Thiophene insertion for continuous modulation of the photoelectronic properties of triphenylamine-based metal-organic frameworks for photocatalytic sulfonylation-cyclisation of activated alkenes

Article abstract of DOI:10.1039/c8nj04151b

Pharmaceutically meaningful conversions like photocatalytic sulfonylation call for the precise control of the photoelectronic properties of photocatalysts. A ligand engineering strategy of inserting different numbers of thiophenes into the scaffold of triphenylamine (TPA)-based ligands was developed to improve the visible light absorption, enhance the oxidation potentials of the ground states, and decrease the reduction potentials of the excited states, providing a powerful tool for continuous modulation of the photoelectronic properties of metal-organic framework (MOF)-based heterogeneous photocatalysts. The ligand inserted with two thiophenes was found to possess a well-balanced photoelectronic performance, endowing the corresponding MOF Zn-BCTA with good visible light-harvesting ability and a moderate excited-state reduction potential with minimal over-potential for the photoinduced generation of sulfonyl radicals while avoiding the competitive over-reduction of the sulfonyl moieties. The breaking of the C₃-symmetry of the TPA-based ligand forged a distorted coordination geometry of the Zn₄O nodes of the MOF to provide potential active sites to facilitate the fixation and activation of α,β-unsaturated carbonyl substrates, and the two-fold interpenetrated frameworks further enhanced the spatial proximity between the encapsulated substrate and the photoredox-active centre. The synergy of the well-tuned photoelectronic properties of Zn-BCTA and the spatial confinement effect within the pores benefited the tandem sulfonylation-cyclisation of unsaturated alkenes in an efficient and diastereoselective mode for the construction of bio-interesting sulphoyl isoquinolinediones/oxindoles.

Full text of DOI:10.1039/c8nj04151b

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Thiophene insertion for continuous modulation
of the photoelectronic properties of
triphenylamine-based metal–organic frameworks
for photocatalytic sulfonylation–cyclisation of
activated alkenes†
Cite this: New J. Chem., 2018,
42, 18448
a
a
a
a
a
Tiexin Zhang, ‡* Yusheng Shi,‡ Sen Zhang, Chen Jia, Cheng He
Chunying Duan*
and
ab
Pharmaceutically meaningful conversions like photocatalytic sulfonylation call for the precise control of
the photoelectronic properties of photocatalysts. A ligand engineering strategy of inserting different
numbers of thiophenes into the scaffold of triphenylamine (TPA)-based ligands was developed to
improve the visible light absorption, enhance the oxidation potentials of the ground states, and decrease
the reduction potentials of the excited states, providing a powerful tool for continuous modulation of
the photoelectronic properties of metal–organic framework (MOF)-based heterogeneous photocatalysts.
The ligand inserted with two thiophenes was found to possess a well-balanced photoelectronic
performance, endowing the corresponding MOF Zn–BCTA with good visible light-harvesting ability and
a
moderate excited-state reduction potential with minimal over-potential for the photoinduced
generation of sulfonyl radicals while avoiding the competitive over-reduction of the sulfonyl moieties.
The breaking of the C -symmetry of the TPA-based ligand forged a distorted coordination geometry of
the Zn O nodes of the MOF to provide potential active sites to facilitate the fixation and activation of
3
4
Received 14th August 2018,
Accepted 11th October 2018
a,b-unsaturated carbonyl substrates, and the two-fold interpenetrated frameworks further enhanced the
spatial proximity between the encapsulated substrate and the photoredox-active centre. The synergy of
the well-tuned photoelectronic properties of Zn–BCTA and the spatial confinement effect within the
pores benefited the tandem sulfonylation–cyclisation of unsaturated alkenes in an efficient and diastereo-
selective mode for the construction of bio-interesting sulphoyl isoquinolinediones/oxindoles.
DOI: 10.1039/c8nj04151b
rsc.li/njc
of aryl sulfonyl radicals with alkenes provides a facile route for
Introduction
the preparation of aryl alkyl sulfones with excellent functional
4
Sulfones are key compounds widely applied in medicines and group tolerance. Moreover, photoredox catalysis was recently
1
5–7
agricultural chemicals. In comparison to classic sulfa drugs employed for the generation of aryl sulfonyl radicals, avoiding
8
containing easily prepared sulfamides, drugs bearing aryl the use of harmful radical initiators and showing potential for
alkyl sulfone scaffolds such as the antipsychotic medication improving pharmaceutical technology with clean photoenergy.
s
Amisulpride (also known as Solian ) are more challenging to Bearing in mind the use of expensive, toxic, and irrecoverable
2
3
access and multistep syntheses, hypertoxic reagents, and poor polypyridyl noble metal complexes as the photocatalysts in the
functional group tolerances are generally encountered during the reported procedures, it is highly desirable to develop the aryl-
introduction of the sulfone moiety. Comparatively, the addition sulfonylation of alkenes in the presence of heterogeneous
photocatalysts constructed from more economic and less harm-
a
ful organic dyes. It should be noted that the electrochemical
demands on arylsulfonyl chlorides for the generation of aryl
sulfonyl radicals raise the requirements for the specific photo-
electronic performance of the photocatalyst. First, the reduction
potential of the excited state of the photocatalyst should cover the
diverse range of electronically differentiated sulfonyl chlorides
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Dalian 116024, China. E-mail: zhangtiexin@dlut.edu.cn, cyduan@dlut.edu.cn;
Fax: +86-411-84986476; Tel: +86-411-84986476
b
Collaborative Innovation Center of Chemical Science and Engineering,
Tianjin 300071, China
†
Electronic supplementary information (ESI) available. CCDC 1861320. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c8nj04151b
9
,10
‡
T. Zhang and Y. Shi contributed equally to this work.
(E1/2 = ꢀ0.44 to ꢀ1.39 V vs. SCE).
More importantly,
1
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the competitive reduction of the sulfonyl moiety vs. the desired
NJC
11
Thermogravimetric analysis (TGA) was carried out with a
bond dissociation of S–Cl under an over-negative electronic Mettler-Toledo TGA/SDTA851 instrument. Powder X-ray diffrac-
potential implies that the over-potential of the reduction potential tion (PXRD) measurements were performed with a PANalytical
of the excited state should be as small as possible. Moreover, the Empyrean X-ray powder diffractometer (Cu Ka radiation, 40 kV,
use of lower energy visible light is strongly recommended to avoid 40 mA). FT-IR spectra were measured as KBr pellets on a JASCO
background reactions and the other destructive pathways caused FT/IR-430. Solid and liquid UV-vis spectra were recorded on a
by the higher energy UV region.
Hitachi U-4100 UV-vis-NIR spectrophotometer and a TU-1900
As inorganic–organic composite materials, metal–organic spectrophotometer, respectively.
frameworks (MOFs) feature the advantages of both hetero-
Solid state cyclic voltammograms were obtained using a
geneous inorganic molecular sieves and homogeneous organic carbon-paste working electrode; a well-ground mixture of each
1
2,13
unimolecules,
and the tenability and flexibility of MOFs bulk sample and carbon paste (graphite and mineral oil) was
allow for the uniform incorporation of photoactive organic dyes set in the channel of a glass tube and connected to a copper
1
4–17
in high densities around the pores,
and the tuning of the wire. A platinum-wire counter electrode and an Ag/AgCl reference
photoelectronic performance of the framework through ligand electrode were utilized. Measurements were conducted using a
1
8,19
engineering.
energy levels of the ground and excited states and the enhance- hexafluorophosphate (0.1 M) at a scan rate of 100 mV s
ment of the visible light harvesting ability of MOFs, continuous Solid and liquid fluorescence spectra were measured on an
To achieve precise control of the shifting three-electrode system in an acetonitrile solution of ammonium
ꢀ1
.
structural modulation of a dye-based ligand scaffold is highly Edinburgh FS920 instrument. The excitation and emission slits
desirable, however, it is still in its infancy. A well-known were both 3 nm wide, and the wavelengths of excitation and
0
00
photoresponsive triphenylamine-based ligand, H TCA (4,4 ,4 - emission were chosen according to each specific case. Time-
3
tricarboxyltriphenylamine, with UV-vis absorption lmax
=
resolved luminescence spectra were measured with an Edinburgh
ca. 350 nm, and a reduction potential of the excited state of FLS920 spectrometer.
2
0
ca. ꢀ2.20 V) has been successfully incorporated into MOFs for
21
Synthetic procedures
photocatalytic organic transformations, and its three aryl moieties
with their C -symmetry provide potential sites for iterative decora-
3
3 2
Synthesis and characterization of ligand H BCTA, 2. Br
tion to achieve continuous modulation of the photoelectronic (1.7 mL, 33.18 mmol) was added dropwise to a solution of
properties. The widely applied modulation method of appending NaOH (3.9 g, 97.51 mmol) in water (16 mL) in an ice bath, the
22
electron-donating groups such as amino/alkyl amino groups
mixture was further stirred for 20 min, then it was transferred
should be excluded since those groups are not compatible with to a constant pressure funnel and added dropwise to a solution of
0
the sulfonyl radical precursor sulfonyl chlorides and the lone pair 1,1 -((((4-acetylphenyl)azanediyl)bis(4,1-phenylene))bis(thiophene-
electrons of the N-containing groups might disturb the photoredox 5,2-diyl))bis(ethan-1-one) (1.61 g, 3.00 mmol) in 1,4-dioxane
cycle. Thus, inserting an electron-rich aromatic moiety like thio- (30 mL). The reaction mixture was heated at 45 1C for 5 h,
phene into the scaffold of H TCA to enlarge the p-system is and after that it was put in an ice bath and an aqueous solution
3
expected to be more practical. Herein, we report a ligand engineer- of saturated hydroxylamine HCl was added to deoxidize exces-
ing strategy of iterative thiophene insertion for continuous modu- sive sodium hypobromite. The solution was acidified with
lation of the photoelectronic properties of triphenylamine-based diluted hydrochloric acid and the precipitated solid was filtered
metal–organic frameworks, with the aim of fine-tuning the photo- and recrystallized from acetic acid to afford the pure product
1
catalytic sulfonylation of alkenes.
(1.48 g, 91%). H NMR (400 MHz, DMSO-d
6
) d 12.32 (br s, 3H),
7
3
7
1
1
.87 (d, J = 8.6 Hz, 2H), 7.75 (d, J = 8.5 Hz, 4H), 7.70 (d, J =
.8 Hz, 2H), 7.53 (d, J = 3.8 Hz, 2H), 7.17 (d, J = 8.5 Hz, 4H) and
1
3
.09 (d, J = 8.6 Hz, 2H). C NMR (101 MHz, DMSO) d 166.8,
Experimental section
Materials and methods
62.8, 150.2, 149.3, 146.4, 134.4, 132.8, 131.0, 128.7, 127.4,
ꢀ1
25.4, 124.4, 124.2 and 121.6. FTIR (KBr pellet; cm ): 3269,
Unless otherwise stated, organic solvents were dried and dis- 2675, 2549, 1677, 1594, 1536, 1508, 1448, 1320, 1275, 1178,
ꢀ
1
tilled prior to use in accordance with standard methods. The 1104, 810 and 752 cm . HRMS (ESI) m/z calcd for C H NO S
6 2
2
9
19
+
other commercially available chemicals were of reagent-grade [M ] 541.0654, found 541.0658.
quality, and were used as received. TLC was carried out on SiO Mono[4-(5-carboxy-2-thienyl)phenyl]bis(4-carboxyphenyl) amine
MCTA, 1) and tris[4-(5-carboxy-2-thienyl)phenyl] amine (H TCTA,
BCTA, 2.
2
(
silica gel 60 F254, Merck), and the spots were located with UV (H
3
3
2
3
light. Flash chromatography was carried out on SiO
2
(silica gel, 3) were prepared using similar procedures to that of H
See the ESI† section for details.
Synthesis of Zn–BCTA. A mixture of H BCTA (8.1 mg,
3
200–300 mesh).
NMR spectra were measured on Bruker Avance 500 WB and
3
Bruker Avance 400 WB spectrometers, and chemical shifts were 0.015 mmol) and Zn(NO ) ꢁ6H O (29.7 mg, 0.1 mmol) was
3
2
2
recorded in parts per million (ppm, d). Elemental analysis was dissolved in 3 mL of DMF in a Teflon-lined steel autoclave
conducted on a Vario EL III elemental analyzer. High resolution and then HOAc (10 mL) was added. The autoclave was sealed
mass spectra were recorded on a LC/Q-TOF mass spectrometer and the resulting mixture was kept in an oven at 100 1C for
(Micromass, England) equipped with a Z-spray ionization source. 3 days. The block yellow crystals for X-ray structural analysis
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1
were collected by filtration. Yield: 65%. H NMR (400 MHz, 2,4,6-collidine (0.50 mmol) by syringe, the reaction mixture was
DMSO-d /DCl): d 7.83 (d, J = 8.8 Hz, 2H), 7.70 (d, J = 9.0 Hz, 4H), stirred and illuminated with visible light using a Xe light with
.49 (d, J = 3.9 Hz, 2H), 7.45 (d, J = 4.3 Hz, 2H), 7.12 (d, J = 8.7 Hz, an ultraviolet cut-off filter (>400 nm) for 24 h. The photocatalyst
H) and 7.06–7.02 (d, J = 8.8 Hz, 2H). Anal. calcd for (Zn O)(C H - was recovered by centrifugation and filtration, and the filtrate
6
7
4
4
29 16
NO S ) (C H NO) (dried vacuum %): C, 51.22; H, 3.09; N, 3.73; S, was concentrated under reduced pressure. The product was
6
2 2
3
7
2
8.55; Found: C, 51.08; H, 3.20; N, 3.87; S, 8.38. IR (KBr): 3036, 1652, isolated by flash chromatography on silica gel from the crude
1
596, 1558, 1534, 1505, 1446, 1395, 1319, 1288, 1273, 1181, 1110, mixture.
ꢀ
1
1044, 1014, 960, 839 and 815 cm
.
For the reactions catalysed by free ligands, the specified
amounts of corresponding photocatalysts were used instead of
Zn–BCTA.
Single-crystal X-ray crystallography
A single crystal of Zn–BCTA with suitable dimensions was
selected and placed in a glass tube filled with the mother liquid
for data collection. The intensity data for the crystal were
collected at 220 K on a Bruker SMART APEX diffractometer
equipped with a CCD area detector and a Mo Ka (l = 0.71073 Å)
Results and discussion
Different numbers of thiophene moieties were inserted into
the scaffold of H TCA, as demonstrated in Scheme 1b, then the
3
2
4
radiation source. The data integration and reduction were
triphenylamine-based ligands decorated with different numbers
of thiophenes were subjected to examination of their photoelec-
tronic properties. As shown in Fig. 1a, the UV-visible absorption
2
5
processed using SAINT software. The structure was solved
2
using direct methods and refined on F with the full-matrix
2
6
least-squares method using the program SHELXL-2017. One of
the carbon atoms in coordinated DMF solvent was disordered
into two parts with the site occupied factor of each part refined
with a free variable value. Except for the disordered carbon atom,
all of the non-hydrogen atoms were refined with anisotropic
thermal displacement coefficients. The hydrogen atoms were
fixed geometrically at calculated positions and allowed to ride on
the parent non-hydrogen atoms. To assist the refinements,
thermal parameters on adjacent atoms of several benzene and
thiophene rings were restrained to be similar. The SQUEEZE
spectra of H
carboxyphenyl)amine, 1), H
phenyl](4-carboxyphenyl)amine, 2), and H TCTA (tris[4-(5-carboxy-
3
MCTA (mono[4-(5-carboxy-2-thienyl)phenyl]bis(4-
3
BCTA (bis[4-(5-carboxy-2-thienyl)-
3
2-thienyl)phenyl]amine, 3) showed sequentially increasing
amounts of red-shift in comparison with that of H TCA, and
3
ligands 2 and 3 bearing bis- and tris-thiophenes exhibited con-
siderable absorptions above 400 nm although their lmax was
still located in the near UV area. The electrochemical measure-
ments of 1 to 3 exhibited redox potentials at 1.12 V, 1.25 V, and
1.36 V (vs. SCE), respectively, assignable to the redox potentials
2
7,28
routine in PLATON
was used to deal with the dissociative
of the radical cation/neutral couples of the substituted triphenyl-
amine moieties (Fig. 1b and e), demonstrating the stronger
oxidative ability of the corresponding radical cations of ligands
solvents within the framework and remove their contribution
to the overall intensity of the data. Crystallographic data of
Zn–BCTA are summarized in Table S1 (ESI†).
Dye uptake experiments
Crystals of Zn–BCTA (27.1 mg, 18.0 mmol) were soaked in a
saturated solution of methylene blue in acetonitrile for 12 h and
the resulting crystals were washed with acetonitrile thoroughly
till the solution became clear, and then briefly dried on filter
paper. The washed samples were digested using minimal
amounts of concentrated aqueous HCl and the resultant clear
solution was diluted to 500 mL with acetonitrile. The same
amount of concentrated aqueous HCl was used when preparing
methylene blue solution in acetonitrile with gradient concen-
trations for the standard curve. The concentration of released
methylene blue was determined by comparing the UV-vis
absorption to a standard curve (Fig. 3a). The amount of
methylene blue taken up by Zn–BCTA was calculated to be
14.1 mg (52%, wt%).
General procedure (GP) for photocatalysed tandem
sulfonylation–cyclisation with Zn–BCTA
Scheme 1 Schematic illustration of ligand decoration for tuning the
photoelectronic properties of the MOF to fulfill the demands for single-
electron reduction of arylsulfonyl chlorides. (a) The possible construction
of a bioactive aryl alkyl sulfone by sulfonyl radical addition onto alkenes
To a pre-dried Pyrex tube equipped with a cooling water system,
Zn–BCTA crystals (0.00625 mmol), the substrate (0.25 mmol),
and TsCl (0.50 mmol) were added. Then the tube was sealed
(
upper) and the electrochemical requirements for the photoreductive for-
mation of sulfonyl radicals and the competitive pathway (lower). (b) Demon-
stration of iterative thiophene insertion for continuous modulation of the
2
with a rubber septum and subjected to three vacuum/N refill
cycles. After adding anhydrous degassed acetonitrile (1 mL) and photoelectronic properties of triphenylamine-based ligands.
1
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bis-thiophene incorporation might distort the coordination geo-
metry of the metal nodes within the MOF, possibly creating
coordination vacancies for fixing substrates in close proximity
to the photocatalytic centres.
The solvothermal reaction of H BCTA and Zn(NO ) ꢁ6H O in
3
3 2
2
DMF at 100 1C for 3 days afforded a new coordination polymer
Zn–BCTA in 65% yield. Elemental analysis (EA) and powder
X-ray diffraction (PXRD) indicated that the bulk sample con-
sisted of a pure phase (Fig. 6b). Thermogravimetric analysis
(
(
TGA) showed an impressive solvent loss of 55 weight percent
wt%) in the temperature range 20 to 190 1C, indicating an open
framework structure (Fig. S5, ESI†). Single-crystal X-ray struc-
tural analysis revealed that this polymer crystallized in the
monoclinic space group C2/c (Table S1, ESI†). The asymmetric
3ꢀ
unit of Zn–BCTA contained a Zn
and two coordinated DMF molecules (Fig. S6, ESI†) and the
framework formula was similar to that of the typical Zn O(L)
L = tritopic ligand) of Zn(II)-MOFs bearing triangular linkers,
4
O cluster, two BCTA moieties
4
3
(
3
3,34
for example, the well-known MOF-177.
In comparison to
Fig. 1 Photoelectronic properties of triphenylamine-based ligands deco-
rated with different numbers of thiophenes. Comparisons of (a) UV-vis
absorptions, (b) cyclic voltammograms (CVs), (c) fluorescence spectra, and
the octahedral Zn O(CO ) cluster of MOF-177 that was com-
4
2 6
2+
2+
posed of four tetrahedrally coordinated Zn ions, three Zn
ions (Zn1, Zn2 and Zn3) of Zn O node of Zn–BCTA also resided in
TCTA, respectively. The inserted arrows of (a) to (d) indicate the shifting tetrahedral environments, each coordinated by three carboxylate
4
(
d) normalized absorption and emission spectra of H
3 3 3
TCA, H MCTA, H BCTA,
and H
3
3ꢀ
directions of the peak signals of the corresponding photoelectronic pro- oxygen atoms from different BCTA moieties and a m
-O atom at
4
2+
perties. (e) Data sheet of the absorption maximum wavelengths (lmax), the
the centre of the cluster. Meanwhile, the fourth Zn (Zn4) ion
0–0
ground state oxidation potentials, the free energy changes (E ) between
adopted an octahedral coordination geometry surrounded by one
m4-O atom, three oxygen atoms of different carboxylates, and
other two carbonyl oxygen atoms from two coordinated DMF
the ground states and the vibrationally related excited states, and the
reduction potentials of the excited states of H
TCTA, respectively.
3 3 3
TCA, H MCTA, H BCTA, and
H
3
2
+
molecules. Each pair of adjacent Zn ions of Zn O were
4
bridged by a carboxylate group of ligand and the central m
bearing more thiophene moieties. Then, the redox potentials of bridge. Within the distorted Zn O tetrahedron around m -O, the
-O to Zn ranged from 1.92 to 2.03 Å, and the
–Zn angles were revealed to range from 108.11 to 112.11
respectively, on the basis of a free energy change (E ) between and deviated slightly from the value for an ideal tetrahedron
4
-O
4
4
+
the excited-state ligand /ligand* couples were calculated to be the distances from m
sequentially decreasing values ꢀ1.86 V, ꢀ1.63 V, and ꢀ1.43 V, Zn–Om
4
4
0–0
29
the ground states and the vibrationally related excited states,
(Fig. 2a). Thus, the torsion of Zn O should be responsible for
4
and the reduction potentials of those excited states seemingly accommodating two additional DMF molecules besides six
fulfilled the demands for photoreducing a series of sulfonyl carboxylates. Moreover, it was believed that the coordinated
chlorides (E1/2 = ꢀ0.44 to ꢀ1.39 V vs. SCE). However, the inter- DMF molecules were replaceable by coordinative organic solvents
35
ligand interactions within the MOFs generally led to variable to expose the Lewis acidic coordination vacancies as the active
degrees of energy loss of the excited states compared to those of sites for the possible fixation and activation of substrates.
the free ligands, making the reduction potentials of the excited
4
The tetranuclear Zn O clusters acted as 6-connected dis-
states of the MOFs not as negative as those of the free ligands, torted octahedral secondary building units (SBUs), and the
3
ꢀ
while red-shifting the maximum absorption peaks in the case of reduced-symmetry tripodal BCTA
ligand rotated the arms
3
0
the MOFs. From consideration of the more and more enlarged with special angles to meet the requirements of the torsional
p-systems from ligands 1 to 3, the p–p interactions between the octahedral geometries of the SBUs (Fig. 2b) and linked three
ligands and the concomitant weakening effect on the photoredu- Zn
4
O clusters through the carboxylate groups to generate a 3D
31
cing abilities were estimated to be more and more significant,
which would suggest that the MOF constructed from H TCTA ca. 22.2 Å (Fig. 2d). Different from the typical pyr, rtl, or
might not be competent at inducing photoreduction of sulfonyl qom topologies of (6,3)-connected MOFs constructed from
framework containing cavities with an opening diameter of
3
6
34
3
3
7
38
32
chlorides with E1/2 at the edge (e.g. ꢀ1.39 V vs. SCE). To balance octahedral Zn
4 2 6 3
O(CO ) clusters and C -symmetric triangular
3
9
the trade-off between visible light harvesting ability and photo- linkers, structural analysis using the TOPOS program indi-
reduction ability of the modified ligands and the prospective cated that Zn–BCTA could be represented as a 3,6-c net and ant
2
4
2
8
MOFs, the bis-thiophene appended ligand H
chosen as an ideal candidate owing to its well-balanced photo- could also be interpreted as the result of the deviation from
electronic properties. We envisioned that the breaking of the -symmetry of the modified ligand H BCTA. It should be
-symmetry of the triphenylamine-based ligand due to this noted that two independent networks are staggered to form a
3
BCTA (2) was topology with the Schl ¨a fli symbol {4 ;6} {4 ;6 ;8 ;10}, which
2
C
3
3
C
3
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Fig. 3 Characterization of the accessible porosity of Zn–BCTA. (a) UV-vis
measurements of methylene blue dye released from Zn–BCTA (blue:
digested sample of Zn–BCTA soaked with MB; black: digested sample
of Zn–BCTA), and (inset) the standard linear relationship between the
absorption and the concentration. (b) Confocal image of a Zn–BCTA
crystal soaked with MB.
the successful uptake of the dye molecules inside the crystals of
4
1
the MOF. The results implied the ability of Zn–BCTA to adsorb
organic substrates within its open channels.
The solid UV-vis absorption spectrum of solid-state Zn–BCTA
exhibited a remarkable red-shift (lmax = 416 nm) and stronger
absorption in the visible light region (400 nm to 500 nm) in
21
3
comparison to that of the H TCA based MOF-150, indicating
the superior visible light harvesting ability for practical appli-
cations (Fig. 4a). Solid-state electrochemical measurements
revealed a redox potential of 1.08 V (vs. SCE) assignable to the
+
Zn–BCTA /Zn–BCTA couple (Fig. 4b). In addition, the redox
+
potential of the excited-state Zn–BCTA /Zn–BCTA* couple was
Fig. 2 Schematic illustration of the crystal structure of Zn–BCTA. The
coordination environments of (a) the node (symmetry code: 1 + x, ꢀy, calculated to be ꢀ1.49 V on the basis of a free energy change
3
ꢀ
0–0
0
.5 + z) and (b) the BCTA bridging ligand of Zn–BCTA. Ball-and-stick
(
E
) between the ground state and the vibrationally related
diagrams of the cage (c) before and (e) after interpenetration. Topological
presentations of Zn–BCTA (d) before and (f) after interpenetration.
Considering the Zn O SBUs as octahedrons and the BCTA crosslinker as
4
triangles, the topology is ant.
20
excited state of 2.57 eV (Fig. 4d). In comparison to MOF-150,
Zn–BCTA exhibited a much milder but still competent ability to
photoreduce sulfonyl chlorides for the formation of sulfonyl
radicals (ꢀ1.49 V of Zn–BCTA vs. ꢀ2.09 V of MOF-150 ), and
meanwhile avoided the competitive pathways under over-
negative electronic potentials. The quenched luminescence
Fig. 5a) and the substantially decreased fluorescence lifetime
3.32 ns to 1.12 ns) (Fig. 5b) of the Zn–BCTA suspension upon
addition of p-toluenesulfonyl chloride (TsCl) indicated a classical
photoinduced electron transfer (PET) process from the excited
2
1
two-fold interpenetrated 3D framework, with the opening
diameter of the 1D channels reduced to ca. 14.3 Å (Fig. 2f).
The interlocking and interweaving structures were envisioned
to not only stabilize the crystalline solid during photocatalytic
applications, but also to lead to closer proximity between the
possibly fixed substrate and the photocatalytic centre to
enhance the reaction efficiency (the nearest inter- vs. intra-
(
(
3ꢀ
network distances between the nitrogen centre of the BCTA
2
+
moiety and the Zn (Zn4) atom coordinated to DMF are 6.90 Å
and 10.62 Å, respectively) (Fig. 2b, c and e). Moreover, the
interpenetration resulting in denser distribution of chromo-
phores was believed to interfere with the energy levels of the
excited states, providing further modulation of the photo-
3
0
electronic performance of the MOF.
The free volume in fully desolvated Zn–BCTA was estimated
2
8
to be approximately 67.9% using PLATON software. Simulta-
neously, a methylene blue dye uptake experiment with Zn–BCTA
4
0
was used to demonstrate the guest accessibility, and the
experiment yielded a methylene blue (MB) dye uptake amount
of ca. 52% of the catalyst weight, as determined from the UV-vis
spectra (Fig. 3a). Confocal laser scanning microscopy of the
crystals soaked with MB gave a strong fluorescence response
Fig. 4 Characterization of the photoelectronic properties of Zn–BCTA.
(
a) Solid-state UV-vis absorption spectra. (b) CVs. (c) Solid-state fluores-
that can be assigned to the emission of MB (Fig. 3b), confirming cence. (d) Normalized absorption and emission spectra.
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loading amounts of TsCl and collidine revealed the necessity
of using 2.0 equiv. of the radical source and additive base, and
either decreased or increased amounts led to lower yields
(entries 8 and 9). The optimal reaction conditions are shown
in entry 5. The following control experiments demonstrated
that the integrity of the photocatalyst, visible light irradiation,
and an inert gas atmosphere were indispensable factors in this
reaction (entries 10, 11, 14 to 17). No desired product was found
in the presence of 1.2 equiv. of the radical scavenger TEMPO
(2,2,6,6-tetramethyl-1-piperidinyloxy), demonstrating a radical
mechanism (entry 19). On the other hand, a comparison of the
photocatalytic performances of free ligands bearing different
numbers of thiophenes further demonstrated the well-balanced
reactivity of the two thiophene incorporated ligand (entries 11
Fig. 5 Characterization of host–guest behavior between the framework
of Zn–BCTA and the substrate or reagent. (a) Fluorescence spectra of to 13). Removal of MOF solids by hot-filtration after 3 h shut
Zn–BCTA upon the addition of TsCl and the corresponding simulated down the reaction immediately (Table 1, entry 18), which
Stern–Volmer curve (inset). (b) The decreased fluorescence lifetime of the
suggested the heterogeneous nature of this reaction. After the
1
Zn–BCTA suspension upon addition of TsCl. (c) H NMR spectrum of the
reaction, the MOF Zn–BCTA could be easily recovered by simple
filtration, and then reused for at least 3 cycles without any
crystals of 1a@Zn–BCTA (digested in DMSO-d
6
/DCl). The peaks marked
with inverted blue triangles represent the aromatic signals of the encap-
sulated substrate 1a. (d) Comparison of the infrared (IR) spectra of 1a (blue remarkable deterioration in activity (Fig. 6a). The fact that the
line), Zn–BCTA (black line), and 1a@Zn–TCTA (red line). The inserted X-ray powder diffraction (XRD) patterns of the photocatalyst
dashed lines indicate the carbonyl peaks of 1a molecules in the corres-
ponding cases.
samples before and after the reactions did not significantly
change indicated the maintaining of the corresponding crystal-
linity of Zn–BCTA (Fig. 6b).
3ꢀ
state of the BCTA moiety to TsCl, which directly yielded active
Then, we investigated the host–guest interactions between
toluenesulfonyl radicals.
the framework of Zn–BCTA and the substrate by soaking the
moieties widely crystals in an acetonitrile solution containing 1a. H NMR
4
2,43
44,45
1
Isoquinolinedione
and oxindole
exist in natural products and pharmaceuticals and show certain analysis of the digested crystals revealed that Zn–BCTA could
biological activities. However, considering the importance of adsorb approximately 1.0 equivalent of 1a per Zn O SBU (Fig. 5c).
4
aryl alkyl sulfones in pharmaceutical fields, it would be inter- The infrared (IR) spectrum of the 1a-impregnated crystals exhibited
ꢀ
1
ꢀ1
ꢀ1
esting to merge an aryl alkyl sulfone and isoquinolinedione/ a 7 cm red-shift (1701 cm to 1694 cm ) in the carbonyl
oxindole within one motif to offer more possibility for drug stretching vibrations of 1a (Fig. 5d), indicating the fixation of 1a
development. Thus, the photocatalytic preparation of aryl- by coordination with its carbonyl group and activation of its
sulfonyl isoquinolinediones/oxindoles was chosen as the model unsaturated alkene moiety. Simultaneously, it was found that
4
6,47
reaction.
The initial exploration toward this tandem sulfonyl one of the two coordinated DMF molecules was replaced by 1a
radical addition–cyclisation process was carried out with (Fig. 5c), demonstrating the feasibility of acetonitrile assisted
N-methacryloyl-N-methylbenzamide 1a (0.25 mmol, 1.0 equiv.) activation of this MOF and also implying the role of the
as the substrate, TsCl 2a (0.50 mmol, 2.0 equiv.) as the arylsulfonyl coordination vacancy as the active site. However, the quality
radical source, 2,4,6-collidine (0.50 mmol, 2.0 equiv.) as the of the 1a-impregnated crystal 1a@Zn–BCTA was not sufficient
4
9,50
additive base, and with a 2.5 mol% loading amount of Zn–BCTA for single crystal X-ray diffraction analysis.
in 1,2-dichloroethane (DCE) at room temperature, using a Xe This heterogeneous photocatalytic approach was extended
light with an ultraviolet cut-off filter (>400 nm) as the visible to the tandem sulfonylation–cyclisation of a series of N-aryl-
light source. The target compound 3a was obtained only in a methacrylamides and N-aroyl-methacrylamides, leading to the
15% isolated yield after 24 h of reaction (Table 1, entry 1). Low formation of the corresponding arylsulfonyl isoquinolinediones/
conversion was also observed in the cases using other uncoor- oxindoles in moderate to high yields as shown in Table 2. When
dinated solvents like toluene (entry 2). Then, some coordinated substrates bearing meta-substituted aryl moieties were used,
solvents such as tetrahydrofuran (THF), ethyl acetate (EtOAc), only one type of regioisomer was obtained, indicating the high
5
1
and especially acetonitrile (MeCN) were found to be capable of regioselectivities of the reactions (Table 2, 3b and 3f). Stereo-
improving the conversion (entries 3 to 5). According to the selective photoredox catalysis has long been hindered due to
4
8
52–54
report of Jeong and coworkers, it has been concluded that the dearth of stereocontrol strategies in radical processes,
coordinative solvents can enhance the dissociation of the especially in the case of there being no covalent or ionic
5
5
coordinated DMF molecules from the zinc nodes, thus pro- binding sites in the substrates. The possibility of an existing
viding more coordination vacancies for fixing the substrates. carbonyl–zinc node interaction encouraged us to explore the
Then, different loading amounts of the photocatalyst were stereocontrol effect of the local confined environment on this
examined, and 2.5 mol% of Zn–BCTA was proven to be more photocatalytic tandem process. The sulfonylation–cyclisation
practical (entries 5 to 7). Furthermore, screening different of vicinal di-substituted electron-deficient olefin 1g and 1h
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a
Table 1 Optimization of the reaction conditions and control experiments
b
Entry
Catalyst (mol%)
Solvent
DCE
TsCl/collidine (x equiv.)
Yield (%)
1
2
3
4
5
6
7
8
9
Zn–BCTA (2.5)
Zn–BCTA (2.5)
Zn–BCTA (2.5)
Zn–BCTA (2.5)
Zn–BCTA (2.5)
Zn–BCTA (1.25)
Zn–BCTA (5)
2
2
2
2
2
2
2
1.2
3
15
o10
27
48
85
55
86
64
76
PhCH
THF
3
EtOAc
CH CN
3
CH
CH
CH
CH
3
CN
CN
CN
CN
3
3
3
Zn–BCTA (2.5)
Zn–BCTA (2.5)
b
Entry
Variants of the optimal conditions
Zn(NO O (5)
Yield (%)
10
11
12
13
14
15
16
17
18
19
3
)
2
ꢁ6H
2
Trace
39
H
H
H
3
3
3
BCTA (2.5)
MCTA (2.5)
TCTA (2.5)
17
26
45
Zn(NO
3
)
2
ꢁ6H
2
O (5) + H
3
BCTA (2.5)
No photocatalyst
Under darkness
Air conditions
Trace
0
0
51
0
3 h first with Zn–BCTA (2.5), then remove the catalyst
With TEMPO (1.2 equiv.) added
a
Reaction conditions: 1a (0.25 mmol, 1.0 equiv.), p-toluenesulfonyl chloride 2a and additive base 2,4,6-collidine (specified amounts), photocatalyst
b
(specified amount calculated based on the BCTA moiety), degassed solvent (1.0 mL), room temperature (r.t.) and an N
2
atmosphere, 24 h. Isolated
yields.
a
Table 2 Investigations on the substrate scope of the activated alkenes
Fig. 6 (a) Histograms of reusing experiments of the MOF Zn–BCTA.
(
(
b) X-ray powder diffraction (XRD) patterns of freshly prepared Zn–BCTA
middle), the recycled catalyst after 3 runs (up), and its simulated pattern
based on the reported single-crystal data (bottom).
afforded the products in the yields 63% and 61%, respectively,
3
2
with diastereoselectivities higher than 95 : 5 (3g and 3h).
In comparison, the use of a free ligand as the catalyst in a
homogeneous control reaction gave diminished conversion and
inferior diastereoselectivity (Table 2, 3g).
As expected, a series of arylsulfonyl chlorides, for which the
reduction potentials of the excited states covered a wide range,
were proven to be applicable to this heterogeneous photo-
catalytic approach (Table 3). Arylsulfonyl chlorides bearing
electron-donating (MeO-, 2e) or electron-withdrawing (F-, 2c;
Br-, 2d) substituents on the benzene ring could be transformed
into the corresponding desired products efficiently in good
yields (Table 3, 3j to 3l). Other carbocycle and heterocycle
a
Optimal reaction conditions as shown in Table 1, entry 5; isolated
yields. Diastereoselectivity (diastereomeric ratio, d.r.) was determined
through H NMR of the crude products. Free ligand H BCTA was used
3
1
b
bearing arylsulfonyl radical sources like naphthalene-1-sulfonyl as the photocatalyst.
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a
Table 3 Investigations on the substrate scope of the sulfonyl chlorides
carbonyl group and drawing the substrate closer towards the
spatially neighboring photocatalytic centre. Upon irradiation
with visible light, the excited state of the BCTA moiety trans-
3
ꢀ
ferred an electron to the arylsulfonyl chloride for the generation
5
6
of an arylsulfonyl radical (Scheme 2, b). The addition of the
5
7
sulfonyl radical to the carbon–carbon double bond of 1a and
5
8,59
the following intramolecular addition
to the aromatic ring
furnished the cyclic radical intermediate A (Scheme 2, c). After
oxidation by the cation radical form of the nitrogen centre of
3ꢀ
BCTA , intermediate A was transformed into the corresponding
cation B with the regeneration of the photocatalytic centre
(Scheme 2, d). Finally, the deprotonation of B assisted by the
additive base yielded the desired product 3a. It was speculated that
the spatial proximity of the docked substrate toward the photo-
catalytic centre within this interpenetrated framework facilitated
60
the photoredox process, and the crowded local environment
61–64
might impose a spatial confined effect
onto the fixed substrate
a
Optimal reaction conditions as shown in Table 1, entry 5; isolated
for premium diastereocontrol of the reaction.
yields.
Conclusions
chloride (2f) and thiophene-2-sulfonyl chloride (2g) afforded
the target compounds in the yields 84% and 93%, respectively
In this study, a ligand engineering strategy of inserting thio-
phenes into the scaffold of triphenylamine-based ligands was
developed as a powerful tool for red-shifting the UV-vis absorp-
tion peaks, improving the oxidation potentials of the ground
states, and decreasing the reduction potentials of the excited
states of the ligands and the MOFs. Thus, it allowed one to
precisely and continuously tune the photoelectronic properties
of MOF-based photocatalysts according to specific demands. In
the case study of the photocatalytic preparation of bio-interesting
sulfonyl isoquinolinediones/oxindoles, a ligand decorated with
two thiophenes was chosen as the candidate, resulting in a good
visible light-harvesting ability and a suitable excited-state redox
potential of the corresponding MOF Zn–BCTA that fulfilled the
requirements for photoineduced generation of sulfonyl radicals
while circumventing the competitive over-reduction of sulfonyl
(3m and 3n). More noticeably, the aliphatic derivative ethane-
sulfonyl chloride (2h) for which the oxidation potential should
be comparable to that of methanesulfonyl chloride (MsCl,
E
1/2 = ꢀ1.39 V), was also amenable to this photocatalytic system
to give the corresponding product 3o in 87% yield.
Based on the above results and previous related reports,
a possible reaction mechanism was proposed as outlined in
Scheme 2. First, MeCN assisted the dissociation of the coordi-
nated DMF on the nodes to create coordination vacancies
(Scheme 2, a) for fixing the substrate by coordinating to the
3
moieties. The deviation from C -symmetry of the modified ligand
forged a distortion of the Zn O nodes to provide potential sites
4
for harnessing substrates. Moreover, the interpenetrated frame-
works facilitated docking of the substrate in closer proximity to
the photoredox-active centre. As a result, this heterogeneous
photocatalytic system exhibited superior efficiency and diastereo-
control to its homogeneous counterpart using a free ligand.
Currently, further extension of this ligand engineering strategy
is being extensively studied in our lab for designing MOF-based
photocatalytic systems from the perspective of pharmaceutical/
fine chemical orientation.
Author contributions
T. Zhang and C. Duan conceived the project. T. Zhang designed
the experiments and wrote the manuscript. T. Zhang, Y. Shi,
S. Zhang, and C. Jia performed the experiments. Y. Shi and C. He
solved and refined the X-ray crystal structures. All the authors
Scheme 2 Schematic illustration of the mechanistic view of this reaction. discussed the results and commented on the manuscript.
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Conflicts of interest
23 X. Guo, L. Zeng, Z. Wang, T. Zhang, C. He and C. Duan,
RSC Adv., 2017, 7, 52907–52913.
There are no conflicts of interest to declare.
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5 SAINT, Data reduction software (version 6.45), Bruker AXS Inc.,
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6 G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem.,
Acknowledgements
We gratefully acknowledge the financial support for this work
from the National Natural Science Foundation of China (21402020,
U1608224, and 21531001), the Fundamental Research Funds for
the Central Universities (DUT16RC(4)09 and DUT18LK50), and the
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