2 Homometallic Alkoxides

Cl
S
N N
OH
O
Cl
Me 3C
NH HN
OH
HO
HO
Cl
O
H H
N N
CMe3
OH HO
Me3C
Scheme 6.5
Cl OH N
HO
CMe 3
N
N
H
N
OH
HO
Metal Aryloxides 451
Changing the diammine backbone and/or introducing substituents at the ˛-carbon or
within the phenoxy ring can easily modify these tetradentate ligands. Ligands derived
from resolved, chiral diammines have been employed in the synthesis of catalysts
for asymmetric epoxidation, and are finding utility in other areas (Scheme 6.5). 60–62
The presence of one or more donor groups in the diammine backbone leads to pentaand
hexadentate ligands, some of which favour the formation of dinuclear complexes
(Scheme 6.5).
2.2.4 Macrocyclic Bis-phenoxides
The pioneering work of Robson and Pilkington showed the metal-mediated template
condensation of 2,6-diformyl-4-methylphenol with 1,3-diaminopropane to produce the
dimetal derivatives of a new macrocyclic bis-phenoxide ligand. 63 These ligands favour
formation of binuclear species with the metal centres bridged by the two phenoxide
oxygen atoms. 64 The parent ligand (Scheme 6.6) has also been synthesized directly
from the organic substrates in the absence of a metal template. 65 This initial work
was followed by the rapid development of related compartmentalized ligands, e.g.
partially and totally saturated analogues, 66 ligands with differing size chelate bridges, 67
nonsymmetric coordination environments for the phenoxide bridged metals, 68 and the
introduction of extra donor groups into the chelate backbone. 69–72 Much of the work in
this area and the related studies utilizing binucleating mono-phenoxides (Section 2.1.3)
has been stimulated by the expectation of mimicking the active sites in various
metalloenzymes. 73