Two series of solvent-dependent lanthanide coordination polymers demonstrating tunable luminescence and catalysis properties

Article abstract of DOI:10.1021/cg500286v

Two series of lanthanide-organic frameworks with the formulas [Ln(BTATB)(DMF)₂(H₂O)]·DMF·2H₂O (Ln = La (1a), Eu (2a), Tb (3a), Er (4a)) and [Ln(BTATB)(H₂O) ₂]·2DMA·4H₂O (Ln = Er (1b), Yb (2b), Lu (3b)), respectively, were solvothermally synthesized from 4,4',4-(benzene-1,3,5- triyltris(azanediyl))tribenzoate (H₃BTATB) and Ln(NO ₃)₃ under DMF or DMA media and characterized by thermogravimetric analyses, IR spectroscopy, X-ray powder diffraction, and single crystal X-ray diffraction. X-ray single-crystal diffraction analyses for these complexes revealed that series A features an interesting 2D interdigitated layer architecture with (6,3) topology. Series B exhibits a 2D bilayer structure. The luminescence properties were studied, and the results showed that complex 3a displayed strong fluorescent emission in the visible region, where the emission intensities of 3a are enhanced upon the addition of Zn ²⁺, demonstrating Zn²⁺-modulated fluorescence. Yb(III) complex 2b emits typical near-infrared luminescence (983 nm) in DMF (λ_ex = 315 nm). The Knoevenagel condensation reaction (benzaldehyde and malononitrile or ethyl cyanoacetate in acetonitrile) was studied using 3a and 2b as catalysts. The results showed that the conversion rates of reactions catalyzed by 3a increased to 99 and 42%, respectively, whereas the reactions catalyzed by 2b exhibited lower conversion rates.

Full text of DOI:10.1021/cg500286v

Article
pubs.acs.org/crystal
Two Series of Solvent-Dependent Lanthanide Coordination Polymers
Demonstrating Tunable Luminescence and Catalysis Properties
Na Wei,† Ming-Yang Zhang,† Xiao-Nan Zhang,† Guang-Ming Li,‡ Xiang-Dong Zhang,†
and Zheng-Bo Han*,†
†College of Chemistry, Liaoning University, Shenyang 110036, P. R. China
‡College of Chemistry and Material Science, Heilongjiang University, Harbin 150080, P. R. China
S
* Supporting Information
ABSTRACT: Two series of lanthanide−organic frameworks
with the formulas [Ln(BTATB)(DMF)2(H2O)]·DMF·2H2O
(Ln = La (1a), Eu (2a), Tb (3a), Er (4a)) and [Ln(BTATB)-
(H2O)2]·2DMA·4H2O (Ln = Er (1b), Yb (2b), Lu (3b)),
respectively, were solvothermally synthesized from 4,4′,4″-
(benzene-1,3,5-triyltris(azanediyl))tribenzoate (H3BTATB)
and Ln(NO3)3 under DMF or DMA media and characterized
by thermogravimetric analyses, IR spectroscopy, X-ray powder
diffraction, and single crystal X-ray diffraction. X-ray single-
crystal diffraction analyses for these complexes revealed that
series A features an interesting 2D interdigitated layer architecture with (6,3) topology. Series B exhibits a 2D bilayer structure.
The luminescence properties were studied, and the results showed that complex 3a displayed strong fluorescent emission in the
visible region, where the emission intensities of 3a are enhanced upon the addition of Zn2+, demonstrating Zn2+-modulated
fluorescence. Yb(III) complex 2b emits typical near-infrared luminescence (983 nm) in DMF (λex = 315 nm). The Knoevenagel
condensation reaction (benzaldehyde and malononitrile or ethyl cyanoacetate in acetonitrile) was studied using 3a and 2b as
catalysts. The results showed that the conversion rates of reactions catalyzed by 3a increased to 99 and 42%, respectively, whereas
the reactions catalyzed by 2b exhibited lower conversion rates.
These excellent properties and extensive applications are
mostly attributed to the distinct structure and composition of this
material. It is well-known that during the self-assembly process
the experimental conditions play a significant role in the growth
of the crystals, such as the reaction temperature, reaction period,
concentrations of the raw materials, and so on.9 Among these
factors, solvent molecules as guests sometime behave like
structure-directing agents during the self-assembly process and
generate new structures in different solvents.10
Herein, we prepared a new tripodal carboxylate ligand with
functional groups, namely, H3BTATB (BTATB = 4,4′,4″-
(benzene-1,3,5-triyltris(azanediyl))tribenzoate). The tripodal
ligand was chosen for the following reasons: (i) a ligand with
longer arms may facilitate the formation of novel unusual
network topologies; (ii) the lanthanide ion is a hard acid and
prefers oxygen to nitrogen donors, so the available amino
functional groups on the ligand may coordinate with other metal
ions after the MOFs are constructed, a characteristic that may
alter their fluorescence; and (iii) the amino groups on the ligand
may impart the material with catalytic activity. As the result of
this rational design, we herein present two series of lanthanide
coordination polymers, [Ln(BTATB)(DMF)2(H2O)]·DMF·
INTRODUCTION
■
For decades, the design and synthesis of novel lanthanide−
organic frameworks (LnOFs) has been a field of rapid growth in
supramolecular and materials chemistry because of their
exceptionally artistic architectures and potential applications,
such as gas adsorption and separation, heterogeneous catalysis,
and luminescence.1 LnOFs offer high photoluminescence
efficiencies, unique line-like emission bands, substantial Stokes
shifts, and long luminescence lifetimes.2 It should be noted that
the unique luminescence properties of lanthanide complexes
result from f−f transitions generated via the “antenna effect”,3
which have made them one of the essential components in the
preparation of advanced materials.4 Recently, studies on
luminescent LnOFs for sensing metal ions have resulted in
their significant development, and some successful fluorescent
probes have been employed to determine the concentration of
metal ions, such as Zn2+, Cu2+, and Fe3+.5
The use of MOFs as catalysts is another hot topic in this field in
addition to their use as luminescent probes. The catalytic activity
of MOF materials results from the metal center in the structure
or the catalytically active functional organic sites on the organic
ligand, such as noncoordinated amino groups.6 Detailed studies
on the catalytic performance of MOFs with catalysis taking place
at the metal center have been reported recently.7 However, only a
few proof-of-concept studies of MOFs with catalysis taking place
at the organic ligand have been reported.8
Received: February 26, 2014
Revised: April 26, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/cg500286v | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Table 1. Crystallographic Data for Series A
1a
2a
3a
4a
empirical formula
formula weight
wavelength (Å)
crystal system
space group
a (Å)
C36H45LaN6O12
892.68
C36H45EuN6O12
905.74
C36H45TbN6O12
912.71
C36H45ErN6O12
921.04
0.710 73
0.710 73
0.710 73
1.541 78
monoclinic
monoclinic
monoclinic
monoclinic
P1
̅
P1
̅
P1
̅
9.9808(5)
P1
̅
10.071(4)
13.401(6)
15.645(7)
106.901(7)
96.009(7)
96.899(7)
1983.8(15)
2
9.9905 (5)
13.2420(6)
15.4464(7)
107.2920(10)
95.8580(10)
97.4250(10)
1913.33(16)
2
10.0286(3)
13.2632(3)
15.4066(5)
107.730(3)
96.065(2)
96.308(2)
1919.04(10)
2
b (Å)
13.2130(7)
15.4078(8)
107.4350(10)
95.8690(10)
97.2850(10)
1901.69(17)
2
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
Dc (mg m−3
)
1.491
1.572
1.594
1.594
μ (mm−1
)
1.144
1.709
1.930
4.654
F(000)
908
924
928
934
range for data collection (deg)
reflections collected
max, min transmission
T (K)
1.61−26.00
10 794
1.40−26.00
10 630
1.40−26.00
10 599
3.04−72.18
13 429
0.7583, 0.6848
293(2)
0.6905, 0.5798
293(2)
0.5763, 0.4943
293(2)
0.4303, 0.2866
293(2)
data/restraints/parameters
7632/0/451
R1 = 0.0521, wR2 = 0.1189
R1 = 0.0674, wR2 = 0.1272
7416/0/451
R1 = 0.0438, wR2 = 0.1097
R1 = 0.0554, wR2 = 0.1155
1.133, −0.903
7379/0/452
R1 = 0.0446, wR2 = 0.0970
R1 = 0.0580, wR2 = 0.1024
0.801, −0.864
7383/0/497
a
final R indices [I > 2σ(I)]
R1 = 0.0400, wR2 = 0.1082
R1 = 0.0426, wR2 = 0.1118
1.835, −0.969
R indices (all data)
largest diff. peak and hole (e Å−3
R1 = ∑∥Fo| − |Fc∥/∑|Fo|; wR2 = ∑[w(Fo − Fc2)2]/∑[w(Fo )2]1/2
)
1.045, −1.170
a
2
2
brown precipitate was collected by centrifugation, washed with water,
and dried under vacuum to yield 2 (8.0 g, 90%). 1H NMR (DMSO-d6):
δ 6.62 (s, 3H), 7.13 (d, 6H), 7.82 (d, 6H), 8.73 (s, 3H).
Preparation of [Ln(BTATB)(DMF)2(H2O)]·DMF·2H2O (Series A; Ln =
La (1a), Eu (2a), Tb (3a), Er (4a)). Complexes 1a−4a were synthesized
by the following procedure: Ln(NO3)3·6H2O (30 mg) was added to a
DMF/H2O (1:1, 3 mL) solution of H3BTATB (15 mg), and the mixture
was stirred for ca. 20 min in air. It was then heated at 105 °C for 2 days to
produce brown block crystals.
2H2O (series A; Ln = La (1a), Eu (2a), Tb (3a), Er (4a)) and
[Ln(BTATB)(H2O)2]·2DMA·4H2O (series B; Ln = Er (1b), Yb
(2b), Lu (3b)), that were generated by changing only the
reaction solvent. More interestingly, the emission intensity of 3a
could be modulated by Zn2+ among other metal ions. 3a and 2b
also show catalytic activity toward Knoevenagel condensation
reactions.
[La(BTATB)(DMF)2(H2O)]·DMF·2H2O (1a). Yield: ca. 68%. Anal.
calcd (%) for 1a C36H45LaN6O12 (%): C, 48.44; H, 5.08; N, 9.41. Found:
C, 48.54; H, 5.11; N, 9.38. IR (KBr, cm−1): 3448 (s), 1654 (s), 1586 (s),
1517 (m), 1392 (vs), 1322 (w), 1261 (w), 1168 (w), 844 (w), 787 (w).
[Eu(BTATB)(DMF)2(H2O)]·DMF·2H2O (2a). Yield: ca. 70%. Anal.
calcd (%) for 2a C36H45EuN6O12 (%): C, 47.74; H, 5.01; N, 9.28.
Found: C, 47.54; H, 5.21; N, 9.01. IR (KBr, cm−1): 3420 (s), 2925 (w),
1654 (s), 1586 (s), 1519 (m), 1396 (vs), 1322 (m), 1261 (w), 1167 (w),
1135 (w), 1101 (w), 844 (w), 786 (m).
[Tb(BTATB)(DMF)2(H2O)]·DMF·2H2O (3a). Yield: ca. 71%. Anal.
calcd (%) for 3a C36H45TbN6O12 (%): C, 47.37; H, 4.97; N, 9.21.
Found: C, 47.21; H, 5.07; N, 9.03. IR (KBr, cm−1): 3425 (s), 2924 (w),
1655 (s), 1586 (s), 1520 (m), 1398 (vs), 1321 (m), 1260 (w), 1166 (w),
1135 (w), 1101 (w), 843 (w), 786 (m).
[Er(BTATB)(DMF)2(H2O)]·DMF·2H2O (4a). Yield: ca. 30%. Anal. calcd
(%) for 4a C36H45ErN6O12 (%): C, 46.95; H, 4.92; N, 9.12. Found: C,
46.71; H, 5.01; N, 9.07. IR (KBr, cm−1): 3428 (s), 2927 (w), 1652 (s),
1587 (s), 1521 (m), 1402 (vs), 1325 (m), 1258 (w), 1164 (w), 1137
(w), 1105 (w), 845 (w), 789 (m).
Preparation of [Ln(BTATB)(H2O)2]·2DMA·4H2O (Series B; Ln = Er
(1b), Yb (2b), Lu(3b)). Complexes 1b−3b were synthesized by the
following procedure: Ln(NO3)3·6H2O (30 mg) was added to a DMA/
H2O (1:1, 3 mL) solution of H3BTATB (15 mg), and the mixture was
stirred for ca. 20 min in air. It was then heated at 105 °C for 2 days to
produce brown block crystals.
EXPERIMENTAL SECTION
■
Materials and Methods. All chemicals used for synthesis were
commercially available reagents of analytical grade and were used
without further purification. C, H, and N microanalyses were carried out
with a PerkinElmer 240 elemental analyzer. The FT-IR spectra were
recorded from KBr pellets in the 4000−400 cm−1 range on a Nicolet
5DX spectrometer. Thermogravimetric analyses (TGA) were recorded
on a PerkinElmer Pyris 1 (25−600 °C, 5 °C min−1, flowing N2(g)). X-
ray powder diffraction (PXRD) patterns were recorded with a Bruker
AXS D8 advanced automated diffractometer with Cu Kα radiation.
Luminescence spectra for the solid and liquid samples were investigated
with a Hitachi F-4500 fluorescence spectrophotometer and Varian Cary
Eclipse Fluorescence spectrophotometer, respectively. The products of
catalysis reactions were monitored by gas chromatography with a SP-
2100A gas chromatograph.
Synthesis of Triethyl 4,4′,4″-(Benzene-1,3,5-triyltris(azanediyl))-
tribenzoate, 1. Concentrated hydrochloric acid (5.2 mL), phloroglu-
cinol (12.6 g, 0.10 mol), 4-aminobenzoic acid ethyl ester (66.0 g, 0.40
mol), and diethylene glycol dimethyl ether (45 mL) were added to a 100
mL round-bottomed flask, and the mixture was stirred at 130 °C for 7 h.
Yellow crystals appeared as the suspension cooled. The crystals were
washed with diethylene glycol dimethyl ether and dried in an oven to
yield 35.2 g of 1 (35.2 g, 70%). 1H NMR (DMSO-d6): δ 1.30 (t, 9H),
4.26 (q, 6H), 6.62 (s, 3H), 7.13 (d, 6H), 8.38 (d, 6H), 8.78 (s, 3H).
Synthesis of H3BTATB, 4,4′,4″-(Benzene-1,3,5-triyltris(azanediyl))-
tribenzoic acid, 2. Compound 1 (10.0 g, 0.019 mol) was suspended in
150.0 mL of THF/MeOH (v/v 1:1) followed by the addition of 60.0 mL
of a 10% NaOH solution. The mixture was stirred overnight. The pH
was adjusted to approximately 3 using hydrochloric acid. The resulting
[Er(BTATB)(H2O)2]·2DMA·4H2O (1b). Yield: ca. 72%. Anal. calcd (%)
for 1b C35H48ErN5O14 (%): C, 45.20; H, 5.20; N, 7.53. Found: C, 45.32;
H, 5.24; N, 7.47. IR (KBr, cm−1): 3399 (s), 2922 (w), 1592 (s), 1518
(m), 1412 (vs), 1318 (m), 1256 (m), 1174 (m), 839 (w), 786 (m).
B
dx.doi.org/10.1021/cg500286v | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
[Yb(BTATB)(H2O)2]·2DMA·4H2O (2b). Yield: ca. 69%. Anal. calcd (%)
for 2b C35H48YbN5O14 (%): C, 44.92; H, 5.17; N, 7.48. Found: C, 44.86;
H, 5.21; N,7.62. IR (KBr, cm−1): 3400 (s), 2922 (w), 1593 (s), 1521
(m), 1413 (vs), 1320 (m), 1256 (w), 1174 (w), 786 (m).
Single-crystal X-ray diffraction analysis revealed that 4a
crystallizes in the triclinic P1 space group. 4a is an interdigitated
̅
metal−organic framework; the asymmetric unit of 4a contains
one crystallographically independent Er(III) center, one BTATB
ligand, two coordinated DMF molecules, one aqua ligand, one
free DMF molecule, and two lattice water molecules. As shown in
Figure 1a, each Er(III) center is nine-coordinate and surrounded
by six carboxylate oxygen atoms from three individual BTATB
ligands, three oxygen atoms from two coordinated DMF
molecules, and one aqua ligand, respectively, forming a tricapped
trigonal-pyramidal geometry. The BTATB ligand adopts a
tri(chelate) coordination mode, and its carboxylate groups are
deprotonated.
[Lu(BTATB)(H2O)2]·2DMA·4H2O (3b). Yield: ca. 67%. Anal. calcd (%)
for 3b C35H48LuN5O14 (%): C, 44.83; H, 5.16; N, 7.47. Found: C, 44.78;
H, 5.23; N, 7.51. IR (KBr, cm−1): 3400 (s), 2921 (w), 1593 (s), 1521
(m), 1415 (vs), 1317 (w), 1255 (w), 1174 (w), 836 (w), 786 (m).
X-ray Crystallography. Crystallographic data of all the complexes
were collected at 173 K with a Apex II diffractometer with Mo Kα
radiation or Cu Kα radiation (λ = 0.71073 or 1.5418 Å) and a graphite
monochromator using the ω scan mode. The structure was solved by
direct methods and refined on F2 by full-matrix least-squares using
SHELXTL.11 Crystallographic data and experimental details for
structural analyses are summarized in Tables 1 and 2. The CCDC
The Er(III) centers are connected by BTATB ligands to form a
2D wave-like layer with a (6,3) topological net (Figure 1b). A
very significant structural feature of 4a is that two adjacent layers
are interdigitated together with the metal ion parts protruding
out of the other layer (Figure 1b,c). The aromatic ring distance
between two stacked BTATB ligands and a pair of interdigitated
layers is 4.11(1) Å, as shown in Figure 1c, and the structure can
be defined as stable MOFs with a framework that is interdigitated
through π−π stacking. The lanthanide(III) ions usually have a
high coordination number because of their large atomic radius,
and they tend to form highly connected complex structures.
Therefore, it is difficult to produce low-connected structures like
a (6,3) net. Although rare-earth coordination polymers with a
(6,3) net have been previously reported, Mak et al. reported a
coordination polymer {[Er(NO3)3(L)1.5]·H2O} (L = 1,2-bis(4-
pyridyl)ethane-N,N′-dioxide) showing a mat-like composite
layer with a (6,3) net,12 and Jin et al. reported three compounds
showing a 2D layer of a (6,3) net.13 Lanthanide coordination
polymers based on oxalate that have a (6,3) net have also been
reported.14 To the best of our knowledge, the LnOF with a (6,3)
net stacked in an interdigitated fashion is unprecedented.15
Furthermore, adjacent interdigitated layers are linked by
hydrogen bonds to form a 3D supramolecular framework
(Figure 1d).
Table 2. Crystallographic Data for Series B
1b
2b
3b
empirical
formula
C35H48ErN5O14
C35H48YbN5O14
C35H48LuN5O14
formula weight
wavelength (Å)
crystal system
space group
a (Å)
930.04
935.82
937.75
0.710 73
monoclinic
0.710 73
monoclinic
0.710 73
monoclinic
P1
̅
P1
̅
P1
̅
11.8978(4)
12.4423(4)
13.2083(5)
84.049(3)
85.054(3)
85.456(3)
1932.65(12)
2
11.8938(4)
12.4541(3)
13.1622(3)
84.035(2)
85.080(2)
85.246(2)
1926.58(9)
2
11.8842(3)
12.4550(3)
13.1712(3)
83.924(2)
84.970(2)
85.196(2)
1925.69(8)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
Dc (mg m−3
)
1.433
1.447
1.617
μ (mm−1
)
2.232
2.488
2.637
F(000)
840
844
952
range for data
collection
(deg)
2.93−26.00
2.91−26.00
2.91−26.00
reflections
collected
13 979
13 112
15 877
Single-crystal X-ray diffraction analysis revealed that 1b
max, min
transmission
0.538, 10.4939
0.5023, 0.4612
0.5985, 0.3983
crystallizes in the triclinic P1 space group, which features a 2D
̅
bilayer structrue. The asymmetric unit of 1b is composed of one
crystallographically independent Er(III) center, one BTATB
ligand, two aqua ligands, two free DMA molecules, and four
lattice water molecules. As shown in Figure 2a, each Er(III)
center is eight-coordinate and surrounded by eight oxygen
atoms, six from the carboxylate atoms of six individual BTATB
ligands via two chelating and one syn−anti O,O′-bridge
coordination modes; the remaining coordination sites are
occupied by two individual aqua ligands. The coordination
geometry around the Er(III) center can be described as a
distorted dodecahedron, with the O−Er−O bond angles ranging
from 54.3(1) to 134.8(1) and the Er−O bond lengths ranging
from 2.209(3) to 2.462(3) Å. There is a unique crystallo-
graphically independent BTATB ligand in 2b, of which two
carboxylate groups bridge two Er(III) centers in chelating
coordination modes, whereas the third carboxylate group bridges
two Er(III) centers in a syn−anti O,O′-bridge coordination
mode; all of the carboxylate groups of the BTATB ligands are
deprotonated. Two near Er(III) centers are bridged by two
carboxylate oxygen atoms of carboxylate groups in syn−syn
fashion to form a Er(III)···Er(III) dimer (Figure 2b). The Er···Er
distance and Er−O−Er bond angles in the dimer are 4.5387(3) Å
and 123.6(4)°, respectively. Four Er(III)···Er(III) dimers are
further linked by two BTATB ligands to form a 2D bilayer
T (K)
293(2)
293(2)
293(2)
data/restraints/
parameters
7585/0/437
7576/0/352
7569/0/496
final R indices
[I > 2σ(I)]
R1 = 0.0590,
wR2 = 0.1420
R1 = 0.0272,
wR2 = 0.0683
R1 = 0.0374,
wR2 = 0.0960
a
R indices (all
data)
R1 = 0.0687,
wR2 = 0.1464
R1 = 0.0310,
wR2 = 0.0700
R1 = 0.0426,
wR2 = 0.0988
largest diff. peak 4.975, −5.681
0.763, −0.772
2.544, −1.045
and hole
(e Å−3
)
a
R1 = ∑∥Fo| − |Fc∥/∑|Fo|; wR2 = ∑[w(Fo − Fc2)2]/∑[w(Fo )2]1/2
2
2
reference numbers are 988282−988285 for 1a−4a and 988286−988288
for 1b−3b. A copy of the data can be obtained free of charge upon
request from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(Fax: +44(1223)336-033; E-mail: deposit@ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
■
The crystals [Ln(BTATB)(DMF)2(H2O)]·DMF·2H2O (Ln =
La (1a), Eu (2a), Tb (3a), Er (4a)) and [Ln(BTATB)(H2O)2]·
2DMA·4H2O (Ln = Er (1b), Yb (2b), Lu (3b)) are two series of
isomorphous coordination frameworks, so 4a and 1b are
employed as representative examples that will be described in
detail.
C
dx.doi.org/10.1021/cg500286v | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 1. (a) Coordination environment of the Er(III) center in 4a. Symmetry mode for series A: x + 1, y − 1, z. Symmetry mode for series B: x + 1, y, z +
1. (b) (6,3) layer of 4a. (c) Top: side view of 2D interdigitated layer architecture of 4a. Bottom: distance between two BTATB ligands. (d) Three-
dimensional stacking plot of 4a. (e) Space-filling representation of the 3D structure of 4a along the a-axis direction (DMF molecules omitted).
structure (Figure 2c,d). Interestingly, the 2D bilayer can be
rationalized as a noninterpenetrating CdI2-type network by
simplifying the BTATB ligand as a three-connected node (vertex
symbol 43) and the Er···Er dimer as a six-connected node (vertex
symbol 466683) (Figure 2e). The CdI2-type network, which can
best of our knowledge, the 2D lanthanide(III) bilayer
coordination polymer with the CdI2-type network is unprece-
dented. Furthermore, adjacent bilayers are linked by hydrogen
bonding to form a 3D supramolecular framework (Figure 2c).
It should be noted that just by changing the solvent under the
same reaction conditions several different compounds were
synthesized with the same ligand and lanthanide ions. 1b was
synthesized in a similar procedure as that of 4a, the difference
being that DMA was used as the solvent instead of DMF.
However, the two complexes display distinct structures. Because
different solvents in the solvothermal conditions can lead to a pH
change of the reaction system, the reaction microenvironment
and the coordination modes of the BTATB ligand change and,
consequently, different structures are generated.
3
be represented by Wells notation {46 }, is one of the Catalan nets
known in inorganic compounds such as metal alkoxides and
hydroxides.16 The main topological character of the CdI2 net is
that it can be regarded as forming by the offset overlap of two
(6,3) nets, but, in fact, it contains only congruent quadrangles
similar to those of the (4,4) net.17 Although coordination
networks with a bilayer network have been reported previously,
Hill et al. reported four lanthanide(III) coordination polymers
with 2D bilayer networks,18 in which three structures have six-
connected topology and a single example has five-connecting
topology, and Wang et al. reported four Zn(II) coordination
polymers that exhibit different bilayer architectures.19 To the
Thermal Stability. The simulated and experimental XRD
patterns of two series of compounds are shown in Figures S1 and
S2, Supporting Information. Their peak positions are almost in
D
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Figure 2. (a) Coordination modes of the BTATB ligand and coordination environment of the Er center in 1b. Symmetry mode of series A: 1 − x, 2 − y, 1
− z. Symmetry mode of series B: 2 − x, 1 − y, −z. C: 1 − x, 2 − y ,−z. (b) Er(III)···Er(III) dimer. (c) and (d) 2D bilayer structure of 1b (yellow
polyhedrons represent the Er dimer). (e) View of the simplified network of 1b.
agreement with each other, indicating the phase purity of the
products. The TGA curve (Figure S3, Supporting Information)
of series A shows the first weight loss of 3.1% at ca. 100 °C, which
corresponds to the loss of two lattice water molecules (calculated
3.94%). A weight loss of 26.9% occurs between 100 and 400 °C,
which corresponds to the loss of one guest DMF, one
coordinated water, and two coordinated DMF molecules
(calculated 27.9%). The residue was Ln2O3 (experimental,
41.2%; calculated, 40.7%). The TGA curve (Figure S4,
Supporting Information) of series B demonstrates that the
weight loss of the free and coordinated solvent molecules was
well-resolved. From 30 to ca. 235 °C, the first weight loss is
18.72%, which corresponds to the weight of two free DMA
molecules (calculated 18.62%). The weight loss of 7.6% between
235 and 444 °C may be attributed to the loss of four lattice water
molecules (calculated 7.7%) and then decomposition of the
complexes upon further heating.
transitions, 5D4 → 7F5 (546 nm) is the strongest. The effects of
a variety of biologically relevant cations, Co2+, Cu2+, Cd2+, Pb2+,
Hg2+, and Zn2+, on the fluorescent intensity of 3a were
investigated, and the result indicates that the emission intensity
of 3a increased significantly upon addition of Zn2+. More
remarkably, the emission intensity of 3a increased gradually upon
addition of 1−3 equiv of Zn2+ with respect to 3a (Figure 3a). The
highest peak at 577 nm is at least twice as intense as the
corresponding band in the solution without Zn2+. The
introduction of other metal ions caused the intensity either to
be unchanged or weakened (Figure 3b).
It is well-known that the luminescent intensity of Ln3+ relies on
the efficiency of the energy transfer from the ligand to the Ln3+
center.20a Chen et al. prepared [Eu(pdc)1.5(DMF)] with Lewis
basic pyridyl sites for sensing Cu2+ and Co2+,5d and Liu et al.
reported Ag+-modulated luminescence for a Eu(III) complex;5c
in this case, the luminescent intensity was enhanced because Ag+
is coordinated by the azacyclic amine group from the tetra(amino
acid) ligand. As with the study of the reported metal-sensitive
luminescent lanthanide probes, the enhancement of the
luminescent intensity in our investigation may result from Zn2+
aggrandizing the antenna efficiency of the BTATB organic
linkers, diminishing the f−f transitions of Tb3+ and leading to
more effective intramolecular energy transfer from the BTATB
ligand to Ln3+ with the addition of Zn2+. The luminescence
properties of 1a were also measured, but no emissions were
observed.
Photoluminescence Properties. Studies on luminescent
lanthanides have concentrated on Sm(III), Eu(III), Tb(III), and
Dy(III) ions in the visible region, so the luminescence properties
of 3a were investigated. The emission spectrum of 3a was
collected in the range 380−700 nm in a methanol suspension and
in the solid state (Figure S5, Supporting Information) at room
temperature under excitation at 360 nm, and it exhibits the
characteristic transition of the Tb3+ ion:20 5D4 → 7FJ (J = 3, 4, 5,
6), namely, 5D4 → 7F6 (491 nm), 5D4 → 7F5 (546 nm), 5D4 →
7F4 (586 nm), and 5D4
→
7F3 (622 nm). Among these
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level, namely, the 2F5/2 level, so its luminescence observed at ca.
980 nm is assigned to the 2F5/2 → 2F7/2 transition.22
Catalysis Properties. The amino-based metal−organic
frameworks are stable, solid, basic catalysts in the Knoevenagel
condensation reaction, and their structural features endow the
MOFs with size- or shape-selective catalysis.23 In order to
investigate the size-selective catalysis of the two series of
complexes, we took a systematic approach by varying the size and
shape of the substrate. The Knoevenagel condensation reaction
of benzaldehyde with each of the active methylene compounds
(malononitrile and ethyl cyanoacetate) was performed with
catalysis by 3a and 2b as representative examples.
A solution of equivalent amounts of benzaldehyde (0.87 nm ×
0.61 nm) and malononitrile (0.69 nm × 0.45 nm) or ethyl
cyanoacetate (1.03 nm × 0.45 nm) was mixed in acetonitrile.
This solution was added at 60 °C to a suspension of 3a or 2b in
acetonitrile (Scheme 1), and the progress of the reaction was
Scheme 1. Knoevenagel Condensation Reaction Catalyzed by
3a and 2b
monitored using gas chromatography (see the Supporting
Information). As shown in Table 3, benzaldehyde (size, 0.87
nm × 0.61 nm; shape, linear) and malononitrile (0.69 nm × 0.45
nm) lead to a 99% conversion when the reaction is catalyzed by
3a. In the case of ethyl cyanoacetate (1.03 × 0.45 nm), 3a
resulted in a 42% conversion. The conversion of the reactions
catalyzed by 2b is 33 and 3%, respectively, which is lower than
that of reactions catalyzed by 3a.
Figure 3. (a) Emission spectra of complex 3a in methanol at room
temperature in the presence of 0−3 equiv of Zn2+ ions with respect to
3a: black, no addition; blue, 1 equiv; red, 2 equiv; green, 3 equiv. 3a was
excited at 360 nm. (b) Room-temperature luminescent intensity of 3a at
546 nm in a methanol suspension upon addition of various metal ions
(excited at 360 nm).
The distinct conversion rates reflect the size and shape
selectivity of the catalyst. 3a may lose coordinated DMF
molecules during the reaction, obtaining 1D microporous
channels (0.75 × 0.64 nm) in the structure (Figure 1e) in
which these catalytic reactions probably take place. If the
reactions were catalyzed only on the external surface of the MOF
crystals, then the difference in the conversion rates between the
two reaction systems would be small. Moreover, as catalysts,
these complexes are easily isolated from the reaction suspension
by a simple filtration and can be recycled with no less activity.
The stability of the catalyst was confirmed by the powder X-ray
diffraction (PXRD) pattern of 3a and 2b after the reactions
(Figure S6 and S7, Supporting Information).
Recently, many scientists have begun to exploit developments
in the detection of near-infrared luminescence to study the
luminescence from Nd(III), Er(III), and Yb(III) complexes,21 so
the near-infrared emission of 2b was measured. When complex
2b was excited at 315 nm in a DMF solution, it led to a typical
metal-centered near-infrared luminescence at 983 nm (Figure 4).
Yb(III) is a special case among the near-infrared-emitting
lanthanide ions because the Yb(III) ion has only one excited 4f
CONCLUSIONS
■
Two series of LnOFs based on the BTATB ligand were
successfully designed and synthesized simply by using different
solvents. Series A and B exhibit a novel 2D interdigitated (6,3)
layer architecture and a 2D bilayer supermolecular structure,
respectively. Their interesting structural features, lanthanide
metal centers, and functional BTATB ligand impart these
materials with excellent properties. 3a exhibits strong lumines-
cence emission in the solid state and in a methanol suspension at
room temperature. 2b exhibits near-infrared luminescence at 983
nm. More interesting is that the luminescence of 3a displayed
high selectivity for Zn2+, which suggests that it may be used as a
luminescent probe for Zn2+. The amide groups on the BTATB
ligand in the structures of 3a and 2b were used as a catalyst for the
selective catalysis of organic molecules in the Knoevenagel
condensation reaction. These results will facilitate the explora-
Figure 4. Near-infrared emission spectrum of 2b in DMF upon
excitation at 315 nm.
F
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Table 3. Knoevenagel Condensation Reaction of Malononitrile and Ethyl Cyanoacetate with Substrates Catalyzed by 3a and 2b
F. R.; Xue, M.; Cui, Y. J.; Qian, G. D. Angew. Chem., Int. Ed. 2009, 48,
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tion of newly functionalized materials through the accessibility of
functional organic linkers and metal components.
ASSOCIATED CONTENT
■
S
* Supporting Information
(6) (a) Burgoyne, A. R.; Meijboom, R. Catal. Lett. 2013, 143, 563.
Simulated and experimental X-ray powder diffraction patterns
for series A and B; TGA curve of series A and B; excitation and
emission spectra of 3a in the solid state at room temperature;
PXRD patterns of simulated and as-synthesized 3a and 2b; GC of
the reactions of benzaldehyde and malononitrile and benzalde-
hyde and ethyl cyanoacetate catalyzed by 3a; and GC of the
reactions of benzaldehyde and malononitrile catalyzed by 2b.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ceshzb@lnu.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was granted financial support from the National
Natural Science Foundation of China (20871063, 21271096),
the Program for Liaoning Excellent Talents in University
(LR2011001), the Liaoning Natural Science Foundation of
China (201102079), and the Liaoning Innovative Team Project
in University (LT2011001).
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