Half-sandwich imido complexes of niobium and tantalum

Abstract

The half-sandwich methylimido complexes [Nb(η-C₅R₅)(NMe)Cl₂](R = H 1a, or Me 1b) have been prepared by treatment of [Nb(η-C₅R₅)Cl₄] with NMe(SiMe₃)₂ in acetonitrile. The sterically hindered alkyl- and aryl-imido analogues [Nb(η-C₅H₅)(NR)Cl₂](R = Bu^t1c, or C₆H₃Prⁱ₂-2,6 1d), [Nb(η-C₅Me₅)(N-C₆H₃Prⁱ₂-2,6)Cl₂]1e and the tantalum compound [Ta(η-C₅Me₅)(N-C₆H₃Prⁱ₂-2,6)Cl₂]1f are obtained by treatment of [M(η-C₅R₅)Cl₄] with two equivalents of NHR(SiMe₃) in chlorocarbon solvent. Crystal structures of 1a, 1c, 1d and 1f show that these complexes are mononuclear with quasi-linear imido ligands. The metal–nitrogen bond distances range from 1.744(3) to 1.780(5)Å consistent with pseudo triple bonds. Compounds 1a–1e react with tertiary phosphines to give eighteen-electron adducts of the type [Nb(η-C₅H₅)(NR)Cl₂(PR′₃)]2a–2e. The crystal structure of [Nb(η-C₅H₅)(NMe)Cl₂(PMe₃)] shows a distorted four-legged piano-stool geometry in which the phosphine binds cis to the methylimido and cis to one of the chloride ligands. Alkoxide and aryloxide derivatives of the type [Nb(η-C₅R₅)(NR′)(OR″)₂] may be prepared via reactions of the dichlorides 1 with 2 equivalents of LiOR″ in Et₂O. Fenske–Hall quantum-chemical calculations indicate that there exists a close relationship between the Nb(η-C₅H₅)(NR) fragment and the bent metallocene moiety M(η-C₅H₅)₂ for the Group 4 elements.