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

The reaction of silylene-bridged bis(fluorenyl)dipotassium salts with neodymium tris(borohydride) afforded new neutral ansa-bis(fluorenyl)silane neodymium borohydrides: (Flu(2)SiR(2))Nd(BH(4))(THF) [R(2) = Me(2), Et(2), (CH(2))(3), Me(Ph), Flu = C(13)H(8)] that were better characterised and more soluble than the previously described anionic [(Flu(2)SiMe(2))Nd(BH(4))(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 [Flu(2)Si(CH(2))(3)]Nd(BH(4))(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 [(Flu(2)SiMe(2))Nd(BH(4))(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.