Chemistry Reference
In-Depth Information
O
O
O
O
O
M
OH
2
Cl
N
N
O
O
Fe
L'
L
Cl
N
N
Fig. 11.12
Metal-di-(
h
2
-peroxo) complex (Mimoun).
O
O
Fig. 11.11
Iron tetra-amide complex (Collins).
to the left and down each row. Nucleophilic d
8
peroxo complexes, which are also catalytic, are
not believed to compare with these in commercial
potential.
Catalysis using tungsten(VI) and molybdenum(VI)
complexes via the formation of peroxo-metal inter-
mediates has been known for over 50 years, but
important advances continue to be made in under-
standing and improving this mode of H
2
O
2
activa-
tion. Peroxo chemistry based on vanadium,
molybdenum and tungsten complexes has been sur-
veyed [49]. It is, in fact, one of the most versatile
systems and comes closest to classical organic
peracids in its range of applications. The key catalytic
intermediate is an h
2
-peroxometal species (one or
two peroxo groups usually are attached in this way)
that acts on the substrate either directly or via pro-
tonation to give the M-O-O-H species (Fig. 11.12).
Substrates may be coordinated to the metal centre
(e.g. in alcohol oxidation, by hydride abstraction) or
un-coordinated (e.g. in epoxidation, according to the
consensus of opinion).
Both molybdenum and tungsten work effectively
in aqueous systems, unlike many catalysts, owing
to their high affinity for H
2
O
2
. If the substrate is
hydrophobic, two-phase systems are commonly
used, the peroxo complex being taken into the
organic phase by suitable ligands and/or cationic
phase-transfer agents. The simple complexes are
moderately active in epoxidation [89], N-oxidation
[90], alcohol oxidation [91], etc., with tungsten
being better except for alcohol oxidation. Activity
sometimes can be raised by increasing temperature
much more robust to oxidation, and recent reports
suggest significant potential of iron amide complexes
[81] as catalysts for H
2
O
2
oxidations (Fig. 11.11).
Hydrogen peroxide can be used for aromatic side-
chain oxidations, catalysed by cobalt and/or man-
ganese compounds in acetic acid, in the presence of
a bromide co-catalyst: this is discussed later.
There have been several attempts at encapsulation
[82] of some of the above types of complex (and
others such as Mn(bpy)
3
) in zeolite cavities (e.g.
zeolite Y) [83], smectite interlayers (e.g. montmoril-
lonite, layered double hydroxide) [84], mesopore
channels (e.g. MCM-41) [85], amorphous silica [86]
or membranes (e.g. polydimethylsiloxane) [87], all
using many synthetic approaches. So far, moderately
good catalysts have resulted in a few cases. Transport
of reactants and products within the support and
space around the active site are common limitations
for 'ship in a bottle' catalysts [88], which are more
difficult to solve than for framework-substituted cat-
alysts. Only quite low loadings can be tolerated, to
ensure adequate mobility. Smectites have the option
of pillaring to increase interlayer volume, and are
thought generally to be under-explored.
3.2 Peroxo-metal systems
This section refers to electrophilic peroxo complexes
of d
0
metals, which are formed by several elements
under Ti, V, Cr and Mn in the Periodic Table, the
relevant oxidation state being favoured as one moves
