Angewandte
Communications
Chemie
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.
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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