Oxygen evolution from BF3/MnO4-

Article abstract of DOI:10.1039/c1cc00019e

MnO₄^- is activated by BF₃ to undergo intramolecular coupling of two oxo ligands to generate O₂. DFT calculations suggest that there should be a spin intercrossing between the singlet and triplet potential energy surfaces on going from the active intermediate [MnO₂(OBF₃)₂]^- to the O...O coupling transition state.

Full text of DOI:10.1039/c1cc00019e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 4159–4161
www.rsc.org/chemcomm
COMMUNICATION
ꢀ
4
Oxygen evolution from BF /MnO w
3
a
a
ab
a
a
Shek-Man Yiu, Wai-Lun Man, Xin Wang, William W. Y. Lam, Siu-Mui Ng,
a
a
a
Hoi-Ki Kwong, Kai-Chung Lau* and Tai-Chu Lau*
Received 3rd January 2011, Accepted 14th February 2011
DOI: 10.1039/c1cc00019e
ꢀ
ꢀ2
MnO4 is activated by BF to undergo intramolecular coupling of
3
However, when BF (>5 ꢂ 10 M in CH CN) was mixed
3
3
ꢀ3
two oxo ligands to generate O . DFT calculations suggest that
2
with KMnO (>5 ꢂ 10 M) at high concentrations in the
4
ꢀ
there should be a spin intercrossing between the singlet and triplet
absence of organic substrates, the purple color of MnO was
4
potential energy surfaces on going from the active intermediate
ꢀ
still discharged rapidly, and gas bubbles could be seen, which
[
MnO (OBF
2
3
)
2
]
to the OꢁꢁꢁO coupling transition state.
2
was shown to be O by GC and GC-MS analysis. The volume
of O evolved under various conditions was measured by a gas
2
burette, and representative data are shown in Table 1.
ꢀ
O–O bond formation involving metal-oxo complexes is a
crucial step in oxygen evolution in biological and chemical
The % yield of O
with [BF
2
(calculated per mol of MnO
4
) increases
) at
3
] (entries 1–4) and levels off to 66% (2.2 ml of O
2
1
systems. In general there are two basic mechanisms for O–O
around 7 equiv. (0.088 M). The yield is not quantitative,
ꢀ
bond formation via metal-oxo species: (1) nucleophilic attack
presumably BF /MnO also oxidizes the solvent CH CN.
4
3
3
n
Similar yields were obtained when Bu N[MnO ] (entry 5) or
of water or hydroxide on metal-oxo species, (2) coupling of
2
4
4
,3
two metal-oxo groups.
The water nucleophilic attack
Ph P[MnO ] (entry 10) was used instead of KMnO , while a
4
4
4
mechanism has been proposed for the oxygen evolving
4
higher yield of 77% was found for Me N[MnO ] (entry 8). The
4
4
–6
complex (OEC) of photosystem II.
Nucleophilic attack of
use of other solvents such as ClCH
2
CH
2
Cl or CF
3
C
6
H
5
hydroxide on Mn(V)-oxo corrole species has recently been
instead of CH
kinetics of O
volume of O
3
CN resulted in much lower yields of O
2
. The
7
demonstrated. This mechanism has also been proposed for
,8
2
evolution have been studied by measuring the
as a function of time. The rate of formation of
3
,9–11
water oxidation by a number of ruthenium catalysts.
2
Oxygen evolution via coupling of metal oxo groups has also
1
2
O follows clean first-order kinetics (Fig. 1). Representative
2–17
been proposed for a number of systems.
Recently, a
kinetic data are shown in Table 2.
ruthenium(II) dihydroxo complex was reported to undergo
4
The first-order rate constant kO2 is independent of [KMnO ]
1
8
ꢀ
3
ꢀ3
ꢀ2
ꢀ1
photochemical Oꢁ ꢁ ꢁO coupling to generate HOOH.
We report herein that in the presence of BF
undergoes a novel intramolecular coupling of two oxo ligands
in CH CN to generate O . Previously we have reported that
(
1 ꢂ 10 –8 ꢂ 10 M) and [BF ] (3 ꢂ 10 –1.2 ꢂ 10 M).
3
3
, MnO
4ꢀ
This is consistent with the mechanism shown in eqn (1) and
2), with saturation kinetics being reached under the relatively
(
3
2
high [BF ] used, and the observed first-order kinetics is due to
3
ꢀ
the oxidation of alkanes by KMnO in CH CN is greatly
4
3
unimolecular decomposition of [2BF ꢁMnO ] . Unfortunately
3
4
1
9
accelerated by BF3.
suggest that at [BF
Kinetic and mechanistic studies
ꢀ
attempts to study the kinetics at lower [BF ] were unsuccessful,
3
3
ꢀ
3
] = (0.1–3) ꢂ 10 M and [MnO
4
] =
ꢀ4
(
0.5–1) ꢂ 10 M in CH
3
CN, a 1 : 1 adduct is formed, i.e.
ꢀ
[
BF
3
ꢁMnO
4
] , which is the active species responsible for the
4ꢀ
Table 1 %a Yield of O
2
obtained by BF
3
/MnO under various
oxidation of C–H bonds. At higher concentrations of BF
ꢀ
3
conditions
2
ꢀ3
(
>5 ꢂ 10ꢀ M) and MnO
4
(>5 ꢂ 10
M), there is
ꢀ
Entry Complex
BF
3
[equiv.] Solvent
CH CN
2
Yield of O [%]
evidence that the predominant species is [2BF
3
ꢁMnO
4
] . DFT
calculations show that the energy barrier for the oxidation of
ꢀ
1
2
3
4
5
6
7
8
9
1
1
K[MnO
4
]
K[MnO4]
1.8
3.6
7.2
14.4
8.0
8.0
8.0
8.0
8.0
8.0
8.0
3
25 ꢃ 5
54 ꢃ 5
66 ꢃ 5
65 ꢃ 5
58 ꢃ 5
33 ꢃ 5
ꢀ
3
CH CN
CH by [2BF ꢁMnO ] is lower than that of [BF ꢁMnO ] by
4
3
4
1 19
.
3
4
K[MnO
4
]
]
CH
CH
CH
3
3
3
CN
CN
CN
ꢀ
about 6 kcal mol
K[MnO
4
n
Bu
Bu
Bu
4
4
4
N[MnO
N[MnO
N[MnO
4
4
4
]
]
]
]
]
n
n
CF
ClCH
CH CN
ClCH CH
CH CN
ClCH CH
3
C
6
H
5
a
2
CH
2
2
2
Cl 15 ꢃ 5
Institute of Molecular Functional Materials and Department of
Biology and Chemistry, City University of Hong Kong,
Tat Che Avenue, Hong Kong, China. E-mail: bhtclau@cityu.edu.hk,
kaichung@cityu.edu.hk; Fax: +(852) 34420522;
Tel: (+852) 34427811
Me
4
Me
4
N[MnO
N[MnO
4
3
77 ꢃ 5
Cl 7 ꢃ 5
64 ꢃ 5
4
2
0
1
Ph
Ph
4
P[MnO
P[MnO
4
]
]
3
4
4
2
Cl 4 ꢃ 5
b
a
College of Chemistry, Sichuan University, Chengdu 610064,
People’s Republic of China
KMnO , 12.6 mM; solvent, 10 ml; BF
4
ꢁCH CN, 1 ml; T = 20 ꢃ 1 1C;
ꢀ
2 4
3
3
time = 10–15 min. % Yield = (mole of O
used) ꢂ 100%.
produced/mole of [MnO ]
w Electronic supplementary information (ESI) available: Experimental
details and DFT calculations. See DOI: 10.1039/c1cc00019e
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4159–4161 4159

View Article Online
Fig. 1 Plot of O
2
ꢀ
production versus time for the reaction between
ꢀ1
3
KMnO
4
(7.6 ꢂ 10 M) and BF
3
ꢁCH
3
CN (1.2 ꢂ 10 M) in CH
3
CN
4 3
Fig. 2 ESI-MS of the reaction mixture of KMnO and BF in
at 20 ꢃ 1 1C. The black line represents the best fit line.
CH CN at 20 1C.
3
3
6
predominant products, and the amount of
much less than that observed.
O
2
should be
(3)
Table 2 Representative first-order rate constants for O
2
production
from BF
3
/KMnO
4
at 20 ꢃ 1 1C in CH
3
CN
a
ꢀ1
18
ꢀ
16
2
16 18
O - O O
[
KMnO
4
]/M
[BF ]/M
3
k
O2 /s
[2BF
ꢁMn O
]
+ H
3
4
ꢀ
3
3
3
3
3
ꢀ1
ꢀ2
ꢀ2
ꢀ1
ꢀ1
ꢀ3
ꢀ2
ꢀ3
ꢀ3
ꢀ2
7
7
7
3
1
.6 ꢂ 10
.6 ꢂ 10
.6 ꢂ 10
.8 ꢂ 10
.9 ꢂ 10
1.2 ꢂ 10
6.2 ꢂ 10
3.1 ꢂ 10
1.2 ꢂ 10
1.2 ꢂ 10
(9.8 ꢃ 0.9) ꢂ 10
(1.0 ꢃ 0.1) ꢂ 10
(9.8 ꢃ 0.7) ꢂ 10
(9.9 ꢃ 0.8) ꢂ 10
(1.1 ꢃ 0.1) ꢂ 10
4ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
In the oxidation of alkanes by BF
19
/MnO , the manganese
ꢀ
3
2 2 3 4
product is MnO . However in O evolution from BF /MnO
in the absence of the organic substrate, no solid or colloidal
MnO2 was observed. The reactant solutions turned from
purple to pink and then colorless. Analysis of the product
solution by ESI/MS (Fig. 2) shows predominantly the Mn(II)
a
Values of ko2 are the average of at least three successive runs.
ꢀ
ꢀ
2
since the yields of O became much lower and the volumes
fluoro species [MnF
3
] ꢁ2BF
3
and [MnF
3
] ꢁ3BF
3
at m/z 248.1
were too small to be measured accurately.
and 316.0, respectively. There is also a minor peak at m/z
K2
Ð
291.0, which can be assigned to the Mn(III) species
ꢀ
2
BF3 þ MnOꢀ
½2BF3 ꢁ MnO4ꢄꢀ
ð1Þ
ð2Þ
4
[Mn(OBF ) F ] ꢁBF . These results are consistent with
2
2
2
3
ꢀ
3
]
unimolecular four-electron decomposition of [MnO
ꢀ
4
ꢁ2BF
KO2
Ð
2BF3 ꢁ MnO4ꢄꢀ
to give O
2
and [Mn(OBF
2
)
F
2 2
] , the Mn(III) species is unstable
½
O2 þ Mn product
and is further reduced to Mn(II).
1
1
8
8
O-labeling experiments have been performed by using 91%
18 18
O-labeled KMn O . KMn O does not exchange its O
DFT calculations have been performed to gain insight into
the mechanism of the BF -activated O evolution from
4
4
3
2
ꢀ
ꢀ
atoms with water for at least 24 h at room temperature. When
MnO4 . The singlet ground state of MnO4 (T symmetry)
d
.1 ꢂ 10ꢀ M of 91% O-labeled KMn O was reacted with
2
18
18
has four identical oxygen atoms and only one BF -coordinated
3
1
intermediate ( AIM1) of [MnO
1
1
6
4
ꢀ
1
ꢀ
.8 ꢂ 10 M BF
3
34
in CH
3
CN, the gas produced consisted of
32
2
(OBF
3
) ]
2
is identified.
3
6
4ꢀ
6%
O
2
, 34%
O
2
and 0%
O
2
(ꢃ5%). For unimolecular
However, the triplet MnO (C2v symmetry) can form three
ꢀ
3
-coordinated intermediates ( AIM1, BIM1 and CIM1)
3
3
decomposition of [2BF
contain around 83%
3
3
ꢁMnO
4
] , the oxygen evolved should
BF
3
6
34
32
ꢀ
O
2
, 16%
% probably arises from exchange of
with trace amount of H O present in CH CN
O
2
and 1%
O
2
. The
for [MnO (Fig. S1, ESIw).
In Fig. 3, the PESs for the singlet and triplet decomposition
2
3
(OBF )
2
]
3
6
observed lower
O
2
ꢀ
ꢀ
[
2BF
3
ꢁMnO
4
]
2
3
pathways A of [2BF
3
ꢁMnO
4
] , which give the most stable
(
although freshly distilled from CaH
2
2
). In another experiment
products, are depicted. The PESs for triplet decomposition
ꢀ
1
.1 ꢂ 10 M of unlabeled KMnO
4
was reacted with 1.8 ꢂ
pathways B and C are given in ESI.w Both the singlet and triplet
decomposition pathways A are similar, except that: (i) the overall
ꢀ1
ꢀ2
18
2
1
0
M BF in CH CN in the presence of 8.8 ꢂ 10 M H
O
3
3
1
8
97% O-enriched), in this case the gas produced consisted of
ꢀ1
(
reaction barrier (59.7 kcal mol
relative to the reactant
3
2
34
36
3
2% O , 47% O and 21% O (ꢃ5%), again suggesting
intermediate) of the singlet pathway is larger than that
ꢀ
2
2
2
1
(16.6 kcal mol ) of the triplet pathway; (ii) the products of
exchange of KMnO with H O. Although KMnO does not
4
2
4
3
the triplet pathway are O
3
the singlet pathway are O
5
and [MnF
ꢀ
ꢀ
exchange with water at room temperature, it is not unreasonable
–
] to exchange rapidly with water since it is
2
2
(OBF
2
(OBF ] ; (iii) the triplet
2
) ] while those of
2
3
and [MnF
for [2BF
3
ꢁMnO
4
2
2
)
2
expected to be much more susceptible to nucleophilic attack
by water at the manganese centre. The observed first-order
pathway is more exothermic than the singlet pathway.
The theoretical calculations indicate that there is a spin
intercrossing between the triplet and singlet PESs going
kinetics for O
that involves nucleophilic attack of H
MnQO. However if this is the case, then the predominant
2
evolution is also consistent with a mechanism
2
O in CH CN on
3
from the adduct intermediates to Oꢁ ꢁ ꢁO coupling TSs. The
ꢀ
singlet MnO
4
3
first attracts two BF molecules to form the
1
8
34
1
product using KMn O should be O (eqn (3)). Moreover, if
4
intermediate AIM1. Through spin intercrossing, the Mn–O
a/b
2
1
oxygen exchange occurs between KMn O and H O before
8
1
bonds of AIM1 are lengthened to form a triplet Mn(V) peroxo
4
34
2
3
2
3
intermediate AIM2 via ATS. During this process, the
3
water nucleophilic attack, then O and O should be the
2
2
4
160 Chem. Commun., 2011, 47, 4159–4161
This journal is c The Royal Society of Chemistry 2011

View Article Online
ꢀ
4
] . The relative energies
Fig. 3 The potential energy profiles of the singlet and triplet decomposition pathways A of the 1 : 2 adduct [2BF
3
ꢁMnO
are the electronic energy with zero-point energy and thermal corrections at 298 K.
O –Mn–O angle decreases from 109.41 to 46.71 and the Mn–O
a/b
3 S. Romain, L. Vigara and A. Llobet, Acc. Chem. Res., 2009, 42,
1944–1953.
a
b
˚
bond lengths increase by B0.2 A whereas the distance between O
a
4
T. J. Meyer, M. H. V. Huynh and H. H. Thorp, Angew. Chem., Int.
Ed., 2007, 46, 5284–5304.
˚
and O contracts to form a peroxo O–O bond (1.412 A). For this
b
decomposition reaction involving intercrossing of PESs, the
3
5 V. L. Pecoraro and W.-Y. Hsieh, Inorg. Chem., 2008, 47, 1765.
6 E. M. Sproviero, J. A. Gason, J. P. McEvog, G. W. Brudvig and
V. S. Batisda, J. Am. Chem. Soc., 2008, 130, 3428.
ꢀ1
activation barrier height (E
barrierless relative to free reactants) or 31.1 kcal mol (relative
a
) of ATS is –3.0 kcal mol
ꢀ
1
(
7
Y. Gao, T. Akermark, J. Liu, L. Sun and B. Akermark, J. Am.
Chem. Soc., 2009, 131, 8726–8727.
1
z
z
ꢀ1
to AIM1) at the B3LYP/6-31+G(d) level. DG and DS (relative
1
to AIM1) are 33.3 kcal mol
ꢀ1
ꢀ1
K ,
and ꢀ7.6 cal mol
8 S. H. Kim, H. Park, M. S. Seo, M. Kubo, T. Ogura, J. Klajn,
D. T. Gryko, J. S. Valentine and W. Nam, J. Am. Chem. Soc.,
2010, 132, 14030–14032.
3
respectively. The ATS has an imaginary frequency of
ꢀ
1
71i cm , which corresponds to the scissoring motion of
4
9
J. J. Concepcion, J. W. Jurss, M. K. Brennaman, P. G. Hoertz,
A. O. T. Patrocinio, N. Y. M. Iha, J. L. Templeton and
T. J. Meyer, Acc. Chem. Res., 2009, 42, 1954–1965; J. A. Gilbert,
D. S. Eggleston, W. R. Murphy, D. A. Geselowitz, S. W. Gersten,
D. J. Hodgson and T. J. Meyer, J. Am. Chem. Soc., 1985, 107,
O –Mn–O . With the continued elongation of Mn–O bonds,
a/b
a
b
3
the peroxo intermediate AIM2 converts into the intermediate
3
ꢀ1
AIM3 with an energy lowering of 35.7 kcal mol . Finally, a
3
ground-state O molecule is dissociated from AIM3, with the
2
3
855–3864.
ꢀ
manganese product being [MnF (OBF
2
2
)
2
] , which was experi-
10 H.-W. Tseng, R. Zong, J. T. Muckerman and R. Thummel, Inorg.
Chem., 2008, 47, 11763–11773.
11 A. Sartorel, M. Carraro, F. Scorrano, R. De Zorzi, S. Geremia,
N. D. McDaniel, S. Bernhard and M. Bonchio, J. Am. Chem. Soc.,
mentally observed by ESI/MS. We have also studied the
–
decomposition channels of MnO
ꢀ
4
without BF
3
and a 1 : 1
adduct [BF
3
ꢁMnO
4
]
(ESIw). Our computational studies show
2
008, 130, 5006–5007; Y. V. Geletii, B. Botar, P. Kogerler,
that the energy barrier for O
ꢀ
2
evolution follows the order
ꢀ
D. A. Hillesheim, D. G. Musaev and C. L. Hill, Angew. Chem.,
Int. Ed., 2008, 47, 3896–3899; L.-P. Wang, Q. Wu and
T. V. Voorhis, Inorg. Chem., 2010, 49, 4543–4553.
ꢀ
[
MnO
4
]
c [MnO
4
ꢁBF
3
]
> [MnO
4
ꢁ2BF
3
–
] .
In summary, we have shown that MnO
4
reacts with excess BF
] which decomposes to give O via a novel
intramolecular OꢁꢁꢁO coupling mechanism. It is worth mentioning
3
1
´
2 C. Sens, I. Romero, M. Rodrıguez, A. Llobet, T. Parella and
J. Benet-Buchholz, J. Am. Chem. Soc., 2004, 126, 7798–7799.
ꢀ
to give [MnO
2
(OBF
3
)
2
2
13 S. Romain, F. Bozoglian, X. Sala and A. Llobet, J. Am. Chem.
Soc., 2009, 131, 2768–2769.
2
4
ꢀ
that the FeO
ion can be readily activated by Brønsted acids to
1
4 F. Bozoglian, S. Romain, M. Z. Ertem, T. K. Todorova, C. Sens,
J. Mola, M. Rodrıguez, I. Romero, J. Benet-Buchholz,
20
liberate O2. Our results should provide insights for the design of
water oxidation catalysts based on metal-oxo species, it also raises
`
X. Fontrodona, C. J. Cramer, L. Gagliardi and A. Llobet,
J. Am. Chem. Soc., 2009, 131, 15176–15187.
5 L. Duan, A. Fischer, Y. Xu and L. Sun, J. Am. Chem. Soc., 2009,
2
+
the possibility that the role of the Ca ion in photosystem II is to
activate the manganese oxo intermediate.
1
1
1
1
131, 10397–10399.
6 J. Nyhlen, L. Duan, B. Akermark, L. Sun and T. Privalov, Angew.
The work described in this communication was supported
by the Hong Kong University Grants Committee Area of
Excellence Scheme (AoE/P-03-08) and General Research
Fund (CityU 101408). We thank one of the reviewers for very
helpful comments.
˚
´
Chem., Int. Ed., 2010, 49, 1773–1777.
7 Y. Surendranath, M. W. Kanan and D. G. Nocera, J. Am. Chem.
Soc., 2010, 132, 16501–16509.
8 S. W. Kohl, L. Weiner, L. Schwartsburd, L. Konstantinovski,
L. J. W. Shimon, Y. Ben-David, M. A. Iron and D. Milstein,
Science, 2009, 324, 74–77.
1
9 W. W. Y. Lam, S. M. Yiu, J. M. N. Lee, S. K. Y. Yau,
H. K. Kwong, T. C. Lau, D. Liu and Z. Y. Lin, J. Am. Chem.
Soc., 2006, 128, 2851–2858.
Notes and references
1
2
R. Eisenberg and H. B. Gray, Inorg. Chem., 2008, 47, 1697–1699.
T. A. Betley, Q. Wu, T. V. Voorhis and D. G. Nocera, Inorg.
Chem., 2008, 47, 1849–1861.
20 J. Lee, D. A. Tryk, A. Fujshima and S. M. Park, Chem. Commun.,
2002, 486–487.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4159–4161 4161