Genuine Heteroleptic Complexes of Early Rare‐Earth Metals: Synthesis, X‐ray Structure, and Their Use for Stereospecific Isoprene Polymerization Catalysis

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Abstract
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: 1 a, Ln = Nd: 1 b) 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 1H NMR spectroscopy and elemental analysis. Complex 1 a has an non-solvated dimeric structure, as indicated by its crystallographic data. The chloroneodymium analogue [Cp*′Nd{(p-tol)NN}(Cl)] (2 b) was only obtained as a part of a mixture. Analysis of crystals of 2 b by X-ray diffraction revealed a molecular structure very similar to that of 1 a. Preliminary isoprene polymerization experiments were carried out with 1 b in the presence of an alkylmagnesium 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.

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