2 Homometallic Alkoxides

Homometallic Alkoxides 57
explain these special characteristics of methoxides again on the basis of steric and
inductive factors. As the molecular weights of the methoxides could not be directly
measured, owing to their insolubility and nonvolatility, Verma and Mehrotra, 270 in an
attempt to sort out the comparative effect of these two factors, synthesized and studied
the properties of heteroleptic ethoxide-methoxides of titanium, [Ti(OC2H5)3(OCH3)],
[Ti(OC2H5)2(OCH3)2]and[Ti(OC2H5)(OCH3)3]. From measurements of the molecular
weights of these derivatives in benzene, the degree of polymerization of titanium
tetramethoxide, Ti(OMe)4 was extrapolated to be four. The single crystal X-ray analysis
has confirmed (Chapter 4) the tetrameric 436 nature of [Ti(OMe)4]4 and this may well
explain its low volatility. However, its low solubility suggests that the small size of
the peripheral methyl groups leads to a high lattice energy. Incidentally, a sparingly
soluble variety of titanium methoxide has been synthesized by Dunn 437 by the reaction
between titanium tetrachloride and methanol in the presence of anhydrous ammonia,
and this derivative has also been shown to be tetrameric in nature.
3.2.3 Alkoxides of Group 1 Metals
Owing to the strongly electropositive nature of alkali metals, their alkoxides would
be expected to be predominantly ionic in character. In fact, sodium ethoxide has been
shown to behave as a strong base136 in ethanol (like sodium hydroxide in water).
Amongst the alkali metals, the lithium derivatives would be expected to be the least
ionic as is shown by their solubility in organic solvents and volatility of lithium tertbutoxide
(110Ž /0.1 mm); 438 alkoxides of other alkali metals tend to be nonvolatile and
decompose on being heated to higher temperatures even under reduced pressure.
Bains438 reported lithium tert-butoxide to be hexameric in cyclohexane. There appears
to be some controversy in the literature about the molecular weight of lithium
tert-butoxide in benzene, the reported degree of polymerization being 4.0, 438 9.0, 439
and 6.0. 440 The last value appears to be most reliable as these investigators440 have
confirmed their findings by mass spectrometry. X-ray structural findings28,29 have
revealed that tetrameric cubanes and hexameric stacks are the most prevalent structural
motifs for molecular alkali metal alkoxides.
In spite of the strongly electropositive character of alkali metals, the importance
of the inductive effect of the alkyl group was shown441 in the comparative molar
conductivities of sodium methoxide (92.0 mhos), ethoxide (45.0 mhos), isopropoxide
(2.5 mhos) and tertiary butoxides (0.05 mhos) in their parent alcohols. Although the
effect could be partially ascribed to the differences in dielectric constants of the
alcohols, yet the sharp break between the conductivities of the isopropoxide and tertbutoxide
could be expected to arise at least partially from the strong CI inductive
effect of the tertiary butyl group, making the sodium–oxygen bond much less ionic in
character.
In contrast to the insoluble, nonvolatile nature of the simple alkoxo-derivatives of
alkali metals, their perfluoro-alkoxo derivatives are comparatively more soluble in polar
solvents like acetonitrile, acetone and ether. Thus the perfluoro tert-butoxides of lithium
and sodium (MOC4Ft 9 ) have been found to be high-melting solids which can be distilled
under reduced pressures. 442 The corresponding potassium perfluoro tert-butoxide could
also be sublimed at 140Ž /0.2 mm pressure. 442 The high volatilities of lithium, sodium,
and potassium tert-butoxides might well arise from low molecular complexities of