Lead-Halide Perovskites for Photocatalytic α-Alkylation of Aldehydes

Article abstract of DOI:10.1021/jacs.8b08720

Cost-effective and efficient photocatalysis are highly desirable in chemical synthesis. Here we demonstrate that readily prepared suspensions of APbBr₃ (A = Cs or methylammonium (MA)) type perovskite colloids (ca. 2-100 nm) can selectively photocatalyze carbon-carbon bond formation reactions, i.e., α-alkylations. Specifically, we demonstrate α-alkylation of aldehydes with a turnover number (TON) of over 52,000 under visible light illumination. Hybrid organic/inorganic perovskites are revolutionizing photovoltaic research and are now impacting other research fields, but their exploration in organic synthesis is rare. Our low-cost, easy-to-process, highly efficient and bandedge-tunable perovskite photocatalyst is expected to bring new insights in chemical synthesis.

Full text of DOI:10.1021/jacs.8b08720

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Communication
Lead-Halide Perovskites for Photocatalytic #-Alkylation of Aldehydes
Xiaolin Zhu, Yixiong Lin, Yue Sun, Matthew C. Beard, and Yong Yan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08720 • Publication Date (Web): 02 Jan 2019
Downloaded from http://pubs.acs.org on January 2, 2019
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Lead-Halide Perovskites for Photocatalytic -Alkylation of
Aldehydes
Xiaolin Zhu†‡, Yixiong Lin†‡, Yue Sun†‡, Matthew C. Beard§, and Yong Yan*†‡
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† Department of Chemistry and Biochemistry, San Diego State University, San Diego, CA 92182 USA
‡ Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102 USA
§ Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401 USA
Supporting Information Placeholder
for photovoltaic applications, they also should be of interest in
photocatalytic applications.26,27
ABSTRACT: Cost-effective and efficient photocatalysis are
highly desirable in chemical synthesis. Here we demonstrate that
readily prepared suspensions of APbBr3 (A
=
Cs or
Here we report a cost-effective, highly-efficient and easy-to-
process photocatalytic system centering on ABX3 Pb-halide
perovskite nanocrystals (NCs) for direct C-C bond formation
reactions. To demonstrate their photocatalytic characteristics, we
explore a model reaction, -alkylation of aldehydes, a widely
employed valuable chemical transformation. Particularly, we
demonstrate that APbBr3 NCs can directly photocatalyze the -
alkylation reaction with high yield and without N2-sparging. By
slight modification of the reaction conditions, we show that our
photocatalytic system can selectively catalyze other important
chemical reactions, such as, sp3 C-couplings and alkyl-halide
reductions. The perovskite photocatalyst studied here are a
versatile material system easily prepared from earth-abundant
elements and low-cost starting materials.10 The as-prepared NCs
are not only stable in common organic solvents (Fig. S1-5), but
also are effective in photocatalysis with a TON of over 52,000 for
-alkylation, three orders of magnitude higher than precious Ir or
Ru catalysts. As a result, the perovskite photocatalyst are much
more economical than Ir/Ru (2-order cost lower, Table S1),
rendering them as new promising candidates for broad application
in organic synthesis.
methylammonium (MA)) type perovskite colloids (ca. 2-100 nm)
can selectively photocatalyze carbon-carbon bond formation
reactions, i.e. -alkylations. Specifically, we demonstrate α-
alkylation of aldehydes with a turnover number (TON) of over
52,000 under visible light illumination. Hybrid organic/inorganic
perovskites are revolutionizing photovoltaic research and are now
impacting other research fields, but their exploration in organic
synthesis is rare. Our low-cost, easy-to-process, highly-efficient
and bandedge-tunable perovskite photocatalyst is expected to
bring new insights in chemical synthesis.
Carbon-carbon (C-C) bond formation is one of the most
fundamental transformations in organic synthesis. Nature is
capable of storing solar energy in chemical bonds via
photosynthesis. The photoconversion process, involves a series of
C-C bond forming photoredox catalytic reactions starting from
CO2 and light.1,2 Tremendous advances in artificial photoredox C-
C bond formations have been made, including the development of
robust and reliable protocols to merge photoredox catalysis with
organocatalysis.3-5 A fundamental aim is the development of new
modes of small molecule activation via cheap, effective and easy-
The exploration of the perovskites’ photocatalytic characteristics
for organic synthesis was inspired by a simple one-pot reaction
(Fig. 1a). After mixing of the readily available starting materials
in an open vial, a one-pot perovskite emissive suspension was
formed due to the solvents’ emulsion/de-emulsion effect.28 To the
resulting suspension organic substrates 2-bromoacetophenone 1a,
and octanal 2a are added at room temperature. Upon blue-LED
illumination, several products are generated, including
dehalogenated acetophenone 3a (yield 76%), sp3 C-coupling
product 4a (8%), and α-alkylation product 5a (7%). Next, we
explore the photocatalytic selectivity towards the desired product.
to-process catalytic systems. Many protocols, including Ru/Ir-
based complexes,3-6 organic dyes,7 semiconductors QDs
etc.,
8,9
can be employed under mild reaction conditions and have broad
substrate scope. However, typically these systems either need
noble metals or require complicated synthetic protocols, both of
which are not desirable.
The recent renaissance in ABX3 hybrid perovskite semiconductors
has revolutionized photovoltaics, enabling solution-processable
solar cells that have now reached 23.3% power conversion
efficiency.10-13 In addition to the exceptional photovoltaic
performance, hybrid perovskite systems have also demonstrated
Gram scale colloids of APbBr3 are readily prepared and are
further isolated via centrifugation. We obtained perovskite
colloids with sizes ranging from 2 ~100 nm (Fig. 1b and S2).
Perovskites’ photoluminescence (PL) is shown in a variety of
solvents (Fig. 1c and S3a). The PL lifetimes in various solvents
are recorded from ~10 to ~100 ns which are long-lived enough to
induce charge transfer in these catalytic systems according to
Nicewize.8 (Fig. 1d and S3b) The lifetimes do not exhibit single
exponential decay times likely resulting from a broad size
distribution.28 Overall, these NCs’ are significantly larger than the
reported Bohr radius of APbBr3, and thus their optical properties
breakthroughs in piezoelectrics,14 high-gain photodetectors,15
17
light-emitting diodes,16 lasers
and transistors.18 The excellent
photovoltaic and optoelectronic performance is attributed to
beneficial opto-electronic properties, such as, strong light
absorption,19 long charge-carrier lifetimes20,21 and long charge-
carrier diffusion lengths.22,23 Our previous work also reported low
trap densities and small surface recombination velocities, leading
to a noticeable enhancement of the photo-generated carrier
lifetime and mobility.19,24,25 Thus, given the beneficial properties
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O
resemble that of their thin film versions.29,30 The colloids display a
major absorption peak at 500 nm and PL at 513 nm in hexane,
indicating a band gap energy of 2.4 eV matching well with bulk
APbBr3. X-ray diffraction (Fig. 1e) confirmed the perovskite
structure.10,11
O
O
O
Hex
Blue LED
Perovskite Photocatalyst
O
H
H
+
Br
+
H
+
base, co-catalyst
solvent, R.T.
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Hex
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1a
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a.Conditions: 1a (0.5 mmol), 2a (1.0 mmol), CsPbBr3 (1.0 mg),
Co-catalyst (20 mol%), 2,6-lutidine (1.0 mmol) and solvent (1
mL) under 455 nm LED illumination at R.T. (Note: no N2
Figure 1. (a) The one-pot reaction demonstration; (b) TEM of
CsPbBr3; (c) Absorption and PL spectra of CsPbBr3 in different
solvents. Inset: under ambient light (left) and 365 nm UV light
(right); (d) PL lifetimes of CsPbBr3 in various solvents; (e) PXRD
of CsPbBr3 and MAPbBr3, * indicated PbBr2 peaks.
b.
1
c.
sparging); Yield determined by H NMR; molecule sieves pre-
dried solvent; d.freshly distilled THF; e.freshly distilled THF
f.
adding 1% water; DIPEA instead of 2,6-lutidine as base; g.with
N2-sparging; h.MAPbBr3 instead of CsPbBr3; i.without light;
j.without CsPbBr3; k.without base.
Selectivity. A high yield of 3a, 4a and 5a can be selectively
obtained under simple alterations of the reaction conditions.
Apparently, 5a is of more synthetic significance towards C-C
formations. Starting with 1a and 2a, in the presence of CsPbBr3
NCs, dicyclohexylamine and 2,6-lutidine without N2-sparging, the
reaction vessel is illuminated with an LED and produces 5a in
75% yield in DCM (Table 1, entry 1). Importantly, freshly
prepared solvents, removing water and stabilizer, suppresses
dehalogenation but encourages 5a formation (entries 2-7).31
Furthermore, using N,N-diisopropyl-ethylamine (DIPEA) (entries
8-9), produces 3a in a yield of 95%, demonstrating a highly
selective alkyl-halide reduction. When the amine is replaced with
Stability and TON. One of major obstacles concerning the
application of Pb-halide perovskites is their instability,
particularly towards moisture.33,34 The situation is quite distinct if
Pb-halide perovskites are applied to organic synthesis. We find a
surprisingly strong stability of CsPbBr3 in organic solvents
indicated by observing the PL spectrum of CsPbBr3 in different
organic solvents for several weeks (MAPbBr3 is less stable, Fig.
S5). The good stability is also corroborated by a large TON. The
CsPbBr3 remain emissive after a typical reaction (Fig. S6)
indicating they likely remain catalytically active. The colloids are
isolated from the previous reaction mixture via centrifuging and
then re-used for a new reaction without any treatment under
identical conditions. The photocatalyst remains active for at least
four cycles (Fig. S7) suggesting a lower limit of the TON to be at
least 52,000 (SI and Table S1). A large aliquot of water will
deactivate and completely dissolve the perovskites, rendering a
desired opportunity to easily separate the photocatalyst from
organic products. Note that the separation of photocatalyst from
photoredox reactions remains an issue using Ru/Ir or organic
dyes.35
6,
(5S)-(−)-2,2,3-trimethyl-5-benzyl-4-imidazolidinone,
an
expensive co-catalyst commonly used in photoredox catalysis,3
surprisingly, the sp3-C coupling product 4a is obtained in 80%
yield (entry 16). Hence, under slight changes in reaction
conditions, we can selectively produce 3a, 4a or 5a. Next, we
screened various amines as co-catalyst (entry 10-18). Primary
amines result in no alkylation products. (entries 14-15) Our results
corroborate previous findings that secondary ammonium salts are
superior to their respective amines for -alkylation.32 Overall,
bis(2-chloroethyl)amine hydrochloride, provides the highest
selectivity and yield, up to 96% for 5a, (entry 10) with substrate
ratio (1a/2a) 1/2 (ratio exploration details in Table S2). Anaerobic
conditions are not required for 5a formation. (entries 10-11; 17-
18) As expected, the control experiments revealed no product in
the absence of perovskite, light, co-catalyst, or base. (entries 19-
22).
Mechanism. A proposed mechanism for these observations is
outlined in Fig. 2a. Photoexcited electrons reduce 1 to form a
radical 7, which is a key step for all three products. In path I and
II, 7 either extracts a hydrogen atom from a sacrificial donor (i.e.
DIPEA) forming 3 or when a sacrificial donor is not present, self-
couples producing 4. Regeneration of the perovskite catalysts can
be achieved via oxidation of the sacrificial donor in I or oxidation
of an aldehyde in II. We also speculate that oxidative quenching
by enamine 8 leads to radical 9 (pathway III). Iminium cation 10
Table 1. Optimization of reaction condition for α-alkylation of
aldehydes.a
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is thus produced via a radical-radical reaction followed by
1
2
3
4
5
6
7
hydrolysis to release the product 5 to regenerate the co-catalyst.
Radical intermediates are the key components for photoredox
catalysis. To further explore our proposed mechanism, radical
trapping experiments were conducted with 1a and enamine 8,
respectively (SI and Figure 2b). Corroborating with perovskite
quenching experiment (enamine, kq = 2.0 ×1011 M-1s-1; 1a, kq =
2.7 ×1010 M-1s-1, SI and Figure S8-10), the radical trapped product
1
TM-1 and TM-2 were all isolated and confirmed by H NMR
from reaction I and II, while no trapped products were detected in
control experiments without CsPbBr3 (Fig. S11-14). Note that
TM-2 has been previously illustrated to form from radical 9
trapped by TEMPO.36 The TEMPO trapped products prove the
direct formation of radicals 7 and 9 as shown in our mechanism,
supporting our proposed closed-cycle mechanism. However, our
observation cannot completely rule out the previous reported
chain reaction mechanism using a molecular photocatalyst.37
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Bandedge Tuning. One promising property of the Pb-halide
perovskite photocatlysts is that their band structure can be easily
tuned.39-41 The excited-state redox potentials, E* of
a
photocatalyst governs organic substrate activation.5 E* can be
defined by: E*ox= Eox - E00 and E*red= Ered + E00, where E is the
potential of the ground-state redox couple and close to the redox
level of the conduction and valence bands, E00 is the energy gap
between the zeroth vibrational levels of the ground and excited
states and is roughly equal to the energy of the PL with error ca.
100 mV.42 Thus E* can be manipulated via perovskite bandedge-
tuning. Halide composition of APbClxBryI3-x-y (0 ≤ x, y ≤3) with a
ratio varying on x, and y, leads to bandgaps covering from 3.2 eV
to 1.5 eV.43-45 Such large bandedge-tuning (1.7 eV), can be easily
reached via simply mixing of different ratio of halides at the
initial mixing, or via anion-exchange after synthesis.10,46,47 Other
catalysts, i.e. Ir/Ru complexes, require significant synthetic efforts
to reach specific E* values. Estimation E* of APbClxBryI3-x-y
(Table S3) leads to perovskites covering almost all known noble-
metal catalysts E*, implying potential for broader substrate
photoactivations via perovskite.
Figure 2. (a) Proposed mechanism for perovskite catalyzed
dehalogenation, sp3C-coupling and α-alkylation. (b) TEMPO
trapped experiment for radical intermediate validation
Reaction Scope. Electron-donating and withdrawing tolerances
were observed on α-bromo carbonyls (5b, 5c, 5e, 5f; 79 to 90%
yield). The extended aromatic rings afforded the desired products
5g (85%) and 5j (82%). Simple α-bromo ester leads to a moderate
alkylation yield (5h 65%, 5l 55%). Overall, aromatic aldehydes
demonstrate slightly less yields, respectively. Product 5k, with 8
chiral centers was also produced with over 56% yield, exhibiting a
general acceptance of our photocatalysis. Moreover, the library of
product 3 and 4 was expanded. High yields of alkyl-halide
reductions (3a-c, 3e above 90%, 3d 79%) with DIPEA are shown
in Table 2. Selective reduction of alkyl-halide over aryl-halide is
also achieved. (3e, 90%) The sp3C-coupling products 4 were also
explored (4a-c, 59-80%, 4d 15%). The lower yield of 3d and 4d is
likely due to the finite absorption and PL of naphthalene which
interferes with the CsPbBr3 NCs absorption, resulting in less-
efficient photocatalysis.
With regards to cost-effective, operational convenience and
possible scale up, it is notable to consider that our perovskite
protocol: (1) only requires readily available non-noble materials,
with 2-orders of magnitude lower costs than Ru/Ir catalyst; (2)
presents high catalytic TON evidenced by 3-orders of magnitude
higher value than Ru/Ir catalysts; (3) only requires minimum
synthetic effort to produce the catalysts; (4) may activate a
broader scope of organic substrates due to easy bandedge-tuning;
(5) is easy-to-process since perovskites are water washable; (6)
only requires visible light; (7) does not require anaerobic
sparging; and (8) does not require heating or cooling.
Table 2. Scope of photocatalytic reductive dehalogenation, sp3-C
couplings, and α-alkylation of aldehydes. 38
In summary, we established
a hybrid halide perovskite
photocatalytic system for organic synthesis. APbBr3 NC colloids
are directly employed in organic solvents to demonstrate highly-
efficient -alkylation of aldehydes, sp3-C couplings and alkyl-
halide reductions. High TON and low-cost garners a significant
progress of the current perovskite system. Easy and wide
bandedge-tuning of perovskites NCs ripostes the key challenge to
activate broader range of organic substrates requiring different
3
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Photoredox Catalysis with Transition Metal Complexes: Applications
energy level from enamine intermediates, alkenes, carbonyls,
halides, acetic acids to amines etc. for C-C, C-O and C-N
formations. The potential broad application of this cost-effective,
easily-prepared, highly-efficient and band-tunable hybrid halide
perovskites may bring in new insights in photocatalysis of organic
reactions.
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ASSOCIATED CONTENT
Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org. Materials and methods, Fig. S1-S44, Table
S1-S3, synthesis procedures, and 1H and 13C NMR spectra.
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AUTHOR INFORMATION
Corresponding Author
*Email: yong.yan@sdsu.edu
ORCID
(12) Best Research-Cell Efficiencies Chart
–
NREL. https://
www.nrel.gov/pv/assets/pdfs/pv-efficiencies-07-17-2018.pdf (accessed
July 17, 2018)
Xiaolin Zhu: 0000-0001-6568-0308
Matthew C. Beard: 0000-0002-2711-1355
Yong Yan: 0000-0001-6361-0541
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, J. S.; Christians, J. A.; Kamat, P. V. Intriguing
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The authors declare the following competing financial interest(s):
A provisional patent application has been filed on the perovskite
catalysts and their use in photocatalytic organic synthesis.
ACKNOWLEDGMENT
This work is supported by NSF under Chemical Catalysis
program, award 1764142 to YY. MCB acknowledges support as
part of the Center for Hybrid Organic Inorganic Semiconductors
for Energy (CHOISE) an Energy Frontier Research Center funded
by the Office of Science, Office of Basic Energy Sciences within
the US Department of Energy. Part of this work was authored by
Alliance for Sustainable Energy, LLC, the manager and operator
of the National Renewable Energy Laboratory under Contract No.
DE-AC36-08GO28308. The views expressed in the article do not
necessarily represent the views of the DOE or the U.S.
Government. The U.S. Government retains and the publisher, by
accepting the article for publication, acknowledges that the U.S.
Government retains
a nonexclusive, paid-up, irrevocable,
worldwide license to publish or reproduce the published form of
this work, or allow others to do so, for U.S. Government
purposes. We also thank M. Uddin and W. Chen for help with
XRD measurements.
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