Chemistry Reference
In-Depth Information
6
Metal Aryloxides
1
INTRODUCTION
The metal - aryloxide bond is not only a component of a rapidly increasing number
of inorganic and organometallic compounds,
1
but also occurs in nature in numerous
metalloproteins.
2
The amino acid residue tyrosine has been shown to bond to metals
through the phenoxide oxygen in the transferrins and other proteins while a key compo-
nent of many siderophores is the catecholate function that strongly binds iron. The
inorganic chemistry of phenolic reagents has a long history. Besides the reaction
of simple phenols with metals the phenoxide group is a critical function in many
complexing agents as exemplified by 8-hydroxyquinoline (oxine), which was one of
the earliest analytical reagents.
3
This and related ligands are useful for the colori-
metric and gravimetric determination of metal ions, as well as for their extraction
from aqueous solution.
3
The phenoxide group is also a constituent of many bi, tri, and
polydentate ligands.
4
As with metal alkoxide chemistry, the utilization of non-protic solvent systems has
expanded the variety of metal aryloxide derivatives that can be isolated, including
organometallic derivatives, and the isolation and characterization of simple aryloxide
compounds of even the most oxophilic metals. In this regard mention should be made
of the work of Funk
et al
. who, in a series of studies dating back to 1937,
5
isolated
many transition metal complexes of phenol and its simple substituted derivatives. More
recently there has been a growing interest in the organometallic chemistry associated
with metal aryloxide compounds. Research has focussed both on the reactivity of
the ligands themselves,
e.g.
the insertion chemistry of the M-OAr bonds, and on the
organometallic reactivity that can be supported by aryloxide ligation.
2
TYPES OF ARYLOXIDE LIGAND
There are a plethora of ligand types that contain at least one phenoxide nucleus for
coordination to metal centres. In this chapter we will initially survey the ligands
depending upon the number of phenoxide units present. The chemistry of some of
these ligand types,
e.g.
catechols, calix[
n
]arenes, and macrocyclic ligands, especially
those containing the salicylaldimine unit, will not be exhaustively reviewed. Instead
reference will be given to existing key works.
