Photocatalytic deoxygenation of N-O bonds with rhenium complexes: From the reduction of nitrous oxide to pyridineN-oxides

Article abstract of DOI:10.1039/d1sc01974k

The accumulation of nitrogen oxides in the environment calls for new pathways to interconvert the various oxidation states of nitrogen, and especially their reduction. However, the large spectrum of reduction potentials covered by nitrogen oxides makes it difficult to find general systems capable of efficiently reducing variousN-oxides. Here, photocatalysis unlocks high energy species able both to circumvent the inherent low reactivity of the greenhouse gas and oxidant N₂O (E⁰(N₂O/N₂) = +1.77 Vvs.SHE), and to reduce pyridineN-oxides (E_1/2(pyridineN-oxide/pyridine) = ?1.04 Vvs.SHE). The rhenium complex [Re(4,4′-tBu-bpy)(CO)₃Cl] proved to be efficient in performing both reactions under ambient conditions, enabling the deoxygenation of N₂O as well as synthetically relevant and functionalized pyridineN-oxides.

Full text of DOI:10.1039/d1sc01974k

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Photocatalytic deoxygenation of N–O bonds with
rhenium complexes: from the reduction of nitrous
oxide to pyridine N-oxides†
Cite this: Chem. Sci., 2021, 12, 10266
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Marianne Kjellberg, Alexia Ohleier, Pierre Thuery,
Emmanuel Nicolas,
´
*
*
Lucile Anthore-Dalion
and Thibault Cantat
The accumulation of nitrogen oxides in the environment calls for new pathways to interconvert the various
oxidation states of nitrogen, and especially their reduction. However, the large spectrum of reduction
potentials covered by nitrogen oxides makes it difficult to find general systems capable of efficiently
reducing various N-oxides. Here, photocatalysis unlocks high energy species able both to circumvent
the inherent low reactivity of the greenhouse gas and oxidant N2O (E0(N2O/N2) ¼ +1.77 V vs. SHE), and
to reduce pyridine N-oxides (E1/2(pyridine N-oxide/pyridine) ¼ ꢀ1.04 V vs. SHE). The rhenium complex
[Re(4,40-tBu-bpy)(CO)3Cl] proved to be efficient in performing both reactions under ambient conditions,
enabling the deoxygenation of N2O as well as synthetically relevant and functionalized pyridine N-oxides.
Received 8th April 2021
Accepted 24th June 2021
DOI: 10.1039/d1sc01974k
rsc.li/chemical-science
important applications is the deoxygenation of C–O bonds of
C1-molecules, which covers a narrow spectrum of reduction
potentials (0.4 V, Fig. 1a).6 The main strategy in optimizing
photo- or electro-catalysts has thus been focused on the selec-
tivity between different products, especially between CO and
HCOOH.7 Nitrogen oxides however cover a wider spectrum of
reduction potentials (over 2.8 V, Fig. 1a) shiing the question
from a search for selective catalysts, to the discovery of versatile
catalytic systems.8
A simple model that can be used to showcase this deoxy-
genation is the ozone-depleting nitrous oxide N2O, since it leads
directly to N2. Although N2O is a strong oxidant (E0(N2O/N2) ¼
+1.77 V vs. SHE),8 homogeneous deoxygenation reactions are
scarcely reported, for it is highly inert and a poorly coordinating
molecule.9 Among the few thermocatalytic deoxygenating
methods, Milstein group disclosed the hydrogenation of N2O to
N2 and H2O catalyzed by a PNP pincer ruthenium complex at
65 ꢁC, under a total pressure of 7 bar (Fig. 1b).10 More recently,
our laboratory proposed the deoxygenation of N2O using dis-
ilanes and a catalytic amount of uoride source under ambient
conditions (Fig. 1b).11 To the best of our knowledge, only
heterogeneous photodeoxygenation of N2O has been so far re-
ported.12 A very recent report by the group of Costentin and
Chardon-Noblat demonstrated the use of ReI complexes able to
catalyse the electroreduction of N2O to N2.13
Introduction
The modern development of intensive soil exploitation and – in
corollary – the massive production of nitrogen-based fertilizers
from atmospheric N2 has enabled us to feed an ever-growing
population.1 Collateral damage, however, includes increased
production and accumulation of noxious nitrogen oxide wastes
such as nitrates NO3ꢀ, nitrites NO2ꢀ or nitrous oxide N2O, their
reduction back to N2 being mostly ensured by the natural
nitrogen cycle.2 As dened by environmental scientists, plane-
tary boundaries for the biogeochemical nitrogen cycle have
indeed already been crossed, the current nitrogen anthropo-
genic xation representing more than twice the estimated
boundary (150 Tg N per year vs. 62 Tg N per year).3 Since
breaking the highly stable N2 bond via the Haber–Bosch process
consumes yearly 2% of worldwide energy, leakage of nitrogen
oxides in the environment also represents a loss of energy.4 In
the search for a more sustainable economy, there is hence
a need for new pathways able to perform efficient interconver-
sion between oxidation states of nitrogen, and especially
reduction of nitrogen oxides to less oxidized molecules.5
Chemically speaking, this raises the issue of deoxygenating
the N–O bond of both inorganic and organic N-oxides. With the
aim to explore new pathways, we decided to harness the
potential of homogeneous photoredox catalysis. One of its most
At the opposite end of the redox scale, pyridine N-oxides
have, in contrast, considerably lower reduction potentials (E1/
2(pyridine N-oxide (1a)/pyridine (2a)) ¼ ꢀ1.04 V vs. SHE, Fig. 1b)
´
Universite Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette CEDEX, France
and they represent
a
much greater challenge.14 These
† Electronic supplementary information (ESI) available: Data relating to the
characterization data of materials and products, general methods, optimization
studies, experimental procedures, gas chromatography, NMR spectra, and CIF
X-ray structures. CCDC 2056048 and 2056049. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/d1sc01974k
compounds are useful intermediates in the synthesis of N-
heteroaromatic-containing pharmaceutical or agrochemical
products, where N-oxides are used for instance as directing
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Fig. 1 Development of a photocatalytic pathway to deoxygenate N–O bonds. (a) Range of potentials involved for the reduction of C–O and
N–O bonds. (b) Existing strategies for N–O bond deoxygenation. (c) Proposed strategy for the deoxygenation of N2O and pyridine N-oxides.
DIPEA, diisopropylethylamine.
groups for C–H bond functionalization.15 In that context, there
is a need for mild methods to post-reduce the N-oxide group and
afford the N-heterocycle. The main reported methods involve
Results and discussion
Design of the system for the photodeoxygenation of N2O
sacricial oxophilic reagents or high energy reductant based on
phosphines,16 silane17 or borane18 derivatives, with thermo-
chemical or (photo)electrochemical activation.19 Nevertheless,
all these methods imply the use of either heating systems or
high energy reductants (with E0 < 0 V), leading to a loss of
energy. There is moreover no report of systems capable of
addressing the wide range of potentials (>2.5 V) required to
reduce both nitrous oxide and pyridine N-oxides.
The potential of [Re(bpy)(CO)3Cl] (Re-1) to photocatalyze the
conversion of N2O to N2 was tested, using Re-1 (5 mol%) as
catalyst in acetonitrile in the presence of triethanolamine
(TEOA) and 1 bar of N2O. Aer 2 h of irradiation with white
LEDs at 20 ꢁC, N2 was formed to our delight in 66% yield
according to GC analysis of the gaseous fraction. This result
represents the rst homogeneous photocatalytic deoxygenation
of N2O to N2 (Table 1, entry 1).
Herein, we demonstrate that a versatile photocatalytic
system based on [Re(bpy)(CO)3Cl]-type complexes20 is able to
efficiently deoxygenate both N2O and pyridine N-oxides under
In the absence of Re-1, no conversion of N2O was observed:
cleavage of the N–O bond does not occur spontaneously under
irradiation (Table 1, entry 2). Other blank experiments
ꢁ
ambient conditions (20 C, 1 bar, Fig. 1c).
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Table 1 Photodeoxygenation of N2O catalyzed by Re-1 and control Table 2 Performances of the photocatalysts in the deoxygenation of
experiments
N2Oa
TOF0 (hꢀ1
)
% N2 (TON)
Time (h)
Entry Deviation from standard conditionsa % H2 % N2 % N2Ob
b
b
Entry
Catalyst
1
2
3
4
5
Re-1
Re-2
Re-3
Re-4
Re-5
1.9
3.7
4.3
2.7
1.1
50 (10)
70 (14)
86 (17)
55 (11)
61 (12)
22
50
115
100
150
1
2
3
4
5
6
7
None
1.5
0
0
91
0
0.2
0
66
2
4
9
8
32
98
96
0
92
63
24
No photocatalyst
No light
No N2Oc
No TEOA
TEA instead of TEOA
DIPEA instead of TEOA
36
76
a
Reaction conditions: N2O (1.0 bar, 1.2 mmol), catalyst (60 mmol,
5 mol%), DIPEA (6.9. mmol, 5.7 equiv.), CH3CN (6 mL), 20 ꢁC.
a
Standard reaction conditions: N2O (1 bar, 100 mmol), Re-1 (5 mmol,
5 mol%), donor (570 mmol, 5.7 equiv.), CD3CN (0.5 mL), 2 h, 20 ꢁC.
b
Determined by GC analysis of the gaseous phase. Percentages
Complexes Re-2 (ref. 22) and Re-3,23 possessing bipyridines
substituted in the 4,40-position with methyl or t-butyl groups,
respectively, were also active in the deoxygenation of N2O, and
led to higher yields in N2 than unsubstituted Re-1, 70% and
86% respectively, corresponding to TONs of 14 and 17. They
also showed higher activity than Re-1: TOF0 reached 3.7 hꢀ1 and
4.3 hꢀ1 for Re-2 and Re-3 respectively vs. 0.7 hꢀ1 for Re-1 (Table
2, entries 1–3). On the other hand, Re-4 (ref. 23) and the novel
Re-5 complex, featuring methyl and bulky mesityl substituents
in the 6 and 60 positions of bipyridines, were less efficient on
a 24 h timescale, leading to moderate yields in N2 of 55% and
61% respectively (Table 2, entries 4 and 5). This behavior is also
present in their initial activities: lower TOF0 (2.7 and 1.1 hꢀ1 for
Re-4 and Re-5 respectively) were measured compared to Re-2
and Re-3.
correspond to the fraction of named gas in the overall analyzed
gaseous phase (corrected with response factors). Under argon (1
bar). TEOA: triethanolamine; TEA: trimethylamine; DIPEA:
diisopropylethylamine.
c
conrmed that no N2 is formed in the absence of light, N2O, or
electron donor (Table 1, entries 3–5).
Interestingly, the reaction performed without N2O afforded
H2 as sole product, as a result of TEOA decomposition in the
presence of the catalyst under irradiation (Table 1, entry 4).
The detection of traces of H2 at the end of the reaction (1.5% of
the gaseous phase, Table 1, entry 1) was thus ascribed to the
use of TEOA. Sacricial amines in photocatalysis are indeed
known to supply the catalytic cycle with electrons and may
serve as a proton source, which can in turn be reduced by the
photocatalyst to H2.6a,21 This prompted us to investigate the
inuence of the sacricial electron donor on the outcome of
N2O photocatalytic reduction. Another tertiary amine, trie-
thylamine (TEA), was used, inducing a drop of the N2O
conversion, the gaseous fraction of N2 aer 2 h decreasing
from 66% to 36% without suppressing the generation of H2
(Table 1, entry 6). Interestingly, when a bulkier tertiary amine,
A more precise monitoring of kinetic proles for each cata-
lyst was also performed (Fig. 2b). The evolution of the N2 yield
over 60 h showed that Re-1 deactivated aer 22 h, sooner than
all the other substituted catalysts, which accounts for the low
yields obtained with Re-1. In contrast, Re-2 and Re-3, presenting
para-substituted bipyridines, showed both high catalytic activity
and increased stability.
A possible explanation of this
phenomenon has been studied in the case of CO2 electro-
reduction by Kubiak et al.: they showed that substituting the
bipyridine ligand in the 4,40-positions provided sufficient steric
hindrance to inhibit the dimerization of a Re(0) reactive inter-
mediate to an inactive bimetallic complex.24,25 The stability
increased with the steric bulk of the substituent. The same
tendency is observed here in the case of the photoreduction of
N2O, and hints towards a similar mechanistic pathway.
Complexes Re-4 and Re-5, substituted in the ortho position,
demonstrated an increased stability compared to Re-1 but,
interestingly enough, the increase in stability brought by
substitution on the bipyridine did not come along with an
increase in activity. These catalysts indeed required respectively
100 h and 150 h to yield 55% and 61% of N2. A structural
explanation could arise from the steric strain between the
substituents on the bipyridine and the equatorial carbonyls in
Re-4 and Re-5.26
¨
diisopropylethylamine (DIPEA, Hunig's base), was used as
electron donor, H2 could not be detected in GC aer 2 hours of
irradiation, and the N2 yield increased up to 76% (Table 1,
entry 7). We therefore selected DIPEA as the sacricial electron
donor for the rest of our study.
Catalyst optimization
When the reaction was scaled up from 0.1 to 1.2 mmol N2O, the
yield with Re-1 as catalyst dropped to 50%, affording a TON of
10 and a TOF0 of 1.9 hꢀ1 (Table 2, entry 1). GC-monitoring
showed no evolution aer 22 h of reaction, indicating poten-
tial deactivation of the catalyst. We hence explored the effects of
substitution on the bipyridine ligand. We synthesized a series of
[Re(bpy)(CO)3Cl]-type catalysts with bipyridines substituted
either on the 4,40 or the 6,60-position, and we studied their
ability to photocatalyze the deoxygenation of N2O (Fig. 2a).
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Fig. 2 Photodeoxygenation of N2O: screening of catalysts. (a) Photocatalysts Re-1–5 used in the study. (b) Kinetic profiles on 1.2 mmol scale.
Reaction conditions: N2O (1.0 bar, 1.2 mmol), catalyst (60 mmol, 5 mol%), DIPEA (6.9 mmol, 5.7 equiv.), CH3CN (6 mL), 20 ꢁC.
Rhenium photocatalysts of type [Re(bpy)(CO)3Cl] are thus reaction times (44–87 h), leading to pyridine (2a) in 85–99%
able to catalyze the deoxygenation of N2O, suggesting that other yield (Table 3, entries 2–5). This stands in contrast with our
N–O bonds could be deoxygenated under similar reaction observations with N2O: here the yields showed steady increase
conditions.
until full conversion with all catalysts. Two explanations may
account for this phenomenon: either the pyridine produced
during the reaction delays catalyst deactivation through the
stabilization of reactive intermediates,27 or the higher oxidative
character of N2O has a detrimental effect on the catalyst
stability. This effect was particularly benecial in the case of the
6,60-dimethylbipyridine rhenium complex Re-4, which led only
to 55% yield of N2 aer 100 h and gave full conversion of pyri-
dine N-oxide (1a) within 55 h. Again, 4,40-substituted catalysts
Re-2 and Re-3 displayed higher catalytic activity (TOF0 ¼ 11 and
16 hꢀ1) than unsubstituted Re-1 and 6,60-substituted Re-4 and
Re-5 (TOF0 ¼ 5.5, 7.5 and 2.6 hꢀ1, respectively), suggesting
similar mechanistic patterns for the deoxygenation of both N2O
and pyridine N-oxide (1a) (Table 3). To study the effect of irra-
diation, a light on/light off experiment was also performed
(Fig. 3a).28 The substrate was only converted when the system
was exposed to light, and the reaction stopped once the system
was in the dark. Continuous irradiation is thus required to
perform the reaction.
Photodeoxygenation of pyridine N-oxides
Encouraged by the success of N2O deoxygenation, and to probe
the versatility of our catalytic system, we considered the possi-
bility of applying this methodology to the deoxygenation of
more challenging N–O bonds across the scale of redox poten-
tials, namely of pyridine N-oxides (E1/2(pyridine N-oxide (1a)/
pyridine (2a)) ¼ ꢀ1.04 V vs. SHE).
The reaction was monitored by NMR using rhenium-based
Re-1–5 photocatalysts. To our delight, a rst attempt using the
tBu-substituted catalyst Re-3 under the reaction conditions
developed for N2O enabled deoxygenation of pyridine N-oxide
(1a) to pyridine (2a) in 82% yield aer 34 h (Table 3, entry 1). The
blank experiments conrmed that all components (light, pho-
tocatalyst, electron donor) were necessary to perform the
reduction (see ESI Table S1†). Remarkably, the other catalysts
also afforded full conversion of compound 1a, albeit with longer
From a kinetic standpoint, the possible improvement of
catalytic performances as pyridine was released was particularly
noticeable in the case of 6,60-substituted catalysts Re-4 and Re-5.
If their initial catalytic activities were low (TOF0 ¼ 7.5 and 2.6
hꢀ1), they showed steady catalytic activity past 2 hours of irra-
diation, while the conversion rates for the other catalysts were
gently decreasing (Fig. 3b). This resulted in Re-4 and Re-5
reaching full conversion before Re-1 and Re-2.
Several para-substituted pyridine N-oxides 1b–f could be
deoxygenated in 77–99% yield using photocatalyst Re-3 (Fig. 4).
The electronic parameters of the substituents affected the
initial reaction rate: the yields aer 30 minutes of irradiation
evolved in line with the electron-withdrawing ability of the
Table
3 Photocatalytic deoxygenation of pyridine N-oxide with
different photocatalystsa
Time to full conversion
(h)
Entry
Cat.
TOF0 (hꢀ1
)
Yield (%)
1
2
3
4
5
Re-3
Re-1
Re-2
Re-4
Re-5
16
5.5
11
7.5
2.6
82
99
85
96
87
34
63
87
55
44
a
Reaction conditions: pyridine N-oxide (100 mmol), catalyst (5 mmol,
5 mol%), DIPEA (570 mmol, 5.7 equiv.), CD3CN (0.5 mL), 20 ꢁC.
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Fig. 3 Photoreduction of pyridine N-oxide. (a) Light and dark experiment with Re-3. Yellow: light on; white: light off. (b) Kinetic profiles of
pyridine N-oxide photoreduction with different catalysts. Reaction conditions: pyridine N-oxide (100 mmol), catalyst (5 mmol, 5 mol%), DIPEA (570
a
1
mmol, 5.7 equiv.), CD3CN (0.5 mL), 20 ꢁC. Determined by H NMR analysis of the crude reaction mixture.
yield, while 1a, 1e and 1f, bearing –H, chloro or cyano groups
(sH ¼ 0, 0.23 and 0.66 respectively) yielded aer 30 min. 26, 30
and 43% of the corresponding pyridines, respectively. This was
also reected in the time to reach full conversion: pyridine N-
oxides 1b-d bearing electron-donating substituents (–NMe2,
–OMe, –Me) in the para position required longer reaction times
(14–126 h) than 1e-f with electron-withdrawing substituents (8
and 0.5 h respectively for the chloro and cyano derivatives). In
particular the conversion of the nitrile derivative 1f was
extremely fast, maximal (43%) aer only 30 minutes of irradi-
ation. Further irradiation however led to over-reduction of 2f to
pyridine 2a (for details see ESI Fig. S1†).
This is the rst photocatalytic system able to perform such
a deoxygenation reaction without additional energy sources. In
contrast, strong reductants based on Hantzch esters or hydra-
zine were used by the groups of Konev and Wangelin19c and
Lee.19d
In order to demonstrate the selectivity of this reaction and its
synthetic utility, we explored the deoxygenation of a more
complex substrate, a 2,6-substituted pyridine N-oxide. Pyridine
N-oxide indeed features an enhanced reactivity compared to the
reduced pyridine, enabling the functionalization of the 2,6-
positions by C–H activation and, as an example, Hiyama et al.
recently reported the synthesis of 2,6-substituted pyridine 3,
based on the oxidation of o-picoline to form the 2-methylpyr-
idine N-oxide 4.30 A Ni-catalyzed coupling reaction leads to 2,6
substituted pyridine N-oxide 5, whose deoxygenation presents
multiple challenges that are chemoselectivity and steric
hindrance. In their publication, the Hiyama group used PCl3 as
a reducing agent to perform such a deoxygenation. We were very
pleased to observe that using our system, Re-3 catalyzed the
deoxygenation with 82% yield aer 8 h of irradiation (Scheme
1). This demonstrates the validity of such an approach for mild
deprotection strategies, which may be extended to other
substrates.
Fig. 4 Photocatalytic reduction of para-substituted pyridine N-oxides.
(top) Scope of substrates (yield, time of irradiation until maximum
yield). Corresponding Hammett constants of the substituents. Reac-
tion conditions: pyridine N-oxide (100 mmol), Re-3 (5 mmol, 5 mol%),
DIPEA (570 mmol, 5.7 equiv.), CD3CN (0.5 mL), 20 ꢁC. Yields were
determined by 1H NMR analysis of the crude reaction mixture.
(bottom) Yields of the photocatalytic deoxygenation of para-
substituted pyridine N-oxides after 30 minutes of irradiation.
substituents as given by their Hammett sigma constants
(Fig. 4b).29 Indeed, compounds bearing electron-donating
substituents were only poorly converted aer 30 minutes: 1b–
d bearing dimethylamino, methoxy or methyl groups (sH
ꢀ0.83, ꢀ0.27 and ꢀ0.17 respectively) reached only 3 to 25%
¼
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Notes and references
1 A. Mosier, J. K. Syers and J. R. Freney, Agriculture and the
nitrogen cycle: Assessing the impacts of fertilizer use on food
production and the environment, Island Press, 2013.
2 (a) N. Lehnert, H. T. Dong, J. B. Harland, A. P. Hunt and
C. J. White, Nat. Rev. Chem., 2018, 2, 278–289; (b) H. Tian,
R. Xu, J. G. Canadell, R. L. Thompson, W. Winiwarter,
P. Suntharalingam, E. A. Davidson, P. Ciais, R. B. Jackson,
G. Janssens-Maenhout, M. J. Prather, P. Regnier, N. Pan,
S. Pan, G. P. Peters, H. Shi, F. N. Tubiello, S. Zaehle,
F. Zhou, A. Arneth, G. Battaglia, S. Berthet, L. Bopp,
A. F. Bouwman, E. T. Buitenhuis, J. Chang,
M. P. Chippereld, S. R. S. Dangal, E. Dlugokencky,
J. W. Elkins, B. D. Eyre, B. Fu, B. Hall, A. Ito, F. Joos,
P. B. Krummel, A. Landol, G. G. Laruelle, R. Lauerwald,
W. Li, S. Lienert, T. Maavara, M. MacLeod, D. B. Millet,
S. Olin, P. K. Patra, R. G. Prinn, P. A. Raymond, D. J. Ruiz,
G. R. van der Werf, N. Vuichard, J. Wang, R. F. Weiss,
K. C. Wells, C. Wilson, J. Yang and Y. Yao, Nature, 2020,
586, 248–256.
3 (a) J. Rockstrom, W. Steffen, K. Noone, A. Persson,
F. S. Chapin 3rd, E. F. Lambin, T. M. Lenton, M. Scheffer,
C. Folke, H. J. Schellnhuber, B. Nykvist, C. A. de Wit,
T. Hughes, S. van der Leeuw, H. Rodhe, S. Sorlin,
P. K. Snyder, R. Costanza, U. Svedin, M. Falkenmark,
L. Karlberg, R. W. Corell, V. J. Fabry, J. Hansen, B. Walker,
D. Liverman, K. Richardson, P. Crutzen and J. A. Foley,
Nature, 2009, 461, 472–475; (b) W. de Vries, J. Kros,
C. Kroeze and S. P. Seitzinger, Curr. Opin. Environ. Sustain.,
2013, 5, 392–402; (c) W. Steffen, K. Richardson,
J. Rockstrom, S. E. Cornell, I. Fetzer, E. M. Bennett,
R. Biggs, S. R. Carpenter, W. de Vries, C. A. de Wit,
C. Folke, D. Gerten, J. Heinke, G. M. Mace, L. M. Persson,
V. Ramanathan, B. Reyers and S. Sorlin, Science, 2015, 347,
1259855.
Scheme
strategy.
1 2-Methylpyridine functionalization via the N-oxide
Conclusions
In summary, a new photochemical method has been developed
to deoxygenate N–O bonds with radically different reduction
potentials, over a 2.8 V window. [Re(4,40-tBu-bpy)(CO)3Cl] (Re-3)
is indeed capable of reducing both N2O under ambient condi-
tions, and pyridine N-oxides 1 and 4 in good to excellent yields.
Those results open new perspectives concerning the photo-
catalytic deoxygenation of nitrogen oxide-containing
compounds. Further mechanistic studies on catalyst Re-3 are
underway in our laboratory. We believe that these will provide
new clues to understanding N–O bonds deoxygenation
chemistry.
Data availability
Crystallographic data for [compound number] has been
deposited at the CCDC under 2056048 and 2056049 and can be
obtained from https://www.ccdc.cam.ac.uk/. The datasets sup-
porting this article have been uploaded as part of the supple-
mentary material.
4 (a) Electrochemical dinitrogen activation: To nd
a
sustainable way to produce ammonia, S. Chen,
S. Perathoner, C. Ampelli and G. Centi, in Horizons in
sustainable industrial chemistry and catalysis, ed. S.
Albonetti, S. Perathoner and E. A. Quadrelli, Elsevier, 2019,
vol. 178, pp. 31–46; (b) O. Elishav, B. Mosevitzky Lis,
E. M. Miller, D. J. Arent, A. Valera-Medina, A. Grinberg
Dana, G. E. Shter and G. S. Grader, Chem. Rev., 2020, 120,
5352–5436.
Author contributions
MK and AO performed the investigations. PT performed the
XRD analyses. EN, LAD and TC supervised the project. All
authors contributed to the writing of the manuscript.
5 J. G. Chen, R. M. Crooks, L. C. Seefeldt, K. L. Bren,
R. M. Bullock, M. Y. Darensbourg, P. L. Holland,
B. Hoffman, M. J. Janik, A. K. Jones, M. G. Kanatzidis,
P. King, K. M. Lancaster, S. V. Lymar, P. Pfromm,
W. F. Schneider and R. R. Schrock, Science, 2018, 360,
eaar6611.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We thank the CEA, CNRS, ERC (Consolidator Grant no. 818260),
and Institut de France for funding. M. K. was supported by
a Master fellowship from the LABEX Charmmmat followed by
a PhD fellowship from ADEME. We thank Thierry Bernard (CEA)
for his assistance in conception and realization of the photo-
chemistry reactor.
6 (a) Y. Yamazaki, H. Takeda and O. Ishitani, J. Photochem.
Photobiol., C, 2015, 25, 106–137; (b) Y. Kuramochi,
O. Ishitani and H. Ishida, Coord. Chem. Rev., 2018, 373,
333–356; (c) C. Chauvier and T. Cantat, ACS Catal., 2017, 7,
2107–2115.
© 2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci., 2021, 12, 10266–10272 | 10271

View Article Online
Chemical Science
Edge Article
7 (a) C. Costentin, M. Robert and J. M. Saveant, Chem. Soc. Rev., 20 J. Hawecker, J. M. Lehn and R. Ziessel, J. Chem. Soc., Chem.
2013, 42, 2423–2436; (b) H. Takeda, C. Cometto, O. Ishitani
and M. Robert, ACS Catal., 2016, 7, 70–88.
8 D. R. Lide, Crc handbook of chemistry and physics, CRC press,
2004.
Commun., 1983, 536–538.
21 (a) J. Hu, J. Wang, T. H. Nguyen and N. Zheng, Beilstein J. Org.
Chem., 2013, 9, 1977–2001; (b) Y. Pellegrin and F. Odobel, C.
R. Chim., 2017, 20, 283–295.
9 (a) V. N. Parmon, G. I. Panov, A. Uriarte and A. S. Noskov, 22 K. Kalyanasundaram, J. Chem. Soc., Faraday Trans., 1986, 82,
Catal. Today, 2005, 100, 115–131; (b) K. Severin, Chem. Soc.
Rev., 2015, 44, 6375–6386.
10 R. Zeng, M. Feller, Y. Ben-David and D. Milstein, J. Am. Chem.
Soc., 2017, 139, 5720–5723.
11 L. Anthore-Dalion, E. Nicolas and T. Cantat, ACS Catal.,
2019, 9, 11563–11567.
12 T. Ming, R. de Richter, S. Shen and S. Caillol, Environ. Sci.
Pollut. Res., 2016, 23, 6119–6138.
2401–2415.
23 V. W.-W. Yam, V. C.-Y. Lau and K.-K. Cheung,
Organometallics, 2002, 14, 2749–2753.
24 (a) J. M. Smieja and C. P. Kubiak, Inorg. Chem., 2010, 49,
9283–9289; (b) J. M. Smieja, E. E. Benson, B. Kumar,
K. A. Grice, C. S. Seu, A. J. Miller, J. M. Mayer and
C. P. Kubiak, PNAS, 2012, 109, 15646–15650; (c)
E. E. Benson, C. P. Kubiak, A. J. Sathrum and J. M. Smieja,
Chem. Soc. Rev., 2009, 38, 89–99; (d) E. E. Benson and
C. P. Kubiak, Chem. Commun., 2012, 48, 7374–7376.
13 R. Deeba, F. Molton, S. Chardon-Noblat and C. Costentin,
ACS Catal., 2021, 11, 6099–6103.
14 T. Kubota and H. Miyazaki, Bull. Chem. Soc. Jpn., 1966, 39, 25 As suggested by a referee, an alternative explanation would
2057–2062.
involve a change in the redox potentials of the catalysts
15 Y. L. Wang and L. M. Zhang, Synthesis, 2015, 47, 289–305.
leading to a change in the efficiency of the electron transfer.
16 E. Howard and W. F. Olszewski, J. Am. Chem. Soc., 1959, 81, 26 As suggested by a referee, addition of chloride anions also
1483–1484.
improves the photocatalytic activity, in agreement with the
ndings of Lehn et al. in the photocatalytic reduction of
CO2. See ref. 27 and ESI† for more details.
17 S. Chandrasekhar, C. R. Reddy, R. J. Rao and J. M. Rao,
Synlett, 2002, 349–351.
18 H. P. Kokatla, P. F. Thomson, S. Bae, V. R. Doddi and 27 J. Hawecker, J. M. Lehn and R. Ziessel, Helv. Chim. Acta,
M. K. Lakshman, J. Org. Chem., 2011, 76, 7842–7848. 1986, 69, 1990–2012.
19 (a) P. Xu and H. C. Xu, Synlett, 2019, 30, 1219–1221; (b) 28 L. Buzzetti, G. E. M. Crisenza and P. Melchiorre, Angew.
Y. Fukazawa, A. E. Rubtsov and A. V. Malkov, Eur. J. Org. Chem., Int. Ed., 2019, 58, 3730–3747.
Chem., 2020, 3317–3319; (c) M. O. Konev, L. Cardinale and 29 L. P. Hammett, J. Am. Chem. Soc., 2002, 59, 96–103.
A. Jacobi von Wangelin, Org. Lett., 2020, 22, 1316–1320; (d) 30 K. S. Kanyiva, Y. Nakao and T. Hiyama, Angew. Chem., Int. Ed.
K. D. Kim and J. H. Lee, Org. Lett., 2018, 20, 7712–7716.
Engl., 2007, 46, 8872–8874.
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© 2021 The Author(s). Published by the Royal Society of Chemistry