Photoaccelerated energy transfer catalysis of the Suzuki-Miyaura coupling through ligand regulation on Ir(iii)-Pd(ii) bimetallic complexes

Article abstract of DOI:10.1039/d0ra08547b

Three bimetallic Ir(iii)-Pd(ii) complexes [Ir(ppy)2(bpm)PdCl2](PF6) (ppy = 2-phenylpyridine, 1), [Ir(dfppy)2(bpm)PdCl2](PF6) (dfppy = (4,6-difluorophenyl)pyridine, 2), and [Ir(pq)2(bpm)PdCl2](PF6) (pq = 2-phenylquinoline, 3) were synthesized by using 2,2′-bipyrimidine (bpm) as a bridging ligand. The influences of the cyclometalated ligand at the Ir(iii) center on the photophysical and electrochemical properties as well as photocatalytic activity for the Suzuki-Miyaura coupling reaction under mild conditions were evaluated. The results revealed that complex 3 enables dramatically accelerating the Suzuki-Miyaura coupling reaction under visible light irradiation at room temperature, due to the effective absorption of visible light and appropriate locus of the excited chromophore. Mechanism studies showed that the chromophore [Ir(pq)2(bpm)] fragment absorbs visible light to produce the triplet excited state centering on the bridging ligand which boosts the formation of electron rich Pd(ii) units and facilitates the oxidative addition step of the catalytic cycle. Simultaneously, the excited chromophore undergoes energy transfer efficiently to the Pd(ii) reaction site to form the excited Pd(ii) species, resulting in enhancement of Pd(ii) reduction steps of the Suzuki-Miyaura coupling reaction and increasing the reactivity of the catalyst. This provides a new strategy for designing photocatalysts for coupling reaction through altering the cyclometalated ligand to modulate the photophysical properties and the cooperation between two metal units.

Full text of DOI:10.1039/d0ra08547b

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Photoaccelerated energy transfer catalysis of the
Suzuki–Miyaura coupling through ligand
Cite this: RSC Adv., 2020, 10, 42874
regulation on Ir(III)–Pd(II) bimetallic complexes†
Su-Yang Yao,
Man-Li Cao* and Xiu-Lian Zhang*
Three bimetallic Ir(III)–Pd(II) complexes [Ir(ppy)
2
(bpm)PdCl
2
](PF
6
)
(ppy
¼
2-phenylpyridine, 1),
[
(
Ir(dfppy)
2
(bpm)PdCl
2
](PF
6
)
(dfppy
¼
2 2 6
(4,6-difluorophenyl)pyridine, 2), and [Ir(pq) (bpm)PdCl ](PF )
0
pq ¼ 2-phenylquinoline, 3) were synthesized by using 2,2 -bipyrimidine (bpm) as a bridging ligand. The
influences of the cyclometalated ligand at the Ir(III) center on the photophysical and electrochemical
properties as well as photocatalytic activity for the Suzuki–Miyaura coupling reaction under mild
conditions were evaluated. The results revealed that complex 3 enables dramatically accelerating the
Suzuki–Miyaura coupling reaction under visible light irradiation at room temperature, due to the
effective absorption of visible light and appropriate locus of the excited chromophore. Mechanism
2
studies showed that the chromophore [Ir(pq) (bpm)] fragment absorbs visible light to produce the triplet
excited state centering on the bridging ligand which boosts the formation of electron rich Pd(II) units and
facilitates the oxidative addition step of the catalytic cycle. Simultaneously, the excited chromophore
undergoes energy transfer efficiently to the Pd(II) reaction site to form the excited Pd(II) species, resulting
in enhancement of Pd(II) reduction steps of the Suzuki–Miyaura coupling reaction and increasing the
reactivity of the catalyst. This provides a new strategy for designing photocatalysts for coupling reaction
through altering the cyclometalated ligand to modulate the photophysical properties and the
cooperation between two metal units.
Received 7th October 2020
Accepted 16th November 2020
DOI: 10.1039/d0ra08547b
rsc.li/rsc-advances
hazardous solvents, and harsh conditions. Thus, the develop-
ment of efficient catalytic system is still an ongoing pursuit.
Introduction
Palladium-catalyzed Suzuki–Miyaura cross-coupling reaction is
Visible-light catalytic reactions have shown great potential in
one of the most powerful protocols for the synthesis of biaryl organic synthesis under mild conditions because of its benign
6
motifs, which are present in the structures of biologically active environmental impact and sustainability. Visible light energy
1
compounds and in advanced materials. Three steps were harvested by photocatalysis enable to cooperate with transition
proposed in the catalytic cycle: oxidative addition, trans- metal efficiently such as the generation of excited-state tradi-
metalation reaction and reductive elimination. Among them, tional transition metal catalysts which exhibit incredible cata-
7
the oxidative addition and/or reductive elimination processes lytic potential and remain unexplored. Therefore, merging
have been regarded as the rate limiting steps in the cycle. chromophore with transition metal catalysts has become
Therefore, signicantly efforts have been devoted during the a powerful strategy for expanding the synthetic application of
past decade to accelerate the reaction via the introduction of visible-light catalysis and has led to the discovery of new reac-
2
electron-rich and bulky ligands, such as phosphine, N- tions, which are not easily accessible in the single catalytic
3
4
5
8
heterocyclic carbenes, palladacycles, and others. During system. Recently, nanostructured Au–Pd catalysts have shown
these processes, the ligand with strong s-donor character high efficient activity for the Suzuki–Miyaura coupling reaction
facilitates the oxidative addition and the bulky ligand enhances under light irradiation due to the electronic heterogeneity at the
9
the reductive elimination of the product. Moreover, the Au–Pd surface and the energy absorbed from the incident light.
10
3 4
electron-rich ligand stabilizes the activated Pd center and Apart from plasmonic metals, mesoporous g-C N , conjugated
11
12
prevents the Pd black formation in the reaction cycle. However, microporous polymers and inorganic semiconductor, well-
most of these protocols suffer from high catalyst loading, established visible light nonmetal photocatalyst, were also
used as photoactive anchoring support for Pd nanoparticles as
a model of a Mott–Schottky accelerated the Suzuki reaction.
Department of Chemistry, Guangdong University of Education, Guangzhou 510303,
Moreover, cyclometalated Ir(III) and polypyridyl Ru(II) complexes
China. E-mail: jsapaper@163.com
have been demonstrated as ideal visible light photosensitizers
†
Electronic supplementary information (ESI) available: UV-visible and emission
due to an intense visible light absorption, long triplet lifetimes
spectra, CV gures and DFT calculation gure. See DOI: 10.1039/d0ra08547b
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Electrospray ionization mass spectra (ESI-MS) were obtained on
1
13
a Thermo LCQ DECA XP mass spectrometer. H and C NMR
spectra were recorded with a Bruker AV 300 or AV 400 spectrometer
using the solvent as an internal standard. Electronic absorption
spectra were obtained on a PERSEE TU-1901 UV-vis spectropho-
tometer. Emission spectra and lifetimes (s) were performed on an
FLS920 uorescence lifetime and steady-state spectrometer at room
r 2
temperature. Quantum yields (F ) in N equilibrated DCM solution
were determined on an FLS920 uorescence spectrophotometer
Scheme 1 Chemical structures of the bimetallic photocatalysts.
using Ru(bpy) (PF ) in N equilibrated dichloromethane (DCM)
3
6 2
2
2
2
and outstanding photostability as well as tuneable photo- solution (F ¼ 0.059) as a standard. Cyclic voltammetric (CV)
r
13
physical properties, and were widely applied in organic experiments were performed with a CH Instruments 730C poten-
6,8
synthesis.
However, the application of these kinds of tiostat. All the measurements were carried out at room temperature
complexes as photocatalysts to accelerate the Suzuki reaction is in acetonitrile solutions with a sample concentration approximately
1
4,15
still scarce.
The photoacceleration reaction mechanism, for 1 mM and using 0.1 M tetrabutylammonium hexauorophosphate
ꢀ1
example through electron or energy transfer (ET or EnT), is still as the supporting electrolyte in 200 mV s scan rate. A 3 mm
16
unclear.
diameter glassy carbon working electrode, a Pt auxiliary electrode,
Inspired by the works of Akita's group in promoted poly- and a Pt-wire reference electrode were used. Oxygen was removed
merization of styrene and vinyl ethers catalyzed by Ru/Ir–Pd from the solutions by bubbling argon for 20 min. All potentials are
17
+
photocatalysts and Yamashita's about enhanced the Suzuki reported relative to the ferrocenium/ferrocene couple (Fc /Fc),
reaction catalyzed by an Ru–Pd bimetallic photocatalyst under which was used as an external standard to calibrate the reference
15
visible-light irradiation, we pay our attention to the Ir–Pd electrode. The photo-oxidation experiments were carried out with
bimetallic photocatalysts. Compared with the Ru(II) polypyridyl a blue light-emitting diode (LED) corn lamp (l ¼ 450–470 nm, 45
complexes, the cyclometalated Ir(III) complexes feature high W) obtained from Shenzhen Taoyuan Technology Co., Ltd as a light
intersystem-crossing (ISC) efficiency, high excited energy and source. A 100 W UV lamp (l ¼ 365 nm) was obtained Guangzhou
13
outstanding photostability. The photocatalysts consisting of Xingchuang Technology Co., Ltd. A green light-emitting diode
a cyclometalated Ir(III) unit as a chromophore and a Pd(II) unit (LED) corn lamp (l ¼ 510–528 nm, 80 W) obtained from Shenzhen
as a reaction site are connected via a bridging ligand, as shown Nuoguan Technology Co., Ltd.
in Scheme 1. Upon irradiation with visible light, the cyclo-
metalated Ir(III) complexes would absorb light to produce the
excited state which would cooperate with Pd(II) reaction site
through ET or EnT process efficiently and thereby impact on its
reaction activity. Here, the synthesis and characterization of Ir–
General procedure for the synthesis of [Ir(C^N)
complexes
2 6
(bpm)](PF )
Pd bimetallic complexes [Ir(ppy)
phenylpyridine and bpm is 2,2 -bipyrimidine, 1), [Ir(dfppy)
2
(bpm)PdCl
2
](PF
6
) (ppy is 2- [Ir(C^N) (MeCN) ](PF ) (0.12 mmol) and bpm (38 mg, 0.24
2
2
6
0
2
mmol) were dissolved into DCM (15 mL). Then, 2 equiv. of KPF6
(
bpm)PdCl
2
](PF
6
) (dfppy is (4,6-diuorophenyl)pyridine, 2), and was added to the solution. The solution was stirred at room
](PF ) (pq is 2-phenylquinoline, 3) are re- temperature for 1 h. The solvent was removed under reduced
[Ir(pq) (bpm)PdCl
2
2
6
ported. Moreover, the photocatalytic activities of these pressure and the raw product was puried by a silica column
complexes for the Suzuki–Miyaura coupling reaction are also chromatography using a DCM/MeOH mixture (100 : 1, v/v) as an
evaluated under visible light irradiation at room temperature. eluent.
By means of altering of the cyclometalated ligand, we nd that
catalyst 3 exhibits highly photocatalytic activity for the Suzuki– C H IrN PF : C 44.83, H 2.76, N 10.46; found: C 44.65, H
2 6
For [Ir(ppy) (bpm)](PF ). Yield, 87%, 83 mg. Anal. calcd for
3
0
22
6
6
+
1
Miyaura coupling reaction under visible light irradiation via 2.82, N 10.81%. ESI-MS: m/z ¼ 658 [M ꢀ PF ] . H NMR (400
6
EnT from the Ir(III) chromophore to the Pd(II) reaction site.
MHz, CD CN-d ): d 9.22 (d, J ¼ 2.6 Hz, 2H), 8.23 (d, J ¼ 5.4 Hz,
3
3
2
7
(
H), 8.10 (d, J ¼ 8.1 Hz, 2H), 7.91 (d, J ¼ 7.9 Hz, 2H), 7.84 (d, J ¼
.7 Hz, 2H), 7.76 (d, J ¼ 5.4 Hz, 2H), 7.68 (t, J ¼ 5.2 Hz, 2H), 7.09
Experimental
Materials and general methods
1
3
t, 4H), 6.96 (t, 2H), 6.29 (d, J ¼ 7.5 Hz, 2H). C NMR (101 MHz,
3 3
CD CN-d ): d 167.09, 162.05, 159.58, 157.68, 149.86, 147.91,
All chemicals were commercially available and used as purchased 144.06, 138.86, 131.52, 130.41, 125.40, 124.96, 123.68, 122.95,
unless otherwise noted. All manipulations were carried out under 119.99.
an Ar atmosphere unless otherwise noted. Complexes [Ru(bpy)
2
(-
2
,
For [Ir(dfppy)
18IrN PF10: C 41.15, H 2.07, N 9.60; found: C 41.01, H
](- 2.32, N 9.31%. ESI-MS: m/z ¼ 730 [M ꢀ PF
PF ), and [Ir(pq) (bpy)](PF ) were synthesized according to the CD CN-d ): d 9.26 (d, J ¼ 4.5 Hz, 2H), 8.36 (d, J ¼ 8.3 Hz, 2H),
2 6
(bpm)](PF ). Yield, 85%, 89 mg. Anal. calcd for
0
bpm)](PF
6
)
2
(bpy is 2,2 -bipyridne), [Ru(bpy)
2
(bpm)PdCl
2
](PF
6
)
C
30
H
6
+
1
[
Ir(ppy) (MeCN)
2
2
](PF ), [Ir(dfppy) (MeCN) ](PF ), [Ir(pq)
6
2
2
6
2
(MeCN)
2
6
] . H NMR (400 MHz,
6
2
6
3
3
1
8–21
literatures.
Column chromatography was performed with silica 8.29 (d, J ¼ 5.3 Hz, 2H), 7.97 (t, 2H), 7.78 (d, J ¼ 5.6 Hz, 2H), 7.72
13
gel (300–400 mesh) under low light. Elemental (C, H, and N) (t, 2H), 7.15 (t, 2H), 6.75 (t, 2H), 5.75 (d, J ¼ 6.8 Hz, 2H). C NMR
analyses were carried out on an Elementar Vario EL analyzer. (101 MHz, CD
3
CN-d
3
): d 164.62, 163.33, 162.55, 162.08, 161.69,
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1
1
60.10, 159.96, 158.14, 151.64, 150.26, 139.85, 125.55, 124.12, Cs
23.95, 123.75, 113.78, 99.15.
2
CO
3
(100 mg, 0.3 mmol) were added to the tube. The tube was
degassed three times, then a solution of 1,2-dichloroethane–
For [Ir(pq) (bpm)](PF ). Yield, 88%, 95 mg. Anal. calcd for ethanol (DCE–EtOH, 2 mL, 1 : 1, v/v) was injected into the tube.
2
6
C H IrN PF : C 50.50, H 2.90, N 9.30; found: C 50.21, H 2.77, N The reaction was conducted at room temperature under Ar
3
8
26
6
6
+
1
9
.37%. ESI-MS: m/z ¼ 759 [M ꢀ PF
6
] . H NMR (400 MHz, atmosphere with irradiation of a 45 W blue LED from ca. 10 cm
CD Cl -d
2
2
2
): d 9.06 (d, J ¼ 2.6 Hz, 2H), 8.55 (d, J ¼ 3.4 Hz, 2H), for an appropriate time (monitored by TLC). The reaction prod-
8
2
7
.40–8.26 (m, 4H), 8.14 (d, J ¼ 7.7 Hz, 2H), 7.82 (d, J ¼ 7.9 Hz, ucts were puried by silica gel chromatography using hexane/
H), 7.68 (t, 2H), 7.44 (t, 2H), 7.29 (t, 2H), 7.18 (d, J ¼ 8.9 Hz, 2H), ethyl acetate (7/1, v/v) as an eluent, which were characterized by
1
3
1
.11 (t, 2H), 6.92 (t, 2H), 6.66 (d, J ¼ 7.7 Hz, 2H). C NMR (101
H NMR (see the ESI†).
MHz, CD Cl -d ): d 169.56, 161.28, 159.67, 154.59, 147.97,
2
2
2
1
1
47.09, 145.26, 140.45, 134.82, 131.39, 131.13, 129.49, 127.93,
27.40, 127.05, 124.73, 124.54, 123.74, 117.81.
Results and discussion
Synthesis of Ir(III)–Pd(II) bimetallic complexes
To observe the impact of cyclometalated ligand on the photo-
physical properties and photocatalytic activity of the Ir(III)–Pd(II)
bimetallic complexes, three bimetallic complexes with different
cyclomelated ligands (ppy, dfppy, and pq) were designed and
synthesized, as shown in Scheme 2. The mononuclear bpm
complexes were synthesized in good yields of 85–88% by reac-
General procedure for the synthesis of Ir–Pd bimetallic
complexes
[
Ir(C^N) (bpm)](PF ) (0.039 mmol) and Pd(MeCN) Cl (10 mg,
2 6 2 2
0.039 mmol) were added to 2 mL DCM. Then the mixture was
stirred at room temperature for an appropriate time (monitored
1
by H NMR). The solvent was removed under reduced pressure,
tion of the precursors [Ir(C^N)
2 2 6
(MeCN) ](PF ) with bpm ligand
affording quantitative product without further purication.
For 1. The reaction time is 3.5 h. Yield, 99%, 36 mg. Anal.
1
at room temperature. The complexes were characterized by H
1
3
NMR, C NMR spectroscopy and mass spectrometry as well as
elemental analysis. Furthermore, the bimetallic complexes were
calcd for C30
H
22Cl
2
IrN
6 6
PF Pd: C 36.73, H 2.26, N 8.57. Found: C
+
1
3
6.55, H 2.34, N 8.69%. ESI-MS: m/z ¼ 837 [M ꢀ PF ] . H NMR
6
obtained by the reaction of bpm complexes with Pd(MeCN) Cl2
2
(
7
¼
400 MHz, CD CN-d ): d 9.46 (d, J ¼ 7.5 Hz, 2H), 8.38 (d, J ¼
3
3
at room temperature in almost quantitative yields. The bime-
tallic complexes were also identied on the basis of NMR
spectroscopy and mass spectrometry.
.2 Hz, 2H), 8.10 (d, J ¼ 8.1 Hz, 2H), 7.98–7.92 (m, 4H), 7.89 (d, J
5.8 Hz, 2H), 7.84 (d, J ¼ 5.8 Hz, 2H), 7.16 (t, 2H), 7.11 (t, 2H),
1
3
6.97 (t, 2H), 6.23 (d, J ¼ 8.2 Hz, 2H). C NMR (75 MHz, CD
d
1
3
CN-
3
): d 166.40, 165.14, 159.42, 157.24, 151.76, 144.03, 143.64,
Photophysical properties
39.23, 131.34, 130.47, 128.36, 124.96, 123.97, 123.67, 119.99.
Table 1 summarizes the main photophysical properties of the
For 2. The reaction time is 6 h. Yield, 99%, 41 mg. Anal. calcd
for C H Cl IrN PF Pd: C 34.22, H 1.72, N 7.98. Found: C mononuclear and bimetallic complexes in DCM solution at
3
0
18
2
6
10
+
1
3
(
8
5
(
4.51, H 1.54, N 7.75%. ESI-MS: m/z ¼ 907 [M ꢀ PF ] . H NMR room temperature. As depicted in Fig. 1, the absorption spectra
6
400 MHz, CD CN-d ): d 9.45 (d, J ¼ 4.1 Hz, 2H), 8.46 (d, 2H), for the mononuclear complexes are comprised of three main
3
3
.35 (d, J ¼ 5.7 Hz, 2H), 8.01 (t, 2H), 7.97 (t, 2H), 7.93 (d, J ¼ absorption bands. An intense band is observed at 250–281 nm,
which can be attributed to the spin-allowed p–p* ligand
.7 Hz, 2H), 7.21 (t, 2H), 6.79 (ddd, J ¼ 12.0, 9.4, 2.3 Hz, 2H), 5.69
1
3
centered (LC) transitions. The moderately intense band at 300–
dd, J ¼ 8.7, 2.4 Hz, 2H). C NMR (101 MHz, CD
3 3
CN-d ):
3
50 nm is mainly attributed to the spin-allowed metal-to-ligand
d 164.80, 164.34, 162.76, 162.37, 161.91, 159.98, 159.66, 157.79,
1
23
charge transfer ( MLCT). While the weaker absorption band at
415 nm for [Ir(ppy) (bpm)](PF ), 390 nm for [Ir(dfppy)
), and 433 nm for [Ir(pq) (bpm)](PF ) mainly come
for C H Cl IrN PF Pd: C 42.22, H 2.42, N 7.77. Found: C 42.31, from the spin-forbidden MLCT transitions according to related
1
9
52.12, 146.85, 140.18, 128.40, 124.41, 123.94, 123.74, 113.94,
9.87.
2
6
2
For 3. The reaction time is 2 h. Yield, 99%, 42 mg. Anal. calcd (bpm)](PF
6
2
6
3
3
8
26
2
6
6
+
1
H 2.71, N 7.85%. ESI-MS: m/z ¼ 935 [M ꢀ PF
MHz, CD Cl -d
): d 9.08 (t, 2H), 8.65 (d, J ¼ 5.6 Hz, 2H), 8.34 (d, J
8.8 Hz, 2H), 8.26 (d, J ¼ 8.8 Hz, 2H), 8.11 (d, J ¼ 7.7 Hz, 2H),
.02 (t, 2H), 7.81 (d, J ¼ 7.7 Hz, 2H), 7.45 (t, 2H), 7.29 (t, 2H), 7.21
t, 2H), 6.96 (d, J ¼ 8.9 Hz, 2H), 6.91 (t, 2H), 6.61 (d, J ¼ 7.6 Hz,
6
] . H NMR (400 reports on [Ir(pq)
2
(bpy-CH
2
NH
2
)](PF
6
)
and [Ir(pq)
2
(bpy)]
0
0
[
B(5FPh) ] (where bpy-CH NH is 4-aminomethyl-4 -methyl-2,2 -
2
2
2
4
2
2
bipyridine and B(5FPh) is tetrakis(2,3,4,5,6-pentauorophenyl)
¼
4
8
(
1
3
2
1
1
H). C NMR (101 MHz, CD Cl -d ): d 168.71, 163.29, 157.88,
2 2 2
56.01, 147.03, 145.04, 144.09, 140.84, 135.10, 132.40, 131.08,
29.57, 128.10, 127.59, 127.38, 127.27, 124.59, 124.51, 117.69.
General procedure for photocatalysis of the Suzuki–Miyaura
coupling reaction
Pd(MeCN) Cl (2.5 mol%) and corresponding [Ir(C^N)2
2
2
(
bpm)](PF ) complex (2.5 mol%) were pre-reacted in 2 mL DCE
6
for corresponding hours in a Schlenk tube, then PPh
3
(5 mol%),
benzene halide (0.15 mmol), arylboronic acid (0.3 mmol), and Scheme 2 Synthesis of Ir(III)–Pd(II) bimetallic complexes.
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Table 1 Photophysical data for mononuclear Ir(III) and their corresponding Ir(III)–Pd(II) complex
a
4
ꢀ1
ꢀ1
b
b
PL [ꢁ10ꢀ3
b
c
c
Complex
l
abs [nm] (3 [10 M cm ])
l
em [nm]
s [ns]
F
]
E
ox [V]
E
red [V]
+
[
[
[
1
2
3
Ir(ppy)
Ir(dfppy)
Ir(pq) (bpm)]
2
(bpm)]
253(2.3), 300(0.86), 379(0.31), 415(sh)
251(2.7), 315(1.1), 362(0.4), 390(sh)
252(2.6), 281(2.6), 350(1.2), 433(0.3)
270(2.2), 312(sh), 375(0.42)
255(sh), 313(1.1), 359(sh)
279(3.4), 330(1.3), 356(1.3), 363(1.3),
700
611
687
556
683
584
7
288
12
n.d.
n.d.
n.d.
6.7
1.30
1.75
1.38
1.38
1.65
1.35
ꢀ1.56, ꢀ1.93
ꢀ1.55, ꢀ2.20
ꢀ1.50, ꢀ1.93
ꢀ2.0, ꢀ2.38
ꢀ2.50
+
2
(bpm)]
107
10.3
0.37
8.1
+
2
0.44
ꢀ1.99, ꢀ2.24
4
27(0.4)
a
ꢀ5
b
ꢀ4
c
In deaerated DCM solution (5 ꢁ 10 M). In deaerated DCM solution (2 ꢁ 10 M, lex ¼ 405 nm). Cyclic voltammograms carried out at a scan
ꢀ1
+
6 3
rate of 200 mV s , versus Fc/Fc using 0.1 M TBAPF as a supporting electrolyte in degassed CH CN.
(
F
PL ¼ 1.03% and t ¼ 12 ns) for [Ir(ppy)
2 6 2
(bpm)](PF ), [Ir(dfppy) (-
bpm)](PF ), and [Ir(pq) (bpm)](PF ), respectively. The changing of
6
2
6
the cyclometalated ligand from ppy to dfppy leads to the emission
wavelength signicantly blue shi from 700 nm to 611 nm and the
triplet state lifetime elongation from 7 ns to 288 ns. These may be
attribute to the introduction of F group on cyclometalated ligand
which affects the energy of essential molecular orbitals and results
in the change of low energy charge transfer states. The emission of
[
2 6 2 6
Ir(ppy) (bpm)](PF ) and [Ir(pq) (bpm)](PF ) can be mainly
3
21
assigned to MLCT (dp(Ir) / p*(bpm)) based transition,
whereas a mixture of MLCT (dp(Ir) / p*(dfppy) and dp(Ir) /
3
p*(bpm)) transitions may show a large contribution to the excited
Fig. 1 UV-vis spectra of mononuclear Ir(III) complexes in DCM solu-
tion (5 ꢁ 10 M).
25,26
state of [Ir(dfppy) (bpm)](PF ).
Comparison with the corre-
ꢀ5
2
6
sponding complexes [Ir(ppy) (bpy)](PF ) (l ¼ 610 nm, F
¼
2
6
em
PL
21
0
.89%, and t ¼ 103 ns), [Ir(dfppy)
2
(bpy)](PF
6
) (lem ¼ 520 nm, FPL
) (lem ¼ 560 nm,
2
7
¼
26%, and t ¼ 561 ns), and [Ir(pq) (bpy)](PF
2
6
23
borate). Compared to [Ir(ppy)
2
(bpm)](PF
6
), the introduction of
21
F
PL ¼ 14%, and t ¼ 474 ns), the emission intensity of the
F groups in phenyl and substitution pyridine with quinoline
leads to a blue and red shi, respectively. The former can be
attributed to the remarkable metal-based MOs stabilization
caused by the introduction electron-withdrawing groups on
phenyl of the cyclometalated ligand. And the main reason for
the latter is the presence of the quinoline moiety which can
stabilize the unoccupied MOs. These are also supported by DFT
mononuclear complexes are decrease due the presence of two sets
of free lone-pairs in the bpm ligand which may cause electronic
repulsion, resulting in thermodynamic quenching of the triplet
excited-state to some extent. The introduction of Pd(II) center to the
mononuclear complex leads to a drastically luminescent quench-
ing compared with the corresponding mononuclear complex (Fig.
3) which is associated with much faster deactivation channels like
+
+
calculations of [Ir(dfppy)
in Scheme S1.† Moreover, the broad MLCT bands for
Ir(ppy) (bpm)](PF ) and [Ir(pq) (bpm)](PF ) complexes suggest
2 2
(bpm)] and [Ir(pq) (bpm)] as shown
electron or energy transfer starting from energy levels above the
3
relaxed MLCT excited state. To comparison, the emission of
[
2
6
2
6
2 6
a mixture of the mononuclear [Ir(pq) (bpm)](PF ) and [Pd(bpm)
that they are capable of harvesting light energy from UV to
visible region.
The absorption spectra of Pd(bpm)Cl , [Ir(pq) (bpm)](PF ) and
2
2
6
its corresponding Ir(III)–Pd(II) complex 3 was shown in Fig. 2.
Compared to the mononuclear Ir(III) complex, binuclear Ir(III)–Pd(II)
complex 3 present a spectrum which is similar to the combination
of the component spectra. However, small changes of the spectrum
around 400 nm and 500 nm imply that underlying new electronic
transitions occurred between the two components in the bimetallic
24
complex. The absorption spectra of other Ir(III)–Pd(II) bimetallic
complexes also exhibit the similar phenomenon as shown in
Fig. S1 and S2.† The emission spectra of the complexes were
measured in degassed DCM solution at room temperature (lex
¼
405 nm), shown in Fig. 3. The mononuclear complexes show an
emission with maximum emission band at 700 nm (FPL ¼ 0.67%
Fig. 2 UV-vis spectra of [Ir(pq)
solution (5 ꢁ 10 M).
2
(bpm)](PF
6
), 3, and Pd(bpm)Cl
2
in DCM
and t ¼ 7 ns), 611 nm (FPL ¼ 10.7% and t ¼ 288 ns), and 687 nm
ꢀ5
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a
Table 2 Optimal photocatalytic conditions
b
Entry
Catalyst
Different condition
Yield (%)
1
2
3
4
5
6
7
8
9
3
3
3
3
3
3
—
3
EtOH
MeCN
DCE
—
63
46
81
93
65
Trace
0
2 3
K CO
Et N
3
—
No PPh
Air
Dark
3
PPh (2 mol%)
UV lamp
Green LED corn lamp
—
—
Fig. 3 The mononuclear and bimetallic complexes in degassed DCM
solution at room temperature (2 ꢁ 10 M, lex ¼ 405 nm).
3
Trace
18
40
81
88
46
86
54
80
59
47
36
41
70
75
89
20
63
ꢀ4
3
3
10
11
12
c
3 (1 mol%)
3
3
1
2
A
2
[Pd(bpm)Cl
2
1
B
C
3
2
d
e
Cl
2
(
] complexes was also observed and exhibited a slight quenching 13
see Fig. S3 in the ESI†), indicating that the intramolecular ET/EnT 14
15
16
17
18
process from the excited state of Ir(III) complex to Pd(II) center is
f
—
15,28
highly efficient.
The quantum yields roughly follow in the case
d
UV lamp
Dark
Dark
Dark
—
of photoluminescence of the binuclear complexes. And the PL
2
]
]
intensive quenching of complexes 1 and 3 where the Ir-based 19
MLCT mainly contributes from the bridged ligand bpm is more 20
3
g
h
i
2
2
2
24
2
1
2
3
effective by a factor of about 20 as compared with complex 2.
—
2 equiv. Et
2 equiv. Et
UV lamp
3
N
N
i
Electrochemistry
3
d
5
[Pd(bpm)Cl
2
Cyclic voltammetry was used to determine the ground state
redox potentials of all the Ir(III) complexes. As listed in Table 1
a
Conditions: 4-bromotoluene (0.15 mmol), phenylboronic acid (0.3
(5 mol%), Cs CO (0.3 mmol) in 4
mL of DCE–EtOH (3 : 1) with Ar atmosphere under a 45 W blue LED
mmol), catalyst (2.5 mol%), PPh
3
2
3
and Fig. S4,† [Ir(ppy)
Ir(pq) (bpm)](PF
) showed a quasi-reversible couple at about corn lamp at room temperature for 5 h. Isolated yield. In 18 h.
1.30 V, +1.75 V, and +1.38 V, respectively, which was assigned
2 6 2 6
(bpm)](PF ), [Ir(dFppy) (bpm)](PF ), and
b
c
[
2
6
d
e
1
00 W UV lamp was used as light source. 80 W green LED corn
+
f
g
lamp was used as light source. A ¼ [Ru(bpy)
2
(bpm)PdCl
C ¼ [Ir(pq) (bpy)](PF
Additional addition 2 equiv. of Et N.
2
](PF
6
)
2
.
B
23b
to Ir(IV/III) oxidation. The more positive oxidation potential for
Ir(dFppy) (bpm)](PF ) can be attributed to the introduction of Pd(bpm)Cl
h
¼
[Ir(pq)
2
(bpm)](PF
6
) + Pd(MeCN)
2
Cl
2
.
2
6
) +
i
[
2
.
3
2
6
electron-withdrawing F groups on phenyl ring of dfppy ligand,
which stabilize the t2g orbital of Ir(III) ion, resulting in more
difficult to lose electron. This is consistent with the observed conditions, affording the corresponding product in a moderate
emission properties. However, the precisely redox data for yield of 63% (entry 1 in Table 2). When MeCN was used as
bimetallic complexes are not obtained because they are elec- a solvent instead of EtOH, the isolated yield decreased to 46%
28
trochemical too unstable to detect (see Fig. S5 in the ESI†).
(entry 2), indicating that a solvent has much impact on the
coupling reaction. When DCE was used, the yield increased to
81% (entry 3 in Table 2). These also inspired us to continuously
Photoacceleration of the Suzuki–Miyaura coupling reaction
optimize the reaction solvent. It was found that a mixture of
DCE–EtOH (3 : 1) is the best one, affording an excellent isolated
yield in 93%. The superior function of the mixed solvent may be
due to the increased solubility of the reactants and base. Since
base has a signicant inuence on the Suzuki–Miyaura
coupling, we then turned our attention to test the impact of
base (entries 4–6). The experiments revealed that Cs CO is the
The high-efficiently intramolecular ET or EnT from the excited
state of the Ir(III) complex to the Pd(II) center encouraged us to
estimate the photoaccelerated Suzuki–Miyaura coupling reac-
tion using these bimetallic complexes as photocatalysts, where
the Ir(III) fragments act as a chromophore and the Pd(II) frag-
ments act as a reaction site. We began our study using 4-bro-
motoluene and phenylboronic acid as model substrates to
screen the reaction conditions for the Suzuki–Miyaura coupling
2
3
best one. K CO is slight inferior, affording a moderate yield in
2
3
6
(
5%, but is superior to Et
entries 6–10) demonstrated that the bimetallic catalyst, base,
and PPh ligand are indispensable to the coupling reaction
under mild conditions. The necessary of PPh ligand may be
3
N. Moreover, the control experiments
reaction in the presence of 3 (2.5 mol%) and PPh
3
(5 mol%) as
catalysts and Cs CO as a base in an environment-friendly
2
3
3
solution EtOH under Ar atmosphere and visible light irradia-
tion at room temperature for 5 h (see Table 2, entry 1). To our
delight, the coupling reaction went smoothly under mild
3
attributed to the effect of bulk and electron richness at the Pd
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center, which can accelerate the rates of both oxidative addition conspicuous difference catalytic reactivity under visible light
and reductive elimination steps. The photocatalytic reaction may attribute to the excited state of the chromophore and the
was signicantly suppressed in air, affording only 18% yield. cooperation between Ir(III)* and Pd(II) unit. Hence, we specu-
This may be ascribed that the excited state of Ir(III) complex was lated that the reason of the dramatically decreased activity of
easily quenched by O
unit to some extent. Meanwhile, the presence of O
2
and failed to cooperate with the Pd(II) complex 2 may be associated with its major contribution of
3
2
also
MLCT (dp(Ir) / p*(dfppy)) transition and the EnT process
impedes the reductive elimination step of catalytic cycle and between two metal units (vide supra). Compared with complex 2,
3
both these factors result in a low reactivity. In summary, visible the MLCT transition of complex 3 is mainly assigned to dp(Ir)
light signicantly accelerated the coupling reaction, increasing / p*(bpm) and leads to an excited state which locate on
the yield from 40% to 93%. When the amount of catalyst was bridging ligand. This kind of locus of excited state on one hand
decreased to 1.0 mol%, the isolated yield decreased to 81% for may increase the charge density of the bridge ligand and boost
1
8 h under the identical conditions (see entry 11). To study the the formation of electron rich Pd unit which could facilitate the
15,28
relationship between light absorption wavelength and catalytic oxidative addition step of catalytic cycle.
On the other hand,
activity, experiments using 100 W UV lamp (l ¼ 365 nm) and the transition direction from Ir(III) unit to the bridging ligand
0 W green LED corn lamp (l ¼ 510–528 nm) as light source which is directly connected with Pd(II) unit could also facilitate
were conducted under the same condition (entries 12 and 13). the EnT from the excited state of chromophore to the reaction
8
17a,28
The yield of product under UV lamp was similar to that under site through the bridged ligand bpm,
resulting in acceler-
blue light. However, the yield of product under green light ation of the Suzuki–Miyaura coupling reaction. Moreover, we
signicantly decreased to 46%. These indicate that the utiliza- also have a research on the catalytic efficiency between different
tions of blue light and ultraviolet light energy of complex 3 are catalytic system. The yields comes to 70% and 75% respectively
similar. However, complex 3 cannot fully harness the green light when a mixture of [Ir(pq)
and lead to the low catalytic reactivity which is in accord with its without pre-reacting to form bimetallic complex), and
UV-vis spectrum. The action spectrum was shown in Fig. S6.† [Ir(pq) (bpy)](PF ) and [Pd(bpm)Cl ] were used as photo-
2 6 2 2
(bpm)](PF ) and Pd(MeCN) Cl (-
2
6
2
Furthermore, the photocatalytic activities of the other bime- catalysts (entries 21 and 22), indicating that visible light
tallic complexes for the Suzuki–Miyaura coupling reaction were signicantly accelerates the Suzuki–Miyaura coupling reaction
also evaluated under the optimal conditions (entries 14–16). (compared with the reaction by using [Pd(bpm)Cl ] as catalyst in
2
When complex 1 and [Ru(bpy) (bpm)PdCl ](PF ) were used the dark, see entry 18) and the cooperation between intra-
2
2
6 2
instead of 3 as catalysts, the yields of the coupling product molecular catalytic system is more efficient compared to the
slightly lowered to 86% and 80%, respectively. These may be intermolecular system (see entry 4). According to the previous
9–15
related to the stronger absorption of complex 3 in the visible studies,
two mechanisms may involve the photoaccelerated
region and the less stability of bimetallic complex 1 compared Suzuki–Miyaura coupling reaction. A 2-electron transfer from
with complex 3. The yield dramatically decreased to 54% when the excited component to the Pd(II) site generates an active
complex 2 was used as a photocatalyst, which was slightly electron rich Pd(0) which facilitates the oxidative addition
1
0–12,15
higher than the catalytic activity of [Pd(bpm)Cl ] (47%, see entry reaction with aryl halides.
1
Alternatively, an EnT from the
8) indicating that no signicantly accelerated coupling reac- triplet excited state of chromophore produces highly energetic
tion was observed in complex 2 under irradiation, even though electrons at the Pd catalytic site, which enhances the coupling
2
9a,9b,14
complex 2 is a more strongly oxidizing photocatalyst. Since the reaction.
To verify our former speculation of the coopera-
absorption intensity of visible region of complex 2 is weaker tion process between two metal units and gain further insight
than other bimetallic Ir–pd complexes, 100 W UV lamp (l ¼ 365 into the photocatalytic coupling reaction, control experiments
nm) was used as a light source to irradiate the reaction, were performed to distinguish the mechanism of the photo-
however, the yield of the product only slightly increased to 59% catalyzed Suzuki–Miyaura coupling reaction. When two equiv.
(entry 17). Therefore, the different absorption intensity in of triethylamine (TEA), which has been demonstrated as an
visible region can be exclude for the main reason to explain the sacricial electron donor for excited cyclometalated Ir(III)
29
low reactivity of complex 2. Moreover, the different chromo- complexes, was added to the reaction solution under the
phore fragments also result in factors such as structure and optimal conditions (see entry 23), the isolated yield was affor-
redox properties of Ir(III) unit which may inuence the catalytic ded in 89%, indicating that there is no obvious suppression of
efficiency without undergoing the photoexcited process. So, it is the coupling reaction. Then we engaged complex 2 which con-
important to consider these factors as potentially reasons for taining a more strongly oxidizing chromophore as catalyst. To
3
the low reactivity of complex 2. To verify these options, reaction our surprise, the addition of Et N signicantly suppressed the
using catalyst complex 2 under the dark was conducted. reactivity of catalyst (see entry 24) indicating that Et N enable to
3
However, the yield of product is similar with the reaction when quench the excited Ir(III)* center thoroughly and impede the
using complex 3 as catalyst under the dark (entries 10 and 19). formation of excited Pd(II)* center through electron transfer
The similar yield was also observed when using complex 1 as from quenched Ir unit to Pd(II) unit, so the ET process may be
30,31,7b
catalyst under the dark (entry 20). These observations imply that ruled out in this case.
Furthermore, the intense light
the different properties of chromophore fragments cannot lead energy was used to nd whether excited Pd(II)* complex can
to the signicant different reactivities of catalysts when the Ir(III) improve the coupling reaction. The reaction by using [Pd(bpm)
unit is in their ground state and the main reason for the Cl
2
] as a photocatalyst under UV lamp irradiation was
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Table 3 Scope of the photoaccelerated Suzuki–Miyaura coupling
a
reaction
Scheme
3 Mechanism of the photoaccelerated C–C coupling
reaction.
a
Conditions: arylbromide (0.15 mmol), arylboronic acid (0.3 mmol), 3
conducted (see entry 25). To our delight, the isolated yield was
indeed increased from 47% to 63% which indicating that the (3 : 1) with Ar atmosphere bunder
formation of a excited Pd(II)* state is available through photo-
excitation which can indeed enhance the reactivity of the
(
2.5 mol%), PPh
3
(5 mol%), Cs
2
CO
3
(0.3 mmol) in 4 mL of DCE–EtOH
45 blue LED at room
a
W
temperature; isolated yield. 4-Acetyl-chlorotoluene and PCy
3
were
ꢂ
c
used instead of 4-acetyl-bromotoluene and PPh
4.0 mol%) and PPh
(8.0 mol%) were used.
3
at 60 C.
3
d
(
3
(8.0 mol%) were used in 24 h. 3 (4.0 mol%)
coupling reaction. This mechanistic proof points to an EnT and PCy
3
process between two metal units.
Therefore, combined with the previous reported mecha-
32,33
indicating that the electronic effect on arylbromide was sensitive
in our case and the electron-decient substitution was benecial
to the Suzuki–Miyaura coupling reaction. However, when the low
activity of 4-acetyl-chlorotoluene was used instead of 4-acetyl-
bromotoluene, the coupling reaction became sluggishly, almost
no product was observed for 12 h. This case was improved by
nism,
proposed on the basis of our experiments, as shown in Scheme
. Under light irradiation, the Ir(III) complex is converted into
a photoaccelerated coupling reaction via EnT was
3
1
a high-energy excited singlet state Ir(III)*, which undergoes
3
intersystem crossing to triplet Ir(III)* quickly since Ir(III)
complex possesses highly efficient spin–orbit coupling. The
3
3 3
using PCy instead of PPh and elevating the reaction tempera-
Ir(III)* can transfer energy to the Pd(II) center to form a triplet
ꢂ
3
ture to 60 C, affording 4e in a moderate yield of 51% for 15 h.
Moreover, the scope of phenylboronic acid was also observed.
The electron-decient substitution 4-F and electron-rich substi-
tution 4-methoxy phenylboronic acids efficiently coupled with
bromotoluene to provide the corresponding products 4f and 4g
in excellent yields of 92% and 93%, respectively, indicating that
the electronic effect of phenylboronic acid was insensitive to the
coupling reaction. Finally, 1-bromonaphthalene and 2-methyl-1-
bromonaphthalene were used as coupling partners with phe-
nylboronic acids, affording the corresponding 4h, 4i, and 4j in
yields of 85%, 78%, and 81%, respectively, in 24 h. Moreover, the
photoaccelerated coupling reaction for the synthesis of
binaphthyl was also observed, because it is a chiral atropisomer
along the biaryl axis and important for many biologically active
products and potential pharmaceuticals. Indeed, when 2-methyl-
excited state Pd(II)*, which enables to enhance Pd(II) reduction
process in the catalytic cycle and increase the formation the C–C
crossing coupling product. The similar process has also been
demonstrated in the C–C and C–N crossing coupling reactions
7b,7d
using Ir(III) and Ni(II) bimolecular photocatalysts.
Further-
more, the light irradiation may also accelerated the oxidative
addition step because the locus of excited state of chromophore
which enable to increase the charge density of the bridge ligand
and boost the formation of electron rich Pd unit and facilitate
9d,15,34,35
the carbon–halogen bond activation.
With the optimal conditions in hand, we further observed the
scopes and limitations of this photoaccelerated Suzuki–Miyaura
coupling reaction under mild conditions (see Table 3). First, the
steric effect on the reaction was investigated. When 2-bromoto-
luene, 3-bromotoluene, and 3,5-dimethyl-bromobenzene were
used as substrates to couple with phenylboronic acid in the
standard conditions, the corresponding desired products 4b, 4c,
and 4d were afforded in 89%, 93%, and 90% for 8 h, respectively,
demonstrating that steric effect of the arylbromide was insensi-
tive to the reaction. When electron-withdrawing 4-acetyl group
was introduced on arylbromide, the corresponding product 4e
was afforded in an excellent isolated yield of 95% for 4 h,
1
-bromonaphthalene and 2-methoxy-1-bromonaphthalene were
used as coupling partners with and 1-naphthylboronic acid in the
presence of 3 (4.0 mol%) and PPh (8.0 mol%) as catalysts, the
3
corresponding products 4k and 4l were afforded in moderate
yields of 57% and 55% for 24 h, respectively. The yield signi-
cantly enhanced to 80% when PCy was used instead of PPh for
3 3
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5
h, indicating that the visible light accelerated protocol can be
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Conclusions
In summary, a series of bimetallic complexes consisting of
a chromophore and a reaction site were synthesized. The
inuences of the cyclometalated ligand at Ir(III) center on the
photophysical properties and the photocatalytic activity were
demonstrated. Through the regulation of cyclometalated
ligands at Ir(III) center, complex 3 could dramatically accelerate
the Suzuki–Miyaura coupling reaction under a mild condition.
The highly effective absorbing of visible light and appropriate
locus of excited chromophore were proved to facilitate the
photoaccelerated reaction. Mechanism studies show that the
light energy absorbed by Ir(III) fragment produces the excited
chromophore and then undergoes EnT efficiently to the reac-
tion site to form a excited Pd(II) center via the bridging ligand
which could accelerate the catalytic activity of complex. This
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