Chemistry of medium-valent vanadium amide complexes.

Abstract

The chemistry of low-valent vanadium was extensively investigated especially in the organometallic field. Apart from cyclopentadienyl and alkoxides (or aryloxides) derivatives, its chemistry is poorly developed. Among the supporting ligands for the stabilization of low-valent vanadium, alkylamides need to be studied in more details. By introducing bulky amides such as R\sb2N\sp- (R = \sp{i}Pr,$ Cy, SiMe\sb3, C\sb6H\sb5,$ 3,5-Me$\sb2$Ph and Ad) in chemistry of vanadium(III) and vanadium(IV), we have opened a new field in the coordination of this early transition metal. Synthetic methodology is focused on the preparation of novel tris(amide) vanadium(III) complexes via transmetallation reaction of VCl\sb3(THF)\sb3$ with R\sb2NLi,$ using the stoichiometry ratio 1:3 and the same reaction conditions. Reaction of VCl\sb3(THF)\sb3$ with $\rm\lbrack(CH\sb3)\sb3Si\rbrack NPhLi$ led to the formation of the corresponding tetrahedral $\rm\{\lbrack(CH\sb3)\sb3Si\rbrack NPh\}\sb3V(THF)$ complex. Conversely, the reaction of CH\sb3C\lbrack(CH\sb2)N(\sp{i}Pr)Li\rbrack\sb3(TIAME)$ with VCl3(THF)$\sb3$ gave the new dinitrogen complex (TIAME)V-N$\sb2$-V(TIAME) whose structure was determined by X-ray analysis. The reaction of VCl$\sb3$(THF)$\sb3$ with (3,5-Me$\sb2$Ph)AdNLi.Et$\sb2$O (Ad = adamantyl) yielded the homoleptic vanadium(III) complex ((3,5-$\rm Me\sb2Ph)AdN\rbrack\sb3V$ which reacts with chalcogens E (E = S, Se) to yield diamagnetic terminal chalcogenide derivatives. The use of low-valent vanadium in the polymerization of olefins is poorly developed apart from acetylacetonato derivatives and chloride derivatives. The complexes of formula $\rm(R\sb2N)\sb2VCl\sb2$ (R = $\rm\sp{i}$Pr, Cy) provide a unique example of oxidative disproportionation. The formation of dicyclahexyl and diisopropyl derivatives is the result of oxidative coupling of two low-valent vanadium entities. The first to be formed is the bis(amido)V$\rm\sp{IV}Cl\sb2$ followed by the unstable $\rm V\sp{II}Cl\sb2$. The stability of the novel vanadium(IV) bisamido complexes allowed us to investigate their reactivity towards olefin polymerization.