Single-Site Cobalt Catalysts at New Zr8(μ2-O)8(μ2-OH)4 Metal-Organic Framework Nodes for Highly Active Hydrogenation of Alkenes, Imines, Carbonyls, and Heterocycles

Article abstract of DOI:10.1021/jacs.6b06759

We report here the synthesis of robust and porous metal-organic frameworks (MOFs), M-MTBC (M = Zr or Hf), constructed from the tetrahedral linker methane-tetrakis(p-biphenylcarboxylate) (MTBC) and two types of secondary building units (SBUs): cubic M₈(μ₂-O)₈(μ₂-OH)₄ and octahedral M₆(μ₃-O)₄(μ₃-OH)₄. While the M₆-SBU is isostructural with the 12-connected octahedral SBUs of UiO-type MOFs, the M₈-SBU is composed of eight M^IV ions in a cubic fashion linked by eight μ₂-oxo and four μ₂-OH groups. The metalation of Zr-MTBC SBUs with CoCl₂, followed by treatment with NaBEt₃H, afforded highly active and reusable solid Zr-MTBC-CoH catalysts for the hydrogenation of alkenes, imines, carbonyls, and heterocycles. Zr-MTBC-CoH was impressively tolerant of a range of functional groups and displayed high activity in the hydrogenation of tri- and tetra-substituted alkenes with TON > 8000 for the hydrogenation of 2,3-dimethyl-2-butene. Our structural and spectroscopic studies show that site isolation of and open environments around the cobalt-hydride catalytic species at Zr₈-SBUs are responsible for high catalytic activity in the hydrogenation of a wide range of challenging substrates. MOFs thus provide a novel platform for discovering and studying new single-site base-metal solid catalysts with enormous potential for sustainable chemical synthesis.

Full text of DOI:10.1021/jacs.6b06759

Article
pubs.acs.org/JACS
Single-Site Cobalt Catalysts at New Zr8(μ2‑O)8(μ2‑OH)4 Metal-Organic
Framework Nodes for Highly Active Hydrogenation of Alkenes,
Imines, Carbonyls, and Heterocycles
Pengfei Ji,† Kuntal Manna,† Zekai Lin, Ania Urban, Francis X. Greene, Guangxu Lan, and Wenbin Lin*
Department of Chemistry, University of Chicago, 929 E. 57th St., Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: We report here the synthesis of robust and porous
metal−organic frameworks (MOFs), M-MTBC (M = Zr or Hf),
constructed from the tetrahedral linker methane-tetrakis
(p-biphenylcarboxylate) (MTBC) and two types of secondary
building units (SBUs): cubic M8(μ2-O)8(μ2-OH)4 and octahe-
dral M6(μ3-O)4(μ3-OH)4. While the M6-SBU is isostructural
with the 12-connected octahedral SBUs of UiO-type MOFs, the
M8-SBU is composed of eight MIV ions in a cubic fashion linked
by eight μ2-oxo and four μ2-OH groups. The metalation of
Zr-MTBC SBUs with CoCl2, followed by treatment with NaBEt3H,
afforded highly active and reusable solid Zr-MTBC-CoH cata-
lysts for the hydrogenation of alkenes, imines, carbonyls, and
heterocycles. Zr-MTBC-CoH was impressively tolerant of a
range of functional groups and displayed high activity in the hydrogenation of tri- and tetra-substituted alkenes with TON > 8000
for the hydrogenation of 2,3-dimethyl-2-butene. Our structural and spectroscopic studies show that site isolation of and open
environments around the cobalt-hydride catalytic species at Zr8-SBUs are responsible for high catalytic activity in the
hydrogenation of a wide range of challenging substrates. MOFs thus provide a novel platform for discovering and studying new
single-site base-metal solid catalysts with enormous potential for sustainable chemical synthesis.
Such steric protection is important for stabilizing weak-field
ligand-coordinated metal catalysts, particularly for late first-row
transition metals in a very weak field coordination environment
consisting of oxygen-donor atoms.7 However, steric protecting
groups often weaken metal−ligand binding and impede the
catalytic activity by preventing challenging hydrogenation sub-
strates, such as tri- and tetra- substituted olefins, from accessing
the catalytic sites.8 Immobilization of catalytic species in
structurally regular porous solid supports can provide catalytic
site isolation without relying on bulky ligands, thus offering an
alternative route to obtaining highly active base metal catalysts.
Significant efforts have been devoted to the development of
zeolite-, silica-, or graphene-supported iron- and cobalt-based
heterogeneous hydrogenation catalysts6a,9 and bare or protected
metallic nanoparticles-based catalysts.10 However, the activities
and stability of these heterogeneous hydrogenation catalysts are
still far from satisfactory.
INTRODUCTION
■
Hydrogenation of unsaturated organic compounds is one of the
most widely practiced metal-catalyzed reactions in organic
synthesis and the chemical industry, with applications in the
production of commodity chemicals, pharmaceuticals, and
agrochemicals.1 For decades, hydrogenation reactions have
relied on precious metal catalysts supported either on solid
surfaces2 or by strong field ligands3 to enable two-electron redox
chemistryoxidative addition and reductive eliminationthat
constitute key bond-breaking and bond-forming steps in the
catalytic cycle. However, the low abundance, high price, and
inherent toxicity of precious metals have led to intense interest in
developing earth-abundant metal catalysts.4 Significant progress
has been made in recent years on the development of single-site
hydrogenation catalysts based on iron, cobalt, nickel, or copper
coordinated with sterically encumbered strong field nitrogen- or
phosphorus-donor ligands.5 However, each homogeneous
base metal catalyst typically only hydrogenates a narrow class
of substrates with limited turnover numbers. Furthermore, few
examples of earth-abundant metal-catalyzed hydrogenation of
imines and heterocycles exist, and all of them require harsh
reaction conditions.6
Metal−organic frameworks (MOFs),11 constructed from metal
cluster secondary building units (SBUs) and organic linkers, have
emerged as a tunable class of porous and crystalline supports for
discovering reusable single-site solid catalysts for organic
transformations.12 By using premetalated organic struts or via
postsynthetic metalation of functionalized bridging linkers,
Homogeneous base metal catalysts typically rely on coor-
dination of sterically bulky chelating ligands to prevent the
formation of catalytically incompetent oligomeric species by
shutting down the intermolecular decomposition pathways.
Received: June 30, 2016
© XXXX American Chemical Society
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MOFs have provided a highly tunable platform to engineer
single-site solid catalysts for many organic transformations that
cannot be performed by traditional porous inorganic materials.13
Furthermore, the SBUs of MOFs consisting of inorganic oxide
clusters can be used as oxygen-donor ligands,14 thus providing an
unprecedented opportunity for developing robust, site-isolated,
base-metal catalysts in a weak field coordination environment.
The synthetic tunability of MOFs makes it possible to fine-tune
electronic and steric properties of catalytic active sites, whereas
the structure regularity and site homogeneity of MOFs greatly
facilitate mechanistic studies of MOF-catalyzed reactions.
We recently designed a highly active and chemoselective
cobalt-hydride catalyst supported on the Zr6(μ3-O)4(μ3-OH)4
node of UiO-68 MOF for benzylic C−H functionalization and
olefin hydrogenation.15 Although the potential of Zr3-μ3-OH as a
ligand site was realized, the μ3-OH site is surrounded by three
phenyl rings around 105° apart from each other, which forms a
very sterically encumbered local environment, thus making the
Zr3-μ4-O-CoH catalyst inactive in the hydrogenation of chal-
lenging substrates, such as tetrasubstituted olefins and hetero-
cycles. Design and synthesis of analogous Zr-based clusters with
open environments around the SBUs would make the metal
node-supported cobalt catalysts more active in the hydro-
genation of challenging substrates and thus potentially applicable
in industrial hydrogenations. Herewe report a new Zr8(μ2-O)8(μ2-
OH)4 MOF node with sterically open Zr2-μ2-OH ligand sites that
support Co-based catalysts for the hydrogenation of a broad
range of unsaturated compounds such as olefins, imines, car-
bonyls, and heterocycles.
RESULTS AND DISCUSSION
■
Synthesis of M-MTBC (M = Zr or Hf) MOFs. Zr-MTBC
was synthesized in 54% yield via a solvothermal reaction between
ZrCl4 and 4′,4‴,4′′‴,4‴‴′-methanetetrayltetrakis([1,1′-biphen-
yl]-4-carboxylic acid) (H4MTBC) in DEF using benzoic acid as
modulator. A single-crystal X-ray diffraction study of Zr-MTBC
indicated that Zr-MTBC crystallizes in the cubic pm3n space group
and revealed the presence of two types of SBUs, Zr8(μ2-O)8
(μ2-OH)4 and the Zr6(μ3-O)4(μ3-OH)4 in 1:3 ratio (Figure 1a).
To our knowledge, the Zr8(μ2-O)8(μ2-OH)4 SBU has not been
synthesized before as either a discrete cluster or as a structural
unit in a MOF.16 In the Zr8(μ2-O)8(μ2-OH)4 SBU, eight ZrIV
ions occupy the eight corners of the cube, while eight μ2-oxo and
four μ2-OH occupy the 12 edges of the cube. The Zr6(μ3-O)4
(μ3-OH)4 unit is isostructural to the SBU of UiO-MOF, with six
ZrIV ions occupying six corners of an octahedron that are held
together by four μ3-oxo and four μ3-OH groups at eight faces of
the octahedron. Solid-state infrared spectrum (IR) spectrum
showed the presence of both the νμ O−H stretching band at
2
3737 cm−1 and νμ O−H stretching band at 3639 cm−1 (Figure 1b).
3
The void space was calculated to be 73.53% by PLATON.
The MOF possessed two kinds of trigonal-bipyramid cavities
Figure 1. (a) The synthesis of M-MTBC (M = Zr or Hf). (b) IR spectrum of Zr-MTBC showing stretching vibration of μ3-OH at 3639 cm−1 from
Zr6-SBU and μ2-OH at 3737 cm−1 from Zr8-SBU. (c) Nitrogen sorption isotherms of Zr-MTBC (77 K). Inset shows pore size distribution in Å. (d) SEM
image of Zr-MTBC.
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of dimensions 24.9 × 21.6 × 35.9 Å and 20.8 × 20.8 × 13.1 Å,
respectively. N2 adsorption isotherm of Zr-MTBC showed a
type I adsorption (77 K, 1 bar) with Brunauer−Emmett−Teller
(BET) surface area of 3700 m2/g (Figure 1c). SEM and TEM
images showed cubic particles of 1−3 μm in length (Figure 1d).
The Hf-MTBC analog was synthesized similarly and charac-
terized by single-crystal X-ray diffraction (Figure S9, SI).
Metalation of Zr-MTBC with CoCl2. Zr-MTBC was treated
Zr-MTBC-CoH proved to be a highly active catalyst for hydro-
genation of a range of alkenes in THF at room temperature.
At a 0.05 mol % Co loading, terminal alkenes containing oxo-,
carboxy-, pyridyl-, or silyl-functionalitiesallyl ether, allyl
acetate, dimethyl itaconate, 2-vinylpyridine, and allyltrimethylsi-
lanewere selectively hydrogenated to afford dipropylether,
propylacetate, dimethyl 2-methylsuccinate, 2-ethylpyridine, and
propyltrimethylsilane, respectively, in quantitative yields (entries
1−6, Table 1). At a 0.05−0.1 mol % Co loading, Zr-MTBC-CoH
was also very active in hydrogenation of trisubstituted alkenes
such as ethyl-3,3- dimethyl acrylate, α-terpinene, trans-α-methyl-
stilbene, and 1-methyl-1-cyclohexene, and corresponding pure
hydrogenated products were obtained in excellent yields by
simple filtration of reaction mixtures followed by removal of the
volatiles (entries 8−12, Table 1). Impressively, Zr-MTBC-CoH
completely hydrogenated tetrasubstituted alkenes such as
2,3-dimethyl-2-butene at room temperature within 48 h to afford
2,3-dimethylbutane with a TON > 8000 (entry 13, Table 1).
Hydrogenation of bulkier tetrasubstituted alkenes such as 2,3,4-
trimethylpent-2-ene could also be achieved at elevated temper-
atures, which facilitated their diffusion through MOF channels as
well as the binding and activation of the substrate at the cobalt-
site (entry 14, Table 1). It has been previously observed that
Zr3(μ4-O)Co site in Zr6-SBU of UiO-68 is inactive in catalyzing
the hydrogenation of bulky and rigid trisubstituted alkenes such
as 1-methyl-1-cyclohexene and tetrasubstituted alkenes.15 There-
fore, we believe that the hydrogenation of these bulky alkenes
occurred exclusively at the Zr2(μ3-O)Co sites in Zr8-SBUs of
Zr-MTBC-CoH. The hydrogenation of 6-methyl-5-hepten-2-
one to 6-methyl-2-heptanone catalyzed by Pd/C or Pd/Al2O3 is a
key step to synthesizing dimethyloctenol (DMOE), an important
fragrance compound.18 At a 0.5 mol % Co loading, Zr-MTBC-
CoH also selectively hydrogenated 6-methyl-5-hepten-2-one at
40 °C to afford 6-methyl-2-heptanone in quantitative yield (entry
15, Table 1). Interestingly, 6-methyl-2-heptanol was obtained
quantitatively upon heating the reaction mixture at 80 °C (entry
16, Table 1).
Importantly, at a 0.1 mol % Co-loading, Zr-MTBC-CoH could
be recovered and reused at least 5 times for the hydrogenation of
1-methylcyclohexene (Figure 3) without loss of MOF crys-
tallinity (Figure 2b). Excellent yields (92−100%) of methyl-
cyclohexane were obtained consistently in the reuse experiments
with no observation of other byproducts (Figure S25, SI).
The PXRD patterns of Zr-MTBC-CoH recovered from the first
and sixth runs remained unchanged from that of pristine
Zr-MTBC-CoH (Figure 2b), indicating the stability of the MOF
under reaction conditions. The heterogeneity of Zr-MTBC-CoH
was confirmed by several experiments. ICP-MS analyses showed
that the amounts of Co and Zr that leached into the supernatant
after the first run were only 1.6% and 0.02%, respectively. More-
over, the rate of hydrogenation was unchanged in the presence of
mercury, and no additional hydrogenation was observed after
removal of Zr-MTBC-CoH from the reaction mixture, which
ruled out the role of the leached Co-nanoparticles or other
Co-species in catalyzing hydrogenation reactions (Figure S24, SI).
Mechanistic Investigation of Zr-MTBC-CoH-Catalyzed
Hydrogenation of 1-Methylcyclohexene. As discussed above,
the treatment of Zr-MTBC-CoCl with NaEt3BH in THF gen-
erated Zr-MTBC-CoH species, which is likely the active cata-
lyst in the hydrogenation reactions. The EXAFS spectrum of
Zr-MTBC-Co recovered from hydrogenation of 1-methylcyclo-
hexene showed the absence of Co−Co scattering from Co
nanoparticles, ruling out the formation of any Co-nanoparticles
n
with 10 equiv of BuLi to deprotonate both the μ2-OH’s in
Zr8-SBU and the μ3-OH’s in Zr6-SBU, then reacted with a CoCl2
solution in THF to afford Zr-MTBC-CoCl as a deep-blue solid
(Figure 2a).
Both the carboxylate groups and the linkers remained intact
1
during lithiation and metalation, as evidenced by a H NMR
spectrum of the digested Zr-MTBC-CoCl in D3PO4/DMSO-d6
(Figure S15, SI) and by the retention of strong carboxylate
carbonyl stretching in the IR spectrum (Figure 1b). The dis-
appearance of both the νμ O−H band (3737 cm−1) and νμ O−H
2
3
band (3639 cm−1) in the IR spectrum indicated that the
metalation occurred at both SBU sites (Figure S12, SI). Induc-
tively coupled plasma-mass spectrometry (ICP-MS) analysis of
the digested MOF revealed complete metalation of all Zr8 and
Zr6 clusters, corresponding to four Co centers per Zr8 or Zr6
node. Crystallinity of the MOF was maintained after metalation,
as indicated by the similarity between the powder X-ray
diffraction (PXRD) patterns of Zr-MTBC and Zr-MTBC-
CoCl (Figure 2b). However, the coordination environments of
the Co centers in Zr-MTBC-CoCl could not be established
by X-ray diffraction due to intrinsic disorder of coordinated
cobalt atoms (Figure S16, SI), similar to our previously reported
examples.15 We instead used X-ray adsorption spectroscopy
(XAS) to investigate Co coordination environments. Four out of
six faces of the Zr8(μ2-O)8(μ2-OH)4 cubic node had a μ2-OH
group that could be lithiated and used for Co binding (Figure 2a).
We envisioned two different Co coordination modes on the Zr8
node: μ2-oxide/μ2-oxo chelation and μ2-oxide/(μ-carboxylate)2
tridentate binding. We realized that the μ2-oxide/μ2-oxo che-
lation binding mode would not be ideal because the μ2-oxide to
μ2-oxo distance was only 2.35 Å, too short for chelation to the
same Co center. Such a structural model does not fit the extended
X-ray adsorption fine structure (EXAFS) data (Figure S19, SI).
In contrast, the μ2-oxide and two μ-carboxylate groups could
coordinate to the same Co center in a stable conformation, with
Co to μ2-oxide distance of 1.88 Å and Co to μ-carboxylate
distance of 2.00 Å, as indicated by the EXAFS fitting result
(Figure 2d). Cobalt coordination on Zr6 node also adopts a
μ2-oxide/(μ-carboxylate)2 tridentate mode, identical to that
observed in the previously studied UiO-68-CoCl system.15
Activation of Zr-MTBC-CoCl with NaBEt3H for Olefin
Hydrogenation. Treating Zr-MTBC-CoCl with 5 equiv of
NaBEt3H in THF generated the cobalt-hydride species
Zr-MTBC-CoH as a black solid. We did not observe the for-
mation of hydrogen gas by GC analysis, indicating H/Cl metathesis
during the activation step. The reaction of Zr-MTBC-CoH with
10 equiv of formic acid at room temperature or with excess
water at 75 °C readily generated an equivalent amount of H2
(Figure S21, SI),17 supporting the identity of Zr-MTBC-CoH.
XANES analysis of Zr-MTBC-CoH suggested a +2 oxidation
state of Co (Figure 2c). EXAFS fitting on Zr-MTBC-CoH
indicated that the Co adopts μ2-oxide/(μ-carboxylate)2
tridentate binding mode, with Co to μ2-oxide distance of 1.83 Å
and Co to η2-carboxylate distance of 1.94, similar to the structure
proposed for Zr-MTBC-CoCl (Figure 2e).
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Figure 2. (a) The metalation of Zr8-SBUs and Zr6-SBUs of Zr-MTBC with CoCl2 to form Zr-MTBC-CoCl. (b) Similarities among the PXRD patterns
simulated from the CIF file of Zr-MTBC (black) and the PXRD patterns of as-synthesized Zr-MTBC (red), Zr-MTBC-CoCl (blue), Zr-MTBC-CoH
samples recovered from hydrogenation of 1-methylcyclohexene after runs 1 (pink) and 6 (green) and after hydrogenation of N-benzylidenebenzenamine
(navy) show the retention of Zr-MTBC crystallinity after metalation and catalysis. (c) XANES spectra of Zr-MTBC-CoCl and Zr-MTBC-CoH are similar
to that of CoCl2, indicating a +2 oxidation state for the Co centers in Zr-MTBC-CoCl and Zr-MTBC-CoH. (d) EXAFS spectra and fits in R-space at the Co
K-edge of Zr-MTBC-CoCl showing the magnitude (hollow squares, black) and real component (hollow squares, blue) of the Fourier transformation.
The fitting range is 1.2−5.6 Å in R space (within the gray lines). (e) EXAFS spectra and fits in R-space at the Co K-edge of Zr-MTBC-CoH showing the
magnitude (hollow squares, black) and real component (hollow squares, blue) of the Fourier transformation. The fitting range is 1.2−5.8 Å in R space
(within the gray lines).
during the catalysis (Figure S22, SI). To further investigate the
mechanism, the rate law was determined by the method of initial
rates (<15% conversion) in THF at room temperature. In order
to avoid complications caused by the presence of two kinds of
Co-centers in Zr-MTBC-CoH, the initial rates were measured
for hydrogenation of 1-methylcyclohexene catalyzed by only
Zr2(μ3-O)Co sites, since Zr3(μ4-O)CoH at the SBUs of UiO-68
was inactive in hydrogenation of 1-methylcyclohexene. The empirical
D
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a
Table 1. Zr-MTBC-CoH-Catalyzed Hydrogenation of Alkenes
a
b
Reaction conditions: 0.25 mg of Zr-MTBC-CoCl, 5 equiv of NaBEt3H (1.0 M in THF) w.r.t. Co, alkene, THF, 40 bar H2, 23 °C. Yields were
c
1
determined by H NMR with mesitylene as the internal standard. Isolated yield in the parentheses. Reaction was performed at 70 °C in toluene.
d
e
f
TON was calculated based on only Zr2(μ3-O)Co sites. Reaction was performed at 40 °C. Reaction was performed at 80 °C in toluene.
rate law showed that the initial rates had a first-order dependence
sought to investigate the hydrogenation of imines and carbonyls.
Zr-MTBC-CoH also displayed excellent activity in catalytic
hydrogenation of imines (Table 2). Though hydrogenation of
imines is an important synthetic route to amines, examples
of base metal catalysts for imine hydrogenation are rare.6b
Our imine hydrogenation reactions were performed in toluene at
80 °C under 40 bar of H2 in the presence of 0.5 mol % Zr-MTBC-
CoH. N-benzylidenebenzenamine was completely hydrogenated
to N-benzylaniline in 5 h. The pure product was isolated in 98%
yield after simple filtration followed by removal of the volatiles
in vacuo (entry 1, Table 1). The Zr-MTBC-CoH recovered after
this reaction remained crystalline, as shown by PXRD (Figure 2b),
and the leaching of Co and Zr into the supernatant was 0.23%
on the cobalt concentrations and PH (Figure 4a) and a zeroth-order
2
dependence on the alkene concentration (Figures 4a and S26, SI).
The activation of H2 at the electron-deficient Co(II)-center via
oxidative addition is unlikely. Our kinetic and spectroscopic data
thus suggest that the insertion of the CC bond of the alkene
into the Co−H bond generates a Co-alkyl intermediate, which
undergoes σ-bond metathesis with H2 in the turnover limiting
step to give an alkane product, simultaneously regenerating the
cobalt-hydride species (Figure 4b).
Zr-MTBC-CoH-Catalyzed Hydrogenation of Imines and
Carbonyls. Prompted by the hydrogenation of the carbonyl
group of 6-methyl-5-hepten-2-one at elevated temperatures, we
E
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Table 2. Zr-MTBC-CoH-Catalyzed Hydrogenation of
a
Imines
Figure 3. Plots of yields (%) of methylcyclohexane at different runs in the
reuse experiments of Zr-MTBC-Co for hydrogenation of 1-methyl-
cyclohexene. The Co loadings were ∼0.1 mol %.
Figure 4. (a) Kinetic plots of initial rates (d[methylcyclohexane]/dt) for
hydrogenation of 1-methylcyclohexene versus concentration of Zr2(μ3-O)
Co and hydrogen pressure for the first 35 min, showing first-order
dependence on both components. (b) Proposed catalytic cycle: the
insertion of the alkene into the Co−H bond gives a Co-alkyl species,
followed by turnover limiting σ-bond metathesis with H2 to generate the
alkane product.
a
Reaction conditions: 0.25 mg of Zr-MTBC-CoCl (0.5 mol % Co),
5 equiv of NaBEt3H (1.0 M in THF) w.r.t. Co, alkene, toluene, 40 bar
b
1
H2, 80 °C. Yields were determined by H NMR with mesitylene as
the internal standard. Isolated yield in the parentheses.
of diffusion of the larger substrate and product through the MOF
channels and less facile binding and activation of the substrate.
Zr-MTBC-CoH is also active in catalyzing hydrogenation of car-
bonyls to their corresponding alcohols in toluene at 90 °C.5b,19 At a
0.5 mol % Co loading, Zr-MTBC-CoH afforded 1-phenylethanol,
1-(4-chlorophenyl)ethanol, and cyclohexanol from correspond-
ing ketone substrates in good yields (entries 8−10, Table 1).
Benzaldehyde was also efficiently reduced to benzyl alcohol in
90% isolated yield.
and 0.08%, respectively. N-(4-chlorobenzylidene)benzenamine,
N-(2-methoxybenzylidene)benzenamine, N-(4-methoxy-
benzylidene)benzenamine, and N-benzylidenebenzylamine
were efficiently reduced within 24 h to afford corresponding
N-benzylanilines in excellent yields (entries 2−5, Table 2).
The hydrogenation of trisubstituted imines, such as (E)-N-(1-
phenylethylidene)aniline, however, required longer reaction
times (entry 7, Table 2), presumably due to the decreased rates
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Zr-MTBC-CoH-Catalyzed Hydrogenation of Hetero-
cycles. The hydrogenation of heterocycles is challenging
due to their resonance stabilization and potential poisoning of
catalysts by substrates and their products. Although significant
progress has been made in developing precious metal-based mole-
cular and heterogeneous catalysts for selective hydrogenation of
N-heteoarenes such as indoles and quinolines, the advancement
of the analogous earth abundant-metal catalysts has lagged
behind.6a,20 Catalytic hydrogenation of O-heteroarenes such as
furans and benzofurans is also significantly under-developed.
Additionally, the hydrogenation of heteroarenes typically requires
harsh reaction conditions, high catalyst loadings, and excess
additives.
At a 0.5 mol % Co loading, Zr-MTBC-CoH-catalyzed hydro-
genation of indole in toluene at 80 °C to affords a mixture of
indoline and 4,5,6,7-tetrahydroindole. Indoline was obtained in
84% isolated yield after preparative thin-layer chromatography
(entry 1, Table 3). Interestingly, hydrogenation of 3-methyl-
indole gave 3-methyl-indoline and 3-methyl-4,5,6,7-tetrahy-
droindole in 46:54 ratio, which indicates that reduction of
the phenyl ring is also possible. Hydrogenation of quinolines
in toluene at 80 °C gave a mixture of two products, 1,2,3,4-
tetrahydroquinoline and 5,6,7,8-tetrahydro-quinoline in a 1:1
ratio. Under identical reaction conditions, the selectivity is highly
dependent on the substitution of the phenyl ring. Electron-
donating substituents at the 6-position of the quinolines favor the
hydrogenation of phenyl ring. For example, the 6-methylquino-
line, 6-methoxyquinoline, and 2,6-dimethylquinoline were
hydrogenated to give 6-methyl-5,6,7,8-tetrahydro-quinoline,
6-methoxy-5,6,7,8-tetrahydro-quinoline, and 2,6-dimethyl-
5,6,7,8-tetrahydro-quinoline, respectively, as the major products
(entries 5−7, Table 3). In contrast, strong electron-withdrawing
substituents disfavor the reduction of the phenyl ring. The
hydrogenation of 2-methyl-6-fluoro-quinoline afforded 2-meth-
yl-6-fluoro-1,2,3,4,-tetrahydro-quinoline exclusively in 72% yield
(entry 8, Table 3). Interestingly, Zr-MTBC-CoH was also a very
active catalyst for hydrogenation of benzofuran. At a 0.2 mol %
Co loading, benzofuran was completely hydrogenated to 2,3-
dihydrobenzofuran in qualitative yield (entry 9, Table 3).
a
Table 3. Zr-MTBC-CoH-Catalyzed Hydrogenation of Heterocycles
a
Reaction conditions: 0.25 mg of Zr-MTBC-CoCl, 5 equiv of NaBEt3H (1.0 M in THF) w.r.t. Co, alkene, toluene (2 mL), 40 bar H2, 80 °C.
b
Yields were determined by 1H NMR with mesitylene as the internal standard. Ratios of the products, as determined by GC-MS, are in the paren-
c
d
theses. Reaction was performed at 100 °C. Yields determined by GC-MS analysis.
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CONCLUSION
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We have synthesized new Zr- and Hf-based mixed-node MOFs
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of active site isolation of and open environments of metal
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distributed single-site catalysts with unprecedented activity and
selectivity.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b06759.
Synthesis and characterization of the H4MTBC ligand,
Zr-MTBC and Zr-MTBC-CoCl, procedures for catalytic
hydrogenation of alkenes, heterocycles and imines, details
for X-ray absorption spectroscopic analysis, crystal
structure figures and crystallographic files of Zr-MTBC,
Hf-MTBC, and Zr-MTBC-CoCl. Crystallographic data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif
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AUTHOR INFORMATION
■
Corresponding Author
*wenbinlin@uchicago.edu
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†These authors contributed equally.
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ACKNOWLEDGMENTS
■
This work was supported by NSF (CHE-1464941). We thank M.
Piechowicz for experimental help. XAS analysis was performed at
Beamline 9-BM, Advanced Photon Source (APS), Argonne
National Laboratory (ANL). Single crystal diffraction studies
were performed at ChemMatCARS, APS, ANL. ChemMatCARS
is principally supported by the Divisions of Chemistry (CHE)
and Materials Research (DMR), NSF, under grant no. NSF/
CHE-1346572. Use of the Advanced Photon Source, an Office of
Science User Facility operated for the U.S. DOE Office of
Science by ANL, was supported by the U.S. DOE under contract
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