections
ꢄ = 10±11ꢁ
ꢅ = 0.084 mm
T = 295 (2) K
Ê
c = 10.546 (2) A
1
ꢃ = 72.95 (3)ꢁ
ꢀ = 86.13 (3)ꢁ
ꢈ = 89.81 (3)ꢁ
3
Ê
Áꢇmax = 0.11 e A
3
Ê
0.14 e A
Áꢇmin
=
Plate, yellow
0.40 Â 0.20 Â 0.20 mm
3
Ê
V = 815.5 (3) A
Compound (2b)
Data collection
Crystal data
Enraf±Nonius CAD-4 diffract-
ometer
ꢄ/2ꢄ scans
3073 measured re¯ections
2858 independent re¯ections
1393 re¯ections with I > 2ꢆ(I)
Rint = 0.086
ꢄmax = 24.97ꢁ
C17H17N3O2
Mr = 295.34
Triclinic, P1
a = 6.1650 (12) A
Ê
b = 7.2730 (15) A
Z = 2
Dx = 1.296 Mg m
Mo Kꢃ radiation
h = 0 ! 10
3
k = 11 ! 11
l = 12 ! 12
Ê
Cell parameters from 24
re¯ections
2 standard re¯ections
every 98 re¯ections
intensity decay: 5%
ꢄ = 10±11ꢁ
ꢅ = 0.087 mm
T = 298 (2) K
Ê
c = 17.158 (3) A
1
ꢃ = 90.70 (3)ꢁ
ꢀ = 100.10 (3)ꢁ
ꢈ = 91.37 (3)ꢁ
Re®nement
Re®nement on F2
R[F2> 2ꢆ(F2)] = 0.080
wR(F2) = 0.201
S = 1.143
H-atom parameters constrained
w = 1/[ꢆ2(Fo2) + (0.1556P)2]
where P = (Fo2 + 2Fc2)/3
(Á/ꢆ)max = 0.001
Square prism, dark yellow
0.40 Â 0.20 Â 0.20 mm
3
Ê
V = 757.1 (3) A
3
Ê
Table 1
Comparison of geometric parameters and characteristics of planarity
2779 re¯ections
211 parameters
Áꢇmax = 0.24 e A
3
Ê
0.21 e A
Áꢇmin
=
ꢁ
Ê
(A, ) for molecules (1a), (2a), (1b), (2b) and (3b).
Table 2
Parameter
(1a)
(2a)
(1b)
(2b)
(3b)
Calculated values of ꢀ (10 51 C m3 V 2) for compounds (1a)±(4b).
N1ÐC7
N1ÐC1
C7ÐC8
C8ÐC9
C9ÐC10
C10ÐC11
C11ÐC12
C12ÐC13
C13ÐC14
C14ÐC15
C10ÐC15
1.286 (2) 1.266 (3)
1.409 (2) 1.411 (3)
1.441 (2) 1.450 (3)
1.348 (2) 1.328 (3)
1.459 (2) 1.462 (3)
1.405 (2) 1.391 (3)
1.376 (2) 1.370 (3)
1.401 (2) 1.379 (3)
1.394 (2) 1.384 (3)
1.389 (2) 1.378 (3)
1.400 (2) 1.386 (3)
1.261 (2) 1.274 (3) 1.281 (4)
1.413 (2) 1.409 (3) 1.404 (4)
1.423 (3) 1.439 (4) 1.415 (5)
1.337 (3) 1.333 (4) 1.336 (5)
1.429 (3) 1.459 (3) 1.439 (5)
1.396 (3) 1.394 (3) 1.390 (5)
1.366 (3) 1.375 (3) 1.354 (5)
1.397 (3) 1.401 (3) 1.422 (5)
1.404 (3) 1.408 (3) 1.398 (5)
1.366 (3) 1.370 (4) 1.352 (5)
1.394 (3) 1.392 (3) 1.409 (5)
(1a)
(2a)
(3a)
(4a)
(1b)
(2b)
(3b)
(4b)
ꢀ
77 (1) 107 (6) 101 (6) 143 (6) 134 (4) 173 (11) 172 (11) 217 (19)
The molecular structures of (1a)±(4b) were calculated by the AM1
semi-empirical quantum-chemical method (Dewar et al., 1985) with
full geometry optimization using the GAMESS program (Schmidt et
al., 1993). Calculations of average molecular hyperpolarizabilities (ꢀ
in 10
C7ÐN1ÐC1ÐC2²
C7ÐN1ÐC1ÐC2³
C8ÐC9ÐC10ÐC15²
C8ÐC9ÐC10ÐC15³
23.5 (2)
29.1
14.2 (2)
19.5
40.2 (4)
38.6
7.7 (4)
10.0
4.9 (3)
30.6
4.0 (4)
14.1
37.3 (4)
36.3
16.0 (4)
15.3
47.6 (5)
41.5
4.1 (6)
0.7
51
C m3 V 2) were carried out with the ®nite ®eld approach
using modi®ed MOPAC (AM1) and HYPER programs (Cardelino et
al., 1991, 1997).
In (1a), (2a) and (2b), H atoms were located from ÁF syntheses
and thereafter re®ned freely. For the remaining structures, (1b) and
(3b), methyl-H atoms were located in a ÁF synthesis and thereafter
re®ned as part of a rigid rotating group, while others were placed in
geometrically calculated positions and re®ned using a riding model.
R.m.s. deviation§
Maximum deviation} 0.214 (1)
0.1000
0.285
0.076
0.548 (3) 0.135 (2) 0.700 (2)
0.437
0.382
0.647 (3)
² According to X-ray data. ³ According to AM1 calculations. § For non-H atoms
and excluding substituents (±NO2, ±CH3, ±NMe2). } Maximum deviation from the
molecular plane is observed for N1 in (1a) and for C5 in the other compounds.
ꢀ
974 Vladimir N. Nesterov et al. Five Schiff bases
Acta Cryst. (2000). C56, 971±975

organic compounds
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We thank the following for support: NASA Alliance for
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Supplementary data for this paper are available from the IUCr electronic
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