2 Homometallic Alkoxides

6 SURVEY OF METAL ARYLOXIDES
6.1 Special Metal Aryloxides
Metal Aryloxides 485
Although technically containing metal–aryloxide functionalities, a number of ligand
types can be considered as special examples of aryloxides. In most of these special cases
the metal–aryloxide bond is either geometrically constrained or electronically attenuated
by other functionalities within the ligand. Detailed discussion of metal catecholates
and related 2-thio and 2-aminophenoxides, pyridones, salicylaldimines, and calixarenes
is omitted from this chapter. Tables 6.5–6.56 contain details of crystallographically
determined structures. These tables focus mainly on simple aryloxide ligands and
exclude the extremely large database of structures (mainly later first row d-block
metals) in which the aryloxide function is part of a multidentate or macrocyclic ligand.
6.1.1 Metal 8-Hydroxyquinolates
Metal derivatives of 8-hydroxyquinoline are an extensive and important class of aryloxide.
Derivatives are known for nearly all metals and are sometimes referred to as
oxinate derivatives. Synthetic methods involve straightforward addition of the parent
quinol to metal halides, alkyls and amides as well as an extensive aqueous chemistry. A
large number of metal “oxinates” have been subjected to single-crystal X-ray diffraction
(Table 6.5). The most common bonding mode is terminal, although there are significant
examples in which the oxygen atom bridges two metal centres and a few examples of
triply bridging quinolates. In nearly all derivatives of 8-hydroxyquinoline the nitrogen
atom is also coordinated, leading to a five-membered chelate ring. Structural studies
show M–O–Ar angles are constrained to values of 110–120 Ž in such bidentate derivatives.
This could possibly restrict the amount of oxygen to metal -donation that can
occur, although the presence of the adjacent pyridine ring will also decrease -donation.
One interesting molecule in this context is [Ti(2Me-8-quin)2(OC6H3Pr i 2 -2,6)2] where
the unrestrained terminal aryloxide bond, 1.819 (7) ˚A [157 Ž ], is significantly shorter
than the quinolate bond length, 1.911 (6) ˚A [123 Ž ].
6.1.2 Metal Salicylaldehydes
A large number of metal derivatives of salicylaldehyde are known. Although the most
extensive use has been made of the parent ligand, recent work has also focussed on
the use of ligands substituted either within the phenoxy ring or at the ˛-carbon atom.
Structural studies (Table 6.6) show that in nearly all cases chelation via the aldehyde
group occurs leading to a constrained M–O–Ar angle of 120–140 Ž . This chelation raises
the question of what is the correct bonding description for these molecules. It is possible
to draw two distinct resonance structures for the six-membered dioxa-metallacycle ring.
O O O O
M
M