Synthesis, Characterization, and Reactivity of <i>ansa-</i>Bis(allyl) Lanthanide Complexes, a New Class of Single-Component Methyl Methacrylate Polymerization Catalysts

The reaction of the dipotassium salt K2{3-(C3H3SiMe3-1)2SiMe2} with LnCl3 in THF leads to the formation of the new ansa-allyl lanthanate complexes [Ln{(η3-C3H3SiMe3)2SiMe2}2{μ-K(THF)}·nTHF]∞ (1, Ln = La, n = 0.5; 2, Ln = Y, n = 1; 3, Ln = Sc, n = 1). Reaction of K2{3-(C3H3SiMe3-1)2SiMe2} with NdI3(THF)3.5 affords THF-free [Nd{(η3-C3H3SiMe3)2SiMe2}2(μ-K)]∞ (4). Treatment of SmI2(THF)2 with Li{3-(C3H3SiMe3-1)2SiMe2}/KOtBu resulted in the samarium(III) complex [Li(OEt2)4][Sm{(η3-C3H3SiMe3)2SiMe2}2] (5), albeit in small yield. Whereas the crystal structure of 1 shows a coordination polymer, with potassium bridging to allyl moieties of two neighboring lanthanide units and close K···CH3Si contacts, complex 5 consists of discrete ions. The complexes 1−5 are all found to be efficient catalysts for the polymerization of methyl methacrylate to high-molecular-weight polymers under a variety of conditions, with 2 at 0 °C giving extremely high turnovers of 86 400 mol of MMA (mol of Y)-1 h-1.