Synthesis and characterization of sterically hindered group IV metallocenes

Three new Ct symmetric heteroannular dialkyl substituted group IV metallocene dichlorides (Cp-R)(2)MCl2 [Cp-R = C5H4C(Me)(2)CHMe2; 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 (CpLi)-Li-R (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 monocylopentadienyl-trihalide 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.