Self-organization of chiral self-complementary europium complex with a new achiral tripod-type ligand: Synthesis and stereochemistry effects to complex luminescence properties

Article abstract of DOI:10.1016/j.molstruc.2007.03.008

Solid complexes of europium picrate with the ligands: 2-(bis-dibutylcarbamoylmethyl-amino)-N,N-dibutyl-acetamid (L₁), 2-{bis-[(ethyl-phenyl-carbamoyl)-methyl]-amino}-N-ethyl-N-phenyl-acetamide (L₂) and 2-(bis-diethylcarbamoyl methyl-amino)-N,N-diethyl-acetamide (L₃) were prepared and characterized by elemental analysis, conductivity measurements and IR spectra. For the [Eu(pic)₃L₁], X-ray measurements show that the screw coordination arrangement of the achiral tripod-type ligand (L₁) around the Eu(III) ion induces the chirality of clockwise (C) and anticlockwise (A) molecular structures. And, the luminescence properties of tripod-type ligands with Eu(III) ion were affected by the complex stereochemistry.

Full text of DOI:10.1016/j.molstruc.2007.03.008

Available online at www.sciencedirect.com
Journal of Molecular Structure 873 (2008) 89–93
www.elsevier.com/locate/molstruc
Self-organization of chiral self-complementary europium complex
with a new achiral tripod-type ligand: Synthesis and
stereochemistry effects to complex luminescence properties
a
a
a,
b
Jiangrong Zheng , Nan Yang , Weisheng Liu *, Kaibei Yu
a
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
b
Chengdu Center of Analysis and Measurement, Academia Sinica, Chengdu 610041, China
Received 18 December 2006; received in revised form 5 March 2007; accepted 6 March 2007
Available online 13 March 2007
Abstract
Solid complexes of europium picrate with the ligands: 2-(bis-dibutylcarbamoylmethyl-amino)-N,N-dibutyl-acetamid (L1), 2-{bis-
[(ethyl-phenyl-carbamoyl)-methyl]-amino}-N-ethyl-N-phenyl-acetamide (L2) and 2-(bis-diethylcarbamoyl methyl-amino)-N,N-diethyl-
acetamide (L3) were prepared and characterized by elemental analysis, conductivity measurements and IR spectra. For the [Eu(pic)3L1],
X-ray measurements show that the screw coordination arrangement of the achiral tripod-type ligand (L1) around the Eu(III) ion induces
the chirality of clockwise (C) and anticlockwise (A) molecular structures. And, the luminescence properties of tripod-type ligands with
Eu(III) ion were affected by the complex stereochemistry.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Europium picrate; Chirality; Tripod-type; Crystal structure; Luminescence properties
1. Introduction
To investigate the self-organization of chiral self-comple-
mentary lanthanide complexes with achiral tripod-type
ligands and the stereochemistry effects on complex lumines-
cence properties, the europium(III) complexes of three tri-
pod-type ligands involving different terminal groups with
the formulas [Eu(pic)3L1] (1), [Eu(pic)3L2] (2) and
[Eu(pic)3L3] (3) were synthesized, where L1 = 2-(bis-dibu-
tylcarbamoylmethyl-amino)-N,N-dibutyl-acetamide, L2=
2-{bis-[(ethyl-phenylcarbamoyl)-methyl]-amino}-N-ethyl-N-
phenyl-acetamide and L3 = 2-(bis-diethylcarbamoylmethyl-
mino)-N,N-diethyl-acetamide.
Since the historic discovery of spontaneous resolution in
ammonium sodium tartrate by Louis Paster, chirality has
been an important topic in chemistry, pharmacy, and living
organisms. Chirality was expressed at the molecular and
supramolecular levels [1–5]. The initial progress in chirality
was in the generation of molecular chirality from the reac-
tion of achiral components, which developed into assem-
bling isolated chiral molecules [6]. Although the law of
physics under specific conditions determines the formation
of chiral molecules, these laws are not yet fully understood
[7]. To realize the synthetic strategy, we have focused on
metal complexes with tripod-type ligands, because the
coordination bonding interactions is regarded as the most
important driving forces in generating chiral compounds.
2. Experimental
2.1. Materials
All the chemical reagents used in this work were analyt-
ical grade. The europium picrate salt [8] and L1–3 [9,10]
were prepared according to literature methods.
*
Corresponding author. Tel.: +86 931 8915151; fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W. Liu).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.03.008

90
J. Zheng et al. / Journal of Molecular Structure 873 (2008) 89–93
Table 1
2.2. Chemical and physical measurements
Crystal data and structure refinement for the complex [Eu(pic)3L1]
Empirical formula
Temperature (K)
Crystal color
C48H66N13O24Eu
293
Yellow
0.20 · 0.20 · 0.30
2738.22
Monoclinic
P21/c
The metal ions were determined by EDTA titration
using xylenal orange as an indicator. C, N and H were
determined using an Elementar Vario EL instrument. IR
spectra were recorded on a Nicolet Avatar 360 FT-IR
instrument using KBr discs in the 400–4000 cmꢀ1 region.
Conductivity measurements were carried out with a
DDSJ-308 type conductivity bridge using 10ꢀ3 mol dmꢀ3
solutions in acetone at 25 ꢁC. Luminescence measurements
were made on a Hitachi F-4500 spectrofluorophotometer
equipped with quartz curettes of 1 cm path length. The
excitation and emission slit widths were 5.0 nm. Time-
resolved fluorescence measurements were performed at
77 K using a Nd-YAG pumped dye laser with RG 10 dye
as the excitation source and a Spex 1403 double grating
monochromater. X-ray measurements were performed on
a P4 four-circle diffractometer with graphite-monochroma-
tized Mo Ka radiation at 293 K, using the x/2h scan mode.
Lorentz and polarization corrections were applied, and
empirical absorption correction was made. A summary of
crystallographic data and details of the structure refine-
ment are listed in Table 1. The structure was solved by
Crystal size (mm3)
Formula weight
Crystal system
Space group
Unit cell dimension
˚
a (A)
23.2890 (10)
19.8820 (10)
28.9870 (2)
90
112.690 (10)
90
12383.1 (15)
4
1.469
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Density (calculated) (g/cm3)
F (000)
5636
0.71073
Radiation, graphite-monochromatized, k
˚
(A)
l (Mo Ka) (cmꢀ1
)
1.101
h Range for data collection
R (all data)
1.80–27.00
R1 = 0.0577,
wR2 = 0.1610a
ꢀ1.17, 1.51
ꢀ3
˚
Largest difference peak and hole [e A
]
a
w = 1/[r2(Fo)2 + (0. 0800p)2 + 1.9500p] where p ¼ ðF o2 þ 2F 2c Þ=3.
R1
O
N
to give transparent yellowish crystals. Yield: 87%. m.p.
73 ꢁC. Anal. data, calc. for C48H66N13O24Eu: C, 42.36;
H, 4.89; N, 13.38; Eu, 11.17. Found: C, 41.96; H, 4.53;
N, 13.16; Eu, 11.35. Main IR (cmꢀ1): 1613 (mC@O), 1275
(mCAO), 1576, 1544 (mas-NO2), 1362, 1324 (ms-NO2) (KBr)
Table 2.
R1
R2
R2
N
N
O
R1
R2
2 was prepared in the similar manner except 2-{bis-
[(ethyl-phenyl-carbamoyl)-methyl]-amino}-N-ethyl-N-phe-
nyl-acetamide (L2) was used. Yield: 91%. m.p. 89 ꢁC. Anal.
data, calc. for C48H42N13O24Eu: C, 43.12; H, 3.17; N,
13.62; Eu, 11.37. Found: C, 43.23; H, 3.33; N, 13.51; Eu,
11.61. Main IR (cmꢀ1): 1615 (mC@O), 1278 (mCAO),
1576, 1542 (mas-NO2), 1365, 1323 (ms-NO2) (KBr).
N
O
L1: R1, R2 = butyl
L2: R1= phenyl, R2= ethyl
L3: R1, R2= ethyl
direct methods and refined by full-matrix least-square tech-
niques with all nonhydrogen atoms treated anisotropically.
All calculations were performed on an Eclipse S/140 com-
puter with the program package SHELXTL [11] version
5.10.
Table 2
˚
Selected bond lengths (A) and bond angles (ꢁ) for [Eu(pic)3L1] (1)
Eu(1)AO(1)
Eu(1)AO(2)
Eu(1)AO(3)
Eu(1)AO(4)
Eu(1)AO(5)
Eu(1)AO(11)
Eu(1)AO(17)
Eu(1)AO(18)
2.383 (4)
2.373 (4)
2.455 (4)
2.373 (4)
2.627 (4)
2.320 (4)
2.604 (4)
2.293 (4)
2.770 (4)
110.14 (4)
105.92 (4)
106.28 (4)
115.92 (7)
112.50 (7)
114.39 (7)
Eu(2)AO(28)
Eu(2)AO(29)
Eu(2)AO(30)
Eu(2)AO(31)
Eu(2)AO(37)
Eu(2)AO(38)
Eu(2)AO(45)
Eu(2)AO(46)
2.424 (6)
2.348 (6)
2.389 (6)
2.394 (5)
2.601 (7)
2.250 (6)
2.307 (7)
2.690 (6)
2.742 (4)
99.75 (5)
110.04 (5)
110.85 (5)
116.91 (8)
112.28 (7)
119.44 (8)
2.3. General preparation of 1, 2 and 3
1: Solution of 2-(bis-dibutylcarbamoylmethyl-amino)-
N,N-dibutyl-acetamide (L1) (0.2 mmol, in 10 ml of ethanol)
was added dropwise to the solution of europium picrate
salt (0.2 mmol, in 5 ml of ethanol). The mixture was stirred
at room temperature for 8 h. The yellowish precipitated
solid complex was filtered, washed with ethanol three times
and dried in vacuo over P4O10 for 48 h. The solid 1 was
recrystallized from CH3OH solution by slow evaporation
Eu(1)AN(4)
Eu(2)AN(17)
Eu(1)AN(4)AC(1)
Eu(1)AN(4)AC(3)
Eu(1)AN(4)AC(5)
N(4)AC(1)AC(2)
N(4)AC(3)AC(4)
N(4)AC(5)AC(6)
Eu(2)AN(17)AC (49)
Eu(2)AN(17)AC(51)
Eu(2)AN(17)AC(53)
N(17)AC(49)AC(50)
N(17)AC(51)AC(52)
N(17)AC(53)AC(54)

J. Zheng et al. / Journal of Molecular Structure 873 (2008) 89–93
91
3 was prepared as similar to 1 and 2 except 2-(bis-diet-
hylcarbamoylmethyl-amino)-N,N-diethyl-acetamide (L3)
was used. Yield: 78%. m.p. 86 ꢁC. Anal. data, calc. for
C36H42N13O24Eu: C, 36.25; H, 3.55; N, 15.27; Eu, 12.74.
Found: C, 36.21; H, 3.50; N, 15.21; Eu, 12.53. Main IR
(cmꢀ1): 1611 (mC@O), 1273 (mCAO), 1577, 1540 (mas-NO2),
1362, 1321 (ms-NO2) (KBr).
3. Results and discussion
3.1. Properties of the complexes
Analytical data for the complexes conform to a 1:3:1
metal-to-picrate-to-L stoichiometry [Eu(pic)3L1–3]. All
complexes are soluble in DMSO, DMF, THF, CHCl3,
CH3CN, methanol and acetone, ethyl acetate, and spar-
ingly soluble in benzene, Et2O and cyclohexane. Conduc-
tivity measurements for these complexes in acetone
solution indicate that three complexes are monelectrolytes,
implying that all of the picrate groups coordinate to Eu(III)
in each complex [12].
Fig. 1. Molecular structures of the C (left) and A (right) and their
schematic representations, in which the curve line from the central tertiary
amine to the coordination oxygen atom through the methylene is
represented by the arrow. The dibutyl groups have been omitted for
clarity.
3.2. IR spectra
Free L1–3 show band at about 1642 cmꢀ1 which may be
assigned to m(C@O). In the IR spectra of complexes, the
bands of m(C@O) of free L1–3 shift about 27–31 cmꢀ1
towards lower wave numbers, indicating the CAO groups
take part in coordination to the metal ions. The OH out-
of-plane bending vibration of the free Hpic at 1151 cmꢀ1
disappears, indicating that the H-atom of the OH group
is replaced by Eu(III). The vibration m(CAO) of the Picꢀ
at 1265 cmꢀ1 is shifted toward higher frequency by 8–
13 cmꢀ1 in the complexes. This is due to the following
two effects. First, the hydrogen atom of the OH group is
replaced by Eu(III), increasing the p-bond character in
the CAO bond. Secondly, coordination of the nitrogen
atom of L–Eu(III) decreases the p-character [13]. Further-
more, the characteristic frequencies (1342, 1555 cmꢀ1) of
free picrate were divided into two double peaks (about
1362, 1325 and 1576, 1545 cmꢀ1, respectively), indicating
that the picrate groups coordinate to the metal ions as
bidentate ligands [14].
angles of N(17)AC(49)AC(50)AO(28), N(17)AC(51)A
C(52)AO(29), and N(17)AC(53)AC(54)AO(30) are
22.82ꢁ, 20.31ꢁ and 16.46ꢁ. While in structure A, the torsion
angles of N(4)AC(1)AC(2)AO(1), N(4)AC(3)AC(4)AO(2)
and N(4)AC(5)AC(6)AO(3) are ꢀ5.17ꢁ, ꢀ27.44ꢁ and
ꢀ10.21ꢁ, respectively.
The complex packing diagram shows that the C and A
structures coexist in the crystal. There is no intermolecular
hydrogen-bond extended structure. In the crystal lattices of
the picrate salt complex (L1), the picrate ions are located in
the cavities, and the homochiral layer consisting of C struc-
tures and the adjacent layer consisting of A structures are
well stacked by adopting the up-and-down layer’s shape
to give a 2D double-layer structure . . .CCCCC. . . and
. . .AAAAA. . . (Fig. 2). The similar structure of magnesium
complex with tripod-type ligand have been reported by He
[15]. Three chains of ligand have two different rounding
directions: C (clockwise) and A (anti clockwise). And, the
C and A structures coexist in the unit cell Fig. 3.
3.3. X-ray crystal structure
The molecular structures and schematic representations
of 1 were shown in Fig. 1, and revealed that each Eu center
binds to one nitrogen atom and three oxygen atoms from
the ligand L1 and five oxygen anions from picrate groups
where one picrate group is monodentate and the remaining
two are bidentate. The complex induces the chirality of C
and A structures due to the screw coordination arrange-
ment of the achiral tripod-type ligand. The values of the
angles formed by the monodentate picrate group and the
center europium ion are different indicating that structures
C and A are not enantiomers. In structure C, the torsion
3.4. The luminescence properties of the complexes
To investigate the influences of the complex fluorescence
by coordination stereochemistry, L2, L3 and their euro-
pium complexes were synthesized by the same methods.
L1–3 form the 1:3:1 (ligand-to-picrate group-to-europium
ion) complex with Eu(III) ion where L1–3 act as tetraden-
tate ligands, the remaining coordination sites are occupied
by picrate groups that could absorb light at k = 437 nm
and transfer energy to Eu(III) ion inducing the europium
characteristic emission at k = 578, 591 and 614 nm. This

92
J. Zheng et al. / Journal of Molecular Structure 873 (2008) 89–93
¦Ë=437nm
NO
2
O
N
N
O
2
N
2
2
O
N
O
NO
O
NO
O
N
2
2
O
O
O
R2
2
L1: R1, R2 = butyl
O
O
N
R2
L2: R1= phenyl, R2= ethyl
L3: R1, R2= ethyl
O
R1
R1
3+
O
N
Eu
R2
R1
N
N
O
λ=614nm
N
Scheme 1. The picrate groups are fluorophores of the complexes which
absorb light at k = 437 nm and induce the europium characteristic
emission.
Fig. 2. The packing diagram of 1 showing that the C and A are arrayed in
an up-and-down fashion to give
a 2D double-layer structure
(. . .CCCCC. . ., . . .AAAAA. . . and . . .CCCCC. . .). The synergistic picrate
groups and the terminal dibutyl groups have been omitted for clarity.
high-frequency NAH oscillators in the final europium com-
plex [16], three secondary amines have been introduced into
the ligand skeletons, which are then transformed into ter-
tiary amide connectors in complexes [17].
700
The terminal group effects of luminescence properties
were investigated by contrasting the luminescence intensity
of 1 with its derivatives (2 and 3). Compound 1 exhibits
about 60-fold luminescence intensity to that of 3 due to
its terminal substituent group’s flexibility and size. For
L1, L2 and L3, the flexibility in the order of L2 < L3 < L1
and the size of space that the terminal groups occupied in
the order of L3 < L1 < L2. The stereochemistry differences
of three terminal groups in final complexes lead to different
luminescence intensity due to their abilities in protecting
europium ions from quenching by high-energy vibration
molecules. These behaviors were also in good agreement
with the results obtained from measuring the luminescence
lifetimes of the complexes (s1: 0.45 ms, s2: 0.42 ms, s3:
0.17 ms, Fig. 4).
600
a
500
400
b
300
200
100
c
0
0.00000
0.00002
0.00004
0.00006
0.00008
0.00010
Concentration of complexes (mol/L)
Fig. 3. The relative luminescence intensity of complexes in CHCl3
solutions with different concentrations. (a) 1, (b) 2, (c) 3. The progressive
4. Summary and conclusions
concentration of complexes induced
a progressive increase in the
luminescence intensity at k = 614 nm. But a small quenching of complexes
luminescence is observed when the concentration reached about
6 · 10ꢀ5 mol/L. This behavior is ascribed to the intermolecular dynamic
quenching via energy transfer, and was found to obey strictly the Stern–
Volmer relationship along the investigated concentration range (up to
5 · 10ꢀ5 mol/L) [18].
In summary, the complex 1 induces the chirality of C
and A structures due to the screw coordination arrange-
ment of the achiral tripod-type ligand and in the unit cell,
the homochiral layer consisting of C structures and the
adjacent layer consisting of A structures are well stacked
by adopting the up-and-down layer’s shape to give a 2D
double-layer structure . . .CCCCC. . . and . . .AAAAA. . ..
Tripodal ligands are able to shield the encapsulated Eu3+
from interaction with the surroundings, and the introduc-
tion of more flexible and larger terminal substituents disfa-
vours the solvent molecule’s coordination to center Eu(III)
ions whose intermolecular energy transfer quench the com-
plex luminescent.
behavior can be explained considering that: (3) the tripodal
ligands L1–3 impose a trigonal cavity-like stereochemistry
on the Eu(III) ion, leaving vacant axial positions. (33) the
oxygen anions of three picrate acids occupy these vacant
positions and transfer electronic energy to Eu(III) ion.
(333) L1–3 coordinate to the metallic site and prevent the
attack from solvent molecules. Scheme 1. shows the mech-
anism of producing the europium characteristic lumines-
cent. Contrast to common series of tetramine ligands,
5. Supplementary data
L1–3 have only one ACH2A between the apex nitrogen atom
and amide group that reducing the size of cavities among
three coordination chains, which improve the ligand ability
to protect the metal center from attacking by solvent mole-
cules. To limit the quenching of Eu(III) luminescence by
Crystallographic data for the structures reported in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Center, CCDC No. 214441. Copies of this
information may be obtained free of charge from the

J. Zheng et al. / Journal of Molecular Structure 873 (2008) 89–93
93
2
Ex 420nm 0.02mJ/p
Ex 420nm 0.02mJ/p
1
Data: row1_w59000
Model: ExpDec1
Data: row1_w6150
Model: ExpDec1
Equation: y = A1*exp(-x/t1) + y0
Weighting:
0.002
0.000
-0.002
-0.0006
-0.0008
-0.0010
-0.0012
Equation: y = A1*exp(-x/t1) + y0
Weighting:
y
No weighting
y
No weighting
Chi^2/DoF
= 2.0802E-9
R^2
= 0.71075
Chi^2/DoF
= 5.2688E-8
R^2
= 0.89422
y0
A1
t1
-0.001 8.9116E-6
0.00029
0.00045
0.00001
0.00005
y0
A1
t1
-0.00236
0.00297
0.00042
0.00004
0.00006
0.00002
-0.0006
0.0000
0.0006
time(s)
0.0012
0.0018
-0.0006
0.0000
0.0006
time(s)
0.0012
0.0018
Data: row_w5920
Model: ExpDec1
Equation: y = A1*exp(-x/t1) + y0
Weighting:
3
Ex 355nm
y
No weighting
-0.0024
Chi^2/DoF
= 2.4538E-9
R^2
= 0.60335
y0
A1
t1
-0.00275
0.00047
0.00017
2.9757E-6
0.00003
0.00002
-0.0027
0.000
0.002
time(s)
Fig. 4. The fluorescence lifetime of 1, 2 and 3. s1: 0.45 ms, s2: 0.42 ms, s3: 0.17 ms.
[5] T. Konno, T. Nagashio, K. Okamoto, J. Hidaka, Inorg. Chem. 31
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[6] I. Katsuki, Y. Motoda, Y. Sunatsuki, N. Matsumoto, T. Nakashima,
M. Kojima, J. Am. Chem. Soc. 124 (4) (2002) 629.
[7] (a) R. Kuroda, S.F. Mason, J. Chem. Soc. Dalton Trans. (1981)
1268;
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.
ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgements
(b) C.P. Brock, J.D. Dunitz, Chem. Mater. 6 (1994) 1118.
[8] D. Li, X. Gan, M. Tan, X. Wang, Polyhedron 23 (1996) 3991.
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We are grateful to the financial support from the NSFC
(Grants 20431010, 20371022 and 20021001), and the Key
Project (01170) of the Ministry of Education of China.
[11] SHELXTL Version 5. 10, Soft reference manual, Brucker Analytical
X-ray System, Madison, Wisconsin, WI, 1997.
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