Anion-Receptor Mediated Oxidation of Carbon Monoxide to Carbonate by Peroxide Dianion

Article abstract of DOI:10.1021/jacs.5b08495

The reactivity of peroxide dianion O₂^2- has been scarcely explored in organic media due to the lack of soluble sources of this reduced oxygen species. We now report the finding that the encapsulated peroxide cryptate, [O₂?mBDCA-5t-H₆]^2- (1), reacts with carbon monoxide in organic solvents at 40 °C to cleanly form an encapsulated carbonate. Characterization of the resulting hexacarboxamide carbonate cryptate by single crystal X-ray diffraction reveals that carbonate dianion forms nine complementary hydrogen bonds with the hexacarboxamide cryptand, [CO₃?mBDCA-5t-H₆]^2- (2), a conclusion that is supported by spectroscopic data. Labeling studies and ¹⁷O solid-state NMR data confirm that two-thirds of the oxygen atoms in the encapsulated carbonate derive from peroxide dianion, while the carbon is derived from CO. Further evidence for the formation of a carbonate cryptate was obtained by three methods of independent synthesis: treatment of (i) free cryptand with K₂CO₃; (ii) monodeprotonated cryptand with PPN[HCO₃]; and (iii) free cryptand with TBA[OH] and atmospheric CO₂. This work demonstrates CO oxidation mediated by a hydrogen-bonding anion receptor, constituting an alternative to transition-metal catalysis.

Full text of DOI:10.1021/jacs.5b08495

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Anion-Receptor Mediated Oxidation of Carbon Monoxide to Carbonate by
Peroxide Dianion
†
†,‡
†
¶
∗,§
Matthew Nava, Nazario Lopez, Peter M u¨ ller, Gang Wu, Daniel G. Nocera, and
∗
Department of Chemistry, Massachusetts Institute of Technology, Cambridge MA, United States
,†
Christopher C. Cummins
†
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2139-4307, ‡Current Address: Centro de Investigaciones Qu ´ı micas, IICBA, Universidad Autonoma del
Estado de Morelos, Avenida Universidad 1001, Colonia Chamilpa, C.P. 62209, Cuernavaca, Morelos
Mexico, ¶Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston ON, Canada K7L3N6,
and §Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge
MA, United States
Received October 10, 2015; E-mail: dnocera@fas.harvard.edu; ccummins@mit.edu
5
with a coordinated carbonyl ligand, and (ii) peroxo metal
6
2
−
has been
complexes that couple with free CO.
Abstract: The reactivity of peroxide dianion O
scarcely explored in organic media due to the lack of soluble
sources of this reduced oxygen species. We now report the
2
A fundamental question that to date has not been ad-
dressed is concerned with the reactivity of peroxide dianion
on its own towards CO; the insolubility of alkali metal per-
oxides in polar aprotic organic media hampers direct testing
of this reaction. Recent experimental and theoretical work
has demonstrated that peroxide and superoxide on the sur-
finding that the encapsulated peroxide cryptate, [O
2
⊂mBDCA-
2−
◦
5
t-H
6
]
(1), reacts with carbon monoxide in organic solvents
at 40 C to cleanly form an encapsulated carbonate. Character-
ization of the resulting hexacarboxamide carbonate cryptate by
single crystal X-ray diffraction reveals that carbonate dianion
forms nine complementary hydrogen bonds with the hexacar-
face of metal oxide materials are able to oxidize CO to CO
2
2
−
7–9
or surface-bound CO
3
.
Ceria, which is used in automo-
2
−
bile catalytic converters, is a prominent example of this type
of reactivity as it has been shown experimentally and com-
putationally to oxidize CO to carbonate upon reaction with
boxamide cryptand, [CO
3
⊂mBDCA-5t-H
6
]
(2), a conclusion
that is supported by spectroscopic data. Labeling studies and
1
7
O solid-state NMR data confirm that two thirds of the oxy-
9
,10
peroxide.
When studying metal oxide systems it is often
gen atoms in the encapsulated carbonate derive from perox-
ide dianion while the carbon is derived from CO. Further evi-
dence for the formation of a carbonate cryptate was obtained
by three methods of independent synthesis: treatment of (i)
difficult to uncouple the reactivity of peroxide dianion with
CO from the reactivity of the supporting metal. In this work,
1
1,12
we report the oxidation of CO by a peroxide dianion
lated in a hydrogen bonding anion receptor [O
iso-
⊂mBDCA-
free cryptand with K
with PPN[HCO ]; and (iii) free cryptand with TBA[OH] and
atmospheric CO . This work demonstrates CO oxidation me-
2
3
CO ; (ii) monodeprotonated cryptand
2
2−
13
(1), resulting in the formation of an encapsulated
5
carbonate cryptate [CO
t-H
6
]
3
2−
3
⊂mBDCA-5t-H
6
]
case of peroxide cryptate 1 and other systems in which an
(2). As in the
2
diated by a hydrogen-bonding anion receptor, constituting an
alternative to transition-metal catalysis.
anion is sequestered in the inner space of hexacarboxamide
1
4
cryptand constructs, the carbonate dianion in 2 is encased
by the sheath of multiple hydrogen-bond donors that con-
stitute the anion receptor’s interior lining.
The reduction of carbon dioxide to carbon monoxide, for-
mate, alcohols or alkanes has garnered considerable atten-
tion, especially when coupled with renewable energy sources
1
5
Exposure of a yellow solution of [(TBA)
◦
2
] [1] to CO (1
atm, 3 h, 40 C) in a THF-DMF mixture (10:1 v:v) results
1
as a means to reduce humanity’s dependence on fossil fuels.
2
in the formation of carbonate adduct [(TBA) ] [2] in quanti-
1
The reverse reaction, oxidation of carbon monoxide to car-
bon dioxide is also an important transformation due to toxic-
ity of CO and the need to mitigate CO from industrial waste
tative yield. The H NMR spectrum of 2 is characterized by
distinct downfield chemical shifts of the internally directed
N-H carboxamide protons of the cryptand as well as the
2
1
streams and vehicle exhaust. This transformation can be
central C-H aryl protons, analogous to the H NMR spec-
effected by soluble metal complexes and heterogeneous sys-
troscopic signatures for 1 and indicative of strong hydrogen
bonding interactions (Figure 1). Proton NMR signals for the
carbonate anion cryptate were observed at 12.2 ppm (N-H
protons); these signals are downfield from those observed for
the free cryptand (8.65 ppm). In addition, the internally di-
rected aryl C-H protons are found at 10.8 ppm in contrast
tems, with gold nanoparticles being the most active catalyst
3
in the oxidation to CO
2
.
systems exist that are capable of oxidizing CO to CO
Numerous other heterogeneous
at
2
room temperature or more commonly at elevated tempera-
tures with an important example being the transition metal
4
1
oxide Hopcalite. Well-defined molecular complexes that can
with the H NMR signals of the peroxide anion cryptate
(10.10 ppm) and the free cryptand (8.60 ppm). Preparation
oxidize CO to carbonate can be divided into two distinct cat-
egories: (i) low oxidation-state metal carbonyl complexes
1
3
of the carbonate anion cryptate from CO and 1 resulted
1
3
that reduce free O
2
to peroxide, which subsequently reacts
in an intense C NMR signal at 172.4 ppm, within the ex-
pected region for carbonate dianion ensconced in an anion
†
16
Massachusetts Institute of Technology
Universidad Autonoma del Estado de Morelos
Queen’s University
receptor, showing that CO is the source of carbon in car-
bonate anion cryptate.
‡
¶
§
Single crystals suitable for an X-ray diffraction study were
obtained by vapor diffusion of diethyl ether into a THF solu-
Harvard University
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Figure 1.
(
H NMR spectrum of [(TBA)2][O2⊂mBDCA-5t-H6]
A) and [(TBA)2][CO3⊂mBDCA-5t-H6] (B) in DMSO-d6 after
◦
exposure to CO gas (40 C, 3 h).
tion of [(TBA)
treatment of a solution of [(TBA)
pseudo-three fold axis of the carbonate anion cryptate indi-
cates that the cryptand retains the propeller like (D ) con-
formation observed in the peroxide dianion cryptate but the
2
] [2]; the sample originated directly from CO
Figure 2. Solid-state structure of [(TBA)2][CO3⊂mBDCA-5t-
2
] [1]. A view along the
6
H ] with thermal ellipsoids plotted at the 50% probability level.
Counter cations, solvent and all hydrogen atoms are omitted for
clarity except for the N-H carboxamide protons engaged in hydro-
gen bonding with the carbonate anion. Selected interatomic dis-
3
tances ( ˚A ) and angles ( ): C81-O(carbonate) 1.291(avg), N5-N5A
◦
2
−
plane of the [CO
pseudo-C axis (the peroxide dianion in 1 lies along the
cryptand pseudo-C axis). The cryptand is slightly more
compressed along its pseudo-C axis in the carbonate anion
3
]
unit is perpendicular to the cryptand
7
.687(2), N1-O5 2.739, N2-O6 2.754(2), N3-O6 2.72(2), N4-O6A
◦
◦
3
2.79(1), O5-C81-O6 120.2 , O6-C81-O6A 119.7 , C9-O6 3.151(1),
C9-O5 3.257, C29-O6 3.14(1) C29-O6A 3.19(1)
3
3
cryptate relative to the peroxide cryptate, highlighting the
conformational flexibility of the hexacarboxamide cryptand,
which adjusts its conformation depending on the shape of
the anion bound in its cavity. A total of six N-H carbox-
amide hydrogen bonds are made to the three oxygen atoms
of the carbonate with N· · · O distances ranging from 2.613
˚ ˚
2.977 A (average distance 2.75 A), characteristic of in-
-
1
7
termediate strength hydrogen bonds. Also evident from
the crystal structure is the presence of bifurcated cryptand
phenylene C-H· · · O interactions with the carbonate oxygens,
as noted previously in other anionic adducts of hexacarbox-
amide cryptand systems.
1
The solid-state O NMR spectra for
1
1,14,18
7
1
7
[
K
2
(DMF)
mBDCA-5t-H
2
][CO
,
O
O
2
⊂mBDCA-5t-H
6
] (3) prepared from
1
7
6
2
and a reducing agent followed by treat-
1
9
ment with CO (Scheme 1) are shown in Figure 4. At 298
K, the O NMR spectrum exhibits a relatively narrow but
featureless peak centered at 150 ppm. This spectral feature
suggests the presence of significant molecular motion for the
encapsulated carbonate anion even in the solid state. To
further probe this molecular motion, O solid-state NMR
spectra were collected at different temperatures. At 223 K,
1
as seen in Figure 4, the O NMR spectrum of 3 now shows
a regular line shape commonly observed in solid-state O
NMR spectra, arising from a combination of second-order
17
Figure 3. Cutway of 2 illustrating the inner hydrogen-bonding
sheath of the hexacarboxamide cryptand and the interactions
with carbonate dianion. NH· · · O hydrogen bonds are indicated
by black lines while bifurcated CH· · · O hydrogen bonds are indi-
cated by light gray lines.
1
7
7
1
7
in reasonably good agreement with the experimental results
discussed above.
2
0
quadrupolar interactions and chemical shift anisotropy.
Analysis of this low-temperature spectrum yielded the fol-
lowing parameters for 3: C
◦
Q
= 7.5 MHz, η
Q
= 0.7, δiso
=
17
23 C, 3 h
+ 2K(TEMPO) −−−−−−→
DMF
mBDCA-5t-H
6
+
O
2
1
70, δ11 = 266, δ22 = 194, δ33 = 50 ppm. These spectral
2
1
◦
parameters are similar to those reported for CaCO
3
and
To further establish the identity of the carbonate
17
23 C, 24 h
(1)
2
2
[K
2
2
(DMF)
(DMF)
3
2
][
O
2
⊂mBDCA-5t-H
6
] + CO −−−−−−−→
Li
anion in the adduct, we used the Amsterdam Density Func-
2
CO
3
.
DMF
1
7
[
K
][CO
O
2
⊂mBDCA-5t-H
6
]
tional (ADF) program to perform quantum chemical com-
⊂mBDCA-
3
17
putations of the O NMR parameters for a [CO
3
Furthermore, as shown in Figure 4, we were able to ana-
2−
17
5
t-H
6
]
model generated from the crystal structure of the
lyze the experimental variable-temperature O NMR spec-
19
anion cryptate. The ADF computational results are: C
7.03 MHz, η = 0.95, δiso = 223, δ11 = 335, δ22 = 222,
33 = 112 ppm. These calculated O NMR parameters are
Q
tra by assuming that the carbonate anion undergoes discrete
jumps about its C3 axis, a dynamic process resembling a
=
δ
Q
1
7
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C, 14 h
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3
[K(DMF)
x
][mBDCA-5t-H
i
5
] + [PPN][HCO
3
] −−−−−−−→
DMF
[
(PPN)K][2]
◦
2
3
C, 48 h
mBDCA-5t-H
6
+ K
2
CO
3
−−−−−−−→ [K
2
(DMF)
x
][2]
ii
DMF
◦
2
3
C, 1 h, air
mBDCA-5t-H
6
+ TBAOH −−−−−−−−−→ [(TBA)
2
][2]
iii
DMF
(
If potassium rather than TBA is used to prepare the car-
1
bonate anion cryptate, the H NMR spectrum taken after
2)
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precipitation of the product with Et O indicates a mixture
of carbonate anion cryptate and free cryptand in typical ra-
tios of 80:20. The formation of free cryptand is likely the re-
sult of carbonate extraction from the cryptand by potassium
ions, explaining the observation (mentioned above) that the
carbonate anion cryptate cannot be prepared in quantitative
yield directly from potassium carbonate. Addition of several
equivalents of potassium triflate to a solution of the tetra-
butylammonium salt of carbonate anion cryptate in DMF-d
7
was also found to induce the formation of free cryptand and
by inference potassium carbonate illustrating the cation’s
role with respect to the stability of the carbonate peroxide
adduct. Tetrabutylammonium salts of the carbonate anion
cryptate are not indefinitely stable in solution, and over time
deprotonated cryptand is observed likely stemming from the
formation of bicarbonate or carbonic acid which decomposes
to release water and CO and may account for the observed
2
water molecule in the crystal structure of the carbonate an-
ion cryptate.
Isotopic labeling studies were conducted to confirm the
origin of the atoms in the carbonate anion cryptate. In par-
ticular, to verify that the proposed reaction of peroxide and
CO occurs and that ensuing carbonate is not a byproduct
of a side reaction or impurity, we sought to confirm that the
carbonate carbon is derived from CO and that two-thirds
of the oxygen atoms are derived from the oxygen atoms in
the peroxide dianion cryptate. The latter was therefore pre-
1
9
Figure 4. Experimental (blue) and simulated (red) solid-state
17
17
O NMR spectra of [K2(DMF)2][CO O2⊂mBDCA-5t-H6] at
different temperatures. All spectra were recorded at 14.1 T.
2
molecular gyroscope. Our analysis of the solid state
3
17
O
NMR data shown in Figure 4 yielded the activation energy
−
1
for such jumps, E
interesting to note that each of the three carbonate oxygen
a
=22±2 kJ mol (see figure S21). It is
˚
atoms form two O · · · H-N hydrogen bonds (ro−n=2.80 A)
inside the cryptand cage. Therefore, in order for the encap-
sulated carbonate to rotate, a total of six O · · · H-N hy-
drogen bonds must break. We also note that the observed
activation energy for the carbonate jumps is smaller than
−1
those found for the three-fold jumps for the -SO
3
group
1
8
pared utilizing
was then exposed to
a labeled carbonate anion cryptate [ CO
2
O and the labeled material so obtained
24
in crystalline sulfonic acids, reflecting the fact that the hy-
drogen bonding strength is greater in the latter cases which
contain more ionic O · · · H-N hydrogen bonds. The current
case represents a very rare instance wherein carbonate an-
1
3
16
C
O. Evidence for the formation of
1
3
18
O
2
⊂mBDCA-5t-
was obtained from the decomposition of the carbonate
adduct by treatment with two equivalents of HBF , in order
to form carbonic acid in situ with ensuing conversion to wa-
2
−
6
H ]
1
7
4
ion dynamics are probed directly by solid-state O NMR
spectroscopy. This also illustrates the unique information
1
3
18
ter and evolved gas identified as CO
2
. Release of
O was confirmed by headspace sampling with
an EI quadrupole mass spectrometer demonstrating that
C O
2
1
7
13
that O NMR spectroscopy can offer, as C NMR would
be insensitive to the aforementioned dynamics.
1
3
18 16
and
C O
The carbonate anion cryptate can also be prepared by
alternative routes in support of the carbonate assignment.
18
2−
13
13 16
2
reacts with CO.
2
O C O was also observed and
in a control experiment, it was found that adding a small
3
Treatment of monodeprotonated cryptand with PPN[HCO ]
1
8
16
amount of water (H
2
O) to O labeled carbonate followed
2
5
(
i)
leads to proton transfer from bicarbonate to the
by acid decomposition resulted in a substantial decrease in
cryptand and subsequent encapsulation of carbonate. In ad-
dition, free cryptand can be treated with excess potassium
carbonate (ii) as a slurry in an organic solvent resulting in
the isolation of the carbonate anion cryptate along with 20%
free cryptand. Excess potassium carbonate is required due
to the presumed equilibrium of solid potassium carbonate
with encapsulated carbonate cryptate. Alternatively, expo-
sure of a solution of free cryptand and TBA[OH] in methanol
to an ambient atmosphere (iii) also led to the clean produc-
tion of the carbonate anion cryptate. Carbonate in the latter
case is presumably derived from nucleophilic attack of hy-
1
6
18
16 18
the C
2 2
O signal and appearance of C O and C O O,
providing a possible explanation for the loss of some of the
O when starting from O labeled peroxide.
While the reaction of CO with peroxide dianion in solu-
1
8
18
tion has not been reported previously, CO has been demon-
2
9–31
upon photoly-
. Detailed kinetic
strated to react with hydrogen peroxide
sis at room temperature to produce CO
2
studies demonstrate that the reaction proceeds first through
O-O bond homolysis of hydrogen peroxide generating hy-
droxy radicals (·OH) which then react with CO under a
radical chain mechanism. Further evidence that the hy-
droxyl radical is able to rapidly oxidize CO was obtained
2
droxide ion on atmospheric CO , as is precedented in anion
1
6,26–28
recognition chemistry.
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through electrochemical studies using boron doped diamond
32
electrodes. In both of the aforementioned examples, per-
oxide dianion is not implicated as the oxidant and thus far
represent the few examples of metal free oxidations of CO
(3) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett.
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(
4) Kireev, A.; Mukhin, V.; Kireev, S.; Klushin, V.; Tkachenko, S.
Russ. J. Appl. Chem. 2009, 82, 169–171.
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3
to yield CO
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ley, D. C.; Whittlesey, M. K. Organometallics 2008, 27,
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M. M.; de Bruin, B. Organometallics 2010, 29, 1629–1641;
The results presented herein demonstrate that peroxide
dianion is able to react with CO without the assistance of a
transition metal. Currently, it is unclear what role cryptand
6
mBDCA-5t-H plays in the transformation besides function-
ing as a solubilizing agent for the peroxide dianion and as
a host for the carbonate dianion reaction product. In situ
generation of hydrogen peroxide via proton transfer from
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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6
7
8
9
0
1
2
3
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5
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7
8
9
0
1
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9
0
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7
8
9
0
(
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3
3
air batteries, where it is the primary discharge product.
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3
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solvent soluble source of peroxide dianion, permitting its in-
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anion receptor. Labeling studies confirm that the resulting
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and CO. Thus, transition metals are not an absolutely re-
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(
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Acknowledgement This research was supported by the
National Science Foundation (NSF) through the Centers for
Chemical Innovation (CCI), Solar Fuels grant CHE-1305124
and U.S. DOE Office of Science under award DE-SC0009565
1
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DGN). Grants from the NSF also provided instrument sup-
port to the DCIF at MIT (Grants CHE-9808061 and DBI-
729592). The X-ray diffractometer was purchased with
2
(
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9
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funding assistance from the NSF (Grant CHE-0946721).
We are grateful to Michael Huynh for his assistance with
NMR calculations, Rebecca de Las Cuevas for assistance
with initial reactivity studies and Ioana Knopf for a gift of
+
(
25) PPN
=
Bis(triphenylphosphine)iminium,
[N(PPh3)2] .
3
6
[PPN][HCO ] is prepared via a literature procedure
3
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(
Bosch Company for partial financial support.
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X-ray diffraction studies and Cartesian coordinates for structures
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ACS Paragon Plus Environment
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