2 Homometallic Alkoxides

Homometallic Alkoxides 39
with organic esters. Lappert 328,329 on the basis of infrared studies has shown that on
coordination of an organic ester with Lewis acids like boron trichloride, the electron
density on the carbonyl oxygen atom appears to be reduced and, hence, that this should
be a more probable donation site for the ester. This mechanism involving coordination
of the carbonyl oxygen of the ester rather than its alkoxo oxygen would appear to be
less prone to steric hindrance than the alcohol interchange, as actually observed by
Mehrotra. 293
2.9 Reactions of Metal Dialkylamides M(NR2)x (R = Me, Et, SiMe3) with
Alcohols (Method I)
2.9.1 Derivatives without Metal–Metal Bonds
Metal dialkylamides are reactive toward alcohols, readily eliminating an amine
according to Eq. (2.101):
M(NR 2⊳x C xR 0 OH ! M(OR 0 ⊳x C xR2NH " ⊲2.101⊳
This method is particularly suitable for those metals which have a greater affinity for
oxygen than for nitrogen. The other advantage of this procedure is the generally higher
volatility of the liberated dialkylamines, which can readily be volatilized out.
The reactions of the type of Eq. (2.101) have often been employed in the synthesis
of metal alkoxides when other routes are either inapplicable or tedious. Historically,
this method was first investigated by Jones et al. 213 for the preparation of uranium
tetralkoxides U(OR)4 (R D Me, Et) from U(NEt)4 and alcohols. For the synthesis
of U(OBu t )4 Jones et al. 213 adopted the modified procedure of interacting uranium
tetrachloride with ammonia and/or potassium amide and tert-butyl alcohol:
UCl4 C 4KNH2 ! [U⊲NH2⊳4] 4ButOH ! U⊲OBu t ⊳4 C 4NH3 "
" 2H2
4K C 4NH3 ⊲liquid⊳ ⊲2.102⊳
Interestingly, the reaction of U(NEt2)4 with excess of tert-butyl alcohol affords the
complex U2(OBu t )8(Bu t OH): 330
2U⊲NEt2⊳4 C 9Bu t OH ! U2⊲OBu t ⊳8⊲Bu t OH⊳ C 8Et2NH " ⊲2.103⊳
Thomas 331 utilized this method for preparation of a number of difficult-to-synthesize
metal alkoxides M(OR)4 (especially when R D Bu t ), e.g. Zr(OBu t )4 from
Zr(NEt2)4, V(OBu t )4 from V(NMe2)4, Cr(OBu t )4 from Cr(NEt2)4, Sn(OBu t ⊳4 and
Sn(OPr i )4.Pr i OH from Sn(NMe2)4.
Thomas 331 prepared tantalum and niobium penta-alkoxides by the method of
Eq. (2.101). For example, the alcoholysis of tris-(dialkylamido) monoalkylimidotantalum
yielded pentaalkoxides very conveniently:
RNDTa(NR 2⊳3 C 5ROH ! Ta(OR) 5 C 3R2NH "CRNH2 " ⊲2.104⊳
However, Nb(NEt2)4 reacted with alcohols to form compounds of the type Nb(OR)4
which were oxidized instantaneously (even after the rigorous exclusion of oxygen)