Chemistry Reference
In-Depth Information
also readily form peroxo complexes with H
2
O
2
but
are generally more selective catalysts with
t
-butyl
hydroperoxide (TBHP) than with hydrogen peroxide
(although, for example, enantioselective S-oxidation
can be achieved with chiral vanadium-complexes
and H
2
O
2
). Indeed, the h
2
-peroxo complexes of vana-
dium and molybdenum are themselves good cata-
lysts for TBHP oxidations.
With H
2
O
2
, vanadium exhibits some one-electron
(V(V)-(IV)) redox chemistry, introducing free-
radical character into its reactions. This makes epox-
idation non-stereoselective and also can change
chemoselectivity. For example, when substituted in
silicalites (see later), titanium (TS-1 and -2) oxidises
toluene mainly at the nucleus to give cresols,
whereas vanadium (VS-1 and -2) oxidises more at
the side chain to give benzylic products—taken as
evidence of parallel electrophilic and radical mecha-
nisms [104]. This radical character can be useful, e.g.
in alcohol oxidations where vanadium systems are
more active than molybdenum, especially towards
primary alcohols. Another chemoselectivity effect in
silicalites concerns allylic alcohols, which are epoxi-
dised mainly by TS-1 but with VS-1 undergo mainly
alcohol oxidation [105].
Oxygen can be used as co-oxidant because the
radical intermediates can capture oxygen from the
atmosphere, even to the extent where H
2
O
2
is act-
ing more as a radical initiator than a stoichiometric
oxidant. The Fenton-like activity of vanadium com-
plexes with azine carboxylic acids (2-picolinic acid,
4-heptyl-2-picolinic acid and pyrazinecarboxylic
acid) has been explored quite thoroughly [106],
even extending to attack on methane [107].
A great deal of work has been reported recently
on mimics of vanadium bromoperoxidase enzymes.
Bromide with V/H
2
O
2
systems can provide an ef-
fective system for halogenation and for hydride
abstractions such as alcohol oxidation at moderate
pH (uncatalysed bromide and H
2
O
2
only work in
strong acid) [108]; molybdenum behaves similarly
[109], as does MTO [110]. There is evidence for a
bound active halogen species in both enzyme and
mimics.
A final practical note is that, owing to the one-
electron chemistry, much more decomposition of
excess H
2
O
2
is caused in vanadium systems than in
W, Mo and Re. This requires either better control of
addition rates or the effective capture and use of the
oxygen generated, with regard for safety issues.
3.3 Polyoxometallates and heteropolyanions
This is a group of polynuclear oxoanion complexes
usually based on tungsten or molybdenum. They
often include structural heteroatoms, which may
be di- to pentavalent, and one or more main atoms
can be substituted by transition metals, giving
additional one- or two-electron redox chemistry.
Common structure types are Keggin (XM
12
) (Fig.
11.14), Wells-Dawson (X
2
M
18
) and 'sandwich'
(M
9
X·Y
4
·XM
9
). They have the attraction of being
fully inorganic and therefore not prone to oxida-
tive degradation, although the equilibria involved in
their formation are subtle and intricate. A bewilder-
ing range of structure options exists, where unit size,
main and heteroatom, substituent, degree of substi-
tution and topomerism can be varied. The catalysis
is based mainly on metal redox/oxo-metal chem-
istry, but peroxo-metal chemistry sometimes can
also be involved.
Adding the Keggin complex [PW
12
O
40
]
3-
catalyses
epoxidations with H
2
O
2
. However, it has been shown
that the active species are the same as in the Ven-
turello system (see earlier), arising from breakdown
of the Keggin structure [111]. This illustrates two
factors involved with polyoxometallate dissociation:
addition of H
2
O
2
itself promotes dissociation because
it is a strong 'ligand' for W and Mo; and dissociation
is a nucleophilic process that occurs readily when
the complex has a relatively low charge. In fact, the
silicon analogue [SiW
12
O
40
]
4-
is much more stable
Fig. 11.14
Ball-and-stick representation of Keggin XM
12
heteropolyacid.
