HALF-SANDWICH IMIDO COMPLEXES OF NIOBIUM AND TANTALUM

The half-sandwich methylimido complexes [Nb(eta-C5R5)(NMe)Cl2] (R = H 1a, or Me 1b) have been prepared by treatment of [Nb(eta-C5R5)Cl4] with NMe(SiMe3)2 in acetonitrile. The sterically hindered alkyl- and aryl-imido analogues [Nb(eta-C5H5)(NR)Cl2] (R = Bu(t) 1c, or C6H3Pri2-2,6 1d), [Nb(eta-C5Me5)(N-C6H3Pri2-2,6)Cl2] 1e and the tantalum compound [Ta(eta-C5Me5)(N-C6H3Pri2-2,6)Cl2] 1f are obtained by treatment of [M(eta-C5R5)Cl4] with two equivalents of NHR(SiMe3) in chlorocarbon solvent. Crystal structure 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) angstrom consistent with pseudo triple bonds. Compounds 1a-1e react with tertiary phosphines to give eighteen-electron adducts of the type [Nb(eta-C5H5)(NR)Cl2(PR'3)] 2a-2e. The crystal structure of [Nb(eta-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(eta-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(eta-C5H5)(NR) fragment and the bent metallocene moiety M(eta-C5H5)2 for the Group 4 elements.