Photocatalytic metal-organic frameworks for the aerobic oxidation of arylboronic acids

Article abstract of DOI:10.1039/c5cc01697e

A photocatalytic Ru complex was incorporated into a Zr(iv)-based metal-organic framework (MOF) via postsynthetic methods. The resulting UiO-67-Ru(bpy)₃ shows efficient and recyclable catalytic activity for the aerobic oxidation of arylboronic acids under near-UV and visible light irradiation.

Full text of DOI:10.1039/c5cc01697e

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Photocatalytic metal-organic frameworks for the
aerobic oxidation of arylboronic acids
Cite this: DOI:
0.1039/x0xx00000x
1
＊
Xiao Yu and Seth M. Cohen
0
0th January 2015,
Accepted 00th January 2015
DOI: 10.1039/x0xx00000x
www.rsc.org/
A photocatalytic Ru complex was incorporated into a Zr( )- cost of these precious metal based photocatalysts, a heterogeneous,
IV
easily reusable system could be of substantial value.
based metal-organic framework (MOF) via postsynthetic
methods. The resulting UiO-67-Ru(bpy)3 shows efficient
and recyclable catalytic activity for the aerobic oxidation of
arylboronic acids under near-UV and visible light
irradiation.
Metalꢀorganic frameworks (MOFs) are an emerging class of
porous materials that have a wide range of applications, such as gas
1
,2
3
4
5
storage/separation, biomedicine, chemical sensors, catalysis,
6
and other technologies. The tunable nature of the organic
components in MOFs allows for significant advantages when
compared to other porous materials, such as zeolites, which cannot
be as readily functionalized. Both preꢀ and postꢀsynthetic methods
have been studied to extend the variety of functionalized MOFs that
7
, 8
can be prepared.
Photoactive MOFs have attracted increasing attention for use in a
9
variety of catalytic applications. Mahata et al. first reported the use
10
of a MOF as a photocatalyst to degrade organic pollutants in 2006.
The majority of studies on photoactive MOFs have focused on
functionalization of MOFs to achieve light harvesting and drive H
2
11
evolution and CO reduction. Also, Li and coꢀworkers incorporated
2
Ru carbonyl complexes into a MOF for photocatalytic CO reduction
2
1
2
under visibleꢀlight irradiation. The ability of amineꢀfunctionalized
MOFs to undergo photoinduced charge separation was demonstrated
in several reports, exhibiting photochemical CO2 reduction
3
Scheme 1. Synthesis of UiOꢀ67ꢀRu(bpy) using three different
synthetic strategies.
13ꢀ17
activities.
organic transformations under light irradiation. Duan and coꢀ
workers incorporated a triphenylamine photoredox group into Znꢀ doping MOFs with Ru and Ir complexes via direct solvothermal
In other studies, MOFs were shown to catalyze
1
8
To produce such a recyclable catalyst, the Lin group reported
19
based MOFs, which can drive a lightꢀdriven αꢀalkylation reaction.
During the last decade, Ru(bpy) and related complexes have been amine coupling, and oxidation of thioanisole. In addition to this
synthesis to produce MOFs that catalyze the azaꢀHenry reaction, an
29
3
2
0
shown to be efficient photocatalysts for organic synthesis. The important report, there remain many other reactions of interest and
Yoon and MacMillan groups first employed Ru(bpy) to perform improvements to the catalyst performance, crystallinity, and loading
3
21
22
[
2+2] cycloadditions and αꢀalkylation of aldehydes, respectively. that are yet to be achieved.
Stephenson and coꢀworkers disclosed a photoredox reductive
MOFs with the ability to catalyze aerobic oxidations have been
2
2
3
4
dehalogenation of activated alkyl halides mediated by Ru(bpy)3.
Ru(bpy) and Ir(bpy) have also been used in azaꢀHenry reactions,
developed in recent years, which utilize molecular oxygen as a green
30, 31
3
3
oxidant.
Herein, we incorporate a Ru phtocatalyst into a robust
2
5
26
aerobic amine coupling,
hydroxylation of arylboronic acids,
UiOꢀ67 (UiO = University of Oslo) framework via postsynthetic
sulfide oxidation, and radical chemistry. Considering the high modification (PSM) to get nearly quantitative metal loadings with
27
28
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retention of crystallinity and porosity. The resulting MOFs exhibit efficient and recyclable heterogeneous photocatalyst for aerobic
efficient photocatalytic activity for aerobic oxidation of arylboronic oxidative hydroxylation of arylboronic acids.
acids to the corresponding phenols under light irradiation.
Importantly, MOFs serve as a matrix to enhance the stability of the
active sites, achieving recyclable catalytic performance over five
cycles without significant loss of activity.
The robust UiOꢀ67 framework, consisting of Zr(IV)ꢀbased
secondary building blocks (Zr O (OH) ) and biphenyl ligands, was
6
4
4
2+
selected as a platform to incorporate [Ru(bpy) (dcbpy)] (bis(2,2'ꢀ
2
bipyridine)(5,5'ꢀdicarboxyꢀ2,2'ꢀbipyridine)ruthenium(II)). Attempts
to directly synthesize UiOꢀ67ꢀRu(bpy) gave low loadings of Ru,
3
2
9
presumably due to the steric bulk of the complex.
employed a postsynthetic exchange (PSE) approach to substitute
the biphenyl ligand in UiOꢀ67 with [Ru(bpy) (H dcbpy)]Cl ;
We also
3
2
2
2
2
however, no enhancement of Ru loading, compared to direct
synthesis, was observed under the PSE conditions used (85 °C for 24
h in DMF, MeCN, or EtOH/H O). Therefore, we turned to PSM to
2
improve the incorporation of the Ru(II) complex (Scheme 1).
Using a mixed ligand strategy, H dcbpy ([2,2'ꢀbipyridine]ꢀ5,5'ꢀ
2
dicarboxylic acid) and H dcbp ([1,1'ꢀbiphenyl]ꢀ4,4'ꢀdicarboxylic
2
33
acid) were used to obtain a mixed MOF containing both ligands.
Solvothermal synthesis using a molar ratio of 1:3 of H dcbpy and
2
H dcbp with ZrCl and benzoic acid (as modulator) in DMF at
2
4
1
20 °C for 24 h gave a UiOꢀ67 derivative containing ~25% of the
2ꢀ
dcbpy ligand (UiOꢀ67ꢀbpy0.25). Postsynthetic modification (PSM,
Scheme 1) via a metalation of this MOF with 0.3 equivalents of
Ru(bpy) Cl in EtOH/H O at 80 °C for 2 h, followed by
2
2
2
centrifugation and washing with fresh EtOH for 3 days, afforded the
desired UiOꢀ67ꢀRu(bpy)3 with ~10% Ru loading (UiOꢀ67ꢀ
[
Ru(bpy)3]0.1). PSM metalation of UiOꢀ67 derivatives containing a
2
ꢀ
higher percentage of dcbpy (50~100%) resulted in a loss of
framework stability, as evidenced by powder Xꢀray diffraction
(PXRD, Fig. S1).
The formation of Ru complexes and the degree of PSM were
1
clearly characterized by H NMR after digesting UiOꢀ67ꢀRu(bpy) in
3
D PO /DMSOꢀd .
This analysis was possible because
3
4
6
Ru(bpy) (dcbpy) remains intact under these MOF digestion
2
conditions.
Integration ꢀof the proton resonances for
2
Ru(bpy) (dcbpy) and dcbp
confirmed the degree of Ru
2
modification, which could be tuned from 2% to 15% by varying the
reaction time from 1~24 h (Fig. 1). PXRD confirmed the retention
of the UiOꢀ67 topology (Fig. 1) after metalation. The TGA trace of
UiOꢀ67ꢀ[Ru(bpy)3]0.1 exhibits a decomposition temperature of ~400
°
C, which is ~100 °C lower than that of the unmetalated MOF (Fig.
1
6
3 4
Figure 1 H NMR (D PO /d ꢀDMSO digested, bottom) and PXRD
S2). In addition, UiOꢀ67ꢀ[Ru(bpy) ] exhibited a BET surface area
of 1803±164 m /g, which is high, but lower than the BET surface
area of the parent MOF UiOꢀ67ꢀbpy0.25 (2425±25 m /g, Fig. S3).
It is well known that phenols are among the most important
intermediates and building blocks in the pharmaceutical and
3
0.1
(top) of UiOꢀ67ꢀRu(bpy) containing different amount of Ru complex.
3
2
2
As a benchmark reaction, phenylboronic acid was chosen as a
substrate. As shown in Table 1, incubating
phenylboronic acid, N,Nꢀdiisopropylethylamine (iPr NEt), and UiOꢀ
a mixture of
34
2
chemical industry.
Arylboronic acids can be hydroxylated by
6
(
7ꢀ[Ru(bpy)3]0.1 as catalyst in MeOH using a photochemical reactor
λ = 365 nm) led to an ~81% yield of phenol after 24 h. Other
solvents, such as DMF, H O, and CH CN produced lower yields
than obtained in MeOH. The overall yield (81%) using UiOꢀ67ꢀ
Ru(bpy)3]0.1 is good, but slightly lower than a homogeneous
reference system (Ru(bpy) Cl , yield ~95%). The lower yield may
be due to incomplete light penetration through the MOF material.
Interestingly, pristine UiOꢀ67ꢀbpy0.25 gave ~22% conversion under
irradiation with UV light after 1 day, indicating a photocatalytic
strong oxidizing agents such as oxone, hydrogen peroxide, or metaꢀ
chloroperoxybenzoic acid (MCPBA), which are usually used in
stoichiometric amounts and carefully controlled to avoid overꢀ
2
3
35ꢀ38
oxidation.
In pursuit of environmentally friendly methods,
[
Cu(II) and Pd(II) catalysts have been investigated for oxidative
hydroxylation of arylboronic acids with molecular oxygen, albeit
using a stoichiometric strong base (KOH or NaOH).
al. reported the photocatalytic hydroxylation of boronic acids with
methylene blue as photosensitizer with high efficiency. Xiao and
coꢀworkers reported photocatalytic aerobic oxidative hydroxylation
mediated by a Ru complex. However, the use of a homogeneous
Ru(bpy)3 catalyst poses challenges including product separation
and high cost. Herein, UiOꢀ67ꢀ[Ru(bpy)3]0.1 is shown to act as an
3
2
3
9ꢀ41
Scaiano et
4
2
43
ability similar to ZrO2.
However, a control experiment with no
photocatalyst showed no conversion upon UV or visible light
irradiation (Table 1, Entry 4). No product was observed when the
reaction was carried out in the absence of light even in the presence
of photocatalyst (Table 1, Entry 5), confirming the photochemical
2
6
2+
nature of this oxidation. O was confirmed to be the oxidizing agent,
2
2
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DOI: 10.1039/C5CC016 E
as a control reaction under an N atmosphere also gave no product Phenylenediboronic acid also proved to be suitable substrate for this
2
(Table 1, Entry 6).
Heterogeneity of UiOꢀ67ꢀRu(bpy)3 was reaction, but with a lower conversion (~20%) for the double
confirmed by filtration of the catalyst after 4 h (4 h yield ~10%), oxidation (Fig. S12). Increasing amount of catalyst and sacrificial
which resulted in no further generation of product after another 44 h agents (iPr NEt) and using pure O instead of air could potentially
2
2
26
of irradiation. This suggests that UiOꢀ67ꢀRu(bpy)3 is a true enhance the yield of these reactions. Finally, the substrate scope
heterogeneous catalyst with no catalytically active species released was extended to the use of phenylboronic acid pinacol ester (Table 2,
into solution.
Entry 7), which is a derivative of phenylboronic acid. The yield for
To examine recyclability, experiments were performed using the this substrate was >90% under both nearꢀUV and visibleꢀlight
same batch of MOF for the oxidation of 4ꢀmethoxyphenylboronic irradiation (Fig S13).
acid for 48 h over five successive catalytic cycles. Between each
In conclusion, an example of a heterogeneous photocatalyst for
run, the catalyst was recovered by centrifugation, washed with the aerobic oxidative hydroxylation of arylboronic acids was
MeOH, and dried under vacuum at room temperature. The catalyst prepared by incorporating polypyridyl ruthenium complexes into a
gave good yields, albeit with slightly lower activity after the fourth UiOꢀ67 MOF via a combination of using a mixed ligand MOF with
run (Fig. S4, S5). The lower yield may be due to some loss in the PSM. The synthesized UiOꢀ67ꢀ[Ru(bpy)3]0.1 photocatalyst is stable
Ru species (see ICPꢀOES results below), or simply due to and active over several cycles, providing a platform to recover and
incomplete recovery of the catalyst materials over several cycles. reuse this precious metalꢀcontaining catalyst.
Importantly, the robust nature of the UiOꢀ67 platform allowed the
Table 2. Scope of substrate conversion using UiOꢀ67ꢀ[Ru(bpy)
catalyst.
3
]0.1 as
photocatalyst to be highly stable even under the mildly basic reaction
conditions required (as confirmed by PXRD, Fig. S6). H NMR
1
showed that there is minimal leaching of the Ru complex from the
MOF after one catalytic run (Fig. S7; although a small degree of
2ꢀ
dcbp ligand was observed in the reaction solution, Fig. S8). After 5
cycles, inductively coupled plasmaꢀoptical emission spectroscopy
a
b
Entry
1
Arꢀ
Yield (%)
Yield (%)
(
ICPꢀOES) confirmed an atomic ratio of 1:0.106 (Zr/Ru), ~10%
lower than fresh UiOꢀ67ꢀ[Ru(bpy)3]0.1 which gave an atomic ratio of
:0.118 (Zr/Ru).
81(7)
80(5)
1
Table 1. Summary of results for the aerobic oxidative hydroxylation of
2
3
74(2)
72(2)
70(2)
a
arylboronic acids using UiOꢀ67ꢀ[Ru(bpy)
3
]0.1 as catalyst.
76(3)
4
5
6
>95
>95
Entry Catalyst
Light
Atmosphere
Yield
Yield
50(5)
20(3)
47(3)
15(2)
b
c
(%)
(%)
1
2
3
4
5
6
UiOꢀ67ꢀ[Ru(bpy)
3
]
0.1
+
+
+
+
−
+
Air
Air
Air
Air
Air
81(7)
>95
22(2)
0
77(3)
Ru(bpy) Cl
3
2
>95
0
7
>95
91(1)
UiOꢀ67ꢀbpy0.25
None
a
b
a, b
1
λ = 365 nm. 23W compact fluorescent bulb.
NMR from three independent experiments.
Yield determined by H
0
This work was supported by a grant from the Division of
Chemistry of the National Science Foundation (CHEꢀ1359906).
We thank Dr. H. Fei (UCSD) for assistance with PXRD and
HNMR analysis, Dr. Y. Su (UCSD) for assistance with mass
spectrometry experiments, Dr. Z. Zhang (UCSD) for assistance
with gas adsorption test, and H. Liu (UCSD) for assistance with
UiOꢀ67ꢀ[Ru(bpy)
3
3
]
]
0.1
0.1
0
0
UiOꢀ67ꢀ[Ru(bpy)
N
2
0
0
1
a
Reaction conditions: phenylboronic acid (0.5 mmol), N,Nꢀ
diisopropylethylamine (0.6 mmol), UiOꢀ67ꢀ[Ru(bpy) 0.1 = [Ru] (5 mol%) in 5
mL MeOH open to air with light irradiation at room temperature for 24 h.
3
]
b
λ
c
b,c
1
=
365 nm. 23W compact fluorescent bulb.
Yield is based on HNMR ICPꢀOES measurements.
analysis.
Notes and references
Department of Chemistry and Biochemistry, University of California, San
The scope of nearꢀUV and visible lightꢀinduced photocatalytic
aerobic oxidative hydroxylation of arylboronic acids is summarized
in Table 2 (Fig. S9ꢀS11). The majority of substrates were oxidized Diego, La Jolla, California 92093, USA. Eꢀmail: scohen@ucsd.edu
to aryl alcohols in good to excellent yields, with conversions under
irradiation with visible light or UV light being very similar. The
slightly lower yields with visible light are likely due to the weaker
visibleꢀlight source. A higher conversion efficiency was observed
when treating with electronꢀrich arylboronic acids (Table 2, Entry 2ꢀ
†
Electronic Supplementary Information (ESI) available: Experimental
details of synthesis and catalysis, additional characterization. See
DOI: 10.1039/c000000x/
1
.
K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch,
4
). (4ꢀFlurophenyl)boronic acid (Table 2, Entry 5) shows lower
26
yield, which is consistent with homogeneous system.
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