The Use of a Vanadium Species As a Catalyst in Photoinduced Water
Oxidation
Marie-Pierre Santoni,*
,†
Giuseppina La Ganga,
†
Viviana Mollica Nardo,
†
Mirco Natali,
‡
Fausto Puntoriero,
†
Franco Scandola,*
,‡
and Sebastiano Campagna*
,†,§
†
Dipartimento di Scienze Chimiche, Universita ̀ di Messina, and Centro Interuniversitario per la Conversione Chimica dell’Energia
Solare (SOLAR-CHEM), sezione di Messina, 98166 Messina, Italy
‡
Dipartimento di Scienze Chimiche e Farmaceutiche, Universita ̀ di Ferrara, and Centro Interuniversitario per la Conversione Chimica
dell’Energia Solare (SOLAR-CHEM), sezione di Ferrara, 44121 Ferrara, Italy
§
ISOF-CNR, 40129 Bologna, Italy
* S Supporting Information
ABSTRACT: The first water oxidation catalyst containing
only vanadium atoms as metal centers is reported. The
compound is the mixed-valence [(V
IV
5
V
V
1
)O
7
(OCH
3
)
12
]
-
species, 1. Photoinduced water oxidation catalyzed by 1, in
the presence of Ru(bpy)
3
2+
(bpy = 2,2′-bipyridine) and
Na
2
S
2
O
8
, in acetonitrile/aqueous phosphate buffer takes
place with a quantum yield of 0.20. A hole scavenging
reaction between the photochemically generated Ru-
(bpy)
3
3+
and 1 occurs with a bimolecular rate constant
of 2.5 × 10
8
M
-1
s
-1
. The time-resolved formation of the
oxidized molecular catalyst 1
+
in bimolecular reactions is
also evidenced for the first time by transient absorption
spectroscopy. This result opens the way to the use of less
expensive vanadium clusters as water oxidation catalysts in
artificial photosynthesis schemes.
A
rtificial photosynthesis, that is the conversion of light
energy into chemical energy, is currently attracting much
interest, for both fundamental and applicative reasons.
1-5
The
achievement of an efficient artificial photosynthesis, aimed to
produce high-energy content chemical species from low-energy
content chemical species using solar energy as the energy
source, would be a sort of Holy Grail of modern science,
2
as it
could solve the problems connected with the intermittency and
low-density of solar energy.
4,5
Moreover, solar energy is diffuse
all over the world, and the social and political problems
somehow connected with localization of fossil energy sources
would be largely alleviated.
4
Any scheme of artificial photosynthesis includes water
oxidation as a crucial step.
6
As a consequence, in the past few
years much work has been devoted to identify new good
catalysts for water oxidation and integrate them into photo-
synthetic schemes. Among molecular catalysts, whereas
probably the most efficient water oxidation molecular catalysts
are ruthenium compounds,
7-10
catalysts based on earth-
abundant metals are particularly attractive. Water oxidation
catalysts containing only earth-abundant metals such as
cobalt,
11
manganese,
12
iron,
13
copper,
14
and molybdenum
15
have been studied. Surprisingly, in spite of the rich redox
chemistry displayed by vanadium compounds,
16
no vanadium
species has been reported to behave as a molecular water
oxidation catalyst yet.
Here we report on the photoinduced water oxidation
obtained by using a sacrificial system made of Ru(bpy)
3
2+
(bpy = 2,2′-bipyridine) as a photosensitizer, Na
2
S
2
O
8
as the
sacrificial acceptor, and a methoxo-polyoxovanadium cluster,
namely (N-Bu
4
)[V
6
O
7
(OCH
3
)
12
](1; Bu = n-C
4
H
9
),
17
as the
water oxidation catalyst. It is the first time, to the best of our
knowledge, that a vanadium species is demonstrated to play the
role of a water oxidation catalyst, and this result introduces the
large and tunable class of polyoxovanadate species into the
field, paving the way for further exploration. Moreover, whereas
in most of the formerly studied polyoxometalated water
oxidation catalysts the polyoxometalated unit(s) were mainly
used as stabilizing units, with the catalytically active metals not
belonging to the polyoxometalated scaffold,
9,11d,h
in the present
case the catalytically active center is the polyoxometalated
framework.
Compound 1 (see Figure 1) has been previously reported by
Daniel and Hartl;
17
it is a cluster formally derived from the
highly symmetrical Lindqvist structure [M
9
O
19
]
n-
,
18
where the
12 μ-bridged oxo ligands are substituted by methoxo ligands.
Alkoxo-polyoxovanadium clusters similar to 1 exhibit a quite
rich redox activity, with a series of thermodynamically stable
redox isomers of general formula [V
IV
n
V
V
6-n
O
7
(OR)
12
]
(4-n)
(R
= alkyl groups), of which several species have been synthesized
and characterized.
17
In particular, compound 1 exhibits six one-
electron redox processes in acetonitrile (although not all of
them are reversible), in the potential window -1.0 ÷ +2.2 V vs
ferrocene/ferrocinium couple.
17b
The whole range of electro-
chemical transformation is included, from full reduction to the
isovalent vanadium(IV) cluster [V
IV
6
O
7
(OCH
3
)
12
]
2-
to com-
plete oxidation to the isovalent vanadium(V) cluster
[V
V
6
O
7
(OCH
3
)
12
]
4+
.
17b
The absorption spectrum of each
redox isomer has also been identified,
17b
evidencing the
presence of intervalence bands, which allowed classification of
the mixed-valence isomers as class II species, according to the
Robin and Day formalism.
19
The redox isomer we prepared,
Received: April 22, 2014
Communication
pubs.acs.org/JACS
© XXXX American Chemical Society A dx.doi.org/10.1021/ja5040182 | J. Am. Chem. Soc. XXXX, XXX, XXX-XXX