Merging N-Hydroxyphthalimide into Metal-Organic Frameworks for Highly Efficient and Environmentally Benign Aerobic Oxidation

Article abstract of DOI:10.1002/chem.202100987

Two highly efficient metal-organic framework catalysts TJU-68-NHPI and TJU-68-NDHPI have been successfully synthesized through solvothermal reactions of which the frameworks are merged with N-hydroxyphthalimide (NHPI) units, resulting in the decoration of pore surfaces with highly active nitroxyl catalytic sites. When t-butyl nitrite (TBN) is used as co-catalyst, the as-synthesized MOFs are demonstrated to be highly efficient and recyclable catalysts for a novel three-phase heterogeneous oxidation of activated C?H bond of primary and secondary alcohols, and benzyl compounds under mild conditions. Based on the high efficiency and selectivity, an environmentally benign system with good sustainability, mild conditions, simple work-up procedure has been established for practical oxidation of a wide range of substrates.

Full text of DOI:10.1002/chem.202100987

Accepted Article
Accepted Article
Title: Merging N-hydroxyphthalimide into Metal−Organic Frameworks
for Highly Efficient and Environmentally Benign Aerobic Oxidation
Authors: Man Wang, Gan Liang, Yunhao Wang, Tao Fan, Baoling
Yuan, Mingxian Liu, Ying Yin, and Liangchun Li
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202100987
Link to VoR: https://doi.org/10.1002/chem.202100987
01/2020

10.1002/chem.202100987
Chemistry - A European Journal
Merging N-hydroxyphthalimide into Metal−Organic Frameworks for
Highly Efficient and Environmentally Benign Aerobic Oxidation
Man Wang, Gan Liang, Yunhao Wang, Tao Fan, Baoling Yuan, Mingxian Liu, Ying Yin,
Liangchun Li*
M. Wang, G. Liang, Y. Wang, T. Fan, B. Yuan, Prof. M. Liu, Y. Yin, Prof. L. Li
Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science
and Engineering, Tongji University, Shanghai 200092, P. R. China
E-mail address: lilc@tongji.edu.cn
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Abstract: Two highly efficient metal−organic framework catalysts TJU-68-NHPI and
TJU-68-NDHPI have been successfully synthesized through solvothermal reactions of which
the frameworks are merged with N-hydroxyphthalimide (NHPI) units for the first time,
resulting in the decoration of pore surfaces with highly active nitroxyl catalytic sites. When
t-butyl nitrite (TBN) is used as co-catalyst, the as synthesized MOFs are demonstrated to be
highly efficient and recyclable catalysts for a novel three-phase heterogeneous oxidation of
activated C−H bond of primary, secondary alcohols and benzyl compounds under mild
conditions. Based on the high efficiency and selectivity, an environmental benign system with
good sustainability, mild condition, simple work-up procedure has been established for
practical oxidation of a wide range of substrates.
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Introduction
Oxidation, as one of the two most significant subjects in chemistry, has always been
attracting great research interests due to the ubiquitous involving in the reactions. Initially the
catalytic oxidation was developed with little environment and economy concerns using toxic
or noble metals, corrosive protic acids, stoichiometric noxious oxidants and hazardous
solvents, etc.[1] After the pioneering research was reported by Sharpless, et al. in 1976 using
the noble Ru catalyst and amine-N-oxides to catalyze very efficient oxidation of various
alcohols in mild condition,[2] significant progresses were made in the next few decades to
develop environmentally benign catalytic oxidation systems such as using H2O as solvent.[3]
Except for the concerns over catalysts, solvents and reaction conditions, developing
heterogeneous reaction system is very crucial for practical applications in virtue of the
tedious purification process resulting from homogeneous reactions generally featuring high
cost, undue energy consumption and challenges for complete removal of hazardous metal
residuals. The heterogeneous systems can render simple filtration to the work-up procedure
not only for facile separation of the product from the reaction mixture but also to collect the
solid catalyst for recycling, which is essential to develop ideal catalytic systems.[4-8]
As a kind of new soft porous crystalline solid, Metal−Organic Frameworks (MOFs),
also known as porous coordination polymers (PCPs),[9] have been emerging and booming in
both the fundamental and application researches because of their enormous diversity in
structures and functionalities.[10-18] MOFs have been considered and proved to be the most
promising materials for building up ideal catalysts due to their characteristics mainly in four
aspects,[19-22] i.e., heterogeneous nature for facile work-up process, large porosity facilitating
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diffusion of the substrates to render high efficiency, incorporation of metal and organic ligand
in one single system with the confinement effect for elevated activity, and strong
designability and functionalizability for specific catalysis such as size and shape
selectivity.[23-29] Based on these concepts, we have developed a new free-radical decorated
MOF (FRMOF) assembling from the copper and aminoxyl radical fused ligand
[4,7-bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl (DPIO)], which indeed showed
prominent performance for highly efficient and selective oxidation of various alcohols.[23]
However, the synthesis of DPIO is rather onerous with the usage of a large amount of oleum
and bromine etc., yet only 9.3% total yield could be achieved.
Instead of the stable aminoxyl type radicals of which the classic representative is
2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), another kind of even more superb radicals
are imidoxyl type, and the representative is phthalimido N-oxyl (PINO) which is not isolable
at ambient temperature but can be readily generated from the stable organic molecule
N-hydroxyphthalimide (NHPI).[30-31] Compared with the TEMPO, the synthesis of NHPI is
much easier via one step condensation reaction of the very cheap o-phthalic anhydride and
hydroxylamine, and the NHPI possesses higher oxidation capability.[32] Moreover, the
catalytic mechanism studies have demonstrated that the key catalytic species of the TEMPO
involved reaction is the oxoammonium salt while in the reactions of NHPI as catalyst the
catalysis is dominated by hydrogen abstracting from C‒H bonds via the generated active
PINO intermediate.[32-36] In practical applications, NHPI has hitherto achieved excellent
catalytic oxidation performance such as photocatalysis and electrocatalysis.[37-53] In order to
avoid the disadvantage of NHPI as homogeneous catalyst, greener heterogeneous catalytic
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systems have been developed by immobilization or confinement of NHPI molecules into
MOFs (solid catalysts) but suffered from the potential pore blocking, NHPI leaching and
harsh catalytic condition problems.[54-60] Compared with the catalysts synthesized by simple
NHPI wet-loading methods or NHPI anchored to the surface of the solid catalysts by
chemical bonding, MOFs constructed by NHPI functionalized ligands ensure the high-density
and regular distribution of catalytic sites as well as sufficient channel space for smooth
diffusion of the reactants that provide an ideal platform for innovation of green, efficient and
heterogeneous oxidation catalysts.
Herein, two Zr−based MOFs incorporated with new organic ligands functionalized with
NHPI units have been successfully synthesized for the first time (Scheme 1 and Figure 1).
The as-synthesized MOFs featuring high surface areas, good stability, uniform pores and
highly dense organo-catalytic sites render us a green catalytic system for cost-effective,
highly efficient and superior recyclable aerobic oxidations of various activated C−H
compounds under mild conditions.
Results and Discussion
Synthesis and Characterization
Development of general method to merge the NHPI unit into the organic ligand is of great
significance because it will pave the way for versatile functional materials covalently
modified by the critical imidoxyl (INO) radicals. However in view of the synthetic route
design it is very challenging to synthesize the ligands H2LNOH (with one NHPI group) and
H2L2NOH (with two NHPI groups), since highly selective condensation of the hydroxylamine
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with carboxylic acid or anhydride is difficult and the general low solubility of the ligands
with imide and carboxylic acid groups makes the purification formidable (Scheme 1 and
Figure 1). Tremendous efforts have been devoted to the traditional strategies such as
N-hydroxyl group protection and de-protection but all failed. Surprisingly, a straightforward
condensation of the hydroxylamine with the corresponding tetracarboxylic acid compound 12
or hexacarboxylic acid compound 15 afforded the target compounds H2LNOH and H2L2NOH in
95.4% and 92.3% yields respectively (Scheme 1, S1 and S2, see the Supporting Information).
Moreover, as shown in Scheme 1, only through three key steps of simple organic reactions
including Suzuki−Miyaura coupling for multi-methyl substituted p-terphenyl 11 and 14,
subsequent KMnO4 oxidation to its multi-carboxylic acids 12 and 15, and selective
condensation to the targets, the total yields of H2LNOH and H2L2NOH can achieve 78.6% and
64.4% respectively without necessitating the synthesis of phthalic anhydride intermediates, of
which not only the synthesis is difficult but also the purification is laborious (see the
Supporting Information). After the ligands were synthesized in gram scales, in consideration
of the outstanding stability of UiO (Univeristy of Olso) MOFs the optimal solvothermal
reaction conditions using these ligands to synthesize the corresponding UiO MOFs were
screened out.[61] It was found that although the solvothermal reactions in the presence of
modulator acetic acid at elevated temperature higher (e.g., 100 °C) could result in highly
crystalline UiO MOFs, the pre-functionalized NHPI groups on MOF linkers were partially
hydrolyzed which could be verified by NMR spectra of the HF digested samples (Figure S1).
After a number of trials, the TJU-68-NHPI with intact NHPI groups could be synthesized by
the solvothermal reaction of H2LNOH ligand with ZrCl4 and acetic acid under 80 °C, while the
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TJU-68-NDHPI with little hydrolyzed imide N-hydroxyl (INOH) groups could be only
prepared by H2L2NOH ligand in the presence of benzoic acid modulator through a specific
programmed temperature process (Figure 1a‒b, and the Supporting Information).[62-63]
Powder X−ray diffraction (PXRD) patterns of the as-synthesized TJU-68-NHPI and
TJU-68-NDHPI are in good agreement with that of the simulated UiO-68 from single-crystal
data, suggesting their high phase purity.[64] Both of the activated TJU-68 MOFs are liable to
gradually lose the crystallinity intensity when exposed in the air moisture accompanied by
color change from white to yellow, but they are very stable in common organic solvents such
as acetonitrile and dichloromethane (Figure 1c‒d).
X
B(OH)2
H3C
CH3
HOOC
COOH
R1
R1
CH3
CH3
KMnO4
Pd(PPh3)4
Na2CO3
H3C
CH3
HOOC
COOH
+
Pyridine
R1
R1
R2
R2
CH3
Br
12, R2 = H (96.7%)
15, R2 = COOH (79.5%)
11, R1 = H (85.2%)
14, R1 = Me (87.7%)
10, X = I, R1 = H
13
, X = Br, R1 = Me
OH
N
O
O
O
O
OH
N
O
O
NH2OH HCl
Pyridine
and
HOOC
COOH
HOOC
COOH
N
OH
H2L
NOH (95.4%)
H2L
2NOH (92.3%)
Scheme 1. Synthetic scheme of ligand H2LNOH and H2L2NOH
.
As shown in the Scanning Electron Microscopy (SEM) imagines, the TJU-68-NHPI and
TJU-68-NDHPI crystals feature very regular octahedron shapes with uniform particle sizes of
ca. 20 µm and 0.50 µm, respectively (Figure 1e‒f). Based on these unambiguous
characterizations the essential difference of TJU-68 to UiO-68 MOFs is the central phenyl
group of the ligand modified by one or two INOH groups at the molecular level.[64] Due to
the features of the organic linkers, the precursors of PINO free radical are periodically
integrated onto the pore surface of the TJU-68 MOFs. It is worth mentioning that this
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approach will give inspiration for conceiving of multifunctional MOFs bearing high-density
active catalytic sites for sensing, catalysis and energy storage, etc.
Figure 1. Synthetic scheme of (a) TJU-68-NHPI and (b) TJU-68-NDHPI. (O, red; C, gray; N,
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blue; H atoms have been omitted). PXRD patterns of (c) TJU-68-NHPI and (d)
TJU-68-NDHPI. Simulated from the UiO-68 (black), as synthesized sample (red), activated
by drying under vacuum at 120 °C for 12 h (blue), re-soaked in CH3CN after the activation
(purple). SEM images of (e) TJU-68-NHPI and (f) TJU-68-NDHPI.
In order to investigate the porosity of activated TJU-68-NHPI and TJU-68-NDHPI, gas
sorption studies of N2 and CO2 were conducted at different temperatures (Figure S2a‒b). For
compounds TJU-68-NHPI and TJU-68-NDHPI, the N2 adsorption isotherms manifest type Ⅰ
with high uptake at low pressure and little hysteresis, indicating the robustness and no strong
interactions between the N2 gas molecules and porous frameworks.[65] The
Brunauer−Emmett−Teller (BET) surface areas of TJU-68-NHPI and TJU-68-NDNPI
calculated from the N2 adsorption isotherms are 2558 m2 g‒1 and 1470 m2 g‒1 respectively,
appreciably decreasing from that of the UiO-68 (without INOH functionalization, 4240 m2 g‒
1).[66] In addition, the pore volumes of TJU-68-NHPI and TJU-68-NDHPI are calculated to be
0.9741 cm3 g‒1 and 0.5996 cm3 g‒1 respectively, which also decrease drastically from that of
UiO-68 (1.82 cm3 g‒1)[66], attributed to the densely functionalized INOH groups and futher
proving the success of this bottom-up functionalization strategy.
Catalytic Performance Tests
The ideal porous structure with high density of accessible organocatalytic sites decorated on
the pore surface and good stability of the TJU-68 MOFs encouraged us to employ them as
heterogeneous catalysts for oxidation reactions. At first, we choose 1-phenylethanol as the
model substrate to screen the optimal reaction condition by changing the additives/initiators
and solvents (Table S1). Without the initiators the oxidation could not proceed under O2
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atmosphere, indicating that the NHPI groups on MOF frameworks could not be directly
activated by the molecular O2 to generate PNIO radicals for oxidation. By addition of various
inorganic and organic initiators including Co(OAc)2, Mn(OAc)2, Co(NO3)2, Cu(OAc)2 and
dimethylglyoxime (DMG), the catalytic reactions gave trace to low yields of the
acetophenone product, which might be attributed to their very slow accessing to the INOH
functional sites. However, facilitated by AIBN additive 52% and 76% moderate yields were
achieved. The yields were further improved to be quantitative when 30 mol% of t-butyl
nitrite (TBN) was used. Not only that, the low boiling point and high volatility (i.e., easy
removability) of the TBN and resultant degraded product t-butyl alcohol could make the
work-up procedure straightforward and eco-friendly.[67] And then, with the TBN as initiator
various common solvents were tested and the relatively green solvent acetonitrile was found
to be optimal (Table S1 and Table 1, entry 1). Finally, the oxidants were examined by using
atmospheric air, pure oxygen and nitrogen gas, affording moderate (63%, 68%), quantitative
and trace amount of the product respectively, which prove that oxygen gas is the terminal
oxidation source (Table 1, entries 1−3). It is worth mentioning that using 5 mol% of the
TJU-68-NDHPI possessing twice of the catalytic site density exhibited similar high
efficiency compared with that of using 10 mol% of the TJU-68-NHPI catalyst, suggesting the
density of the catalytic site is appropriately proportional to the catalytic efficiency in this
heterogeneous system (Table 1, entries 1−2). The reaction in the absence of MOF catalyst
afforded 8% yield induced by TBN (Table 1, entry 4). The attempts to a homogeneous
catalysis by the ligands led low yields (42% and 36%) probably attributed to the insufficient
solubility of the H2LNOH and H2L2NOH in acetonitrile (Table 1, entry 5). In addition, to
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evaluate the effect of Zr metal, catalytic reactions in the presence of ligand and ZrCl4 were
performed to give a complex mixture with the yields of target acetophenone in 76% and 82%,
respectively, further indicating the superiority of MOF catalysts (Table 1, entry 6 and Figure
S3). Based on the above screening experiments, the optimal condition for this
heterogeneously catalytic system is that 10 mol% of TJU-68-NHPI or 5 mol% of
TJU-68-NDHPI with 30 mol% TBN under the atmospheric pressure of O2 gas in acetonitrile
at 80 °C for 5 h.
Table 1. Catalytic Oxidation of 1a with difference initiators.[a]
OH
O
catalyst
initiator
(30 mol%)
(5 or 10 mol%),
O2 (balloon), CD3CN, 80 C, 5 h
Entry
1
Initiator
TBN
Catalyst
Yield[b] (%)
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
/
99
99
63
2[c]
TBN
68
Trace
Trace
8
3[d]
TBN
TBN
TBN
4
H2LNOH
42
5[e]
H2L2NOH
36
H2LNOH + ZrCl4
H2L2NOH + ZrCl4
76
6[f]
TBN
82
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[a] Reaction conditions: 0.5 mL CD3CN, 0.075 mmol 1-phenylethanol, TJU-68-NHPI (10
mol%)/TBN (30 mol%) or TJU-68-NDHPI (5 mol%)/TBN (30 mol%). [b] Determined by
1H NMR spectroscopy and GC−MS using nitrobenzene or naphthalene as internal standard.
[c] Reaction carried out under air. [d] Reaction carried out under nitrogen. [e] Reaction
conditions: 0.5 mL CD3CN, 0.075 mmol 1-phenylethanol, H2LNOH (10 mol%)/TBN (30
mol%) or H2L2NOH (5 mol%)/TBN (30 mol%). [f] Reaction conditions: 0.5 mL CD3CN,
0.075 mmol 1-phenylethanol, H2LNOH (10 mol%)/ZrCl4 (10%)/TBN (30 mol%) or H2L2NOH
(5 mol%)/ZrCl4 (5%)/TBN (30 mol%).
Under the optimized reaction condition a broad scope of secondary alcohols have been
explored and oxidized to the corresponding ketone products (Table 2). Notably, a series of
substituted secondary alcohols containing aromatic and aliphatic alcohols can be successfully
oxidized by TJU-68 MOFs to desired products with excellent yields (92% − 100%, Table 2,
entries 1−16). The longer or cyclic alkyl chains make the oxidation of benzylic alcohols
slightly adverse. However excellent yields (95% − 99%) can still be obtained by prolonging
the reaction time to 5 h (Table 2, entries 1−4). On the basis of quantitative yields of the
corresponding ketones, the reaction times of phenyl secondary alcohols substituted with
electron-donating groups are shorter than that of electron-withdrawing group substituted ones
(3 h vs 4 h, Table 2, entries 5−8). The extended π−conjugation all enable the oxidation
reactions to accomplish in shorter times (Table 2, entries 9−10). It is notable that the
diphenylethanediol 1k featuring diol groups smoothly underwent the bis-oxidation to the
benzil 2k in 95% and 92% yields respectively, necessitating reaction times over 10 h (entry
11). A variety of other linear or cyclic aliphatic secondary alcohols can also be oxidized to the
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corresponding ketones in longer reaction times (8 h or 10 h), affording very good yields
(Table 2, entries 12−16). Strikingly, the oxidation of secondary alcohols by this TJU-68
MOFs system reveals broad substrate applicability, excellent yield, outstanding selectivity
and much higher efficiency compared with previously documented MOF catalytic oxidation
systems, which generally demand much longer reaction time with lower conversion
efficiency.[68-69]
Table 2. TJU-68-NHPI (TJU-68-NDHPI) Catalyzed Oxidation of the Secondary Alcohols.[a]
Entry
1
Alcohol
Product
Catalyst
Time (h) Yield[b] (%)
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
4
4
4
4
5
5
5
5
3
3
3
3
99
99
98
97
96
95
97
95
99
99
99
99
1a
2a
2
3
4
5
6
1b
2b
1c
2c
1d
2d
1e
2e
1f
2f
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TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
TJU-68-NHPI
TJU-68-NDHPI
4
4
100
100
100
100
98
99
7
1g
2g
4
8
1h
2h
4
4
9
1i
1j
2i
4
3
98
97
10
11
2j
3
12
10
10
10
8
95
92
1k
2k
92
94
12
1l
2l
89
92
13
1m
2m
8
8
95
90
14
15
1n
2n
8
8
94
93
1o
2o
8
8
97
98
16
1p
2p
8
[a] Reaction conditions: 0.5 mL CD3CN, 0.25 mmol alcohol, TJU-68-NHPI (10 mol%)/TBN
1
(30 mol%) or TJU-68-NDHPI (5 mol%)/TBN (30 mol%). [b] Determined by H NMR
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spectroscopy and GC−MS using nitrobenzene or naphthalene as internal standard.
Furthermore, the primary benzyl alcohols can also be oxidized efficiently by this
catalytic system. As shown in Table 3, all the oxidations catalyzed by TJU-68-NHPI achieved
very high conversion with good to excellent selectivity. It was found that aromatic primary
alcohols regardless of the electron-donating and electron-withdrawing substituents are
oxidized to the corresponding aldehydes as the major products and the carboxylic acid as the
minor products (Table 3). Neither decreasing the reaction temperature nor increasing the
catalyst and TBN amounts gave improved selectivity to the aldehyde or acid product.
However, the higher electron-donating ability of the substituents, the alcohol substrates is
more propitious to be oxidized to the carboxylic acid products. The trimethoxybenzyl alcohol
3d could be completely oxidized to give 43% of acid product 5d (Table 3, entry 4). The
oxidation of the substrates with electron-withdrawing groups (i.e., NO2, F, Cl, Br, I) always
leads to excellent conversion, however it is not inevitable that the stronger the
electron-withdrawing ability of the substrates, the higher the selectivity of conversion to
carboxylic acid products (Table 3, entries 5−10). The nitro substituted benzyl alcohol 3e was
oxidized in high conversion of 98% and selectivity of 89% to the aldehyde 4e (Table 3, entry
5), whereas the strong electron-withdrawing group fluoro substituted benzyl alcohol 3f was
oxidized in 99% conversion with low selectivity of 50% to the acid 5f (Table 3, entry 6). The
oxidation of furan-2-ylmethanol 3l resulted in a moderate conversion of 76% with supreme
selectivity of 99% to the aldehyde 4l (Table 3, entry 12). The thiophen-2-ylmethanol 3m was
readily oxidized in 99% conversion to 84% yield of the aldehyde 4m and 16% yield of the
acid 5m (Table 3, entry 13). In addition, the chemo-selectivity of this oxidation system was
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investigated by 3n with primary and secondary alcohol groups, and the results revealed that
the primary alcohol was more easily oxidized than the secondary alcohols (43% vs 32%,
Table 3, entry 14). Albeit the reaction conditions were desperately optimized by prolonging
the reaction time, decreasing the dosage of the catalyst and additive, etc., the oxidation of
primary alcohols to achieve excellent conversion as well as selectivity was elusive because
the degree of the oxidation strongly depends on the oxidation capability of the catalytic
systems. Compared with the homogeneous oxidation catalyzed by NHPI which can only
oxidize the primary benzyl alcohols to the aldehydes, our heterogeneous oxidation systems
exhibited higher oxidation capability to partly over-oxidize them to the corresponding acids,
probably due to the confinement effect of the TJU-68 MOFs.[38] Base on the same INOH
functional groups, similar oxidations of all the primary alcohols catalyzed by 5 mol% of
TJU-68-NDHPI were implemented, affording comparable conversion and selectivity to that
of TJU-68-NHPI (Table S3).
Table 3. TJU-68-NHPI Catalyzed Oxidation of the primary alcohols.[a]
Conversion[b] Selective[b]
Entry
Substrate
Product
(%)
(%)
51
4a
1
97
3a
49
5a
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90
4b
5b
2
3
4
99
OH
3b
O
10
84
16
4c
97
3c
5c
56
4d
100
3d
3e
43
89
11
50
50
77
23
73
27
74
26
5d
4e
5
6
7
8
9
98
99
93
99
99
5e
4f
3f
5f
4g
3g
5g
4h
3h
5h
4i
3i
5i
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82
18
72
28
4j
10
11
99
80
3j
5j
4k
3k
5k
4l
99
0
12
13
76
99
3l
5l
84
16
4m
3m
5m
43
32
25
4n
4n′
4n″
14
82
3n
[a] Reaction conditions: 0.5 mL CD3CN, 0.075 mmol alcohol, TJU-68-NHPI (10 mol%)/TBN
1
(30 mol%). [b] Determined by H NMR spectroscopy and GC−MS using nitrobenzene or
naphthalene as internal standard.
In order to further unveil the oxidation capability of this catalytic system, a variety of
substrates with less activated benzyl C−H bonds were subjected to this oxidation system and
the results are presented in Table 4. The ethylbenzene 6a was smoothly oxidized to the
acetophenone 7a in 10 h in good yield and conversion (Table 4, entry 1). In particular,
oxidation of the diphenylmethane derivatives 6b, 6c, 6d successfully produced the
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corresponding ketones in quantitative yield with superb selectivity (Table 4, entries 2−4). The
benzyl ether derivatives could be readily oxidized quantitatively but the selectivity varied
from 36% to 97% (Table 4, entries 5−7).
Table 4. TJU-68-NHPI (TJU-68-NDHPI) Catalyzed Oxidation of Benzyl C−H bonds.[a]
Time
(h)
Conversion[b] Selective[b]
Entry Substrate
Product
Catalyst
(%)
71
76
(%)
80
86
95
97
99
98
99
94
97
96
36
58
74
81
TJU-68-NHPI
10
1
6a
7a
TJU-68-NDHPI 10
TJU-68-NHPI 10
TJU-68-NDHPI 8
TJU-68-NHPI
TJU-68-NDHPI 6
TJU-68-NHPI
94
93
2
3
4
5
6
7
6b
7b
8
98
99
6c
7c
8
96
97
6d
7d
TJU-68-NDHPI 6
TJU-68-NHPI
TJU-68-NDHPI 8
TJU-68-NHPI
TJU-68-NDHPI 6
TJU-68-NHPI
TJU-68-NDHPI 6
8
97
98
6e
7e
8
100
100
91
95
6f
7f
8
6g
7g
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[a] Reaction conditions: 0.5 mL CD3CN, 0.20 mmol substrate, TJU-68-NHPI (20
mol%)/TBN (50 mol%) or TJU-68-NDHPI (10 mol%)/TBN (50 mol%). [b] Determined by
1H NMR spectroscopy and GC−MS using nitrobenzene or naphthalene as internal standard.
Heterogeneity and Recyclability
The most important advantage of these MOF catalysts constructed by bottom-up strategy is
their sophisticated integration of the merits of homogeneous and heterogeneous catalysts, i.e.,
comparable or even higher efficiency than the homogeneous catalysts, good recyclability and
resultant simple purification procedure as the heterogeneous catalysts, etc. The heterogeneous
characteristic of this catalytic system was elucidated by control experiments. As shown in
Figure 2a, 1-phenylethanol was oxidized smoothly in 2 hours to give 38% conversion. The
reaction after catalyst filtration kept on heating for another 2 hours and gave an almost
constant conversion of 40%, indicating the reaction of filtrate mixture ceased after the MOF
catalyst was filtered out, while the parallel reaction without performing catalyst filtration
afforded a conversion of 99%, proving that the reaction can only proceed with the MOF
catalyst (see the Supporting Information). Additionally, the recyclability of the TJU-68 MOF
catalysts was also evaluated by consecutive runs in the same reaction condition using the
catalyst recycled via centrifugation. After ten cycles of the consecutive reactions, still a yield
of no less than 88% could be obtained (Figure 2b). Furthermore, the Zr contents in the filtrate
of the first and fifth cycles were detected to be 0.00638 and 0.00627 wt% of the total amount
respectively by inductively coupled plasma−optical emission spectroscopy (ICP−OES)
analyses, indicating negligible metal leaching to the reaction solution (Table S2, entries 1−2).
The carboxylic acid product in this catalytic oxidation system resulted in a little Zr leaching
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(0.185 wt%, Table S2, entries 3) but the catalyst was not poisoned to lose its catalytic
capability. The PXRD patterns of the TJU-68 MOFs before and after the catalytic reactions
are in good agreement and without appreciable decreasing of the intensity, suggesting good
stability of these TJU-68 MOF catalysts in this aerobic oxidation condition (Figure 2c−d).
Moreover, the outstanding selectivity, efficiency, conversion and volatile solvent and side
products of this heterogeneous catalysis system endowed us with an economic and
eco-friendly product purification process. Taking the oxidation of alcohol 1f as an example,
after the general reaction procedure for 3 hours, the MOF catalyst was collected by simple
centrifugation or filtration, and the filtrate was concentrated to recover the reaction solvent
acetonitrile and dried under vacuum to give the product 2f in 91% yield (theoretically shall be
99%) with over 99% purity, avoiding the tedious purification by chromatography or high
energy-consuming distillation process (Figure S4−6).
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Figure 2. (a) Plots of conversions versus reaction time of the 1-phenylethanol oxidation
catalyzed by TJU-68-NHPI. (b) Recycle tests of TJU-68-NHPI (green bars) and
TJU-68-NDHPI (purple bars). PXRD profiles of the TJU-68-NHPI (c) and TJU-68-NDHPI
(d) before and after the catalytic reactions.
Mechanistic Studies
The reaction mechanism of this NHPI catalyzed oxidation has been intensively studied and
70-71]
demonstrated in various systems, mainly for homogeneous ones.[38,
In this
heterogeneous system, intrinsically the mechanism is similar (Figure 3). First of all, under
elevated temperature the TBN will decomposed to generate NO species which will directly
react with molecular oxygen gas to produce the very active NO2 oxidant.[72] And then the
small NO2 molecules will readily diffuse into the solution and MOF pores, transforming the
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INOH groups on the pore surface to the INO radical active sites, that is, conversion of the
TJU-68-NHPI to TJU-68-PINO. Simultaneously the NO2 molecules are reduced to NO
molecules furnishing the other half of the cycle in the gas phase. The high density INO
radicals can efficiently abstract the hydrogen of activated C−H bonds on the alcohols or
benzylic positions to generate C radical intermediates RI1, which subsequently activate the
molecular O2 to form the α–hydroxyl peroxyl radical intermediates RI2. Finally, the
metastable RI2 convert to the stable products by redox decomposition. The product
aldehydes (R2 = H) are potentially over-oxidized to the corresponding acids via the same
pathway. To examine the involvement of the radicals in this oxidation system, 0.5 equivalents
of radical scavenge 2,6-diterbutul-4-methylphenol was added to the oxidation reaction of
1-phenylethanol resulting in little conversion (3%) to the acetophenone product (see the
Supporting Information). A straightforward evidence for the existence of critical radical
species was obtained by electron paramagnetic resonance (EPR) spectra of the solid catalyst
in the reaction system, showing a free radical signal at g = 2.0073 which is an essential
characteristic of PINO radical in the pore.[73-75] The broadening of the EPR spectra was
observed and indicated that the very active PINO radicals were successfully generated and
confined on the TJU-68 frameworks under the reaction conditions (Figure S7a‒b).[68] Most
strikingly, this is a novel three-phase heterogeneous catalytic system first stated in this work,
manifesting very high efficiency, outstanding selectivity, facile work-up procedure with green
chemistry characteristics (Figure 3).
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Figure 3. Proposed mechanism of the oxidation catalyzed by the TJU-68-NHPI/TBN/O2
system.
Conclusions
In conclusion, a straightforward synthetic strategy has been developed to merge the NHPI
unit as radical precursor sites into the organic ligand for bottom-up construction of novel
free-radical TJU-68 MOFs. By comprehensive studies on the MOF characteristics and
optimal reaction condition and mechanism, an unprecedented three-phase heterogeneous
system has been established for catalytic oxidation of a wide scope of primary, secondary
alcohols and benzyl compounds. The highly dense radical sites orderly decorated on the pore
surfaces render the catalytic oxidation high efficiency, excellent selectivity, mild condition
and environmental friendliness with molecular O2 as terminal oxidant. Moreover, these MOF
catalysts with satisfactory stability can catalyze the oxidation for ten cycles with little
decrease of the conversions. The universal synthetic method reported here to functionalize the
MOFs with the critical NHPI units will amalgamate the active radical chemistry with
versatile MOF materials. Further efforts towards development of sophisticated MOFs with
synergic effect of the organic catalytic sites and open metal sites for ideal catalysis are
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currently underway.
Experimental Section
Materials and Methods
All the solvents used in both synthesis and performance testing were purchased and used
without further purification. The synthetic procedures of the ligands for the MOFs were
described in the Supporting Information. Nuclear magnetic resonance (NMR) data were
obtained by Bruker ARX400 and ARX600 spectrometer (400 MHz/600 MHz). Powder
X−ray diffraction (PXRD) patterns were collected on a Bruker D8 Advance diffractometer at
40 mA, 40 kV for Cu Kα (λ = 1.5418 Å) at room temperature with 2° min−1 at the range of 4
− 40°. Scanning Electron Microscopy (SEM) images were performed on a Hitachi S−4800
instrument. Gas adsorption measurements were conducted using the adsorption equipment
TRISTAR 3020. The reaction products of aerobic oxidation were analyzed in Gas
Chromatography Mass Spectrometry (GC−MS, SHIMADZU GCMS−QP2010 SE W). The
Zr4+ concentrations were analyzed by an inductively coupled plasma−optical emission
spectroscopy (ICP−OES, PerkinElmerOptima 8300). Electron Paramagnetic Resonance (EPR)
spectrum were obtained by Bruker A300 spectrometer at the following experimental
parameters: frequency, 9.852609 GHz; power, 18.26 mW; modulation amplitude, 0.1 G;
modulation frequency, 100 kHz; sweep Time, 41.94 s; center field, 3510 G; sweep width, 50
G.
Synthesis of TJU-68-NHPI
A mixture of ZrCl4 (3.00 mg, 0.01 mmol) and H2LNOH (5.00 mg, 0.01 mmol) in DMF (2.5
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mL) was ultrasonically dissolved in a 10 mL Pyrex vial. Acetic acid (150 μL) was added to
the reaction mixture followed by sonication. The clear solution was heated at 80 °C in an
oven for 3 days to give white powder after filtration (yield 72%, based on the ligand).
Synthesis of TJU-68-NDHPI
A mixture of ZrCl4 (2.40 mg, 0.01 mmol) and benzoic acid (0.10 g, 0.82 mmol) in DMF (3
mL) was dissolved in a 10 mL Pyrex vial by shaking. H2L2NOH (5.00 mg, 0.01 mmol) and
deionized water (12 μL) were added subsequently to the reaction mixture followed by
sonication. The clear solution was heated at 70 °C for 48 h, then temperature was elevated to
85 °C for 24 h, finally the temperature was increased to 100 °C and kept for another 48 h.
The resulting white powder was obtained by filtration (yield 68%, based on the ligand).
Catalytic Reactions
To a 10 mL glass tube was added the alcohol (75.00 μmol), catalyst (7.50 μmol of
TJU-68-NHPI or 3.75 μmol of TJU-68-NDHPI, or 7.50 μmol of other catalyst), tert-butyl
nitrite (22.50 μmol) and 0.5 mL CD3CN. The glass tube was sealed with rubber stopper and
connected to an O2 balloon through a long needle. After smoothly discharging the air in the
glass tube by oxygen bubbling, the reaction mixture was fixed in a thermoshaker for and
heated at 80 °C with a shaking speed of 900 rpm for a set time, and the conversions were
1
checked by H NMR spectra and GC−MS using nitrobenzene or naphthalene as internal
standard.
Supporting Information
Supporting information for this article is available on the WWW under
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https://doi.org/10.1002/chem.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(Grant Nos. 21501135, 21875165), the Fundamental Research Funds for the Central
Universities, and the Recruitment Program of Global Experts of China.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: N-hydroxyphthalimide, metal−organic frameworks, heterogeneous catalysis,
aerobic oxidation, nitroxyl radical
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