Metal-Organic Framework Nodes Support Single-Site Magnesium-Alkyl Catalysts for Hydroboration and Hydroamination Reactions

Article abstract of DOI:10.1021/jacs.6b03689

Here we present the first example of a single-site main group catalyst stabilized by a metal-organic framework (MOF) for organic transformations. The straightforward metalation of the secondary building units of a Zr-MOF with Me₂Mg affords a hi

Full text of DOI:10.1021/jacs.6b03689

Communication
pubs.acs.org/JACS
Metal−Organic Framework Nodes Support Single-Site Magnesium−
Alkyl Catalysts for Hydroboration and Hydroamination Reactions
†
†
†
Kuntal Manna, Pengfei Ji, Francis X. Greene, and Wenbin Lin*
Department of Chemistry, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States
*
S Supporting Information
20
tionation reactions. Herein, we report a simple strategy to
stabilize active heteroleptic Mg-alkyl species at secondary
building unit (SBUs) of TPHN-MOF (TPHN = 4,4′-bis-
ABSTRACT: Here we present the first example of a
single-site main group catalyst stabilized by a metal−
organic framework (MOF) for organic transformations.
The straightforward metalation of the secondary building
(
carboxyphenyl)-2-nitro-1,1′-biphenyl) to afford highly active
and reusable single-site solid catalysts for hydroboration of
carbonyl and imine compounds and for hydroamination of
aminoalkenes. Site isolation within the MOF stabilizes catalyti-
cally active heteroleptic Mg-alkyl species by shutting down
Schlenk-type intermolecular ligand redistribution reactions
units of a Zr-MOF with Me Mg affords a highly active and
2
reusable solid catalyst for hydroboration of carbonyls and
imines and for hydroamination of aminopentenes.
Impressively, the Mg-functionalized MOF displayed very
4
high turnover numbers of up to 8.4 × 10 for ketone
(
Figure 1).
hydroboration and could be reused more than 10 times.
MOFs can thus be used to develop novel main group solid
catalysts for sustainable chemical synthesis.
TPHN-MOF of UiO-69 topology was synthesized via a
solvothermal reaction between ZrCl and H TPHN in the
4
2
14d
presence of DMF and trifluoroacetic acid in 95% yield. The
metalation of Zr (μ -OH) sites in SBUs of TPHN-MOF was
3
3
performed by treating TPHN-MOF with Me Mg in THF at
2
espite their abundance and biocompatibility, alkaline earth
room temperature to afford Mg-functionalized MOF material
(TPHN-MOF-MgMe) as a yellow solid (Figure 2a). During the
metalation reaction, an equivalent amount of methane was
generated, which was identified and quantified by GC analysis
(Figure S4, Supporting Information [SI]). The disappearance of
D
metals such as Mg and Ca have found limited application in
catalytic processes. Developing catalysts that use these metals can
be challenging because of the propensity of heteroleptic
complexes to undergo Schlenk-type and/or irreversible ligand
redistribution reactions that result in catalytically incompetent
−1
the νμ3‑OH band (∼3629 cm , KBr) in the infrared spectrum of
MOF-MgMe indicated that the metalation occurred at Zr (μ -
1
homoleptic complexes. To counter such associative intermo-
3
3
lecular ligand redistribution reactions, bulky chelating ligands,
OH) sites (Figure S5, SI). Inductively coupled plasma mass
spectrometry (ICP-MS) analysis of the digested MOF-MgMe
revealed 100% metalation at the Zr (μ -OH) sites, correspond-
2
such as β-diketiminate, aminotroponates, aminotropon-
3
4
5
iminates, pybox, silylamido phenolate, and tris(oxazolinyl)-
3
3
6
borate, have been explored in the preparation of main group
ing to four Mg centers per Zr node. Crystallinity of TPHN-
6
catalysts for a range of organic transformations, such as
MOF was maintained upon metalation, as shown by the
similarities in the PXRD patterns of TPHN-MOF and MOF-
MgMe (Figure 2b). SEM images showed that TPHN-MOF has
particle sizes of ∼2−3 μm (Figure S9, SI). The single-crystal X-
ray diffraction study revealed that MOF-MgMe crystallizes in the
2
c,3,7
8
9
hydroamination,
hydrosilylation, hydroboration, hydro-
1
0
4
phosphination, Friedel−Crafts alkylation, and ring-opening
2
a
polymerization. Although sterically encumbered ligands
stabilize the heteroleptic species, they also often impede their
catalytic activity by slowing down the binding and activation of
substrates. In contrast, the immobilization of heteroleptic
̅
6 3 4 3 4
Fm3m space group, with the Zr (μ -O) (μ -OH) SBUs
connected by the TPHN bridging linkers to afford the 12-
connected fcu topology. However, due to crystallographic
disorder of the MgMe moiety, the Mg coordination environ-
ments in MOF-MgMe could not be established by X-ray
crystallography (Figure S10, SI). The four MgMe moieties are
1
1
alkaline earth metal species in porous solid supports, such as
metal−organic frameworks (MOFs), can provide site isolation
without relying on bulky ligands and thus provide an alternative
route for obtaining active catalysts.
MOFs have recently emerged as an interesting class of highly
randomly distributed among the eight μ -oxo positions. In
3
addition, every μ -O-MgMe moiety is in close proximity to six
porous molecular materials with many applications, including gas
4
1
2
13
14
15
carboxylate oxygen atoms; one or two of the carboxylate oxygen
storage, separation, catalysis, nonlinear optics, biomedical
1
6
17
18
atoms could possibly tilt the Mg atom off the C -axis through
imaging, chemical sensing, drug delivery, and solar energy
3
1
9
weak coordination. These disorders lead to low site occupancy of
harvesting. MOFs provide a highly tunable platform for the
engineering of single-site solid catalysts for organic trans-
formations that cannot be performed by traditional porous
inorganic materials. We recently showed that highly active base-
metal catalysts can be stabilized in bipyridyl-based MOFs via
active site isolation, while analogous homogeneous catalysts
undergo rapid intermolecular deactivation via ligand dispropor-
1
/6 for the MgMe moiety, if we assume Mg coordinates to two
carboxylate groups, thus making Mg atoms impossible to observe
in the Fm3m space group (Figure S10, SI).
̅
Received: April 10, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b03689
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Journal of the American Chemical Society
Communication
Table 1. TPHN-MOF-Mg-Catalyzed Hydroboration of
a
Ketones and Aldehydes
Figure 1. Schematic showing the beneficial effect of active site isolation
within a MOF on stabilizing heteroleptic Mg-species by preventing
Schlenk-type ligand redistribution reactions.
a
Reaction conditions: 1.0 mg of MOF-MgMe, carbonyl substrate,
b
1
hexanes, HBpin (1.1 equiv), 23 °C. Yield was determined by H
NMR with mesitylene as the internal standard. Isolated yields are in
parentheses. THF was used as the solvent.
c
Figure 2. (a) Synthesis of TPHN-MOF and metalation of its SBUs with
Me Mg to form MOF-MgMe. (b) Similarities among the PXRD
2
patterns simulated from the CIF file (SI) of TPHN-MOF (black) and
the PXRD patterns of MOF-MgMe (green), MOF-Mg samples
recovered from hydroboration of acetophenone after run 1 (red) and
after run 11 (blue), and after hydroboration of N-benzylidene-
benzenamine show the retention of TPHN-MOF crystallinity after
metalation and catalysis.
Mg afforded borate ester products from a range of carbonyl
substrates, including alkyl, halogenated, and alkoxy-function-
alized aryl ketones and aldehydes in essentially quantitative yields
(
Table 1). Pure hydroboration products were obtained by simply
removing the solid catalyst via centrifugation followed by
removal of the organic volatiles. Impressively, a turnover number
(
TON) of 84 000 was obtained for hydroboration of
TPHN-MOF-Mg displayed excellent activity in the hydro-
boration of a wide range of carbonyl compounds with
pinacolborane (Table 1). The hydroboration reactions were
performed by treating ketones or aldehydes with equimolar
HBpin in the presence of 0.1−0.01 mol % MOF-Mg in hexane or
THF at room temperature. At a 0.05 mol % Mg loading, MOF-
acetophenone (entry 2, Table 1). Linear and cyclic aliphatic
ketones were efficiently reduced in excellent yields (entries 8−
10, Table 1). In addition, α,β-unsaturated carbonyl substrates
such as cyclohexenone, trans-crotonophenone, and trans-
cinnamaldehyde were reduced selectively at the carbonyl, leaving
the CC bond intact (entries 10, 11, and 17, Table 1).
B
DOI: 10.1021/jacs.6b03689
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Journal of the American Chemical Society
Communication
Table 3. TPHN-MOF-Mg-Catalyzed Hydroamination of
a
Aminoalkenes
a
Reaction conditions: MOF-MgMe (1.0 mol % Mg), aminoalkene,
b
1
benzene, 80 °C. Yields were determined by H NMR with mesitylene
as the internal standard. Isolated yields are in parentheses.
experiments. ICP-MS analyses showed that the amounts of Mg
and Zr leaching into the supernatant after the 1st run were only
1
.6% and 0.02%, respectively, and after the 11th run were 1.3%
Figure 3. (a) Plot of yields (%) of borate ester at different runs in the
reuse experiments of TPHN-MOF-Mg (0.2 mol % Mg) for hydro-
boration of acetophenone. (b) Kinetic plots of initial rates (d-
and 0.02%, respectively. Moreover, no further hydroboration was
observed after removal of MOF-Mg from the reaction mixture,
which rules out the role of the leached Mg species in catalyzing
hydroboration reactions (Figure S12, SI). An additional control
experiment using the unmetalated TPHN-MOF did not afford
the hydroboration product, indicating that the catalysis occurred
at the supported Mg centers at the SBUs.
[
acetophenone]/dt) for hydroboration of acetophenone versus [Mg]
and [acetophenone]initial for the first 10 min, showing the first-order
dependence on both components. (c) Proposed catalytic cycle for
TPHN-MOF-Mg-catalyzed hydroboration reactions.
Two types of pathways were proposed for the Mg-catalyzed
hydroboration of carbonyls: (a) σ-bond metathesis involving
Table 2. TPHN-MOF-Mg-Catalyzed Hydroboration of
Imines
a
9
insertion of a CO into a Mg−H bond or (b) zwitterionic
21
mechanism without involvement of Mg−H bond species. The
mechanism of MOF-Mg-catalyzed hydroboration reactions was
investigated by spectroscopic techniques, gas quantification, and
kinetic studies. The treatment of MOF-MgMe (1 mol % Mg)
with acetophenone followed by addition of HBpin (PhCO-
Me:HBpin = 1.2:1.0) in benzene immediately generated
MeBpin, and hydroboration reaction occurred without a
detectable induction period. After completion of the hydro-
boration reaction, Zr (μ -O)-Mg(OCHMePh) was identified
3
4
the intermediate (Figure S15, SI). In addition, Zr (μ -O)-
3
4
Mg(OMe), prepared by treatment of MOF-MgMe with 10 equiv
wrt Mg) of MeOH was also active in catalyzing hydroboration
(
of acetophenone. We thus infer that the rapid reaction between
Zr (μ -O)-MgMe with 1 equiv of HBpin afforded the Zr (μ -O)-
3
4
3
4
MgH intermediate, which is likely the active catalyst for
hydroboration reactions. The empirical rate law, determined by
the method of initial rates (<10% conversion), showed that the
hydroboration of acetophenone catalyzed by MOF-Mg has a
first-order dependence on the Mg and acetophenone concen-
trations (Figure 3b) and a zeroth-order dependence on the
HBpin concentration (Figure S16, SI). Based on our
experimental observations, the proposed pathways for hydro-
boration of carbonyls likely involved the insertion of CO into
a
Reaction conditions: 1.0 mg of MOF-MgMe (0.5 mol % Mg), imine,
b
1
HBpin, heptane, 70 °C. Yields were determined by H NMR with
mesitylene as the internal standard. 0.05 mol % Mg was used.
c
Remarkably, at a 0.2 mol % catalyst loading, MOF-Mg could
be recovered and reused for hydroboration of acetophenone at
least 10 times (Figure 3a) without loss of MOF crystallinity
the [Mg]−H bond in the turnover-limiting step to give the Zr -
3
1
2
(
Figure 2b). Excellent yields (95−100%) of borate ester were
(μ -O)-Mg(OCHR R ) intermediate. The reaction of Zr -(μ -
4
3
4
1
2
obtained consistently in the reuse experiments with no
observation of other byproducts. The PXRD patterns of MOF-
Mg recovered from the 1st and 11th runs remained unchanged
from that of pristine MOF-Mg (Figure 2b), indicating the
stability of the MOFs under reaction conditions. The
heterogeneity of MOF-Mg was confirmed by the following
O)-Mg(OCHR R ) species with HBpin furnished the borate
ester product and regenerated Zr (μ -O)-MgH in the cycle
3
4
(Figure 3c).
Similar to the hydroboration of carbonyls, TPHN-MOF-Mg is
also an active catalyst in the hydroboration of imines to give N-
22
borylamines. At a 0.05 mol % Mg loading, TPHN-MOF-Mg-
C
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Journal of the American Chemical Society
Communication
catalyzed hydroboration of N-benzylideneaniline with HBpin in
heptane at 70 °C for <12 h afforded N-benzyl-N-pinacolboryl-
aniline in quantitative yield (entry 1, Table 2). MOF-Mg
recovered after this reaction remained crystalline, as shown by
PXRD (Figure 2b), and the leaching of Mg and Zr into the
supernatant was 2.2% and 0.07%, respectively. The hydro-
boration of substituted imines, however, required higher catalyst
loading and longer reaction times (entries 2−5, Table 2),
presumably due to the decreased rates of diffusion of the larger
substrates and products through the MOF channels.
supported by the U.S. DOE under Contract No. DE-AC02-
06CH11357.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b03689.
■
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AUTHOR INFORMATION
Corresponding Author
wenbinlin@uchicago.edu
■
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*
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Author Contributions
K.M., P.J., and F.X.G. contributed equally.
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†
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by National Science Foundation (NSF,
grant no. CHE-1464941). We thank Z. Lin, D. Micheroni, and C.
Poon for experimental help. Single-crystal diffraction studies
were performed at ChemMatCARS, APS, Argonne National
Laboratory (ANL). ChemMatCARS is principally supported by
the Divisions of Chemistry and Materials Research, NSF, under
grant no. NSF/CHE-1346572. Use of the Advanced Photon
Source, an Office of Science User Facility operated for the U.S.
Department of Energy (DOE) Office of Science by ANL, was
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and More; Wiley-VCH: Weinheim2014; pp 1135−1258. (d) Crimmin,
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3
D
DOI: 10.1021/jacs.6b03689
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX