2 Homometallic Alkoxides

448 Alkoxo and Aryloxo Derivatives of Metals
the ortho position has been explored. One development has been the synthesis of
2-hydroxyphenyl-bis(pyrazolyl)methane ligands. These mono-anionic groups, termed
“heteroscorpionate” ligands, have some of the coordination characteristics of traditional
tris(pyrazolyl)borates but contain a metal–aryloxide bond. 23–26
2.1.2.1 Cyclometallated aryloxides and ligands chelated through carbon
The simple coordination of aryloxides to metal centres is sometimes followed by
carbon–hydrogen bond activation, which leads to new oxa-metallacycle rings such as
those shown in the top part of Scheme 6.2. The cyclometallation of simple phenoxide
as well as various 2-alkyl- and aryl-phenoxides leads to 4-, 5-, and 6-membered
metallacycles. 27,28 It is also possible for o-aryl substituents to chelate to metal centres
via -arene interactions. 29 The intramolecular dehydrogenation of alkyl groups 30 or
hydrogenation of aryl rings can lead to aryloxides chelated to the metal through -
bonding interactions with the resulting unsaturated olefin or diene functional group. 31
Ligands such as 2-allylphenol can bind simply through oxygen, chelate via an 2 -olefin
interaction, 32 or be converted to an 3 -allyl chelate. 33
O
Bu t
M
Pr i
Me
O
M
O M
Me
Me
O M
Me
Ph
Pr i
Pr i
O
M
O M
O
M
Scheme 6.2
2.1.2.2 Mono-salicylaldimines
By far the largest group of chelating mono-aryloxides are those derived from 2-formyl
phenols and their condensation with primary amines (Scheme 6.3). 34 The resulting
ligands chelate through the phenoxide and the imine functions to generate fivemembered
chelate rings. The ligands are highly amenable to tuning of their steric
and electronic properties through variation of the substituents on nitrogen, at the ˛carbon
or within the phenoxide ring. The condensation of 2-formyl-phenols with more
complex amines has been exploited to yield mono-aryloxide ligands that can chelate
through a multitude of neutral and anionic functional groups.
2.1.3 Binucleating Mono-aryloxides
Important classes of mono-aryloxides are those containing chelating groups on either
side of a central phenoxide function. The geometrical constraints of such ligands
Me
Me
Me