Design, synthesis, characterization, and catalytic properties of a large-pore metal-organic framework possessing single-site vanadyl(monocatecholate) moieties

Article abstract of DOI:10.1021/cg400500t

Through a combination of protecting groups, postsynthesis deprotection, and postsynthesis metallation, a homogeneously inaccessible, single-site vanadyl(monocatecholate) moiety can be incorporated into the dipyridyl struts of a Zn-based, pillared paddlewheel MOF. The resulting MOF, which has large pores, exhibits catalytic activity in the benzylic oxidation of tetralin in the presence of tert-butylhydroperoxide.

Full text of DOI:10.1021/cg400500t

Article
pubs.acs.org/crystal
Design, Synthesis, Characterization, and Catalytic Properties of a
Large-Pore Metal-Organic Framework Possessing Single-Site
Vanadyl(monocatecholate) Moieties
Huong Giang T. Nguyen, Mitchell H. Weston, Amy A. Sarjeant, Daniel M. Gardner, Zhi An,
Raanan Carmieli, Michael R. Wasielewski, Omar K. Farha,* Joseph T. Hupp,* and SonBinh T. Nguyen*
Department of Chemistry and the Institute of Catalysis for Energy Processes, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208, United States
S
* Supporting Information
ABSTRACT: Through a combination of protecting groups,
postsynthesis deprotection, and postsynthesis metallation, a
homogeneously inaccessible, single-site vanadyl-
(monocatecholate) moiety can be incorporated into the
dipyridyl struts of a Zn-based, pillared paddlewheel MOF.
The resulting MOF, which has large pores, exhibits catalytic
activity in the benzylic oxidation of tetralin in the presence of
tert-butylhydroperoxide.
postsynthetically.19,22 Herein, we extend this MOF scaffold to
display a novel vanadyl(monocatecholate) motif that is
INTRODUCTION
■
As a class of porous and crystalline coordination polymers,
catalytically active in a single-site fashion. Unlike metallo-
metal-organic frameworks (MOFs)1−4 are highly promising
porphyrins and metallosalens, this monocatecholate motif is
catalyst scaffolds due to their uniform and well-defined
structures and tailorable micropore environments.5,6 Because
inaccessible in solution because the low-steric coordination
environment of the catechol ligand tends to overwhelmingly
MOFs are microporous materials comprising metal nodes
favor coordinatively saturated bis- and tris-chelate binding
linked together by organic struts, both opportunistic and
modes.34 We hypothesize that the pillared paddlewheel MOF
designed catalysis have been demonstrated at unsaturated metal
nodes, at organocatalyst- or metal-complex-tethered organic
scaffold, constructed from orthogonal carboxylate and dipyridyl
struts, would be ideal platforms for spatially isolating catechol-
linkers, as well as through catalysts physically encapsulated by
the micropores.5−11 While the majority of catalytically active
functionalized struts, which can then be postsynthetically
metallated to achieve well-defined metal motifs with unsatu-
MOFs reported to date contain active metal centers or
rated (or labile) coordinative sites capable of novel catalytic
complexes, these motifs are often pre-existing components of
behaviors.35,36 To achieve MOFs with large pores and
apertures, we employ a series of protected catechol-function-
alized dipyridyl strut L1 in combination with the catenation-
suppressing37 dibromotetratopic ligand L2. The resulting
MOFs can then be deprotected postsynthetically and
metallated with vanadium(IV) ions to afford our desired
catalytically active MOF materials. To prevent opportunistic
catalysis by metal nodes,38 we select redox-inert Zn ions as the
structural metal ions.
the secondary building units (SBUs) of the MOFs,12−14
heterogenized homogeneous catalytic metal complexes,15−24
or encapsulated metal complexes25−27 or clusters.8,28 Examples
of MOFs featuring unique metal coordination environments
that are inaccessible in solution (or otherwise) remain rare;29,30
however, their investigation offers unique opportunities for the
development of MOF-based catalysts that can access novel
activity or mechanistic pathways.
We and others have previously demonstrated that the single-
site activity of homogeneous metalloporphyrin,18,21,22,31−33
chiral metallosalen,15,17,20 and metalloBINOL16,23,24 catalysts,
along with their chemo- and enantioselectivities,6,11 can be
integrated with the shape- and size-selectivity of the MOF
environments. Specifically, we have shown that dipyridyl-
functionalized analogues of homogeneous porphyrin18,22,31 and
salen15,17 complexes can be synthesized and readily deployed as
struts in permanently microporous Zn-based pillared-paddle-
wheel MOFs that are catalytically active and can be modified
Because catechol groups readily chelate to Zn ions during
crystal growth to form bis- or tris-catecholate homogeneous
complexes34 or amorphous coordination networks,39 we started
our MOF synthesis with protected catechol struts L1. We note
that previous attempts, by our groups and others,30 to grow
Received: April 6, 2013
Revised: June 22, 2013
Published: June 25, 2013
© 2013 American Chemical Society
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to give 95% deprotection and minimal deprotection, respectively.
Note: Stirring does lead to fragmentation of the crystals.
MOFs from struts possessing naked catechol moieties have
been unsuccessful; even in the best-case scenario where the
desired paddlewheel structure can be formed from the
unprotected catechol-containing struts, these moieties would
readily bind to the residual Zn ions in solution, and the
resulting (catecholato)zinc moieties would preclude metallation
with other catalytically active metal ions of interest.
2. From BOC-CatBrO MOF. Inside a 6 dram vial, DMF-soaked
crystals of BOC-CatBrO MOF (∼50 mg) were solvent-exchanged to
1,2-dichlorobenzene over a period of 3 days with solvent replacement
every 24 h. After the last solvent exchange, the whole vial was capped,
and the sample was then heated at 140 °C in a silicone oil bath for 24
h to quantitatively yield free-catechol-bearing CatBrO MOF.
Realizing that many deprotection protocols40 involve harsh
conditions that would degrade most MOFs, we selected
protecting groups that can be removed under mild conditions
and without compromising the integrity of pillared paddlewheel
MOFs. Postsynthesis deprotection41−45 studies of MOFs have
employed various protecting groups, including fluoride-,46,47
photo-,30,48−50 and thermo-labile51,52 protecting groups, such as
trimethylsilyl-, o-nitrobenzyl- (oNBn-), and tert-butoxycarbonyl
(BOC), respectively, to mask functional groups that can
potentially interfere with de novo MOF synthesis. Thus, we
synthesized a series of catechol-containing dipyridyl struts
(L1b−d) where the catechol moieties are protected with tert-
butyldimethylsilyl (TBS), oNBn, and BOC groups and used
them together with L2 in our MOF synthesis (Scheme 1). As a
model, we also synthesized a highly stable MOF from the
methyl (Me)-protected derivative L1a.
1
Complete deprotection was observed through H NMR analysis of a
sample of MOF that has been dissolved in a 1:9 v/v mixture of
concentrated HCl (aq)/DMSO-d6.
Synthesis of V-CatBrO MOF. Inside a 6 dram vial, a sample of
CatBrO MOF (∼50 mg) was solvent-exchanged to tetrahydrofuran
(THF) over a period of 2 days with solvent replacement every 12 h.
The solvent was then decanted quickly, and to the still-solvated
CatBrO MOF was added a solution of VO(acac)2 (50 mg, 189 μmol)
in THF (5 mL). The reaction vial was then capped and heated at 50
°C in a silicone oil bath for 24 h. The resulting V-CatBrO MOF was
isolated via decantation, soaked in fresh THF for 1 day at 50 °C, and
then soaked in fresh DMF for 3 days at 50 °C to remove any excess
metal ions. ICP analysis indicates that V-CatBrO MOF has a V/Zn
ratio of 0.35
catechol groups.
0.019, corresponding to ∼70% metallation of the
Catalytic Oxidation of Tetralin. In a 5 mL microwave vial
(capacity designates the amount of solution that can be safely loaded),
catalyst (V-CatBrO MOF or BOC-CatBrO MOF (4.3 mg (9.6 mg of
wet sample based on TGA determination of the amount of solvent
uptake), 2.5 μmol of V for V-CatBrO MOF) or VO(acac)2 (0.66 mg,
2.5 μmol)), tetralin (34.1 μL, 33.1 mg, 0.25 mmol), and
tribromobenzene (20 mg, 0.064 mmol, as an internal standard) were
combined in chlorobenzene (3 mL). tert-Butyl hydroperoxide (0.043
mL of a 5−6 M solution in nonane; a 6 M solution is assumed for
TON calculation) was slowly added dropwise over 10 s. The reaction
vial was sealed with a Teflon-lined cap and allowed to shake at 200
rpm and 50 °C in a Thermolyne Type 17600 aluminum heating block
(Thermolyne, Dubuque, IA) mounted on a Thermolyne Type 65800
shaker (Thermolyne, Dubuque, IA). Aliquots from the reaction
mixture (∼0.1 mL) were regularly collected using a syringe and diluted
to 1 mL with dichloromethane in a GC vial before being analyzed with
gas chromatography.
Scheme 1. A Schematic Illustration of the Solvothermal
Synthesis of Zn-Pillared Paddlewheel PG-CatBrO-MOFs
Gas chromatography was performed on an Agilent Technologies
6890N Network GC system equipped with an FID detector and HP-5
capillary column (30 m × 320 μm × 0.25 μm film thickness). Analysis
parameters were as follows: initial temperature = 50 °C, initial time = 3
min, ramp = 10 °C/min, final temperature = 200 °C, final time = 10
min. Elution times (min) = 12.7 (tetralin); 15.8 (tetralol); 16.2 (α-
tetralone); 16.9 (tribromobenzene, internal standard); oxidation
product concentration was calculated on the basis of calibration
curves using tribromobenzene as the internal standard.
Determination of Potential Catalyst Leaching via Filtration.
At the 4 h point into a typical oxidation run (see above), the reaction
mixture was removed from the shaker, and catalyst crystals were
allowed to settle at the bottom of the microwave vial. The supernatant
was gently removed via a syringe equipped with a fine needle, leaving
behind the catalyst, and filtered through a 0.1 μm filtration disk into a
new microwave vial. The filtrate was allowed to react further at 50 °C
on the shaker and monitored by gas chromatography by removing
aliquots from the reaction mixture (∼0.1 mL) using a syringe. Each
aliquot is diluted with dichloromethane to 1 mL in a GC vial before
being analyzed.
Single-Crystal Structure Determination. A single crystal of
C56H30Br2N2O10Zn2 [Me-CatBrO-MOF, designated as n1694 in the
CIF], isolated from a sample of freshly grown MOFs in DMF, was
mounted in inert oil and transferred to the cold gas stream of a Bruker
Kappa APEX CCD area detector equipped with a Cu Kα microsource
and MX optics. Absorption correction was carried out using SADABS-
2008/1 (Bruker, 2008) (R(int) was 0.1061 before and 0.0755 after
correction). The ratio of minimum to maximum transmission is
0.7660. The λ/2 correction factor is 0.0015. See Table S1 in the
Supporting Information for additional information.
EXPERIMENTAL SECTION
■
Synthesis of PG-CatBrO MOF (PG = Protecting Group). In a
typical synthesis, 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)-
benzene (42 mg, 58.6 μmol) and Zn(NO3)2·6H2O (34 mg, 114
μmol) were dissolved via sonication in N,N-dimethylformamide
(DMF, 8 mL) in an 8 dram vial. L1a−d (60 μmol) was added to the
mixture and vortexed until fully dissolved (care was taken to not
sonicate these ligands as the sonication energy may be enough to
deprotect them). The solution was evenly divided among three 1 dram
vials and heated at 75 °C for at least 24 h to give plate-like crystals that
were kept in fresh DMF. See Figure S21 for photographs of Me-
CatBrO MOF (clear, colorless crystals), BOC-CatBrO MOF (amber
crystals), oNBn-CatBrO MOF (yellow crystals), and TBS-CatBrO
MOF (dark amber crystals).
Synthesis of CatBrO MOF. 1. From oNBn-CatBrO MOF. Inside a
4 dram vial, DMF-soaked crystals of oNBn-CatBrO MOF (∼50 mg)
were solvent-exchanged to ethyl acetate over a period of 3 days with
solvent replacement every 24 h. A portion of the oNBn-CatBrO MOF
(∼20 mg) was then transferred into a 1 × 1 × 3 cm quartz UV cuvette,
capped with a Teflon plug, and irradiated with a Black-ray longwave
ultraviolet lamp (model B 100 AP, UVP, San Gabriel, CA, λ = 365 nm,
115 V, 60 Hz, 2.5A, distance ∼10 cm) for 24 h with or without stirring
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A single crystal of C54H36N6O20Zn2 [BOC-CatBrO MOF,
designated as n2053 in the CIF], isolated from a sample of freshly
grown MOFs in DMF, was mounted in inert oil and transferred to the
cold gas stream of a Bruker Kappa APEX CCD area detector equipped
with a Cu Kα microsource and Quazar optics. Absorption correction
was carried out using SADABS-2008/1 (Bruker, 2008) (wR2(int) was
0.0639 before and 0.0533 after correction). The ratio of minimum to
maximum transmission is 0.8948. The λ/2 correction factor is 0.0015.
See Table S2 in the Supporting Information for additional information.
RESULTS AND DISCUSSION
■
MOF Synthesis and Characterization. PG-CatBrO
MOFs were solvothermally synthesized from Zn(NO3)2·6H2O,
L2, and L1a−d in DMF (Scheme 1) to give plate-like colorless
to amber crystals. TGA analyses of these as-synthesized MOFs
suggest that they have large solvent-accessible pores (∼45−55
wt % of trapped DMF solvent) and high thermal stability
(decomposition at 450 °C) (Figure S22 in the Supporting
Information).
Figure 2. PXRD patterns of simulated (bottom to top) PG-CatBrO
MOFs (blue), Me-CatBrO MOF (red), BOC-CatBrO MOF (green),
TBS-CatBrO MOF (purple), and oNBn-CatBrO MOF (orange).
integration of the dipyridyl protons against those of L2. For
digested oNBn-CatBrO MOF, a ∼1.3:1 molar ratio of L1c:L2
could be obtained if integration was based on the benzylic
protons of the nitrobenzyl groups (Figure S32 in the
Supporting Information). For BOC-CatBrO MOF, digestion
in a milder solution of concentrated HCl:DMSO-d6 (1:9 v/v)
was a good alternative, showing a (0.6 0.1):1 ratio for L1d:L2
(Figure S33 in the Supporting Information; see Figure S34 in
the Supporting Information for deviation in ratio after
deprotection).
Unfortunately, only the structures of Me-CatBrO MOF and
BOC-CatBrO MOF were fully determined; that of NBn-
CatBrO MOF was only partially resolved, and that of TBS-
CatBrO MOF could not be obtained due to weak diffraction.
Figure 1 shows the full structures of Me- and BOC-CatBrO
Postsynthesis Deprotection of PG-CatBrO MOFs.
1
Surprisingly, the H NMR spectrum of concentrated D2SO4-
digested TBS-CatBrO MOF (Figure S35 in the Supporting
Information) shows that the TBS ether was deprotected in situ
during MOF synthesis, an observation that is supported by the
absence of Si in its ICP-OES data. ICP analysis of a dry sample
of TBS-CatBrO MOF further indicated that the deprotected
catechol groups are also chelated to Zn (16.1 wt % Zn expected
for Zn-CatBrO MOF, 17.8 wt % Zn observed). As Kitagawa
and co-workers,43 and later Rankine et al.,44 have seen similar in
situ deprotection of acetyl esters during MOF synthesis, it is
possible that the slightly acidic solvothermal conditions that we
used to synthesize TBS-CatBrO MOF could be responsible for
the in situ desilylation. Because our goal was to obtain free
catechol moieties that could be metallated with VIV ions
postsynthesis and the deprotection of Me-CatBrO MOF would
require a vigorous and framework-degrading reaction with
BBr3, we explored the photolabile oNBn- and the thermolabile
BOC-protecting groups.
Figure 1. Ball-and-stick depiction of the structures of Me-CatBrO
MOF (left) and BOC-CatBrO MOF (right) as determined by single-
crystal X-ray diffraction. The BOC groups are disordered and are only
shown in representative locations, reflecting their 50% occupancy
probability (i.e., they occupy two of the four positions around that
central phenyl ring of the dipyridyl struts). Gray = C, red = O, blue =
N, yellow = Zn. All H atoms have been omitted for clarity.
Following a procedure reported by Cohen and co-workers,30
a sample of oNBn-CatBrO MOF in EtOAc was irradiated at
365 nm for 24 h. Unfortunately, complete photodeprotection,
as determined by NMR spectroscopic analysis of concentrated
D2SO4-digested MOF samples, was not achieved when the
crystals were not stirred, probably due to a combination of light
scattering by the MOF crystals and incomplete light
penetration. Stirring the crystals led to 95% deprotection
(Figure S36 in the Supporting Information), but also broke the
sample down to powdery microcrystallites (Figure S38 in the
Supporting Information). Thus, while the photochemical
deprotection of the photolabile oNBn protecting group does
not chemically degrade the MOF, mechanical stress leading to
crystal fragmentation was unavoidably incurred. Although
smaller crystallites will increase the percentage of external
surface area relative to internal pore surface, and may be
advantageous in cases where catalysis is mostly limited to
MOFs, which consist of L1 pillars linking xy-oriented 2-D
sheets of Zn(II) dimers bridged by ligand L2. The available
single-crystal X-ray diffraction data for oNBn-CatBrO MOF
showed that it crystallizes in the same space group Pmmm as
Me- and BOC-CatBrO MOFs and share the same unit cell (a
= 11; b = 16; c = 23 Å). In addition, the PXRD patterns of all
four MOFs agree with the simulated pattern for Me-CatBrO
MOF, indicating that they are isostructural (Figure 2).
Consistent with the formula [Zn2(L1a)(L2)], obtained from
X-ray diffraction, the NMR spectra of a sample of concentrated
D2SO4-digested Me-CatBrO MOF showed a 1:1 ratio of
L1a:L2 (Figure S31 in the Supporting Information). Unfortu-
nately, D2SO4 digestion of the remaining PG-CatBrO MOFs
partially degraded the acid-labile protecting groups in L1b−d
struts and decomposed the redox-active catechol group, making
it difficult to obtain quantitative stoichiometric data through
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surface, they also diminish the ability to distinguish surface
versus internal catalysis. As such, a protecting group that can be
removed without leading to crystal fragmentation is preferred
when size-, shape-, or enantioselectivities offered by the pores
of the MOF are desired.
the solvent-wetted (i.e., unactivated) MOF crystals at a
temperature that is similar to that of the BOC deprotection,
we cannot completely distinguish these two processes by TGA.
However, it is possible that the volume vacated by the BOC
groups in CatBrO MOF is replaced by solvent molecules,
resulting in a mass loss similar to that of BOC-CatBrO MOF.
Metallation of Catechol Group. Given that the
deprotection of BOC-CatBrO MOF afforded unfragmented
crystals of CatBrO MOF, we proceed to metallate it with
VO(acac)2 in THF to give the metallated derivative V-CatBrO
MOF, with metal loadings as high as 0.72 V/catechol (average
= 0.35 V/Zn, expected 0.5 V/Zn) as determined by ICP-OES
analysis (Scheme 2). The TGA profile of the metallated MOF
appears nearly unchanged from the parent MOF, indicating
retention of large solvent-accessible pores that can facilitate
catalysis (Figure S44 in the Supporting Information). Metal-
Fortunately, the thermolabile BOC protecting group does
offer a facile path for “traceless” deprotection,51 as shown by
the TGA profile of L1d (Figure S39 in the Supporting
Information), which indicates ∼35% mass loss starting at 130
°C, corresponding to the thermolysis of the BOC groups. As
the TGA plot of BOC-CatBrO MOF activated at room
temperature under vacuum (to preserve the thermolabile BOC
groups) shows an initial mass loss step (∼12%, Figure S40 in
the Supporting Information), which we attributed to the BOC
group (theoretical BOC group mass % = 15%), and the
components of PG-CatBrO MOFs are thermally stable up to
450 °C (see TGA discussion above), this relatively low
deprotection temperature should not pose a problem. Indeed,
the BOC groups could be removed by carrying out the
deprotection of BOC-CatBrO MOF in 1,2-dichlorobenzene at
t
lation in MeOH, DMF, BuOH, and dioxane did not work as
well, yielding materials with either low metal loadings or
reduced crystallinity (Table S4 and Figure S43 in the
Supporting Information). As a control, subjecting BOC-
CatBrO MOF to the same metallation procedure in THF
resulted in low metal loading (0.07 V/Zn), further confirming
the importance of the catechol moiety for binding the vanadyl
ion.
1
140 °C. Removal of the BOC group was confirmed by the H
NMR spectrum of a concentrated HCl/DMSO-d6-digested
MOF sample (Figure 3). In addition, the FTIR spectrum of
Analysis of V-CatBrO MOF using X-ray photoelectron
spectroscopy (XPS) and electron paramagnetic resonance
(EPR) spectroscopy indicates the presence of a OVIV species
with V2p binding energy of 516 eV (Figure S45 in the
Supporting Information) and EPR signals that are comparable
to those reported for solid-supported vanadyl species (see
Figure S46 in the Supporting Information for more details).
Because the chelating catecholate ligand is dianionic, complex-
ation of the OVIV moiety to it balances out the remaining
charges so that additional coordinated ligand, if any, must be
neutral. While it is possible that one or both of the neutral
acetylacetonate (Hacac) ligands remain bound to the V center
after metallation, we were unable to confirm their presence (or
absence) using FTIR spectroscopy (Figure S48 in the
Supporting Information) given the overlapping peaks in the
1500−1600 cm−1 region of the spectra of the MOFs with those
expected from the CC bond of the conjugated α-carbonyl
enol (∼1526 cm−1) and CO (∼1562 cm−1) of the acac
(quoted values are for VO(acac)2). Indeed, the spectra of
CatBrO MOF and the metallated V-CatBrO MOF are quite
similar. However, the FTIR spectrum of a physical mixture of
CatBrO MOF and VO(acac)2 (0.35 mol %, to simulate the
theoretical loading of one acac per vanadyl ion in MOF)
exhibits a noticeable growth in the 1526−1560 cm−1 region,
relative to the adjacent 1620 cm−1 peak, that is not seen in the
spectrum V-CatBrO MOF, suggesting that either Hacac is not
present in V-CatBrO MOF or that FTIR is insensitive to the
small amount of Hacac in the sample. Nonetheless, even if a
small amount of Hacac is still bound to the V center, we expect
it to be labile enough for displacement by substrates or solvents
during catalysis, leading to accessible open metal sites for
catalysis.
1
Figure 3. H NMR spectra in concentrated HCl/DMSO-d6 (1:9 v/v)
of (a) L1d, (b) BOC-CatBrO MOF, and (c) CatBrO MOF,
highlighting the disappearance of the tert-butyl protons at 1.5 ppm
(boxed region) in (c). The complex splitting pattern of the aromatic
protons in (b) indicates a possible mixture of di- and monoprotected
L1d as well as deprotected L1d in the BOC-CatBrO MOF. Integration
of tert-butyl protons in (b) relative to aromatic protons of L1d
indicates 1/4 of BOC groups are cleaved. See Figure S33 in the
Supporting Information for further analysis.
CatBrO MOF indicated the complete disappearance of the
CO stretch of the BOC group around 1775 cm−1 after
thermal treatment (Figure S42 in the Supporting Information).
The PXRD pattern of the deprotected CatBrO MOF (Figure
S43 in the Supporting Information), which is quite similar to
that of the starting materials, showed that the crystallinity of the
framework was maintained. This is further supported by the
observation that the TGA profile of CatBrO MOF is also very
similar to that of BOC-CatBrO MOF, showing ∼55 wt % loss
before 450 °C (Figure S44 in the Supporting Information).
Because the trapped DMF solvent molecules are removed from
Although a crystal structure of V-CatBrO MOF could not be
obtained, its PXRD pattern indicates that the crystallinity of the
starting BOC-CatBrO MOF was maintained throughout the
two postsynthesis modification steps (Figure S43 in the
Supporting Information). While similar sequences of mod-
ification have been carried out on derivatives of UMCM-1,30
the catalytic utility of the resulting metallated MOFs was not
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Scheme 2. A Schematic Illustration of the Synthesis of V-CatBrO MOF via Postsynthesis Deprotection Followed by
a
Metallation
a
The [VO] notation represents a generic vanadyl ion and indicates the possibility that other coordinated ligands (solvent or neutral
acetylacetonate) may exist around the V center.
demonstrated. However, Rosseinsky and co-workers have
demonstrated that V-modified MOF, obtained by postsynthesis
incorporation of VO(acac)2 into salicylaldehyde-funcationalized
IR-MOF-3, is catalytically active for cyclohexene oxidation.53
Thus, given the large pores of V-CatBrO MOF and the
unsaturated (labile) coordination environment around its
oxidation-capable V centers, we hypothesized that it would
also be catalytically active in oxidation catalysis, particularly
towards relatively large substrates. We chose to explore the
benzylic oxidation of tetralin given that large porous MOFs
such as MIL-101 have been shown to catalyze this reaction with
high selectivity.54,55
Catalysis. As expected, V-CatBrO MOF is catalytically
active for the benzylic oxidation of tetralin in the presence of
tert-butyl hydroperoxide (TBHP) oxidant (Figure 4). At 50 °C
in chlorobenzene, tetralin was oxidized to mainly tetralol and
tetralone (∼1:3 molar ratio) at 45% total conversion by 24 h
(Figure 4). This activity is comparable to that of the
homogeneous VO(acac)2, whose products comprise more of
the overoxidized product tetralone (1:11 tetralol:tetralone
ratio) at comparable overall total conversion (Figure 4). The
PXRD pattern of V-CatBrO MOF after catalysis indicated that
crystallinity is partially maintained, with great reduction in the
intensity of the lowest angle peak and the appearance of
Figure 4. The reaction profile for the oxidation of tetralin into tetralol
and tetralone using VO(acac)2 or V-CatBrO MOF catalyst in the
presence of TBHP. Reactions were carried out in chlorobenzene at 50
°C and at a 100:100:1 (or 0.75) molar ratio of tetralin:TBHP:catalyst.
Exposing tetralin to TBHP alone under the same conditions does not
additional peaks/noise (Figure S50 in the Supporting
Information). Nonetheless, the porosity of the crystals as
determined at TGA was preserved (Figure S51 in the
Supporting Information), and the crystal size was unaffected
by catalysis (Figure S52 in the Supporting Information).
Noncoordinating solvent such as chlorobenzene is the best
medium for catalysis with V-CatBrO MOF because acetonitrile
caused significant leaching of vanadium ions in our hands.
Catalyst-filtration test (Figure 4) showed that the catalysis by
V-CatBrO MOF in chlorobenzene is mostly heterogeneous
result in oxidation. Catalysis profiles are designated as follow: solid
lines, V-CatBrO MOF; dashed lines, the filtered-off (abbreviated as f)
supernatant at 4 h in the reaction initially catalyzed by V-CatBrO
MOF; dotted lines, homogeneous VO(acac)2 (abbreviated as h).
Products designation: tetralone, green circles; tetralol, purple triangles.
CONCLUSIONS
■
By using a combination of protecting groups, postsynthesis
deprotection, and postsynthesis metallation, we were able to
extend the pillared paddlewheel MOF platform system to
display a novel and catalytically active vanadyl-
(monocatecholate) motif that is inaccessible homogeneously.
In our hands, the thermolabile BOC group offered traceless
deprotection of BOC-CatBrO MOF to give crystalline CatBrO
MOF with large pores that can be loaded with a high density of
vanadyl ions. The resulting metallated MOF can catalyze the
benzylic oxidation of a large substrate such as tetralin in the
with minimal contribution from any leached metal ions.56
A
control experiment using “metallated” BOC-CatBrO MOF
(with a low 0.07 V/Zn ratio, see above) showed no catalytic
activity at 4 h and only minimal activity after 24 h (only 4%
tetralone and 2% tetralol yields, see Figure S49 in the
Supporting Information), further supporting our hypothesis
that the catalytic activity observed in V-CatBrO MOF is
primarily due to the (catecholate)VO moiety.
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ASSOCIATED CONTENT
* Supporting Information
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Materials and methods, ligand synthesis, NMR spectra of
ligands, photographic images of MOFs, selected single-crystal
X-ray diffraction data, TGA, CO2 isotherms, NMR spectra of
digested MOFs, ICP-OES data, PXRD patterns, and FTIR,
XPS, and EPR spectra of PG-CatBrO MOF, CatBrO MOF,
and V-CatBrO MOF. This material is available free of charge
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AUTHOR INFORMATION
Corresponding Author
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467-3347 (S.T.N.). Fax: 847-491-7713. E-mail: o-farha@
northwestern.edu (O.K.F.); j-hupp@northwestern.edu
(J.T.H.); stn@northwestern.edu (S.T.N.).
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Author Contributions
M.H.W., O.K.F., J.T.H., and S.T.N. conceived the initial
experiment. H.G.T.N. and M.H.W. carried out all of the
experiments and measurements. A.A.S. solved the single crystal
structures. Z.A. performed XPS measurements. M.R.W.
supervised the EPR experiments and analysis performed by
D.M.G. and R.C. O.K.F., J.T.H., and S.T.N. supervised the
project. H.G.T.N. and M.H.W. wrote the initial drafts of the
paper. H.G.T.N. and S.T.N. finalized the manuscript with
contributions from all authors. All authors have given approval
to the final version of this manuscript.
Funding
This work was supported by the Chemical Sciences, Geo-
sciences, and Biosciences Division, Office of Basic Energy
Sciences, U.S. Department of Energy (grant DEFG02-
03ER15457 to the Institute of Catalysis for Energy Processes
at Northwestern University (assistantship for H.G.T.N.) and
grant no. DE-FG02-99ER14999 to M.R.W.). D.M.G. was
supported by the Department of Defense through the National
Defense Science & Engineering Graduate Fellowship
(NDSEG) Program. R.C. was supported as part of the
ANSER Center, an Energy Frontier Research Center funded
by the U.S. Department of Energy (DOE), Office of Science,
Office of Basic Energy Sciences, under award number DE-
SC0001059.
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Notes
The authors declare no competing financial interest.
(33) Meng, L.; Cheng, Q.; Kim, C.; Gao, W.-Y.; Wojtas, L.; Chen, Y.-
S.; Zaworotko, M. J.; Zhang, X. P.; Ma, S. Angew. Chem., Int. Ed. 2012,
51, 10082−10085.
ACKNOWLEDGMENTS
We thank the reviewers of the initial version of this manuscript
for suggestions that greatly strengthened the final work.
■
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