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

摘要

Genuine heteroleptic neody-mium and samarium complexes of formula [CpLn{(p-tol)NN}(BH_4)] (Cp=C_5Me_4(nPr),(p-tol)NN=(p-tol)-NC(Me)CHC(Me)N(p-tol),Ln-Sm:la,Ln - Nd:1b)have been synthesized for the first time.These unprecedented homologues of early lanthano-cenes are prepared by a metathetic reaction between their monocyclopenta-dienylbisborohydrido precursors with the corresponding potassium diketimi-nate.Both complexes were obtained in good yields and were characterized by ~1H NMR spectroscopy and elemental analysis.Complex la has an non-sol-vated dimeric structure,as indicated by its crystallographic data.The chloro-neodymium analogue [CpNd{(p-tol)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 with1b 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-poly-isoprene with more than 98 % regularity.The control of the polymer structure is related to the steric hindrance around the lanthanide atom.