Synthesis, structure and photoluminescence of two porous metal-organoboron frameworks with rtl topology

Article abstract of DOI:10.1007/s11426-011-4355-2

Rigid trigonal tris(3-pyridylduryl)borane L was synthesized through four steps in good overall yield from readily available 1,2,4,5-tetramethylbenzene and was used to construct two porous cadmium (II) complexes Cd(L)X ₂?G (X = Cl, Br; G = guset

Full text of DOI:10.1007/s11426-011-4355-2

SCIENCE CHINA
Chemistry
September 2011 Vol.54 No.9: 1430–1435
doi: 10.1007/s11426-011-4355-2
• ARTICLES •
· SPECIAL TOPIC · Coordination Polymer
Synthesis, structure and photoluminescence of two porous
metal-organoboron frameworks with rtl topology
LIU Yan, XU Xin, XUAN WeiMin, ZHU ChengFeng & CUI Yong*
State Key Laboratory of Metal Matrix Composites; School of Chemistry and Chemical Technology,
Shanghai Jiao Tong University, Shanghai 200240, China
Received April 14, 2011; accepted May 19, 2011
Rigid trigonal tris(3-pyridylduryl)borane L was synthesized through four steps in good overall yield from readily available
1,2,4,5-tetramethylbenzene and was used to construct two porous cadmium (II) complexes Cd(L)X2·G (X = Cl, Br; G = guset
molecules), 1 (Cd(L)Cl2·EtOH·iPrOH·3H2O) and 2 (Cd(L)Br2·MeOH·C7H8·3H2O), under mild reaction conditions. 1 and
2 are isostructural and featured with 3D porous metal-organoboron framewoks with rtl topology, in which L acts as a
3-connected node while a dicadmium motif serves as 6-connected node. In addition, 1 and 2 exhibit strong photoluminescence
in the visible region and 1 shows moderate adsorption ability of carbon dioxide at 273 K.
boron, metal-organic frameworks, coordination polymer, photoluminescence
rials [12, 13]. We have recently showed that rational com-
1 Introduction
bination of three-coordinated boron with pyridine-based
coordination units will yield a C3-symmetric tris(pyridyldu-
ryl)borane that could be served as building block to fabri-
cate a series of chiral octupolar NLO-active solids based on
3D coordination networks [14] and three porous networks
exhibiting remarkable multiple (n,3) topology isomerism
and excellent antibacterial activities and durability[15].
Encouraged by the successful construction of the afore-
mentioned functional porous metal-organoboron frame-
works and to further extend our work in this field, herein we
design and synthesize a new C3-symmetric tris(pyridyldu-
ryl)borane ligand L and employ it to assemble with CdCl2
and CdBr2 to afford two isostructural porous metal-
organoboron frameworks 1 and 2. Photoluminescence and
gas adsorption were studied to test their potential applica-
tions as optics and adsorbents.
Coordination-driven self-assembly method provides unique
opportunities to design and prepare ordered arrays of mole-
cules and clusters, and has led to significant progress in the
Metal-organic frameworks (MOFs) [1, 2]. MOFs are at-
tracting increasing attention not only because of their intri-
guing topologic structures but also due to their excellent
properties and potential promising applications in catalysis,
adsorption, separation, optics, magnetism and so on [3–7].
Pyridine-based ligands have been proved very useful build-
ing blocks to form 3D MOFs and numerous novel structures
with fascinating properties have been constructed with this
kind of ligands [8, 9]. On the other hand, three-coordinated
boron, with its vacant p-orbital, is a useful π-acceptor in
conjugated organic molecules [10, 11]. The inherently elec-
tron deficiency can lead to significant delocalization when
conjugated with an adjacent organic π-system. Therefore,
three-coordinated boron contained motifs are good candi-
dates for construction of electrooptical and electronic mate-
2 Experimental
2.1 Materials and apparatus
*Corresponding author (email: yongcui@sjtu.edu.cn)
All of the chemicals were commercially available, and used
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com www.springerlink.com

Liu Y, et al. Sci China Chem September (2011) Vol.54 No.9
1431
without further purification. Elemental analyses of C, H and
2.3 Synthesis of ligand
N were performed with an EA1110 CHNS-0 CE elemental
analyzer. The IR (KBr pellet) spectra were recorded (400–
4000 cm−1 region) on a Nicolet Magna 750 FTIR spectrom-
eter. Thermogravimetric analyses (TGA) were carried out in
an N2 atmosphere with a heating rate of 10 °C min−1 on a
STA449C integration thermal analyzer. The fluorescence
spectra were carried out on an LS 50B Luminescence Spec-
trometer (Perkin Elmer, Inc., USA). High pressure adsorp-
tion measurements were carried out on a BELSORP-HP
apparatus. The adsorption isotherm of CO2 was measured
using a volumetric adsorption system. Before the adsorption
measurement, the sample was activated at 373 K under
vacuum (<10−3 torr) for 24 h.
The ligand was synthesized in 75% yield by Suzuki cou-
pling between 3-pyridylboronic acid and tris(iododuryl)-
borane according to a reported procedure [14], which was
obtained in three steps in good overall yield from readily
available 1,2,4,5-tetramethylbenzene (Scheme 1). HNMR
(CDCl3) δ (ppm): 8.59 (m, 3H), 8.41 (s, 3H), 7.49 (d, 3H),
1
7.37 (m, 3H), 2.08 (s, 18H), 1.85 (s, 18H).
2.2 Crystallographic measurements and structure de-
termination
Single-crystal XRD data for the compounds was collected
on a Bruker SMART Apex II CCD-based X-ray diffractom-
eter with Mo-Kα radiation (λ = 0.71073 Å) for 1 and 2. The
two structures were solved by direct methods with
SHELXS-97 and refined with SHEXLX-97 [16]. All the
non-hydrogen atoms were refined by full-matrix techniques
with anisotropic displacement parameters. The hydrogen
atoms were geometrically fixed at calculated positions at-
tached to their parent atoms, and treated as riding atoms.
The crystallographic data and other pertinent information of
complexes 1 and 2 are summarized in Table 1, and the se-
lected bond lengths and bond angles are given in Table 2.
Scheme 1 Synthesis of ligand L.
2.4 Synthesis of complex 1
The ligand (0.0032 g, 0.05 mmol), CdCl2·2.5H2O (0.034g,
0.15 mmol), toluene (0.1 mL), PrOH (0.5 mL) and EtOH
(0.5 mL) were added to a small vial that was sealed and
heated to 353 K for 3 d and then cooled to room tempera-
i
Table 1 Crystal data for the title compound
Complex
Empirical formula
Formula weight
1
2
C49H66BCdCl2N3O5
971.16
C53H48BBr2CdN3O4
1073.97
0.71073
0.71073
Radiation (MoKα) (Å)
Crystal system, space group
T (K)
monoclinic, P21/c
293(2)
monoclinic, P21/c
298(2)
a (Å)
b (Å)
c (Å)
β (°)
V (Å3)
Z
11.794(2)
25.683(5)
17.275(4)
103.10(3)
5096.5(2)
4
12.125(9)
26.21(2)
17.467(1)
104.727(1)
5369(7)
4
Dc (g cm−3)
μ (MoKα)(cm−1)
1.266
1.938
0.578
2032
1.194
2168
F(000)
3.09 to 25.00
1.55 to 25.00
θ range for data collection (°)
Index ranges
−12 ≤ h ≤ 14, −30 ≤ k ≤ 30, −20 ≤ l ≤ 20
38365/8915 (Rint = 0.0548)
1.023
−13 ≤ h ≤ 14, −22 ≤ k ≤ 31, −19 ≤ l ≤ 20
24591/9216 (Rint = 0.0997)
0.918
Reflections collected/unique
Goodness-of-fit on F2
R, wR (I>2σ (I))
R1 = 0.0642, wR2 = 0.1698
R1 = 0.0640, wR2 = 0.1473

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Liu Y, et al. Sci China Chem September (2011) Vol.54 No.9
Table 2 Selected bond lengths (Å) and bond angles (°)
Complex
Bond
1
Complex
Bond
2
Dist.
Dist.
Cd(1)-N(2)#1
2.353(5)
2.355(5)
2.411(4)
2.5761(15)
2.6187(16)
2.6405(15)
2.6405(15)
2.353(5)
2.355(5)
(°)
Br(1)-Cd
2.7579(17)
2.7720(19)
2.6939(16)
2.375(6)
2.381(7)
2.437(6)
2.7720(19)
2.381(7)
2.375(6)
(°)
Cd(1)-N(3)#2
Br(1)-Cd#1
Cd(1)-N(1)
Br(2)-Cd
Cd(1)-Cl(2)
Cd-N(3)#2
Cd(1)-Cl(1)
Cd-N(2)#3
Cd(1)-Cl(1)#3
Cd-N(1)
Cl(1)-Cd(1)#3
Cd-Br(1)#1
N(2)-Cd(1)#4
N(2)-Cd#3
N(3)-Cd(1)#2
N(3)-Cd#4
Angle
Angle
N(2)#1-Cd(1)-N(3)#2
N(2)#1-Cd(1)-N(1)
N(3)#2-Cd(1)-N(1)
N(2)#1-Cd(1)-Cl(2)
N(3)#2-Cd(1)-Cl(2)
N(1)-Cd(1)-Cl(2)
N(2)#1-Cd(1)-Cl(1)
N(3)#2-Cd(1)-Cl(1)
N(1)-Cd(1)-Cl(1)
Cl(2)-Cd(1)-Cl(1)
N(2)#1-Cd(1)-Cl(1)#3
N(3)#2-Cd(1)-Cl(1)#3
N(1)-Cd(1)-Cl(1)#3
Cl(2)-Cd(1)-Cl(1)#3
Cl(1)-Cd(1)-Cl(1)#3
Cd(1)-Cl(1)-Cd(1)#3
174.43(17)
93.28(16)
87.02(16)
88.11(12)
86.32(13)
94.71(12)
89.68(12)
90.71(12)
172.44(12)
92.34(5)
89.89(12)
95.67(13)
85.07(11)
177.98(5)
87.99(5)
92.01(5)
Cd-Br(1)-Cd#1
N(3)#2-Cd-N(2)#3
N(3)#2-Cd-N(1)
N(2)#3-Cd-N(1)
N(3)#2-Cd-Br(2)
N(2)#3-Cd-Br(2)
N(1)-Cd-Br(2)
N(3)#2-Cd-Br(1)
N(2)#3-Cd-Br(1)
N(1)-Cd-Br(1)
Br(2)-Cd-Br(1)
N(3)#2-Cd-Br(1)#1
N(2)#3-Cd-Br(1)#1
N(1)-Cd-Br(1)#1
Br(2)-Cd-Br(1)#1
Br(1)-Cd-Br(1)#1
92.33(6)
176.2(2)
91.2(2)
86.7(2)
89.64(17)
87.43(18)
95.30(16)
92.81(17)
90.21(18)
86.62(16)
176.86(4)
90.88(15)
91.52(16)
174.02(16)
90.33(6)
87.67(6)
Symmetry transformations used to generate equivalent atoms for 1: #1 x, −y+1/2, z-1/2 #2 −x+1, −y, −z+2; #3 −x, −y, −z+1; #4 x, −y+1/2, z+1/2.
Symmetry transformations used to generate equivalent atoms for 2: #1 −x+1, −y+1, −z; #2 x, −y+3/2, z-1/2; #3 −x+2, −y+1, −z+1; #4 x, −y+3/2, z+1/2.
ture. Colorless block-like crystals of 1 were obtained and
the yield based on Cd salt is up to 78.3%. Anal. calcd (%)
for C49H66BCdCl2N3O5: C, 60.60; H, 6.85; N, 4.33. Found:
C, 60.26; H, 6.82; N, 4.26. IR (KBr, cm–1): 3419 (s), 2923
(m), 2367 (w), 1575 (w), 1478 (w), 1417 (m), 1396 (m),
1301 (w), 1267 (s), 1193 (m), 1032 (w), 952 (s), 800 (w),
717 (s), 642 (m).
3 Results and discussion
3.1 Synthesis and characterization
Two Cadmium (II) Complex complexes, 1 and 2, were syn-
thesized under mild reaction conditions. The bulk product
of them was obtained by repeating the experiment and the
resulting crystals were collected by filtration, washed sever-
al times with MeOH, and dried under vacuum for 2 h. The
phase purity of the bulk samples of them was established by
comparison of their observed and simulated X-ray powder
diffraction patterns (Figure 1).
The thermal stability of 1 and 2 was investigated on
crystalline samples under a N2 atmosphere from 40 to
700 °C. TGA result analysis of complex 1 shows that an
initial weight loss of 10.78% from 40 to 66 °C is probably
2.5 Synthesis of complex 2
The ligand (0.0032 g, 0.05 mmol), CdBr2·4H2O (0.052 g,
0.15 mmol), toluene (0.1 mL), MeOH (0.5 mL), EtOH (0.5
mL) and DMSO (0.1 mL) were added to a small vial that
was sealed and heated to 333 K for 12 h and then cooled to
room temperature. Colorless block-like crystals of was ob-
tained and the yield based on Cd salt is only 11.6%. Anal.
calcd (%) for C53H48BBr2CdN3O4: C, 59.27; H, 4.50; N, 3.91.
Found: C, 58.96; H, 4.53; N, 4.02. IR(KBr, cm−1): 3422 (m),
2890 (w), 1624 (m), 1560 (m), 1542 (m), 1478 (s), 1458 (m),
1418 (s), 1394 (s), 1298 (w), 1262 (s), 1192 (m), 1090 (w),
1050 (w), 1030 (m), 950 (s), 850 (m), 798 (m), 752 (w), 716
(s), 640 (m).
i
due to trapped guest alcohol (EtOH and PrOH) molecules
(calculated 10.9%). When the temperature reaches 140 °C,
the residual guest water molecules almost completely dis-
appear with a further weight loss of 5.45% (calcd 5.56%).
The weight loss of 7.35% in the range of 140 to 365 ºC is
corresponding to the loss of two coordinated chlorine atoms
(calculated 7.45%). From then on, its framework starts to
decompose. The MeOH and water guest molecules residing

Liu Y, et al. Sci China Chem September (2011) Vol.54 No.9
1433
in complex 2 completely disappear around 145 °C and the
toluene molecules in 2 may be removed in the process of
washing and collection. The weight loss of 12.96% in the
range of 145 to 383 °C is corresponding to the loss of two
coordinated bromide atoms (calcd 13.11%), after which, its
framework starts to decompose (Figure 2).
groups from three L ligands and three halogen atoms, two
of which(Cl1) bridge two cadmium atoms to form a binu-
clear Cd2Cl2 unit (Figure 4). The bond angles around the
Cd(1) atom range from 86.32(2) to 177.98(5)°, and the
Cd(1)–N and Cd–Cl bond lengths in the range of 2.353(5)
and 2.641(2) Å, respectively. Rectangular binuclear Cd2Cl2
unit is almost coplanar, in which the bond angles of
Cl1-Cd1-Cl1(87.99°) and Cd1-Cl1-Cd1(92.01°) are close to
90°, and the distance between the two non-bonded Cd atoms
is 3.784(7)Å.
The dicadmium motif in 1 is linked to six ligands L and
each L is linked to three dicadmium motifs to generate a 3D
framework with a rtl topology. The topology of 1 can be
described as a binodal (4.62)2(42.610.83) net if the dicadmium
motif is treated as a six-connected node and the ligand as a
three-connected node (Figure 5). The network contains 1D
channels with an opening of ~4.45 Å×2.1 Å along the a
3.2 Structural description
Single-crystal structure analysis reveals that 1 crystallizes in
the monoclinic space group P2(1)/c with Z = 4 and exhibits
a 3D framework. As shown in Figure 3, the basic building
i
unit of complex 1 contains one Cd(L)Cl2 unit, one PrOH,
one EtOH and three H2O guest molecules. The coordination
environment of the central boron atom of each L is com-
pletely trigonal planar with three duryl groups arranged in a
propellerlike fashion. The dihedral angles between the bo-
ron planes and the duryl planes are from 52.36° to 58.81°,
and those between the duryl planes and the outer pyridyl
planes are from 63.43° to 83.75°. Each Cd center adopts a
distorted octahedron by coordinating to three pyridine
Figure 3 Asymmetric unit of complex 1 (H atoms and guest molecules
Figure 1 Experimental and simulated powder XRD patterns of 1 and 2,
were omitted for clarity).
respectively.
Figure 4 Binuclear unit in 1.
Figure 2 Thermal analysis curves of the two cadmium complexes.

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Liu Y, et al. Sci China Chem September (2011) Vol.54 No.9
axis, suggesting potential adsorption and separation of gases
with small kinetic diameter such as hydrogen, methane and
carbon dioxide. The volume occupied by the lattice solvent
molecules in 1 is 1592Å3 per unit cell, which is 31.2% of the
total crystal volume (calculated by the program PLATON
[17]). High-pressure gas adsorption measurements were per-
formed to investigate the CO2 uptake capacity of 1 (Figure 6).
After activation at 100 °C under vacuum, 1 can take up 7.5
wt% (38 cm3 g−1 STP) of CO2 at 20 bar and 273 K.
Single-crystal X-ray structure determination revealed that
2 is isostructural to 1, but with different coordinated halo-
gen atoms and guest molecules (one toluene, one MeOH
and three water molecules). Besides this, there are also
some minor differences in the corresponding bond lengths,
bond angles and packing mode. The dihedral angles be-
tween the duryl planes and the boron plane range from
50.8° to 57.79° and that between the duryl planes and the
outer phenyl planes is from 62.23° to 83.97°. Additionally,
The bond angles around the Cd atom range from 86.62(2) to
176.86(4)°, and the Cd–N and Cd–Br bond lengths range
from 2.375(6) and 2.7720(2) Å. More detailed data are pre-
sented in Table 2. Calculations with the PLATON program
indicated that 2 has about 33.9% of total volume occupied
by solvent molecules.
Figure 7 Fluorescent emission spectra of 1, 2 and L.
2 are dominated by broad emission bands centered at 412
nm and 405 nm, respectively (Figure 7). Compared with the
free ligand, the emissions of 1 and 2 show red-shift of about
20 and 14 nm, respectively. The slight difference for 1 and
2 may arise from the variation of halogen atoms, which is
probably due to different coordination environments around
the central metal ions caused by anions, because fluorescent
behavior is closely related with the local chemical environ-
ments around metal centers [18]. All the emission peaks may
be assigned to the ligand-centered π→π* or n→π* process.
3.3 Photoluminescence property
Upon excitation at 320 nm, the emission spectra of 1 and
4 Conclusions
In summary, we have synthesized two Cd-based porous
metal-organoboron frameworks, 1 and 2, from a rational
designed C3-symmetric tris(pyridylduryl)borane multiden-
tate ligands. 1 and 2 are isostructural and display rtl topol-
ogy. As expected, the strong photoluminescence was found
for 1 and 2 and can be tuned by the different coordinated
halogen atoms. In addition, the porosity of 1 was verified by
moderate uptake of carbon dioxide at 273 K.
Figure 5 (a) 3D structure of 1 along the a axis; (b) scheme showing the
(4.62)2(42.610.83) topology of 1.
This work was supported by the National Natural Foundation of China
(21025103 & 20971085), the National Basic Research Program of China
(2007CB209701 & 2009CB930403), and Shanghai Science and Technolo-
gy Committee (10DJ1400100) and the Key Project of State Education
Ministry.
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