Two ZnII metal-organic frameworks with coordinatively unsaturated metal sites: Structures, adsorption, and catalysis

Article abstract of DOI:10.1002/asia.201200601

Assembly of Zn(NO₃)₂ with the tripodal ligand H ₃TCPB (1,3,5-tri(4-carboxyphenoxy)benzene) affords two porous isoreticular metal-organic frameworks, [Zn₃(TCPB)₂· 2DEF]· 3DEF (1) and [Zn₃(TCPB)₂·2H ₂O]· 2H₂O·4DMF (2). Single-crystal X-ray diffraction analyses reveal that 1 crystallizes in the monoclinic space group P2₁/c and possesses a 2D network containing 1D microporous opening channels with an effective size of 3.0×2.9 A², whereas 2 crystallizes in the trigonal space group P3c1 and also possesses a 2D network containing 1D channels, with an effective aperture of 4.0×4.0 A². TOPOS analysis reveals that both 1 and 2 have a (3,6)-connected network topology with the Schlaefli symbol of (4 ³·6¹²) (4³)₂. According to the variable-temperature powder X-ray diffraction patterns, the solid phase of 1 can be converted into that of 2 during a temperature-induced dynamic structural transformation, thus indicating that the framework of 2 represents the most thermally stable polymorph. Desolvated 2 exhibits highly selective adsorption behaviors toward H₂/N₂, CO₂/N₂, and CO₂/CH₄; furthermore, it displays size-selective catalytic activity towards carbonyl cyanosilylation and Henry (nitroaldol) reactions. Going through channels: Two robust porous metal-organic frameworks (MOFs) containing 1D channels have been synthesized from Zn²⁺ ions and the semi-rigid tripodal ligand 1,3,5-tri(4-carboxyphenoxy)benzene. The desolvated MOFs present selective adsorption behaviors toward H₂/N₂, CO₂/N₂, and CO₂/CH₄, and exhibit size-selective catalytic activity towards carbonyl cyanosilylation and the Henry reaction. Copyright

Full text of DOI:10.1002/asia.201200601

FULL PAPER
DOI: 10.1002/asia.201200601
Two ZnII Metal-Organic Frameworks with Coordinatively Unsaturated Metal
Sites: Structures, Adsorption, and Catalysis
Xiao-Ming Lin,[a] Ting-Ting Li,[a] Yi-Wei Wang,[a] Li Zhang,*[a] and Cheng-Yong Su*[a, b]
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Chem. Asian J. 2012, 7, 2796 – 2804

ꢀ
Abstract: Assembly of Zn
(NO3)2 with
trigonal space group P3c1 and also pos-
diffraction patterns, the solid phase of
1 can be converted into that of 2
during a temperature-induced dynamic
structural transformation, thus indicat-
ing that the framework of 2 represents
the most thermally stable polymorph.
Desolvated 2 exhibits highly selective
adsorption behaviors toward H2/N2,
CO2/N2, and CO2/CH4; furthermore, it
displays size-selective catalytic activity
towards carbonyl cyanosilylation and
Henry (nitroaldol) reactions.
the tripodal ligand H3TCPB (1,3,5-
tri(4-carboxyphenoxy)benzene) affords
two porous isoreticular metal-organic
sesses a 2D network containing 1D
channels, with an effective aperture of
4.0ꢁ4.0 ꢂ2. TOPOS analysis reveals
that both 1 and 2 have a (3,6)-connect-
ed network topology with the Schlꢃfli
symbol of (43·612) (43)2. According to
the variable-temperature powder X-ray
frameworks,
[Zn3ACTHNGUTERN(UNG TCPB)2·2DEF]·
3DEF (1) and [Zn3(TCPB)2·2H2O]·
2H2O·4DMF (2). Single-crystal X-ray
diffraction analyses reveal that 1 crys-
tallizes in the monoclinic space group
P21/c and possesses a 2D network con-
taining 1D microporous opening chan-
nels with an effective size of 3.0ꢁ
2.9 ꢂ2, whereas 2 crystallizes in the
Keywords: adsorption · catalysis ·
Henry reaction
·
metal-organic
frameworks · X-ray diffraction
Introduction
catalysts.[3,4] Moreover, metals and ligands are strongly
bound within the framework to prevent its decomposition in
solution.
As one type of metal-organic materials, metal-organic
frameworks (MOFs) have gained current attention mainly
because of their potential applications in gas adsorption and
heterogeneous catalysis.[1] MOFs adsorb gas principally by
physisorption, and two strategies can be envisioned to reach
stronger physisorption energy: 1) smaller pore sizes, being
slightly larger than the kinetic diameter of gas molecules, so
that the gas molecules can simultaneously interact with
more pore walls; and 2) incorporation of coordinatively un-
saturated metal sites that can bind gas molecules, so that the
gas molecules interact with the framework strongly; more-
over, these sites could selectively adsorb certain compon-
tents from gas mixtures based on their different polarizabili-
ty and/or quadrupole moment.[2]
One of our goals is to design and synthesize robust MOFs
containing coordinatively unsaturated metal sites that can
display selective adsorption and catalytic abilities.[5] Herein
we select a semi-rigid tripodal ligand of H3TCPB (1,3,5-
tri(4-carboxyphenoxy)benzene; Scheme 1) due to the fol-
lowing considerations: 1) the multifunctional carboxylate
groups have good prospects for the construction of well-de-
fined porous MOFs; and 2) the incorporation of flexible
ether groups to the rigid benzene spacers could generate
subtle structural diversifications owing to the conformation-
al freedom in the assembly process. Two porous isoreticular
Zn-MOFs, [Zn
ylformamide and [Zn CTHUNGTRENNUNG
(TCPB)2·2DEF]·3DEF (1; DEF=N,N’-dieth-
3A(TCPB)2·2H2O]·2H2O·4DMF (2),
On the other hand, MOFs can work as catalysts through:
1) the metallic components, which either provide the coordi-
natively unsaturated nodes or form the active metal com-
plexes incorporated into the linker ligand; and 2) ligands
that are functionalized with organic groups. In the former
case, MOFs can act as Lewis acid catalysts because of the
Lewis acidity of metal cations, whereas in the latter case
MOFs can behave as Lewis base catalysts due to the Lewis
basicity of the functional groups such as pyridine, amide,
and amine. As crystalline MOFs are generally poorly solu-
ble in common solvents, they can be used as heterogeneous
have been constructed, of which phase 1 can be converted
into phase 2 upon heating. Desolvated 2 exhibits a highly se-
lective adsorption behavior toward H2/N2, CO2/N2, and CO2/
CH4;[6,7] moreover, it displays size-selective catalytic activity
towards carbonyl cyanosilylation[8] and Henry (nitroaldol)
reactions[9]
.
[a] X.-M. Lin, T.-T. Li, Y.-W. Wang, Prof. Dr. L. Zhang,
Prof. Dr. C.-Y. Su
MOE Laboratory of Bioinorganic and Synthetic Chemistry/KLGHEI
of Environment and Energy Chemistry
State Key Laboratory of Optoelectronic Materials and Technologies
School of Chemistry & Chemical Engineering
Sun Yat-sen University, Guangzhou 510275 (China)
Fax : (+86)20-8411-5178
Scheme 1. 1,3,5-Tri(4-carboxyphenoxy)benzene (H3TCPB).
E-mail: zhli99@mail.sysu.edu.cn
cesscy@mail.sysu.edu.cn
[b] Prof. Dr. C.-Y. Su
Results and Discussion
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
Syntheses and Crystal Structures
A solvothermal reaction of Zn
DEF at 858C afforded colorless block-shaped crystals of
ACHTUNGTNER(NNUG NO3)2·6H2O and H3TCPB in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200601.
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Cheng-Yong Su et al.
FULL PAPER
[Zn
(TCPB)2·2DEF]·3DEF (1). Single-crystal X-ray diffrac-
gy of the MoS2-H structure with the Schlꢃfli symbol of
(43·612) (43)2 (Scheme 2).[10] As seen from Figure 1b, two
tion studies reveal that 1 crystallizes in the monoclinic space
group P21/c (Table 1), and the asymmetric unit contains two
TCPB3À ligands, three Zn2+ ions, two coordinated DEF, and
TCPB3À ligands doubly connect three Zn
3ACHTNUTRGNEUNG(O2CR)6 clusters
with an eclipsed arrangement, which generates a p···p stack-
ing interaction with the distance of 3.6 ꢂ between the two
Table 1. Crystallographic data for 1 and 2.
MOF
1
2
Empirical formula
C79H85N5O23Zn3
1668.63
monoclinic
P21/c
16.5826(3)
28.3958(6)
17.8352(3)
90
C54H34O20Zn3
1198.97
Formula weight
Crystal system
Space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
trigonal
ꢀ
P3c1
16.7057(4)
16.7057(4)
14.3418(5)
90
a [8]
Scheme 2. View of the schematic 2D network with the net topology com-
prising the Schlꢃfli symbol of (43.612) (43)2.
b [8]
108.980(2)
90
7941.6(3)
4
1.396
3472
1.707
28163
1.081
0.0830, 0.1972
0.1080, 0.2109
90
120
3466.28(17)
2
1.145
1456
1.721
11970
1.037
g [8]
V [ꢂ3]
Z
central phenyl rings (Figure S1a,b in the Supporting Infor-
mation). Thus, the formed 2D networks are stacked in an
offset fashion, with the distance of 5.8 ꢂ between two cen-
tral phenyl rings from two adjacent layers (Figure S1c,d in
the Supporting Information). Although the -(AB)n- stacking
modes between the 2D layers significantly reduce the cross-
ing window size, 1D channels are still formed along the c-
axis with an effective size of 3.0ꢁ2.9 ꢂ2 (after considering
the van der Waals radii), in which the void space is filled
with DEF molecules (Figure 1c). Removal of the guest mol-
ecules reveals that the effective free volume of 1 is 49% of
the crystal volume (3889.1 ꢂ3 of the 7941.6 ꢂ3 unit cell
volume), as calculated by PLATON analysis.
1cald [gcmÀ3
]
F
A
]
Reflections collected
GOF
R1, wR2 indices [I>2s (I)]
R1, wR2 indices (all data)
0.0376, 0.1137
0.0423, 0.1172
three lattice DEF molecules (Figure 1a). The Zn(1) atom
links Zn(2) and Zn(3) atoms through six ligands, forming
a trinuclear cluster of Zn
tance of 3.639 ꢂ, the Zn(1)···Zn(3) distance of 3.629 ꢂ, and
the aZn(2)-Zn(1)-Zn(3) angle of 177.68. Each Zn3A(O2CR)6
cluster is linked with six other Zn3A(O2CR)6 clusters by six 3-
3ACHTUGNRTEN(NUNG O2C)6 with the Zn(1)···Zn(2) dis-
U
Interestingly, the solvothermal reaction of Zn-
G
ACHTUNGTRENNUNG
connecting TCPB3À ligands, thus leading to a 2D (3,6)-con-
nected net with a mesh size of 12.9ꢁ13.8 ꢂ2 (atom-to-atom
separation, Figure 1b). To the best of our knowledge, such
doubly connecting 2D network represents a rare net topolo-
CHTUNGTRENNUNG
tural determination of 2 shows that the crystal system and
space group are obviously different from those of 1, with 2
crystallizing in the higher symmetric trigonal space group
ꢀ
P3c1. The Zn(1) atom, being
located at an inversion center,
is surrounded by six oxygen
atoms from six equivalent
TCPB3À ligands, whereas the
Zn(2) atom is fourfold coordi-
nated, with a coordination ge-
ometry similar to that in 1,
with the coordinating DEF re-
placed by H2O (Figure 1d).
The 2D network of 2 has a sim-
ilar (3,6)-connected net topolo-
gy (14.0ꢁ14.0 ꢂ2, Figure 1e).
The distance between the cen-
tral phenyl rings is 3.3 ꢂ
within the same layer, while
the interplanar distance be-
tween two adjacent layers is
3.9 ꢂ, which is obviously short-
er than 5.8 ꢂ in 1, thus indicat-
Figure 1. Crystal structures of 1 (a–c) and 2 (d–f). a,d) The coordination environment of the Zn2+ ions. b,e) Po-
lyhedral view of the 2D network along the c-axis. c,f) A space-filling view of the porous network along the c-
axis, showing the 1D channels. Solvent molecules are omitted for clarity.
ing
a
stronger interplanar
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Chem. Asian J. 2012, 7, 2796 – 2804

Two ZnII Metal-Organic Frameworks with Coordinatively Unsaturated Metal Sites
packing in 2 (Figure S1e,g in the Supporting Information).
The offset packing of the 2D networks also affords 1D mi-
crochannels along the c-axis. The effective aperture is 4.0ꢁ
4.0 ꢂ2 (Figure 1 f) and thus obviously larger than that of 1.
Removal of the guest molecules reveals that the effective
free volume of 2 is 40% of the crystal volume (1396.5 ꢂ3 of
the 3466.3 ꢂ3 unit cell volume), as calculated by PLATON
analysis.
Based on the above description of the syntheses and crys-
tal structures of 1 and 2, we could conclude that the solvents
play an important role in influencing the crystallization of
MOFs, whereas the semi-rigidity of TCPB3À ligand has
strong ability to affect the connectivity and topology of the
coordination networks. MOFs 1 and 2 feature the same
(3,6)-connected 2D net topology but display different mo-
lecular and lattice symmetry. The latter has a higher point
and lattice symmetry, resulting in a highly regular net ar-
rangement and lattice packing, and therefore results in
larger network meshes and 1D lattice channels. Neglecting
the coordinating solvent molecules, 2 can be regarded as the
high symmetric polymorph of 1 attained by rearrangement
and sliding of the 2D networks of 1 in the crystal lattice to
achieve high symmetry (see below). This can be seen from
the twisting angles of the 4-carboxyphenoxy groups versus
the central benzene ring (see Table S2 in the Supporting In-
formation), which vary from 76.9 to 89.68 in 1, but are con-
stant with a value of 68.98 in 2.
Figure 2. The TG curves of 1, 2, and desolvated 2.
range of 30–1808C. As shown in Figure 3, the patterns of
1 remain unchanged below 608C; however, another set of
patterns gradually appears upon further increase in the tem-
perature. At about 808C, a new well-resolved pattern is
From the viewpoint of potential applications, as long as
the coordination frameworks of 1 and 2 are retained upon
the removal of solvent molecules, they could be used in se-
lective adsorption and Lewis acid catalysis, on the basis of
the following arguments: 1) they have a large effective free
volume (1, 49%; 2, 40%) so that the cavities could accom-
modate guest molecules; 2) they have micropores (1, 3.0ꢁ
2.9; 2, 4.0ꢁ4.0 ꢂ2) so that they could selectively adsorb
gases of small kinetic diameters (e.g., H2, 2.89; CO2, 3.30 ꢂ)
through a size-exclusion effect; 3) they contain the trinu-
clear cluster Zn3ACHTUNGTRENNUNG(O2CR)6CAHUTNGTRENN(GUN solvent)2, which could generate co-
ordinatively unsaturated metal sites (CUMs) upon removal
of the coordinating solvent molecules, so that they could se-
lectively adsorb specific molecules through adsorbate–ad-
sorbent interactions between CUMs and the gas molecules;
and 4) the CUMs of Zn2+ could function as Lewis acid cata-
lysts.[8d,9c,11]
Thermal Stability and Dynamic Framework Transformation
To examine the thermal stability, thermal gravimetric (TG)
curves of 1 and 2 were recorded. As shown in Figure 2, the
TG curves of 1 and 2 in the range of 30-2508C show a total
loss of 29.9 and 23.1%, respectively, which corresponds to
the loss of all solvent molecules (calcd. 30.3 and 23.9%, re-
spectively). Above 4008C, an abrupt weight loss appears,
which is indicative of the decomposition of the frameworks.
To determine whether the frameworks could be retained
upon heating, variable-temperature powder X-ray diffrac-
tion (VT-PXRD) measurements were carried out in the
Figure 3. Variable-temperature PXRD patterns of
(bottom). Inset: Comparison of PXRD patterns of heated 1 with 2.
1
(top) and
2
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Cheng-Yong Su et al.
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formed, thereby indicating the formation of a new phase.
This new phase is stable up to 1808C. In comparison, the
main diffraction peaks of 2 remain unchanged at 30–1808C,
thus indicating that its framework is stable in this tempera-
ture range. A noticeable observation is that the new pattern
of 1 above 808C is identical to that of 2. This means that the
framework of 1 has turned into 2 through a temperature-in-
duced dynamic structural transformation process.[12] There-
fore, 2 should represent the most thermally stable phase of
the 2D (3, 6)-connected network packing, while 1 is the
metastable polymorph. Heating could cause rearrangement
and sliding of the 2D layers in the solid state to reach the
high symmetric and stable polymorphic phase.
for 10 hours under vacuum. As verified by TG analysis
(Figure 2), no weight loss occurs below 4008C, indicating
that both coordinating and free solvent molecules have been
completely removed. Desolvated 2 remains crystalline with
PXRD patterns resembling the diffraction patterns of the
single crystal, only showing a slight broadening of some dif-
fraction peaks (Figure S2 in the Supporting Information),
suggesting that the framework is robust upon removal of co-
ordinating water molecules.
To evaluate the storage capability of desolvated 2, differ-
ent gas sorption studies were conducted. Gas adsorption–de-
sorption isotherms for desolvated 2 are shown in Figure 4
and in Figure S4 in the Supporting Information. In the case
of N2 (77 K) or CH4 (273 K), only a small loading occurs.
For H2, the sorption isotherm reveals a storage capacity of
up to 0.80 wt% (90.0 cm3 gÀ1, STP) at 77 K, and it displays
a significant hysteresis loop. The H2 adsorption value is
Sorption Properties
Based on the above discussion, 2 constitutes the most ther-
mally stable framework and the solvent molecules can be
completely evacuated to leave a robust porosity; hence, we
selected 2 to study the gas/vapor adsorption property. The
bulk sample of desolvated 2 can be obtained by soaking as-
prepared 2 in methanol for three days and heating at 1308C
comparable to that of Zn4O
ACHTUNGTERN(NNUG tmbdc)3 (IRMOF-18,
0.89 wt%) but less than those of other Zn4O-based MOFs
such as Zn4OACHTUNTRGENNUG(bdc)3 (MOF-5, 1.32 wt%) and Zn4OACHTUNGTRENNUNG(btb)2
(MOF-177, 1.25 wt%) under similar conditions.[2a,13] The
CO2 uptake capacities at 1 atm are 21.7 wt% (141.2 cm3 gÀ1,
Figure 4. Gas adsorption/desorption isotherms of CO2, H2, and N2 at low temperatures (a), of CO2 and CH4 at 273 K (b), of methanol, ethanol and 1-
propanol vapor at 298 K (c), and of water vapor at 298 K (d). Solid symbols, adsorption; open symbols, desorption.
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Two ZnII Metal-Organic Frameworks with Coordinatively Unsaturated Metal Sites
STP), 11.4 wt% (65.0 cm3 gÀ1, STP), and 4.8 wt%
(25.4 cm3 gÀ1, STP) at 195 K, 273 K, and 298 K, respectively.
These values are better than those of MOF-5 (6.2 wt%,
273 K; 3.2 wt%, 298 K) and MOF-177 (3.6 wt%, 298 K) at
1 atm.[13] Based on the CO2 adsorption isotherm at 195 K,
the Brunauer–Emmett–Teller surface area of 2 is calculated
to be about 573 m2 gÀ1, a value much smaller than those of
MOF-5 (3320 m2 gÀ1) and MOF-177 (4508 m2 gÀ1).[13] As
demonstrated above, the framework displays preferential
adsorption of H2 and CO2 over N2 at low temperatures. In
addition, desolvated 2 exhibits selective adsorption of CO2
over CH4 at 273 K. In light of the selective adsorption of H2
over N2 as well as CO2 over CH4, 2 may be applied in hy-
drogen enrichment of the H2/N2 exhaust in ammonia synthe-
sis, natural gas purification, and landfill gas separation, re-
spectively.
The principal mechanisms underlying selective gas ad-
sorption in rigid MOFs are based on a size-exclusion effect
and/or adsorbate–adsorbent interactions. According to the
Horvath–Kawazoe (HK) pore distribution plot of desolvat-
ed 2, the pore diameter of the sample is approximately
4.3 ꢂ (Figure S5 in the Supporting Information). This value
is consistent with the single-crystal analysis and slightly
larger than the kinetic diameter of N2 (3.64 ꢂ), H2 (2.89 ꢂ),
CO2 (3.30 ꢂ), and CH4 (3.76 ꢂ). In our case, the preferen-
tial adsorption of H2 over N2 might be due to the size-exclu-
sion effect, whereas the selective sorption of CO2 over N2
and CH4 might be attributed to a combined effect of size
and of host–guest interactions between CO2 molecules of
a significant quadrupole moment (13.4ꢁ10À40 Cm2) and the
coordinatively unsaturated Zn2+ sites on the host frame-
work.
Desolvated 2 was also subjected to vapor sorption of alco-
hols and water. The sorption isotherms of alcohols present
typical type-I curves with apparent hysteresis loops (Fig-
ure 4c). Approximately 6.2 MeOH, 3.9 EtOH, or 1.8 PrOH
molecules can be absorbed per formula unit at 0.95 atm. It
is clear that the adsorption of alcohol molecules is size-de-
pendent. The vapor-loaded sample powders still display the
characteristic PXRD patterns, thus indicating that the
framework remains stable after alcohol adsorption (Fig-
ure S3d in the Supporting Information). The sorption iso-
therms of water vapor show a hysteretic adsorption behav-
ior, with an uptake of 10.66 wt% (133 cm3 gÀ1, STP) at P/
Po =0.95 (Figure 4d). The water uptake increases abruptly
between 0.08 and 0.13 atm, with an uptake of 6.9 and
48.3 cm3 gÀ1, respectively. It is noted that dehydated 2 takes
Scheme 3. Illustration of the exterior and interior catalytic sites in desol-
vated 2.
posed on the external surface of the framework (blue
arrows) and the other is located on the second layers adja-
cent to the external surface (off-white arrows). Single-crystal
structural analysis shows that the mesh size of 2 is 14.0ꢁ
14.0 ꢂ2 (Figure 1e). Therefore, as long as the substrates are
smaller in size than the meshes, they could access both the
exterior and the interior ZnII CUMs. Nevertheless, smaller-
sized substrates could enter the meshes more easily and
have more chances to access the interior ZnII CUMs. As
a result, 2 might display size-selective catalysis. To character-
ize the possible acid-type catalytic behaviors of the coordi-
natively unsaturated Zn-MOF of 2, carbonyl cyanosilylation
in the presence of desolvated 2 was performed. The addition
of CNÀ to a carbonyl compound to form a cyanohydrin is
À
one of the fundamental C C bond formation reactions in or-
ganic chemistry.[8]
Since 2 is insoluble in most organic solvents (e.g., n-
hexane, dichloromethane, tetrahydrofurane, ethyl acetate,
acetonitrile, and ethanol), we used a 1:2 molar ratio of ben-
zaldehyde and cyanotrimethylsilane in n-hexane at room
temperature to explore the heterogeneous catalysis of desol-
vated 2. We found that a loading of 2.5 mol% of desolvated
2 leads to 100% conversion of benzaldehyde after a reaction
time of 13 hours. By contrast, the removal of 2 by filtration
after 2 hours stopped the reaction, resulting in only 7% ad-
ditional conversion upon stirring for another 12 hours (Fig-
ure S6 in the Supporting Information). The leached Zn con-
tent in the filtrate was measured by inductively coupled
plasma–atomic emission spectroscopy (ICP–AES), and only
a trace amount of Zn (0.1%) was found in the filtrate. In
addition, the reaction in the presence of a mixture of
5 mol% H3TCPB and 7.5 mol% ZnACHTNUTRGNEUNG(NO3)2·6H2O only gives
a conversion of benzaldehyde of 8% under similar condi-
tions. These results evidently demonstrate that 2 acts as an
effective heterogeneous catalyst. The solids of 2 can be
easily isolated from the reaction suspension by simple filtra-
tion and can be reused at least four times with only a slight
decrease in the reactivity (Figure 5). Judging from the
PXRD pattern, the framework of 2 is intact after the fourth
run (Figure S3 in the Supporting Information). According to
our results presented here, the activity of 2 is much higher
up 2.1 water molecules per [Zn3ACHTNUTRGNEUNG(TCPB)2] unit between
these two pressures, thereby indicating coordination of the
water molecules to the Zn ions, which matches well with the
single-crystal X-ray structure.
Size-Selective Catalysis
than that of Cu
Cr3O(H2O)2F(BDC)3 (MIL-101), which is so far the most
powerful Lewis acid catalyst of a MOF. A small amount of
3ACHTUNGTNER(NUNG btc)2 (HKUST-1) but lower than that of
As illustrated in Scheme 3, there are two kinds of ZnII
CUMs that could act as catalytically active sites: one is ex-
A
ACHTUNGTRENNUNG
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Cheng-Yong Su et al.
FULL PAPER
the nitroalkanes, which again verifies that the interior ZnII
sites could advance the reaction and endow size-selectivity
to this catalytic reaction.
Conclusions
Figure 5. Recycling tests for carbonyl cyanosilylation catalyzed by desol-
vated 2.
In summary, we have presented the syntheses and crystal
structures of two new Zn-MOFs containing 1D channels,
[Zn
(TCPB)2·2DEF]·3DEF
(1)
and
[Zn3
MIL-101 (0.5 mol%) could promote the reaction with 100%
conversion in 3 hours,[8c] whereas only a moderate yield of
50% was obtained after 48 h in the presence of 5 mol%
HKUST-1.[8b]
To probe whether the interior coordinatively unsaturated
ZnII sites (Scheme 3) contribute to the catalysis, substrates
of increasing dimension were tested. As can be seen in
Table 2, the reaction with benzaldehyde (8.9ꢁ6.7 ꢂ2) was
ACHTUNGTNERN(UNG TCPB)2·2H2O]·2H2O·4DMF (2). Upon heating above 808C,
the framework of 1 converts into that of 2. Fortunately, the
framework of 2 is robust enough to maintain after removal
of solvent molecules, and exhibits a highly selective adsorp-
tion behavior toward H2/N2, CO2/N2, and CO2/CH4. More-
over, the coordinatively unsaturated ZnII sites in the frame-
work can serve as Lewis acid catalytic centers for carbonyl
cyanosilylation and the Henry reaction. Both catalytic sys-
tems are dependent on the size of the substrates. The Zn-
MOF catalyst can be recovered and maintains its activity in
subsequent runs. Further study of the applications of this
robust porous Zn-MOF is in progress.
Table 2. Results for carbonyl cyanosilylation catalyzed by desolvated
2.[14]
Experimental Section
Entry
1
Aldehyde
Molecular size
Conversion [%]
100
General Information
All reagents and solvents were commercially available and used as re-
ceived. 1,3,5-Tri(4-carboxyphenoxy)benzene (H3TCPB) was prepared as
described.[15] Infrared spectra were collected from KBr pellets in the
region of 4000–400 cmÀ1 on a Nicolet/Nexus-670 FT-IR spectrometer.
1H NMR spectra were recorded with a Varian Mercury Plus 300 MHz
spectrometer. X-ray powder diffraction patterns were measured on
a Bruker D8 Advance diffractometer at 40 kV and 40 mA with a Cu
target tube and a graphite monochromator. Thermogravimetric analyses
(TGA) were performed on a Netzsch Thermo Microbalance TG 209 F3
2
85
3
65
8
Tarsus from room temperature to 9008C with
a heating rate of
108CminÀ1 under flowing nitrogen. The elemental analyses were ob-
tained with a Perkin–Elmer 240 elemental analyzer. The sorption iso-
therms were measured with a Quantachrome Autosorb-iQ-MP gas sorp-
tion analyzer.
4[a]
Synthesis of [Zn
(TCPB)2·2DEF]·3DEF (1)
(NO3)2·6H2O (60 mg, 0.2 mmol) and H3TCPB (50 mg,
[a] Catalyzed by a mixture of 5 mol% H3TCPB and 7.5 mol% Zn-
(NO3)2·6H2O.
ACHTUNGTRENNUNG
A mixture of ZnACHTUNGTRENNUNG
0.1 mmol) in 3 mL DEF was sealed in a Teflon-lined autoclave and
heated under autogenous pressure at 858C for three days. After cooling
to room temperature, colorless block-shaped crystals of 1 were obtained
and washed twice with DEF. Yield: 60 mg (75% based on the ligand); IR
(KBr): n˜ =3419(br), 2976(m), 1652(s), 1593(s), 1501(m), 1390(w), 1264(s),
1219(s), 1159(m), 1117(m), 1007(m), 853(w), 782(m), 707(m), 646(m),
453 cmÀ1 (w); elemental analysis calcd (%) for C79H85N5O23Zn3: C 56.86,
H 5.13, N 4.20; found: C 56.78; H 5.21; N 4.25.
accomplished with 100% conversion after 13 hours. By con-
trast, the conversions for 1-naphthaldehyde (9.6ꢁ8.6 ꢂ2)
and 4-phenylbenzaldehyde (13.1ꢁ6.7 ꢂ2) are reduced to
85% and 65% under the same reaction conditions, respec-
tively. These size-selective behaviors support the assumption
that catalysis is promoted by the interior catalytic sites and
not only by the exterior ones.
To examine the versatility of the heterogeneous Zn-MOF
2 catalytic system, the Henry (nitroaldol) reaction was also
investigated. Although this reaction is known as one of the
Synthesis of [Zn
(TCPB)2·2H2O]·2H2O·4DMF (2)
(NO3)2·6H2O (30 mg, 0.1 mmol) and H3TCPB (25 mg,
A mixture of ZnACHTUNGTRENNUNG
0.05 mmol) in 4 mL DMF was sealed in a Teflon-lined autoclave and
heated under autogenous pressure at 858C for three days. After cooling
to room temperature, colorless block-shaped crystals of 2 were obtained
and washed with DMF. Yield: 30 mg (76% based on the ligand); IR
(KBr): n˜ =3393(br), 1661(m), 1595(s), 1402(m), 1392(s), 1224(s),
1160(m), 1121(m), 1009(m), 855(w), 815(w), 783(w), 707(w), 650(m),
457 cmÀ1 (w); elemental analysis calcd (%) for C66H66N4O26Zn3: C 51.90,
H 4.36, N 3.67; found: C 52.42, H 4.32, N 3.65.
À
most powerful reactions forming a new C C bond, few re-
ports on MOF-catalyzed Henry reactions exist.[9] As shown
in Table S3 in the Supporting Information, the conversion of
substrate systematically depends on the molecular size of
2802
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Chem. Asian J. 2012, 7, 2796 – 2804

Two ZnII Metal-Organic Frameworks with Coordinatively Unsaturated Metal Sites
X-Ray Structure Determination
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S Ultra diffractometer equipped with CuKa radiation (l=
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by the direct method and refined by full-matrix least-squares on F2 using
SHELXL programs.[17] All non-hydrogen atoms were refined anisotropi-
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to the fact that all guest solvent molecules of complex 1 are located in
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mal displacements; therefore, constrained refinements were applied by
using DELU/SIMU to approximate to isotropic behaviors. In complex 2,
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from the data set using the SQUEEZE routing of PLATON and refined
further using the data generated.[18] A summary of the crystal structure
refinement data are shown in Table 1, and selected bond angles and dis-
tances are listed in Table S1 in the Supporting Information. The contents
of the solvent region in 2 are not represented in the unit cell contents in
the crystal data. CCDC 879380 (1) and CCDC 879381 (2) contain the
supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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Gas Sorption Measurements
Prior to the sorption examination, the sample was activated by the fol-
lowing procedure: the as-prepared sample was soaked in methanol for
three days, during which methanol was refreshed five times, and then the
sample was placed under vacuum at 1308C for 10 h. Gas sorption iso-
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Typical Procedure for Carbonyl Cyanosilylation
Before the reaction, the as-made samples were activated and treated in
the same way as described for the sorption measurements. Desolvated 2
(7.3 mg, 2.5 mol%) was added to a mixture of benzaldehyde (26.5 mg,
0.25 mmol) and cyanotrimethylsilane (49.6 mg, 0.5 mmol), and then the
resulting reaction mixture was stirred at room temperature for 13 h. The
conversions were determined by 1H NMR analysis.
Typical Procedure for Henry (Nitroaldol) Reaction
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Received: July 5, 2012
Published online: October 4, 2012
2804
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 2796 – 2804