Structural analysis of zincocenes with substituted cyclopentadienyl rings

New zincocenes [ZnCp’₂] (2–5) with substituted cyclopentadienyl ligands C₅Me₄H, C₅Me₄^tBu, C₅Me₄SiMe₂^tBu and C₅Me₄SiMe₃, respectively, have been prepared by the reaction of ZnCl₂ with the appropriate Cp’-transfer reagent. For a comparative structural study, the known [Zn(C₅H₄SiMe₃)₂] (1), has also been investigated, along with the mixed-ring zincocenes [Zn(C₅Me₅)(C₅Me₄SiMe₃)] (6) and [Zn(C₅Me₅)(C₅H₄SiMe₃)] (7), the last two obtained by conproportionation of [Zn(C₅Me₅)₂] with 5 or 1, as appropriate. All new compounds were characterised by NMR spectroscopy, and by X-ray methods, with the exception of 7, which yields a side-product (C) upon attempted crystallisation. Compounds 5 and 6 were also investigated by ¹³C CPMAS NMR spectroscopy. Zincocenes 1 and 2 have infinite chain structures with bridging Cp’ ligands, while 3 and 4 exhibit slipped-sandwich geometries. Compounds 5 and 6 have rigid, η⁵/η¹(σ) structures, in which the monohapto C₅Me₄SiMe₃ ligand is bound to zinc through the silyl-bearing carbon atom, forming a Zn-C bond of comparable strength to the Zn-Me bond in ZnMe₂. Zincocene 5 has dynamic behaviour in solution, but a rigid η⁵/η¹(σ) structure in the solid state, as revealed by ¹³C CPMAS NMR studies, whereas for 6 the different nature of the Cp’ ligands and of the ring substituents of the η¹-Cp’ group (Me and SiMe₃) have permitted observation for the first time of the rigid η⁵/η¹ solution structure. Iminoacyl compounds of composition [Zn(η⁵-C₅Me₄R)(η¹-C(NXyl)C₅Me₄R)] resulting from the reactions of some of the above zincocenes and CNXyl (Xyl=2,6-dimethylphenylisocyanide) have also been obtained and characterised.