Structure and fluorescence of three new homodinuclear lanthanide (III)/Schiff-base compounds [{Ln(clapi)}2]·2CH3CN {Ln=La, Ce, Eu; H3clapi = 2-(5-chloride-2-hydroxyphenyl)-1,3-bis [4-(5-chloride-2-hydroxyphenyl)-3-azabut-3

Article abstract of DOI:10.1007/s10870-008-9351-9

Three new homodinuclear lanthanide(III)/Schiff-base compounds [{Ln(clapi)}₂]·2CH₃CN {Ln = La 1, Ce 2, and Eu 3; H₃clapi = 2-(5-chloride-2-hydroxyphenyl) - 1,3-bis [4-(5-chloride-2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidaz

Full text of DOI:10.1007/s10870-008-9351-9

J Chem Crystallogr (2008) 38:595–599
DOI 10.1007/s10870-008-9351-9
ORIGINAL PAPER
Structure and Fluorescence of Three New Homodinuclear
Lanthanide (III)/Schiff-base Compounds [{Ln(clapi)}2] Á 2CH3CN
{Ln=La, Ce, Eu; H3clapi = 2-(5-Chloride-2-hydroxyphenyl)-1,3-
bis [4-(5-chloride-2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-
imidazolidine}
Zhang Lijuan Æ Zhou Yunshan Æ Jiang Fei
Received: 29 October 2006 / Accepted: 27 March 2008 / Published online: 9 April 2008
Ó Springer Science+Business Media, LLC 2008
Abstract Three new homodinuclear lanthanide(III)/Schiff-
base compounds [{Ln(clapi)}2] Á 2CH3CN {Ln = La 1, Ce
2, and Eu 3; H3clapi = 2-(5-chloride-2-hydroxyphenyl)––
1,3-bis [4-(5-chloride-2-hydroxyphenyl)-3-azabut-3-enyl]-
1,3-imidazolidine} have been prepared successfully by tem-
plate procedure and characterized by elemental analysis, IR
spectra, and the dinuclear cerium compound (2) selected as a
representative was structurally characterized. The crystal
be employed in the assembly of coordination architec-
tures directed by lanthanide cations [1, 2] which can in
reverse promote Schiff-base condensation and can give
access to complexes of otherwise inaccessible ligands [3].
Schiff-base and lanthanide complexes are now exten-
sively used as catalysts for RNA hydrolysis [4, 5], active
agents in cancer radiotherapy [6], contrast agents for
NMR imaging [7, 8], luminescent probes in visible and
near IR domain [9, 10]. Among them, dinuclear lantha-
nide and Schiff-base complexes have been a hot topic
and extensively reported because of their potential
applications in biology and medicine [11–15], chemistry
and technology [16]. For example, luminescent dinuclear
lanthanides ions complexes of Schiff-base have been
exploited to study metal–metal coupling processes and
interactions, chemical environment of coordinated metal
ions, the local site symmetry and fluxionality of the
ligand frameworks [17, 18].
˚
system is triclinic with space group P-1, a = 10.588 A,
˚
˚
b = 11,521(2) A, c = 13.979(3) A, a = 109.66(3)°, b =
3
˚
97.77(3)°, c = 106.33(3)°, V = 1486.2(5) A and Z = 1.
In compound 2, each Ce atom is coordinated to four N and
four O atoms from two clapi3- ligands, forming a distorted
square antiprism. Two phenol oxygen atoms from the middle
arms of the two heptadentate ligands as l2-bridging connect
the two Ce atoms. Compounds 1, 2 and 3 in CH2Cl2 exhibit
strong blue fluorescence at room temperature, compound 3
has no typical red emission of Eu3+ at room temperature
while exhibits strong red fluorescence at 77 K in CH2Cl2.
In the course of a program to synthesize dinuclear lan-
thanide Schiff-base complexes, we are interested in the
formation of dinuclear lanthanide/N4O3 heptadentate Schiff-
base complexes. It has been found that complexes formed by
tripodal heptadentate N4O3-type Schiff-base coordinated to
Ln3+ are normally highly unstable and very sensitive to
moisture and metal ion-promoted hydrolysis of the imine
C=N linkages [11, 12], therefore it is important to find a
solution to get stable dinuclear lanthanide/N4O3 heptadentate
Schiff-base complexes. In this respect, Orvig [11] and Kahwa
[12] tried a template method using a new type of N4O3
Schiff-base containing imidazolidine and synthesized suc-
cessfully stable homodinuclear lanthanide(III)/containing
imidazolidine N4O3 Schiff-base complexes [{La(brapi)}2] Á
2CHCl3 and [{Sm(api)}2], respectively, while our efforts
yielded a family of air-stable homodinuclear lanthanide(III)/
Schiff-base complexes [{Ln(clapi)}2] Á 2CH3CN {Ln=La 1,
Keywords Crystal structure Á Fluorescence Á
Homodinuclear compounds Á Ln(III) Á Schiff-base
Introduction
Schiff-base ligands with N, O donor sets are very
attractive because of their versatile structures and
remarkable ability to coordinate metal cations, and may
Z. Lijuan Á Z. Yunshan (&) Á J. Fei
College of Science and State Key Laboratory of Chemical
Resource Engineering, Beijing University of Chemical
Technology, Beijing 100029, P.R. China
e-mail: zhouys@mail.buct.edu.cn
123

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J Chem Crystallogr (2008) 38:595–599
Ce 2, Eu 3; H3clapi = 2-(5-chloride-2-hydroxyphenyl)-1,3-
bis [4-(5-chloride-2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-
imidazolidine}.
Table 1 Crystal data and structure refinement for 2
Empirical formula
Formula weight
Temperature
C58H54N10Ce2Cl6O6
1480.05
In this paper, we present the synthesis, structural char-
acterization and fluorescence of the new complexes. The
crystal structure and fluorescence of the stable homodinu-
clear cerium(III)/Schiff-base complex [{Ce(clapi)}2]Á
2CH3CN (2) was reported here for the first time. Impor-
tantly, [{Eu(clapi)}2] Á 2CH3CN (3) exhibits very strong
red fluorescence from Eu3+ at 77 K, but very weak at room
temperature.
293(2) K
˚
0.71073 A
Wavelength
Crystal system
Space group
Triclinic
P-1
Unit cell dimensions
˚
a = 10.558(2) A
a = 109.66(3)°
b = 97.77(3)°
c = 106.33(3)°
˚
b = 11.521(2) A
˚
c = 13.979(3) A
3
˚
Volume
1486.2(5) A
Z
1
Experimental
Density (calculated)
Absorption coefficient
F(000)
1.654 Mg/m3
1.841 mm-1
738
Materials
Crystal size
0.3 9 0.2 9 0.2 mm3
Eu(NO3)3 Á 6H2O was prepared by neutralization of 99.9%
Eu2O3 with HNO3 acid. All the other chemicals are of
reagent grade. H3clapi was prepared by condensation
of stoichiometric 5-chloride-salicylaldehyde and triethyl-
enetetramine.
Theta range for data collection
Index ranges
1.60–25.00°
0 B h B 12, -13 B k B 13,
-16 B l B 16
Reflections collected
5538
Independent reflections
Completeness to theta = 25.00
Absorption correction
5226 [R(int) = 0.0531]
99.8%
General and Physical Measurements
Psi-scan
Max. and min. transmission
Refinement method
0.67 and 0.45
Full-matrix least-squares on F2
Infrared spectra were recorded on a Nicolet 170SX RT-IR
spectrophotometer, elemental analysis was performed on a
Perkin-Elmer 240C analytical instrument and fluorescence
spectra on an Aminco Bowman Series 2 luminescence
spectrometer.
Data/restraints/parameters
Goodness-of-fit on F2
5226/0/355
1.138
Final R indices [I[2sigma(I)]
R indices (all data)
R1 = 0.0621, wR2 = 0.1620
R1 = 0.0917, wR2 = 0.1862
-3
˚
Largest diff. peak and hole
2.377 and -2.216 e A
Single Crystal X-ray Crystallography of 2
The data for compound 2 were collected by x/scan on a
CAD4SDP4 four-circle single crystal X-ray diffractometer
with graphite-monochromated Mo-Ka radiation (k =
0.4695 g (3 mmol) 5-chloride-salicylaldehyde in methanol
(100 mL) was added 1 mmol Ln(NO3)3 Á 6H2O (Ln = La,
Ce, or Eu) in methanol (20 mL) and then the resulting
solution was refluxed for 1 h. Excess triethylenetetramine
(4 mmol) was added and the solution was allowed to reflux
for two days. The initial yellow solution turned slowly into
a deep yellow or red color on addition of triethylenetetra-
mine. Crude yellow precipitates of 1 and 3 or red
precipitate of 2 were collected (30–40% yield) and dried
in vacuum. Air-stable single crystals were obtained by
re-crystallization of the corresponding crude products in
CH3CN/CH2Cl2. (Found: C, 47.22; H, 3.69; N, 9.52; La,
19.13%. Calc. for C58H54N10La2Cl6O6 1: C, 47.12; H,
3.66; N, 9.48; La, 18.82%; Found: C, 47.21; H, 3.72; N,
9.39; Ce, 19.05%. Calc. for C58H54N10Ce2Cl6O6 2: C,
47.06; H, 3.65; N, 9.47; Ce, 18.93%; Found: C, 46.53; H,
3.65; N, 9.36; Eu, 19.95%. Calc. for C58H54N10Eu2Cl6O6
3: C, 46.24; H, 3.58; N, 9.30; Eu, 20.33%.)
˚
0.71073 A) at 293(2) K. Empirical absorption corrections
were made from w-scan data using the program SHEL-
XTL-93 at the data reduction stage. The structure was
solved by direct methods and refined by Full-matrix least
square on F2 (SHELXTL V5.1). Anisotropic thermal fac-
tors were assigned to all the non-hydrogen atoms. Relevant
details of the structure determination are presented in
Table 1. The selected bond distances and angles are pre-
sented in Table 2. Crystallographic data in CIF format have
been deposited at the Cambridge Crystallographic Data
Centre with CCDC No. 153094.
Preparation of [{Ln(clapi)}2] Á 2CH3CN (1, 2 and 3)
In a typical procedure, the title compounds were prepared
by the following template method: To a solution of
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J Chem Crystallogr (2008) 38:595–599
597
Table 2 Selected bond lengths
˚
Ce(1)–O(1)
2.324(7)
2.333(6)
2.433(5)
2.449(5)
2.574(8)
127.9(2)
69.5(2)
Ce(1)–N(2)#1
2.584(7)
[A] and angles [°] for 2
Ce(1)–O(2)#1
Ce(1)–N(4)#1
2.849(7)
2.849(7)
2.852(7)
4.0245(15)
83.3(2)
Ce(1)–O(3)
N(4)–Ce(1)#1
Ce(1)–O(3)#1
Ce(1)–N(3)
Ce(1)–N(1)
Ce(1)–Ce(1)#1
O(1)–Ce(1)–O(2)#1
O(2)#1–Ce(1)–N(2)#1
O(3)–Ce(1)–N(2)#1
O(3)#1–Ce(1)–N(2)#1
N(1)–Ce(1)–N(2)#1
O(1)–Ce(1)–N(4)#1
O(2)#1–Ce(1)–N(4)#1
O(3)–Ce(1)–N(4)#1
O(3)#1–Ce(1)–N(4)#1
N(1)–Ce(1)–N(4)#1
N(2)#1–Ce(1)–N(4)#1
O(1)–Ce(1)–N(3)
O(2)#1–Ce(1)–N(3)
O(3)–Ce(1)–N(3)
O(1)–Ce(1)–O(3)
O(2)#1–Ce(1)–O(3)
O(1)–Ce(1)–O(3)#1
O(2)#1–Ce(1)–O(3)#1
O(3)–Ce(1)–O(3)#1
O(1)–Ce(1)–N(1)
O(2)#1–Ce(1)–N(1)
O(3)–Ce(1)–N(1)
O(3)#1–Ce(1)–N(1)
O(1)–Ce(1)–N(2)#1
O(3)#1–Ce(1)–N(3)
N(1)–Ce(1)–N(3)
N(2)#1–Ce(1)–N(3)
N(4)#1–Ce(1)–N(3)
146.5(2)
143.9(2)
85.6(2)
140.3(2)
114.2(2)
89.2(2)
69.0(2)
84.3(2)
70.1(2)
109.8(2)
82.23(19)
69.78(19)
147.9(2)
64.3(2)
74.1(3)
112.6(2)
141.5(2)
73.3(2)
82.5(2)
109.1(2)
87.1(2)
64.4(2)
Symmetry transformations used
to generate equivalent atoms: #1
-x + 1, -y + 2, -z
149.1(2)
145.67(19)
68.79(19)
Results and Discussion
IR Spectra of the Title Compounds
The IR spectra of the title compounds show that the peak
due to O–HÁÁÁN stretching vibration from 3300 to
2100 cm-1 in H3clapi is disappeared in compound 1, 2 and
3, and the peak due to C=N stretching vibration in H3clapi
shifts from 1632 to 1623 cm-1 in compound 1, 2 and 3.
The loss of intramolecular hydrogen bonds and the for-
mation of the new compounds 1, 2 and 3 is the main reason
for the latter shift.
Structure of 2
Fig. 1 ORTEP view of the asymmetric unit in compound 2, showing
the atom-labeling scheme with 30% probability ellipsoids (all
hydrogen atoms are omitted for clarity)
Compound 2 crystallized in triclinic space group P–1,
˚
˚
˚
a = 10.588 A, b = 11,521(2) A, c = 13.979(3) A, a =
109.66(3)°, b = 97.77(3)°, c = 106.33(3)°, V = 1486.2(5)
3
˚
A and Z = 1. The asymmetric unit of 2, presented in Fig. 1,
such coordination polyhedra of Ce edge-share through
˚
contains 41 non-hydrogen atoms. There are one Ce, three O,
three Cl, twenty-nine C and five N atoms, which are crys-
tallographically independent. Each Ce atom is coordinated to
four N and four O atoms with Ce–O distances in the range of
O(3)ÁÁÁO(3a) (2.764 A), as shown in Figs. 2 and 3, with an
˚
intramolecular CeÁÁÁÁÁÁCe distance of 4.024 A. Of the four
oxygen atoms connected to Ce atom, two are from terminal
phenols and the other two, as l2-bridging, from the middle
arms of the two heptadentate ligands. Of the four nitrogen
atoms, two are from imidazolidine parts and the other two
from imines (see Fig. 3). The average Ce–N (N atoms from
˚
2.324(7)–2.449(5) A (av. Ce–O = 2.385 A) and Ce–N dis-
˚
˚
˚
tances from 2.574(8) to 2.852(7) A (av. Ce–N = 2.715 A).
The O–Ce–O bond angles are distributed in the range
69.0(2)–146.5(2)° (av. O-Ce–O = 109.4°), O–Ce–N bond
angles in the range 68.79(19)–141.5(2)° (av. O–Ce–
N = 93.1°), N–Ce–N bond angles in the range 64.3(2)–
149.1(2)°(av. N–Ce–N = 110.1°), thus each Ce3+ centre has
an approximate square antiprismatic geometry, and two
˚
imidazolidine) bond lengths (2.851 A) are significantly
˚
longer than that of Ce–N (N atoms from imine) (2.573 A)
and the average Ce–O (O atoms from the middle arms of the
˚
two heptadentate ligands) bond lengths (2.441 A) are longer
than that of Ce–O (O atoms from the terminal phenols) bond
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J Chem Crystallogr (2008) 38:595–599
Excitation Emission
300
350
400
450
500
550
λ (nm)
Fig. 2 The approximate square antiprismatic geometry around each
Ce3+ ion in 2
Fig. 4 The room temperature excitation and emission spectra of 2
(kem = 462 nm, kexc = 394 nm) (short dash line) and H3clapi
(kem = 463 nm, kexc = 393 nm) (solid line) at a concentration of
5 9 10-5 M in CH2Cl2, respectively
Emission
Excitation
(a)
(b)
300
350
400
450
500
(nm)
550
600
650
Fig. 3 ORTEP view of the sandwich dimmer in 2, showing the atom-
labeling scheme with 30% probability ellipsoids (hydrogen atoms and
CH3CN was omitted for clarity)
λ
Fig. 5 The excitation and emission spectra of 1 and 3 at room
temperature (a) The excitation and emission spectra of 1 (5 9
10-5 M) in CH2Cl2 (kem = 462 nm, kex = 394 nm); (b) The excita-
tion and emission spectra of
(kem = 462 nm, kex = 394 nm)
˚
lengths (2.328 A). The four nitrogenatoms of each ligand are
˚
coplanar within max. deviation of 0.0098 A and the middle
3
(5 9 10-5 M) in CH2Cl2
phenolate rings inclined 90.3° to this plane, while the two
terminal phenolate rings are inclined to the four nitrogen
coplanar by ca.13.4°, which is smaller than 25° reported in
reference 13, the change probably reflects the influence of the
presence of chloride atoms in the phenol ring.
462 nm, which is also observed in the free ligand under
394 nm excitation. The indication of intensive band at
462 nm is also found in spectrum for either compounds 1
or 3 (Fig. 5) in CH2Cl2 at room temperature except for lack
of the peak at 320 nm in the excitation spectrum of 2.
Unfortunately, no typical red emission of Eu3+ is observed
at room temperature in compound 3. However, at 77 K the
emission at 618 nm (Fig. 6) which is the typical transition
of 5D0?7F2 for Eu3+ ion is inspected by excitation of light
in the range 325–400 nm (kmax = 360 nm). Meanwhile
it also shows a broad poorly resolved peak in the range
400–500 nm in the emission spectrum of compound 3.
Additionally, no splitting peak at 580 nm for the forbidden
5D0?7F0 transition indicates that there is only one site for
Fluorescence of the Title Compounds
Figure 4 shows the fluorescent spectra of compound 2 and
H3clapi (5 9 10-5 M) in CH2Cl2 at room temperature. The
band at 394 nm observed for the compound in both exci-
tation spectra for the ligand and compound 2 is presumably
from the electronic states of ligand [12], and the new band
at 320 nm for compound 2 is mainly ascribed to spin
allowed transition from 4f to 5d of Ce3+ ion [19]. Under
394 nm excitation for 2, an intensive band recorded at
123

J Chem Crystallogr (2008) 38:595–599
599
exhibits strong red fluorescence at 77 K. It is highly likely
that similar dinuclear lanthanide/N4O3 heptadentate Schiff-
base complexes can also be prepared and can be extended
to (1) a variety of Schiff-base ligands, (2) different trivalent
lanthanide cations, including mixed metals and (3) differ-
ent solvent molecules like CH3CN which significantly
affect properties as the compounds because, for example,
solvation may affect the lanthanide(III) ion site symmetry
while O–H and C–H stretching vibrations can quench Ln3+
emission. Work on this theme is currently underway.
Excitation
Emission
Acknowledgements Many thanks to professor Xiaozeng You of
Coordination Chemistry Institute, Nanjing University, P.R. China for
his kind help.
300 350 400 450 500 550 600 650 700 750
(nm)
λ
Fig. 6 The excitation and emission spectra of
(kem = 618 nm, kex = 360 nm
3 at 77 K.
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It is clear that the cooperative assembly of lanthanide ions
and N4O3-type heptadentate Schiff-base offer a rich
chemistry as exemplified by the isolation of the title
compounds which broaden the present understanding and
provide insights toward the synthesis of novel structures of
dinuclear lanthanide/N4O3 heptadentate Schiff-base com-
plexes with designed functional groups and consequently
desired properties, for example, compounds 1, 2 and 3 in
CH2Cl2 exhibit strong blue fluorescence at room temper-
ature, while compound 3 has no typical red emission of
Eu3+ ion at room temperature, but compound 3 in CH2Cl2
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123