N-Confused Porphyrin Metal Complexes with an Axial Pyridine Directly Tethered from an Inner Carbon: A Bioinspired Ligand as a Versatile Platform for Catalysis

Article abstract of DOI:10.1002/ejic.201701494

Bioinspired pentadentate ligand N-confused porphyrin (NCP) bearing a 2-mercaptopyridine group and its Ru^II and Co^III complexes were synthesized. Their structures were revealed by single-crystal X-ray crystallographic analysis. Installation of an axial thiopyridine ligand shifts the redox potentials negatively and enhances the catalytic activity largely, which was demonstrated in the cyclopropanation reaction using the Co complex.

Full text of DOI:10.1002/ejic.201701494

A Journal of
Accepted Article
Title: N-Confused Porphyrin Metal Complexes with an Axial Pyridine
Directly Tethered from an Inner Carbon: A Bioinspired Ligand as
Versatile Platform for Catalysis
Authors: Takaaki Miyazaki, Takaaki Yamamoto, Shunichi Mashita,
Yuya Deguchi, Kazuki Fukuyama, Masatoshi Ishida, Shigeki
Mori, and Hiroyuki Furuta
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701494
Link to VoR: http://dx.doi.org/10.1002/ejic.201701494

10.1002/ejic.201701494
European Journal of Inorganic Chemistry
COMMUNICATION
N-Confused Porphyrin Metal Complexes with an Axial Pyridine
Directly Tethered from an Inner Carbon: A Bioinspired Ligand as
Versatile Platform for Catalysis
Takaaki Miyazaki, Takaaki Yamamoto, Shunichi Mashita, Yuya Deguchi, Kazuki Fukuyama, Masatoshi
Ishida, Shigeki Mori, and Hiroyuki Furuta*
Abstract: Bioinspired pentadentate ligand, N-confused porphyrin
(NCP) bearing a 2-mercaptopyridine group (1), and its RuII and CoIII
complexes, Ru(CO)-1a and Co(NO2)-1, were synthesized and the
structures were revealed by single crystal X-ray analysis. Installation
of an axial thiopyridine ligand negatively shifted the redox potentials
and enhanced the catalytic activity largely, which was demonstrated
in the cyclopropanation reaction using the Co complex.
tautomeric resonance effect; the distinct 3H-tautomer having two
NHs and one CH in the core can serve as a trianionic ligand in
metal complexation (Figure 1a). In contrast, the 2H-tautomer has
one NH and one CH in the core, and one NH at the periphery,
which ordinary provides a dianionic coordination environment.
Furthermore, the unsymmetrical structures of metallo-NCPs
enabled the highly stereo-selective cyclopropanation of alkenes
by rhodium-NCP6 and oxygen atom transfer by rhenium-NCP.7
Introduction
Mimicking the active sites of heme enzymes has been considered
as one of the promising strategies for development of the
metalloporphyrin-based catalysts.1 The original family of enzymes
is responsible for various organic transformation reactions in
biological systems under mild conditions.1d,2 A key component of
the active sites of heme enzymes is the axial donor moiety derived
from the histidine (imidazole) or cysteine (thiol) residue, which
plays an important role in tuning the catalytic activity (Figure 1b).
In particular, the donating property of axially coordinated ligands
largely facilitates the dioxygen activation on the iron center in the
tetrapyrrolic macrocycles.1-3 On this basis,
a variety of
biomimetic/bioinspired model complexes such as imidazole-
tethered metalloporphyrins have been synthesized,1,3 however,
the limited synthetic accessibility often hampers a wide-scale use
of such catalysts.
Figure 1. (a) NH-tautomerism-coupled transformation of N-confused porphyrin
metal complexes; (b) Active site of horseradish peroxidase (HRP) enzyme
(PDB-1H5L); (c) Schematic illustration of the biomimetic heme-type model
complex studied in this work.
Within the scope of artificial mutants of metalloporphyrin
catalysts, we have been working on N-confused porphyrin
(termed as NCP) since its discovery.4 NCP is an isomer of
porphyrin containing an inverted pyrrole subunit, and from the
viewpoint of catalytic property, corresponding metallo-NCPs have
shown the advantage of stabilizing high-valent metal ions due to
the strong s-donation of the carbon atom present in the NNNC
coordination environment.5 This is realized by the unique NH
In this study, we have designed novel pentadentate NCP
ligands with an axial pyridine moiety utilizing the regioselective
substitution reaction at the inner carbon of the NCP core. This
modification induced the formation of another type of tautomeric
form, 3H’-tautomer, possessing one NH and one CH2 (sp3 carbon
atom) in the core (Figure 1a).8 Notably, the NCP metal complexes
M-1 (e.g., Ru(CO)-1 and Co(NO2)-1, as representatives for
ruthenium(II) and cobalt(III) complexes, respectively) with an axial
2-thiopyridine unit covalently linked to the inner carbon atom were
prepared in a short-step from the NCP ligand 2 (Figure 1c). Such
axially modified complexes could work as a new type of
bioinspired catalyst.9 Herein, we examined the cyclopropanation
of styrene using the cobalt catalysts to evaluate the effect of
introduced axial 2-thiopyridine ligand intramolecularly, because
the catalytic activity of cobalt(II) NCPs for the cyclopropanation
reaction was already shown by Zeigler and co-workers.10 As a
result, installation of the axial ligand was revealed to accelerate
the catalytic reaction significantly.
[*]
Dr. T. Miyazaki,
Education Center for Global Leaders in Molecular Systems for
Devices, Kyushu University, Fukuoka 819-0395 (Japan)
Dr. T. Yamamoto, S. Mashita, Y. Deguchi, K. Fukuyama,
Dr. M. Ishida, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry, Graduate School of
Engineering, Center for Molecular Systems, Kyushu University,
Fukuoka 819-0395 (Japan)
E-mail: hfuruta@cstf.kyushu-u.ac.jp
http://www.cstf.kyushu-u.ac.jp/~furutalab/
Dr. S. Mori
Advanced Research Support Center, Ehime University
Matsuyama 790-8577 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
1
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10.1002/ejic.201701494
European Journal of Inorganic Chemistry
COMMUNICATION
Next, Ru(CO)-3a was treated with an excess amount of 2-
mercaptopyridine, then a new complex Ru(CO)-1a, in which the
adjacent sulfur atom is connected to the inner carbon atom of
NCP, was formed (step b in Scheme 1). The X-ray structure
shows the presence of an axial 2-thiopyridine moiety connected
to the sp3 inner carbon atom (Figure 2a, Table S2).11 This
modification lead to a stronger bonding of the pyridine ligand as
inferred from the shorter bond length between the ruthenium
center and pyridine nitrogen in Ru(CO)-1a than that of Ru(CO)-
Results and Discussion
Synthesis of the complex Ru(CO)-1a is simple and
straightforward. At first, treatment of freebase NCP 2a with
Ru3(CO)12 in chlorobenzene at 140 °C and succeeding addition of
pyridine at room temperature afforded Ru(CO)-3a with an axial
pyridine ligand (step a in Scheme 1). The 1H NMR spectrum
displayed the multiplet signals at the aromatic region from d 7.46
to 7.88 ppm along with the characteristic peripheral NH signal of
the confused pyrrole ring at 9.38 ppm in CDCl3, which indicates
the formation of 2H-type tautomer (Figure S6). The characteristic
upfield signals at 4.70 (ortho-), 6.09 (meta-), and 6.69 (para-) ppm
could be assigned as the coordinated pyridine molecule on the
ruthenium center. The explicit structural evidence of Ru(CO)-3a
came from the X-ray crystallographic analysis (Figure S5b, Table
S2).11 Like the parent ruthenium tetraphenylporphyrin complex,12
Ru(CO)-3a adopts an octahedral geometry and the ruthenium
metal cation is accommodated at the equatorial NNNC
coordination site with CO and pyridine at the both axial positions.
The complex is neutral and the valence state of the ruthenium
center is +2 (d6), which is consistent with the diamagnetic
character as shown in the NMR spectrum.
3a (2.172(3)
Å vs 2.200(5) Å, respectively) (Figure S5).
Interestingly, due to the formation of 5-member ring, the pyridine
ring of Ru(CO)-1a aligns along the C–Ru–N axis, whereas that of
Ru(CO)-3a is tilted by ca. 33° to the axis. The bond lengths
around the inner carbon atom of Ru(CO)-1a are elongated
compared to those of Ru(CO)-3a (C1–C2 = 1.472(5) vs 1.409(7),
C2–C3 = 1.479(5) vs 1.399(9), C2–Ru = 2.118(3) vs 2.015(5), and
C2–S = 1.843(5) Å) and appropriate feature in the bond angles
for sp3 carbon were observed (approximately 109° for ÐC3–C2–
S). Consequently, the distinct 18p-conjugated circuit in the
annulenic core of Ru(CO)-1a can be inferred from NMR
spectroscopy (Figure S7). Namely, the representative features,
the lack of outer NH signal and appearance of peripheral a-CH
signal of the confused pyrrole ring in the typical aromatic region
(9.35 ppm), were observed in CDCl3. In addition, the axially
coordinated pyridine-CH signals are significantly upfield shifted to
6.05 (para-), 5.41 (meta-), 5.27 (meta-), and 2.59 (ortho-) ppm
due to the diatropic ring current effect of the macrocycle. The 13
C
NMR spectrum also supports the proposed structure; the sp3
hybridized carbon signal was observed at 44.9 ppm (Figure S8).
Therefore, the complex Ru(CO)-1a is composed of a 3H’-type
resonance form of NCP.
Figure 2. X-ray structures of (a) Ru(CO)-1a and (b) Co(NO2)-1b. Hydrogen
atoms were omitted for clarity. Thermal ellipsoids are shown at the 30%
probability level.
The aforementioned protocol was applicable to the
synthesis of cobalt complex in a one-step fashion. That is,
treatment of 2a with 2-mercaptopyridine in the presence of
Co(NO3)2·6H2O in refluxing THF afforded the corresponding
complex (Co(NO2)-1a) in 56% yield (step c in Scheme 1).
Likewise, the derivative bearing meso-(4-methoxylphenyl)-
substituents (Co(NO2)-1b) could be synthesized and the structure
was successfully proved by X-ray analysis (Figure 2b, Table
S3).11 A cobalt atom is located in the NNNC coordination sphere
with a nitrite anion as an axial ligand. As with Ru(CO)-1a, the
covalently tethered thiopyridine moiety is present, forming the
octahedral geometry. The resulting d6 cobalt(III) complex with
(a) i) Ru3(CO)12, chlorobenzene, reflux, N2, 18 h; ii) pyridine, rt, 1 h, 71%; (b) 2-
mercaptopyridine, 1,2-dichloroethane, reflux, 18 h, 69%; (c) Co(NO3)2·6H2O, 2-
mercaptopyridine, NaNO2, THF, reflux, 1 day, 56% for Co(NO2)-1a, 40% for
Co(NO2)-1b; (d) Ni(acac)2, CHCl3, reflux, N2, 1 day, 80% for Ni-3a, 82% for Ni-
3b; (e) 2,2’-dithiopyridine, MeCN, reflux, 3 h, 66% for 1a, 39% for 1b; (f)
Co(NO3)2·6H2O, THF, reflux, N2, 2 h, 63% for Co(NO2)-1a, 47% for Co(NO2)-
1b.
Scheme 1. Synthetic routes for ruthenium and cobalt complexes of NCP with
an axial 2-thiopyridine group tethered from the inner carbon.
2
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10.1002/ejic.201701494
European Journal of Inorganic Chemistry
COMMUNICATION
3H’-form structure was again inferred from 1H and 13C NMR
spectroscopy (Figures S13 and S14).
potentials of Co(NO2)-1a are affected similarly. Accordingly, the
axial thiopyridine donor induced lowering of the oxidation
potentials, which infers its potency as oxidation and/or carbene
insertion catalysts.15,16
In an effort to verify the versatile coordination ability of the
pentadentate NCP ligand, the freebase ligand 1 bearing a 2-
thiopyridine unit was separately prepared by heating the nickel(II)
NCP complexes (Ni-2a and Ni-2b) with 2,2’-dithiopyridine in
acetonitrile (step e in Scheme 1). The structure of 1a was
unambiguously characterized by X-ray crystallography (Figure 3,
Table S3).11 The pyridine moiety is linked at the inner carbon and
the confused pyrrole moiety is inclined at 38° from a mean-plane
consisting of four meso-carbon atoms due to the steric congestion.
Unlike the case of Co(NO2)-1a, the 3H-type tautomeric structure
with a modified sp2-inner carbon atom is retained as inferred from
the ÐC–N1–C bond angle (106.9°) of the confused pyrrole ring as
well as the 1H NMR spectrum. Reflecting the deshielding effect of
the NCP ring, the proton signals of the pyridyl moiety atop were
also upfield shifted (Figure S9). As expected, the resulting
Table 1. Yields and stereo-selectivity for cyclopropanation of styrene with
EDA
Catalyst (0.5 mol%)
toluene, rt, 1 h
O
COOEt
trans/cis
+
N2
OEt
EDA
dimer (%)
Entry
catalyst
cp (%)[a]
trans/cis[a]
[a,b]
1
2
Co(NO2)-1a
Co-1ared
70
-
83/17[c]
thiopyridine-functionalized NCP ligand
1
afforded the
78 (80)[d]
11
92/8 (92/8)[e]
corresponding cobalt(III) complexes, Co(NO2)-1a/1b in good
yields (step f in Scheme 1).13
3
4
Co-1bred
Co-4
76
31
6
12
9
90/10
92/8
5
CoII-TPP
Co-1ared
-
72/28
89/11
6[f]
39
10
[a]
The yields and trans/cis ratios were determined by GLC using tridecane
as an internal standard. [b] The dimer is diethyl maleate ester (Z form). [c] At
80 °C, 6 h. Isolated yield. The trans/cis ratios were determined by 1H
[d]
[e]
NMR in CDCl3. [f] 0.1 mol% of cobalt catalyst. cp: cyclopropanated product.
Figure 3. X-ray structures of 1a; (a) side and (b) top views. Hydrogen atoms
were omitted for clarity. Thermal ellipsoids are shown at the 30% probability
level.
The electrochemical properties of Ru(CO)-1a and Co(NO2)-
1a were investigated using cyclic voltammetry in CH2Cl2
containing 0.1 M n-tetrabutylammonium hexafluorophosphate
(TBAPF6). Two reversible oxidation peaks at 0.34 and 0.79 V and
an
irreversible
reduction
peak
at
–1.39
V
(vs
ferrocene/ferrocenium couple, Fc/Fc+) were observed for
Ru(CO)-1a (Figure S4, Table S1). In comparison with Ru(CO)-3a,
the overall potentials are positively shifted due to the intrinsic
electronic effect of the NCP ligand tautomers (3H’-form vs 2H-
form). The resulting HOMO-LUMO gap of 1.73 eV for Ru(CO)-1a
is larger than that of Ru(CO)-3a (1.58 eV), which is consistent with
the blue-shifted longest-wavelength transition in the absorption
spectra (Figure S1). In the case of Co(NO2)-1a, on the other hand,
three irreversible oxidation peaks at 0.67, 0.79, and 1.07 V and
four irreversible reduction peaks at –1.10, –1.26, –1.50, and –1.70
V were observed (Figure S4, Table S1). Although details of the
irreversibility of Co(NO2)-1a on CV is unclear at this moment, it is
probably related to the affinity strength between the cobalt center
and the axial ligands. The first reduction potential is considerably
negatively-shifted compared to that of cobalt(III) tetrakis(4-
Figure 4. The time course of cyclopropanation reactions (2, 5, 10, 20, 30, 45,
and 60 min) with Co-1ared (blue), Co-4 (red), and CoII-TPP (green).
To illustrate the catalytic capability of metalated 1,
cyclopropanation of styrene was investigated using cobalt
complexes as a test reaction (Table 1). At first, Co(NO2)-1a was
subjected to the reaction (conditions: catalyst (0.5 mol%), styrene
(5 equiv), ethyl diazoacetate (EDA, 1 equiv), aerobic, in toluene).
At room temperature, the reaction did not proceed. When the
reaction temperature was raised at 80 °C for 6 h, cyclopropane
product was obtained in 70% yield (trans/cis = 83/17, entry 1).
Next, Co(NO2)-1 was pretreated with 0.5% Na2S2O4 aq in
carboxyphenyl)porphyrin chloride, –0.56
V
(in 0.1
M
TBAPF6/DMSO),14 which suggests the strongly electron-donating
nature of the 2-thiopyridine moiety in Co(NO2)-1a. The oxidation
3
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10.1002/ejic.201701494
European Journal of Inorganic Chemistry
COMMUNICATION
dichloromethane to form the reduced species, Co-1ared and Co-
1bred, and subjected to the catalytic reactions at room temperature
for 1 h. For comparison, cobalt(II) complex of the N-methylated N-
confused tetraphenylporphyrin with pyridine as an axial ligand,
Keywords: N-confused porphyrin • pentadentate • catalyst
•cyclopropanation• acceleration effect
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CoII(N-MeNCTPP)(py)
(Co-4),10
and
cobalt(II)
tetraphenylporphyrin (CoII-TPP) were used as reference catalysts.
The time course of cyclopropanation reactions with Co-1red, Co-
4, and CoII-TPP catalysts were also followed (Figure 4). As a
result, Co-1red exhibited 78% yield with 92/8 trans/cis-selectivity
(entry 2) and, to our surprise, the reaction was almost completed
within 5 min. Similar result was obtained with Co-1bred (76%,
90/10) (entry 3). In the case of Co-4, the trans-selectivity was
same as that of Co-1red, whereas the yield was low (31%, 92/8)
(entry 4).10 CoII-TPP showed the lesser catalytic activity as well
as trans-selectivity under the same conditions (6%, 72/28) (entry
5). Furthermore, the catalytic reaction of Co-1red proceeded even
with 0.1 mol% of Co-1ared (TON = 390, entry 6). Notably, Co-1red
showed an excellence catalytic activity and stability under the air,
whereas the cyclopropanation reaction of olefins with EDA is
usually carried out under anaerobic condition. Particularly, the
rate of reaction was dramatically increased compared with Co-4
and CoII-TPP.
a) J. T. Groves, T. E. Nemo, R. S. Meyers, J. Am. Chem. Soc. 1979, 101,
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J. Am. Chem. Soc. 2003, 125, 11822–11823.
The attempt to clarify the structure of the initially reduced
species (Co-1ared), which we assume the nitrite-free CoII-1a, is so
far unsuccessful. The mass spectra indicated at least the
thiopyridine group was retained in the catalyst during the catalytic
reaction. We believe the coordinated thiopyridine ligand tethered
to the NCP skeleton affects the catalytic ability through electron-
donation to the metal center and protects the inner carbon, which
is rather reactive in an sp2 form of the ordinary NCP metal
complexes.17
[6]
[7]
[8]
T. Niino, M. Toganoh, B. Andrioletti, H. Furuta, Chem.
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Only a few examples of 3H’-forms have been reported. See, a) I. Schmidt,
P. Chmielewski, Z. Ciunik, J. Org. Chem. 2002, 67, 8917–8927; b) I.
Schmidt, P. Chmielewski, J. Inorg. Chem. 2003, 42, 5579–5593; c) C.-H.
Hung, W.-M. Ching, G.-F. Chang, C.-H. Chuang, H.-W. Chu, W.-Z. Lee,
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2536–2540.
Conclusions
In summary, the bioinspired pentadentate NCP ligand 1 and its
metal complexes Ru(CO)-1 and Co(NO2)-1 were synthesized
with simple procedures in good yields, and the structures were
revealed by X-ray analysis. Co-1red exhibited an excellent
[9]
catalytic activity, especially,
a large acceleration in the
cyclopropanation reaction of styrene with EDA. Because of the
facile synthesis and good stability and its intrinsic nature to
stabilize higher oxidation states of metals, it appears that the
present bioinspired pentadentate NCP ligand would provide an
ideal platform for the metal catalysts in a variety of reactions.
[10] Catalytic cyclopropanation of styrene with EDA using cobalt NCPs was
investigated previously. Co-4 (1.0 mol%) was reported to show the
catalytic activity (86%, 93:7) at room temperature for 20 h under nitrogen
atmosphere. K. B. Fields, J. T. Engle, S. Sripothongnak, C. Kim, X. P.
Zhang, C. J. Ziegler, Chem. Commun. 2010, 47, 749–751.
[11] Crystallographic data for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre (CCDC
1579795 (1a), CCDC 1579796 (Co(NO2)-1b), CCDC 1579797 (Ru(CO)-
1a) and CCDC 1579798 (Ru(CO)-3a)) and data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/ data_request/cif.
Acknowledgements
The present work was supported by JSPS KAKENHI Grant
Numbers JP15K13646, JP16K05700, JP17H05377. The financial
support from a bilateral program between JSPS and the National
Research Foundation (NRF) of South Africa is also acknowledged.
[12] R. G. Little, J. A. Ibers, J. Am. Chem. Soc. 1973, 95, 8583–8590.
[13] Under the reaction conditions, the nitrate ion of cobalt salt could be
reduced to the nitrite. The FTIR spectrum of Co(NO2)-1a showed the
typical stretching mode of nitrite anion (nNO; 1311, 1405 cm–1). T. S.
Kurtikyan, S. R. Eksuzyan, J. A. Goodwin, G. S. Hovhannisyan, Inorg.
Chem. 2013, 52, 12046–12056.
Conflict of interest
The authors declare no conflict of interest.
4
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10.1002/ejic.201701494
European Journal of Inorganic Chemistry
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45, 15746–15761; b) S. M. Ujwaldev, K. S. Sindhu, A. P. Thankachan,
G. A. Anilkumar, Tetrahedron 2016, 72, 6175–6190; c) C.-M. Che, J.-S.
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K. M. Kadish, K. M. Smith, R. Guilard) World Scientific, Singapore, 2010,
295–362.
5
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10.1002/ejic.201701494
European Journal of Inorganic Chemistry
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Bioinspired pentadentate ligand, N-
confused porphyrin (NCP) bearing a
2-mercaptopyridine group (1), and
its RuII and CoIII complexes,
Ru(CO)-1a and Co(NO2)-1, were
synthesized and the structures were
revealed by single crystal X-ray
analysis. Installation of an axial 2-
Takaaki Miyazaki, Takaaki
Yamamoto, Shunichi Mashita, Yuya
Deguchi, Kazuki Fukuyama,
Masatoshi Ishida, Shigeki Mori, and
Hiroyuki Furuta*
Page No. – Page No.
N-Confused Porphyrin Metal
thiopyridine
ligand
negatively
Complexes with an Axial Pyridine
Directly Tethered from an Inner
Carbon: A Bioinspired Ligand as
Versatile Platform for Catalysis
shifted the redox potentials and
largely enhanced the catalytic
activity for the cyclopropanation
reaction with the Co complex.
6
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