Neutral ansa-bis(fluorenyl)silane neodymium borohydrides: synthesis, structural study and behaviour as catalysts in butadiene–ethylenecopolymerisation

Abstract

The reaction of silylene-bridged bis(fluorenyl)dipotassium salts with neodymium tris(borohydride) afforded new neutral ansa-bis(fluorenyl)silane neodymium borohydrides: (Flu₂SiR₂)Nd(BH₄)(THF) [R₂ = Me₂, Et₂, (CH₂)₃, Me(Ph), Flu = C₁₃H₈] that were better characterised and more soluble than the previously described anionic [(Flu₂SiMe₂)Nd(BH₄)₂]⁻. The X-ray structures of three of these complexes were determined, and their solid-state geometrical parameters are very similar, despite the ring strain introduced by the silacyclobutane bridge in [Flu₂Si(CH₂)₃]Nd(BH₄)(THF). The main geometrical features were satisfactorily reproduced by DFT calculations. The catalytic activity of the title complexes in ethylene–butadiene copolymerisation reactions was assessed and compared to that of the reported activity of [(Flu₂SiMe₂)Nd(BH₄)₂]⁻ under similar conditions. From these results it can be concluded that the cyclo-copolymerisation of ethylene with butadiene is characteristic of a catalyst featuring silylene-bridged bis(fluorenyl) ligands around neodymium, and appears to be independent of the substituents at the silicon atom.