Organolanthanide Complexes Supported by Thiazole-Containing Amidopyridinate Ligands: Synthesis, Characterization, and Catalytic Activity in Isoprene Polymerization

Neutral bis­(alkyl)-organolanthanide complexes supported by tridentate {<i>N</i><sup>–</sup>,<i>N</i>,<i>N</i>} monoanionic 5-methylthiazole- or benzothiazole-amidopyridinate ligands have been prepared and completely characterized: (L<sup>Thia</sup>Me<sub>2</sub>)­Ln­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub> [Ln = Lu<sup>3+</sup> (<b>3</b>), Er<sup>3+</sup> (<b>7</b>), Yb<sup>3+</sup> (<b>8</b>)] and (L<sup>BnTh</sup>Me<sub>2</sub>)­Lu­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub> (<b>5</b>). Similarly to related Y<sup>3+</sup> systems, the nature of the thiazole unit controls the ultimate catalyst stability in solution. In the diamagnetic Lu<sup>3+</sup> complex <b>5</b>, a progressive and complete rearrangement of its metal coordination sphere takes place through a metal-to-ligand alkyl migration with subsequent benzothiazole ring-opening and generation of the Lu<sup>3+</sup> mono­(alkyl)-arylthiolate species stabilized by a tetradentate {<i>N</i><sup><i>–</i></sup><i>,N,N,S</i><sup><i>–</i></sup>} dianionic ligand. On the other hand, the 5-methylthiazole-containing complexes <b>3</b>, <b>7</b>, and <b>8</b> showed no evidence of any ligand rearrangement. Complexes <b>3</b>–<b>8</b> have been tested as homogeneous catalysts in isoprene (IP) polymerization, after activation with selected organoborates. Binary systems <b>3</b>/TB and <b>7</b>/TB [TB = tritylium tetrakis­(pentafluorophenyl)­borate] show the highest activity and living character toward IP polymerization, affording polymers with relatively high <i>trans</i>-1,4-selectivity (up to 76.4%), moderate molecular weights (<i>M</i><sub>n</sub> up to 146 000 g/mol), and narrow polydispersities (<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub>). Depending on the rare-earth ion of choice, a prevalent <i>trans</i>-1,4 (Lu<sup>3+</sup>, Er<sup>3+</sup>, Yb<sup>3+</sup>; up to 76.4%) or a dominant 3,4 (Y<sup>3+</sup>; 92.7%) polymer structure is observed. The influence of the ligand type, metal ion, and activator(s) on the ultimate catalyst activity and selectivity is discussed.