Copper(i) pyrimidine-2-thiolate cluster-based polymers as bifunctional visible-light-photocatalysts for chemoselective transfer hydrogenation of α,β-unsaturated carbonyls

Article abstract of DOI:10.1039/d1ra01102b

The photoinduced chemoselective transfer hydrogenation of unsaturated carbonyls to allylic alcohols has been accomplished using cluster-based MOFs as bifunctional visible photocatalysts. Assemblies of hexanuclear clusters [Cu6(dmpymt)6] (1, Hdmpymt = 4,6-dimethylpyrimidine-2-thione) as metalloligands with CuI or (Ph3P)CuI yielded cluster-based metal organic frameworks (MOFs) {[Cu6(dmpymt)6]2[Cu2(μ-I)2]4(CuI)2}n (2), {[Cu6(dmpymt)6]2[Cu2(μ-I)2]4}n (3), respectively. Nanoparticles (NPs) of 2 and 3 served both as photosensitizers and photocatalysts for the highly chemoselective reduction of unsaturated carbonyl compounds to unsaturated alcohols with high catalytic activity under blue LED irradiation. The photocatalytic system could be reused for several cycles without any obvious loss of efficiency.

Full text of DOI:10.1039/d1ra01102b

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Copper(I) pyrimidine-2-thiolate cluster-based
polymers as bifunctional visible-light-
Cite this: RSC Adv., 2021, 11, 14899
photocatalysts for chemoselective transfer
hydrogenation of a,b-unsaturated carbonyls†
a
David James Young,b Ji Long Maa and Guo Quan Shaoa
*
Meng Juan Zhang,
The photoinduced chemoselective transfer hydrogenation of unsaturated carbonyls to allylic alcohols has
been accomplished using cluster-based MOFs as bifunctional visible photocatalysts. Assemblies of
hexanuclear clusters [Cu6(dmpymt)6] (1, Hdmpymt ¼ 4,6-dimethylpyrimidine-2-thione) as metalloligands
with CuI or (Ph3P)CuI yielded cluster-based metal organic frameworks (MOFs) {[Cu6(dmpymt)6]2[Cu2(m-
I)2]4(CuI)2}n (2), {[Cu6(dmpymt)6]2[Cu2(m-I)2]4}n (3), respectively. Nanoparticles (NPs) of 2 and 3 served
both as photosensitizers and photocatalysts for the highly chemoselective reduction of unsaturated
carbonyl compounds to unsaturated alcohols with high catalytic activity under blue LED irradiation. The
photocatalytic system could be reused for several cycles without any obvious loss of efficiency.
Received 9th February 2021
Accepted 14th April 2021
DOI: 10.1039/d1ra01102b
rsc.li/rsc-advances
(MOFs) offer an exceptional opportunity as potential scaffolds
for non-noble photocatalytic materials. There are two main
Introduction
strategies to achieve photocatalytically active MOFs. One
approach is to introduce both the photon absorption and
catalytic center into the coordinated frameworks of the MOF.28
These photoactive MOFs based on a ligand-to-metal charge
transfer (LMCT) mechanism are not readily tunable to visible
light absorption. The second approach is the incorporation of
the desired catalytic species (molecular catalysts, metal nano-
particle etc.) as a co-catalyst into a visible-light photoactive MOF
with visible-light absorbent components (e.g. porphyrins, 2-
aminoterephthalates, perylene diimides, organic dyes, metal-
complexes), yielding a dual-catalyst system.29,30 These dual-
catalysts mostly require cumbersome preparation methods. An
alternative strategy, single-step construction of visible-light
The chemoselective hydrogenation of unsaturated carbonyl
compounds is a preferred method for the synthesis of allylic
alcohols, a class of important chemical, pharmaceutical and
agrochemical intermediates.1–4 However, reduction of the C]C
bond is both thermodynamically and kinetically favored over
the reaction of the C]O bond,5–9 thereby creating a synthetic
challenge.10–17 Extensive effort has been devoted to developing
catalytic systems that enable to efficiently and selectively
hydrogenate unsaturated carbonyls to the corresponding
unsaturated alcohols. However, current methods oen use
expensive transition metal catalysts, or toxic and hard to sepa-
rate phosphine organic ligands at elevated reaction tempera-
tures (Table 1).18–20 The design of a catalytic system that
efficiently promotes chemoselective transfer hydrogenation of
unsaturated carbonyls with alcohol activated by visible light at
room temperature is highly desired.21–26 We are aware of only
one report that is a visible light photoactive noble metal Au/SiC
catalyst, achieving selective hydrogenation of cinnamalde-
hyde.27 To the end, the utilization of efficient and robust earth-
abundant catalysts with visible-light activity for this trans-
formation is a long-term target. Metal–organic frameworks
photoactive MOFs using
a bifunctional photocatalyst as
a building block/linker serving both as a photosensitizer and as
a photocatalyst, is a promising approach because it is simpler
and easier-to-use. Only a few MOFs have been doped with
a bifunctional visible light photocatalyst based on (bpy)Re(CO)3
(bpy ¼ 2,20-bipyridine),31 (bpy)Ru(CO)2Cl2,32 Ir(bpy)3,33 or (bpy)
PtCl2 (ref. 34) functional groups for CO2 reduction and
hydrogen production. Interestingly, the resulting solid MOF
materials displayed higher photocatalytic activity than the cor-
responding homogeneous catalysts. We postulated that
a bifunctional photocatalyst as a building block, using as both
visible-light photosensitizer and transfer-hydrogenation cata-
lyst, could be developed to heterogeneously catalyze reduction
of unsaturated carbonyls to allylic alcohols under visible light.
Typical metal–ligand bifunctional H–M–NH catalysis have
provided an efficient method for this reaction.35–37 Very recently,
aCollege of Traditional Chinese Medicine, Bozhou University, Bozhou 236800, Anhui,
People's Republic of China. E-mail: 751011300@qq.com
bFaculty of Science and Engineering, University of the Sunshine Coast, Maroochydore
DC, Queensland, 4558, Australia
† Electronic supplementary information (ESI) available: Experimental details, 1H
and 13C NMR of products, crystallographic data, PXRD patterns, uorescence
spectra, etc. CCDC 2061814 and 2061807. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/d1ra01102b
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Table 1 Examples for the selective hydrogenation C]O bond of cinnamaldehyde
Reaction conditions
Entry
Catalyst
T (ꢀC)
PH (bar)
t (h)
Yield (%)
Selec. (%)
Ref.
2
1
2
3
Pt/YCo0.3Fe0.7O3
Ir/H–MoOx
Co@CN-900
Cu6(dmpymt)6
Au/SiC
90
100
80
100
20
20
20
20
1
0
0
0.5
2
48
10
4
93.8
93
98
98
100
94
94.9
93
99
99
100
100
18
19
20
38
27
—
4
5a
6a
3
0
24
a
Reactions were promoted under visible light irradiation.
we have found that Cu(I) pyrimidine-2-thiolate clusters can four (in 3) of six dmpymt ligands in the [Cu6(dmpymt)6] units
efficiently catalyze transfer hydrogenation reactions via a coop- take a m-(k1-N,k1-N0,k3-S) mode to bridge four Cu(I) atoms. Other
erative metal hydride and ligand proton transfer mechanism.38 dmpymt ligands bridge three Cu(I) atoms via m-(k1-N,k3-S)
Cu(I) N-heterocycle thiolate clusters are potentially attractive coordination fashion. If each “Cu2(m-I)2” or “CuI” is considered
precursors for the construction of bifunctional photocatalyst
MOFs based on the following considerations: (i) metal thiolate
complexes can be used as co-catalysts for the production of
hydrogen compatible with a photosensitizer,39–45 (ii) Cu(I) N-
heterocycle thiolates absorb visible light and are intensely
luminescent with long uorescent lifetimes,46–48 (iii) they can
smoothly achieve such conversion at 100 ꢀC,38 (iv) some clusters
such as [Cu6(dmpymt)6] with uncoordinated N atoms can be
used as metalloligands for the construction of MOFs. We have
investigated this possibility using the hexanuclear Cu(I)
pyrimidine-2-thiolate cluster [Cu6(dmpymt)6] (1) as a precursor
to construct cluster-based coordination complexes {[Cu6(-
dmpymt)6]2[Cu2(m-I)2]4(CuI)2}n (2) and {[Cu6(dmpymt)6]2[Cu2(m-
I)2]4}n (3) from CuI and (Ph3P)CuI, respectively. Nanoparticles of
2 and 3 can efficiently catalyse transfer hydrogenation of
unsaturated carbonyl compounds to the corresponding unsat-
urated alcohols under the irradiation of blue LEDs using 2-
propanol as the hydrogen source.
Results and discussion
Syntheses and characterization
As remarked above, the coordinative N atom in [Cu6(dmpymt)6]
(1) can further coordinate with metal ion. At the outset, we
carried out reaction of 1 with CuI in MeCN and CHCl3 at room
temperature to result in the formation of one 3D cluster-based
polymer {[Cu6(dmpymt)6]2[Cu2(m-I)2]4(CuI)2}n (2) in 75% yield.
Solvothermal reaction of [Cu6(dmpymt)6] with (Ph3P)CuI in
MeCN/toluene at 120 ꢀC yielded several crystals of a 3D complex
{[Cu6(dmpymt)6]2[Cu2(m-I)2]4}n (3) in 89% yield. Single-crystal
analysis revealed that 2 crystallized in the tetragonal space
group P41212, 3 crystallizes in the monoclinic space group C2/c.
In 2, each “Cu6(dmpymt)6” unit is interconnected with the other
four through “Cu2(m-I)2” linkages forming a 2D layer, which is
Fig. 1 (a) The minimum structure unit in 2; (b) [Cu6(dmpymt)6] and
further connected to equivalent layers via N–Cu–N bonds to
[Cu(m-I)]2 form a two-dimensional network structure extending along
form a 3D net (Fig. 1). Compound 3 also consists of a single 3D
the bc surface in 2; color codes: Cu (cyan), I (pink), S (yellow), N (blue),
C (black); (c) view of the 3D network of 2. (d) Schematic view of the
network in which each “Cu6(dmpymt)6” unit, serving as
a tetrahedral node, interconnects with the other four nearby
units through N-{Cu(m-I)2Cu}-N bridges (Fig. 2a). All six (in 2) or green represents Cu2I2 linking; violet represents [CuI] units.
topological net of 2. Red represents 4-connected Cu6(dmpymt)6;
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as a 2-connecting node, then each “Cu6(dmpymt)6” is simplied
as 6- or 4-connecting node and the 3D topologies can be
described using the {(8)(8)(812$123)} (2) (Fig. 1d) and
4
2
¨
{(8)(8 $12 )} (3) (Fig. 2b) Schlai symbols. In 2, Cu/Cu of
˚
contacts of 2.5460(19) A in “Cu2(m-I)2” unit is longer than the
˚
corresponding one in 3 (2.501(2) A). For “Cu6(dmpymt)6” unit,
˚
˚
Cu/Cu of distances in 2 are 2.7767(18) A, 2.7957(18) A, 2.998(2)
˚
˚
A, which are comparable with those in 3 (2.663(3) A, 2.7825(19)
˚
˚
A, 2.963(3) A). The measured X-ray powder patterns of 2 and 3
show a good agreement with those of the corresponding
simulated ones (Fig. S1 and S2†). Complexes 2 and 3 are air and
moisture stable and insoluble in common organic solvents such
as CHCl3, MeOH, MeCN, DMF and DMSO. Complexes 2 and 3
are thermally stable up to 270 ꢀC, and 3 has the better ther-
mostability (Fig. S3†).
Fig. 3 SEM images of nanostructures obtained from 2 (a) and 3 (b).
N2 adsorption–desorption measurements supported this
hypothesis (Fig. S5†).
Nanoparticles of 2 were generated by mixing a solution of
[Cu6(dmpymt)6] with CuI in MeCN and CHCl3 for several
minutes. Nanowires of 3 were prepared by mixing a solution of
[Cu6(dmpymt)6] with (Ph3P)CuI and PVP in MeCN and toluene,
followed by heating to 120 ꢀC for 24 h in a glass pressure tube.
Their XRD patterns of as-synthesized nanoparticles 2 and 3
match their crystallographic data well, respectively. Scanning
electron microscopy (SEM) images (Fig. 3a and b) show that
nanoparticles of 2 consist of an approximate spherical structure
with the diameter about 50–100 nm. NPs 3 grow into nanowires
(z50–100 nm in width; tens of micrometers in length). As
shown in Fig. S4,† the peaks at 931.9 eV (2) or 932.2 eV (3) and
951.8 eV (2)/952.1 eV (3), can be attributed to Cu 2p3/2 and Cu
2p1/2 of Cu+, respectively. The observed type III isotherm in the
Optical properties
Complex 1 exhibits an ultraviolet and visible light absorption
behavior (Fig. S6†). Complexes 2 and 3 show similar absorption
bands, but wider and red shied relative to that of 1. Upon the
excitation at l ¼ 420 nm, 1 exhibits largely enhanced photo-
luminescence (quantum yield QY ¼ 36.23%, emission centered
at l ¼ 742 nm), whereas the CHCl3 solution of [Cu6(dmpymt)6]
is only weakly luminescent (QY ¼ 4.37%). These emissions are
most probably arising from a ligand-to-metal charge-transfer
(LMCT) and/or metal-to-ligand charge-transfer (MLCT),
possibly involving metal-centered (ds/dp) states.49,50 The solid-
state emission spectra of 2 and 3 were investigated at room
temperature with excitation at 420 nm (Fig. S7†). The room
temperature emission of 2 is similar to that of [Cu6(dmpymt)6]
(lex ¼ 742 nm) while complex 3 exhibited a broad emission over
the range 550–850 nm with a maxima at ca. 677 nm. The
emission of 3 is blue-shied compared to those of 1 and 2. Their
quantum yields of 2 and 3 are 42.47% (2) and 55.28% (3),
respectively, which are much higher than that of 1. The pho-
toluminescence lifetimes of 1, 2 and 3 in the solid state were
measured to be 3.6, 4.1 and 7.1 ms, respectively. These results
indicate that the assemblies of cluster could expand its
absorption region and improve its luminescent properties.
What's more, the transient photocurrent curves for NPs 2 and 3
were also investigated (Fig. S8†), which shows that the stable
photocurrent value for NPs 3 was higher, suggesting its fastest
charge separation efficiency.
Catalytic properties
The band gap energies (Eg) of 1–3 were determined using the
Kubelka–Munk method based on their diffuse reectance
spectra (Fig. S9†). The energy band gaps obtained by extrapo-
lation of the linear portion of the absorption edges are 2.20 eV (1
and 2) and 2.02 eV (3). These narrow band gaps encouraged us
to investigate their visible light photocatalytic activity, which
can be evaluated to catalyze the selective transfer hydrogenation
of unsaturated carbonyls. We chose cinnamaldehyde (4a) as our
model substrate to optimize the reaction conditions. As shown
in Table S3,† incubating a mixture of 4a, NaOH as a base and
NPs of 3 as a catalyst in isopropanol under visible light afforded
Fig. 2 (a) View of the 3D network of 3. (b) Schematic view of the
topological net of 3. See Fig. 1 for color codes.
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Table
2
Selective photocatalytic hydrogenation of unsaturated Table 2 (Contd.)
carbonylsa
Entry Main product
Conv.b (%) Select.b (%) Yieldc (%)
Entry Main product
Conv.b (%) Select.b (%) Yieldc (%)
20
72(85)d
85(83)d
61(68)d
1
2
100
100
94
91
94
91
a
General conditions: unsaturated carbonyls (0.2 mmol), 3 (6 mg),
NaOH (20 mol%), i-PrOH/MeCN (8 mL, 2/1), room temperature, blue
LEDs (455 nm, 26 W), 24 h, N2. Yields determined by HPLC with
b
c
d
biphenyl as an internal standard. Isolated Yields. Reaction time ¼
3
4
5
6
100
83(90)
95
94
93
36 h in parentheses.
92(91)
93
75(79)
88
full conversion of 4a to the target product cinnamyl alcohol (5a)
in 80% yield, the saturated aldehyde of 3-phenylpropanal (6a) in
6% yield and saturated alcohol of 3-phenylpropan-1-ol (7a) in
14% yield aer 24 h (Table S3,† entry 1). When this reaction was
carried out in MeOH or EtOH, the conversion and selectivity of
4a became lower (entries 2 and 3). When the reaction was per-
formed in i-PrOH–MeCN mixture, the selectivity of 5a was
increased signicantly with high activity (entries 4–7). The i-
PrOH/MeCN volume ratio of 2 : 1 provided highest selectivity
to 5a with highly catalytic activity (entry 6). Control experiments
showed that reaction hardly proceeded in the absence of cata-
lyst, base or light (entries 8–10). Other bases KOH, t-BuOK,
NaOMe, Cs2CO3, Na2CO3, K2CO3, K3PO4 exhibited reduced
selectivity (entries 11–19) relative to NaOH. The catalytic
performances of other two compounds were also investigated.
NPs 3 exhibited higher selectivity and activity toward reduction
of 4a to 5a in comparison with 1 and 2. It is notable that the
nely ground synthesized crystals of 2 or 3 as photocatalyst gave
the conversion in 84% or 87% under the same conditions. As
such, the visible-light-catalyzed chemoselective reduction of 4a
to 5a was able to efficiently proceed under the optimized
conditions, i.e., 3 as a both photosensitizer and photocatalyst
and 20 mol% NaOH as a base, in i-PrOH–MeCN mixture solu-
tion, irradiated by blue LEDs for 24 h at room temperature.
With the optimized conditions in hand, our initial study
focused on the chemoselective reduction of a,b-unsaturated
aldehydes (Table 2). Pleasedly, the transfer hydrogenation of
cinnamaldehyde type substrates showed photoinduced selec-
tive hydrogenation with high selectivity toward the production
of corresponding unsaturated alcohol (entries 1–10). The
electron-withdrawing substituent slightly reduced the conver-
sion of cinnamaldehydes (entries 4–7 and 10), but had little
inuence on the selectivity to the unsaturated alcohols. What's
more, the reduction of heteroatom-containing a,b-unsaturated
aldehyde resulted in the C]O bond-selective hydrogenation to
a,b-unsaturated alcohol in good yield (85%). The aliphatic
unsaturated aldehyde exhibited lower activity and selectivity
than the aromatic unsaturated aldehydes (entries 12–13). The
catalyst system can also be applied to selective hydrogenation of
a,b-unsaturated ketones. NPs 3 can catalyse hydrogenation of
benzylideneacetone with high activity, obtaining the corre-
sponding a,b-unsaturated alcohols in excellent yields. p-
95
95
89
7
8
86(92)
93
93(92)
91
78(82)
83
9
88
91
82
10
11
73(85)
90
90(91)
91
65(74)
85
12
13
84(92)
80(90)
86(83)
84(82)
67(72)
62(70)
14
15
16
87
92
85
84
87
86
75
80
72
17
18
19
89
82
75
84
86
92
75
70
68
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substituted benzylideneacetones were selectively transformed
to the corresponding unsaturated alcohols, providing good
yields (entries 14–19). a,b-unsaturated ketone without a conju-
gated aromatic ring was also converted, but the selectivities was
lower (entry 20) than those of a,b-unsaturated ketones with
a conjugated aromatic ring.
Conclusions
In summary, we developed a straightforward synthesis of
bifunctional cluster-based MOFs 2 and 3 through the reaction
of a preformed cluster [Cu6(dmpymt)6] as a metalloligand with
CuI and (Ph3P)CuI. 2 and 3 serve both as a photosensitizer and
a co-catalyst. Under the irradiation of blue LEDs, they display
high photocatalytic activity toward the transfer hydrogenation
of unsaturated carbonyl compounds to unsaturated alcohols
with high selectivity at room temperature. This work also clearly
demonstrates that the synthetic methodology may be applied to
other bifunctional clusters to yield recyclable heterogeneous
photocatalysts with enhanced catalytic activities. Further
studies are in progress to design robust and efficient MOFs-
based photocatalysts for visible light-driven transfer hydroge-
nation reactions.
The reusability of the catalyst 3 is also investigated for the
selectivity hydrogenation of cinnamaldehyde to cinnamyl
alcohol. Aer the rst reaction cycle, the catalyst is directly
separated from the reaction mixture by centrifugation and
washed with MeCN, H2O and Et2O. The isolated catalyst is used
for the second cycle by adding 4a and NaOH in MeCN/i-PrOH
under the blue LEDs irradiation. Aer the catalytic reaction,
catalyst 3 was collected by ltration, washed with MeCN, H2O
and Et2O, dried in air and then characterized by PXRD. The
PXRD patterns were in good agreement with the simulated ones
generated from the single crystal data of 3, suggesting that the
original framework structure of 3 was retained during the
catalysis (Fig. S10†). Aer the hydrogenation reaction was per-
formed for the fourth cycle, the SEM images of the resulting
catalytic species revealed the obvious shape changes of NPs 3
(Fig. S11†), resulting in poor hydrogenation activity, but still
with high selectivity. When the h cycle was performed for
longer time (36 h), about 82% of 4a was converted to 5a. The
catalyst deactivation was witnessed aer each run (Fig. 4). This
result also suggested that photocatalysis is very likely to be
limited to the surface layers of NPs 3. We propose the mecha-
nism involving a metal–ligand bifunctional mechanism.51–56 As
shown in scheme S1,† the proton from i-PrOH is delivered to the
dmpymt ligand, yielding Cat II. Meanwhile, the b-elimination of
acetone from Cu isopropoxide intermediate (I) produces Cat I,
which is subsequently protonated by alcohol to obtain
a hydride/proton complex Cat II. In order to investigate the role
of the NH or SH group, we modied Hdmpymt by methylation,
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by Bozhou University and Natural
Science Research Key Projects of Anhui Province Universities
(KJ2020A0774).
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