Generation and Confinement of Long-Lived N-Oxyl Radical and Its Photocatalysis

Article abstract of DOI:10.1021/jacs.7b12928

Generation of controllable carbon radical under the assistance of N-oxyl radical is an efficient method for the activation of C-H bonds in hydrocarbons. We herein report that irradiation of α-Fe₂O₃ and N-hydroxyphthalimide (NHPI) under 455 nm light generates phthalimide-N-oxyl radical (PINO), which after being formed by oxidation with holes, is confined on α-Fe₂O₃ surface. The half-life time of the confined radical reaches 22 s as measured by in situ electron paramagnetic resonance (EPR) after the light being turned off. This allows the long-lived N-oxyl radical to abstract the H from C-H bond to form a carbon radical that reacts with molecular oxygen to form R₃C-OO· species, decomposition of which leads to oxygenated products.

Full text of DOI:10.1021/jacs.7b12928

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Communication
Generation and Confinement of Long-
lived N-oxyl Radical and its Photocatalysis
Chaofeng Zhang, Zhipeng Huang, Jianmin Lu, Nengchao Luo, and Feng Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12928 • Publication Date (Web): 24 Jan 2018
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Journal of the American Chemical Society
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Generation and Confinement of Long-lived N-oxyl Radical and
its Photocatalysis
Chaofeng Zhang,a Zhipeng Huang,a,b Jianmin Lu,a Nengchao Luo,a,b and Feng Wanga*
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a State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Phys-
ics, Chinese Academy of Sciences, Dalian 116023, China
b University of Chinese Academy of Sciences, Beijing 100049, China
Supporting Information Placeholder
Metal oxides are frequently used catalysts in organic
ABSTRACT: Generation of controllable carbon radical un-
der the assistance of N-oxyl radical is an efficient method for
reactions.12 When used as photocatalysts, pristine metal
oxide surface is usually lack of photocatalytic center to
the activation of C–H bonds in hydrocarbons. We herein
adsorb and cleave the C–H bond of hydrocarbons into a
carbon radical. Besides, the wide band gap and/or fast
recombination of photogenerated hole (h+) and electron
(e−) remains obstacles for photo harvesting. Solving these
issues by introducing defect level13 and substrate modifi-
cation12a, 12b have been studied. In addition, adding free
photosensitizer to introduce e− or adding h+ acceptor to
report that irradiation of α-Fe2O3 and N-hydroxyphthalimide
(NHPI) under 455 nm light generates phthalimide-N-oxyl
radical (PINO*), which after being formed by oxidation with
holes, is confined on α-Fe2O3 surface. The half-life time of
the confined radical reaches 22 seconds as measured by in
situ electron paramagnetic resonance (EPR) after the light
being turned off. This allows the long-lived N-oxyl radical to
abstract the H from C–H bond to form a carbon radical that
reacts with molecular oxygen to form R3C–OO• species, de-
composition of which leads to oxygenated products.
transfer surface h+ can restrain their recombination.12b, 12d,
14
The h+ of some metal oxides is able to oxidize
ROH/RNH2 to RO+H/RN+H2.12a, 12c Then we hypothesize
that the h+ of oxides with appropriate valence band po-
tential can also oxidize R2NOH to R2NO•, which can pro-
mote the H-abstraction of C–H bond to a carbon radical.
Further exploration found α-Fe2O3 was the desired semi-
conductor. A representative R2NOH structure, NHPI, can
adsorb on α-Fe2O3 surface and promote the separation of
h+ and e− by reaction with h+. The generated N-oxyl radi-
cal is confined on α-Fe2O3 surface and has an unexpected
long life time.
Selective oxidation of C(sp3)–H bonds of hydrocarbons
with molecular oxygen (O2) is an essential tool in organic
synthesis and industrial chemistry.1 However, due to the
3
spin-flip restriction between ground triplet state Σ-gO2
and ground state substrate,2 O2 is generally unreactive
under mild conditions,3 and thus transition-metal cata-
3
lysts are used to activate Σ-gO2 to various active O* spe-
cies.4 Alternatively, the C–H bond can be first H-
abstracted to form a carbon radical, which then reacts
with molecular oxygen to produce oxygenated com-
pounds.5 A representative example is to employ N-oxyl
radicals (R2NO•) to abstract H of C–H bond.6 But the gen-
eration of N-oxyl radicals from N-hydroxyl compounds
(R2NOH) requires radical initiators, such as azocom-
pounds, peroxides, metal salts, NOx, and quinones.7
Therefore, exploring new methods for the generation of
N-oxyl radical from N-hydroxyl compound is very im-
portant for C–H bond oxidation.8
Table 1. Photocatalytic oxidation of ethylbenzene a
GC selectivities
Additive
Conv.
(%)
(%)
2
Entry
Catalyst
(0.1 mmol)
1
-
3
1
α-Fe2O3
-
0
74
0
-
0
-
-
2
3b
α-Fe2O3 + NHPI
α-Fe2O3 + NHPI
NHPI
-
99
-
1
Free N-oxyl radical is short-lived and over-reactive,9
which brings out side reactions via the uncontrollable
chain oxidation of hydrocarbon.10 A long-lived but still
active N-oxyl radical maybe beneficial for C–H bond acti-
vation.11 A possible idea to extend the life time of N-oxyl
radical is to avoid the recombination of R2NO• and the
abstracted hydrogen of N-hydroxyl compounds. Based on
this idea, confining and stabilizing the N-oxyl radical on
solid support or catalyst that can also conveniently pro-
mote the generation of R2NO• from R2NOH is proposed in
the present study.
-
-
4
-
-
0
-
-
-
5c
6c
7c
8c
9c
α-Fe2O3 + NHPI
α-Fe2O3 + NHPI
α-Fe2O3 + NHPI
α-Fe2O3 + NHPI
α-Fe2O3 + NHPI
55
0
96
-
2
-
2
BHT
-
Na2S
0
-
-
-
(NH4)2C2O4
AgNO3
BQ
0
-
-
-
trace
2
43
47
99
81
0
17
10c α-Fe2O3 + NHPI
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a Reaction conditions: α-Fe2O3 10 mg, ethylbenzene 0.1 mmol, NHPI 3
mg, CH3CN 1.0 mL, O2 1 atm, blue LED light (455 nm, 18 W), 40 °C, 10
h; symbol - means no additive or no product; b no light; c 4 h.
phenylethoxy)isoindoline-1,3-dione (Figure S1), which
indicated the role of PINO radical in the H-abstraction at
the benzylic site.16 Above experimental results also sug-
gest a relationship between photo-generated h+ and PINO
generation.
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7
8
Although α-Fe2O3 has a band gap of 2.20 eV, with a 0.28
eV conduction band (CB) and 2.48 eV valence band (VB),
which could be a photocatalyst under visible light, α-
Fe2O3 alone could not catalyze the ethylbenzene oxida-
tion under 455 nm irradiation (Table 1, entry 1). A combi-
nation of α-Fe2O3 and NHPI can promote the reaction
with a 73% acetophenone yield (entry 2). Control experi-
ments confirm the reaction is a photocatalytic process
(entry 3), and both NHPI and α-Fe2O3 are necessary (en-
tries 1, 2 and 4).
In-situ EPR was used to explore the generation of PINO
radical under visible irradiation. Tests with and without
455 nm irradiation could not generate PINO from NHPI
(Figure 1a and 1b). An obvious triplet EPR signal appeared
in the combination of α-Fe2O3 and NHPI under 455 nm
irradiation (Figure 1c and 1d). The g factor of this triplet
signal is 2.0093, indicating the PINO* radical is formed
(g=2.0073).7a When ethylbenzene was added in the above
solution (Figure 1e), the signal of PINO* radical decreased
by 60%, inferring the reaction between PINO* radical and
ethylbenzene. When the EPR test of NHPI and α-Fe2O3
was carried out in Ar (Figure 1f), PINO* radical was still
generated, showing that the formation of PINO* did not
need oxygen. This phenomenon concludes the key role of
h+ on the generation of N-oxyl radical.
According to Pedulli’s report,17 the electron-
withdrawing carbonyl groups reduce the mesomeric
structures of N-oxyl radical with charge separation
(Scheme S1), which makes PINO less stable and increases
the R2NO–H bond dissociation enthalpy (BDE) of NHPI.
Besides, NHPI tends to generate an intramolecular hy-
drogen bond, which is absent in the PINO radical. Given
the fact that the BDE of R2NO–H bond of NHPI in
CH3CN is about 368.4 kJ·mol-1 (3.82 eV),17 it is hard for the
direct homolysis of R2NO–H bond under 455 nm irradia-
tion (about 2.7 eV). Instead, NHPI may first adsorb and
get activated on α-Fe2O3 surface, followed by being oxi-
dized by holes to the confined PINO* radical. This postu-
lation is experimentally confirmed by the photocatalytic
oxidation of a sterically-hindered substrate, triphenylme-
thane (Ph3C-H). Only 3% conversion was obtained under
the identical conditions (Scheme S2A). In comparison,
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The
inhibition
effect
of
2,6-di-tert-butyl-4-
methylphenol (BHT), as a radical scavenger, indicates
that this reaction contains a carbon radical intermediate
(entry 6). The detection of butane-2,3-diyldibenzene un-
der Ar condition suggests that the radical may be 1-
phenethyl carbon radical (Figure S1). The necessity of h+
of α-Fe2O3 in the reaction was confirmed by adding sodi-
um sulfide or ammonium oxalate as hole capture reagent
(entries 7-8).15 Adding AgNO3 as the e− capture reagent
•−
and 1,4-benzoquinone (BQ) as the capture reagent for O2
species could not inhibit the reaction (entries 9 and 10),15a
in comparison with the reaction without additive (entry
5). These experiments conclude that the oxidation reac-
tion requires the photo-generated h+ rather than e−.
(a) NHPI (Air)
(b) NHPI + h
ν
(Air)
-Fe
-Fe
(c) NHPI +
(d) NHPI +
α
2
O
3
(Air)
α
2
O
3
+ hν (Air)
(e) NHPI +
α-Fe2O3
+ h + EB (Air)
ν
the
combined
homogeneous
catalyst
of
NaNO2/DDQ/NHPI, which generates unconstrained
o
PINO radical,7b, 7c gave 98% conversion at 100 C in 3 h
(f) NHPI +
α-Fe2O3
+ h (Ar)
ν
(Table S1, entry 1). We attribute the difference to that the
formed PINO* is confined on α-Fe2O3 surface. As such,
the close contact between the N-oxyl radical and C(sp3)–
H bond of triphenylmethane is impeded (Figure S2). This
confinement effect may partly explain the g-factor of the
generated PINO* is slightly less than that of the free
PINO.
g = 2.0093
1.992 1.998 2.004 2.010 2.016 2.022 2.028
g Factor
Figure 1. EPR measurements. Conditions: solvent CH3CN,
455 nm blue LED light, and ambient air unless noted.
Details can be found in the SI. (a) NHPI; (b) (a) + 10 min
light; (c) NHPI + α-Fe2O3; (d) (c) + 10 min light; (e) (c) +
ethylbenzene + 10 min light; (f) (c) under Ar atmosphere
and 10 min light.
The state of adsorbed NHPI* on α-Fe2O3 surface was
confirmed by FTIR (Figure 2). The apparent red shift of
C=O vibration at 1711 cm-1 of free NHPI to 1681 cm-1 of the
adsorbed NHPI* indicates the interaction between car-
bonyl groups and α-Fe2O3 surface. The red-shift of aro-
matic ring skeleton vibration (1483 cm-1 to 1467 cm-1; 1466
cm-1 to 1446 cm-1) and the appearance of peaks between
1526 and 1585 cm-1 signifies the adsorption of aromatic
ring on α-Fe2O3 surface. FTIR results suggest NHPI mole-
cules lie down on α-Fe2O3 surface, facilitating electron
Although, the 455 nm irradiation (about 2.7 eV) may
induce the separation of h+ and e− in α-Fe2O3 (band gap
2.20 eV), the h+ of the α-Fe2O3 alone could not cleave the
Cα–H bond of ethylbenzene to a carbon radical. However,
adding NHPI can trigger the oxidation. The controlled
experiment without O2 offered
a product of 2-(1-
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transfer. It should be noted that the direction of electron
ing (Figure 4). (i) The 455 nm light irradiation generates
h+ and e- in α-Fe2O3. (ii) The h+ of α-Fe2O3 oxidizes the
absorbed NHPI* to PINO* and H+ species. (iii) The H+
shifts to CB band, where reacts with O2 and get reduced
by e- to form H2O via a possible H2O2 intermediate,19 and
the detailed discussion can be found in the SI. (vi) The
confined PINO* then abstracts an H from C–H bond and
restores to the adsorbed NHPI* for the next catalytic cycle.
(v) It is known process that PhCH•CH3 radical reacts with
O2 to form PhCH(OO•)CH3 and then transforms to
PhCH(OOH)CH3, whose selective decomposition leads to
PhCOCH3.6b
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2
3
4
5
6
7
8
transfer from catalyst to these functional groups is differ-
ent from the one from NHPI* to α-Fe2O3 h+ during R2NO–
H oxidation under light.
(a)
NHPI
1466
1483
Aromatic rings
zone
1711
9
Carbonyl group
(b)
1446
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*
1467
*
*
1526
1585
NHPI + α-Fe2O3
1681
Carbonyl group
1950
1800
1650
1500
1350
Wavenumber (cm-1)
Figure 2. FTIR spectra of (a) NHPI and (b) adsorbed
NHPI* on α-Fe2O3.
In situ EPR tests were conducted to study the behav-
ior of confined PINO* (Figure S3). When the mixture of
NHPI and α-Fe2O3 was irradiated with 455 nm light, the
PINO radical triplet signal immediately appeared and
remained as along as the light was on. When the light was
turned off, the EPR signal decayed to zero in more than
200 seconds. The half-life time of PINO* was measured to
be 22 seconds (Figure 3 and Table S2, see the calculation
of half-time), which was quite different from the free but
shorted-lived PINO.9, 18 Baciocchi in his works mentioned
that the PINO signal of time-resolved spectrum is stable
on the millisecond time scale.18 Although the confinement
effect makes PINO* a long-lived N-oxyl radical, given the
EPR characterization obtained above (Figure 1d and 1e),
the PINO* is still active in abstracting H from hydrocar-
bon substrate.
Figure 4. Proposed mechanism of ethylbenzene photoxi-
dation over α-Fe2O3 and NHPI.
We also explored the feasibility of this catalytic system
in the photo-oxidation of other substrates at C(sp3)–H
bond position (Table 2). The substrates with an R–CH2–R
structure tended to give ketone products (entries 1-4).
The reaction occurs at Cα site for Ar–CαH2–R. Side methyl
or methylated benzene Ar-CH3 gave acids as the main
products (entries 5 and 6).
Table 2. Photoxidation of C(sp3)‒H bonds with NHPI
a
and α-Fe2O3
light on
light off
1.0
0.5
0.0
200 400 600 800 1000 1200 1400 1600 1800
Time (s)
Figure 3. In situ EPR light on-off tests. Experimental con-
ditions: 10 mg NHPI and 10 mg α-Fe2O3 are dispersed in
1.0 mL acetonitrile under Ar. The value of EPR signal
(Figure S3) nearby G=3314.62 marks the relative change of
PINO*. All signals are normalized to the highest signal
intensity.
a
Reaction condition: substrate 0.05 mmol, NHPI 3.0 mg, α-Fe2O3 10
mg, acetonitrile 1.0 mL, O2 1 atm, 40 °C, blue LED light (455 nm, 18
W).
In conclusion, we here report a novel strategy of the
generation and confinement of N-oxyl radical on α-Fe2O3
surface. Combined characterizations and measurements
Based on above results, the photo-oxidation of
ethylbenzene to acetophenone may proceed as the follow-
3
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Page 4 of 5
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of hole and electron on α-Fe2O3 surface by consuming the
hole of α-Fe2O3 by virtue of the transformation of NHPI*
to a confined PINO* radical. This confined and long-lived
PINO* radical is required for efficient oxidation of hydro-
carbons with molecular oxygen to oxygenated compounds.
This study offers a new way of utilizing organic radical in
selective oxidation.
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ASSOCIATED CONTENT
Supporting Information.
Experiment process, controlled experiments and discussions.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
14. Olshansky, J. H.; Ding, T. X.; Lee, Y. V.; Leone, S. R.;
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wangfeng@dicp.ac.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the Strategic Priority Research
Program of the Chinese Academy of Sciences (Grant
No.XDB17020000), National Natural Science Foundation of
China (21690082, 21690084 and 21690080).
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