Studies on lanthanide complexes of the tripodal ligand bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine. Crystal structures and luminescence properties

Article abstract of DOI:10.1039/b003566l

A new unsymmetric tripodal ligand bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine (L) bearing two kinds of chromophores benzimidazole and pyridine has been synthesized, and two series of lanthanide(in) complexes [LnL(NO3)3]-C2H5OH (Ln = La, Sm, Eu, Gd, Tb or Tm) and [LnLCl3(H2O)]-C2H5OH (Ln = La, Sm, Eu, Tb or Ho) have been characterized by elemental analyses, FAB mass spectra, 'H NMR, conductivity measurements, and IR spectra. The crystal and molecular structures of the three complexes [SmL(NO3)3]-C2HjOH 1, [GdL(NO3)3]-C2H5OH 2, and [SmLCl^HjO^^HjOH 3 have been determined by X-ray diffraction analysis. 1 and 2 are isomorphous with the central metal ion co-ordinated by the four nitrogen atoms of the ligand and six oxygen atoms of three chelating nitrate groups. In complex 3, besides the four nitrogen atoms of the ligand and three chlorine atoms, there is one H2O molecule co-ordinated. Conductivity studies on the [LnL(NO3)3]-C2H5OH complexes in methanol showed that two of the nitrate anions dissociate to give 2:1 electrolytes. Comparison of the emission lifetimes of the Eu3+ and Tb3t complexes in CH3OH and CD3OD gives the average value for q, the number of co-ordinated methanol molecules, of 3.4, suggesting the formation of species [LnL(NO3)(CH3OH)3][NO3]2. The emission quantum yields of the nitrate-coordinated series are substantially higher than those of the chloride series, as shown by luminescence studies in MeCN. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b003566l

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Studies on lanthanide complexes of the tripodal ligand
bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine.
Crystal structures and luminescence properties†
Xiao-Ping Yang,ab Cheng-Yong Su,*a Bei-Sheng Kang,*a Xiao-Long Feng,a Wang-Leng Xiaoc
and Han-Qin Liua
a School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275,
P. R. China. E-mail: cesscy@zsu.edu.cn
b State Key Laboratory of Rare Earth Materials Chemistry and Applications,
Peking University, Beijing 100871, P. R. China
c Institute of Laser and Spectroscopy, Zhongshan University, Guangzhou 510275, P. R. China
Received 4th May 2000, Accepted 27th July 2000
First published as an Advance Article on the web 5th September 2000
A new unsymmetric tripodal ligand bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine (L) bearing two kinds
of chromophores benzimidazole and pyridine has been synthesized, and two series of lanthanide() complexes
[LnL(NO3)3]ؒC2H5OH (Ln = La, Sm, Eu, Gd, Tb or Tm) and [LnLCl3(H2O)]ؒC2H5OH (Ln = La, Sm, Eu, Tb or Ho)
have been characterized by elemental analyses, FAB mass spectra, 1H NMR, conductivity measurements, and IR
spectra. The crystal and molecular structures of the three complexes [SmL(NO3)3]ؒC2H5OH 1, [GdL(NO3)3]ؒC2H5OH
2, and [SmLCl3(H2O)]ؒC2H5OH 3 have been determined by X-ray diffraction analysis. 1 and 2 are isomorphous with
the central metal ion co-ordinated by the four nitrogen atoms of the ligand and six oxygen atoms of three chelating
nitrate groups. In complex 3, besides the four nitrogen atoms of the ligand and three chlorine atoms, there is one H2O
molecule co-ordinated. Conductivity studies on the [LnL(NO3)3]ؒC2H5OH complexes in methanol showed that two
of the nitrate anions dissociate to give 2:1 electrolytes. Comparison of the emission lifetimes of the Eu3ϩ and Tb3ϩ
complexes in CH3OH and CD3OD gives the average value for q, the number of co-ordinated methanol molecules, of
3.4, suggesting the formation of species [LnL(NO3)(CH3OH)3][NO3]2. The emission quantum yields of the nitrate-co-
ordinated series are substantially higher than those of the chloride series, as shown by luminescence studies in MeCN.
complexes.1,3 Based on the inherent character of the benzimid-
Introduction
azole group which allows easy derivation and potentially strong
There have been vigorous studies on the chemistry of multi-
dentate ligands with lanthanide() ions in the past decade.
π–π stacking interaction to form supramolecular structures,18,19
we have investigated lanthanide complexes with tripodal lig-
Much of this has been spurred by their potential uses as
ands incorporating benzimidazole rings.20–22 The luminescent
supramolecular devices,1–3 fluorescent sensors, or luminescent
lanthanide complexes of a symmetric tripodal N-donor ligand
probes.4–7 In order to obtain strongly luminescent complexes,
tris(benzimidazol-2-ylmethyl)amine (ntb) have been prepared,
the chromophoric ligands which chelate to the lanthanide
indicating that such ligands with benzimidazole groups are
metals should be able (i) to absorb energy and transfer it effi-
good candidates to obtain high luminescence.20 In an effort to
ciently to the central metal and (ii) to encapsulate and protect
explore these kind of complexes in more detail to inspect the
ligand-to-metal energy transfer efficiency, herein we describe a
new unsymmetric tripodal ligand bis(2-benzimidazolylmethyl)-
(2-pyridylmethyl)amine (L, Scheme 1) bearing two kinds of
the lanthanide ion from the solvent molecules.1 For these
purposes, macrocyclic, macrobicyclic (cryptand) and podand-
type ligands have extensively been used.8–11
Work on the applications of lanthanide complexes as
chromophores, benzimidazole and pyridine, and two series of
contrast agents for magnetic resonance imaging (MRI),
its lanthanide complexes.
luminescent stains of fluoroimmunoassays, and catalysts for
the selective cleavage of RNA and DNA requires close
Experimental
General comments
and tunable control of the co-ordination sphere to enhance
specific structural and electronic properties.12–14 As contrasting
agents, the tripodal ligands derived from Schiff-base conden-
sation with tris(2-aminoethyl)amine (tren) have attracted much
attention.15–17 With three suitably designed arms, the tripodal
ligands could provide hosts for the lanthanide ions and well
encapsulate them, and by deliberate incorporation of appro-
priate multiple absorption groups suitable for energy transfer
they could be used to develop strongly luminescent lanthanide
Hydrated lanthanide nitrates and chlorides were prepared by
dissolving the corresponding oxides (99.1%) in 50% nitric acid
and 40% hydrochloric acid, respectively. o-Phenylenediamine,
iminodiacetic acid and 2-pyridylmethyl chloride hydrochloride
were obtained from Aldrich and used without further purifi-
cation. Elemental analyses (C, H, N) were carried out on a
Varian EL elemental analyser. 1H NMR spectra were recorded
on a Varian INOVA 500 spectrometer, UV spectra on a
SHIMADZU UV-2501PC spectrophotometer, infrared spectra
from 4000 to 400 cmϪ1 on a Bruker EQINOX 55 FT-IR and
fast atom bombardment (FAB) mass spectra on a VG ZAB-HS
† Electronic supplementary information (ESI) available: hydrogen
bonded network in complex 1, 1H NMR spectra, decay curves and
selected bond angles for 1–3. See http://www.rsc.org/suppdata/dt/b0/
b003566l/
DOI: 10.1039/b003566l
J. Chem. Soc., Dalton Trans., 2000, 3253–3260
This journal is © The Royal Society of Chemistry 2000
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7.74 (t, 1 H) and 8.51 (d, 1 H). νmax/cmϪ1 3058, 1621s, 1594,
1443s, 1382, 1274 and 743s.
Bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine
(L).
An 200 ml volume of 6 M HCl solution containing dipotassium
N-(2-pyridylmethyl)iminodiacetate (3.00 g, 0.01 mol) and
o-phenylenediamine (2.16 g, 0.02 mol) was refluxed for 72 h.
A spoonful of active carbon was added after lowering the
temperature and the solution continually refluxed for 5 min.
After being filtered, the solution was treated with an excess
of NH4OH to give a brown residue which was collected by
filtration and dissolved in 40 ml of CHCl3. After filtration, the
solution was washed three times with 20 ml of 0.1 M
NaOH solution. Then, the CHCl3 was removed at reduced
pressure to afford a solid that was recrystallized from ethanol to
give the pure product as a yellow solid (1.53 g, 42%). The IR,
MS (FAB), elemental analysis and 1H NMR data were identical
to those obtained by method 1.
Syntheses of complexes
[LnL(NO3)3]ؒC2H5OH (Ln ؍ La, Sm, Eu, Gd, Tb or Tm). To
a clear solution of L (0.018 g, 0.05 mmol) in ethanol (5 ml) was
added Ln(NO3)3ؒ5H2O (0.05 mmol) in 5 ml of ethanol. The
resulting solution was left standing overnight at room temper-
ature to afford a pale yellow solid which was filtered off, washed
three times with ethanol and diethyl ether successively, and
dried over CaCl2 for 1 day to give the product in 50–70%
yield. Pale yellow single crystals were obtained by vapor-phase
diffusion of diethyl ether into the ethanol solution.
Scheme 1 Synthetic routes for the preparation of the ligand L.
instrument. Conductivity measurements were carried out with
a DDS-11 conductivity bridge for 10Ϫ3 mol dmϪ3 solutions in
CH3CN and CH3OH.
[LnLCl3(H2O)]ؒC2H5OH (Ln ؍ La, Sm, Eu, Tb or Ho). In a
similar method, a solution of LnCl3ؒ6H2O (0.05 mmol) in 5 ml
of ethanol was added to L (0.018 g, 0.05 mmol) in ethanol
(5 ml) and the mixture treated as above. The yield was in
the range 40–70%. Pale yellow single crystals were obtained as
above.
Synthesis of ligand
The ligand bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine
(L) was prepared by the following synthetic routes.
FAB MS, IR and elemental analysis data for all complexes
are summarized in Table 1.
Method 1. Bis(2-benzimidazolylmethyl)amine. Reaction was
run according to ref. 23, and the compound obtained as a white
solid, yield 29%. δH ((CD3)2SO, 298 K) 4.01 (s, 4 H), 7.13 (m, 4
H) and 7.51 (m, 4 H). νmax/cmϪ1 3395 (br), 3184, 1616s, 1539,
1440, 1307 and 744s.
Photophysical studies
Absorption spectra were obtained on a SHIMADZU UV-
2501PC spectrophotometer, emission and excitation spectra
on a HITACHI 850 spectrometer. Fluorescence quantum
yields were determined by using [Ru(bipy)3]Cl2 (bipy = 2,2Ј-
bipyridine; Φ = 0.028 in water)24 as standard for the Eu3ϩ com-
plex, and quinine sulfate (Φ = 0.546 in 0.5 mol dmϪ3 H2SO4)25
for the Tb3ϩ complex. The luminescence decays were recorded
using a pumped dye laser (Lambda Physics model FL2002) as
the excitation source. The nominal pulse width and the line-
width of the dye-laser output were 10 ns and 0.18 cmϪ1, respec-
tively. The emission of a sample was collected by two lenses into
a monochromator (WDG30), detected by a photomultiplier
and processed by a Boxcar Average (EGG model 162) in line
with a microcomputer. The number of co-ordinated solvent
molecules (q) for the Eu3ϩ and Tb3ϩ complexes was calculated
from Horrocks equation q = n (τHϪ1 Ϫ τDϪ1) where τH is the
lifetime in the protonated solvent (water or CH3OH), τD that
in the corresponding deuteriated solvent, and the value of n is
1.05 (Eu3ϩ in H2O/D2O), 4.2 (Tb3ϩ in H2O/D2O), 2.1 (Eu3ϩ in
CH3OH/CD3OD), or 8.4 (Tb3ϩ in CH3OH/CD3OD).1
Bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine (L). To
a solution of bis(2-benzimidazolylmethyl)amine (2.77 g, 0.01
mol) in methanol (150 ml) was added K2CO3 (2.00 g, 0.014 mol)
and the mixture stirred and heated for 10 min. 2-Pyridylmethyl
chloride hydrochloride (3.28 g, 0.02 mol) in 50 ml of methanol
was added dropwise in 30 min and the resulting solution stirred
and heated to reflux under nitrogen for 24 h. After filtration, the
filtrate was reduced in volume at low pressure to leave a brown
residue which was dissolved in 60 ml of CHCl3 and filtered. The
filtrate was washed with 20 ml of 0.1 M NaOH solution three
times and the CHCl3 removed at low pressure to leave a solid
that was recrystallised from ethanol to give 0.56 g of product
(15%). νmax/cmϪ1 3220 (br), 3145 (br), 3054, 1622, 1594, 1588,
1538, 1484, 1434s, 1350, 1272, 999 and 746s. FAB MS and
elemental analysis data are summarized in Table 1.
Method 2. Dipotassium N-(2-pyridylmethyl)iminodiacetate.
Iminodiacetic acid (2.66 g, 0.02 mol) was dissolved in an
aqueous solution (20 ml) of KOH (2.24 g, 0.04 mol) and
2-pyridylmethyl chloride hydrochloride (3.28 g, 0.02 mol) in an
aqueous solution (20 ml) of KOH (1.12 g, 0.02 mol) added with
stirring. After addition of 1.38 g (0.01 mol) of K2CO3, the
resulting red solution was stirred at 80 ЊC for 18 h. The solvent
was then removed at reduced pressure and 20 ml of methanol
were added to the resulting dark purple residue. After filtration,
the dark purple filtrate was concentrated at reduced pressure to
precipitate the crude product which was recrystallized from
ethanol to give the purple product (4.69 g, 78%). δH ((CD3)2SO,
298 K) 4.08 (s, 2 H), 4.49 (s, 4 H), 7.23 (t, 1 H), 7.70 (d, 1 H),
X-Ray crystallography
Details of crystallographic parameters, data collection and
refinements are listed in Table 2. The coordinates of the non-
hydrogen atoms were refined anisotropically, while hydrogen
atoms were included in the calculation isotropically but not
refined. The ethanol solvate molecules in complexes 1 and 2
exhibit disorder over two positions and were refined isotropic-
ally with half site occupancy. In 3 the two carbon atoms of the
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Table 1 Mass, elemental analytical, IR spectral and conductivity data for complexes
Elemental analyses (%)a
IR (νmax/cmϪ1
)
ΛM
Ω
/
c
Ϫ1 cm2
b
FAB MS, m/z
C
H
N
νC᎐N
νN–O
molϪ1
L
369 [M ϩ H]ϩ
71.93(71.72) 5.48(5.47) 23.08(22.81) (1622,1594)
38.63(38.99) 3.61(3.54) 17.21(17.05) (1623,1610)
38.12(38.39) 3.41(3.49) 17.47(16.79) (1622,1605)
38.59(38.31) 3.63(3.48) 16.92(16.75) (1621,1605)
38.06(38.04) 3.37(3.46) 16.57(16.64) (1623,1609)
37.58(37.96) 3.36(3.45) 16.13(16.60) (1623,1608)
37.74(37.46) 3.43(3.41) 16.20(16.38)
41.84(42.53) 3.89(4.16) 12.35(12.40) (1624,1609)
41.85(41.82) 4.37(4.09) 12.09(12.19) (1622,1609)
41.98(41.73) 3.95(4.08) 12.14(12.16) (1623,1607)
[LaL(NO3)3]ؒC2H5OH
[SmL(NO3)3]ؒC2H5OH
[EuL(NO3)3]ؒC2H5OH
[GdL(NO3)3]ؒC2H5OH
[TbL(NO3)3]ؒC2H5OH
[TmL(NO3)3]ؒC2H5OH
[LaLCl3(H2O)]ؒC2H5OH
[SmLCl3(H2O)]ؒC2H5OH
[EuLCl3(H2O)]ؒC2H5OH
[M Ϫ EtOH Ϫ NO3 ϩ H]ϩ
(1295,1463)
(1295,1458)
(1295,1458)
(1300,1475)
(1298,1470)
187.2
191.2
209.1
201.7
219.9
221.7
653 [M Ϫ EtOH Ϫ NO3 ϩ 2H]ϩ
577 [M Ϫ EtOH Ϫ H2O Ϫ Cl]ϩ
591 [M Ϫ EtOH Ϫ H2O Ϫ
Cl ϩ H]ϩ
[TbLCl3(H2O)]ؒC2H5OH
[HoLCl3(H2O)]ؒC2H5OH
41.58(41.31) 3.83(4.04) 12.31(12.04) (1623,1606)
41.22(40.96) 4.36(4.01) 11.82(11.94)
a Data in brackets are calculated values. b Imidazole ring. c Measured at 25 ЊC.
ethanol molecule were disordered between two slightly different
locations with total occupancy of one, individually. Neutral
atom scattering factors were taken from Cromer and Waber.26
CCDC reference number 186/2119.
See http://www.rsc.org/suppdata/dt/b0/b003566l/ for crystal-
lographic files in .cif format.
Results and discussion
Synthesis
Two different routes were designed to synthesize the ligand as
shown in Scheme 1, using the same starting materials with
reversed reaction processes and the final yields were quite dif-
ferent. In route 1, where condensation precedes substitution,
although an excess of 2-pyridylmethyl chloride hydrochloride
(ii) was used (A:ii = 1:2), the yield of the second step was quite
low (15%) and 78% of bis(2-benzimidazolylmethyl)amine was
recovered at the end. It is possible that the steric hindrance
resulting from free rotation of the two large benzimidazole
groups prevents approach of the pyridylmethyl group. Change
of the reaction conditions, i.e. the basicity of the reaction or the
solvent, did not improve the yield. The yield of the first step was
also not high (29%). In order to improve the total yield, route 2
was designed. The steric effect in the first step was small for the
2-pyridylmethyl group to react with the iminodiacetic acid and
a good yield of B was obtained (78%). The final yield was 32%
based on iminodiacetic acid, as compared to that of route 1,
which was only 4%. Pure product was obtained after recrystal-
lization and it was fairly soluble in many organic solvents such
as methanol, acetone, acetonitrile and benzene.
In order to study how the anions control the reactions, both
Ln(NO3)3 and LnCl3 were treated respectively with L in a 1:1
molar ratio in ethanol. Elemental analyses and FAB MS
confirmed the 1:1 ligand:metal ratio for all complexes. On the
basis of the similarity of their IR spectra (see later), it is
assumed that the rest of the complexes have the same formula
[LnL(NO3)3]ؒC2H5OH or [LnLCl3(H2O)]ؒC2H5OH (Table 1). In
the FAB MS spectra of the complexes, besides the solvate
ethanol molecule, some base ions lost one internal anion.
Fig. 1 Molecular structure of the complex [GdL(NO3)3]ؒC2H5OH
showing the atom labeling scheme. All hydrogen atoms have been
omitted for clarity.
NO3Ϫ groups. In the crystal structure of 3, besides the four nitro-
gen atoms of the ligand and three chloride ions, there is one
water molecule co-ordinated, resulting in an 8-co-ordinated
metal ion. As mentioned earlier, nitrate-based lanthanide com-
plexes have high co-ordination numbers as the donor atoms
of bidentate nitrate are closer together than those of two
independent monodentate ligands such as ClϪ, due to the small
bite angle in an NO3Ϫ group.
For all the complexes, the ligand L exhibits a tripodal
co-ordination mode, with three imino nitrogen (two from benz-
imidazolyl and one from pyridyl group) and one amino nitro-
gen atom as donors. Selected bond lengths are given in Table 3
(selected bond angles in Tables S1 and S2 in ESI). The average
Ln–Nimine distances (2.589 (1), 2.565 (2) and 2.620 (3) Å) are
significantly shorter than Ln–Namine distances (2.725 (1), 2.696
(2) and 2.700 (3) Å), respectively, probably due to steric
Crystal structures
requirements.
A
similar phenomenon was reported for
The crystallographic data for the complexes 1–3 are listed in
Table 2 and their structures and atomic numbering schemes
shown in Figs. 1, 2, and 3, respectively. Each complex is com-
posed of a neutral [LnL(NO3)3] (Ln = Sm or Gd) or [SmLCl3-
(H2O)] entity and a solvated ethanol molecule. Complexes 1
and 2 are isomorphous with the central metal ion 10-co-
ordinated, being linked to the four nitrogen atoms of the tetra-
dentate ligand L and to the six oxygen atoms of three bidentate
[Gd(L1)]2ؒ(2CHCl3)
(HL1 = tris[2-(2-hydroxybenzylamino)-
ethyl]amine).16 The average Ln–O distances in complexes 1 and
2 are 2.534 (1) and 2.512 (2) Å, respectively, comparable to
those in the tripodal analogue [Ln(ntb)(NO3)3]ؒH2O (Ce–O
2.597, Er–O 2.473),21 taking into account the difference in ionic
radii.
It is interesting that the configuration of the ligand in com-
plexes 1 and 2 is quite different from that in 3. In the former the
J. Chem. Soc., Dalton Trans., 2000, 3253–3260
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Table 2 Crystal data and structure refinement for complexes 1, 2 and 3
1
2
3
Empirical formula
Formula weight
C24H26N9O10Sm
750.89
C24H26GdN9O10
757.79
C24H28Cl3N6O2Sm
689.22
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
a/Å
b/Å
c/Å
10.3670(10)
26.500(2)
11.4070(10)
108.77
2967.1(4)
4
10.3130(10)
26.376(2)
11.3680(10)
109.000(10)
2923.8(4)
4
11.6120(10)
10.9130(10)
21.5700(10)
100.19
2690.3(4)
4
β/Њ
V/Å3
Z
µ/mmϪ1
2.048
2.339
2.514
T/K
Rint
293(2)
0.0329
293(2)
0.0549
293(2)
0.0318
No. reflections/observed
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
6424/4952
0.0343
0.0834
6372/4084
0.0381
0.0617
5870/4666
0.0241
0.0489
Table 3 Selected bond lengths (Å) for complexes 1, 2 and 3
1
2
3
M(1)–O(2)
M(1)–O(3)
M(1)–O(5)
M(1)–O(6)
M(1)–O(8)
M(1)–O(9)
M(1)–O(1W)
M(1)–Cl(1)
M(1)–Cl(2)
M(1)–Cl(3)
M(1)–N(1)
M(1)–N(3)
M(1)–N(5)
M(1)–N(6)
2.491(3)
2.581(3)
2.549(3)
2.510(3)
2.511(3)
2.559(3)
2.465(3)
2.558(3)
2.543(3)
2.482(3)
2.486(3)
2.537(3)
2.4943(18)
2.7055(7)
2.6967(7)
2.8184(7)
2.558(2)
2.575(2)
2.728(2)
2.700(2)
2.550(3)
2.547(3)
2.669(3)
2.725(3)
2.531(3)
2.525(3)
2.640(4)
2.696(3)
Fig. 2 View of the fan-like configuration of the ligand in [SmL-
(NO3)3]ؒC2H5OH showing the atom labeling scheme. All hydrogen
atoms and nitrate groups have been omitted for clarity.
Scheme 2 Fan-like (a) and pincer-like (b) configurations of the
ligand (benzimidazolyl, ; pyridyl, ).
(Scheme 2, b). Consequently, the pyridyl ring inclines to the
benzimidazolyl ring containing N(1), and the centroid-to-
centroid distances (3.956, 7.100, 7.884 Å) and the Nimine–Ln–
Nimine angles (64.46, 93.37, 127.15Њ) are quite different from
each other (Fig. 3).
It is noteworthy that in complex 3 the dihedral angle between
the pyridyl ring and the adjacent benzimidazolyl ring is 33.7Њ,
together with the center-to-center distance of 3.96 Å, implying
weak π–π interaction between the two rings.27 In the meantime,
weak π–π stacking interaction exists intermolecularly as shown
in Fig. 4. The vertical distance between two offset benzimid-
azole rings which are exactly parallel is 3.53 Å. These intra- and
inter-molecular interactions offset the steric effect of the two
neighboring rings and stabilize the pincer-like configuration.
On the other hand, as the pyridyl ring is close to one of the
benzimidazole groups there is more space available to allow a
water molecule to enter the co-ordination sphere. In complexes
1 and 2, besides ligand L, the co-ordination environment of the
metal ion is blocked by three bidentate nitrate ligands and a
water molecule cannot enter the co-ordination cavity. Since co-
ordinated solvent molecules, especially water or alcohol, can
efficiently quench lanthanide luminescence, the ability to satisfy
the co-ordination requirements of the lanthanide() centre
with ten donors without additionally bound solvent molecules
Fig. 3 Molecular structure of the complex [SmLCl3(H2O)]ؒC2H5OH
showing the atom labeling scheme. All hydrogen atoms have been
omitted for clarity.
ligand exhibits a fan-like configuration (Scheme 2, a), with the
three heterocyclic rings deflecting to the same direction and
apart from each other at similar distances (Fig. 2). The dis-
tances between the centroids of the three rings are 6.808, 6.879,
and 7.315 Å for complex 1 (or 6.783, 6.841, and 7.287 Å for 2),
and the angles Nimine–Ln–Nimine (99.43, 100.36, 102.66 (1) or
99.59, 100.47, 103.44Њ (2)) are also similar. However, in com-
plex 3, the pyridyl ring deflects to the opposite direction to the
two benzimidazolyl rings, forming a pincer-like configuration
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Fig. 4 The intra- and inter-molecular π–π stacking interactions
between two molecules in the complex [SmLCl3(H2O)]ؒC2H5OH.
becomes an important criterion in the design of supra-
molecular photonic devices.1
The hydrogen bonds play important roles in the crystal pack-
ing of all complexes. In 1 (or 2), atoms O(7) and O(9) of one
nitrate group act as hydrogen bond acceptors to form O ؒ ؒ ؒ
H–C(12) (O ؒ ؒ ؒ H, 2.53 (1) and 2.57 Å (2), O ؒ ؒ ؒ H–C, 129.8 (1)
and 129.9Њ (2)) and O ؒ ؒ ؒ H–N(4) (O ؒ ؒ ؒ N, 3.07 (1) and 3.07 Å
(2), O ؒ ؒ ؒ H–N, 164.9 (1) and 166.7Њ (2)), respectively, with a
neighboring molecule, where C(12) and N(4) are the hydrogen
bond donors. In the mean time, O(1), O(3), C(4) and N(2) are
involved in a cyclic hydrogen bonding mode with N(2), C(4),
O(3) and O(1) atoms, respectively, of a third neighboring
molecule (O ؒ ؒ ؒ N, 3.03 (1) and 3.04 Å (2), O ؒ ؒ ؒ H–N, 160.3 (1)
and 159.8Њ (2); O ؒ ؒ ؒ H, 2.42 (1) and 2.43 Å (2), O ؒ ؒ ؒ H–C, 128
(1) and 127.6Њ (2)), thus generating a two-dimensional layer as
shown in Fig. 5(a). In addition, such layers are cross-linked by
O ؒ ؒ ؒ H–C (O ؒ ؒ ؒ H, 2.36 (1) and 2.37 Å (2), O ؒ ؒ ؒ H–C, 164.6
(1) and 166Њ (2)) formed between O(1) and C(6) belonging to
adjacent layers, resulting in a three-dimensional open network
(Fig. S1 in ESI). Extended channels are formed within the net-
work along the c axis in which disordered ethanol molecules are
accommodated. The ability of carbon atoms to act as proton
donors in hydrogen bonds has been the subject of argument for
many years and Taylor and Kennard28 provided conclusive
evidence of the existence of C–H ؒ ؒ ؒ Y (Y = N, O, P, Cl or Br)
in crystals in 1982. They showed that C–H ؒ ؒ ؒ Y contacts are
electrostatic with the C–H–Y angle in the range of 90–180Њ, the
distance between the donor and acceptor being shorter than the
sum of their van der Waals radii. The O ؒ ؒ ؒ H distances found
in 1 and 2 (2.36–2.58 Å) fall well within the estimated range of
2.16–2.65 Å.28 Hydrogen bonding between aromatic hydrogen
and electronegative atoms was also reported by Atwood et al.29
The crystal packing in 3 is dominated by Cl ؒ ؒ ؒ H–O and
Cl ؒ ؒ ؒ H–N hydrogen bondings. Each Cl(3) atom is connected
to O(1w) (Cl ؒ ؒ ؒ O 3.22 Å, Cl ؒ ؒ ؒ H–O 141.8Њ) and N(4)
(Cl ؒ ؒ ؒ N 3.20 Å, Cl ؒ ؒ ؒ H–N 151.6Њ) belonging to different
molecules thus generating a two-dimensional layer as shown in
Fig. 5(b). The crystal structure of 3 is stacked by such layers
within which the disordered ethanol molecules are located.
Fig. 5 Hydrogen-bonding networks in the complexes: (a) 2-D layers
generated by O ؒ ؒ ؒ H–C and O ؒ ؒ ؒ H–N hydrogen bonds in 1; (b) 2-D
layers generated by Cl ؒ ؒ ؒ H–O and Cl ؒ ؒ ؒ H–N hydrogen bonds in 3.
its counterpart for the “free” ligand. For all the lanthanide com-
plexes, broad bands at about 3450 cmϪ1 occur from the O–H
stretching vibrations of ethanol and water. In addition, the
absence of bands at 1380, 820, and 720 cmϪ1 in the spectra of
[LnL(NO3)3]ؒC2H5OH indicates that free nitrate groups (D3h)
are absent, instead two intense absorptions associated with the
asymmetric stretching appear in the ranges 1295–1300 (ν4) and
1458–1475 (ν1) cmϪ1, clearly establishing that the NO3Ϫ groups
(C2ν) are co-ordinated.31 This is in agreement with the results of
the structural analyses discussed above. The close similarity of
the infrared spectra within each series [LnL(NO3)3]ؒC2H5OH
and [LnLCl3(H2O)]ؒC2H5OH suggests the same co-ordination
behaviors in the respective complexes.
1H NMR and conductivity studies in solution
The 1H NMR spectra of the “free” ligand L and the
diamagnetic lanthanum() complex [LaL(NO3)3]ؒC2H5OH
were measured in CD3CN and DMSO-d6. The chemical shift
data are given in Table 4 and 1H NMR spectra shown in Fig. 6.
In CD3CN, due to the electronic delocalization in imidazole
rings and free rotation of the three arms, H2 and H5 in the
“free” ligand have the same environments and show one signal
(δ 7.59). However, the three arms of the ligand become more
rigid after co-ordination in complex [LaL(NO3)3]ؒC2H5OH and
will be influenced by the co-ordinated N1 and N3 which become
more electropositive, and the resonance of H5 shifts to lower
field (δ 7.95), as compared to that of H2 (δ 7.50) which is farther
away from N1 and N3. For the four H3 and H4 hydrogens, the
effect of the co-ordinated N1 and N3 nitrogen atoms becomes
weaker and cannot be differentiated on the NMR timescale. In
Infrared spectra
The “free” ligand exhibits two absorption bands at 1622 and
1594 cmϪ1 which are assigned to the C᎐N stretchings in the
᎐
imidazole ring.30 The lanthanide complexes also exhibit these
two bands. The high-energy band remains unchanged while the
low-energy band blue shifts to about 1608 cmϪ1 as compared to
J. Chem. Soc., Dalton Trans., 2000, 3253–3260
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Table 4 1H NMR data for the ligand and complex [LaL(NO3)3]ؒC2H5OH in CD3CN and DMSO-d6 (m, multiplet; s, singlet; d, double; t, triplet)
Compounds
δH10
δH1
δH3,4
δH8
δH6
δH2,5
δH7
δH9
La
3.91 (s)
4.28 (m)
3.92 (s)
3.95 (s)
4.03 (s)
4.31 (d)
4.06 (s)
4.07 (s)
7.20 (m)
7.28 (m)
7.17 (m)
7.16 (m)
7.25 (t)
7.41 (t)
7.26 (dd)
7.24 (dd)
7.48 (d)
7.45 (d)
7.70 (d)
7.61 (d)
7.59 (m)
(7.50m, 7.95m)
7.57 (m)
7.69 (t)
7.89 (t)
7.77 (td)
7.75 (td)
8.61 (d)
8.75 (d)
8.52 (d)
8.52 (d)
[LaL(NO3)3]ؒC2H5OHa
Lb
[LaL(NO3)3]ؒC2H5OHb
7.55 (m)
a Measured in CD3CN. b Measured in DMSO-d6.
Fig. 6 1H NMR spectra of the ligand L (a) and [LaL(NO3)3]ؒC2H5OH
(b) at 25 ЊC in CD3CN.
Fig. 7 Absorption spectra.
DMSO-d6 the complex [LaL(NO3)3]ؒC2H5OH shows an identi-
1
cal H NMR spectrum to that of the “free” ligand, indicating
that the ligand L has dissociated (Fig. S2 in ESI). Owing to the
1
solvent effect, the H NMR spectrum of free L in CD3CN is
slightly different from that in DMSO-d6.
The conductivities of the complexes [LaL(NO3)3]ؒC2H5OH
were measured in CH3OH and CH3CN. The molar conduc-
tivities in CH3OH lie in the range 180–230 ΩϪ1 cm2 molϪ1 (Table
1), indicating that all complexes are 2:1 electrolytes.32 However,
in CH3CN, the molar conductances of these complexes are
much smaller (≈100 times) than those in methanol, indicating
that the complexes remain intact and therefore neutral.
1
From the results of H NMR and conductivity measure-
ments, it is seen that, in δ-donor solvents, there is likely competi-
tion between the solvent and the ligand for the lanthanide ion.
In the strongly polar DMSO the complex tends to decompose
to form solvated Ln3ϩ species,33 while in the moderately polar
CH3CN the complexes remain intact. In CH3OH the ligand
still co-ordinates, although two nitrates dissociate. A similar
phenomenon was found in heptadentate N4O3 Schiff base
and ntb lanthanide complexes.15,20
Fig. 8 The emission spectra of the ligand ( ؒ ؒ ؒ ), [EuL(NO3)3]ؒ
C2H5OH (- - - -; 5D0 → 7FJ, J = 1, 2, 3 or 4) and [TbL(NO3)3]ؒ
C2H5OH (——; 5D4 → 7FJ, J = 3, 4, 5 or 6).
Photophysical properties of the ligand and its Eu3؉ and Tb3؉
complexes
correspond to the singlet and triplet states.3,18,35 Excitations in
the ligand-based π–π* transitions cause the structured emission
of lanthanide complexes while the ligand luminescence is com-
pletely quenched showing that ligand-to-metal energy transfer
occurs.36 The ability to transfer energy from ligand-centered to
metal-centered is important in the design of lanthanide()
supramolecular photonic devices.1,37 The emission spectra of
complexes [EuL(NO3)3]ؒC2H5OH and [TbL(NO3)3]ؒC2H5OH
are shown in Fig. 8, and the emission quantum yields (Φ) are
listed in Table 5. The stronger and better resolved spectrum of
the Tb3ϩ complex (Φ = 0.38) showed the expected sequence of
5D4 → 7FJ transitions (J = 6, 5, 4 or 3) and no further
splitting is visible due to the limitation of the equipment. In
a typical spectrum (Φ = 0.031, relatively weak) of the Eu3ϩ
As the complexes are stable in CH3CN the photophysical
investigations were carried out in this solution. Free L displays
three strong absorption bands in the UV spectral region at 220,
276 and 283 nm, arising from π–π* transitions together with
a broad low band around 245 nm (Fig. 7).34 These bands
are strongly perturbed upon co-ordination to Ln3ϩ in [LnL-
(NO3)3]ؒC2H5OH and [LnLCl3(H2O)]ؒC2H5OH (Ln = Eu or Tb)
and blue shifts are found for the last two bands, implying
co-ordination of the ligand. These complexes display almost
identical spectral profiles and the absorption spectra of [EuL-
(NO3)3]ؒC2H5OH and [TbL(NO3)3]ؒC2H5OH are also shown in
Fig. 7.
The luminescence spectra of the ligand and its Eu3ϩ and Tb3ϩ
complexes were recorded at room temperature. Excited by the
absorption band at 276 nm, the “free” ligand exhibits two
broad emission bands (λmax = 376 and 504 nm) which may
5
complex, the relative intensity of D0 → 7F2 is more intense
than that of 5D0 → 7F1, showing that the Eu3ϩ ion does
not lie in a centrosymmetric co-ordination site.20
3258
J. Chem. Soc., Dalton Trans., 2000, 3253–3260

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Table 5 Luminescence lifetime data (τ) and derived solvation values (q) for the Eu3ϩ and Tb3ϩ complexes
τ/ms
q
Complex
Φ
CH3CN
CH3OH
CD3OD
CH3CN–D2O
water
CH3OHb
[EuL(NO3)3]ؒC2H5OH
[TbL(NO3)3]ؒC2H5OH
[EuLCl3(H2O)]ؒC2H5OH
[TbLCl3(H2O)]ؒC2H5OH
0.031
0.38
0.001
0.18
0.71
1.14
0.31
0.80
0.36
0.95
—
0.81
1.57
—
—
—
0.25
1.05
—
—
0.8
1.3
3.2
3.5
—
—
—
—
a The number of co-ordinated water molecules for complexes [LnLCl3(H2O)]ؒC2H5OH (Ln = Eu or Tb) in CH3CN. b The number of co-ordinated
methanol molecules for complexes [LnL(NO3)3]ؒC2H5OH (Ln = Eu or Tb) in CH3OH.
The values of Φ for [LnLCl3(H2O)]ؒC2H5OH are lower than
those for [LnL(NO3)3]ؒC2H5OH (Ln = Eu or Tb) which is evi-
dent from the fact that the co-ordinated water molecule in the
former partially quenches the luminescence and decreases the Φ
value. Through vibronic coupling with the vibrational states of
the O–H oscillators, efficient non-radiative deactivations take
place in the lanthanide active state.37 At the same time, by
comparing the emission lifetimes of the complexes in CH3CN
before and after addition of a few drops of D2O which will
exchange with the water molecule, the effect of the co-ordinated
water on the luminescence intensity of [LnLCl3(H2O)]ؒC2H5OH
was checked.38 The metal-centered emission lifetimes were
determined by monitoring the decay of the 5D0 → 7F2 transi-
tion (615 nm) for the Eu3ϩ complex, and the 5D4 → 7F5 tran-
sition (550 nm) for the Tb3ϩ complex in CH3CN and CH3CN–
D2O. The lifetimes of metal-centered emissions become longer
after treatment with D2O, and use of the Horrocks equation
gives the number of co-ordinated water molecules (q) (see Table
5).1,37 Considering the error in q ( 0.5), the values of 0.8 for the
Eu3ϩ complex and 1.3 for Tb3ϩ suggest one water molecule is
co-ordinated in CH3CN, in agreement with the crystallographic
analyses.
the fact that dissociation of nitrates in methanol permits
solvent-based quenching, with average effective solvation
number (q) of 3.4. In CH3CN the emission quantum yields
of [LnL(NO3)3]ؒC2H5OH are higher than those of [LnLCl3-
(H2O)]ؒC2H5OH (Ln = Eu or Tb) where there is one co-
ordinated water molecule which could partially quench the
luminescence.
Acknowledgements
Financial support from the National Natural Science
Foundation of China, the Natural Science Foundation of
Guangdong Province and from the State Key Laboratory
of Rare Earth Materials Chemistry and Applications, Peking
University, is greatly appreciated.
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