Postsynthetic Oxidation of the Coordination Site in a Heterometallic Metal-Organic Framework: Tuning Catalytic Behaviors

Article abstract of DOI:10.1021/acs.chemmater.0c01267

Postsynthetic modification (PSM) in metal-organic frameworks (MOFs) can introduce multiple functionalities and alter the structural function for the desired application. However, the PSM of the coordination site faces the challenges of structural collapse or incompatibility between the original metal site and the newly formed coordination group. Herein, we developed a novel concept of introducing primary and secondary nodes, coexisting in a water-stable, Zr-based heterometallic MOF, [Zr6(μ3-OH)8(OH)8][Cu4I(L1)4]2 (1-SH-a, H2L1 = 6-mercaptopyridine-3-carboxylic acid). The postsynthetic oxidation at the coordination site was successfully achieved in the secondary nodes [Cu4I(L1)4]4-, while the robust primary nodes [Zr6(μ3-OH)8(OH)8]8+ stabilized the whole framework to form [Zr6(μ3-OH)8(OH)8][(Cu0.44ICu0.56II(OH)0.56)4(L2)4]2 (1-SO3H, H2L2 = 6-sulfonicotinic acid). Attempts to directly synthesize 1-SO3H through the reactions of Zr(IV), Cu(II), and H2L2 failed. Furthermore, the PSM of 1-SH-a to form 1-SO3H was utilized to tune the catalytic behaviors toward the styrene oxide ring-opening reaction to give an improved regioselectivity of the primary alcohol (A) of ～99% compared with that of 1-SH-a (～71%).

Full text of DOI:10.1021/acs.chemmater.0c01267

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Article
Postsynthetic Oxidation of Coordination Site in a Heterometallic
Metal-Organic Framework: Tuning Catalytic Behaviors
Yi Han, Michael A. Sinnwell, Robert G. Surbella III, Wenjuan Xue, Hongliang Huang, Jian
Zheng, Bo Peng, Gaurav Verma, Yang Yang, Lili Liu, Shengqian Ma, and Praveen K. Thallapally
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.0c01267 • Publication Date (Web): 20 May 2020
Downloaded from pubs.acs.org on May 20, 2020
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Chemistry of Materials
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Postsynthetic Oxidation of Coordination Site in a
Heterometallic Metal-Organic Framework: Tuning Catalytic
Behaviors
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Yi Han,*,† Michael A. Sinnwell,‡ Robert G. Surbella III,‡ Wenjuan Xue,§ Hongliang Huang,§ Jian Zheng,‡ Bo
Peng,‡ Gaurav Verma,# Yang Yang,‡ Lili Liu,‡ Shengqian Ma,# Praveen K. Thallapally*,‡
† Key Laboratory of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science
and Technology, Qingdao, Shandong 266042, P. R. China.
‡ Pacific Northwest National Laboratory, Richland, WA 99352, United States.
§ State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, P. R.
China.
# Department of Chemistry, University of South Florida, Tampa, Florida 33620, United States.
ABSTRACT: Postsynthetic modification (PSM) in metal-organic frameworks (MOFs) can introduce multiple functionalities and alter the
structural function for the desired application. However, the PSM of the coordination site faces the challenges of structural collapse or
incompatibility between the original metal site and the newly formed coordination group. Herein, we developed a novel concept of
introducing “primary” and “secondary” nodes, coexisting in a water-stable, Zr-based heterometallic MOF, [Zr6(μ3-
OH)8(OH)8][CuI4(L1)4]2 (1-SH-a, H2L1 = 6-mercaptopyridine-3-carboxylic acid). The post-synthetic oxidation at the coordination site was
successfully achieved in the “secondary” nodes [CuI4(L1)4]4-, while the robust “primary” nodes [Zr6(μ3-OH)8(OH)8]8+ stabilized the
whole framework to form [Zr6(μ3-OH)8(OH)8][(CuI0.44CuII0.56(OH)0.56)4(L2)4]2 (1-SO3H, H2L2 = 6-sulfonicotinic acid). Attempts to directly
synthesize 1-SO3H under the reactions of Zr(IV), Cu(II), and H2L2 failed. Furthermore, PSM of 1-SH-a to form 1-SO3H was utilized to
tune the catalytic behaviors toward the styrene oxide ring-opening reaction to give an improved regioselectivity of primary alcohol (A) of
~99% compared with 1-SH-a (~71%).
PSM of MOFs can be broadly divided into three major
INTRODUCTION
divisions: unique reactions in terms of (i) metal or linker
exchange,5-6
Postsynthetic modification (PSM) of porous materials such as
mesoporous silica and porous carbons has been successfully used
to control and analyze their organization and functionalization.1,2
In the past decades, much interest has been focused on tailoring
metal-organic frameworks (MOFs) using PSM.3-4 PSM has been
extensively used and has become much sought-after compared to
de novo synthesis because PSM offers the ability to: (i) facilitate
the introduction of a wide variety of chemical functionalities, (ii)
maximize the rejection of undesirable byproducts in unpredictable
ways, and (iii) endow functions that are thermally labile, which
are thwarted almost in solvothermal conditions. Accordingly,
MOFs have been vested with significant multi-functionalities, and
thus adapting the modified functions in response to exploration
for the associated applications.
(ii)
covalent
modification,7
and
(iii)
oxidation/reduction.8-10 Most of these modifications are carried
out on the organic components of the framework. Specifically,
linker exchange in readily accessible isoreticular MOF analogues
leads to progressively diverse functionalities and properties.11-15
By contrast, covalent modification allows for chemically
modifying the organic struts to either introduce the functionality
or functionalize the inherent reactive tags16-19 (Scheme 1). On the
other hand, PSM on the coordination site has not yet been
exploited by the MOF community, possibly due to the structural
collapse under the conditions required or incompatibility between
the original metal and the newly formed coordination group.
The coordination ability of thiol and sulfonic groups has been
popularized by their extensive use in materials science.20,21
Generally, thiol preferentially coordinates with soft Lewis acids
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(i.e., Cu(I), Ag(I), Au(III)), while the sulfonic group shows (98+%, Alfa Aesar), isopropanol (99.6%, ARCOS), 1-bromo-
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8
affinity for the harder ones (i.e., Cu(II), Ni(II), Fe(II)). In addition,
the vulnerability of thiol compounds to oxidize to the sulfonate
derivatives assisted by H2O2 is well documented even in porous
supports, such as thiol grafted single-walled carbon nanotubes.22
The unique use in coordination chemistry and ready conversion
toward thiol and sulfonic groups would enable the preparation of
a specific thiolated MOF and permit the formation of the isolated
sulfonate one employing postsynthetic oxidation.
3,5-difluorobenzene (98%, Alfa Aesar), tetrahydrofuran (THF)
(99.9%, Sigma-Aldrich), methanol (95%, Fisher), ethanol (95%,
Fisher). 6-sulfonicotinic acid (H2L2) was synthesized according to
the reported literature.23
Synthesis of 1-SH. A mixture of ZrCl4 (100 mg), CuI (142.5 mg),
and H2L (61.6 mg) in presence of benzoic acid (2 g) dissolved in
N,N-dimethylformamide (DMF, 10 mL) and acetonitrile (CH3CN,
5 mL) in a 20 mL vial was heated at 120 °C for 96 h. Yellow
single crystals in 56% yield (based on H2L) were harvested.
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Scheme 1. Generic schemes for (a) linker exchange, (b) covalent
PSM, and (c) coordination site PSM.
Synthesis of 1-SH-a. As-synthesized single crystals of 1-SH was
thoroughly washed by fresh DMF and then soaking in acetone for
12 h to produce single crystals of 1-SH-a. Elemental analysis (EA)
Herein, we report a water-stable heterometallic Zr-MOF
[Zr6(μ3-OH)8(OH)8][CuI4(L1)4]2·xguest (1-SH, H2L1
(%):
Calcd
for
C57H62Cu8N8O37S8Zr6
=
[Zr6(μ3-
=
6-
OH)8(OH)8][CuI4(L1)4]2 · 3(C3H6O)· 2H2O: C, 24.77; H, 2.26; N,
4.06; S, 9.28; Found: C, 25.01; H, 1.99; N, 4.02; S, 9.32. The
resulting molecular formula was derived from the combination of
EA and thermogravimetric analysis (TGA). The solid was
continued to soaking in acetone for 5 days, during which acetone
was decanted and freshly replenished two times, and then dried at
120 °C under vacuum for 12 h to yield activated sample.
mercaptopyridine-3-carboxylic acid), which can undergo a guest-
dependent structural transformation to form 1-SH-a. As expected,
in the presence of H2O2, the coordination site modification was
successfully achieved in 1-SH-a by postsynthetic oxidation of L12-
to the corresponding L22- (H2L2 = 6-sulfonicotinic acid) (Scheme
1c). The resulting 1-SO3H was determined by powder X−ray
diffraction (PXRD), X−ray photoelectron spectroscopy (XPS),
and nuclear magnetic resonance (1H NMR). Significantly, the
styrene oxide ring-opening reactions catalyzed by 1-SH-a and 1-
SO3H were thoroughly investigated to compare their catalytic
behaviors derived from their modified organic fragments and
copper species, indicating that the functionalization of 1-SH-a to
form 1-SO3H tuned the catalytic behavior to give an improved
regioselectivity of primary alcohol (A) of ~99% compared with 1-
SH-a (~71%).
Synthesis of 1-SO3H. As-synthesized single crystals of 1-SH-a
(~ 30 mg) were immersed in a mixture of THF (4 mL)/H2O2 (0.1
ml) for 12 h, during which THF/H2O2 was decanted and freshly
replenished two times. The resulting green crystals was washed
by THF and dried in air. EA (%): Calcd for C63H74Cu8N8O38S8Zr6
=
[Zr6(μ3-
OH)8(OH)8][(CuI0.44CuII0.56(OH)0.56)4(L2)4]2·3(C4H8O)·(C3H6O)·2H
2O: C, 26.42; H, 2.60; N, 3.91; S, 8.96; Found: C, 26.22; H, 3.01;
N, 3.88; S, 9.10. Inductively coupled plasma atomic emission
spectroscopy (ICP-AES) confirmed the ratios of Zr/Cu upon
digestion of 1-SO3H in dilute H2O/HNO3, showing the
concentrations of Zr and Cu are 49.8 and 49.6 ppm, respectively,
corresponding to the molar ratio of Zr/Cu to be ~ 0.7, which is
close to the ratio of Zr/Cu (0.75) in 1-SH. The activated sample
was obtained by soaking in acetone for 5 days and dried at room
temperature under vacuum for 24 h.
Figure 1. Rational design of BUT-52 and 1-SH on the basis of In(III)
2-
or Zr(IV), Cu(I), and the in-situ synthesized L1 .
EXPERIMETAL SECTION
Unless otherwise noted, all starting materials were directly
purchased from commercial suppliers and used without further
purification. ZrCl4 (>99.5+%, Alfa Aesar), CuI (99.0%,
ARCOS), 6,6’-Dithiodinicotinic Acid (H2L) (>98.0%, TCI), 6-
Mercaptopyridine-3-carboxylic acid (H2L1) (90%, Sigma-Aldrich),
Benzoic Acid (>99%, TCI), Hydrogen Peroxide Solution (H2O2)
(30 wt. % in H2O, ACS reagent, Sigma-Aldrich), N,N-
Dimethylformamide (DMF) (>99.8%, GR ACS), acetonitrile
(CH3CN) (99.8%, Sigma-Aldrich), dichloromethane (CH2Cl2),
(99.5+%, Alfa Aesar), acetone (99.5%, Fisher) styrene oxide
RESULTS AND DISCUSSION
Solvothermal reactions of ZrCl4, CuI, 6,6’-Dithiodinicotinic
(H2L) and benzoic acid in a mixture of DMF/CH3CN at 120 °C
afforded yellow single crystals of 1-SH. Single-crystal X-ray
diffraction (SCXRD) reveals that 1-SH crystallizes in the
tetragonal space group P42/mnm and has a three-dimensional flu
topology, which consists of the 8-connected [Zr6(μ3-
OH)8(OH)8]8+ clusters (denoted “primary” nodes) linked by
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Chemistry of Materials
tetrahedral [CuI4(L1)4]4- metalloligands (denoted “secondary”
DMSO-d6 solution (3). Note: We found there is trace H2L2 in
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nodes). Similar to our previously reported framework BUT-52,24
both H2L1 are originated from the cleavage of S−S bond during
the solvothermal reaction and are allowed to form distinct cuprous
sulfur cluster-based heterometallic MOFs (Figure 1). Unlike the
6-connected [CuI6(L1)6]6- moiety observed in BUT-52, in 1-SH, a
4-connected [CuI4(L1)4]4- tetrahedra with D2d symmetry is achieved
(Figure. S1). Moreover, an open dodecahedral cageis observed
with dimensions of about 22.8×22.8×29.3Å3, which is constructed
by six [Zr6(μ3-OH)8(OH)8]8+ and eight [CuI4(L1)4]4- tetrahedra as
vertices and twenty-four L12- linkers as edges (Figure S2).
commercially available H2L1 from Sigma-Aldrich Co. (purity: 90%).
Due to the combination of exceptional thermal and chemical
stability of 1-SH-a and the nature of vulnerability of thiol
compounds to oxidize to the sulfonate derivatives, the exploration
for postsynthetic oxidation of coordination site seemed to be very
promising. Therefore, as-synthesized 1-SH-a was treated in a
mixed solution of THF/H2O2 (v/v = 4/0.1) at room temperature
for 12 h to produce 1-SO3H (Figure S7). XPS data showed the
sulfur 2p region of 1-SH-a showed that the peak centered at
~162.3 eV corresponded to a reduced oxidation state of sulfur as
in L12-, while in 1-SO3H, this peak is absent and a new
characteristic band at ~167.6 eV formed, which is assigned to a
higher oxidation state of sulfur (L22-) (Figure. 3a).30 Similarly, an
intense asymmetrical Cu 2p3/2 photoelectron peak together with
shakeup satellite peaks indicated mixed valence state of Cu in
green crystals of 1-SO3H (Figure S7), which can be deconvoluted
into two contributions located at 932.1 and 934.6 eV,
corresponding to Cu(I) 2p3/2 and Cu(II) 2p3/2, respectively.
Integration of the Cu(I) 2p3/2 and Cu(II) 2p3/2 peaks with Cu(II)
satellite peaks gives a calculated ratio of Cu(II)/Cu(I) to be ~
0.56/0.44 in 1-SO3H. In 1-SH-a, a strong characteristic band at
932.1 eV is consistent with Cu(I) 2p3/2, indicating that the Cu
species possess 1+ oxidation state (Figure 3b).31 Further, 1H NMR
upon digestion of 1-SO3H in dilute DCl/D2O and DMSO-d6
solution confirmed the degree of conversion by oxidation. It is
obvious that the appearance of pure resonances centered at δ
8.91, 8.21, and 7.73 ppm are shown, corresponding to different
hydrogen signals in H2L2 (Figure 3c). The conversion from -S- to -
SO3- highlights the fact that the oxidation can access both the
exterior and the interior of the MOF crystal, contributing to
overall modification of the organic linkers. FT-IR spectra also
indicated that the characteristic S−O stretches of R-SO3- in 1-
SO3H were evident at 1244 and 1201 cm−1,21 which were not
absent in 1-SH-a (Figure S8). Continued oxidation of 1-SO3H for
another 12 h can lead to 1-SO3H-a with an increased ratio of
Cu(II)/Cu(I) being ~0.78/0.22, however, causing partial structural
collapse (Figures 3b and S4). Despite the incomplete conversion
of Cu(I), the percent sulfonation of 1-SH-a is 100% during the
oxidation, indicating the first example of coordination site
modification through PSM in MOF materials. In addition, no
obvious reversible process from -SO3- to -S- was observed when
soaking 1-SO3H (~ 30 mg) in a saturated THF/H2O (2 ml/2 ml)
solution of reduced agent, such as NaNO2 for 10 days (Figure
S11). Attempts to directly synthesize 1-SO3H under the reactions
of Zr(IV), Cu(I)/Cu(II), and pure H2L2, however, failed.
Furthermore, BUT-52 was also investigated under the same
condition for this modification. The PXRD patterns indicated that
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Figure 2. Experimental and simulated PXRD patterns of 1-SH and 1-
SH-a; inset is the structural transformation from 1-SH to 1-SH-a.
An interesting feature in the PXRD pattern of dried bulk 1-SH
after acetone washing resulted in the peak shift, compared to the
calculated pattern from the single crystal data, inferring a guest-
dependent structural transformation (Figure 2). The size and shape
of single crystals after acetone treatment remains unaltered
indicative of a single-crystal to single-crystal transformation
(Figure. S3). This prompted us to obtain the transformed structure
(1-SH-a) using SCXRD. 1-SH-a exhibits very similar unit cell
parameters (Table S1), but with a shrink angle in the window
compared to that of 1-SH (Figure 2 insert). Significantly, the
calculated PXRD pattern matched well with the as-synthesized 1-
SH-a. Unlike the varied flexibility presented by the previously
reported Zr-MOFs in response to guest molecules,25,26 1-SH was
found to exclusively maintain its contracted conformation upon
immersion in different solvents, showing no guest solvent
dependence (Figure S4). TGA and PXRD indicated that 1-SH-a
did not decompose until 350°C in air (Figures S4 and S5). Upon
acetone exchange and activation at 120°C, 1-SH-a showed no N2
uptake at 77 K, which is most likely due to the insufficient
activation and pore size selectivity.27 However, effective CO2
adsorption at 273 K was measured with maximum capacity of
40.1 cm3 g-1, which showed type I adsorption isotherm (Figure
S6). The chemical stability of 1-SH-a was measured by soaking in
water for 12 h, where the crystallinity and porosity were
maintained as evidenced by PXRD and CO2 adsorption (Figures
S5 and S6). These results demonstrate that the high connectivity
“primary” (Zr6) nodes28 and robust “secondary” Cu4S4 nodes
(pKa (Cu2S) = 47.6)29 in 1-SH-a are responsible for the excellent
thermal and chemical stability.
Figure 3. XPS spectra in the regions of (a) Cu 2p and (b) S 2p for 1-
1
SH-a and 1-SO3H. (c) H NMR spectrum of commercially available
H2L1 (1), as-synthesized H2L2 (2) and DCl/D2O digested 1-SO3H in
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the material lost crystallinity, albeit the color change from red to reaction is hardly processed in the absence of catalyst, due to the
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green was observed (Figure S4 and S7). Clearly, the access of
robust “primary” nodes seems extremely critical for
stabilization of the whole framework, while postsynthetic
oxidation can occur in “secondary” nodes.
poor nucleophilicity of alcohols.40,41 As shown in Scheme 2a, the
reaction was carried out at 55 °C with 1 mol% MOF catalyst. In
accordance with the two possible sites of nucleophilic attack of
isopropanol on styrene oxide, two products are possible: the
primary alcohol (A) and the secondary alcohol (B). After 8 h, the
reactions led to a >90% conversion of styrene oxide, and the final
conversion can reach ~100% after 24 h for both catalysts (Scheme
2b and Table S2). PXRD indicated that the crystallinity of both
MOFs remained intact after catalysis (Figure S4). Recycling tests
with three consecutive runs were performed without any
significant loss of their catalytic activities to indicate the catalysts
can be reused (Table S2).
Figure 4. (a) PXRD patterns of simulated or as-synthesized 1-SH, 1-
SH-a, and 1-SO3H. (b) DFT optimized structure of 1-SO3H. (Only the
optimized unit of the R-SO3-CuII is shown; the remaining atoms and
hydrogen atoms in 1-SO3H are omitted for clarity.). C, black; O, red;
N, blue; S, yellow; H, gray; CuII, cyan.
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PXRD revealed that the pattern of 1-SO3H was not consistent
with 1-SH-a, but changed to match the PXRD pattern of 1-SH
(Figure
4a).
The
resulting
formula
of
[Zr6(μ3-
OH)8(OH)8][(CuI0.44CuII0.56(OH)0.56)4(L2)4]2·3(C4H8O)·(C3H6O)·2H2
O (1-SO3H) was defined from the combination of PXRD, XPS,
1H NMR, IR spectra, ICP-AES, EA and TGA along with the
reasonable deduction of oxidation states based on L12- and Cu(I)
(Section VI†).32 Although scanning electron microscopy (SEM)
confirmed that the shapes of single crystals of 1-SO3H remain, the
structure of 1-SO3H could not be determined by SCXRD due to
the cracking of crystals (Figures S3 and S9), leading to very weak
diffraction. Thus, we modeled 1-SO3H using the framework of 1-
SH as a constraint and geometrically optimized the models using
periodic density functional theory (DFT) calculations using the
CP2K code (Section VII †).33 The newly formed sulfonate anions
coordinate in a bidentate fashion to Cu(II), either in a bridging
mode or in chelating and bridging modes to two Cu(II) ions. Cu(II)
was then allowed to coordinate to N atom of pyridine unit, and
further coordinated to one -OH terminal to form 4- or 5-
coodinated species (Figure 4b). TGA shows that 1-SO3H is not
thermally stable and the activated sample was achieved by the
exchange with acetone and dried at room temperature under
vacuum for 24 h (Figure S5). 1-SO3H exhibits CO2 adsorption at
273 K with maximum capacity of 32.0 cm3 g-1 (Figure S6).
Figure 5. Plausible mechanisms for styrene oxide ring-opening
reaction catalyzed by different catalytic sites.
Interestingly, for 1-SH-a tested, both products A and B were
achieved with ~71% of the regioselectivity towards primary
alcohol (A), whereas 1-SO3H gave a regioselectivity of ~99%
(Scheme 2c, Fig. S13-14 and Table S2). The regioselectivity of 1-
SH-a (71%) and 1-SO3H (99%) toward primary alcohol (A) was
almost identical throughout the reaction course. This observation
can be further rationalized by a first-order parallel reaction model
(Section VIII †). From mechanistic point of view, generally, the
ring-opening reaction of epoxides can proceed by either SN1 or
SN2 mechanisms, depending on the nature of the catalyst and
reaction conditions. Accordingly, plausible reaction mechanisms
are proposed: (1) When styrene oxide undergoes in presence of an
acidic catalyst, ring-opening occurs by an SN1 mechanism, and
the more-substituted carbon is the site of attack, leading to
product A. (2) Conversely, when initiating with a basic catalyst,
the reaction takes place by a mechanism with substantial SN2
character, and the less-substituted carbon is the site of
nucleophilic attack. As a result, product B predominates.42 As
shown in Figure 5, 1-SH-a possesses [Zr6(μ3-OH)8(OH)8]8+ (Zr6
nodes) and cuprous sulfur clusters (R-S-CuI), in which there is Zr6
nodes-based BrФnsted acid available to protonate the oxygen
atom in styrene oxide to create a good leaving group, resulting in
product A (I). This matched well with the product catalyzed by
the previously reported benchmark Zr-MOFs.40,41 For R-S-,
divalent organic sulfides are well-known strong Lewis bases and
have stronger electron-donating ability. Thus, a deprotonated,
negatively charged alkoxide is preferentially created. An alkoxide
is a poor leaving group, making the ring unlikely to open without
a “push” from the nucleophile, and the reaction is very likely
to proceed by an SN2 mechanism (II). Clearly, this illuminates
that both products A and B were observed when using BrФnsted
Scheme 2. (a) Styrene oxide ring-opening reaction with
isopropanol, (b) conversion and (c) regioselectivity of A catalyzed
by 1-SH-a and 1-SO3H.
Compared with ideal 12-connected Zr-MOFs, such as UiO-type
series,34 1-SH-a and 1-SO3H with lower connectivity (8-connected)
can be seen with “inherent defects”, and the terminal
hydroxide groups (Zr-OH) can be considered as catalytic
BrФnsted acid sites.35-39 To test the catalytic activities of 1-SH-a
and 1-SO3H, ring-opening of styrene oxide with a monohydric
alcohol (i.e. isopropanol) was selected as a model reaction. This
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AUTHOR INFORMATION
acid sites and strong Lewis bases dual-functionalized 1-SH-a.
1
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After postsynthetic oxidation, 1-SH-a was completely accessible
to form 1-SO3H, with retained Zr6 nodes and newly formed R-
SO3-CuII0.56-OH0.56 units. Cu(II)-MOFs were found to be effective
for the ring-opening reaction of styrene oxide with unsaturated
coordinated Cu(II) sites as Lewis acids.43-45 In this case, three
types of plausible mechanisms could probably be adopted
simultaneously and give an integrative contribution to the
catalytic performance in 1-SO3H. Besides mechanism I, Lewis
acidic Cu(II) binds to styrene oxide to produce an enhanced
electrophilic nature of the carbon attached to the phenyl group
through an acid–base interaction. Subsequently, the carbon with
a partial positive charge is attacked by isopropanol to give product
A (III). In addition, R-SO3- moieties might be protonated by
inherent -OH from Zr-OH or CuII-OH in 1-SO3H and form strong
BrФnsted acidic R-SO3H sites for alcoholysis of epoxides (IV).46
Nevertheless, only product A is exclusively obtained within the
above three proposed mechanisms, which is consistent with the
experimental data observed in this work.
Corresponding Author
Yi Han–Key Laboratory of Eco-chemical Engineering,
College of Chemistry and Molecular Engineering, Qingdao
University of Science and Technology, Qingdao 266042,
China;
ocrid.org/0000-0003-0395-1111;
Email:
roberthan@yeah.net
9
Praveen K. Thallapally–Pacific Northwest National
Laboratory, Richland, Washington 99352, United States;
Email: praveen.thallapally@pnnl.gov
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Author
Michael A. Sinnwell–Pacific Northwest National Laboratory,
Richland, Washington 99352, United States
Robert G. Surbella III–Pacific Northwest National Laboratory,
Richland, Washington 99352, United States
Wenjuan Xue–State Key Laboratory of Separation Membranes
and Membrane Processes, Tianjin Polytechnic University, Tianjin
300387, China
Hongliang Huang–State Key Laboratory of Separation
Membranes and Membrane Processes, Tianjin Polytechnic University,
Tianjin 300387, China
CONCLUSION
Jian Zheng–Pacific Northwest National Laboratory, Richland,
Washington 99352, United States
In conclusion, a water-stable heterometallic Zr-MOF with
“primary” and “secondary” nodes has been reported, and
the guest-dependent structural dynamic was studied by SCXRD.
The presence of “primary” nodes in the framework plays a
vital role in the improvement of structural stability. Subsequently,
postsynthetic oxidation induced coordination site modification
was successfully observed in “secondary” nodes. In addition,
the catalytic behavior toward the styrene oxide ring-opening
reaction after PSM was tuned, and plausible mechanisms were
also unambiguously verified. The present work not only
demonstrates a suitable MOF model for investigating the
coordination site modification but also provides a potential
prediction on the unique design of heterogeneous catalysts at the
molecular level. Furthermore, postsynthetic oxidation of
coordination site is expected to become a promising strategy for
the introduction of multi-functionalities in MOFs for advanced
applications.
Bo Peng–Pacific Northwest National Laboratory, Richland,
Washington 99352, United States
Gaurav Verma–Department of Chemistry, University of South
Florida, Tampa, Florida 33620, United States
Yang Yang–Pacific Northwest National Laboratory, Richland,
Washington 99352, United States
Lili Liu–Pacific Northwest National Laboratory, Richland,
Washington 99352, United States
Shengqian Ma–Department of Chemistry, University of South
Florida, Tampa, Florida 33620, United States
ACKNOWLEDGMENT
This work was supported by the U.S. DOE, Office of Science, Basic
Energy Sciences, Division of Materials Sciences and Engineering,
under Award KC020105-FWP12152. Department of Energy by
BattellePNNL is a multiprogram national laboratory operated for the
U.S. Memorial Institute under Contract DE-AC05-76RL01830. Y.H.
acknowledges the support of the NSF of China (No. 21601102), the
NSF of Shandong Province (No. ZR2016BQ34), the “International
Postdoctoral Exchange Fellowship Program" (No. 20170035), and the
Scientific and Technical Development Project of Qingdao (No.18-2-
2-32-jch). RGS is grateful for the support of the Linus Pauling
Distinguished Postdoctoral Fellowship program at PNNL.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information including synthesis of MOFs, PXRD,
FT-IR, TGA, SEM, 1H NMR, XPS, catalysis reaction, DFT
calculation and crystal data is available free of charge on the ACS
Publications website.
REFERENCES
Crystallographic data for 1-SH and 1-SH-a (CIF)
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