2 Homometallic Alkoxides

112 Alkoxo and Aryloxo Derivatives of Metals
In these cases also, the forward reaction could be pushed further by fractionating
out the more volatile alcohol (ROH) liberated during the reaction.
The silanolysis reactions appear to be subject to less steric hindrance than the analogous
alcoholysis reactions. For example, the final products in the reactions of Al(OPr i )3
with Me3COH and Me3SiOH 668 have been reported to be Al2⊲OPr i ⊳⊲OCMe3⊳5 and
Al2⊲OSiMe3⊳6, respectively. However, the reaction of Al(OPr i )3 with HOSi(OBu t )3
ledtotheformationofAl2⊲OPr i ⊳2[OSi⊲OBu t ⊳2]4 only. 669
A complicating factor affecting the reactions of metal alkoxides with silanols is the
condensation tendency of silanols to yield hexa-alkyldisiloxanes and water (Eq. 2.200);
this may be to some extent avoided by adding the silanol slowly to the reaction mixture.
R 0 3 Si—O H C HO —SiR0 3 ! R0 3 Si—O—SiR0 3 C H2O ⊲2.200⊳
In spite of this difficulty the reactions of a wide range of metal(loid) alkoxides with
silanols (Eq. 2.199) under controlled conditions have been successfully employed for
the synthesis of a range of metal siloxides such as Ti⊲OR⊳3fOSi⊲OBu t ⊳3g (R D Pr n , 670
Bu t ), 671 Ti⊲OPr i ⊳fOSi⊲OBu t ⊳3g3, 672 Ti⊲OPr i ⊳2fOSi⊲OBu t ⊳3g2. 672
Attempts to prepare niobium pentakis(trimethylsiloxide) by the reaction of Nb(OEt)5
with Me3SiOH did not yield the expected product; 6 instead the sublimed (in vacuo)
material corresponded in analysis to [Nb⊲OSiMe3⊳4]2O. As compared to nonvolatile
titanium tetrakis(triphenysiloxide), the trimethylsiloxide derivatives of titanium and
zirconium as well as tantalum pentakis(trimethylsiloxide) could be purified in vacuo
by either distillation or sublimation. 678
The metal tetrasiloxides differ from the analogous alkoxides in their degree of
polymerization and volatility. This is primarily due to the smaller steric effect of
trialkylsiloxo compared with tertiary alkoxo groups. Furthermore, trialkylsilanols are
more acidic than alcohols owing to the presence of (p-d) bonding in the Si–O bond,
and consequently the trialkylsiloxide derivatives may experience a difference in degree
of covalency compared with tertiary alkoxides. However, steric factors appear to offer
the most plausible explanation for higher association in metal siloxides.
Metal siloxides are more resistant to hydrolysis than their alkoxide analogues; this
salient difference in hydrolytic stability of metal siloxides may be ascribed to the
water-repelling property of trialkylsiloxo groups.
As discussed already (Section 2.8), metal alkoxides undergo transesterification reactions
with organic as well as silyl esters as illustrated by Eqs (2.201), (2.202), and
(2.203):
M⊲OR⊳x C yCH3COOR 0 ⇀
↽ M⊲OR⊳x y⊲OR 0 ⊳y C yCH3COOR ⊲2.201⊳
M⊲OR⊳x C yCH3COOSiR 0 3
⇀
↽ M⊲OR⊳x y⊲OSiR 0 3 ⊳y C yCH3COOPr i
where M D a wide range of s-, p-, d-, and f-block metals.
M⊲OPr i ⊳5 C 5CH3COOC6H5
where M D Nb, Ta.
! M⊲OC6H5⊳5 C 5CH3COOPr i
⊲2.202⊳
⊲2.203⊳