Oxidation of adamantane catalysed by imidazolylporphyrinatoiron(III) complexes and structural studies of 5-coordinating iron(III) porphyrin

Article abstract of DOI:10.1016/j.molcata.2007.12.016

Oxidation of adamantane with phenylperacetic acid was carried out in the presence of three imidazolyltriarylporphyrinatoiron(III) complexes having pentafluorophenyl, phenyl, and mesityl (2,4,6-trimethylphenyl) groups as meso-substituents and three corresponding tetraarylporphyrinatoiron(III) complexes. The yield of 1- and 2-adamantanols was 76% in the case of chloro-5-(1-methyl-2-imidazolyl)-10,15,20-tri(pentafluorophenyl)porphyrinatoiron(III) (ImTPFPP-Fe(III)Cl), whereas the yield was only 26% in the case of chloro-5,10,15,20-tetra(pentafluorophenyl)porphyrinatoiron(III) in the presence of 100 eq. N-methylimidazole. The apparent effect of the appended imidazolyl group is discussed in terms of a 5-coordinated dimer of ImTPFPP-Fe(III)Cl, which was observed in the ¹H and ¹⁹F NMR, and UV-vis spectra.

Full text of DOI:10.1016/j.molcata.2007.12.016

Available online at www.sciencedirect.com
Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
Oxidation of adamantane catalysed by imidazolylporphyrinatoiron(III)
complexes and structural studies of 5-coordinating iron(III) porphyrin
Yuji Miyazaki, Akiharu Satake∗, Yoshiaki Kobuke∗∗
The Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Takayama 8916-5, Ikoma, Nara 630-0192, Japan
Received 10 September 2007; received in revised form 17 November 2007; accepted 13 December 2007
Available online 23 December 2007
Abstract
Oxidation of adamantane with phenylperacetic acid was carried out in the presence of three imidazolyltriarylporphyrinatoiron(III)
complexes having pentafluorophenyl, phenyl, and mesityl (2,4,6-trimethylphenyl) groups as meso-substituents and three corresponding
tetraarylporphyrinatoiron(III) complexes. The yield of 1- and 2-adamantanols was 76% in the case of chloro-5-(1-methyl-2-
imidazolyl)-10,15,20-tri(pentafluorophenyl)porphyrinatoiron(III) (ImTPFPP–Fe(III)Cl), whereas the yield was only 26% in the case of
chloro-5,10,15,20-tetra(pentafluorophenyl)porphyrinatoiron(III) in the presence of 100 eq. N-methylimidazole. The apparent effect of the appended
imidazolyl group is discussed in terms of a 5-coordinated dimer of ImTPFPP–Fe(III)Cl, which was observed in the 1H and 19F NMR, and UV–vis
spectra.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Iron porphyrin; Oxidation; Peroxy acid; Imidazole coordination; Supramolecule
1. Introduction
number of catalytic oxidation with the use of peracids and per-
oxide [5].
One-oxygen oxidation of inactive C H bonds has been of
great interest in both enzymatic reactions and synthetic organic
chemistry [1]. Various metal catalysts have been developed
for this purpose and discussed in relation to the excellent
activation methods in natural systems [2]. Peroxidases and
cytochrome P-450 have been investigated most extensively
as one-oxygen oxidation catalysts. Mimics of their structure
and function have been examined for nearly three decades,
and their development has been a continuing target of active
research [3]. The following two basic strategies are proposed
for active catalysts: (1) A proximal imidazole group introduced
in an iron porphyrin assists heterolytic O–O bond cleavage
by the push effect [4]; (2) substituents of meso-aryl groups
greatly affect the reactivity. For example, pentafluorophenyl and
2,6-dichlorophenyl groups significantly improve the turnover
We report here a new iron(III) porphyrin catalyst 1 that has
one 2-imidazolyl group and three pentafluorophenyl groups at
meso positions (Fig. 1). Complex 1 is expected to form its dimer
2, representing the otherwise difficult-to-obtain 5-coordinated
iron(III) species. We previously reported such complemen-
tary dimers for zinc(II) [6], magnesium(II) [7], cobalt(II), and
cobalt(III) imidazolylporphyrin [8], but not iron(III) porphyrin.
Catalyst 1 shows good activity for oxidation of adamantane
with phenylperacetic acid (PPAA) as the oxidant. We also report
structural studies of 5-coordinated dimer 2 by UV–vis as well as
1H and 19F NMR spectroscopy in conjunction with elucidation
of the structure of the active catalyst.
2. Experimental
2.1. Materials
∗
∗∗
Corresponding author. Tel.: +81 743 72 6113; fax: +81 743 72 6119.
Corresponding author. Present address: Institute of Advanced Energy, Kyoto
All the commercially available chemicals were used directly
unless otherwise described. 1-Methylimidazole and CH2Cl2
were distilled over CaH2. Phenylperacetic acid (PPAA) was
prepared according to the literature method [9]. 2,4,6-Tri-(tert-
butyl)phenol (TBPH) was purified by recrystallization from hot
University, Gokasho, Uji 611 0011, Japan. Tel.: +81 774 38 4581;
fax: +81 774 38 4577.
E-mail addresses: satake@ms.naist.jp (A. Satake), kobuke@ms.naist.jp,
kobuke@iae.kyoto-u.ac.jp (Y. Kobuke).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.12.016

130
Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
Fig. 1. Chloro(imidazolylporphyrinato)iron(III) 1 and its dimer 2.
ethanol. Tetra(pentafluorophenyl)porphyrin [10], tetraphenyl-
porphyrin [11], and tetra(mesityl)porphyrin [12] were prepared
according to the corresponding literatures.
then a porphyrin mixture (82.5 mg) containing predominantly
the target was followed. Since the mixture included not only
the target but also many byproducts having similar polari-
ties, the free base porphyrin was once converted to the zinc
form which was less polar than the byproducts due to com-
plementary coordination [7]. A methanol solution saturated
with zinc acetate dihydrate (2.0 mL) was added to the chlo-
roform solution (50 mL) of the material and stirred at rt for
5 h. The reaction mixture was washed with saturated aqueous
NaHCO3 solution and brine, dried over anhydrous Na2SO4, and
evaporated under reduced pressure. This residue was purified
by silica gel column chromatography eluting with benzene to
give a zinc porphyrin mixture (66.6 mg). The methanol solu-
tion (2.0 mL) of 35% aqueous HCl (0.5 mL) was added to the
chloroform solution (50 mL) of the zinc porphyrin, and stirred
at rt for 5 h. The reaction mixture was washed with saturated
aqueous NaHCO3 solution and brine, dried over anhydrous
Na2SO4, and evaporated under reduced pressure. The residue
was purified by silica gel column chromatography eluting with
chloroform/acetone (9/1) to give pure free base porphyrin as pur-
ple solids (61.5 mg, 0.0692 mmol, 8.1%). 1H NMR (600 MHz,
CDCl3) δ −2.91 (s, 2H, NH), 3.47 (s, 3H, imidazole-Me), 7.52
(d, J = 1.2 Hz, 1H, imidazole), 7.70 (d, 1H, J = 1.2 Hz, imida-
zole), 8.88 (br, 2H, pyrrole), 8.91 (br, 4H, pyrrole), 8.94 (br,
2H, pyrrole); 19F NMR (564 MHz, CDCl3) δ −162.43 (ddd,
J = 24.3, 20.9, 7.3 Hz, 2F, phenyl-m), −162.27 (ddd, J = 24.3,
24.3, 10.2 Hz, 1F, phenyl-m), −162.07 (ddd, J = 24.3, 24.3,
10.2 Hz, 1F, phenyl-m), −161.93 (ddd, J = 24.3, 20.9, 7.3 Hz,
2F, phenyl-m), −152.20 (td, J = 20.9, 10.7 Hz, 2F, phenyl-p),
−152.16 (td, J = 24.3, 7.3 Hz, 1F, phenyl-p), −137.60 (dd,
J = 24.3, 10.7 Hz, 2F, phenyl-o), −137.51 (dd, J = 24.3, 7.3 Hz,
1F, phenyl-o), −137.04 (dd, J = 24.3, 10.7 Hz, 2F, phenyl-o),
−137.03 (dd, J = 24.3, 7.3 Hz, 1F, phenyl-o); MALDI-TOF MS
m/z 889.6 (M + H+), Calcd for C42H15F15N6, 888.1; HRMS m/z
889.1198 (M + H+), Calcd for C42H16F15N6, 889.1197; UV–vis
(CHCl3) λmax (Abs. ratio) 415(1), 507(0.074), 585(0.024),
637(0.0029) nm; fluorescence (λEx 415 nm, CHCl3) λEm 641,
710 nm.
2.2. Instruments
1H and 19F NMR spectra were measured on a JEOL JNM-
ECP 600 spectrometer in CDCl3 or acetonitrile-d3. 1H chemical
shifts were referenced to TMS as the internal standard in the
case of CDCl3 solution, and to the residual protons (1.96 ppm)
in the case of acetonitrile-d3 solution. 19F chemical shifts were
referenced to trifluoroacetic acid (−76.5 ppm) as the external
standard. UV–vis spectra were measured by a Shimadzu UV-
3000 PC spectrometer. Fluorescence spectra were recorded on a
Hitachi F-4500 spectrometer. MALDI-TOF mass spectra were
measured on a Perseptive Biosystems Voyager DE-STR or Shi-
madzu AXIMA with dithranol as a matrix. High-resolution mass
spectra (FAB method, m-NBA as a matrix) were measured on
a JEOL MStation. TLC was operated on glass plates coated
with 60 F254 (Merck) silica gel. Column chromatography was
undertaken using a column packed with silica gel 60 N (Kanto
Chemical, spherical, neutral, 63–210 ␮m). Gas chromatography
was carried out on a Shimadzu GC-14B gas chromatograph with
an FID detector using a 0.25 mm × 30 m dimethylpolysiloxane
capillary column (DB-1, J&W Scientific).
2.3. Porphyrin synthesis
2.3.1. 5-(1-Methyl-2-imidazolyl)-10,15,20-
tris(pentafluorophenyl)porphyrin,
ImTPFPPH2
1-Methyl-2-imidazolecarboxaldehyde (93.6 mg, 0.85 mmol,
1.0 eq.) and pentafluorobenzaldehyde (500 mg, 2.55 mmol,
3.0 eq.) were dissolved in refluxing propionic acid (50 mL).
Pyrrole (228 mg, 3.40 mmol, 4.0 eq.) was then added quickly
to the boiling solution. The mixture was heated under reflux
for 4 h. The solvent of the reaction mixture was removed by
distillation under reduced pressure. The ethyl acetate solution
(100 mL) of the residue was washed with saturated aqueous
NaHCO3 solution and brine, dried over anhydrous Na2SO4, and
evaporated under reduced pressure. The crude material included
tetraarylporphyrin, target monoimidazolylporphyrin, and other
multi-imidazolylporphyrins. The residue was purified by silica
gel column chromatography eluting with chloroform/acetone
(10/1). The least polar tetraarylporphyrin was eluted first, and
2.3.2.
5-(1-Methyl-2-imidazolyl)-10,15,20-triphenylporphyrin,
ImTPPH2
1-Methyl-2-imidazolecarboxaldehyde (5.0 g, 45.4 mmol,
1.0 eq.) and benzaldehyde (16.9 g, 159 mmol, 3.5 eq.) were dis-
solved in refluxing propionic acid (700 mL). A solution of

Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
131
pyrrole (13.7 g, 204 mmol, 4.5 eq.) in propionic acid (300 mL)
2.3.4. Chloro[5-(1-methyl-2-imidazolyl)-10,15,20-
tripentafluorophenylporphyrinato]iron(III),
ImTPFPP–Fe(III)Cl
was then added slowly to the solution under reflux. After the
addition, the reaction mixture was refluxed for further 30 min.
Most of the solvent of reaction mixture was removed with
distillation under reduced pressure. The ethyl acetate solution
(300 mL) of the residue was washed with saturated aqueous
NaHCO3 solution and brine, dried over anhydrous Na2SO4, and
evaporated under reduced pressure. The residue was purified
by silica gel column chromatography eluting with chloro-
form/acetone (9/1) to give a purple solids (1.55 g, 2.51 mmol,
5.5%). 1H NMR (600 MHz, CDCl3) δ −2.77 (s, 1H, NH),
3.44 (s, 3H, imidazole-Me), 7.47 (d, J = 1.8 Hz, 1H, imidazole),
7.67 (d, J = 1.8 Hz, 1H, imidazole), 7.73–7.81 (m, 9H, phenyl),
8.15–8.26 (m, 6H, phenyl), 8.78 (d, J = 4.2 Hz, 2H, pyrrole), 8.84
(s, 4H, pyrrole), 8.90 (d, J = 4.2 Hz, 2H, pyrrole); MALDI-TOF
MS m/z 619.2 (M + H+), Calcd for C42H30N6, 618.3; HRMS
m/z 618.2534 (M+), Calcd for C42H30N6, 618.2532; UV–vis
(CHCl3) λmax (Abs. ratio) 418(1), 515(0.054), 550(0.019),
588(0.018), 644(0.0086) nm; fluorescence (λEx 418 nm, CHCl3)
λEm 647, 711 nm.
ImTPFPPH2 (61.5 mg, 0.0692 mmol, 1.0 eq.) and excess
iron(II) chloride tetrahydrate (138 mg, 0.692 mmol, 10 eq.)
were dissolved in acetonitrile (20 mL) at rt. The mixture
was heated under reflux for 5 h. The solvent was evaporated
under reduced pressure. The chloroform solution (50 mL) of
the residue was washed with 1 M HCl and brine, dried over
anhydrous Na2SO4, and evaporated under reduced pressure.
The product was purified by reprecipitation with chloro-
form/hexane. The pure compound was obtained as dark
green solids (53.3 mg, 0.055 mmol, 79%). MALDI-TOF MS
m/z 942.7(M+), 978.2 (M + Cl + H+), 1885.0 (2M + H+), Calcd
for C42H13F15FeN6, 942.0; HRMS m/z 943.0394 (M + H+),
Calcd for C42H14F15FeN6, 943.0390; UV–vis (acetonitrile)
λmax nm (M−1 cm−1) 395 (65,900), 505 (8800), 632 (4300),
UV–vis (acetonitrile + 70 eq. 1MeIm) λmax nm (M−1 cm−1) 411
(124,000), 542 (8500), UV–vis (CHCl3) λmax (Abs. ratio)
357(0.55), 413(1), 505(0.12), 626(0.053) nm.
2.3.5. Chloro[5-(1-methyl-2-imidazolyl)-10,15,20-
trisphenylporphyrinato]iron(III),
2.3.3.
5-(1-Methyl-2-imidazolyl)-10,15,20-trimesitylporphyrin,
ImTMPH2
ImTPP–Fe(III)Cl
Reaction conditions were almost same as ImTPFPP–
Fe(III)Cl except for CHCl3 as solvent instead of acetonitrile.
From ImTPPH2 (200 mg, 0.323 mmol, 1.0 eq.), the title com-
pound was obtained as black solids (194 mg, 0.274 mmol, 84%).
1H NMR (600 MHz, CDCl3) δ −5.88 (br-s, 3H, phenyl), 6.71
(br-s, 2H, phenyl), 6.92 (br-s, 3H, imidazole-Me), 7.11 (br-s, 1H,
phenyl), 8.85 (br-s, 3H, phenyl), 11.00 (br-s, 3H, phenyl), 11.93
(br-s, 1H, imidazole), 12.23 (br-s, 3H, phenyl), 14.37 (br-s, 1H,
imidazole), 76.83 (br-s, 2H, pyrrole), 77.88 (br-s, 2H, pyrrole),
82.07 (br-s, 4H, pyrrole); MALDI-TOF MS m/z 672.5 (M+),
708.0 (M + Cl + H+), 1345.6 (2M + H+) Calcd for C42H28FeN6,
672.2; HRMS m/z 673.1795 (M + H+), Calcd for C42H29FeN6,
673.1804; UV–vis (CHCl3) λmax (Abs. ratio) 379(0.54), 417(1),
510(0.11), 580(0.025) nm.
Since the title compound was scarcely obtained by Adler
method using benzaldehyde and imidazolecarboxaldehyde in
propionic acid, modified dipyrromethane method was applied.
Mesityldipyrromethane [13] (528 mg, 2.0 mmol, 2.0 eq.) and
mesitaldehyde (148 mg, 1.0 mmol, 1.0 eq.) were dissolved in
chloroform (100 mL). BF3 OEt2 (260 mg, 2.0 mmol, 2.0 eq.)
was added at rt and stirred for 1 h. At this stage, tetrapyrrol
was detected by MALDI-TOF mass. This reaction mixture was
passed through a column filled with solid NaHCO3 eluting with
chloroform (100 mL) to neutralize the acid. To the eluent, 1-
methyl-2-imidazolecarboxaldehyde (110 mg, 1.0 mmol, 1.0 eq.)
and trifluoroacetic acid (228 mg, 2.0 mmol, 2.0 eq.) was added,
and the mixture was stirred for 5 h. The mixture was washed
with saturated aqueous NaHCO3 solution and brine, dried over
anhydrous Na2SO4, and evaporated under reduced pressure. The
residue was roughly purified by silica gel column chromatogra-
phy eluting with chloroform/acetone (9/1) to give a porphyrin
mixture (41.2 mg) containing predominantly the target. For fur-
ther purification, the mixture was converted to zinc porphyrin
similarly to the case of ImTPFPPH2. After purification, the zinc
porphyrin (31 mg) was demetalated to give the title compound
2.3.6. Chloro[5-(1-methyl-2-imidazolyl)-10,15,20-
trismesitylporphyrinato]iron(III),
ImTMP–Fe(III)Cl
Reaction
conditions
were
almost
same
as
ImTPFPP–Fe(III)Cl except for toluene for reaction solvent,
and diethylether/hexane for reprecipitation. From ImTMPH2
(20 mg, 0.0268 mmol, 1.0 eq.), the title compound was obtained
1
1
ImTMPH2 as purple solids (27 mg, 0.0362 mmol, 3.6%). H
as light brown solids (18.8 mg, 0.0225 mmol, 84%). H NMR
NMR (600 MHz, CDCl3) δ −2.56 (s, 1H, NH), 1.79 (s, 6H,
mesityl-Me), 1.84 (s, 3H, mesityl-Me), 1.89 (s, 6H, mesityl-
Me), 1.93 (s, 3H, mesityl-Me), 2.62 (s, 9H, mesityl-Me), 3.47
(s, 3H, imidazole-Me), 7.23–7.26 (m, 2H, mesityl), 7.27–7.29
(m, 4H, mesityl), 7.45 (s, 1H, imidazole), 7.64 (s, 1H, imida-
zole), 8.63(br-s, 2H, pyrrole), 8.65(br-s, 2H, pyrrole), 8.72(br-s,
2H, pyrrole); MALDI-TOF MS m/z 745.2 (M + H+), Calcd for
(C51H48N6) 744.4; UV–vis (CHCl3) λmax (Abs. ratio) 420(1),
516(0.052), 550(0.016), 589(0.018), 645(0.0078) nm; fluores-
cence (λEx 420 nm, CHCl3) λEm 647, 714 nm.
(600 MHz, CDCl3) δ −3.16 (br-s, 9H, mesityl-Me), 3.77 (s,
3H, imidazole-Me), 3.88 (br-s, 9H, mesityl-Me), 6.16 (br-s, 9H,
mesityl-Me), 11.53–15.08 (m, 8H, mesityl-Me, imidazole), 3.16
(br-s, 9H, mesityl-Me), 75.88 (br-s, 2H, pyrrole), 77.17 (br-s,
2H, pyrrole), 81.00 (br-s, 2H, pyrrole), 81.64 (br-s, 2H, pyrrole);
MALDI-TOF MS m/z 798.4 (M+), 834.3 (M + Cl + H+), 1668.7
(2M + H+), Calcd for C51H46FeN6, 798.3; HRMS m/z 799.3218
(M + H+), Calcd for C51H47FeN6, 799.3213; UV–vis (CHCl3)
λmax (Abs. ratio) 377(0.52), 418(1), 508(0.12), 662(0.027)
nm.

132
Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
2.3.7. Perchloro[5-(1-methyl-2-imidazolyl)-10,15,20-
triphenylporphyrinato]iron(III),
ImTPP–Fe(III)ClO4
CAUTION! Although we encountered no problems, care
should be taken when using the potentially explosive perchlorate
compounds.
ImTPP–Fe(III)Cl (164 mg, 0.232 mmol, 1.0 eq.) was dis-
solved in THF (20 mL). AgClO4 (96.2 mg, 0.464 mmol, 2.0 eq.)
was added to the solution, and the mixture was stirred at rt
for 1 h. Precipitates were filtered on membrane filter (0.1 ␮m,
polyvinylidene fluoride), and the filtrate was evaporated. The
residue was purified by reprecipitation with hot toluene to give
red black solids (199 mg, 0.258 mmol, 95%). MALDI-TOF MS
m/z 672.2 (M+), 1345.2 (2M + H+), Calcd for C42H28FeN6,
672.2; UV–vis (CHCl3) λmax (Abs. ratio) 400(1), 523(0.081),
654(0.014) nm.
(0.10 mM), TBPH (20 mM), PPAA (1.0 mM), diethyleneglycol
diethyl ether (0.50 mM) in CH2Cl2 (1 mL). The mixture was
stirred at rt. After 20 min, the reaction was quenched by adding
diphenyl sulfide (4.0 mM in CH2Cl2, 0.5 mL, 2.0 ␮mol), and the
mixture was stirred for 5 min. The products were quantified with
referring to the internal standard (diethyleneglycol diethyl ether)
by gas chromatography. The data are listed in Table 1.
2.4.2. Hydroxylation of adamantane with PPAA
Adamantane (27.2 mg, 200 ␮mol) and iron(III) porphyrin
(0.10 ␮mol) was placed in a test tube fitted with a rubber sep-
tum. The inside was replaced with Ar atmosphere. A mixture
(0.2 mL) of PPAA (5.0 mM, 1.0 ␮mol) and diethyleneglycol
diethyl ether (2.50 mM, 0.5 ␮mol) in CH2Cl2 was added. The
final concentration of each component becomes as follows:
iron(III) porphyrin (0.10 mM), adamantane (200 mM), PPAA
(1.0 mM), diethyleneglycol diethyl ether (0.50 mM) in CH2Cl2
(1 mL). The mixture was stirred at rt. After 20 min, the reaction
was quenched by adding diphenyl sulfide (4.0 mM in CH2Cl2,
0.5 mL, 2.0 ␮mol), and the mixture was stirred for 5 min. The
products were quantified with referring to the internal standard
(diethyleneglycol diethyl ether) by gas chromatography. The
data are listed in Table 2.
2.3.8. Perchloro[5-(1-methyl-2-imidazolyl)-10,15,20-
trismesitylporphyrinato]iron(III),
ImTMP–Fe(III)ClO4
In accordance with the procedure for ImTPP–Fe(III)ClO4,
the title compound (11.6 mg, 0.0129 mmol, 90%) was
obtained as red brown solids from ImTMP–Fe(III)Cl (12 mg,
0.0144 mmol). MALDI-TOF MS m/z 798.4 (M+), Calcd for
C51H46FeN6, 798.3; UV–vis (CHCl3) λmax (Abs. ratio) 396(1),
517(0.071), 689(0.0076) nm.
2.5. 1MeIm titration to ImTPFPP–Fe(III)Cl in acetonitrile
2.5.1. UV–vis measurements
ImTPFPP–Fe(III)Cl solution in acetonitrile (2.8 × 10−5 M,
4.0 mL) was placed in a quartz glass cuvette (band path 1 cm),
and 1-methylimidazole (1MeIm) (0–70 eq.) was added to the
solution. Each mixture was stirred for 2 min at rt and UV–vis
spectrum of the solution was measured. The spectra are shown
in Fig. 4.
2.4. Catalytic reaction
2.4.1. Catalytic oxidation of TBPH with PPAA
Iron(III) porphyrin (0.10 ␮mol) was placed in a test tube fitted
with a rubber septum. The inside was replaced with Ar atmo-
sphere. A TBPH solution (25 mM in CH2Cl2, 0.8 mL, 20 ␮mol)
was added to the test tube. Subsequently, a mixture (0.2 mL)
of PPAA (5.0 mM, 1.0 ␮mol) and diethyleneglycol diethyl ether
(2.50 mM, 0.5 ␮mol) in CH2Cl2 was added. The final concentra-
tion of each component becomes as follows: iron(III) porphyrin
2.5.2. NMR measurements
ImTPFPP–Fe(III)Cl
solution
in
acetonitrile-d3
(1.0 × 10−2 M, 0.5 mL) was placed in an NMR tube, and
1MeIm (1, 2, and 5 eq.) was added to the solution. Each
Table 1
TBPH oxidation with PPAA catalysed by iron porphyrinsa
Run
Catalyst
1MeIm (eq.)
Ph2S O (yield, %)
Products (yield, %)
PAA
PhCHO
PhCH2OH
1
2
3
4
5
6b
7
8
9
10
11
12
13
ImTPFPP–Fe(III)Cl
TPFPP–Fe(III)Cl
TPFPP–Fe(III)Cl
ImTPP–Fe(III)Cl
TPP–Fe(III)Cl
0
0
100
0
0
0
100
0
0
100
0
0
0
0
0
0
0
–
0
0
0
0
0
0
0
88
21
90
99
48
9
90
96
55
90
95
64
98
0
6.2
0
0
4.7
–
0
0
4.4
0
4.8
64
1.4
0
33
24
0
1.4
30
1.0
2.5
23
0.5
c
TPP–Fe(III)Cl
TPP–Fe(III)Cl
ImTMP–Fe(III)Cl
TMP–Fe(III)Cl
TMP–Fe(III)Cl
ImTPP–Fe(III)ClO4
TPP–Fe(III)ClO4
TPP–Fe(III)ClO4
1.2
8.7
0.9
100
a
These reactions were carried out in CH2Cl2 at 25 ◦C under argon for 20 min. [Fe(Por)] = 0.10 mM; [PPAA] = 1.0 mM; [TBPH] = 20 mM.
Reported in Ref. 3d, this reaction was carried out in benzene at 25 ◦C under argon for 10 min. [Fe(Por)] = 0.10 mM; [PPAA] = 1.0 mM; [TBPH] = 10 mM.
No benzaldehyde, but toluene (61%) was detected. Yields are determined by GC based on PPAA.
b
c

Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
133
Table 2
Hydroxylation of adamantane with PPAA by iron porphyrin catalysisa
Run
Catalyst
1MeIm (eq.)
Ph2S O (yield, %)
Products (yield, %)
Products (yield, %)
PAA
PhCHO
PhCH2OH
1-ol
2-ol
Total
1
2
3
4
5
6b
7
8
9
10
11
12
13
ImTPFPP–Fe(III)Cl
TPFPP–Fe(III)Cl
TPFPP–Fe(III)Cl
ImTPP–Fe(III)Cl
TPP–Fe(III)Cl
0
0
100
0
0
0
100
0
0
100
0
0
0
95
0
0
0
–
0
0
0
0
0
0
0
88
97
96
67
57
–
91
75
60
88
93
66
91
5.3
0
1.6
0
65
4.9
22
29
13
11
18
19
28
8.5
46
15
12
11
0
4.2
4.2
1.8
2
2.1
4.6
8.6
1.8
10
2.6
1.2
70
4.9
26
33
15
13
20
24
37
10
56
18
13
2.7
8.1
19
13c
2.7
14
0.5
2.4
6.0
c
TPP–Fe(III)Cl
TPP–Fe(III)Cl
–
0.6
5.4
10
0.8
0.6
8.7
0.9
ImTMP–Fe(III)Cl
TMP–Fe(III)Cl
TMP–Fe(III)Cl
ImTPP–Fe(III)ClO4
TPP–Fe(III)ClO4
TPP–Fe(III)ClO4
25
5.8
6.2
21
100
7.3
a
These reactions were carried out in CH2Cl2 at 25 ◦C under argon for 20 min. [Fe(Por)] = 0.10 mM; [PPAA] = 1.0 mM; [adamantane] = 200 mM.
Reported in Ref. 3d, this reaction was carried out in benzene at 25 ◦C under argon for 10 min. [Fe(Por)] = 1.0 mM; [PPAA] = 1.0 mM; [adamantane] = 500 mM.
Sum of benzaldehyde and benzylalcohol; yields are determined by GC based on PPAA.
b
c
mixture was kept standing for 2 min at rt and 1H and 19F
NMR spectra were measured. The spectra are shown in
Figs. 5 and 6. In the absence of 1MeIm: H NMR (600 MHz,
trimethylphenyl)) groups as meso-substituents were newly
prepared as oxidation catalysts. The pentafluorophenyl group
is highly electron-deficient, whereas the mesityl group is
electron donating compared with the phenyl group. Therefore,
the electronic effect of the peripheral aryl groups can be
examined using these three porphyrins. The free base por-
phyrins, ImTPFPPH2 and ImTPPH2, were synthesized by
condensation of imidazolecarboxaldehyde, pyrrole, and the cor-
responding arylaldehyde (Adler’s method). Pure ImTPPH2 was
obtained by SiO2 chromatography. In the case of ImTPFPPH2,
however, a byproduct with the same polarity accompanied the
target free base porphyrin. The pure ImTPFPPH2 was obtained
by converting the free base porphyrin into its zinc complex.
Because the polarity of the complementary coordination dimer
of the zinc complex on TLC becomes much lower than that
of the free base while the byproduct maintains its polarity,
they can be easily separated by SiO2 column chromatography.
Finally, the pure free base, ImTPFPPH2, was obtained by
demetallation with hydrochloric acid solution (Scheme 1).
Synthesis of mesityl porphyrin, ImTMPH2, is more difficult.
An Adler method and a normal dipyrromethane method using
one kind of acid, either BF3 Et2O or trifluoroacetic acid (TFA),
did not give ImTMPH2. Finally, stepwise reactions using two
kinds of acid succeeded in giving the ImTMPH2. Mesityl
dipyrromethane (2 eq.) and mesitylaldehyde (1 eq.) were reacted
in the presence of BF3 Et2O first to give a non-cyclic tetrapyrrole
1
acetonitrile-d3) δ −5.4 (br-s, 3H, Im-Me), 10.5 (s, 1H, Im-H),
11.0 (s, 1H, Im-H), 80.7 (br-s, 2H, pyrrole), 81.6 (br-s, 2H,
pyrrole), 82.8 (br-s, 2H, pyrrole), 83.1 (br-s, 2H, pyrrole); 19F
NMR (564 MHz, acetonitrile-d3) δ −160.8 (m, 6F, phenyl-m),
−154.3 (s, 1F, phenyl-p), −154.1 (s, 2F, phenyl-p), −127.6
1
(br-s, 6F, phenyl-o). After adding 5 eq. of 1MeIm: H NMR
(600 MHz, acetonitrile-d3) δ −19.1 (s, 2H, pyrrole), −15.4 (s,
2H, pyrrole), −14.4 (s, 2H, pyrrole), −8.0 (br-s, 2H, pyrrole),
19.8 (s, 3H, 1MeIm-Me), 20.0 (s, 3H, 1MeIm-Me); 19F NMR
(564 MHz, acetonitrile-d3) δ −165.98 (m, 2F, phenyl-m),
−165.63 (dd, J = 18.6, 18.6 Hz, 2F, phenyl-m), −165.43 (dd,
J = 18.6, 18.6 Hz, 2F, phenyl-m), −156.95 (t, J = 18.6 Hz, 1F,
phenyl-p), −156.50 (t, J = 18.6 Hz, 2F, phenyl-p), −143.59
(d, J = 18.6 Hz, 1F, phenyl-o), −142.47 (d, J = 18.6 Hz, 1F,
phenyl-o), −142.19 (d, J = 18.6 Hz, 2F, phenyl-o), −141.91 (d,
J = 18.6 Hz, 2F, phenyl-o).
3. Results
3.1. Synthesis of iron(III) porphyrins
Three chloro(imidazolylporphyrinato)iron(III)s having
tripentafluorophenyl, triphenyl, and trimesityl (tri(2,4,6-
Scheme 1. Preparation of pure ImTPFPPH2.

134
Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
Scheme 2. Preparation of ImTMPH2.
that had three mesityl groups. Formation of the tetrapyrrole was
monitored by MALDI-TOF mass spectrometry. When the for-
mation was almost complete, the mixture was passed through
a column packed with NaHCO3 to remove BF3 Et2O. Then,
addition of imidazolecarboxaldehyde and TFA to the mixture
followed by oxidation with chloranil gave ImTMPH2. It is note-
worthy that mesitylaldehyde was activated by BF3 Et2O but
not by TFA, whereas BF3 Et2O deactivates imidazolecarbox-
aldehyde by precipitate formation. Replacing BF3 Et2O with
TFA is essential to synthesize ImTMPH2. Pure ImTMPH2 was
obtained by a similar method to that of ImTPFPPH2 via the zinc
complex (Scheme 2).
Iron(III) complexes of the free base imidazolylporphyrins
were obtained by treatment with FeCl2. The corresponding
chloro(tetraarylporphyrinato)iron(III)s were also prepared as
reference compounds for the oxidation catalyst. Iron(III) per-
chlorate porphyrins were prepared from the corresponding
chloride complexes by treatment with AgClO4 (Fig. 2).
3.2. Heterolytic O–O bond cleavage of peroxy acid
Phenylperacetic acid (PPAA) as a ligand of iron(III) por-
phyrin is a sensitive probe to evaluate the pathway leading to
either heterolytic or homolytic cleavage [3c,3d] (Scheme 3). It
is transformed into phenylacetic acid (PAA) via a heterolytic
O–O bond cleavage, whereas benzyl alcohol is produced via
homolytic cleavage. Benzyl alcohol is sometimes oxidized fur-
ther to benzaldehyde. Therefore, the ratio of PAA to the sum
of benzyl alcohol and benzaldehyde becomes a measure of the
reaction pathways leading to either heterolytic or homolytic
cleavage, respectively. High-valent iron porphyrin species, com-
pounds I and II, formed via heterolytic and homolytic cleavages,
respectively, are known to be reduced by 2,4,6-tri-t-butylphenol
(TBPH) to regenerate the starting iron(III) porphyrin. Therefore,
Fig. 2. Structures of porphyrins.
the reaction was undertaken in the presence of a large excess of
TBPH.
The reaction was carried out in the presence of iron(III)
porphyrin (0.10 mM), PPAA (1.0 mM), and TBPH (20 mM)
in CH2Cl2 under an Ar atmosphere. After 20 min, unreacted
PPAA was quenched with diphenyl sulfide (Ph2S) to determine
the consumption of PPAA. The amounts of diphenylsulfox-
ide (Ph2S O), PAA, benzaldehyde, and benzyl alcohol were
determined by gas chromatography. The yields based on PPAA
are shown in Table 1. Three iron(III) imidazolylporphyrin
Scheme 3. Reaction pathway of PPAA and TBPH in the presence of iron(III) porphyrin.

Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
135
chlorides having tri(pentafluorophenyl) (ImTPFPP–Fe(III)Cl,
run 1), triphenyl (ImTPP–Fe(III)Cl, run 4), and trimesityl
(ImTMP–Fe(III)Cl, run 8) substituents were employed. The
corresponding tetraaryl iron(III) porphyrins without the imida-
zolyl group were used in the absence (runs 2, 5, and 9) and
presence of 100 eq. of N-methylimidazole (runs 3, 7, and 10) for
comparison. Results for the perchlorate complexes of phenyl-
substituted porphyrins are listed in runs 11–13.
Because no diphenylsulfoxide was detected after 20 min for
all runs, PPAA was consumed quantitatively within the initial
20 min. Three iron(III) imidazolylporphyrin chlorides gave PAA
predominantly (88% for ImTPFPP (run 1), quantitative for
ImTPP (run 4), 96% for ImTMP (run 8)) whereas significant
amounts of benzyl alcohol and benzaldehyde were produced
for cases of non-imidazolylporphyrins (runs 2, 5, and 9). These
results indicate that heterolytic cleavage predominates for the
former cases, and that considerable homolytic cleavage occurred
in the latter cases. When 100 eq. of N-methylimidazole were
added to the non-imidazolylporphyrins (runs 3, 7, and 10), the
PAA formation was increased to 90% for all three cases, clearly
suggesting that imidazole assists heterolytic cleavage of the
O O bond. Exchange of the counter ion by perchlorate ion has
a minimal effect and maintains the product percentages of the
chloride ion cases (runs 11–13 in comparison with runs 4, 5 and
7).
active as the oxidant of adamantane, and the minimal imidazolyl
ligand attached to the porphyrin is effective as the electron-
deficient system to make this system the best among those
investigated. When ImTPP–Fe(III)Cl and ImTMP–Fe(III)Cl
are used, smaller percentages of heterolytic cleavage lead to
adamantane oxidation (runs 4 and 8). In contrast to the tetraaryl
cases (runs 2, 5, and 9), yields of adamantanols decrease by
increased electron donation from aryl substituents (run 4 vs.
run 8). These results suggest that the coordination from the
imidazole substituent becomes less significant for cases where
electron density of the central iron(III) is increased by the push
effect from the peripheral aryl groups. The favourable effect of
imidazole substitution is lost for the TMP cases (run 8 vs. run
9), where the peripheral mesityl substituents have the dominant
effect of electron donation to the central iron(III). When perchlo-
rate ion was used as the counter ion instead of chloride ion for
ImTPP, the conversion increased to 56% from 33%, suggesting
that iron(III) porphyrin of cationic nature is a better catalyst (run
11 vs. run 4). Addition of external N-methylimidazole (100 eq.)
to any type of iron(III) tetraarylporphyrin does not improve the
oxidation of adamantane very much, even though heterolytic
cleavage becomes dominant. Apparently, other pathways make
significant contributions to the consumption of compound I in
the presence of excess imidazole.
In the following sections, we will analyse the structure of
ImTPFPP–Fe(III)Cl in detail using UV–vis and NMR spectra.
3.3. Catalytic oxidation of adamantane with PPAA
3.4. UV–vis study of ImTPFPP–Fe(III)Cl
Oxidation of adamantane is also a sensitive probe to evaluate
the characteristics of compound I formed by heterolytic O O
bond cleavage. Therefore, a mixture of adamantane (200 mM),
iron(III) porphyrin (0.10 mM), and PPAA (1.0 mM) in CH2Cl2
was stirred under an Ar atmosphere. After 20 min, diphenyl sul-
fide was added to quench the remaining PPAA. Products were
analysed by gas chromatography. All of the data are listed in
Table 2. The most striking difference from the data of Table 1
was observed for run 2, where diphenylsulfoxide was detected
almost quantitatively, suggesting almost no reaction of PPAA,
even after 20 min. This result is consistent with the observation
that O O bond cleavage of the peroxo complex of electron-
deficient PTFPP–Fe(III) is sluggish in the absence of imidazole
[14], and the direct hydroxylation of adamantane with the per-
oxo complex is also slow, unlike epoxidation of alkenes [3e].
Increasing the electron-donating character of the porphyrin lig-
and promotes O O bond cleavage by the “push effect”. Thus,
the yields of adamantanols increase in the order of electron-
donating character of the peripheral aryl groups (TPP–FeCl (run
5) < TMP–FeCl (run 9)). In the presence of an external imida-
zole ligand with TPFPP–FeCl, PPAA was consumed completely
within 20 min, predominantly through a heterolytic cleavage
mechanism (run 3). This is consistent with previous reports
claiming the importance of the push effect by a proximal ligand
[3d,14]. Although the heterolytic cleavage was dominant in run
3, conversion of adamantane oxidation was only 26%. In the case
ofImTPFPP–Fe(III)Cl(run1), theyieldofheterolyticcleavage
corresponds almost to the conversion of adamantane oxida-
tion. This result indicates that most of compound I in run 1 is
UV–vis spectra of ImTPFPP–Fe(III)Cl and TPFPP–
Fe(III)Cl in acetonitrile are shown in Fig. 3. The latter spec-
trum is typical of Fe(III)Cl porphyrin, which has an LMCT
band at 350 nm and a Soret band at 408 nm. The spec-
trum of ImTPFPP–Fe(III)Cl, however, shows a broad Soret
band at 395 nm without showing an LMCT band. The char-
acteristic broad Soret band is similar to those observed for
complementary dimers of Co(III) imidazolylporphyrin [8] and
Rh(III) pyridylporphyrin [15]. To confirm the formation of
the dimeric structure, N-methylimidazole was titrated into the
solution (Fig. 4). The broad Soret band was red-shifted and
Fig. 3. UV–vis spectra of ImTPFPP–Fe(III)Cl (solid line) and
TPFPP–Fe(III)Cl (doted line) in acetonitrile at rt. Cell length = 1 cm.

136
Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
The characteristic UV–vis spectrum of the dimer from
ImTPFPP–Fe(III)Cl was observed only in acetonitrile (ε
(dielectric constant [19]) 36.64). In dioxane (ε 2.21), benzene (ε
2.28), chloroform (ε 4.81), ethyl acetate (ε 6.08), THF (ε 7.52),
acetone (ε 21.0), and benzonitrile (ε 25.9), spectral patterns were
similar to that of TPFPP–Fe(III)Cl and regarded as represent-
ing predominantly the monomeric chloro complex. In methanol
(ε 32.35) and DMF (ε 37.06), monomeric 6-coordinated com-
plexes coordinated by the two solvent molecules were observed.
In addition, ImTPP–Fe(III)Cl and ImTMP–Fe(III)Cl exist as
monomeric forms, even in acetonitrile. Aryl substituents and
solvent strongly affect the dimer formation tendency. This will
be discussed later.
3.5. 1H and 19F NMR study of ImTPFPP–Fe(III)Cl
Fig. 4. UV–vis spectral change of ImTPFPP–Fe(III)Cl dimer to monomer on
the addition of 1MeIm in acetonitrile (ImTPFPP–Fe(III)Cl: 2.8 × 10−5 M,
1MeIm: from 0 to 70 eq.).
1H and 19F NMR spectra of ImTPFPP–Fe(III)Cl in CD3CN
(10 mM as a monomeric unit) are shown in Figs. 5 and 6. In the
1H NMR spectra, characteristic ␤ proton signals appeared in
the region 90–70 ppm, which correspond to the high-spin state
of Fe(III) porphyrin [20]. Imidazolyltriarylporphyrin generally
shows three kinds of ␤ protons: those nearest to the imidazolyl
group, the next nearest, and the others, in a 1:1:2 ratio. There-
fore, Fig. 5a seems to include two kinds of species, three major
peaks at 83.0, 81.6, and 80.7 ppm (marked as x) and minor
peaks at 75.8 and 73.8 ppm (filled circles) (the other is prob-
ably overlapped with the major peaks). In the 19F NMR spectra,
the two species are separated more clearly. The major peaks
appear around −128, −154.2, and 161 ppm, and the minor ones
around −108, −155, and −162 ppm. Judging from the charac-
teristic ortho fluorine signal at −108 ppm, the latter is assigned
as the peak of monomeric Fe(III)Cl–porphyrin. Therefore, the
former peak is assigned temporarily as the 5-coordinating dimer.
When 1 eq. of 1-methylimidazole was added to the mixture, the
major signals disappeared; however, the minor ones remained in
both the 1H and the 19F NMR spectra (Figs. 5b and 6b). Instead
of the major peaks, peaks due to low-spin species appeared at
sharpened, and the Q-bands also changed, passing through
isosbestic points. The converged spectrum was almost identi-
cal with the 6-coordinating complex prepared by the addition
of N-methylimidazole to TPFPP–Fe(III)Cl in acetonitrile.
These results suggest that the original acetonitrile solution of
ImTPFPP–Fe(III)Cl is composed predominantly of a comple-
mentary 5-coordinating dimer.
The
original
spectrum
of
ImTPFPP–Fe(III)Cl
(2.8 × 10−5 M in acetonitrile) converged to the final spec-
trum after the addition of 70 eq. of N-methylimidazole. If
we assume the following equilibria and complete dimer
formation at the initial state [16], the self-association constant
of ImTPFPP–Fe(III)Cl, K0, is estimated from competitive
coordination experiments
[P2]
[P]2
2P ꢀ P2
K0 =
(1)
(2)
[PL2]
[P][L]2
P + 2L ꢀ PL2
Ka =
1
−8.0, −14.4, −15.4, and −19.1 ppm (␤-Hs) in the H NMR
[PL2]2
[P2][L]4
spectra and −141.9 (o), −142.2 (o), −142.5 (o), −143.6 (o),
−156.5 (p), −157.0 (p), and −165.4 (m), −165.6 (m), −166.0
(m) in the 19F NMR spectra. All of the signals were con-
verted finally to a new species (open circles) after the addition
of 5 eq. of 1-methylimidazole. The new species is assigned
as a monomeric 6-coordinated bisimidazole adduct. 19F NMR
spectra also show the conversion of high-spin species into the
bisimidazole adduct. All NMR and UV–vis absorption spectra
and titration experiments with 1-methylimidazole consistently
lead to the conclusion that ImTPFPP–Fe(III)Cl exists as a pre-
dominantly high-spin dimer in acetonitrile, as shown in Fig. 1.
P2 + 4L ꢀ 2PL2
Kb =
(3)
(1)
Ka2
K0 =
Kb
where P, P2, L, and PL2 are the monomeric chloride complex,
dimer, imidazole, and 6-coordinated bisimidazole complex,
respectively.
Kaꢀ (9.12 × 107 M−2) obtained by the titration of N-
methylimidazole into TPFPP–Fe(III)Cl may represent the
approximate value of Ka [17]. Kb was obtained as
5.75 × 109 M−3 from the above titration experiment of
ImTPFPP–Fe(III)Cl. Then, K0 was calculated to be
1.45 × 106 M−1. This is much smaller than the extremely
large equilibrium constant of zinc imidazolylporphyrins
4. Discussion
4.1. Formation of 5-coordinating Fe(III) porphyrin complex
(K0 > 1011
M
−1) [18], but large enough to maintain the dimeric
structure in dilute micromolar solutions. Probably, charge repul-
sion between the two cationic Fe(III) porphyrins in the dimer
contributes to the decrease in the self-association constant.
Iron(III) porphyrin pentacoordinated with an imidazole
derivative has been claimed to be an active form of various
cytochromes for performing enzymatic reactions [21]. How-

Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
137
Fig. 5. 1H NMR (600 MHz) spectral change of ImTPFPP–Fe(III)Cl: 1.0 × 10−2 M taken in acetonitrile-d3 solution at the addition of various amounts of
1MeIm: (a) 0.0 eq.; (b) 1.0 eq.; (c) 2.0 eq.; (d) 5.0 eq. Marks x, filled circle, and open circle were assigned as 5-coordinating ImTPFPP–Fe(III) porphyrin dimer,
ImTPFPP–Fe(III)Cl, and 6-coordinated bisimidazole adduct of ImTPFPP–Fe(III) porphyrin, respectively.
ever, the 5-coordinated complex was obtainable under restricted
conditions of controlled supply of ligands from environmental
peptide side chains, as in enzymatic systems. Synthesis of such a
complex is not easy in homogeneous media because the associa-
tion constant of the sixth coordination of imidazole is larger than
that of the fifth coordination. Even the addition of 1 eq. of imida-
zole to chloro(porphyrinatoiron(III)) produces a 1:1 mixture of
6-coordinated bisimidazole adduct and starting chloro complex.
Nakamura reported selective synthesis of mono(imidazole)-
ligated (meso-tetramesitylporphyrinato)iron(III) using a weakly
coordinating anion such as perchlorate ion (ClO4−) instead of
chloride ion. In their method, bulky substituents on imidazole
and the peripheral four aryl groups are essential [22]. As a coor-
dination ligand, 4,5-dichloroimidazole and 2-alkylimidazoles,
such as 2-(i-propyl)imidazole and 2-ethylimidazole, were effec-
tive, but too bulky 2-(tert-butyl)imidazole did not give the
corresponding 5-coordinated complex. In our present case, 5-
coordinated iron(III) porphyrin was derived from the chloride
form, and the bulky mesityl group was rather catalytically
ineffective. Therefore, the method suggests a new strategy
for constructing a catalytically active 5-coordinated complex
by dimerization. Because acetonitrile and pentafluorophenyl
groups are essential to obtain the 5-coordinated dimer, synthetic
conditions for the dimer formation are still limited. A delicate
balance between the affinity of counter anion and imidazole
substituent is still important as the environmental conditions.
Probably, polar acetonitrile effectively solvates the chloride ion,
but does not coordinate the cationic iron(III) porphyrin. The
more coordinating methanol and DMF act as competing lig-
ands towards imidazole in the presence of a large excess and
prevent the dimerization. It is noted that it is not essential for
the catalytic reaction to keep the 5-coordinated dimer struc-
ture as the dominant species. Although no dimer structure was
observed in CH2Cl2 by UV–vis spectroscopy, the effect of the
Fig. 6. 19F NMR (564 MHz) spectral change of ImTPFPP–Fe(III)Cl: 1.0 × 10−2 M taken in acetonitrile-d3 solution on the addition of various amounts of
1MeIm: (a) 0.0 eq.; (b) 1.0 eq.; (c) 2.0 eq.; (d) 5.0 eq. Marks x, filled circle, and open circle were assigned as 5-coordinating ImTPFPP–Fe(III) porphyrin dimer,
ImTPFPP–Fe(III)Cl, and 6-coordinated bisimidazole adduct of ImTPFPP–Fe(III) porphyrin, respectively.

138
Y. Miyazaki et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 129–139
imidazolyl group was clearly operative in the catalytic oxida-
tion in CH2Cl2. Unfortunately, catalytic oxidation of TBPH and
adamantane could not be undertaken in acetonitrile because of
the low solubility of the substrates.
which hydroxylation of adamantane was performed. This result
indicates that it is not very important to keep the dimer structure
in the whole catalytic cycle, and ligation of the transient species
in the catalytic cycle is active for the reaction. Complementary
coordination is concluded to be a unique method to perform a
single imidazolyl ligation to iron(III) porphyrin.
4.2. Activity of catalyst
An efficient push effect by the proximal imidazole and thio-
late ligands has been claimed to be important in heterolytic O O
bond cleavage reactions [23]. In fact, the addition of 100 eq. of
external N-methylimidazole improves yields of PAA, as shown
in Tables 1 and 2. However, yields of adamantanols were inferior
to those of heterolytic cleavage. This suggests that excess imi-
dazole inhibits the oxidation reaction somewhere in the catalytic
cycle. A great advantage of the use of imidazolyl-appended
iron(III)porphyrinresultsfromthepointthatexactly1 eq. ofimi-
dazole is enough to coordinate because of its strong coordination
by complementarity. Therefore, high percentages of consumed
PPAA successfully oxidized adamantane. Imidazole effect was
reported by Nam in catalytic epoxydation of cyclohexene with
H2O2 in the presence of (meso-tetraarylporphyrinato)iron(III)
[24]. The yields of oxidized product depend strongly on the
electron-donating ability of the added imidazoles, and 5-chloro-
1-methylimidazole showed the best performance. Considering
with Nakamura’s report [21], both electron-donating ability and
bulkiness of imidazole are important factors. Performance of
catalytic oxidation in our system may be modified further by
introduction of a substituent on the imidazolyl part.
Acknowledgements
This work was supported by Young Scientists (B) (17750091)
(AS) and Grants-in-Aid for Scientific Research (A) (YK) from
the Japan Society for the Promotion of Science (JSPS).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.molcata.2007.12.016.
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Halogenated catalysts are known as oxidation-resistant mate-
rials [5]. Turnover numbers in the catalytic oxidation are
generally larger than for the corresponding non-fluorinated cata-
lysts. Furthermore, ImTPFFP–Fe(III)Cl can be prepared easily
from commercially available products. This is another advan-
tage from the synthetic viewpoint. These considerations suggest
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5. Conclusion
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Using the probe reaction of adamantane oxidation with
PPAA, the catalytic ability of imidazolyl-appended iron(III)
porphyrins was examined. ImTPFPP–Fe(III)Cl showed the
best performance in producing adamantanols. From the com-
parison among different iron(III) porphyrins, we conclude that
the pentafluorophenyl groups in ImTPFPP–Fe(III)Cl have two
roles: (1) the electron-withdrawing nature decreases the elec-
tron density of the central iron(III) making it more amenable
to accepting imidazolyl ligation, which assists the cleavage of
O O bond of phenylperacetate ligand to generate the active
high-valent oxo-iron species; and (2) halogenation substitution
increases resistance to decomposition by the high-valent oxo-
iron species.
The imidazolyl ligation was directly observed spectroscop-
ically using UV–vis as well as 1H and 19F NMR as the
complementary dimer of ImTPFPP–Fe(III)Cl in acetonitrile.
Stable formation of the dimer is highly solvent dependent. Only
in acetonitrile was a clear dimer structure observed; however,
the monomeric chloride complex was dominant in CH2Cl2 in
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