Iridium complex-linked porous organic polymers for recyclable, broad-scope photocatalysis of organic transformations

Article abstract of DOI:10.1039/c9gc03688a

Two rigid porous organic polymers (Ir-POP-1 and Ir-POP-2) were prepared from the coupling reactions of tetraphenylmethane tetraborate and two [Ir(ppy)2(dtbbpy)]⁺-based bitopic linkers and applied as heterogeneous visible-light photocatalysts for organic transformations. Ir-POP-2 was found to exhibit high catalytic activity for a wide range of organic reactions, which include Smiles-Truce rearrangement of alkyliodides, desulfurative conjugate addition to Michael acceptors, and aerobic oxidations of sulfides and arylboronic acids. For all the transformations, Ir-POP-2 could achieve heterogeneous photocatalytic efficiency that rivals that of the homogeneous prototype iridium complexes. This remarkably high photocatalytic performance has been attributed to the large pore size of the conjugated backbone. The new heterogeneous photocatalyst was also highly stable to achieve good recyclability for all the studied reactions and could be reused eight to nineteen times.

Full text of DOI:10.1039/c9gc03688a

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PAPER
Iridium complex-linked porous organic polymers
for recyclable, broad-scope photocatalysis of
organic transformations†
Cite this: Green Chem., 2020, 22,
136
Zi-Yue Xu, Yi Luo, Dan-Wei Zhang, * Hui Wang, Xing-Wen Sun * and
Zhan-Ting Li
*
Two rigid porous organic polymers (Ir-POP-1 and Ir-POP-2) were prepared from the coupling reactions
of tetraphenylmethane tetraborate and two [Ir(ppy)2(dtbbpy)]+-based bitopic linkers and applied as
heterogeneous visible-light photocatalysts for organic transformations. Ir-POP-2 was found to exhibit
high catalytic activity for a wide range of organic reactions, which include Smiles–Truce rearrangement of
alkyliodides, desulfurative conjugate addition to Michael acceptors, and aerobic oxidations of sulfides and
arylboronic acids. For all the transformations, Ir-POP-2 could achieve heterogeneous photocatalytic
efficiency that rivals that of the homogeneous prototype iridium complexes. This remarkably high photo-
catalytic performance has been attributed to the large pore size of the conjugated backbone. The new
heterogeneous photocatalyst was also highly stable to achieve good recyclability for all the studied reac-
tions and could be reused eight to nineteen times.
Received 27th October 2019,
Accepted 22nd November 2019
DOI: 10.1039/c9gc03688a
rsc.li/greenchem
Sonogashira cross-coupling reaction to introduce Ir(ppy2)
(bpy)+ complexes into a cross-linked polymer for hetero-
Introduction
The past decade has witnessed the rapid development of geneous photocatalysis of the aza-Henry reaction with four
visible light photoredox catalysis of organic reactions due to cycles of the catalyst.4a Recently, Han and co-workers demon-
its mild reaction conditions and the potential of harvesting strated that [Ir(ppy)2(bpy)]+-cross-linked polycarbazole net-
solar energy for chemical transformations.1 In a general works could work as heterogeneous photocatalysts for the oxi-
working model, this strategy utilizes metal complexes and dation of sulfides, hydroxylation of arylboronic acids, and
organic dyes to engage in single-electron transfer processes cross-dehydrogenative coupling reactions.4b For sulfide oxi-
with organic substrates upon photoexcitation with visible dation, the catalyst realizes five cycles of high conversion.
light. The most widely used metal complex photocatalysts are Given that loading precious-metal catalysts to solid supports
polypyridyl complexes of ruthenium and iridium.2 In compari- represents one of the most efficient strategies for their
son with ruthenium complexes, a library of ligands with dis- green and cost-lowering applications,5 it is highly valuable to
crete electronic properties has been developed for iridium,3 develop new iridium complex-incorporated recyclable photo-
which allows for the design of tunable iridium-based photo- catalysts to expand their catalysis for important organic
redox catalysts. Most of the reported studies have focused on transformations.
their applications in homogeneous organic transformations.
In the past decade, conjugated porous polymers and related
In contrast, examples of heterogeneous reactions catalyzed by structures have emerged as promising platforms for the con-
iridium complexes immobilized to rigid porous polymers or struction
of
recyclable
heterogeneous
catalysts.6
frameworks are very rare.4 In 2011, Lin and co-workers Tetrakisphenylmethane and its derivatives are synthetically
reported the first example which involved the utilization of the accessible and easily allow cross-coupling polymerization and
exhibit permanent backbone porosity. Incorporation of multi-
pyridyl-iridium complexes into tetrakisphenyl-methane-based
polymers through conjugated cross-linkers would lead to
highly stable heterogeneous photocatalysts that maintain the
porosity of the backbones and enable the dispersion of the
catalytic sites.7 Herein we present the synthesis of two highly
stable [Ir(ppy)2(dtbbpy)]+ (ppy: phenylpyridine, dtbbpy: 4,4′-di
(tert-butyl)-2,2′-bipyridine)-linked porous organic polymers,
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Fudan University, 2205 Songhu Lu, Shanghai 200438, China.
E-mail: zhangdw@fudan.edu.cn, sunxingwen@fudan.edu.cn, ztli@fudan.edu.cn
†Electronic supplementary information (ESI) available: Synthesis and character-
ization, TGA, FT-IR, SEM, PXRD, EDS, CV, fluorescence and recycling experi-
ments (images, spectra and tables). CCDC 1961159–1961161. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c9gc03688a
136 | Green Chem., 2020, 22, 136–143
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Ir-POP-1 and Ir-POP-2, from Pd-catalyzed coupling reactions of
Results and discussion
a tetrakisphenyl-methane borate and two [Ir(ppy)2(dtbbpy)]+-
derived bromides. We demonstrate that the iridium com- The synthetic routes for Ir-POP-1 and Ir-POP-2 are shown in
plexes in the new porous coordination polymers can be Scheme 1. The dichloro-bridged iridium(III) dimers 2a and 2b
applied as heterogeneous photoredox catalysts for four dis- were first prepared from the reaction of 1a or 1b with iridium
crete organic transformations with previously unattainable chloride in methoxyethanol and water. The two dimers were
recyclability.
then treated with 3 in dichloromethane and methanol to
Scheme 1 The synthesis of polymers Ir-POP-1 and Ir-POP-2 and complexes Ir-Ph-1 and Ir-Ph-2.
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Fig. 1 The crystal structure of complexes (a) 4a, (b) 4b, and (c) Ir-Ph-2
(CCDC deposit no. 1961159–1961161†).
afford [Ir(ppy)2(dtbbpy)]+ complexes 4a and 4b. Finally, the
two iridium complexes were coupled with tetraborate 5 in
dioxane and water to produce Ir-POP-1 and Ir-POP-2 after
being treated with hot water and organic solvents to remove
soluble species. Starting from 6a and 6b, iridium complexes
Ir-Ph-1 and Ir-Ph-2 were also prepared as controls. Single crys-
tals suitable for X-ray diffraction analysis were obtained for
complexes 4a, 4b and Ir-Ph-2 by slow evaporation of their
solution in a suitable solvent. All their crystal structures
showed that the two 2-phenyl-substituted pyridine ligands
were oriented in the opposite direction of the octahedral
complex core (Fig. 1). Thus, complexes 4a and 4b resembled
rod-like 4,4′-dibromobiphenyl or longer analogues in their
coupling reactions with 5 to form rigid porous polymers
Ir-POP-1 and Ir-POP-2. Both polymers were insoluble in
common solvents including water, DMF, DMSO, acetonitrile
and acetone, whereas the controls Ir-Ph-1 and Ir-Ph-2 were
soluble in the above organic solvents. Thermogravimetric
Fig. 2 (a) The XPS survey spectra and (b) the Ir 4f spectra of polymers
Ir-POP-1 and Ir-POP-2.
analyses revealed that the two polymers were stable up to (Ir 4f5/2: 64.8 eV and Ir 4f7/2: 62.0 eV), which indicated the +3
350 °C (10% weight loss) (Fig. S1, ESI†).
oxidation state of iridium atoms (Fig. 2b).4b,10
Fourier transform infrared spectroscopy of polymers
The permanent porosities of Ir-POP-1 and Ir-POP-2 were
Ir-POP-1 and Ir-POP-2 showed the absence of the C–B (around investigated by nitrogen sorption measurement at 77 K. Their
1020 cm−1) or B–O (around 1320 cm−1) vibration absorption of Brunauer–Emmett–Teller (BET) surface areas were determined
the borate group8 and the C–Br (around 1068 cm−1) vibration to be 29 and 124 m2 g−1, respectively (Fig. 3). The value of Ir-
absorption of 4a or 4b 8b (Fig. S2 and S3, ESI†), indicating that POP-2 was significantly larger than that of Ir-POP-1, which was
the precursors were consumed completely in the coupling consistent with the elongated linkers of its backbone and
reactions. Scanning electron microscopy (SEM) images and suggested that Ir-POP-2 might exhibit higher photocatalytic
powder X-ray diffraction (PXRD) patterns indicated that Ir- activity.
POP-1 existed as amorphous solids (Fig. S4 and S5, ESI†),
With the two highly stable porous Ir-complex-incorporated
whereas Ir-POP-2 featured spherical aggregates with sizes polymers in hand, we investigated their potential for hetero-
around 1 μm (Fig. S4 and S5, ESI†). Energy dispersive X-ray geneous visible light photocatalysis of organic transform-
spectroscopy (EDS) confirmed the existence of the C, N, F, P ations. The intramolecular Smiles–Truce rearrangement reac-
and Ir elements (Fig. S6 and S7, ESI†). X-ray photoelectron tion of aryl amines was first studied (Table 1). Stephenson
spectroscopy (XPS) further confirmed the composition of the et al. recently reported that this reaction can be realized homo-
above elements (Fig. 2a). The oxygen peak may be attributed to geneously in acetonitrile through visible light mediated photo-
the partial oxidation of iridium during oxidative polymeriz- catalysis with the prototype iridium complex [Ir(ppy)2(dtbbpy)]
ation and the coordinated solvent molecules.9 The local spec- PF6.11 The product is a useful substrate for the synthesis of tetra-
trum of the Ir 4f peak showed a division of two emission peaks hydrothienoazepinone, which has received significant atten-
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realize quantitative transformation through homogeneous
photocatalysis (entries 1 and 2, Table 1) in 1.5 hours. This reac-
tion time was substantially shorter than that (23 hours)
required by the prototype complex [Ir(ppy)2(dtbbpy)]PF6 for
quantitative conversion of 8a into 9a, suggesting that the two
aryl units of Ir-Ph-1 and Ir-Ph-2 increased their catalytic
activity. Insoluble IR-POP-2, which contained an identical
molar amount of the Ir complex, catalyzed the reaction quanti-
tatively in a heterogeneous manner in 24 hours (entry 4,
Table 1), which was very close to that required by the prototype
complex [Ir(ppy)2(dtbbpy)]PF6, indicating that the pores of
Ir-POP-2 are large enough to avoid significant obstruction by the
backbone for the catalytic process. Under the same conditions,
the reaction catalyzed by Ir-POP-1 afforded 9a in 52% yield
(entry 3, Table 1), which may be attributed to its smaller poro-
sity that did not allow free access of the substrate molecules to
the Ir complexes in the polymer. The Ir catalyst was indispens-
able for the reaction, and decreasing the loading of the Ir
complex (0.5 mol%) also caused significant reduction of the
reaction yield (entries 5 and 6, Table 1). With the use of a
smaller amount of DIPEA or a light source of lower power, the
yield decreased significantly (entries 7–10, Table 1). The reac-
tion could occur in other organic solvents (entries 11–15,
Table 1), including N,N-dimethylacetamide (DMF), N,N-di-
methylacetamide (DMAC), dimethyl sulfoxide (DMSO), acetone
or N,N′-dimethylpropyleneurea (DMPU). However, the yield
was generally notably lower than that in MeCN. With NEt3,
tetramethylethylenediamine (TMEDA), Hantzsch ester (5.0) or
1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) as a base, the reac-
tion also afforded a lower yield of 9a or even did not occur
(entries 16–19, Table 1).
After 6 hours and with Ir-POP-2 (1.0 mol%) as the catalyst,
the reaction of 8a afforded 9a in 56% yield. After seven cycles,
9a was still formed in 50% yield (Fig. S9, ESI†). Inductively
coupled plasma-optical emission spectroscopy (ICP-OES)
experiment revealed that Ir-POP-2 had a loading of 73.9 wt%
for [Ir(ppy)2(dtbbpy)]+ catalytic center, which was very close to
the calculated value of 75.0 (wt%). After recycling, the Ir
complex only had ≤0.4% leaching. These results clearly sup-
ported the high stability of the new heterogeneous catalyst.
The above optimized conditions were then applied for a
variety of structurally related starting materials (Scheme 2). It
was found that tosyl (Ts), trifluorotosyl (TsF3, for 9b and 9g)
and tert-butyloxycarbonyl (Boc, for 9c) all could be used as a
protecting group (PG) for this transformation, but tosyl was
the most suitable. Both (CH2)3 and (CH2)4 linkers could
achieve good to excellent reaction yields, and the substrates
bearing a –(CH2)4– group at the 4- and 5-positions of the thio-
phene ring also gave high yields of the corresponding products
(9d and 9f). When the positions of the two substituents were
Fig. 3 N2 adsorption and desorption isotherms of Ir-POP-1 and
Ir-POP-2 at 77 K.
Table 1 Optimization of the photocatalytic Smiles–Truce rearrange-
ment of alkyliodide 8a a
Entry Catalyst [mol%] Base [equiv.]
Solvent
Yieldb [%]
1c
2c
3
4
5
6
7
8
Ir-Ph-1 (1.0)
Ir-Ph-2 (1.0)
Ir-POP-1 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (0.5)
—
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
Ir-POP-2 (1.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (3.0)
—
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
DIPEA (5.0)
NEt3 (5.0)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
DMAC
DMSO
Acetone
DMPU
MeCN
MeCN
>99
>99
52
>99
57
n.d.
11
n.d.
9
n.d.
65
59
80
22
20
68
26
40
n.d.
n.d.
9d
10e
11
12
13
14
15
16
17
18
19
20f
TMEDA (5.0)
Hantzsch ester (5.0) MeCN
DBU (5.0)
DIPEA (5.0)
MeCN
MeCN
a Standard conditions: 8a (48 mg, 0.1 mmol, 1.0 equiv.), Ir photo-
catalyst (1 μmol, 1.0 mol%), base (0.5 mmol, 5.0 equiv.), two 34 W blue
LED lamps, 25 °C. b Yield determined by GC using dodecane as the
internal standard. c Reaction time: 1.5 h. d Using one 23 W compact
fluorescent lamp (CFL) as the light source. e No light source. f 2,2,6,6-
Tetramethylpiperidinyl-1-oxide (TEMPO, 1.0 equiv.) was added as a
free radical scavenger.
tion in drug development.11a We first studied the rearrange- exchanged, the corresponding starting materials still under-
ment reaction of model molecule 8a to produce 9a, using the went the rearrangement reaction to afford the corresponding
reported reaction conditions with Ir-POP-1, Ir-POP-2, Ir-Ph-1 or products (9h and 9i) in high yields. The anisole derivative also
Ir-Ph-2 as the photocatalyst. Under standard conditions, both underwent a similar rearrangement. However, the yield of the
control complexes Ir-Ph-1 (E1/2(M/M−) = −1.50 V vs. SCE) and reaction was considerably lowered (9j). In contrast, the reaction
Ir-Ph-2 (E1/2(M/M−) = −1.53 V vs. SCE) (Fig. S8, ESI†) could of the cyclopentene and cyclohexene-derived substrates
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Scheme 2 Substrate scope for the Ir-POP-2-catalyzed rearrangement
of alkyliodides 8a–m to form the related alkylamines 9a–m. The yield
represents the isolated one, and the value in the bracket represents the
GC yield. Standard conditions:
8 (0.1 mmol, 1.0 equiv.), Ir-POP-2
(1 μmol, 1.0 mol%), DIPEA (85 μL, 0.5 mmol, 5.0 equiv.), 25 °C.
afforded the corresponding products (9k–m) in good to excel-
lent yields. The polymer Ir-POP-1 could catalyze all the above
reactions. However, the yields were considerably lower
(Fig. S10, ESI†). The recyclability of Ir-POP-2 for the reaction of
8a and 8d was further investigated and 90% conversion was
achieved after eight or five cycles, respectively (Fig. 4a and b).
Reducing the amount of Ir-POP-2 to 0.5 mol% (entry 5,
Table 1) led to lowered yields, but the catalyst could exhibit
good recyclability (five times, in 57%, 56%, 57%, 54%, and
53% yields), reflecting the high stability of the polymeric
catalyst.
With the addition of 1.0 equiv. of TEMPO as a radical trap-
ping agent,12 the reaction was inhibited completely (entry 20,
Table 1). Stern–Volmer quenching experiments indicated that
DIPEA rather than 8a was the appropriate quencher for the
photosensitizer (Fig. S11, ESI†).13 Cyclic voltammetry (CV)
experiments revealed that the redox potential of the Ir-complex
species (E1/2(IrIII/IrII) = −1.53 V vs. SCE for Ir-Ph-2 and E1/2(IrIII/
IrII) = −1.50 V vs. SCE for Ir-Ph-1) matched well with that of 8a
(E1/2 = −1.26 V vs. SCE) (Fig. S8, ESI†). Because Ir-POP-1 and Ir-
Fig. 4 Recyclability of Ir-POP-2 in the reactions for the generation of
(a) 9a, (b) 9d, (c) 12a, (d) 14a and 15a, and (e) 17a. The catalysts were
recycled from the reaction mixture by centrifugation, washed with
acetonitrile and dichloromethane and dried, and reused in a fresh reac-
tion solution.
POP-2 contained the Ir-Ph-1 and Ir-Ph-2 units, respectively, all (Fig. S12a, ESI†), while a leaching experiment supported the
the above results supported that the reaction mediated by Ir- heterogeneity of the procedure (Fig. S12b and S13, ESI†).14
POP-1 and Ir-POP-2 proceeded also through a radical mecha-
Catalytic activities of polymers Ir-POP-1 and Ir-POP-2 were
nism as reported previously for the homogeneous photo- also investigated for intermolecular reactions. Qin et al.
catalytic reaction.11a On–off experiments confirmed that a con- recently reported the visible-light photoredox catalysis for the
tinuous light source was indispensable for the reaction conjugate addition reaction between N-benzoyl alkylsulfina-
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Table 2 Photocatalytic oxidation of sulfides catalyzed by Ir-POP-2 a
mides and Michael acceptors in the presence of
[Ir(ppy)2(dtbbpy)]PF6,15 which provides a new synthetic utility
of N-acyl alkylsulfinamides as alkyl radical precursors. We first
conducted the reaction of 10a and 11a in the presence of Ir-
Ph-1, Ir-Ph-2, Ir-POP-1 and Ir-POP-2. The heterogeneous photo-
catalytic activity of Ir-POP-2, with an identical molar amount of
the iridium complex, was comparable to that of homogeneous
Ir-Ph-1 or Ir-Ph-2, whereas the photocatalytic activity of
Ir-POP-1 was considerably lower (Table S1, ESI†). Reaction con-
ditions were then screened with Ir-POP-2 as the heterogeneous
photocatalyst (Tables S2 and S3, ESI†), and the optimized reac-
tion conditions were applied for alkyl sulfinamides 10a and
10b and a variety of Michael acceptors 11a–f to form the
corresponding products 12a–l (Scheme 3). The reaction toler-
ated different kinds of Michael acceptors including 2-methyl-
enemalonate diesters, acrylic acids, acrylates and α- and
β-methylene butyrolactones. In most cases, radical addition
products were obtained in good to excellent yields. The recycl-
ability of Ir-POP-2 for the photocatalytic reaction of 10a and
Entry R1
R2
Time [h] Conversionb [%] Selectivity [14 : 15]c
1
2
3
4
H
Me 10
>99
94
95
96
95
87
<1
<1
94 : 5
92 : 2
90 : 5
90 : 6
91 : 4
94 : 3
—
OMe Me 20
Me
Cl
Br
H
H
H
Me 18
Me 20
Me 10
Bn 10
Me 25
Me 25
5
6
7d
8e
—
a Standard conditions: 13 (0.5 mmol, 1.0 equiv.), Ir-POP-2 (5 μmol,
1.0 mol%), air, two 34 W blue LED lamps, 25 °C. b Determined by 1H
NMR. c Determined by 1H NMR. d No light source. e No photocatalyst.
11a to produce 12a was further investigated. After seven cycles, (Fig. S15a, ESI†), whereas a leaching experiment supported
12a still could be obtained in 91% yield (Fig. 4c). Stern–Volmer that the reaction was catalyzed heterogeneously by the in-
quenching experiments indicated that the deprotonated sulfo- soluble polymer catalyst, rather than the possibly available
namide 10a could be oxidized by polymers Ir-POP-1 and trace amount of iridium species dissolved in the solvent
Ir-POP-2 to generate tert-butyl radicals which subsequently (Fig. S15b and S16, ESI†).
coupled with 11a to afford 12a (Fig. S14, ESI†), as reported for
Given the remarkably high activity of Ir-POP-2 as a hetero-
the homogeneous reactions.15 An on–off experiment con- geneous photocatalyst, its further application for heterogeneous
firmed the indispensability of continuous irradiation oxidation of sulfides was also performed (Table 2). All the
studied substrates 13a–f, which bear an electron-donating group
(EDG) such as the methoxy or methyl group or an EWG such as
chlorine or bromine, were transformed into the corresponding
sulfoxides 14a–f in excellent yields, with high selectivity over the
corresponding sulfone products 15a–f. Again, light irradiation
and the photocatalyst were indispensable for all the transform-
ations (entries 7 and 8, Table 2). The recyclability of Ir-POP-2
was studied for the oxidation of 13a. After six cycles, 91% con-
version of the substrate was still realized (Fig. 4d) and the cata-
lyst could be recycled unprecedently 9 times.4b
The aerobic oxidation of arylboronic acids represents a
mild approach for the preparation of phenols.16 Recently, Han
et al. reported that this reaction could be conducted hetero-
geneously with iridium complex-based conjugated polymers as
photocatalysts.4b However, the recyclability of the polymeric
catalysts was not demonstrated. We found that, in the presence
of DIPEA as the sacrificial electron donor, Ir-POP-2 could
efficiently catalyze this transformation in acetonitrile.
Phenylboronic acid and the derivatives 16a–e that bear an EDG
or EWG all underwent quantitative depletion to afford the
phenols in excellent yields (Scheme 4), although a recent
report showed that the introduction of an EDG group into the
benzene ring would notably decrease the transformation.4b
Scheme 3 Substrate scope for the Ir-POP-2-catalyzed conjugate Moreover, the reaction of 2-naphthylboronic acid 16f also gave
addition between N-benzoyl alkylsulfinamides 10a and b and Michael
acceptors 11a–f to form 12a–l. The yield represents the isolated one
and the value in the bracket represents the GC yield. EWG = electron-
withdrawing group. Typical conditions: 10 (0.36 mmol, 1.8 equiv.), 11
rise to naphthalene-2-ol 17f in a very high yield (96%). A study
of the transformation of 16a revealed high recyclability of
Ir-POP-2 (Fig. 4e), which could realize 90% conversion after
thirteen cycles and retained considerable activity after nine-
teen cycles.
(0.20 mmol, 1.0 equiv.), Ir-POP-2 (2 μmol, 1.0 mol%), K2HPO4 (125 mg,
0.72 mmol, 3.6 equiv.), 25 °C.
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Scheme 4 Photocatalytic hydroxylation of arylboronic acids catalyzed
by Ir-POP-2. The yields were determined by 1H NMR. Standard con-
ditions: 16 (0.1 mmol, 1.0 equiv.), Ir-POP-2 (1 μmol, 1.0 mol%), DIPEA
(33 μL, 0.2 mmol, 2.0 equiv.), air, 25 °C.
Conclusions
We have developed a convenient strategy for the preparation of
iridium(III)-complex-connected porous organic polymers. One
of the polymers, Ir-POP-2, exhibits highly efficient hetero-
geneous photocatalytic activity for broad-scope organic trans-
formations including intramolecular Smiles–Truce rearrange-
ment of alkyliodides, desulfurative conjugate addition for the
C–C bond formation and aerobic oxidations of sulfides and
arylboronic acids to form sulphones and phenols. For all the
transformations, the new porous photocatalyst exhibits very
high or previously unattainable recyclability. It is worth noting
that, for all the organic transformations, Ir-POP-2 containing
an identical amount of the iridium complex exhibits hetero-
geneous photocatalytic activities that are comparable with
those of the homogeneous prototype complexes. Thus, we
propose that the new catalytic polymer possesses pores that
are large enough to allow electron transfer between the
embedded, excited iridium complexes and discrete organic
substrates. The work demonstrates that transition metal
complex-embedded porous organic polymers can achieve very
high stability of the backbones and high recyclability, a key
feature for sustainable heterogeneous catalysis. Future studies
will focus on the preparation of transition metal complex-
embedded porous organic polymers that are expected to
achieve large pores and the utility of highly modifiable conju-
gated linkers for loading chiral catalysts for important asym-
metric catalysis.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grants 21921003 and 21890732).
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