Heterogeneous Photocatalytic Click Chemistry

Article abstract of DOI:10.1021/jacs.6b06922

Copper-doped semiconductors are designed to photoassist the alkyne-azide cycloaddition catalysis by Cu(I). Upon irradiation, injection of electrons from the semiconductor into copper oxide nanostructures produces the catalytic Cu(I) species. The new catalysts are air- and moisture-tolerant and can be readily recovered after use and reused several times.

Full text of DOI:10.1021/jacs.6b06922

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Communication
Heterogeneous Photocatalytic Click Chemistry
Bowen Wang, Javier Esteban Durantini, Jun Nie, Anabel E. Lanterna, and Juan C. Scaiano
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06922 • Publication Date (Web): 27 Sep 2016
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Heterogeneous Photocatalytic Click Chemistry
Bowen Wang‡a,b, Javier Durantini‡a, Jun Nie*b, Anabel E. Lanterna*a and Juan C. Scaiano*a
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a Department of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and Innovation, University of Ot-
tawa, Ottawa, Ontario, Canada K1N 6N5.
b State Key Laboratory of Chemical Resource Engineering, Faculty of Science, Beijing University of Chemical Technology.
4
Supporting Information Placeholder
11
Cu on TiO2 for water splitting, CO2 photoreduction or for
ABSTRACT: Copper-doped semiconductors are designed to
photo-assist the alkyne-azide cycloaddition catalysis by Cu(I).
Upon irradiation, injection of electrons from the semiconduc-
tor into copper oxide nanostructures produces the catalytic
Cu(I) species. The new catalysts are air and moisture tolerant
and can be readily recovered after use and reused several
times.
12
biomedical research, but to the best of our knowledge, not to
photoinitiated catalytic organic reactions. In a related article
Bowman et al.9 explored the use of suspended TiO2 to reduce
Cu(II) in solution; while the system worked, it was dismissed as
impractical for the imaging applications under study. In a re-
cent review, Pale et al. covered numerous methods involving
heterogeneous catalysis of click chemistry that have been, or
13
are currently being developed; surprisingly, none of them
involves photo-activated catalysis.
Click chemistry (as in Scheme 1) is typically a thermal process
efficiently catalyzed by Cu(I) or by copper nanostructures. In
This work was undertaken with the goal of developing a hy-
brid catalyst as it is illustrated in Figure 1 for TiO2 (it is similar
for Nb2O5) decorated with copper nanoparticles, mainly pre-
sent as oxides (vide infra). When supported on TiO2, the con-
duction band of CuO is about 0.3 eV lower than for TiO2
(note that exact gap will depend on particle size), making the
1
2
a recent publication we demonstrated that the process involves
3
‘truly’ heterogeneous catalysis as opposed to examples where
heterogeneous systems act as suppliers for soluble catalysts; in
such cases the advantages of heterogeneous chemistry are de-
4
feated by the leaching of active materials. A commercial cop-
14
electron transfer shown quite favorable. For both semicon-
5
per-on-charcoal catalyst is also available for click chemistry,
ductors, excitation in the band gap region (normally in the
UVA region, see Figure S1) promotes a valence band electron
to the conduction band. Under normal circumstances elec-
tron-hole recombination is a relatively fast process unless ei-
which in spite of its usefulness in organic chemistry, contains
6
less than 0.003% active surface. We report here on a hetero-
geneous, photoactivated catalyst for the click reaction, specifi-
cally for the Huisgen cycloaddition of azides and terminal al-
kynes. Further, this makes catalyst separation and re-use
straightforward. In fact, there have been a few interesting re-
ports of “photo-click” chemistry. Some of them involve photo-
ther electron or hole is trapped; in our case CuO on the sur-
12;14-16
face can trap the electron, yielding Cu(I), Figure 1.
Elec-
tron donors can trap the hole and in the process hinders the
recombination; amines and alcohols can perform this role (vide
infra). In other systems the electron can be trapped by O2 and
7
activation of an organic reagent, while others use organic
photoreducing agents to convert soluble Cu(II) to Cu(I), the
the hole by water in well understood processes, particularly in
8;9
active catalyst. The latter approach can afford more flexibil-
12;17
the case of TiO2,
involving Nb2O5.
although there are also a few examples
There are examples where taking ad-
ity, as it could be largely independent of the detailed structure
of the substrates.
16,18
12
vantage of the reduced recombination rates, the electron, the
hole, or both are trapped by species in solution, including the
N
Y
N
Cu+
N
14
solvent itself. The quantum yields of these processes are lim-
Y
+
X
N N N
ited to <1, as one photon can only cause one electron to be
promoted and at best one chemical change can take place
involving the surface of these materials. In our system the ef-
fect of excitation is amplified, as photoexcitation leads to the
generation of one catalytic site on the surface (i.e., a Cu(I) site)
capable of multiple catalytic events until recombination inexo-
rably takes place.
X
Scheme 1: Copper-catalyzed azide–alkyne cycloaddition
reaction (CuAAC).
The use of light combined with reusable heterogeneous cata-
lysts reduces adverse environmental effects and thus accom-
modates some of the key strategies of green chemistry.
In order to test the validity of our hypothesis, we prepared
CuOx@TiO2 and CuOx@Nb2O5 and used the reaction be-
tween 1a and 2a as a model system (Table 1). The synthesis of
nanoparticles decorating the semiconductors was achieved in-
situ using benzoin I-2959 as a source of reducing ketyl radicals,
In this contribution we report that semiconductors with band
gaps within the 3-3.5 eV range (such as TiO2 and Nb2O5) dec-
orated with CuOx nanostructures serve as effective photoacti-
vated click catalysts. Earlier reports include a catalyst with
19
as shown in scheme S1. This normally produces the metal
10
copper hydroxide supported on alumina and titanium oxide,
nanoparticles and in the case of copper, air exposure oxidizes
them to CuO, a process that has been characterized earlier.
but where thermal click-reactions required 60 °C under oxy-
gen-free atmosphere. Other articles describe applications of
14
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Page 2 of 6
Table 1. Solvent and the electron donor (ED) effect on
the photocatalytic CuAAC using CuOx @Nb2O5 as cata-
lyst.
The early generation of low oxidation states of copper (Cu(0)
or Cu(I)) is clearly evidenced by the brownish color of the ma-
terials, which in the case of TiO2 is only visible before the
samples are exposed to air, then they turn to light gray, char-
acteristic of supported CuO (Figure S2). The XPS spectrum of
CuOx@TiO2 shows the characteristic satellites peaks of
1
2
3
4
CuOx@Nb2O5
N
ED
N
N3
N
5
Solvent, air
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1a
2a
3a
Cu(II) between 945-940 eV further confirming the presence
6
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9
of CuO. In the case of CuOx@Nb2O5 more reduced Cu spe-
cies are present initially (Figure S3), although differentiation
between Cu(I) and Cu(0) is not accurate by this technique.
Cu(I), if present, was below the limit of detection of our Ra-
man spectrometer. The Cu loading has been determined by
ICP-OES analysis and is 2.51 and 2.04 wt% for
CuOx@Nb2O5 and CuOx@TiO2, respectively. The analysis of
SEM and STEM images indicates that nanoparticles deposited
on TiO2 are considerably smaller than those on Nb2O5 (See
SI).
% Yield of 3a
Solvent
ED
Light[a]
Dark
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TEA
TEA
TEA
1
2
3
4
5
6
7
CH3CN
H2O
EtOH
THF
THF
THF
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35
34
90
83
96
58
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4
5
15
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4
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TEA
PMDTA
2-propanol[b]
--
THF
ND
Reaction conditions: 1.2 mmol% Cu, azide/alkyne/ED (1:1:1), RT, 6 h.
N,N’,N”,N”-Pentamethyldiethylenetriamine (PMDTA), Triethylamine
1
[a]
(TEA). Yields were calculated by H NMR in CDCl3. UVA irradiation
at 21.4 W/m2. [b] 5 mmol.
100
80
60
40
20
0
Figure 1. Proposed mechanism of electron transfer from
the excited semiconductor to CuO nanoparticles form-
ing catalytic Cu(I).
0
4
8
12
Time (h)
16
20
24
In order to evaluate the efficiency of the photocatalytic activity
of CuOx@Nb2O5 for the copper catalyzed azide–alkyne cy-
cloaddition (CuAAC) a series of experiments were performed
to optimize the reaction conditions (Table 1). The reaction was
tested with different polar solvents and comparing the use of
amines and alcohols as co-catalysts. For most CuAAC reac-
tions the addition of amines is anticipated to protect and stabi-
lize copper (I) from oxidation.13;22 We believe the main role of
the amine here is to act as a hole-scavenger and hence the
reaction is expected to work with other sacrificial electron do-
nors (ED). Indeed, for CuOx@Nb2O5 the reaction works very
well when amine is replaced by 2-propanol, although the same
is not true for CuOx@TiO2, where the alcohol effectiveness as
a hole scavenger is inferior (Figure S9). There are literature
precedents showing the superior catalytic activity of Nb2O5
Figure 2. Kinetic study of the photocatalytic reaction of
1a and 2a in THF in the presence (black) and in the ab-
sence (blue) of TEA using CuOx@Nb2O5 as photocata-
lyst.
Overall, the reaction conditions that best suit both materials
(Table 1, entry 4 and Figure S9) were chosen to study their
catalytic activity. The kinetic study of the reaction between 1
and 2 is shown in Figure 3. Interestingly, under dark condi-
tions the catalysts show slightly different activity; CuOx@TiO2
is inactive, while CuOx@Nb2O5 shows a small but not negligi-
ble amount of products, probably due to the nature of the
catalytic copper species present in the latter. The presence of
some reduced Cu species, as confirmed by XPS analysis,
should favor some reaction under dark conditions for
CuOx@Nb2O5. Both catalysts reach ca. 90 % yield after 5 h
UVA irradiation. If the light is turned off, the catalysis yields
are reduced, suggesting that Cu(I) transient needs continued
irradiation for its activity level to be maintained (Figure 4).
Although greatly reduced, some reactivity remains for some
time after the light is turned off.
23
towards alcohol oxidation. Experiments performed in the
absence of an ED can also lead to good yields while the reac-
tion rate slows down as shown in Figure 2 (and Figure S12).
Control experiments in the absence of catalyst or in the pres-
ence of TiO2 or Nb2O5 were also performed but no click reac-
tion was detected.
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ing the catalytic cycle active as shown in the inset in Figure 5.
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1
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3
4
5
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8
100
80
60
40
20
0
e-
Cu2O
O2
e-
CuO
Cu0
O2
Opened
to air
9
10
11
12
0
2
4
Time (h)
6
8
Figure 3 Kinetic study of the photocatalytic reaction of
1a and 2a in THF under dark (£) and upon UVA irradia-
tion (r & s). Black: CuOx@TiO2. Blue: CuOx@Nb2O5.
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Figure 5. Study of the photocatalytic reaction of 1a and
2a in THF in Ar (black) and in air (blue) using
CuOx@Nb2O5. Red dots: yields obtained when system is
exposed to air after 8 h of reaction under Ar. Inset: Role
of the O2 in the Cu redox cycle.
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80
60
40
20
0
Table 2 shows a study of the scope of this reaction varying the
alkyne and the azide. Both catalyst show great versatility to-
wards substituted phenyl alkynes with electron donor or elec-
tron withdrawing groups in position para, although TiO2 seems
to work slightly better than Nb2O5 with ortho-substituted reac-
tants. Further, the reaction can be extended for use with ali-
phatic alkynes, with TiO2 being quite effective, even with
bulky reactants (cf. entries 9 and 11). In the case of the azides
studied, the catalysts proved useful towards different electronic
demands, and only the aryl azide was inactive.
CuO @Nb2O5
CuO @TiO2
x
x
Catalyst
Figure 4. Yields obtained for each material after irradi-
ation for 1 h (blue), 3 h (blue stripes) and 1 h followed
for 2 h in dark (black). Notice that for CuOx@TiO2 16%
yield was produced in the dark after UV exposure, while
for CuOx@Nb2O5 the value is within experimental error.
Finally, the reusability (Figure 6) of both catalysts was tested
for the reaction of 1a and 2a in THF in air upon UVA irradia-
tion for 6 h, when the high yield plateau is reached for both
TiO2 and Nb2O5-supported catalysts (Figure 3). We note that
both catalysts show great stability after at least four reusability
cycles. This stability can be explained by the continuous re-
generation of Cu(I) upon light irradiation. ICP analysis of the
crude of the reaction shows barely 0.01 and 0.002 % of the
available copper for the case of CuOx@Nb2O5 and
CuOx@TiO2, respectively. These numbers suggest the release
of Cu species during reaction is negligible, in accordance with
the good performance of both catalysts during several catalytic
cycles.
We were surprised to discover that a “control” experiment at
525 nm leads to click products, albeit in yields after 4 h about
4 times lower than excitation at 368 nm. This inspired explor-
atory experiments performed at longer wavelengths (594, 660
and 740 nm). With the exception of excitation at 740 nm, all
LEDs can cause reaction, even after just 4h, although with
decreasing yields at the longer wavelengths. This is summa-
rized in the table S1 in the SI. Control experiments at 525 nm
showed that CuO and Cu2O have no photocatalytic activity
(See table S1). The lack of activity showed by Cu2O may be
due to the small surface area available and CuO contamina-
tion (See figure S7 and S8). We speculate that under visible
light irradiation, absorption by the smaller CuO particles (and
thus with a larger band gap) leads to oxide mediated electron
transfer to larger CuO particles, while the hole ultimately re-
sides in the oxide support. This electron shuttle is illustrated in
Figure S11 for CuOx@TiO2 and leads to Cu(I) in the larger
CuO particles. Interestingly, in the mechanism proposed, pol-
ydispersity ultimately helps the photocatalytic process. The
practical result is that the catalysts can be activated with visible
light, and provides wavelength tunability of light-induced Cu-
100
80
60
40
20
0
1
2
3
4
Catalytic Cycle
Figure 6. Reusability of CuOx@TiO2 (black) and
CuOx@Nb2O5 (blue) as per conditions in table 2, entry 1.
24
AAC reaction without the use of organic photoinitiators.
In summary, we developed a new method to photocatalyze
CuAAC under mild conditions and easy product isolation. We
describe the use of two semiconductors as supports for copper
nanoparticles, both showing excellent performance. Nb2O5 is a
largely unexplored material sometimes employed as a strong
Interestingly, under argon the reaction stops at half of its nor-
mal yield (Figure 5 and Figure S11). Opening the vessel after
reaching the plateau leads to increase the reaction yield up to
almost 90 % within 2 h of irradiation. This is likely due to an
over reduction of the catalyst to Cu(0). The presence of O2
may play a role in the oxidation of both Cu(I) and Cu(0) keep-
25
acid, but rarely as a photocatalyst. In our system, the light is
used to inject electrons to copper oxide from the semiconduc-
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tor. The catalyst can be reused several times because of the
Page 4 of 6
ACKNOWLEDGMENT
1
2
3
4
5
6
7
8
continuous generation of Cu(I) species under excitation of the
semiconductor band gap. The catalyst is moisture tolerant and
needs air and room temperature for optimal performance. The
scope of the reaction has demonstrated the versatility of the
catalysts extending their use to aliphatic alkynes.
We thank the Natural Sciences and Engineering Research Coun-
cil, the Canada Foundation for Innovation and the Canada Re-
search Chairs program. BW thanks Beijing University of Chemi-
cal Technology for an International Joint Graduate-Training
Program Scholarship.
N3
N3
N3
N3
N3
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* E-mail: titoscaiano@mac.com
* E-mail: anabel.lanterna@icloud.com
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‡These authors contributed equally.
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TOC abstract
85x34mm (300 x 300 DPI)
ACS Paragon Plus Environment