Doping metal-organic frameworks for water oxidation, carbon dioxide reduction, and organic photocatalysis

Article abstract of DOI:10.1021/ja203564w

Catalytically competent Ir, Re, and Ru complexes H₂L ₁-H₂L₆ with dicarboxylic acid functionalities were incorporated into a highly stable and porous Zr₆O ₄(OH)₄(bpdc)₆

Full text of DOI:10.1021/ja203564w

ARTICLE
pubs.acs.org/JACS
Doping MetalꢀOrganic Frameworks for Water Oxidation,
Carbon Dioxide Reduction, and Organic Photocatalysis
Cheng Wang, Zhigang Xie, Kathryn E. deKrafft, and Wenbin Lin*
Department of Chemistry, CB #3290, University of North Carolina, Chapel Hill, North Carolina 27599, United States
S Supporting Information
b
ABSTRACT: Catalytically competent Ir, Re, and Ru complexes H2L1ꢀ
H2L6 with dicarboxylic acid functionalities were incorporated into a highly
stable and porous Zr6O4(OH)4(bpdc)6 (UiO-67, bpdc = para-biphenyl-
dicarboxylic acid) framework using a mix-and-match synthetic strategy.
The matching ligand lengths between bpdc and L1ꢀL6 ligands allowed the
construction of highly crystalline UiO-67 frameworks (metalꢀorganic
frameworks (MOFs) 1ꢀ6) that were doped with L1ꢀL6 ligands. MOFs
1ꢀ6 were isostructural to the parent UiO-67 framework as shown by
powder X-ray diffraction (PXRD) and exhibited high surface areas ranging
from 1092 to 1497 m2/g. MOFs 1ꢀ6 were stable in air up to 400 °C and
active catalysts in a range of reactions that are relevant to solar energy utilization. MOFs 1ꢀ3 containing [Cp*IrIII(dcppy)Cl]
(H2L1), [Cp*IrIII(dcbpy)Cl]Cl (H2L2), and [IrIII(dcppy)2(H2O)2]OTf (H2L3) (where Cp* is pentamethylcyclopentadienyl,
dcppy is 2-phenylpyridine-5,40-dicarboxylic acid, and dcbpy is 2,20-bipyridine-5,50-dicarboxylic acid) were effective water oxidation
catalysts (WOCs), with turnover frequencies (TOFs) of up to 4.8 hꢀ1. The [ReI(CO)3(dcbpy)Cl] (H2L4) derivatized MOF 4
served as an active catalyst for photocatalytic CO2 reduction with a total turnover number (TON) of 10.9, three times higher than
that of the homogeneous complex H2L4. MOFs 5 and 6 contained phosphorescent [IrIII(ppy)2(dcbpy)]Cl (H2L5) and
[RuII(bpy)2(dcbpy)]Cl2 (H2L6) (where ppy is 2-phenylpyridine and bpy is 2,20-bipyridine) and were used in three photocatalytic
organic transformations (aza-Henry reaction, aerobic amine coupling, and aerobic oxidation of thioanisole) with very high activities.
The inactivity of the parent UiO-67 framework and the reaction supernatants in catalytic water oxidation, CO2 reduction, and
organic transformations indicate both the molecular origin and heterogeneous nature of these catalytic processes. The stability of the
doped UiO-67 catalysts under catalytic conditions was also demonstrated by comparing PXRD patterns before and after catalysis.
This work illustrates the potential of combining molecular catalysts and MOF structures in developing highly active heterogeneous
catalysts for solar energy utilization.
’ INTRODUCTION
new mild, clean, and atom-efficient methodologies for organic
synthesis.9
Sunlight provides the most attractive and abundant renewable
energy source to meet mankind’s future energy needs.1 Chemists
have long been interested in building artificial photosynthetic
systems to harvest solar energy and to enable visible-light-driven
organic synthesis.2 Although the first artificial photodriven water
splitting was demonstrated with a solid state semiconductor,3
significant recent efforts have been devoted to exploring molec-
ular systems for energy harvesting, due to the ease of their
structure/property tuning. Numerous molecular systems have
now been designed to serve as individual functional components
in an artificial photosynthetic device. For example, efficient
molecular antenna have been constructed from cyclic tetrapyr-
roles and [Ru(bpy)3]2+ (bpy = 2,20-bipyridine) or [Ir(ppy)2-
(bpy)]+ (ppy = 2-phenylpyridine) derivatives.4 Molecular cata-
lysts have also been developed for a range of energy-storing
reactions, such as water oxidation,5 hydrogen production,6,7 and
carbon dioxide reduction.7,8 In addition, molecular phosphors
such as [Ru(bpy)3]2+ and [Ir(ppy)2(bpy)]+ have been used to
perform visible-light-driven organic transformations, leading to
One of the remaining challenges in artificial synthesis is the
integration of individual components in a structurally controlled
manner to create an efficient functional device. The photoelec-
trochemical cell (PEC) provides a promising design which
harvests molecular excitations via electron injection into the
conduction band of a semiconductor.4,5 Molecular catalysts
attached to the electrode surface then use the separated electrons
and holes to drive energy-storing chemical reactions. Self-assem-
bly of large molecules into nano- or microscale structures has also
been identified as a potential strategy to achieve an integrated
light-harvesting system.4,10
Metalꢀorganic frameworks (MOFs) have recently emerged
as an interesting class of porous solids that can be constructed
from a variety of molecular complexes11ꢀ15 and explored for a
range of applications in gas storage,16,17 compound separation,18,19
chemical sensing,20ꢀ23 nonlinear optics,24 biomedical imaging,25ꢀ27
Received: April 18, 2011
Published: July 22, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Doped UiO-67
drug delivery,28ꢀ30 and heterogeneous catalysis.31ꢀ35 We are inter-
ested in utilizing MOFs as a platform to integrate individual
functional components for solar energy harvesting. For example,
we recently demonstrated efficient energy migration in Os-doped
Ru(II)-bpy MOFs where the Ru(II)* excited states transferred
energy to lower-lying (∼0.5 eV) Os(II) trap sites via multiple
Ru(II)*f Ru(II) hops.36 Herein, we report the development of the
first MOF-based heterogeneous catalytic systems for water oxida-
tion, photocatalytic CO2 reduction, and visible light-driven organic
photocatalysis.
Catalytic water oxidation constitutes a key half reaction in
artificial photosynthesis.4,5 A large number of homogeneous
water oxidation catalysts (WOCs) have recently been developed
based on dimeric Ru complexes,37,38 monomeric Ru and Fe
complexes,39ꢀ41 monomeric Ir complexes,42ꢀ44 and polyoxo-
metalates with a Ru4O4 or a Co4O4 core.45,46 These molecular
WOCs are highly tunable with high catalyst activity and stability.
On the other hand, heterogeneous WOCs based on iridium
oxide47ꢀ49 and cobalt oxide/phosphate50,51 particles can be
readily interfaced with electrodes or photosensitizers to achieve
electrocatalytic or photocatalytic water oxidation. We believe it is
beneficial to incorporate the molecular WOCs into framework
structures. Unfortunately, most MOF structures tend to lack
stability under water oxidation reaction conditions.52 The UiO
family of MOFs based on Zr6O4(OH)4(CO2)12 secondary
building units (SBUs) and dicarboxylate bridging ligands repre-
sent an interesting exception and is very stable in water.53,54 We
successfully incorporated three iridium-based WOCs, [IrIII(Cp*)-
(dcppy)] (H2L1, where Cp* = pentamethylcyclopentadienyl, dcppy =
2-phenylpyridine-5,40-dicarboxylic acid), [IrIII(Cp*)(dcbpy)]+
(dcbpy = 2,20-bipyridine-5,50-dicarboxylic acid) (H2L2), and
[IrIII(dcppy)2(H2O)]+ (H2L3), into the Zr6O4(OH)4(bpdc)6
(UiO-67, bpdc = para-biphenyldicarboxylate) framework
(MOFs 1ꢀ3) and demonstrated catalytic water oxidation by
these highly stable MOFs.
In photosynthesis, the reducing equivalents resulting from
water oxidation reactions in Photosystem II are used to drive
CO2 reduction in Photosystem I.55 Photochemically reducing
CO2 into a source of fuel offers an attractive way to both
harvest energy from sunlight and alleviate the rise of atmospheric
CO2 concentrations.8 A number of molecular photocatalysts,
including cobalt/nickel tetraaza-macrocyclic compounds,56ꢀ60
iron/cobalt metalloporphyrins,61ꢀ65 and ReI(CO)3(bpy)X com-
plexes,66ꢀ70 have been examined for CO2 reduction in recent
years. We successfully incorporated ReI(CO)3(dcbpy)Cl (H2L4)
into the UiO-67 framework to afford a heterogeneous photo-
catalyst for CO2 reduction using visible light.
Organic transformations driven by visible light are gaining
increasing interest from synthetic chemists because of generally
mild reaction conditions, atom efficiency, and the potential to
mediate thermodynamically uphill reactions.71 Photocatalysts
are often required in visible-light-driven organic reactions since
the majority of organic substrates in these reactions do not
readily absorb photons in the visible region. [Ru(bpy)3]2+ and
[Ir(ppy)2(bpy)]+ have been reported recently as photoredox
catalysts in a variety of new photocatalytic organic reactions,
such as [2 + 2] cycloaddition,72 tin-free dehalogenation,73
aza-Henry reactions,74 aerobic amine coupling,75,76 sulfide and
alcohol oxidation,77,78 olefin epoxidation,79 functional group
transformation,80 asymmetric organophotoredox catalysis,81
and radical chemistry.82 Because these photocatalysts contain
precious metals, it is highly desirable to develop recyclable and
reusable heterogeneous photocatalytic systems based on molec-
ular phosphors. We incorporated [IrIII(ppy)2(dcbpy)]Cl (H2L5)
and [RuII(bpy)2(dcbpy)]Cl2 (H2L6) into the UiO-67 framework
and demonstrated the applications of these doped MOFs as
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Figure 1. (a) Structure model of MOF-1 showing doping of the L1 ligand into the UiO-67 framework. (b) SEM micrograph of intergrown nanocrystals
of MOF-1. (c) PXRD patterns for UiO-67 and MOFs 1ꢀ6. (d) Nitrogen adsorption isotherms of MOFs 1ꢀ6 at 77 K.
highly active heterogeneous catalysts for photochemical aza-
Henry reactions between tertiary amines and nitroalkanes, aero-
bic amine coupling, and sulfide photo-oxidations.
(H2L5) was synthesized by allowing [IrCl(ppy)2]2 to react
with 4,40-(EtO2C)2-bpy, followed by base-catalyzed hydrolysis.
The Ru complex [Ru(bpy)2(dcbpy)]Cl2 (H2L6) was synthe-
sized by reacting (2,20-bipyridine)-5,50-dicarboxylic acid with
Ru(bpy)2Cl2. Complexes H2L1ꢀH2L6 were characterized by
NMR spectroscopy, and the new compounds H2L1, H2L3, and
H2L5 were also characterized by mass spectrometry.
’ EXPERIMENTAL SECTION
Detailed experimental procedures can be found in the Supporting
Information.
Reactions of ZrCl4 and metal complexes H2L1ꢀH2L6 in N,N0-
dimethylformamide (DMF) failed to produce crystalline UiO
frameworks, presumably due to the steric demand of the L1ꢀL6
ligands. Structure modeling studies indicated that the steric bulk
of the L1ꢀL6 ligands precluded the formation of UiO frame-
works based on Zr6O4(OH)4(CO2)12 SBUs and pure L1ꢀL6
ligands. We hypothesized that the L1ꢀL6 ligands could instead
be doped into the framework of UiO-67 (Zr6(μ3-O)4-
(μ3ꢀOH)4(bpdc)6) by taking advantage of matching ligand
lengths between bpdc and L1ꢀL6. Such a substitution strategy
not only can allow for the incorporation of a sterically demanding
bridging ligand into a parent framework but also allows for
’ RESULTS AND DISCUSSION
Synthesis of Metal Complex Derivatized MOFs by Doping
the UiO-67 Framework. The Ir complexes [IrCp*Cl(dcppy)]
(H2L1) and [IrCp*Cl(dcbpy)]Cl (H2L2) were synthesized by
allowing [IrCp*Cl2]2 to react with 5,40-(EtO2C)2-ppy or
4,40-(EtO2C)2-bpy, followed by base-catalyzed hydrolysis.
The complex [Ir(dcppy)2(H2O)2](OTf) (H2L3) was synthe-
sized by treating [Ir(dcppy)2]2Cl2 with AgOTf. The Re complex
[Re(CO)3(dcbpy)Cl] (H2L4) was synthesized by a reaction
between (2,20-bipyridine)-5,50-dicarboxylic acid and pentacarbo-
nylchloro rhenium(I). The Ir complex [Ir(ppy)2(dcbpy)]Cl
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retention of the porosity of the parent framework to facilitate
substrate diffusion for efficient catalysis.
of ∼200 nm in dimensions (Figure 1b and Figure S3 [SI]). Metal
complex contents in MOFs 1ꢀ6 were established by inductively
coupled plasma mass spectrometry (ICP-MS) analysis. MOFs
1ꢀ6 were found to contain 2ꢀ8 wt % of the L1ꢀL6 ligands
(Table 1). The formulas Zr6(μ3-O)4(μ3-OH)4(bpdc)6ꢀx(L)x of
MOFs 1ꢀ6 were supported by thermogravimetric analysis
(TGA), showing 61ꢀ63% weight loss for the organic linkers
(Figure S4 [SI]). Permanent porosities of MOFs 1ꢀ6 were
demonstrated by N2 adsorption at 77 K (Figure 1d). Type I
isotherms were obtained for all of the six MOFs with BET surface
areas ranging from 1092 to 1497 m2/g, indicating microporous
structures (Table 1). Pore size distribution (calculated by the HK
method) centering at 6.7 Å perfectly agrees with that of the
structure model (Figure S5 [SI]).
Metal complex doped UiO-67 (MOFs 1ꢀ6) with the Zr6-
(μ3-O)4(μ3-OH)4(bpdc)6ꢀx(L)x formula were synthesized by
treating ZrCl4 with a combination of H2bpdc and ligands H2L1ꢀ
H2L6 in DMF at 100 °C (Scheme 1). Crystallinity of the MOFs
could be enhanced by adding acetic acid to the reaction mixture,
which presumably stabilized soluble Zr4+ species by acetate
coordination and slowed down the formation of amorphous
zirconium oxides/hydroxides. Synthetic conditions were opti-
mized to obtain highly crystalline powdery samples of MOFs
1ꢀ6 (Supporting Information [SI]), which are isostructural with
the parent framework UiO-67, based on the similarity of their
powder X-ray diffraction (PXRD) patterns (Figures 1a and 1c).
SEM images of the samples showed intergrown nanocrystals
Water Oxidation Catalysis Using MOFs 1ꢀ3. Water oxida-
tion catalytic activities of MOFs 1ꢀ3 were examined with Ce4+
(cerium ammonium nitrate, CAN) as an oxidant (Figure 2a). As
shown in Table 2, MOFs 1ꢀ3 are highly effective water oxidation
Table 1. Ligand Doping Level and BET Surface Area of
MOFs 1ꢀ6
MOF x
catalysts with turnover frequencies (TOFs) as high as 4.8 hꢀ1
.
Lx ligand wt %a
BET surface area (m2/g)b
The catalytic activity must come from the doped L1ꢀL3 since the
parent UiO-67 did not catalyze water oxidation. The hetero-
geneous nature of MOFs 1ꢀ3 was verified by the reusability of
MOFs 1ꢀ3 for water oxidation (Figure 2b) and the lack of
catalytic activity for the supernatants of the water oxidation
mixtures (Figure 2c), which contained no Ir as determined by
ICP-MS. Furthermore, the solids recovered from the reactions
exhibited the same PXRD patterns as those of the pristine MOFs
1ꢀ3 (Figure 2d), supporting the stability of the UiO-67 frame-
work under the present water oxidation conditions.
1
2
3
4
5
6
7.7
8.1
6.0
4.2
2.0
3.0
1254
1497
1410
1092
1194
1277
a Determined by ICP-MS. b BET surface area calculations were based on
the adsorption isotherms using P/P0 from 0.005 to 0.1.
Figure 2. (a) Plots of O2 evolving turnover number (O2-TON) vs time for MOFs 1ꢀ3 and the homogeneous H2L1ꢀH2L3. (b) Plots of O2-TON vs
time for reuse experiments of MOFs 1ꢀ3. (c) The amount of detected O2 vs time with undoped UiO-67 and supernatant solutions of MOFs 1ꢀ3
reaction mixtures. The amount of O2 generated by MOF 3 was also plotted for comparison. (d) PXRD patterns of MOFs 1ꢀ3 after catalytic reaction and
that simulated from the UiO-67 structure.
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Table 2. TOFs for MOFs 1ꢀ3 Catalyzed Water Oxidationa
activity of less than 1/30 of that of the parent [Ir(Cp*)(ppy)Cl]
complex (Figure S7 [SI]). The coordination of PPh3 apparently
prevents the water coordination to the Ir center which is key to
the water oxidation reaction. We then proceeded to carry out the
poisoning experiment on MOF 1 with PPh3. After being treated
with PPh3, MOF 1 completely lost its WOC activity (Figure S8
[SI]). Considering the much larger size of PPh3 than the pore
size of the MOF 1, the PPh3 molecules can only have access to
the Ir sites near the surface of the MOF particles, leading to
selective surface poisoning of the WOC. The complete shutdown
of the WOC activity can only occur if the Ce4+-driven water
oxidation reactions exclusively take place near the surface of the
MOF particles. This set of control experiments thus unambigu-
ously demonstrated that the MOF catalysts exhibit a lower WOC
activity than the homogeneous counterparts due to the inability
of the CAN molecules to enter the MOF channels.
catalyst
TOF (hꢀ1
)
catalyst
TOF (hꢀ1
)
c
MOF 1b
MOF 2b
MOF 3b
4.8
1.9
0.4
H2L1
37.0
15.7
6.2
c
H2L2
c
H2L3
a TOF is defined as the number of evolved oxygen molecules per
catalytic site per hour over the first 3 h. CAN concentration
62.3ꢀ67.1 mM, pH = 1. b Heterogeneous catalysis was carried out with
3.2ꢀ7.4 mg of MOFs 1ꢀ3 (equivalent to 0.5ꢀ1.0 μmol of Ir WOCs).
TOFs were calculated based on the Ir complex doping levels determined
by ICP-MS. c Homogeneous catalysis with H2L1ꢀH2L3 was carried out
with 1.5ꢀ7.5 ꢁ 10ꢀ5 M catalyst.
Table 3. Investigations of MOF 4 as a Photocatalyst for Light-
Driven CO2 Productiona
Total WOC turnover numbers of MOFs 1ꢀ3 were deter-
mined from UVꢀvis absorption of the residual Ce4+ ions after
water oxidation reactions of 12 days. The oxygen sensor can
accurately detect O2 in the gas phase only for a few hours and
is not suitable for the longer time-scale experiments. A control
experiment using the undoped UiO-67 MOF in the 12-day
reactions showed no significant change of the concentration
of Ce4+, confirming the validity of this method. The TONs
were estimated to be 1513, 1312, and 2152 for MOFs 1ꢀ3,
respectively.
We do not believe that the water oxidation activity of MOFs
1ꢀ3 comes from IrO2 nanoparticles that could result from the
decomposition of H2L1ꢀH2L3. Control experiments using pre-
synthesized IrO2 nanoparticles under the same conditions used
with the MOFs indicated that the IrO2 nanoparticles were a
highly active WOC with a TOF of ∼150 hꢀ1. However, the IrO2
nanoparticles were unstable under the reaction conditions with
their catalytic activity lasting for less than 30 min. The addition of
more Ce4+ to the reaction mixture did not produce more O2. The
facts that the MOF catalysts could be reused and MOFs 1ꢀ3
each exhibited different water oxidation activity also argue against
the possibility that in situ generated IrO2 nanoparticles are
responsible for water oxidation. The complete poisoning of
MOF 1 by PPh3 also supported the molecular origin of the
WOC activity in the MOFs. X-ray photoelectron spectroscopic
analyses of the fresh and recovered MOFs were inconclusive, as
the Ir 4f binding energies of the L1ꢀL3 ligands were too close to
those of IrO2 (Figure S6 [SI]).
entry
photocatalyst
reaction time (h) H2-TONb CO-TONc
1
MOF 4
6
6
6
6
0.5
0.5
0.6
0.1
5.0
6.9
0
2 (reuse1)d MOF 4
3 (reuse2)d MOF 4
4
supernatant
0
after MOF filtration
5
6
MOF 4
20
6
2.5
0.5
0.8
0.1
0.6
0.3
1.0
0.02
0
10.9
2.5
L4
7 (reuse1)e L4
8 (reuse2)f L4
6
0.07
6
0
9
L4
20
6
3.5
5.6
7.0
0
10
11
12g
13h
14
Re(CO)3Cl(bpy)
Re(CO)3Cl(bpy)
MOF 4
20
6
MOF 4
6
0
undoped UiO-67
6
0
0
a The reaction vials were placed 10 cm in front of a 450 W Xe-lamp with a
300 nm cutoff filter, with magnetic stirring. b H2-TON is defined as the
number of evolved hydrogen molecules per catalytic site. c CO-TON is
defined as the number of evolved CO molecules per catalytic site. d The
MOF solids were recovered by centrifugation for reuse in new
catalytic runs. e The reaction solution was degassed with CO2 before a
second photocatalytic run. f 100 μL of TEA was added to the reaction
solution, and the solution was then degassed with CO2 before a third
photocatalytic run. g Without CO2 h Without light.
Photocatalytic CO2 Reduction Using MOF 4. The Re-based
L4 ligand can serve as an active catalyst for photochemical CO2
reduction. Photocatalytic CO2 reduction activity of MOF 4 was
examined in CO2-saturated acetonitrile (MeCN), using triethy-
lamine (TEA) as a sacrificial reducing agent. Immediately after
irradiating the reaction mixture, the MOF 4 color changed from
orange to green, suggestive of catalytic turnovers. As shown in
Table 3, the MOF catalyst selectively reduced CO2 to CO under
light, as determined by gas chromatography (GC). Under the
reaction conditions (MeCN/TEA = 20/1, regular MeCN and
TEA, saturated with CO2 gas), the molar ratio of the CO and H2
production was around 10 during the first six hours. When the
photocatalytic CO2 reduction was carried out with CD3CN as
the solvent, no formic acid or methanol product was detected by
1H NMR spectroscopy. The CO-TONs reached 5.0 after the first
six hours (Table 3, entry 1). No CO generation was observed in
the absence of CO2 under the same reaction conditions (Table 3,
entry 12), ruling out the possibility that the detected CO could
Comparisons of water oxidation TOFs for MOFs 1ꢀ3 to
those of corresponding homogeneous catalysts H2L1ꢀH2L3
provide important insights into the reaction processes. MOFs
gave lower TOFs than their homogeneous counterparts (only
6.4ꢀ12.9% of the homogeneous catalyst activities, Table 2). This
level of activity can be accounted for by the Ir catalysts on the
MOF particle surface. CAN is apparently too large (∼11.3 Å in
diameter for the cerium nitrate anions from the crystal structure
of CAN) to enter the MOF channels (∼6.7 Å in diameter).
A surface poisoning experiment was performed to shed light
on the low activity of the MOF catalysts. We first found that
treatment of the homogeneous [Ir(Cp*)(ppy)Cl] catalyst with
triphenylphosphine (PPh3) effectively shut down the WOC
activity. We isolated the new Ir complex [Ir(Cp*)(ppy)-
(PPh3)]Cl by treating [Ir(Cp*)(ppy)Cl] with triphenylpho-
sphine in ethylacetate at rt for 6 h. The [Ir(Cp*)(ppy)(PPh3)]Cl
complex was an inactive WOC, exhibiting a negligible WOC
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Figure 3. (a) Plots of CO evolution turnover number (CO-TON) versus time in the photocatalytic CO2 reduction with MOF 4 (blue square) and
homogeneous H2L4 (red circle). (b) FT-IR of as-synthesized MOF 4 (blue) and MOF 4 after photocatalysis (red). (c) PXRD patterns of MOF 4 after
catalysis (black), as-synthesized (red), and simulated from the UiO-67 structure (blue). (d) UVꢀvis diffuse reflectance spectra of as-synthesized MOF 4
(black) and MOF 4 after photocatalysis (red).
have resulted from the decomposition of the L4 ligand. The
photocatalytic nature of the reaction was proved by the fact that
no CO was generated in the dark (Table 3, entry 13). The
inactivity of the parent UiO-67 framework in this reaction
confirmed that the [ReI(dcbpy)(CO)3Cl] moiety was respon-
sible for the catalytic CO2 reduction (Table 3, entry 14).
solid when compared to those of the as-synthesized MOF 4
(Figure 3b), further supporting the decomposition of the L4
ligand by losing the Re-carbonyl moieties.
Photocatalytic CO2 reduction was also conducted with the
homogeneous ligand L4 under the same conditions. Upon
irradiation, the reaction mixture also turned from orange to
green color immediately. After a six hour reaction, the solution
color turned to yellowish gray, and GC analysis indicated a
moderate CO-TON of 2.5 (Table 3, entry 6). The reaction
mixture was almost inactive in the second photocatalytic run,
even after resaturating the solution with CO2 (Table 3, entry 7).
Adding more TEA to the solution did not regenerate the catalytic
activity (Table 3, entry 8). All of these observations indicated that
the Re L4 ligand decomposed during the catalytic turnovers. The
overall CO-TON for the homogeneous H2L4 ligand was esti-
mated to be 3.5 based on the 20 h reaction (Table 3, entry 9). A
time-dependent catalytic activity experiment was performed by
analyzing the CO production at different time points by GC
(Figure 3a). Although the homogeneous H2L4 was more active
than the MOF 4 catalyst in the first two hours, the MOF 4
catalyst retained activity over a longer reaction time to yield a
higher total TON. The MOF 4 catalyst thus exhibited much
higher total TONs than the homogeneous system, presumably as
a result of the catalyst stabilization by the MOF framework.
Reactions using Re(CO)3(bpy)Cl as catalyst were performed
to test our experimental setup and reaction conditions (Table 3,
entries 10 and 11). A total TON of 7.0 was obtained after 20 h
of irradiation, which is comparable to the previously reported
value.69
We also tested the recyclability of the MOF 4 catalyst in light-
driven CO2 reduction. The solid in the reaction mixture was
recovered via centrifugation and reused in additional runs of
catalytic reactions. However, after two six-hour reaction runs, the
catalyst became inactive in CO generation, but a small amount of
H2 was still detected (Table 3, entries 2 and 3). The supernatant
of the MOF 4 reaction mixture showed no CO generation
activity but slight H2 generation activity (Table 3, entry 4).
The total CO-TON of the MOF 4 catalyst was estimated to be
10.9 from the 20 h reaction (Table 3, entry 5). During the 20 h
reaction, 43.6% of the Re had leached into the supernatant, as
determined by ICP-MS. In contrast, only 3.5% of the Zr was
detected in the supernatant by ICP-MS. PXRD of the recovered
solid indicated that the framework structure of MOF 4 remained
intact (Figure 3c). These results suggest that the Re leached into
solution via the detachment of Re-carbonyl moieties from the
dcbpy group in the MOF 4 framework and not by MOF 4
dissolution. Consistent with this, the recovered MOF 4 lost the
1
UVꢀvis peak at 412 nm that is characteristic of the MLCT
absorption of the Re(CO)3(bpy)Cl species (Figure 3d). The
intensities of the IR peaks corresponding to the CO stretching
vibrationsoftheL4 ligand at ∼2025 cmꢀ1(A0), ∼1923 cmꢀ1(A0),
and ∼1900 cmꢀ1 (A00) significantly decreased in the recovered
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Table 4. MOF 5 and MOF 6 Catalyzed aza-Henry Reactionsa
Table 5. Reuse of MOF 5 and MOF 6 in aza-Henry reactionsa
conv. (%)b
catalyst
substrate
1st run
2nd run
3rd run
MOF-5
MOF-5
MOF-5
MOF-6
MOF-6
MOF-6
1a
2a
3a
1a
2a
3a
59
62
96
86
68
97
57
68
93
69
71
93
59
68
95
62
66
95
catalyst/conv. (%)b
c
c
entry substrate no catalyst MOF 5 L5-Et2
MOF 6 L6-Et2
a The reactions were carried out with 1 mol % catalyst loading, 5 cm in
front of a 26 W fluorescent lamp for 12 h. b Conversion yields were
determined by 1H NMR.
1
2
3
1a
2a
3a
19
17
28
59
62
96
99
90
86
68
97
97
88
>99
>99
a The reactions were carried out with 1 mol % catalyst loading, 5 cm in
front of a 26 W fluorescent lamp for 12 h. b Conversion yields were
determined by 1H NMR. c As the H2L5 acid ligand has very low solubility
in nitromethane, the diethyl esters of L5 and L6 ligands L5-Et2 and L6-Et2
were used in the homogeneous control experiments instead.
experiment was carried out to prove the heterogeneity of the MOF
catalyst. Substrate 3a was first used in the MOF 6 catalyzed aza-
Henry reaction, and 95% conversion was achieved after 12 h. The
MOF catalyst was then removed by filtering through Celite, and
another substrate 1a was added to the supernatant solution. After
stirring the solution under light for 12 h, only 22% conversion was
observed for the second substrate. This low conversion, comparable
to that of the background reaction, proved that the supernatant of
the MOF 6 reaction mixture is inactive in photocatalysis, supporting
the heterogeneous nature of the MOF photocatalysts. A further
examination of the supernatant by ICP-MS showed no observable
leaching of Ru to the solution during the reaction. The MOF 5and 6
catalysts were also recovered from the reaction mixture by centri-
fugation and reused three times without loss of activity (Table 5). In
addition, PXRD patterns of MOF 5and 6after the reactions showed
no deterioration of the crystallinity (Figure 4).
We performed another control experiment to demonstrate the
need of MOF permanent porosity in catalyzing the photo-
driven aza-Henry reaction. Amorphous nanoparticles were
synthesized under conditions similar to those of MOF 6
except that wet DMF was used in the synthesis and glacial
acetic acid was not added. The resultant material was non-
porous as indicated by N2 adsorption measurement (with a
negligible BET surface area of 46 m2/g, Figure S19 [SI]) and
amorphous by PXRD (Figure S20 [SI]). The L6 ligand weight
percentage in this material was found to be higher than that of
MOF 6 by ICP-MS measurements (7.0 wt %). However, the
nonporous nanoparticles did not catalyze the aza-Henry
reaction using 1a as the substrate (18% conversion, corre-
sponding to background reaction) at the same catalyst loading
as MOF 6. This observation unambiguously supported that
the photoredox step of the reaction happened inside the
channels of MOF 5 and MOF 6. Although the substrate
amines are relatively large compared to the size of channels
in these MOFs, it is still possible for them to move through the
MOF channels as a result of favorable interactions between
the amine substrates and the MOF framework (e.g., πꢀπ
stacking interactions).86 Alternatively, it is possible that the
aza-Henry reaction can be mediated by photochemically
generated singlet oxygen,87 which will allow the photocata-
lysis with substrates much larger than the open channels since
only O2 molecules need to diffuse through the MOF channels.
As surface photocatalytic sites of these systems have minor
contributions to the overall photocatalysis (<12% as indicated
by the water oxidation activity shown above), microporosity is
a prerequisite for the high activity of these doped MOFs.
Previous mechanistic studies on the [Re(CO)3(bpy)Cl]-cat-
alyzed CO2 reduction suggested both a unimolecular pathway
involving a [ReI(bpy)(CO)3(COOH)] intermediate83 and a
bimolecular pathway involving a CO2-bridged Re dimer
[(CO)3(bpy)ReI](CO2)[ReI(bpy)(CO)3]84,85 or outer-sphere
redox reactions between two Re molecules69 in the catalytic
cycle. The present MOF 4 catalyzed CO2 reduction can only
occur via the unimolecular mechanism as a result of the
immobilization of the L4 catalyst in the MOF framework.
Interestingly, the incorporation of L4 into the MOF 4 framework
not only led to higher CO2 reduction TONs but also shed light
on the CO2 reduction reaction mechanisms and photocatalyst
decomposition pathways.
Photocatalytic Organic Transformations using MOF 5 and
MOF 6. Catalytic activities of Ir(ppy)2(bpy)+-based MOF 5 and
Ru(bpy)32+-based MOF 6 toward photocatalytic aza-Henry
reactions were evaluated with tetrahydroisoquinoline (1a) as
the amine substrate and CH3NO2 as solvent. The reaction was
carried out in the presence of air with a common fluorescent lamp
(26 W) as the light source. The reaction was stopped after 12 h,
and the MOF catalysts were filtered off. Conversions of the
reactions were determined by integrating the peaks of 1H NMR
spectra of the crude reaction mixtures (SI). As shown in Table 4
(entry 1), both MOF 5 and MOF 6 were highly effective
photocatalysts for the aza-Henry reaction between 1a and
nitromethane, with 59% and 86% conversions, respectively.
The MOF 5 and MOF 6 catalysts also effectively catalyzed the
aza-Henry reactions between nitromethane and bromo- and
methoxy-substituted tetrahydroisoquinoline (2a and 3a) with
high efficiency (Table 4, entries 2 and 3). A number of control
experiments were carried out to demonstrate the heterogeneous
and photocatalytic nature of the reactions. The reaction of 1a
in the dark yielded negligible amounts of aza-Henry products
(<5%), demonstrating the necessity of light in this reaction.
On the other hand, the background reaction in the absence of
the catalysts but in the presence of light showed only 19%
conversion after 12 h (Table 4, entry 1), indicating that the MOF
played a catalytic role in the reactions. These observations are
consistent with those of the homogeneous catalytic system
reported by Stephenson and co-workers.74 In addition, a crossover
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Figure 4. (a) PXRD patterns of MOF 5: as-synthesized (red), after aza-Henry reaction (black), and simulated from the UiO-67 structure (blue).
(b) PXRD patterns of MOF 6: after sulfide oxidation (pink), after amine coupling (green), after aza-Henry reaction (blue), as-synthesized (red), and
simulated from the UiO-67 structure (black).
Table 6. Photocatalytic Aerobic Amine Coupling Reactionsa
Table 7. Photo-Oxidation of Thioanisole
entry
catalyst/reaction condition
L6
time (h)
conversion %
1
2
3
4
5
6
7
22
22
22
22
22
22
22
72
73
0
MOF-6
entry
catalyst
MOF 6
substrate
yield %b
no catalyst, with light
no light, with L6
1
2
3
4
5c
6
7
1c
1c
1c
1c
1c
2c
3c
83
80
96
8
0
MOF 6 (reuse)
L6
no light, with MOF 6
L6 under N2 protection
MOF 6, under N2 protection
0
0
no catalyst
MOF 6
0
6
MOF 6
90
three substrates have also been monitored by GC analysis at
different time points, which gave similar initial reaction rates for
the three different substrates. The absence of size selectivity of
the initial reaction rates is consistent with the singlet oxygen-
mediated reaction mechanism. The seemingly lower finial yield
of 3d is due to its slow decomposition on MOF 6, presumably
catalyzed by the Lewis acidic [Zr6(μ3-O)4(μ3-OH)4(CO2)12]
building block.
MOF 6
46
a Reactions were carried out with 1 mol % catalyst loadings, 5 cm in front
of a 300 W Xe lamp for 1 h. b Yields were determined by 1H NMR based
on the product/[product + starting material] ratio. c Without light.
2+
We also demonstrated the applicability of the Ru(bpy)3
-
based MOF 6 as a photocatalyst in other light-driven reactions.
As shown in Table 6, MOF 6 efficiently catalyzed aerobic
oxidative coupling of a series of primary amines with 46ꢀ90%
conversions in three hours (Table 6, entries 1, 6, and 7). The
conversion of substrate 1c using the MOF 6 was comparable to
that using the homogeneous molecular catalyst (Table 6, entry 3).
The recyclability and reusability of MOF 6 were also evaluated
in this reaction using 1c as the substrate. The recovered catalyst
from simple filtration showed no deterioration of conversion %
(Table 6, entry 2) and retained the crystallinity of the pristine
sample (Figure 4b). The background reaction in the absence of
the catalyst but in the presence of light showed only ∼8%
conversion (Table 6, entry 4), verifying that MOF 6 played a
catalytic role in the reactions. On the other hand, the reaction of
1c in the dark yielded negligible amounts of coupling products
(Table 6, entry 5), demonstrating the necessity of light in
this reaction. These observations are consistent with those
reported by Lang et al.75 and Su et al.76 using carbon nitride
and TiO2 as photocatalysts. Time-dependent conversions of the
The photocatalyzed aerobic oxidation of thioanisole was also
examined, using MOF 6 as the photocatalyst. Photocatalytic
aerobic oxidation of sulfide to sulfoxide has been reported
2+
before with Ru(bpy)3 in acetonitrile but only when a lead
ruthenate pyrochlore mineral was added as an electron
shuttle.77 We found that by changing the solvent to methanol
the photocatalyzed sulfide oxidation occurred without an
electron relay (Table 7, entry 1). It is highly likely that the
thioanisole oxidation reaction is mediated by the photochemi-
cally generated singlet oxygen.88ꢀ90 As shown in Table 7, with
methanol as the solvent, MOF 6 catalyzed the selective aerobic
oxidation of thioanisole to methyl phenyl sulfoxide. No sulfone
1
(the possible overoxidized byproduct) was detected by H
NMR, demonstrating a high degree of selectivity of this reac-
tion. The conversion % after 22 h is comparable to that of
the corresponding homogeneous catalytic system (Table 7,
entries 1 and 2). A control experiment with no photocatalyst
but with light showed no appreciable conversion of the sulfide
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Journal of the American Chemical Society
ARTICLE
(Table 7, entry 3). No sulfoxide products were detected when
the reactions were carried out in the absence of light but in the
presence of MOF 6 or L6 (Table 7, entries 4 and 5). O2 was
shown to be the oxidizing agent since no conversion of sulfide
to sulfoxide was observed when the reaction was carried
out under N2 protection (Table 7, entries 6 and 7). The PXRD
pattern of the MOF 6 catalyst after the reaction was identical to
that of the pristine MOF 6, indicating its stability under the
reaction conditions (Figure 4).
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’ CONCLUSIONS
We have successfully incorporated Ir, Re, and Ru complexes
into the UiO framework by a mix-and-match strategy. These
stable and porous metal complex-derivatized doped MOFs are
highly effective catalysts for a range of reactions related to solar
energy utilization. MOFs 1ꢀ3 were used in catalytic water
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tranformations: aza-Henry reaction, aerobic amine coupling, and
aerobic thioanisole oxidation. Stability of these MOF catalysts
under the reaction conditions was verified by comparing PXRD
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these catalysts can not only facilitate catalyst recycling and reuse
but also provide mechanistic insights into the reactions, as in the
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’ ASSOCIATED CONTENT
(28) Horcajada, P.; Chalati, T.; Serre, C.; Gillet, B.; Sebrie, C.; Baati,
T.; Eubank, J. F.; Heurtaux, D.; Clayette, P.; Kreuz, C.; Chang, J. S.;
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S
Supporting Information. Detailed experimental proce-
b
dures and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(30) Rieter, W. J.; Pott, K. M.; Taylor, K. M.; Lin, W. J. Am. Chem.
Soc. 2008, 130, 11584.
’ AUTHOR INFORMATION
(31) Ma, L.; Abney, C.; Lin, W. Chem. Soc. Rev. 2009, 38, 1248.
(32) Ma, L. F., J. M.; Abney, C.; Lin, W. Nat. Chem. 2010, 2, 838.
(33) Song, F.; Wang, C.; Falkowski, J. M.; Ma, L.; Lin, W. J. Am.
Chem. Soc. 2010, 132, 15390.
Corresponding Author
wlin@unc.edu
(34) Cho, S. H.; Ma, B.; Nguyen, S. T.; Hupp, J. T.; Albrecht-
Schmitt, T. E. Chem. Commun. 2006, 2563.
(35) Banerjee, M.; Das, S.; Yoon, M.; Choi, H. J.; Hyun, M. H.; Park,
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(36) Kent, C. A.; Mehl, B. P.; Ma, L.; Papanikolas, J. M.; Meyer, T. J.;
Lin, W. J. Am. Chem. Soc. 2010, 132, 12767.
’ ACKNOWLEDGMENT
We acknowledge sole financial support from the UNC EFRC:
Solar Fuels and Next Generation Photovoltaics, and Energy
Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences
(DE-SC0001011).
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