Dalton
Transactions
PAPER
Cite this: Dalton Trans., 2020, 49,
6065
Received 8th February 2020,
Accepted 7th April 2020
DOI: 10.1039/d0dt00475h
rsc.li/dalton
Catalytic dioxygen reduction mediated by a
tetranuclear cobalt complex supported on a
stannoxane core†
Anirban Chandra,
a
Stefan Mebs,
b
Subrata Kundu,
a
Uwe Kuhlmann,
c
Peter Hildebrandt,
c
Holger Dau
b
and Kallol Ray *
a
The synthesis, spectroscopic characterization (infrared, electron paramagnetic resonance and X-ray
absorption spectroscopies) and density functional theoretical calculations of a tetranuclear cobalt
complex Co
4
L1 involving a nonheme ligand system, L1, supported on a stannoxane core are reported.
Co
4
L1, similar to the previously reported hexanuclear cobalt complex Co
6
L2, shows a unique ability to
catalyze dioxygen (O
2
) reduction, where product selectivity can be changed from a preferential 4e
−
/4H
+
dioxygen-reduction (to water) to a 2e
−
/2H
+
process (to hydrogen peroxide) only by increasing the temp-
erature from −50 to 30 °C. Detailed mechanistic insights were obtained on the basis of kinetic studies on
the overall catalytic reaction as well as by low-temperature spectroscopic (UV-Vis, resonance Raman and
X-ray absorption spectroscopies) trapping of the end-on μ-1,2-peroxodicobalt(III) intermediate 1. The
Co
4
L1- and Co
6
L2-mediated O
2
-reduction reactions exhibit different reaction kinetics, and yield different
ratios of the 2e
−
/2H
+
and 4e
−
/4H
+
products at −50 °C, which can be attributed to the different stabilities
of the μ-1,2-peroxodicobalt(III) intermediates formed upon dioxygen activation in the two cases. The
deep mechanistic insights into the transition-metal mediated dioxygen reduction process that are
obtained from the present study should serve as useful and broadly applicable principles for future design
of more efficient catalysts in fuel cells.
Introduction
Significant, attention has been focused in recent years on the
synthesis of transition metal based dendrimer structures
owing to their diverse applications in various fields.
1
In par-
ticular, these dendrimers, in many cases, allow synergistic
interactions between the individual transition metal centers in
carrying out a variety of important transformations. The orga-
nooxotin clusters are in particular attractive because of the
diversity of arrangements that they adopt, such as ladder,
O-capped, cube, butterfly, drum, one, two and three-dimen-
sional structures, (1D, 2D, and 3D).
2–6
Furthermore, incorpor-
ation of redox-active transition-metal centers into the stannox-
ane clusters has previously led to the demonstration of impor-
tant reactivity patterns.
7,8
For example, an extensive coopera-
tive effect between the Cu centers was observed during the
cleavage of supercoiled DNA catalyzed by a hexanuclear Cu–
porphyrin complex, supported on a stannoxane core.
7a
In our
group we have previously demonstrated the ability of a non-
heme stannoxane based hexanuclear ligand system to undergo
O–O bond formation
7b
and O–O bond cleavage reactions,
8
when bound to iron(II) and cobalt(II) centers, respectively. In
the present manuscript we report the synthesis, characteriz-
ation and X-ray structure of a tetranuclear stannoxane based
non-heme ligand system (L1), and a detailed kinetic study of
the catalytic dioxygen reduction reaction mediated by the
corresponding cobalt complex Co
4
L1. Notably, catalytic
reductions of O
2
to water or H
2
O
2
have tremendous technologi-
cal significance.
9–12
However, in contrast to biology, where
cheap and readily available transition-metal complexes of Fe,
and Cu are employed for O
2
reduction,
13
high loadings of a
precious metal like platinum is warranted for achieving
appreciable reactivity during abiological O
2
-reduction reac-
tions.
12d
Thus the present study is relevant to the ongoing
research activities that are being dedicated towards the devel-
opment of O
2
reduction catalysts based on nonprecious
†Electronic supplementary information (ESI) available. CCDC 818335. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0dt00475h
a
Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Straße 2,
Fax: +4930 2093 7387; Tel: +49 30 2093 7385
b
Freie Universität Berlin, FB Physik, Arnimallee 14, D-14195 Berlin, Germany
c
Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 135,
10623 Berlin, Germany
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metals.
14
Furthermore, it provides deep mechanistic insights
into the factors that control two- vs. four-electron reductions of
O
2
, thereby providing useful and broadly applicable principles
for the future design of more efficient O
2
reduction catalysts.
Results and disscussion
Synthesis and characterisation of L1
The condensation reaction (Schemes S1 and S2†) of equimolar
amounts of di-n-butyltin oxide and 4-(1,3-bis(2-pyridylmethyl)-
2-imidazolidinyl)benzoic acid in toluene afforded L1 as a pale
yellow solid. The molecular structure of L1 shows that a planar
Sn
4
O
2
core supports the four metal-binding sites (Fig. 1: top).
This is in contrast to the situation reported earlier for the hexa-
nuclear non-heme ligand system L2, where six metal-binding
sites were located in a wheel-like arrangement around a
central Sn
6
O
6
prismane core (Fig. 1: bottom). The stannoxane
core in Ligand L1 adopts a ladder framework with two central
and two terminal tin atoms. The tetranuclear structure of L1 is
maintained in solution.
119
Sn NMR spectrum of L1 exhibits
two sharp singlets of equal intensity at −210.82 ppm and
−213.81 ppm (Fig. S1†), which is the characteristic signature
for a planar Sn
4
O
2
core.
2–6
The infrared spectrum shows four
vibrations at 1622 cm
−1
, 1591 cm
−1
, 1569 cm
−1
, and
1545 cm
−1
for the carboxyl absorptions (ν
COO
), and one strong
band at 682 cm
−1
assigned to ν
Sn−O
for the Sn
4
O
2
core
(Fig. S2†).
Synthesis and characterization of Co
4
L1
The reaction of L1 with 4 equiv. of Co(CF
3
SO
3
)
2
in acetone
yields Co
4
L1 as a dark yellow powder in 70% yield (Scheme 1).
The C, H, and N content of Co
4
L1, determined by elemental
analysis, established the presence of four cobalt atoms per
tetrameric ligand, with two triflates associated with each
cobalt (see ESI†).
The infrared spectrum of Co
4
L1 depicts the characteristic
vibrations of the Sn
4
O
2
core at 1625 cm
−1
, 1593 cm
−1
,
1572 cm
−1
, 1549 cm
−1
, and 682 cm
−1
(Fig. S2†). These
vibrations are only slightly shifted relative to that of L1, which
reveals that the tetranuclear arrangement is also maintained
in Co
4
L1.
Electronic and structural information of Co
4
L1 were
obtained from X-ray absorption spectroscopy (XAS) in conjunc-
tion with density functional theory (DFT) calculations. The
near edge structure (XANES) was used for determination of the
oxidation states, whereas the extended fine structure (EXAFS)
unraveled the local site geometries around the Co atoms. The
spectra are displayed in Fig. 2, the corresponding fit values are
collected in Table 1. The XANES spectrum of Co
4
L1 (blue
trace) is displayed together with spectra from Co reference
compounds of known oxidation states (Co
2+
,Co
2.66+
,Co
3+
), see
Fig. 2a; it is consistent with a Co
2+
oxidation state in Co
4
L1
Fig. 1 Comparison of the distances between the metal binding sites in
L1 (top) and L2 (bottom). X-ray crystal structure of L1 and L2 with 30%
ellipsoid probability of the atoms. Hydrogen atoms and the n-butyl
(
n
Bu-) groups on the tin atoms have been omitted for clarity. Color
code: nitrogen-blue; carbon-grey; oxygen-red; tin-green.
Scheme 1 Synthesis of the tetra-nuclear cobalt(II) complex (Co
4
L1) from the tetra-nuclear stannoxane ligand (L1) and the formation of the
cobalt(III)–peroxo complex (1).
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(Fig. 2b). The EXAFS of Co
4
L1 could be well fitted by four
shells, with one shorter N-shell with coordination number (N)
of 5, a longer N/O-shell with N= 1, and two C-shells with N=3
and 2 (Fig. 2c and d). Attempts to fit Co
4
L1 with a sum of N=5
in the first two shells (instead of 6) significantly worsen the fit
parameters. In principle, there are up to nine C-atoms within a
radius of 3.5 Å around the Co-atom, however, due to the pro-
nounced inhomogeneity of the Co–C distances, these shells
may partially cancel each other out. The average Co–O/N dis-
tance is found to be 2.17 Å.
Since there are no X-ray diffraction (XRD) structures avail-
able for Co
4
L1 and the Co atoms are mainly surrounded by O,
N and C atoms with similar scattering properties, the EXAFS
fits may suffer from non-uniqueness and misinterpretations.
In order to reduce this problem as well as to obtain suitable
phase functions for the fits, DFT calculations were conducted
in the experimentally observed (from EPR; Fig. S3†)S= 3/2
spin state for a series of potential structural variants of the
monomeric subsection of the organic ligands, starting from
the modified XRD structure of the tetrameric stannoxane
ligand (see Fig. 3 and S4†). This approach is justified as there
are no intra-molecular electronic interactions detectable
between adjacent Co(II) sites, as evident from the X-band EPR
spectrum of Co
4
L1, which exhibits a major axial signal with
effective g′
⊥
= 4.01 and g′
∥
≈2.0 corresponding to the S=
3/2 ground state (Fig. S3†). Structural variants include the
Table 1 EXAFS fit parameters for Co
4
L1 and 1
Model Shell N
a
R(Å) Err σ(Å) Err
Co
4
L1 Co–N 5 2.14 0.01 0.056 0.007
R
f
= 18.4 Co–N 1 2.34 0.05 0.056
E
0
= 0.74 Co–C 3 2.94 0.03 0.056
d
av
= 2.17 Å Co–C 2 3.06 0.05 0.056
1Co–N 4 1.91 0.01 0.036 0.009
R
f
= 15.0 Co–O 1 2.02 0.03 0.036
E
0
= 2.82 Co–C 3 2.78 0.03 0.036
d
av
= 1.93 Å Co–C 2 2.92 0.05 0.036
a
Value kept constant in the final refinement. Amplitude reduction
factor S
02
= 0.95. Nrepresents the EXAFS coordination number, Rthe
absorber-backscatter distance and σthe Debye Waller parameter.
Fig. 2 (a) Co K-edge XANES spectra of reference compounds (black:
Co(OH
2
)
6
(NO
3
)
2
, dark grey: Co
3
O
4
spinel, grey: CoOOH) and samples
(blue: Co4L1, red: 1). The horizontal line at 0.5 is used for determination
of the oxidation states. (b) Oxidation states of samples Co
4
L1 and 1
derived from linear fit lines extracted from the 0.5 edge-rise positions of
reference compounds (black line and squares) and derived from the
integral method (grey line and squares). Both methods result in equal
oxidation state assignments. (c) k-Space EXAFS spectra (k
3
weighted,
colored) and respective fits (grey). (d) Fourier transform of the EXAFS
spectra (colored) and fits (grey).
Fig. 3 (a) The DFT calculated coordination environment of the individ-
ual Co-centers in Co
4
L1, which matches best with the experimental
data. The straight green lines reveal the N-capped O
2
N
3
H–octahedron
environment of the central Co(II) center, by taking into account the
agostic C–H⋯Co interaction. (b) DFT model of 1with fixed carboxylate
C⋯C atom distance of 6.43123 Å according to the XRD structure of the
free ligand. In 1,L1 acts as a tetradentate ligand with one of the nitrogen
atoms staying away from the coordination environment of Co.
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coordination of triflate (OTf) and/or solvent acetone molecules
in cis-ortrans-orientations, and with or without inherent
molecular symmetry, (see legend of Fig. S4†). Four of the six
DFT models show hexa-coordinated Co(II) (no. 1, 2, 5, and 6),
and the other two show penta-coordination (no. 3 and 4; see
Table S1†). Since unrestrained EXAFS fits of Co
4
L1 clearly indi-
cate hexa-coordination, the corresponding DFT models are
considered to be closer to the actual structure of Co
4
L1. The
average Co–X (X = N, O) bond distances, however, vary in the
narrow range of 2.09 to 2.19 Å in all six models, which can
hardly be discriminated by EXAFS, but all of them are close to
the 2.17 Å obtained from the experiments.
The lowest molecular energy is obtained for the hexa-co-
ordinated model 5, followed by penta-coordinated model 3,
which is only 5 kJ mol
−1
higher in energy. Fine structural
details are visible in the EXAFS wave, which promises more
insight into the real structure than geometric and energetic
considerations alone. Accordingly, EXAFS was calculated for
all six small DFT models and compared to the experimental
spectra of Co
4
L1, see Fig. S5 in the ESI.†Here again, model 5
apparently gives the best match to the spectrum of Co
4
L1,fol-
lowed by the other hexa-coordinate models 2, 6 and 1,
whereas the two penta-coordinate models 3 and 4 give the
worst match. Taking all results into account –geometry,
energy and EXAFS –DFT model 5 seems to be the closest
representative for the structure underlying in the experi-
mental data of Co
4
L1 we have so far. In this model, the six O-
and N-atoms bound to the central Co-atom are aligned in a
low symmetrical fashion, which might be described as quad-
ratic-pyramidal (O
2
N
3
) with one (extra) N-atom below but
close to the quadratic plane, (see Fig. 3). However, these
results seemingly are in contradiction with the XANES spec-
trum of Co
4
L1, which looks like typical octahedral or trigo-
nal–bipyramidal (i.e. high local coordination symmetry) com-
pounds, e.g. the hexa–aqua Co(II) compound used for refer-
ence (black line in Fig. 2a).
The answer to this riddle might be the potentially under-
rated electronic and steric effect of the agostic proton in DFT
model 5, which is part of the carboxylated phenyl group, see
Fig. 3. Since the organic ligand system has only limited flexi-
bility, an unoccupied coordination site can be filled by a
C–H⋯Co contact, which changes the picture. Taking the
H-atom into account, the coordination geometry is rather an
N-capped O
2
N
3
H–octahedron, see green lines in Fig. 3, than a
square pyramid. In order to understand this in more detail,
the electronic situation of model 5 was thoroughly analyzed by
means of Real-Space Bonding Indicators (RSBIs) extracted
from the computed electron density (ED, Fig. S6†). Fig. S6b†
shows the spin-density, the majority of which is localized at
the Co-atom, as expected, and with minor contributions at all
six non-H-atoms. Bond topological analysis of the ED accord-
ing to the Atoms-In-Molecules (AIM)
15
theory, however, finds
bond critical points (bcp) and thus bond paths to all seven O-,
N- and H-atoms, see Fig. S6c.†AIM theory also provides
atomic basins. Mapping the ED distribution on them discovers
bonding regions and strength of chemical interactions. The
AIM atomic Co basin has the basic shape of a cube (typical for
octahedral ligand sphere) with one edge cropped by the
capping N-atom, see Fig. S6d.†The more interesting point,
however, is that the shape of the basin is also flat along the
Co⋯H axis, although the agnostic interaction is quite weak
(only little ED accumulation on the respective cube face). This
“regular shape”of the Co-atom is also visible applying the
Non-Covalent interactions Index (NCI),
16
which uncovers non-
covalent bonding aspects of strong medium and even very
weak atom–atom contacts, see Fig. S6e.†Ring-shaped blue-
colored NCI basins indicate dominating covalent bonding
aspects (one O, one N), whereas disc-shaped blue-colored NCI
basins indicate dominating non-covalent bonding aspects (one
O, three N). The agostic Co⋯H contact is represented by a flat
and extended greenish-blue colored NCI basin, being typical
for weak non-covalent interactions, such as H⋯H or metallo-
philic contacts. AIM and NCI are complemented by the
Electron Localizability Indicator (ELI-D),
17
which dissects real-
space into regions/basins of (non-) bonding electron pairs,
resembling in a way the Lewis-picture of chemical bonding. An
iso-surface representation is shown in Fig. S6f.†Highlighted
(solid, green) are the six non-bonding d-electron ELI-D basins
of the Co-atom, which altogether form a regular polyhedron in
order to minimize electron–electron repulsion to the electron
pairs from the electron donating ligand atoms, according to
the well-known “key-lock”arrangement in transition metal
chemistry.
Co
4
L1 catalyzed dioxygen reduction reaction
The evaluation of the catalytic activity of Co
4
L1 towards
oxygen reduction was carried out using the Fukuzumi and
Guilard’smethod;
18
decamethyl–ferrocene was employed as a
one electron donor, triflic (TfOH) or fluoroboric (HBF
4
) acids
were used as proton source, and, in their presence, O
2
was set
to react with a catalytic amount of Co
4
L1 in acetone. The
occurrence of the oxygen reduction reaction was proved by
the formation of decamethylferrocenium ion (Fc*
+
)witha
characteristic absorption band at 780 nm (Fig. 4; ε
780 nm
=
520 M
−1
cm
−1
).
19,20
Notably, the rate and yield of formation
of Fc*
+
is not significantly affected by the nature of the
proton source (TfOH or HBF
4
Fig. S7d†), thereby suggesting
that the conjugate bases (OTf
−
or BF
4−
) play no major role in
controlling the efficiency of the O
2
-reduction reactions.
However, the concentration of Fc*
+
formed in the complex
Co
4
L1-catalyzed reduction of O
2
by Fc* is dependent on the
temperature at which the reactions were performed (Fig. 4
bottom, S7a–c†). At 30 °C 0.35 mM of Fc*
+
ion is generated in
the reaction, which corresponds approximately twice that of
the O
2
concentration (0.18 mM). Thus, only two-electron
reduction of O
2
occurs at 30 °C. With decreasing temperature,
theamountofFc*
+
generated from O
2
reduction increases,
presumably because of the increasing contribution of the
four-electron reduction of O
2
. At 25 °C the amount of Fc*
+
formed is 0.44 mM, which is 2.5 times that of the O
2
concen-
tration. The mechanism shifts predominantly to a four-elec-
tron reduction process at −50 °C; the amount of Fc*
+
gener-
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ated is 0.62 mM, which represents 3.45 equiv. relative to the
initial concentration of O
2
(0.18 mM).
1
H-NMR spectrum (Fig. 5a) of the reaction mixture of Co
4
L1
(0.04 mM), Fc* (3 mM), TFA (10 mM) and O
2
(0.18 mM) at
25 °C in d
6
-acetone further confirms the change in mechanism
from a predominantly 2e
−
reduction of O
2
(to H
2
O
2
)at30°Cto
a4e
−
reduction to H
2
Oat−50 °C. The solution after the Co
4
L1
catalysed O
2
reduction at 30 °C shows a signal at 3.88 ppm,
whose position is upshifted relative to the signal at 3.93 ppm
obtained for an authentic H
2
O
2
/H
2
O mixture (70 weight %
H
2
O
2
basis) in d
6
-acetone at 25 °C. Note that the signal corres-
ponding to a H
2
O
2
/H
2
O mixture undergoes a downshift with
increasing amounts of water in the mixture; pure water shows
a signal at 4.61 ppm. Thus for a catalytic reaction at 30 °C, a
>70% H
2
O
2
concentration can be inferred. When the catalysis
is performed at 25 °C the signal gets downshifted to 3.99 ppm,
thereby confirming the presence of the higher amount of
water as the 4e
−
reduction product. The resultant solution
after the catalytic O
2
-reduction reaction at −50 °C shows a
signal at 4.20 ppm, which lies between the signals at 4.44 ppm
and 4.05 ppm obtained for authentic 15% and 30% H
2
O
2
/H
2
O
mixtures, respectively. A turnover number (TON) of 28.7 during
a lapse of 2000 s was determined in acetone at −50 °C. The
TON decreased linearly with increasing temperature to a value
of 9 at 25 °C (Fig. S8†).
Reaction of Co
4
L1 with dioxygen to form 1
An acetone solution of Co
4
L1, when treated with O
2
saturated
acetone at −50 °C, results in the formation of an orange
species 1with an intense absorption maximum λ
max
(ε
max
,
M
−1
cm
−1
) centered at 464 nm (12 200 M
−1
cm
−1
). As the temp-
erature is increased, the absorption band at 464 nm due to 1is
decreased (Fig. 6: top one). This process is reversible in the
temperature range −50 to 30 °C. The resonance Raman (rR)
spectrum (Fig. 6: bottom one) of 1in acetone-d
6
displays two
isotopically sensitive vibrational bands at 862 (O–O stretching
mode of a peroxo ligand) and 595 cm
−1
(Co–O stretching
mode), which are downshifted to 808 and 561 cm
−1
, respect-
ively, in
18
O
2
prepared samples.
XAS studies were also performed to probe the oxidation
state and the coordination environment of Co in 1.The
XANES spectra of 1when compared with that of Co
4
L1 and
other reference compounds reveals an almost complete oxi-
dation from Co
2+
to Co
3+
during the transformation of Co
4
L1
to 1. Additionally, the edge shape of 1shows minor altera-
Fig. 4 Top: Spectral changes associated with the catalytic reduction of
dioxygen by Co
4
L1 (0.04 mM) in the presence of Fc* and TFA. Bottom:
Time profiles of the formation of Fc*
+
monitored at 780 nm during the
reduction of O
2
(0.18 mM) by Fc* (3 mM) and Co
4
L1 (0.04 mM) in the
presence of TFA (10 mM) in acetone at 30 °C, 25 °C as well as at −50 °C.
Table 2 Temperature dependence of the catalytic O
2
reductions
mediated by Co
4
L1 and Co
6
L2
Temp. Catalyst
used mM Equiv. of O
2
reduced %ofH
2
O
2
formed
(from NMR)
−50
0
CCo
4
L1 0.01 3.46 20%
+25
0
C 0.01 2.5 64%
+30
0
C 0.01 1.92 74%
−50
0
CCo
6
L2 0.01 3.80 0
+25
0
C 0.01 1.88 70%
Fig. 5
1
H-NMR spectra of the products formed during the complex
Co
4
L1 catalyzed dioxygen reduction reactions at 25 °C, 30 °C and
−50 °C [Co
4
L1 (0.01 mM), Fc* (3 mM), TFA (10 mM) and O
2
(0.18 mM)] in
acetone-d
6
and comparison with authentic samples containing 70%,
30%, 15% and 0% (weight percent H
2
O
2
basis) H
2
O
2
/H
2
O mixtures. All
the
1
H-measurements were performed at 25 °C. The results obtained for
Co
4
L1 are then compared with that for Co
6
L2 in Table 2.
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tionsincomparisonwithCo
4
L1 (rise of the pre-edge –see
inset of Fig. 2a, lower steepness of the edge rise, formation of
a shoulder in the edge) indicating a slightly lower coordi-
nation symmetry and coordination number in 1(Fig. 2,
Table1).TheEXAFSof1reveals a 5 coordinate geometry at
Co with 4 short Co–N/O distances at 1.91 Å, and one long Co–
N/O distance at 2.02 Å. The average Co–O/N distances of
2.17 Å for Co
4
L1 and 1.93 Å for 1, is a consequence of the
different oxidation states and the lower steric requirements of
five versus six ligands attached to the Co-atom. Notably, the
470 nm absorption band, ν(O–O) of 862 cm
−1
and ν(Co–O) of
595 cm
−1
and the Co–O
2
distance of 1.91 Å of 1matches the
spectroscopic properties of the previously reported end-on
μ-1,2-peroxo-dicobalt(III) complexes.
21
DFT calculations also
support an S=0end-onμ-1,2-peroxo-dicobalt(III) assignment
of 1; the calculated metrical parameters very well match with
the experiments.
Reactivity of 1 with protons and electrons
1exhibited different reactivities with proton and electron
donors depending on the reaction temperature. At −50 °C no
reaction of 1with Fc* was observed in the absence of TfOH.
Similarly, no reaction of 1with TfOH was observed in the
absence of Fc*. However, in the presence of both TFA and Fc*
1underwent fast decay, presumably by a proton coupled elec-
tron transfer (PCET) mechanism to form water as the major
product (Fig. 7 top). At 25 °C in the absence of TFA, no
reduction of 1by Fc* was observed, very similar to our find-
ings at −50°C.However,inpresenceofTFA,eveninthe
absence of Fc*, fast decay of 1was observed (Fig. 7 bottom),
with the release of H
2
O
2
by a proton transfer (PT)
mechanism.
The temperature dependence of the PT and PCET pro-
cesses will be the controlling factor in determining the temp-
erature dependence of the 4e
−
/4H
+
vs.2e
−
/2H
+
reductions of
dioxygen mediated by 1. We therefore compared the tempera-
ture-dependence of the PCET and PT processes of 1at various
temperatures (Fig. 7, 8 and S9†). PCET rates were determined
at −50 °C, −40 °C, −30 °C and −20 °C under the condition [1]
≪[Fc*] ≪[TFA] to ensure pseudo first-order kinetics (Fig. 7
top; at these temperatures PT rates are negligible). Similarly,
PT rates were determined at 20 °C, 22 °C, 25 °C, 30 °C and
32 °C under the condition [1]≪[TFA] (Fig. 7 bottom). PT is
found to vary with temperature at a much more drastic rate
relative to that of PCET, and it becomes the predominant
mechanism for the reduction of 1at temperatures >11 °C
(Fig. 8a).
Fig. 7 Top: Changes in the absorption band associated with the reac-
tion of 1(0.015 mM) with TFA (3 mM) and Fc* (0.15 mM) at −50 °C;
Inset: The pseudo-first order decay of the absorption band at 464 nm as
a function of time (left) and the linear dependence (right) of the pseudo-
first order rate constants (k
obs
) on Fc* concentrations (0.13–0.38 mM)
that led us to determine the second order rate constant value, k’
2
.
Bottom: Changes in the absorption band associated with the reaction of
1(0.015 mM) with TFA (0.75 mM) at +25 °C; inset: the pseudo-first order
decay of the absorption band at 464 nm as a function of time (left) and
the linear dependence (right) of the pseudo-first order rate constants
(k
obs
) on TFA concentrations (0.75–2.7 mM) led us to determine the
second order rate constant value, k’
2
.
Fig. 6 Top: Absorption spectra showing the reversibility of dioxygen
binding to Co
4
L1. Bubbling O
2
into an acetone solution of Co
4
L1
(0.02 mM) produces 1(in high yield) at −50 °C (orange, solid line).
Increasing the temperature up to 25 °C produces the blue solid spec-
trum. After recooling to −50 °C the orange solid spectrum can be
regenerated. Bottom: Resonance Raman spectra of 1-
16
O (red trace),
1-
18
O (black trace) with 514 nm laser excitation in acetone-d
6
at −40 °C.
Solvent bands are marked by “blue color”.
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Conclusions
In our previous study
8
we reported the synthesis and character-
ization of a hexanuclear cobalt complex Co
6
L2 involving a
nonheme ligand system, L2, supported on a Sn
6
O
6
stannoxane
core (Fig. 1: bottom), whose cobalt complex acts as a unique
catalyst for dioxygen reduction, whose selectivity can be
changed from a preferential 4e
−
/4H
+
dioxygen-reduction (to
water) to a 2e
−
/2H
+
process (to hydrogen peroxide) only by
increasing the temperature from −50 to 25 °C. Herein, we
report the synthesis and characterization of a tetranuclear
Co
4
L1 complex, supported on the stannoxane core, and
compare its dioxygen reduction ability with that of Co
6
L2
(Fig. 8). The temperature dependence of the product selectivity
of the catalytic dioxygen reduction is still observed in Co
4
L1;
however, some subtle differences are noted relative to Co
6
L2,
which can be attributed to the different nuclearity and Co–Co
distances in the two cases (Fig. 1).
In L2 all the six plausible metal binding sites are equidi-
stant from each other at 11.365 Å. This is in contrast to L1,
where only two of the four metal binding sites are in close
proximity to each other. The more symmetric nature of L2
ensures an efficient cooperative dioxygen binding in Co
6
L2
relative to Co
4
L1, which results in the lower stability of 1com-
pared to that of the corresponding {[L2(Co
III
(O
2
)Co
III
)
3
]}
12+
complex 2that is formed upon dioxygen activation of Co
6
L2.
The faster self-decay rate (1 × 10
−4
s
−1
for 2vs.2×10
−3
for 1at
25 °C), as well as the 16 cm
−1
downshift in the Co–O vibration
energy (ν(Co–O) for 1is 595 cm
−1
and 611 cm
−1
for 2)
8
in 1
relative to 2, is consistent with the lower stability of 1.
Accordingly, as previously observed, the high enthalpic stabi-
lity of 2makes its formation at −50 °C highly favored that
leads to the complete oxygenation of Co
6
L2. Complex 2then
undergoes O–O bond cleavage via a PCET mechanism to yield
water as the sole product under catalytic turnover conditions.
The rate constant of the reaction was found to be independent
of the O
2
concentration; the kinetic equation at −50 °C for
Co
6
L2 is
d½Fc=dt¼kobs½Co6L2
kobs ¼kcat½Fc½TFA
where “k
cat
”is the third-order rate constant for the catalytic
4e
−
-reduction of O
2
by Fc* at −50 °C and k
obs
is the pseudo
first-order rate constant. In contrast, an equilibrium binding
of O
2
occurs for Co
4
L1,evenat−50 °C, so that the rate of the
catalytic reaction shows a linear dependence on the O
2
concen-
tration (Fig. S10†). The rate equation for Co
4
L1 is
d½Fc=dt¼kobs½Co4L1
kobs ¼kcat½Fc½TFA½O2
where “k
cat
”is the fourth-order rate constant for the catalytic
4e
−
-reduction of O
2
by Fc* at −50 °C and k
obs
is the pseudo
first-order rate constant. Furthermore, Co
4
L1 catalysed O
2
reduction yields 15–30% H
2
O
2
at −50 °C, in contrast to Co
6
L2
for which no H
2
O
2
production could be detected at this temp-
erature. However the rate constant of the two-electron O
2
reduction at +25 °C is a fourth-order process for both Co
6
L2
and Co
4
L1 (Fig. S11†).
The constraints imposed by the stannoxane core ensure
entropic instability of both 1and 2. This is mainly because of
the large reduction in the Co–Co distances that is associated
with their formation. Although experimental determination of
the Co–Co distances in Co
4
L1,Co
6
L2,1and 2was not possible,
approximate shortening of ∼2.4 Å (from a distance of 6.82 Å in
L1 to the DFT calculated distance of 4.48 Å in 1) and ∼7Å
(from a distance of 11.36 Å in L2 to the DFT calculated dis-
tance of 4.48 Å in 2) can be predicted for dioxygen binding at
Co
4
L1 and Co
6
L2 complexes, respectively. This would impose a
large strain on the μ-1,2-peroxo-dicobalt(III) cores in 1and 2,
which would attribute to their instability at higher tempera-
tures upon protonation leading to the formation of H
2
O
2
as
the major product. Thus for both Co
4
L1 and Co
6
L2, an equili-
brium binding of O
2
will take place at 25–30 °C, such that only
a small portion of Co
4
L1 and Co
6
L2 will be converted to 1and
2, respectively. This would also explain the experimentally
observed direct correlation of the reaction rates to oxygen con-
centration at 25–30 °C in both cases.
In summary, the Co
4
L1 complex like the previously reported
Co
6
L2 complex is a unique catalyst for dioxygen-reduction
reaction, whereby the product selectivity can be changed from
Fig. 8 Comparison of the temperature-dependence of the PCET vs.PT
rate constants for (a) 1and (b) 2.
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a predominant 4e/4H
+
reduction process (to water) at −50 °C
to a 2e
−
/2H
+
process at 25–30 °C. μ-1,2-peroxo-dicobalt(III)
complexes 1and 2are proposed as plausible reactive inter-
mediates, which are reduced to H
2
O by a PCET mechanism at
−50 °C, or to H
2
O
2
by a proton transfer mechanism at
25–30 °C. For both 1and 2, the PT rates are found to vary dras-
tically with temperature relative to the PCET rates, and PT
becomes the predominant mechanism at 11 °C for 1and at
19.5 °C for 2. The ∼10 °C reduction in the transition
temperature for 1can be attributed to its reduced stability rela-
tive to 2, as also evident from the faster self-decay rate and
lower ν(Co–O) vibration energy in 1relative to 2. This study,
therefore, underlines the importance of subtle electronic and
steric changes in the reactivity of the biologically relevant
metal–dioxygen intermediates, and how they can control the
2e
−
/2H
+
vs.4e
−
/4H
+
product selectivity in catalytic dioxygen
reductions.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was funded by the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation) under Germany’s
Excellence Strategy –EXC 2008 –390540038 –UniSysCattoK.R.,
P. H. and H. D. and the Heisenberg-Professorship to K. R.
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