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}2HB­(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 (1JSiH) in 1H NMR spectra of 2a and 3a, the presence of one set of C6F5 signals
in the 19F NMR spectra of 2a and 3a, the detection of only m-F and p-F resonances in the 19F NMR spectra of 2b–d and 3b–d,
two νSiH 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 1a 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 AliBu3, 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 AliBu3.