2 Homometallic Alkoxides

Homometallic Alkoxides 107
[Al⊲OPr i ⊳⊲O2CR⊳2]2 C 2RCOOH slow
! 2Al⊲O2CR⊳3 C 2Pr i OH " (2.184)
Similarly, preparation of anhydrous and/or unsolvated lanthanide tris-acetylacetonates
remained a challenge till 1965, as the efforts to remove ligated water from Ln(ˇdik)3.2H2O
(obtained from aqueous media) led to hydrolyzed products. However, pure
tris-ˇ-diketonates of lanthanum, 638 praseodymium, and neodymium 635 could be easily
synthesized by the reactions of their alkoxides (methoxides/isopropoxides) with 3 equivalents
of acetylacetone (acacH):
La⊲OMe⊳3 C 3acacH ! La⊲acac⊳3 C 3MeOH " ⊲2.185⊳
Ln⊲OPr i ⊳3 C 3acacH ! Ln⊲acac⊳3 C 3Pr i OH " ⊲2.186⊳
where Ln D Pr, Nd.
In spite of the fact that a wide variety of structurally interesting mixed
ligand-alkoxide derivatives of a large number of metals and metalloids are
known, X-ray structural data are available only in a very few cases such
as Ce2⊲OPr i ⊳6⊲ -OC2H4NMeC2H4NMe2⊳2, 640 [TifOCH⊲CF3⊳2g2⊲OEt⊳2⊲HOEt⊳]2, 272
[Fe⊲OMe⊳2⊲OCH2Cl⊳]10, 641,642 [Al⊲OSiMe3⊳2⊲acac⊳]2, 643 [Al⊲OPr i ⊳2⊲acac⊳]3, 643
[Al⊲OPr i ⊳⊲Et2acac⊳2]2, 643 and [Sn⊲OBu t ⊳3⊲OAc⊳⊲py⊳]. 644
The reactions of metal alkoxides appear to be generally influenced by steric factors.
For example, reactions with highly branched alcohols (Bu t OH, Am t OH) are generally
slower and may be even sterically hindered in some cases as is illustrated 293 by
Eqs (2.187)–(2.189):
Al⊲OEt⊳3 C 2Bu t OH facile
! Al⊲OEt⊳⊲OBu t ⊳2 C 2EtOH " ⊲2.187⊳
2Al⊲OEt⊳⊲OBu t ⊳2 C Bu t OH slow
! Al2⊲OEt⊳⊲OBu t ⊳5 C EtOH " ⊲2.188⊳
Al2⊲OEt⊳⊲OBu t ⊳5 C Bu t OH negligible
! Al2⊲OBu t ⊳6 C EtOH ⊲2.189⊳
In the following pages the general reactions of metal alkoxides with a wide variety
of reagents are discussed under the following headings, with special reference to the
synthesis and salient features of the new products isolated:
(1) Hydrolysis reactions (Section 4.2)
(2) Reactions with alcohols, phenols, and silanols (as well as organic and silyl esters)
(Section 4.3)
(3) Reactions with alkanolamines (Section 4.4)
(4) Reactions with ˇ-diketones and ˇ-ketoesters (Section 4.5)
(5) Reactions with carboxylic acids and acid anhydrides (Section 4.6)
(6) Reactions with glycols (including some special types of polyhydroxy reagents)
(Section 4.7)
(7) Reactions with oximes and N,N-diethylhydroxylamine (Section 4.8)
(8) Reactions with Schiff bases and ˇ-ketoamines (Section 4.9)
(9) Reactions with thiols (Section 4.10)
(10) Reactions with halogens, hydrogen halides, acyl halides, and metal halides
(Section 4.11)