2 Homometallic Alkoxides

Homometallic Alkoxides 19
2.3 Reactions of Metal Atom Vapours with Alcohols (Method C)
Although the development of metal atom vapour technology over the past three decades
has shown tremendous utility for the synthesis of a wide range of organometallic
compounds (many of which were inaccessible by conventional techniques), 97 the use
of this technique for the synthesis of metal alkoxides and related derivatives does not
appear to have been fully exploited. 98 In 1990, Lappert et al. 99 demonstrated the utility
of this technique for the synthesis of M—O—C bonded compounds by the isolation
of alkaline earth metal aryloxides.
2.4 Direct Reactions of Metal Halides with Alcohols (Method D)
By far the most common synthetic technique for metal alkoxides (Eq. 2.18) is the
replacement of halides from an appropriate metal halide by alkoxo groups.
MCln C ⊲x C y⊳ROH ⇀
↽ MCln x (OR) x (ROH) y C xHCl " ⊲2.18⊳
Halides of alkaline earth, lanthanide, actinide, and later 3d (Mn, Fe, Co, Ni) metals on
interactions with alcohols form crystalline molecular adducts like MgBr 2 .6MeOH, 100
CaBr2.6MeOH, 100 LnCl3.3Pr i OH 101–103 where Ln is a lanthanide metal,
ThCl4.4EtOH, 104,105 MCl2.2ROH (M D Mn,Fe,Co,Ni;RD Me, Et, Pr n ,Pr i ). 106 Apart
from the alkaline earth metal (Ca, Sr, Ba) halides, all of these undergo alcoholysis in
the presence of a suitable base to yield the corresponding homoleptic alkoxide or
chloride-alkoxide derivatives (Sections 2.5.1, 2.5.2, and 2.5.3).
Interesting variations in the extent of alcoholysis reactions of tetravalent metal (Ti,
Zr, Th, Si) chlorides may be represented 107 by Eqs (2.20–2.23), to which CCl4 has
been added for comparison.
CCl4 C ROH (excess) !no reaction ⊲2.19⊳
SiCl4 C 4ROH !Si(OR) 4 C 4HCl " ⊲2.20⊳
TiCl4 C 3ROH (excess) !TiCl2(OR) 2.ROH C HCl " ⊲2.21⊳
2ZrCl4 C 6ROH (excess) !ZrCl2(OR) 2.ROH
C ZrCl3(OR).2ROH C 3HCl " ⊲2.22⊳
ThCl4 C 4ROH (excess) !ThCl4.4ROH ⊲2.23⊳
Depending on the nature of the metal (M), the initial metal chloride (MCln) ora
product MClx y(OR) y forms an addition complex with alcohol molecules (ROH)
without enough perturbation of electronic charges for the reaction to proceed further.
The reactions of metal tetrachlorides MCl4 (M D Ti, Zr, Th) towards ethyl alcohol
show a gradation TiCl4 > ZrCl4 > ThCl4. 107
Although no clear explanation is available for the varying reactivity of different metal
chlorides with alcohols, it is interesting to note that final products of similar compositions
have been isolated in the reactions of tetraalkoxides of these metals with HCl.
For example, the reaction of Ti⊲OPr i ⊳4 with HCl leads finally to Ti⊲OPr i ⊳2Cl2.Pr i OH
(Section 4.11.2).