Pentamethylcyclopentadienyl imido compounds of chromium

Abstract

The interaction of the pentamethylcyclopentadienylchromium(III) complex [CrBr(µ-Br)(η-C₅Me₅)]₂ with alkyl- and aryl-lithium amido compounds, LiNHR, gave products whose nature depends on the reactants and on reaction conditions. For the aryl (R) 2,6-Prⁱ₂C₆H₃ both [(η-C₅Me₅)Cr(µ-NR)(µ-NHR)(µ-Br)Cr(η-C₅Me₅)]1 and [Cr(η-C₅Me₅)(µ-NR)]₂2 were obtained, whereas LiNH(C₆H₁₁) gave [CrBr(η-C₅Me₅)(µ-NC₆H₁₁)]₂5. Use of R = 2,4,6-Bu^t₃C₆H₂ gave [Cr(η-C₅Me₅)(µ-NR)]₂3. The unisolable chromium(III) intermediate can be trapped by interaction of [CrBr₂(η-C₅Me₅)]₂ and LiNHR (R = 2,6-Prⁱ₂C₆H₃) in the presence of diphenylacetylene to give Cr(η-C₅Me₅)(NR)(PhC≡CPh)6. A comparable reaction but using LiNHBu^t gave only the dimer (η-C₅Me₅)BrCr(µ-NHBu^t)₂Cr(NBu^t)(NHBu^t)7. Trapping by various other ligands failed but the interaction in the presence of 2,6-xylyl isocyanide gave a chromium(III) chelate 8 of a quinoline diamide where an isopropyl group has been removed from the original C₆H₃Pr₂ⁱ moiety. The crystal structures of 1–7 and 8 have been determined. The triply bridged complex 1, which has a confacial bitetrahedral structure (assuming C₅Me₅ takes one co-ordination site) has a Cr^III⋯ Cr^III distance of 2.649(5)Å(average over two molecules), whilst the doubly bridged (Cr^III)₂ complexes 2 and 3, each of which is centrosymmetric, have Cr^III⋯ Cr^III distances of 2.591(1) and 2.564(1)Å. In compound 5, also showing centrosymmetric bis(imido) bridging, the Cr^IV⋯ Cr^IV distance is 2.974(5)Å. In 6 the acetylene is symmetrically bihapto and the imido function bonds in a linear manner. Compound 7 has dissimilar chromium atoms, formally assigned as Cr^III, Cr^IV, with bridging amido functions. The Cr ⋯ Cr distance is 2.936(2)Å.