2 Homometallic Alkoxides

26 Alkoxo and Aryloxo Derivatives of Metals
The reactions in THF have been reported to be cleaner with higher yields in all the
cases when n D 1–4. The formation of heterobimetallic alkoxides, Na2Ce(OR)6 and
NaCe2(OR)9 was also reported, when n D 6 or 4.5, respectively.
The above novel synthesis of cerium(IV) tert-butoxide nitrate complexes appears
to be the only report using a nitrate salt as the starting material for the preparation
of tertiary butoxo derivatives. It may, therefore, be worthwhile to investigate similar
routes for alkoxo (particularly t-butoxo) derivatives of other metals (especially in their
higher oxidation states) starting with their nitrate salts. The chances of success in these
efforts may be higher in cases where the nitrate groups are bonded predominantly in
a monodentate manner.
The X-ray crystallographically characterized 165 tetrameric thorium(IV) isopropoxide
complex, Th4(OPr i )16(py)2, has been prepared in 80% yield by the (1:4) reaction of
ThBr4(thf)4 with KOPr i in THF followed by addition of excess pyridine (py):
ThBr4⊲thf⊳4 C 4KOPr i
⊲i⊳ THF
1
! 4
⊲ii⊳ py
[Th4⊲OPr i ⊳16]⊲py⊳2 C 4KBr # ⊲2.52⊳
2.5.2.3 d-Block metals
Titanium tetraethoxide was first obtained pure by Bischoff and Adkins 166 by the reaction
of sodium ethoxide with titanium tetrachloride (Eq. 2.53).
TiCl4 C 4NaOEt ! Ti⊲OEt⊳4 C 4NaCl # ⊲2.53⊳
By contrast, the alkali alkoxide route appears to be inapplicable for the synthesis of
zirconium tetra-alkoxides or niobium (tantalum) penta-alkoxides, as these tend to form
heterobimetallic alkoxides with alkali metal alkoxides (Chapter 3, Section 3.2.1.1),
which volatilize out during final purification, whereas alkali titanium alkoxides, even
if formed, dissociate readily to give volatile titanium alkoxides.
The alkali alkoxide method has been extended 167 to the preparation of alkoxides
of the hexanuclear niobium and tantalum cluster units, e.g., [M6X12](OMe)2.4MeOH
(where M D Nb or Ta and X D Cl or Br), and M2[Ta2Cl12](OMe)6.6MeOH.
The chromium(IV) alkoxides, Cr(OR)4 (where R D OBu t or 1-adamantoxide), can
be prepared by the reaction of CrCl3(thf)3 with 4 equivalents of the corresponding K
(or Na) alkoxide in the presence of cuprous chloride in THF:
CrCl3⊲thf⊳3 C 4MOR C CuCl THF
! Cr⊲OR⊳4 C 4MCl #CCu # ⊲2.54⊳
⊲M D K, 168 R D Bu t ; M D Na, 169 R D 1-adamantyl)
The sodium method has been successfully employed for the synthesis of a triply
metal–metal bonded fluoroalkoxide of molybdenum (Eq. 2.55). 170
Mo2Cl6⊲dme⊳ C 6NaOCMe2CF3 ! Mo2⊲OCMe2CF3⊳6 C 6NaCl # ⊲2.55⊳
where dme D 1,2-dimethoxyethane.
The reaction of WCl4(PMe2Ph)2 with TlOCH2CF3 in 1:2 molar ratio gives
W(OCH2CF3)Cl2(PMe2Ph)2, which has been studied by spectroscopic and X-ray
diffraction methods. 171