Photoimmobilized Ni Clusters Boost Photodehydrogenative Coupling of Amines to Imines via Enhanced Hydrogen Evolution Kinetics

Article abstract of DOI:10.1021/acscatal.0c00282

Imines are important precursors for pharmaceutical, agricultural, and synthetic chemistry. The state-of-art synthesis of imines via condensation of amines with aldehydes or ketones often uses homogeneous catalysts and dehydrating agents to promote the elimination of water, which requires huge manpower input for the late-stage purification process and is usually environmentally unfriendly. Photocatalytic synthesis of imines from amines oxidation via the release of hydrogen (H₂) is of great promise due to the mild reaction characteristics; however, the efficiency of such a reaction lags due to the missing designed photocatalyst owing to the ambiguous reaction mechanism. Here, we demonstrate that by constructing in situ photoimmobilized Ni clusters on the CdS photocatalyst, the generation of imines is dramatically improved with the rapid release of molecular H₂ under visible light illumination. Mechanistic investigation reveals that the adsorption of photogenerated hydrogen atoms during the dehydrogenation of amines is significantly weakened on Ni clusters, thus resulting in fast C-N coupling kinetics for the generation of imines. The photocatalyst presents stable performance with high efficiency. A remarkably apparent quantum efficiency (AQE) of ?44% is realized under 420 nm irradiation for the conversion of 4-methoxybenzylamine within six consecutive runs. Furthermore, a series of primary and secondary amines bearing different functional groups (i.e., heterocyclic, aliphatic, N-heterocycles) that are synthetically challenging by the condensation process can be selectively converted to the corresponding imines, featuring its application prospect.

Full text of DOI:10.1021/acscatal.0c00282

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Article
Photo-Immobilized Ni Clusters Boost Photo-Dehydrogenative Coupling
of Amines to Imine via Enhanced Hydrogen Evolution Kinetics
Yi Huang, Cuibo Liu, Mengyang Li, Huizhi Li, Yongwang Li, Ren Su, and Bin Zhang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c00282 • Publication Date (Web): 27 Feb 2020
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Photo-Immobilized Ni Clusters Boost Photo-
Dehydrogenative Coupling of Amines to Imine via
Enhanced Hydrogen Evolution Kinetics
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Yi Huang,†,§ Cuibo Liu,†,§ Mengyang Li,† Huizhi Li,† Yongwang Li,# Ren Su,#,ǁ,* and Bin Zhang†,‡,*
†Institute of Molecular Plus, Department of Chemistry, School of Science, Tianjin University,
Tianjin, 300072, P. R. China
‡Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Collaborative Innovation Center
of Chemical Science and Engineering, Tianjin, 300072, P. R. China
#SynCat@Beijing, Synfuels China Technology Co. Ltd., Leyuan South Street II, No. 1, Yanqi
Economic Development Zone C#, Huairou District, Beijing, 101407, P. R. China
ǁSoochow Institute for Energy and Materials InnovationS (SIEMIS), Key Laboratory of Advanced
Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University,
Suzhou, 215006, P. R. China
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ABSTRACT Imines are important precursors for pharmaceutical, agricultural and synthetic
chemistry. The state-of-art synthesis of imines via condensation of amines with aldehydes or
ketones often uses homogeneous catalysts and dehydrating agents to promote the elimination of
water, which requires huge manpower input for late-stage purification process and is usually
environmental unfriendly. Photocatalytic synthesis of imines from amines oxidation via releasing
of hydrogen (H2) is of great promising due to the mild reaction characteristics; however, the
efficiency of such reaction is lagged behind due to the missing of designed photocatalyst owing to
the ambiguous reaction mechanism. Here, we demonstrate that by constructing an in-situ photo-
immobilized Ni clusters on CdS photocatalyst, the generation of imines is dramatically improved
with the rapid release of a molecular H2 under visible light illumination. Mechanistic investigation
reveals that the adsorption of photo-generated hydrogen atoms during the dehydrogenation of
amines is significantly weakened on Ni clusters, thus resulting in a fast C-N coupling kinetics for
the generation of imines. The photocatalyst presents a stable performance with high efficiency. A
remarkably apparent quantum efficiency (AQE) of ~44% is realized under 420 nm irradiation for
the conversion of 4-methoxybenzylamine within six consecutive runs. Furthermore, a series of
primary and secondary amines bearing different functional groups (i.e., heterocyclic, aliphatic, N-
heterocycles) that are synthetically challenging by condensation process can be selectively
converted to corresponding imines, featuring its application prospect.
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KEYWORDS: photocatalysis, hydrogen evolution, acceptor-free dehydrogenation, Ni clusters,
two-dimensional materials
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Imines, as a category of important precursors that contains C=N bond, have been widely used in
the synthesis of quinolines, cucurbiturils, and oxaziridine, which are of great importance in
pharmaceutical, agricultural, and synthetic chemistries.1,2 Especially, molecules bearing the
special C=N moiety often display a wide range of pharmacological activities (i.e., anti-
inflammatory, anti-depressant, and anti-cancer) or find important applications in the preparation
of liquid crystals and conductive polymers.3 Traditional syntheses of imines are usually relying on
the condensation of amines with aldehydes or ketones through releasing of a water molecule.4-6 In
order to push the reaction equilibrium for imine formation, dehydrating agents (i.e., molecular
sieves and strong acids) and expensive catalysts (i.e., Sheppard amidation reagent) are always
required.7,8 There also exist some more specialized protocols for the synthesis of imines, however,
toxic reactants and harsh reaction conditions limit their applications.9,10 In addition, high
temperatures and dean starks are usually needed to remove the generated water for a full
conversion of imines. These processes not only lead to low selectivity and poor functional group
tolerance, but also pose difficulties in late-stage purification of imines for the removal of
dehydrating agents, by-products and catalyst residuals. Therefore, the development of a mild,
green and sustainable method for efficient imine synthesis is highly desirable.
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Utilizing solar energy as driving force for synthetic chemistry has become an evolving research
field in photocatalysis.11-19 In comparison with photocatalytic H2 production and CO2 reduction,
photocatalytic synthesis provides an economical solution to convert cheap chemicals to value-
added products.20-27 In addition, photocatalytic syntheses are normally performed under ambient
conditions, which offer an ideal platform for the synthesis of metastable chemicals compared to
that of thermal catalytic processes. Photosynthesis of imine stands out as a green approach, as
amine can be oxidized by the photogenerated holes and undergo a consecutive dehydrogenative
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coupling.4,28 Photocatalytic dehydrogenation of amines under oxygen (O2) atmosphere has been
successfully realized by several groups.4,29-32 Here, the reduction half-reaction employs molecular
O2 as the electron acceptor, which is reduced by the surface adsorbed H atoms that are released
from the amino groups by photogenerated electrons. Such a process calls for a fast kinetics of O2
reduction reaction to consume hydrogen to push the reaction equilibrium towards imine
formation,33,34 which makes the design of photocatalysts complicated (i.e., expensive cocatalysts).
Increasing the O2 pressure may be a solution to improve the reaction rates to some extent, however,
other unexpected issues (i.e., side reactions, deep oxidation) may arise during the conversion of
amines with vulnerable functional groups.
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Photocatalytic imine synthesis via acceptor-free dehydrogenation (AD) of amines is therefore
crucially desired to overcome the aforementioned disadvantages.35-38 In addition, the AD process
also provides a platform to collect H2 as a byproduct owing to the oxidant-free environment. The
photocatalytic dehydrogenative coupling of amine can also be considered ralevent to
photocatalytic alcohol splitting, which has been widely used as electron donor in hydrogen
production.39-41 However, such a process is characterized by poor efficiency due to more
complicated reaction mechanisms. In general, catalytic AD reactions mainly consist of
dehydrogenative activation of the substrates, bonds formation of the dehydrogenated
intermediates, and liberation of gaseous products (H2 and NH3).36,39-42 Therefore, an efficient
photocatalyst for imine synthesis from acceptor-free dehydrogenation of amine requires
optimization of each elementary step in the AD process. It has been suggested that the initial step
of such a coupling reaction involves the formation of N-centred radicals via deprotonation of
amines by photogenerated holes, which is eventually transformed into C-centred radicals.22,43
Unfortunately, the reaction mechanism remains incomplete as the reduction half-reaction has been
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merely studied, which limits the rational design of efficient photocatalyst materials for imine
synthesis.
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Scheme 1. Nickel-decorated ultrathin CdS nanosheets as efficient photocatalysts for
dehydrogenative coupling of amines to imines.
Herein, we demonstrate a promoted dehydrogenative coupling of amines into corresponding
imines by employing nickel-decorated ultrathin CdS nanosheets (Ni/CdS) as photocatalyst under
visible-light illumination and deaerated conditions. As presented in Scheme 1, the presence of Ni
clusters is essential in promoting the desorption of surface adsorbed hydrogen atoms (Hads), which
are produced during photo-dehydrogenation of amines to the formation of C-centred radicals. This
accelerated removal of Hads provides more accessible sites on CdS to allow C-H and N-H bond
dissociation to form aldimine intermediate continuously, which eventually boosts the nucleophilic
attack of N moiety in another amine for the generation of coupling product accompanied with the
releasing of ammonia (NH3). With this design strategy, a wide range of primary and secondary
amines with diverse functional groups can be converted into corresponding imines with excellent
yields, thus exhibiting promising potential for practical applications.
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Figure 1. Microstructural analysis of the photocatalyst. (a) HRTEM of the Ni/CdS photocatalyst.
(b-d) HAADF and EDX elemental mapping images of the photocatalyst. The dashed square in (b)
indicates the position of the EDX elemental mapping region.
Two-dimensional (2D) ultrathin CdS nanosheets are synthesized according to our previous work
(Figure S1),44 whereas the loading of Ni clusters is realized by a photo-deposition process using
NiCl2 as the precursor under deaerated conditions. Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images display that the sheet-like morphology of CdS
can be maintained after photo-immobilized Ni clusters (Figure S2a and b). No obvious change is
observed in the X-ray diffraction (XRD) patterns of pristine CdS and Ni/CdS (Figure S2c),
indicating that the structure of CdS is preserved after Ni modification. High-resolution
transmission electron microscopy (HRTEM) image further reveals that the Ni clusters are
uniformly deposited on the 2D structured CdS (Figure 1a). The dark domains with diameters
ranging from 2-4 nm show lattice spacing of 0.23 nm, which can be assigned to the (010) plane of
metallic Ni. Meanwhile, the majority region with a lattice spacing of 0.33 nm corresponds to the
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(002) plane of layered CdS. High-angle annular dark-field (HAADF) and the associated energy
dispersive X-Ray Spectroscopy (EDX) element mappings of a selected region display the
homogeneous distribution of Ni on the CdS photocatalyst (Figure 1b-d), which facilitates the
transfer of photogenerated H atoms and charge carriers.
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UV-Vis diffuse reflectance spectra (DRS) show that the absorption edge of both pristine CdS
and Ni/CdS is ~ 505 nm, corresponding to an optical bandgap of 2.46 eV (Figure 2a and Figure
S3). The flat-band potential is estimated to be -0.75 V (vs. normal hydrogen electrode, NHE, pH
= 7) according to the Mott−Schottky analysis (Figure 2b), which corresponds to a conduction band
minimum of ~-0.75 V and a valence band maximum of ∼+1.71 V vs. NHE (inset of Figure 2b).
This suggests that the photogenerated electrons can transfer to Ni to reduce protons45,46 and holes
are capable of oxidizing amines, respectively.31 It is also worth noting that the presence of Ni
clusters remarkably reduces the radiative recombination of charge carriers according to the
photoluminescence spectroscopy (PL, Figure 2c), as also evidenced by the photos taken in a glove-
box under 420 nm irradiation (inset of Figure 2c) and photocurrent measurement (Figure S4).
Figure 2. Optical property and band position of the photocatalyst. (a) UV-Vis DRS of CdS and
Ni/CdS. (b) Mott-Schottky plot of ultrathin CdS NSs. The inset shows the estimated band structure
of CdS and redox potential of amine. (c) PL spectra of CdS and Ni/CdS. The insets display the
photographs of CdS (upper) and Ni/CdS (lower) under 420 nm illumination in glove-box.
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The photocatalytic performance of Ni/CdS in the dehydrogenative coupling of amine are
evaluated using 4-methoxybenzylamine (1a) as the model substrate at room temperature in
anhydrous N,N-dimethylformamide (DMF) solution under deaerated conditions. A specialized
cell that is equipped with a water-cooling jacket is used as the reactor to avoid light-induced
increase of temperature (Figure S5a). No reaction is observed under irradiation in the absence of
catalyst, nor under dark with catalyst (Figure S5b), suggesting that the amine coupling reaction is
truly a photocatalytic process. The concentrations of amine and Ni2+ are adjusted to optimize the
efficiency of the photocatalytic coupling reaction (Figure S6). Figure 3a shows the time course of
1a consumption and production of corresponding imine (N-benzylidenebenzylamine, 2a) upon
visible light irradiation (λ ≥ 420 nm). Both exponential decay of 1a and an increase of 2a suggest
a typical pseudo-first-order reaction kinetics, and imine with a nearly 100 % yield is achieved
within 4 h illumination. Meanwhile, the evolution of H2 takes place simultaneously with a molar
ratio of n(H2)/n(2a) close to 1 (Figure 3b), suggesting a stoichiometric selective dehydrogenative
coupling process. Since the light intensity is 50 mW, we estimate that such high performance
corresponds to an AQE of ~44% at 420 nm, which is a very competitive photocatalyst system for
organic transformation or H2 evolution in recently reported works.22,26,47 In comparison, the H2
evolution over Ni/CdS within the initial 1 h is about 18 times higher than that of pristine CdS (inset
of Figure 3b). Moreover, the Ni clusters anchored on CdS via the photo-deposition process is more
efficient in promoting the photocatalytic activity compared to that of the randomly loaded Ni via
classical deposition method (labeled as r-Ni/CdS) and CdS loaded with other Ni-based co-catalysts
(inset of Figure 3b). In addition, the durability of Ni/CdS photocatalyst has been also evaluated
using 1a as the model compound. As shown in Figures 3c and 3d, the substrate 1a (2 mmol) can
be fully converted to 2a within 4 h irradiation for six consecutive runs. Meanwhile, a constant
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amount of H2 evolved within each cycle also confirms the excellent stability of the Ni/CdS, which
reveals the promising potential for applications.
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Figure 3. Performance of the Ni/CdS photocatalyst. (a and b) Evolution of reactant (1a) and
products (2a and H2). Reaction conditions: 4 mg of CdS, 0.24 mM of Ni2+, 2 mmol of 1a under
deaerated conditions in 10 mL DMF. The inset shows the H2 evolution within an initial 1 h of CdS
modified by different co-catalysts. (c and d) Stability test of the Ni/CdS photocatalyst. 2 mmol of
1a is added for each cycle. The blue bars mark the irradiation period.
We have then examined the general applicability of our Ni/CdS photocatalyst for selective
dehydrogenation of a series amines with different functional groups (Table 1). Both primary
amines (Table 1, entries 1-9) and secondary amines (Table 1, entries 10-12) can be converted into
the corresponding imines with high yields and selectivity. For aromatic primary amines with
electron-donating and electron-withdrawing groups on aryl ring, the coupling rate follows the
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order of p-OCH3 > p-CH3 > p-H > p-F > p-Cl (Table 1, entries 1-5), which is in line with their H2
evolution rates and the derived Hammett plot (Figure S7). In addition, a higher formation rate of
imine and H2 is achieved for para-substituted benzylamine relative to the meta-isomer (Table 1,
entries 1 and 6). Heterocyclic primary amines with O and S moieties (1g and 1h) are also good
candidates to deliver corresponding imine products with excellent efficacy (Table 1, entries 7-8).
Interestingly, the dehydrogenative coupling of aliphatic amines (1i) has been successfully realized
using our Ni/CdS photocatalyst (Table 1, entry 9). To further display the universality, secondary
amines are also evaluated (Table 1, entries 10-12). Substrate 1j can be proceeded smoothly to
produce 2j with a yield of ~96% (Table 1, entry 10). In the case of dibenzylamine (1k), excellent
selectivity to the corresponding imine (2k) with reasonable yield can be achieved (Table 1, entry
11). Especially, the N-heterocycle containing compounds, which are potential organic hydrogen
storage materials,22,48 are also good candidates to engage in the photocatalytic dehydrogenation
transformation by releasing H2. For example, selective semi-dehydrogenative conversion of
tetrahydroisoquinolines (1l) is facilely realized to deliver dihydroisoquinoline (2l) in good yield.
The product 2l exhibits highly potential applications in the pharmaceutical industry and is difficult
to be synthesized via traditional thermal-catalytic strategy.48,49
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Table 1. Substrate scope of the selective dehydrogenation of amines to imines over Ni/CdS.[a]
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6
7
Yield [%]
[b]
Entry
Substrate
Product
Sel. [%]
[c]
[d]
Imine
H
2
8
9
Photocatalytic dehydrogenative coupling of primary amines to imines
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NH
2
R
1
2
3
4
5
1a, R = p-OMe
1b, R = p-Me
1c, R = H
2a, R = p-OMe
2b, R = p-Me
2c, R = H
99
96
99
99
99
99
99
99
98
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>99
>99
>99
>99
>99
1d, R = p-F
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2e, R = p-Cl
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>99
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O
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NH
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2g
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S
S
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93
94
>99
>99
NH
N
2h
N
2
1h
NH
2
6
7
7
2i
1i
Photocatalytic dehydrogenation of secondary amines to imines
N
H
[e]
N
10
11
96
84
97
81
98
1j
2j
OMe
OMe
[e]
NH
N
>99
1k
2k
2l
MeO
MeO
[e]
12
96
95
95
NH
N
1l
[a] Reaction conditions: amines (0.4 mmol), CdS (2 mg), Ni2+ (0.24 mM) in 5 mL DMF at room
temperature, irradiated by a 300 W Xe lamp (λ ≥ 420 nm). [b] Identified by gas chromatograph-
mass spectrometer (GC-MS). [c] Determined by gas chromatography (GC) using bromobenzene
as an internal standard. [d] Determined by GC using methane as an internal standard. [e] 0.2 mmol
of substrate.
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To demonstrate the photocatalytic mechanism, we have first employed in-situ electron spin
resonance (ESR) to monitor the effect of Ni cluster in the photocatalytic dehydrogenative coupling
of amines with the presence of a spin trapping reagent (5,5-dimethyl-1-pyrroline N-oxide, DMPO),
as shown in Figure 4a. No radical species is formed under dark conditions for both catalysts,
suggesting that the dehydrogenative coupling of amines is a photocatalytic process. Upon
irradiation, the C-centred α-amine radical is gradually evolved and eventually trapped by DMPO,
as evidenced by the 1:1:1:1:1:1 pattern (αN=14.5, αH=19.8).22 This suggests that both pristine CdS
and Ni/CdS can initiate the deprotonation of amines by photogenerated holes. We have further
integrated the signals of the trapped radical and analyzed the effect of Ni clusters in the formation
kinetics, as depicted in Figure 4b. A zero-order kinetics is observed for both pristine and Ni/CdS,
suggesting that the reaction is limited by the number of active sites on the catalyst surface rather
than the concentration of the reactant. Upon Ni decoration, only a slight increase in the evolution
rate of C-centred α-amine radical is observed compared to that of pristine CdS, indicating that the
Ni clusters show a negligible effect in promoting the deprotonation of amines.
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Figure 4. Promotional mechanism analysis. (a) In-situ ESR spectra of the fresh CdS and Ni/CdS
(0.43 g·L-1) recorded in dark and under illumination in presence of benzylamine (0.1 M) and
DMPO (0.1 M) in anaerobic DMF. (b) Evolution of integrated ESR signals of the C-centred radical
as a function of irradiation time.
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Since H2 and NH3 are expected to be produced via the photo-reduction and oxidation half-
reactions, we have further probed the effect of Ni in influencing the releasing of gaseous products
during photocatalytic dehydrogenative amine coupling by employing in-situ mass spectrometry
(MS). Here, the coupling reaction is performed in anhydrous DMF solution under deaerated
conditions (Figures 5a and 5b). While pristine CdS nanosheets show a very slow H2 evolution rate
of 3.7 μmol∙h-1 upon irradiation, the loading of Ni significantly boosts the releasing of molecular
H2 with a rate of 75 μmol∙h-1. The 20-fold enhancement in the H2 evolution rate also agrees well
with the consumption rate of amine determined by gas chromatography (GC) analysis. Note that
the presence of Ni results in the immediate evolution of H2 upon irradiation, whereas an induction
period of ~90 min is required on the pristine CdS, indicating a promoted effect of Ni incorporation.
Simultaneously, NH3 evolution is also observed upon irradiation on both photocatalysts. Here a
similar rate of both samples is associated with the high solubility of NH3 in DMF. While pristine
CdS only produce NH3 for ~90 min and gradually stopped, the Ni/CdS photocatalyst produces
NH3 continuously within the whole radiation course, suggesting that the presence of Ni promotes
the desorption of NH3 to avoid surface poisoning. Interestingly, the evolution of H2 on pristine
CdS starts after ammonia evolution stopped, again indicating that the desorption of photogenerated
hydrogen atoms is crucial to complete the catalytic cycle. In addition, we have further confirmed
the origin of evolved H2 using deuterated benzylamine using GC by employing helium as the
carrier gas (Figure 5c). Since the thermal conductivity of gases follows the order of H2 > He >
D2,50,51 the positive signal at retention time of ~1.4 min suggests the evolution of D2 that comes
solely from deuterated amine rather than the solvent.
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Figure 5. Promotional mechanism analysis. (a and b) Evolution of gaseous products (H2 and NH3)
during amine dehydrogenation under visible light irradiation determined by in-situ mass
spectrometry (420 nm LED, 75 mW·cm-2). (c) GC analysis of dehydrogenation product during
photocatalytic amine coupling using normal and deuterated benzylamine. (d-f) High-resolution
XPS of Cd 3d and Ni 2p of fresh photocatalyst, with dibenzylamine (BA) adsorbed, and exposed
to irradiation (1 h), respectively.
The promotional effect of Ni in the photocatalytic dehydrogenative coupling of amines has been
further revealed by X-ray photoelectron spectrometry (XPS, Figures 5d-f and Figure S8). As
displayed in Figure 5d, the Cd 3d5/2 of freshly prepared pristine CdS is located at 404.2 eV, which
is in good agreement of reported values.44 As expected, the dosing of amine (10 μL) shows no
obvious shift of the peak position. Interestingly, an obvious shift of the Cd 3d5/2 peak to 403.7 eV
is observed after irradiation. This may attribute to the surface anchored Hads from dehydrogenation
of amine,52 which is in line with the in-situ MS analysis. Meanwhile, the S 2p peaks of pristine
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CdS remain unchanged after exposed irradiation (Figure S9), suggesting the stability of CdS
during photocatalytic reaction with the presence of dibenzylamine. In contrast, the Cd 3d5/2 peak
of Ni/CdS shows no obvious shift after the dosing of amine and irradiation (Figure 5e), suggesting
that the photogenerated hydrogen atoms recombine and desorb from the surface, which is
supported by the rapid H2 evolution (Figure 5a). Meanwhile, a spontaneous change of Ni is
observed upon irradiation with the presence of amine, as shown in Figure 5f. Since it has been
shown that metal cations can be reduced and deposited on the semiconductor photocatalyst by the
photo-generated electrons with the presence of hole scavengers (i.e., alcohols, amines),45,53 we
assign the evolved Ni 2p3/2 peak at lower binding energy (852.4 eV) to metallic Ni supported on
CdS.45
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Based on the above results and discussions, a mechanism for photo-immobilized Ni clusters
promoted transformation of amines to imines is proposed (Figure 1). Since the reaction products
and radicals are identical for both CdS and Ni/CdS photocatalysts, we propose that the Ni clusters
only promote the reaction kinetics without changing the reaction pathways. Upon illumination, the
photogenerated electrons via excitation of CdS are initially injected to Ni2+, resulting in the
deposition of Ni clusters on the surface of CdS. Meanwhile, the photogenerated holes are
transferred to the surface of CdS, initiating the oxidation of amines via the deprotonation process
with the formation of C-centred α-amine radicals to form aldimine intermediates and release
protons. The abstracted protons are further reduced by the photogenerated electrons and produce
Hads that stick on the catalyst surface. While pristine CdS and Ni/CdS show similar formation rate
of dehydrogenated amine radical (Figure 4), the Ni/CdS shows a significant fast kinetics in
desorption of hydrogen (Figure 5). This suggests that the Ni clusters are the active sites to boost
the recombination of Hads and release as H2, thus allowing robrust liberation of the active sites on
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CdS for continuous photo-dehydrogenation of amine compared to the case of pristine CdS (Figure
S10).33,54-56 Eventually, the aldimine intermediate reacts with anther amine molecule to produce
imine accompanied by the release of NH3.
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In summary, we have reported a strategy to design efficient photocatalyst for promoted
dehydrogenative coupling of amines to corresponding imines under visible-light conditions. By
photo-deposition of Ni clusters on CdS, the desorption of Hads in the form of molecular H2 can be
significantly enhanced, thus liberating the active sites on CdS to promote the catalytic cycle. The
Ni/CdS photocatalyst exhibits excellent stability with a remarkable AQE of 44% under 420 nm
irradiation. Furthermore, it is highly tolerant of a wide range of primary and secondary amines to
the formation of corresponding imines with excellent yields, thus providing a promising and
economic route for the synthesis of value-added organic compounds.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
Experimental details, materials characterizations, photocatalytic measurements, and additional
characterizations and measurements (PDF)
AUTHOR INFORMATION
Corresponding Author
*Email: bzhang@tju.edu.cn
suren@suda.edu.cn
Author Contributions
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B.Z. and R.S. conceived and supervised the project. Y.H. designed the experiments and performed
synthesis and characterization experiments. C.L. performed and analyzed the data of GC-MS. M.L.
assisted the isotopic labeling experiment. R.S. and Y.L. designed and carried out the in-situ MS
and XPS. M.L. and H.L. carried out the in-situ ESR. Y.H., B.Z., R.S., and C.L. co-wrote the paper.
All authors analyzed the data and discussed the results. §(Y.H. and C.L) These authors contributed
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equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the financial support from the National Natural Science Foundation of China (Nos.
21871206, 21422104, and 21972100) and the China Postdoctoral Science Foundation (Nos.
2018M630269 and 2019T120182).
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