One-dimensional Salen-type Chain-like lanthanide(III) coordination polymers: Syntheses, crystal structures, and fluorescence properties

Article abstract of DOI:10.1002/zaac.201000366

Four salen-type lanthanide(III) coordination polymers [LnH ₂L(NO₃)₃(MeOH)_x]_n [Ln = La (1), Ce (2), Sm (3), Gd (4)] were prepared by reaction of Ln(NO ₃)₃·6H₂O with H₂L [H ₂L = N,Na-bis(salicylidene)-1,2-cyclohexanediamine]. Single-crystal X-ray diffraction analysis revealed that H₂L effectively functions as a bridging ligand forming a series of 1D chain-like polymers. The solid-state fluorescence spectra of polymers 1 and 2 emit single ligand-centered green fluorescence, whereas 3 exhibits typical red fluorescence of Sm^III ions. The lowest triplet level of ligand H₂L was calculated on the basis of the phosphorescence spectrum of Gd^III complex 4. The energy transfer mechanisms in the lanthanide polymers were described and discussed.

Full text of DOI:10.1002/zaac.201000366

ARTICLE
DOI: 10.1002/zaac.201000366
One-dimensional Salen-type Chain-like Lanthanide(III) Coordination
Polymers: Syntheses, Crystal Structures, and Fluorescence Properties
Gui-Ling Wang,*[a] Yong-Mei Tian,[a,b] Dian-Xue Cao,[a] Yan-Shuang Yu,[b] and
Wen-Bin Sun*[b]
Keywords: Coordination polymer; Fluorescence; Lanthanides; Salen-type ligands; X-ray diffraction
Abstract. Four salen-type lanthanide(III) coordination polymers cence spectra of polymers 1 and 2 emit single ligand-centered green
[LnH2L(NO3)3(MeOH)x]n [Ln = La (1), Ce (2), Sm (3), Gd (4)] were fluorescence, whereas 3 exhibits typical red fluorescence of SmIII ions.
prepared by reaction of Ln(NO3)3·6H2O with H2L [H2L = N,N'-bis(sali- The lowest triplet level of ligand H2L was calculated on the basis of
cylidene)-1,2-cyclohexanediamine]. Single-crystal X-ray diffraction the phosphorescence spectrum of GdIII complex 4. The energy transfer
analysis revealed that H2L effectively functions as a bridging ligand mechanisms in the lanthanide polymers were described and discussed.
forming a series of 1D chain-like polymers. The solid-state fluores-
are so far rare as to the diverse coordination modes of the
Introduction
Self-assembled lanthanide(III) coordination complexes have
recently attracted much attention because of their potential use
numerous salen-type ligands.
Our recent studies have focused on the use of a variety of
salen-type ligands to coordinate to LnIII atoms to improve lan-
in materials science in the field of luminescence, magnetism,
thanide luminescence.[6] As part of our continuing studies we
gas absorption, and sensing.[1,2] For the lanthanide ions, the f–
f transitions are parity forbidden and the absorption coeffi-
herein present the syntheses, crystal structures, and photophys-
ical
properties
of
four
lanthanide
polymers
cients are usually very low. To efficiently tune emissions of
lanthanide ions, suitable chromophores have been used to act
as antennas or sensitizers, which can transfer absorbed energy
efficiently to the lanthanide ion.[3] Therefore, the multidentate
N-donor and O-donor organic compounds are usually used as
ligands to construct lanthanide complexes. Salen-type ligands,
as important multidentate ligands, have played an essential role
in tuning bioactivity and magnetic performance of organic lan-
thanide complexes.[4] However, their applications in moderat-
ing luminescence properties of lanthanide ions are seldom doc-
umented due to the difficulty in controlling the coordination
environments of lanthanide(III) ions, which display high coor-
dination numbers and flexible arrangements.[5] In addition,
structurally characterized salen-type lanthanide complexes es-
pecially with salen-type ligands deriving from cyclic diamine
[LnH2L(NO3)3(MeOH)x]n [Ln = La (1), Ce (2), Sm (3), Gd (4)]
constructed by salen-type ligand with cyclohexane-1,2-diam-
ine.
Results and Discussion
Synthesis
The salen-type ligand H2L [N,N'-bis(salicylidene)-1,2-cyclo-
hexanediamine] was prepared according to the literature.[6b,7]
Polymers 1–4 were synthesized by reaction of equivalent
amounts of ligand and lanthanide nitrate in methanol solution
(Scheme 1). To ensure the slow formation of crystalline mate-
rial, the crystalline products were dissolved in methanol solu-
tion and the diethyl ether was allowed to diffuse slowly. Crys-
tals of the polymers formed at the interface of the two solvents
within days.
* Prof. G.-L. Wang
E-Mail: wangguiling@hrbeu.edu.cn
* Dr. W.-B. Sun
Fax: +86-451-86608042
Crystal Structure
E-Mail: wenbinsun@yahoo.cn
[a] College of Material Science and Chemical Engineering
Harbin Engineering University
Nantong St. 145
Single-crystal X-ray diffraction analysis reveals that poly-
mers 1–3 have a similar structure. The structures are described
by taking compound 1 as an example. It crystallizes in the
monoclinic space group P2/c. A perspective view of 1 with
atom-labeling scheme is given in Figure 1. The unit cell con-
tains one [LnH2L(NO3)3(MeOH)2] unit. An infinite 1D poly-
meric chain structure results from the bridging ligand H2L be-
tween each two LnIII ions through two Ln–O (phenol) bonds.
Harbin 150001, P. R. China
[b] Key Laboratory of Functional Inorganic Material Chemistry
(HLJU)
Ministry of Education, School of Chemistry and Materials
Science
Heilongjiang University
No. 74, Xuefu Road, Nangang District
Harbin 150080, P. R. China
Z. Anorg. Allg. Chem. 2011, 637, 583–588
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G.-L. Wang, W.-B. Sun et al.
ARTICLE
Scheme 1. Synthesis of the polymers 1–4.
Each LnIII ion, which adopts a distorted hexadecahedral ar-
rangement (Figure 2), is decacoordinated with two phenol oxy-
gen atoms from the ligands, two methanol oxygen atoms, and
six oxygen atoms from three bidentate nitrate groups.
Table 1. Selected bond lengths /Å and bond angles /° for polymers 1–3.
Ln
1
2
3
Ln–O(MeOH) (average)
Ln–O(phenolic) (average)
Ln–O(nitrate) (average)
O(phenolic)–Ln–O(phenolic)
O(MeOH)–Ln–O(MeOH)
Coordination number
2.625
2.420
2.683
158.44
65.42
10
2.603
2.397
2.663
158.30
65.38
10
2.448
2.315
2.517
157.00
9
These structures are similar to the first neutral salen-type
coordination polymer [La(H2salen)(NO3)3(MeOH)2]n
Figure 1. Perspective view of the 1D chain like coordination polymer 1.
[H2salen = N,N'-ethylene bis(salicylideneimine)].[9] Along the
chain structure, the benzene rings and the cyclohexanediamine
rings located on the same side are parallel and all lanthanum
atoms are in a line. The dihedral angle of the two benzene
rings in the ligand is 70.281°. Moreover, in complex 1, there
are two parallel polymeric chains extended along the a axis,
where the adjacent chains are connected by O6–H16···O7
[2.816(6) Å] (Figure 3). The oxygen atom O6 is the third un-
coordinated atom in the bidentate chelating nitrate group, and
the oxygen atom O7 comes from the methanol molecules.
Figure 2. View of the coordination polyhedron of LaIII and SmIII ions
in 1 and 3.
Selected bond lengths and bond angles for compounds 1–3
are listed in Table 1. The phenol Ln–O bond is the shortest
with an average length of 2.420 Å in 1. The two Ln–O(metha-
nol) bond lengths have a length of 2.625 Å and the Ln–O(ni-
trate) distance is the longest with an average value of 2.683 Å.
Besides, the average C–O(phenol) bond length is notably
shorter than that of the related free ligand.[8] This is believed
to be due to the coordination of the phenol oxygen atom to
lanthanide(III) ion. A tendency of bond lengths and angles
shortening is observed, owing to lanthanide contraction. The
smaller lanthanide(III) ions have a shorter bond between oxy-
gen and lanthanide(III) ion. Besides, 1 and 2 are both decaco-
ordinated with distorted hexadecahedral arrangement and 3 has
a nonacoordinate tricapped trigonal prismatic arrangement
(Figure 2).
Figure 3. Schematic representation for complex 1 viewed along a axis,
the chains are self-assembled through hydrogen bonds (O–H···O,
dashed lines).
IR Spectroscopy
The infrared spectra of compounds 1–4 are similar, the broad
band of the O–H stretching vibration of ligand at ca.
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Z. Anorg. Allg. Chem. 2011, 583–588

1D Salen-type Chain-like Lanthanide(III) Coordination Polymers
2890 cm–1 (on which the CH modes are superimposed) is re-
placed by a band at ca. 3416 to 3476 cm–1 It is assigned to
the N–H vibration in C=N+–H groups, which results from the
lanthanide(III) ions ligating the phenol oxygen atoms and the
hydrogen protons transfer to the imine C=N group. The typical
N–H bonds are also observed in the crystal structure (Fig-
ure 3). This neutral zwitterionic (O–, NH+)2 groups have been
previously observed in some Schiff base lanthanide
complexes.[10] Additionally, the C=N stretching vibration in
the ligand is shifted to higher wavenumber after complexation
(1641 cm–1). It has been noted that coordination of a Schiff
base ligand to a metal ion through the phenol oxygen atoms
increases the wavelength of ν(C=N) with respect to the free
ligand value, whereas coordination via the phenol oxygen atom
and the imino nitrogen atoms causes a shift towards lower
ν(C=N) values.[10] Moreover, the characteristic stretching vi-
Figure 4. Normalized excitation and emission spectra of H2L and pol-
ymer 1–3.
–1
brations bands (ν1 to ν4) of the NO3 group are observed, and
the different in wavenumber between ν1 and ν2 is ca. 200 cm–1,
typical for bidentate chelating nitrate (monodentate nitrate
groups display a much smaller splitting).
The emission spectrum of 3 mainly shows five narrow bands
at 532, 567, 601, 646, and 701 nm excitation at 390 nm (Fig-
ure 4). They are attributed to the characteristic π → π* emis-
Luminescent Properties
All absorption spectra in methanol at room temperature ex-
hibited similar bands below 360 nm, which can be assigned to
π–π* transition of the ligand. Besides, a new absorption band
centered at 242 nm was observed, which is attributed to the
electron transfer from ligand to metal (LMCT).[9,10] The solid-
state fluorescence spectra of the polymers were measured at
room temperature (Figure 4). In contrast to the two emitting
bands at 544 and 573 nm from the free ligand, complex 1 emits
one strong band centered at 542 nm exited at 470 nm in the
solid state. Since there are no 4f electrons and the energy level
of exited states is below the triplet levels of the ligand for LaIII
4
6
4
6
4
6
sion of ligand, G5/2 → H5/2, G5/2 → H7/2, G5/2 → H9/2
,
4
6
and G5/2 → H11/2 transition of the SmIII ion, respectively. It
is different from the salen-type complexes
[Sm(H2L2)(NO3)3(MeOH)]n [H2L2 = N,N'-bis(salicylidene)
propane-1,2-diamine][6a] and [Sm(H2salen)1.5(NO3)3]n,[6c] in
which the emission band at ca. 644 nm from the transition of
6
electronic dipole (4G5/2 → H9/2) is the strongest. It is similar
to the complex Sm(DPAP)3·12H2O (HDPAP = 6-diphenylami-
necarbonyl-2-pyridinecarboxylic acid),[14] in which the emis-
sion band around 600 nm originating from the transition of
6
ions, the energy absorbed by the ligand H2L cannot be trans- magnetic dipole (4G5/2 → H7/2) is the strongest. These are
ferred to LaIII ions. The absorbed energy can only relax through
its own lower energy levels. The intense blue fluorescence
emission of 1 is attributed to the enhanced π–π* electron transi-
tion of the ligand. Thus, 1 emits ligand-centered fluorescence.
The half-band width of 1 is 80 nm. It is markedly wider than
our formerly reported lanthanum-containing salen-type com-
plex [La(H2L1)1.5(NO3)3]n (H2L1 = N,N'-bis(salicylidene)pro-
pane-1,2-diamine).[6a] However, it is notably narrower than
those for some ligand-centered lanthanide complexes
La(PMIP)3(TPPO)2 (PMIP = 1-phenyl-3-methyl-4-isobutyryl-
5-pyrazoloneate, TPPO = triphenyl phosphane oxide) (116 nm)
attributed to the different symmetry of SmIII ions in these
complexes.[14]
Triplet State of the Ligand and Energy Transfer
It is known that the intermolecular transfer efficiency has a
close relationship to the energy gap between the lowest triplet
energy level (T1) of the ligand and the lowest excited state
level of LnIII ions.[15] The corresponding gadolinium complex
was usually selected as a model complex for the determination
of the triplet state energies of the organic ligands owing to their
high phosphorescence-fluorescence ratio compared to those of
the other LnIII complexes and GdIII is able to sensitize the
phosphorescence emission of ligand. The phosphorescence
spectrum of [GdH2L(NO3)3(MeOH)2]n (4) was measured at
77 K in a mixed solution of ethanol and methanol (v/v = 1:1)
(as shown in Figure 5) and the triplet state energy of H2L can
be determined to be 20725 cm–1, which was calculated from
the shortest-wavelength phosphorescence band.
and La(PMIP)3(Phen)2 (Phen
=
1,10-phenanthroline).[11]
The emission spectrum of 2 shows the similar bands with poly-
mer 1. In 2, a markedly strong single band is observed with a
narrower half-band width of 27 nm. This is most likely ascribed
to either ligand-centered fluorescence with d–f transition from
CeIII ions or both at the same time. On comparing with com-
plexes Alq3 and Gaq3 (q = 8-hydroxyquinoline)[12,13] with li-
gand-centered fluorescence that are used as electrolumines-
cence or photoluminescence materials, the ligand-centered
fluorescence for lanthanide complexes deserve further
exploring.[11]
Z. Anorg. Allg. Chem. 2011, 583–588
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585

G.-L. Wang, W.-B. Sun et al.
ARTICLE
ligating to the SmIII atom, which usually quenches the lumines-
cence. The energy intersystem crossing (ISC) of the ligand to
their triplet states (T1) occurs not efficiently, finally, the weaker
fluorescence centered at 532 nm from the ligand was also ob-
served in the emission of polymer 3.
Conclusions
The salen-type ligand with cyclic diamine N,N'-bis(salicylid-
ene)-1,2-cyclohexanediamine (H2L) is able to ligate lanthanide
ions to form a series of 1D chain-like coordination polymers.
The solid-state fluorescent spectra suggest that the ligand un-
dergoes an efficient ligand-to-metal energy transfer (antenna
effect) that enhances the characteristic metal-centered fluores-
cence of SmIII ions. The fluorescence of 3 is correlated either
to the energy match between T1 of ligand and lowest excited
state of SmIII ion or the structure of the lanthanide complexes.
Obviously, an encapsulated lanthanide-centered structure con-
structed by this ligand could result in a more efficient fluores-
cence. Further research on lanthanide-centered fluorescent ma-
terial is currently underway.
Figure 5. Phosphorescence spectrum of polymer 4.
As a efficient organic ligand, the gap ΔE(T1 – LnIII) should
be intermediate. Too big or too small would decrease the effi-
ciency of energy transfer.[16] It has been established that
ΔE(T1
–
5D4) = 2400 300 cm–1 for the organic ligand at
room temperature would more efficiently sensitize lumines-
cence of the TbIII ion. Whereas ΔE(T1 – D1) > 4000 cm–1 for
5
the EuIII complex or ΔE(T1 – D1) < 1500 cm–1 for the TbIII
5
Experimental Section
complex would much more reduce the fluorescence yield.[15,16]
The energy gap (ΔE) between the triplet state of H2L and the
resonance energy level of SmIII is 2825 cm–1. Therefore, strong
luminescence is observed for 3. They are close to that of com-
plex Sm(DPAP)3·12H2O (2778 cm–1).[14]
Materials and Methods
Elemental (C, H, and N) analyses were performed with a Perkin–Elmer
2400 analyzer. IR spectra were determined with a Perkin–Elmer Spec-
trum one FT-IR Spectrometer. UV/Vis spectra were recorded with a
Shimadzu UV2240 spectrophotometer. Fluorescence spectra were
taken with a Perkin–Elmer LS-55 fluorescence photometer at room
temperature. Phosphorescence spectra were taken at 77 K in a mixed
solution of ethanol and methanol (v/v = 1:1).
To describe the mechanism, a diagram describing the energy
transfer is shown in Figure 6. The sensitization pathway in
luminescent polymer 3, in general, consists of excitation of the
ligand into their excited single states (S1), subsequent intersys-
tem crossing (ISC) of the ligand to their triplet states (T1), and
6
energy transfer (ET) from the triplet state to the G5/2 level of
Preparation of [LnH2L(NO3)3(MeOH)x]n (1–4)
SmIII ion, followed by internal conversion to the emitting 6Hj,j.
Consequently, polymer 3 mainly exhibits the characteristic red
fluorescence of SmIII ion when a transition to the ground state
occurs. However, according to the structure of 3, the chain-
like open structure could not prevent small solvents (MeOH)
Polymers 1–4 were prepared by similar procedures. To a MeOH solu-
tion (10 mL) of H2L (0.338 g, 1.0 mmol) was slowly added a MeOH
solution (10 mL) of Ln(NO3)3·6H2O (0.434 g, 1.0 mmol) at ambient
temperature. A yellow precipitate formed immediately. After stirring
for 5 h, a yellow solid was collected by filtration and washed with
cold MeOH. Single crystals were obtained by slow diffusion of ethyl
ether into a methanol solution of the powder sample over one week.
[LaH2L(NO3)3(MeOH)2]n (1): Yield 78 %. Elemental analysis:
C22H32LaN5O13 (713.42): calcd. C 37.04; H, 4.52; N, 9.82 %; found:
C, 36.98; H, 4.46; N, 9.29 %. Selected IR (KBr, selected data): ν =
˜
3416 ν(C=N–H+), 1642 ν(C=N), 1310 ν(Cph–O), 1476 (ν1), 1278 [ν2],
1026 (ν3), 820 (ν4) (ν1–ν4, bidentate chelating NO3 group) cm–1. UV/
–
Vis (MeOH): λmax (ε): 212 (2.73), 244 (0.81), 317 (0.23) nm.
[CeH2L(NO3)3(MeOH)2]n (2): Yield 75 %. Elemental analysis:
C22H31CeN5O13 (713.62): calcd. C 37.03; H, 4.38; N, 9.81 %; found:
C, 36.95; H, 4.27; N, 9.37 %. IR (KBr, selected data): ν = 3476 ν(C=
˜
N–H+), 1644 ν(C=N), 1482 (ν1), 1292 [ν2, ν(Cph–O) and/or ν(NO3 )],
–
1028 (ν3), 815 (ν4), (ν1–ν4, bidentate chelating NO3– group) cm–1. UV/
Vis (MeOH): λmax (ε): 211 (2.50), 243 (0.93), 319 (0.22) nm.
[SmH2L(NO3)3MeOH]n (3): Yield 80 %. Elemental analysis:
C21H26N5O12Sm (690.82): calcd. C 36.51; H, 3.79; N, 10.14 %; found:
Figure 6. Schematic energy level diagram and the energy transfer
process.
586
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 583–588

1D Salen-type Chain-like Lanthanide(III) Coordination Polymers
Table 2. Crystallographic data for polymers 1–3.
Crystal data
1
2
3
Empirical formula
C22H32LaN5O13
713.44
C22H30CeN5O13
712.63
C21H26N5O12Sm
690.83
Formula weight
Temperature /K
293(2)
293(2)
293(2)
Wavelength /Å
0.71073
0.71073
0.71073
Crystal system
Monoclinic
P2/c
Monoclinic
P2/c
Triclinic
¯
space group
P1
a /Å
10.050(3)
8.736(3)
10.038(4)
8.757(3)
9.7875(10)
10.1978(10)
15.0709(15)
78.7090(10)
1377.8(2)
2
b /Å
c /Å
16.537(7)
100.740(16)
1426.5(9)
2
16.528(5)
100.680(2)
1427.6(9)
2
β /°
V /Å3
Z
Calculated density /Mg·m–3
Absorption coefficient /mm–1
F(000)
1.661
1.658
1.665
1.569
1.666
2.199
720
718
690
θ Range for data collection /°
Limiting indices
3.21 to 27.48
–11 ≤ h ≤ 13,
–10 ≤ k ≤ 11
–21 ≤ l ≤ 21
13173
3.11 to 27.46
–12 ≤ h ≤ 13,
–11 ≤ k ≤ 10,
–21 ≤ l ≤ 21
13566
2.44 to 25.00
–11 ≤ h ≤ 11,
–12 ≤ k ≤ 7,
–17 ≤ l ≤ 17
7693
Reflections collected
Unique reflection
R(int)
3217
3258
4762
0.0269
0.0253
0.0264
Goodness-of-fit on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
Largest peak and hole /e·Å–3
0.987
1.055
1.119
0.0507, 0.1305
0.0546, 0.1345
3.414 and –1.342
0.0246, 0.0583
0.0268, 0.0591
0.542 and –0.693
0.0415, 0.0895
0.0642, 0.1001
1.178 and –1.020
C, 36.35; H, 3.59; N, 9.97 %. IR (KBr, selected data): ν = 3461 ν(C= longjiang University Science Fund for Young Scholars (Y. M. Tian,
˜
N–H+), 1641 ν(C=N), 1482 (ν1), 1277 [ν2, ν(Cph–O) and/or ν(NO3 )], QL201021) and The Open Experiment of (HLJU) Undergraduate (W.
–
1030 (ν3), 808 (ν4), (ν1–ν4, bidentate chelating NO3– group) cm–1. UV/ B. Sun) and Heilongjiang University (No.s Hdtd2010-08, 20100261-
Vis (MeOH): λmax (ε): 212 (2.35), 240 (1.32), 327 (0.18) nm.
63).
[GdH2L(NO3)3MeOH]n (4): Yield 77 %. Elemental analysis:
C21H26GdN5O12 (697.71): calcd. C 36.15; H, 3.76; N, 10.04 %; found:
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˜
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Acknowledgement
The authors gratefully acknowledge financial support from the Educa-
tional Committee of Heilongjiang Province (No. 11551336) and Hei-
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Received: October 10, 2010
Published Online: December 7, 2010
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Z. Anorg. Allg. Chem. 2011, 583–588