Reduction of Dioxygen by Radical/B(p-C6F4X)3 Pairs to Give Isolable Bis(borane)superoxide Compounds

Article abstract of DOI:10.1002/anie.201709309

Triplet dioxygen was reduced by TEMPO or trityl radicals in the presence of two molar equivalents of the strong B(p-C₆F₄X)₃ (X: F or H) boron Lewis acids under mild conditions to give the bis(borane)superoxide systems 2. T

Full text of DOI:10.1002/anie.201709309

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Accepted Article
Title: Reduction of Dioxygen by Radical/B(p-C6F4X)3 Pairs to Give
Isolable Bis(borane)superoxide Compounds
Authors: Gerhard Erker, Xin Tao, Constantin G. Daniliuc, Oliver Janka,
Rainer Pöttgen, Robert Knitsch, Michael R. Hansen, Hellmut
Eckert, Maximilian Lübbesmeyer, Armido Studer, and Gerald
Kehr
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709309
Angew. Chem. 10.1002/ange.201709309
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Reduction of Dioxygen by Radical/B(p-C6F4X)3 Pairs to Give Isolable
Bis(borane)superoxide Compounds
Xin Tao,[a] Constantin G. Daniliuc,[a] Oliver Janka,[b] Rainer Pöttgen,[b] Robert Knitsch,[c] Michael R.
Hansen,[c] Hellmut Eckert,[c,d] Maximilian Lübbesmeyer,[a] Armido Studer,[a] Gerald Kehr[a] and Gerhard
Erker*[a]
Dedication ((optional))
Abstract: Triplet dioxygen was reduced by TEMPO or trityl radicals
in the presence of two molar equivalents of the strong B(p-C6F4X)3
(X: F or H) boron Lewis acids under mild conditions to give the
bis(borane)superoxide systems 2. The sensitive radical anion
species were isolated and characterized including X-ray crystal
structure analysis and EPR spectroscopy.
/[B]2O2•− pair being at a value close to the TEMPO/TEMPO+ pair.
This let us speculate that it might be possible to reduce triplet
oxygen[11] by suitable persistent radicals to the superoxide state
(or beyond) in the presence of the strong B(C6F5)3 Lewis acid.
This actually turned out to be the case.
The reduced forms of dioxygen, superoxide [O2]•− and peroxide
[O2]2– are of great importance for chemical transformations as
well as reactive species in biological system.[1] There is a rich
chemistry of the alkali metal superoxides and of metal complexes
with peroxo or superoxo ligands.[2-4]
Boranes were reported to undergo a variety of reactions with
dioxygen species.[5] The reaction of triethylborane with triplet
dioxygen is a typical example, which provides a method of
generating ethyl radicals.[6] Bourissou et al reported about the
addition/insertion reaction of singlet dioxygen to a phosphane/
borane pair,[7] and there were several ways to peroxy boron
compounds reported.[8] Agapie et al recently described the
reduction of dioxygen by two molar equivalents of ferrocenes in
the presence of the strong boron Lewis acid B(C6F5)3 to yield the
bis-ferrocenium bis(borane)peroxides 1a,b.[9,10] The cyclic
2-
voltamogram of 1a showed the redox potential of the [B]2O2
Scheme 1. Reduction of dioxygen in the presence of strong boron Lewis acids
to bis(borane)peroxide/-superoxide compounds.
[a]
Dr. X. Tao, Dr. C. G. Daniliuc, M.Sc. M. Lübbesmeyer, Prof. Dr. A.
Studer, Dr. G. Kehr, Prof. Dr. G. Erker*
Organisch-Chemisches Institut
Westfälische Wilhelm-Universität Münster
Corrensstraße 40, 48149 Münster, Germany
E-mail: erker@uni-muenster.de
We exposed a mixture of TEMPO with two molar equivalents of
B(C6F5)3[12] in deuterated dichloromethane to O2 (1.5 bar) at room
temperature. This led to an immediate color change from orange
to dark brown. The 1H NMR spectra (see the Supporting
Information for details) indicated the formation of a paramagnetic
reaction product. From the concentrated solution, dark brown
crystals of compound 2a precipitated during 48 h at 35oC; they
were isolated in 88% yield. The X-ray structure analysis revealed
the formation of the bis(borane)superoxide radical anion with one
oxoammonium countercation, [C5H6Me4N=O]+{O2[B(C6F5)3]2}•−
and two dichloromethane solvent molecules. The O1-O2 bond in
compound 2a.(2 CD2Cl2) is significantly shorter than the oxygen-
oxygen linkage in the B-O-O-B bis(borane)peroxide compound
1a (Δd = 0.18 Å).[9] The B1-O1/ B2-O2 distances are markedly
longer than those of 1a (average Δd = 0.11 Å), and the B-O-O-
angles are slightly larger by ca. 7°.[9] (the 2a ·2 CH2Cl2 structure
is depicted in the Supporting Information).
[b]
[c]
[d]
Dr. O.Janka, Prof. Dr. R. Pöttgen
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelm-Universität Münster
Corrensstraße 28/30, 48149 Münster, Germany
M.Sc. R. Knitsch, Prof. Dr. M. R. Hansen, Prof. Dr. H. Eckert
Institut für Physikalische Chemie
Westfälische Wilhelm-Universität Münster
Corrensstraße 28, 48149 Münster, Germany
Prof. Dr. H. Eckert
Instituto de Fisica, Sáo Carlos
Universidade de Sáo Paulo
CP 369, 13560-970, Sáo Carlos, S.P. Brasil
Supporting information for this article is given via a link at the end of
the document.CCDC deposition numbers are 1570215 to 1570219.
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We performed the reaction of TEMPO with O2 and two molar
equivalents of B(C6F5)3 in CDCl3 at room temperature and
obtained solvent free single crystals of 2a. The O1-O2 distance
in 2a is close to that of the trimethylphenylammonium superoxide
(of 1.332(2) Å) described by Jansen et al. which contains the
unperturbed O2•− superoxide radical anion [13-15] [see Fig. 1 and
Table 1; for further details see the Supporting Information].
addition to the typical NMR features of the trityl cation [e.g. 13C:
δ = 209.9 (233 K)] we observed a broad 19F NMR singlet at ca. δ
= 158.5 similar as it was observed in compound 2a [see the
Supporting Information for experimental details and the depicted
NMR spectra]. The X-ray crystal structure analysis of compound
2c confirmed the formation of the [Ph3C]+ {O2[B(C6F5)3]2}•−
bis(borane)superoxide radical anion/trityl cation salt (see Figure
2 and Table 1).
Figure 2. Molecular structure of compound 2c (thermal ellipsoids are shown
at the 15% probability level; hydrogen atoms are omitted for clarity).
Figure 1. Molecular structure of compound 2a (thermal ellipsoids are shown
at the 30% probability level; hydrogen atoms are omitted for clarity).
Table 1. Selected structural parameters of the bis(borane)superoxide
compounds 2.[a]
The analogous reaction of a 1:2 mixture of the TEMPO radical
and B(p-C6F4H)3 with O2 gas (1.5 bar) in CD2Cl2 at r.t. gave
[16]
dark brown crystals of 2b in ca. 90% yield. The X-ray crystal
structure analysis confirmed the formation of the analogous B-O-
O-B bis(borane)superoxide radical anion/oxoammonium salt
[C5H6Me4N=O]+ {O2[B(p-C6F4H)3]2}•− with similar structural
parameters as compound 2a (for further details see Table 1 and
the Supporting Information).
compound
cation
2a·(2 CH2Cl2)
TEMPO+
1.305(4)
1.582(6)
1.581(6)
114.6(3)
115.1(3)
2a
2b
2c
TEMPO+
1.325(2)
1.608(3)
1.604(3)
116.8(2)
112.4(2)
TEMPO+
1.280(4)
1.581(6)
1.578(6)
116.0(3)
115.7(3)
Ph3C+
O1-O2
1.310(6)
1.590(9)
1.576(9)
114.0(5)
114.2(5)
B1-O1
B2-O2
B1-O1-O2
B2-O2-O1
B1-O1-O2-B2
177.3(3)
-178.1(2)
169.8(4)
179.2(5)
[a] Bond lengths in (Å), angles in (°).
Scheme 2. Reaction of Gomberg dimer with dioxygen in the presence of
B(C6F5)3.
The effective magnetic moments were determined for the
compounds 2a (eff = 1.60(1) B), 2b (eff = 1.51(1) B), and 2c
(eff = 1.54(1) B) (for details see the Supporting Information).
The values are slightly lower than the expected value for a free
single electron (eff = 1.73 B), which might be due to impurities
present in these very sensitive materials.[13d,e]
We characterized compounds 2a and 2c by EPR spectroscopy.
The X-band EPR spectrum of the oxoammonium cation/bis-
(borane)superoxide radical anion salt (see Figure 3) shows a
seven-line signal centered at g = 2.01101. The multiplicity
confirms the influence of two symmetry-equivalent boron nuclei
with dominating contribution of the 11B-11B isotopologues.
Simulation of the spectra gave a 11B hyperfine coupling constant
Trityl radical, available by in situ equilibration from the Gomberg
dimer,[17] is known to react with O2 giving bis(triphenyl-
methyl)peroxide (3).[18] We mixed the Gomberg dimer with four
equivalents of B(C6F5)3 in CD2Cl2 at room temperature, and
confirmed by NMR spectroscopy that no reaction had occurred.
We then exposed this mixture to O2 (1.5 bar, r.t.) (Scheme 2).
Within ca. 30 min the color of the reaction mixture had changed
from orange to dark yellow. Diffusion of pentane to the reaction
solution gave compound 2c as dark yellow crystals, which we
isolated in ca. 40% yield. Compound 2c was characterized by
elemental analysis, spectroscopy and X-ray diffraction. In
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of 3.23 G (9.1 MHz) for compound 2a. Compound 2c showed
analogous EPR behavior [g = 2.01101, septet, A(11B) = 3.26 G
(9.2 MHz), for further details see the Supporting Information)].
cation/trityl radical redox feature (0.16 V) and the
bis(borane)superoxide radical anion/bis(borane)-peroxide signal
at +0.22 V.[9] We note from an independent experiment (for
details see the Supporting Information) that the latter value is
almost identical with the oxoammonium/ TEMPO redox potential,
which is in accord with the apparently facile back-electron
transfer under suitable conditions (see above).[20]
Figure 3. X-band EPR spectrum of
a M CH2Cl2 solution of the
10-2
oxoammonium cation/bis(borane)superoxide radical anion salt 2a;
experimental (bottom) and simulated spectrum (top).
The bis(borane)superoxide compounds 2 are rather labile in
•−
solution at ambient temperature. Free superoxide O2 can
undergo back electron transfer to oxoammonium cation to give
TEMPO and O2,[19] and so can apparently the
bis(borane)superoxides 2. Treatment of 2a with tetrahydrofuran
(THF) or dimethylsulfoxide (DMSO) resulted in the rapid
formation of the corresponding donor solvent-borane adducts
with liberation of a gas, probably dioxygen. The formation of
TEMPO was observed by EPR spectroscopy in both reactions.
The reaction of the trityl compound 2c with DMSO gave a slightly
different outcome. We observed the formation of the
(dimethylsulfoxide)B(C6F5)3 Lewis adduct and trityl peroxide
(Ph3C-O-O-CPh3 3)[18] as major components (see Scheme 3).
We assume that in this case liberated dioxygen was trapped by
trityl radical, in situ formed by back electron transfer.
Figure 4. Cyclic voltamogram of the trityl/bis(borane)superoxide system 2c in
dichloromethane (0.1 M [Bu4N][PF6]) in the presence of a slight excess of
decamethylferrocene.
Treatment of the in situ generated compound 2c with an
additional molar equivalent of trityl radical led to the formation of
the trityl cation/bis(borane)-peroxide dianion salt 1c, featuring
tetra-coordinate borate NMR signals (Scheme 4). (11B: δ = 2.1,
Δδ19Fm,p = 2.9 ppm {O2[B(C6F5)3]2}2–, for the depicted NMR
spectra and experimental details see the Supporting Information)
Workup gave compound 1c as a yellow amorphous solid in ca.
49%
yield.
Single
crystals
were
obtained
from
dichloromethane/pentane by the diffusion method. The X-ray
structure analysis (see the Supporting Information for details and
the depicted molecular structure) revealed that compound 1c
features a B-O-O-B bis(borane)-peroxide dianion with two trityl
counter cations similar to that previously observed by Agapie et
al in the respective ferrocenyl systems[9] [d(O-O): 1.488(2) Å
(1c); 1.485(2) Å (1a)].
Scheme 3. Reaction of bis(borane)superoxide 2a THF or DMSO.
Direct characterization of the sensitive superoxide radical salts
by cyclic voltammetry proved difficult. Therefore, we added a
slight excess of decamethylferrocene to the solution of 2c, which
served as a pre-reductant and an internal standard. The cyclic
voltamogram (Figure 4) showed the strong signal of the Fc*+/Fc*
redox couple at 0.59 V (vs. Fc/Fc+), followed by the weak trityl
Scheme 4. Reduction of compound 2c by trityl radical.
Our study has shown that triplet dioxygen is readily reduced to
the superoxide stage by treatment with suitable radicals in a 1:1
stoichiometry, here TEMPO or the trityl radical, in the presence
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(with X: F or H) to give the bis(borane)superoxide compounds 2.
The isolation and characterization of these rather sensitive
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electrophilic bulky boranes to serve as reagents in small
molecule activation and suitable stabilization of their reaction
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Acknowledgements
Financial support from the European Research Council and the
Deutsche Forschungsgemeinschaft is gratefully acknowledged.
RK thanks the Fonds der Chemischen Industrie for a stipend. We
cordially thank Prof. Hans J. Schäfer for helping us with the CV
experiments.
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Keywords: dioxygen • superoxide • peroxide • radicals • boron
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