Preparation, crystal structure and luminescent properties of the 3-D netlike supramolecular lanthanide picrate complexes with 2,2′-[1,2- phenylenebis(oxy)]bis(N-benzylacetamide)

Article abstract of DOI:10.1016/j.ica.2005.10.004

Solid complexes of lanthanide picrates with a new podand-type ligand, 2,2′-[(1,2-phenylene)bis(oxy)]bis(N-benzylacetamide) (L) have been prepared and characterized by elemental analysis, conductivity measurements, IR, electronic and ¹H NMR spectroscopies. The crystal and molecular structures of the complex NdL(Pic)₃ have been determined by single-crystal X-ray diffraction. The crystal structure shows that the Nd(III) ion is coordinated with four oxygen atoms of the ligand L and six oxygen atoms of three bidentate picrates. Furthermore, the NdL(Pic)₃ complex units are linked by the intermolecular hydrogen bonds to form a three-dimensional (3-D) netlike supermolecule. Under excitation, Eu complex exhibited characteristic emissions. The lowest triplet state energy level of the ligand indicates that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.

Full text of DOI:10.1016/j.ica.2005.10.004

Inorganica Chimica Acta 359 (2006) 1207–1214
www.elsevier.com/locate/ica
Preparation, crystal structure and luminescent properties of
the 3-D netlike supramolecular lanthanide picrate complexes with
2,20-[1,2-phenylenebis(oxy)]bis(N-benzylacetamide)
a
a
a,
a
a
b
Kuan-Zhen Tang , Jian Zhang , Yu Tang *, Wei-Sheng Liu , Min-Yu Tan , Yu-Xi Sun
a
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, PR China
b
Department of Chemistry, Qufu Normal University, Qufu 273165, PR China
Received 4 August 2005; received in revised form 6 October 2005; accepted 6 October 2005
Available online 8 November 2005
Abstract
Solid complexes of lanthanide picrates with a new podand-type ligand, 2,20-[(1,2-phenylene)bis(oxy)]bis(N-benzylacetamide) (L) have
1
been prepared and characterized by elemental analysis, conductivity measurements, IR, electronic and H NMR spectroscopies. The
crystal and molecular structures of the complex NdL(Pic)3 have been determined by single-crystal X-ray diffraction. The crystal structure
shows that the Nd(III) ion is coordinated with four oxygen atoms of the ligand L and six oxygen atoms of three bidentate picrates. Fur-
thermore, the NdL(Pic)3 complex units are linked by the intermolecular hydrogen bonds to form a three-dimensional (3-D) netlike super-
molecule. Under excitation, Eu complex exhibited characteristic emissions. The lowest triplet state energy level of the ligand indicates
that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Lanthanide picrate complexes; Podand; Crystal structure; Luminescent properties
1. Introduction
serve as sensitizers (antenna effect) [4]. The role of the
ligands is on one hand to collect the photons provided by
the light source in order to allow an energy transfer to
the energy levels of the Ln(III) ion, and on the other hand
to shield it against the solvent in order to avoid non-radio-
active deactivation processes. It is generally accepted that
the energy transfer from ligand to Ln(III) ion occurs from
the lowest triplet state energy level (T1) of the ligand to the
resonance level of Ln(III) [5]. This energy transfer process
is one of the most important processes having influence on
the luminescence properties of the lanthanide complexes.
Podand-type ligands have drawn much attention in recent
years, mainly due to their selective coordinating capacity
and hard binding sites, therefore, stabilizing their com-
plexes, acquiring novel coordination structure and shield-
ing the encapsulated ion from interaction with the
surroundings [6]. Among numerous podands which have
demonstrated their potential use in functional supramolec-
ular chemistry [7], amide type podands are important for
Many lanthanide complexes are noted for their lumines-
cence arising from f–f transitions, which result in emission
bands with extremely narrow bandwidth and no theoretical
cap on the quantum efficiency [1]. This makes lanthanide
ions very attractive for a variety of applications, such as
chromophores for LEDs and as probes and labels in a vari-
ety of biological and chemical applications [2]. And the
probes based on europium and terbium ions are of special
interest because of the particularly suitable spectroscopic
properties of these ions, such as their large Stokes shift
and narrow emission profiles [1a,3]. Since the f–f transi-
tions are spin- and parity-forbidden, the excited state of
the lanthanide ion is populated through intramolecular
energy transfer from the ligand or ligands, which therefore
*
Corresponding author. Tel.: +869318912552; fax: +869318912582.
E-mail address: tangyu@lzu.edu.cn (Y. Tang).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.10.004

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K.-Z. Tang et al. / Inorganica Chimica Acta 359 (2006) 1207–1214
HN
O
O
O
NH2
O
O
+
K2CO3,KI
Cl
Cl
Cl
NH
CH3COCH3
OH
OH
O
HN
Scheme 1. The synthetic route for the ligand L.
preparing the lanthanide complexes possessing strong lumi-
nescent properties.
(25 cm3), then anhydrous potassium carbonate (2.4 g,
17.40 mmol) and a small quantity of KI were added. The
reaction mixture was stirred and refluxed for 5 h. After
cooling down, the mixture was filtered and the solvent
was distilled off. Then the resulted solid was recrystallized
with acetone to get the ligand L; Yield: 75%. m.p. 114–
115 ꢁC; 1H NMR (CDCl3, 300 MHz) d: 4.42 (d,
J = 6.0 Hz, 4H), 4.50 (s, 4H), 6.96 (s, 2H), 6.88–7.03 (m,
4H), 7.17–7.31 (m, 10H); Anal. Calc. for C24H24N2O4: C,
71.27; H, 5.98; N, 6.93. Found: C, 70.94; H, 5.90; N, 7.09%.
Our group is interested in the supramolecular coordina-
tion chemistry of lanthanide ions with amide type podands
that have strong coordination capability to the lanthanide
ions and terminal group effects [8]. We have designed a ser-
ies of amide type podands having both selective ability to
coordinate lanthanide ions and enhanced luminescence of
lanthanide complexes by providing some of proper conju-
gate absorption groups suitable for energy transfer [9]. In
the present work, we designed and synthesized a new amide
type podand ligand 2,20-[(1,2-phenylene)bis(oxy)]bis(N-
benzylacetamide) (L) (Scheme 1) with two relatively long
‘‘arms’’, and studied the crystal structure and luminescent
properties of lanthanide complexes with this new ligand.
The crystal structure of the complex NdL(Pic)3 shows that
the Nd(III) ion could be effectively encapsulated and pro-
tected by the coordinated ligands. And the two nitrogen
atoms of the amide groups in the ligand arms are located
in the outer part of the whole ligand molecule, which is
very important for the assembly of the complex molecule
units into a three-dimensional (3-D) netlike supermolecule
by the intermolecular hydrogen bonds. Under excitation,
Eu complex exhibited characteristic emissions. And the
lowest triplet state energy level of the ligand which was cal-
culated from the phosphorescence spectrum of the Gd
complex at 77 K indicates that the triplet state energy level
of the ligand matches better to the resonance level of
Eu(III) than Tb(III) ion.
2.3. Preparation of the complexes
A solution of 0.1 mmol L in 10 cm3 of ethyl acetate was
added drop wise to a solution of 0.1 mmol lanthanide
picrates (Ln = La, Pr, Nd, Eu, Gd, Tb, Er) in 10 cm3 of
ethyl acetate. The mixture was stirred at room temperature
for 4 h. Then diethyl ether was added to the mixture and
the precipitated solid complex was filtered, washed with
ethyl acetate and diethyl ether and dried in vacuo over
P4O10 for 48 h and submitted for elemental analysis; Yield:
60%. Analytical data and molar conductance values of the
complexes are given in Table 1. All the complexes are yel-
low powders and stable in air. Single crystals of neodym-
ium complex were grown from ethyl acetate and
chloroform (V:V = 1:3) mixed solution with slow evapora-
tion at room temperature. After ca. 3 weeks, transparent
yellow crystals were formed from the solution.
2.4. Chemical and physical measurements
2. Experimental
The Ln(III) ion was determined by EDTA titration
using xylenol-orange as an indicator. Carbon, nitrogen
and hydrogen were determined using an Elementar Vario
EL (see Table 1). The IR spectra were recorded in the
4000–400 cmꢀ1 region using KBr pellets and a Nicolet
2.1. Materials
Lanthanide picrates [10] and chloracetobenzylamine [11]
were prepared according to the literature methods. Other
chemicals were obtained from commercial sources and
used without further purification.
1
Nexus 670 FT-IR spectrometer (see Table 4). H NMR
spectra were measured on a Varian Mercury 300 spectrom-
eter in CDCl3 and CD3COCD3 solutions, with TMS as
internal standard. Electronic spectra were recorded with a
Varian Cary 100 spectrophotometer in acetonitrile solu-
tion. Luminescence spectra and phosphorescence spectra
2.2. Preparation of the ligand L
The synthetic route for the ligand is shown in Scheme 1.
Chloracetobenzylamine (0.96 g, 8.73 mmol) and o-dihydr-
oxybenzene (3.2 g, 17.44 mmol) were added to acetone
were obtained on
spectrophotometer.
a
Hitachi F-4500 fluorescence

K.-Z. Tang et al. / Inorganica Chimica Acta 359 (2006) 1207–1214
1209
Table 1
Analytical and molar conductance data of the complexes (calculated values in parentheses)
Complexes
%C Found (Calc.)
%H Found (Calc.)
%N Found (Calc.)
%Ln Found (Calc.)
Km (cm2 Xꢀ1 molꢀ1
)
LaL(Pic)3
PrL(Pic)3
NdL(Pic)3
EuL(Pic)3
GdL(Pic)3
TbL(Pic)3
ErL(Pic)3
a
41.35 (41.09)
41.22 (41.02)
41.34 (40.91)
40.98 (40.66)
40.85 (40.49)
40.72 (40.43)
40.74 (40.16)
2.35 (2.46)
2.20 (2.46)
2.30 (2.45)
2.25 (2.44)
2.30 (2.43)
2.22 (2.42)
2.35 (2.41)
12.88 (12.55)
12.95 (12.53)
12.85 (12.50)
12.75 (12.42)
12.12 (12.37)
12.55 (12.35)
12.60 (12.27)
11.50 (11.31)
11.75 (11.46)
11.90 (11.70)
11.99 (12.25)
12.38 (12.62)
12.56 (12.74)
13.01 (13.32)
50.2a, 35.5b
51.5a, 37.0b
50.6a, 32.7b
50.9a, 33.9b
52.5a, 38.3b
50.4a, 32.8b
52.8a, 33.6b
Measured in acetonitrile.
b
Measured in acetone.
2.5. Crystal structure determination
[12,13] (see Table 2). Selected bond lengths and angles
are presented in Table 3. The complex NdL(Pic)3 crystal-
lizes in the monoclinic system, space group P21/a, with lat-
X-ray diffraction data for a crystal of the neodymium
complex with dimensions 0.14 · 0.13 · 0.08 mm were per-
formed with graphite-monochromated Mo Ka radiation
on a Bruker APEX area-detector diffractometer and were
collected by the x–2h scan technique. The crystal structures
were solved by direct methods. All non-hydrogen atoms
were refined anisotropically by full-matrix least-squares
methods on F2. Primary non-hydrogen atoms were solved
by direct method and secondary non-hydrogen atoms were
solved by difference maps. The hydrogen atoms were added
geometrically and not refined. All calculations were per-
formed using the programs SHELXS-97 and SHELXL-97
˚
˚
tice parameters a = 12.293(14) A, b = 34.259(4) A, c =
˚
12.298(14) A, b = 112.710(2)ꢁ, F(000) = 2476, GOF = 1.019,
R1 = 0.0555, wR2 = 0.1078, Z = 4. qcalc = 1.714 g cmꢀ3
V = 4777.6(9) A , k = 0.71073 A, T = 295 K.
,
3
˚
˚
3. Results and discussion
Analytical data for the newly synthesized complexes,
listed in Table 1, indicate that the seven picrates complexes
conform to a 1:1:3 metal-to-ligand-to-picrate stoichiometry
LnL(Pic)3 (Ln = La, Pr, Nd, Eu, Gd, Tb, Er).
All the complexes are soluble in DMF, DMSO, metha-
nol, ethanol, acetonitrile, ethyl acetate and acetone, but
slightly soluble in THF and CHCl3. The molar conduc-
tances of the complexes in acetonitrile and acetone (see
Table 1) indicate that all complexes act as non-electrolytes
[14], implying that all picrates groups are in coordination
sphere.
Table 2
Crystal data and structure refinement for the complex NdL(Pic)3
Complex
NdL(Pic)3
Empirical formula
Formula weight
Colour
C42H30N11O25Nd
1233.01
yellow
Temperature (K)
295(2)
˚
Wavelength (A)
Radiation
0.71073
Mo Ka
3.1. Crystal structure of Nd L(Pic)3
Crystal system
Space group
Unit cell dimensions
monoclinic
P21/a
A summary of crystallographic data and details of the
structure refinements are listed in Table 2. The selected
bond lengths and bond angles are given in Table 3.
˚
a (A)
12.293(14)
34.259(4)
12.298(14)
112.710(2)
4777.6(9)
4
1.714
1.191
2476
0.14 · 0.13 · 0.08
1.19–25.50
ꢀ14 6 h 6 14; ꢀ41 6 k 6 40;
ꢀ14 6 l 6 14
8871
5919 (0.0918)
multi-scan
full-matrix least-squares on F2
8871/162/700
1.019
R1 = 0.0555, wR2 = 0.1078
0.811 and ꢀ0.771
˚
b (A)
The single-crystal X-ray analysis of the complex
NdL(Pic)3 reveals that each Nd atom is coordinated with
10 oxygen donor atoms. Six of them belong to three biden-
tate picrate groups and the remaining four are from the tet-
radentate ligand L (Fig. 1). The coordination polyhedron
around Nd is a distorted bicapped square antiprism
(Fig. 2). The ligand wraps around the metal ion with its
oxygen atoms and forms a ring-like coordination structure
[15] together with the Nd atom. The four oxygen atoms of
the ligand (O(1), O(2), O(3), O(4)) are not quite coplanar,
their deviation from the mean plane being in the range
˚
c (A)
b (ꢁ)
Volume (A )
3
˚
Z
Dcalc (g cmꢀ3
)
l (Mo Ka) (mmꢀ1
F(000)
)
Crystal size (mm)
Range (ꢁ)
Index ranges
Reflection collected
Independent reflections (Rint
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices
)
˚
˚
0.067–0.121 A. The mean absolute deviation is 0.0935 A
from the mean plane. The neodymium atom lies out of this
˚
plane by 0.795 A. The Nd–O (C@O) distances (mean
2.426 A) are significantly shorter than the Nd–O (C–O–
C) distances (mean 2.719 A). This suggests that the Nd–O
˚
˚
ꢀ3
˚
Largest difference peak and hole (e A
)
(C@O) bond is stronger than the Nd–O (C–O–C) bond.

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K.-Z. Tang et al. / Inorganica Chimica Acta 359 (2006) 1207–1214
Table 3
˚
Selected bond lengths (A) and angles (ꢁ) for the complex NdL(Pic)3
Nd1–O1
Nd1–O4
Nd1–O12
Nd1–O20
2.438(5)
2.415(5)
2.304(5)
2.674(6)
Nd1–O2
Nd1–O5
Nd1–O13
2.662(5)
2.357(5)
2.843(6)
Nd–O3
Nd1–O6
Nd1–O19
2.776(5)
2.594(5)
2.379(5)
O1–Nd1–O2
O1–Nd1–O5
O1–Nd1–O13
O2–Nd1–O3
O2–Nd1–O6
O2–Nd1–O19
O3–Nd1–O5
O3–Nd1–O13
O4–Nd1–O5
O4–Nd1–O13
O5–Nd1–O6
O5–Nd1–O19
O6–Nd1–O13
O12–Nd1–O13
O13–Nd1–O19
60.70(16)
81.76(18)
65.95(17)
55.96(14)
68.12(16)
74.10(16)
118.72(15)
175.58(15)
74.07(18)
125.17(17)
65.69(17)
155.80(18)
114.97(18)
61.28(18)
114.99(17)
O1–Nd1–O3
O1–Nd1–O6
O1–Nd1–O19
O2–Nd1–O4
O2–Nd1–O12
O2–Nd1–O20
O3–Nd1–O6
O3–Nd1–O19
O4–Nd1–O6
O4–Nd1–O19
O5–Nd1–O12
O5–Nd1–O20
O6–Nd1–O19
O12–Nd1–O19
O13–Nd1–O20
110.84(15)
68.10(18)
121.06(17)
113.78(16)
146.04(18)
73.74(17)
65.33(16)
63.61(15)
74.60(18)
89.43(18)
84.12(19)
126.51(18)
127.52(17)
75.74(19)
62.86(18)
O1–Nd1–O4
O1–Nd1–O12
O1–Nd1–O20
O2–Nd1–O5
O2–Nd1–O13
O3–Nd1–O4
O3–Nd1–O12
O3–Nd1–O20
O4–Nd1–O12
O4–Nd1–O20
O5–Nd1–O13
O6–Nd1–O12
O6–Nd1–O20
O12–Nd1–O20
O19–Nd1–O20
141.40(18)
126.38(18)
69.42(17)
128.55(16)
119.78(16)
59.25(15)
121.26(17)
113.46(16)
80.98(19)
148.89(17)
64.43(18)
145.20(19)
132.75(18)
78.84(19)
62.78(17)
Fig. 1. ORTEP diagram (30% probability ellipsoids) showing the coordination sphere of NdL(Pic)3.
In the complex, requirement of high coordination num-
ber of the lanthanide ion is satisfied by the tetradentate
ligand and three bidentate picrate groups. Since coordi-
nated solvent molecules, especially water or alcohol, can
efficiently quench lanthanide luminescence, the ability of
the ligands to satisfy the coordination requirements of
the Ln(III) centre with high coordination number becomes
an important criterion in the design of supramolecular
photonic devices [16].
The conformation of the ligand L in the complex is
interesting. The two arms of the ligand are so long that
their terminal groups can ‘‘fold back’’. Through intramo-
lecular p–p interaction [17], two benzyl groups of the two
arms wrap around a coordinated picrate, one above and
the other below, forming the ‘‘folding-shape’’ of the
ligand (Fig. 3). These three almost parallel phenyl rings
˚
˚
are separated by 3.808 A (a and b) and 3.051 A (a and
c) (vertical distance) and the dihedral angle between a
and b is 27.2ꢁ and between a and c is 12.0ꢁ. Thus, the
two nitrogen atoms of the amide groups in the ligand,
namely, N(1) and N(2), are located in the outer part
of the whole ligand molecule (Fig. 3), which is very
important for the assembly of the complex molecule
units into the supermolecule by the intermolecular hydro-
gen bonds.
In the complex, atoms N(1) and N(2) of the ligand L act
as hydrogen bond donors to form Oꢁ ꢁ ꢁH–N with the
oxygen atoms O(14), O(16) and O(18) of a neighbouring

K.-Z. Tang et al. / Inorganica Chimica Acta 359 (2006) 1207–1214
1211
Fig. 3. The intramolecular p–p stacking interactions in the complex
molecule.
Fig. 2. Coordination polyhedron of Nd ion.
˚
molecule [O(14)ꢁ ꢁ ꢁH(1), 2.09 A and O(14)ꢁ ꢁ ꢁH(1)–N(1),
as shown in Fig. 4(b) to form a three-dimensional (3-D)
netlike supermolecule.
˚
˚
168ꢁ; O(16)ꢁ ꢁ ꢁH(2), 2.46 A, O(18)ꢁ ꢁ ꢁH(2), 2.57 A and
O(16)ꢁ ꢁ ꢁH(2)–N(2), 149ꢁ, O(18)ꢁ ꢁ ꢁH(2)–N(2), 135ꢁ], thus,
generating a two-dimensional (2-D) supramolecular layer
as shown in Fig. 4(a). In addition, the layers are linked
by intermolecular hydrogen bond O(7)ꢁ ꢁ ꢁH(39)–C(39)
3.2. Infrared spectra
The most important IR peaks of the ligand and the com-
plexes LnL(Pic)3 are reported in Table 4.
˚
[O(7)ꢁ ꢁ ꢁH(39), 2.59 A and O(7)ꢁ ꢁ ꢁH(39)–C(39), 170ꢁ] [18]
Fig. 4. Hydrogen bonding networks in the complexes: (a) 2-D layers generated by the intermolecular Oꢁ ꢁ ꢁH–N hydrogen bonds [symmetry codes for
O(14)ꢁ ꢁ ꢁH(1)–N(1), O(16)ꢁ ꢁ ꢁH(2)–N(2) and O(18)ꢁ ꢁ ꢁH(2)–N(2) are: ꢀ1/2 + x, 1/2 ꢀ y, z; 1 ꢀ x, ꢀy, 1 ꢀ z; and ꢀ1 + x, y, z, respectively]; (b) The
intermolecular Oꢁ ꢁ ꢁH–C hydrogen bonds [symmetry code is x, y, ꢀ1 + z] between the 2-D layers (some atoms of the ligand are omitted for clarity and the
hydrogen bonds are indicated by the yellow dashed lines).

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K.-Z. Tang et al. / Inorganica Chimica Acta 359 (2006) 1207–1214
Table 4
The IR spectra data (cmꢀ1
)
Compounds
m(C@O)
m(C–O–C)
m(C–O)
mas(–NO2)
ms(–NO2)
HPic
1265s
1555m
1342s
L
1660s
1616s
1616s
1615s
1617s
1616s
1617s
1617s
1053m
1020m
1018m
1020m
1024m
1017m
1021m
1020m
LaL(Pic)3
PrL(Pic)3
NdL(Pic)3
EuL(Pic)3
GdL(Pic)3
TbL(Pic)3
ErL(Pic)3
1271m
1272m
1270m
1272m
1271m
1270m
1270m
1574m, 1541m
1570m, 1543m
1577m, 1542m
1574m, 1542m
1573m, 1544m
1575m, 1540m
1576m, 1540m
1362m, 1331m
1362m, 1334m
1361m, 1331m
1360m, 1334m
1362m, 1333m
1357m, 1331m
1362m, 1333m
1
The ‘‘free’’ ligand L exhibit two absorption bands at
1660 and 1053 cmꢀ1 which are assigned to m(C@O) and
m(C–O–C), respectively. In the complexes, the bonds shift
by ca. 45 and 30 cmꢀ1 towards lower wave numbers, thus
indicating that the C@O and ether O-atoms take part in
coordination to the metal ions. The larger shifts m(C@O)
in the spectra of the complexes suggest that the Ln–O (car-
bonyl) bond is stronger than the Ln–O (ether) bond. And
this is in agreement with the crystal structure analysis.
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 Ln(III). The vibration m(C–O) of
Picꢀ at 1265 cmꢀ1 is shifted toward higher frequency by ca.
5 cmꢀ1 in the complexes. This is due to the following two
effects. First, the hydrogen atom of the OH group is
replaced by Ln(III), increasing the p-bond character in
the C–O bond. Secondly, coordination of the oxygen atom
of L to Ln(III) decreases the p-character. The free HPic has
mas(–NO2) and ms(–NO2) at 1555 and 1342 cmꢀ1, respec-
tively, which split into two bands at ca. 1575, 1542 cmꢀ1
and ca. 1360, 1330 cmꢀ1, respectively, in the complexes.
This indicates that O-atoms in the nitro group of Picꢀ take
part in coordination [19]. On the basis of the similarity of
their IR spectra, it may be assumed that the complexes
have the similar structures.
The H signal of the OH group in the free HPic disap-
pears in the complex, indicating that the H-atom of the
OH group is replaced by Ln(III). The benzene ring protons
of the free HPic exhibit a singlet at 9.12 ppm. Upon coor-
dination, the signal moves to higher field. Only one singlet
is observed for the benzene ring protons of the three coor-
dination picrate groups, indicating fast exchange among
the groups in solution [20].
3.4. Electronic spectra
The electronic spectra in the visible region of the Ln(III)
complexes exhibit alternations in intensity and shifts in
position of the absorption bands relative to the corre-
sponding Ln(III) aquo ions. The shift has been attributed
by Jørgensen to the effect on the crystal field of interelec-
tronic repulsion between the 4f electrons, and is related
to the covalent character of the metal–ligand bond,
assessed by SinhaÕs parameter (d), the nephelauxetic ration
(b) and the bonding parameter (b1/2) [21]. Absorption spec-
tra of the Nd(III) complex was registered in acetonitrile
solution at room temperature and the covalent parameters
were calculated (Table 5). The values of b, which are less
than unity, and positive values of d and b1/2 indicate that
the metal–ligand bonds present some covalent character
[22].
1
3.3. H NMR spectra
3.5. Luminescence properties of the complexes
The 1H NMR spectra of the ‘‘free’’ ligand L and the dia-
magnetic Ln(III) complex LaL(Pic)3 were measured in
CD3COCD3. Owing to the solvent effect, the 1H NMR
spectrum of free L in CD3COCD3 is slightly different from
that in CDCl3 (vide ante). In CD3COCD3, the spectrum of
L exhibits two unsolved multiplets at ca. 7.25 and
7.01 ppm, a singlet at 4.59 and a doublet at 4.43 ppm
(J = 6 Hz), assigned to –C6H5–, –C6H4–, –C(O)–CH2–,
–N–CH2–, respectively. Upon coordination, the signal of
–C(O)–CH2– is shifted by 0.15 ppm and those of –C6H4–
protons by only 0.03 ppm. This is probably due to the
inductive effect of Ln–O(L) bonds and a change in the con-
formation of the ligand in the complexes. The larger shift
for –C(O)–CH2– protons than for –C6H4– protons
indicates that the Ln–O(C@O) bond is stronger than the
Ln–O(C–O–C) bond.
Monitored by the emission band at 610 nm, EuL(Pic)3
exhibits broad excitation bands around 400 nm. The lumi-
nescence emission spectra of the ligand L and EuL(Pic)3 in
solid state (the excitation and emission slit widths were
2.5 nm, Fig. 5(a)) and in ethyl acetate, acetone, acetonitrile,
ethanol and methanol solutions (concentration: 1.0 ·
Table 5
Electronic spectral data and covalent parameters of Nd complex
Complex
Frequency (cmꢀ1
)
Assignment
Covalent parameters
4
NdL(Pic)3
19048
17212
13405
12484
4I9/2 ! G9/2
b = 0.9924
d = 0.7658
2
! G7/2
4
! S3/2
b
1/2 = 0.06164
2
! H9/2

K.-Z. Tang et al. / Inorganica Chimica Acta 359 (2006) 1207–1214
1213
Eu(III) (17331 cmꢀ1) and 5D4 (20500 cmꢀ1) of Tb(III).
Thus, the absorbed energy could be transferred from ligand
to the Eu or Tb ions. And we may deduce that the triplet
state energy level T1 of this ligand L matches better to
the lowest resonance level of Eu(III) (Dm = 4549 cmꢀ1
)
than Tb(III) (Dm = 1380 cmꢀ1) ion, because such small Dm
(T1–5D4) could result in the non-radioactive deactivation
of the terbium emitting state via a back-energy transfer
process (T1 Tb(5D4)) and quench the luminescence of
the Tb complex [5c]. Actually, we do not observe the lumi-
nescence of the Tb complex at room temperature in solid
state or in solutions.
It could be seen from Fig. 5(b) that in ethyl acetate solu-
tion the Eu complex has the strongest luminescence, and
then in acetone, acetonitrile, ethanol and methanol. This
is due to the coordinating effects of solvents, namely sol-
vate effect [24]. Together with the raising coordination abil-
ities of ethyl acetate, acetone, acetonitrile, ethanol and
methanol for the lanthanide ions, the oscillatory motions
of the entering molecules consume more energy which the
ligand triplet level transfer to the emitting level of the lan-
thanide ion. Thus, the energy transfer could not be carried
out perfectly.
4. Conclusions
In summary, seven new lanthanide picrate complexes
LnL(Pic)3 (Ln = La, Pr, Nd, Eu, Gd, Tb, Er) of a amide
type podand ligand 2,20-[(1,2-phenylene)bis(oxy)]bis(N-
benzylacetamide) (L) were obtained and structurally char-
acterized. The crystal structure of the complex NdL(Pic)3
shows that the Nd(III) ion is coordinated with four oxy-
gen atoms of the ligand L and six oxygen atoms of three
bidentate picrates. The two arms of the ligand are so
long that their terminal groups can ‘‘fold back’’. Thus,
the two nitrogen atoms of the amide groups in the ligand
are located in the outer part of the whole ligand mole-
cule, which is very important for the assembly of the
complex molecule units into the supermolecule by the
intermolecular hydrogen bonds. It is noteworthy that
the ligand shield Ln3+ using its two long arms, thus,
the solid Eu complex does possess comparatively strong
luminescence at room temperature. And the lowest triplet
state energy level of the ligand indicates that the triplet
state energy level of the ligand matches better to the res-
onance level of Eu(III). To sum up, we designed a new
podand ligand with two long arms, which can strongly
bind Ln(III) ions to form elegant supramolecular archi-
tecture and sensitize Eu(III) emission.
Fig. 5. Emission spectra of the Eu complex: (a) solid state; (b) in different
solutions at room temperature.
10ꢀ3 mol Lꢀ1, the excitation and emission slit widths were
5.0 nm, Fig. 5(b)) were recorded at room temperature. It
can be seen from Fig. 5(a) that the Eu complex shows
strong emission when excited with 400 nm in the solid state.
This indicates that the ligand L is a comparative good
organic chelator to absorb energy and transfer them
to Eu ion. The most intensity ratio value g(5D0 !
7
7F2/5D0 ! F1) is 9.9, showing that the Eu(III) ion does
not lie in a centro-symmetric coordination site [23], in
agreement with the crystal structure analysis.
An intramolecular energy transfer from the triplet state
of the ligand to the resonance level of the Ln(III) ion is one
of the most important processes having influence on the
Ln(III) luminescence properties of Ln(III) chelates [5b].
A triplet excited state T1 which is localized on one ligand
only and is independent of the lanthanide nature [5b]. In
order to acquire the triplet excited state T1 of the ligand
L, the phosphorescence spectrum of the Gd(III) complex
was measured at 77 K in a methanol–ethanol mixture
(V:V = 1:1). The triplet state energy level T1 of the ligand
L, which was calculated from the shortest-wavelength
phosphorescence band [5a], is 21880 cmꢀ1. This energy
level is above the lowest excited resonance level 5D0 of
5. Supplementary data
Crystallographic data for the structure reported in this
paper have deposited with the Cambridge Crystallographic
Data Centre and allocated the deposition number CCDC
279698. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge

1214
K.-Z. Tang et al. / Inorganica Chimica Acta 359 (2006) 1207–1214
(c) C.S. Hamann, A.V. Zelewsky, A. Barbieri, F. Barigelletti, G.
Muller, J.P. Riehl, A. Neels, J. Am. Chem. Soc. 126 (2004) 9339.
[7] (a) C.Y. Su, Y.P. Cai, C.L. Chen, M.D. Smith, W. Kaim, H.C. zur
Loye, J. Am. Chem. Soc. 125 (2003) 8595;
CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: deposit@
ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
(b) C.Y. Su, Y.P. Cai, C.L. Chen, F. Lissner, B.S. Kang, W. Kaim,
Angew. Chem., Int. Ed. Engl. 41 (2002) 3371.
Acknowledgements
[8] B. Tummler, G. Maass, F. Vo¨gtle, J. Am. Chem. Soc. 101 (1979)
¨
This work was supported by the National Natural Sci-
ence Foundation of China (Project 20401008) and the Re-
search Foundation for the Young Teachers Possessing
DoctorÕs Degree of Lanzhou University.
2588.
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