Synthesis and Characterization of Sterically Hindered Group IV Metallocenes

Three new C₂ symmetric heteroannular dialkyl substituted group IV metallocene dichlorides (Cp^R)₂MCl₂ [Cp^R = C₅H₄C(Me)₂CHMe₂; M = Zr (2), Hf (3), Ti (4)] were synthesized as potential catalytic precursors for the dehydropolymerization of silanes to polysilanes. The hindered complexes were generated in fair yield by reaction of 2 equiv of the alkyllithium reagent Cp^RLi (1) and the corresponding metal halide, MCl₄·2THF for 2 and 3 and MCl₃·3THF for 4. Crystal structures for compounds 2−4 were obtained and established a pseudotrans configuration of the ancillary ligands in the solid state. The structural features adopted by these compounds to accommodate the large substituents are described with comparisons to known metallocenes. Monoalkyl-substituted metallocenes Cp(Cp^R)MCl₂, M = Ti (5), Zr (6), were also synthesized from 1 and the corresponding monocylopentadienyltrihalide species CpMCl₃. Characterization of 2−6 by various NMR techniques in conjunction with solid-state analysis of 2, 3, and 4 permitted interpretation of an interligand mechanism for the observed NOE signals involving the distal protons of the substituted cyclopentadienyl ligand and portions of the alkyl cyclopentadienyl substituent.