Synthesis, Stability and Aggregation State

For a long time it was thought that binary complexes of transition metals containing only o-alkyl- or aryl ligands could be thermodynamically stable only if they are coordinatively saturated (18 electron rule). A series of illuminating papers by Wilkinson 26), Lappert27) and others 28) dispelled this fallacy. It became clear that transition metal alkyls can decompose via kinetically low energy pathways such as P-hydride elimination or abstraction, but rarely via primary metal-carbon homolysis. Mechanisms and theories of decomposition have been proposed and reviewed 29). 

It was not until the pioneering studies of Bestian, Clauss and Beermann at Farbwerke Hoechst, that a number of methyltitanium alkoxides and chlorides were prepared in high yields, isolated and characterized 31). Later, additional examples and improvements were reported 1,19,28 32). Methyltitanium triisopropoxide 6 is readily prepared by quenching methyllithium with 5, decanting from the inorganic precipitate and distilling at 0.01 torr/50 °C (95% yield)31b,32). 

Homologs of 5, e.g., ethyl and n-butyl derivatives 9 and 10 are more sensitive, but can be handled in solution21 300. However, this does not apply to branched derivatives such as isopropyltitanium triisopropoxide, which appear to be unstable even in solution due to P-Hydride elimination. Moderate to pronounced degrees of thermal stability are characteristic of compounds in which p-hydrogens are not easily abstracted due to strain in the corresponding olefin (e.g., 1135) or 12 36) or in which they are lacking (e.g., 13 37)). Tetramethyltitanium 14 has no P-hydrogen atoms but is 

Burger et al. have shown that the presence of amino ligands at the metal enhances the thermal stability of primary, secondary and tertiary alkyl as well as allyl and vinyl titanium compounds dramatically 39). For example, n-propyltitanium tris(di-ethylamide) 16 can be isolated in pure form by sublimation at 60 °C/10-3 torr (68 %)39b . 

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