Synthesis, and characterization of low-valent titanium complexes.

Abstract

The chemistry of low-valent titanium is poorly developed apart from ubuquitous cyclopentadienyl derivatives. Among the possible supporting ligands for stabilizing low-valent transition-metal complexes, are alkylamides. By introducing bulky amides such as R$\rm \sb2 N\sp-$ $\rm (R\ =\ Me\sb3 Si,\ \sp{i}Pr,\ Cy)$ in the chemistry of Ti(III), we have opened a new field in the coordination and organometallic chemistry of this early transition metal. Synthetic methodology is focused on the preparation of novel bis(amido) Ti(III) complexes via transmetallation reaction of $\rm TiCl\sb3 THF\sb3\ with\ R\sb2 NLi,$ using different stoichiometric ratios, reaction conditions and in the some cases the presence of chelating legands, such as TMEDA. The $\rm (R\sb2 N)\sb2 Ti(\mu$-$\rm Cl)\sb2(TMEDA)\ (R=\ \sp{i}Pr\ 4,\ Cy\ 5)$ complexes provide a unique example of reversible reductive coupling of pyridine. The formation of 1,2,3,4-tetraphenyl-1-butene 9, promoted by diisopropyl derivative 4, is the result of reductive coupling followed by the transfer of two molecules of hydrogen. The large steric bulk of the amido ligands and their electronic flexibility allowed us to investigate novel bis(amido) Ti(III) complexes in further ligand replacement reactions with different functions, such as hydrides, borohydrides and alkyls.