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ChemComm
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DOI: 10.1039/C6CC04173F
Journal Name
COMMUNICATION
[Ru(bpy)3](ClO4)3 (10 mM) in 1:1 H2O/CF3CH2OH solution exhibits following. The redox potential for the twoꢀelectron reduction of
two major peaks at m/z 512.0 and 628.1, respectively. The peak at [MnVII(N)(O)(CN)4]2− to [MnV(N)(CN)4]2ꢀ shown in equation 6 is pH
m/z 512.0 is assigned to the ion {(PPh4)[Mn(N)(CN)4]}−. The other dependent. At high pH the potential is lower; hence the rate of its
peak at m/z 628.1 corresponds to the manganese oxo species formation (k in scheme 1) is expected to be faster. This accounts for
{(PPh4)[Mn(N)(O)(CN)4]}−
•
CF3CH2OH (Figure 2, inset
Figure S4 ). When H218O(90% 18Oꢀlabeled) was used, a new peak at 1). However, at pH > 6.8 the TON decreases sharply, presumably
m/z 630.0 appeared, consistent with the species because the Mn(VII) active intermediate becomes less stable at
{(PPh4)[Mn(N)(18O)(CN)4]}−
CF3CH2OH (Figure 2, inset b). This higher pH.
a and the increase in TON of styrene epoxidation from pH 5 – 6.8 (Figure
•
manganese(VII) nitrido oxo species has also been previously
observed using H2O2 as oxidant.22
[MnV(N)(CN)4(H2O)]2− + 2[Ru(bpy)3]3+
[MnVII(N)(O)(CN)4]2− + 2[Ru(bpy)3]2+ + 2H+ (3)
→
(PPh4)[MnV(N)(CN)4]-
512.0
a)
H2O
b) H218O
[MnVII(N)(O)(CN)4]2− + RCH2OH →
[MnV(N)(OH)(CN)4]3− + RCHOH+
628.1
628.1
630.0
(4)
(5)
(6)
[MnV(N)(OH)(CN)4]3− + RCHOH+→
[MnV(N)(OH2)(CN)4]2− + RCHO
627 628 629 630 631 632 627 628 629 630 631 632 633
m/z (amu)
m/z (amu)
[MnVII(N)(O)(CN)4]2− + 2H+ + 2e→
[MnV(N)(CN)4(H2O)]2−
-
(PPh4)[MnVII(N)(O)(CN)4]
(CF3CH2OH)
628.1
.
In conclusion, we have demonstrated efficient photocatalytic
oxidation of a variety of alkenes and alcohols by [Mn(N)(CN)4]2ꢀ in
aqueous solution without the need of any organic solvent. This is in
contrast to the photocatalysis by manganese prophyrins, where only
water soluble substrates were used.17 Also, in photocatalysis by a
nonꢀporphyrin Mn catalyst, 5% CH3CN has to be added and only
alcohol substrates were investigated.18 Our results further
demonstrate that MnV(N) is a useful platform for the construction of
highly efficient thermal and photochemical oxidation catalysts.
The work was supported by Hong Kong University Grants
Committee Area of Excellence Scheme (AoE/Pꢀ03ꢀ08) the Shenzhen
500
520
540
560
580
600
620
640
m/z (amu)
Figure 2 Mass spectrum of a mixture of 1 (5 mM) and [Ru(bpy)3](ClO4)3 (10
mM) in CF3CH2OH/H2O (1:1 v/v). Insets show the expanded spectra of the
peak at m/z 628.1: a) using CF3CH2OH/H2O as solvent; b) using
CF3CH2OH/H218O as solvent.
A proposed mechanism for the photocatalytic oxidation is shown
in Scheme 1. In water the manganese(V) nitrido complex is probably
present as a sixꢀcoordinate complex with a weakly bound water
molecule trans to the nitrido ligand, i.e. [Mn(N)(CN)4(H2O)]2−. This
Science
and
Technology
Research
Grant
(JCYJ20150601102053067).
intermediate
is
oxidized
to
a
MnVII=O
species,
[MnVII(N)(O)(CN)4]2−, by [RuIII(bpy)3]3+ (equation 3) that is
generated by irradiation of [RuII(bpy)3]2+ with visible light in the
presence of [CoIII(NH3)5Cl]2+ as quencher. This protonꢀcoupled
electron transfer (PCET) step is probably rateꢀdetermining with a
high activation barrier, which would account for the low quantum
yield for the photocatalytic process. The MnVII=O species then
oxidizes organic substrates to yield various products. In the case of
alkenes, epoxides are formed by Oꢀatom transfer from MnVII=O. In
Notes and references
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L. Hammarström and S. HammesꢀSchiffer, Acc. Chem. Res., 2009, 42,
1859.
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4
R. Eisenberg, Science, 2009, 324, 44.
M. Haumann, P. Liebisch, C. Müller, M. Barra, M. Grabolle and H. Dau,
Science, 2005, 310, 1019.
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6
C. W. Hoganson and G. T. Tabcock, Science, 1997, 277, 1953.
T.ꢀC. Weng, W.ꢀY. Hsieh, E. S. Uffelman, S. W. GordonꢀWylie, T. J.
Collins, V. L. Pecoraro and J. E. PennerꢀHahn, J. Am. Chem. Soc., 2004,
126, 8070.
the oxidation of alcohols,
a twoꢀelectron hydrideꢀabstraction
mechanism is proposed (equations 4 and 5) based on the observed
selective formation of cyclobutanone from cyclobutanol, and the
large KIE of 3.5 obtained in the competitive oxidation of cꢀC6H11OH
and cꢀC6D11OD.
7
8
A. S. Borovik, Chem. Soc. Rev., 2011, 40, 1870.
(a) G. Yin, Coord. Chem. Rev., 2010, 254, 1826. (b) K. P. Bryliakov and
E. P. Talsi, Coord. Chem. Rev., 2014, 276, 73. (c) C. Miao, B. Wang, Y.
Wang, C. Xia, Y.ꢀM. Lee, W. Nam and W. Sun, J. Am. Chem. Soc. 2016,
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Caruthers, S. Hong, W. Nam and M. M. AbuꢀOmar, J. Am. Chem. Soc.
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Wçrner, C.V. Sastri and P. Comba, Angew. Chem. Int. Ed. 2015, 54,
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9
H. Inoue, M. Sumitani, A. Sekita and M. Hida, J. Chem. Soc., Chem.
Commun., 1987, 1681.
Scheme 1 Proposed photocatalytic oxidation cycle, S represents substrate
10 (a) W. Chen, F. N. Rein and R. C. Rocha, Angew. Chem. Int. Ed., 2009,
48, 9672. (b) W. Chen, F. N. Rein, B. L. Scott and R. C. Rocha, Chem.
Eur. J., 2011, 17, 5595.
and SO represents oxidized product.
11 (a) D. Kalita, B. Radaram, B. Brooks, P. P. Kannam and X. Zhao,
ChemCatChem, 2011, 3, 571. (b) W. M. Singh, D. Pegram, H. Duan, D.
The observed pH dependence can be accounted for by the
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