The effect of axial ligand on the reactivity of oxomanganese(V) corrole

Article abstract of DOI:10.1246/cl.2007.274

The effect of axial ligand on the reactivity of oxomanganese(V) 5,10,15-tris(pentafluorophenyl)corrole (tpfc)Mn_VO had been investigated by kinetic method. It was found the reactivity of (tpfc)Mn ^VO towards styrene could be greatly en

Full text of DOI:10.1246/cl.2007.274

274
Chemistry Letters Vol.36, No.2 (2007)
The Effect of Axial Ligand on the Reactivity of Oxomanganese(V) Corrole
Hai-Yang Liu,ꢀ1;2 Hua Zhou,1 Lan-Ying Liu,1 Xiao Ying,3 Huan-Feng Jiang,1 and Chi-Kwong Changꢀ2
1Department of Chemistry, South China University of Technology, Guangzhou 510641, P. R. China
2Department of Chemistry, The Hong Kong University of Science and Technology, Hong Kong, P. R. China
3Department of Applied Physics, South China University of Technology, Guangzhou 510641, P. R. China
(Received October 23, 2006; CL-061251; E-mail: chhyliu@scut.edu.cn; chang@ust.hk)
The effect of axial ligand on the reactivity of oxomanga-
O
nese(V) 5,10,15-tris(pentafluorophenyl)corrole (tpfc)MnVO
had been investigated by kinetic method. It was found the reac-
tivity of (tpfc)MnVO towards styrene could be greatly enhanced
by axial coordination interaction. Catalytic experiments indicat-
ed that the turnover frequency (TOF) of (tpfc)MnIII-catalyzed
epoxidation of styrene with PhIO was increased about 7 times
in the presence of imidazole (catalyst/ligand, 1:10). Theoretical
calculations showed that axial coordination will lower the bind-
ing energy of Mn=O bond in (tpfc)MnVO.
MnIII
MnV
+[O]
O
MnIII
N
MnV
+ [O]
N
= tpfc
N
H
N
H
BEMn=O = E[Mn(V)O Corrole] - ( E[Mn(III) Corrole] + E[O]
)
Scheme 1.
of the ligands. Fryxelius et al.9 reported manganese 5,10,15-
tri(4-nitrophenyl)corrole [t(4-NO2P)C]MnIII(Py) could not cata-
lyze the oxidation of stilbene with PhIO as terminal oxidant.
The reactivity of MnVO corroles observed by different
research groups is somewhat puzzling. As MnVO corrole may
play a key role in the catalytic oxidation process, to explore
the reactivity of MnVO corroles at different conditions is certain-
ly helpful in understanding the mechanism of manganese
corrole-catalyzed oxidation reactions. We herein wish to report
the effect of axial ligand on the reactivity of MnVO corrole.
5,10,15-Tris(pentafluorophenyl)corrole (tpfc) can be easily
prepared by Gross’s solvent-free method,10 and its oxomanga-
nese(V) complex (tpfc)MnVO is even stable enough to run a
flash chromatography for purification.7 For convenience, we
choose (tpfc)MnVO (see Scheme 1) as model MnVO corrole
and imidazole as axial ligand. (tpfc)MnIII can form 1:1 complex
with imidazole, the binding constant turned out to be 2:05 ꢁ 105
Mꢂ1 (25 ꢃ 0:1 ꢄC) in CH2Cl2. In UV–vis spectra of (tpfc)MnIII,
a strong absorption appeared at 484 nm with the addition
of imidazole (Figure S1).14 When PhIO was added to (tpfc)-
MnIII(Im) in CH2Cl2, the colour of solution would gradually
turns from green to red, which is similar to (tpfc)MnIII case,2
indicating the formation of (tpfc)MnVO(Im). The UV–vis spec-
tra of (tpfc)MnVO(Im) is nearly the same to that of (tpfc)MnVO.
However, the self decay rate of (tpfc)MnVO(Im) is much faster
than (tpfc)MnVO, and it will finally return to (tpfc)MnIII(Im)
in itself decay reaction as indicated by UV–vis spectra
(Figure S2).14 In the presence of styrene, the decay of (tpfc)-
MnVO(Im) can be remarkably accelerated further (Figure S3).14
To make a quantitative comparison, a (tpfc)MnVO solution
of ca. 5:0 ꢁ 10ꢂ3 M was prepared for kinetic study.15 At
25(ꢃ0:1) ꢄC, the self decay reaction constant of (tpfc)MnVO
in CH2Cl2 is 2:07 ꢁ 10ꢂ4 sꢂ1, and (tpfc)MnVO returned to
(tpfc)MnIII completely after 300 min. The self decay reaction
constant of (tpfc)MnVO will increase to 9:30 ꢁ 10ꢂ4 sꢂ1 in
the presence of imidazole (0.3 M), and (tpfc)MnVO became
(tpfc)MnIII(Im) atfer 70 min. This suggests axial binding of
imidazole can significantly affects the stability of (tpfc)MnVO.
Pseudo-first-order reaction rate constant between (tpfc)MnVO
In recent years, study on corrole chemistry has received ex-
tensive interests.1 Although it has been demonstrated that man-
ganese corroles can catalyze the oxidation of alkene2 and sul-
fide,3 the mechansim of manganese corrole-catalyzed oxidation
reaction is not fully understood so far. In the first investigation
of manganese corrole-catalyzed oxidation of alkenes with PhIO,
Gross et al.2 had succesfully isolated oxomanganese(V) 5,10,15-
tris(pentafluorophenyl)corrole (tpfc)MnVO. The lack of reactiv-
ity of (tpfc)MnVO towards alkenes led to the proposal that
the active oxidant in the catalytic reactions was possibly
a higher valent MnVIO species generated by disproportion of
MnVO complex. ꢀ-Octakis(4-tert-butylphenyl)corrolazine man-
ganese(III) complex (TBP8CZ)MnIII is an active catalyst for the
oxidation of PhSMe or epoxidation of stilbene with PhIO, but
oxomanganese(V) corrolazine (TBP8CZ)MnVO was also found
not reactive to alkenes.4 18O-Labeling experiments indicated
that a new ‘‘third oxidant’’ (TBP8CZ)MnVO(PhIO) was involved
in the catalytic oxidation process. Thus, Goldberg and Kerber5
had proposed the possibility that (tpfc)MnVO(PhIO) might be
the true oxidant in the (tpfc)MnIII-catalysed oxidation of alkenes
with PhIO. To examine the role of (tpfc)MnIII as catalyst in the
epoxidation of alkenes, Collman et al.6 had performed a compet-
itive epoxidation of styrene/cyclooctene with different ArIO ter-
minal oxidants (Ar = C6H5, C6F5, mesityl), and found the ratio
of epoxides varied with the property of ArIO. It is suggested that
both MnVO and MnIII—OIAr intermediates were the active
oxidants in the catalytic processes. Chang et al.7 had prepared
a perfluorinated manganese corrole (F8tpfc)MnIII, and found
the isolated (F8tpfc)MnVO is much more reactive than
(tpfc)MnVO in the direct reaction with cyclooctene. Recently,
Newcomb et al.8 had checked the reactivity of several MnVO
corroles bearing different substituents, (BPFMC)MnVO,
(TPC)MnVO, and (tpfc)MnVO (BPFMC = 5,15-bis(pentafluo-
rophenyl)-10-(p-methoxyphenyl)corrole; TPC = 5,10,15-tri-
phenylcorrole). The order of reactivity of these MnVO corroles
was found to be TPC > BPFMC > TPFC in self-decay reac-
tions or in the reactions with substrates. This is inverted from
the expected MnVO reactivity based on the electron-demand
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
275
PhIO was added to the catalyst–substrate reaction mixtures,
solutions would gradually turn from green to red colour. In the
presence of imidazole, the solution would return green colour
in 30 min, and the catalyst returned to (tpfc)MnIII(Im) as detect-
ed by UV–vis spectra. But reaction mixture would keep red
colour for more than 12 h without imidazole axial ligand.
Catalytic experimental results indicate the oxygen atom transfer
(OAT) rate between (tpfc)MnVO and styrene can be remarkably
enhanced by axial ligand. The axial ligand induced rate acceler-
ation of catalytic oxidation of substrates by metalloporphyrins is
also well known.13
In summary, we have demonstrated that the reactivity of
(tpfc)MnVO towards alkenes could be strongly enhanced by
axial ligand. The axial coordination interaction would weaken
Mn=O bond, raise the energy of HOMO and LUMO, as well
as alter the HOMO distribution of (tpfc)MnVO significantly.
Scheme 2.
and styrene (0.3 M) in CH2Cl2 is 3:24 ꢁ 10ꢂ4 sꢂ1 (25 ꢃ 0:1 ꢄC).
It will increase up to 31:3 ꢁ 10ꢂ4 sꢂ1 in the presence of imida-
zole (0.3 M). Such a remarkable increase of the pseudo-first-or-
der reaction rate constant between (tpfc)MnVO and styrene in
the presence of imidazole indicates the reactivity of (tpfc)MnVO
could be greatly enhanced by coordination of an axial ligand.
To further confirm the reactivity of (tpfc)MnVO could be en-
hanced by axial ligand, we have performed a theoretical calcula-
tions. The binding energy of MnV=O bond in (tpfc)MnVO and
(tpfc)MnVO(Im) is defined as shown in Scheme 1. Energies of
all MnIII (S ¼ 5)11 and MnVO (S ¼ 1)12 corrole species were
calculated by B3PW91 method at STO-3Gꢀ level with Gaussian
03 program. The calculated binding energy of MnV=O in
(tpfc)MnVO and (tpfc)MnVO(Im) is ꢂ103:2 and ꢂ80:4 kcal/
mol respectively. This indicates that (tpfc)MnVO is much more
stable than (tpfc)MnVO(Im), that is (tpfc)MnVO(Im) is a more
reactive oxidant than (tpfc)MnVO. The energies of HOMO and
LUMO of (tpfc)MnVO(Im) are higher than that of (tpfc)MnVO.
Calculated LUMOs of (tpfc)MnVO and (tpfc)MnVO(Im) are
mainly localized on corrole nucleus and oxygen atom. Interest-
ingly, the distribution of HOMO in (tpfc)MnVO(Im) is quite
different from that in (tpfc)MnVO. HOMO of (tpfc)MnVO is
localized on ꢁ carbons of corrole ring and manganese atom,
while HOMO of (tpfc)MnVO(Im) is distributed extensively on
manganese atom, axial ligand nitrogen atom, oxygen atom,
and corrole nucleus (Scheme 2, Figure S4).14 This indicates
the axial coodination interaction could not only affect the energy
level of MnVO corroles, but also the HOMO distributions.
The direct evidences that the reactivity of (tpfc)MnVO could
be greatly increased by axial ligand also come from the catalytic
oxidation of styrene by (tpfc)MnIII with PhIO as terminal oxi-
dant in the presence of imidazole. Catalytic experiments were
carried out at following conditions: Styrene:PhIO:catalyst =
500:50:1 in 2-mL CH2Cl2 (catalyst, 2.4 mmol) under aerobic
conditions at room temperature; Yield was determined by GC
after 30 min based on the PhIO used; Axial ligand imidazole
was added with a styrene/PhIO/catalyst/Im molar ratio of
500:50:1:10 in 2-mL CH2Cl2 (catalyst, 2.4 mmol); Turnover
frequency (TOF) was calculated according to the reaction rate
of the first 30 min.
This work was supported by Natural Science Foundation of
China (Nos. 20332030 and 20572027), Research Grant Council
of Hong Kong and SRF for ROCS, SEM.
References and Notes
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14 Supporting Information is also available electronically on the
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15 A (tpfc)MnIII solution of 5:0 ꢁ 10ꢂ3 M in CH2Cl2 was used
to prepare (tpfc)MnVO according to the method described
in Ref. 7.
The main product of catalytic oxidation reaction is styrene
oxide (Figure S5).14 TOF of (tpfc)MnIII-catalyzed oxidation of
styrene to styrene oxide is quite low (4.8 hꢂ1), but it will increase
to 34.5 hꢂ1 in the presence of imidazole. Such an increase in
TOF basically matches the kinetic data mentioned above. When
Published on the web (Advance View) January 18, 2007; doi:10.1246/cl.2007.274