2 Homometallic Alkoxides

Homometallic Alkoxides 127
Interesting structures for oxime and N,N-diethylhydroxylamine derivatives of metals
and metalloids634,635 have been suggested, mainly on the basis of molecular weight
measurements and spectroscopic studies, but none of these have been so far elucidated
by X-ray crystallography.
For example, MONMON six-membered ring structures have been suggested835 for
Me2MONDCMe2 (M D Al, Ga, In, Tl). However, mass spectral studies of trimethyltin
cyclohexanone oximate gave evidence for the dimeric units having four-membered
cyclic bridges836 O
O N
Sn Sn rather than the six-membered ring Sn Sn In
O
N O
view of the variety in the ligating modes of oximes and hydroxylamines as shown in
Fig. 2.12, X-ray crystallographic studies of homoleptic derivatives and mixed ligandalkoxide
compounds assume considerable importance and may be expected to reveal
some exciting structural features, some of which have already been established for the
products:
[f 5-C5H5)2Gd( - 2-ONDCMe2⊳g2], 837 ( 5-C5H5)(OC)2Mo( 2-ONDCR2), 838 and
(OC)4Fe( -NO)( -NR)Fe(CO)4 (where R represents DCMe2). 839
N-Methylhydroxylamine (MeNHOH) acts as a bidentate ligand through both N
and O atoms to form three-membered chelate rings (Fig. 2.12d), which has been
demonstrated by X-ray crystallography. 840,841 N,N-Diethylhydroxylamine and N,N-dit-butylhydroxylamine
have also been shown to bind molybdenum(IV) 842 and palladium(II),
843 respectively, in this manner.
4.9 Reactions with Schiff Bases and b-Ketoamines
Similar to reactions of metal alkoxides with mono- and bidentate hydroxylic reagents
described in earlier sections, their reactions with Schiff bases and ˇ-ketoamines offer
a variety of synthetic and structural possibilities, as illustrated in the following brief
account.
Schiff bases involve essentially the azomethine group in addition to some functional
groups such as —OH, —SH, —COOH, present in the ortho position of the phenyl
group bonded either to azomethine carbon or nitrogen or to both. 843 Schiff bases
derived from ˇ-diketones and related ligands are known as ˇ-ketoamines. The Schiff
bases (including ˇ-ketoamines) are obtained by the condensation of reagents such as
salicylaldehyde, 2-hydroxy-1-naphthaldehyde, 2-hydroxy acetophenone, 1-hydroxy-2acetonaphthone,
or ˇ-diketones and related ligands with primary amines, diamines,
hydrazines, monoalkanolamines, or 2-aminopyridine. The resulting ligands bind the
metals through the available NO, NNO, ONNO, ONS donor sites in the ligand and
depending upon the number of functional groups and coordinating centres, these may be
classified as (i) monofunctional bidentate, (ii) bifunctional tridentate, and (iii) bifunctional
tetradentate ligands (Fig. 2.15), and so on.
Although many routes for the synthesis of metal complexes of Schiff bases and ˇketoamines
are available, 844–846 the facile reactivity of metal alkoxides has been utilized
for the synthesis of homo- and heteroleptic Schiff base and ˇ-ketoamine derivatives of
metals and metalloids with advantage. The most interesting results concern the Schiff
bases and ˇ-ketoamines shown in Fig. 2.15, and other ligands obtained by suitably
modifying them.