New Schiff base zinc(II) complexes exhibiting second harmonic generation

Article abstract of DOI:10.1039/b104360a

Four bis(salicylaldiminato)zinc(II) complexes have been synthesized from four Schiff bases obtained by the condensation of chiral (R)-(+)-1-phenylethylamine with salicylaldehyde (1), 5-nitrosalicylaldehyde (2), 3,5-dichlorosalicylaldehyde (3) and 5-methox

Full text of DOI:10.1039/b104360a

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New Schiff base zinc(II) complexes exhibiting second harmonic
generation
Cara Evans† and Dominique Luneau*
Laboratoire de Chimie Inorganique et Biologique (UMR 5046), DRFMC, CEA-Grenoble,
1
7 rue des Martyrs, 38054 Grenoble cedex 09, France. E-mail: luneau@drfmc.ceng.cea.fr
Received 17th May 2001, Accepted 17th October 2001
First published as an Advance Article on the web 6th December 2001
Four bis(salicylaldiminato)zinc() complexes have been synthesized from four Schiff bases obtained by the
condensation of chiral (R)-(ϩ)-1-phenylethylamine with salicylaldehyde (1), 5-nitrosalicylaldehyde (2),
3
,5-dichlorosalicylaldehyde (3) and 5-methoxysalicylaldehyde (4). The complexes crystallize in the
noncentrosymmetric space groups P2 2 2 (1), P2 (2), and C2 (3, 4). The geometry around the zinc() metal
1
1
1
1
center is pseudo-tetrahedral with two oxygen and two nitrogen atoms from the ligands and has the Λ absolute
configuration. Powders of the four compounds exhibit second harmonic generation (SHG) of intensity
between that of 3-methyl-4-nitropyridine-1-oxide (POM) and N-(4-nitrophenyl)-(S)-prolinol (NPP).
Introduction
ture but not the NLO properties of compound 1 were pub-
23
lished several years ago. Our aim was to characterize the SHG
properties of 1 and to investigate the effect of varying its sub-
stituents [R = R = H (1); R = NO , R = H (2); R = R = Cl
1
–3
In the last decade a growing number of reviews and public-
ations have highlighted the utility of organometallic complexes,
in which organic chromophores are bound to metal centers,
1
2
1
2
2
1
2
(
3); R = OMe, R = H (4)] (Fig. 1). The choice of zinc() as the
4–20
1 2
for second harmonic generation (SHG).
Molecular polar-
metal center was guided by transparency considerations. An
important requirement for SHG applications is transparency in
a large spectral range in order to avoid absorption of the second
izabilites (β) are frequently larger for the metallic complex than
for the free chromophore (ligand) because of metal-to-ligand or
ligand-to-metal charge transfer and because of the involvement
24
harmonic. In this regard Zn(), Cd() and Hg() are interest-
7,8,10,12,18
of the orbitals on the metal.
Another appealing aspect
13–15,18
ing because of the lack of d–d transitions.
Furthermore,
for the design of nonlinear optical (NLO) properties is that
metal centers may serve as anchors in the engineering of three-
dimensional geometries (tetrahedral, octahedral . . .), giving rise
other work has shown that, from a series of complexes with
different metal centers, those with Zn() generally exhibit the
8,18
largest hyperpolarizability.
10,21
to octupolar molecules.
Moreover, the combination of
organic and inorganic elements affords materials of relatively
high mechanical and thermal stability, as is also observed for
organic chromophores in inorganic host matrixes.
22
Results
Second-order NLO properties
All compounds 1–4 give strong SHG signals when incident light
is from a 1.064 µm laser. The second harmonic intensities of all
four compounds are between those of POM and NPP and
increase as 2 < 1 < 3 < 4.
Description of the structures
Views of the molecular structures of compounds 2–4 are shown
in Fig. 2–4, respectively. Selected bond lengths (Å) and angles
(deg.) for compounds 1–4 are listed in Table 1. In the four
complexes 1–4 the Zn() metal center is in a pseudo-tetrahedral
environment with two bidentate Schiff base ligands (L1–4) act-
ing as chelates through their phenolate oxygen and azometh-
ine nitrogen atoms (Fig. 1–4). All compounds crystallize in
noncentrosymmetric space groups and exhibit the same con-
figuration around the metal center. When the geometry is
pseudo-tetrahedral, two different optical isomers, Λ and ∆ are
normally possible. However, for compounds 2–4 we found only
the Λ configuration. This configuration was also found for 1,
and circular dichroism spectra of this material in chloroform
have demonstrated that the Λ configuration is preponderant in
II
Fig. 1 Chemical structure of the complexes Zn (L ) (n = 1–4). R =
n
2
1
R = H (1); R = NO , R = H (2); R = R = Cl (3); R = OMe, R = H (4).
2
1
2
2
1
2
1
2
However, despite the large molecular hyperpolarizabilities of
many organometallic compounds, few organometallic crystals
exhibit SHG (nonzero second-order susceptibility χ ). This
is because the molecules are arranged centrosymmetrically in
the solid state. One possible way to control the crystallization
of organometallic complexes in noncentrosymmetric space
groups is to incorporate chiral chromophores. Following this
approach we report herein four zinc complexes (1–4) that
exhibit SHG. The complexes were obtained from chiral Schiff
bases as schematized in Fig. 1. The synthesis and crystal struc-
2
19
23
solution. The preference for the Λ(R,R) configuration in com-
pounds 1–4 is thought to be driven by the minimization of
steric hindrance between the two ligands in the coordination
†
Present address: Department of Chemical Engineering, Rm. 3357,
Engineering II, University of California Santa Barbara, Santa Barbara,
CA 93106, USA.
23
sphere.
DOI: 10.1039/b104360a
J. Chem. Soc., Dalton Trans., 2002, 83–86
This journal is © The Royal Society of Chemistry 2002
83

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nearly tetrahedral with a slight distortion toward a cis-planar
geometry.
2
crystallizes in the P2 monoclinic space group. As for 1, the
1
asymmetric unit comprises two different molecules, A and B.
The dihedral angles between the two planes O ZnN1 and
1
O ZnN2 are 80.47Њ and 94.61Њ, respectively, in molecule A
2
and molecule B. The conformation around the zinc is nearly
tetrahedral with distortion toward a cis-planar geometry for
molecule A and a trans-planar geometry for molecule B.
3
crystallizes in the C2 monoclinic space group. The asym-
metric unit is comprised of a half molecule with the zinc()
metal ion located on the two-fold axis bisecting the N–Zn–N
and O–Zn–O angles. The dihedral angle between the two planes
O ZnN is 77.07Њ. The conformation around the zinc has a
1
1
strong distortion toward a cis-planar geometry.
crystallizes in the C2 monoclinic space group. As in the case
4
of 3, the asymmetric unit comprises half a molecular complex.
The Zn metal ion is in a special position on the two fold axis
bisecting the N–Zn–N and O–Zn–O angles. The dihedral
angle between the two planes O ZnN is 84.67Њ, corresponding
1
1
to a slight distortion of the geometry from tetrahedral toward
cis-planar.
Spectroscopic properties
II
The UV-visible spectra of the solids (KBr pellets) and solutions
Fig. 2 View of the molecular structures A and B for complex Zn (L )
2
2
(
2). Ellipsoids are drawn at the 30% probability level.
(CH
absorption maxima for the lowest optical transitions are below
00 nm (Table 2). Compound 2 is blue-shifted compared to 1
2 2
Cl , acetone) of complexes 1–4 are very similar. The
4
because of the acceptor effect of the nitro substituent. The
maxima of 3 and 4 are red-shifted relative to 1 because the
chloro or methoxy substituents act as donors in addition to
the phenylato oxygen.
Discussion
All compounds 1–4 give strong SHG signals when illuminated
3ϩ
with a Nd : YAG laser (λ = 1.064 µm). Second harmonic
signal intensities are between those of POM and NPP and
increase as 2 < 1 < 3 < 4. This trend is surprising; we had
expected compound 2 to give the strongest SHG signal. The
intensity of the second harmonic depends on the structure of
both the molecules and the crystals they form. In the ligand of
II
Fig. 3 View of the molecular structure for complex Zn (L ) (3).
3
2
Ellipsoids are drawn at the 30% probability level.
1
there is no substituent, and charge transfer (CT) is along
the O(phenolato) CN(imino) direction. The substituents
NO , Cl, and OMe in compounds 2–4 tune the CT of 1. In
2
the ligand of 2 there are two CT directions: O
NO and
2
O
CN(imino). The large value of β calculated for the
25
4
-nitrophenolate anion would predict a significant contri-
bution to the hyperpolarizability of 2 from O NO CT.
However, its effect is weakened by the O CN CT. In the
2
chromophores of 3 and 4, on the other hand, the chloro and
methoxy substituents have cooperative effects for the charge
transfer directions, which are Cl
MeO CN and O CN (4).
CN and O
CN (3) and
In considering the effect of crystallization of the chromo-
phores, we assume that the Zn() ion has an identical impact on
the molecular hyperpolarizabilities of compounds 1–4 and that
its main role is structural anchorage of the chromophore. Then
only the polarizabilities of the chromophores and their orient-
ations in point groups 2 and 222 should be taken into account.
Compounds 2–4 crystallize in point group 2, for which four
II
Fig. 4 View of the molecular structure for complex Zn (L ) (4).
4
2
Ellipsoids are drawn at the 30% probability level.
2
coefficients dijk of the χ tensor are nonzero. 1, on the other
23
1
crystallizes in the P2 2 2 orthorhombic space group. The
hand, crystallizes in point group 222, for which only one coef-
1
1 1
2
26
asymmetric unit comprises two independent molecules, A and
B, which show slight differences in the Zn() coordination
sphere (Table 1). For both molecules there is a pseudo-C2 axis
running through the Zn() ion and bisecting the N–Zn–N
and O–Zn–O angles. In molecules A and B the dihedral
angles between the two planes O ZnN1 and O ZnN2 are
ficient of χ is nonzero. In the crystal structure of 2, the two
chromophores of molecule B are nearly in opposition; the angle
between the two O
NO charge transfer directions is 7Њ. Such
2
a situation may contribute to cancellation of the d coefficients
ijk
2
of the χ tensor. This factor, in addition to molecular charge
transfer considerations, may explain why 2 exhibits the lowest
1
2
8
1.7Њ and 84.9Њ, respectively, so that the geometry around Zn is
SHG signal.
8
4 J. Chem. Soc., Dalton Trans., 2002, 83–86

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Table 1 Selected bond lengths (Å) and angles (deg.) for 1–4 (# is for the symmetry related position Ϫx, y, Ϫz)
1
A
1B
2A
2B
3
4
Zn–O1
Zn–O2
Zn–N1
Zn–N2
1.907(4)
1.929(4)
1.992(4)
1.992(5)
1.916(4)
1.902(3)
1.996(4)
1.986(3)
1.924(2)
1.937(2)
2.017(2)
2.010(2)
1.927(2)
1.925(2)
1.999(3)
1.997(2)
1.918(7)
2.01(1)
1.914(2)
2.017(2)
O –Zn–O
117.7(2)
97.6(2)
111.6(2)
113.1(2)
114.84(9)
95.49(8)
104.8(1)
96.60(8)
1
2
O –Zn–O #
130.1(4)
93.1(3)
110.5(3)
117.3(1)
96.39(8)
113.00(8)
1
1
O –Zn–N
1
1
O –Zn–N #
1
1
2
1
2
O –Zn–N
112.0(2)
110.8(2)
94.6(2)
95.5(2)
97.1(2)
113.5(2)
126.6(2)
111.3(1)
109.39(9)
94.17(9)
132.70(9)
118.28(9)
117.2(1)
97.22(9)
122.6(1)
1
O –Zn–N
2
O –Zn–N
2
N –Zn–N
1
125.8(2)
2
N –Zn–N #
122.5(5)
122.3(1)
1
1
Table 2 Absorption maxima (λmax) of compounds 1–4 in the solid
for C H N O Zn: C, 59.66; H, 4.34; N, 9.28; O, 15.89; Zn,
30 26 4 6
phase (KBr pellets), and in acetone and dichloromethane (nm).
1
0.83. Found: C, 59.09; H, 4.07; N, 9.14; Zn, 10.43.
Ϫ1
Molecular extinction coefficients (εmax) are given in parenthesis (M
Ϫ1
cm
)
II
Zn (L ) , bis{2-[(R)-(؉)-1-phenylethyliminomethyl]-4,6-di-
3
2
chlorophenolato-N,O}zinc(II) (3). The same procedure as for (1)
afforded the condensation of (R)-1-phenylethylamine (0.48 g,
Solid
λmax
Acetone
λmax (εmax
CH Cl
2 2
λmax (εmax)
Complex
)
4
mmol) with 3,5-dichlorosalicylaldehyde (0.76 g, 4 mmol). As
for 2, slow cooling of the reaction mixture gave only a micro-
crystalline powder. Pale yellow parallelepiped single crystals
suitable for X-ray structure determination were obtained after
recrystallization from dichloromethane. Mp 240 ЊC. Anal. Calc.
for C H N O Cl Zn: C, 55.29; H, 3.71; N, 4.30; O, 4.91; Cl,
1
2
3
4
379
362
394
398
372 (8400)
361 (33000)
386 (13300)
394 (3100)
374 (8300)
361 (37000)
389 (13000)
395 (3000)
30
24
2
2
4
2
1.76; Zn, 10.03. Found: C, 54.66; H, 3.49; N, 4.29; Cl, 21.14;
Zn, 9.62.
Conclusion
II
This paper reports the synthesis and characterization of four
new Schiff base Zn() complexes that exhibit intense second
harmonic generation. As expected from the chirality of the
ligands, these materials crystallize in noncentrosymmetric
space groups. Only one optical isomer, Λ, is observed. The
compounds are light-colored and show a large range of trans-
parency above 400 nm. The various substituents tune the SHG
properties and also the thermal stability of the compounds, as
reflected by their melting points.
Zn (L ) , bis{2-[(R)-(؉)-1-phenylethyliminomethyl]-4-meth-
4
2
oxyphenolato-N,O}zinc(II) (4). The same procedure was used as
for (1) to accomplish the condensation of (R)-1-phenyl-
ethylamine (0.48 g, 4 mmol) with 5-methoxysalicylaldehyde
(0.61 g, 4 mmol). Single crystals suitable for X-ray structure
determination were obtained by slow cooling of the reaction
mixture. Mp 100 ЊC. Anal. Calc. for C H N O Zn: C, 66.96;
32
32
2
4
H, 5.62; N, 4.88; O, 11.15; Zn, 11.39. Found: C, 66.62; H, 5.44;
N, 4.99; Zn, 11.54.
Crystallography
Experimental
Data were collected at room temperature (298 K) with a Bruker
SMART CCD diffractometer equipped with a normal focus
X-ray tube having a molybdenum target. The data were pro-
cessed through the SAINT reduction software. Empirical
absorption corrections were carried out by SADABS. The
Synthesis of the complexes
All reagents and solvents were reagent grade and were used
without further purification. The complexes were synthesized
using procedures reported for 1.
27
28
23
2
structures were solved and refined on F using the SHELXTL
29
software. All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. The hydrogen atoms were included
in the final refinement model in calculated positions with iso-
tropic thermal parameters. Crystal structure and refinement
data for compounds 1–4 are summarized in Table 3.
CCDC reference numbers 165067–165069.
II
Zn (L ) , bis{2-[(R)-(؉)-1-phenylethyliminomethyl]phenolato-
1
2
N,O}zinc(II) (1). A solution of (R)-1-phenylethylamine (0.48 g,
mmol) and salicylaldehyde (0.48 g, 4 mmol) was refluxed in
ethanol (25 mL) for 2 h. To the resulting yellow solution were
added powdered NaHCO (0.34 g, 4 mmol) and zinc() acetate
dihydrate (0.44 g, 2 mmol), and the mixture was refluxed for 2 h.
Light yellow parallelepiped crystals were obtained by slow cool-
ing of the solution. Mp 190 ЊC. Anal. Calc. for C H N O Zn:
C, 70.11; H, 5.49; N, 5.45; O, 6.23; Zn, 12.72. Found: C, 70.28;
H, 5.43; N, 5.49; Zn, 12.90.
4
3
See http://www.rsc.org/suppdata/dt/b1/b104360a/ for crystal-
lographic data in CIF or other electronic format.
30
28
2
2
Optical measurements
The efficiency of the crystals for frequency doubling was
30
determined by the Kurtz and Perry powder test using a pulsed
II
3ϩ
Zn (L ) , bis{2-[(R)-(؉)-1-phenylethyliminomethyl]-4-nitro-
Nd : YAG laser (λ = 1.06 µm). Prior to measurement the
2
2
phenolato-N,O}zinc(II) (2). The same procedure as for (1)
afforded the condensation of (R)-1-phenylethylamine (0.48 g,
mmol) with 5-nitrosalicylaldehyde (0.67 g, 4 mmol). Contrary
to 1, slow cooling of the resulting solution gave only a micro-
crystalline powder. Pale yellow needle-like single crystals suit-
able for X-ray structure determination were obtained after
recrystallization from dichloromethane. Mp 226 ЊC. Anal. Calc.
crystalline compounds were ground into powders, sieved to
an average particle size between 250–300 µm, and packed into
pellets of 2 mm path length. The references used were urea,
POM [3-methyl-4-nitropyridine-1-oxide] and NPP [N-(4-nitro-
phenyl)-(S)-prolinol]. Absorption spectra of KBr pellets and of
solutions in acetone and dichloromethane were obtained on a
Varian Cary5E spectrometer.
4
J. Chem. Soc., Dalton Trans., 2002, 83–86
85

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Table 3 Summary of data collection and refinement for crystals 1–4
a
1
2
3
4
Formula
M
T /K
C H N O Zn
513.91
293(2)
C H N O Zn
603.92
298(2)
C H N O Cl Zn
651.68
298(2)
C H N O Zn
573.97
298(2)
3
0
28
2
2
30 26
4
6
30 24
2
2
4
32 32
2
4
Cryst. syst.
Space group
a/Å
b/Å
c/Å
Orthorhombic
Monoclinic
P21
Monoclinic
C2
Monoclinic
C2
P2 2 2
1
1 1
17.738(4)
29.632(9)
9.968(2)
90
5239(2)
8
15.4512(8)
10.1073(5)
18.0725(9)
90.541(1)
2822.3(2)
4
13.928(2)
10.949(2)
10.208(2)
118.246(3)
1371.4(4)
2
17.569(2)
8.758(8)
11.981(1)
130.283(1)
1406.4
2
β/deg.
3
V/Å
Z
µ/mm
d/g cm
Ϫ1
0.98
1.303
0.920
1.421
1.318
1.578
0.921
1.421
Ϫ3
λ/Å
R(F)
R (F )
w
0.71073
0.0725
0.71073
0.0311
0.0880
0.71073
0.0859
0.2006
0.71073
0.0312
0.0835
b
2
c
a
b
c
2
2
o
2
c
2
4 1/2
o
Crystal data obtained from ref. 23. I > 2σ(I ), R(F) = Σ||F | Ϫ |F ||/Σ|F |. All data, R (F ) = Σ[w(F Ϫ F ) /ΣwF ]
.
o
c
o
w
1
3 Y.-p. Tian, C.-y. Duan, C.-y. Zhao, X.-z. You, T. C. W. Mak and
Z.-y. Zhang, Inorg. Chem., 1997, 36, 1247.
Acknowledgements
This work was supported by the Centre National de la
Recherche Scientifique (CNRS), the Commissariat à l’Energie
Atomique (CEA), and the Université Joseph-Fourier. The
authors thank Dr. René Masse for encouragement and fruitful
discussions. C. E. is grateful for a Chateaubriand Fellowship
from the French government.
14 C. C. Evans, R. Masse, J. F. Nicoud and M. Bagieu-Beucher,
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5 H. Zhang, D. E. Zelmon, G. E. Price and B. K. Teo, Inorg. Chem.,
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1
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J. Chem. Soc., Dalton Trans., 2002, 83–86