Zwitterionic Trivalent (Alkyl)Lanthanide Complexes in Ziegler-type Butadiene Polymerization

Abstract

<p>The organolanthanide complexes Ln{C(SiHMe₂)₃}₃ (Ln = La, <strong>1a</strong>; Ce, <strong>1b</strong>; Pr, <strong>1c</strong>; Nd, <strong>1d</strong>) react with one or two equiv. of B(C₆F₅)₃ to yield the well-defined zwitterionic species Ln{C(SiHMe₂)₃}₂HB(C₆F₅)₃ (Ln = La, <strong>2a</strong>; Ce, <strong>2b</strong>; Pr, <strong>2c</strong>; Nd, <strong>2d</strong>) or LnC(SiHMe₂)₃{HB(C₆F₅)₃}₂(Ln = La, <strong>3a</strong>; Ce, <strong>3b</strong>; Pr, <strong>3c</strong>; Nd, <strong>3d</strong>), respectively. These complexes are shown to contain labile, bridging Si–H⇀Ln and <em>o</em><strong>-</strong>F->Ln interactions based upon the observation of low one-bond silicon-hydrogen coupling constants (¹J_SiH) in ¹H NMR spectra of <strong>2a</strong> and <strong>3a</strong>, the presence of one set of C₆F₅ signals in the ¹⁹F NMR spectra of <strong>2a</strong> and <strong>3a</strong>, the detection of only m-F and p-F resonances in ¹⁹F NMR spectra of <strong>2b-d</strong> and <strong>3b-d</strong>, two <em>ν</em>_SiH bands in IR spectra of <strong>2a-d</strong>, and X-ray crystallography analyses of <strong>2b</strong> and <strong>3d</strong>. In addition, a hexametertoluene molecule is coordinated to neodymium in <strong>3d</strong>. Reactions of <strong>1a</strong> and (AlMe₃)₂ yield labile adducts with an approximate stoichiometry of <strong>1a</strong>·3AlMe₃. Exchange between free and bound AlMe₃ groups was observed in EXSY NMR experiments with greater than 3 equiv. of AlMe₃. Compounds <strong>2a-d</strong> and <strong>3a-d</strong>, in the presence of Al<em>i</em>Bu₃, are precatalysts for polymerization of butadiene. The neodymium alkyl <strong>3d</strong> has the highest activity of the series, and its performance is consistent with chain transfer reactions with Al<em>i</em>Bu₃.</p>