Synthesis and Characterization of Sterically Hindered Group IV Metallocenes
journal contributionposted on 15.01.1999, 00:00 by Brian J. Grimmond, Joyce Y. Corey, Nigam P. Rath
Three new C2 symmetric heteroannular dialkyl substituted group IV metallocene dichlorides (CpR)2MCl2 [CpR = 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 CpRLi (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(CpR)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.