Synthesis and structural characterization of Group 4 metallocene complexes that contain remote carboxamide functionalities at their Cp-side chains

Abstract

Treatment of the levulinic acid amides CH₃COCH₂CH₂CONR₂7 [–NR₂ = NMe₂ (a), NEt₂ (b), pyrrolidino (c)] with cyclopentadiene in the presence of excess pyrrolidine gave the corresponding pentafulvene derivatives 8(a–c). Their subsequent reaction with [Me₂CuLi] followed by hydrolysis furnished the corresponding C-methylated cyclopentadienes 9(a–c), each obtained as a mixture of isomers (9-A and 9-B). Deprotonation of the cyclopentadiene units in these compounds was achieved by treatment with lithium diisopropylamide (LDA) to yield the substituted lithium cyclopentadienide reagents [(C₅H₄)–CMe₂CH₂CH₂CONR₂]Li (10a–c), that were resistant to intramolecular cyclization under the applied reaction conditions. The transmetalation reaction of 10a with FeCl₂ furnished the ferrocene derivative [(C₅H₄)–CMe₂CH₂CH₂CONMe₂]₂Fe (11a). Treatment of 10a–c with CpTiCl₃ gave the Group 4 bent metallocene complexes Cp[(C₅H₄)–CMe₂CH₂CH₂CONR₂]TiCl₂ (12a–c). Similarly, the reaction of 10b with CpZrCl₃ led to formation of the complex Cp[(C₅H₄)CMe₂CH₂CH₂CONEt₂]ZrCl₂ (13b). Treatment of 12a with Meerwein's reagent [(Et₃O⁺)(BF₄⁻)] resulted in chloride abstraction with formation of the metallocene cation complex 16 (with BF₄⁻ anion), in which, contrary to the 16-electron systems 12 and 13, the carboxamido oxygen atom intramolecularly coordinates to the electron deficient Group 4 metal cation. The organic reagents 8a and 9a as well as the transition metal complexes 11a, 13b, and 16[BF₄⁻] were characterized by X-ray crystal structure analyses.