Insights into Catalytic Hydrolysis of Organophosphonates at M-OH Sites of Azolate-Based Metal Organic Frameworks

Article abstract of DOI:10.1021/jacs.1c03901

Organophosphorus nerve agents, a class of extremely toxic chemical warfare agents (CWAs), have remained a threat to humanity because of their continued use against civilian populations. To date, Zr(IV)-based metal organic framework (MOFs) are the most pre

Full text of DOI:10.1021/jacs.1c03901

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Insights into Catalytic Hydrolysis of Organophosphonates at M−OH
Sites of Azolate-Based Metal Organic Frameworks
Mohammad Rasel Mian, Haoyuan Chen, Ran Cao, Kent O. Kirlikovali, Randall Q. Snurr,
Timur Islamoglu,* and Omar K. Farha*
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ABSTRACT: Organophosphorus nerve agents, a class of
extremely toxic chemical warfare agents (CWAs), have remained
a threat to humanity because of their continued use against civilian
populations. To date, Zr(IV)-based metal organic framework
(MOFs) are the most prevalent nerve agent hydrolysis catalysts,
and relatively few reports disclose MOFs containing nodes with
other Lewis acidic transition metals. In this work, we leveraged this
synthetic tunability to explore how the identity of the transition
metal node in the M-MFU-4l series of MOFs (M = Zn, Cu, Ni,
Co) influences the catalytic performance toward the hydrolysis of
the nerve agent simulant dimethyl (4-nitrophenyl)phosphate
(DMNP). Experimental studies reveal that Cu-MFU-4l exhibits
the best performance in this series with a half-life for hydrolysis of
∼2 min under these conditions. In contrast, both Ni- and Co-MFU-4l demonstrate significantly slower reactivity toward DMNP, as
they both fail to surpass 30% conversion of DMNP after 1 h under analogous conditions. Further modification of the active site
within Cu-MFU-4l is possible, and we found that although the identity of the anion coordinated to the Cu(II)−X (X = Cl−,
−
−
HCOO−, ClO4 , NO3 ) active site has little influence on the catalytic performance, reduction of the Cu(II) sites yields nodes that
contain Cu(I) ions in a trigonal geometry with open metal sites, leading to remarkable catalytic activity with a half-life for hydrolysis
less than 2 min. Computational studies indicate the Cu(I) sites exhibit stronger binding affinities than Cu(II) to both water and
DMNP, which corroborates the experimental results.
nodes comprised of inorganic ions or clusters that are joined
by organic linkers to form periodic two- and three-dimensional
frameworks,11,12 and they offer potential solutions to unique
challenges in many fields, including gas storage and
separation,13−15 purification,16 sensing,17 drug delivery and
imaging,18 and catalysis.19 To date, the majority of MOFs
employed for the catalytic hydrolysis of organophosphorus
nerve agents and nerve agent simulants, which are molecules
with similar functionality that exhibit lower toxicity, have
featured nodes comprised of the Lewis acidic Zr(IV)-based
metal-oxide clusters (Zr-MOFs).8,20,21 These nodes feature
bridging Zr−OH−Zr moieties that are similar to those found
in the Lewis acidic Zn−OH−Zn active sites in enzymes that
are capable of hydrolyzing organophosphorus nerve agents,
such as phosphotriesterase (PTE),22,23 and strong Zr(IV)−O
INTRODUCTION
■
Despite international bans prohibiting their use, chemical
warfare agents (CWAs) have still been deployed against both
civilian and military populations within the past few years.1,2
Organophosphorus nerve agents, including soman (GD), sarin,
cyclosarin, and VX, comprise a particularly toxic class of
CWAs, and their continued use has necessitated materials
capable of safely detoxifying these deadly compounds.3,4 In
nature, many enzymes can efficiently hydrolyze organo-
phosphorus substrates at active sites that are comprised of
Lewis acidic metal−oxy/hydroxy moieties, but the insufficient
stability of enzymes outside of buffered solutions has limited
their implementation in practical applications, such as on filters
used in masks.5 Using the enzymatic active sites as inspiration,
researchers set out to synthesize materials that incorporated
Lewis acidic metal-oxide species and also demonstrated
improved stability in a broader range of conditions relative
to that of enzymes.
Received: April 13, 2021
Published: June 23, 2021
In this regard, metal−organic frameworks (MOFs) have
emerged as a tunable platform of state-of-the-art heteroge-
neous catalysts that feature both excellent performance and
high stability in a wide range of conditions.6−10 MOFs feature
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bonds between the nodes and the carboxylate linkers engender
exceptional stability to Zr-MOFs.6,24,25 Importantly, the most
effective Zr-MOF catalysts are capable of hydrolyzing nerve
agents almost instantaneously in basic aqueous solutions.26
Despite the synthetic tunability inherent to these materials,
relatively few studies have explored the use of MOFs with
nodes composed of Lewis acidic transition metals other than
Zr(IV).
In this regard, the Zn-based MOF MFU-4l offers an
excellent platform to study how the metal node influences
catalytic activity, as isoreticular structures can be obtained
through straightforward transmetalation reactions. Specifically,
the 6-connected Zn5 node in MFU-4l features four Zn(II) ions
that are each tetrahedrally coordinated by three triazole-based
linkers and one Cl− ion, and one central Zn(II) ion that is
octahedrally coordinated by six linkers (Figure 1b), resulting in
tions yield the isoreticular structures M-MFU-4l (M = Cu, Ni,
Co), and all analogues demonstrate good stability in basic
aqueous solutions, providing a platform to probe the catalytic
reactivity of each transition metal in this hydrolysis reaction.
Importantly, this system eliminates the effects of particle size,
pore aperture, and topology on catalytic activity since the
transmetalated products should retain these properties from
the parent MOF, allowing for direct comparisons between the
different metal ions. We found that Cu-MFU-4l exhibits the
best performance toward DMNP hydrolysis among this series,
with a half-life for hydrolysis of ∼2 min in basic aqueous
solutions, while both Ni- and Co-MFU-4l are unable to achieve
full conversion of DMNP after 1 h under analogous conditions.
Next, we explored the influence of the anion bound to the
nodes in Cu-MFU-4l and found similar catalytic performance
for all derivatives, suggesting rapid anion displacement and
subsequent formation of Cu−OH active sites under the basic
reaction conditions. In contrast, the loss of formate anions
bound to the Cu(II) nodes under vacuum at 180 °C generates
Cu(I) ions in a trigonal geometry with open metal sites,
leading to a remarkable catalytic performance with a half-life
for DMNP hydrolysis less than 2 min, which places it among
the best performing MOF-based catalysts employed under
similar reaction conditions.28−30 Finally, density functional
theory (DFT) calculations provide insight for the observed
differences in reactivity and suggest that Cu(I) exhibits
stronger binding interactions than Cu(II) toward water and
DMNP, which corroborates the experimental results.
RESULTS AND DISCUSSION
■
To begin our investigation, we synthesized Zn-MFU-4l
through the solvothermal reaction between ZnCl2 and
bis(1H-1,2,3-triazolo[4,5-b],[4,5-i])dibenzo[1,4]dioxin)
(H2BTDD) in N,N-dimethylformamide (DMF) at 140 °C for
24 h.27,31 Upon isolation from the reaction mixture, Zn-MFU-
4l provides a convenient access point for the other three M-
MFU-4l analogues, where M = Co, Ni, and Cu, as the M-
MFU-4l topology can not be accessed de novo for these three
derivatives (Figure 2b).32 Specifically, Zn-MFU-4l and MCl2
(M = Co, Ni, or Cu) were combined in a vial with DMF or
N,N-dimethylacetamide (DMA) and heated overnight at 60−
140 °C (see the Supporting Information for complete synthetic
details).32 Transmetalation from Zn-MFU-4l was confirmed
using inductively coupled plasma-optical emission spectrosco-
py (ICP-OES), which provides M:Zn ratios of 4:1, 4:1, and
2.9:2.1 for Co-, Ni-, and Cu-MFU-4l, respectively. For Co- and
Ni-MFU-4l, a 4:1 ratio of M:Zn suggests all four tetrahedrally
coordinated Zn(II) ions are successfully transmetalated from
the Zn5 nodes, and the octahedrally coordinated Zn(II) ion at
the center of the node likely remains (Figure 2a,b). In contrast,
∼2.9 Cu(II) ions are successfully installed in Cu-MFU-4l,
suggesting ∼1.1 Zn(II) ions remain in the tetrahedral sites
within the nodes. Powder X-ray diffraction (PXRD) patterns
indicate the transmetalated products are crystalline and phase
pure (Figure S1), and scanning electron microscopy (SEM)
images confirm that the cubic morphology is retained following
transmetalation (Figure S2). Finally, analysis of N2 adsorp-
tion−desorption isotherms collected at 77 K confirms the
permanent porosity of the materials following transmetalation
(Figure S3). Pore size distributions based on DFT indicated
micropores at 13−14 Å (Figure S4). Next, we investigated the
oxidation state of the metal ions in M-MFU-4l using X-ray
photoelectron spectroscopy (XPS), and XPS peaks for Cu, Ni,
Figure 1. Comparison between the active site in carbonic anhydrase
(a) and MFU-4l (b). Brown, Zn; gray, C; blue, N; and red, O.
Hydrogen atoms are omitted for clarity.
a framework that is exceptionally stable in basic aqueous
solutions.27 Under these basic conditions, the Cl− ions
coordinated to Zn(II) readily exchange to OH− groups in
situ to form Zn(II)−OH active sites that are analogous to
those found in the enzyme carbonic anydrase (CA), which is
capable of hydrolyzing carboxylate and phosphonate esters
(Figure 1).27 Notably, MFU-4l possesses nodes that contain
single-site Zn(II) ions, which contrasts with the bimetallic Zr−
OH−Zr active sites found in Zr-MOFs in which two adjacent
metals can cooperate.
In this work, we leverage the synthetic tunability of MOFs to
explore how the identity of the single-metal active site
influences the hydrolysis of the nerve agent simulant DMNP.
Starting from Zn-MFU-4l, appropriate transmetalation reac-
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Figure 2. (a) Cl− ion is replaced with OH− after base treatment. (b)
Scheme for the postsynthetic modification of MFU-4l. Brown, Zn;
dark green, M (Co, Ni, Cu), light green, Cl, gray, C; blue, N; and red,
O. Hydrogen atoms are omitted for clarity. (c) Hydrolysis reaction of
the phosphonate-based nerve agent simulant DMNP (dimethyl(4-
nitrophenyl) phosphonate).
Figure 3. (a) Cl− ions are replaced by OH− after basic treatment (EM
buffer, pH = 10.5). (b) Hydrolysis profiles of DMNP catalyzed by Zn-
MFU-4l (black), Cu-MFU-4l (red), Ni-MFU-4l (blue), and Co-
MFU-4l (pink) with 6 mol % catalyst loading and without catalyst
(green) in the presence of EM buffered solution.
and Co in M-MFU-4l are observed around 933, 855, and 780
eV for 2p3/2 and 955, 873, and 795 eV for 2p1/2, respectively
(Figure S5). The satellite shake up signals for the 2p3/2 peaks
are observed at 943, 882, and 786 eV for Cu, Ni, and Co,
respectively.33−36 Combined, these results indicate the
presence of Cu(II), Ni(II), and Co(II) ions within Cu-, Ni-,
and Co-MFU-4l, respectively. Moreover, all four M-MFU-4l
derivatives exhibit a peak at roughly 198 eV in the Cl scan that
is indicative of a tetrahedral coordination geometry, confirming
the M(II) ions (M = Zn, Cu, Ni, and Co) in these sites are
coordinated to three BTDD2− linkers and one Cl− anion.
With the M-MFU-4l series (M = Co, Ni, Cu, Zn) in hand,
we then sought insight into the reactivity of these materials
when employed as catalysts toward the hydrolysis of the nerve
agent simulant DMNP under basic aqueous conditions.
Notably, M-MFU-4l features spatially isolated transition
metal ions that serve as single atom active sites for catalysis
under these reaction conditions, which enables a direct
comparison between each transition metal. Hydrolysis
reactions were conducted at room temperature with 6 mol %
M-MFU-4l catalyst loading in pH 10.5 solutions with N-
ethylmorpholine (EM) used as the base (Figure 2c), and
reaction progress was monitored using 31P NMR spectroscopy
(see Supporting Information). Specifically, the peak that
corresponds to DMNP, which appears at roughly −4.4. ppm,
disappears as the reaction proceeds, and a new peak appears at
about 2.8 ppm that corresponds to the benign hydrolysis
product dimethyl phosphate (Figure S6). The relative ratios
between these two peaks are used to construct kinetic profiles
that allow for comparisons between the performance for each
catalyst. It should be noted that no other byproducts were
observed in the 31P NMR spectroscopic data for all reactions
performed in this study (Figure S6). In the hydrolysis profiles
depicted in Figure 3, Cu- and Zn-MFU-4l both exhibit
excellent performance as ∼80% of DMNP conversion is
achieved after 5 min, while Co- and Ni-MFU-4l are much less
reactive with <10% DMNP conversion observed at the same
time point. A first-order kinetic equation was used to calculate
the initial reaction rate constant (k)37 and half-life (t1/2) for
each reaction. Hydrolysis of DMNP with Cu-MFU-4l yields k
and t1/2 values of 0.30 0.02 min−1 and ∼2 min, respectively
(Figure S7). Notably, a comparison between the calculated k
values for Ni-MFU-4l (0.016 0.002 min−1) and Co-MFU-4l
(0.006 0.001 min−1) with that of Cu-MFU-4l reveals a 20-
and 50-fold improvement in the rate, respectively, for the Cu-
based material relative to that of the Ni- and Co-based
analogues (Table 1) (Figure S8 and S9).
Table 1. Comparison between Performance of M-MFU-4l
Catalysts for the Hydrolysis of DMNP
MOFs
Initial rate (min−1
)
t1/2(min)
Zn-MFU-4l
0.33 0.03
0.30 0.02
0.50 0.06
0.016 0.002
0.006 0.001
∼2
∼2
<2
44
116
Cu(II)-MFU-4l
Cu(I)-MFU-4l
Ni-MFU-4l
Co-MFU-4l
Moreover, complete DMNP conversion is observed after
about 20 min when Cu-MFU-4l is employed as the catalyst,
which contrasts the 10% and 25% DMNP conversion observed
for Co- and Ni-MFU-4l, respectively, after reaction times of 1
h (Figure 3). We found similar reactivity with the reuse of the
Cu-MFU-4l for DMNP hydrolysis (Figure S10). In addition,
the parent Zn-MFU-4l does not achieve full DMNP
conversion even after a reaction time of 1 h, which further
highlights the excellent performance observed for Zn-MFU-4l
under these conditions.
ICP-OES analysis of the solutions in the catalytic reaction
mixtures for Co- and Ni-MFU-4l reveals no detectable Co or
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Ni ions, and the same analysis on the solid portions of the
reaction mixtures indicates that the ratio of M:Zn remains 4:1
for both Co- and Ni-MFU-4l, confirming that the poor
reactivity for these materials is not due to decomposition of the
framework but rather the inherent properties of the transition
metal in each active site. Due to the incomplete trans-
metalation for Cu-MFU-4l, each node contains roughly 2.9 Cu
and 1.1 Zn active sites, offering the possibility that the
improved catalytic reactivity observed for Cu-MFU-4l can
potentially be attributed to a pathway involving both Cu(II)
and Zn(II) ions. We performed the hydrolysis reaction using a
catalyst loading of 1.5 mol % of Zn-MFU-4l, which has
approximately the same number of available Zn(II) active sites
as does a loading of 6 mol % of Cu-MFU-4l, and found that
25% DMNP conversion is observed after 25 min (Figure S11).
Moreover, the initial rate for this reaction using Zn-MFU-4l is
calculated to be 0.05 min−1, which is six times lower than that
of Cu-MFU-4l (0.30 min−1). Overall, these results suggest that
the excellent reactivity of Cu-MFU-4l is due to Cu(II) active
sites rather than Zn(II) active sites.
Under the basic reaction conditions employed during
hydrolysis, the Cl− ions coordinated to the nodes of M-
MFU-4l are rapidly exchanged with OH− ions to form M−OH
sites (Figure 3a), which was confirmed by diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTs) and XPS
(Figure S12, S13). Next, we investigated the pKa values of
these metal-bound water molecules using acid−base titra-
tions38 and determined values of 7.7, 6.8 8.3 and 8.5, for Zn,
Cu, Ni, and Co, respectively, in M-MFU-4l (Figure S14).
Importantly, the pKa of Zn-bound water in CA is determined
to be ∼7.5, which is comparable to the value obtained here for
Zn-MFU-4l.39 These values provide insight into both the
Lewis acidity of metal ions and the relative strength of the
coordination bond between the OH− groups and the M(II)
active sites (M = Zn, Cu, Ni, and Co). For example, the higher
pKa values for Co−OH and Ni−OH relative to those of Cu−
OH and Zn−OH indicate stronger OH− binding for Cu and
Zn, which suggests these metal ions are relatively more Lewis
acidic than the Ni and Co ions and is consistent with the better
catalytic performance observed for Cu- and Zn-MFU-4l toward
DMNP hydrolysis than that of the Co-MFU-4l analogue.39,40
In addition to the relative strength of the coordination bond
between the metal site and the hydroxyl ligand, the
coordination geometry also plays a significant role in the
catalytic performance, which will be discussed later.
Figure 4. (a) Postsynthetic modifications of Cu-MFU-4l described
through anion exchange. (b) Hydrolysis profiles of DMNP catalyzed
by Cu-MFU-4l (6 mol % catalyst loading) with different type of
counteranion in the presence of N-ethylmorpholine buffered solution.
indicating the anions are all exchanged to OH− anions in situ
under the basic reaction conditions and corroborates the
similar kinetic profiles observed in Figure 4b.
Next, to further explore the effect of the coordination
environment, oxidation state, and geometry of the metal ions
on catalysis, we prepared the Cu(I) analogue, Cu(I)-MFU-4l,
and studied its performance toward DMNP hydrolysis. Cu(I)-
MFU-4l is obtained following treatment of Cu(II)-MFU-4l-
formate at 180 °C under dynamic vacuum, which results in the
thermal decomposition of formate ions and subsequent
reduction of Cu(II) to Cu(I) (Figure 5a).41 This trans-
formation is accompanied by a distinct color change, as the
green Cu(II)-MFU-4l-formate slowly changes to the beige
Cu(I)-MFU-4l. Additionally, the reduction to Cu(I) should be
accompanied by the formation of an open metal site with a
trigonal geometry due to loss of the charge-balancing
anion.41−43 Indeed, a significant increase in the BET surface
area from 3400 m2/g to 4000 m2/g is observed following the
reduction of Cu(II)-MFU-4l to Cu(I)-MFU-4l (Figure S17).
Pore size distributions based on DFT indicated micropores at
14 Å (Figure S18). Additionally, the Cu(II) satellite peaks at
943 eV in the 2p scan of XPS spectrum are no longer present
in the XPS spectrum of Cu(I)-MFU-4l, providing further
evidence for the successful reduction of Cu(II) sites to Cu(I)
(Figure S19).
Remarkably, Cu(I)-MFU-4l demonstrates even better
activity toward DMNP than its Cu(II) analogue with 80%
conversion observed after 3 min and complete conversion
achieved within only 10 min under similar reaction conditions
(Figure 5b). With a calculated initial rate of 0.50 0.06 min−1
and a half-life <2 min (Figure S20), which is almost double the
initial rate relative to the parent MOFs, Cu(I)-MFU-4l is one
of the best MOF-based hydrolysis catalysts under basic
aqueous conditions reported to date (Table S1).21 Addition-
ally, decreasing the catalytic loading of Cu(I)-MFU-4l to 3 mol
As the reactive M−OH active sites are generated in situ
following an anion exchange reaction in the basic reaction
mixture (Figure S12), we explored the influence of the anion
on the catalytic activity of Cu-MFU-4l, which exhibits the best
performance among this series. The use of LiX salts (X =
HCOO−, ClO4 , NO3 ) enabled access to the desired anion-
exchanged Cu-MFU-4l-X materials. Following the structural
characterization of the resulting materials using PXRD, N2
adsorption−desorption isotherms, and XPS (Figures S15, S16,
S5), we conducted catalytic hydrolysis experiments and found
no significant difference in the performance between the
various Cu-MFU-4l-X materials, including the parent Cu-
MFU-4l-Cl (Figure 4). The catalysts were washed with acetone
several times after the reactions were completed, and PXRD
analysis of these materials reveals that crystallinity is
maintained. Additionally, DRIFTS spectra obtained for all
materials following catalysis show a peak around 3694−3700
cm−1 (Figure S13), which is attributed to the OH stretch,
−
−
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Figure 6. DMNP binding to Cu(I) active site in Cu(I)-MFU-4l (a)
and Cu(II) active site in Cu(II)-MFU-4l. Color code: Zn (brown); O
(red); C (gray); H (light pink); P (deep red); N (blue); Cu (green).
Most hydrogen atoms are omitted for clarity.
observed for the former material. Additionally, the phenyl
ring in DMNP is well positioned to interact with one of the
benzotriazolate rings in the linker via π−π stacking in both
systems (Figure 6), which enhances DMNP binding and may
play a role in the superior catalytic activity toward DMNP
hydrolysis for the MFU-4l family.
CONCLUSION
Figure 5. (a) Synthesis of Cu(I)-MFU-4l from Cu(II)-MFU-4l-
formate. (b) Hydrolysis profiles of DMNP catalyzed by Cu(II)-MFU-
4l (red) and Cu(I)-MFU-4l with 6 mol % catalyst loading in the
presence of EM buffered solution.
■
We leveraged the tunability within MOFs to generate four
isostructural catalytic materials that are capable of hydrolyzing
the organophosphorus nerve agent simulant DMNP in basic
aqueous solutions. The active sites in these materials are
composed of single metal ions, and importantly, for the first
time, this study provides insight into how the identity of the
transition metal influences the reactivity toward this trans-
formation. Specifically, we explored late first row transition
metals within the M-MFU-4l series (M = Co, Ni, Cu, or Zn),
which are accessible from straightforward transmetalation
reactions from Zn-MFU-4l, to create materials with high
concentrations of well-defined and site-isolated M(II)-based
active sites. Following extensive structural characterization of
these materials, we performed catalytic experiments that reveal
a half-life for DMNP hydrolysis of ∼2 min for Cu(II)-MFU-4l,
which places it among the best performing DMNP hydrolysis
catalysts reported to date; in contrast, DMNP conversion fails
to surpass 30% after 1 h when either Co- or Ni-MFU-4l are
employed under analogous conditions. Acid−base titrations
indicate that Cu-MFU-4l has the lowest pKa among this series
and suggest that the Cu(II)−OH active sites are the relatively
most Lewis acidic, which is consistent with this MOF
demonstrating the best catalytic performance. Modification
of the anion bound to the Cu(II) sites in Cu(II)-MFU-4l has a
negligible effect on the catalytic performance, indicating that
the anion is rapidly displaced under the basic reaction
conditions to form Cu(II)−OH sites in situ. Thermal
treatment of Cu(II)-MFU-4l under dynamic vacuum leads to
the decomposition of formate ions and subsequent reduction
of Cu(II) to Cu(I), which is accompanied by loss of the
charge-balancing anion and generation of an open metal site at
each node. Importantly, Cu(I)-MFU-4l demonstrates even
better performance toward DMNP hydrolysis than its Cu(II)
analogue, and the half-life of <2 min observed for the former
material further highlights the opportunities available to tune
the catalytic properties of this class of materials. Computa-
tional studies corroborate these experimental results and
% results in a t1/2 of ∼4 min and a calculated initial rate
constant of 0.15 0.02 min−1 (Figure S20). While Cu(I) sites
are oxidized to Cu(II), the structural integration of the
framework is maintained as confirmed by PXRD and SEM
image (Figures S21, S22).
To explain why DMNP hydrolysis is faster on Cu(I)-MFU-
4l than on Cu(II)-MFU-4l, we carried out density functional
theory (DFT) calculations using the Gaussian 16 (Rev.A03)44
package to study the interaction between DMNP and the
nodes. The M06-L functional45 with the def2-SVP basis set46
was used and the DFT-D3 dispersion correction47 with zero
damping48 was applied (see the Supporting Information for
details). As shown in Table 2, these calculations suggest that
Table 2. Enthalpy and Binding Free Energies (in kJ/mol)
for Water and DMNP Molecules Bound to Cu(I)-MFU-4l
and Cu(II)-MFU-4l
ΔH (kJ/mol)
ΔG (kJ/mol)
Metal
DMNP
Water
Diff.
DMNP
Water
Diff.
Cu(I)
Cu(II)
−135.5
−113.6
−48.0
−42.8
−87.5
−70.8
−70.8
−29.3
−9.9
4.7
−60.9
−34.0
DMNP binds stronger to Cu(I)-MFU-4l than to Cu(II)-MFU-
4l. This is likely because Cu(I) has a more “open” coordination
environment than Cu(II), which has the charge-balancing −
OH ligand on it (Figure 6). If the potentially competitive
binding between water and DMNP is considered, Cu(I)-MFU-
4l still has a stronger differential binding energy of DMNP
versus water than Cu(II)-MFU-4l does (Table 2). This
suggests that Cu(I)-MFU-4l has a higher ability to bind and
activate DMNP compared to Cu(II)-MFU-4l, which is
consistent with the higher experimental catalytic activity
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suggest that Cu(I) has a stronger differential binding strength
(DMNP vs water) than Cu(II), which may explain why Cu(I)
exhibits the best catalytic performance. Overall, these results
establish a platform for the development of highly effective
organophosphorus nerve agent hydrolysis catalysts beyond the
ubiquitous Zr-MOF that currently dominate this field. As the
identity of the transition metal-based active sites at the nodes
dramatically influences the catalytic performance of these
materials, this work provides a blueprint for the development
of advanced heterogeneous catalysts with a similar level of
tunability to that of their homogeneous counterparts.
ACKNOWLEDGMENTS
■
The authors acknowledge the support from the Defense Threat
Reduction Agency under Award Number HDTRA1-18-1-
0003. K.O.K. gratefully acknowledges support from the IIN
Postdoctoral Fellowship and the Northwestern University
International Institute for Nanotechnology. This work made
use of the Integrated Molecular Structure Education and
Research Center (IMSERC) NMR facility at Northwestern
University, which has received support from NSF CHE-
1048773, the Soft and Hybrid Nanotechnology Experimental
(SHyNE) Resource (NSF ECCS2025633) and Northwestern
University. We also thank the EPIC facility of Northwestern
University’s NUANCE Center, which has received support
from the SHyNE Resource (NSF ECCS-2025633), the IIN,
and Northwestern’s MRSEC program (NSF DMR-1720139).
The authors appreciate Northwestern University Quantitative
Bioelement Imaging Center for ICP-OES measurements.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c03901.
■
sı
Instrumentation, syntheses of linker and M-MFU-4l,
hydrolysis procedure details, PXRD, SEM image, N2
isotherm, pore size distribution, NMR spectra, DRIFTs
spectra, XPS analysis, titration, 1st order fitting for
DMNP hydrolysis (PDF)
REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
■
Omar K. Farha − International Institute of Nanotechnology
and Department of Chemistry and Department of Chemical
& Biological Engineering, Northwestern University, Evanston,
Illinois 60208, United States; orcid.org/0000-0002-
9904-9845; Email: o-farha@northwestern.edu
Timur Islamoglu − International Institute of Nanotechnology
and Department of Chemistry, Northwestern University,
Evanston, Illinois 60208, United States; orcid.org/0000-
0003-3688-9158; Email: timur.islamoglu@
northwestern.edu
Authors
Mohammad Rasel Mian − International Institute of
Nanotechnology and Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0002-3034-8054
Haoyuan Chen − Department of Chemical & Biological
Engineering, Northwestern University, Evanston, Illinois
60208, United States; orcid.org/0000-0002-8634-4028
Ran Cao − International Institute of Nanotechnology and
Department of Chemistry, Northwestern University,
Evanston, Illinois 60208, United States; orcid.org/0000-
0002-7013-2848
Kent O. Kirlikovali − International Institute of
Nanotechnology and Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0001-8329-1015
Randall Q. Snurr − Department of Chemical & Biological
Engineering, Northwestern University, Evanston, Illinois
60208, United States; orcid.org/0000-0003-2925-9246
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c03901
Notes
The authors declare the following competing financial
interest(s): OKF and RQS have a financial interest in the
start-up company NuMat Technologies, which is seeking to
commercialize metal-organic frameworks.
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