2 Homometallic Alkoxides

Homometallic Alkoxides 119
with the earlier conclusion(s) on the effect of steric factors on the molecular association
of analogous derivatives, [Al⊲OSiPh3⊳2⊲acac⊳] becomes monomeric with a tetrahedral
structure. 729
Dhammani et al. 730–732 have carried out the reactions shown in Eqs (2.224)–(2.226),
characterizing similar products by colligative, IR and NMR ( 1 H, 13 Cand 27 Al) measurements:
fAl⊲CH3COCHCOR⊳⊲OPr i ⊳2g2 C Ph3SiOH
Pr i OH
! [⊲CH3COCHCOR⊳2Al⊲ -OPr i ⊳2Al⊲OSiPh3⊳⊲OPr i ⊳] ⊲2.224⊳
fAl⊲CH3COCHCOR⊳⊲OPr i ⊳2g2 C 2Ph3SiOH
2Pr i OH
! [⊲CH3COCHCOR⊳2Al⊲ -OPr i ⊳2Al⊲OSiPh3⊳2] ⊲2.225⊳
fAl⊲CH3COCHCOR⊳⊲OPr i ⊳2g2 C HOSiPh2OH
2Pr i OH
! [⊲CH3COCHCOR⊳2Al⊲ -OPr i ⊳2Al⊲OSiPh2O⊳] ⊲2.226⊳
where R D CH3, OC2H5, C6H5.
Stepwise reactions of isopropoxides of other tervalent metals (lanthanides, 733–735
gallium, 736 and antimony 737 ) have also been carried out by similar procedures and a
number of isopropoxide-ˇ-diketonates as well as tris-ˇ-diketonates have been characterized
by physico-chemical techniques.
By contrast, homoleptic tetrakis-ˇ-diketonates of titanium and tin(IV) could not be
prepared by this route; reactions of Ti(OR)4 and Sn(OR)4 (with R D Et, Pr i ) with
excess of ˇ-diketones/ˇ-ketoesters yield bisalkoxide bis-ˇ-diketonate derivatives only.
The reactions of titanium alkoxides with ˇ-diketones and ˇ-ketoesters in 1:1 and 1:2
molar ratios have been investigated by several workers; 738–740 the 1:2 products were
characterized as monomeric derivatives, but there has been a difference of opinion about
the dimeric or monomeric nature of 1:1 products which awaits further investigation.
In another investigation, 741 Ti(OEt)2(acac)2 and Ti⊲OPr i ⊳2⊲acac⊳2 wereshowntobe
monomeric volatile products, in which the alkoxide component(s) have been shown to
be highly reactive. In addition to facile replacement by hydroxylic reagents,
Ti(OEt)2(acac)2 was found to react with 2 mol HCl, to yield a monomeric volatile
product TiCl2(acac)2 which could be converted into Ti(OEt)2(acac)2 by reaction with
EtOH in the presence of anhydrous NH3; this led to a correction 742–745 of the wellquoted
trimeric nature of the covalent TiCl2(acac)2 as fTi⊲acac⊳3g2TiCl6 746 on the basis
of its reaction with FeCl3 to yield a product of the formula fTi⊲acac⊳3gfFeCl4g, a species
the actual nature of which awaits further elucidation by more refined physico-chemical
investigations (crystal structure or more refined NMR investigations).
In reactions of titanium alkoxides with benzoylacetone, 747 methylacetoacetate 748
and ethylacetoacetate, bis-ˇ-diketonate or ketoester derivatives were the final products
even when excess of these ligands was used. The nonreplaceability of the third
or fourth alkoxy groups with ˇ-diketones or ketoesters may most probably be due
to the preferred coordination number of 6 for titanium in the bis-derivatives. The
trialkoxide monomethylacetoacetate disproportionated to the bis-derivative and tetraalkoxide,
when heated under reduced pressure, whereas bis-derivatives distilled out