2 Homometallic Alkoxides

Industrial Applications 673
compounds were considerably more volatile than the trinuclear tert-butoxides and the
diglyme complexes sublimed at ca. 120 Ž C in vacuo without loss of diglyme. However,
it is noteworthy that thermal decomposition of fluorinated alkoxy compounds may
produce metal fluoride or oxyfluoride instead of metal oxide although this may be
prevented by incorporating oxygen or water vapour into the system.
Purdy and George 15 have obtained the volatile copper compounds Cu4fOC⊲CF3⊳3g7
(sublimes at room temperature in vacuo) and [CuOC⊲CF3⊳3]n (sublimes 40–50 Ž C
in vacuo) and the remarkably volatile barium copper heterometal alkoxide
Ba[CufOCMe⊲CF3⊳2g3]2 (sublimes 70–90 Ž C in vacuo). The X-ray crystal structure of
the barium copper complex shows the presence of a monomeric molecule containing
trigonal planar three-coordinated Cu(II) and with the barium atom closely coordinated
to four alkoxide oxygens and more weakly by intramolecular interactions with
eight fluorines. Other volatile copper compounds are the Cu(I) mixed ligand species
[Cu⊲OBu t ⊳x ⊲OR⊳1 x ] (R D C⊲CF3⊳3, CMe⊲CF3⊳2, CH⊲CF3⊳2; x ¾ 0.5) 16 and the
copper(II) mixed alkoxides [Cu4⊲OBu t ⊳6fOC⊲CF3⊳3g2]. 17a The X-ray crystal structure
of the tetranuclear copper(II) complex showed a linear tert-butoxy-bridged species with
the two internal copper atoms in distorted tetrahedral coordination and the two outer
copper atoms in trigonal planar coordination involving two bridging tert-butoxides
and one terminal perfluoro-tert-butoxide. The MOCVD process has been used for the
deposition of ZrO2, 17b PbTiO3 17c and SrTiO3. 17d
2.2 Spray Coating and Flash Evaporation of Metal Alkoxide Solutions
As an alternative process to MOCVD, which requires a volatile metal alkoxide, there is
the technique of depositing a film of metal alkoxide onto a substrate from solution in a
volatile solvent followed by thermolysis. Thus oligomeric metal alkoxides [M⊲OR⊳x ]n
that may not be appreciably volatile can by suitable choice of the alkyl group R be made
soluble in volatile organic solvents. This method is especially useful for depositing a
multicomponent heterometal oxide because the stoichiometry can be determined by the
initial concentration of each component metal alkoxide. This technique is similar in
principle to the sol–gel technique which is favoured for the formation of bulk materials
(Section 3).
A number of applications of this technique have been reported. For example,
single layer or multi-layer coatings of SiO2/metal oxide (metal D aluminium, titanium,
or zirconium) have been produced from solutions of hydrolysed alkoxysilane/metal
alkoxide mixtures for protection of electronic devices. 18 Alkoxysiloxy transition metal
complexes [MfOSi⊲OBu t ⊳3g4] (MD Ti, Zr or Hf) have been shown to be single-source
precursors for low-temperature (ca 150 Ž C) formation of MSi4O10 oxide materials. 19
Thin porous membranes of mixed TiO2/SiO2 oxides have been produced from
the alkoxides to form the tubular channels of a ceramic cartridge for tangential
microfiltration. 20 Other applications are the manufacture of crystalline fine TiO2
particles, 21 the deposition of nonreflective or selectively reflective films of TiO2 on
glass, 22 and the deposition of a thin film of metal oxide (Al, Zr, Ti, Si, Y, or Ce)
on stainless-steel or nickel supports to improve the bonding to -Al2O3 catalyst. 23
Although small particles of TiO2 (