Aluminum Hydroxide Secondary Building Units in a Metal-Organic Framework Support Earth-Abundant Metal Catalysts for Broad-Scope Organic Transformations

Article abstract of DOI:10.1021/acscatal.9b00259

The intrinsic heterogeneity of alumina (Al₂O₃) surface presents a challenge for the development of alumina-supported single-site heterogeneous catalysts and hinders the characterization of catalytic species at the molecular level as well as the elucidation of mechanistic details of the catalytic reactions. Here we report the use of aluminum hydroxide secondary building units (SBUs) in the MIL-53(Al) metal-organic framework (MOF) with the formula Al(μ₂-OH)(BDC) (BDC = 1,4-benzenedicarboxylate) as a uniform and structurally defined functional mimic of Al₂O₃ surface for supporting Earth-abundant metal (EAM) catalysts. The μ₂-OH groups in MIL-53(Al) SBUs were readily deprotonated and metalated with CoCl₂ and FeCl₂ to afford MIL-53(Al)-CoCl and MIL-53(Al)-FeCl precatalysts which were characterized by powder X-ray diffraction, nitrogen sorption, elemental analysis, density functional theory, and extended X-ray fine structure spectroscopy. Activation with NaBEt₃H converted MIL-53(Al)-CoCl to MIL-53(Al)-CoH which effectively catalyzed hydroboration of alkynes and nitriles and hydrosilylation of esters. X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy (XANES) indicated the presence of Al^III and Co^II centers in MIL-53(Al)-CoH while deuterium labeling studies suggested σ-bond metathesis as a key step for the MIL-53(Al)-CoH-catalyzed addition reactions. MIL-53(Al)-FeCl competently catalyzed oxidative Csp³-H amination and Wacker-type alkene oxidation. XANES analysis revealed the oxidation of Fe^II to Fe^III centers in the activated MIL-53(Al)-FeCl catalyst and suggested that oxidative Csp³-H amination occurs via the formation of Fe^III-O^tBu species by single electron transfer between Fe^II centers in MIL-53(Al)-FeCl and (^tBuO)₂ with concomitant generation of 1 equiv of ^tBuO· radical, C-H activation through hydrogen atom abstraction to generate alkyl radicals, protonation of Fe^III-O^tBu by aniline to generate MIL-53(Al)-Fe^III-anilide, and finally C-N coupling between the Fe^III-anilide and alkyl radical to form the Csp³-H amination product and regenerate the Fe^II catalyst. These highly active single-site MOF-based solid catalysts were readily recovered and reused up to five times without significant decrease in catalytic activity. This work thus demonstrates the great potential of using the aluminum hydroxide SBUs in MOFs to support EAM catalysts for important organic transformations.

Full text of DOI:10.1021/acscatal.9b00259

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Article
Aluminum Hydroxide Secondary Building Units in a
Metal-Organic Framework Support Earth-Abundant Metal
Catalysts for Broad Scope Organic Transformations
Xuanyu Feng, Pengfei Ji, Zhe Li, Tasha Drake, Pau Oliveres,
Emily Y Chen, Yang Song, Cheng Wang, and Wenbin Lin
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00259 • Publication Date (Web): 06 Mar 2019
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ACS Catalysis
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Aluminum Hydroxide Secondary Building Units in a Metal-
Organic Framework Support Earth-Abundant Metal Catalysts
for Broad Scope Organic Transformations
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Xuanyu Feng,†,1 Pengfei Ji,†,1 Zhe Li,†,1,2 Tasha Drake,1 Pau Oliveres,1 Emily Y. Chen,1 Yang Song,1
Cheng Wang,2 and Wenbin Lin*,1,2
1Department of Chemistry, The University of Chicago, 929 E 57th St, Chicago, IL 60637, USA
2College of Chemistry and Chemical Engineering, iCHEM, State Key Laboratory of Physical Chemistry of Solid Surface
Xiamen University, Xiamen 361005, China
ABSTRACT: The intrinsic heterogeneity of alumina (Al2O3) surface presents a challenge for the development of alumina-supported single-
site heterogeneous catalysts and hinders the characterization of catalytic species at the molecular level as well as the elucidation of
mechanistic details of the catalytic reactions. Here we report the use of aluminum hydroxide secondary building units (SBUs) in the MIL-
53(Al) metal-organic framework (MOF) with the formula Al(μ2-OH)(BDC) (BDC = 1,4-benzenedicarboxylate) as a uniform and structurally-
defined functional mimic of Al2O3 surface for supporting Earth-abundant metal (EAM) catalysts. The μ2-OH groups in MIL-53(Al) SBUs
were readily deprotonated and metalated with CoCl2 and FeCl2 to afford MIL-53(Al)-CoCl and MIL-53(Al)-FeCl precatalysts which were
characterized by powder X-ray diffraction, nitrogen sorption, elemental analysis, density functional theory, and extended X-ray fine structure
spectroscopy. Activation with NaBEt3H converted MIL-53(Al)-CoCl to MIL-53(Al)-CoH which effectively catalyzed hydroboration of
alkynes and nitriles and hydrosilylation of esters. X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy (XANES)
indicated the presence of Al3+ and Co2+ centers in MIL-53(Al)-CoH while deuterium labeling studies suggested -bond metathesis as a key
step for the MIL-53(Al)-CoH catalyzed addition reactions. MIL-53(Al)-FeCl competently catalyzed oxidative Csp3-H amination and Wacker-
type alkene oxidation. XANES analysis revealed the oxidation of Fe2+ to Fe3+ centers in the activated MIL-53(Al)-FeCl catalyst and
suggested that oxidative Csp3-H amination occurs via the formation of FeIII–OtBu species by single electron transfer between FeII centers in
MIL-53(Al)-FeCl and (tBuO)2 with concomitant generation of one equiv of tBuO· radical, C-H activation through hydrogen atom abstraction
to generate alkyl radicals, protonation of FeIII–OtBu by aniline to generate MIL-53(Al)-FeIII-anilide, and finally C-N coupling between the
FeIII-anilide and alkyl radical to form the Csp3-H amination product and regenerate the FeII catalyst. These highly active single-site MOF-
based solid catalysts were readily recovered and reused up to five times without significant decrease in catalytic activity. This work thus
demonstrates the great potential of using the aluminum hydroxide SBUs in MOFs to support EAM catalysts for important organic
transformations.
KEYWORDS: Metal-Organic Framework; Earth-Abundant Metal Catalyst; Single-site Solid Catalyst; Hydrofunctionalization; C-H
Amination; Wacker-Type Alkene Oxidation
INTRODUCTION
defect surface Al sites cause undesired reforming and
hydration/dehydration process particularly at elevated
As one of the most abundant, widely used, and thoroughly studied
metal oxide supports, alumina (Al2O3), especially γ-Al2O3, has
drawn great interest in industrial research studies, due to its high
surface area (80-250 m2/g), acid/base chemistry, thermal and
chemical stability, and abundance of surface hydroxy (OH)
groups.1-4 Previous studies have demonstrated the ability of γ-
Al2O3 in supporting either metallic nanoparticles for catalytic
methanation5-7 and dehydrogenation reactions8-10 or organometallic
species for olefin polymerization,11-12 C-H activation,13-15 and
metathesis reactions.16-18 However, unlike the thermodynamically
stable phase α-Al2O3, which has a compact and crystalline structure,
γ-Al2O3 features different aluminum (Al) coordination numbers
(AlIV and AlVI) and varied surface OH coordination modes
temperatures,3 leading to insufficient binding strength between
active species and oxide support and consequently decreased
catalytic activity. Thus, new Al-based materials with uniform OH
groups are highly desired for mimicking γ-Al2O3 in supporting
single-site catalysts and providing better understanding of γ-Al2O3-
supported commercial catalytic systems.
Constructed from metal cluster secondary building units (SBUs)
and organic linkers, MOFs have provided a unique and highly
tunable platform for supporting active species.20-24 Benefitting
from large pore/channel sizes, thermal stability, and predictable
structures, MOF-based materials have out-performed traditional
heterogeneous catalysts in both activity and selectivity in a number
of reactions.25-32 Moreover, the isolation of active sites in MOFs
provides unique opportunities to obtain solution-inaccessible
molecular catalysts in MOF structures.33-36 The inorganic nodes of
many MOFs feature functional OH groups to provide a mimic of
(terminal-, 2-, and 3-) to the Al centers (Figure 1a).1, This
4
intrinsic heterogeneity of γ-Al2O3 surface presents a challenge for
the uniform modification of catalytically active species, the
characterization of these active species, and the elucidation of
reaction mechanisms.19 Furthermore, non-uniform OH groups and
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metal oxides/hydroxides. The crystallinity of MOF frameworks
nitriles as well as hydrosilylation of esters. MIL-53(Al)-FeCl
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and the homogeneity of SBUs render the OH groups unique
supports for metal complexes to afford single-site solid catalysts.
In particular, recent works have demonstrated the anchoring of
Earth-abundant metal (EAM) species to Zr-oxo/hydroxo and Ti-
oxo/hydroxo SBUs to provide novel single-site solid catalysts for
a number of important organic transformations and the control of
catalytic activity and selectivity by fine-tuning electronic and steric
properties of SBUs.37-40 SBU-supported single-site EAM catalysts
thus present a cost-effective and sustainable strategy for the
development of solid catalysts with well-defined catalytic species
for the synthesis of commodity and fine chemicals.
efficiently catalyzed oxidative C-H amination and Wacker-type
alkene oxidation reactions. MIL-53(Al) possesses several
advantages over traditional alumina supports including a higher Al
to OH ratio (1:1), uniform and structurally defined μ2-OH groups,
and all μ2-OH groups being accessible via the porous MOF
structure (Figure 1b), allowing for
a greater density of
functionalization with metal complexes and easier investigation of
catalytically active sites and reaction mechanisms. Compared to
commonly studied divalent Zn, Cd, and Cu MOFs, Al-based MOFs
feature higher thermal stability (over 500 °C) due to the stronger
coordination of Al to carboxylate linkers.42-45 The perforated
structure of uniform 1-D channels in MIL-53(Al) allows for facile
substrate diffusion to access the catalytic sites to effect reduction
and oxidation reactions with diverse mechanisms. Moreover,
compared to the well-studied Zr3-OH sites, Al2-OH moieties are
expected to be more electron donating after deprotonation. MIL-
53(Al)-supported EAM catalysts thus represent a versatile and
cost-effective alternative to traditional metal oxide heterogeneous
catalysts.
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RESULT AND DISCUSSIOIN
AlIV
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AlVI
Synthesis and Characterization of MIL-53(Al)-CoCl and MIL-
53(Al)-FeCl
γ-alumina(110) termination
(b)
MIL-53(Al) with the formula Al(OH)(BDC) was synthesized in 85%
yield through a solvothermal reaction between AlCl3·6H2O and
H2BDC in N,N-dimethylformamide (DMF) in 18 h (Figure S1, SI).
The previous synthesis of MIL-53(Al) from Al(NO3)3·9H2O and
H2BDC in H2O led to trapping of H2BDC in the MOF channels via
H-bonding between free ligands and bridging μ2-OH groups,41
likely due to the insolubility of H2BDC in water. In contrast,
thermal gravimetric analysis (TGA) results and FT-IR spectra
indicated that MIL-53(Al) synthesized from AlCl3·6H2O in DMF
are free of trapped H2BDC and features open channels and uniform
free μ2-OH groups for anchoring catalytically active metal centers
(Figure S2, S3, SI).46 MIL-53(Al) was first treated with
LiCH2Si(CH3)3 to generate Al(OLi)(BDC) via deprotonation, and
then metalated with 1 equiv. of CoCl2 or FeCl2. After removing
unreacted metal salts via extensive washing with tetrahydrofuran
(THF), MIL-53(Al)-CoCl and MIL-53(Al)-FeCl were obtained as
light blue and light brown solids, respectively. Powder X-ray
diffraction (PXRD) patterns of the metalated MOFs remained
unchanged from that of as-synthesized MIL-53(Al), indicating the
maintenance of crystallinity after metalation with Co and Fe
(Figure 2b). The Co and Fe contents in the metalated MOFs were
determined to be 0.15 Co per Al and 0.23 Fe per Al, respectively,
by inductively coupled plasma-mass spectrometry (ICP-MS)
analysis of digested samples. Nitrogen sorption isotherms of MIL-
53(Al)-CoCl and MIL-53(Al)-FeCl indicated highly porous
structures with Brunauer-Emmett-Teller (BET) surface areas 2066
and 2001 m2/g, respectively (Figure 2c). Pore size distribution
analyses by density functional theory (DFT) showed a pore size of
8 Å for both metalated MOFs, matching well with that expected
from the crystal structure (Figure 2d).
0.9 nm
1.2 nm
well-definedand uniform μ2-OHsin Al-BDC
Figure 1. (a) The presence of five main types of OH groups on the
(110) surface of -Al2O3. (IV and VI stand for coordination
numbers of 4 and 6 for Al sites.) (b) The structure of MIL-53(Al)
features large one-dimensional channels (1.2 × 0.9 nm2) and a high
density of uniform 2-OH groups.
Herein, we report the use of MIL-53(Al), which is composed of
one-dimensional Al-hydroxo chains linked together by 1,4-
benzenedicarboxylate (BDC) bridging ligands,41 as a mimic for the
alumina surface to anchor Co and Fe complexes for a broad range
of organic transformations. The isolated μ2-OH groups were first
deprotonated to give μ2-O- and then metalated with Co and Fe
complexes to afford MIL-53(Al)-supported precatalysts.
Following activation with a hydride source, the resultant MIL-
53(Al)-CoH effectively catalyzed hydroboration of alkynes and
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Figure 2. (a) The preparation of MIL-53(Al)-CoCl from MIL-53(Al) via sequential deprotonation and metalation. (b) PXRD patterns of as-
synthesized MIL-53(Al) (red), MIL-53(Al)-CoCl (blue), MIL-53(Al)-FeCl (navy), indicating that crystallinity of the MOF was maintained
after metalation. PXRD patterns of MIL-53(Al)-CoH (magenta) and after catalytic alkyne hydroboration are unchanged from that of MIL-
53(Al)-CoCl, indicating the MOF is stable toward activation and under catalytic reactions. (c) N2 sorption isotherms of MIL-53(Al)-CoCl
(navy) and MIL-53(Al)-FeCl (wine). (d) Pore size distributions of MIL-53(Al)-CoCl (navy) and MIL-53(Al)-FeCl (wine), both showing a
uniform pore size of 8 Å. (e) EXAFS spectra (gray circles) and fits (navy solid line) in R-space at the Co K-edge adsorption of MIL-53(Al)-
CoCl. The Co coordination mode was calculated by DFT.
We performed DFT calculations using the B3LYP level of theory
to optimize the coordination environments of Co and Fe in
metalated MOFs. The Co coordination converged at a distorted
square pyramidal geometry with one anionic bridging oxo (from
the deprotonation of the 2-OH group), two carboxylate oxygen,
one chloride, and one THF molecule to afford the [(μ2-
O-)(carboxylate-O)2CoCl(THF)] species. The Co-(μ2-O-) distance
is 1.91 Å, while the Co-(carboxylate-O) distances are longer at 2.21
Å and 2.58 Å, respectively. The Co-Cl distance is 2.33 Å and the
Co-(THF-O) distance is 2.06 Å (Table S4, SI). The calculated
model fitted well to the Co K-edge absorption of the extended X-
ray fine structure spectroscopy (EXAFS) data of MIL-53(Al)-CoCl
with a R factor of 0.0045 (Figure 2e, Table S1, SI). EXAFS fitting
gave a Co-(μ2-O-) distance of 1.87 Å, and Co-(carboxylate-O)
distances of 1.96 Å and 2.32 Å, a Co-Cl distance of 2.28 Å, and a
Co-(THF-O) distance of 2.02 Å. DFT calculations indicated a
similar distorted square pyrimidal geometry for Fe centers in MIL-
53(Al)-FeCl. The calculated Fe-(μ2-O-) distance of 1.91 Å, Fe-
(carboxylate-O) distances of 2.32 and 2.41 Å, Fe-Cl distance of
2.31 Å, and Fe-(THF-O) distance of 2.14 Å (Table S6, SI) match
well with those determined by EXAFS fitting (1.91, 2.39, 2.48,
2.13, and 2.11 Å, respectively). The R-factor of the EXAFS fitting
is 0.015 (Figure S14, Table S3, SI).
similarity of PXRD patterns. X-ray photoelectron spectroscopy
(XPS) was used to determine the oxidation states of Al and Co
centers in MIL-53(Al)-CoH. MIL-53(Al)-CoH displayed strong
2p3/2 and 2p1/2 peaks at 778.3 and 794.3 eV along with strong 2p3/2
and 2p1/2 shake-up peaks at 783.0 and 800.2 eV for Co centers,
indicating typical CoII species (Figure 3d). MIL-53(Al)-CoH
showed one set of strong 2p3/2 and 2p1/2 peaks at 71.5 and 71.0 eV
for Al centers, which are characteristic of AlIII species (Figure 3e).
These results indicate that neither AlIII or CoII species were reduced
by NaBEt3H during the chloride/hydride exchange process.
X-ray absorption near-edge spectroscopy (XANES) analysis
supported the Co oxidation state assignment by XPS. The pre-K
edge features of MIL-53(Al)-CoCl and MIL-53(Al)-CoH are
identical to that of CoCl2, indicating the Co2+ centers before and
after chloride/hydride exchange (Figure 3c). The presence of only
Co2+ centers in MIL-53(Al)-CoH ruled out the possibility of
forming Co nanoparticle during the NaBEt3H treatment. The
absence of Co nanoparticle was also supported by the significant
misfit of MIL-53(Al)-CoH EXAFS data using a model containing
5% of Co nanoparticle (Figure S11, SI). Treatment of MIL-53(Al)-
CoH with excess amount of trifluoroacetic acid (TFA) generated
1.10±0.03 equiv. of H2 w.r.t. Co, indicating the presence of Co-H
species in the MOF (Figure S12, SI). EXAFS fitting was also
performed at the Co K-edge for MIL-53(Al)-CoH. The EXAFS
spectrum was well fitted with a Co coordination environment
calculated by DFT, indicating the formation of (2-O)(O-
carboxylate)2CoH(THF) species in MIL-53(Al)-CoH (Figure 3b
and Table S2. SI).
Structure and Electronic Properties of MIL-53(Al)-CoH
Upon treatment with NaBEt3H, the color of MIL-53(Al)-CoCl
changed from light blue to black, suggesting the formation of MIL-
53(Al)-CoH via chloride/hydride exchange. The MOF crystallinity
was maintained after chloride/hydride exchange based on the
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(a)
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Figure 3. (a) Activation of MIL-53(Al)-CoCl to form MIL-53(Al)-CoH with NaBEt3H in THF. (b) EXAFS fitting of MIL-53(Al)-CoH
shows the Co coordination environment as (2-O)(O-carboxylate)2CoH(THF). (c) XANES pre-edge features of MIL-53(Al)-CoCl (navy)
and MIL-53(Al)-CoH (red) as compared to that of CoCl2 (black), indicating the Co2+ oxidation state before and after NaBEt3H treatment.
(d) Co 2p XPS spectra of MIL-53(Al)-CoH (blue circle) and the fitting result (gray solid line) indicate the Co2+ oxidation state after NaBEt3H
treatment. (e) Al 2p XPS spectra of MIL-53(Al)-CoH (black circle) and fitting result (gray solid line) indicate the Al3+ oxidation state after
NaBEt3H treatment.
MIL-53(Al)-CoH Catalyzed Hydroboration of Alkynes
the reactivity, but the substrates with the methyl group in ortho and
meta positions required twice the catalyst loading. Halogen atoms
did not interfere with the hydroboration reaction. Under un-
optimized conditions, MIL-53(Al)-CoH catalyzed hydroboration
of aliphatic alkynes to give exclusively E-alkenylboronate esters in
47-57% of yields.
Alkenylboronate esters are important starting materials for the
synthesis of a wide range of organic compounds via cross-coupling
reactions.47-49 Transition metal catalyzed hydroboration of alkynes
provides the simplest and most atom-efficient synthetic route to
alkenylboronate esters.50-53 Several complexes of EAMs, e.g.,
Fe,54-56 Co,57-58 and Cu,59-60 have recently been shown to catalyze
hydroboration of alkynes, but these reactions either require high
catalyst loadings or the use of additives.
As a solid catalyst, MIL-53(Al)-CoH was readily recycled and re-
used for hydroboration of phenylacetylene. At a 1.0 mol% Co
loading, MIL-53(Al)-Co catalyzed five consecutive runs of
phenylacetylene hydroboration without a significant drop in yields
(Figure S23, SI). The leaching of Co and Al into the supernatant
after the hydroboration was determined to be 0.3% and 0.8%,
respectively, by ICP-MS analysis. Moreover, control experiments
using Co nanoparticles with the same amount of Co gave very low
yields of the products (Table S8, SI), further confirming single-site
supported Co-H complex as the true catalytic species.
MIL-53(Al)-CoH was found to be a highly effective catalyst for
hydroboration of alkynes. Screening of reaction conditions (Table
S7, SI) revealed that, at 0.2 mol% of catalyst loading, MIL-53(Al)-
CoH catalyzed the solvent-free reaction of phenylacetylene with
1.2 equiv pinacolborane (HBpin) at 90 °C for 22 h to give the E-
alkenylboronate in 85% yield, with a turnover number (TON) of
425. No conversion was found in the control experiment without
adding the MOF. Addition of 2 equiv. HBpin did not lead to further
hydroboration of the alkenylboronate ester. A TON of 1428 was
achieved when the reaction was carried out with 0.035 mol % MIL-
53(Al)-CoH at 90 oC (Table S7, SI). PXRD pattern of MIL-53(Al)-
CoH recovered from the hydroboration reaction was similar to that
of as-synthesized MIL-53(Al)-CoH (Figure 2b), indicating the
stability of MOF framework during hydroboration reaction.
Table 1. MIL-53(Al)-CoH catalyzed hydroboration of alkynesa
MIL-53(Al)-CoH-catalyzed hydroboration has a broad substrate
scope and exhibits good functional group tolerance. (Table 1) Both
electron donating group (i.e., CH3, OCH3) and electron
withdrawing group (i.e., F) work well under standard reaction
conditions. The methyl group on the para position did not impact
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0.2 mol % MIL-53(Al)-Co
90 oC, neat, 22 h
H
MIL-53(Al)-CoH in hydroboration of nitriles. Reduction of nitriles
HBpin
Bpin
1
2
3
4
5
6
7
8
R
to amines with high activity and selectivity is important for the
production of many dyes, agrochemicals, and pharmaceutical
compounds.62 Hydroboration of nitriles also provides a route to
protected amines for further functionalization.63-65 Treatment of
benzonitrile with 2.1 equiv. of HBpin in the presence of 1.0 mol%
MIL-53(Al)-CoH at 90 °C for 2 days afforded the fully
hydroborated product in 73% yield. No semi-hydroborated or other
by-products were observed. Under this un-optimized condition, a
wide range of nitriles were hydroborated with HBpin to afford fully
hydroborated products in 58% to 91% yields (Table 2). The
hydroboration reaction works for aromatic nitriles containing
electron-withdrawing groups, electron-donating groups, and
halogens as well as aliphatic nitriles (Table 2).
R
H
O
O
Bpin =
B
H
H
H
H
Bpin
Bpin
Bpin
H3C
H3C
Bpin
CH3
85 %
74 %
Cl
66 %b
64 %b
9
H
H
H
10
11
12
13
14
15
16
17
Bpin
Bpin
Bpin
H3CO
F
48 %c
74 %
H
73 %
Table 2. MIL-53(Al)-CoH catalyzed hydroboration of nitrilesa
H
H
1.0 mol % MIL-53(Al)-CoH
H
H
N
Bpin
2 HBpin
O
Bpin
R
N
Bpin
Bpin
Bpin
Si
H3C CH3
R
90 oC, neat, 2 days
4
53 %b
47 %
57 %
H
H
H H
H
H
H
H
aStandard condition: 1.00 mmol alkyne substrate, 1.20 mmol HBpin, 0.2
mol% Co, 90 °C, 22 h; Yield was determined by 1H NMR using mesitylene
as internal standard. b0.4 mol% Co. c1.0 mol% Co.
H3C
Bpin
Bpin
Bpin
Bpin
N
N
N
N
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Bpin
Bpin
Bpin
Bpin
Br
F
91 %
71 %
62 %
73 %
We also conducted deuterium labeling experiments to gain further
insight into MIL-53(Al)-Co catalyzed hydroboration of alkynes.
After hydroboration of phenylacetylene-d1 with HBpin, H NMR
O
O
H
H
H
H
H H
H
H
1
Bpin
Bpin
Bpin
Bpin
N
Bpin
N
Bpin
N
O
N
Bpin
analysis of the product (53% yield) revealed exclusive deuterium
labeling at the 1-position of trans-2-phenylvinylboronic acid
pinacol ester (Figure 4a), indicating no H/D exchange between the
alkyne and Co centers.61 Based on the deuterium labeling result and
the Co2+ oxidation state in 53(Al)-CoH, we propose a reaction
pathway involving σ-bond metathesis as a key step for the
hydroboration of terminal alkynes (Figure 4b). The coordination of
an alkyne to the Co center of the active catalyst MIL-53(Al)-CoH
is followed by migratory insertion of the hydride to the triple bond
to form the Co-alkenyl intermediate, which undergoes σ-bond
metathesis with HBpin to give the alkenylboronate ester product
and regenerate the Co-H catalyst (Figure 4b).
Bpin
58 %
89 %
66 %
61 %
aStandard condition: 0.60 mmol nitrile, 1.26 mmol HBpin, 1.0 mol% Co,
90 °C, 2 days; Yield was determined by 1H NMR using mesitylene as
internal standard.
MIL-53(Al)-CoH Catalyzed Hydrosilylation of Esters
Beside hydroboration reactions, hydrosilylation provides another
reductive pathway to introduce silyl groups into unsaturated bonds
for further functionalization.66-68 In contrast to extensively studied
hydrosilylation of ketones and aldehydes,69-73 hydrosilylation of
less reactive substrates such as amides and esters remains a
challenge, especially for EAM catalysts.74-76 We investigated
hydrosilylation of esters using MIL-53(Al)-CoH as catalyst.
Impressively, at 0.1 mol % loading of the MOF catalyst, simply
stirring an equimolar mixture of methyl benzoate and phenylsilane
D
H
(a)
0.2 mol % MIL-53(Al)-CoH
90 oC, neat, 22 hrs
Bpin
HBpin
+
D
Isotopic purity > 98%
(PhSiH3) at room temperature over 18
h afforded the
hydrosilylation products with complete conversion. 1H NMR
spectroscopy identified the hydrosilylation products as a mixture of
PhSi(OCH3)2(OCH2C6H5) (42 mol%), PhSi(OCH3)(OCH2C6H5)2
(45 mol%), and PhSi(OCH2C6H5)3 (13 mol%) in addition to the by-
product PhSi(OCH3)3. Treatment of this mixture with 2 M NaOH
in 1:1 MeOH/H2O (v/v) afforded benzyl alcohol in 92% isolated
yield. No conversion was observed in the background reaction
without the MOF catalyst.
(b)
H
Bpin
H
H
H
[Co]II
Ph
Ph
H
Bpin
Cl
[Co]II
MIL-53(Al)-CoH catalyzed hydrosilylation of esters exhibited a
broad substrate scope. (Table 3) Substituted benzoates,
dibenzoates, and acetate esters were readily hydrosilylated in the
presence of 0.1 mol% MIL-53(Al)-CoH. For less reactive aliphatic
esters, such as methyl octanoate and methyl phenylacetate,
quantitative hydrosilylation was achieved in the presence of 1.0
mol% MIL-53(Al)-CoH. Due to the mild reaction conditions, MIL-
53(Al)-CoH catalyzed hydrosilylation showed good functional
group tolerance to esters containing reducible groups such as
alkynyl and bromo groups. More importantly, MIL-53(Al)-CoH
catalyzed hydrosilylation of esters containing a pyridyl or
thiophenyl moiety, which usually poisons catalysts due to strong
coordination to the metal centers. Upon basic workup,
Ph
Ph
H
[Co]II
H
H
[Co]II
H
Figure 4. (a) Deuterium labeling experiment and (b) proposed
mechanism of MIL-53(Al)-CoH catalyzed hydroboration of
alkynes.
MIL-53(Al)-CoH Catalyzed Hydroboration of Nitriles
Encouraged by the success in hydroboration of alkynes, we tested
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t
t
corresponding alcohols were obtained in 71% and 93% yield.
concomitant generation of one equiv of BuO· radical. The BuO·
radical subsequently underwent C-H activation through hydrogen
atom abstraction (HAT). Interestingly, FeIII pre-edge features of
MIL-53(Al)-FeIIIOtBu were maintained after its treatment with
aniline, indicating a simple anion exchange with aniline to generate
MIL-53(Al)-FeIII-aninide. The final step of the reaction involved
C-N coupling between the FeIII-anilide and alkyl radical with
concomitant SET to the anilide to the FeIII center to regenerate the
FeII catalyst (Figure 5b).
1
2
3
4
5
6
7
8
Table 3. MIL-53(Al)-CoH catalyzed hydrosilylation of estersa
0.1~1.0 mol %
MIL-53(Al)-CoH
R1
OH
O
2 M NaOH
Si(OCH2R1)n(OR2)3-n
n = 0,1,2,3
R2
+
R1
O
1 equiv. PhSiH3,
R2 OH
r.t., neat/THF, 18 h
O
O
O
O
O
O
O
O
O
N
9
O
100 % (71 %)
O
100 % (92 %)
92 % (71 %)
(a)
100 % (78 %)b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
O
O
O
0.15
1.5
O
O
6
100 %
100 % (83 %)b
O
100 % (84 %)c
100 % (91 %)
O
0.10
O
O
O
100 % (80 %)c
O
O
O
S
100 % (93 %)c
Br
1.0
100 % (73 %)c
100 % (79 %)c
0.05
a Standard condition: 1.0 mmol ester, 1.0 mmol PhSiH3, 0.1-1.0 mol% Co,
r.t., 18 h; For solid substrates, 0.3 mL THF was added as solvent;
Hydrosilylation yields are based on 1H NMR integration with isolated
yields of alcohols in parenthesis. b0.5 mol% Co. c1.0 mol% Co.
0.00
0.5
7105
7110
7115
FeCl2
FeCl3
MIL-53(Al)-FeCl Catalyzed C-H Amination
MIL-53(Al)-FeCl
t
MIL-53(Al)-FeOBu
MIL-53(Al)-FeAni
To further explore the potential of MIL-53(Al) hydroxyl groups as
a support for base metal catalysts, we introduced Fe centers into
this MOF. MIL-53(Al)-FeCl was similarly prepared as a pale
yellow solid through deprotonation of μ2-OH sites in MIL-53(Al)
with LiCH2SiMe3 followed by metalation with FeCl2. We
hypothesized that the FeII centers can undergo the FeII/FeIII redox
process to enable the application of MIL-53(Al)-FeCl in catalyzing
challenging reactions through single electron transfer (SET)
processes.
0.0
7100
7120
Energy (eV)
7140
(b)
[Fe]II
tBuO OtBu
Ph
R
N
H
Catalytic formation of C-N bonds through Csp3-H amination using
Earth-abundant and environmental friendly first-row transition
metals (e.g., Fe, Cu) has attracted significant research interest.77-79
We tested the catalytic performance of MIL-53(Al)-FeCl in Csp3-
H amination using aniline as the nitrogen source. At 5.0 mol%
loading of MIL-53(Al)-Fe, heating a mixture of aniline and indane
in the presence of 1.5 equiv. of (tBuO)2 at 105 °C gave the desired
amination product, N-phenyl-2,3-dihydro-1H-inden-1-amine, in
63% yield. The protocol converted tetralin to the Csp3-H amination
product in 70% yield. Indane and tetralin were also aminated with
different aniline derivatives containing 2,4,6-substitutents and 4-
substituent to afford desired products in moderate to good yields
(15-70% yields for 5 mol% catalyst loading, Table 4). Lower yields
of desired Csp3-H amination products were obtained for bulky
anilines such as 2,4,6-trimethylaniline and 2,4,6-trichloroaniline,
likely as a result of steric hindrance around the Fe centers. Higher
yields of amination products were generally obtained for para-
substituted anilines with electron-withdrawing groups (Cl and Br)
in comparison with those with electron-donating groups (CH3).
RH
tBu O
R
- tBuOH
[Fe]III
NH
Ph
OtBu
[Fe]III
tBu OH
Ph NH2
Figure 5. (a) XANES pre-edge features of MIL-53(Al)-FeIICl
(blue), MIL-53(Al)-FeIIIOtBu (red), and MIL-53(Al)-FeIII-anilide
(navy) as compared to those of FeCl2 (green) and FeCl3 (wine). (b)
Proposed mechanism for MIL-53(Al)-FeCl catalyzed Csp3-H
amination involving the FeII/FeIII cycle.
Table 4. MIL-53(Al)-FeCl catalyzed C-H aminationa
H
NH2 5.0 mol % MIL-53(Al)-FeCl
R
+
HN
1.5 eq tBuOOtBu, 105 o
neat, 18 h
C
n = 1,2
R
MIL-53(Al)-FeCl catalyzed Csp3-H amination was proposed to
proceed through radical-mediated C-H activation followed by C-N
cross coupling between the alkyl radical and FeIII-anilide,
analogous to the mechanism proposed in the literature for CuI-
catalyzed CH amination reactions.80-81 XANES analysis was
carried out to identify the oxidation state of Fe centers in the
activated MIL-53(Al)-FeCl catalyst (Figure 5a). As-prepared MIL-
53(Al)-FeCl exhibited similar pre-edge features to FeCl2. After
treatment with (tBuO)2, the MOF showed similar pre-edge features
to FeCl3, indicating the formation of FeIII–OtBu species via SET
between FeII centers in MIL-53(Al)-FeCl and (tBuO)2 with
n = 1,2
R
R
R
R
R
R
HN
HN
HN
HN
R
R
R = CH3
70 %
R = CH3
R = Cl
R = Br
R = H
R = CH3
R = Cl
49 %
57 %
55 %
R = H
R = CH3
R = Cl
30 %
51 %
51 %
63 %
41 %
45 %
R = Cl
31 %
R = Br
15 %
a
Standard condition: 0.32 mmol anilines substrate, 6.4 mmol indane or
tetralin, 5 mol% Fe, 105 °C, 18 h; Yield was determined by H NMR or
1
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GC-MS analysis using mesitylene as internal standard.
alkynes, aromatic alkynes (phenylacetylene) and aliphatic alkynes
1
2
3
4
5
6
7
8
(ethynylcyclohexane) achieved moderate to good yields at 0.2 mol%
MIL-53(Al)-CoH loading, but the same loading of UiO-68-CoH
afforded the aromatic and aliphatic products in 16% and 8% yields,
respectively. Significant drops of catalytic activities were also
observed in the hydroboration of aromatic and aliphatic nitriles.
Moreover, UiO-68-CoH was completely inactive for the
hydrosilylation of esters, with >99% of starting substrates
recovered (Figure 6).
MIL-53(Al)-FeCl Catalyzed Wacker-Type Alkene Oxidation
MIL-53(Al)-FeCl is also highly effective in catalyzing Wacker-
type alkene oxidation reactions. Wacker-Tsuji oxidation directly
converts terminal alkenes into carbonyl-containing compounds
using a wide range of oxidants.82-83 However, precious metal
catalysts (e.g., PdCl2) along with stoichiometric Cu salt83-85 or
“unusual” oxidants, such as hypervalent iodine86 and 1,4-
benzoquinoline,87-88 are needed in Wacker oxidation reactions to
achieve high chemo-selectivity, significantly restricting their broad
applications. Recently, Han and co-workers89 reported an Fe-based
catalyst for Wacker-type oxidation under ambient air conditions.
The reaction was believed to go through the FeII/FeIII cycle. We
hypothesized that MIL-53(Al) with isolated μ2-OH sites could
provide an excellent support for developing single-site solid
catalyst for Wacker-type olefin oxidation.
100
9
MIL-53(Al)-CoH
UiO-68-CoH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
80
60
40
20
0
After screening different silanes and reaction temperatures (Table
S9, SI), we found that MIL-53(Al)-FeCl showed high catalytic
activity for the oxidation of a wide range of styrene derivatives
using ambient air as the oxidant and 1,1,1,3,5,5,5-
heptamethyltrisiloxane (HMTS) as the additive. (Table 5) At 3.0
mol% loading of MIL-53(Al)-FeCl (w.r.t. Fe), acetophenone was
obtained in 92% yield from the oxidation of styrene at 60 °C for 12
h. No reaction occurred in the absence of either the MOF, silanes,
or the air (Table S9, SI). Substrate scope of this reaction was broad
and extended to styrenes containing both electron-donating groups
such as methyl, methoxyl(at either para- or meta-position), and t-
butyl groups and electron-withdrawing groups such as fluoro,
chloro, and bromo substituents. In all cases, acetophenone
derivatives were obtained in very high yields (90-100%). MIL-
53(Al)-FeCl was also readily recovered and used in three
consecutive run of oxidation reactions with no significant drop in
reaction yields (Figure S44, SI). Only 0.3% of Fe leached into the
supernatant as determined by ICP-MS, indicating the stability of
the MOF catalyst.
a
b
c
d
e
f
d
e
f
a
b
H
Bpin
N
HBpin
HBpin
HBpin
Bpin
HBpin
CN
O
Bpin
H
H
H
O
H
PhSiH3
PhSiH3
OH
OH
Bpin
H
c
O
H
H
CN
Bpin
N
OEt
Bpin
N
N
Figure 6. Hydroboration of alkynes and nitriles and hydrosilylation
of esters with different MOF SBU supported Co-hydride catalysts.
Reaction conditions: a,b) 0.2 mol% of MOF-CoH catalyst, alkynes
(1.0 mmol), HBpin (1.2 mmol), 90 °C, 22 h. c,d) 1.0 mol% of
MOF-CoH catalyst, nitriles (0.6 mmol), HBpin(1.26 mmol), 90 °C,
2 days. e,f) 0.1 mol% of MOF-CoH catalyst, esters (1.0 mmol),
PhSiH3 (1.0 mmol), r.t., 18 h.
Table 5. MIL-53(Al)-FeCl catalyzed Wacker-type alkene
oxidationa
O
3.0 mol % MIL-53(Al)-FeCl
R
R
5 equiv. HMTS, 60 oC,
EtOH, air, 12 h
H
O
O
Si
Si Si
HMTS =
To understand the reason for such drastic differences in
hydrofunctionaliztion activities, DFT calculations were carried out
on the two MOF SBU supported Co-hydride catalytic systems.
With the same overall charge and CoII spin state, MIL-53(Al)-CoH
features a more electron-rich hydride site with a NBO charge of -
0.441 on the hydride. In comparison, UiO-68-CoH has a NBO
charge of -0.354 on the hydride (Figure 7, Table S11, S12). We
believe that electron-rich oxo sites in MIL-53(Al)-CoH afford a
reactive Co-hydride for insertion to the unsaturated bonds to lead
to excellent catalytic performance in hydrofunctionalization
reactions. Upon dissociation of THF, the Co centers in MIL-
53(Al)-CoH also have more open coordination site for substrate
binding to further facilitate hydrofunctionalization reactions.
O
O
O
O
O
O
92 %
O
92 %
O
100 %
O
100 %
O
F
t-Bu
Br
Cl
95 %
91 %
90 %
90 %
a
Standard condition: 0.20 mmol alkenes substrate, 1.0 mmol HMTS, 3
mol% Fe, 1.0 mL dry EtOH, 60 °C, 12 h; Yield was determined by GC-MS
analysis using mesitylene as internal standard.
UiO-68-CoH
MIL-53(Al)-CoH
Catalytic Performance Comparison between MIL-53(Al)-CoH and
UiO-68-CoH
-0.441
-0.354
H
OTHF
H
To further demonstrate the outstanding catalytic performance and
the unique electronic property of EAM catalysts supported on Al-
OH chain SBUs, several control reactions were conducted using
the well-established UiO-68-Co MOF catalyst at the same Co
loading.28 Significantly lower yields of hydroboration and
hydrosilylation products were observed when UiO-68-CoH was
used in place of MIL-53(Al)-CoH. For the hydroboration of
Co
Co
O
O
O
O
O
O
O
O
O
O
O
O
Al
O
O
O
-1.287
Al
-0.975
Zr
Zr
Zr
O
O
O
O
O
O
O
O
Al2-O-CoH
Zr3-O-CoH
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Figure 7. NBO charges of Co centers and hydride and bridging oxo
operated for the U.S. DOE Office of Science by ANL, was
1
2
3
4
groups in MIL-53(Al)-CoH and UiO-68-CoH.
supported by the U.S. DOE under Contract No. DE-AC02-
06CH11357. Z. Li acknowledges financial support from the China
Scholarship Council and the National Science Foundation of China
(21671162).
CONCLUSION
5
6
7
8
MIL-53(Al) with one-dimensional Al-OH chain SBUs was used as
a structural and functional mimic of γ-Al2O3 surface to support
Earth-abundant Co and Fe catalysts. Deprotonation of the μ2-OH
groups in MIL-53(Al) followed by metalation with CoCl2 and
FeCl2 afforded MIL-53(Al)-CoCl and MIL-53(Al)-FeCl
precatalysts which showed impressive catalytic activities in distinct
reductive addition and oxidative amination/alkene oxidation
reactions. MIL-53(Al)-CoCl was activated by NaBEt3H to afford
MIL-53(Al)-CoH for reductive hydrofunctionalization reactions,
including hydroboration of alkynes to generate E-
alkenylboronates, hydroboration of nitriles to give N,N-
difunctionalized amines, and hydrosilylation of esters followed by
hydrolysis to afford corresponding alcohols. Due to the site-
isolation effect and unique coordination environment, this highly
stable and solution-inaccessible CoII hydride species exhibits
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unprecedented
catalytic
activities
in
reductive
hydrofunctionalization reactions. On the other hand, due to the
FeII/FeIII redox property, MIL-53(Al)-FeCl catalyzed interesting
oxidative transformations, including Csp3-H amination and
Wacker-type alkene oxidation reactions. Spectroscopic studies
indicated the involvement of the FeII/FeIII redox process in
oxidative transformations. Compared to traditional γ-Al2O3-
supported metal catalysts, structurally-defined and single-site of
MIL-53(Al)-M (M = Co, Fe) solid catalysts allow for molecular
level understanding of coordination environments and electronic
structures of the catalytically active site as well as the probing of
reaction mechanisms in unprecedented details. Moreover, control
experiments combined with computational studies showed that
such Al2-O- sites provide electron-rich Co-H sites to facilitate
hydrofuctionalization
reactions.
The
establishment
of
structure/activity relationships in MOF-supported catalysts
promises to facilitate their rational optimization to afford cost-
effective and sustainable solid catalysts for the practical synthesis
of commodity and fine chemicals.
ASSOCIATED CONTENT
Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org. Synthesis and characterization of MIL-53(Al),
MIL-53(Al)-CoCl, MIL-53(Al)-CoH and MIL-53(Al)-FeCl;
reaction procedure and product characterization of catalytic
reactions, including cobalt catalyzed alkyne hydroboration, nitrile
hydroboration, ester hydrosilylation, and iron catalyzed C-H
amination and alkene oxidation.
AUTHOR INFORMATION
†X. Feng, P. Ji, and Z. Li contributed equally to this work.
Corresponding Author Email: wenbinlin@uchicago.edu
ORCID
ACKNOWLEDGMENT
Organometallic
Chemistry
and
Periodic
Calculations.
This work was supported by NSF (CHE-1464941). XAS analysis
was performed at Beamline 10-BM , supported by the Materials
Research Collaborative Access Team (MRCAT). Use of the
Advanced Photon Source, an Office of Science User Facility
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