Catalytic solvolytic and hydrolytic degradation of toxic methyl paraoxon with la(catecholate)-functionalized porous organic polymers

Article abstract of DOI:10.1021/cs4001738

Two robust catechol-functionalized porous organic polymers (catPOPs) with different T_d-directing nodes were synthesized using a cobalt-catalyzed acetylene trimerization (CCAT) strategy. Postsynthesis metallation was readily carried out with La(

Full text of DOI:10.1021/cs4001738

Research Article
pubs.acs.org/acscatalysis
Catalytic Solvolytic and Hydrolytic Degradation of Toxic Methyl
Paraoxon with La(catecholate)-Functionalized Porous Organic
Polymers
Ryan K. Totten, Mitchell H. Weston, Jin Kuen Park, Omar K. Farha,* Joseph T. Hupp,*
and SonBinh T. Nguyen*
Department of Chemistry and the International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208-3113, United States
S
* Supporting Information
ABSTRACT: Two robust catechol-functionalized porous
organic polymers (catPOPs) with different Td-directing nodes
were synthesized using a cobalt-catalyzed acetylene trimeriza-
tion (CCAT) strategy. Postsynthesis metallation was readily
carried out with La(acac)3 to afford catalytically active La-
functionalized catPOPs for the solvolytic and hydrolytic
degradation of the toxic organophosphate compound methyl
paraoxon, a simulant for nerve agents.
KEYWORDS: porous organic polymers, catechol, catalysis, organophosphates, lanthanum
ions while their isolation inside the POP micropore precludes
the formation of stable bis- and tris-chelation motifs that are the
dominant species in a homogeneous environment.23 The result
is metal centers with empty or labile coordination environ-
ments (solvents or water) that can be used to bind and
recognize small molecules.
INTRODUCTION
■
Over the past decade, porous organic polymers (POPs) have
emerged as a new class of materials with promising applications
in gas storage,1−7 chemical separation,3,8−11 and heterogeneous
catalysis.12−17 The directed assembly of one or more rigid
organic building blocks into a microporous network using
stable organic bonds can afford chemically and thermally stable
materials with both highly accessible internal surface areas and a
wide range of chemical functionalities.7 Of particular interest to
us is the design of POPs suitable for applications that combine
molecular recognition and catalysis. Specifically, the integration
of coordinatively unsaturated metal ions into POPs should
afford micropore environments that are capable of binding and
transforming small-molecule substrates.17 Together with the
known ability of nanometer-sized cavities to sequester reactants
and enhance their in-pore concentrations beyond that in
solution,18,19 this should lead to enhanced catalysis.
While metal ions have been incorporated into POPs
possessing bipyridyl ligands and used for catalysis,16,20 this
strategy necessitates the use of charge-compensating anions,
which can fill valuable pore volumes and block coordination
sites.12,13,15,16 To generate POPs that can stabilize metal ions
featuring high degrees of unsaturation, we have focused instead
on the use of negatively charged ligands as building blocks. In
addition to porphyrin-based systems,13 we have been able to
integrate catechol-bearing building blocks into POPs and
successfully form POP-stabilized monocatecholated metal
complexes with a series of divalent metals (CuII, MgII, MnII,
and ZnII) that demonstrate enhanced hydrogen sorption
capabilities21 and removal of toxic chemicals.22 The dianionic
catecholate ligands offset the cationic charges of divalent metal
Given our interest in the solvolytic degradation of phosphate
triesters and its relevance in the decomposition of toxic
organophosphate-based nerve agents,19,24 we reasoned that a
Lewis-acidic metal ion bound inside a catechol-functionalized
POP would be a potent, reusable catalyst for these
reactions.25,26 While both AlIII and ZnII complexes have been
used in the supramolecularly catalyzed methanolysis of
phosphate esters,19,24 we choose to incorporate LaIII ions into
our catechol POPs (catPOP) given its strong oxophilic nature
and its ability to coordinate up to 9 ligands.27 We hypothesized
that (monocatecholate)La moieties stabilized inside a POP
cavity would be able to bind phosphate triesters together with a
large number of hydroxylated reagents, such as methanol and
water, inside a micropore and accelerate its solvolytic
decomposition. In addition to the dianionic catecholate ligand,
the third anionic ligand of the LaIII ion (i.e., MeO− or HO−) or
other protonated ligands (MeOH or H2O) can serve as a pool
of nucleophiles in the solvolysis/hydrolysis of the coordinated
phosphate ester. We note that the high activity of LaIII ion in
the hydrolytic cleavage of DNA28 and solvolysis of phosphate
triesters,29−31 has been demonstrated.
Received: March 4, 2013
Revised: April 7, 2013
© XXXX American Chemical Society
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Scheme 1. Synthesis of Catechol-Containing POPs A2B1 and A2C1 Using a Cobalt-Catalyzed Acetylene Trimerization Strategy
a
(CCAT) and Their Subsequent Metallation with LaIII Ions
a
The POPs shown above are idealized representations of a completely formed network. However, different substitution patterns (1,3,4 vs 1,3,5) may
be present as well as olefins/dienes and unreacted acetylene groups because of incomplete polymerization. For the A2B1 composition, solid-state 13
C
NMR analysis suggested that this material is free of olefin/diene groups. See the Supporting Information by Weston et al.21 for additional discussion.
with a quick spray of acetone (∼5 s) to prevent excessive
MeOH evaporation, and quickly opened for aliquot (100 μL)
sampling before being put back on the shaker. The sampled
aliquot was diluted with MeOH to 25 mL in a volumetric flask
and analyzed by UV−vis spectroscopy. The conversion of
PNPDMP as a function of reaction time was obtained by
monitoring the increase of absorbance of p-nitrophenol at 311
nm.
Hydrolysis of p-Nitrophenyl Dimethyl Phosphate
(PNPDMP) by LaA2C1 under Shaking Conditions. On
the benchtop, PNPDMP (19 mg, 25 mM), LaA2C1 (4 mg, 6
mol % LaIII) and 4-ethylmorpholine buffer solution (93 mM, 30
vol % EtOH, pH 10, 3 mL)34 were placed in a 2 dram vial. The
reaction vial was sealed with a Teflon-lined cap and allowed to
shake at 200 rpm and 60 °C in a Thermolyne Type 17600
aluminum heating block (Thermolyne, Dubuque, IA) mounted
on a Thermolyne Type 65800 shaker (Thermolyne, Dubuque,
IA). Periodically, the reaction vial was removed from the shaker
and quickly opened for aliquot (100 μL) sampling before being
put back on the shaker. The sampled aliquot was diluted with
the buffer solution to 25 mL in a volumetric flask and analyzed
by UV−vis spectroscopy. The conversion of PNPDMP as a
function of reaction time was obtained by monitoring the
increase of absorbance of the p-nitrophenolate ion at 406 nm.
2. EXPERIMENTAL SECTION
Synthesis of Polymers. POP A2B1 was synthesized
following a previously reported procedure.21
POP A2C1. In a nitrogen-filled glovebox, a 5 mL microwave
vial (capacity designates the amount of solution that can be
safely loaded) equipped with a magnetic stir bar was charged
with monomers A32 (38 mg, 0.18 mmol) and C33 (54 mg, 0.09
mmol) in the same optimal ratio found for A2B1.21 This
mixture was dissolved in dry 1,4-dioxane (4 mL) and
Co2(CO)8 (30 mg, 0.09 mmol) was added. The solution was
stirred for 5 min prior to being sealed and removed from the
glovebox. The reaction mixture was then stirred at 110 °C in a
silicone oil bath for 3 h, during which time a black precipitate
formed. Upon cooling to room temperature, the microwave vial
was opened and the precipitate was isolated by filtration,
washed with H2O (30 mL), and stirred in conc. aqueous
hydrochloric acid (10 mL) for 2 h. The remaining black
polymer was filtered, washed with H2O (30 mL) and MeOH
(30 mL), dried over dynamic vacuum, and activated under a
stream of nitrogen at 150 °C for 12 h to afford A2C1 (80 mg,
97%).
Representative Procedure for the Metallation of
Polymers. POP LaA2B1. A2B1(100 mg) was added to a
solution of La(acac)3·H2O (260 mg, 0.57 mmol) in MeOH (10
mL) and stirred at 50 °C for 18 h. The suspension was filtered
and the solid polymer was extracted for 12 h with MeOH using
a Soxhlet apparatus. The remaining material was filtered and
activated under a stream of nitrogen at 150 °C for 12 h.
Representative Procedure for the Methanolysis of p-
nitrophenyl Dimethyl Phosphate (PNPDMP) by La-
(catecholate)-Functionalized POPs under Shaking Con-
ditions. On the benchtop, a 2 dram vial was charged with
PNPDMP (19 mg, 25 mM), La(catecholate)-functionalized
POP catalyst (4 mg for LaA2C1, 6 mol % LaIII), and methanol
(3 mL). The reaction vial was sealed with a Teflon-lined cap
and allowed to shake at 200 rpm (see section S5 in the SI for a
discussion on the importance of this protocol) and 60 °C in a
Thermolyne Type 17600 aluminum heating block (Thermo-
lyne, Dubuque, IA) mounted on a Thermolyne Type 65800
shaker (Thermolyne, Dubuque, IA). Periodically, the reaction
vial was removed from the shaker, cooled down to below 50 °C
3. RESULTS AND DISCUSSION
In exploring the use of La-functionalized catPOPs for
phosphate ester hydrolysis, we wanted to investigate how the
substrate accessibility of the micropore reaction environment,
and thus its catalytic activity, can be tuned by incorporating
different organic linkers. Previously,21 we have demonstrated
that the copolymerization of an orthoester-protected 1,4-
diethynyl-2,3-dihydroxybenzene35 (see Scheme S1 in the SI for
its synthesis) with a Td-directing tetrakis(4-ethynyl)methane
monomer36 (see Scheme S2 in the SI for its synthesis) in
different monomer ratios resulted in a family of porous
polymers with tunable surface areas and pore size distributions.
One member of this family (A2B1, Scheme 1, R = Carbon, )
was then metallated with divalent metals that readily bind small
molecules such as H2.21 For the degradation of large phosphate
ester substrates, we hypothesized that using the larger Td-
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Table 1. Pore, Surface, and Catalytic Properties of Catechol-Containing POPs
total pore
BET surface
area (m2/g)
theoretical metal
loading (wt %)
actual metal
loading (wt %)
volume
dominant pore
diameter (Å)
observed initial
relative initial rate vs
a
,
b
c
entry
POP
A2B1
(cm3/g)
rate (M s−1
)
uncatalyzed reaction
1
2
3
4
1050
265
0.51
0.09
0.59
0.28
12
11
13
11
2
2
2
2
LaA2B1
A2C1
23
20
22
16
7.0 × 10−7
1.7 × 10−6
41
1165
650
LaA2C1
100
a
b
Reaction conditions: PNPDMP (25 mM), [LaIII] (1.5 mM, 6 mol %), MeOH, 60 °C. Initial rates were measured up to 10% conversion and
c
corrected against background reactions. Reaction conditions for the uncatalyzed reaction: PNPDMP (25 mM), MeOH, 60 °C. Initial rate was also
measured up to 10% conversion.
directing tetrakis(4-ethynyl)adamantyl monomer37 (Scheme 1,
R = adamantyl; see Scheme S3 in the SI for its synthesis) would
result in a POP with higher surface area, more accessible pores,
and thus increased catalytic activity. In addition, the more
hydrophobic adamantyl node could further enhance the
solvophobic encapsulation of the hydrophobic PNPDMP
substrate in polar media and increase the catalysis rate.19,24
As shown in Table 1, polymer A2C1 has a BET surface area
that is about 10% higher than that for A2B1 (1165 vs 1050 m2/
g, respectively; see also Figure 1). Both catPOPs were smoothly
Figure 2. Reaction profiles for the methanolysis of PNPDMP in the
presence of 6 mol % of either LaA2C1 (blue diamonds) or LaA2B1
(red squares).
Table 2. Observed Initial Rates in the Methanolysis of
PNPDMP by LaA2B1 and LaA2C1
LaIII
observed initial rate
leaching
relative rate vs uncat
reaction
a
b
c
catalyst
cycle
(M s−1
)
(%)
Figure 1. Nitrogen isotherms measured at 77 K for polymers A2C1
(purple cirlces), A2B1 (green triangles), LaA2C1 (blue diamonds), and
LaA2B1 (red squares). Closed symbols, adsorption; open symbols,
desorption.
LaA2B1
LaA2C1
1
1
2
3
7.0 × 10−7
1.7 × 10−6
7.1 × 10−7
2.9 × 10−7
7.0 × 10−8
10.1
0.1
0.6
4.3
1.4
41
100
42
17
4
d
4
metallated in the presence of excess La(acac)3 to afford the
corresponding La-functionalized catPOPs with close to a 1:1
catechol:metal stoichiometry. Although the nonmettalated
POPs show a small difference in BET surface areas, LaA2C1
shows a surprisingly ∼2.5-fold higher surface area over LaA2B1
(650 vs 265 m2/g) and a 3-fold higher methanol uptake at 0.80
a
Reaction conditions: PNPDMP (25 mM), [LaIII] (1.5 mM, 6 mol %),
b
MeOH, 60 °C. Initial rates were measured up to 10% conversion and
corrected against background reactions. Percentage of the initial metal
loading. After Soxhlet extraction with THF.
c
d
P
(partial)/P(relative) (Figure S18 in SI). Given that the pore
LaIII content as determined by ICP-OES analysis of the product
solutions) leached from LaA2B1 during the reaction, in contrast
to the negligible leaching in the case of LaA2C1 (Table 2).
Although the BET-derived pore size distributions of these two
materials are quite similar (Figures S14 and S15 in the SI), the
leaching differences can be explained if the local environments
around the La(catecholate) moieties are different enough to
cause instability. Supporting this speculation is the observation
that the acac counterion is not visible in the solid-state IR
spectra of LaA2B1, but is observable in that for LaA2C1 (see
section S13 in the SI).
diameters are similar for both materials, (Figures S14 and S15
in the SI, pore size distributions were calculated using nitrogen
and argon measurements respectively) the larger surface area
and methanol-uptake capacity for LaA2C1 indicate its greater
pore accessibility, which should make it a better catalyst for the
solvolysis/hydrolysis of a phosphate triester.
The methanolysis of p-nitrophenyl dimethyl phosphate
(PNPDMP), a toxic pesticide and simulant for chemical
warfare agents,38 is enhanced by both LaA2B1 and LaA2C1
(Figure 2). As expected, significantly enhanced activity was
observed with POP LaA2C1, which has much higher specific
surface area than LaA2B1: the initial methanolysis rate for the
former is ∼2.5 fold faster than that for the latter (Table 2).
Surprisingly, a significant amount of LaIII (10% of the overall
The degree of LaIII leaching was further corroborated by
monitoring the catalytic activities of the solution phases in the
methanolysis of PNPDMP by LaA2C1 and LaA2B1. After each
reaction reached 30% conversions (∼2 h for LaA2C1 and ∼3 h
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for LaA2B1), the POP particles were removed by filtering and
the PNPDMP concentrations in the solutions were further
monitored. The background-corrected rate of methanolysis is
∼1.7-fold higher in the filtrate for LaA2B1 than that for LaA2C1
(Figure S12 in the SI), which is consistent with the former
containing a larger amount of leached LaIII ions. The activity in
the filtrate is much less than that for an equivalent amount of
La(acac)3 (see section S6 in the SI), suggesting that the leached
LaIII is a much less-active form than La(acac)3 monomer.
Motivated by the promising catalytic activity of LaA2C1, we
examined its recyclability in the methanolysis of PNPDMP
(Table 2). Surprisingly, the initial methanolysis rate drops
significantly with each cycle, even though little leaching of LaIII
is observed. This phenomenon can be attributed to a number of
factors: (1) the substrate/products are retained in the pores
during catalysis, thus blocking new substrate from entering; (2)
the framework of the POP is degraded/relaxed, which inhibits
substrate access to the catalytic sites;39 or (3) the encapsulated
La(catecholate) moieties are degraded into less-active forms
during catalysis.
degradation products, it is conceivable that the LaIII ion could
be cleaved from the catechol in the presence of MeOH to form
less active species that are trapped in the micropores. Such an
explanation is consistent with both the low LaIII leaching and
the decrease in the surface area of the POP. Together with the
build-up of substrate in the pores, these data indicate that the
drop in catalytic activity after each cycle of methanolysis is due
to a combination of La(catecholate) degradation as well as a
decrease in transport through the pores of LaA2C1 because of
substrate sequestration.
As both A2C1 and LaA2C1 can take up a significant amount
of water vapor (see Figure S19 in SI, section 14), we
hypothesize that LaA2C1 would be active in the hydrolysis of
PNPDMP, which is a more practical route toward the
destruction of nerve agents.40 Under pH 10 buffered conditions
and in 30 vol % EtOH, which helps to solubilize the nerve
agent simulant, the hydrolysis of PNPDMP in the presence of
LaA2C1 is 12 times faster than the uncatalyzed reaction (Figure
4). In contrast to the catalyzed methanolysis of PNPDMP,
To evaluate the possibility that substrate/products may be
clogging the pores, LaA2C1 was Soxhlet-extracted with THF for
24 h after the third cycle. Surprisingly, evaluation of the filtrate
by UV−vis spectroscopy revealed a significant amount of
“adsorbed” PNPDMP substrate (2.7 mg from 6.6 mg of
isolated materials, or a 2.4 substrate:La molar ratio). While we
cannot determine if the adsorbed substrate is bound to the
oxophilic LaIII ion, its sequestration in the pores might be
limiting transport between the pores and the external solution,
thereby contributing to the reduction of catalytic activity.
Unfortunately, the catalytic activity of the Soxhlet-extracted
LaA2C1 did not improve, as the initial rate for the “fourth” cycle
was only 4× faster than that for the uncatalyzed reaction.
To evaluate factors 2 and 3, we “aged” LaA2C1 in methanol
at 60 °C and in the absence of PNPDMP for four days, and
then remeasured its surface area by N2 gas adsorption (see
Section S8 in the SI). To our surprise, the BET surface area
greatly decreased (650 m2/g as-synthesized vs 180 m2/g aged,
Figure 3). However, conducting the same aging experiment on
the nonmetallated POP A2C1 produced no significant change in
surface area (1170 m2/g as-synthesized vs 1080 m2/g aged).
This result led us to conclude that the degradation of supported
La(catecholate) moieties into less-active forms must be an
important contributor to the successive decrease in catalysis
rate. Although we do not know the exact nature of the
Figure 4. Reaction profiles for the first three hydrolysis cycles of
PNPDMP in the presence of 6 mol % LaA2C1 in 4-ethyl morpholine
buffer solution (93 mM, 30 vol % EtOH, pH 10) using shaking
conditions. First cycle, blue diamonds; second cycle, red squares; third
cycle, purple triangles.
LaA2C1 retains significantly more catalytic activity over three
cycles of hydrolysis: the initial rate only drops 40% after the
first cycle and remains the same for the second and third cycles,
showing good recyclability (Table 3). Soxhlet extraction of the
LaA2C1 catalyst after each cycle revealed essentially no trapping
Table 3. Observed Initial Rates in the Hydrolysis of
PNPDMP by LaA2B1 and LaA2C1
observed rate
LaIII leaching relative rate vs uncat
c
a
b
catalyst
cycle
(M s−1
)
(%)
reaction
LaA2B1
LaA2C1
1
1
2
3
1.1 × 10−6
1.1 × 10−6
6.7 × 10−7
6.7 × 10−7
0.1
0.1
0.1
0.2
12
12
7
7
a
Reaction conditions: PNPDMP (25 mM), [LaIII] (1.5 mM, 6 mol %),
4-ethyl morpholine buffer solution (93 mM, 30 vol % EtOH, pH 10),
b
60 °C. Initial rates were measured up to 10% conversion and
c
Figure 3. BET isotherms for as-synthesized LaA2C1 (blue diamonds)
and after aging (red squares) at 60 °C in methanol for four days.
corrected against background reactions. Percentage of the initial metal
loading.
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of the substrate (see section S11 in the SI) and the amount of
La leaching from LaA2C1 is again negligible. Together, these
data suggest that under the basic buffered aqueous conditions,
transport of the substrates through the cavities of LaA2C1 is
more facile and the LaIII sites are more stable against
decomposition. The basic aqueous environment of the solution
must have preferentially solubilized both the substrate and the
acidic hydrolyzed products, keeping them from clogging the
pores. Interestingly, PNPDMP hydrolysis by LaA2B1 under the
same conditions yields similar turnover number with minimal
leaching (Table 3).
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: o-farha@northwestern.edu (O.K.F.); j-hupp@
northwestern.edu (J.T.H.); stn@northwestern.edu (S.T.N.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this work is provided by DTRA
(agreement HDTRA1-10-1-0023). Instruments in the North-
western University Integrated Molecular Structure Education
and Research Center (IMSERC) were purchased with grants
from NSF-NSEC, NSF-MRSEC, Keck Foundation, the state of
Illinois, and Northwestern University.
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