Syntheses and structural systematics of trialkylphosphine complexes of open titanocenes, zirconocenes and hafnocenes

Abstract

The synthesis and characterization of the open hafnocene, Hf(2,4-C₇H₁₁)₂(PMe₃) (C₇H₁₁ = dimethylpentadienyl), is reported. Additionally, a much improved synthetic procedure has been developed for Hf(2,4-C₇H₁₁)₂(PEt₃). Structural data have been obtained for these complexes, and for Ti(2,4-C₇H₁₁)₂(PEt₃) and Zr(2,4-C₇H₁₁)₂(PMe₃), thereby allowing for detailed comparisons between all M(2,4-C₇H₁₁)₂(PX₃) species (M = Ti, Zr, Hf; X = Me, Et). The presence of the coordinated phosphines led in all cases to the adoption of the expected syn-eclipsed geometries, with the phosphines positioned by the open dienyl edges. These phosphine ligands lead to substantial alterations of the bonding patterns in these species, relative to ligand-free complexes. Most notably, the shortest M–C distances involve the central dienyl carbon atoms. Additionally, the data reveal high degrees of steric crowding within these complexes, especially for the weakly bound Ti(2,4-C₇H₁₁)₂(PEt₃), and also demonstrate that significant deformations which have taken place within the dienyl ligands were substantially determined by the relative sizes of the metal centers.