Dinuclear rare-earth picrate complexes based on a multidentate amide ligand: Syntheses, crystal structures, and luminescence properties

Article abstract of DOI:10.1080/00958972.2012.731503

A new amide-based multidentate ligand, N,N′-1,2-ethanediyl-bis{2-[(N, N-diethylcarbamoyl)- methoxy]benzamide} (L) reacts with M(Pic) ₃·6H₂O to give rare-earth picrate complexes [M ₂L₂(Pic)₄(H₂O)₂](Pic) ₂ (M = La (1), Nd (2), Eu (3), Gd (4), Tb (5), Dy (6), Yb (7), Y (8)). X-ray single-crystal diffraction analyses indicate that dinuclear complexes 3·2C₄H₈O₂, 6·2C ₄H₈O₂, and 8·2CH₃CN are isomorphous. Each metal is nine-coordinate by four oxygen atoms of two ligands, four oxygen atoms of two bidentate picrates, and one water molecule with a distorted monocapped square antiprism. With hydrogen bonds between the free picrate anions and the coordination cations the complexes exhibit 2-D layers. The luminescent properties of 3 [Eu₂L₂(Pic) ₄(H₂O)₂](Pic)₂ are described and factors that influence luminescent intensities are also discussed.

Full text of DOI:10.1080/00958972.2012.731503

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Dinuclear rare-earth picrate complexes
based on a multidentate amide ligand:
syntheses, crystal structures, and
luminescence properties
Wei Wang a , Yong Huang a , Tong Li a , Guang-Nong Lu a , Ying
Yang a , Ning Tang a & Hai-Bin Song b
a Key Laboratory of Nonferrous Metals Chemistry and Resources
Utilization of Gansu Province, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou 730000, Gansu, P.R.
China
b The State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, P.R. China
Accepted author version posted online: 18 Sep 2012.Version of
record first published: 03 Oct 2012.
To cite this article: Wei Wang , Yong Huang , Tong Li , Guang-Nong Lu , Ying Yang , Ning Tang & Hai-
Bin Song (2012): Dinuclear rare-earth picrate complexes based on a multidentate amide ligand:
syntheses, crystal structures, and luminescence properties, Journal of Coordination Chemistry,
65:22, 4041-4053
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Journal of Coordination Chemistry
Vol. 65, No. 22, 20 November 2012, 4041–4053
Dinuclear rare-earth picrate complexes based on a multidentate
amide ligand: syntheses, crystal structures, and luminescence
properties
WEI WANGy, YONG HUANG*y, TONG LIy, GUANG-NONG LUy,
YING YANGy, NING TANG*y and HAI-BIN SONGz
yKey Laboratory of Nonferrous Metals Chemistry and Resources Utilization of
Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou 730000, Gansu, P.R. China
zThe State Key Laboratory of Elemento-Organic Chemistry, Nankai University,
Tianjin 300071, P.R. China
(Received 1 January 2012; in final form 27 August 2012)
A new amide-based multidentate ligand, N,N0-1,2-ethanediyl-bis{2-[(N,N-diethylcarbamoyl)-
methoxy]benzamide} (L) reacts with M(Pic)3 ꢀ 6H2O to give rare-earth picrate complexes
[M2L2(Pic)4(H2O)2](Pic)2 (M ¼ La (1), Nd (2), Eu (3), Gd (4), Tb (5), Dy (6), Yb (7), Y (8)).
X-ray single-crystal diffraction analyses indicate that dinuclear complexes 3 ꢀ 2C4H8O2,
6 ꢀ 2C4H8O2, and 8 ꢀ 2CH3CN are isomorphous. Each metal is nine-coordinate by four
oxygen atoms of two ligands, four oxygen atoms of two bidentate picrates, and one water
molecule with a distorted monocapped square antiprism. With hydrogen bonds between the
free picrate anions and the coordination cations the complexes exhibit 2-D layers. The
luminescent properties of 3 [Eu2L2(Pic)4(H2O)2](Pic)2 are described and factors that influence
luminescent intensities are also discussed.
Keywords: Rare-earth complex; Dinuclear complex; Crystal structure; Luminescence properties
1. Introduction
Rare-earth complexes attract attention due to their unique luminescence properties
(long, millisecond-order lifetime, large Stokes’ shift and narrow emission bands) [1] and
their important applications as labels and sensors in materials and probes in biological
systems [2–4]. However, rare-earth ions have poor direct light absorption as a result of
the parity-forbidden 4f–4f transitions [5]. As a result, luminescence intensity of rare-
earth complexes is strongly dependent on the efficiency of ligand absorption in the UV
region, the efficiency of ligand to metal energy transfer, and the efficiency of rare-earth
luminescence, the well-known ‘‘antenna effect’’ [6]. Rare-earth ions adopt a range of
coordination numbers between 6 and 12 to form flexible coordination geometries; the
photoluminescence intensity of rare-earth complexes shows hypersensitivity to
*Corresponding authors. Email: huangyong@lzu.edu.cn; tangn@lzu.edu.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.731503

4042
W. Wang et al.
coordination environment and molecular structures [1, 7]. Therefore, to synthesize an
ideal multidentate organic ligand which coordinates with rare-earth ions, improving the
absorption and forming new structures are a challenge. Bifunctional ligands such as
macrocycles [8, 9], podand-type ligands [10], carboxylic acid derivatives [11, 12], and
ꢀ-diketone derivatives [13, 14] have been synthesized and used as antennas to improve
the poor absorption of rare-earth ions. High denticity acyclic polyethers encapsulate the
metal ion and protect it from solvent interactions, offering many advantages in
extraction and analysis of rare-earth ions [15]. Nevertheless, to achieve good
luminescence and optimal binding, it is also important to get the right balance between
flexibility and rigidity [16].
Recently, we have designed a series of functional acyclic polyethers having both
selective ability to coordinate rare-earth ions and enhanced luminescence of rare-earth
complexes, by selecting amide as conjugate groups and providing a protective
environment for the rare-earth ion. Also, picrates as the bulky counter anions have
been used to influence structures of rare-earth complexes and the triplet state energy
level of the ligands [17, 18]. As part of our systematic studies, a new, doubly
functionalized multidentate ligand, N,N0-1,2-ethanediyl-bis{2-[(N,N-diethylcarbamoyl)-
methoxy]benzamide} (L) and the corresponding rare-earth picrate complexes
(scheme 1) have been prepared. The coordination and luminescence properties of
rare-earth picrate complexes show that europium complex emits the intrinsic spectrum
of Eu(III) ions under excitation of ultraviolet light, and the solvents affect the
luminescence.
2. Experimental
2.1. Materials and methods
Rare-earth picrates M(Pic)3 ꢀ 6H2O [19], N,N0-1,2-ethanediyl-bis(2-hydroxybenzamide)
[20] and N,N-diethyl-2-chloroacetamide [21] were prepared according to literature
methods. Other reagents purchased commercially were of A.R. grade and used without
purification. Carbon, hydrogen, and nitrogen analyses were obtained using an
Elementar Vario EL elemental analyzer. The rare-earth ions were determined by
EDTA titration using xylenol orange as indicator. Conductivity measurements were
carried out in 10ꢁ3 mol ꢀ cmꢁ3 solutions at 25ꢂC using a DDS-307 conductivity meter.
IR spectra from 4000 to 400 cmꢁ1 were measured using KBr pellets on a Nicolet Nexus
1
670 FTIR spectrometer. H NMR spectra were recorded on a Varian Mercury 400 BB
Scheme 1. The synthetic route of L and rare-earth picrate complexes 1–8.

Rare earth picrates
4043
using CDCl3 as solvent and TMS as internal standard. The excitation and luminescence
spectra were performed on a Hitachi F-4500 spectrophotometer.
2.2. Synthesis of the ligand L
N,N0-1,2-ethanediyl-bis(2-hydroxybenzamide) (2.40 g, 8.0 mmol) and sodium hydroxide
(0.64 g, 16.0 mmol) in DMF (100 cm3) were heated to 80ꢂC and stirred for 0.5 h. Then a
solution of N,N-diethyl-2-chloroacetamide (2.63 g, 17.6 mmol) and KI (catalytic
amount) in DMF (50 cm3) was added dropwise. The reaction mixture was stirred at
80ꢂC for an additional 15 h and then concentrated in vacuum. The resultant solid was
recrystallized with ethanol twice to give 2.95 g of colorless prism crystals. Yield: 70%;
m.p. 168–171ꢂC. 1H NMR (CDCl3, ppm): ꢁ 9.55 (s, 2H, –CO–NH–), 6.91–8.12 (m, 8H,
Ar–H), 4.90 (s, 4H, CH2–O), 3.83 (m, 4H, CH2–CH2–N), 3.27–3.34 (q, 8H, CH3–CH2–
N), 1.04–1.20 (d and t, 12H, –CH3). IR (KBr, cmꢁ1): 3305(s, ꢂ(N–H)), 3075(m, Ph–H),
2969(m, C–H), 2927(m, C–H), 1659(s, C¼O), 1627(s, C¼O), 1599(m, C¼C), 1540(m),
1487(m), 1248(m, Ar–O–C), 1107(m), 1068(m), 871(m), 758(m). Anal. Calcd (%) for
C28H38N4O6: C, 63.86; H, 7.27; N, 10.64; O, 18.23. Found (%): C, 63.83; H, 7.20;
N, 10.72; O, 18.25.
2.3. Syntheses of 1–8
A solution of 0.2 mmol rare-earth picrates M(Pic)3 ꢀ 6H2O (M ¼ La, Nd, Eu, Gd, Tb,
Dy, Yb, Y) in 10 cm3 of ethyl acetate was added to a solution of 0.2 mmol L in 10 cm3
of chloroform. The mixture was stirred for 6 h at room temperature. The precipitated
yellow solid was separated from the solution by suction filtration, purified by washing
several times with ethyl acetate and chloroform, and then dried for 24 h in a vacuum
over P4O10 at room temperature (yield 80%). Elemental analyses are in good agreement
with stoichiometries of all complexes.
2.3.1. [La2L2(Pic)4(H2O)2](Pic)2 (1). Anal. Calcd for C92H92La2N26O56 (%): C, 40.39;
H, 3.39; N, 13.31; O, 32.75; La, 10.16. Found (%): C, 40.46; H, 3.67; N, 13.12; O, 32.85;
La, 10.30. IR (KBr, cmꢁ1): 3325(m, ꢂ(O–H, N–H)); 3090(m, Ph–H), 2972(m, C–H),
2930(m, C–H), 1626(s, ꢂ(C¼O)), 1613(s, ꢂ(C¼O)); 1575(s, ꢂas(–NO2)),
1543(s, ꢂas(–NO2)); 1488(m), 1360(s, ꢂs(–NO2)), 1331(s, ꢂs(–NO2)); 1274(s, ꢂ(C–O));
1245(m, ꢂ(Ar–O–C)), 1221(m, ꢂ(Ar–O–C)), 1077(m), 1046(m), 845(m), 753(m), 710(m).
2.3.2. [Nd2L2(Pic)4(H2O)2](Pic)2 (2). Anal. Calcd for C92H92N26Nd2O56 (%): C,
40.24; H, 3.38; N, 13.26; O, 32.62; Nd, 10.50. Found (%): C, 40.13; H, 3.29; N, 13.43;
O, 33.04; Nd, 10.11. IR (KBr, cmꢁ1): 3320(m, ꢂ(O–H, N–H)); 3094(m, Ph–H), 2977(m,
C–H), 2935(m, C–H), 1623(m, ꢂ(C¼O)), 1614(s, ꢂ(C¼O)); 1574(s, ꢂas(–NO2)), 1541(s,
ꢂas(–NO2)); 1490(m), 1360(s, ꢂs(–NO2)), 1330(s, ꢂs(–NO2)); 1272(s, ꢂ(C–O)); 1246(m,
ꢂ(Ar–O–C)), 1220(m, ꢂ(Ar–O–C)), 1080(m), 1048(m), 845(m), 755(m), 710(m).
2.3.3. [Eu2L2(Pic)4(H2O)2](Pic)2 (3). Anal. Calcd for C92H92Eu2N26O56 (%): C,
40.01; H, 3.36; N, 13.19; O, 32.44; Eu, 11.00. Found (%): C, 39.96; H, 3.37; N,

4044
W. Wang et al.
13.26; O, 32.50; Eu, 10.91. IR (KBr, cmꢁ1): 3328(m, ꢂ(O–H, N–H)); 3094(m, Ph–H),
2977(m, C–H), 2935(m, C–H), 1628(m, ꢂ(C¼O)), 1614(s, ꢂ(C¼O)); 1574(s, ꢂas(–NO2)),
1540(s, ꢂas(–NO2)); 1490(m), 1361(s, ꢂs(–NO2)), 1330(s, ꢂs(–NO2)); 1272(s, ꢂ(C–O));
1247(m, ꢂ(Ar–O–C)), 1220(m, ꢂ(Ar–O–C)), 1080(m), 1048(m), 847(m), 755(m), 710(m).
2.3.4. [Gd2L2(Pic)4(H2O)2](Pic)2 (4). Anal. Calcd for C92H92Gd2N26O56 (%): C,
39.86; H, 3.34; N, 13.14; O, 32.32; Gd, 11.34. Found (%): C, 39.65; H, 3.67; N, 13.08;
O, 32.58; Gd, 11.02. IR (KBr, cmꢁ1): 3317(m, ꢂ(O–H, N–H)); 3088(m, Ph–H), 2970(m,
C–H), 2930(m, C–H), 1626(s, ꢂ(C¼O)), 1614(s, ꢂ(C¼O)); 1574(s, ꢂas(–NO2)), 1542(s,
ꢂas(–NO2)); 1488(m), 1360(s, ꢂs(–NO2)), 1330(s, ꢂs(–NO2)); 1272(s, ꢂ(C–O)); 1248(m,
ꢂ(Ar–O–C)), 1220(m, ꢂ(Ar–O–C)), 1078(m), 1047(m), 842(m), 754(m), 710(m).
2.3.5. [Tb2L2(Pic)4(H2O)2](Pic)2 (5). Anal. Calcd for C92H92N26O56Tb2 (%): C,
39.81; H, 3.34; N, 13.12; O, 32.28; Tb, 11.45%. Found (%): C, 39.54; H, 3.23; N,
12.95; O, 32.90; Tb, 11.38%. IR (KBr, cmꢁ1): 3318(m, ꢂ(O–H, N–H)); 3087(m, Ph–H),
2970(m, C–H), 2928(m, C–H), 1627(s, ꢂ(C¼O)), 1614(s, ꢂ(C¼O)); 1576(s, ꢂas(–NO2)),
1542(s, ꢂas(–NO2)); 1488(m), 1361(s, ꢂs(–NO2)), 1330(s, ꢂs(–NO2)); 1271(s, ꢂ(C–O));
1248(m, ꢂ(Ar–O–C)), 1221(m, ꢂ(Ar–O–C)), 1076(m), 1045(m), 846(m), 755(m), 711(m).
2.3.6. [Dy2L2(Pic)4(H2O)2](Pic)2 (6). Anal. Calcd for C92H92Dy2N26O56 (%):
C, 39.71; H, 3.33; N, 13.09; O, 32.19; Dy, 11.68. Found (%): C, 39.80; H, 3.17;
N, 13.01; O, 32.19; Dy, 11.83. IR (KBr, cmꢁ1): 3307(m, ꢂ(O–H, N–H)); 3088(m, Ph–H),
2970(m, C–H), 2928(m, C–H), 1626(s, ꢂ(C¼O)), 1615(s, ꢂ(C¼O)); 1573(s, ꢂas(–NO2)),
1545(s, ꢂas(–NO2)); 1487(m), 1360(s, ꢂs(–NO2)), 1330(s, ꢂs(–NO2)); 1274(s, ꢂ(C–O));
1248(m, ꢂ(Ar–O–C)), 1222(m, ꢂ(Ar–O–C)), 1078(m), 1046(m), 842(m), 752(m), 710(m).
2.3.7. [Yb2L2(Pic)4(H2O)2](Pic)2 (7). Anal. Calcd for C92H92N26O56Yb2 (%):
C, 39.41; H, 3.31; N, 12.99; O, 31.95; Yb, 12.34. Found (%): C, 39.45; H, 3.58;
N, 13.17; O, 32.31; Yb, 12.49. IR (KBr, cmꢁ1): 3312(m, ꢂ(O–H, N–H)); 3091(m, Ph–H),
2973(m, C–H), 2930(m, C–H), 1625(s, ꢂ(C¼O)), 1614(s, ꢂ(C¼O)); 1573(s, ꢂas(–NO2)),
1542(s, ꢂas(–NO2)); 1487(m), 1361(s, ꢂs(–NO2)), 1332(s, ꢂs(–NO2)); 1272(s, ꢂ(C–O));
1248(m, ꢂ(Ar–O–C)), 1221(m, ꢂ(Ar–O–C)), 1077(m), 1050(m), 845(m), 758(m), 709(m).
2.3.8. [Y2L2(Pic)4(H2O)2](Pic)2 (8). Anal. Calcd for C92H92N26O56Y2 (%): C, 41.93;
H, 3.52; N, 13.82; O, 33.98; Y, 6.75. Found (%): C, 41.67; H, 3.58; N, 13.68; O, 34.58;
Y, 6.49. IR (KBr, cmꢁ1): 3305(m, ꢂ(O–H, N–H)); 3094(m, Ph–H), 2975(m, C–H),
2932(m, C–H), 1627(s, ꢂ(C¼O)), 1615(s, ꢂ(C¼O)); 1573(s, ꢂas(–NO2)),
1543(s, ꢂas(–NO2)); 1488(m), 1361(s, ꢂs(–NO2)), 1331(s, ꢂs(–NO2)); 1273(s, ꢂ(C–O));
1248(m, ꢂ(Ar–O–C)), 1220(m, ꢂ(Ar–O–C)), 1078(m), 1051(m), 844(m), 759(m), 708(m).
2.4. Single-crystal structure determination
Determinations of the unit cell and crystallographic data collection were performed
˚
with graphite-monochromated Mo-Ka radiation (ꢃ ¼ 0.71073 A) at 293(2)/113(2) K on
a BRUKER SMART 1000 diffractometer equipped with a CCD camera. The !–’ scan

Rare earth picrates
4045
technique was employed. SAINT [22] was used for integration of the diffraction
profiles. The structures were solved primarily by direct methods and secondly by
Fourier difference techniques and refined by full-matrix least-squares using the
SHELXS program of the SHELXTL package [23]. Metals in each complex were
located from the E-maps, and other non-hydrogen atoms were located in successive
difference Fourier syntheses and refined with anisotropic thermal parameters on F2.
Hydrogen atoms were set in calculated positions and refined as riding with a common
fixed isotropic thermal parameter (hydrogen atoms of coordinated water were located
using the difference Fourier method).
3. Results and discussion
3.1. Syntheses of complexes
The synthetic routes for the multidentate amide L and 1–8 are shown in scheme 1. All
the picrate complexes are air stable, yellow powders, and soluble in DMSO, DMF,
methanol, ethanol, acetone, acetonitrile, and ethyl acetate, slightly soluble in diethyl
ether and chloroform, but insoluble in benzene and water. All the picrate complexes are
characterized by elemental and spectral analyses. The analytical data for the complexes
indicate that all conform to a 2 : 2 metal-to-ligand stoichiometry, which is in accord with
the results of crystal structure determinations. The molar conductance values of the
complexes in DMF (106–143 S cm2 molꢁ1) indicate the presence of a 1 : 2 type electrolyte
[24], implying that two picrates are counter anions outside the coordination sphere.
3.2. Descriptions of crystal structures
Crystals of 3 ꢀ 2C4H8O2, 6 ꢀ 2C4H8O2, and 8 ꢀ 2CH3CN were grown by slow evaporation
from methanol and ethyl acetate (or acetonitrile) mixed solution at room temperature
over a period of several weeks. The crystallographic data and structure determination
parameters are summarized in table 1 and selected bond lengths and angles are listed in
table 2. The structure analyses show that the dinuclear complexes are isomorphous.
Each complex has one [M2L2(Pic)4(H2O)2]2þ (figure 1), two free picrates as counter
anions, and two solvents (ethyl acetate or acetonitrile) linked by van der Waals’ forces.
The ligands are tetradentate and coordinate to one metal ion by two carbonyl and one
phenoxy oxygen atoms and to another metal ion by another carbonyl. The three
complexes show nine coordination with oxygen donors, four from two L, four from two
bidentate picrates, and the last from one water molecule. The coordination polyhedron
is a distorted monocapped square antiprism in which O6, O4, O8, O3A and O7, O9, O15,
O16 define, respectively, the upper and lower tetragonal faces of the antiprism, the
former being capped by O5 (figure 2, complex 3 ꢀ 2C4H8O2 for an example).
With smaller central metal the average distances between the metal ions and the
˚
˚
coordinated oxygen become slightly shorter (Eu, 2.462 A; Dy, 2.422 A; Y, 2.415 A). The
˚
˚
˚
M–O (C¼O) distances [mean to Eu 2.313(2)–2.366(2) A, to Dy 2.280(2)–2.327(3) A and
˚
to Y 2.2667(17)–2.314(3) A] are much shorter than the M–O (C–O–C) [mean to Eu
˚
2.811(2) A, to Dy 2.798(2) A and to Y 2.7759(19) A] distances, suggesting that the M–O
˚
˚
(C¼O) bond is stronger than the M–O (C–O–C). The M–O distances agree with those

4046
W. Wang et al.
Table 1. Crystallographic data for 3ꢀ2CH3COOC2H5, 6ꢀ2CH3COOC2H5, and 8ꢀ2CH3CN.
Parameters
3ꢀ2CH3COOC2H5
6ꢀ2CH3COOC2H5
8ꢀ2CH3CN
Block
Yellow
0.32 ꢃ 0.12 ꢃ 0.10
C96H98N28O56Y2
2717.84
Crystal shape
Color
Block
Yellow
0.24 ꢃ 0.20 ꢃ 0.18
C100H108Eu2N26O60
2938.04
Block
Yellow
0.14 ꢃ 0.16 ꢃ 0.10
C100H108Dy2N26O60
2959.12
Size (mm3)
Formula
Formula weight
Crystal system
Space group
Triclinic
P-1
Triclinic
P-1
Triclinic
P-1
ꢂ
˚
Unit cell dimensions (A, )
a
b
c
ꢅ
ꢀ
ꢆ
12.9244(13)
12.9846(13)
20.779(2)
93.285(2)
107.671(2)
109.814(2)
3075.5(5), 1
1.586
12.758(3)
12.845(3)
20.548(5)
93.136(3)
107.751(2)
109.805(2)
2970.1(12), 1
1.654
12.307(3)
12.575(2)
20.451(4)
93.6070(10)
106.960(2)
108.327(2)
2832.0(9), 1
1.594
3
˚
Volume (A ), Z
Calculated density (Mg mꢁ3
)
Absorption coefficient (mmꢁ1
F(000)
)
1.121
1496
1.363
1502
1.135
1396
2ꢇ range for data collection (ꢂ)
Index ranges
4.7–52.64
ꢁ13 ꢄ h ꢄ 15;
ꢁ15 ꢄ k ꢄ 14;
ꢁ24 ꢄ l ꢄ 17
15,769
10,780
9119
293(2)
0.0235
3.4–55.8
ꢁ14 ꢄ h ꢄ 15;
ꢁ14 ꢄ k ꢄ 15;
ꢁ24 ꢄ l ꢄ 18
21,691
10,419
9290
113(2)
0.0451
3.46–55.76
ꢁ15 ꢄ h ꢄ 16;
ꢁ15 ꢄ k ꢄ 16;
ꢁ26 ꢄ l ꢄ 21
25,424
13,277
9617
113(2)
0.0414
Measured reflections
Unique reflections
Reflection used [I 4 2ꢈ(I)]
Temperature (K)
Rint
R1
wR2[I 4 2ꢈ(I)]
0.0335
0.0736
1.035
0.0368
0.0758
1.041
0.0519
0.1075
1.016
Goodness-of-fit on F2
found in similar complexes and the changes are related to balance between flexibility
and rigidity of the ligand containing different functional groups. Complex 3 ꢀ 2C4H8O2,
for example, has average Eu–O (C¼O) bond length slightly shorter than that of [EuL]þ
˚
(2.369 A,
L ¼ 3,3,7,7-tetra[N-ethyl-N-phenyl(acetamide)-2oxymethyl]-5-oxanonane)
˚
[25] and [Eu(L)2(H2O)2(CF3SO3)]3þ (2.386(3)–2.463(3) A, L ¼ N,N,N ,N -tetraisopro-
0
0
pylpyridine-2,6-dicarboxamide) [26], but Eu–O (C–O–C) bond length is much longer
than that of [EuL]þ (2.486 A) [25] mentioned above.
˚
In each coordination cation, there exist intramolecular face-to-face ꢄꢀ ꢀ ꢀꢄ stacking
interactions between phenylꢂrings of two ligands, as shown in figure 3. In 3, the dihedral
˚
angle between planes is 8.00 with a centroid distance of 3.963 A, while in 6 the dihedral
ꢂ
ꢂ
˚
angle is 7.55 with a centroid distance of 3.918 A, and in 8 the dihedral angle is 9.42 with
˚
a centroid distance of 3.885 A. The amides form intramolecular hydrogen bonds with
both the carbonyl [N(2)–H(2)ꢀ ꢀ ꢀO1] and phenoxy [N(2)–H(2)ꢀ ꢀ ꢀO(2)]. Coordinated H2O
forms hydrogen bonds with the carbonyl [O(7)–H(7A)ꢀ ꢀ ꢀO(1)] (figure 3 and table 3,
3 ꢀ 2C4H8O2 for an example). Eu(1)ꢀ ꢀ ꢀEu(1A), Dy(1)ꢀ ꢀ ꢀDy(1A), Y(1)ꢀ ꢀ ꢀY(1A) separa-
˚
˚
˚
tions of 8.009 A, 8.321 A, and 8.347 A, respectively, are also in agreement with those of
dinuclear complexes which have special properties for efficient energy conversion and
transfer [7, 27, 28].

Rare earth picrates
4047
ꢂ
˚
Table 2. Selected bond distances (A) and angles ( ) for 3, 6, and 8.
Complex 3ꢀ2CH3COOC2H5
Bond lengths
Eu(1)–O(4)
Eu(1)–O(6)
Eu(1)–O(9)
Eu(1)–O(5)
Eu(1)–O(8)
2.313(3)
2.365(2)
2.576(3)
2.811(2)
2.337(3)
Eu(1)–O(3A)
Eu(1)–O(15)
Eu(1)–O(16)
Eu(1)–O(7)
2.334(3)
2.387(3)
2.590(3)
2.447(2)
8.009
Eu(1)–Eu(1A)
Bond angles
O(4)–Eu(1)–O(3A)
O(5)–Eu(1)–O(8)
O(4)–Eu(1)–O(7)
O(3A)–Eu(1)–O(9)
O(15)–Eu(1)–O(9)
O(15)–Eu(1)–O(16)
O(4)–Eu(1)–O(5)
O(6)–Eu(1)–O(5)
77.55(9)
75.38(8)
80.63(8)
76.76(9)
67.79(8)
65.20(8)
61.82(7)
59.00(8)
O(8)–Eu(1)–O(3A)
O(8)–Eu(1)–O(6)
O(3A)–Eu(1)–O(7)
O(15)–Eu(1)–O(7)
O(8)–Eu(1)–O(9)
O(9)–Eu(1)–O(7)
O(16)–Eu(1)–O(7)
O(3A)–Eu(1)–O(5)
80.05(9)
75.40(9)
83.05(8)
82.35(9)
65.75(8)
69.06(8)
68.13(8)
76.46(8)
Complex 6ꢀ2CH3COOC2H5
Bond lengths
Dy(1)–O(2)
Dy(1)–O(6)
Dy(1)–O(8)
2.280(3)
2.327(3)
2.531(3)
2.798(2)
2.301(2)
Dy(1)–O(1A)
Dy(1)–O(14)
Dy(1)–O(15)
Dy(1)–O(28)
Dy(1)–Dy(1A)
2.288(2)
2.348(2)
2.539(2)
2.382(3)
8.321
Dy(1)–O(5)
Dy(1)–O(7)
Bond angles
O(2)–Dy(1)–O(1A)
O(5)–Dy(1)–O(7)
O(2)–Dy(1)–O(28)
O(1A)–Dy(1)–O(8)
O(14)–Dy(1)–O(8)
O(15)–Dy(1)–O(14)
O(2)–Dy(1)–O(5)
O(6)–Dy(1)–O(5)
78.24(8)
74.20(8)
80.57(9)
75.74(8)
67.58(8)
66.39(8)
62.05(8)
59.36(7)
O(7)–Dy(1)–O(1A)
O(7)–Dy(1)–O(6)
O(1A)–Dy(1)–O(28)
O(14)–Dy(1)–O(28)
O(7)–Dy(1)–O(8)
O(8)–Dy(1)–O(28)
O(15)–Dy(1)–O(28)
O(1A)–Dy(1)–O(5)
79.90(9)
74.45(9)
82.93(9)
83.20(9)
66.65(8)
68.98(9)
67.87(9)
75.90(8)
Complex 8ꢀ2CH3CN
Bond lengths
Y(1)–O(2)
Y(1)–O(6)
Y(1)–O(8)
Y(1)–O(5)
Y(1)–O(7)
2.2667(17)
2.314(2)
2.4862(19)
2.7759(19)
2.3101(19)
Y(1)–O(1A)
Y(1)–O(14)
Y(1)–O(15)
Y(1)–O(28)
Y(1)–Y(1A)
2.3059(18)
2.3503(18)
2.5375(19)
2.384(2)
8.347
Bond angles
O(2)–Y(1)–O(1A)
O(5)–Y(1)–O(7)
O(2)–Y(1)–O(28)
O(1A)–Y(1)–O(8)
O(14)–Y(1)–O(8)
O(15)–Y(1)–O(14)
O(2)–Y(1)–O(5)
O(6)–Y(1)–O(5)
77.34(6)
72.96(6)
80.29(7)
76.84(6)
68.61(6)
66.39(6)
62.35(6)
59.67(6)
O(7)–Y(1)–O(1A)
O(7)–Y(1)–O(6)
O(1A)–Y(1)–O(28)
O(14)–Y(1)–O(28)
O(7)–Y(1)–O(8)
O(8)–Y(1)–O(28)
O(15)–Y(1)–O(28)
O(1A)–Y(1)–O(5)
79.04(7)
73.18(7)
84.91(7)
82.02(7)
66.90(7)
70.20(7)
67.93(7)
75.30(6)
The packing diagram of 3 ꢀ 2C4H8O2, 6 ꢀ 2C4H8O2, and 8 ꢀ 2CH3CN exhibit a 2-D
supramolecular architecture arising from C–Hꢀ ꢀ ꢀO and O–Hꢀ ꢀ ꢀO hydrogen bonds, as
shown in figure 4. Taking 3 as an example (table 3), the C–Hꢀ ꢀ ꢀO interactions can be
depicted by Cꢀ ꢀ ꢀO distances and the C–Hꢀ ꢀ ꢀO angle ꢇ [29, 30]. ‘‘Free’’ picrates form
hydrogen bonds with benzamide nitrogen [N(3)–H(3)ꢀ ꢀ ꢀO(22)] and the coordinated
oxygen of H2O [O(7)–H(7A)ꢀ ꢀ ꢀO(22)]. O(23) of ‘‘free’’ picrate forms a double-linked
C–Hꢀ ꢀ ꢀO hydrogen bond with phenyl C(4) [with H(4)] of neighboring molecules along

4048
W. Wang et al.
Figure 1. ORTEP diagram (20% probability ellipsoids) showing [M2L2(Pic)4(H2O)2]2þ of 3 (M ¼ Eu), 6
(M ¼ Dy), and 8 (M ¼ Y).
Figure 2. Coordination polyhedron of Eu1 ion.
ꢂ
˚
the a-axis; Cꢀ ꢀ ꢀO distances of 3.386(6) A and the C–Hꢀ ꢀ ꢀO angle ꢇ of 150.7 indicate a
strong interaction, leading to a 1-D chain. O(27) of the same ‘‘free’’ picrate forms
another double-linked C–Hꢀ ꢀ ꢀO hydrogen bond with phenyl C(16) [with H(16)] of
˚
another neighboring molecule; Cꢀ ꢀ ꢀO distances of 3.354(6) A and C–Hꢀ ꢀ ꢀO angle ꢇ of
140.9ꢂ form a 2-D sheet along the ab-plane by interweaving oxygen of free picrate with
four neighboring molecules.

Rare earth picrates
4049
Figure 3. The intramolecular ꢄ–ꢄ interactions and hydrogen bonds (indicated by dashed lines) in
[M2L2(Pic)4(H2O)2]2þ
.
Table 3. Hydrogen bonds for [Eu2L2(Pic)4(H2O)2](Pic)2 (A, ꢂ).
˚
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
ff(D–Hꢀ ꢀ ꢀA)
N(2)–H(2)ꢀ ꢀ ꢀO(1)
N(2)–H(2)ꢀ ꢀ ꢀO(2)
N(3)–H(3)ꢀ ꢀ ꢀO(22)a
O(7)–H(7A)ꢀ ꢀ ꢀO(1)b
O(7)–H(7B)ꢀ ꢀ ꢀO(22)a
0.86
0.86
0.86
0.79
0.81
0.93
0.93
2.45
1.88
2.24
2.06
2.01
2.54
2.58
3.126(5)
2.576(5)
2.885(4)
2.837(4)
2.816(4)
3.386(6)
3.354(6)
135.7
136.4
131.6
172.4
178.8
150.8
140.9
c
C(4)–H(4)ꢀ ꢀ ꢀO(23)0
d
C(16)–H(16)ꢀ ꢀ ꢀO(27)0
b
d
Symmetry codes: a1 ꢁ x, 1 ꢁ y, 1 ꢁ z; ꢁx, 2 ꢁ y, 1 ꢁ z; cx, 1 þ y, z; ꢁ2 þ x, y, z.
Figure 4. 2-D sheet generated by the intermolecular hydrogen bonds in 3 (the O(N)–Hꢀ ꢀ ꢀO and C–HꢀꢀꢀO
hydrogen bonds are indicated by dashed lines).

4050
W. Wang et al.
3.3. IR spectra
The complexes have similar IR spectra, suggesting that they have similar coordination.
Characteristic absorptions of free ligand due to ꢂ(Ar–C¼O) are at 1659 cmꢁ1, ꢂ(CH2–
C¼O) at 1627 cmꢁ1, and ꢂ(Ar–O–C) at 1248 cmꢁ1, respectively. After coordination,
(Ar–)C¼O bands of ligands shift to lower wavenumbers, (CH2–)C¼O bands become
weak and new strong broader absorptions appear at 1614 cmꢁ1, indicating that oxygen
atoms of all (Ar–)C¼O bonds and part of (CH2–)C¼O bonds coordinate. Ar–O–C
bands become weak and some ꢂ(Ar–O–C) absorptions shift 25–29 cmꢁ1 to lower
wavenumbers, indicating that oxygen atoms of ether bonds of L take part in
coordination. The band at 1265 cmꢁ1 for ꢂ(C–O) of Picꢁ shifts to higher frequency by
ca 7 cmꢁ1 and the bands of free HPic due to ꢂas(–NO2) and ꢂs(–NO2) appear at 1555 and
1342 cmꢁ1, which split into two bands at ca 1574, 1542 cmꢁ1 and ca 1361, 1330 cmꢁ1
,
respectively, in the complexes. These indicate that some oxygen atoms in the C–O bands
and in the nitro groups of Picꢁ participate in coordination [31]. Broad band at
3320 cmꢁ1 indicates the presence of water, in agreement with the elemental analysis and
crystal structure determination.
3.4. Luminescent properties
Among all eight complexes only the Eu(III) complex shows strong luminescent emission
when excited in the solid state at room temperature and no ligand-based emission
bands, indicating that the ligand is a good chelator to absorb and transfer energy to Eu
5
7
5
7
ions (figure 5). The D0! F2 transition is much more intense than the D0 ! F1
transition; the intensity ratio ꢉ(5D0 ! F2/5D0 ! F1) is 8.0, indicating that Eu3þ ions
do not lie in a centro-symmetric coordination site [32], in agreement with the crystal
structure analysis.
7
7
Based on the antenna effect, the intensity of the luminescence of lanthanide
complexes is related to the efficiency of the intra-molecular energy transfer between the
triplet levels of the ligand and the emitting level of lanthanide ions, which depends on
the energy gap between the two levels. As all the complexes are isostructural, the energy
of the ligand-centered triplet state does not depend significantly on the metal and the
energy absorbed by the ligands could not be transferred to the lowest excited state of the
1000
500
0
560 580 600 620 640 660 680
Wavelength/nm
Figure 5. Emission spectra of the Eu complex in the solid state (excited at 420 nm).

Rare earth picrates
4051
Gd3þ ion (about 32,000 cmꢁ1). The phosphorescence spectrum of
4
[Gd2L2(Pic)4(H2O)2](Pic)2 was clearly observed at 77 K in a methanol–ethanol mixture
(V : V ¼ 1 : 1) to acquire the triplet excited state of L. The phosphorescence spectra
demonstrated that the triplet energy levels (T1) of L and picrate are 23,810 cmꢁ1
(420 nm) and 18,248 cmꢁ1 (548 nm), respectively, both higher than the lowest excited
resonance level 5D0 of Eu(III) (17,300 cmꢁ1), confirming the suitability of the ligands as
sensitizers for europium complexes [33]. The triplet state energy level T1 of the ligand is
also higher than the lowest excited resonance level 5D4 of Tb(III) (20,500 cmꢁ1), but the
luminescence of the Tb complex is not observed at room temperature. Energy is
transferred from the triplet state of L to that of picrate and then transferred from the
triplet state of picrate to the emitting level of rare-earth ions. The triplet state energy
level of picrate (18,248 cmꢁ1, 548 nm) is lower than the lowest excited resonance level
5D4 of Tb(III) (20,500 cmꢁ1). Therefore the intramolecular energy transfer between
triplet levels of the ligand and the emitting level of terbium ions has been quenched by
the picrate groups.
The influence of solvent on the luminescence intensities of the europium complex has
also been studied. Luminescence spectra in ethyl acetate, tetrahydrofuran, dioxane,
acetonitrile, acetone, methanol, and dimethylformamide solutions (concentration:
1.0 ꢃ 10ꢁ5 mol ꢀ cmꢁ3, the emission slit widths were 10 nm, figure 6) are recorded at
room temperature. The fluorescence intensities of 3 in other solutions are lower than
that in ethyl acetate and become weaker from THF, dioxane, MeCN, acetone,
methanol to DMF due to the coordinating effects of solvents [34]. The rare-earth
complexes have a dinuclear structure due to the balance between flexibility and rigidity
of the amide-based acyclic polyether ligand and bulky picrate anions, but this structure
could not prevent solvent from entering. Together with coordination abilities of THF,
dioxane, MeCN, and DMF for lanthanides, the oscillatory motions of entering
molecules induce non-radiative deactivation of the ligand triplet level transfer to the
emitting level of the europium ion. Thus, the energy transfer could not be carried out
perfectly. All of these indicate that solvents have strong coordination effect and the
- Ethyl acetate
- Tetrahydrofuran
- Dioxane
200
100
0
- Acetonitrile
- Acetone
- Methanol
- DMF
600
650
Wavelength/nm
700
Figure 6. The emission spectrum of [Eu2L2(Pic)4(H2O)2](Pic)2 in ethyl acetate, THF, dioxane, MeCN,
acetone, methanol, and DMF solutions at room temperature. Concentration: 1.0 ꢃ 10ꢁ5 mol ꢀ cmꢁ3
.

4052
W. Wang et al.
environment plays an important role in determining the fluorescence intensity of the
complexes [17, 35].
4. Conclusion
New dinuclear rare-earth picrate complexes, [M2L2(Pic)4(H2O)2](Pic)2, where M ¼ La3þ
(1), Nd3þ (2), Eu3þ (3), Gd3þ (4), Tb3þ (5), Dy3þ (6), Yb3þ (7), Y3þ (8), have been
prepared and characterized. The amide-based acyclic polyether forms stable complexes
with rare-earth picrates and IR spectral changes were observed after coordination.
X-ray structure analyses show that the dinuclear complexes are isomorphous and each
has one coordination cation and two free picrates as the counter anions. With hydrogen
bonds between them the complexes exhibit a 2-D net supermolecule. The Eu(III)
complex shows strong luminescence emission in the solid state at room temperature,
indicating that the ligand is a good chelator to sensitize Eu(III) emission. The influence
of solvent on the fluorescence intensities indicates that solvent and environment play
important roles in determining the fluorescence intensity of the complexes.
Supplementary material
CCDC 600837, 610281, and 610284 contain the supplementary crystallographic data
for 3ꢀ2C4H8O2, 6ꢀ2C4H8O2, and 8ꢀ2CH3CN, respectively. These data can be obtained
free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif. Supplementary material associated with this article can be
found in supporting information section.
Acknowledgments
The work was supported by the Fundamental Research Funds for the Central
Universities (lzujbky-2011-114), the National Natural Science Foundation of China for
Personnel Training (J1103307), and the National Natural Science Foundation of China
(21171077, 51074083).
References
[1] L.-Y. Zhu, X.-F. Tong, M.-Z. Li, E.-J. Wang. J. Phys. Chem. B, 105, 2461 (2001).
[2] J. Kido, Y. Okamato. Chem. Rev., 102, 2357 (2002).
[3] J.-C.G. Bunzli, C. Piguet. Chem. Soc. Rev., 34, 1048 (2005).
¨
[5] T. Justel, H. Nikol, C. Ronda. Angew. Chem. Int. Ed., 37, 3084 (1998).
¨
[4] J.-C.G. Bunzli, A.-S. Chauvin, H.K. Kim, E. Deiters, S.V. Eliseeva. Coord. Chem. Rev., 254, 2623 (2010).
¨
[6] N. Sabbatini, M. Guardigli, J.-M. Lehn. Coord. Chem. Rev., 123, 201 (1993).
[7] M. Elhabiri, R. Scopelliti, J.-C.G. Bunzli, C. Piguet. J. Am. Chem. Soc., 121, 10747 (1999).
¨

Rare earth picrates
4053
[8] R.S. Dickins, J.A.K. Howard, C.W. Lehmann, J. Moloney, D. Parker, R.D. Peacock. Angew. Chem. Int.
Ed. Engl., 36, 521 (1997).
[9] B. Alpha, R. Ballardini, V. Balzani, J.-M. Lehn, S. Perathoner, N. Sabbatini. Photochem. Photobiol., 52,
299 (1990).
[10] P.L. Jones, A.J. Amoroso, J.C. Jeffery, J.A. Mccleverty, E. Psillakis, L.H. Ress, M.D. Ward. Inorg.
Chem., 36, 10 (1997).
[11] S.-S. Xiao, X.-J. Zheng, S.-H. Yan, X.-B. Deng, L.-P. Jin. CrystEngComm, 12, 3145 (2010).
[12] W.-T. Chen, Z.-L. Yao. J. Coord. Chem., 64, 996 (2011).
[13] D.-J. Qian, W.-N. Zhang, Z. Chen, J.V. Houten. Spectrochim. Acta, Part A, 56, 2645 (2000).
[14] F.-H. Liu, Y.-B. Zhou. Inorg. Chem. Commun., 13, 1410 (2010).
[15] W.P.-W. Lai, W.-T. Wong, B.K.-F. Li, K.-W. Cheah. New J. Chem., 26, 576 (2002).
[16] K.R. Adam, I.M. Atkinson, J. Kim, L.F. Lindoy, O.A. Matthews, G.V. Meehan, F. Raciti,
B.W. Skelton, N. Svenstrup, A.H. White. J. Chem. Soc., Dalton Trans., 2388 (2001).
[17] W. Wang, Y. Huang, N. Tang. Spectrochim. Acta, Part A, 66, 1058 (2007).
[18] W.-N. Wu, F.-X. Cheng, L. Yan, N. Tang. J. Coord. Chem., 61, 2207 (2008).
[19] K. Nakagawa, K. Amita, H. Mizuno, Y. Inoue, T. Hakushi. Bull. Chem. Soc. Japan, 60, 2037 (1987).
[20] J.B. Gandhi, N.D. Kulkarni. J. Chem. Res. (S), 12, 488 (1994).
[21] J. Speziale, P.C. Hamm. J. Am. Chem. Soc., 78, 2556 (1956).
[22] SAINT. Software Reference Manual, Bruker AXS, Madison, WI (1998).
[23] G.M. Sheldrick. SHELXTL NT Version 5.1, Program for Solution and Refinement of Crystal Structures,
University of Gottingen, Germany (1997).
¨
[24] W.J. Geary. Coord. Chem. Rev., 7, 81 (1971).
[25] K.-W. Lei, W.-L. Liu, M.-Y. Tan. Spectrochim. Acta, Part A, 66, 118 (2007).
[26] T.L. Borgne, J.-M. Be
Trans., 3856 (2003).
[27] G. Bernardinelli, C. Piguet, A.F. Williams. Angew. Chem. Int. Ed., 12, 1622 (1992).
´
nech, S. Floquet, G. Bernardinell, C. Aliprandini, P. Bettens, C. Piguet. Dalton
[28] C. Piguet, J.-C.G. Bunzli, G. Bernardinelli, G. Hopfgartner, A.F. Williams. J. Am. Chem. Soc., 115, 8197
¨
(1993).
[29] G.R. Desiraju. Acc. Chem. Res., 29, 441 (1996).
[30] T. Steiner. Angew. Chem. Int. Ed., 41, 48 (2002).
[31] S.-X. Liu, W.-S. Liu, M.-T. Tan, K.-B. Yu. J. Coord. Chem., 10, 391 (1996).
[32] A.F. Kirby, D. Foster, F.S. Richardson. Chem. Phys. Lett., 95, 507 (1983).
[33] P. He, H.-H. Wang, S.-G. Liu, J.-X. Shi, G. Wang, M.-L. Gong. Inorg. Chem., 48, 11382 (2009).
[34] H.Q. Liu, T.C. Cheung, C.M. Che. Chem. Commun., 1039 (1996).
[35] D.-J. Zhou, Q. Li, C.-H. Huang, G.-Q. Yao, S. Umetani, M. Matsui, L.-M. Ying, A.-C. Yu, X.-S. Zhao.
Polyhedron, 16, 1381 (1997).