Journal of the American Chemical Society
Article
4
5,46
species reducible at more positive potentials.
The peak
nearly 0.5 V (Scheme S1, SI). Such reduction will appear as a
−
currents on the first cycle vary linearly with the square root of
the sweep rate (ν = 0.05−2 V s ), as expected for diffusion-
controlled processes with unaltered electron stoichiometry
single 2e reduction at the averaged reduction potential of
−
1
+
−1.50 V Fc /Fc, in good agreement with the experimentally
+
observed potential of −1.56 V Fc /Fc.
Figure S1, SI). If the sweeping is extended to two cycles, a
The trace crossing observed for 1a, 4, 6, and 7 on the
oxidative sweep is assigned to a parent−child reaction between
new reduction directly associated with the oxidation process
apparent in the case of 1a, 2, and 4−7 (Figure S2, SI). For 1a,
this peak appears at the same reduction potential as that of 1b,
i.e., the cationic Mn (bpy )(CO) . Importantly, this new peak
disappears at the expense of the original reduction peak on the
−
0 −
the 2e reduction product [Mn ] and the initial catalyst
I
0 −
I
Mn Br. An electron transfer from [Mn ] to Mn Br produces
Mn and [Mn Br] , which readily expels Br to form another
equivalent of Mn . At the applied potential, Mn can diffuse to
the electrode surface and be reduced. Although this reaction i−s
thermodynamically unfavorable, the rapid dissociation of Br
from [Mn Br] along with the electrochemical reduction of
Mn drives it. For 2, 3, and 8, the same mechanism explains
0
0
−
−
I
1
+
3
0
0
I
1
first sweep pertaining to Mn (bpy )(CO) Br upon addition of
3
+
−
0
−
4
0
Scheme 2 outlines a mechanism that explains these main
features and is in general agreement with previous stud-
the electrochemical behavior with the difference that the
reduction peaks fuse together. For 2, this is because the
0
Scheme 2. Proposed Mechanism for the Reduction of fac-
Mn (N∧N)(CO) Br Complexes in Acetonitrile (under Ar)
reduction of Mn becomes more difficult than the reduction of
I
a
I +
[Mn ] . And for 3 and 8 this is because the initial reduction of
3
the precatalysts occurs at potentials similar to the reduction of
0
Mn .
Electrocatalytic Reduction of CO . Figures 3, S6, and S7
2
(
SI) show voltammograms recorded for complexes 1−8 in
50
CO -saturated MeCN ([CO ] ≈ 0.28 M) with 2,2,2-
2
2
trifluoroethanol (TFE) as proton source. The potentials for
the catalytic reduction (Ecat/2), determined as the half-wave
potential, are enumerated in Table 1. They belong to, as will
be seen, a high overpotential (HO) reduction pathway.
51
+
Moreover, they fall within ±50 mV of −2.02 V vs Fc /Fc,
implying that variations in activity or selectivity of the catalysts
considered cannot be explained by potential differences.
Interestingly, a small plateau current is present at a significantly
a
Bipyridine and CO ligands are omitted for clarity
less negative potential for catalysts 1a, 1b, 3, and 5−8, with
cat/2
groups of Rochford and Nippe have observed and discussed a
+
E
ranging from −1.56 to −1.77 V vs Fc /Fc. Previously, the
1
8,47,48
I
ies.
On the first sweep, the reduction of fac-Mn (N∧N)-
I
(
CO) Br (denoted Mn Br) is accompanied by a fast
low overpotential (LO) reduction pathway for the CO
2
3
29,47
−
−1
0
dissociation of Br (k > 400 s ) to give Mn . The latter is
reduction with Mn(N∧N) complexes.
dis
I
instantly reduced, being easier to reduce than Mn Br. Thus,
Figures 3e and S8a (SI) show a plot of icat/i
p
vs [TFE] for
the first reduction wave becomes a single 2e− process, as
confirmed by bulk electrolysis of 1a, 2, and 4 (Figure S4, SI).
Infrared spectroelectrochemistry (IR-SEC) of 1a further
the HO pathway, where icat is the plateau current observed
after addition of TFE to a CO -saturated solution and i is the
2
p
peak current observed in the absence of TFE under Ar. In
general, the catalytic rate becomes faster with [TFE] for all
complexes, but 1a, 1b, and 6 outperform the remaining
0
−
verified the formation of [Mn ] with CO stretching bands
ferrocenium/ferrocene (Fc /Fc) (Figure S5, SI), in agreement
−
1
+
catalysts at the highest [TFE]. Notably, icat/i
for 1a, 1b, 6, and
p
with the IR spectra of other doubly reduced Mn(N∧N)-
7 is linearly dependent on [TFE] in the range from 0.05 to 2.0
18,29,49
2
(
CO) X complexes.
Note that we label this species as
M. Since (icat/i
(TOF), as discussed later, the TOF becomes second-order in
p
) is proportional to the turnover frequency
3
0
−
[
Mn ] to indicate that the reduction is ligand-based and the
metal maintains a Mn(0) state.
When the potential direction is reversed, [Mn ] is oxidized
in a 2e process to yield [Mn ] via Mn . Evidence for the
involvement of two electrons in the oxidation wave is not only
given by the equivalent size of the current signal but also by its
p
0
−
0.5
linearly correlated to [TFE] at low [TFE] (Figure S8, SI),
corresponding to a first-order dependency on [TFE]. At high
concentrations, the rate becomes independent of [TFE], i.e.,
zeroth-order dependency. In that instance, the rate-controlling
step does not include protonation, which, otherwise, is a
−
I +
0
narrow peak width, Ep/2 − E , of 33 ± 5 mV, which is
p
approximately half of the 60 ± 3 mV obtained for the
corresponding reversible one-electron process of ferrocene
used as reference. The oxidative wave represents a case of
prevalent feature for the conversion of CO to CO with similar
2
18
complexes.
Next, bulk electrolysis was performed to determine the
product selectivity. All experiments were conducted in an H-
cell with two compartments separated by a glass frit with 1 mM
of the given catalyst in a CO -saturated 0.2 M Bu NBF /
0
−
inverted potentials, where the oxidation of [Mn ] is more
difficult than that of its one-electron oxidation product Mn .
DFT calculations confirm the potential inversion and the
assignments of the metal oxidation states. Specifically, the
reduction of the [Mn ] species of 1b, where the OTf was
replaced by an acetonitrile solvent molecule,
calculated to take place at standard potentials of −1.77 and
−
0
2
4
4
MeCN solution containing 2.0 M TFE in the cathodic
I +
−
1
+
compartment. An electrode potential of −2.17 V vs Fc /Fc was
8,29
was
maintained throughout the 1 h electrolysis corresponding to
the HO reduction pathway. The expectation from previous
+
I
1.23 V vs Fc /Fc, thus showing a potential inversion of
work on Mn (bpy) complexes is that the primary product is
4
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J. Am. Chem. Soc. 2020, 142, 4265−4275