Characterization of a Rh(III) porphyrin-CO complex: Its structure and reactivity with an electron acceptor

Article abstract of DOI:10.1039/c5dt01453k

To analyse the electrocatalytic oxidation of carbon monoxide by Rh porphyrins, we isolated a CO-adduct of Rh octaethylporphyrin, and examined its properties and reactivity by IR, NMR, and X-ray crystallographic analyses. The results indicate that the CO adduct of Rh octaethylporphyrin is vulnerable to nucleophilic attack by H₂O. The CO-adduct was easily oxidized by an electron acceptor (1,4-naphthoquinone) to generate CO₂. This indicates that CO is sufficiently activated in the CO complex of Rh octaethylporphyrin to reduce an electron acceptor. This mechanism is in contrast to that for the CO oxidation by Pt-based electrocatalysts.

Full text of DOI:10.1039/c5dt01453k

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Characterization of a Rh(III) porphyrin–CO
complex: its structure and reactivity with an
electron acceptor†
Cite this: Dalton Trans., 2015, 44,
13823
Received 17th April 2015,
Accepted 26th June 2015
a
a
a
b
Shin-ichi Yamazaki,* Masaru Yao, Masafumi Asahi, Hiroyasu Sato,
b
a
DOI: 10.1039/c5dt01453k
Akihito Yamano and Tsutomu Ioroi
www.rsc.org/dalton
To analyse the electrocatalytic oxidation of carbon monoxide by
For this purpose, we turned our attention to Rh porphyrin
Rh porphyrins, we isolated a CO-adduct of Rh octaethylporphyrin, catalysts. It has been demonstrated that Rh porphyrins can
3
,4
and examined its properties and reactivity by IR, NMR, and X-ray catalyze CO at low overpotentials. We have shown that Rh
crystallographic analyses. The results indicate that the CO adduct porphyrins with hydrophilic groups (carboxy and sulfo groups)
of Rh octaethylporphyrin is vulnerable to nucleophilic attack by exhibit better activity, and catalyze the CO oxidation at much
4
b–d
H
2
O. The CO-adduct was easily oxidized by an electron acceptor lower overpotentials than Pt–Ru catalysts.
State-of-the-art
(
1,4-naphthoquinone) to generate CO . This indicates that CO is Rh porphyrin catalysts can oxidize CO below 0.1 V vs. a revers-
2
4
c
sufficiently activated in the CO complex of Rh octaethylporphyrin ible hydrogen electrode (RHE). A combined catalyst of a Rh
to reduce an electron acceptor. This mechanism is in contrast to porphyrin and a Pt–Ru catalyst tolerates a high concentration
4
d
that for the CO oxidation by Pt-based electrocatalysts.
of CO better than a Pt–Ru catalyst alone. A PEFC that uses a
Rh porphyrin as an anode catalyst delivered high power
In the improvement of stationary polymer electrolyte fuel cells
−2
−2
4a
(
95 mA cm , 44 mW cm ) when CO was supplied as a fuel.
(PEFCs), the development of CO-tolerant anode catalysts is
This high current means that Rh porphyrins can be used in
the severe environments of PEFCs.
very important, since Pt-based anode catalysts are strongly
poisoned by a small amount of CO, which is contained in the
Compared to a lot of findings on the catalytic activity and
application of Rh porphyrins, the information on the reaction
pathway of the CO electro-oxidation is not abundant. Hendrik-
1
anode gas. To date, a wide variety of CO-tolerant anode cata-
2
lysts have been developed. Pt–Ru catalysts, which interact very
little with CO, are currently used as CO-tolerant anode cata-
lysts. However, even with state-of-the-art Pt–Ru catalysts,
PEFCs are still vulnerable to high concentrations of CO
−
sen et al. proposed a mechanism of [RhCl (CO) ] -catalyzed
2
2
CO oxidation by NO. They showed that the coordinated CO par-
5
ticipates in the catalytic reaction. With regard to the (electro)
(>500 ppm).
catalytic CO oxidation by Rh porphyrins, it has been postulated
The electrochemical oxidation of CO is a key reaction for
4b,6
that CO is coordinated to a Rh porphyrin.
The CO complex
the development of such anode catalysts, since this reaction
can decrease the CO concentration in an anode catalyst.
However, current Pt–Ru catalysts require high overpotentials
for the CO oxidation and hence the reaction hardly occurs at
the anode potentials in PEFCs (<0.1 V). Catalysts that promote
the CO oxidation at low overpotentials should help enhance
CO-tolerance.
is believed to react with water to generate a carboxy intermedi-
ate, which in turn may give electrons to an electron acceptor to
afford CO-free Rh porphyrin and CO . Also in CO electro-oxi-
2
dation by Co porphyrin, a similar mechanism may be
7
presented.
III
Thus, the Rh porphyrin–CO complex is supposed to be an
important intermediate for catalytic CO oxidation. This
8
complex was reported more than 40 years ago. The IR and UV
III
spectra of CO coordinated to Rh
porphyrins were
8
–10 1
examined.
H-NMR was also measured with a CO complex
with Rh (OEP)(Cl). However, more information on the pro-
perties of the Rh porphyrin–CO complex is not available,
while metalloradical CO complexes of Rh porphyrin and
a
Research Institute of Electrochemical Energy, Department of Energy and
III
10a
Environment, National Institute of Advanced Industrial Science and Technology
III
(AIST), 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan.
II
11
E-mail: s-yamazaki@aist.go.jp; Fax: +81-72-751-9629; Tel: +81-72-751-9653
b
Rigaku Corporation, 3-9-12 Matubara, Akishima, Tokyo 196-8666, Japan
I
12
Rh carbonyl complexes have been intensively investigated.
†
Electronic supplementary information (ESI) available: Details of experimental
information, UV-vis spectra, IR spectra, and X-ray crystallography. CCDC
059137. For ESI and crystallographic data in CIF or other electronic format see
III
We are also interested in the reactivity of the Rh por-
phyrin–CO complex. Although the reactivities of the CO adduct
1
8
b,9
13
DOI: 10.1039/c5dt01453k
with alkoxide
and CO in the presence of KOH were exam-
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ined, the reactivity of the complex with an electron acceptor C atom. This near-linearity of Rh–C–O (173.4(12)°) is typical of
has not been examined. The reactivity with an electron accep- carbon monoxide. This is in contrast to the bent structure of
1
5
III
tor would be significant for elucidating the (electro)catalytic Rh–C–O in Rh(OEP)–CHO (129.6(5)°). In [Rh (OEP)(CO)-
CO oxidation. (Cl)], the axial ligands (Cl and CO) occupy equivalent sym-
From these backgrounds, we tried to examine the properties metrical positions at the same probability. Therefore, the
III
of a Rh porphyrin–CO complex, and investigated the reactiv- detailed bond distances of Rh–C and C–O remain unclear due
ity of the complex toward an electron acceptor. In this work, a to the existence of Cl. It is very difficult to discuss the elec-
III
Rh porphyrin–CO complex was characterized by X-ray crystal- tronic state of CO coordinated to Rh only from the results of
lography, NMR, and IR. We also examined the reactivity of the the X-ray crystallographic analysis. Then, we analysed the
1
3
CO complex with an electron acceptor (1,4-naphthoquinone complex with C-NMR and IR.
NQ)). For the analysis of NMR,
1
3
(
CO was introduced into
III
III
III
13
Rh octaethylporphyrin chloride ([Rh (OEP)(Cl)], Chart 1) [Rh (OEP)(Cl)] in a CDCl
3
solution. The C-NMR spectrum
was recorded after the reaction, and the result is shown in
CO ( C, 99%) was purchased from Cambridge Isotope Labo- Fig. 2 (δ 160.2(d, J = 58.9 Hz), 143.1, 139.2, 98.1, 20.1,
1
4
was synthesized as described in the literature (see the ESI†).
1
3
13
1
3
ratories, Inc. NQ was purchased from Tokyo Kasei Corp. Other 18.5 ppm). The introduction of CO generated a split peak at
regents were obtained commercially, and used as received. 160.2 ppm. This signal is reasonably ascribed to a CO mole-
First, we analysed the reaction between [Rh (OEP)(Cl)] and cule coordinated to the Rh octaethylporphyrin. The coupling
CO in CH Cl with UV (Fig. S1†). On introduction of CO, the constant of 58.9 Hz is similar to the values reported for Rh–C
spectrum of [Rh (OEP)(Cl)] underwent red shift, indicating bonds.
III
2
2
III
16,17
This indicates that CO is coordinated to Rh
that [Rh (OEP)(Cl)] reacts with CO in CH Cl . Similar spectral through the C atom, supporting the results of crystallographic
shifts have been reported in the literature.
III
2
9
2
,10a,c
analysis. A dimer of Rh(OEP) ([Rh(OEP)]
2
) has been reported to
III
The product of the reaction between [Rh (OEP)(Cl)] and react with CO to generate [Rh(OEP)]
2
6
–CO, Rh(OEP)–CO–Rh-
1
13
CO was subjected to X-ray crystallographic analysis. The result (OEP), or Rh(OEP)–CO–CO–Rh(OEP). However, the C-NMR
III
(
2
Fig. 1) clearly shows that [Rh (OEP)(CO)(Cl)] (Chart 1) is gen- signals of the terminal CO in [Rh(OEP)] –CO were not observed
III
erated from the reaction between [Rh (OEP)(Cl)] and CO. CO due to the quick exchange of CO with dissolved CO. In con-
and Cl coordinate to the Rh atom as axial ligands. There is no trast, the terminal CO in [Rh (OEP)(CO)(Cl)] exhibited strong
anion other than Cl in the crystal of [Rh (OEP)(CO)(Cl)]; this peaks at δ 160.2, which indicates that the terminal CO does
indicates that Rh is trivalent. The distances and the angle of not dissociate in CDCl within the NMR timescale.
III
−
III
3
III
Rh–C–O strongly suggest that CO coordinates to Rh via the
We also examined the IR spectra of [Rh (OEP)(CO)(Cl)]
and [Rh (OEP)( CO)(Cl)]. IR analyses of the complex between
Rh porphyrins and CO have already been conducted by several
III
13
4
b,8,9,10a
researchers including our group.
Also with regard to
III
the complex between [Rh (OEP)(Cl)] and CO, an IR analysis
1
0a
was conducted by Thackray et al.
crystalline powder is shown in Fig. S2.† The peak (2086 cm
coincides with the reported value for a Rh-OEP and CO com-
The IR spectrum of this
−
1
)
1
0a
13
plex.
toward a lower wavenumber (2039 cm ). The observed iso-
topic shift (47 cm−1) is very close to the value (45 cm−1
When CO was used, the peak underwent a shift
−1
)
Chart 1 Chemical structures of the compounds used in this study.
III
Fig. 1 X-ray crystallographic structure of [Rh (OEP)(CO)(Cl)] with 50%
probability ellipsoids. Hydrogen atoms and an uncoordinated solvent
molecule are omitted for clarity.
13
III
13
Fig. 2
A
3
C-NMR spectrum of [Rh (OEP)(Cl)( CO)] in CDCl .
13824 | Dalton Trans., 2015, 44, 13823–13827
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obtained by the harmonic oscillator approximation of the C–O
1
8
stretching bond. This also supports that the peak is derived
III
from CO. The wavenumber of CO in [Rh (OEP)(CO)(Cl)] is
much higher than those of CO-complexes of metallopor-
1
8
phyrins such as Fe porphyrins. This indicates that π back-
donation from Rh to CO is very weak compared to those for
1
9
other metals. Weak π back-donation is also observed in a
1
5
formyl complex of Rh porphyrin.
Anson et al. analysed electrochemical CO oxidation by Co
octaethylporphyrin, and postulated that CO coordinates to Co
7
via σ-donation from C to Co in the reaction. Our results
strongly suggest that the contribution of π-back-donation to Fig. 3 A proposed reaction mechanism of catalytic CO oxidation by
III
the CO–Rh bond is small. This situation should decrease the [Rh (OEP)(Cl)].
electron density on the C atom, and hence the C atom will be
2
vulnerable to nucleophilic attack by H O. NMR spectra indi-
III
cate that [Rh (OEP)(CO)(Cl)] is stable from kinetic viewpoints,
but the coordinated CO is considered to be activated for CO
oxidation.
carboxy intermediate to regenerate the CO-free Rh porphyrin
step (C)). An axial ligand might change from Cl if other ligand
candidates exist in the system, since Cl is once dissociated in
step (B) in the mechanism.
(
−
Overall, the results indicate that CO coordinates to
III
III
[
[
Rh (OEP)(Cl)] to form [Rh (OEP)(CO)(Cl)], and CO on the
NQ (δ 8.05–8.08, δ 7.90–7.94, and δ 7.10) was also detected
III
Rh (OEP)(CO)(Cl)] would be feasible to nucleophilic attack
III
after the reaction between NQ and [Rh (OEP)(CO)(Cl)]. Even
III
by H
2
O. However, it remains unclear whether [Rh (OEP)(CO)-
III
though an equivalent amount of [Rh (OEP)(CO)(Cl)] was
(Cl)] undergoes oxidation by an electron acceptor in the pres-
added, NQ remained after the reaction. The ratio of peaks at
δ 7.10 (NQ) and δ 6.74 (NQred) indicates that the amount of NQ
ence of water. Therefore, we next examined the reaction of
III
[
Rh (OEP)(CO)(Cl)] with an electron acceptor to check this
is ca. 73% of the sum of NQ and NQred. This suggests that
possibility. We chose 1,4-naphthoquinone (NQ, Chart 1) as an
III
[
Rh (OEP)(CO)(Cl)] is decomposed before the reaction with
1
electron acceptor. The reaction was analysed by H-NMR. This
NQ in the mixed solvent containing water. A possible reason is
the dissociation of CO due to the ligation of H O (the conver-
electron transfer needs water, and hence a mixed solvent of
acetone-d6 (0.9 mL) and water (0.1 mL) was used for this
experiment as a solvent.
2
III
III
sion from [Rh (OEP)(CO)(Cl)] to [Rh (OEP)(H O)(Cl)]) in the
2
mixed solvent.
NQ (4.4 μmol) gave signals at δ 8.05–8.08 (m, C–H),
Finally, we examined a catalytic CO oxidation by NQ in the
7
.90–7.94 (m, C–H), and 7.10 (s, C–H) in the mixed solvent.
The reduced form of NQ (NQred, Chart 1), which was prepared
by the H /Pt catalyst system, gave signals at δ 9.13 (s, O–H),
III
presence of a small amount of [Rh (OEP)(Cl)]. The reaction is
denoted as follows:
2
δ 8.13–8.16 (m, C–H), 7.42–7.46 (m, C–H), and 6.74 (s, C–H).
On the addition of [Rh (OEP)(CO)(Cl)] (4.4 μmol) to the
NQ (4.4 μmol) solution, clear peaks at δ 9.19 (s, O–H),
CO þ NQ þ H2O ! CO2 þ NQred
ð1Þ
III
2
0
NQ (10 mM) was dissolved in a mixed solvent of acetone
δ 8.13–8.16 (m, C–H), 7.42–7.46 (m, C–H), and 6.74 (s, C–H) (4.5 mL) and water (0.5 mL). The solution was sealed with a
III
emerged. These peaks can be ascribed to the reduced form of rubber cap. CO was passed through this solution. [Rh (OEP)-
NQ (NQred). The peaks of NQred were not observed when CO (Cl)] (0.1 mM, 0.5 μmol) was injected to the solution. The gas
gas was passed through the solution of NQ without the Rh por- phase above the solution was subjected to analysis by gas
phyrin. These results indicate that NQ was reduced by chromatography after the reaction. The generation of CO
2
III
[
Rh (OEP)(CO)(Cl)] in the presence of water. This reaction (50 μmol) was clearly observed 1 hour after the addition of
III
21
III
strongly suggests that a CO-adduct of a Rh porphyrin is an [Rh (OEP)(Cl)]. This result indicates that [Rh (OEP)(Cl)]
intermediate of (electro)catalytic CO oxidation by the Rh por- catalyses CO oxidation by NQ (Fig. 3). A similar catalytic oxi-
phyrins. The generation of [Rh (OEP)(CO)(Cl)] and the elec- dation (CO oxidation by indigo carmine) has already been rea-
tron transfer from [Rh (OEP)(CO)(Cl)] to NQ would be a main lized in aqueous solution using a water-soluble Rh porphyrin.
pathway of the CO oxidation by Rh porphyrins. The possible pathway of CO oxidation by Rh porphyrin (CO
A conceivable overall catalytic reaction cycle is depicted in activation and the reaction between the CO-adduct and an
III
III
6
3
,5–7
Fig. 3. This cycle is similar to the previously reported ones.
electron acceptor) is totally different from that for CO oxi-
III
2a–c
Step (A) represents the coordination of CO on [Rh (OEP)(Cl)], dation by Pt-based electrocatalysts.
A water molecule is sup-
and steps (B) and (C) correspond the reaction between posed to be activated for the oxidation of CO by Pt catalysts. In
III
[
Rh (OEP)(CO)(Cl)] and NQ. A carboxy intermediate would be contrast, in the CO oxidation of a Rh porphyrin, CO is acti-
3
,5–7
involved in the reaction as reported previously.
The nucleo- vated with the coordination on the Rh atom. The activation of
philic addition of H
2
O to CO coordinated to the Rh porphyrin a water molecule is not necessary for the CO oxidation. This
generates this carboxy intermediate (step (B)). NQ oxidizes the reaction mechanism would be responsible for low overpoten-
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tials for electrocatalytic CO oxidation by a Rh porphyrin, since
activation of a water molecule needs large overpotentials.
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(
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III
We obtained [Rh (OEP)(CO)(Cl)] from the reaction of
III
[
Rh (OEP)(Cl)] with CO in a non-aqueous solvent (CH Cl ),
2 2
and analysed the CO-adduct by IR, NMR, and X-ray crystallo-
graphy. The results showed that back-donation from Rh to C is
relatively low among metalloporphyrin–CO complexes. This
suggests that CO on the Rh porphyrin easily undergoes nucleo-
philic attack by H
2
O. This property is correlated with the
(
electro)catalytic oxidation of CO by Rh porphyrins. Actually,
5 D. E. Hendriksen and R. Eisenberg, J. Am. Chem. Soc., 1976,
98, 4662–4664.
III
[Rh (OEP)(CO)(Cl)] reduced NQ, an electron acceptor, in
acetone-d containing water. Thus, CO is activated on a Rh
porphyrin for the oxidation by an electron acceptor.
6
6 J. C. Biffinger, S. Uppaluri, H. Sun and S. G. DiMagno, ACS
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1
4,22,23
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9
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Acknowledgements
We wish to thank Ms M. Ichimura, Ms M. Yoshioka, and Ms
S. Matsuyama for providing stellar technical assistance. We are
grateful to Dr T. Sugino (AIST), Dr S. Takeda (AIST), and
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1
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The amount of CO
44.6 μmol from that in gas phase based on Henry’s
law.
2
in solution was calculated to be
2
1
2
0 The slight difference in the chemical shifts would reflect 22 L. Liu, M. Yu, B. B. Wayland and X. Fu, Chem. Commun.,
the difference in the environment around phenolic
2010, 46, 6353–6355.
hydroxyl groups in the mixed solvent.
1 The amount of CO is the sum of those in solution
2
and those in gas phase. The amount in gas phase
was measured to be 5.3 μmol by gas chromatography.
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2
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