Dual Exchange in PCN-333: A Facile Strategy to Chemically Robust Mesoporous Chromium Metal-Organic Framework with Functional Groups

Article abstract of DOI:10.1021/jacs.5b07373

A facile preparation of a mesoporous Cr-MOF, PCN-333(Cr) with functional group, has been demonstrated through a dual exchange strategy, involving a sequential ligand exchange and metal metathesis process. After optimization of the exchange system, the functionalized PCN-333(Cr), N₃-PCN-333(Cr) shows well maintained crystallinity, porosity, as well as much improved chemical stability. Because of the exceptionally large pores (～5.5 nm) in PCN-333(Cr), a secondary functional moiety, Zn-TEPP with a size of 18 ? × 18 ?, has been successfully clicked into the framework. In this article, we have also analyzed kinetics and thermodynamics during dual exchange process, showing our attempts to interpret the exchange event in the PCN-333. Our findings not only provide a highly stable mesoporous Cr-MOF platform for expanding MOF-based applications, but also suggest a route to functionalized Cr-MOF which may have not been achievable through conventional approaches.

Full text of DOI:10.1021/jacs.5b07373

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Article
Dual Exchange in PCN-333: A Facile Strategy to Chemically Robust
Mesoporous Chromium Metal–Organic Framework with Functional Groups
Jihye Park, Dawei Feng, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07373 • Publication Date (Web): 28 Aug 2015
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Journal of the American Chemical Society
Dual Exchange in PCN-333: A Facile Strategy to Chemically
Robust Mesoporous Chromium Metal–Organic Framework
with Functional Groups
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Jihye Park, Dawei Feng, and Hong-Cai Zhou*
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Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, USA
Supporting Information Placeholder
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ABSTRACT: A facile preparation of a mesoporous Cr-MOF, PCN-333(Cr) with functional group, has been demonstrated
through a dual exchange strategy, involving a sequential ligand exchange and metal metathesis process. After optimiza-
tion of the exchange system, the functionalized PCN-333(Cr), N3-PCN-333(Cr) shows well maintained crystallinity, porosi-
ty, as well as much improved chemical stability. Due to the exceptionally large pores (~5.5 nm) in PCN-333(Cr), a second-
ary functional moiety, Zn-TEPP with a size of 18 Å × 18 Å, has been successfully clicked into the framework. In this article,
we have also analyzed kinetics and thermodynamics during dual exchange process, showing our attempts to interpret the
exchange event in the PCN-333. Our findings not only provide a highly stable mesoporous Cr-MOF platform for expand-
ing MOF-based applications, but also suggest a route to functionalized Cr-MOF which may have not been achievable
through conventional approaches.
most reported mesoporous MOFs suffer from weak chemical
INTRODUCTION
As an emerging class of inorganic–organic hybrid materials,
stability, mainly arising from the labile M–L bond in those
frameworks.8 Also, reticular chemistry which usually involves
metal–organic frameworks (MOFs) have gained growing
attention as advanced porous materials.1 The unique combi-
the extension of linkers to obtain larger pores tends to weak-
en the stability of frameworks.9 Therefore, extremely inert
nations of organic/inorganic building blocks which compose
M–L bond is highly encouraged to increase the stability of
these materials allow diverse functions leading to various
applications.2 Among many features of MOFs, mesoporosity
MOFs with mesopores.
is of great interest due to the prospect of hosting large guest
Based on literature reporting, Cr(III) based clusters might
molecules such as organometallic complexes, nanoparticles,
be strong candidates to serve as an inorganic node to support
and enzymes, creating diverse applications with MOFs.3
mesoporous MOFs as compared to other trivalent metal spe-
Aside from such encapsulation within the large pores, mo-
cies due to the kinetic inertness of Cr(III).7a,10 However, such
lecular level design of organic ligands provides tailor-made
inert Cr–carboxylate bonding also impedes the direct synthe-
structures and functionalities for targeted applications.4
sis of chromium MOFs (Cr-MOFs), resulting in few Cr-MOF
examples.7a,10a,10c,10d Although postsynthetic metal metathesis
It would be ideal if a mesoporous MOF could be directly
has led to obtaining MOFs which cannot be synthesized di-
synthesized from the ligand with a predesigned functionality.
rectly,11 there is only one successful example where almost
It has been shown, however, that in many cases this does not
complete exchange, via a microporous Cr(II)-MOF interme-
lead to a desired product due to the changes in molecular
diate, has yielded a Cr(III) MOF.12 Moreover, the successful
geometry of the ligand upon addition of functional group.5
synthesis of Cr-MOFs typically involves water as a solvent,
As a classic tactic for the aforementioned difficulty, postsyn-
where solubility of larger organic ligands required to achieve
thetic exchange has greatly promoted the functionalization
larger pores can be rather poor.7a,10 In particular, mesoporous
of MOFs.5b,6 However, most parent MOFs require that
structures are commonly limited to specific topologies, re-
postsynthetic processes occur under mild conditions to
quiring a subtle control of synthesis to have the expected
maintain their integrity. Despite the advances of postsyn-
configuration of both inorganic and organic building
thetic exchange and solvent assisted linker exchange, intro-
blocks,10c,13 which often limits the variation of synthetic con-
ditions in Cr-MOF system.
duction of various kinds of reactive functional groups is still
challenging. Also, such methods usually involve labile metal–
ligand (M–L) bonds in the MOF, suggesting a latent instabil-
In addition to the synthetic difficulties, the functionaliza-
ity of the parent material. Therefore, the resulting product
tion of Cr-MOFs has been a great challenge. For instance,
with postsynthetically inserted functional ligand still needs
functionalization through the ligand design in one of the
to overcome the stability issue.
most famous mesoporous MOFs, MIL-101(Cr) (MIL stands for
Matérial Institut Lavoisier), has been limited to a small sub-
With growing attention, MOFs have been utilized in various
set of functional groups, such as –Br, –NO2, and –SO3H,
applications, including catalysis, sensing, biomedical imaging,
as well as the areas of gas storage and separation.2a-e,2g-j As
while relatively practical functional groups in organic reac-
tions (e.g., –OH, –NH2) on the ligands often undergo decom-
MOF applications have gained broader impacts in materials
science, it gradually requires even more sophisticated design
of materials encompassing their compatible stability in tar-
position under the hydrothermal condition (~200 °C) re-
quired for synthesis.14 Although postsynthetic modification
could somewhat alleviate such difficulties, the conflict be-
geted applications (i.e., aqueous stability for biomedical ap-
tween the harsh reaction conditions and inherent stability of
plications), as well as pore size and shape. As a result, de-
the parent MOF still hampers the diversification of function-
mands for framework robustness have increased in pursuit of
applicability of MOF in more complex systems.7 However,
alization. In comparison, postsynthetic ligand exchange may
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be more suitable to diversify the library of functionalized Cr-
reactive functional groups were introduced into PCN-333 to
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MOFs. However, the inertness of Cr-MOF also impedes effi-
cient postsynthetic ligand exchange processes.6g,6x
covalently anchor the guest molecules. Despite the successful
introduction of various functional groups into PCN-333,
however, its innate stability at the expense of the extended
ligand to achieve larger pore still remains a challenge for its
utilization in harsh chemical conditions for MOFs (e.g., wa-
ter) due to the relative lability of the Fe(III)– or Sc(III)–
carboxylate bonds in aqueous environment. Having reviewed
the reported stability of Cr-MOFs, we conceived that PCN-
333(Cr) with covalent anchors might serve as a better plat-
form for the utilizations in harsh environment. However, a
direct synthesis of PCN-333(Cr) is not trivial due to the con-
flict between the hydrothermal synthetic condition required
for the synthesis of Cr-MOFs and the poor solubility of TATB
in water. Moreover, even if PCN-333(Cr) was obtained, it
would be challenging to introduce functional group into the
framework through the postsynthetic ligand exchange, be-
cause the kinetic inertness of Cr(III) prevents an effective
ligand exchange process. Therefore, a stepwise exchange was
considered to obtain functionalized PCN-333(Cr).
Herein we present a stepwise exchange strategy of both
ligands and metals, namely dual exchange in PCN-333(Fe) for
the preparation of functionalized PCN-333(Cr) (PCN stands
for Porous Coordination Network). After the dual exchange
process, functionalized PCN-333(Cr) shows well preserved
crystallinity, porosity, as well as enhanced chemical stability.
Understanding of chemical dynamics along with the dual
exchange in PCN-333(Fe) may allow for a generalized route
to functionalized Cr-MOFs, which will exhibit a maintained
structural integrity of parent MOF with desired functional
groups and enhanced stability. Furthermore, having studied
the incorporation of various functional groups in PCN-
333(Fe), dual exchange shows a great potential to employ
many different functional groups in the highly stable Cr-
MOF platform.
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RESULTS AND DISCUSSION
2. Optimization for Metal Metathesis in PCN-333.
Having determined that a functionalizable PCN-333(Cr)
might serve as a useful platform by providing exceptional
stability and diverse functionality with large room for chemi-
cal reactions within the pores, we designed a procedure to
achieve the targeted product, namely dual exchange. Possible
routes to functionalized PCN-333(Cr) via dual exchange were
conceived as two main schemes as shown in Scheme 1: (1)
Metal exchange followed by ligand exchange and (2) Ligand
exchange followed by metal exchange.
Prior to examining a better strategy to obtain functionalized
PCN-333(Cr), PCN-333(Sc) and PCN-333(Fe) were tested for a
Cr(III) metathesis to choose the most suitable base MOF for
successful dual exchange. PCN-333 was synthesized as previ-
ously reported. Because Cr(III) salts often yield insoluble
formates due to DMF decomposition with long reaction time
at high temperature, the reaction time was limited to pre-
serve crystallinity and porosity while maintaining a high
metal exchange ratio. After optimization for the reaction
time, the following Cr(III) metathesis in PCN-333(M) was
performed for 1 h by replacement of CrCl3∙6H2O stock solu-
tion in DMF after the first 30 min of reaction.
Figure 1. (a) Ligands in MIL-100 (BTC) and PCN-333
(TATB). (b) The largest cage in PCN-333. (c) Supertetrahe-
dron in PCN-333.
1. PCN-333: An Ideal Scaffold for Functionalizable Cr-
MOF with Mesoporosity.
Our group recently reported a mesoporous MOF, PCN-333,15
constructed from trivalent metal ions and isoreticular struc-
ture to MIL-100 (Supporting Information Section 3).7a Due to
the larger size of composing ligand 4,4′,4″-s-triazine-
2,4,6-triyl-tribenzoate (TATB), extended version of benzene-
1,3,5-tricarboxylate (BTC), PCN-333 exhibits larger pores (~5.5
nm) than that of MIL-100 (~2.9 nm), which allow the incor-
poration larger guest species as well as faster diffusion and
provide sufficient room for chemical reactions in the meso-
pores (Figure 1). Having utilized the structural support of
PCN-333, we recently studied a facile route to functionalize
PCN-333(M) (M = Fe(III), Sc(III)) via postsynthetic ligand
exchange.16 As a result of the ligand exchange, a variety of
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tios than those at lower temperature (64.8% for PCN-
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333(Sc/Cr) and 99.8% for PCN-333(Fe/Cr)). In particular,
Cr(III) metathesis from PCN-333(Fe) showed near complete
exchange along with well-maintained crystallinity and poros-
ity, which were confirmed by PXRD (Figure 2c) and N2 sorp-
tion at 77 K, respectively (Figure 3). On the other hand, the
diffraction of PCN-333(Sc) almost completely disappeared
after the Cr(III) metathesis. It is reasoned that these results
are due to the discrepancy between reaction rates of dissocia-
tion of leaving metal species in the framework and incoming
metal species from the solution. For instance, in our previous
study of ligand exchange in PCN-333, the successful ligand
exchange was carried out at low temperature (85 °C), which
suggests it gives enough energy to overcome the energy bar-
rier of Sc(III)–carboxylate or Fe(III)–carboxylate bond disso-
ciations within the framework. Closely looking at the metal
metathesis process, the M–L bond dissociation is required for
both incoming metal complex and metals in the parent MOF
to accomplish the overall exchange process. Even though the
temperature is sufficient enough for the ligand dissociation
in the framework, much slower ligand dissociation in the
incoming Cr(III) complex can still be a rate limiting step.
Therefore, the Cr(III) metathesis at low temperature was not
successful. On the other hand, when the given energy is
enough for breaking Cr(III)–X (i.e., Cl-) bond from Cr(III)
salts and following association with the open carboxylates in
the framework after Fe(III) dissociation for successful ex-
change at high temperature, Cr(III) exchange reached to
higher exchange ratio in both cases (Figure 2a). When it
comes to the retention of framework structure, however, the
metathesis in PCN-333(Fe) was superior to PCN-333(Sc), pre-
sumably due to the relative robustness of PCN-333(Fe) result-
ing from the slower Fe(III)–carboxylate dissociation rate than
that of PCN-333(Sc) allowing compatible exchange environ-
ment with Cr(III) association. Particularly, in the metal me-
tathesis of labile PCN-333(Sc) platform, the framework would
undergo significant destruction by free Cr(III) and decom-
posed reactive species from DMF before obvious metal me-
tathesis happened.12 After discovering the previous result,
PCN-333(Fe) was chosen as a platform for studying the dual
exchange.
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3. Analysis of Dual Exchange.
Figure 2. (a) Exchanged Cr(III) ratios in PCN-333(Sc) (left)
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Following up on our previous study, exploring the chemical
dynamics of ligand exchange in PCN-333,16 we further sought
to expand our observations and rationalizations focused on
changes of entropy and enthalpy during the dual exchange.
As was discussed in the previous study, the entropy change
of the system (∆ꢀꢁꢂꢁ), comprising a solid-state MOF and ex-
changing solution during the ligand exchange, was assumed
to be largely dependent on ∆ꢀ in solution, as ∆ꢀ in the solid
state MOF may be negligible (Supporting Information Sec-
and PCN-333(Fe) (right) after metal metathesis at different
temperatures for 1 h. (b-c) Photographs and PXRD patterns
of PCN-333(Sc) (b) and PCN-333(Fe) (c) after metal metathe-
sis at different temperatures for 1 h.
To investigate an optimal exchange reaction temperature,
firstly, PCN-333(Sc) and PCN-333(Fe) were incubated for 1 h
(by changing stock solution after first 30 min) in Cr(III) stock
solution at 85 °C and 150 °C in pre-heated ovens. Interesting-
ly, both PCN-333(Sc) and PCN-333(Fe) showed no apparent
color changes upon metathesis at 85 °C (Figures 2b,c). Ener-
gy-dispersive X-ray (EDX) spectra revealed low exchange
ratios of both samples (37.0% for PCN-333(Sc/Cr) and 23.0%
for PCN-333(Fe/Cr)), and especially poor powder X-ray dif-
fraction (PXRD) patterns of PCN-333(Sc) (Figure 2). Notably,
PCN-333(Sc) showed slightly higher Cr(III) exchange ratio
than that of PCN-333(Fe) at 85 °C, while showing no PXRD
pattern, which suggests its lability. Whereas at 150 °C, both
samples generally showed much higher Cr(III) exchange ra-
tion 9). Thus the ligand exchange process in PCN-333 was
ꢄꢅ
driven by a drastic entropy increase ꢃ∆ꢀ ꢆ 0) upon the
ꢁꢂꢁ
provision of excess amount of the exchanging ligands (Figure
S11). Similarly, the entropy change of the system during the
ꢇꢇ
metal metathesis ꢃ∆ꢀ ꢈ follows the same concept; and thus
ꢁꢂꢁ
ꢇꢇ
it yields a positive ∆ꢀ
due to the excess input of Cr(III) in
ꢁꢂꢁ
solution to replace Fe(III) in the framework. In ꢀꢁꢂꢁ stand-
point, therefore, both ligand exchange and metal metathesis
should have entropy contributions for the desired exchanges
from the excess of exchanging entries.
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Figure 4. (a) Entropy change of system during the metal
metathesis. (b) Changes in enthalpy and (c) Gibbs free ener-
gy of system during the metal metathesis in PCN-333.
Understanding of ∆ꢉꢁꢂꢁ during metal metathesis can be in-
terpreted by focusing on electrostatic interaction and orbital
interaction upon the exchange event. Firstly, the Cr(III) has a
smaller ionic radius than that of high spin Fe(III), leading to
a larger Z/r value.17 Considering carboxylate (L) is also a
harder Lewis base compared to ligands, X, when Cr(III) co-
ordinates to the carboxylates in the framework, the electro-
static interaction becomes stronger, providing larger enthal-
py change upon metathesis compared to Fe(III).18
Aside from the electrostatic interaction, the orbital interac-
tion also changes for each metal species along with the me-
tathesis. To compare the changes in orbital interaction,
which contributes to ∆ꢉꢁꢂꢁ, ligand field theory (LFT) was
applied to simplify the given scenario. Although the actual
coordination environment for the M(III) (M = Cr or Fe) in
the framework gives rise to a C4v symmetry due to the two
different axial ligands and that for MX6 species in the solu-
tion may even vary, herein we assumed Oh symmetry for
both cases to simplify the analysis of the contributions from
Figure 3 (a) Photographs of PCN-333(Fe) and PCN-333(Cr)
exchanged at 150 °C. (b) N2 sorption isotherm of PCN-333(Fe)
and PCN-333(Cr) at 77 K.
When it comes to the enthalpy analysis of the ligand ex-
change process in PCN-333, as the degree of exchange in-
creases the enthalpy of the system (Hsys) also increases (ΔHsys
ꢆ 0) due to the insertion of BTB (1,3,5-benzenetribenzoate)
derivatives with unfavored conformation in the framework
(Figure S10). For this reason, although the large ∆ꢀꢁꢂꢁ at the
early stage of ligand exchange could drive the exchange reac-
tion forward, the system establishes an equilibrium with a
certain ratio of ligand exchange (Supporting Information
Section 9).
the coordinating ligands to ∆ꢉꢁꢂꢁ
.
However, metal metathesis between Fe(III) and Cr(III) ac-
companies the changes in M–L bond nature, comprising the
framework, whereas the bond nature in the ligand exchange
remains the same as Fe(III)–L bond (L represents carbox-
ylate) in the framework. Meanwhile, metal metathesis also
involves changes of enthalpy in solution. For example, Fe(III)
ions, coming out from the framework replace Cr(III) in the
provided CrX6 complex (X= Cl-, DMF), resulting in FeX6 spe-
cies in solution. Consequently, there are two processes in-
volving enthalpy changes of the system: (1) ΔHFe, correspond-
ing to the enthalpy change of Fe(III) from the MOF to the
solution; (2) ΔHCr corresponding to the enthalpy change of
Cr(III) species from solution to the MOF. Thus, the enthalpy
Figure 5. Analysis of the orbital interaction contribution to
ΔHsys using ligand field theory with a simplified coordination
environment (Oh) for the metal metathesis process in PCN-
333.
change of the system can be represented as: ΔHsys = ΔHFe
ΔHCr.
+
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Scheme 1. The energy diagrams of two possible dual exchange routes to functionalized PCN-333(Cr).
os, suggesting indeed Cr(III)–X bond has to be sufficiently
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Considering ligands X in solution are relatively weaker field
ligands than carboxylate (L),19 and the metal-carboxylate (M–
L) bonds are dominant in the framework, the changes in the
coordinating ligands upon metathesis will have impacts on
∆ꢉꢁꢂꢁ. For example, when Cr(III) is exchanged from solution
into the framework, the d orbital splitting energy changes
from ∆ꢊꢋ to ∆ꢌꢊ (Figure 5). As Cr(III) has d3 configuration,
such coordination environment change gives out a net ligand
field stabilization energy (LFSE) of ꢍ1.2ꢃ∆ꢌꢊꢍ∆ꢊꢎꢈ. Therefore,
from the Cr(III) standpoint, as ∆ꢄꢊ is larger, the system gets
more stabilized. On the other hand, because Fe(III) adopts
high spin d5 configuration with weak field ligands, the LFSE
is canceled out from its two electrons in eg orbitals, resulting
in no stabilization impacts from the exchange event. Overall,
the system gets more stabilized after metal metathesis from
the orbital interaction aspect. In short, while both electro-
static interaction and orbital interaction go through the
changes during the metal metathesis, the absolute value of
ΔHCr is more likely to be larger than that of ΔHFe, making the
ΔHsys negative and the overall metal metathesis from PCN-
333(Fe) to PCN-333(Cr) an exothermic process. Thus, the
metal metathesis from PCN-333(Fe) to PCN-333(Cr) can be
considered as a both entropically and enthalpically favored
process.
activated by high temperature. Such high temperature not
only provided sufficient energy for Cr(III)–X bond dissocia-
tion, but also allowed a short reaction time, avoiding a long
exposure of MOF in the reactive environment which further
guarantees the framework intactness. Moreover, owing to the
high reaction temperature, the entropy contribution to ΔGsys
is magnified from the TΔS term, which pushes the exchange
more forward. In particular, our optimized metathesis in-
volves the exchange of fresh the CrCl3∙6H2O solution after
first 30 min of exchange; and thus this can remove the Fe(III)
in the solution to facilitate an entropy driven process, further
driving the reaction forward to the almost fully exchanged
PCN-333(Cr).
4. Functionalization of PCN-333(Cr) through Dual Ex-
change.
Considering the previous results, schematic energy diagrams
representing possible dual exchange pathways are summa-
rized in Scheme 1. Pathway 1 (path 1) shows a case where the
dual exchange to get the functionalized PCN-333(Cr) in an
order of ligand exchange followed by metal metathesis. As
shown in path 1, each step involves kinetically and thermo-
dynamically favored process as analyzed the given tempera-
ture was sufficient to drive the reactions forward. From the
framework standpoint, path 1 involves a dissociation of Fe-
carboxylate bonds at each step, which takes less energy than
breaking Cr–carboxylate bond.
According the findings in the previous section, the metal
metathesis is optimized at high temperatures (150 °C). Inter-
estingly, we found the metatheses of from Sc(III) to Cr(III)
and Fe(III) to Cr(III) at 85 °C showed very low exchange rati-
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Figure 6. (a) N3-BTB used for ligand exchange and photographs of N3-PCN-333(Fe), N3-PCN-333(Cr), and N3-PCN-333(Cr) after
click reaction. (b) N2 sorption isotherm of N3-PCN-333(Fe), N3-PCN-333(Cr), and N3-PCN-333(Cr) after click reaction. (c) PXRD
patterns of N3-PCN-333(Fe) and (d) N3-PCN-333(Cr) after different treatments.
On the other hand, path 2, in which the functionalization
(ligand exchange) is done after Cr-MOF formation (metal
metathesis), requires a large amount of energy to overcome
the barrier of Cr–carboxylate bond dissociation for the ligand
exchange step. Therefore, to obtain functionalized PCN-
333(Cr), thermodynamically favored path 1 was chosen for
the double exchange in the following experiments. However,
to confirm our hypothesis, path 2 was also examined for its
feasibility for dual exchange. Having prepared PCN-333(Cr)
by the optimized condition as previously described in section
2, firstly a ligand exchange in PCN-333(Cr) was tested via
path 2 as shown in Scheme 1. For an incoming ligand, one of
BTB derivatives having an azide group (N3-BTB; shown in
Figure 6a) was chosen as a model inserting ligand because of
its versatile utilities in many applications and allowance of
click reaction after successful implantation of functional
group, so as to confirm the feasibility of a secondary chemi-
cal reaction onto dual exchanged PCN-333(Cr). The ligand
exchange followed the optimized condition (85 °C, 12 h) from
our previous study wherein the exchange condition guaran-
tees the crystallinity and porosity.16 As a result of dual ex-
change following the path 2, though crystallinity of PCN-
333(Cr) was retained after going through the ligand ex-
the ligand exchange from TATB to N3-BTB in PCN-333(Fe)
was performed as previously stated. 1H NMR study confirmed
that the ligand exchange ratio in PCN-333(Fe) showed a simi-
lar N3-BTB ratio to the previous report (~30%) (Figure S2).
Also, the N3-BTB inserted PCN-333(Fe) (N3-PCN-333(Fe))
showed well retained crystallinity and porosity (Figures 6
and S9). In addition, a characteristic stretching band of the
azide group at 2106 cm-1 was observed from infrared (IR)
spectrum of N3-PCN-333(Fe) (Figure S7). As a second step of
dual exchange in path 1, the previously optimized condition
for metal metathesis was then applied to obtain the azide
functionalized PCN-333(Cr) (N3-PCN-333(Cr)). After 1 h of
Cr(III) metathesis, the dual exchanged sample was prepared
accompanying with an obvious color change from bright
yellow to green (Figure 6a). EDX results revealed that ~97%
of Fe(III) in N3-PCN-333(Fe) was exchanged to Cr(III) with-
out compromising its crystallinity (Figure 6 and Supporting
1
Information Section 8). Additionally, H NMR analysis of
supernatant of Cr(III) exchange reaction showed no detecta-
ble TATB or N3-BTB ligands leaching out from the parent
MOF, which further suggests there is minimal destruction of
framework as supported by PXRD.As shown in Figure 6b, the
porosity of N3-PCN-333(Cr) was almost perfectly retained
compared to its starting material, N3-PCN-333(Fe). According
to DFT pore size distribution, a reduced pore size of the
smallest pore in both N3-PCN-333(Fe) and N3-PCN-333(Cr)
was observed (Figure S4). This suggests the presence of func-
tional group on the N3-BTB affected the smallest cage most.
However, as we analyzed in the previous study, the middle-
1
change, H NMR analysis of supernatant of reaction revealed
that no TATB released from PCN-333(Cr) under the given
condition, as expected (Figure S1).
Having demonstrated path 2 is not suitable to realize dual
exchange on the PCN-333(Fe) platform, we then sought to
optimize path 1 to obtain the dual exchanged product. Again
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sized cages (~4.5 nm) and the largest cages (~5.5 nm) re-
ure 7b. The color of sample changed from green to dark pur-
1
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mained less affected due to their extralarge size. Thus, dual
exchanged PCN-333(Cr) also showed well preserved charac-
teristics of mesoporosity after going through a sequential
exchange of ligands and metals. IR spectrum of N3-PCN-
333(Cr) clearly showed the azide stretching band, which fur-
ther confirms the undamaged functional group after the dual
exchange process (Figure S8). In addition, after the metal
metathesis from Fe(III) to inert Cr(III) of azide functional-
ized PCN-333, significantly enhanced stability than that of
the parent material was observed. For instance, PXRD pat-
terns showed that crystallinity of N3-PCN-333(Cr) remains
intact after submersion in deionized water for 7 days whereas
that of N3-PCN-333(Fe) showed gradually decreasing crystal-
linity upon the identical treatment. Moreover, N3-PCN-
333(Cr) showed well maintained crystallinity in harsh condi-
tions, such as in 1 M HCl and 10 mM NaOH after treatment
for 24 h, while N3-PCN-333(Fe) was dissolved under the same
conditions (Figures 6c,d and Figure S12). These findings sug-
gest that dual exchange can be used as a means to prepare
functionalized Cr-MOF. Expectedly, our results show suc-
cessful dual exchange process via path 1, but not via path 2.
This may seem different from a work by Cohen and cowork-
ers that has studied a similar concept, called tandem ex-
change,6j which is the only example involving both ligand
exchange and metal metathesis within one framework yet to
our best knowledge. Tandem exchange allowed for the prep-
aration of microporous heterometallic zeolitic imidazolate
frameworks (ZIFs), where two routes (e.g., different sequenc-
es of exchanges similar to Scheme 1) showed very similar
results out of any pathway because of the similar lability of
Zn–imidazolate and Mn–imidazolate bonds. Nonetheless,
considering the different M–L bond natures and structures of
each MOF, our rationalizations still validate the unfeasibility
of dual exchange via path 2, while supporting path 1 in the
current work and the results of Cohen’s work. As we have
demonstrated the successful ligand exchange in PCN-333(Fe)
with a variety of BTB derivatives, other functional groups
could also be introduced into PCN-333(Cr). Therefore, dual
exchange strategy not only provides a facile route to obtain
extremely robust Cr-MOFs, but also allows a generalized
functionalization method of highly stable Cr-MOFs, which
can vastly expand the scope of MOF applications.
ple upon the anchorage of Zn-TEPP in PCN-333(Cr), indicat-
ing successful incorporation of the strongly colored porphy-
rin (Figure 6a). In addition, the PXRD pattern of the clicked
MOF also showed unaltered diffraction peaks, suggesting the
robustness of PCN-333(Cr) scaffold (Figure S9). In parallel,
porosity of N3-PCN-333(Cr) was also examined. Figure 6b
showed reduced N2 uptake, which distinctly indicates the
inserted TEPP does take up the corresponding pore volume
in the MOF. Nevertheless, even after the incorporation of
such large entity, the material still retains sufficient porosity
(pore volume 1.60 cm3/g) which may allow a volumetric ca-
pacity for further generations of guest molecules inclusion in
PCN-333(Cr). As the size of TEPP is larger than the smallest
cages (11 Å) in PCN-333, but much smaller than the middle
sized cages (45 Å) and the largest cages (55 Å), theoretically
Zn-TEPP can only occupy the middle sized cages and the
largest cages. Expectedly, the pore size distribution showed
the decreased size of middle cages and largest cages, as well
as significantly decreased pore volume of those two cages
while the smallest cage is not much affected after N3-BTB
insertion, which matches well with our size analysis (Figure
S5). Next, we further sought to visualize positions of Zn-
TEPP in the PCN-333(Cr). As shown in Figure 7c, SEM/EDX
mapping of this sample shows the inserted Zn-TEPP mole-
cules via click reaction are well distributed throughout the
crystals, indicating the exchanged ligands are well dispersed
and the click reaction occurred evenly in the MOF through
the efficient diffusion utilizing its large pores. Considering an
ideal atomic ratio between M(III) and ligands in PCN-333(M)
is 3:2 by structure, the ratio of ~5:1 between Cr(III) and N3-
BTB is expected in the framework, based on the ~30% of lig-
and exchange. In accordance with this analysis, the EDX re-
sults of clicked-PCN-333(Cr) matched well (Cr:Zn = 4.86:1),
which further supports most azide group in the MOF reacted
with the large guest molecule, TEPP, as well as successful
incorporation of N3-BTB (Table S1 in Supporting Infor-
mation).
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5. Click Reaction in N3-PCN-333(Cr).
Having dual exchanged MOF prepared, a click reaction was
performed on N3-PCN-333(Cr) to demonstrate the introduc-
tion of large entity. To illustrate our concept, meso-tetra(4-
ethylphenyl)porphyrin (TEPP) was synthesized due to its
two-dimensional large size (18 Å × 18 Å), which will demon-
strate the feasibility of covalent anchorage of a large guest
molecule in the dual exchanged platform. The center of
TEPP was metallated with Zn(II) aiming to achieve infor-
mation about its spatial distribution by scanning electron
microscopy/energy-dispersive
X-ray
spectroscopy
(SEM/EDX). With this in mind, the click reaction between
the N3-PCN-333(Cr) and Zn-TEPP was performed in the pres-
ence of CuI in DMF (60 °C, 28 h) with stirring. The comple-
tion of reaction was determined by the disappearance of the
azide band at 2106 cm-1 in the IR spectrum as shown in Fig-
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ments. Sample was activated by solvent exchange (in several
1
2
3
cycles using fresh acetone and hexanes), followed by degas-
sing at elevated temperature (150 °C) for 2 h. Details are pro-
vided in the Supporting Information.
4
5
6
7
8
9
Synthesis of PCN-333(Fe). H3TATB (60 mg), anhydrous
FeCl3 (60 mg), and trifluoroacetic acid (0.6 mL) were dis-
solved in 10 mL DEF. The mixture was heated in 150 °C oven
for 12 h until a brown precipitate formed. The resulting
brown precipitate was centrifuged and washed with fresh
DMF five times.
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Synthesis of PCN-333(Sc). H3TATB (80 mg) and
ScCl3∙6H2O (200 mg) were dissolved in 10 mL DMF. The mix-
ture was heated in 150 °C oven for 2 h until a white precipi-
tate formed. The resulting white precipitate was centrifuged
and washed with fresh DMF five times.
General procedure for ligand exchange of PCN-333(Fe).
First, as-synthesized PCN-333(Fe) (ca. 50 mg) was thoroughly
washed with hot DMF and the isolated sample was then in-
cubated with a stock solution of N3-BTB (50 mg in 10 mL of
DMF) at 85 °C for 12 h.
General procedure for the preparation of PCN-333(Cr):
Approximately 50 mg of as synthesized PCN-333(M) sample
is initially immersed in the CrCl3∙6H2O stock solution (200
mg in 10 mL of DMF) in 20 mL vial in a 150 °C oil bath. After
30 min of first incubation, the solution was decanted after
centrifugation and additional 30 min of reaction was per-
formed with new CrCl3∙6H2O stock solution. Resulting prod-
uct was then thoroughly washed with DMF five times. (Note:
To preserve the morphology of the exchanged product, the
reaction was not stirred, but refluxed. However, we observed
the exchange underwent slightly faster with stirring to yield
similar exchanged metal ratio. In addition, according to our
control experiment (pure CrCl3∙6H2O in DMF), when heating
at 150 °C is longer than 3 h, green precipitates gradually
formed. Therefore, it is suggested not to leave the exchange
reaction in mother solution too long at high temperature, to
prevent possible formation of insoluble Cr formates or other
unknown, which are likely to affect porosity and crystallinity
of the product.)
Figure 7. (a) TEPP used for the click reaction in N3-PCN-
333(Cr). (b) IR spectra of N3-PCN-333(Cr) before and after the
click reaction. (c) SEM-EDX mapping of N3-PCN-333(Cr)
after the click reaction.
Table 1. Summary of dual exchange
Pore
volume
(cm3/g)
N2 uptake
(cm3/g)a
Exchanged
Cr (%)b
Entry
PCN-333(Fe)
PCN-333(Cr)
1985
1838
1663
1707
3.07
2.84
2.57
2.64
n/a
~99
n/a
~97
N3-PCN-333(Fe)
N3-PCN-333(Cr)
Clicked-PCN-
333(Cr)
1033
1.60
n/a
General procedure for metal metathesis of N3-PCN-
333(Fe). Approximately 10 mg of N3-PCN-333(Fe) samples are
initially immersed in the CrCl3∙6H2O stock solution (40 mg
in 2 mL of DMF) at 150 °C oven. After 30 min of first incuba-
tion, the solution was decanted after centrifugation and addi-
tional 30 min of reaction was performed with new
CrCl3∙6H2O stock solution. Resulting product was then thor-
oughly washed with DMF five times.
aN2 sorption was measured at 77 K. bBased on EDX results.
EXPERIMENTAL SECTION
Instrumentation. Nuclear magnetic resonance (NMR)
spectra were recorded on Varian Inova 500 spectrometer
unless otherwise noted. Powder X-ray diffraction (PXRD)
was carried out on a Bruker D8-Focus Bragg-Brentano X-ray
powder diffractometer equipped with a Cu sealed tube (λ =
1.54178) at 40 kV and 40 mA. Fourier transform infrared (FT-
IR) measurements were performed on a Shimadzu IR Affini-
ty-1 spectrometer. Scanning electron microscope (SEM) was
performed on QUANTA 450 FEG and energy dispersive X-ray
spectroscopy (EDS) was carried out by X-Max20 with Oxford
EDS system equipped with X-ray mapping. N2 adsorption-
desorption isotherms at 77 K were measured by using a Mi-
MOF digestion. Approximately 10 mg of sample was di-
gested with 37% HCl, refluxed overnight, and washed with
water until a neutral pH was reached. DMSO-d6 (0.5 mL) was
1
added to dissolve the ligands. The H NMR spectrum (500
MHz) was collected at room temperature (~ 21 °C).
Supernatant analysis. Upon completion of treatment, the
supernatant was removed to examine TATB or N3-BTB re-
leased from the parent MOF. The aliquot of supernatant was
filtered through a syringe filter to exclude the possible pres-
ence of MOF crystals remaining and the ligands in superna-
tant were recovered by acidification with few drops of 1 M
crometritics ASAP 2420 system.
A high-purity grade
(99.999%) of gas was used throughout the sorption experi-
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HCl, followed by washing with water. The resulting precipi-
Prof. Dr. Lixian Sun from Guilin University of Electronic
Technology and Ms. Ying-Pin Chen for the experimental
helps, and also Mr. Zachary Perry for helpful discussion.
tates were dried and analyzed by 1H NMR spectroscopy.
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4
5
6
7
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Click reaction. TEPP (50 mg) was added to a mixture of
N3-PCN-333(Cr) (30 mg) and CuI (5 mg) in DMF (7 mL) in a
round bottom flask. The reaction mixture was stirred at 60
°C for 28 h. The resulting precipitate was collected by cen-
trifugation, washed thoroughly with DMF followed by ace-
tone and hexanes, and dried to afford a dark purple solid in
quantitative yield.
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CONCLUSION
In summary, a dual mode of postsynthetic exchange (dual
exchange), involving a sequential exchange of ligands and
metals in PCN-333(Fe) has been studied for the preparation
of functionalized PCN-333(Cr). These results illustrate a pos-
sible step forward for the functionalization of mesoporous
Cr-MOFs that allow covalent anchors for guest molecules,
sufficient room (extralarge pore) for the introduction of sec-
ondary functionality, and exceptional stability from inert
Cr(III). In addition, our rational design and analyses of dual
exchange process give a comprehensive understanding of
chemical dynamics in PCN-333 during dual exchange, follow-
ing up our previous study of ligand exchange in this platform.
The dual exchanged Cr-MOF, N3-PCN-333(Cr) showed main-
tained integrity of the parent MOF, functionality, and en-
hanced chemical stability. Meanwhile, dual exchange strate-
gy may allow incorporation other wanted functional groups
into PCN-333(Cr) platform, as noted. These findings have
enabled the preparation of functionalized PCN-333(Cr) with
a design flexibility to be utilized as a stable platform for a
variety of desired applications by exploiting facile functional-
ization, enhanced chemical stability, and extremely large
pores which will allow for more possible chemistry with
MOFs.
ASSOCIATED CONTENT
Supporting Information
Full details for sample preparation and characterization re-
sults. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
zhou@mail.chem.tamu.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
The syntheses of MOFs and their characterizations were
supported as part of the Center for Gas Separations Relevant
to Clean Energy Technologies, an Energy Frontier Research
Center funded by the U. S. Department of Energy, Office of
Science, and Office of Basic Energy Sciences under Award
Number DE-SC0001015, and by part of The Welch Founda-
tion under Award Number A-1725. H.-C. Z. was supported by
Texas A&M University. The authors gratefully acknowledge
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