Titanium(III)-Oxo Clusters in a Metal-Organic Framework Support Single-Site Co(II)-Hydride Catalysts for Arene Hydrogenation

Article abstract of DOI:10.1021/jacs.7b11241

Titania (TiO₂) is widely used in the chemical industry as an efficacious catalyst support, benefiting from its unique strong metal-support interaction. Many proposals have been made to rationalize this effect at the macroscopic level, yet the underlying molecular mechanism is not understood due to the presence of multiple catalytic species on the TiO₂ surface. This challenge can be addressed with metal-organic frameworks (MOFs) featuring well-defined metal oxo/hydroxo clusters for supporting single-site catalysts. Herein we report that the Ti₈(μ₂-O)₈(μ₂-OH)₄ node of the Ti-BDC MOF (MIL-125) provides a single-site model of the classical TiO₂ support to enable Co^II-hydride-catalyzed arene hydrogenation. The catalytic activity of the supported Co^II-hydride is strongly dependent on the reduction of the Ti-oxo cluster, definitively proving the pivotal role of Ti^III in the performance of the supported catalyst. This work thus provides a molecularly precise model of Ti-oxo clusters for understating the strong metal-support interaction of TiO₂-supported heterogeneous catalysts.

Full text of DOI:10.1021/jacs.7b11241

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Article
Titanium(III)-Oxo Clusters in a Metal-Organic Framework Support
Single-Site Co(II)-Hydride Catalysts for Arene Hydrogenation
Pengfei Ji, Yang Song, Tasha Drake, Samuel S. Veroneau, Zekai Lin, Xiandao Pan, and Wenbin Lin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11241 • Publication Date (Web): 06 Dec 2017
Downloaded from http://pubs.acs.org on December 6, 2017
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Titanium(III)-Oxo Clusters in a Metal-Organic Framework Support
Single-Site Co(II)-Hydride Catalysts for Arene Hydrogenation
Pengfei Ji,a Yang Song,a Tasha Drake,a Samuel S. Veroneau,a Zekai Lin,a Xiandao Panb,* and Wenbin
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Lina,*
aDepartment of Chemistry, University of Chicago, 929 E 57th St, Chicago, IL 60637, USA
bState Key Laboratory of Bioactive Substances and Functions of Natural Medicines, Institute of Materia Medica, Chinese
Academy of Medical Sciences & Peking Union Medical College, Beijing 100050, China
ABSTRACT: Titania (TiO2) is widely used in the chemical industry as an efficacious catalyst support, benefiting from its unique
strong metalꢀsupport interaction. Many proposals have been made to rationalize this effect at the macroscopic level, yet the underꢀ
lying molecular mechanism is not understood due to the presence of multiple catalytic species on TiO2 surface. This challenge can
be addressed with metalꢀorganic frameworks featuring wellꢀdefined metal oxo/hydroxo clusters for supporting singleꢀsite catalysts.
Herein we report that the Ti8(ꢀ2ꢀO)8(ꢀ2ꢀOH)4 node of the TiꢀBDC MOF (MILꢀ125) provides a singleꢀsite model of the classical
TiO2 support to enable CoIIꢀhydride catalyzed arene hydrogenation. The catalytic activity of the supported CoIIꢀhydride is strongly
dependent on the reduction of the Tiꢀoxo cluster, definitively proving the pivotal role of TiIII in the performance of the supported
catalyst. This work thus provides a molecularly precise model of Tiꢀoxo clusters for understating the strong metalꢀsupport interacꢀ
tion of TiO2ꢀsupported heterogeneous catalysts.
Introduction
Metal oxides have found broad applications in the chemical
industry as catalyst supports by using surface oxo/hydroxo
groups to coordinate to catalytic metal centers. Such surface
oxo/hydroxo ligands are readily available, thermally robust,
and chemically stable, but lack tunability of their electronic
and steric properties when compared to organic ligands that
are used to support homogeneous catalysts. In this context,
TiO2 is one of the most efficacious metal oxide catalyst supꢀ
ports, due to its negligible toxicity, low price, high stability,
and most interestingly, tunable electronic property through the
titanium redox activity.1ꢀ2 Previous research into TiO2ꢀ
supported catalysts has uncovered the strong metalꢀsupport
interaction (SMSI) effect, which occurs when metallic or metꢀ
al oxide catalysts are immobilized on TiO2 followed by therꢀ
mal treatment.3ꢀ5 The SMSI effect has been shown to signifiꢀ
cantly impact catalytic activity and selectivity. Although many
proposals have been put forth to explain this effect at the macꢀ
roscopic level, including TiO2 migration,6 local TiIV reduction
to TiIII,5, 7 substrate activation with the surface Oꢀvacancies,8ꢀ9
there is still not a clear understanding of the SMSI effect at the
molecular level. The difficulty in understanding the effects of
metal oxide supports partly arises from the presence of multiꢀ
ple species, due to the catalyst binding to different crystal facꢀ
es, different binding modes within one certain crystal face, and
pervasive defects at the molecular level (Figure 1a).10ꢀ12 Thus,
it is highly desirable to design structurally defined Tiꢀoxo
surfaces to support singleꢀsite catalytic metal centers in order
to precisely pinpoint the effects of the Tiꢀoxo support on cataꢀ
lytic activity and selectivity.
Figure 1. (a) Schematic representation of possible faces on the
anatase phase of TiO2 (left) and surface structures of the (101)
face (top right) and (100) face (bottom right). The first layer of
oxygen atoms on each of the faces are highlighted in red. All of
these faces can undergo surface metalation leading to multiple
catalytic sites. (b) Ti8 SBUs in the Ti8ꢀBDC MOF have one possiꢀ
ble binding site only and support the formation of singleꢀsite cataꢀ
lysts.
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Figure 2. (a) Depiction of the tetradentate binding pocket of the Ti8 node and the cobalt coordination environment within the binding
pocket. (b) TEM image of Ti8ꢀBDCꢀCoCl, showing the same morphology as pristine Ti8ꢀBDC. The scale bar is 1 ꢀm. (c) Nitrogen sorption
isotherms of Ti8ꢀBDC and Ti8ꢀBDCꢀCoCl. The BET surface area of Ti8ꢀBDCꢀCoCl (1479 m2/g) is slightly lower than that of Ti8ꢀBDC
(1552 m2/g) due to the increase of molecular weight after metalation. (d) The similarity of PXRD patterns of Ti8ꢀBDC (red), Ti8ꢀBDCꢀ
CoCl (blue), Ti8ꢀBDCꢀCoH (pink), and Ti8ꢀBDCꢀCoH recovered from benzene hydrogenation (navy) to the simulated PXRD pattern for
Ti8ꢀBDC (black) indicates that the crystallinity of the MOF was maintained after metalation, reduction, and catalysis.
Over the past two decades, metalꢀorganic frameworks
(MOFs) have emerged as a highly tunable platform for the
design of singleꢀsite solid catalysts.13ꢀ15 Compared to traditionꢀ
al heterogeneous metal oxideꢀbased catalysts, porous MOF
catalysts allow uniform functionalization of the interior chanꢀ
nels and pores to afford more efficient singleꢀsite solid cataꢀ
lysts. MOFs share a similarity to industrially important zeolite
catalysts that are characterized by large internal surface areas
and uniform pore sizes,16 but can be constructed from a variety
of organic linkers and inorganic nodes to afford an infinite
array of structurally defined singleꢀsite solid catalysts. For
example, many types of MOF catalysts have been prepared
through channel entrapment,17ꢀ19 organic linker functionalizaꢀ
tion,20 and, most recently, inorganic node functionalization.21ꢀ
H2 and the spillover of electrons to reduce TiIV to TiIII on Tiꢀ
oxo SBUs.26 The reduction of Tiꢀoxo SBUs tuned the electronꢀ
ic properties of CoIIꢀH catalysts to effect the hydrogenation of
arenes and heteroarenes with exceptional activities.
Results and Discussion
Structure and Co Binding Mode of Ti8-BDC-CoCl. The
Ti8ꢀBDC MOF, with the formula of Ti8O8(OH)4(BDC)6, was
prepared solvothermally from Ti(OiPr)4, H2BDC, methanol,
and dimethylformamide (DMF) according to the literature
procedure.25 Ti8ꢀBDC was first deprotonated with LiCH2SiMe3
to generate a Ti8O8(OLi)4(BDC)6 intermediate, and then metaꢀ
lated with 1 equiv. of CoCl2 to generate Ti8ꢀBDCꢀCoCl as a
turquoise solid (Figure 2a). The Co content in the metalated
MOF was determined to be 0.7 Co per Ti8 node by inductively
coupled plasmaꢀmass spectrometry (ICPꢀMS). Transmission
electron microscopy (TEM) imaging showed that Ti8ꢀBDCꢀ
CoCl particles perfectly maintained the plateꢀlike morphology
of Ti8ꢀBDC (Figure 2b and Figure S1, SI). The porosity of Ti8ꢀ
BDCꢀCoCl was evaluated by nitrogen sorption isotherms to
afford a BrunauerꢀEmmettꢀTeller (BET) surface area of 1479
m2/g, comparable to that of Ti8ꢀBDC (1552 m2/g, Figure 2c).
24
In particular, MOF nodes with oxo/hydroxo functionalities
have recently been used to support earthꢀabundant metals to
generate highly active metal oxideꢀsupported catalysts for a
range of organic transformations. MOF node supported cataꢀ
lysts are truly singleꢀsite and tunable, promising a unique apꢀ
proach to systematic investigation of the electronic and steric
effects of metalꢀoxo supports on catalytic performances.
In this work, we used Tiꢀoxo/hydroxo secondary building
unit (SBU) of the titaniumꢀbased MOF, MILꢀ125 (Ti8ꢀBDC,
BDC is 1,4ꢀbenzenedicarboxylate),25 to support CoIIꢀH cataꢀ
lysts for highly effective arene hydrogenation (Figure 1b). Ti8ꢀ
BDC features a nanoꢀsized octaꢀtitanium cluster with structurꢀ
ally defined bridging hydroxides and oxides, serving as an
excellent molecular model to study the electronic effects of
TiO2 on the catalytic performance of supported catalysts. We
observed Coꢀmediated dihydride reductive elimination to form
The Co coordination environment of Ti8ꢀBDCꢀCoCl was
first optimized by density functional theory (DFT) calculation
using the Perdew–Burke–Ernzerhof (PBE) functional.27ꢀ28
DFT optimization converged at a geometry with the octahedral
cobalt ion sitting in the Ti8 octagonal plane and coordinating
to two anionic bridgingꢀoxo (
tions and two neutral bridgingꢀoxo (
ꢀ
2ꢀOꢀ) groups in the axial posiꢀ
ꢀ2ꢀO) groups in the equaꢀ
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cies reported in the literature.30ꢀ32 The second set of broad sigꢀ
torial positions. The Ti8 node thus acts as a tetradentate oxoꢀ
based ligand to form the [( 2ꢀO)2(
2ꢀOꢀ)2CoCl(THF)]ꢀ species.
The average calculated Coꢀ( 2ꢀO)
2ꢀOꢀ) distance and Coꢀ(
1
2
3
4
5
6
7
8
nals, with an average gꢀvalue of 2.32, is consistent with those
of highꢀspin d7 CoII species reported in the literature.33 Co 2p
XPS data of the reduced MOF displayed strong 2p3/2 and 2p1/2
peaks at 781.4 eV and 797.4 eV along with strong 2p3/2 and
2p1/2 shakeꢀup peaks at 786.8 eV and 803.4 eV, which are
characteristic of a highꢀspin CoII species (Figure 4c).33 We
thus inferred that the reduction occurred at the Ti centers and
formulated the reduced CoꢀTi8 system as TiIII2TiIV6ꢀBDCꢀ
CoIIH. Highꢀspin CoII centers indicate the weak field nature of
the oxo ligands, which is different from those of the widely
investigated Cꢀ, Nꢀ, or Pꢀbased pincer ligands.34ꢀ35 Such oxoꢀ
based ligands are difficult to access in molecular systems due
to their tendency to bridge multiple metal centers. Co Kꢀedge
XANES spectra of Ti8ꢀBDCꢀCoCl, TiIII2TiIV6ꢀBDCꢀCoH, and
TiIV8ꢀBDCꢀCoH were compared against CoCl2, where the preꢀ
edge of the three MOFs aligned well to that of CoCl2, indicatꢀ
ing CoII centers in all three MOFs (Figure 4d). EPR, XPS, and
XANES results thus indicated that the TiIV centers, not the CoII
centers, were reduced during the treatment of Ti8ꢀBDCꢀCoCl
with NaBEt3H.
ꢀ
ꢀ
ꢀ
ꢀ
distance were 2.05 Å and 2.27 Å, respectively, consistent with
the general trend of shorter MꢀO distances formed by the aniꢀ
onic oxo ligand than the neutral oxo ligand. The calculated
structural model fitted well to the extended Xꢀray fine strucꢀ
ture (EXAFS) data of Co in Ti8ꢀBDCꢀCoCl, with average exꢀ
perimental Coꢀ(ꢀ ꢀ
2ꢀOꢀ) distance and Coꢀ( 2ꢀOꢀ) distance of
2.03 Å and 2.25 Å, respectively (Figure 3a). These Coꢀ(OTi)
bond distances are also consistent with those reported in the
literature.29
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To quantify the amount of TiIII species per Ti8 cluster, the
reduced MOF was titrated with ferrocenium hexafluorophosꢀ
phate (FcPF6, 1 mM in CH2Cl2).36 Gas chromatography (GC)
quantification indicated the formation of 2.07±0.11 equiv. of
ferrocene with respect to Co. The Co centers in the oxidized
MOF maintained CoII oxidation state as indicated by Kꢀedge
XANES analysis (Figure 4d). Also, hydrides attached to CoII
centers are generally known to be poorly reducing through
electron transfer reactions.37 We thus conclude that the Fc+
oxidation reaction only occurs at TiIII centers, suggesting a
Figure 3. EXAFS spectra (gray circles) and fits (black solid line)
in Rꢀspace at the Co Kꢀedge adsorption of Ti8ꢀBDCꢀCoCl (a) and
Ti8ꢀBDCꢀCoH (b). The scattering paths of Co to THFꢀoxygen
(wine), axial oxo (red), equatorial oxo (pink), chloride (green),
hydride (gray) and titanium (purple) are shown in dashed lines.
The EXAFS fitting results agree well with DFT calculated bond
distances.
TiIII2TiIV composition of the Coꢀmetalated Ti8 node (Figure
6
5a). EXAFS fitting revealed that the Co coordination enviꢀ
ronment in the MOF after ferrocenium oxidation, TiIV8ꢀBTCꢀ
CoH, was similar to that in TiIII2TiIV6ꢀBDCꢀCoH (Figure 5b).
Thus TiIV8ꢀBTCꢀCoH was used as a control for both spectroꢀ
scopic analyses and catalytic reactions in subsequent studies.
The partial reduction of TiIV to TiIII was also evidenced by
Structure and Electronic Properties of TiIII2TiIV6-BDC-
CoIIH. Upon treatment with NaBEt3H, the color of Ti8ꢀBDCꢀ
CoCl immediately changed from turquoise to navy and then to
black with concomitant vigorous evolution of H2, suggesting
Coꢀmediated electron transfer from the hydride to the CoꢀTi8
system via reductive elimination of H2 (Figure 4a). Electron
paramagnetic resonance (EPR), Xꢀray photoelectron spectrosꢀ
copy (XPS), and Xꢀray absorption nearꢀedge spectroscopy
(XANES) were employed to determine whether the reduction
occurred at the Co centers or at the Ti centers. The EPR specꢀ
tra of the reduced sample in toluene clearly displayed two sets
of intense signals (Figure 4b). The first sharp isotropic signal,
with a gꢀvalue of 1.94, is in agreement with those of TiIII speꢀ
XPS and XANES analysis. The TiIV node in Ti8ꢀBDC disꢀ
8
played 2p3/2 and 2p1/2 peaks at 458.8 eV and 464.6 eV, respecꢀ
tively, which provided the benchmark for Ti 2p XPS analysis.
The Ti 2p3/2 and 2p1/2 binding energies for TiIII2TiIV6ꢀBDCꢀ
CoH shifted lower to 458.4 eV and 464.3 eV, respectively,
consistent with partial reduction of TiIV centers (Figure 4f).
Fitting of the 2p3/2 peak with TiIII and TiIV components conꢀ
verged at 1:3 ratio, with binding energy of 458.0 eV and 458.5
eV, respectively. The two weak signals at 461.2 eV and 466.9
eV are likely TiIII shakeꢀup satellites, which are characteristic
of TiIII centers with unpaired d electrons.38ꢀ39
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Figure 4. (a) Proposed reduction of Ti8ꢀBDCꢀCoCl to TiIII2TiIV6ꢀBDCꢀCoH with NaBEt3H via electron spillover from Co to Ti through
reductive elimination of H2. (b) EPR spectra of TiIII2TiIV6ꢀBDCꢀCoH at 80 K (black), 130 K (red), and 293 K (blue). The signal from TiIII
is shaded in yellow, and the signals from CoII are shaded in blue. (c) Co 2p XPS spectra of TiIII2TiIV6ꢀBDCꢀCoH displayed 4 peaks at 781.4
eV (2p3/2), 786.8 eV (2p3/2 shakeꢀup), 797.4 eV (2p1/2) and 803.4 eV (2p1/2 shakeꢀup). (d) Co Kꢀedge XANES spectra of Ti8ꢀBDCꢀCoCl
(blue), TiIII2TiIV6ꢀBDCꢀCoH (red) and TiIV8ꢀBDCꢀCoH (black) compared against CoCl2 (magenta). The preꢀedge of the three MOFs
aligned well to that of CoCl2, showing CoII centers in all three MOFs. (e) XANES analysis of TiIV8ꢀBDCꢀCoCl (blue), TiIII2TiIV6ꢀBDCꢀ
CoH (red), and TiIV8ꢀBDCꢀCoH (black) at the Ti Kꢀedge preꢀedge region. (f) Ti 2p XPS spectra of TiIV8ꢀBDC (bottom) and TiIII2TiIV6ꢀ
BDCꢀCoH (top). The experimental data (gray circles) fit well (gray bold line) with a 3:1 ratio of TiIV and TiIII species.
The XANES preꢀedge region of TiIII2TiIV6ꢀBDCꢀCoH
showed an overall 0.3 eV shift to lower energy compared to
that of TiIV8ꢀBDCꢀCoCl, due to the presence of TiIII centers
which enables the lower energy 1s→3d1 transition in addition
to the 1s→3d0 transition (Figure 4e).39 Furthermore, the preꢀ
edge region of Ti8ꢀBDCꢀCoH has higher overall intensity than
that of the Ti8ꢀBDCꢀCoCl, presumably due to geometry distorꢀ
tion of [TiO6]3ꢀ or [TiO6]2ꢀ octahedra caused by partial reducꢀ
tion, which leads to higher probability of spinꢀforbidden 1s to
3d transition.40 In contrast, TiIV8ꢀBTCꢀCoH displays identical
preꢀedge signals as Ti8ꢀBDCꢀCoCl, supporting the assignment
of the aforementioned features to a change in the Ti oxidation
state.
Figure 5. (a) Oxidation of TiIII to TiIV with FcPF6 through an
electron transfer reaction. (b) EXAFS spectrum (gray circles) and
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CoCl, with average calculated Coꢀ(ꢀ ꢀ2ꢀ
2ꢀOꢀ) distance and Coꢀ(
fit (black solid line) in Rꢀspace at the Co Kꢀedge adsorption of
TiIV8ꢀBDCꢀCoH. The scattering paths of Co to THFꢀoxygen
(wine), axial oxo (red), equatorial oxo (pink), hydride (gray), and
1
2
3
4
5
6
7
8
O) distance of 1.94 Å and 2.10/2.41 Å, respectively. The calꢀ
culated structural model was used to fit the Co Kꢀedge EXAFS
data, affording an experimental Coꢀ(ꢀ
2ꢀOꢀ) distance and Coꢀ
titanium (purple) are shown in dashed lines
.
(ꢀ
2ꢀOꢀ) distance of 2.00 Å and 2.17/2.48 Å, respectively. The
The Co coordination environment and electronic structure
in TiIII2TiIV6ꢀBDCꢀCoH were studied through DFT calculation
using the PBE functional. The Co center in TiIII2TiIV6ꢀBDCꢀ
CoH maintained a similar coordination geometry to that in Ti8ꢀ
BDCꢀ
DFT structure of CoII centers thus agreed well with that of
EXAFS fitting (Figure 3b).
Table 1 | Optimization of TiIII2TiIV6-BDC-CoH Catalyzed Arene Hydrogenationa
9
Entry
Reaction
Catalyst (mol % Loading)
T (oC)
120
140
80
Time (h)
1.5
Yield
100 %
53 %
100 %
66 %
42 %
76 %
0 %
TON
≥2000
10526
≥2000
330
TOF (hꢀ1)
1333
7017
111
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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
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2b
3
4
5
6
7
TiIII2TiIV6ꢀBDCꢀCoH (0.05 %)
TiIII2TiIV6ꢀBDCꢀCoH (0.005 %)
TiIII2TiIV6ꢀBDCꢀCoH (0.05 %)
TiIII2TiIV6ꢀBDCꢀCoH (0.2 %)
TiIII2TiIV6ꢀBDCꢀCoH (0.005 %)
TiIII2TiIV6ꢀBDCꢀCoH (0.05 %)
TiIV8ꢀBDCꢀCoH (0.05 %)
1.5
18
25
18
18
160
120
120
1.5
8400
1520
0
5600
1013
0
1.5
1.5
aReaction conditions: Parr high pressure reactor, freshly prepared TiIII2TiIV6ꢀBDCꢀCoH, neat arene substrate, 50 bar H2 unless specified.
b120 bar H2.
TiIII2TiIV6-BDC-CoIIH Catalyzed Arene Hydrogenation.
Arene hydrogenation is of great importance to the industrial
production of commodity and fine chemicals, such as cycloꢀ
hexane analogues, polyester monomers, and plasticizers. With
increasingly stringent environmental and health regulations
over carcinogenic aromatic compounds,41 dearomatization
reactions, notably arene hydrogenations, are playing a bigger
role in diesel fuel refining/upgrading42 and polymer producꢀ
tions.43 Conventional arene hydrogenation catalysts include
Ni, Ru, Pd, and Pt nanoparticles supported on metal oxides or
carbon materials.44 However, the exact structures and reaction
mechanisms of these catalysts remain elusive due to the presꢀ
ence of multiple catalytic species and involvement of multimeꢀ
tallic catalytic pathways.45
Benzene and toluene were chosen as model compounds to
optimize hydrogenation reaction conditions. At 120 °C under
50 bar H2, only 0.05 mol% loading of the TiIII2TiIV6ꢀBDCꢀCoH
catalyst (based on Co) was needed to completely hydrogenate
benzene to produce cyclohexane in 100% yield in 1.5 h, afꢀ
fording a turnover frequency (TOF) of 1333 hꢀ1 (Table 1, entry
1). At 140 °C and 120 bar H2, an impressive TOF of 7017 hꢀ1
was obtained with a catalyst loading of 0.005 mol% (Table 1,
entry 2). To our knowledge, this is one of the highest TOFs
reported for a benzene hydrogenation catalyst, assuming every
supported metal to be catalytically active.50 Exceptionally high
TOFs were also obtained with substituted arenes. At 160 °C
and 50 bar H2, toluene was hydrogenated to methylcyclohexꢀ
ane in 42% yield in 1.5 h, with a TOF of 5600 hꢀ1 (Table 1,
entry 5). TiIII2TiIV6ꢀBDCꢀCoH also exhibited high activity at
room temperature, hydrogenating benzene to cyclohexane in
66 % yield at 0.2 mol% of catalyst loading (Table 1, entry 4).
TiIII2TiIV6ꢀBDCꢀCoH was reused for at least 6 times for benꢀ
zene hydrogenation without a decrease in yields, highlighting
the robustness and recyclability of this catalyst (Figure S15,
SI). Mercury test of TiIII2TiIV6ꢀBDCꢀCoH catalyzed benzene
hydrogenation ruled out the involvement of Co nanoparticles
(Figure S16, SI). High thermal stability of TiIII2TiIV6ꢀBDCꢀ
CoH is particularly interesting in terms of potential industrial
application considering the exothermic nature of arene hydroꢀ
genation reactions.
Although many smallꢀmolecule coordination complexes were
reported as homogeneous arene hydrogenation catalysts, most
of them have been shown to be precursors to nanoparticle
catalysts, evidenced by their long induction periods, short life
times, and mercury deactivation.46 Arenes are difficult to hyꢀ
drogenate with a single metal center, due to their inertness and
relatively weak coordination to metal centers. To date, there
are few examples of singleꢀsite arene hydrogenation catalysts
based on late transition metals47 or early transition metals.48ꢀ49
TiIII2TiIV6ꢀBDCꢀCoH is a MOFꢀbased singleꢀsite solid catalyst
which cannot undergo a bimetallic decomposition due to acꢀ
tive site isolation, representing a unique platform to study nonꢀ
nanoparticle catalyzed arene hydrogenations.
Table 2 | TiIII2TiIV6-BDC-CoH Catalyzed Arene Hydrogenationa
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aReaction conditions: Parr high pressure reactor, freshly prepared TiIII2TiIV6ꢀBDCꢀCoH, neat arene substrate, 50 bar H2.
To investigate the impact of metalꢀsupport interactions on
the catalyst performance, TiIV8ꢀBDCꢀCoH, generated by FcPF6
oxidation (see above), was also tested for toluene hydrogenaꢀ
tion. At 0.05 mol% catalyst loading and under identical reacꢀ
tion conditions, TiIII2TiIV6ꢀBDCꢀCoH converted toluene to
cyclohexane in 76 % yield while TiIV8ꢀBDCꢀCoH displayed no
catalytic activity (Table 1, entries 6ꢀ7). This experiment indiꢀ
cates that the reduction of TiIV support to TiIII2TiIV is crucial
useful in industrially relevant sizeꢀselective arene hydrogenaꢀ
tions.
TiIII2TiIV6ꢀBDCꢀCoH also tolerates a variety of polar substitꢀ
uents on arenes. Arenes with amino groups, including aniline,
αꢀmethylbenzylamine, and Nꢀmethylbenzylamine, were all
hydrogenated in high yields without substrate/product inhibiꢀ
tion. Aryl ethers, such as anisole and 1,4ꢀdimethoxybenzene,
and benzylic ethers, such as dibenzyl ether, were all hydroꢀ
genated to corresponding cyclohexyl ethers in good yields
without ether hydrogenolysis. The hydrogenation of dialkyl
phthalates and terephthalates to corresponding aliphatic esters
models an important industrial process for producing green
plasticizers, which cannot be achieved with other reported
arene hydrogenation catalysts due to electron deficiency and
steric hindrance.51 TiIII2TiIV6ꢀBDCꢀCoH hydrogenated diethyl
phthalate to produce diethyl cyclohexaneꢀ1,2ꢀdicarboxylate in
31% yield at 160 °C.
TiIII2TiIV6-BDC-CoIIH Catalyzed Heteroarene Hydro-
genation. Heteroarene hydrogenation is an efficient method to
generate saturated heterocycles, which are very common strucꢀ
tural motifs in pharmaceuticals and biologically active natural
products.52ꢀ53 However, compared to other unsaturated comꢀ
pounds, heteroarenes are generally very difficult to hydrogenꢀ
ate due to aromaticity stabilization and heteroatom inhibition.
TiIII2TiIV6ꢀBDCꢀCoH was highly active for the hydrogenation
of a variety of heteroarenes (Table 3). At 0.05 mol% loading,
TiIII2TiIV6ꢀBDCꢀCoH catalyzed pyridine perhydrogenation to
piperidine in 94% yield without forming any ringꢀcleavage
products, making it one of the most active and selective cataꢀ
lysts for pyridine reduction. More hindered pyridines, such as
2,6ꢀlutidine and 2,4,6ꢀcollidine, were also hydrogenated in
almost quantitative yields to the corresponding piperidines.
8
6
for the catalytic activity. TiIII2TiIV provides a more electronꢀ
6
donating oxoꢀbased ligand to afford electronꢀrich highꢀspin
CoII centers which enable efficient H2 cleavage and arene
dearomatization. This result offers a molecular level underꢀ
standing for the role of reduced Ti in the SMSI effect which is
often observed in TiO2ꢀsupported catalysts.
TiIII2TiIV6ꢀBDCꢀCoH catalyzed arene hydrogenation exhibitꢀ
ed an exceptionally broad substrate scope (Table 2). Monoꢀ
substituted benzenes, including toluene, ethylbenzene, cuꢀ
mene, and nꢀpropylbenzene, were readily hydrogenated to
corresponding saturated hydrocarbons in almost quantitative
yields and high turnover numbers (TONs). Disubstituted benꢀ
zenes, including pꢀ, mꢀ, and oꢀxylenes, were quantitatively
hydrogenated to corresponding dimethylcyclohexanes with
TONs of ≥2000. Mesitylene was hydrogenated to 1,3,5ꢀ
trimethylcyclohexane in 84% yield at 160 °C. Impressively, at
0.2 mol% catalyst loading, polyarenes, including naphthalene
and fluorene, were perhydrogenated in 100% and 85% yields,
respectively. One unique feature of MOF catalysts is the sizeꢀ
exclusion effect, originating from the uniform shape and size
of MOF channels/cages. In contrast to fluorene, anthracene
was perhydrogenated in only 2% conversion under identical
reaction conditions. TiIII2TiIV6ꢀBDCꢀCoH is thus potentially
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Impressively, for the hydrogenation of 2,6ꢀlutidine, the cataꢀ
of metal oxo clusters for understating the strong metalꢀsupport
interaction of TiO2ꢀsupported heterogeneous catalysts. The
singleꢀsite nature of MOF nodeꢀsupported catalysts should
allow detailed spectroscopic and computational investigations,
thus promising a deep understanding of key parameters influꢀ
encing catalytic performances and rational design of effective
arene hydrogenation catalysts to meet the ever increasing enviꢀ
ronmental and health regulations for the chemical industry.
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lyst was recycled 10 times without any decrease in yields
(Figure S20, SI), supporting the heterogeneity and robustness
of the MOF catalyst. Quinoline derivatives, including 3ꢀ
methylquinoline and 2,6ꢀdimethylquinoline, were perhydroꢀ
genated to the corresponding decahydroquinolines in over
90% yields. For substrates with a strong electron withdrawing
group, such as 6ꢀchloroquinoline, hydrogenation occurred only
at the nonꢀsubstituted ring, generating the tetrahydroquinoline
product. Other heterocyclic systems, such as indole and benzoꢀ
furan, were also semiꢀhydrogenated to indoline and dihydroꢀ
benzofuran in good yields.
Experimental Methods
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Synthesis of Ti8-BDC-CoCl. In an N2ꢀfilled glovebox,
LiCH2SiMe3 (1.0 M in pentane, 2.3 mL, 10 equiv. to Ti8 node)
was added dropwise to a suspension of Ti8ꢀBDC (500 mg,
29% solvent content by TGA, 0.23 mmol Ti8 node) in 15 mL
hexanes. After being stirred at 25 °C for 6 h, the resulting
lightꢀyellow solid was collected through centrifugation, and
washed with hexanes six times. The lithiated Ti8ꢀBDC was
then transferred to a vial containing 11.5 mL of a CoCl2 soluꢀ
tion in THF (20 mM). This mixture was stirred for 12 h at 25
°C and the turquoise solid was then centrifuged and washed
with THF five times. The Ti8ꢀBDCꢀCoCl was freezeꢀdried in
benzene and stored in the glovebox for further use. ICPꢀMS
analysis showed a Ti/Co ratio of 11.4, indicating 0.7 Co per
Ti8 node. PXRD, TEM and BET showed the crystallinity of
MOF was maintained after metalation.
Synthesis of TiIII2TiIV6-BDC-CoH. In an N2ꢀfilled glove
box, Ti8ꢀBDCꢀCoCl (10.0 ꢀmol of Co) was charged to a 1.5
mL centrifuge tube and dispersed in 1.0 mL of toluene. Naꢀ
BEt3H (0.1 mL, 0.1 mmol, 1.0 M solution in toluene) was then
added dropwise to the suspension. The color of the MOF
changed immediately from turquoise to deep blue and then to
black while vigorously evolving H2 gas. The resulting suspenꢀ
sion was kept at r.t. for 30 min to ensure complete reduction.
The black solid was then centrifuged out of suspension and
washed three times with toluene to remove excess NaBEt3H
and other byproducts (NaCl and BEt3).
Oxidation of TiIII2TiIV6-BDC-CoH to TiIV8-BDC-CoH. A
ferrocenium hexafluorophosphate (FcPF6) solution (1 mM in
CH2Cl2) was added to TiIII2TiIV6ꢀBDCꢀCoH (10 ꢀmol of Co) in
3 mL aliquots. Upon this addition, the suspension was vorꢀ
texed and its immediately changed from deep blue to yellow,
indicating rapid reduction of ferrocenium to ferrocene by TiIII.
The suspension was then centrifuged and the yellow supernaꢀ
tant was combined in a separate vial. This reduction process
was repeated until no color change (from deep blue to yellow)
could be observed within 10 min. Upon completion, 8 equiv.
of mesitylene w.r.t. Co were added to the combined supernaꢀ
tant as an internal standard, and the amount of ferrocene was
analyzed by GC.
A typical procedure for TiIII2TiIV6-BDC-CoH catalyzed
hydrogenation of arenes and heteroarenes. In an N2ꢀfilled
glove box, Ti8ꢀBDCꢀCoCl (5 ꢀmol of Co) in 1.0 mL heptane
was charged into a glass vial. NaBEt3H (10 equiv. to Co, 50
μL, 1.0 M in toluene) was then added to the vial and the mixꢀ
ture was stirred for 30 min. The solid was then centrifuged,
washed with heptane three times, and transferred to a Parr
reactor with a glass liner using benzene (0.89 mL, 10 mmol).
The reactor was sealed in the glovebox, and then charged with
hydrogen to 50 bar. After stirring at 120 °C for 18 h, the presꢀ
Table 3 | Ti8-BDC-CoH Catalyzed Heteroarene Hydrogena-
tiona
aReaction condition: Parr high pressure reactor, freshly prepared
TiIII2TiIV6ꢀBDCꢀCoH, neat heteroarene substrate, 50 bar H2.
Conclusions
The Ti8(ꢀ2ꢀO)8(ꢀ2ꢀOH)4 node of the Ti8ꢀBDC MOF (MILꢀ
125) was shown to provide an excellent molecular model of
TiO2 to support a singleꢀsite CoIIꢀhydride catalyst for arene
hydrogenation. Upon deprotonation, the Tiꢀoxo clusters of Ti8ꢀ
BDC coordinated to CoII centers as a tetradentate ligand to
afford Ti8ꢀBDCꢀCoCl without the complication of forming
multiꢀCo oligomers due to the MOF site isolation effect.
Treatment of Ti8ꢀBDCꢀCoCl with NaBEt3H led to partial reꢀ
duction of the Tiꢀoxo support via electron spillover from Co to
Ti by reductive elimination of H2 to form TiIII2TiIV6ꢀBDCꢀ
CoIIH whose electronic property was established by EPR,
XPS, XANES, DFT, and ferrocenium oxidation. TiIII2TiIV6ꢀ
BDCꢀCoIIH features electronꢀrich high spin CoII centers with
electronꢀdonating TiIIIꢀoxo ligands and is one of few highly
active molecular, nonꢀnanoparticle catalysts for arene and
heteroarene hydrogenation. Interestingly, the corresponding
oxidized MOF TiIV8ꢀBDCꢀCoIIH is totally inactive in arene
hydrogenation, proving the crucial role of Tiꢀoxo cluster reꢀ
duction on the catalytic activity of supported CoIIꢀH species.
The Ti8ꢀBDC MOF thus provides a molecularly precise model
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sure was released and the MOF catalyst was removed from the
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reaction mixture via centrifugation. The supernatant was then
analyzed by 1H NMR, revealing a 100% yield of cyclohexane.
ICPꢀMS analysis of the supernatant showed <0.1% leaching of
Co from the TiIII2TiIV6ꢀBDCꢀCoH catalyst.
ASSOCIATED CONTENT
Supporting Information. Synthesis and characterization of
Ti8ꢀBDCꢀCoCl, TiIII2TiIV6ꢀBDCꢀCoH, TiIV8ꢀBDCꢀCoH; Reacꢀ
tion procedures and product characterizations for catalytic
hydrogenation of arenes and heteroarenes; This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*wenbinlin@uchicago.edu
*xdp@imm.ac.cn
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manuꢀ
script.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by National Science Foundation (CHEꢀ
1464941). Xꢀray absorption spectroscopy analysis was performed
at Beamline 9ꢀBM, supported by the Materials Research Collaboꢀ
rative Access Team (MRCAT). 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 No. DEꢀAC02ꢀ06CH11357. We thank Guangxu
Lan for help with TEM and Dr. Alexander S. Filatov for help with
XPS.
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