2 Homometallic Alkoxides

70 Alkoxo and Aryloxo Derivatives of Metals
In spite of a considerable amount of work, the mechanism of the thermal decomposition
of boron alkoxides is not yet fully understood; it probably occurs via a carbonium
ion species as the thermal decomposition of boron tris(tetrahydrofurfuroxide) yields
2,3-dihydropyran which is also obtained by vapour-phase dehydration of tetrahydrofurfuryl
alcohol. 478 Similarly, decomposition of free tert-butyl methylcarbinol has
been found to yield the same product as that of its corresponding boron alkoxide
derivative; 478 this led Dupuy and King 479 to assume that the thermal decomposition
of boron alkoxides might proceed via an acid catalysed mechanism rather than via a
pyrolytic cis-elimination mechanism.
The thermochemistry of boron alkoxides was studied by Charnley et al., 480 which
led to an estimate of the average B–O bond dissociation energies for boron alkoxides
in the range of 110 š 5kcalmol 1 ; these results are consistent with the order: B–F >
B–N > B–O > B–Cl, reported earlier by Sidgwick. 481 In another study 482 the reported
B–O bond dissociation energy ¯D(B–OR) of B(OMe)3, B(OEt)3, B(OPr n )3, were 118.0,
117.7, 119.0 š 2kcalmol 1 , respectively.
Fenwick and Wilson 482 have measured the vapour pressures of boron triphenoxide
and substituted triphenoxides manometrically and found that the results were in good
agreement with the equation, log p mm D a b/T (where p is the pressure in mm at
which boiling point T was observed and a and b are constants) over a wide range of
temperatures.
Aluminium alkoxides are thermally stable and even the insoluble [Al(OMe)3]n may
be sublimed at 240 Ž C under high vacuum. 275 The higher alkoxides are all soluble, distillable
products and the melting points of the solids increase with increasing complexity
of the alkyl chain. 293,294
In view of the large variations reported in the boiling points and molecular weights
of aluminium alkoxides by various authors, 71,484–487 Mehrotra 293 in 1953 measured the
boiling points of a number of aluminium alkoxides at different pressures (Table 2.15)
(in the range 0.1–10.0 mm pressures) and observed that the boiling points measured in
the pressure range 2.0 to 10.0 mm follow the equation, log p D a b/T . The constants
a and b, the latent heats of vaporization, 1H , and entropies of vaporization 1S5
at 5 Ž C are reported in Table 2.16 for a pressure of 5.0 mm, along with the boiling
points and molecular complexities (n) of various alkoxides. Measurements of the same
compounds were carried out by Wilhoit 488 isotenoscopically in 1957 and these data
are also included in the table.
The data in Table 2.16 show that the latent heats of vaporization, 1H , and entropies
of vaporization, 1S , decrease regularly with decreasing polymerization of aluminium
alkoxides. The entropies of vaporization reported for aluminium alkoxides are slightly
higher than those observed for monomeric tert-butoxides of titanium and zirconium
Table 2.15 Boiling points of aluminium alkoxides at different pressures
Al(OR)3
B.p. ( Ž C/mm)
Al(OEt)3 162/1.3 169/1.5 171/2.2 181/4.2 184/5.0 188/6.0 190/6.8 197/10.0
Al(OPr i )3 106/1.5 111/2.3 118/3.8 122/4.6 124/5.2 131/7.5 132/8.3 135/10.0
Al(OPr n )3 205/1.0 211/1.9 215/2.0 228/4.0 233/6.9 238/6.9 239/7.5 245/10.0
Al(OBu n )3 242/0.7 258/2.7 262/3.6 272/6.0 276/7.5 281/8.8 284/10.0 —
Al(OBu t )3 134/0.25 151/1.3 156/2.0 164/3.5 170/4.6 177/6.6 181/8.0 185/10.0