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-But3C6H2 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 LiNHBut gave only the dimer (η-C5Me5)BrCr(µ-NHBut)2Cr(NBut)(NHBut)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 CrIII⋯ CrIII distance of 2.649(5)Å(average over two molecules), whilst the doubly bridged (CrIII)2 complexes 2 and 3, each of which is centrosymmetric, have CrIII⋯ CrIII distances of 2.591(1) and 2.564(1)Å. In compound 5, also showing centrosymmetric bis(imido) bridging, the CrIV⋯ CrIV 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 CrIII, CrIV, with bridging amido functions. The Cr ⋯ Cr distance is 2.936(2)Å.