2 Homometallic Alkoxides

Homometallic Alkoxides 23
obtain only thorium trialkoxide monochlorides owing to the partial replacement of chlorides.
These workers observed that the alcoholic solutions of Th(OEt) 4 or Th⊲OPr i ⊳4
were alkaline to thymolphthalein. On the other hand anhydrous ammoniacal alcohols
were acidic to this indicator. Thus thorium tetra-alkoxides tend to be more basic than
ammonia and the following feasible equilibria (Eqs 2.35 and 2.36) may be responsible
for the formation of Th(OR) 3Cl instead of the expected tetra-alkoxides.
Th(OR) 4 C NH4 C ↽ ⇀
C
Th(OR) 3 C NH3 C ROH ⊲2.35⊳
C
Th(OR) 3 C Cl ↽ ⇀ Th(OR) 3Cl ⊲2.36⊳
However, it was observed 141 that treatment of alcoholic solutions of thorium tetrachloride
with sodium alkoxides gave thorium tetra-alkoxides.
In search of a convenient method for the synthesis of tetra-alkoxides of cerium(IV)and
plutonium(IV), which do not form stable chlorides, the complex chlorides ⊲C5H6N⊳2MCl6
(M D Ce(IV), Pu(IV); C5H6N D pyridinium) method proved to be convenient starting
materials:
⊲C5H6N⊳2MCl6 C 6NH3 C 4ROH ! M(OR) 4 C 6NH4Cl # 2C5H5N ⊲2.37⊳
where M D Ce 143 or Pu 144 and R D Pr i .
Bradley et al. 145 had earlier reported that dipyridinium hexachlorozirconate
⊲C5H6N⊳2ZrCl6, which can be prepared from the commonly available ZrOCl2.8H2O,
also reacted smoothly with alcohol in the presence of ammonia to form the tetraalkoxides
Zr(OR) 4.
During an attempt to prepare tetra-tert-alkoxides of zirconium and cerium by the
reactions of ⊲C5H6N⊳2MCl6 (M D Zr, Ce) with tert-butyl alcohol, Bradley and coworkers
143,144 had noticed the formation of MCl⊲OBu t ⊳3.2C5H5N as represented by
Eq. (2.38):
⊲C5H6N⊳2MCl6 C 3Bu t OH C 5NH3
! MCl⊲OBu t ⊳3.2C5H5N C 5NH4Cl # ⊲2.38⊳
As the product reacts with primary alcohols (Eq. 2.39) in the presence of ammonia to
give heteroleptic alkoxides, M(OR)⊲OBu t ⊳3, steric reasons have been suggested as a
possible explanation for the partial replacement reactions with tert-butyl alcohol:
MCl⊲OBu t ⊳3.2C5H5N C EtOH C NH3 ! M(OEt)⊲OBu t ⊳3 C 2C5H5N C NH4Cl #
⊲2.39⊳
It is, however, somewhat intriguing that dipyridinium hexachloro derivatives of
zirconium and cerium146 undergo complete replacement with Cl3C.CMe2OH, which
should apparently be an even more sterically hindered alcohol than ButOH: ⊲C5H6N⊳2MCl6 C 4Cl3C.CMe2OH C 6NH3
! M⊲OCMe2CCl3⊳4 C 2C5H5N C 6NH4Cl # ⊲2.40⊳
Reactions of MCl4 (M D Se, Te) with a variety of alcohols (MeOH, EtOH, CF3CH2OH,
Bu t CH2OH, Me2CHOH) in 1:4 molar ratio in THF using Et3N as a proton acceptor afford
corresponding tetra-alkoxides. 146a