Ruthenium(II)-cored supramolecular organic framework-mediated recyclable visible light photoreduction of azides to amines and cascade formation of lactams

Article abstract of DOI:10.1016/j.cclet.2019.03.056

Ru(bpy)₃]²⁺-cored supramolecular organic framework SMOF-1, assembled from a [Ru(bpy)₃]²⁺-derived hexaarmed molecule and cucurbit[8]uril, has been demonstrated to heterogeneously catalyze visible light-induced reduction of phenyl, benzyl, 2-phenylethyl and 3-phenylpropyl azides in acetonitrile to produce the corresponding amines in good to high yields. For the last two kinds of azides that bear a CO₂Me group at the para-position of the benzene ring, cascade reactions take place to generate the corresponding lactams in high yields. Compared with homogeneous control [Ru(bpy)₃]Cl₂, SMOF-1 exhibits remarkably increased photocatalysis activity as a result of synergistic effect of the [Ru(bpy)₃]²⁺ units that form cubic cages to host the azide molecules and related intermediates. Moreover, SMOF-1 displays high recyclability and considerable photocatalysis activity after 3 to 12 runs.

Full text of DOI:10.1016/j.cclet.2019.03.056

Accepted Manuscript
Title: Ruthenium(II)-cored supramolecular organic
framework-mediated recyclable visible light photoreduction
of azides to amines and cascade formation of lactams
Authors: Yi-Peng Wu, Meng Yan, Zhong-Zheng Gao, Jun-Li
Hou, Hui Wang, Dan-Wei Zhang, Junliang Zhang, Zhan-Ting
Li
PII:
DOI:
Reference:
S1001-8417(19)30152-4
https://doi.org/10.1016/j.cclet.2019.03.056
CCLET 4892
To appear in:
Chinese Chemical Letters
Please cite this article as: Wu Y-Peng, Yan M, Gao Z-Zheng, Hou J-Li,
Wang H, Zhang D-Wei, Zhang J, Li Z-Ting, Ruthenium(II)-cored supramolecular
organic framework-mediated recyclable visible light photoreduction of azides to
amines and cascade formation of lactams, Chinese Chemical Letters (2019),
https://doi.org/10.1016/j.cclet.2019.03.056
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Communication
Ruthenium(II)-cored supramolecular organic framework-mediated recyclable visible
light photoreduction of azides to amines and cascade formation of lactams
Yi-Peng Wu, Meng Yan, Zhong-Zheng Gao, Jun-Li Hou*, Hui Wang, Dan-Wei Zhang, Junliang Zhang*,
Zhan-Ting Li*
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Centre of Chemistry for Energy
Materials (iChEM), Fudan University, Shanghai 200433, China
 Corresponding authors.
E-mail addresses: junliangzhang@fudan.edu.cn (J. Zhang), ztli@fudan.edu.cn (Z.T. Li).
Graphical Abstract
Porous [Ru(bpy)3]2+-cored supramolecular metal organic framework can efficiently catalyze visible light photoreduction of various
azides to afford amines and, through cascade reactions, lactams.
ARTICLE INFO
Article history:
Received 27 February 2019
Received in revised form 19 March 2019
Accepted 21 March 2019
Available online
ABSTRACT
Ru(bpy)3]2+-cored supramolecular organic framework SMOF-1, assembled from a [Ru(bpy)3]2+-derived hexaarmed molecule and cucurbit[8]uril, has
been demonstrated to heterogeneously catalyze visible light-induced reduction of phenyl, benzyl, 2-phenylethyl and 3-phenylpropyl azides in
acetonitrile to produce the corresponding amines in good to high yields. For the last two kinds of azides that bear a CO2Me group at the para-position
of the benzene ring, cascade reactions take place to generate the corresponding lactams in high yields. Compared with homogeneous control
[Ru(bpy)3]Cl2, SMOF-1 exhibits remarkably increased photocatalysis activity as a result of synergistic effect of the [Ru(bpy)3]2+ units that form cubic
cages to host the azide molecules and related intermediates. Moreover, SMOF-1 displays high recyclability and considerable photocatalysis activity
after 3 to 12 runs.
Keywords:
Supramolecular organic framework
Visible light photocatalysis
Recyclability
Azide reduction
Amine
Lactam
In recent years, photoredox catalysis has received great attention as a powerful strategy for the activation of organic molecules [1].
Metal complexes, organic dyes and other small molecules have been used to convert visible light into chemical energy by engaging in

single-electron transfer with discrete organic substrates [2-4]. In this category, [Ru(bpy)3]2+ and its derivatives have been demonstrated
as a family of most robust catalysts to mediate the transfer of electrons between different substrates and intermediates [5]. Covalent
attachment of the [Ru(bpy)3]2+ moiety to solid porous materials such as regular metal-organic framework or supramolecular organic
framework (SOF) has been revealed to achieve both high catalysis efficiency and recyclable use of the catalysts [6,7]. We have
previously developed a general self-assembly strategy for the generation of SOFs from multiarmed building blocks and cucurbit[8]uril
(CB[8]) in water [8]. By using a hexaarmed [Ru(bpy)3]2+-derived precursor, we have assembled the first [Ru(bpy)3]2+-cored
supramolecular metal organic framework (SMOF-1) [9], which adsorbs anionic Wells–Dawson-type polyoxometalate clusters to
enable visible light photocatalysis of the reduction of proton to hydrogen gas.
Reduction of organic azides is one of the most important protocols for the preparation of amines owing to the easy accessibility of
azides from respective halides, alcohols, and sulfonates [10]. Several groups have investigated homogeneous visible light
photocatalysis of the reduction of phenyl azides to the corresponding amines [11]. We recently reported that [Ru(bpy)3]2+-attached
tetraphenylmethane-derived SOF (SOF-CH=N-[Ru(bpy)3]) could function as heterogeneous recyclable catalyst for this conversion
[9]. We herein describe the visible light-induced recyclable heterogeneous photocatalysis of SMOF-1 for the reduction of phenyl,
benzyl and 2-phenylethyl azides to the corresponding amines. We further demonstrate that benzyl and 2-phenylethyl azides that bear a
methyl ester group at the ortho-position of the benzene ring undergo can cascade reactions to afford the corresponding lactams in good
to excellent yields.
SMOF-1 contains a [Ru(bpy)3]2+ core whose 2,2'-bipyriding ligands are covalently modified to form peripheral arms for the
formation of the regular porous framework [9]. Because the dynamic hydrazine bond of SOF-CH=N-[Ru(bpy)3] underwent partial
decomposition during the recycling photocatalysis [12], we investigated the catalysis efficiency of SMOF-1 for the reduction of phenyl
azides. Previous study with SOF-CH=N-[Ru(bpy)3] showed that phenyl azides that bear electron-withdrawing groups on the benzene
ring gave rise to higher yield of amines [9]. Thus, four substrates that bear electron-withdrawing groups were used for the present
study. As for earlier reports, the reactions were conducted in the presence of Hantzsch ester (H), i-Pr2NEt (A) and HCO2H (F) and the
flask was irradiated with a 50 W compact fluorescent light (CFL). The reaction conditions were first screened at room temperature
using 4-cyanophenyl azide 1X (R = 4-CN) as model substrate. In solvents of low polarity such as n-hexane or dichloromethane
(DCM), no or low yield of 4-cyano-substituted amine was formed (entries 1 and 2, Table 1). In acetonitrile, the reaction occurred
smoothly. After 9 h, thin layer chromatography (TLC) showed that the azide had consumed completely (entry 3, Table 1), and the yield
of the amine was determined to be 96% by HPLC. The reaction conditions, that is, SMOF-1 (0.1%), H (1.5 equiv.), A (10 equiv.) and
F (10 equiv.) in acetonitrile, were thus chosen for other substrates. In the mixture of acetonitrile and water (1:1, v/v), the reaction took
place much more quickly, and after 1 h the azide was consumed to give the amine in 95% yield (entry 15, Table 1). This result might
be attributed to the partial dissolution of SMOF-1 because the solution was orange, which should correspond to the absorption of the
[Ru(bpy)3]2+ complex. With sodium ascorbate as reducing agent [13], the amine was obtained in 43% yield (entry 16, Table 1). When
the reaction was applied to a 26 W CFL, only a trace of the amine was formed (entry 17, Table 1). Other azides such as those with 3-
CN or 4-CO2Me being on the benzene ring could also be reduced under similar conditions to afford the corresponding amine
derivatives in excellent yields (entries 18 and 19, Table 1).
The reaction was also investigated with control photocatalyst [Ru(bpy)3]Cl2. Compared with the above results (entries 2-4, Table 1),
this homogeneous complex catalyst (0.1 mole% relative to the azide) exhibited notably decreased activity. After 9 h, the yield of the
amine was 17% and 56%, respectively, when the reaction was conducted in dichloromethane or acetonitrile, whereas in binary
acetonitrile and water (1:1), the reaction afforded 70% of the amine after 1 h. Considering the heterogeneity of SMOF-1, we attributed
its increased catalysis ability to its porous structure, through which the eight [Ru(bpy)3]2+ units that formed a cubic cage could jointly
mediate the reduction of an azide and the intermediates that were located in the same cage (Fig. 1, vide infra).
Visible light-induced photocatalysis of SMOF-1 for benzyl azides 3X (n = 1) to form the corresponding amines 4X (n = 1) were
then conducted in acetonitrile under the conditions shown in entry 3, Table 1. The results are shown in Table 2. For all the six
investigated substrates, longer time was needed for their complete consumption, which is consistent with their decreased ability of
accepting electron as compared with phenyl azides. For benzyl azide and its derivatives that bear the CN, CO2Me, Cl or Me group at
the para-position of the benzene ring (entries 2 and 6-9, Table 2), the reaction afforded the amines in high yields (84-92%), whereas
for the azide that bears OMe (entry 9, Table 2), a typical electron-rich group, at the para-position of the benzene ring, the
corresponding amine was obtained in relatively low yield of 64%, reflecting that strong electron-rich group is unfavourable for the
reaction by reducing the electron-accepting ability of the azide. With [Ru(bpy)3]Cl2 of the same molar amount as catalyst, after 30
hours of irradiation, the reaction of (4-cyanophenyl)methyl azide (3X, R = CN, n = 1) gave rise to the corresponding amine in only
10% yield, which again supported the increased activity of the [Ru(bpy)3]2+ incorporated in the framework.
Encouraged by the above results for benzyl azides, we also studied the photoreduction of 2-phenylethyl azides 3X (n = 2) under the
above reaction conditions for benzyl azides. After about 30 h, the substrates were consumed and all the reactions gave rise to high
yields (78%-94%) (entries 10,11 and 14-16, Table 2), even though the reaction yield of the substrate that has a MeO group at the para-
position of the benzene ring was also the lowest. When the identical reaction conditions were applied for 3-phenylpropyl azide 3X (n =
3), the corresponding amine was obtained in 73% yield after long time of irradiation (70 hours). All the results showed the generality

of the photocatalysis approach for the reduction of aliphatic azides to amines. For 2-phenylethyl azide, when [Ru(bpy)3]Cl2 was used,
the amine was generated in the very low yield of 6%.
It has been reported that methyl 2-(aminomethyl) and 2-(2-aminoethyl)benzoates undergo intramolecular substitution to yield the
corresponding lactams [14,15], a family of organic compounds that may exhibit important biological activity [16]. We thus further
studied the cascade reaction of azides 5X that bear a CO2Me group at the ortho-position of their benzene ring. Although the reaction
times were different for the azides to be consumed, all the reactions of the five studied azides, that bears no or one electron-deficient or
rich group, produced the expected lactams with very high yields (90%-93%) (entries 1, 4, 14, 23 and 24, Table 3). When control
[Ru(bpy)3]Cl2 was used as catalyst, after irradiating for 40, 30, 30 or 70 h, the four azides (5X: n = 1, R = H, CN, Br, and n =2, R = H)
could afford the corresponding lactams in 3%, 2%, 2% or 8% yield, respectively.
The recyclability of the SMOF-1 catalyst was further studied for several azides. The catalysts were recovered by simply centrifuging
and then removing the solution and used for the next step. For 1X (R = 4-CN) (entries 3-14, Table 1), the catalyst was still able to
exhibit considerable activity after 12 runs, even though the reaction needed longer time, which well reflected the high stability of
SMOF-1 as a heterogeneous catalysis. Synchrotron X-ray powder diffraction experiment for the recovered SMOF-1 sample after 12
runs revealed a broad but discernible peak (Fig. S1 in Supporting information), which corresponded to the {100} spacing of the
calculated value of the modelled cubic framework [9]. This observation supported that the sample still kept its regularity after repeated
irradiation. The slow decrease of the catalysis activity with the increase of the run time may be mainly attributed to its partial loss
during the recovery process, even though the decomposition of the framework or the complex molecule cannot be excluded.
The recycling use of SMOF-1 was then investigated for several other azides. Generally, repeated use of the recovered catalysis
required elongated irradiation for achieving high yield of the corresponding products. For 3X (R = CN, n = 1) (entries 2-5, Table 2),
after 4 runs, the yield of the amine was still as high as 87% by elongating the irradiation time from 30 hours to 70 hours. For 3X (R =
CN, n = 2) (entries 11-13, Table 2), the catalyst could also be repeatedly used. However, after 3 runs, the yield of the amine decreased
considerably from 92% to 73%. Concerning the cascade reactions of the CO2Me-bearing azides, the reaction 5X (R = H, n = 1) gave
rise to the lactam in very high comparable yields for 3 runs (entries 1-3, Table 3). For 5X (R = CN, n =1), the first 3 runs also afforded
the lactam product in comparable yields (entries 4-6, Table 3), but further repeated use of the recovered catalyst led to continued
decrease of the reaction yield from 67% (run 4) to 30% (run 10) (entries 7-13, Table 3). For 5X (R = Br, n =1), the first 5 runs could
obtain high yield of the lactam (entries 14-18, Table 3), and from the sixth to the tenth, the yield decreased substantially from 66% to
20% (entries 19-22, Table 3). For 5X (R = H, n = 2), 7 runs were conducted and the reactions all generated the lactam in high yield
(entries 24-30, Table 3).
The mechanism of [Ru(bpy)3]2+-catalysed photoreduction of organic azides to amines in organic solvents in the presence of i-Pr2NEt
has been investigated previously by Chen and co-workers [11a]. It is reasonable to propose that the present reactions of azides 1X, 3X
and 5X proceeded through the same mechanism. The increased activity of the [Ru(bpy)3]2+ units of SMOF-1, as compared with
[Ru(bpy)3]Cl2 control, reflects the synergistic feature of the eight [Ru(bpy)3]2+ units that form one cubic cage (Figure 1), which
facilitated electron transfer from [Ru(bpy)3]+ to azide. The result also implied that porous SMOF-1 is highly transparent and thus could
allow for efficient penetration of visible light.
In conclusion, we have demonstrated that [Ru(bpy)3]2+-cored supramolecular organic framework can be used as heterogeneous
catalysis for the visible light-induced photoreduction of various organic azides to the corresponding amines, which can undergo
cascade reactions to afford discrete lactams through subsequent ester substitution. Generally, the framework-styled catalyst exhibited
very good recyclability, which can allow for more than 10 runs of repeating use and considerable retainment of the photocatalysis
activity. Moreover, compared with the homogeneous control [Ru(bpy)3]Cl2, the new framework-styled catalyst exhibits remarkably
increased catalysis activity due to the synergistic effect of the [Ru(bpy)3]2+ units that form the cubic cages to host the substrates or
intermediates. Thus, this new self-assembled photocatalysis system will be further explored for many other important reactions.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 21432004 and 21890732). We also thank
Shanghai Synchrotron Radiation Facility (beamlines BL16B1 and BL14B1) for providing the beam time for this work.
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Fig. 1. Proposed synergistic effect of the [Ru(bpy)3]2+ units of SMOF-1 that form the cubic cages for the photoreduction of organic azides to amines.

Table 1
Visible light-induced reduction of phenyl azides 1X to amines 2X using SMOF-1 as photocatalyst. a.
Entry Recycling
R
Solvent (molar
equiv.)
DCM/n-hexane
(1:1)
Time 2X Yield
(h)
9
No.
(%)b
trace
1c
4-CN
2
DCM
9
20
3
4
5
6
7
8
9
10
11
1
2
3
4
5
6
7
8
9
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
9
96
93
91
90
88
86
83
82
83
10
10
10
10
10
10
12
12
12
13
14
15
10
11
12
MeCN
MeCN
MeCN
MeCN/water
(1:1)
12
15
20
1
77
72
70
95
16d
MeCN
9
43
17e
18
19
MeCN
MeCN
4-CO2Me MeCN
9
12
12
trace
91
92
3-CN
a Reaction conditions: 0.1% molar amount of [Ru(bpy)3]2+ relative to the azides. Using a 50 W compact fluorescent light (CFL) located 20 cm away from the
reaction flask. The temperature of the reaction solution was kept at 25 °C by being exposed to a high-power fan.
b HPLC yield.
c A (2 equiv.), F (2 equiv.) in the reaction.
d Conducted in the presence of sodium ascorbate (10 equiv.) and with no H, A, or F.
e Using a 26 W CFL.

Table 2
Visible light-induced reduction of aliphatic azides 3X to amines 4X using SMOF-1 as photocatalyst.
Entry Recycling
R
n
Time
(h)
30
30
35
50
70
30
40
40
40
30
30
40
70
30
30
30
70
30
Yield
(%)a
89
92
88
89
87
92
84
87
64
92
92
86
73
94
81
78
73
6
No.
1
H
CN
1
1
2
3
4
5
1
2
3
4
6
7
8
9
CO2Me
Cl
Me
MeO
H
CN
1
1
1
1
2
2
10
11
12
13
14
15
16
17
18b
1
2
3
Cl
2
2
2
3
2
Me
MeO
H
H
a HPLC yield.
b With [Ru(bpy)3]Cl2 as photocatalyst.

Table 3
Visible light-induced reduction and subsequent intramolecular substitution of CO2Me-bearing aliphatic azides 5X to lactams 6X.
Entry
1
2
3
4
5
6
7
8
Recycling No.
R
H
n
1
Time (h)
40
50
50
30
50
80
90
110
110
160
180
200
200
Yield (%)a
1
2
3
1
2
3
4
5
6
7
8
9
10
93
87
88
90
92
88
67
61
39
43
41
27
30
CN
1
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
1
2
3
4
5
6
7
8
9
Br
1
30
70
90
110
140
240
270
300
320
50
93
90
88
90
90
66
60
40
20
91
92
94
94
92
93
91
84
MeO
H
1
2
1
2
3
4
5
6
7
70
90
110
170
170
200
260
a HPLC yield.