Catalytic Activation of N2O at a Low-Valent Bismuth Redox Platform

Article abstract of DOI:10.1021/jacs.0c10092

Herein we present the catalytic activation of N2O at a BiI→BiIII redox platform. The activation of such a kinetically inert molecule was achieved by the use of bismuthinidene catalysts, aided by HBpin as reducing agent. The protocol features remarkably mild conditions (25 °C, 1 bar N2O), together with high turnover numbers (TON, up to 6700) and turnover frequencies (TOF). Analysis of the elementary steps enabled structural characterization of catalytically relevant intermediates after O-insertion, namely a rare arylbismuth oxo dimer and a unique monomeric arylbismuth hydroxide. This protocol represents a distinctive example of a main-group redox cycling for the catalytic activation of N2O.

Full text of DOI:10.1021/jacs.0c10092

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Catalytic Activation of N2O at a Low-Valent Bismuth Redox Platform
̈
Yue Pang, Markus Leutzsch, Nils Nothling, and Josep Cornella*
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ABSTRACT: Herein we present the catalytic activation of N2O at a BiI⇄BiIII redox platform. The activation of such a kinetically
inert molecule was achieved by the use of bismuthinidene catalysts, aided by HBpin as reducing agent. The protocol features
remarkably mild conditions (25 °C, 1 bar N2O), together with high turnover numbers (TON, up to 6700) and turnover frequencies
(TOF). Analysis of the elementary steps enabled structural characterization of catalytically relevant intermediates after O-insertion,
namely a rare arylbismuth oxo dimer and a unique monomeric arylbismuth hydroxide. This protocol represents a distinctive example
of a main-group redox cycling for the catalytic activation of N2O.
itrous oxide (N2O) is known to be a potent greenhouse
gas, with much greater warming potential than CO2.
1
N
The concentration of this gaseous molecule in the atmosphere
has significantly increased in the modern era as a result of
human activities, thus generating an environmental threat.1b In
this sense, the development of catalytic strategies for the
decomposition of N2O has recently drawn much attention.2
From the chemical standpoint, N2O is a thermodynamically
powerful O atom transfer reagent, and indeed, Nature has
evolved a powerful enzymatic pathway for converting N2O into
N2 in microbial denitrification processes.3 Yet, N2O is
kinetically inert, which poses challenges for its catalytic
activation with artificial systems.2c In contrast to heteroge-
neous catalysts,2a,b,d a handful of examples are known to be
capable of activating N2O based on homogeneous catalysts
(Figure 1A),2e mainly consisting of transition metals (Ru,4
Rh,5 Co,6 and others7).
With the aim of emulating transition-metal-like reactivity, a
growing number of organo-main-group compounds have been
shown to activate small molecules.8 Among them, many low-
valent p-block compounds circumvented the kinetic barrier for
Figure 1. (A) Homogeneous N2O activation at centers of elements:
N2O activation and formed structurally unique O-containing
red (stoichiometric), blue (catalytic). (B) Catalytic N2O deoxygena-
compounds.9 Additionally, frustrated Lewis pairs (FLPs)10 and
tion at a Bi(I)⇄Bi(III) redox platform.
N-heterocyclic carbenes (NHCs)11 have also been shown to
capture N2O and eventually cleave the N−O bond. Recently,
its demonstrated ability to access various oxidation states.14
degradation of N2O with disilanes initiated by a fluoride anion
One-electron BiII⇄BiIII has been described by Coles15 and
has also been achieved,12 representing alternative pathways
based on main-group systems. However, despite these
precedents, catalytic redox processes for the activation of
N2O by a main group compound still remain elusive.
recently by Lichtenberg,16 whereas, two-electron Bi redox
catalysis (BiI⇄BiIII and BiIII⇄BiV) has recently been reported
by us.17−19 We have previously demonstrated that the N,C,N-
chelated bismuthinidene complexes, originally reported by
Based on earlier precedents, activation of N2O could be
20
Dostal, provide a privileged platform for BiI⇄BiIII redox
́
facilitated by low-valent main group species, through the
formation of an M−O bond with release of N2.9 If catalytic
turnover is to be achieved, such M−O species should be
reduced satisfactorily back to the starting low-valent main
group catalyst. Although this process is well-established for
high-valent oxo compounds,13 access to low-valent counter-
parts is nontrivial. Nevertheless, bismuth presents itself as an
interesting candidate to address such a challenging idea, due to
Received: September 21, 2020
https://dx.doi.org/10.1021/jacs.0c10092
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cycling. Herein, we demonstrate that N,C,N-chelated bismu-
thinidenes are able to catalyze N2O deoxygenation in the
presence of pinacolborane (HBpin) (Figure 1B). The catalytic
system features the activation of N2O at remarkably mild
conditions (25 °C, 1 bar) with high TON (up to 6700) and
TOF. Ligand design and structural analysis on the
bismuthinidene catalysts enabled the full characterization of
key BiIII−oxo intermediates by NMR, X-ray, and HRMS.
To interrogate the reactivity of bismuthinidenes with N2O,
we initially subjected complex 1 to a N2O atmosphere (1 bar)
in THF-d8 at −78 °C (Scheme 1). The green solution slowly
scalable procedure to obtain 4 and 5: reduction of the parent
arylbismuth dichlorides 2 and 3 with Cp2Co afforded 4 and 5
after simple filtration as dark purple and red-purple solids
respectively in very high yields (Figure 2A).21 X-ray
crystallography revealed that, in spite of the steric bulkiness
of the m-Tp groups, the bond lengths and angles resemble
those reported for 1 and related ketimine-N,C,N-complexes of
bismuth (Figure 2B).20
When 4 was exposed to a N2O atmosphere at room
temperature, the color slowly changed from dark purple to pale
1
yellow and evolution of N2 was observed (Figure 3A). H
Scheme 1. Oxidation of Bismuthinidene 1 with N2O
turned pale yellow with concomitant evolution of gas. Analysis
of the head space by GC-TCD identified the formation of N2
during the reaction.21 1H NMR analysis at −40 °C revealed
complete consumption of 1 after 45 min and the formation of
a major species containing intact N,C,N-ligand scaffold and a
C−Bi bond.21 ESI-HRMS analysis of this mixture clearly
suggested the formation of dimeric arylbismuth oxides
[(ArBiO)2+H+, calcd 937.33012, found 937.33070]. Such
species was found to be dynamic in solution and thermally
unstable, preventing its characterization by crystallographic
techniques. The observed behavior for this species is consistent
with other related Ar−BiIII oxo or sulfido dimers.22−24
In order to shed light on the possible structure of this
species, the tBu on imines was replaced with m-terphenyl (m-
Tp, 4 and 5, Figure 2A), which has been previously utilized to
stabilize reactive organobismuth compounds such as Bi−H,25
BiBi,26 and Bi−OH.27 Due to the sensitivity of aryl-
(ket)imines to metal hydrides,28 we developed a facile and
Figure 3. (A) Oxidation of bismuthinidine 4 with N2O. (B) ORTEP
drawing of 6, with ellipsoids drawn at the 50% probability level. H
atoms, the toluene molecules and the enantiomer of 6 in the unit cell
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Bi1−C1 2.300(5), Bi1−O1 2.103(3), Bi1−O2 2.120(3), Bi1−N1
2.672(4), O1−Bi1−O2 79.70(13). (C) Dynamic imine coordina-
tion.21
NMR analysis at −50 °C confirmed the full consumption of 4
after 40 min and indicated the formation of a single species
with an asymmetric N,C,N-pincer backbone. Crystals suitable
for X-ray crystallography were obtained by slow diffusion of n-
pentane into a concentrated toluene solution of the reaction
mixture at −78 °C. The crystal structure unequivocally
determined the presence of the dimeric mono-organobismuth
oxide 6, which features two μ-oxo bridge moieties (Figure 3B).
Examples of mono-organobismuth(III) oxides are
rare.22,23,29 Due to the high polarity of the Bi−O bond and
the large difference in orbital size between Bi and O, these
oxides readily undergo dimerization or polymerization.30 To
the best of our knowledge, only two crystal structures of
dimeric mono-organobismuth oxides have been reported: a
Figure 2. (A) Preparation of bismuthinidenes 4 and 5. (B) ORTEP
drawing of 4 and 5, with ellipsoids drawn at the 50% probability level.
H atoms of 4 and 5 as well as distortions of 4 are omitted for clarity.
Ar = m-Tp. Selected bond lengths (Å): for 4, Bi1−C1 2.1487(19),
Bi1−N1 2.4601(15), Bi1−N2 2.5066(15), N1−C7 1.301(3), N2−C8
1.300(2); for 5, Bi1−C1 2.1503(18), Bi1−N1 2.4621(15), Bi1−N2
2.4552(16), N1−C7 1.301(2), N2−C9 1.305(2).
B
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syn-23 and an anti-isomer.22,31 Here, 6 represents an anti-
isomer with a slightly asymmetric Bi2O2 core (Figure 3B). As a
result of the weak coordination of N1 to Bi1 [Bi1−N1,
2.672(4) Å], the Bi1−O2 distance [2.120(3) Å] is marginally
longer than Bi1−O1 [2.103(3) Å]. Interestingly, one m-Tp
group in each half of the complex points away from the central
Bi. Although H27 could not be refined unambiguously, the
short C27−O2 distance (3.104 Å) strongly indicated a
hydrogen bonding between H27 (and its symmetric H) and
O2. 1H NMR at −50 °C reveals dramatically different chemical
shifts for both imines (8.11 and 10.04 ppm), thus endorsing
the hydrogen-bonding proposed. Yet, the dynamic imine
coordination was indicated by the exchange peaks in ROESY-
NMR and convergence of these imine peaks at higher
temperatures as shown in VT-NMR data (Figure 3C). On
the other hand, DOSY-NMR experiments suggested that the
Bi2O2 ring of 6 was preserved in solution and no dissociation
occurred.21 The structure of 6 suggests that similar species are
formed when 1 is oxidized with N2O (Scheme 1).
At this point, we speculated that the dimeric nature of 6
could be the result of a rapid dimerization of a monomeric
terminal Ar−BiO compound. Based on previous examples,32
we speculated that replacement of the imines with ketimines
would favor the isolation of a monomeric species via
tautomerization processes. To entertain this hypothesis, 5
was subjected to N2O in THF-d8 (Figure 4A). Similar to 6, 1H
NMR of the resulting orange-red solution indicated the
formation of one single species. X-ray crystallography
unequivocally determined that 7 was a monomeric organo-
bismuth hydroxide (Figure 4B). The high quality of the
crystals allowed the unambiguous assignment of the positions
of the H1, H8a, H8b, and H10 (3 H). One of the Me groups in
the ketimines converted into a CH2, resulting in a reduction of
the C−C length in C7−C8 [1.3552(16) Å], consistent with a
double bond.33 The longer C7−N1 and Bi1−N2 distances
[1.3832(15) and 2.6117(9) Å] compared to C9−N2 and Bi1−
N1 [1.2848(14) and 2.2319(9) Å] also manifest the presence
of an amido bond in one of the arms of the pincer.34
Interestingly, the OH points to a phenyl group of a m-Tp with
a short H−phenyl centroid distance of 2.622 Å, indicative of a
weak OH···π interaction.27 Due to the high tendency to form
oxides or clusters through dehydration, reports on well-defined
organobismuth hydroxides are limited;24a,27,35 yet, 7 is
noticeably stable.
The monomeric compound 7 represents a tautomeric form
of a monomeric Ar−BiO, an elusive species which has yet to
be reported. In the same way, 6 can be conceived as the result
of a fast dimerization process of two molecules of monomeric
Ar−BiO. The formation of hydroxide 7 highlights the high
basicity of the O atom in Ar−BiO, which could be better
described as a polarized BiO bond: Ar−Bi+−O−. Therefore,
it is reasonable to assume that both 6 and 7 are fingerprints for
the transient generation of such elusive species (int-I, both
resonance structures depicted; Scheme 2), which rapidly
dimerizes or tautomerizes to the more stable compounds 6 and
7.
Scheme 2. Postulated Intermediates during Oxidation of
Bi(I) with N2O
Having identified the intermediacy of Bi−O bonds after
N2O activation, we explored the reduction of 6 and 7 to Bi(I)
to sustain a putative catalytic cycle. Among other uses,36
HBpin has been utilized as a deoxygenation agent for amine
and phosphine oxides37 as well as for the catalytic reduction of
CO2.38 Inspired by this reactivity, we treated 6 with 2.0 equiv
of HBpin, which resulted in immediate formation of a dark
purple solution (Scheme 3). Bismuthinidene 4 formed in 79%
1
yield judging by H NMR. Similarly, the reduction of 7 gave
78% of 5. Meanwhile, ca. 1 equiv of HBpin was converted to a
mixture of HO−Bpin (8) and (pinB)2O (9).39
At this point, we decided to merge this reactivity to unfold a
catalytic system for the activation of N2O with Bi(I)
compounds. Blank experiments demonstrated that no reaction
occurs in the absence of Bi(I) (Table 1, entry 1). Catalytic
N2O deoxygenation with HBpin proceeded smoothly at room
temperature in the presence of 1 mol % of 4 or 5, with the
TON reaching 54 and 89, respectively (entries 2 and 3). The
higher efficiency of 5 over 4 could be ascribed to the higher
stability of oxobismuth species 7 compared to 6. To our
Figure 4. (A) Oxidation of bismuthinidine 5 with N2O; (B) ORTEP
drawing of 7, with ellipsoids drawn at the 50% probability level. H
atoms except H1, H8s, and H10s and the enantiomer of 7 in the unit
cell are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Bi1−C1 2.1869(11), Bi1−O1 2.0984(10), Bi1−N1 2.2319(9),
Bi1−N2 2.6117(9), N1−C7 1.3832(15), N2−C9 1.2848(14), C7−
C8 1.3552(16), C9−C10 1.4969(16), C1−Bi1−O1 94.68(4).
C
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a
counterparts, unveiling an alternative opportunity for catalytic
N2O transformations.
Scheme 3. Reduction of 6 and 7 with HBpin
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c10092.
■
sı
Experimental procedures and analytical data (1H, 13C,
and 11B NMR, HRMS) for new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Josep Cornella − Max-Planck-Institut fu
Mulheim an der Ruhr 45470, Germany; orcid.org/0000-
0003-4152-7098; Email: cornella@kofo.mpg.de
̈
r Kohlenforschung,
̈
Authors
Yue Pang − Max-Planck-Institut fur Kohlenforschung, Mulheim
̈ ̈
an der Ruhr 45470, Germany; orcid.org/0000-0002-5152-
6876
a
Reaction conditions: 6 (17.8 μmol) or 7 (35.7 μmol), HBpin (71.4
Markus Leutzsch − Max-Planck-Institut fu
Mulheim an der Ruhr 45470, Germany; orcid.org/0000-
0001-8171-9399
̈
r Kohlenforschung,
μmol), mesitylene (35.7 μmol, 1.0 equiv) in 1.25 mL of THF-d8 at 25
°C.
̈
̈
̈
Nils Nothling − Max-Planck-Institut fur Kohlenforschung,
Table 1. Bi(I)-Catalyzed N2O Deoxygenation with HBpin
Mulheim an der Ruhr 45470, Germany; orcid.org/0000-
̈
0001-9709-8187
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c10092
Notes
The authors declare no competing financial interest.
Crystallographic data for compounds 2−7 can be obtained free
of charge from www.ccdc.cam.ac.uk under reference numbers
2031447, 2031446, 2031445, 2031448, 2031449, and
2031450, respectively.
a
b
Based on HBpin. Calculated by 1H NMR using mesitylene as
c
internal standard. Determined by disappearance of the characteristic
color of Bi(I).
ACKNOWLEDGMENTS
Financial support for this work was provided by Max-Planck-
Gesellschaft, Max-Planck-Institut fu
■
̈
r Kohlenforschung and
delight, when 1 was revisited as catalyst, a dramatic rate
enhancement was observed; the reaction was complete in 3
min with vigorous release of N2 gas (entry 4). The high
reactivity of 1 permitted lowering the catalyst loading to 0.01
mol % (entries 4 to 7). Unprecedentedly, the TON reached to
6700 at 0.01 mol % catalyst loading (entry 7). In addition, the
TOF was estimated to be 52 min−1 at 0.1 mol % catalyst
loading (entry 5).21 Since dimerization and tautomerization
have already been shown to proceed really fast, we believe that
species similar to 6 and 7 could probably be involved in the
catalytic cycle. The mild conditions and high catalytic
efficiency contrast with the elevated temperatures, high catalyst
loadings, and/or prolonged reaction times usually required for
transition metals.
In conclusion, this work demonstrates the capacity of
bismuthinidenes to catalytically activate N2O in a Bi(I)⇄
Bi(III) redox platform. The synthesis of sterically congested
bismuthinidenes using m-Tp substituents on the imines
permitted isolation and characterization of catalytically relevant
species such as bismuth oxide dimer 6 and bismuth hydroxide
7. Bis-imine and bis-ketimine N,C,N-chelated bismuthinidenes
provide the first main-group redox platform for catalytic N2O
decomposition. The ambient conditions and the very high
catalytic efficiency make this system akin to transition-metal
Fonds der Chemischen Industrie (FCI-VCI). This project
has received funding from European Union’s Horizon 2020
research and innovation programme under Agreement No.
850496 (ERC Starting Grant, J.C.). We thank Prof. Dr. A.
Fu
thank the MS, GC, and X-ray departments of Max-Planck-
Institut fur Kohlenforschung for analytic support. We thank
̈
rstner for insightful discussions and generous support. We
̈
Dr. R. Goddard for X-ray crystallographic analysis. Y.P. thanks
CSC for a PhD scholarship.
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