Synthesis, characterization, Co-S bond reactivity of a vitamin B 12 model complex having pentafluorophenylthiolate as an axial ligand

Article abstract of DOI:10.1039/c3dt00042g

Heptamethyl (aquo)(pentafluorophenylthiolate)cobyrinate perchlorate, [(H₂O)(C₆F₅S)Cob(iii)7C₁ester] ClO₄, was synthesized as a B₁₂ model complex having a thiolate ligand in the axial position. The axial ligand change in heptamethyl (diaquo)cobyrinate diperchlorate, [(H₂O)₂Cob(iii)7C ₁ester](ClO₄)₂, from H₂O to C ₆F₅S^- afforded the B₁₂-thiolate complex. The B₁₂-thiolate model complex was characterized by UV-vis, NMR and ESI-mass spectroscopies. The coordination of C₆F ₅S^- to the cobalt center affected the spectroscopic properties of the corrin ring through the electronic interaction between the axial ligand (C₆F₅S^-) and the equatorial ligand (corrin). The photolysis of the B₁₂-thiolate model complex led to the homolytic cleavage of the Co(iii)-S bond to form the Co(ii) complex and the phenyl thiyl radical. The thermolysis of the B₁₂-thiolate model complex also led to the homolytic cleavage of the Co(iii)-S bond. Furthermore, the reactivity of the Co(iii)-S bond of the B₁₂-thiolate model complex was applied to the catalytic oxidation of C₆F₅SH to C₆F₅S-SC₆F₅.

Full text of DOI:10.1039/c3dt00042g

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Synthesis, characterization, Co–S bond reactivity of a
vitamin B12 model complex having
Cite this: Dalton Trans., 2013, 42, 6410
pentafluorophenylthiolate as an axial ligand
Keishiro Tahara,a,b Ayaka Matsuzaki,a Takahiro Masuko,a Jun-ichi Kikuchib and
Yoshio Hisaeda*a,c
Heptamethyl (aquo)(pentafluorophenylthiolate)cobyrinate perchlorate, [(H2O)(C6F5S)Cob(III)7C1ester]-
ClO4, was synthesized as a B12 model complex having a thiolate ligand in the axial position. The axial
ligand change in heptamethyl (diaquo)cobyrinate diperchlorate, [(H2O)2Cob(III)7C1ester](ClO4)2, from
H2O to C6F5S− afforded the B12–thiolate complex. The B12–thiolate model complex was characterized by
UV-vis, NMR and ESI-mass spectroscopies. The coordination of C6F5S− to the cobalt center affected the
spectroscopic properties of the corrin ring through the electronic interaction between the axial ligand
(C6F5S−) and the equatorial ligand (corrin). The photolysis of the B12–thiolate model complex led to the
homolytic cleavage of the Co(III)–S bond to form the Co(II) complex and the phenyl thiyl radical. The
thermolysis of the B12–thiolate model complex also led to the homolytic cleavage of the Co(III)–S bond.
Furthermore, the reactivity of the Co(III)–S bond of the B12–thiolate model complex was applied to the
catalytic oxidation of C6F5SH to C6F5S–SC6F5.
Received 5th January 2013,
Accepted 12th February 2013
DOI: 10.1039/c3dt00042g
www.rsc.org/dalton
radical pair is considered to be an analogue of the key inter-
mediates in adenosylcobalamin (Ado)-dependent enzymes
Introduction
Cobalamins (B12) having a thiolate ligand have attracted such as ribonucleoside triphosphate reductase (RTPR).24 In
increasing attention in recent years. The first example of isol- their pioneering work regarding the RTPR model system, pre-
able B12–thiolate complexes is glutathionylcobalamin liminary examinations of the reactivity of the Co(III)–S bond of
(GluSCbl), which has glutathionate in the upper axial C6F5SCbl were reported.23,25 However, there is limited infor-
position.1–4 GluSCbl is an important intermediate in the bio- mation on the reactivity of the Co(III)–S bond of C6F5SCbl,
synthesis of the coenzyme forms of B12, adenosylcobalamin which is only from their work. The cleavage form (homolysis
and methylcobalamin.5 The structure,6–8 reactivity,9–12 compu- or heterolysis) of the Co(III)–S bond of C6F5SCbl is still not fully
tational study,13,14 potential biological roles15–18 and use as clarified due in part to the difficulty to identify the cobalamin
pharmacological antioxidants19 of GluSCbl have been inten- species of the reaction product.
sively studied. Cobalamins having thiolate ligands of amino
The investigation of the reactivity of the Co(III)–S bond of
acids, peptides and captopril have also been reported.20–22 The the isolable B12–thiolate complexes is significantly important
second example of isolable B12–thiolate complexes is (penta- due to its relevance to the B12 functions in vivo. In the cobala-
fluorophenylthiolato)cobalamin (C6F5SCbl), which has penta- min systems, both the equatorial (corrin) and lower axial (5,6-
fluorophenylthiolate (C6F5S−) in the axial position.23 The B12– dimethylbenzimidazole, DMB) ligands would affect the reactiv-
thiolate complex was synthesized and characterized in detail ity of the Co(III)–S bond of the B12–thiolate complex. In
by Finke et al. C6F5SCbl has attracted significant interest as a addition, the coordination equilibrium of DMB between the
precursor to the thiyl radical and Cob(II)alamin pair. The base-on and base-off forms should be considered for the inves-
tigation of the reactivity of the Co(III)–S bond. Therefore,
simple B12 model complexes are useful in order to clarify the
aDepartment of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu University, Fukuoka 819-0395, Japan.
intrinsic effect on the reactivity of the Co(III)–S bond. Removing
the lower axial ligand of B12 is an effective approach for focus-
ing on the corrin ring. This would help to identify the cobalt
species of the reaction product by spectroscopic methods such
as UV-vis and ESR spectroscopies. We have been dealing with
a hydrophobic B12 model complex, heptamethyl cobyrinate
E-mail: yhisatcm@mail.cstm.kyushu-u.ac.jp; Fax: +81-92-802-2827;
Tel: +81-92-802-2826
bGraduate School of Materials Science, Nara Institute of Science and Technology
(NAIST), 8916-5, Takayama, Ikoma Nara 630-0192, Japan
cCenter for Molecular Systems (CMS), Kyushu University, Fukuoka 819-0395, Japan
6410 | Dalton Trans., 2013, 42, 6410–6416
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with 30% (w/w) aqueous perchloric acid (60 mL), then the
CH2Cl2 extract was washed with distilled water (50 mL). The
extract was concentrated and the target compound was
obtained upon the addition of n-hexane as a red powder.
Yield: 15.1 mg (76%). 1H-NMR (CDCl3): δ 3.43, 3.55, 3.65, 3.67,
3.73, 3.81 (s, 21H, –(CO2CH3)7, 6.26 (s, 1H, C(10)–H). Found: C,
49.36; H, 6.34; N, 4.36. Calc. for C52H77Cl2CoN4O24: C, 49.10;
H, 6.10; N, 4.40. UV-vis (CH2Cl2): [λmax/nm] (εmax), 348 (1.4 ×
104), 485 (7.0 × 103), 516 (4.8 × 103). ESI-MS: (M − 2ClO4 − H)+,
1071.60 (calcd for M − 2ClO4 − H, 1071.46).
Synthesis of [(H2O)(C6F5S)Cob(III)7C1ester]ClO4 (5)
A CH2Cl2 solution (100 mL) of [(H2O)2Cob(III)7C1ester](ClO4)2
(9.9 mg, 8.8 × 10−6 mol) was deoxygenated by bubbling nitro-
gen gas. The following procedures were carried out in the
dark. To the solution, a CH2Cl2 solution (436 μL) of C6F5SH
(1.82 × 10−2 M) was added. After 5.5 h of stirring the combined
solution, the reaction solution was concentrated and the target
compound was obtained upon the addition of n-hexane as a
yellow green powder. Yield: 8.7 mg (83%). 1H-NMR (CDCl3):
δ 3.61, 3.63, 3.71, 3.72, 3.75, 3.79 (s, 21H, –(CO2CH3)7), 5.17
(1H, C(10)–H). 19F-NMR (CDCl3): δ −162.38 (m-F-PhS), −151.32
(p-F-PhS), −125.88 (o-F-PhS). UV-vis (CH2Cl2): [λmax/nm] (εmax),
262 (1.7 × 104), 301 (1.7 × 104), 350 (1.1 × 104), 432 (1.0 × 104),
634 (1.4 × 103). HR-ESI-MS: (M − 2ClO4 − H2O)+, 1235.4098
(calcd for M − 2ClO4 − H2O, 1235.4096).
Chart 1
perchlorate 1, [Cob(II)7C1ester]ClO4, which has ester groups in
place of the peripheral amide moieties of the cobalamins26 as
shown in Chart 1 and succeeded in performing various cata-
lytic reactions.27,28 To investigate the intrinsic corrin framework
effect on the reactivity of the Co(III)–S bond, we synthesized a
hydrophobic B12 model complex having C6F5S− as an axial
ligand (5) in this study. We characterized the B12–thiolate model
complex 5 by UV-vis, NMR and ESI-mass spectroscopies.
Photolysis and thermolysis of compound 5 were successfully fol-
lowed by UV-vis and NMR spectroscopies, and the reactivity and
the cleavage form of the Co(III)–S bond of 5 were unambiguously
clarified. Furthermore, the reactivity of the Co(III)–S bond of the
B12 model complex was applied to the catalytic oxidation of a
thiol (C6F5SH) to disulfide (C6F5S–SC6F5).
Photolysis and thermolysis of [(H2O)(C6F5S)Cob(III)7C1ester]-
ClO4 (5)
A CHCl3 solution of 5 was added to a quartz cell, degassed by
freeze–pump–thaw cycles and irradiated by a 200 W tungsten
lamp with a cutoff filter (λ > 420 nm) at ambient temperature.
The photolysis of 4 was monitored by UV-vis absorption
spectrometry.
A CHCl3 solution of 5 was added to a quartz cell and
degassed by freeze–pump–thaw cycles. The CHCl3 solution of
5 was set in the sample holder of the UV-vis spectrometer kept
at 50 °C by a temperature controller. The thermolysis of 5 was
monitored by UV-vis absorption spectrometry. A CDCl3 solu-
tion of 5 and C6F6 as the internal standard was prepared in a
glove box and transferred to an NMR tube. The CDCl3 solution
of 5 was kept at 50 °C by the temperature controller of the
NMR spectrometer. The thermolysis of 5 was also monitored
by 19F-NMR spectrometry.
Experimental
Materials and methods
All chemicals were of reagent grade and used as received.
[(H2O)(CH3)Cob(III)7C1ester]ClO4 (3) was synthesized by pre-
viously reported methods.29 The UV-vis absorption spectra
1
were measured using a Hitachi U-3900H spectrometer. The H
and 19F NMR spectra were recorded by Bruker Avance 500 and
Bruker AX 300 spectrometers, respectively. The chemical shifts
(in ppm) were referenced relative to tetramethylsilane and C6F6
1
for H and 19F-NMR, respectively. The elemental analysis was
obtained from the Service Center of Elementary Analysis of
Organic Compounds at Kyushu University. The high resolution
electrospray ionization (HR-ESI) mass spectra were obtained
using a JEOL JMS-T100CS. The GC-MS data were obtained
using a Shimadzu GC-QP5050A equipped with a J&W Scientific
DB-1 column (length 30 m; ID 0.25 mm; film 0.25 μm).
Catalytic oxidation of C6F5SH to C6F5S–SC6F5
A CH2ClCH2Cl solution of C6F5SH (2.5 × 10−4 mol) and com-
pound 4 (2.5 × 10−6 mol) was heated at 50 °C under aerobic
conditions. The reaction solution was analyzed by GC-MS. The
Synthesis of [(H2O)2Cob(III)7C1ester](ClO4)2 (4)
A CH2Cl2 solution (100 mL) of [(H2O)(CH3)Cob(III)7C1ester]- product was identified by GC-MS comparisons with the corre-
ClO4 (18.3 mg, 1.57 × 10−5 mol) was irradiated by a 200 W sponding authentic sample. The conversion and product yield
tungsten lamp with a cutoff filter (λ > 420 nm) for 15 min were quantified by GC-MS comparisons to phenylbenzene as
along with bubbling air. The resulting solution was treated the internal standard.
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Results and discussion
Synthesis and characterization of the diaquo complex 4
We synthesized heptamethyl diaquocobyrinate diperchlorate 4,
[(H2O)2Cob(III)7C1ester](ClO4)2, from heptamethyl aquomethyl-
cobyrinate perchlorate 3, [(H2O)(CH3)Cob(III)7C1ester]ClO4, as
shown in Scheme 1. A CH2Cl2 solution of the methylated
cobalt complex was first irradiated with a 200 W tungsten
lamp with a cutoff filter (λ > 420 nm) along with bubbling air.
The Co(III)–C bond of the Co(III)–CH3 complex 3 was homolyti-
cally cleaved upon visible-light irradiation to form the methyl
radical and the Co(II) complex 1.29,30 It was demonstrated that
the resulting Co(II) complex is oxidized by oxygen to form the
Co(III) species when the homolysis is performed in CH2Cl2.30
In contrast, the oxygenation of the Co(II) complex to the Co(III)
species does not occur when the homolysis is performed in
CH3OH.29,30 In the present study, we performed the homolysis
in CH2Cl2 with bubbling air for the effective oxygenation to
the Co(III) complex.
The target product 4 was obtained through the treatment
with aqueous perchloric acid and identified as [(H2O)2Cob(III)-
7C1ester](ClO4)2 by NMR, UV-vis absorption, ESI-MS and
elemental analysis. The proton peak of the Co(III)–CH3 dis-
1
appeared in the H-NMR spectrum of compound 4, indicating
Fig. 1 UV-vis spectra of (a) [(H2O)2Cob(III)7C1ester](ClO4)2 4 (4.6 × 10−5 M) and
(b) [(H2O)(C6F5S)Cob(III)7C1ester]ClO4 5 (4.5 × 10−5 M) in CH2Cl2.
that the described method for the demethylation of the cobalt
center in 3 is effective. In the UV-vis spectrum of 4, absorption
bands arising from the π–π* transitions of the conjugated
corrin ring were observed at 516, 485 and 348 nm as shown in
Fig. 1a. These bands were assigned to the αβ and γ bands
which are typical for cobalamins and their derivatives.31–33
These bands differ from the bands of the Co(III)–CH3 complex
3 at 264, 305, 373 and 457 nm.29 The exchange of the axial
ligand from CH3 to H2O leads to the drastic UV-vis spectral
ligands in the axial positions. The ligand exchange from H2O
to thiolate was next investigated.
Synthesis and characterization of the B12–thiolate model
complex 5
change. This result indicates the electronic interaction We synthesized [(H2O)(C6F5S)Cob(III)7C1ester](ClO4) 5 by the
between the axial ligand and the corrin ring through the axial ligand exchange of compound 4 from H2O to C6F5S− in
cobalt center in the B12 model complex, which was consistent CH2Cl2 as shown in Scheme 1. The addition of an equivalent
with the cobalamin systems.31–33
of C6F5SH vs. 4 in the non-coordinating organic solvent in the
It should be noted that the homolysis of the Co(III)–CH3 dark led to the successful ligand exchange from H2O to C6F5S−
complex bond followed by the oxygenation of the cobalt center followed by general workup procedures. The ESI mass spec-
is a more effective method for the synthesis of 4 than the trometry of the obtained product afforded a positive ion mass
ligand exchange of [(CN)(H2O)Cob(III)7C1ester]ClO4 2 from with a peak cluster at m/z 1235.4098, which has a mass value
CN− to H2O. This is because removing the cyanide ligand from content with the ion [M − ClO4 − H2O]+ as shown in Fig. 2.
the solution is obviously difficult due to the strong affinity of This result confirms the coordination of the C6F5S− ligand to
the B12 derivative to CN−. Therefore, we chose the method the cobalt center and the formation of a B12 derivative having
accompanied by the valence change of the cobalt center. Thus, a Co–S bond.
compound 4 was available as a precursor having labile aqua
The obtained thiolate complex 5 was also characterized by
1H-NMR spectrometry. The vinyl proton peaks of the C10 posi-
tion of compounds 4 and 5 were observed at 6.26 and
5.17 ppm, respectively. The coordination of C6F5S− to the
cobalt center led to an upfield shift of the vinyl proton
through the electronic interaction between the axial ligand
and the corrin ring. Such a cis-influence had also been
reported in the 1H-NMR spectra of cobalamin systems.34
The thiolate complex 5 was also characterized by UV-vis
absorption spectrometry. The UV-vis spectrum of 5 was domi-
nated by absorption bands at 262, 301, 350 and 432 nm as
Scheme 1
6412 | Dalton Trans., 2013, 42, 6410–6416
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2
The benzimidazole ligand increases the energy of the Co 3dz
orbital. This probably shifts the LMCT corrin π → Co 3dz2 tran-
sition to a higher energy region where it would be hidden
among many intense bands. Therefore, the LMCT band at
634 nm in 5 is a feature of heptamethyl cobyrinate having a
thiolate ligand. It should be noted that this finding came from
the use of the B12 model complex which does not have the
lower ligand of cobalamins.
Cleavage of the Co–S bond of the B12–thiolate model complex 5
The reactivity of the Co–S bond of 5 was next investigated.
A CHCl3 solution of 5 was degassed and irradiated by a 200 W
tungsten lamp with a cutoff filter (λ > 420 nm). Upon
irradiation by visible light for 80 min, a UV-vis spectral change
was observed as shown in Fig. 3. This result clearly indicates
that the thiolate complex 5 is a photo-sensitive compound.
The obtained UV-vis spectrum after the visible light irradiation
has absorption bands at 264, 316 and 468 nm. According to
the similarity of this spectrum to that of 1 with absorption
bands at 266, 316 and 468 nm,26 the product of the photolysis
of 5 is identified as the Co(II) complex 1. The photolysis of 5
was also monitored by ESR spectrometry. Upon irradiation by
visible light for 80 min, the ESR signal ascribed to a Co(II) low
spin of 126 appeared as shown in Fig. 4b. These results suggest
that the photolysis of 5 leads to the homolytic cleavage of the
Co(III)–S bond to form the Co(II) complex 1 and, based on the
mass and charge valance, the phenyl thiyl radical as shown in
Scheme 2. The very weak ESR signal was detected before
visible light irradiation (Fig. 4a). This is probably because the
unavoidable photolysis of a small amount of 5 proceeded due
to the imperfect protection from room lights while the
degassed solution of 5 was prepared. The photolysis of 5 was
also performed in the presence of α-phenyl-tert-butyl-nitrone
(PBN) as a spin trap reagent. However, no radical species was
detected by ESR spectrometry. This is probably due to the fast
electron transfer from PBN to the radical species to form a
thiolate anion. This is consistent with previous reports of the
fast electron transfer from spin trap reagents to the radical
species to form thiolate anions.40
Fig. 2 ESI mass spectrum of the B12–thiolate model complex 5. Inset: experi-
mental (left) and theoretical (right) isotopic distributions for the B12–thiolate
complex.
shown in Fig. 1b. These bands differ from the bands of the
precursor 4, which are designated as αβ and γ bands and are
typical for cobalamin systems as shown in Fig. 1a. Such an
obvious spectral change indicates that the C6F5S− ligand has a
significant influence on the π–π* transitions of the corrin ring
through the cobalt center in compound 5. The influence of the
C6F5S− ligand on the spectroscopic properties of the corrin
ring was also observed in the chemical shift of the vinyl proton
of the C10 position in the 1H-NMR spectrum as discussed
above.
Interestingly, the UV-vis spectrum of 5 was dominated by
another absorption band at 634 nm, whose extinction coeffi-
cient is about an order of magnitude less than those of other
bands as shown in Fig. 1b. The usual cobalamins do not have
absorption bands in the red region. However, Chemaly has
recently reported that, in aqueous solutions, cobalamins
having I−, Br−, Cl− and SC(NH2)2 in the upper position have
broad absorption bands at 749, 716, 687 and 675 nm, respecti-
vely.35 These absorption bands were designated “band A” and
assigned by the author to ligand-to-metal charge transfer
(LMCT) transitions from a π orbital in the corrin ring to the
2
Co 3dz orbital.35 The wavelength shift for band A correlates
with the spectrochemical series.36–39 Both 5 and the SC(NH2)2–
cobalamin have S donor ligands which are electrically neutral
organic compounds. Thus, the weak absorption band at
634 nm of 5 is assigned to an LMCT transition (corrin π →
2
Co 3dz ) according to its similarity to that of the SC(NH2)2–
cobalamin. The S donor ligands are placed near the bottom of
the spectrochemical series,36–39 and the ligand field strength
is relatively weak for the S donor ligands. Thus, the C6F5S−
2
ligand affects the Co 3dz orbital in energy for 5, resulting in
2
the relatively low energy of the LMCT corrin π → Co 3dz tran-
sition corresponding to the red region. On the other hand,
such an apparent LMCT transition corresponding to the red
region was not observed in the UV-vis spectrum of C6F5S−–
cobalamin reported by Finke et al.23 C6F5S−–cobalamin has a
benzimidazole as a σ-donating ligand in the lower position.
Fig. 3 UV-vis spectra of the B12–thiolate model complex 5 (4.6 × 10−5 M)
before and after visible light irradiation (λ > 420 nm) in CHCl3.
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Fig. 6 19F-NMR spectra of the B12–thiolate model complex 5 before (top) and
after (bottom) heating at 50 °C in CDCl3 containing C6F6 as the internal
standard.
Fig. 4 ESR spectra of the B12–thiolate model complex 5 (1.1 × 10−3 M) before
(a) and after (b) visible light irradiation (λ > 420 nm) in CHCl3 at 100 K.
Scheme 3
Scheme 2
thermolysis of 5 was identified as C6F5S–SC6F5. This suggests
the involvement of the phenyl thiyl radical in the thermolysis
of 5. Combined with the former spectrometric study, a plaus-
ible mechanism is shown in Scheme 3. The thermolysis of 5
leads to the homolytic cleavage of the Co(III)–S bond to form 1
and the phenyl thiyl radical.
It was demonstrated that thiolatocobalamines of gluta-
thione, N-acetylcysteine, homocysteine and captopril decom-
posed to aquacobalamin under acidic conditions.12 It should
be mentioned that, in the present study, the reactivity of the
Co(III)–S bond was investigated using the hydrophobic B12
model complex in a non-aqueous solvent. In addition, we
chose the particular thiolate group C6F5S−, which has high
resistance to protonation (pKa = 2.68).41 Because the pKa value
of ligands decreases upon binding to cobalamins, that of
C6F5S− is expected to decrease upon binding to the B12 model
complex 4.16 The ligand–solvent (C6F5S−–CH2Cl2) combination
is considered to offer a benefit to isolate 5 and investigate the
Co(III)–S homolysis probably without the protonation at S of 5
and the following decomposition.
Fig. 5 UV-vis spectra of the B12–thiolate model complex 5 (5.0 × 10−5 M)
before and after heating at 50 °C in CHCl3.
Next, the thermolysis of 5 was investigated. A CHCl3 solu-
tion of 5 was degassed and heated at 50 °C. Upon heating for
4 h, a UV-vis spectral change was observed as shown in Fig. 5.
This result clearly indicates that the thiolate complex 5 is a
thermo-sensitive compound. The obtained UV-vis spectrum
after heating suggests that the reaction solution contained the
Co(II) and Co(III) species of the B12 derivative. This is probably
because the formed Co(II) species by thermolysis would be par-
tially oxidized to the Co(III) species by contaminated oxygen
during the relatively long heat treatment.
Catalytic oxidation of C6F5SH to C6F5S–SC6F5
The catalytic activity of compound 4 for the oxidation of
C6F5SH was investigated under aerobic conditions. The result
is summarized as shown in Table 1. In the presence of com-
pound 4, the thiophenol C6F5SH was selectively oxidized to the
corresponding disulfide, C6F5S–SC6F5, under reflux without
any evidence for the formation of the corresponding sulfonic
acid (entry 1 in Table 1). On the other hand, under controlled
blank conditions, there was no oxidation of C6F5SH to C6F5S–
SC6F5 in the absence of 4 (entry 2 in Table 1). These results
The thermolysis of 5 was also monitored by 19F-NMR spec-
trometry. Upon heating for 7 h, shifts in the 19F peak were
observed as shown in Fig. 6. This result confirmed the thermo-
sensitivity of 5. According to the comparison of the 19F peaks
of the authentic sample of C6F5S–SC6F5, the product of the
6414 | Dalton Trans., 2013, 42, 6410–6416
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Straightforward Synthesis”, No. 23105537, from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of
Table 1 Catalytic oxidation of C6F5SH to C6F5S–SC6F5 mediated by compound
4a
Japan and
a Grant-in-Aid for Challenging Exploratory
Research, No. 24655134, from the Japan Society for the Pro-
motion of Science (JSPS), SEKISUI “Innovations Inspired by
Nature” Support Program, and the Kyushu University P&P
Program. The authors would like to thank Ms Yoshiko Nishi-
kawa (NAIST) for the ESI-MS measurements.
Product yield of
Turnover
numbersc
Entry
Catalyst
C6F5S–SC6F5 b (%)
1
2
Compound 4
None
17
0
17
0
a Conditions: [4] = 0.25 mM, [C6F5SH] = 25 mM, at 50 °C for 24 h
in CH2ClCH2Cl. b Product was quantified by GC-MS. c Turnover
numbers based on the initial concentration of 4.
Notes and references
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2 D. Dolphin and A. W. Johnson, J. Chem. Soc., 1965,
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revealed that the diaquo Co(III) complex 4 mediates the oxi-
dation of C6F5SH to C6F5S–SC6F5.
Combined with previously obtained results, a plausible
mechanism for the catalytic oxidation is proposed as follows.
The thiolate C6F5S− binds to the diaquo Co(III) complex 4 to
afford the thiolate complex 5. The Co(III)–S bond of 5 is homo-
lytically cleaved upon heating to form the Co(II) complex 1 and
the phenyl thiyl radical. The corresponding disulfide, C6F5S–
SC6F5, is formed by the dimerization of the radical species.
The Co(II) complex 1 is oxidized by oxygen to the Co(III)
species, which is further bound by C6F5S−.
4 P. Y. Law and J. M. Wood, J. Am. Chem. Soc., 1973, 95,
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Heptamethyl (diaquo)cobyrinate diperchlorate, [(H2O)2(C6F5S)-
Cob(III)7C1ester](ClO4)2, was synthesized and characterized as
a precursor to a B12 model complex having a thiolate ligand in
the axial position. An axial ligand change in the diaquo
complex from H2O to C6F5S− afforded heptamethyl (aquo)-
(pentafluorophenylthiolate)cobyrinate perchlorate, [(H2O)-
(C6F5S)Cob(III)7C1ester](ClO4). The B12–thiolate complex was
characterized by UV-vis, NMR and ESI-mass spectroscopies.
The coordination of C6F5S− to the cobalt center affected the
spectroscopic properties of the corrin ring through the elec-
tronic interaction between the axial ligand (C6F5S−) and the
equatorial ligand (corrin). The photolysis of the B12–thiolate
complex led to the homolytic cleavage of the Co(III)–S bond to
form the Co(II) complex and the phenyl thiyl radical. The
thermolysis of the B12–thiolate complex also led to the homoly-
tic cleavage of the Co(III)–S bond. Furthermore, the reactivity of
the Co(III)–S bond of the B12–thiolate complex was applied to
the catalytic oxidation of C6F5SH to C6F5S–SC6F5.
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Acknowledgements
This work was partially supported by a Grant-in-Aid for Scienti- 20 R. K. Suto, N. E. Brasch, O. P. Anderson and R. G. Finke,
fic Research on Innovative Areas “Molecular Activation toward
Inorg. Chem., 2001, 40, 2686–2692.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6410–6416 | 6415

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