Synthetic, Reactivity, and Structural Studies on Half‐Sandwich (η5‐C5Me5)Be and Related Compounds: Halide, Alkyl, and Iminoacyl Derivatives

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Abstract
The half‐sandwich compounds [(η5‐C5Me5)BeX] (X=Cl, 1 a; Br, 1 b), readily prepared from the reaction of the halides BeX2 and M[C5Me5] (M=Na or K), are useful synthons for other (η5‐C5Me5)Be organometallic compounds, including the alkyl derivatives [(η5‐C5Me5)BeR] (R=Me, 2 a; CMe3, 2 b; CH2CMe3, 2 c; CH2Ph, 2 d). The latter compounds can be obtained by metathetical exchange of the halides 1 with the corresponding lithium reagent and exhibit NMR signals and other properties in accord with the proposed formulation. Attempts to make [(η5‐C5Me5)BeH] have proved fruitless, probably due to instability of the hydride toward disproportionation into [Be(C5Me5)2] and BeH2. The half‐sandwich iminoacyl [(η5‐C5Me5)Be(C(NXyl)Cp′)] and [(η5‐C5Me4H)Be(C(NXyl)Cp′)]3, 6 where Xyl=C6H3‐2,6‐Me2 and Cp′=C5Me5 or C5Me4H, are formed when the beryllocenes [Be(C5Me5)2], [Be(C5Me4H)2], and [Be(C5Me5)(C5Me4H)] are allowed to react with CNXyl. Isolation of three different iminoacyl isomers from the reaction of the mixed‐ring beryllocene [(η5‐C5Me5)Be(η1‐C5Me4H)] and CNXyl, namely compounds 5 a, 5 b, and 6, provides compelling evidence for the existence in solution of different beryllocene isomers, generated in the course of two very facile processes that explain the solution dynamics of these metallocenes, that is the 1,5‐sigmatropic shift of the Be(η5‐Cp′) unit around the periphery of the η1‐Cp′ ring, and the molecular inversion rearrangement that exchanges the roles of the two rings.