Structural and catalytic performance of a polyoxometalate-based metal-organic framework having a lanthanide nanocage as a secondary building block

Article abstract of DOI:10.1021/ic901504q

A polyoxometalate-based lanthanide-organic framework was achieved using the {[Ho₄(dpdo)₈(H₂O)₁₆BW ₁₂O₄0] (H₂O)₂}⁷+ nanocage as a secondary building block fo

Full text of DOI:10.1021/ic901504q

1280 Inorg. Chem. 2010, 49, 1280–1282
DOI: 10.1021/ic901504q
Structural and Catalytic Performance of a Polyoxometalate-Based Metal-Organic
Framework Having a Lanthanide Nanocage as a Secondary Building Block
Dongbin Dang,†,‡ Yan Bai,‡ Cheng He,† Jian Wang,† Chunying Duan,*,† and Jingyang Niu‡
†State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China, and
‡Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan
University, Kaifeng 475004, China
Received July 28, 2009
A polyoxometalate-based lanthanide-organic framework was
achieved using the {[Ho4(dpdo)8(H2O)16BW12O40] (H2O)2}7þ
nanocage as a secondary building block for the heterogeneous
catalysis of phosphodiester cleavage in an aqueous solution.
have attracted considerable interest in the fields of solid
acid catalytic, electronic, and magnetic materials.3-6 Also,
because of the large number of potential coordination sites
and the relatively weak coordination ability of POMs,
embedding them into the nanocage or framework of the
Werner-type solids might be an efficient approach to creating
POM-based coordination polymers with novel structures
and interesting properties, especially with homogeneous or
heterogeneous catalytic functions.7-9
Recently, the chemistry of hybrid solid metal-organic
frameworks (MOFs) constructed from organic linkers and
metal nodes has received much attention, owing to the fine-
desired properties by judicious choice of the building blocks.
While the ability of these MOFs to incorporate functional
groups makes them excellent candidates as heterogeneous
catalysts, it remains a great challenge to engineer a strong
Lewis acid in MOFs to find applications in many of the
processes that are currently catalyzed by zeolites.1,2 Poly-
oxometalates (POMs) represent a large class of inorganic oxo
clusters that contain early transition metals. Because of the
versatile chemical, structural, and electronic properties, they
On the other hand, the development of rational app-
roaches to the design of catalysts for hydrolysis of phosphate
diesters is a subject continually attracting interest in bio-
organic chemistry.10 Complexes of lanthanide ions have
proven to be among the most effective synthetic hydrolases
reported to date besidesthose employing alkaline-earth metal
ions such as Ca2þ and Mg2þ. The Lewis acidity of the tri- and
tetravalent lanthanide ions, together with their high coordi-
nation numbers, fast ligand-exchange rates, and absence of
accessible redox chemistry, provides an opportunity to mimic
the activity of hydrolases to bind and activate phosphate
*To whom correspondence should be addressed. E-mail: cyduan@
dlut.edu.cn.
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r
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Published on Web 01/20/2010
2010 American Chemical Society

Communication
Inorganic Chemistry, Vol. 49, No. 4, 2010 1281
esters for cleavage.11 However, there are only a few examples
of heterogeneous catalysts for hydrolysis of phosphate die-
sters in an aqueous solution,12 and no lanthanide-based
MOF catalysts have been reported for phosphodiester clea-
vage in a heterogeneous fashion, to the best of knowledge.
Despite that, heterogeneous catalysis exhibits significant
potential advantages such as ease of separation, efficient
recycling, and minimization of metal traces in the product.13
To seek efficient catalysts for hydrolysis of phosphate
diesters, herein we report a POM-based MOF having a
lanthanide-based nanocage as a secondary building block.
It is anticipated that the weak conjugate bases BW12O405- as
counteranions could enhance the Lewis acidity of the lantha-
nide compound, which is one of the key factors in accelerat-
ing the rate of phosphate ester hydrolysis for the metal
complexes. 4,40-Bipyridine-N,N0-dioxide was selected to act
as the longer spacer ligand not only because of the excellent
hard acid/hard base complementarity of the lanthanide
cations and the N-oxide donor but also because of the small
steric size of this ligand, which avoids crowding at the metal
centers and encourages high connectivity and a large volume
of voids.14
Figure 1. One-dimensional ribbon structural pattern of compound 1
composed of the {[Ho4(dpdo)8(H2O)16BW12O40](H2O)2}7þ nanocages
for catalysis of phosphodiester bond cleavage in an aqueous solution.
The holmium ions, oxygen atoms, nitrogen atoms, phosphorus atoms,
and carbon atoms are drawn in green, red, blue, yellow, and gray,
respectively.
Compound {[Ho4(dpdo)8(H2O)16BW12O40] 2H2O}(BW12-
3
O40)2 (H1.5pz)2 (H2O)11 (1; Figures 1 and 2) was assembled
3
3
through a hydrothermal synthesis method using HoCl3
6H2O, 4,40-bipyridine-N,N0-dioxide hydrate (dpdo), HoH2B-
3
W12O40 nH2O, and hexahydropyrazine (pz) as the reaction
3
origins.15 It crystallizes in a triclinic space group P1.16 Four
HoIII ions connected by four bridged dpdo ligands, alterna-
tively, consolidate the main skeleton of the nanocage
{[Ho4(dpdo)8(H2O)16](H2O)2}12þ. The Ho Ho separation
3 3 3
˚
in the equatorial plane is 13.3 A. Two terminal dpdo ligands
are toward one side of the rhombus, with the symmetry-
related pair positioned on the other side. Each pair of
Figure 2. Molecular structure of the {[Ho4(dpdo)8(H2O)16BW12O40]-
(H2O)2}7þ nanocage showing the proposed rationale for the disparate
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(14) (a) Hill, R. J.; Long, D. L.; Champness, N. R.; Hubberstey, P.;
˚
reactivity of compound 1 toward BNPP. Selected bond distances (A):
Ho(1)-O(dpdo) 2.31(1), Ho(1)-O(H2O) 2.40(1), Ho(2)-O(dpdo) 2.29(2),
and Ho(2)-O(H2O) 2.36(2) on average (symmetry code: A, 1 - x, 1 - y,
1 - z).
terminal dpdo ligands positioned on one side forms hydrogen
bonds with one water molecule to further complete the
cagelike molecular cavity. The O O separations of the
3 3 3
˚
hydrogen-bonding interaction are 2.67(1) and 2.58(1) A for
O(17W) O(66) and O(17W) O(68A), respectively.
3 3 3
3 3 3
There are two crystallographic independent HoIII centers
in a crystal of compound 1. Ho(1) and Ho(2) are coordinated
in distorted trigondodecahedra with three and five water
molecules, respectively. The coordination geometry of the
lanthanide ions is well consistent with that of the DNA-Foot
printing active cyclen-based lanthanide(III) complexes. The
water molecules occupy several coordination sites and have
the potential to act as removable labile ligands, allowing for
substrate binding and the simultaneous provision of a metal-
bound nucleophile.10a One BW12O405- anion bridging Ho(1)
and Ho(1A) centers is found to fill the three-dimensional
body with metal-ligand interactions between Ho(1) and the
terminal oxygen atoms of the anion. The Lewis acidity of
lanthanide ion Ho(1) is enhanced directly; thus, compound 1
has the potential to exhibit excellent catalytic properties of
hydrolysis for modes of natural phosphate esters. Adjacent
cages are linked together through coordination bonds
between oxygen atoms of the bridging dpdo ligands and
€
Schroder, M. Acc. Chem. Res. 2005, 38, 337–350. (b) Wei, M. L.; He, C.; Hua,
W. J.; Duan, C. Y.; Li, S. H.; Meng, Q. J. J. Am. Chem. Soc. 2006, 128, 13318–
13319.
(15) Compound 1 was prepared from a hydrothermal reaction by putting
R-HoH2BW12O40 nH2O, HoCl3 6H2O, 4,40-bipyridine-N,N0-dioxide hy-
3
3
drate, hexahydropyrazine, and water with a molar ratio of 1:3:3:7:1600 into
a Teflon-lined stainless steel autoclave and keeping the resulting mixture
under autogenous pressure at 120 °C for 3 days. Yellow crystals suitable for
single-crystal X-ray structure analysis were isolated and washed with
distilled water (about 70% yield based on HoH2BW12O40 nH2O). Anal.
3
Calcd for C128H175B3Ho4N28O172W36: C, 12.63; H, 1.45; N, 3.22; B, 0.27; W,
54.39; Ho, 5.42. Found: C, 12.55; H, 1.50; N, 3.20; B, 0.25; W, 54.19; Ho,
5.52. IR (KBr, cm-1): three characteristic asymmetric vibrations from
heteropolyanions, ν(W-Oc) 809, ν(B-Oa) 905, ν(B-Od) 955; four char-
acteristic vibrations from dpdo molecules, ν(N-O) 1227, ν(ring) 1473,
ν(C-H, in-plane) 1180, ν(N-O) 839.
(16) Crystal data of 1 for C128H175B3Ho4N28O172W36, Mr = 12 168.71,
˚
˚
triclinic, space group P1 with a = 18.922(1) A, b = 20.038(1) A, c =
20.789(1) A, V = 6160.4(3) A ; Z = 1; Fcalcd = 3.280 g cm-3; T = 293(2) K.
The final refinement gave R1 = 0.0598, wR2 = 0.1270, and GOF = 1.03 for
11 697 observed reflections with I > 2σ(I).
3
˚
˚

1282 Inorganic Chemistry, Vol. 49, No. 4, 2010
Dang et al.
phosphate remains. Interestingly, crystals of compound 1
were easily isolated from the reaction suspension by filtration
alone and reused three times, displaying only a slight decrease
in activity. The exact index of the X-ray diffraction (XRD)
patterns of compound 1 after the catalytic reaction supports
that the crystallographic data characteristic of compound 1
are maintained directly. The effect of the pH on the cleavage
reaction was also investigated.19 In a comparison under
optimum conditions, kobs is only slightly decreased with a
decrease or increase of the pH of the solution, and the fastest
cleavage is observed at pH 4.0 (Table S4 in the Supporting
Information).
Figure 3. Family of UV-visible absorbance spectra of a 4-nitropheno-
xide anion formed from the cleavage of BNPP in H2O at 50 °C. Inset:
Kinetic plot of hydrolysis of BNPP using compound 1 as the catalyst. A is
the observed absorbance, and AL is the final absorbance at the end of the
reaction.
From a structural point of view, the large size and high
coordination number of Ho3þ, the small steric size of the
dpdo ligand, and the steric constraints of the formation of
cagelike molecular structures avoid crowding at the metal
centers and encourage high connectivity. The presence of
water molecules as the labile ligands allows for substrate
binding and the simultaneous provision of a metal-bound
two Ho(2) atoms from different cages, generating a one-
dimensional ribbon composed of the nanocages. Adjacent
ribbons are linked together through intermolecular hydrogen
5-
nucleophile. The directed coordination of the BW12O40
bonds and π π-stacking interactions in the crystal. Free
anion to the lanthanide ions results in a significant feature of
the nanosize compound with the activity of the acid surface
(Figure 1), benefitting facilitation of the transesterification
step by virtue of its additional labile ligand sites, conceivable
through the provision of a hydroxide ligand.10a In the mean-
time, the surface properties of the nanocages also meet a
number of criteria of artificial phosphoesterases such as an
overall positive charge, the presence of coordinated water,
and the presence of functional groups for acid catalysis.17,20
In conclusion, a POM-based lanthanide MOF exhibiting a
heterogeneous catalytic performance for phosphodiester bond
cleavage in an aqueous solution was achieved using the nano-
cage {[Ho4(dpdo)8(H2O)16BW12O40](H2O)2}7þ as a second-
ary building block. Phosphodiester bond cleavage promoted
by the compound under heterogeneous conditions suggests a
pseudo-first-order hydrolytic cleavage reaction. Studies invol-
ving other types of lanthanide polymers containing POMs as
artificial phosphoesterases are currently underway.
3 3 3
BW12O405- anions and solvent water molecules are found to
fill the pores of the crystals.
The heterogeneous catalysis of compound 1 for phospho-
diester bond cleavage is evidenced. An aqueous solution of
Bis(4-nitrophenyl) phosphate (BNPP, 10 mM, pH 4.0) was
stirred at 50 °C, with a polycrystalline sample of 1 (15 mol %)
being suspended as the heterogeneous catalyst. Cleavage of
BNPP was followed by monitoring of the increase in absor-
bance at 400 nm due to formation of an 4-nitrophenoxide
anion, generally, and no obvious pH change was observed
during the catalytic processes. As can be seen in Figure 3, a
plot of the reaction progress log[1/(AL - A)] against time fits
well with the pseudo-first-order rate equation, giving a rate
constantkobs of 2.35 ((0.04) ꢀ 10-6 s-1 and a half-life time of
5-
2.95 ꢀ 105 s, respectively.17 Because the free BW12O40
anions (in sodium salt) do not exhibit any significant catalytic
activity for hydrolytic cleavage of the phosphodiester, the
possiblecatalytic activity of compound 1 should beattributed
to the lanthanide MOF. Furthermore, the conversion of
BNPP is higher with a certain number of moles of the hetero-
geneous catalyst than it is in prior studies using a similar
number of moles of a homogeneous catalyst,18 indicative of a
rapid acceleration of the cleavage of BNPP.
Acknowledgment. We gratefully acknowledge financial
support from the National Natural Science Foundation
of China, the Key Project of Chinese Ministry of Educa-
tion (Grant 207068), and the China Postdoctoral Science
Foundation (Grant 20080431135).
BNPP cleavage progress was also monitored by 31P NMR.
During the course of the reaction, there are three 31P NMR
signals corresponding to inorganic phosphate (δ 0.04),
p-nitrophenyl phosphate (δ -4.81), and BNPP (δ -11.21).
At the end of the reaction, only the signal due to inorganic
Supporting Information Available: Physical measurements,
crystallographic data in CIF format, IR, TGA, and XRD
patterns of compound 1. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) Hydrolysis of BNPP in the absence of a metal complex was almost
negligible, compared to that of the catalytic reaction; however, the final value
of kobs was also adjusted for the background cleavage.
(19) The pH of the solutions was adjusted with hydrochloric acid and
NaOH and measured before and after the hydrolysis reaction.
(20) Williams, N. H.; Takasaki, B.; Wall, M.; Chin, J. Acc. Chem. Res.
1999, 32, 485–493.
(18) Takasaki, B. K.; Chin, J. J. Am. Chem. Soc. 1993, 115, 9337–9338.