Exploiting parameter space in MOFs: A 20-fold enhancement of phosphate-ester hydrolysis with UiO-66-NH2

Article abstract of DOI:10.1039/c4sc03613a

The hydrolysis of nerve agents is of primary concern due to the severe toxicity of these agents. Using a MOF-based catalyst (UiO-66), we have previously demonstrated that the hydrolysis can occur with relatively fast half-lives of 50 minutes. However, these rates are still prohibitively slow to be efficiently utilized for some practical applications (e.g., decontamination wipes used to clean exposed clothing/skin/vehicles). We thus turned our attention to derivatives of UiO-66 in order to probe the importance of functional groups on the hydrolysis rate. Three UiO-66 derivatives were explored; UiO-66-NO₂ and UiO-66-(OH)₂ showed little to no change in hydrolysis rate. However, UiO-66-NH₂ showed a 20 fold increase in hydrolysis rate over the parent UiO-66 MOF. Half-lives of 1 minute were observed with this MOF. In order to probe the role of the amino moiety, we turned our attention to UiO-67, UiO-67-NMe₂ and UiO-67-NH₂. In these MOFs, the amino moiety is in close proximity to the zirconium node. We observed that UiO-67-NH₂ is a faster catalyst than UiO-67 and UiO-67-NMe₂. We conclude that the role of the amino moiety is to act as a proton-transfer agent during the catalytic cycle and not to hydrogen bond or to form a phosphorane intermediate. This journal is

Full text of DOI:10.1039/c4sc03613a

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Exploiting parameter space in MOFs: a 20-fold
enhancement of phosphate-ester hydrolysis with
UiO-66-NH2†
Cite this: DOI: 10.1039/c4sc03613a
a
a
a
a
Michael J. Katz, Su-Young Moon, Joseph E. Mondloch, M. Hassan Beyzavi,
a
ab
ac
Casey J. Stephenson, Joseph T. Hupp* and Omar K. Farha*
The hydrolysis of nerve agents is of primary concern due to the severe toxicity of these agents. Using a
MOF-based catalyst (UiO-66), we have previously demonstrated that the hydrolysis can occur with
relatively fast half-lives of 50 minutes. However, these rates are still prohibitively slow to be efficiently
utilized for some practical applications (e.g., decontamination wipes used to clean exposed clothing/
skin/vehicles). We thus turned our attention to derivatives of UiO-66 in order to probe the importance of
functional groups on the hydrolysis rate. Three UiO-66 derivatives were explored; UiO-66-NO
UiO-66-(OH) showed little to no change in hydrolysis rate. However, UiO-66-NH showed a 20 fold
increase in hydrolysis rate over the parent UiO-66 MOF. Half-lives of 1 minute were observed with this
MOF. In order to probe the role of the amino moiety, we turned our attention to UiO-67, UiO-67-NMe
and UiO-67-NH . In these MOFs, the amino moiety is in close proximity to the zirconium node. We
observed that UiO-67-NH is a faster catalyst than UiO-67 and UiO-67-NMe . We conclude that the role
2
and
2
2
2
Received 21st November 2014
Accepted 3rd February 2015
2
2
2
DOI: 10.1039/c4sc03613a
of the amino moiety is to act as a proton-transfer agent during the catalytic cycle and not to hydrogen
bond or to form a phosphorane intermediate.
www.rsc.org/chemicalscience
Introduction
Phosphate-based compounds are key ingredients in biological
systems. The cleavage and formation of P–O bonds are
1,2
responsible for converting ATP to ADP and vice versa. It is thus
not surprising that organophosphorous compounds such as the
ones shown in Fig. 1 are capable of disrupting biological func-
2–5
tions.
Sarin, for example, is a phosphate-based chemical
warfare agent that has unfortunately shown resurgence in
4,6,7
recent times.
The P–F bond is easily cleaved followed by the
formation of a strong acetylcholinesterase inhibitor due to the
6,8
favorable P–O bond formed; this oen leads to asphyxiation.
Other organophosphorous containing compounds, such as
methyl-paraoxon, are phosphate-based pesticides. Methyl par-
aoxon's toxicity is due to the ease in which the P–O group of the
nitrophenol can undergo transesterication under physiolog-
ical conditions. Interestingly, due to the high-use of these
pesticides, enzymes have evolved as a response to these
a
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, USA. E-mail: j-hupp@northwestern.edu; o-farha@northwestern.edu
b
Chemical Science and Engineering Division, Argonne National Laboratory, 9700 S.
Fig. 1 (a) Molecular structure of various nerve agents as well as the
pesticide methyl-paraoxon. The fragment shown in red is the desired
leaving group during hydrolysis. (b) Catalytic hydrolysis reaction for the
hydrolysis of methyl-paraoxon.
Cass Avenue, Argonne, Illinois 60439, USA
c
Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi
Arabia
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4sc03613a
1
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pesticides. Ultimately, this has led to enzymes, both natural and
9
synthetic, which have a high specicity and efficacy towards
this family of nerve agents. Thus, given the importance of
phosphate esters in biological systems, it is not surprising that
there has been much work done in studying enzymes, proteins,
and small molecules which are capable of performing phos-
phate-ester hydrolysis on nitrophenol-containing pesticides/
nerve-agent simulants.
Transition-metal, or rather Lewis-acid, hydrolysis is ubiqui-
10–16
tous in the eld of phosphate-hydrolysis.
Taking a lesson
from nature, many transition-metal complexes employ
1
0,11,17–20
hydroxide bridged dimers as catalysts.
For example, the
II
II
family of phosphoesterase enzymes utilize a Zn –OH–Zn active
site which is oen further supported by a bridging carboxylate.
Fig. 2 Molecular structure of the node of UiO-66 showing four of the
twelve bound carboxylates (left) and the connectivity of the octahedral
21
Given that early row transition metals are oen found in bio- pore in UiO-66 (right). X
1
¼ X
2
¼ H for UiO-66, X
1
¼ NO
2
and X
2
¼ H
for UiO-66-NO
2
, X
1
¼ X
2
¼ OH for UiO-66-(OH)
2
, and X
1
¼ NH
2
and X
2
logical materials due to bioavailability, researchers such as
Brown have utilized coordination complexes with more Lewis-
¼
H for UiO-66-NH .
2
III
IV
acidic centres such as La and Zr as catalysts for phosphate-
11,14,22
ester hydrolysis.
Our group has also been investigating the role of Lewis-
Cr-MIL-101 is also capable of hydrolysing phosphate esters
with a half-life of nearly 3 hours.
Our initial success with UiO-66 was based on the observation
that enzymatic systems use bimetallic Lewis-acidic metal
centres bridge by a hydroxide. Along the same line, we continue
with the biomimetic approach by looking at the necessary
III
II
acidic centres such Al and Zn as catalysts for the degrada-
2
3–26
tion of phosphates such as methyl-paraoxon.
We observed
III
24
26
that Al –porphyrin dimers and tetramers signicantly
II
outperformed their Zn counterparts; half-lives of 10 hours
were observed for the methanolysis of methyl- and phenyl-
16,21,55–59
III
components of the mechanism.
Raushel, for example,
paraoxon by these Al -complexes. In-line with these mate-
has used a combination of experimental and computational
chemistry to probe the mechanism (Fig. 3). They, and others,
have shown the importance of a proximal aspartate and histi-
dine. Both moieties act as a proximal base, transferring
protons to and from these moieties at key portions of the
catalytic cycle. In the absence of these moieties, the hydrolysis
rates were diminished.
With this mechanistic insights in-mind, we hypothesized
2
that UiO-66-NH would be a faster catalyst than UiO-66. To that
end, we investigate the role of UiO-66-NH2 for its ability to
enhance the reaction rate of our catalysis. As shown below,
UiO-66-NH is capable of decreasing the half-life by roughly
rials, we began looking at porous-organic polymers (POPs) as
2
3,25
potential catalysts.
These POPs offered the combined
advantage of homogeneous catalysis (e.g., well dened cata-
60
III
23
II
25
lysts such as Al porphyrins and La catecholates ) and
heterogeneous catalysis (e.g., insoluble catalyst that can be
easily removed).
We observed two key design features in the aforementioned
systems: (i) strong Lewis-acidic metals were key for fast turn
over, and (ii) the use of simple synthesis, such as those used in
the formation of POPs, are attractive heterogeneous catalysts.
Thus, we turned our attention to another class of porous
materials, namely, metal–organic frameworks (MOFs).
MOFs are porous materials formed from inorganic
2
20 fold.
1
2+
cationic nodes (e.g., Zr
6
O
4
(OH)
4
) and anionic bridging
2
7
ligands (e.g., 1,4-benzenedicarboxylate). In addition to our
Experimental
two design principles above, MOFs are ordered/crystalline
materials which are needed for rapid screening and hypoth- All reagents were purchased from commercial sources and used
2
8
esis driven research. Due to their high porosity as well as without
further
purication.
UiO-66,
UiO-66-NH2,
2
9–32
thermal and chemical stability,
materials in applications such as gas-storage,
MOFs are promising UiO-66-(OH) , UiO-66-NO , UiO-67, UiO-67-NMe , UiO-67-NH ,
2
2
2
2
3
3–35
chemical and methyl-paraoxon were synthesized according to literature
and light harvest- procedures (see the ESI† for powder X-ray diffractograms and N2
3
6–41
42,43
44–47
separations,
sensing,
In keeping with our aforementioned design principle, isotherms).
we have shown that UiO-66 (Fig. 2), a MOF containing de-solvated by supercritical CO
catalysis,
4
8–51
52,61
ing.
2 2
UiO-67, UiO-67-NMe , and UiO-67-NH were
26,52
2
.
IV
IV
12+
highly Lewis-acidic Zr ions, in the form of Zr O (OH)
Hydrolysis experiments were carried out at room tempera-
clusters, which can be easily formed in high yield using a ture as described previously. Briey, to a solid sample of UiO-66
6
4
4
5
2
variety of different synthetic procedures, had a half-life (2.5 mg, 6 mol%, 0.0015 mmol; 0.045 mol% of active surface
of 50 minutes for the hydrolysis of methyl-paraoxon at sites) in an Eppendorf tube was added an aqueous solution of
5
3
room temperature. This half-life is among the fastest, N-ethyl-morpholine (0.45 M); the N-ethyl-morpholine is used as
5
4
non-enzymatic, half-lives reported to date. Hatton et al. have a buffer to limit pH changes. The resulting mixture was stirred
III
also demonstrated that the Lewis-acidic Cr
centre in for 30 minutes to nely disperse the MOF particles. To the
suspension was then added methyl-paraoxon (6.2 mg,
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Fig. 4 (a) UV-Vis trace of the hydrolysis of methyl-paraoxon as a
function of time using UiO-66. The arrows denote the disappearance
of the starting material at 275 nm and the appearance of the product at
4
07 nm. (b) Hydrolysis rate of UiO-66 (blue circles), UiO-66-NO
2
(red
(pink
triangles), UiO-66-(OH)
stars).
2 2
(green squares), and UiO-66-NH
Fig. 3 One proposed enzymatic mechanism by Raushel for the
hydrolysis of paraoxon. Note the presence of proximal bases such as
aspartate carboxylate and a histidine imidazole.
Results and discussion
21
As we have previously demonstrated, the hydrolysis of methyl-
paraoxon with UiO-66 is the fastest MOF-based catalysts
53
reported to date. The ease in which alternative linkers can be
0.025 mmol). Periodic monitoring was carried out by removing
29,31,52,63–72
incorporated into the framework
makes UiO-66 an
a 20 mL aliquot from the reaction mixture and diluting with an
aqueous solution of N-ethyl-morpholine (10 mL, 0.45 M) prior to
UV-Vis measurements (Varian Cari 5000) (Fig. 4a). Progress of
the reaction was monitored by following the p-nitrophenoxide
absorbance at 407 nm to avoid overlapping absorptions with
other species. No spectral evidence for the p-nitrophenol was
observed at this pH (10.2). All background reactions were
carried out under identical reaction conditions without the
MOF catalyst.
attractive platform for generating a large parameter space which
can be used to easily probe a mechanism. As illustrated in
Fig. 4b and Table 1, just as the enzymatic hydrolysis is enhanced
by the presence of a proximal anchored base, the hydrolysis rate
of UiO-66-NH is 20 times greater than that of UiO-66. In order
2
to compare the amino moiety in our system with the aspartate
and histidine moieties in the enzymatic system, we also
73
2 2
synthesized UiO-66-NO and UiO-66-(OH) . These two MOFs are
capable of hydrogen bond donating and receiving, similar to
Initial rates were determined using the method of initial
UiO-66-NH , but they cannot act as a Brønsted base like the
62
2
rates. Polynomial ts of order 3–5 were used with the lowest
observed correlation coefficient of 0.98.
amino moiety. As illustrated in in Fig. 4 and Table 1, there is no
signicant difference between the initial rates of UiO-66-(OH)
2
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Table 1 Initial rates and turn-over frequencies (TOFs) for UiO-66, UiO-66-(OH)
2
, UiO-66-NO
2
, UiO-66-NH
2
ꢀ
1
ꢀ1
a
ꢀ1
MOF
Initial rate (mM s
)
Half-life (min)
TOFall (s
)
TOFsurface (s
)
UiO-66
UiO-66-(OH)
UiO-66-NO
UiO-66-NH
0.010
0.0063
0.0070
0.20
35
60
45
1
0.0077
0.0047
0.0052
0.15
1.0
0.62
0.70
20
2
2
2
a
4
00 nm particles of UiO-66 are synthesized. Due to the aperture size of UiO-66 and the relatively larger kinetic diameter of methyl-paraoxon, only
the surface sites (ca. 0.75% of the catalyst loading) are catalytically active.
Table 2 Initial rates and turn-over frequencies (TOFs) for UiO-67,
UiO-67-NMe
2
, and UiO-67-NH
2
Initial rate Half-life
ꢀ1
ꢀ1
a
ꢀ1
MOF
(mM s
)
(min)
TOFall (s
)
TOFsurface (s
)
Full catalyst loading
UiO-67
UiO-67-NMe
UiO-67-NH
0.058
0.078
0.067
4.5
2
2
0.038
0.052
0.044
5.1
7.0
6.0
2
2
Half catalyst loading
UiO-67
UiO-67-NMe
UiO-67-NH
0.018
0.049
0.057
15
7
3.5
0.024
0.032
0.075
3.2
4.3
10
2
2
a
4
00 nm particles of UiO-67 are synthesized; as with UiO-66, only the
surface sites are expected to be catalytically active.
2
Fig. 6 Hydrolysis rate of UiO-67 (blue), UiO-67-NH (red) and UiO-
Fig. 5 Connectivity of the octahedral pore in UiO-67 (right). X ¼ H for 67-NMe
2
(green) at full catalyst loading (1.5 mmole) with respect to
UiO-66 (top) and at half catalyst loading (0.75 mmole) with respect to
UiO-67, X ¼ NH
2
for UiO-67-NH
2
, and X ¼ N(CH
3
)
2
for UiO-67-NMe
2
.
UiO-66 (bottom).
and UiO-66-NO ; the initial rates for the catalytic hydrolysis by
2
ꢀ
1
UiO-66-(OH)2 and UiO-66-NO2 (0.0063–0.0070 mmol
s
(Fig. 5), the functional groups point away from the node. Under
respectively) are slightly slower than that of UiO-66 (0.010 mmol these conditions, the amino moiety can only act as a Brønsted-
ꢀ
1
s
). Based on these observations, we concluded that the amino base around the outer sphere of the active site.
moiety acts as a base and not as a hydrogen-bonding moiety. In comparison with UiO-66, UiO-67 (Fig. 6a) is a faster
Furthermore, considering there is no difference between catalyst. We suspect that the longer distance between nodes
UiO-66, UiO-66-NO (an electron withdrawing group) and prevents steric crowding around neighbouring nodes allowing
2
74
UiO-66-(OH) (a steric electron donating group), there is little to for a more active material. Nevertheless, as with UiO-66,
2
no electronic effect and only a slight steric effect from the linker UiO-67-NH and UiO-67-NMe are faster than UiO-67 (Fig. 6,
2
2
on the rate limiting step of the hydrolysis reaction (Table 1).
In order to further investigate the role of the amino moiety, enhancement (1.2 times) is not 20 times as observed between
we turned our attention to UiO-67 and its derivatives. In UiO-67 UiO-66 and UiO-66-NH . Due to the fact that UiO-67-NH and
Table 2). Due to the fast hydrolysis of UiO-67, the rate
2
2
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UiO-67-NMe
2
show nearly identical kinetic traces (Fig. 6), we
broken (e.g., Me, F, or iso-propyl group in Sarin).
Alternatively, specicity may refer to the specic phosphate
targeted (i.e., some enzymes are only efficient at
hydrolyzing one specic nerve agent and not all nerve
agents equilly). Lastly, given the stereogenic phosphorous
centre, speciciy may refer to the specic enantiomer (e.g.,
Sarin), or pairs of enantiomers (e.g., Soman).
suspect that a different part of the mechanism is now limiting.
To illustrate this, the catalyst loading was decreased by a factor
of two (Fig. 6b). The hydrolysis rate of UiO-67 decreased by a
factor of 3.2 making UiO-67-NH
Furthermore, the hydrolysis rate of UiO-67-NMe
more than that of UiO-67-NH (Fig. 6b); these results support
our hypothesis that UiO-67-NH and UiO-67-NMe are limited 10 A. A. Neverov, C. T. Liu, S. E. Bunn, D. Edwards, C. J. White,
2
2
3.2 times faster than UiO-67.
2
decreased
2
2
by another step in the catalytic cycle when compared to normal
catalyst loadings.
S. A. Melnychuk and R. S. Brown, J. Am. Chem. Soc., 2008,
130, 6639.
Thus, given the results above, we propose that the role of 11 J. S. W. Tsang, A. A. Neverov and R. S. Brown, Org. Biomol.
amino moiety in these derivatives is to act as a Brønsted-base Chem., 2004, 2, 3457.
whereby a proton is transferred to and from the base during the 12 S. A. Melnychuk, A. A. Neverov and R. S. Brown, Angew.
catalytic cycle. This is supported by the UiO-67-NMe hydrolysis Chem., Int. Ed., 2006, 45, 1767.
which is slower than the UiO-67-NH
hydrolysis likely due to the 13 F. Aguilar-P ´e rez, P. G ´o mez-Tagle, E. Collado-Fregoso and
fact that the dimethylamino moiety is a stronger base and thus A. K. Yatsimirsky, Inorg. Chem., 2006, 45, 9502.
it is harder to deprotonate the moiety in order to regenerate the 14 T. Liu, A. A. Neverov, J. S. W. Tsang and R. S. Brown, Org.
2
2
catalyst.
Biomol. Chem., 2005, 3, 1525.
1
5 E. Stulz, H. B. B u¨ rgi and C. Leumann, Chem.–Eur. J., 2000, 6,
5
23.
Conclusions
1
6 E. Kimura, Curr. Opin. Chem. Biol., 2000, 4, 207.
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were able to enhance the hydrolysis rate. A rate enhancement of 19 K. L. Klinkel, L. A. Kiemele, D. L. Gin and J. R. Hagadorn,
up to 20 times was observed for UiO-66. Due to a careful
comparison between UiO-66 and UiO-67 derivatives, we propose 20 K. L. Klinkel, L. A. Kiemele, D. L. Gin and J. R. Hagadorn,
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2
date. With these results in hand, we are currently testing the
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Acknowledgements
2
O.K.F. and J.T.H. gratefully acknowledges DTRA for nancial
support (grant HDTRA-1-10-0023). The authors would like to
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similar.
5
Chem. Sci.
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