2 Homometallic Alkoxides

118 Alkoxo and Aryloxo Derivatives of Metals
R 00 D alkoxy or aryloxy group) contains a reactive hydroxy group, which is prone to
react readily with metal alkoxides to yield alcohol(s) and a variety of interesting (in
terms of compositions, structures, volatility, reactivity, and applicability) homo- and
heteroleptic derivatives of a wide variety of metals and metalloids. 629,723,724 Typical
general reactions are shown in Eqs (2.218) and (2.219):
M⊲OR⊳x C yR 0 COCH2COR 00 ! M⊲OR⊳x y⊲R 0 COCHCOR 00 ⊳y C yROH "
⊲2.218⊳
M⊲OR⊳x C yR 0 COCH2COOR 00 ! M⊲OR⊳x y⊲R 0 COCHCOOR 00 ⊳y C yROH "
⊲2.219⊳
The importance of this synthetic route has been shown in the preparation of anhydrous
tris-ˇ-diketonates of lanthanides, the hydrated forms of which obtained from aqueous
solutions tend to decompose into hydroxy-derivatives. By using isopropoxides or ethoxides
as starting materials and removing the liberated alcohol by fractionation as a lower
boiling azeotrope with a solvent such as benzene, volatile stable intermediate products
may be obtained. The alkoxide groups in these mixed derivatives are much more reactive
than the ˇ-diketonate ligands, as illustrated by Eqs (2.220)–(2.223): 725–727
benzene
! ⊲RO⊳3 x Al⊲OC⊲R 0 ⊳CHCOR 00 ⊳x C xROH "
Al⊲OR⊳3 C xR 0 COCH2COR 00
⊲2.220⊳
where R D Et, Pri and R0 , R00 D CH3; or R0DCH3, R00 D C6H5 or R0 D CH3,
R00 D OC2H5; x D 1–3.
Al⊲OPr i ⊳3 x ⊲ˇ-dik⊳x C ⊲3 x⊳Bu t OH
benzene
! Al⊲OBu t ⊳3 x ⊲ˇ-dik⊳x C ⊲3 x⊳Pr i OH " ⊲2.221⊳
Al⊲OPr i ⊳3 x ⊲ˇ-dik⊳x C ⊲3 x⊳R3SiOH/R3SiOOCCH3
benzene
! Al⊲OSiR3⊳3 x ⊲ˇ-dik⊳x C ⊲3 x⊳Pr i OH "/CH3COOPr i " ⊲2.222⊳
where R D CH3, C2H5; ˇ-dik D acetylacetonate; x D 1, 2.
Al⊲OPr i ⊳3 x ⊲ˇ-dik⊳x C ⊲3 x⊳LH ! Al⊲ˇ-dik⊳x ⊲L⊳3 x C ⊲3 x⊳Pr i OH "
⊲2.223⊳
where LH is another ˇ-diketone, ˇ-ketoester or a ligand such as 8-hydroxyquinoline.
Aluminium tris-ˇ-diketonates are monomeric in nature and have 728 an octahedral
geometry. The crystal structure for the [⊲Et2acac⊳2Al⊲ -OPr i ⊳2Al⊲Et2acac⊳2]⊲Et2acac D
3,5-heptanedione⊳ dimer shows 726 two octahedral aluminium atoms with isopropoxy
bridging. The crystal structure of trimeric fAl⊲OPr i ⊳2⊲acac⊳g3 can be represented 729 by
[⊲acac⊳2Al⊲ -OPr i ⊳2Al⊲acac⊳⊲ -OPr i ⊳2Al⊲OPr i ⊳2] with an interesting distribution of the
ligand moieties on the three aluminium atoms. It has been shown 729 that the analogous
fAl⊲OSiMe3⊳2⊲acac⊳g2 is dimeric and instead of a symmetrical f⊲acac⊳⊲OSiMe3⊳Al⊲ -
OSiMe3⊳2Al⊲OSiMe3⊳⊲acac⊳g-type structure with both aluminium atoms in the
pentacoordinated state, it contains an unsymmetrical structure of the type [⊲acac⊳2Al⊲ -
OSiMe3⊳2Al⊲OSi-Me3⊳2] in which one of the aluminium atoms is hexacoordinated and
the other aluminium atom is tetracoordinated with four (Me3SiO) groups. In conformity