Undercoordinated neodymium and samarium hydrides and alkyls: Their behaviour and stability in solution. Application in isoprene polymerization

Abstract The dimeric alkyl rare earth complexes [Cp′ 2 SmMe] 2 and [Cp′ 2 NdMe] 2 (Cp′=C 5 H 4 tBu) can be isolated in the solid state, whereas formation of the hydride [Cp′ 2 NdH] 2 does not occur and the hydride [Cp′ 2 SmH] 2 is only depicted in solution. Neodymium hydrides are characterized in solution as solvated (trisalkylboron) complexes of general formula Cp′ 2 NdL x HBEt 3 . By hydrogenolysis of a convenient alkyl derivative, a transient hydride Cp Q 2 NdH is formed (Cp Q =C 5 H 4 CH 2 CH 2 OCH 3 ). This hydride is not stable and rapidly rearranges into the corresponding triscyclopentadienyl derivative Cp Q 3 Nd. It can be trapped by a hindered ketone such as pivalone, leading to the alkoxide Cp Q 2 NdOCH(tBu) 2 . The complex [Cp′ 2 NdMe] 2 is not stable in THF and the analogous Cp Q 2 NdMe complex cannot be obtained from the corresponding chloride, using a solution of MeLi as alkylating reagent. Bimetallic Cp′ 2 NdMe 2 Li is stable. Anionic allyl complexes of samarium and neodymium of general formula (C 5 H 4 CMe 2 ) 2 Ln(C 3 H 5 ) 2 Li(OR 2 ) 2 were synthesized. These complexes are efficient catalysts for the trans 1–4 polymerization of isoprene.