2 Homometallic Alkoxides

Homometallic Alkoxides 15
reactivity sequence of alcohols, MeOH > EtOH > Pr i OH > Bu t OH, towards an alkali
metal. This order of reactivity is understandable from an electronic viewpoint which
predicts a decrease in the acidity of the hydroxyl hydrogen in the same order.
2.1.1.2 Group 2 metals (Be, Mg, Ca, Sr, Ba)
Group 2 metals, being less electropositive than group 1 metals, react sluggishly even
with sterically compact alcohols and require a catalyst (iodine or mercury(II) chloride)
particularly in cases of lighter group 2 metals (Be and Mg) 30–34 to yield insoluble,
polymeric, and nonvolatile metal dialkoxides.
The reaction of magnesium with methanol had been reported 26 to form solvates of different
compositions: Mg⊲OCH3⊳2.3CH3OH and Mg⊲OCH3⊳2.4CH3OH, 35,36 which have
been shown by X-ray diffraction studies to have the compositions Mg⊲OCH3⊳2.2CH3OH 37
and Mg⊲OCH3⊳2.3.5CH3OH, 38 respectively.
With sterically less demanding alcohols, alkoxides of the heavier alkaline earth
metals (Ca, Sr, Ba) [M(OR) 2]n ⊲R D Me, Et, Pr i ⊳ had been prepared by a number
of workers 39–44 by reactions of metals with alcohols. These are also oligomeric or
polymeric, and nonvolatile.
Interest in the synthesis and chemistry of soluble and volatile alkaline earth metal
alkoxides experienced a sudden upsurge in the 1990s, 21–23 owing to the discovery of
superconducting ceramics 45,46 containing Ba and Ca.
Reactions of sterically demanding monodentate alcohols 47 with heavier alkaline earth
metals (M 0 ) have been reported to yield soluble derivatives:
M 0 C 2R 0 OH ! M 0 ⊲OR 0 ⊳2 C H2 " (2.3)
M 0 D Ba; R 0 D CMe3, CEt3, CHMe2, CH⊲CF3⊳2. M 0 D Ca, Sr; R 0 D C⊲CF3⊳3.
By contrast, reaction of barium granules with Ph3COH does not appear to take place,
even in the presence of I2 or HgCl 2 as a catalyst, in refluxing tetrahydrofuran (THF)
over three days. However, the same reaction in the presence of ammonia as a catalyst
yields X-ray crystallographically characterized dimeric derivative [H3Ba6⊲O⊳⊲OBu t ⊳11
⊲OCEt2CH2O⊳(thf) 3]. 48 It may be inferred that ammonia reacts initially with barium
to form Ba⊲NH2⊳2, which undergoes proton transfer and anion metathesis to yield the
desired alkoxide derivative.
Although the reactions of heavier alkaline earth metals with alcohols are generally
straightforward, yielding the expected homoleptic derivatives, in some instances it
has been reported that the reaction follows a different course to yield an intriguing
product as in the case of the formation of X-ray crystallographically characterized 49
oxo-alkoxide cluster of the composition H3Ba6O⊲OBu t ⊳11⊲OCEt2CH2O⊳(thf), in the
reaction of Ba with Bu t OH in THF. The reasons for the formation of such an unusual
product in a simple reaction of the above type (Eq. 2.3) are not yet well understood,
but it tends to indicate that either adventitious hydrolysis or alkene/ether elimination
may be the main factor. Furthermore, the formation of ⊲OCEt2CH2O⊳ ligated product
in this reaction indicates that the diolate ligand is probably formed in a side-reaction
involving the solvent tetrahydrofuran molecules.
2-Methoxyethanol (a chelating alcohol) has been shown 50 to react with calcium
filings in refluxing n-hexane to yield an X-ray crystallographically authenticated product