Iron(III) 5,15-diazaporphyrin catalysts for the direct oxidation of C(sp3)-H bonds

Article abstract of DOI:10.1021/acs.inorgchem.0c02166

5,15-Diazaporphyrins are porphyrin analogues with imine-type sp²-hybridized nitrogen atoms at the meso-positions. Even though these compounds are more electron-deficient than regular porphyrins, the use of iron diazaporphyrins as catalysts has not been reported. Herein, we disclose the synthesis, structure, and electronic properties of iron(III) 5,15-diazaporphyrins. We evaluate their structures and electronic natures by X-ray analysis and electrochemical analyses. We also demonstrate that chloroiron(III) 5,15-diazaporphyrins exhibit high catalytic activity in the direct oxidation of alkanes due to their intrinsic electron-deficient nature. On the basis of stoichiometric reactions of iron(III) diazaporphyrin with iodosylbenzene as an oxidant, it was possible to demonstrate the existence of an iodosylbenzene-iron diazaporphyrin adduct reaction intermediate that serves as a reservoir to generate oxo-iron species.

Full text of DOI:10.1021/acs.inorgchem.0c02166

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Iron(III) 5,15-Diazaporphyrin Catalysts for the Direct Oxidation of
C(sp3)−H Bonds
Tsubasa Nishimura, Takahisa Ikeue, Osami Shoji, Hiroshi Shinokubo, and Yoshihiro Miyake*
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ABSTRACT: 5,15-Diazaporphyrins are porphyrin analogues with imine-type sp2-hybridized nitrogen atoms at the meso-positions.
Even though these compounds are more electron-deficient than regular porphyrins, the use of iron diazaporphyrins as catalysts has
not been reported. Herein, we disclose the synthesis, structure, and electronic properties of iron(III) 5,15-diazaporphyrins. We
evaluate their structures and electronic natures by X-ray analysis and electrochemical analyses. We also demonstrate that
chloroiron(III) 5,15-diazaporphyrins exhibit high catalytic activity in the direct oxidation of alkanes due to their intrinsic electron-
deficient nature. On the basis of stoichiometric reactions of iron(III) diazaporphyrin with iodosylbenzene as an oxidant, it was
possible to demonstrate the existence of an iodosylbenzene−iron diazaporphyrin adduct reaction intermediate that serves as a
reservoir to generate oxo-iron species.
INTRODUCTION
■
As aliphatic C−H bonds are ubiquitous in organic molecules,
their direct oxidation represents a promising strategy for the
development of energy-saving alternatives for fossil-fuel based
feedstocks in the chemical industry as well as for the late-stage
diversification of pharmaceutical and bioactive molecules.1
Metalloporphyrins have attracted considerable attention as
model complexes for heme in cytochrome P450 and been
extensively explored as catalysts in the direct oxidation of
saturated C−H bonds.2,3 Previous research into the catalytic
activity of metalloporphyrins has revealed that the introduction
of electron-withdrawing groups onto porphyrin ligands
Figure 1. Chloroiron(III) porphyrins (1a, 1b) and 5,15-diazapor-
phyrins (2a−2c).
substantially enhances the reactivity of oxo-metalloporphyrins
and diminishes the oxidative degradation of these complexes.4,5
Nam and Kim have reported that chloroiron(III) 5,10,15,20-
tetrakis(pentafluorophenyl)porphyrin (1b) exhibits higher
catalytic activity in the oxidation of alkanes than the
corresponding tetrakis(mesityl)porphyrin complex (1a) (Fig-
ure 1a).4b Fine-tuning of the electronic nature of ligands
represents an essential strategy to increase catalytic turnover.
However, methods to access electron-deficient porphyrinic
ligands have generally been limited to the introduction of
Received: July 21, 2020
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electron-withdrawing substituents at the meso- and β-positions
of porphyrins.
state (S = 5/2).11c The temperature-dependent magnetic
susceptibility measurements using SQUID (Figure S2)
indicated that the effective magnetic moments (μeff) of 2a,
2b, and 2c at 300 K are 5.35, 5.24, and 5.51 μB, respectively.
These results are consistent with those of the EPR measure-
ments.
Recrystallization of 2a from chlorobenzene/hexane afforded
single crystals suitable for an X-ray diffraction analysis (Figure
2). The mean plane deviation of the diazaporphyrin ligand in
Recently, the meso-modification of porphyrinoids by the
introduction of heteroatoms has attracted considerable
attention as an effective method for giving the significantly
electronic perturbation to regular porphyrins.6 This knowledge
caused us to focus on the meso-modification as a useful
protocol for modulating catalysts in the direct oxidation of
saturated C−H bonds. 5,15-Diazaporphyrin is an 18π
porphyrinoid, whose electron-deficient nature is revealed to
be due to the imine-type sp2-hybridized nitrogen atoms at two
of the meso-positions by electrochemical and photophysical
measurements in recent days (Figure 1b).7−9 Consequently,
we anticipated that metal complexes with 5,15-diazaporphyrin
ligands could exhibit high catalytic activity. Herein, we describe
the synthesis, structures, and electronic properties of
chloroiron(III) 5,15-diazaporphyrinates 2 and report their
use as catalysts for the direct oxidation of alkanes.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. The syn-
thesis of chloroiron(III) 5,15-diazaporphyrinates 2 is summar-
ized in Scheme 1. Treatment of free-base 5,15-diazaporphyrin
Figure 2. (a) Top view and (b) side view of the molecular structure of
2a in the crystalline state. Hydrogen atoms and mesityl groups in part
b are omitted for clarity, and thermal ellipsoids are shown at 50%
probability. (c) Selected bond lengths and cavity area of 2a.
Scheme 1. Synthesis of Chloroiron(III) 5,15-
Diazaporphyrins 2a−2c
2a (0.0749 Å) reflects the highly planar conformation of the
ligand. The Fe atom of 2a adopts a square-pyramidal
coordination geometry. The distance between the Fe atom
and the mean plane of diazaporphyrin ligand in 2a (0.623 Å) is
longer than that in FeCl-TPP (0.359 Å) (Figure 2b).13 The
bond lengths between Fe and the nitrogen atoms of the pyrrole
rings (2.026(1)−2.038(1) Å) and the area of the cavity (7.729
Å2) in 2a are shorter and smaller than those of FeCl-TPP
(2.052 and 8.116 Å2), indicating that the inner cavity of 2a is
smaller than that of FeCl-TPP. The intermediate spin
character of 2 can be rationalized in terms of the
destabilization of the dx2‑y2 and dπ(dxz, dyz) orbitals due to
the small cavity in 2.11e
Electronic Properties. Electrochemical analyses of 1a and
2 were performed in CH2Cl2 using 0.1 M Bu4NPF6 as the
supporting electrolyte (Table 1). Complex 2a exhibited first
3a with 20 equiv of FeCl2·4H2O in o-dichlorobenzene at 160
°C for 4 h provided the corresponding μ-oxo dimer after
treatment with aqueous NaOH. The reaction of the μ-oxo
dimer with aqueous hydrochloric acid afforded chloroiron(III)
complex 2a in 72% yield. The iron complexes 2b and 2c, which
contain pentafluorophenyl and 2,6-dichlorophenyl groups,
respectively, were prepared in a similar fashion and obtained
a
Table 1. Redox Potentials of 1a and 2 in CH2Cl2
a
a
Eox (V)
Ered (V)
b
b
2a
2b
2c
1a
0.91
−0.65
−0.54
−0.61
−1.13
b
1.09
b
b
0.97
1
b
in good yield. The H NMR spectra of 2a−2c exhibited two
0.57
a
broad peaks at approximately 70−80 ppm, which were
Solvent: CH2Cl2; supporting electrolyte: Bu4NPF6 (0.1 M); working
assigned to the β-protons of the pyrrole units.10
electrode: platinum; counter electrode: platinum wire; reference
electrode: Ag/Ag+; scan rate: 500 mV s−1. All potentials are
referenced to the potential of the ferrocene/ferrocenium couple.
The spin states of 2a−2c were evaluated using two methods:
EPR spectroscopy and measurement of their effective magnetic
moments by SQUID magnetometry. The EPR spectra of
CH2Cl2 solutions of 2a−2c frozen at 4 K are shown in Figure
S1. On the basis of the EPR simulation data, the g⊥ values of
2a, 2b, and 2c were determined to be 5.89, 5.90, and 5.87,
respectively. These g⊥ values clearly suggest that the spin-states
of 2a−2c are spin-admixed states (S = 5/2 and 3/2) in which
the intermediate spin state (S = 3/2) is the minor
component.11,12 In sharp contrast, chloroiron(III) 5,10,15,20-
tetraphenylporphyrinate (FeCl-TPP) adopts a pure high-spin
b
Determined by differential pulse voltammetry.
oxidation (0.91 V) and reduction potentials (−0.65 V) that
were higher than those of 1a. The introduction of the electron-
withdrawing pentafluorophenyl and 2,6-dichlorophenyl groups
on the diazaporphyrin ligands induced positive shifts of the
oxidation and reduction potentials. To clarify the assignment
of the redox potentials, we investigated the solvent dependence
of the redox events (Table 2). Kadish has reported that the
B
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a
Table 2. Solvent Dependence of the Reduction Potential of
2a
Table 3. Catalytic Oxidation of Ethylbenzenes with PhIO
a
a
solvent
Ered (V)
b
CH2Cl2
DMF
DMSO
−0.65
−0.51
−0.26
a
TON
Supporting electrolyte: Bu4NPF6 (0.1 M); working electrode:
b
c
d
platinum; counter electrode: platinum wire; reference electrode:
entry
cat.
4 (R)
5
6
total
A/K
Ag/Ag+; scan rate: 500 mV s−1. All potentials are referenced to the
1
2
3
4
5
6
7
8
9
2a
2b
2c
1a
1b
2a
2a
2a
2a
4a (H)
4a (H)
4a (H)
4a (H)
103
92
102
35
135
114
139
107
52
88
42
67
17
48
41
87
84
47
191
134
169
52
183
155
226
191
99
2
4
3
4
6
6
3
3
2
b
potential of the ferrocene/ferrocenium couple. Determined by
differential pulse voltammetry.
coordination of solvents to the iron center in porphyrin
complexes significantly affects the reduction of Fe(III) to
Fe(II).14 The first reduction potential of 2a shifted positively
with increasing solvent polarity as has previously been
observed for other iron porphyrin complexes. These results
clearly suggest that the first reduction potential of 2
corresponds to the reduction of Fe(III) to Fe(II).
Next, we performed spectroelectrochemical measurements
on 2a to identify the one-electron-oxidized species. When the
electrochemical oxidation of 2a was conducted at a potential
higher than the first oxidation potential, a new absorption band
appeared at approximately 600−700 nm and several isosbestic
points were observed (Figure 3). Matano has reported that a
4a (H)
4b (OMe)
4c (Ph)
4d (Cl)
4e (NO2)
a
All reactions between 4 (5.0 mmol) and PhIO (1.0 mmol) were
carried out in the presence of the indicated catalyst (2.0 μmol) in
b
benzene (5 mL) at room temperature for 3 h. TON for 5 = (moles
c
of 5)/(moles of catalyst). TON for 6 = (moles of 6) × 2/(moles of
catalyst). A/K = (moles of 5)/(moles of 6).
d
oxidation of 4a, while iron porphyrin 1a exhibited a
substantially decreased TONs (Table 3, entries 2−4). We
also monitored the time course of the oxidation of 4a using 1a
and 2a under otherwise identical conditions (Figure 4). Not
Figure 3. Spectroscopic changes in the electrochemical oxidation of
2a in CH2Cl2, from 0 min (red line) to 60 min (blue line) in intervals
of 5 min.
Figure 4. Time profiles for the oxidation of ethylbenzene catalyzed by
1a (black circles) and 2a (red circles). The reaction of 4a (5.0 mmol)
with PhIO (1.0 mmol) was conducted in the presence of 1a or 2a (2.0
μmol) in benzene (5 mL) at 15 °C.
nickel(II) diazaporphyrin π-radical cation displays a character-
istic absorption band at approximately 600−700 nm.7b
Consequently, we concluded that the first single-electron
oxidation occurs on the diazaporphyrin ligand to provide the
corresponding diazaporphyrin π-radical cation. In their
entirety, these electrochemical measurements (Table 1 and
Figure 3) demonstrate an increased electron deficiency at both
the iron center and the ligand in 2a as compared to 1a, which
suggests enhanced Lewis acidity at the iron center as well as
increased tolerance of the ligand against oxidative degradation.
Catalysis. We then investigated the oxidation of ethyl-
benzene (4a) with iodosylbenzene (PhIO) catalyzed by
chloroiron(III) porphyrins 1 and chloroiron(III) diazapor-
phyrins 2 (Table 3). The use of 2a as a catalyst provided
alcohol 5a and ketone 6a with TONs of 103 and 88,
respectively (Table 3, entry 1). The use of mCPBA (m-
chloroperbenzoic acid) as an oxidant was not effective (TON
for 5a: 28, TON for 6a: 23). When we employ O2 and H2O2 in
place of PhIO, no formation of 5a and 6a was observed.
Complexes 2b and 2c showed similar catalytic activity in the
only did 2a provide 5a and 6a more rapidly than 1a, but the
duration of its catalytic activity was also longer. On the basis of
these results, we concluded that the higher TON with 2a
originates from both the high initial reaction rate and the
stability of the catalytically active species. The activity of
electron-deficient iron porphyrin 1b was similar to that of 2a,
suggesting that the imine-type nitrogen atoms in the
diazaporphyrin ligand act as an efficient electron-withdrawing
core (Table 4, entry 5). Ethylbenzenes (4b−4e) with various
substituents at the para-position were also suitable for this
catalytic system (Table 4, entries 6−9).
Subsequently, we investigated the scope of the present
catalytic oxidation (Table 4). The oxidation of tetralin (4f)
afforded 5f and 6f in good yield (Table 4, entry 1). When
cyclooctane (4g) was used as a substrate in the presence of 2a,
the corresponding products (5g and 6g) were obtained with
TONs of 122 and 45, respectively (Table 4, entry 2). The use
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a
diazaporphyrin π-radical cation shown in Figure 3. These
results suggest the formation of a reaction intermediate other
than the iron diazaporphyrin π-radical cation. Importantly, this
intermediate is very stable in benzene: The absorption
spectrum showed no change after 30 min at 15 °C (Figure
S5). This observation is noteworthy as oxo-iron porphyrin π-
radical cations are generally short-lived.
Table 4. Catalytic Oxidation of Alkanes 4 with PhIO
Several groups have reported that the reaction of iron
porphyrin complexes with iodosylarenes (ArIO) affords the
corresponding iron porphyrin−iodosylarene adducts, which
subsequently furnish oxo-iron porphyrin π-radical cations as
reactive intermediates through the rapid elimination of aryl
iodides (ArI).15,16 Nam has reported that the reaction of the
oxo-iron porphyrin π-radical cation with an excess of aryl
iodide induces the reversible formation of the porphyrin−
iodosylarene adduct; the use of an electron-deficient porphyrin
complex is essential in this context.15a On the basis of our
current findings and these previous reports, we propose that
iodosylbenzene adduct 7a is generated in the reaction of 2a
with PhIO (Scheme 2). In fact, the ESI-MS spectrum of the
a
All reactions between 4 (5.0 mmol) and PhIO (1.0 mmol) were
carried out in the presence of the indicated catalyst (2.0 μmol) in
b
benzene (5 mL) at room temperature for 3 h. TON for 5 = (moles
of 5)/(moles of catalyst). TON for 6 = (moles of 6) × 2/(moles of
catalyst). 2c (0.04 μmol) was used. A/K = (moles of 5)/(moles of
Scheme 2. Equilibrium between the Iodosylbenzene Adduct
and Oxo-Iron Diazaporphyrin π-Radical Cation
c
d
e
f
6). Selectivity for tertiary C−H bonds/secondary C−H bonds (3°/
2°) = (moles of 5ha)/(moles of 5hb) × 3 (nomalized on a per-
hydrogen basis).
of 2c for the oxidation of 4g increased the TON (Table 4,
entry 3). In the oxidation of adamantane (4h), 1-adamantyl
alcohol (5ha) was obtained as the major product, together
with the secondary C−H-bond-oxidized products 5hb and 6hb
(Table 4, entries 4−6). While catalysts 2a and 2c both showed
good catalytic activity, the total TON reached 4235 in the
presence of 0.04 μmol of 2c.
Reaction Intermediates. To investigate the reaction
intermediates, a stoichiometric reaction between 2a and
PhIO was carried out. The reaction of 2a with an excess of
PhIO in benzene at 15 °C was monitored via the UV−vis
absorption spectra of the resulting solution after the removal of
the remaining PhIO by filtration. Upon treatment of 2a with
PhIO, the intensity of the two absorption bands of 2a at 550
and 656 nm decreased, while that of the peak at 601 nm
increased simultaneously (Figure 5a). The features of this
spectrum are substantially different from that of the iron
solution supported the formation of 7a (Figure S6). In the
present system, 7a is stabilized due to the enhanced Lewis
acidity of the iron center induced by the diazaporphyrin ligand.
The stoichiometric reaction between 2a and iodosylbenzene
provided further support for the formation of 7a. The
absorption bands of 7a and 2a decreased and increased,
respectively, upon addition of an excess of 4f to the solution of
7a (Figure 5b). The formation of 5f and 6f was detected by
GC analysis. This result clearly suggests that an equilibrium
between 7a and the oxo-iron diazaporphyrin π-radical cation
8a exists, and that adduct 7a is the predominant intermediate
and serves as a stable reservoir for the catalytically active
species 8a (Scheme 2). Additionally, the reaction between 7a
and 4f in the presence of H218O yielded 18O-enriched 5f
(Figure S7). The incorporation of 18O from H218O also
supports the formation of 7a, as the oxygen-exchange reaction
of metal−iodosylbenzene adducts with water occurs read-
ily.15a,17 Consequently, we concluded that the reversible and
gradual formation of 8a from 7a prevents the degradation of
the complexes and thus enhances the duration of the catalytic
activity of 2a.
CONCLUSIONS
■
We successfully used chloroiron(III) 5,15-diazaporphyrins as
catalysts for the direct oxidation of C(sp3)−H bonds.
Spectroscopic measurements of the stoichiometric reaction of
an iron diazaporphyrin complex with iodosylbenzene revealed
that the iodosylbenzene−iron diazaporphyrin adduct acts as a
stable reservoir of oxo-iron diazaporphyrin π-radical cations.
Electrochemical measurements also support the conclusion
that the high Lewis acidity of this iron diazaporphyrin complex
Figure 5. (a) Absorption spectra of 2a (black line) and 7a (red line)
in benzene at 15 °C. (b) Change in the spectrum of 7 in benzene at
15 °C in the presence of an excess of 4f, from 0 min (red line) to 30
min (blue line) in intervals of 3 min.
D
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stabilizes the iodosylbenzene−iron diazaporphyrin adduct. We
found that the reversible formation of the oxo-iron
diazaporphyrin π-radical cations as an active species is an
important factor in the high catalytic activity of this iron
diazaporphyrin complex. The present results demonstrate that
the meso-modification of the porphyrin skeleton offers a novel
direction for fine-tuning porphyrinic ligands for applications in
catalysis.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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Experimental procedures as well as spectroscopic and X-
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Accession Codes
CCDC 2012719 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Yoshihiro Miyake − Department of Molecular and
Macromolecular Chemistry, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japan; orcid.org/
0000-0003-0247-531X; Email: miyake@chembio.nagoya-
u.ac.jp
Authors
Tsubasa Nishimura − Department of Molecular and
Macromolecular Chemistry, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japan
Takahisa Ikeue − Department of Chemistry, Graduate School of
Natural Science and Technology, Shimane University, Matsue
690-8540, Japan
Osami Shoji − Department of Chemistry, Graduate School of
Science, Nagoya University, Nagoya 464-8603, Japan;
orcid.org/0000-0003-3522-0065
Hiroshi Shinokubo − Department of Molecular and
Macromolecular Chemistry, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japan; orcid.org/
0000-0002-5321-2205
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Complete contact information is available at:
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Funding
The authors declare the absence of any potential competing
financial interests.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Grant-in-Aid for Scientific
Research on Innovative Areas “Precisely Designed Catalysts
with Customized Scaffolding (No. 2702)” (JSPS KAKENHI
grant 16H01013) from MEXT, Japan. T.N. expresses his
gratitude for a JSPS Research Fellowship for Young Scientists
(JP19J15248).
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