2 Homometallic Alkoxides

R1
OH O
R2
R3NH2 −H2O Scheme 6.3
R1
OH N
R2
R3
Metal Aryloxides 449
typically preclude chelation of all donor groups to a single metal centre. Instead
these ligands form highly stable dinuclear complexes in which two metal centres are
bridged by the phenoxide oxygen atom and, typically, other groups. 35 Two common
synthetic pathways to ligands of this type proceed via either 2,6-diformyl- or 2,6di(chloromethyl)phenols
(Eq. 6.3) as intermediates. 36
CH3
Cl OH Cl
+ 2HN(CH2py) 2
+ 2NEt3
− 2[HNEt 3][Cl]
CH3
(pyCH2) 2N OH N(CH2py) 2
(6.3)
Non-symmetrical ligands have also been designed such that they impose differing
coordination environments about each metal centre. 37
2.2 Bis-aryloxides
2.2.1 Catechols and Related Ligands
The study of metal derivatives of 1,2-dihydroxybenzene (catechol) is an important
area of inorganic chemistry. 38 This importance is emphasized not only by the presence
of catecholate binding sites in metalloproteins (e.g. enterobactin, which mediates iron
uptake by Escherichia coli and related bacteria), 39 but also by the existence of enzymes
such as catechol dioxygenases, which are involved in oxidative degradation of aromatic
hydrocarbons. 40,41 It is also sometimes possible to generate identical metal complexes
using orthoquinones as the ligand precursor. Hence the relative importance of the
dioxy, quinone, and intermediate semi-quinone forms tends to dominate discussions
of the spectroscopy, structure, electrochemistry, and reactivity of metal complexes of
these ligands. 38 The chemistry of catechols containing alkyl and halide substituents as
well as derivatives of other aromatic hydrocarbons such as 9,10-dihydroxyphenanthrene
has been extensively investigated. 38
A similar situation can occur for hydroquinone (1,4-dihydroxybenzene). However,
in this case the ligand can coordinate to two metal centres. 42 In one study this ability
was utilized to produce interesting covalent three-dimensional networks. 43
Although not technically bis-aryloxides, 2-thio- and 2-amidophenoxides have the
aryloxide bonds considerably perturbed by the ortho heteroatom in a fashion similar
to that in catediolates.