Using Postsynthetic X-Type Ligand Exchange to Enhance CO2Adsorption in Metal-Organic Frameworks with Kuratowski-Type Building Units

Article abstract of DOI:10.1021/acs.inorgchem.1c01077

Postsynthetic modification methods have emerged as indispensable tools for tuning the properties and reactivity of metal-organic frameworks (MOFs). In particular, postsynthetic X-type ligand exchange (PXLE) at metal building units has gained increasing attention as a means of immobilizing guest species, modulating the reactivity of framework metal ions, and introducing new functional groups. The reaction of a Zn-OH functionalized analogue of CFA-1 (1-OH, Zn(ZnOH)4(bibta)3, where bibta2- = 5,5′-bibenzotriazolate) with organic substrates containing mildly acidic E-H groups (E = C, O, N) results in the formation of Zn-E species and water as a byproduct. This Br?nsted acid-base PXLE reaction is compatible with substrates with pKa(DMSO) values as high as 30 and offers a rapid and convenient means of introducing new functional groups at Kuratwoski-type metal nodes. Gas adsorption and diffuse reflectance infrared Fourier transform spectroscopy experiments reveal that the anilide-exchanged MOFs 1-NHPh0.9 and 1-NHPh2.5 exhibit enhanced low-pressure CO2 adsorption compared to 1-OH as a result of a Zn-NHPh + CO2 ? Zn-O2CNHPh chemisorption mechanism. The MFU-4l analogue 2-NHPh ([Zn5(OH)2.1(NHPh)1.9(btdd)3], where btdd2- = bis(1,2,3-triazolo)dibenzodioxin), shows a similar improvement in CO2 adsorption in comparison to the parent MOF containing only Zn-OH groups.

Full text of DOI:10.1021/acs.inorgchem.1c01077

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Using Postsynthetic X‑Type Ligand Exchange to Enhance CO2
Adsorption in Metal−Organic Frameworks with Kuratowski-Type
Building Units
Caitlin E. Bien,† Zhongzheng Cai,† and Casey R. Wade*
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ABSTRACT: Postsynthetic modification methods have emerged as
indispensable tools for tuning the properties and reactivity of
metal−organic frameworks (MOFs). In particular, postsynthetic X-
type ligand exchange (PXLE) at metal building units has gained
increasing attention as a means of immobilizing guest species,
modulating the reactivity of framework metal ions, and introducing
new functional groups. The reaction of a Zn−OH functionalized
analogue of CFA-1 (1-OH, Zn(ZnOH)4(bibta)3, where bibta2−
=
5,5′-bibenzotriazolate) with organic substrates containing mildly
acidic E−H groups (E = C, O, N) results in the formation of Zn−E
species and water as a byproduct. This Brønsted acid−base PXLE
reaction is compatible with substrates with pKa(DMSO) values as
high as 30 and offers a rapid and convenient means of introducing
new functional groups at Kuratwoski-type metal nodes. Gas adsorption and diffuse reflectance infrared Fourier transform
spectroscopy experiments reveal that the anilide-exchanged MOFs 1-NHPh0.9 and 1-NHPh2.5 exhibit enhanced low-pressure CO2
adsorption compared to 1-OH as a result of a Zn−NHPh + CO2 ⇌ Zn−O2CNHPh chemisorption mechanism. The MFU-4l
analogue 2-NHPh ([Zn5(OH)2.1(NHPh)1.9(btdd)3], where btdd2− = bis(1,2,3-triazolo)dibenzodioxin), shows a similar
improvement in CO2 adsorption in comparison to the parent MOF containing only Zn−OH groups.
INTRODUCTION
■
The past two decades have witnessed tremendous growth in
the study of metal−organic frameworks (MOFs) as they have
permeated nearly every chemical subfield and continued to
spark the scientific imagination.1,2 Their modularity and
astounding porosity have propelled the synthesis of thousands
of new structures for fundamental studies and potential
applications in gas storage and separation,3−6 catalysis,7−9
drug delivery,10 energy storage and electronic devices,11,12 and
a host of others. MOFs represent a bridge between
homogeneous, molecular chemistry and solid-state materials
but offer more than a simple translation of the molecular
properties. Indeed, new or unexpected properties often arise
from the framework microenvironment and site isolation.13−24
Postsynthetic modification (PSM) has emerged as an
essential tool for tuning the properties and reactivity of
MOFs.25−29 A variety of different PSM methods have been
developed, but they share the common objective of furnishing
MOFs with structural or chemical features that are not readily
accessible using direct synthetic methods. PSM methods can
be broadly categorized by the type of reaction (exchange,
removal, and addition/insertion) and target component (linker
or metal node) (Figure 1). Exchange reactions in which
structural components (linkers or metal ions) are replaced
Figure 1. Summary of common PSM methods.
Special Issue: Postsynthetic Modification of Metal-
Organic Frameworks
Received: April 7, 2021
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A

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Figure 2. Selected examples of PXLE.64,83,93,100
without dissolution of the framework have been developed to
the point that they are now routine. Postsynthetic metal
exchange (PSME) provides access to metal nodes with variable
bi- or multimetallic compositions.30,31 Similarly, linker
exchange allows for substitution using extrinsic linkers of
similar size or shape but varying ancillary functional
groups.32,33 Heterogeneous reactions can also be performed
at organic linkers or metal nodes to insert new functional
groups or modify existing ones. Covalent modification of
organic linkers remains one of the most common and
straightforward methods of inserting new functional groups.34
This approach usually relies on linkers with reactive groups
such as amines or aldehydes and benefits from well-established
synthetic organic methods.
PSM of MOFs by removal or exchange of ancillary ligands at
metal nodes is also quite common. The most prevalent
example is the removal of nonstructural L-type ligands (e.g.,
water or N,N-dimethylformamide) using heat and/or
vacuum.35 The resulting Lewis acidic, coordinatively unsatu-
rated metal sites have become a hallmark functional group for
MOF-based adsorbents and catalysts.36 Labile solvent ligands
can also be exchanged to graft new functional groups at metal
nodes. One of the earliest examples of this approach involved
the substitution of axial aqua ligands with pyridine at the
dicopper paddlewheel nodes of HKUST-1.37 L-type ligand
exchange or “dative PSM” at metal nodes has subsequently
been employed to immobilize species that modify the guest
adsorption properties,38,39 introduce chirality,40,41 or serve as
catalytically active sites.42,43 Recently, diamine-grafted MOFs
have received considerable attention as adsorbents for selective
CO2 capture.44−52 Most notably, Long and co-workers have
shown that amine-grafted variants of Mg2(dobpdc) [dobpdc4−
= 4,4′-dihydroxy-(1,1′-biphenyl)-3,3′-dicarboxylate] exhibit
steplike CO2 adsorption, resulting from a cooperative
chemisorption mechanism.53 Carbamate formation via CO2
insertion into an Mg−N bond is accompanied by proton
transfer to a neighboring diamine ligand, and this process
propagates to form one-dimensional ammonium carbamate
chains.
There are numerous examples of postsynthetic exchange of
charge-balancing anions residing within the pores of cationic
MOFs.54−63 However, inner-sphere, postsynthetic X-type
ligand exchange (PXLE) at MOF metal nodes has been less
studied, partly as a result of the relative paucity of materials
containing exchangeable X-type ligands. Hupp and co-workers
pioneered solvent-assisted ligand incorporation (SALI) as a
means of immobilizing nonstructural carboxylate or phospho-
nate groups at linker-deficient 6- or 8-connected Zr6 nodes
(Figure 2a).64 The method relies on acid−base chemistry to
substitute terminal OH−/H2O ligands at Zr sites of the nodes,
and the mild conditions have allowed for the immobilization of
a wide range of functional molecules.65−71 Yaghi and co-
workers have used a similar PXLE approach to bind small
molecules at the Al-based metal nodes of chiral MOF-520. The
immobilization method, termed coordinative alignment,
reduces the degrees of freedom of the guest species and
facilitates structural characterization via single-crystal X-ray
diffraction of MOF crystals.72,73 Linker installation or
“retrofitting” strategies involving PXLE at undercoordinated
Zr or Al metal nodes bearing labile monotopic ligands have
also been developed.18,74,75
MOFs assembled from bis(benzotriazolate) linkers have also
emerged as platforms for using PXLE to modify gas adsorption
and catalytic properties. The high donor atom to charge ratio
of the benzotriazolate groups tends to favor the formation of
Kuratowski- or chain-type metal nodes with charge compensat-
ing anions. M2X2(BBTA) (BBTA2− = benzo(1,2-d:4,5-d′)-
bistriazolate) and M2X2(BTDD) [BTDD2− = bis(1,2,3-
triazolato)dibenzodioxin] adopt honeycomb structures with
chains of metal ions coordinated by three benzotriazolate
nitrogen atoms, two bridging halide or hydroxide anions (X),
and a terminal solvent ligand (Figure 2b).76−78 Li and co-
workers showed that PXLE of the μ-Cl− ligands in
Co2Cl2(BBTA) (MAF-X27-Cl) for μ-OH− results in improved
B
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Figure 3. Postsynthetic ligand exchange of Zn−OAc to generate Zn−OH in 1-OAc and reversible CO2 binding.
electrocatalytic activity toward water oxidation.79 Subsequent
reports from the groups of Snurr and Long showed that Co2(μ-
OH)2(BBTA) strongly chemisorbs O2 at the coordinatively
unsaturated Co2+ sites, while Co2Cl2(BBTA) exhibits only
weak O2 physisorption.80,81 A concurrent density functional
theory (DFT) computational study explored how varying the
bridging anion in the M2X2(BBTA) series of MOFs could be
used to optimize the reactivity of putative metal−oxo species
molecules such as O2, N2, and H2. CuI-MFU-4l has also
recently been exploited for olefin separation98,99 and NO
disproportionation.102
Recently, our group and others have used PXLE to generate
nucleophilic transition-metal hydroxide groups (M−OH,
where M = Co, Zn, and Ni) at the peripheral metal sites of
CFA-1, MFU-4, and MFU-4l.95−97,103 The resulting M−OH
species resemble the active sites of α-carbonic anhydrases and
are highly reactive, allowing the MOFs to adsorb large
amounts of CO2 at low partial pressures via a M−OH +
CO2 ⇌ M−O2COH chemisorption mechanism. The CO2
capacity and isosteric heat of adsorption in these materials can
be modulated by using PSME to vary the metal composition.96
In addition, spectroscopic data and computational studies have
revealed that inter-cluster hydrogen bonding interactions play
a key role in CO2 capture in the CFA-1 analogue 1-OH
(Figure 3). CFA-1 contains crystallographically distinct,
peripheral metal sites (ZnA and ZnE), and the distance
between the ZnE centers of neighboring metal nodes (∼7.7 Å)
enables the formation a hydrogen-bonded bicarbonate dimer
upon CO2 adsorption. Such metal-based, synergistic binding
mechanisms are rare in MOFs since metal sites/clusters are
usually separated by relatively large distances.
While PXLE is gaining attention, it remains less explored
than other PSM methods. When available at a MOF metal
node, X-type ligands offer a handle for precise placement of
ligand-based functional groups and greater stability against
volatilization than dative PSM approaches. The combined
reactivity of the framework metal and X-type ligand can also be
exploited to generate highly reactive species or enable metal−
ligand cooperativity for selective gas adsorption processes or
new catalytic applications.104−106 Herein we show that the
Zn−OH groups in 1-OH act as strong Brønsted bases and
undergo PXLE with a range of substrates containing mildly
acidic E−H (E = C, N, O) bonds. The reaction of 1-OH with
aniline (PhNH2) results in the formation of Zn−NHPh
species, and CO2 adsorption measurements for the resulting
MOFs, 1-NHPhx (x = 0.9, 2.5), show steep, low-pressure
uptake exceeding that of the parent 1-OH. In situ diffuse-
reflectance infrared Fourier transform spectroscopy (DRIFTS)
experiments reveal that the Zn−NHPh groups in 1-NHPhx
react with CO2 to generate zinc carbamate (Zn−O2CNHPh)
species that have a higher CO2 affinity than the parent Zn−
toward C−H bond activation.82 Dinca and co-workers have
̌
also used PXLE to prepare a series of anion-exchanged variants
of Ni2X2BTDD (X = Cl, F, Br, OH) and investigate the effect
of bridging anion identity on the hydrophobicity and water
adsorption properties (Figure 2b).83 The resulting materials
showed remarkable differences in the relative humidity at
which the water adsorption steps occurred.
The zinc benzotriazolate MOFs MFU-4, MFU-4l, and CFA-
1 (CFA = Coordination Framework Augsburg University and
MFU = Metal−organic Framework Ulm University) are
assembled from discrete [Zn5X4(bt)6] metal nodes (X−
=
Cl−, CH3CO2 ; bt− = benzotriazolate), also referred to as
Kuratowski clusters.84−86 Each node is comprised of an
octahedrally coordinated, central Zn site and four peripheral
metal sites in pseudotetrahedral coordination geometries
capped by a terminal X-type ligand (Figure 2c). These
materials are chemically and thermally robust and highly
amenable to postsynthetic metal and ligand exchange. As a
result, PSM methods have been used to elaborate this small
family of MOFs as catalysts for olefin polymerization87−92 and
C−H bond oxidation93,94 as well as adsorbents for selective
CO2 and olefin capture.95−99 In an early study, Volkmer and
co-workers screened PXLE reactions at the terminal Co−Cl
sites of CoII-exchanged MFU-4l using alkali metal salts to carry
out anion metathesis.93 Nearly complete substitution occurred
−
−
−
−
with a variety of different anions, including NO2 , NO3 , N3 ,
NCO−, HCO2 , and F−. More recently, the same group
−
showed that organozinc and organolithium reagents could be
used to functionalize the peripheral Zn/Co sites of MFU-4l
with alkyl and acetylide ligands.100 PXLE reactions in MFU-4l
have also facilitated subsequent modifications and new types of
reactivity. For example, thermolysis of MFU-4l analogues
containing Zn−O2CH and Cu−O2CH groups was found to
generate Zn−H and coordinatively unsaturated CuI sites,
respectively.101 The latter were shown to strongly bind small
C
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OH groups. CO2 adsorption and DRIFTS studies with an
anilide-exchanged analogue of MFU-4l (2-NHPh) further
support the enhanced reactivity of the Zn−NHPh groups.
RESULTS AND DISCUSSION
■
The ligand-exchanged MOFs 1-E were generated by treating 1-
OH with an excess (∼5 equiv per Zn−OH group) of acidic E−
H (E = C, N, O) substrates in a dichloromethane or 1,4-
dioxane solution (Table 1). 1-NHPh0.9 was prepared similarly
Table 1. Products (1-E) of Brønsted Acid−Base PXLE
Reactions
Figure 4. DRIFTS spectra of 1-CH2NO2, 1-OPh, 1-CCPh, 1-
NHPh2.5, and 1-OH.
1-E
1-OH
formula
BET surface area (m2 g−1
)
acetylide species generated at the metal nodes of MFU-4l.100
The ν(O−H) band arising from the Zn−OH groups remains
present in the DRIFT spectra of 1-NHPh0.9 and 1-NHPh2.5,
albeit with lower relative intensity compared to 1-OH. This
observation is consistent with the incomplete incorporation of
Zn5(OH)4(bibta)3
Zn5(CH2NO2)4(bibta)3
Zn5(OPh)4(bibta)3
Zn5(CCPh)4(bibta)3
Zn5(NHPh)2.5(OH)1.5(bibta)3
2326
2013
1443
1470
2005
c
1-CH2NO2
b
1-OPh
c
1-CCPh
1-NHPh2.5
1-NHPh0.9
b
b
1
Zn−NHPh groups, as determined from the acid-digested H
Zn5(NHPh)0.9(OH)3.1(bibta)3
2070
NMR spectra. The DRIFT spectra also contain ν(CN) and
ν(N−H) bands attributable to the anilide group at 1292 and
3391 cm−1, respectively. The presence of a single ν(N−H)
band supports N−H bond activation and the formation of a
metal anilide species rather than coordination of the neutral
amine. In the latter case, two ν(N−H) bands would be
expected in the IR spectrum.110
pKa values of the substrates in DMSO.111,112 1H NMR spectroscopy
was used to establish the empirical formula. IR spectroscopy was
used to establish the empirical formula.
a
b
c
using 0.25 equiv of aniline per Zn−OH group. The Brønsted
acid−base reactions result in PXLE at the Zn−OH sites with
concomitant formation of H2O. Powder X-ray diffraction
(PXRD) patterns of the products indicate that no major
structural changes occur as a result of the postsynthetic
The 1H NMR and DRIFT spectra obtained after treating 1-
OH with neat acetonitrile [pKa(DMSO) = 31.3] show no
evidence of a PXLE reaction to form zinc cyanomethide
species (Figures S10 and S17). This observation reveals the
upper limit of the basicity of the Zn−OH groups in 1-OH,
which falls just short of the pKa(DMSO) value of 32 for H2O.
Metal aqua complexes are typically more acidic than
uncoordinated H2O, but the magnitude of the decrease in
pKa is strongly influenced by the metal identity, coordination
environment, and charge.113 In fact, molecular complexes with
terminal M−OH (M = Ni, Cu, Au) groups are well-known to
activate weakly acidic C−H bonds with pKa(DMSO) values
greater than 30.114−119 In the present case, the strong Lewis
acidity of Zn2+, which would normally lead to less basic Zn−
OH groups, may be counteracted by the strong benzotriazolate
donor ligands and MOF microenvironment. Indeed, Gascon
and co-workers found that MOF-immobilized aniline groups
exhibit enhanced basicity compared to the homogeneous
molecular analogue.120 Consequently, the Brønsted acid−base
PXLE reactions shown here for 1-OH represent the first
examples of activation of weakly acidic E−H functional groups
by terminal Zn−OH species.
1
reactions (Figure S1). Acid-digested H NMR spectra were
used to quantify the extent of PXLE in 1-OPh, 1-NHPh0.9, and
1-NHPh2.5. Integration of the spectrum measured for 1-OPh
gives a H2bibta/PhOH ratio of 1:1.3, confirming that
phenoxide is exchanged at all four peripheral Zn−OH sites
of the metal nodes (Figure S11). The spectra of 1-NHPh0.9
and 1-NHPh2.5 show H2bibta/PhNH2 ratios of 1:0.3 and
1:0.82, indicating that the PXLE reactions incorporate ∼0.9
and ∼2.5 anilide groups per cluster (Figures S12 and S13).
Despite numerous attempts to digest 1-CH2NO2 and 1-CCPh
under different conditions, 1H NMR spectra suitable for
quantifying the ligand exchange could not be obtained owing
to acid-induced side reactions (Figures S15 and S16).
However, the DRIFT spectra of 1-CH2NO2, 1-CCPh, and 1-
OPh show nearly complete disappearance of the ν(O−H)
bands corresponding to the Zn−OH groups, supporting
quantitative ligand exchange (Figures 4 and S6−S8). The
DRIFT spectrum of 1-CH2NO2 also contains a band at 1600
cm−1 assigned as the ν(CN) mode of the nitromethanate
ligand. The frequency of this band is consistent with significant
CN double-bond character and similar to that reported for
κ2-O,O-nitromethanate complexes.107−109 The DRIFT spec-
trum of 1-OPh shows new bands corresponding to ν(C−H)
modes of the phenoxide group at 2989 and 3010 cm−1, ν(C
C) modes at 1591 and 1491 cm−1, and a C−O stretch at 1291
cm−1. 1-CCPh exhibits a ν(CC) band at 2119 cm−1 that is
in good agreement with those observed for related zinc
Samples of 1-E were desolvated by heating at 100 °C under
vacuum. The Brunauer−Emmett−Teller (BET) surface areas
calculated from N2 adsorption isotherms are all lower than that
of 1-OH, consistent with the incorporation of larger X-type
ligands (Table 1 and Figures S3 and S4). CO2 adsorption
isotherms measured at 300 K for 1-CH2NO2, 1-OPh, and 1-
CCPh show linear adsorption profiles indicative of only weak
physisorptive interactions (Figure S18). On the other hand, 1-
D
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NHPh0.9 and 1-NHPh2.5 exhibit steep, low-pressure CO2
uptake similar to that of 1-OH (Figures 5 and S19). The
increase in the molecular weights of 1-NHPh0.9 and 1-
NHPh2.5, plotting the CO2 uptake on a per formula unit basis
(mmol of CO2 per mmol of MOF) better highlights the
differences. At 1 mbar, 1-NHPh0.9 and 1-NHPh2.5 adsorb 3.18
and 3.31 mmol mmol−1 of CO2, respectively, which are
considerably higher than the 2.58 mmol mmol−1 adsorbed by
1-OH. The theoretical density of Zn−X (X = OH, NHPh)
groups in the MOFs is 4 mmol mmol−1, meaning that the
chemisorption sites reach saturation beyond 100 mbar of CO2
pressure. The enhanced low-pressure CO2 adsorption in 1-
NHPh0.9 and 1-NHPh2.5 should arise from a greater
thermodynamic driving force for reaction of the Zn−NHPh
groups with CO2 to form Zn−O2CNHPh species compared to
the Zn−OH + CO2 ⇌ Zn−O2COH chemisorption reaction.
Unfortunately, attempts to measure variable-temperature
isotherms for 1-NHPh0.9 and 1-NHPh2.5 and calculate isosteric
heats of adsorption have been hindered by their poor thermal
stability. Repeated thermal activation leads to a gradual decline
in the CO2 capacity and crystallinity (Figures S20 and S21).
Consequently, DFT calculations have been carried out on
model Kuratowski clusters to gain insight into the relative CO2
binding energies of Zn−NHPh and Zn−OH groups (see the
Supporting Information for details). Indeed, the CO2 binding
energy calculated for the formation of Zn−O2CNHPh species
(82.90 kJ mol−1) is found to be considerably greater than that
for the Zn−O2COH groups (47.48 kJ mol−1).
Variable-temperature DRIFTS experiments have been used
to gain further insight into the reaction of 1-NHPh0.9 and 1-
NHPh2.5 with CO2 (Figure 6). At room temperature, the CO2-
loaded MOFs 1-NHPh0.9-CO2 and 1-NHPh2.5-CO2 exhibit IR
bands at 2650, 1584, 1500, and 1342 cm−1 that match those of
the hydrogen-bonded ZnE−HCO3 species in 1-OH (Figure
3).95 Accordingly, the presence of these bands indicates that
pairs of Zn−OH groups remain at the ZnE sites of the anilide-
exchanged MOFs. In addition, weak bands appear at 1414 and
1664 cm−1 in the DRIFT spectrum of 1-NHPh0.9-CO2 under a
Figure 5. Semilogarithmic plots of CO2 adsorption isotherms (300 K)
measured for 1-OH, 1-NHPh2.5, and 1-NHPh0.9 plotted in terms of
(a) mmol of CO2 per g of MOF and (b) mmol of CO2 per mmol of
MOF.
isotherms for the anilide-functionalized MOFs are distinctly
steeper in the low-pressure region with slightly higher
gravimetric capacities up to 10 mbar of CO2. Because the
PhNH−/OH− ligand exchange results in a considerable
Figure 6. (a and b) Evolution of the DRIFT spectra of 1-NHPh0.9-CO2 and 1-NHPh2.5-CO2 upon heating under a N2 atmosphere. The
highlighted regions are bands corresponding to ZnE−HCO3 (gray), Carbamate 1 (red), and Carbamate 2 (green) species. (c) Temperature-
dependent evolution of selected IR bands in 1-NHPh2.5‑CO2.
E
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CO2 atmosphere but quickly vanish when the sample is placed
under flowing N2 (Figure S28). These bands are nearly
identical in frequency and behavior with those observed for the
ZnA−O2COH sites in 1-OH, which are isolated and unable to
engage in intercluster hydrogen-bonding interactions (Figure
S27). However, they are absent in the spectrum of 1-NHPh2.5-
CO2 measured under CO2, indicating complete PhNH−/OH−
exchange at the ZnA sites of this material (Figure S29). Further
inspection of the DRIFT spectra reveals a new set of bands at
1613 and 1472 cm−1 that can be assigned as asymmetric and
symmetric ν(C−O) modes of a distinct Zn−O2CNHPh
species (Carbamate 1). This species also gives rise to a
carbamate ν(C−N) + δ(N−H) (amide II) mode at 1516 cm−1
and a sharp ν(N−H) band at 3462 cm−1. The latter is blue-
shifted by 71 cm−1 compared to the ν(N−H) band found in 1-
NHPh2.5 (3391 cm−1). Insertion of CO2 into the metal amide
bonds of small-molecule complexes to form metal carbamates
has been shown to result in a similarly blue-shifted ν(N−H)
band.121 The DRIFT spectrum of 1-NHPh2.5-CO2 clearly
shows the presence of at least one additional Zn−O2CNHPh
species (Carbamate 2) characterized by a pair of asymmetric
and symmetric ν(C−O) bands at 1598 and 1448 cm−1 and a
broad ν(N−H) band at 3280 cm−1. The IR bands assigned to
Carbamate 2 also appear in the DRIFT spectrum of 1-
NHPh0.9-CO2 but with lower relative intensity.
The presence of the distinct Carbamate 1 and Carbamate 2
species is supported by the evolution of the corresponding
DRIFTS bands upon heating the CO2-loaded MOFs to 150 °C
under a N2 flow. The most striking observation in the variable-
temperature spectra is the difference in the rates of
disappearance of the carbamate ν(N−H) bands at 3462 and
3280 cm−1 (Figure 6c). In the spectrum of 1-NHPh2.5-CO2,
the former almost completely devolves before the onset of the
disappearance of the band at 3280 cm−1. This behavior clearly
indicates that Carbamate 2 exhibits much stronger CO2
binding than Carbamate 1. The same trend is observed for
the bands corresponding to Carbamate 1 and Carbamate 2 in
the 1200−1700 cm−1 range, but the overlap in this region
makes them more difficult to discern. At the same time, the
features associated with the hydrogen-bonded Zn−HCO3
species appear to devolve in the lower-temperature regime
with Carbamate 1.
spectrum of 1-NHPh2.5-CO2 signals the presence of unreacted
Zn−OH groups. which may arise from the formation of
unsymmetrical ZnE−O2CNHPh···HO−ZnE pairs. Conse-
quently, while the assignment of the Carbamate 2 species to
the ZnE sites is well-founded, the identity of the hydrogen-
bond partner remains ambiguous. Nevertheless, the variable-
temperature DRIFTS data clearly show enhanced CO2 binding
at both the ZnA−NHPh and ZnE−NHPh sites. While the
ZnA−O2COH groups in the parent 1-OH readily desorb CO2
at room temperature, the ZnA−O2CNHPh species only
devolve upon heating. Similarly, the hydrogen-bonded ZnE−
O2CNHPh species are much more robust than the ZnE−
O2COH congeners and remain intact until nearly 150 °C.
To further probe the effect of Zn−NHPh groups on CO2
adsorption and support the assignments above, an anilide-
functionalized MFU-4l analogue, 2-NHPh, was synthesized
using the same PXLE procedure as that employed for 1-
NHPh2.5. MFU-4l contains Kuratowski-type metal nodes, but
the Zn−X sites are crystallographically equivalent without the
possibility of intercluster hydrogen-bonding interactions
between bicarbonate ligands (Figure 2). As a result, 2-OH
exhibits a lower CO2 capacity and affinity than 1-OH.97 The
PXRD pattern of 2-NHPh matches that of parent MOF, and
1
the acid-digested H NMR spectrum shows a H2btdd/PhNH2
ratio of 1:0.63, indicating the incorporation of ∼1.9 anilide
groups per cluster. The N2 adsorption isotherm measured for
2-NHPh gave a calculated BET surface area of 2816 m2 g−1,
which is lower than that of the parent MFU-4l (3770 m2 g−1)
but consistent with the presence of the larger anilide ligands
(Figure S5). CO2 adsorption isotherms (300 K) show that 2-
NHPh adsorbs a greater amount of CO2 than 2-OH along the
entire pressure range up to 1 bar (Figure 7a), with an
adsorption capacity of 0.95 mmol g−1 at 1 mbar. Notably, 2-
NHPh displays better thermal stability than the 1-NHPhx
derivatives, and only a small decrease in the CO2 capacity was
observed after several adsorption−activation cycles (Figure
S22). Consequently, variable-temperature CO2 isotherms have
been used to calculate the isosteric heat of adsorption (Qst) as
a function of CO2 loading (Figure S26). The observed
maximum of 65 kJ mol−1 reflects a strong chemisorption
mechanism but is only slightly higher than the value previously
reported for 2-OH (63 kJ mol−1).97
The isolated Zn−O2CNHPh species formed in 2-NHPh-
CO2 should be expected to exhibit an IR spectroscopic
signature similar to that of ZnA−O2CNHPh groups in 1-
NHPh-CO2. This is indeed the case, and 2-NHPh-CO2 gives
rise to a single set of new carbamate bands at 1442, 1513,
1606, and 3464 cm−1 that closely match those observed for 1-
NHPhx-CO2 (Figures 7b and S31). Consistent with our
previous findings, the Zn−OH groups remaining in 2-NHPh
exhibit weak CO2 binding, and bands assigned to the
corresponding Zn−HCO3 species at 1661, 1425, and 1207
cm−1 quickly disappear when the atmosphere in the sample
chamber is switched from CO2 to flowing N2.97 However, the
IR spectral features corresponding to the Zn−O2CNHPh
groups are stable under a N2 atmosphere and only begin to
devolve upon heating (Figure S31).
The relatively weak CO2 binding and sharp, blue-shifted
ν(N−H) band (3462 cm−1) observed for Carbamate 1 elicit its
association with isolated ZnA−O2CNHPh sites. This assign-
ment is further supported by a comparison with an anilide-
functionalized MFU-4l analogue (vide infra). Accordingly, the
broad ν(N−H) band of Carbamate 2 (3280 cm−1) and its
greater CO2 affinity are consistent with intercluster hydrogen-
bonding interactions resembling those at the ZnE−OH sites in
1-OH. A similar broadening and red-shifting of the ν(N−H)
bands has been observed for hydrogen-bonded alkyl carbamate
dimers.122 However, the situation at the ZnE sites is obfuscated
by the three possible ligand combinations within each pocket
(Figure S33). The DRIFTS data clearly show that some
fraction of sites are occupied by ZnE−OH pairs that result in
hydrogen-bonded ZnE−O2COH···HOCO2−ZnE species. The
remaining pockets should contain ZnE−NHPh with neighbor-
ing ZnE−OH or ZnE−NHPh groups. Statistically, the high
anilide loading in 1-NHPh2.5 and observation of ZnE−
O2COH···HOCO2−ZnE species necessitates the presence of
ZnE−NHPh···PhNHCO2−ZnE pairs. However, a slightly red-
shifted, residual ν(O−H) band at 3280 cm−1 in the DRIFT
CONCLUSIONS
■
The Zn−OH groups in 1-OH are unexpectedly strong bases
capable of activating E−H functional groups with pKa(DMSO)
values approaching 30. The resulting Brønsted acid−base
PXLE reactions offer an efficient and robust method for
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and pulse shapes.123 Briefly, spectra were collected using selective
pulses of 1 ms with the transmitter frequency set to the center of the
solvent resonance. The recycle delay between scans was 2 s, 16K
1
points were collected, and the acquisition time was 0.7 s. For H
NMR spectra, the solvent resonance was referenced as an internal
1
standard. Samples were digested for solution-state H NMR analysis
by suspending ∼10 mg of solid in CF3CO2H (0.5 mL) or H2SO4 (1−
3 drops) and gently heating the suspensions until all of the solids
dissolved. Deuterated dimethyl sulfoxide (DMSO-d6; 0.1−1 mL) was
then added to provide a lock signal for shimming.
Gas Adsorption Isotherms. Single-component gas adsorption
isotherms were measured using a Micromeritics 3Flex surface
characterization analyzer. All measurements were performed using
ultrahigh purity grade gases (99.999%) purchased from Praxair (N2,
NI 5.0UH-K; CO2, CD 5.0LS-K). Prior to analysis, samples (100−
200 mg) were transferred to oven-dried and tared sample tubes
equipped with TranSeals (Micrometrics) and heated to 100 or 150 °C
(1 °C min−1) under vacuum until the outgas rate was less than 0.0033
mbar min−1. BET surface areas were calculated from the N2
adsorption isotherms (77 K) by fitting the data to the BET equation,
with the appropriate pressure range (0.0001 ≤ P/P0 ≤ 0.1)
determined by the consistency criteria of Rouquerol.124 A Micro-
metrics thermocouple-controlled heating mantle was used to maintain
the sample temperature for isotherms measured between 300 and 373
K.
DRIFTS Measurements. DRIFT spectra were recorded using a
Bruker Tensor spectrometer equipped with a liquid N2-cooled MCT
detector and a Praying Mantis diffuse-reflectance accessory (Harrick
Scientific). Background spectra were collected with anhydrous KBr.
Samples were diluted with anhydrous KBr and loaded into the sample
cup of a high-temperature reaction chamber fitted with KBr windows.
The chamber was sealed and placed under a flow of dry N2 gas.
Variable-temperature DRIFT spectra were measured periodically,
while heating the samples from 25 to 150 °C. For in situ CO2
adsorption experiments, the chamber was sealed, placed under a flow
of N2 gas, and heated from 28 to 150 °C while the DRIFT spectra
were monitored to confirm activation. Upon cooling, the sample flow
gas was switched to pure CO2 using a three-way valve, and DRIFT
spectra were recorded until the sample reached equilibrium. The
sample flow gas was then switched back to N2, and DRIFT spectra
were measured periodically while the sample was heated from 28 to
150 °C.
Synthesis of 1-CH2NO2. In a N2-filled glovebox, a 20 mL
scintillation vial was charged with 1-OH (0.176 g, 0.64 mmol of Zn−
OH) and a 0.5 M solution of nitromethane (6.4 mL, 3.2 mmol) in
dichloromethane. The suspension was kept at room temperature for
72 h. The solution was decanted, and the MOF was washed with 3 ×
20 mL of dichloromethane. Elem anal. Calcd for
Zn5(CH2NO2)4(bibta)3 (Zn5C40H26N22O8): C, 37.84; H, 2.06; N,
24.27. Found: C, 37.72; H, 2.20; N, 23.60.
Synthesis of 1-OPh. In a N2-filled glovebox, a 20 mL scintillation
vial was charged with 1-OH (0.0925 g, 0.34 mmol of Zn−OH) and a
0.5 M solution of phenol (3.4 mL, 1.7 mmol) in 1,4-dioxane. The
suspension was kept at room temperature for 72 h. The solution was
decanted, and the MOF was washed with 3 × 20 mL of dioxanes.
Elem anal. Calcd for Zn5(OPh)4(bibta)3·2H2O (Zn5C60H42N18O6):
C, 50.11; H, 2.94; N, 17.53. Found: C, 49.43; H, 2.46; N, 17.69.
Synthesis of 1-PhCCH. In a N2-filled glovebox, a 20 mL
scintillation vial was charged with 1-OH (0.1503 g, 0.54 mmol of Zn−
OH) and a 0.5 M solution of phenylacetylene (5.5 mL, 2.7 mmol) in
1,4-dioxane. The suspension was kept at room temperature for 16 h.
The solution was decanted, and the MOF was washed with 3 × 20
mL of dioxanes. Elem anal. Calcd for Zn5(CCPh)4(bibta)3·5H2O
(Zn5C68H48N18O5): C, 53.58; H, 3.17; N, 16.54. Found: C, 53.05; H,
2.37; N, 17.46.
Figure 7. (a) Semilogarithmic plots of CO2 adsorption isotherms
(300 K) measured for 2-NHPh and 2-OH. (b) Room temperature
DRIFT spectra measured for 2-NHPh after exposure to CO2 and
subsequent purging with N2. The bands highlighted in gray and red
are associated with Zn−HCO3 and Zn−O2CNHPh species,
respectively.
functionalizing Kuratowski-type metal nodes with X-type
ligands that modulate gas adsorption or catalytic properties.
CO2 adsorption studies with 1-NHPh0.9 and 1-NHPh2.5
demonstrate the utility of this approach, and the partial
incorporation of Zn−NHPh groups increases low-pressure
CO2 uptake in comparison to the parent 1-OH. In situ
DRIFTS experiments reveal that CO2 reacts with the Zn−
NHPh groups to form two distinct zinc carbamate species,
both of which exhibit greater CO2 affinities than their Zn−OH
congeners. The Brønsted acid−base PXLE method described
here for 1-OH resembles SALI developed for Zr-based
MOFs.64 However, the ability to apply both PSME and
PXLE in MOFs containing Kuratowski-type nodes presents
unique opportunities to exploit metal-based reactivity and
design metal−ligand cooperativity.
EXPERIMENTAL SECTION
■
General Considerations. 1-OH and 2-OH were synthesized
according to reported procedures.95,97 Nitromethane (Acros Organic,
99+%), aniline (Mallinckrodt, ACS grade), and phenylacetylene (Alfa
Aesar, 98+%) were dried over CaH2, distilled, and stored in a N2-filled
glovebox. Phenol (Alfa Aesar, 99+%) was used as received.
Tetrahydrofuran, 1,4-dioxane, acetonitrile, and dichloromethane
were degassed by sparging with ultrahigh-purity argon and dried via
passage through columns of drying agents using a solvent purification
system from Pure Process Technologies. All other solvents and
reagents were purchased from commercial suppliers and used as
received. Routine powder X-ray diffraction patterns for phase
identification were collected on a Rigaku Miniflex 600 diffractometer
using nickel-filtered Cu Kα radiation (λ = 1.5418 Å). Elemental
microanalyses were performed by Robertson Microlit Laboratories,
Ledgewood, NJ. Solution-state NMR spectra were measured using a
Bruker DPX 400 MHz spectrometer. 1H NMR spectra were collected
using 180° water-selective excitation sculpting with default parameters
Synthesis of 1-NHPh2.5. In a N2-filled glovebox, a 20 mL
scintillation vial was charged with 1-OH (0.1165 g, 0.42 mmol of Zn−
OH) and a 0.5 M solution of aniline (4.2 mL, 2.1 mmol) in 1,4-
dioxane, resulting in a rapid color change of the solid from beige to
yellow. The suspension was kept at room temperature for 72 h. The
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solution was decanted, and the MOF was washed with 3 × 20 mL of
1,4-dioxane. Elem anal. Calcd for Zn5(NHPh)2.5(OH)1.5(bibta)3·
4H2O (Zn5C51H42.5N20.5O5.5): C, 45.12; H, 3.16; N, 21.15. Found: C,
44.05; H, 2.50; N, 21.59.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01077.
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Analytical data, attenuated-total-reflectance IR and
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AUTHOR INFORMATION
■
Corresponding Author
Casey R. Wade − Department of Chemistry and Biochemistry,
The Ohio State University, Columbus, Ohio 43210, United
States; orcid.org/0000-0002-7044-9749;
Email: wade.521@osu.edu
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Authors
Caitlin E. Bien − Department of Chemistry and Biochemistry,
The Ohio State University, Columbus, Ohio 43210, United
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Zhongzheng Cai − Department of Chemistry and
Biochemistry, The Ohio State University, Columbus, Ohio
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Author Contributions
†These authors contributed equally to this work.
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ACKNOWLEDGMENTS
■
This work was supported by an Early Career Faculty grant
(Grant 80NSSC18K1504) from NASA’s Space Technology
Research Grants Program and The Ohio State University
Materials Research Seed Grant Program, funded by the Center
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