A photosensitizing decatungstate-based MOF as heterogeneous photocatalyst for the selective C-H alkylation of aliphatic nitriles

Article abstract of DOI:10.1039/c6cc00862c

The efficient photosensitizing of decatungstate-based MOF with 1D channels was achieved via in situ synthesis under solvothermal conditions for light driven acceleration of β- or γ-site C-H alkylation of aliphatic nitriles. The high catalytic efficiency, excellent size selectivity, high stability and good recyclability of the photocatalyst offer an environmentally-friendly route for widening the scope of accessible nitriles in both laboratory and industry.

Full text of DOI:10.1039/c6cc00862c

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DOI: 10.1039/C6CC00862C
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Photosensitizing decatungstate-based MOF as heterogeneous
photocatalyst for the selective C–H alkylation of aliphatic nitriles
Received 00th January 20xx,
Accepted 00th January 20xx
Dongying Shi, Cheng He, Wenlong Sun, Zheng Ming, Changgong Meng and Chunying Duan*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The efficiently photosensitizing decatungstate-based MOF with 1D light as a renewable source of energy.8 Of these inorganic
channels was achieved via in situ synthesis under solvothermal photosensitizers, decatungstate exhibits high photocatalytic
conditions for light driven acceleration of β- or γ-site C–H activity toward the activation of C–H bonds in various classes
alkylation of aliphatic nitriles. The high catalytic efficiency, of organic molecules,9 including aldehydes, amides, and even
excellent size selectivity, high stability and good recyclability of alkanes. The low stability and the high sensitivity to
the photocatalyst offers an environmental-friendly route for temperature and acidity compared than other POM species
widening the scope of accessible nitriles in both the laboratory make the heterogenization of [W10O32]4– anions in the pores
and industry.
such as these MOF-based materials is highly desirable in case
of the possibility to provide an efficient and recyclable
photocatalytic system.
Polyoxometalates (POMs) are a large diversity of nanoscale
anionic metal oxide clusters that have oxygen-enriched surface
and the unique redox catalytic properties.1 They are well-
known oxidation catalysts and solid acids in several chemical
industries.2 To improve their catalytic behavior for the
practical application, several strategies were developed to
incorporate POMs within different kinds of surface of
materials.3 Of the strategies reported, the incorporation of
POMs in the pores of metal–organic frameworks (MOFs)
exhibits several advantages.4 The mild synthetic conditions,
tunable porosity, wide range of structural motifs and thermal
stability render them a promising family of catalytic materials.5
However, due to the intrinsic constraints of the self-assembly
synthetic procedure, most of the reported POMOFs are based
on classical POMs (e.g., Keggin-, Dawson- and Lindquist-type
POMs).6 One of the challenges is to incorporate these POMs
with rich catalytic or photocatalytic properties within in the
pore of MOFs to improve the stability and practical application
of these interesting and appealing structural motifs.
Scheme 1 Schematic representation of the photocatalytic reaction and the crystal-type
I to crystal-type II transformation induced by catalytic substrate encapsulation.
By incorporating photosensitizing [W10O32]4– anions within
the pores of copper-based MOFs, we developed an approach
to immobilize decatungstate for light driven acceleration of β-
or γ-site C–H alkylation of aliphatic nitriles (Scheme 1). The
assembled decatungstate-based MOF exhibits one-dimen-
sional hydrophilic/ hydrophobic channels that enable the
adsorption of the reaction substrates. The high stability of the
frameworks even in low pH value and the simply embedded of
the photoactive decatungstate within the pores prevent
catalyst leaching and nonuniformity of the active sites during
the heterogenizing processes. As aliphatic nitriles play an
important role in both the laboratory and industry, the direct
functionalization of their C–H bonds would offer an attractive
route for widening the scope of accessible nitriles.10 The
potential constraint of decatungstate-based MOFs relevant to
On the other hand, visible-light photocatalysis with POMs
for direct functionalization of C–H bonds under mild conditions
continues to be a topic of intense research.7 It moves toward a
“sustainable chemistry” opening to new synthetic routes that
require mild temperature and pressure conditions and use of
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116024, P. R. China. E-mail: cyduan@dlut.edu.cn
†
Electronic Supplementary Information (ESI) available: Experimental details,
characterization data, as well as additional tables and figures. CCDC 1443253 and
1443255. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x
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the confined space of the pores would provide additional organic substrates, the synthetic approach provides an
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DOI: 10.1039/C6CC00862C
selectivity for the β- or γ-site C–H alkylation of aliphatic opportunity to stabilize the highly active functional groups
nitriles.
even in the low pH aqueous systems and at the same time
enables the combination of photocatalysis and configuration
constraints within the confined space for a synthetic organic
transformation in a heterogeneous manner.
The UV–vis absorption spectrum of DT–BPY in the solid
state (Fig. 2a) exhibited an intense absorption at wavelengths
less than 400 nm; this absorption is assigned to the oxygen-to-
tungsten charge transfer transition.11 In addition, the
absorption at 626 nm corresponds to the nitrogen-to-copper
charge transfer transition. The [W10O32]4– typically exhibits an
absorption spectrum in the near-UV region that partially
overlaps the UV solar emission spectrum, opening the
possibility to carry out benign solar-photo assisted
applications.12 Upon excitation at this absorption band, the
fluorescence emission spectrum of free DT–BPY exhibited an
intense luminescence band at approximately 459 nm (Fig. 2b).
Noticeable, the progressive addition of isovaleronitrile to the
CH3CN suspension of DT–BPY caused substantial luminescence
quenching. This quenching process is attributed to the directly
electron transfer from the isovaleronitrile to the excited state
of the decatungstate moieties. It suggests the possibility of the
reactive excited state of [DT–BPY]* to oxidize isovaleronitrile
for the generation of alkyl radicals, which is further used for
the functionalization of nitrile substrates.
Fig. 1 The 3D open-framework structure of DT–BPY, which is generated by the
covalent linking of tetra-Cu clusters and BPY bridges (drawn as a ball-and-stick model)
4–
with [W10O32
]
embedded (drawn as purple polyhedra), highlighting the 1D channel
(drawn as a yellow pillar) with dimensions of 13.3 Å × 6.8 Å. Symmetry operation: A: –x,
y, 1 – z; B: –0.5 + x, –0.5 + y, z; C: 0.5 – x, –0.5 + y, 1 – z.
The novel MOF of [Cu4(BPY)6Cl2(W10O32)]·3H2O (referred to
as DT–BPY; CCDC No. 1443253) was synthesized by a
solvothermal reaction of (TBA)4[W10O32], Cu(ClO4)2·6H2O and
the ligand 4,4′-bipyridine (BPY) at pH 2.3 with a yield of 51%.
Elemental analyses and powder X-ray analysis indicated the
pure phase of its bulk sample. Single-crystal structural analysis
revealed the presence of free decatungstate anions embedded
in the pores of the copper-bipyridine framework. Two
crystallographically independent copper ions were alternatively
connected by μ3-chlorine bridges to produce a planar tetra-Cu
cluster (Fig. 1a). The Cu(1) atom exhibited a five-coordinate
square-pyramidal geometry with four nitrogen atoms from
four BPY ligands positioned in the basal plane and a chlorine
anion occupied the vertex position. The Cu(2) atom adopted a
four-coordinate tetrahedral geometry with two nitrogen
atoms of BPY ligands and two μ3-chlorine bridges. These tetra-
Cu clusters were connected using the BPY ligands to form 2D
bilayer grids; adjacent sheets were further connected together
by other BPY bridges to generate a 3D Cu–BPY framework (Fig.
1b). The photoactive [W10O32]4– polyoxoanions were
Fig. 2 (a) Solid-state UV–vis absorption spectra of DT–BPY and TBADT. (b) Family of
emission spectra of DT–BPY (0.1% by weight) in CH3CN suspension upon addition of
isovaleronitrile up to 0.50 mM, excitation at 376 nm. (c–d) 1H NMR in D2SO4/DMSO and
solid state IR spectra of DT–BPY (i) and (iii), respectively, and of DT–BPY impregnated in
embedded in the pores of Cu–BPY via electrostatic attraction a CH3CN solution of isovaleronitrile (ii) and (iv), respectively, the blue and red triangle
represented the signals of isovaleronitrile in the NMR and IR spectra, respectively.
to generate a DT–BPY POMOF with 13.3 Å × 6.8 Å channel (Fig.
1c and 1d). It should be noted that this is the first example of
Detailed structural analysis using the PLATON software
suggests that the effective free volume of DT–BPY without the
solvent molecules is 22.8%.13 The pores of DT–BPY thus are
able to adsorb isovaleronitrile and acrylonitrile, which are the
possible substrates for the C–H functionalization of aliphatic
porous decatungstate-based three-dimensional MOFs, as the
high sensitivity of decatungstate to the temperature and
acidity make the incorporation and stabilization quite difficult.
No significant interactions relevant to the decatungstate
anions were observed to inactivate the surface oxygen atoms
of the free photocatalyst, suggesting the possibility of
maintaining the photocatalytic activity of decatungstate within
the pores of the MOF. Because decatungstate exhibits high
photoreactivity in various catalytic oxidation processes of
1
nitriles. As shown in Fig. 2c and 2d, the H NMR spectrum of
DT–BPY solids immersed in isovaleronitrile indicated that DT–
BPY could adsorb approximately 1 equiv of isovaleronitrile per
[W10O32]4– anion. The IR spectrum of DT–BPY impregnated
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with isovaleronitrile revealed a C≡N stretching vibration at reaction was conducted in the dark or in the absence of DT–
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2240 cm–1. The red shift from 2246 cm–1 (free isovaleronitrile) BPY catalyst, which suggests that boDthOI:t1h0e.103li9g/hCt6CaCn0d086th2Ce
suggested the adsorption and the activation of isovaleronitrile photocatalyst are required for efficient conversion to the
in the pores of the MOFs. We hypothesized that these alkylation products.14 Notably, solid DT–BPY was easily
substrates could enter the pores of the MOF and that the isolated from the reaction suspension by simple filtration and
enforced spatial proximity between the adsorbed substrate was subsequently reused at least three times, with a slight
and the photoactive center facilitated the in situ generation of decrease in its reactivity and selectivity (from 81% to 80% yield
the active alkyl radicals from the aliphatic nitriles.
after three cycles). The indexing of XRD patterns of DT–BPY
collected by filtration from the reaction mixture demonstrated
the maintenance of the crystallinity (Fig. S3, ESI†).
Interestingly, the efficiency of the heterogeneous catalyst in
one round catalytic processes is comparable to that of the
homogeneous decatungstate catalyst.15 The recyclability of our
catalyst render our synthetic approach superior to other
relevant heterogenizing methods.
Table 1 Selective C–H alkylation of aliphatic nitriles with DT–BPY as a heterogeneous
photocatalysta
Fig.
3
Color changes of reaction system for DT–BPY indicating the blue color
characteristic of H+[DT–BPY]– [(a)-Reaction time for 0 h, (b)-Reaction time for 0~36 h,
(c)-After the reaction, the mixture was stirred under air atmosphere].
Importantly, in contrast to the smooth reactions of
substrates 1–7, the site-selective reaction in the presence of
bulky propen-amide(N,N-bis[3,5-bis(1,1-dimethylethyl)phenyl]
-2-propenamide)
8 gave less than 5% conversion under the
same reaction conditions. The negligible adsorption resulting
from immersion of DT–BPY into an acetonitrile solution of
substrate
8
coupled with the fact that the size of substrate
8
8
was larger than the openning of the pores16 revealed that
was too large to be absorbed into the pores. Furthermore,
these results suggest that the photocatalytic site-selective
reaction indeed occurred in the pores of the POMOF and not
on its external surface.
a Reaction conditions: aliphatic nitrile (2.5 mmol), alkene (0.5 mmol), photocatalyst
of DT–BPY (1 mol %), TBA (4 mol %), acetonitrile (1 mL), irradiation by a 500 W Xe
b
lamp at room temperature for 36 h. Yields of product isolated after flash
chromatography on SiO2. c Not detected
From a mechanistic viewpoint (Scheme S1, ESI†), the
reactive excited state of [DT–BPY]*, which is characterized by
a hole in an O-based wO and efficiently abstracts a hydrogen
from the β- or γ-C–H bond of aliphatic nitriles to form alkyl
radicals.17 The alkyl radicals then combine with the electron-
deficient alkene to form the adduct radical. Back-hydrogen
As shown in Table 1, the catalytic reaction was initially
examined using isovaleronitrile and acrylonitrile as the
alkylation partners, along with a Xe lamp (500 W) as the light
source. The loading 1% percent of mole ratio of the catalyst
lead to 83% in yield for β-substituted isovaleronitrile) after 36
h of irradiation. The result revealed the successful execution of
our POMOF design, showing a high site-selective efficiency.
Other substrates (e.g., valeronitrile, isocapronitrile and
adiponitrile) were also found to be prone to react with
acrylonitrile under the same conditions. DT–BPY thus
represents the first example of a POM-based heterogeneous
photocatalyst for this important site-selective alkylation
reaction. A number of control experiments were conducted to
elucidate the heterogeneous and photocatalytic natures of the
reactions. Almost no conversion was observed when the
atom transfer from the reduced form of H+[DT–BPY]– gives the
18
alkylated product and regenerates the starting DT–BPY
.
Obviously, the occurrence of the reaction is indicated by the
shift to the blue color characteristic of H+[DT–BPY]– (Fig. 3).19
Interestingly, the quality of the acrylonitrile impregnated
crystals [Cu4(BPY)6Cl2(W10O32)]·C3H3N·H2O (referred to as
AN@DT–BPY; CCDC No. 1443255) was sufficient for the
subsequent X-ray crystallographic analysis. The same space
group and almost the same cell dimensions between the
impregnated crystal and the original crystal confirmed that the
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4
5
(a) C. Zou, Z. Zhang, X. Xu, Q. Gong, J. Li and C.-VDie.wWArutic,leJ.OAnlmine.
framework was maintained during the substrates adsorption
(Fig. 4). Detailed structural analysis revealed that molecules of
acrylonitrile were positioned near the decatungstate anions,
and that weak C–HO hydrogen bonds (2.854 Å) were
observed between the acrylonitrile and decatungstate anions.
Luminescence titration confirmed the occurrence of direct PET
from the substrate to the excited state MOF as well as the
formation of active alkyl radicals; however, the enforced
spatial proximity between the encapsulated substrate and the
photoactive center further facilitated the in situ combination
of an active alkyl radical with an electron-deficient alkene to
form the adduct radical and the in situ back-hydrogen transfer
from the reduced form of MOF to give the alkylated product
and to regenerate the original DT–BPY. The high catalytic
efficiency, excellent size selectivity and good recyclability
render our synthetic approach superior over other hetero-
genization methods for the stabilizing and incorporating
decatungstate on the surface of the supports.
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In summary, we reported
a
new example of
photosensitizing decatungstate-based MOF with 1D channels
via in situ synthesis under solvothermal conditions. The MOF
exhibits remarkable photocatalytic activities for the β- or γ-site
C–H alkylation of aliphatic nitriles under mild conditions, which
can offer an environmental-friendly route for widening the
scope of accessible nitriles in both the laboratory and industry.
This work was financially supported by the National
Natural Science Foundation of China (21531001 and
21421005).
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