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

The dimeric alkyl rare earth complexes [Cp'2SmMe]2 and [Cp'2NdMe]2 (Cp'=C5H4tBu) can be isolated in the solid state, whereas formation of the hydride [Cp'2NdH]2 does not occur and the hydride [Cp'2SmH]2 is only depicted in solution. Neodymium hydrides are characterized in solution as solvated (trisalkylboron) complexes of general formula Cp'2NdLxHBEt3. By hydrogenolysis of a convenient alkyl derivative, a transient hydride CpQ2NdH is formed (CpQ=C5H4CH2CH2OCH3). This hydride is not stable and rapidly rearranges into the corresponding triscyclopentadienyl derivative CpQ3Nd. It can be trapped by a hindered ketone such as pivalone, leading to the alkoxide CpQ2NdOCH(tBu)2. The complex [Cp'2NdMe]2 is not stable in THF and the analogous CpQ2NdMe complex cannot be obtained from the corresponding chloride, using a solution of MeLi as alkylating reagent. Bimetallic Cp'2NdMe2Li is stable. Anionic allyl complexes of samarium and neodynium of general formula (C5H4CMe2)2Ln(C3H5) 2Li(OR2)2 were synthesized. These complexes are efficient catalysts for the trans 1-4 polymerization of isoprene. (orig.)