Chemistry Reference
In-Depth Information
acting as a catalyst is the simple square planar Rh(I) complex [RhCl
P(C
6
H
5
)
3
}
3
],
Wilkinson's catalyst, used for the hydrogenation of terminal alkenes (see Figure 6.1).
It involves an intermediate where both hydrogen and the alkene are coordinated, allowing
an intramolecular reaction between these to form an alkane product - effectively a reaction
of coordinated ligands, but one where the product can depart, so that the original complex
is reformed, allowing the process to occur again, a key requirement for catalysis.
There are a number of others in common industrial use, such as those used for polymer-
ization of alkenes. One example of an organometallic homogeneous catalyst is the Zr(IV)
complex [Zr(CH
3
)(
{
5
-Cp)
2
X], which operates by binding alkene monomers to the metal
prior to addition to a growing carbon chain. A similar coordination of substrate is involved
in the use of [Co(CO)
4
H] as catalyst in the hydroformylation of alkenes to aldehydes.
Likewise, the [Rh(CO)
2
I
2
]
−
ion, formed
in situ
, catalyses the carbonylation of methanol to
acetic acid.
Another catalyst type is the heterogeneous catalyst, which remains as a solid and pro-
motes chemistry at the surface. To function well, they require high surface areas per unit
mass. Metal oxides and hydroxides are common examples. A vanadium(V) oxide is em-
ployed in the formation of ammonia from nitrogen and hydrogen under elevated temperature
and pressure, for example. Polyoxometallate metal clusters, which are oxo-ligand coordi-
nation complexes employing dominantly O
2
−
and HO
−
as ligands, have some catalytic
roles.
The two types of catalysts merge with the development of what are called 'tethered'
catalysts, where a homogeneous catalyst is amended so that it is able to be attached
covalently to an inert surface, such as silica. This is also called a supported catalyst. Having
the active component available as part of a solid can assist processes where carrying the
catalyst forward in solution to another stage of the process may lead to contamination or
catalyst destruction. Further, surface attachment also can alter catalytic activity favourably
in certain cases.
9.3.2.2
Complexes as Nanomaterials
Nanotechnology is one of the new frontiers of chemistry - the development of new mate-
rials with well-defined structure within the size range of from 1 to
100 nm. In particular,
the properties of the nanomaterial should differ from those of both conventional molec-
ular compounds and bulk solids, and it this difference that is the key to their attraction
and potential applications. This definition can include metal complexes, particularly larger
polymetallic systems. Of course, many biomolecules fall within the definition of nanoma-
terials in terms of size, but the interest in the field at present lies in the development of
synthetic more than natural materials.
Fabrication of nanomaterials divides into two approaches - 'top down', which relies on
starting with bulk materials and processing them to yield nanoscale materials, and 'bottom
up', which employs atomic and molecular species aggregated to form larger nanoscale
materials. It is the latter of these two approaches that may involve coordination chemistry.
A simple example may suffice to display this in application. It is possible to produce
(cation)
+
[AuCl
4
]
−
as a finely-divided particulate by choosing an appropriate cation that
limits complex solubility in the chosen solvent. In the presence of a thiol (RSH), substitution
of chloride ligands on the gold(III) by thiolate (RS
−
) ligands can occur. When the resultant
gold(III) thiolate is treated with a suitable reducing agent, the Au(III) is reduced to form
nanoscale metallic gold particles that remain coated with thiol, of type
∼
Au
x
(RSH)
y
}
.This
{
