Presence versus Proximity: The Role of Pendant Amines in the Catalytic Hydrolysis of a Nerve Agent Simulant

Article abstract of DOI:10.1002/anie.201712645

Amino-functionalized zirconium-based metal-organic frameworks (MOFs) have shown unprecedented catalytic activity compared to non-functionalized analogues for hydrolysis of organophosphonate-based toxic chemicals. Importantly, the effect of the amino group on the catalytic activity is significantly higher in the case of UiO-66-NH₂, where the amino groups reside near the node, compared to UiO-67-m-NH₂, where they are directed away from the node. Herein, we show that the proximity of the amino group is crucial for fast catalytic activity towards hydrolysis of organophosphonate-based nerve agents. The generality of the observed amine-proximity-dictated catalytic activity has been tested on two different MOF systems which have different topology. DFT calculations reveal that amino groups on all the MOFs studied are not acting as Br?nsted bases; instead they control the microsolvation environment at the Zr₆-node active site and therefore increase the overall catalytic rates.

Full text of DOI:10.1002/anie.201712645

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International Edition: DOI: 10.1002/anie.201712645
German Edition: DOI: 10.1002/ange.201712645
MOF Catalysts
Hot Paper
Presence versus Proximity: The Role of Pendant Amines in the
Catalytic Hydrolysis of a Nerve Agent Simulant
Timur Islamoglu, Manuel A. OrtuÇo, Emmanuel Proussaloglou, Ashlee J. Howarth,
Nicolaas A. Vermeulen, Ahmet Atilgan, Abdullah M. Asiri, Christopher J. Cramer, and
Omar K. Farha*
À
Abstract: Amino-functionalized zirconium-based metal-
organic frameworks (MOFs) have shown unprecedented
catalytic activity compared to non-functionalized analogues
for hydrolysis of organophosphonate-based toxic chemicals.
Importantly, the effect of the amino group on the catalytic
activity is significantly higher in the case of UiO-66-NH2,
where the amino groups reside near the node, compared to
UiO-67-m-NH2, where they are directed away from the node.
Herein, we show that the proximity of the amino group is
crucial for fast catalytic activity towards hydrolysis of organo-
phosphonate-based nerve agents. The generality of the
observed amine-proximity-dictated catalytic activity has been
tested on two different MOF systems which have different
topology. DFT calculations reveal that amino groups on all the
MOFs studied are not acting as Brønsted bases; instead they
control the microsolvation environment at the Zr6-node active
site and therefore increase the overall catalytic rates.
the P F bond found in nerve agents, such as Sarin and Soman,
has been shown to be effective for mitigating toxicity.[5] While
a number of different metal oxides are capable of catalyzing
À
the hydrolysis of this P F bond, the development of advanced
materials, and specifically catalysts capable of further ena-
bling this reaction, are still needed.[6]
Metal–organic frameworks (MOFs) are self-assembled,
and atomically precise porous materials comprising metal
ions/clusters and organic linkers.[7] MOFs have been
employed as catalysts and catalyst supports, in part because
of their ordered nature, structural tunability, and permanent
porosity.[8] We and others have shown that fine-tuning pore
size,[9] particle size,[10] metal node connectivity[11] and chemical
composition[12] can favorably affect the activity of MOF-based
catalysts in the hydrolysis of a nerve agent simulant, dimethyl
4-nitrophenyl phosphate (DMNP). Importantly, DMNP has
been shown to be effective for mimicking the reactivity of
organophosphonate-based nerve agents (Scheme 1).[13]
N
erve agents are organophosphonate compounds that
inhibit acetylcholinesterase (AChE)-an enzyme responsible
for the breakdown of neurotransmitters.[1] Organophospho-
nate-based nerve agents bind covalently to the esteratic site of
AChE resulting in a build-up of neurotransmitters, which
leads to muscle fasciculation and eventual death by asphyx-
iation.[2] Although the production, stockpiling, and use of
chemical weapons has been banned by an international
convention,[3] there are still reports of chemical warfare
agents, including nerve agents, being used today.[4] Develop-
ing materials for the simple and fast abatement of these toxic
chemicals therefore remains a very important challenge. In
regards to nerve agent detoxification, selective hydrolysis of
Scheme 1. Hydrolysis reaction of phosphonate-based nerve agent
simulant (dimethyl 4-nitrophenyl phosphonate, DMNP).
Among the linker-appended functional groups screened,
primary amine bearing MOFs have shown unprecedented
activity in the hydrolysis of DMNP. We have demonstrated
that, while keeping other factors constant, installing a primary
amine group on the benzene-1,4-dicarboxylic acid (BDC)
linkers of UiO-66 (Figure 1) can accelerate the catalytic
hydrolysis of DMNP by 20 times compared to the parent
UiO-66 (Figure 2 and Table 1).[12a] We originally attributed
this activity to synergistic effects between the amine group on
the linker and the ZrIV node which could act as a proximal
base and Lewis acid, respectively. We hypothesized that this
synergy would result in enhanced catalytic activity towards
phosphate ester bond hydrolysis since the phosphotriesterase
enzyme that performs this reaction in nature also contains
a Lewis acidic active site with a proximal base. In addition to
the promising activity demonstrated by UiO-66 and its amino
functionalized derivative, we have also shown that using UiO-
67 (Figure 1), a MOF which is isostructural to UiO-66 but
contains 4,4’-biphenyldicarboxylic acid linkers and larger
pores, results in a further enhancement in hydrolysis reaction
[*] Dr. T. Islamoglu, E. Proussaloglou, Dr. A. J. Howarth,
Dr. N. A. Vermeulen, A. Atilgan, Prof. O. K. Farha
Department of Chemistry
Northwestern University
2145 Sheridan Road, Evanston, IL 60208-3113 (USA)
E-mail: o-farha@northwestern.edu
Dr. M. A. OrtuÇo, Prof. C. J. Cramer
Department of Chemistry, Supercomputing Institute, and Chemical
Theory Center, University of Minnesota
Minneapolis, MN 55455 (USA)
Prof. A. M. Asiri, Prof. O. K. Farha
Department of Chemistry, Faculty of Science King Abdulaziz,
University Jeddah
Jeddah 21589 (Saudi Arabia)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201712645.
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rate compared to UiO-66 (Figure 2 and Table 1).[9,12a]
This reaction rate enhancement is to be expected given
that diffusion of the simulant DMNP in UiO-67 is
expected to be faster compared to that in UiO-66.[14]
Since primary amines have been shown to have
a favorable effect on DMNP hydrolysis and the
apertures of UiO-67 are known to promote DMNP
diffusion, the next step was to test the activity of amine
functionalized UiO-67 against DMNP. It should be
noted that we previously tested the activity of UiO-67-
m-NH2 for DMNP hydrolysis at a catalyst loading of
6 mol%. Herein we chose to use 3 mol% catalyst
loading since at lower catalyst loading more data
points can be collected before DMNP is consumed
giving rise to a more accurate conversion versus time
profile. As found previously at higher catalyst load-
ings, we observe enhanced activity for UiO-67-m-NH2
compared to UiO-67 where the half-life for DMNP
conversion dropped from 17.9 min to 6.8 min (Table 1
and Figure 2). At these same catalyst loadings
(3 mol%), the incorporation of primary amines has
a much more drastic effect on the activity of UiO-66
where the half-life decreased from 63.5 min to 3.1 min
from UiO-66 to UiO-66-NH2, respectively. Given that
both UiO-66-NH2 and UiO-67-m-NH2 contain an
aromatic amine with comparable pKa value, the differ-
ence between the catalytic activity of these two MOFs
must be more than simply a buffering effect. Thus, we
turned our attention to the active site of our catalyst-
the Zr6 cluster node of the MOF. The major difference
in the chemical environment surrounding the active
site of each MOF is that the NH2 group resides in the
vicinity of the node in the case of UiO-66-NH2 while it
is directed away from the node in the case of UiO-67-
m-NH2 (Figure 1). It is well-documented that the
chemical and/or physical environment around a cata-
lyst active site can have significant effects on catalytic
rates.[15] Therefore, we speculated that the effect of the
amine group in UiO-67-m-NH2 is less pronounced
compared to UiO-66-NH2 due to the proximity of the
amine to the Zr6 node in UiO-66-NH2. To test this
hypothesis, we synthesized UiO-67-o-NH2 where the
amine is in close proximity to the node as in the case of
UiO-66-NH2. Hydrolysis experiments confirmed our
hypothesis where an almost four-fold enhancement in
catalytic activity is observed when the amine is located
in the ortho position of the linker (i.e. closer to the
node) versus the meta position. Additionally, UiO-67-
o-NH2 presents a ten-fold increase in catalytic activity
compared to the parent UiO-67. To test the generality
of the observed catalytic activity driven by the location
of the amine, we have also designed another series of
Zr-based MOFs with -NH2 groups installed in different
locations. The new system is based on NU-1000,
a MOF with csq-topology containing mesoporous
(ca. 30 ꢀ) hexagonal channels and microporous
(12 ꢀ) triangular channels connected by 10 ꢀ ꢁ 8 ꢀ
orthogonal windows which allow for facile diffusion of
substrate/product. NU-1002-o-NH2 showed a three-
Figure 1. A) Structural representation of UiO-67-o-NH2 and linkers for UiO-66
and UiO-67 series MOFs. B) Structural representation of NU-1002-o-NH2 and
linkers for NU-1000 series MOFs (m=meta, o=ortho).
Figure 2. A)–D) Hydrolysis profile of DMNP with UiO-66, UiO-67 and NU-1000
series MOFs using 3 mol% catalyst loading. B) and D) are the same conversion
profiles as (A) and (C), respectively, expanding the first 10 min for clarity. See
Experimental Section in the Supporting Information for details of reaction
conditions.
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Table 1: Initial rates and half-lives for MOFs for hydrolysis of DMNP.[a]
center with the cluster then surrounded by an implicit
solvation model for all optimized species (i.e., a continuum-
cluster approach). We emphasize that with these calculations,
we aim primarily to describe qualitative trends as quantitative
accuracy is necessarily limited in models addressing reactivity
at large length and time scales with sizable microsolvation
shells. We anticipate the cluster nature of the computational
models used for the UiO-66 family to be transferable to UiO-
67 analogues and other Zr6-based MOFs exposing similar
reactive sites (e.g., NU-1000[21] and MOF-808[22]).
MOFs
Surface Area Pore Size Half-life Initial Rate
[m2 gÀ1
]
[ꢀ]
[min]
[mmsÀ1
]
UiO-66
UiO-66-NH2
UiO-67
UiO-67-m-NH2
UiO-67-o-NH2
NU-1000
NU-1002-m-NH2 1700
NU-1002-o-NH2 1720
1570
1350
2400
2020
2050
2100
10.9/15.9 63.5
10.9/14.8 3.1
11.8/21.6 17.9
0.005
0.093
0.016
0.042
0.163
0.079
0.103
0.250
11.8/21.6
11.8/21.6
11.8/29.5
12.7/27.3
12.7/27.4
6.8
1.8
3.6
2.8
1.2
The hydrolysis mechanism entails coordination of
DMNP,[23] water attack at phosphorus, elimination of
ArOH, and decoordination of the hydrolyzed product.[11b,24]
Recent DFT calculations have suggested that water attack is
rate determining,[24a] and so we focus on this step. We consider
as zeroes of energy, DMNP bound to a defective node and
surrounded by four water molecules. For UiO-66-NH2,
Figure 3 shows the free energies and transition-state (TS)
structures for the nucleophilic attack of water. In TS1
(17.0 kcalmolÀ1, Figure 3 left), the amino group deprotonates
water, forming an incipient nucleophilic hydroxo group that
readily attacks the DMNP bound to the node. In lower-energy
TS2, however (9.5 kcalmolÀ1, Figure 3 right), the amino
group is instead a spectator, and the deprotonation of
nearby water is promoted by a hydroxo bound to Zr. For
both TS1 and TS2, all three remaining water molecules are in
the same position forming an H-bonding network between
the amino and m3-OH groups. Therefore, the comparison
between TS1 and TS2 mostly reflects energetic changes
associated with the relative basicity of the different proton
acceptors. The free energy difference of 7.5 kcalmolÀ1 favor-
ing TS2 indicates that the amino group does not play a role as
a Brønsted base, consistent with the low basicities of anilines
in general.[25] Instead, we infer that the amino groups finely
tune the solvent coordination shell around the defect site, thus
increasing the nucleophilicity of reacting water molecules.
[a] See experimental section in the Supporting Information for details of
reaction conditions (m=meta, o=ortho).
fold enhancement compared to NU-1000 and more than two-
fold enhancement compared to NU-1002-m-NH2 in the
catalytic hydrolysis of DMNP (Figure 2 and Table 1). These
results emphasize that the observed difference in the catalytic
activity is mainly due to the proximity of the amine rather
than a simple pore confinement effect. Moreover, the fast
hydrolysis rate demonstrated by NU-1002-o-NH2 (t1/2
=
1.2 min with 3 mol% cat. loading) places it among the best
heterogeneous catalysts developed for DMNP hydrolysis.[5a,-
b,6a,11a,16] It is worth noting that UiO-67-o-NH2 and NU-1002-
o-NH2 present moderate enhancement in catalytic activity
compared to UiO-66-NH2, which can be attributed to a lower
local amine concentration around the Zr6 node in the former.
Experimental N2 adsorption–desorption isotherms, corre-
sponding pore size distribution analysis, and powder X-ray
diffraction (PXRD) data confirm the structure of each MOF
and that each framework series has comparable BET surface
areas while SEM images show comparable particle sizes
within the same series of MOF family (Figure S10–S14 in the
Supporting Information). Diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) experiments performed on
amine functionalized MOFs show asymmetric and sym-
metric amino group stretching bands which suggests that
free amino groups are present in the MOF (Figure S15). It
is important to note that, in homogeneous systems,
proximal aromatic amines have been shown to facilitate
the delivery of phosphate ester substrates to the nearby
catalytic active site via the formation of H-bonds between
the amine and phosphate ester.[17] As a result, significantly
faster hydrolysis rates for phosphonate ester species have
been achieved by using catalysts bearing amino groups as
opposed to their parent structures.[18]
To reveal the influence of the amino functionality
(Table 1), we modeled[19] the hydrolysis reaction coordi-
nate at the density functional level of theory (DFT) using
the M06-L functional (see Supporting Information for
details). Although pristine Zr6 nodes (as in defect-free
UiO-66) are fully coordinated with twelve linkers, it is
defective nodes with missing linkers that are responsible
for catalytic reactivity. From periodic DFT calculations of
[20]
UiO-66-NH2
we designed a cluster model with one
missing linker to study the local reactivity of the node
(Figure S17). To properly account for water solvent, we
include four explicit water molecules near the reaction
Figure 3. Transition state structures for the water addition step in UiO-66-
NH2. DGwater in kcalmolÀ1. ArO=4-nitrophenoxide.
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This indirect effect of amino groups is in line with other recent
Conflict of interest
computational studies in related MOF systems.[20,26]
To further validate this hypothesis, we investigated the
water addition step for UiO-66 and UiO-66-OH. According
to previous experimental studies[12a] the initial rates for
hydrolysis follow the trend UiO-66-NH2 > UiO-66 > UiO-
66-OH. In line with experiment, our calculations via corre-
sponding TS2-type transition states predict relative activation
free energies of 9.5 (UiO-66-NH2), 11.3 (UiO-66), and 13.3
(UiO-66-OH) kcalmolÀ1 (Figure S18). The results presented
for UiO-66-NH2 and UiO-66 relate to those found for UiO-
67-o-NH2 and UiO-67 derivatives (Table 1). As for UiO-67-
m-NH2, similar although less dramatic effects are expected
due to the greater distance between the meta-amino sub-
stituent and the active site of the node. Modeling UiO-67-m-
NH2, however would require the use of a large microsolvation
shell such that statistical sampling intractable at the fully
quantum mechanical level would become necessary, and thus
we have not attempted to address this given the relatively
small free energy differences observed experimentally.
In conclusion, we tackled, for the first time, understanding
the role of amino functionalities on MOFs for the hydrolysis
of a nerve agent simulant. Owing to the tunability of MOFs at
the atomic level, we were able to show that not only the
presence but also the proximity of the amino group is
important for effective chemical detoxification of a phos-
phate-ester based nerve agent simulant. Our calculations rule
out amino groups as Brønsted bases and suggest that more
subtle changes in microsolvation around the defect sites likely
govern the reactivity trends. Unraveling the mechanistic role
of functionalized linkers will help guide the design of more
efficient MOF catalysts for nerve agent destruction.
The authors declare no conflict of interest.
Keywords: amino groups · heterogeneous catalysis · hydrolysis ·
metal–organic frameworks · nerve agents
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Threat Reduction Agency (HDTRA1-18-1-0003) for the
catalysis study and Army Research Office-STTR (W911SR-
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Manuscript received: December 8, 2017
Accepted manuscript online: January 4, 2018
Version of record online: && &&, &&&&
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Angew. Chem. Int. Ed. 2018, 57, 1 – 6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
MOF Catalysts
Proximity De-pendant: The role of amino
groups on the hydrolysis of a nerve agent
simulant, DMNP, has been explored.
Experiments and theoretical calculations
reveal that not only the presence but also
the proximity of the amino group is
important for effective chemical detoxifi-
cation of DMNP. NU-1002-o-NH2, a Zr-
based mesoporous metal–organic
T. Islamoglu, M. A. OrtuÇo,
E. Proussaloglou, A. J. Howarth,
N. A. Vermeulen, A. Atilgan, A. M. Asiri,
C. J. Cramer, O. K. Farha*
&&& — &&&
Presence versus Proximity: The Role of
Pendant Amines in the Catalytic
Hydrolysis of a Nerve Agent Simulant
framework (MOF) with pendant amines
demonstratest1/2 of 1.2 min with only
3 mol% catalyst loading.
6
www.angewandte.org
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
These are not the final page numbers!