Synthesis and structure of acyclic bis(ketenimine) complexes of zirconium

Mono- and disubstituted bis(ketenimine) complexes of zirconium (1 and 2, respectively) can be readily prepared by reaction of these ligands (3) with Zr(NMe)(4) or, in some cases, the homoleptic tetrabenzyl derivative ZrBn4. Treatment of 1 (Ar = Ph, p-CF3Ph; X = NMe2) with Me2NH . HCl and of 2(Ar = Ph, X = NMe2) with either Me2NH . HCl or TMSCl provides the corresponding chloro derivatives in high yield. Alkyl derivatives of 2 (X = Me, Bn) can be readily prepared by reaction of the corresponding chloro derivatives with Grignard or organolithium reagents. Monocyclopentadienyl bis(ketenimine) complexes 4 (Ar Ph, p-CF3Ph; Cp = eta(5)-C5H5, eta(5)-C9H7; X = Cl) were prepared from 1 (Ar = Ph, p-CF3Ph; X = Cl) by reaction with CpLi or IndLi in high yield, and methyl derivatives 4 (Ar = Ph, p-CF,Ph; Cp = eta(5)-C5H5, eta(5)-C9H7; X = Me) were accessible by treatment of the chloro precursors with methyllithium. The X-ray structure of 2(Ar = Ph; X = NMe2) reveals a distorted-octahedral geometry in which the bis(ketenimine) ligands are sigma-bound to the metal and the dimethyl-amido groups are cis to one another, and the corresponding dichloro derivative also adopts a similar structure in the solid state, whereas the bis(ketenimine) ligands adopt a distorted, eta(5) binding mode in the monoindenyl derivative 4 (Ar = Ph; Cp = eta(5)-C9H7' X = Cl). Enantiomers of 2 (Ar = Ph, X = Cl) readily interconvert in solution via a Bailar-twist mechanism, as revealed by variable-temperature H-1 NMR spectroscopic studies; activation parameters for this process were determined (Delta H++ = 9.0 +/- 0.45 kcal mol(-1); Delta S++ = -9.9 +/- 1.0 cal mol(-1) K-1).