Genuine heteroleptic complexes of early rare-earth metals: Synthesis, x-ray structure, and their use for stereospecific isoprene polymerization catalysis

Genuine heteroleptic neodymium and samarium complexes of formula [Cp*'Ln{(p-tol)NN}(BH4)] (CP*' = C5Me4(nPr), (p-tol)NN = (p-tol)-NC(Me)CHC(Me)N(p-tol), Ln = Sm: 1a, Ln = Nd: 1b) have been synthesized for the first time. These unprecedented homologues of early lanthanocenes are prepared by a metathetic reaction between their monocyclopentadienylbisborohydrido precursors with the corresponding potassium diketiminate. Both complexes were obtained in good yields and were characterized by H-1 NMR spectroscopy and elemental analysis. Complex la has an non-solvated dimeric structure, as indicated by its crystallographic data. The chloroneodymium analogue [Cp*'Nd{(ptol)NN}(Cl)] (2b) was only obtained as a part of a mixture. Analysis of crystals of 2b by X-ray diffraction revealed a molecular structure very similar to that of la. Preliminary isoprene polymerization experiments were carried out with 1b in the presence of an alkyl-magnesium coactivator. The resulting bimetallic Nd/Mg system behaves as an efficient and highly stereospecific catalyst with the synthesis of trans-1,4-polyisoprene with more than 98% regularity. The control of the polymer structure is related to the steric hindrance around the lanthanide atom.