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 = But1c, 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.