Communications
À
oxide radical anion (O2 ), hydrogen peroxide (H2O2), singlet
further supported the chemical stability of the meso-CuS
catalyst (Figure S13). To determine the reusability of the
catalyst, the material was retrieved from each cycle and washed
with acetonitrile before the next use. This process was
continued for five cycles until the TOF decreased to ~30%
(Figure S14, a). Similar reusability tests under nitrogen environ-
ment exhibited a fast drop of TOF (~80% after four cycles)
(Figure S14, a), however, the activity was somewhat recoverable
as evidenced by the 5th cycling of this catalyst under an oxygen
environment. This validated the significance of oxygen in the
protocol. Moreover, to witness the change in copper (of meso-
CuS) during the reaction, electron paramagnetic resonance
(EPR) spectra of the catalyst before use and after the 5th cycle
were compared. This showed a drastic drop in the intensity of
the EPR peak, which can be attributed to the transformation of
Cu2+ to Cu+ during the reaction (Figure S14, b). This was likely
caused by the reduction of Cu2+ by the amines (step B–C,
Scheme 1) (consistent with its excellent reducibility as shown in
Figure S4, a). Whereas, the singlet oxygens probably extracted
electrons from the surface adsorbed amines (D, Scheme 1)
leading to the formation of surface adsorbed imines (E,
Scheme 1). Molecular nitrogen failed to do so and caused a
dramatic decrease of TOF (Figure S14, a). Therefore, this catalyst
is truly stable, and reusable under optimized (aerobic) reaction
conditions.
These experimental findings led us to propose a copper
sulfide driven photo-catalyzed reaction mechanism where the
in-situ formation of Cu+ species leads to the activation of
singlet oxygens which couple amines to imines (Scheme 1).
Photo-excitation of Cu+ generated electrons and an equal
number of holes in the system. These holes facilitated the
adsorption of amines on the catalysts and electrons were
transferred to the singlet oxygens. In the presence of light, the
molecular oxygen species is assumed to form singlet oxygen,[39]
which upon extraction of electrons from the substrate formed
superoxide radical anions and intermediate D. Further rear-
rangement of protons and electrons in intermediates D and E,
yielded the desired product (imine) F. To validate the trans-
formation of intermediate D to E to be the rate-determining
step, the KH/KD of the step was evaluated. A KH/KD value of 1.76
�2, known as the primary kinetic isotope effect, identified this
H-removal step from α-CH2 as the rate-limiting process (Fig-
ure S15).
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oxygen (1O2) and hydroxyl radical ( OH)) were detected in the
reaction mixture using UV-vis and fluorescence spectroscopy in
the presence of different indicators (see Note S5).[34,35] Among
these, superoxide radical anions and singlet oxygen (produced
by the reaction between superoxide anion (O2.À ) electron hole
(h+) on the surface)[33,36,37] are prone to abstract electrons,
which is required to drive the reaction forward.[33] Hence, to
identify the ROS responsible for this process, specific quenchers
were used (Table S6). In the presence of p-benzoquinone (p-
BQ), the superoxide radical anion quencher, no significant drop
in conversion (93%) was observed as compared to the
unquenched system (97%) (Table S6). However, the conversion
decreased significantly to 37% in the presence of sodium azide
(NaN3), a singlet oxygen quencher (Table S6). This indicated the
importance of singlet oxygen in catalyzing the meso-CuS driven
photocatalytic transformation of amines to imines. This led us
to propose a reaction mechanism using singlet oxygen, which
initiated the reaction by abstracting electrons from the
substrate and produced superoxide radical anions. These super-
oxide radical anions are reported to further abstract protons
and electrons to form H2O2 and OH radicals (Scheme 1),[38]
whose presence is in agreement with our ROS determination
study (Table S5, Figure S12).
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The major challenges of these heterogeneous catalysts are,
i) the leaching of active metal species in solution, and ii) poor
reusability of the catalyst due to the deactivation of active sites
caused by adsorption or aggregation. To determine the
possibility of leaching,
performed and compared with the time-dependent plot
obtained for 10 mg catalyst under an oxygen environment at
a hot filtration experiment was
°
40 C (Figure S13). The hot-filtration of the catalyst was done for
separation from the filtrate in the middle of a reaction (after
one hour, at about 12% conversion). Continuous sampling from
the filtrate after hot-filtration exhibited no significant change in
the conversion. This indicated that there was no considerable
leaching of active metal species during the reaction, which
To further evaluate the mechanism of the proposed
reaction, quantum mechanical computations were performed
on the system using density functional theory (DFT) as
implemented in the Vienna Ab Initio Simulation Package (VASP)
code (Note S4). According to these DFT calculations, the
transformation of intermediate D to E, which corresponds to
CÀ H bond dissociation from α-CH2, was the maximum energy
demanding process throughout the cycle. This supported our
experimentally determined studies (via KH/KD) of the rate-
determining step.
In short, we demonstrated a unique method of preparing
mesoporous CuS nanocrystals using a hydrothermal method.
The excellent photocatalytic activity of transforming amines to
imines with >99% selectivity, chemical stability and reusability
Scheme 1. The overall mechanism of amines to imines formation.
ChemCatChem 2019, 11, 1–5
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