Integration of Earth-Abundant Photosensitizers and Catalysts in Metal-Organic Frameworks Enhances Photocatalytic Aerobic Oxidation

Article abstract of DOI:10.1021/acscatal.0c05053

We report here the construction of two metal-organic frameworks (MOFs), Zr6-Cu/Fe-1 and Zr6-Cu/Fe-2, by integrating earth-abundant cuprous photosensitizers (Cu-PSs) and Fe catalysts for photocatalytic aerobic oxidation. Site isolation and pore confinement stabilize both Cu-PSs and Fe catalysts, while the proximity between active centers facilitates electron and mass transfer. Upon visible light irradiation and using O2 as the only oxidant, Zr6-Cu/Fe-1 and Zr6-Cu/Fe-2 efficiently oxidize alcohols and benzylic compounds to afford corresponding carbonyl products with broad substrate scopes, high turnover numbers of up to 500 with a 9.4-fold enhancement over homogeneous analogues, and excellent recyclability in four consecutive runs. Control experiments, spectroscopic evidence, and computational studies revealed the photo-oxidation mechanism: Oxidative quenching of [Cu-PS]? by O2 affords [CuII-PS], which efficiently oxidizes FeIII-OH to generate a hydroxyl radical for substrate oxidation. This work highlights the potential of MOFs in promoting earth-abundant metal-based photocatalysis.

Full text of DOI:10.1021/acscatal.0c05053

pubs.acs.org/acscatalysis
Research Article
Integration of Earth-Abundant Photosensitizers and Catalysts in
Metal−Organic Frameworks Enhances Photocatalytic Aerobic
Oxidation
Xuanyu Feng,§ Yunhong Pi,§ Yang Song, Ziwan Xu, Zhong Li, and Wenbin Lin*
Cite This: ACS Catal. 2021, 11, 1024−1032
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: We report here the construction of two metal−organic frameworks (MOFs), Zr6-
Cu/Fe-1 and Zr6-Cu/Fe-2, by integrating earth-abundant cuprous photosensitizers (Cu-PSs) and
Fe catalysts for photocatalytic aerobic oxidation. Site isolation and pore confinement stabilize both
Cu-PSs and Fe catalysts, while the proximity between active centers facilitates electron and mass
transfer. Upon visible light irradiation and using O2 as the only oxidant, Zr6-Cu/Fe-1 and Zr6-Cu/
Fe-2 efficiently oxidize alcohols and benzylic compounds to afford corresponding carbonyl
products with broad substrate scopes, high turnover numbers of up to 500 with a 9.4-fold
enhancement over homogeneous analogues, and excellent recyclability in four consecutive runs.
Control experiments, spectroscopic evidence, and computational studies revealed the photo-
oxidation mechanism: oxidative quenching of [Cu-PS]* by O2 affords [CuII-PS], which efficiently
oxidizes FeIII−OH to generate a hydroxyl radical for substrate oxidation. This work highlights the
potential of MOFs in promoting earth-abundant metal-based photocatalysis.
KEYWORDS: aerobic oxidation, photoredox catalysis, metal−organic framework, heterogeneous catalysis, earth-abundant metal,
cuprous photosensitizer, site isolation
cesses.19−27 Although semiconductor-based photocatalysts
INTRODUCTION
■
can address some of these challenges, they tend to have a
With increasing emphasis on environmental sustainability and
distribution of active centers, which makes it difficult to study
green chemistry,1,2 O2 has established its irreplaceable role as a
the reaction mechanism in detail and tune the active sites for
green, cheap, and abundant oxidant for the synthesis of
enhanced reactivity or selectivity.14,28−32 Thus, there is a
oxygen-containing chemical feedstocks.3 The development of
strong need for developing well-defined photocatalytic systems
efficient and cost-effective catalytic systems for aerobic
based on inexpensive earth-abundant metals (EAMs).33−40 We
oxidation processes has become an important research topic,
envisioned that metal−organic frameworks (MOFs), a class of
crystalline and porous molecular solid materials, could provide
a tunable platform to design efficient photocatalytic aerobic
oxidation systems based on EAMs. MOFs have recently been
shown to stabilize homogeneously inaccessible EAM com-
plexes for catalytic or photocatalytic applications through
active site isolation and pore confinement.18,41−45 Meanwhile,
high local concentrations and proximity of multiple active sites
in MOFs can enhance electron and mass transfer, which is
critically important in photocatalytic processes.46−48 The well-
defined active centers in MOF catalysts allow an in-depth
understanding of reaction mechanisms to facilitate catalyst
optimization.
with significant progress made in recent years.4−9 However, to
activate the triplet O2 and control the radical reaction during
the oxidation process, traditional transition-metal-catalyzed
aerobic oxidation reactions typically rely on hash reaction
conditions and (sub)stoichiometric amounts of expensive
radical initiators, which limits their practical applications.10−13
In contrast, by utilizing light to provide energy for the
activation of ground-state O2 and to facilitate electron transfer
between the substrates and oxidants, photocatalytic aerobic
oxidation reactions can be performed under ambient temper-
ature and atmospheric pressure without cocatalysts, thus
providing an attractive platform for the development of
novel catalytic systems for environmentally benign synthetic
processes.14−18
Despite the advantages offered by and the progress made in
photocatalytic aerobic oxidation systems, significant challenges
remain and limit their widespread applications. For example,
reliance on rare, expensive, and toxic noble metals, such as Ru,
Ir, Os, and Pt, for efficient photosensitization presents a
significant roadblock for practical photocatalytic pro-
Received: November 18, 2020
Revised: December 25, 2020
Published: January 7, 2021
https://dx.doi.org/10.1021/acscatal.0c05053
© 2021 American Chemical Society
ACS Catal. 2021, 11, 1024−1032
1024

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
We previously showed that MOFs stabilize cuprous
photosensitizers (Cu-PSs) as potent photoreductants.49 As
Cu-PSs are also effective photo-oxidants50,51 and Fe catalysts
have been successfully used in many homogeneous oxidation
reactions,9,52−54 we surmised that earth-abundant Cu-PSs and
Fe catalysts could be integrated into MOFs for photocatalytic
oxidation using O2 as the oxidant. The MOF catalysts with
both Cu-PSs and Fe catalysts, Zr6-Cu/Fe-1 and Zr6-Cu/Fe-2,
were highly active in photocatalytic aerobic oxidation of
alcohols and benzylic compounds to selectively afford
corresponding carbonyl products with turnover numbers
(TONs) as high as 500. No additives were required and the
aerobic process could proceed smoothly at room temperature.
Due to the stabilization of both EAM complexes by the MOF
framework and enhanced electron transfer between proximate
Cu-PSs and Fe catalysts, Zr6-Cu/Fe-1 showed a nearly tenfold
enhancement in TONs than the homogeneous analogue. In-
depth mechanistic studies by combining control experiments,
spectroscopic data, and computational results revealed that,
upon light irradiation and with O2 as the oxidant, [CuII-PS]
efficiently oxidizes FeIII−OH to generate a hydroxyl radical as
the key reactive species for substrate oxidation (Figure 1).
(diphenylphosphino)xanthene, H2bpydc = 2,2′-bipyridine-
5,5′-dicarboxylic acid) and Cu(H2bpydc)(POP)(PF6) (POP
= bis(2-diphenylphosphino)phenyl ether) were successfully
incorporated into UiO-type MOFs through solvothermal
reactions of H2bpydc, [Cu(H2bpydc)(XantP)](PF6), or [Cu-
(H2bpydc)(POP)](PF6) and Zr6-Mc in oxygen-free N,N-
dimethylformamide (DMF) to afford Zr6-Cu-1 and Zr6-Cu-2,
respectively (Figure 2a). Zr6-Cu/Fe-1 and Zr6-Cu/Fe-2 were
obtained as light olive solids through subsequent metalation of
the bipy sites in Zr6-Cu-1 and Zr6-Cu-2, respectively, with
FeCl2. Powder X-ray diffraction (PXRD) and transmission
electron microscopy (TEM) studies showed that Zr6-Cu-1,
Zr6-Cu-2, Zr6-Cu/Fe-1, and Zr6-Cu/Fe-2 all adopted UiO-67-
type topology with near-octahedral morphologies of ∼40 nm
in dimensions (Figures 2b,c, S7, and S8). Such nanosizes and
uniform morphologies are expected to facilitate light
penetration and substrate diffusion as well as efficient
adsorption of oxygen for photocatalytic aerobic processes. N2
sorption measurements showed Brunauer−Emmett−Teller
surface areas of 930, 1192, and 2563 m2/g for Zr6-Cu/Fe-1,
Zr6-Cu/Fe-2, and UiO-67-bpy,56 respectively (Figure 2d). H
1
NMR spectra of digested MOFs revealed a 1:5 [Cu(H2bpydc)-
(XantP)]+ to H2bpydc ratio in Zr6-Cu-1 (17% Cu loading)
and a 1:4 [Cu(H2bpydc)(POP)]+ to H2bpydc ratio in Zr6-Cu-
2 (20% Cu loading, Figure S6). Inductively coupled plasma-
mass spectrometry (ICP-MS) measurements showed Fe
loadings of 63 and 60% in Zr6-Cu/Fe-1 and Zr6-Cu/Fe-2
(relative to bipy), respectively.
X-ray absorption spectroscopy was used to determine the
electronic properties and coordination environments of Cu
and Fe centers in Zr6-Cu/Fe-1 and Zr6-Cu/Fe-2. X-ray
absorption near-edge structure (XANES) spectroscopy in-
dicated CuI oxidation state in both Zr6-Cu/Fe-1 and Zr6-Cu/
Fe-2 by comparing the pre-edge features with corresponding
Cu salts, all displaying a similar pre-edge peak corresponding
to the spin-allowed 1s → 4p electronic transition at 8981−
8983 eV (Figure S13). Extended X-ray absorption fine
structure (EXAFS) features of Zr6-Cu/Fe-1 and Zr6-Cu/Fe-
2 at both Cu K-edge and Fe K-edge were well fit with the
reported crystal structures of corresponding CuI and FeII
complexes with nearly identical coordination environments.
Specifically, for Zr6-Cu/Fe-1, EXAFS results revealed that the
tetrahedral CuI center coordinates to one bidentate bipy with
Cu−N distances of 1.98 and 2.05 Å, and one bidentate XantP
with Cu−P distances of 2.25 and 2.25 Å (Figure 2e), while the
tetrahedral FeII center coordinates to one bidentate bipy with
the same Fe−N distance of 2.08 Å and two equivalent chloride
groups with Fe−Cl distance of 2.25 Å (Figure S14). Zr6-Cu/
Fe-2 adopted similar tetrahedral CuI centers with one
bidentate bipy and one bidentate POP and tetrahedral FeII
centers with one bidentate bipy and two chloride groups
(Figures S12 and S15). No EXAFS features corresponding to
metallic nanoparticles were detected at either Cu K-edge or Fe
K-edge. These results confirm the successful integration of Cu-
PSs and Fe catalysts with well-defined electronic and local
coordination structures into the UiO frameworks.
Figure 1. MOFs integrate earth-abundant Cu-PSs and Fe catalysts for
photocatalytic aerobic oxidations.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of MOF Catalysts. We
targeted cuprous diimine−diphosphine photosensitizing com-
plexes for the construction of visible-light-responsive MOF
catalysts. The initial attempt of using ZrCl4 as the Zr source
failed to give the targeted MOF due to the decomposition of
C u - P S s b y t h e h i g h l y L e w i s a c i d i c Z r C l 4 .
Zr6O4(OH)4(McO)12 cluster (Zr6-Mc, Mc = methacrylate),55
which was compatible with the cuprous complex, was applied
as an alternative Zr source. After optimization, Cu-PSs
Cu(H2bpydc)(XantP)](PF6) (XantP = dimethyl-4,5-bis-
Photocatalytic Aerobic Oxidation Reactions. As a
proof of concept, we focused on the oxidation of alcohols to
aldehydes and the direct oxidation of benzylic C−H bonds to
aromatic carbonyl derivatives to test the photocatalytic
performance of our EAM MOF catalysts. The carbonyl
products, including aldehydes, ketones, and esters, are
important feedstocks for pharmaceutical synthesis and
1025
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
Figure 2. (a) Synthesis of MOF-based photocatalytic aerobic oxidation systems Zr6-Cu/Fe-1 and Zr6-Cu/Fe-2 with Zr6-Mc as the Zr source. (b)
PXRD patterns of Zr6-Cu-1 (orange), Zr6-Cu/Fe-1 (olive), and Zr6-Cu/Fe-1 (AR, violet) and the simulated pattern of UiO-67 (black). (c) TEM
images of Zr6-Cu-1 and Zr6-Cu/Fe-1. (d) N2 sorption isotherms of Zr6-Cu/Fe-1 (orange), Zr6-Cu/Fe-2 (olive), and UiO-67-bpy (gray). (e)
EXAFS spectra (gray solid line) and fits (black circles) of Zr6-Cu/Fe-1 in R-space at the Cu K-edge.
a
Table 1. Photocatalytic Aerobic Oxidation of 4-Methylbenzyl Alcohol by MOF and Homogeneous Catalytic Systems
entry
catalyst
yield (24 h) (%)
TON
1
2
3
4
0.5 mol % Zr6-Cu/Fe-1
0.5 mol % Zr6-Cu/Fe-2
0.2 mol % Zr6-Cu/Fe-1
98 (73% in 6 h)
196
164
340
0
82
68
0
0.5 mol % Zr6-Cu/Fe-1 (dark)
5
1.5 mol % Zr6-Fe
9
−
6
0.5 mol % Zr6-Cu-1
1
−
7
none
0
−
8
UiO-67-bpy
0
−
9
10
11
0.5 mol % [Cu(H2bpydc)(XantP)](PF6) + 1.5 mol % Fe(bpy)Cl2
1.5 mol % Fe(bpy)Cl2
0.5 mol % [Cu(H2bpydc)(XantP)](PF6)
18
5
0
36
−
−
a
Unless noted, reactions were conducted with 0.05 mmol 4-methylbenzyl alcohol in 1.0 mL of DMSO under light irradiation for 24 h. Catalyst
loadings were based on Cu-PS for dual-metallated MOFs.
commodity chemicals.57,58 Traditional synthesis protocols
typically relied on elevated reaction temperatures or radical
initiators, which put a significant burden on the cost and
environment.12,59
Merging of Cu-PSs and Fe catalysts in robust MOFs resulted
in efficient photocatalytic aerobic oxidation of alcohols to
corresponding aldehydes under mild reaction conditions and
without any additives. In the presence of 0.5 mol % of Zr6-Cu/
Fe-1 and irradiation with a 13.9 W 350−700 nm solid-state
plasma light source (equipped with a cooling fan), a solution of
4-methylbenzyl alcohol in dimethylsulfoxide (DMSO) was
efficiently oxidized to 4-methylbenzaldehyde in 73% yield in 6
h and 98% yield in 24 h under ambient O2 atmosphere and at
room temperature (Table 1). No significant overoxidized
products were detected. A slightly lower 4-methylbenzaldehyde
yield of 82% was obtained for Zr6-Cu/Fe-2 in 24 h. Further
lowering the Zr6-Cu/Fe-1 loading to 0.2 mol % led to a TON
of 340, exhibiting a 9.4-fold enhancement over the
homogeneous control using [Cu(H2bpydc)(XantP)](PF6)
and Fe(4,4′-bipyridine)Cl2 in a 1:3 mole ratio (18% yield
with a TON of 36). This enhancement of photocatalytic
activity can be attributed to active center stabilization and
confinement as well as the close proximity (<1 nm) between
MOF-incorporated Cu-PS and Fe catalyst. As a heterogeneous
catalyst, Zr6-Cu/Fe-1 was recovered and reused in four
consecutive runs without a significant decrease in photo-
catalytic activity (Figure S20). PXRD studies indicated the
maintenance of the crystallinity of both Zr6-Cu/Fe-1 and Zr6-
1026
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
a
Table 2. Zr6-Cu/Fe-1 Catalyzed Photocatalytic Aerobic Oxidation of Alcohols and Benzylic Compounds
a
Reactions were conducted with 0.05 mmol substrate in 1.0 mL of DMSO and irradiated for indicated periods of time.
Cu/Fe-2 after photocatalysis (Figures 2b and S7). ICP-MS
revealed minimal leaching of Cu (0.8%) and Fe (0.7%) into
the supernatant after one reaction run.
temperature and atmospheric pressure, suggesting the potential
utility of this process.
Besides the outstanding performance in photocatalytic
aerobic oxidation of alcohols, Zr6-Cu/Fe-1 was also highly
active for direct aerobic oxidation of benzylic compounds to
afford aromatic carbonyl derivatives (Table 2b). Upon
photoirradiation, 4-methylanisole was quantitively oxidized to
4-methoxylbenzaldehyde in the presence of 0.5 mol % Zr6-Cu/
Fe-1 within 24 h. Several substrates containing benzylic C−H
bonds, including 4-ethylanisole, indane, tetralin, fluorene, and
xanthene, were all successfully oxidized to afford desired
ketones in 60−99% yields under similar reaction conditions.
Higher photocatalytic activity was observed for substrates with
adjacent oxygen atoms, e.g., phthalane and isochromane, to
afford lactones in excellent yields with lower catalyst loadings.
Zr6-Cu/Fe-1 catalyzed photocatalytic aerobic oxidation of a
wide scope of alcohols (Table 2a). Under standard conditions,
Zr6-Cu/Fe-1 converted various primary and secondary
benzylic alcohols to corresponding carbonyl compounds.
Both electron-donating groups, e.g., methyl, methoxyl, and
hydroxymethyl, and electron-withdrawing groups, e.g., chloride
before trifluoromethyl, were well tolerated in the photo-
catalytic processes with good to excellent yields. Moreover,
aliphatic alcohols were also smoothly and selectively oxidized
to corresponding aliphatic aldehydes, ketones, or α,β-
unsaturated aldehydes without interference from the unsatu-
rated bonds. All of the reactions were conducted at room
1027
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
Figure 3. (a) CV curves of [Cu(H2bpydc)(XantP)]+ (orange) and [Cu(H2bpydc)(POP)]+ (olive). (b) Normalized XANES features at Fe K-edge
in Zr6-Cu/Fe-1 (navy), Zr6-Cu/Fe-1 (AR, olive), FeCl2 (black), and FeCl3 (red). (c) CV curve of Zr6-Fe coated on the electrode. (d) EPR signal
of the BMPO-•OH adduct generated by Zr6-Cu/Fe-1 in H2O/toluene under N2.
Mechanistic Studies. The well-defined active site
structures of the MOF catalysts provide an opportunity for
in-depth mechanistic studies. To gain insights into the Zr6-Cu/
Fe-1-catalyzed photocatalytic alcohol to aldehyde trans-
formation, we performed several control experiments with 4-
methylbenzyl alcohol. When O2 was replaced by air and N2,
the yields of 4-methylbenzaldehyde dropped from 73 to 32 and
0%, respectively (Table S6), demonstrating the crucial role of
O2 as the oxidant. The removal of either light source, Cu-PS,
or Fe catalyst significantly decreased the yields of 4-
methylbenzaldehyde or completely shut down the reaction
(Table 1). A significantly lower yield of 4-methylbenzaldehyde
(4%) was found when 4 Å molecular sieve was added as a
water absorbent (Table S6), indicating the involvement of
water in the photo-oxidation process. A low yield of 7% 4-
methylbenzaldehyde was obtained with H2O2 as the oxidant
(Table S6), ruling out the contribution of the Fenton reaction.
To further identify the role of potential reactive oxygen
species (ROS) in the photocatalytic oxidation process, specific
ROS quenching agents were added to Zr6-Cu/Fe-1-catalyzed
oxidation of 4-methylbenzyl alcohol. In the presence of 50 mM
suggesting the in situ generated hydroxyl radical from water as
the key ROS for photocatalytic oxidation. Furthermore, a
radical clock experiment with 4-penten-1-ol as a substrate
afforded 87% cyclic products along with 13% primary
aldehydes, confirming the alkoxyl radical as the photo-
oxidation intermediate (Figure S19).63
Spectroscopic evidence was obtained to shed light on the
electron-transfer pathways across the metal centers. Zr6-Cu/
Fe-1 and Zr6-Cu/Fe-2 displayed characteristic [Cu(diimine)-
(diphosphine)]+ luminescence at 491 and 489 nm, respectively
(Figure S23),64 which correspond to ground-state to excited-
state energy gaps of 2.52 eV for Zr6-Cu/Fe-1 and 2.54 eV for
Zr6-Cu/Fe-2. These emissions were effectively quenched by
the increasing concentration of O2, indicating the efficient
electron transfer between [Cu-PS]* and O2 (Figure S23).
Furthermore, CV curves of [Cu(H2bpydc)(XantP)]+ and
[Cu(H2bpydc)(POP)]+ showed similar one-electron oxidation
peaks at +1.58 and +1.55 V vs NHE for the [CuII-PS]/[CuI-
PS] couple (Figure 3a), indicating the strong oxidation power
of both [CuII-PS] species. XANES analysis gave the FeIII
oxidation state for Zr6-Cu/Fe-1 (AR) recovered from
photocatalytic oxidation, suggesting the oxidation of FeII
precatalyst during the photocatalytic oxidation process (Figure
3b). The CV curve of the MOF-stabilized Fe catalyst, Zr6-Fe,
displayed a reversible one-electron oxidation peak at +0.42 V
vs NHE and an irreversible one-electron oxidation peak at
1,4-benzoquinone (as an O2 scavenger)60 or 50 mM NaN3
−
(as an 1O2 scavenger),61 no obvious decrease of 4-
methylbenzaldehyde yields was detected (Table S6). However,
the addition of hydroxyl radical capturing agent TEMPO (50
mM)62 completely shut down the reaction (Table S6), thus
1028
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
Figure 4. Gibbs free energy changes for the photocatalytic oxidation steps on the Fe centers via the metal−oxo pathway or the hydroxyl radical
pathway as calculated by DFT.
+1.43 V vs NHE (Figure 3c). The first peak can be assigned to
the reversible FeII/FeIII couple, while the second peak can be
attributed to the further irreversible oxidation of the FeIII
species. Both Cu-PSs in the MOFs have enough oxidation
potential to drive such steps for subsequent substrate
oxidation.
and BMPO were added to a suspension of Zr6-Cu/Fe-1 in
H2O/toluene under N2 and light irradiation (Figure 3d). EPR
signals from other BMPO-radical adducts were also evident in
the spectrum, which were likely generated from the reactions
•
between highly reactive OH and organic solvents.
Based on these control experiments, computational results,
and spectroscopic evidence, we propose a dual Cu−Fe
photocatalytic cycle for Zr6-Cu/Fe-1 catalyzed aerobic
oxidation (Figure 5). Upon photoirradiation, the [CuI-PS] in
the MOF is first excited to [Cu-PS]*, which is subsequently
Density functional theory (DFT) calculations with the
B3LYP functional were carried out to gain insights into the
second electron oxidation process on the FeIII centers and the
subsequent substrate oxidation. As shown in Figure 4, two
possible transformation pathways were proposed: (a) the
metal−oxo pathway with the involvement of a key FeIV−oxo
intermediate (IN-FeO) before the hydrogen atom transfer
(HAT) process or (b) the hydroxyl radical pathway with the
generation of •OH from the oxidation of FeIII−OH
intermediate followed by HAT between the highly reactive
•OH and the substrate. With the energy input from the oxidant
[Cu(H2bpydc)(XantP)]2+, the hydroxyl radical pathway from
IN-FeOH to IN-Fe required a ΔG of +39.1 kcal/mol, while the
metal−oxo pathway required a much higher ΔG of +78.5 kcal/
mol for the generation of IN-FeO. Although both processes
were driven by exothermic HAT between the active
intermediates and alcoholic substrates, the hydroxyl radical
pathway is favored to occur to form alkoxyl radicals.
•−
oxidized by O2 to generate O2 and [CuII-PS]. The strongly
oxidizing [CuII-PS] efficiently oxidizes [FeIII−OH], which is
Finally, electron paramagnetic resonance (EPR) experiments
were conducted to detect the in situ generated ROS. The
•−
generation of O2 was confirmed by detecting the character-
•−
istic EPR signal for the BMPO-O2 adduct when BMPO was
added to the suspension of Zr6-Cu-1 in anhydrous MeOH/
toluene in air with irradiation (Figure S25). We also detected
the characteristic quartet EPR signal for the BMPO-•OH
adduct at g = 2.006 when Ag+ (sacrificial electron acceptor)
Figure 5. Proposed mechanism for photocatalytic aerobic oxidation of
alcohols to generate alkoxyl radicals with Zr6-Cu/Fe-1.
1029
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
formed by deprotonation of H2O on the acidic FeIII center, to
Authors
•
•
generate highly reactive OH. Through HAT, OH rapidly
oxidizes the alcohol to generate alkoxyl radical species as the
key intermediate, which subsequently transforms into the
aldehyde product. In the presence of H2O, the generated
[FeIII] undergoes hydrolysis to afford [FeIII−OH] for the next
oxidation cycle. Akin to natural enzymes, the product
selectivity of the MOFs is likely controlled by the uniform
and small hydrophobic channels, which facilitate substrate and
product diffusion and minimize overoxidized products.
Xuanyu Feng − Department of Chemistry, The University of
Chicago, Chicago, Illinois 60637, United States;
orcid.org/0000-0003-1355-1445
Yunhong Pi − Department of Chemistry, The University of
Chicago, Chicago, Illinois 60637, United States; School of
Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou 510640, China
Yang Song − Department of Chemistry, The University of
Chicago, Chicago, Illinois 60637, United States;
orcid.org/0000-0003-4212-8814
•
Similarly, OH can efficiently oxidize alkanes with benzylic
C−H bonds to generate benzylic radicals, which are further
converted to the corresponding carbonyl products (Figure
S26).
Ziwan Xu − Department of Chemistry, The University of
Chicago, Chicago, Illinois 60637, United States
Zhong Li − School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640,
China; orcid.org/0000-0002-2796-9300
CONCLUSIONS
■
In summary, we hierarchically integrated earth-abundant
cuprous photosensitizers and Fe catalysts into two robust
MOF catalysts for visible-light-driven aerobic oxidation.
Spectroscopic evidence shows that the well-defined Cu-PSs
and Fe catalysts in the MOFs are significantly stabilized by site
isolation and pore confinement to afford robust and reusable
heterogeneous EAM-based photocatalysts. The close proximity
between Cu-PSs and Fe catalysts further enhances electron and
mass transfer during the photocatalytic process to significantly
increase catalytic performance. Under mild reaction conditions
and without additives, Zr6-Cu/Fe-1 displays excellent activity
in photocatalytic aerobic oxidation of alcohols and benzylic
compounds to corresponding carbonyl products with broad
substrate scopes, high TONs, which are nearly an order of
magnitude higher than those of their homogeneous counter-
parts, and excellent recyclability. Benefitting from uniform
active centers, a combination of control experiments,
spectroscopic evidence, and computational results established
the mechanism of the aerobic photo-oxidation process: [CuI-
PS] is first photoexcited to [Cu-PS]* and then oxidized by O2
to generate [CuII-PS]. As a potent oxidant, [CuII-PS] oxidizes
in situ generated FeIII−OH to afford a highly reactive hydroxyl
radical, which further oxidizes alcohols and benzylic C−H
bonds to corresponding carbonyl products through a radical
process. To the best of our knowledge, this work reports the
first example of MOF-based dual-EAM catalysts for photo-
catalytic oxidation processes, thus highlighting the great
potential of MOFs in stabilizing EAM complexes for
photocatalytic organic synthesis.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c05053
Author Contributions
§X.F. and Y.P. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSF (CHE-1464941) and the
University of Chicago. 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 operated for
the U.S. DOE Office of Science by ANL, was supported by the
U.S. DOE under Contract No. DE-AC02-06CH11357. The
computational study was completed with resources provided
by the University of Chicago’s Research Computing Center.
Y.P. acknowledges financial support from the China Scholar-
ship Council.
REFERENCES
■
(1) Anastas, P. T.; Warner, J. C. Green chemistry. Frontiers 1998,
640.
́
(2) de Marco, B. A.; Rechelo, B. S.; Totoli, E. G.; Kogawa, A. C.;
Salgado, H. R. N. Evolution of green chemistry and its multidimen-
sional impacts: A review. Saudi Pharm. J. 2019, 27, 1−8.
(3) Anastas, P. T.; Kirchhoff, M. M. Origins, Current Status, and
Future Challenges of Green Chemistry. Acc. Chem. Res. 2002, 35,
686−694.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05053.
■
(4) Wang, Q.; Zhang, M.; Chen, C.; Ma, W.; Zhao, J. Photocatalytic
Aerobic Oxidation of Alcohols on TiO2: The Acceleration Effect of a
Brønsted Acid. Angew. Chem., Int. Ed. 2010, 49, 7976−7979.
(5) Hoover, J. M.; Steves, J. E.; Stahl, S. S. Copper(I)/TEMPO-
catalyzed aerobic oxidation of primary alcohols to aldehydes with
ambient air. Nat. Protoc. 2012, 7, 1161−1166.
sı
Synthesis and characterization of Zr6-Cu-1, Zr6-Cu-2,
Zr6-Cu/Fe-1, and Zr6-Cu/Fe-2; reaction procedures
and supplementary experiment results of photocatalytic
aerobic oxidation; and supplementary photophysical and
electrochemical data (PDF)
(6) Zhang, P.; Gong, Y.; Li, H.; Chen, Z.; Wang, Y. Solvent-free
aerobic oxidation of hydrocarbons and alcohols with Pd@N-doped
carbon from glucose. Nat. Commun. 2013, 4, No. 1593.
(7) Liu, J.; Zhang, X.; Yi, H.; Liu, C.; Liu, R.; Zhang, H.; Zhuo, K.;
Lei, A. Chloroacetate-Promoted Selective Oxidation of Hetero-
benzylic Methylenes under Copper Catalysis. Angew. Chem., Int. Ed.
2015, 54, 1261−1265.
AUTHOR INFORMATION
Corresponding Author
■
́
(8) Zhang, W.; Fernandez-Fueyo, E.; Ni, Y.; van Schie, M.; Gacs, J.;
Wenbin Lin − Department of Chemistry, The University of
Chicago, Chicago, Illinois 60637, United States;
orcid.org/0000-0001-7035-7759; Email: wenbinlin@
uchicago.edu
Renirie, R.; Wever, R.; Mutti, F. G.; Rother, D.; Alcalde, M.;
Hollmann, F. Selective aerobic oxidation reactions using a
combination of photocatalytic water oxidation and enzymatic
oxyfunctionalizations. Nat. Catal. 2018, 1, 55−62.
1030
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
̈
̈
(9) Gudmundsson, A.; Schlipkoter, K. E.; Backvall, J.-E. Iron(II)-
Catalyzed Biomimetic Aerobic Oxidation of Alcohols. Angew. Chem.,
Int. Ed. 2020, 59, 5403−5406.
(29) Yuan, B.; Chong, R.; Zhang, B.; Li, J.; Liu, Y.; Li, C.
Photocatalytic aerobic oxidation of amines to imines on BiVO4 under
visible light irradiation. Chem. Commun. 2014, 50, 15593−15596.
(30) Lang, X.; Leow, W. R.; Zhao, J.; Chen, X. Synergistic
photocatalytic aerobic oxidation of sulfides and amines on TiO2
under visible-light irradiation. Chem. Sci. 2015, 6, 1075−1082.
(31) Lang, X.; Zhao, J.; Chen, X. Visible-Light-Induced Photoredox
Catalysis of Dye-Sensitized Titanium Dioxide: Selective Aerobic
Oxidation of Organic Sulfides. Angew. Chem., Int. Ed. 2016, 55, 4697−
4700.
(32) Wang, Z.; Lang, X. Visible light photocatalysis of dye-sensitized
TiO2: The selective aerobic oxidation of amines to imines. Appl.
Catal., B 2018, 224, 404−409.
(33) Pirtsch, M.; Paria, S.; Matsuno, T.; Isobe, H.; Reiser, O.
[Cu(dap)2Cl] As an Efficient Visible-Light-Driven Photoredox
Catalyst in Carbon−Carbon Bond-Forming Reactions. Chem. - Eur.
J. 2012, 18, 7336−7340.
(34) Hernandez-Perez, A. C.; Vlassova, A.; Collins, S. K. Toward a
Visible Light Mediated Photocyclization: Cu-Based Sensitizers for the
Synthesis of [5]Helicene. Org. Lett. 2012, 14, 2988−2991.
(35) Gualandi, A.; Marchini, M.; Mengozzi, L.; Natali, M.; Lucarini,
M.; Ceroni, P.; Cozzi, P. G. Organocatalytic Enantioselective
Alkylation of Aldehydes with [Fe(bpy)3]Br2 Catalyst and Visible
Light. ACS Catal. 2015, 5, 5927−5931.
(36) Shanmugam, S.; Xu, J.; Boyer, C. Exploiting Metalloporphyrins
for Selective Living Radical Polymerization Tunable over Visible
Wavelengths. J. Am. Chem. Soc. 2015, 137, 9174−9185.
(37) Higgins, R. F.; Fatur, S. M.; Shepard, S. G.; Stevenson, S. M.;
(10) Minisci, F.; Recupero, F.; Pedulli, G. F.; Lucarini, M. Transition
metal salts catalysis in the aerobic oxidation of organic compounds:
Thermochemical and kinetic aspects and new synthetic developments
in the presence of N-hydroxy-derivative catalysts. J. Mol. Catal. A:
Chem. 2003, 204−205, 63−90.
(11) Wendlandt, A. E.; Suess, A. M.; Stahl, S. S. Copper-Catalyzed
Aerobic Oxidative C−H Functionalizations: Trends and Mechanistic
Insights. Angew. Chem., Int. Ed. 2011, 50, 11062−11087.
(12) Parmeggiani, C.; Cardona, F. Transition metal based catalysts
in the aerobic oxidation of alcohols. Green Chem. 2012, 14, 547−564.
(13) Cao, Q.; Dornan, L. M.; Rogan, L.; Hughes, N. L.; Muldoon,
M. J. Aerobic oxidation catalysis with stable radicals. Chem. Commun.
2014, 50, 4524−4543.
(14) Lang, X.; Ma, W.; Chen, C.; Ji, H.; Zhao, J. Selective Aerobic
Oxidation Mediated by TiO2 Photocatalysis. Acc. Chem. Res. 2014,
47, 355−363.
(15) Muhldorf, B.; Wolf, R. C−H Photooxygenation of Alkyl
̈
Benzenes Catalyzed by Riboflavin Tetraacetate and a Non-Heme Iron
Catalyst. Angew. Chem., Int. Ed. 2016, 55, 427−430.
(16) Jin, Y.; Ou, L.; Yang, H.; Fu, H. Visible-Light-Mediated Aerobic
Oxidation of N-Alkylpyridinium Salts under Organic Photocatalysis. J.
Am. Chem. Soc. 2017, 139, 14237−14243.
(17) Zhang, X.; Rakesh, K. P.; Ravindar, L.; Qin, H.-L. Visible-light
initiated aerobic oxidations: a critical review. Green Chem. 2018, 20,
4790−4833.
́
Boston, D. J.; Ferreira, E. M.; Damrauer, N. H.; Rappe, A. K.; Shores,
(18) Xu, C.; Pan, Y.; Wan, G.; Liu, H.; Wang, L.; Zhou, H.; Yu, S.-
H.; Jiang, H.-L. Turning on Visible-Light Photocatalytic C−H
Oxidation over Metal−Organic Frameworks by Introducing Metal-
to-Cluster Charge Transfer. J. Am. Chem. Soc. 2019, 141, 19110−
19117.
(19) Baranoff, E.; Collin, J.-P.; Flamigni, L.; Sauvage, J.-P. From
ruthenium(ii) to iridium(iii): 15 years of triads based on bis-
terpyridine complexes. Chem. Soc. Rev. 2004, 33, 147−155.
(20) Iqbal, N.; Choi, S.; You, Y.; Cho, E. J. Aerobic oxidation of
aldehydes by visible light photocatalysis. Tetrahedron Lett. 2013, 54,
6222−6225.
(21) Koike, T.; Akita, M. Visible-light radical reaction designed by
Ru- and Ir-based photoredox catalysis. Inorg. Chem. Front. 2014, 1,
562−576.
(22) Wu, C.-K.; Liou, T.-J.; Wei, H.-Y.; Tsai, P.-S.; Yang, D.-Y.
Visible light photoredox catalysis: aerobic oxidation of perimidines to
perimidinones. Tetrahedron 2014, 70, 8219−8225.
(23) Yoo, W.-J.; Tsukamoto, T.; Kobayashi, S. Visible-Light-
Mediated Chan−Lam Coupling Reactions of Aryl Boronic Acids
and Aniline Derivatives. Angew. Chem., Int. Ed. 2015, 54, 6587−6590.
(24) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Dual Catalysis Strategies
in Photochemical Synthesis. Chem. Rev. 2016, 116, 10035−10074.
(25) Toth, B. L.; Tischler, O.; Novak, Z. Recent advances in dual
transition metal−visible light photoredox catalysis. Tetrahedron Lett.
2016, 57, 4505−4513.
(26) Tao, C.; Wang, B.; Sun, L.; Liu, Z.; Zhai, Y.; Zhang, X.; Wang,
J. Merging visible-light photoredox and copper catalysis in catalytic
aerobic oxidation of amines to nitriles. Org. Biomol. Chem. 2017, 15,
328−332.
M. P. Uncovering the Roles of Oxygen in Cr(III) Photoredox
Catalysis. J. Am. Chem. Soc. 2016, 138, 5451−5464.
(38) Nicholls, T. P.; Constable, G. E.; Robertson, J. C.; Gardiner, M.
G.; Bissember, A. C. Brønsted Acid Cocatalysis in Copper(I)-
Photocatalyzed α-Amino C−H Bond Functionalization. ACS Catal.
2016, 6, 451−457.
(39) Zhang, Y.; Petersen, J. L.; Milsmann, C. A Luminescent
Zirconium(IV) Complex as a Molecular Photosensitizer for Visible
Light Photoredox Catalysis. J. Am. Chem. Soc. 2016, 138, 13115−
13118.
́
(40) Call, A.; Casadevall, C.; Acuna-Pares, F.; Casitas, A.; Lloret-
̃
Fillol, J. Dual cobalt−copper light-driven catalytic reduction of
aldehydes and aromatic ketones in aqueous media. Chem. Sci. 2017, 8,
4739−4749.
(41) Zhang, T.; Manna, K.; Lin, W. Metal−Organic Frameworks
Stabilize Solution-Inaccessible Cobalt Catalysts for Highly Efficient
Broad-Scope Organic Transformations. J. Am. Chem. Soc. 2016, 138,
3241−3249.
(42) Zhai, Q.-G.; Bu, X.; Zhao, X.; Li, D.-S.; Feng, P. Pore Space
Partition in Metal−Organic Frameworks. Acc. Chem. Res. 2017, 50,
407−417.
(43) Baek, J.; Rungtaweevoranit, B.; Pei, X.; Park, M.; Fakra, S. C.;
Liu, Y.-S.; Matheu, R.; Alshmimri, S. A.; Alshehri, S.; Trickett, C. A.;
Somorjai, G. A.; Yaghi, O. M. Bioinspired Metal−Organic Framework
Catalysts for Selective Methane Oxidation to Methanol. J. Am. Chem.
Soc. 2018, 140, 18208−18216.
(44) Li, J.; Liao, J.; Ren, Y.; Liu, C.; Yue, C.; Lu, J.; Jiang, H.
Palladium Catalysis for Aerobic Oxidation Systems Using Robust
Metal−Organic Framework. Angew. Chem., Int. Ed. 2019, 58, 17148−
17152.
(45) Yang, X.; Xu, Q. Encapsulating Metal Nanocatalysts within
Porous Organic Hosts. Trends Chem. 2020, 2, 214−226.
(46) Hod, I.; Sampson, M. D.; Deria, P.; Kubiak, C. P.; Farha, O. K.;
Hupp, J. T. Fe-Porphyrin-Based Metal−Organic Framework Films as
High-Surface Concentration, Heterogeneous Catalysts for Electro-
chemical Reduction of CO2. ACS Catal. 2015, 5, 6302−6309.
(47) Zhu, Y.-Y.; Lan, G.; Fan, Y.; Veroneau, S. S.; Song, Y.;
Micheroni, D.; Lin, W. Merging Photoredox and Organometallic
Catalysts in a Metal−Organic Framework Significantly Boosts
́
́
(27) Stephenson, C.; Yoon, T.; MacMillan, D. W. C.; Prier, C. K.;
MacMillan, D. W. C. Dual Photoredox Catalysis: The Merger of
Photoredox Catalysis with Other Catalytic Activation Modes. In
Visible Light Photocatalysis in Organic Chemistry; Wiley-VCH:
Weinheim, 2018; pp 299−333.
(28) Cherevatskaya, M.; Neumann, M.; Fu
Kummel, S.; Dankesreiter, S.; Pfitzner, A.; Zeitler, K.; Konig, B.
Visible-Light-Promoted Stereoselective Alkylation by Combining
Heterogeneous Photocatalysis with Organocatalysis. Angew. Chem.,
Int. Ed. 2012, 51, 4062−4066.
̈
ldner, S.; Harlander, C.;
̈
̈
1031
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
Photocatalytic Activities. Angew. Chem., Int. Ed. 2018, 57, 14090−
14094.
(48) Feng, L.; Wang, Y.; Yuan, S.; Wang, K.-Y.; Li, J.-L.; Day, G. S.;
Qiu, D.; Cheng, L.; Chen, W.-M.; Madrahimov, S. T.; Zhou, H.-C.
Porphyrinic Metal−Organic Frameworks Installed with Brønsted Acid
Sites for Efficient Tandem Semisynthesis of Artemisinin. ACS Catal.
2019, 9, 5111−5118.
(49) Feng, X.; Pi, Y.; Song, Y.; Brzezinski, C.; Xu, Z.; Li, Z.; Lin, W.
Metal−Organic Frameworks Significantly Enhance Photocatalytic
Hydrogen Evolution and CO2 Reduction with Earth-Abundant
Copper Photosensitizers. J. Am. Chem. Soc. 2020, 142, 690−695.
(50) Hernandez-Perez, A. C.; Collins, S. K. A Visible-Light-
Mediated Synthesis of Carbazoles. Angew. Chem., Int. Ed. 2013, 52,
12696−12700.
(51) Zhou, K.; Yu, Y.; Lin, Y.-M.; Li, Y.; Gong, L. Copper-catalyzed
aerobic asymmetric cross-dehydrogenative coupling of C(sp3)−H
bonds driven by visible light. Green Chem. 2020, 22, 4597−4603.
(52) Liu, L.-w.; Wang, Z.-z.; Zhang, H.-h.; Wang, W.-s.; Zhang, J.-z.;
Tang, Y. Iron-catalyzed aerobic C−H functionalization of pyrrolones.
Chem. Commun. 2015, 51, 9531−9534.
(53) Hu, P.; Tan, M.; Cheng, L.; Zhao, H.; Feng, R.; Gu, W.-J.; Han,
W. Bio-inspired iron-catalyzed oxidation of alkylarenes enables late-
stage oxidation of complex methylarenes to arylaldehydes. Nat.
Commun. 2019, 10, No. 2425.
(54) Jiang, X.; Liu, J.; Ma, S. Iron-Catalyzed Aerobic Oxidation of
Alcohols: Lower Cost and Improved Selectivity. Org. Process Res. Dev.
2019, 23, 825−835.
(55) Kickelbick, G.; Wiede, P.; Schubert, U. Variations in capping
the Zr6O4(OH)4 cluster core: X-ray structure analyses of [Zr6-
(OH)4O4(OOC−CHCH2)10]2(μ-OOC−CHCH2)4 and Zr6-
(OH)4O4(OOCR)12(PrOH) (RPh, CMeCH2). Inorg. Chim.
Acta 1999, 284, 1−7.
(56) Li, L.; Tang, S.; Wang, C.; Lv, X.; Jiang, M.; Wu, H.; Zhao, X.
High gas storage capacities and stepwise adsorption in a UiO type
metal−organic framework incorporating Lewis basic bipyridyl sites.
Chem. Commun. 2014, 50, 2304−2307.
́ ́
(57) Munoz Solano, D.; Hoyos, P.; Hernaiz, M. J.; Alcantara, A. R.;
̃
́
Sanchez-Montero, J. M. Industrial biotransformations in the synthesis
of building blocks leading to enantiopure drugs. Bioresour. Technol.
2012, 115, 196−207.
(58) Amslinger, S. The Tunable Functionality of α,β-Unsaturated
Carbonyl Compounds Enables Their Differential Application in
Biological Systems. ChemMedChem 2010, 5, 351−356.
(59) Mallat, T.; Baiker, A. Oxidation of Alcohols with Molecular
Oxygen on Solid Catalysts. Chem. Rev. 2004, 104, 3037−3058.
(60) Pelaez, M.; Falaras, P.; Likodimos, V.; O’Shea, K.; de la Cruz,
A. A.; Dunlop, P. S. M.; Byrne, J. A.; Dionysiou, D. D. Use of selected
scavengers for the determination of NF-TiO2 reactive oxygen species
during the degradation of microcystin-LR under visible light
irradiation. J. Mol. Catal. A: Chem. 2016, 425, 183−189.
(61) Franco, R.; Panayiotidis, M. I.; Cidlowski, J. A. Glutathione
depletion is necessary for apoptosis in lymphoid cells independent of
reactive oxygen species formation. J. Biol. Chem. 2007, 282, 30452−
30465.
(62) Voest, E. E.; van Faassen, E.; Marx, J. J. M. An electron
paramagnetic resonance study of the antioxidant properties of the
nitroxide free radical tempo. Free Radicals Biol. Med. 1993, 15, 589−
595.
(63) Hartung, J.; Gallou, F. Ring Closure Reactions of Substituted 4-
Pentenyl-1-oxy Radicals. The Stereoselective Synthesis of Function-
alized Disubstituted Tetrahydrofurans. J. Org. Chem. 1995, 60, 6706−
6716.
̈
(64) Zhang, Y.; Schulz, M.; Wachtler, M.; Karnahl, M.; Dietzek, B.
Heteroleptic diimine−diphosphine Cu(I) complexes as an alternative
towards noble-metal based photosensitizers: Design strategies,
photophysical properties and perspective applications. Coord. Chem.
Rev. 2018, 356, 127−146.
1032
https://dx.doi.org/10.1021/acscatal.0c05053
ACS Catal. 2021, 11, 1024−1032