Angewandte Chemie International Edition
10.1002/anie.202110190
RESEARCH ARTICLE
To quantify the catalytic product, controlled potential
electrolysis (CPE) experiments were then performed both in DMF
electron-reduced species, Fe(0)TPP, which reacts with CO
2
(Scheme 1a, EEEC mechanism). In contrast, our results indicate
that the two-electron-reduced species, Fe(I)TPP, reacts with CO
in MeCN (Scheme 1b, EEC mechanism). The binding constant of
CO
to Fe(I)TPP in MeCN (KCO2) was calculated to be 2.58 M–1
and MeCN with 1.0 M TFE and 0.1 M TBAP under CO
2
at –2.35
2
+
V vs. Fc/Fc . In CPE experiments, tetra-n-butylammonium
acetate (TBAA) was added in the anodic chamber (for details of
the experimental setup, see Figure S3) to promote an oxidation
process on the Pt counter electrode. TBAA reacts at anode to
2
(see the SI (P.S27) for details of the calculation).
Subsequently, UV-visible absorption spectroelectrochemistry
(SEC) measurements were performed to further clarify the
reaction of Fe(I)TPP with CO in the presence of MeCN. Figure
2
S7a shows the UV-visible absorption spectra of the Fe(II)TPP
species generated in DMF by applying a potential at E = –1.20 V.
2
consume the holes and to produce CO and ethane via Kolbe
20
reaction . In the CPE of FeTPP-Cl in DMF, the total amount of
charge passed over a period of 60 min was 3.9 C when 0.01 mM
of catalyst was used (Figure 2c, red line), and CO (4.5 μmol) was
formed with a Faradaic efficiency (FE) of 23%. The similar
catalytic performance was also obtained in the CPE of FeTPP-
2
The spectra measured under Ar and CO were almost identical,
and two Q-bands were observed at 569 and 609 nm. A scan to
the negative potential region showed changes in the UV-visible
absorption spectra with the occurrence of isosbestic points, and
three Q-bands (538, 569, and 609 nm) were observed when E =
–1.70 V. Notably, the features of the reduction-induced spectral
ClO
4
in DMF; 2.6 C of charge has passed, and the formation of
(FE: 17.4%) was detected (Figure S4, No.11
CO (FE: 21.9%), H
2
and Table S4, No.11), indicating that catalytic performance is not
affected by counter anions. Upon increasing the catalyst
concentration to 0.50 mM, CO (27.0 μmol) was formed with a total
charge and FE of 5.7 C and 92%, respectively. On the other hand,
change measured under Ar and CO
and 3b). These results indicate that Fe(I)TPP does not react with
CO in DMF, which is consistent with the cyclic voltammetry (CV)
2
were identical (Figures 3a
in the CPE of FeTPP-ClO
4
in MeCN, the total amount of charge
2
was 33.3 C and the FE was 98% (170.0 μmol), even at a low
catalyst concentration (0.01 mM). These results clearly
results. Similarly, Fe(II)TPP was also generated by applying a
potential at E = –1.20 Vin the presence of MeCN,21 and UV-visible
absorption spectra with two Q-bands (569 and 609 nm) were
demonstrate that the electrocatalytic activity of FeTPP for CO
reduction is significantly enhanced in MeCN. Note that the CPE
of FeTPP-ClO in MeCN using 0.5 mM of catalyst afforded the
2
obtained under both Ar and CO
2
(Figure S7b). However, the
upon
4
changes in the UV-visible spectra under Ar and CO
2
similar amounts of products (Table S4, No.10), indicating that the
catalytic process is deaccelerated when higher concentration (0.5
mM) of catalyst is used in MeCN.
scanning the potential to the negative potential region were quite
different (Figures 3c and 3d). While two Q-bands (569 and 609
nm) were observed at E = –1.70 V under Ar, only one Q-band
To further verify the effect of MeCN, the reaction mechanism of
2
(538 nm) was observed under CO . These results strongly
CO
voltammograms of FeTPP-Cl in DMF and FeTPP-ClO
were measured in the absence of a proton source under both Ar
and CO . As shown in Figure S5a, the cyclic voltammograms of
2
reduction was investigated. Initially, the cyclic
indicate that in the presence of MeCN, Fe(I)TPP rapidly reacts
-
4
in MeCN
with CO
2
to form Fe(II)TPP-CO
2
. It should be also noted that the
in MeCN and FeTPP-Cl in
DMF also indicate the interaction of Fe(I)TPP with CO in MeCN.
comparison of the CVs of FeTPP-ClO
4
2
2
FeTPP-Cl in DMF displayed almost identical redox peaks at −1.51
Under Ar, the difference in redox potentials attributed to
V for the Fe(II)/Fe(I) redox couple, indicating the lack of reaction
Fe(I)/Fe(0) process is 0.08 V (–2.06 V in MeCN and –2.14 V in
between the Fe(I) species and CO
voltammograms of FeTPP-ClO
features when measured under different conditions: under Ar,
there was a reversible redox wave at −1.43 V attributed to the
Fe(II)/Fe(I) redox couple (Figure S5b, red line ), while under CO
2
in DMF. In contrast, the cyclic
in MeCN exhibited distinct
2
DMF). On the other hand, under CO in the presence of TFE, the
4
difference in the onset potentials for catalytic current is 0.15 V (–
1.91 V in MeCN and –2.06 V in DMF), which is significantly larger
than that observed under Ar. These results support our proposed
catalytic cycle that the reduction of Fe(II)TPP-CO -
triggers the
2
,
2
there was an increase in the cathodic current (Figure S5b, blue
line). The same phenomenon was also observed in the cyclic
voltammograms measured in the MeCN-DMF mixed solvent
system (Figure S5c). These results suggest that FeTPP reacts
catalysis in MeCN whereas the reduction of Fe(I)TPP triggers the
catalysis in DMF (for detailed discussion, see the SI (P.S15)).
Based on the aforementioned results, a plausible catalytic cycle
in MeCN was proposed, as shown in Scheme S1.
with CO
FeTPP undergoes one-electron reduction thrice to form the three-
2
as Fe(I) in MeCN. As reported previously,4–6 in DMF,
(a)
+
-
2-
-
+ e-
+ e-
+ CO2
e-
+
-
- e-
- e-
- CO
e-
2
E
E
E
C
(b)
+
-
e-
e-
E
+ e-
+ CO2
+
-
E: Electron transfer
C: Chemical reaction
- e-
- CO2
E
C
Scheme 1. Proposed mechanism for the formation of the metallocarboxylate intermediate of FeTPP (a) in DMF via the EEEC mechanism (b) in MeCN via the EEC
mechanism.
3
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