A ruthenium bisoxazoline complex as a photoredox catalyst for nitro compound reduction under visible light

Article abstract of DOI:10.1039/c9dt00428a

An unreported ruthenium(ii) complex containing bisoxazoline ligands has been synthesized and characterized. To test the catalytic ability of the ruthenium complex, the synthesis of anilines from nitro compounds in the presence of a mild reducing agent sodium borohydride and visible light has been developed. Mechanistic studies involving the experiment and DFT calculations suggest that the reaction could involve a radical pathway with the assistance of a photoredox catalyst.

Full text of DOI:10.1039/c9dt00428a

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DOI: 10.1039/C9DT00428A
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ARTICLE
Ruthenium Bisoxazoline Complex as Photoredox Catalyst for
Nitro Compound Reduction under Visible Light
Wei-Guo Jia*a, Ming-Xia Chenga, Li-Li Gaoa, Siu Min Tan,b Chao Wang,b Xiaogang Liu,b
Richmond Lee*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unreported ruthenium(II) complex containing bisoxazoline can be easily modified through the use of different amino alcohol
ligands has been synthesized and characterized. To test the precursors. Both the electronic and steric properties of ancillary
catalytic ability of the ruthenium complex, synthesis of anilines ligands could significantly influence the catalytic activities of the
from nitro compounds in the presence of a mild reducing agent metal center.15 To the best of our knowledge, the use of ruthenium
sodium borohydride and visible light has been developed. complexes possessing bisoxazoline ligands for photoredox catalysis
Mechanistic studies involving experiment and DFT calculations, has not been reported in the literature. This situation motivates us to
suggest that the reaction could involve radical pathway with the investigate ruthenium complexes with bisoxazolines ligands in
assistance of the photoredox catalyst.
search of novel photocatalyst. Thus we endeavored to synthesize and
characterize ruthenium-bisoxazoline complex. We further
a
In the last few years, photoredox catalysis has emerged as a powerful
tool for numerous organic transformations due to the mild reaction
subjected the synthesized ruthenium complex as catalyst in the
hydrogenation of nitro compounds to aromatic anilines with sodium
borohydride reducing agent under visible light. High yields were
obtained. This catalytic system was found to be robust toward the
reduction of various nitro compounds with different substituents on
the aromatic ring. When compared with commercially available
photocatalysts, the catalytic performance of the ruthenium-
bisoxazoline complex was better.
conditions,
efficiency,
economical,
and
environmentally
friendliness.1-8 The most widely used catalysts are ruthenium- or
iridium based transition metal complexes, eosin Y, acridinium salts
organic molecules and inorganic semiconductors such as CdS and
TiO2 graphitic carbon nitride.9-10 Photocatalytic systems rely on
electron transfer or energy transfer between the photoexcited
catalysts and the substrates.
The 4,4,4’,4’-tetramethyl-2,2’-bisoxazole (DMOX) ligand and
[Ru(DMOX)2Cl2] precursor were synthesized according to
reported literature procedures and characterized by
spectroscopy.16-17 The ruthenium complex 1 was prepared by the
reaction of [Ru(DMOX)2Cl2] with bpy (2,2’-bipyridine) in refluxing
Among the transition metal catalyst complexes, the original
[Ru(bpy)3]2+ (where bpy is 2,2’-bipyridine) complex remains one
of the most commonly used photocatalysts due to its outstanding
photophysical and chemical properties.1, 11-13 However, there are still
limitations in the scope of substrates which can undergo
photocatalytic transformations with the [Ru(bpy)3]2+ catalyst.9 The
design and synthesis of new photocatalysts is necessary to expand
the scope of substrates and types of transformations. The ideal
photoredox catalyst should possess good absorption, high quantum
yield, long lifetime, reversible electrochemical behavior and be
easily tuned.14 Bisoxazolines are important ligands with broad
applications in homogeneous catalysis and asymmetric synthesis.15
Typically, the steric and electronic properties of the bisoxazolines
-
ethylene glycol for 6 h, followed by PF6 anion exchange and
isolated in high yield (90%). The structure of ruthenium complex
1 was confirmed by IR, NMR spectroscopy as well as mass
spectrometry.
1
The H NMR spectrum of complex 1 in DMSO-d6 showed
characteristic resonances of the bisoxazoline and bipy ligands.
A surprising characteristic of this spectrum, in comparison to
that of the [Ru(DMOX)2Cl2] precursor, was the chemical shifts
of the methyl hydrogen atoms of the bisoxazoline rings. The
eight methyl groups of the bisoxazolines were found to be
magnetically equivalent, resonating as four singlets at -0.54-1.6
ppm, consistent with the cis structure of the [Ru(DMOX)2Cl2]
precursor, which is due to the chirality and C2 symmetry of the
complex. These higher-field shifts for the methyl in the
bisoxazoline ring were due to the π-ring current effect of the
nearby co-ligating nitrogen-heteroaromatic chelate and weak
a. The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui
Laboratory of Molecular-Based Materials (State Key Laboratory Cultivation Base),
College of Chemistry and Materials Science, Anhui Normal University, Wuhu,
241002, China E-mail: wgjiasy@mail.ahnu.edu.cn.
b. Science and Mathematics Cluster, Singapore University of Technology and Design,
8 Somapah Rd Singapore 487372 E-mail: richmond_lee@sutd.edu.sg.
Electronic Supplementary Information (ESI) available: Additional copies of NMR
spectra. (CCDC 1891168). For ESI and crystallographic data in CIF or other
electronic format See DOI: 10.1039/x0xx00000x
-
hydrogen bonding with PF6 anion.18 Similar patterns appear in
This journal is © The Royal Society of Chemistry 20xx
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the spectra of [Ru(bipy)2(i-Pr-biox)][BF4]2 and [Ru(phen)2(i-Pr- better than Ru(bpy)3Cl2, indicating 1 as a potential photoredox
View Article Online
biox)][BF4]2.19 Positive-ion electrospray ionization mass catalyst.
DOI: 10.1039/C9DT00428A
Complex 1 is stable at room temperature as a solid and in MeCN
solution in the dark. In the solid state, complex 1 was stable for
months at room temperature when exposed to the air atmosphere at
298K, as determined by 1H NMR. However, the complex 1
decomposes under the irradiation with 36 W white LED (Figure S9-
S11). The absorption spectra of complex 1 gradually decreases at
287 nm and 451 nm and increases at 236 nm with increasing
irradiation time without any change in the spectral shape, indicating
the complex decomposes under the irradiation with 36 W white
LED. We suspect that this instability is in part due to the dissociation
of the ligands or undergo torsional motion to break planarity. The
complex 1 shows strong luminescence at 610 nm upon excitation at
the 470 nm (Figure S12 and S13). The average lifetime of complex 1
is 5.48 ns in the solid state, which was modelled using double
exponentials successfully. (SI equation 1 and 2). However, the
lifetime of complex 1 in the solutions is longer than that of in the
solid. The lifetimes are varying between 155 and 319 ns, depending
upon the solvents (TableS4). Moreover, complex 1 exhibits the
longest lifetime (319 ns in DCM) among the series of solvents. Next,
we studied the quenching behaviour of complex 1 fluorescence in
the presence of the quencher nitrobenzene in MeCN/EtOH (v/v =
1:19 10-4 M) at room temperature (Figure S14). The fluorescence
intensity gradually decreases with increasing quencher concentration,
without any change in the spectral shape, indicating fluorescence
quenching without exciplex formation. The lifetimes of complex 1
decrease with increasing quencher concentration (Table S5). The
quenching process indicated that the excited state of complex 1
could activate nitrobenzene in the photocatalytic chemical reaction.
spectroscopic (ESI-MS) studies further supported the formation
of ruthenium complexes. For complex 1, the ESI-MS spectrum
for showed peaks at m/z 795.1702 and 325.1079, corresponding
to [M-PF6]+ (calcd for [M-PF6]+ 795.1799) and [M-2PF6]2+
(calcd for [M-2PF6]2+ 325.1076), respectively.
Single crystals suitable for X-ray diffraction were obtained by
slow solvent evaporation from a MeCN/Et2O solution of complex 1.
Crystallographic data are provided in Table S1 with selected bond
distances and angles in Table S2. The molecular structure of 1 is
shown in Fig. 1.
Fig. 1 Molecular structure of complex 1 with thermal ellipsoids drawn at the
30% level. All hydrogen atoms and PF6- anion are omitted for clarity.
The crystal structures were solved with monoclinic P121/n1
group. As shown in Fig. 1, the crystal of complex 1 consists of
-
one molecular cation and two PF6 anions. The Ru is
surrounded by six nitrogen atoms, four nitrogen atoms from the
ancillary bisoxazoline ligand and two nitrogen atoms from the
bipyridine ligand. The crystal structure reveals that the
ruthenium center adopts a similar octahedral geometry through
the coordination of two ancillary bisoxazoline and bidentate
N,N ligand which is common for ruthenium complexes with
polypyridines ligands.20-22 The Ru-N bond distances in 1 are in
the range of 2.042(3)-2.157(3) are comparable to those of
ruthenium complexes containing polypridines and bisoxazoline
ligands.23-24 The Ru-N (bisoxazoline) average distances
(2.131aev Å) are longer than those of Ru-N (bipy) distances
(2.047aev Å). The bite angles are 84.365º, 89.511º (bisoxazoline
and bipy plane) and 72.714º (bisoxazoline) in 1.
The absorption and emission spectra complex 1 were recorded
in MeCN/EtOH (v/v=1:19, 1×10-4 M) and show ligand centered
(π→π*) absorptions in the UV region as well as an MLCT band
(d(Ru) → π*(N^N), λ = 452 nm) (Figure S4 and S5). The
complex 1 has a higher MLCT extinction coefficient than the
complex Ru(bpy)3Cl2 (1.471×104 L mol-1 cm-1 vs 1.279×104 L
mol-1 cm-1, Figure S6). To gain further insights about redox
properties of complex 1, electrochemical methods were used.
The oxidation wave, corresponding to RuIII/II couple, is
observed occurring at 1.36 V versus Ag/AgCl, significantly
shifted from the same couple in [Ru(bpy)3]Cl2 at 1.05 V (SI,
Figure S7).25 The two reduction potentials are at -1.29 V and -
1.60 V (Table S3). From both absorption and emission spectra,
the electrochemical behavior for complex 1 is comparable or
Table 1. Optimization of reaction conditions for 1-bromo-4-nitrobenzene reduction
under visible light
NH2
NO2
Br
photocatalyst (1 mol%)
solvent, NaBH4, 3h,
36 W white LEDs, RT
Br
Entrya
Photocatalyst
complex 1
[Ru(bpy)3]Cl2
Eosin B
Solvent
EtOH
EtOH
EtOH
EtOH
MeOH
DCM
IPA
Conversionb (%)
1
2
3
4
5
6
7
8
99
29
25
25
22
22
27.4
7
Eosin Y
complex 1
complex 1
complex 1
complex 1
ACN
9
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
NRc
NRd
tracee
37.5f
28g
23h
10
11
12
13
14
complex 1
complex 1
complex 1
complex 1
complex 1
a: Reaction conditions: 1-bromo-4-nitrobenzene (0.1 mmol), photocatalyst (1 mol
%), NaBH4 (10 equiv), solvent (2 mL), 36 W white LED, room temperature; b:
Conversion were determined by GC; c: Without photocatalyst; NR: no reaction; d:
Without NaBH4; e: Without visible light; f: N2; g: O2; h: 1.5 eq TEMPO.
2 | J. Name., 2012, 00, 1-3
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Table 2 Screening of substrates reduction catalyzed by complex 1. a,
setup for this approach. Gratifyingly, the expected reduction
View Article Online
DOI: 10.1039/C9DT00428A
product 4-bromoaniline was obtained in 96% yield with 1
mol% 1 when the reaction was irradiated with 7W blue LEDs
in 2 h at room temperature. Interestingly, when the reaction
mixture was irradiated with 36 W white LED instead of blue
LEDs, the desired product was obtained in quantitative yield in
less than 3 h (Table 1, entry 1). The use of other photoredox
catalysts such as ruthenium complex [Ru(bpy)3]Cl2 and organic
photoredox catalysts eosin B or eosin Y under similar
conditions resulted in lower yields of the product (Table 1,
entries 2-4). Changing the solvent from EtOH to MeOH, DCM
(dichloromethane), IPA (isopropanol), or ACN (acetonitrile)
also resulted in lower product formation (Table 1, entries 5-8).
When the reactions were done without any one of the three
important conditions, i.e. photocatalyst, NaBH4, or visible light
no trace product was found (Table 1, entries 9-11). These
control experiments suggest that the ruthenium complex plays a
crucial role in this photocatalytic reduction. By running the
reaction under O2 atmosphere or with 1.5 eq TEMPO, the
product yield diminished by about 70%. With the optimized
reaction conditions, we started to apply this methodology to an
expanded substrate scope. Nitroarenes bearing an electron-
withdrawing or -donating group underwent smooth reduction to
give the corresponding aniline derivatives in high yields. It is
noteworthy that halo-substituted nitroarenes could be used to
generate halo-substituted anilines, which could be used for
further transformations such as cross-coupling reactions of
arylboronic acids with aryl halides (Table 2, entries 2-6). When
a strong electron-donating group -OH was introduced, the
reaction was also successfully completed within 3 hours (Table
2, entries 8-9). In addition, when a -CH3 substituent is
introduced on the aromatic ring, high yields are maintained
without prolonging the reaction time, even though the -CH3 is
electron-donating (Table 2, entries 10-12). Functional group
such as -CHO was reduced, while -OMe and -CH2OH were
unreacted under the same conditions (Table 2, entries 13-16).
Heterocycles also performed well. Good yield was obtained for
2-chloro-5-nitropyridine after 3 h (Table 2, entry 17), however,
a longer reaction time and stronger light source (7 W blue LED)
were needed for 5-nitroindole substrate (Table 2, entry 18).
NH2
1
NO2
89%
Cl
Cl
2
3
NH2
NH2
NO2
NO2
94%
95%
Cl
Cl
4
5
6
7
(93%)
Cl
Br
I
NH2
NH2
NH2
NH2
Cl
Br
I
NO2
NO2
NO2
NO2
96%
99%
92%
97%
(92%)
(90%)
NC
NC
(95%)
HO
HO
8
9
NH2
NH2
NO2
NO2
94%
94%
99%
HO
HO
10
NO2
NH2
11
12
NH2
NH2
NH2
NH2
99%
97%
NO2
NO2
NO2
NO2
O
O
98%
99%
13
14
HOH2C
HOH2C
OHC
HOH2C
15
NO2
NH2
NH2
NH2
99%
97%
16
17
OHC
Cl
NO2 HOH2C
NH2
NO2
Complex 1 (1 mol%)
N
N
Yield
89%
Cl
NO2
NO2
95%
EtOH, NaBH4, r.t.
I
I
36 W white LED, 24h
85% sunlight, 8h
4.5 mmol 1.12 g
18
HN
HN
98%c
NH2
Fig. 2 Gram scale reactions
To demonstrate the synthetic utility of this method, the reaction
was performed on gram scale. Under standard conditions, p-
iodonitrobenzene was reduced to p-iodoaniline in 89% yield after 24
h (Fig. 2). By replacing the 36 W white LEDs with sunlight
(Summer, Wuhu, China), the reaction time was shortened to 8 h with
a similar yield of 85%.
a: Reaction conditions: nitroarene (0.1 mmol), photocatalyst 1 (1 mol %), NaBH4
(10 equiv), EtOH (2 mL), 36 W white LEDs, 3 h, at room temperature; b:
Conversion were determined by GC, isolated yields were in parenthesis; c: 3 h, 7
W blue LEDs.
We began our studies with 1-bromo-4-nitrobenzene as the
model substrate and NaBH4 as the reductant using ruthenium
complex photocatalyst in ethanol. The reaction was set up on
the benchtop without degassing, illustrating the ease of reaction
Proposed Mechanism
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3
Subsequently the triplet state Ru2+ can act as a reductant or
Ru-1
Rubpy
ISC
View Article Online
205.1
197.3
T2
T1
204.5
*1Ru2+
S1
(34.8 cm-1
(0.49 cm-1
)
9
DOI: 10.1039/C9DT00428A
oxidant (see Scheme 2 proposed mechanism). Single electron
transfer (SET) from 3Ru2+ to 4-bromonitrobenzene results in the
formation of nitroarene radical anion b and oxidation of the Ru
catalyst to the doublet complex Ru3+. Borohydride BH4 is
known to undergo hydrogen atom transfer, in the process
)
188.4
S1
3Ru2+
ISC
187.6T3
183.2 T2
(48.4 cm-1
(24.8 cm-1
)
*1Ru2+
2
-
)
T
178.0
1
(13.8 cm-1
)
3Ru2+
•– 29
hv
generating borane radical anion BH3
.
Hydrogen atom
-
hv
transfer from BH4 to the nitroarene radical anion b generates
nitroarene anion c. Concomitant electron transfer between 2Ru3+
and BH3 radical anion regenerates 1Ru2+ and BH3 and
•–
subsequent proton transfer from ethanol to nitroarene anion c
followed by elimination of water forms 4-bromonitrosobenzene
d and the [EtO-BH3]- complex. Further reduction of 4-
bromonitrosobenzene d leads to the observed product, 4-
bromoaniline.
0.0
0.0
S0
1Ru2+
1Ru2+
S0
Computational method:
M06L/DKH-def2-TZVP+DKH-TZVP//M06-L/def2-SVP
36 W white LEDs, RT
solvent, NaBH4, 2h,
Br
NO2
Br
NO
NH2
Br
photocatalyst (2 mol%)
a
Scheme 1 Energies of the singlet and triplet excited states of Ru-1 and Rubpy
based on S0 optimized electronic structures. Values are in kJ mol-1.
1Ru2+
d
hv
*Ru2+
3Ru2+
Trace amounts of azoxybenzene and azobenzene were detected
by GC-MS, which also suggests the ruthenium-catalyzed one-
electron reduction through a photo-induced metal to ligand
electron transfer, followed by electron transfer from metal
complex to nitrobenzene. Nitrosobenzene is tentatively formed
via proton and hydrogen transfer as reported.26-27
EtO BH3 + H2O
SET
EtOH
2Ru3+
Br
NO2
Br
NO2H
+
BH3
Computational modelling was performed to shed insights
into the photochemistry of complexes Ru-1 and Ru-bpy
(Scheme 1). The S0 electronic structures were used as an
approximation to calculate inter-system crossing since it is
computationally demanding to use TD-DFT for optimizing all
of the excited electronic structures. Given that the S1 and T1
excitations both involve MLCT, the relaxation energies of S1
and T1 should be comparable. 28 The vertical single-triplet gap
is thus comparable as the adiabatic one, indicating that our
calculations based on the S0 molecular geometry is a reasonable
approximation. The TD-DFT calculations reveal that the singlet
excited S1 state’s relative energy with respect to S0 for Ru-1 is
188.4 kJ mol-1, much lower compared to Ru-bpy’s S1 relative
energy of 204.5 kJ mol-1. The S1 state could further undergo
intersystem crossing (ISC) to the triplet excited states in Ru-1
and Ru-bpy. Our calculations show that for Ru-1, three triplet
states (T1—T3) sit below S1, with appreciable spin orbit
coupling (SOC) constants, suggesting that ISC to these triplet
states is probable. In particular, S1 and T3 are energetically
closest (S1-T3 energy gap = 0.8 kJ mol-1) with a strong SOC of
48.4 cm-1 in Ru-1, indicating highly efficient S1 to T3
transitions. In contrast, ISC transition from S1 to T1 in Ru-bpy
is quite unlikely due to weak SOC of 0.5 cm-1. While ISC from
S1 to T2 in Ru-bpy possesses a large SOC of 34.8 cm-1, this
value is ~28% lower than the S1-T3 SOC in Ru-1. Furthermore,
in Ru-bpy the energy level of S1 is slightly lower than that of
T2 (by 0.6 kJ mol-1), suggesting ISC from S1 to T2 is also
energetically unfavorable. Taken together, these calculations
show that the transition to the triplet states in Ru-1 is easier
than that in Ru-bpy.
-
+ BH4
b
c
Scheme 2 Proposed mechanism of the photocatalytic reduction reaction of
nitroarene.
In conclusion, we have synthesized and characterized a novel
ruthenium(II)-based photoredox complex bearing bisoxazoline
and bipyridine ligands and determinated its absorption and
emission spectrum, fluorescent lifetime, quantum yield and
reversible redox behavior. The ruthenium complex
was used,
together with sodium borohydride under visible light, for the
reduction of a variety of nitro compounds and proved to be more
efficient and robust than the parent [Ru(bpy)3]2+. The reduction
reaction offers varied functional group compatibility and broad
substrate scope, providing aromatic anilines in excellent yields. The
successful synthesis of ruthenium complex with bisoxazolines
could give an inspiration in the development of an efficient
photoredox catalysts in organic transformation.
Financial support from the National Nature Science Foundation of
China (21102004) and Natural Science Foundation of Anhui
Province (1708085MB44) to W. J., Singapore University of
Technology and Design (T1SRCI17126, T1MOE1706 and SUTD-
MIT IDC grant IDG31800104) and computational resources from
National Supercomputing Centre (Singapore) to X. L. and R. L. are
all gratefully acknowledged. We would like to thank Dr. Xiao-Wang
Liu, Ying-Hua Zhou and Li Zhang for the measurement and
discussion.
Conflicts of interest
There are no conflicts of interest to declare.
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DOI: 10.1039/C9DT00428A
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TOC
A novel ruthenium(II) complex containing bisoxazoline and bipyridine ligands has been synthesized and characterized, which
shows high catalytic activities for nitro compounds in the presence of sodium borohydride and visible light.
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