Cycloheptatri-ene and -enyl derivatives of niobium

Abstract

The new compounds [Nb(η⁶-C₇H₈)(PMe₃)₂Cl₂]1, [Nb(η-C₇H₇)(η⁴-C₇H₈)(PMe₃)]2 and [Nb(η-C₇H₇)(CO)₃]4 have been prepared. The 17-electron compound 1 has been crystallographically characterised and contains mutually trans tertiary phosphine and chloro ligands. Compound 2 contains the first crystallographically characterised η⁴-cycloheptatriene ligand, the structure of which is discussed in relation to its ¹³C NMR spectrum and the use of inverse-mode (¹H-observed)¹³C–¹H shift correlation NMR spectroscopy. Treatment of 1 with magnesium butadiene reagent gives [Nb(η-C₇H₇)(η⁴-C₄H₆)(PMe₃)]3 and photolysis of 4 in the presence of cycloheptatriene gives [Nb(η-C₇H₇)(η-⁴-C₇H₈)(CO)]5. Compound 2 is protonated readily to yield [Nb(η-C₇H₇)(η⁵-C₇H₉)(PMe₃)][PF₆]6, and reacts with iodomethane to yield paramagnetic [Nb(η-C₇H₇)(PMe₃)₂I]7 which is isomorphous with the analogous 16-electron compound [Zr(η-C₇H₇)(PMe₃)₂I], but has an unusually long Nb–I bond. Compound 4 reacts with Me₂PCH₂CH₂PMe₂(dmpe) to give the monomeric [Nb(η-C₇H₇)(dmpe)(CO)]9. The analogous compound [Nb(η-C₇H₇)(PMe₃)₂(CO)]10 is prepared by reaction of 4 with PMe₃. The paramagnetic sandwich complexes [Nb(η-C₅H₅)(η-C₇H₇)]11 and [Nb(η-C₅H₄Me)(η-C₇H₇)]12 react with 1 equivalent of allyl bromide to yield the diamagnetic bent-sandwich species [Nb(η-C₅H₅)(η-C₇H₇)Br]13 and [Nb(η-C₅H₄Me)(η-C₇H₇)Br]14 respectively.