Zwitterionic Trivalent (Alkyl)lanthanide Complexes in Ziegler-Type Butadiene Polymerization

The organolanthanide complexes Ln{C-(SiHMe2)(3)}(3) (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) react with 1 or 2 equiv of B(C6F5)(3) to yield the well-defined zwitterionic species Ln{C(SiHMe2)(3)}(2)HB(C6F5)(3) (Ln = La, 2a; Ce, 2b; Pr, 2c; Nd, 2d) or LnC(SiHMe2)(3){HB(C6F5)(3)}(2) (Ln = La, 3a; Ce, 3b; Pr, 3c; Nd, 3d), respectively. These complexes are shown to contain labile, bridging Si-H -> Ln and o-F -> Ln interactions based upon the observation of low one-bond silicon-hydrogen coupling constants ((1)J(SiH)) in H-1 NMR spectra of 2a and 3a, the presence of one set of C6F5 signals in the F-19 NMR spectra of 2a and 3a, the detection of only m-F and p-F resonances in the F-19 NMR spectra of 2b-d and 3b-d, two v(SiH) bands in the IR spectra of 2a-d, and X-ray crystallography analyses of 2b and 3d. In addition, a hexahaptotoluene molecule is coordinated to neodymium in 3d. Reactions of la and (AlMe3)(2) yield labile adducts. With greater than 3 equiv of AlMe3, exchange between free and bound AlMe3 groups was observed in EXSY NMR experiments. Compounds 2a-d and 3a-d, in the presence of AliBu(3), are precatalysts for polymerization of butadiene. The neodymium alkyl 3d has the highest activity of the series, and its performance is consistent with chain-transfer reactions with AliBu(3).