2 Homometallic Alkoxides

Heterometallic Alkoxides 197
In addition to hetero-bimetallic and -trimetallic alkoxides, some heterotetrametallic
alkoxides have also been reported 9 as illustrated already by Eqs (3.76) and (3.77).
In spite of considerable initial scepticism, the synthesis of an increasing
number of well-characterized heterometallic alkoxides is now well established and
attempts are being directed towards the synthesis of “single-source” precursors for
preparation of mixed metal–oxide ceramic materials by the sol–gel or MOCVD
processes. 12,20,22,24–27,43 Before proceeding to a brief account of some novel types of
heterometal glycolate and aminoalkoxide derivatives, it is appropriate to mention four
review articles 47,163–165 summarizing the state of knowledge about the conventional
heterometallic alkoxide systems at the end of the twentieth century.
2.4 Synthesis of Heterometallic Glycolate Alkoxide Derivatives
The facile dissolution of alkaline earth metals in alcohols in the presence of alkoxides
of metals like aluminium (Section 2.1.2) has been ascribed to the formation of
alcohol adducts of the metal alkoxides R O Al(OR)3 rendering the proton of
H
the adduct alcohol more labile. A similar type of lability of protons of glycol molecules
coordinated in derivatives like titanium trisglycolate166 has been utilized43,167,168 for
synthesis of bi- and triheterometallic glycolate alkoxides by reactions of the types
shown in Eqs (3.87)–(3.89):
M(OPr i )4 +
3G OH
OH
benzene
G
+ 4Pri O
M
O
G OH ↑
O
H
2 O
(1) Soluble
where M = Ti, Zr, Hf, Sn(IV); G OH
= 2-methylpentane-2,4-diol.
OH
(1) + 2Al(OPr i ) 3
(1) + 2Nb(OPr i ) 5
[{Al(OPri ) 2} 2 ] + 2Pri M OH ↑
O G
(2) Soluble
[{Nb(OPr i )4}2
(3.87)
O 3 (3.88)
M O G
O 3
] + 2Pr i OH ↑
(3.89)
These two reactions can also be carried out in two consecutive stages of 1:1
molar reactions with Al⊲OPr i ⊳3 and Nb⊲OPr i ⊳5, yielding a heterotrimetallic derivative
{M O [{Al(OPr G
O
i )2}{Nb(OPri )4}
}].
3