Bis(thiophenolate)pyridine Pincer Ligands and Trivalent Zirconocene Complexes Relevant to Earlv Transition Metal Polymerization Catalysts
<p>Two major topics are covered: the first chapter is focused on the development of post-metallocene complexes for propylene polymerization. The second and third chapters investigate the consequences of diisobutylaluminum hydride (HAl<sup>i</sup>Bu<sub>2</sub>) additive...
| Summary: | <p>Two major topics are covered: the first chapter is focused on the development of post-metallocene complexes for propylene polymerization. The second and third chapters investigate the consequences of diisobutylaluminum hydride (HAl<sup>i</sup>Bu<sub>2</sub>) additives in zirconocene based polymerization systems.</p>
<p>The synthesis, structure, and solution behavior of early metal complexes with a new tridentate LX<sub>2</sub> type ligand, bis(thiophenolate)pyridine ((SNS) = (2-C<sub>6</sub>H<sub>4</sub>S)<sub>2</sub>-2,6-C<sub>5</sub>H<sub>3</sub>N) are investigated. SNS complexes of Ti, Zr, and Ta having dialkylamido coligands were synthesized and structurally characterized. The zirconium complex, (SNS)Zr(NMe<sub>2</sub>)<sub>2</sub>, displays C<sub>2</sub> symmetry in the solid state. Solid-state structures of tantalum complexes (SNS)Ta(NMe<sub>2</sub>)<sub>3</sub> and (SNS)TaCl(NEt<sub>2</sub>)<sub>2</sub> also display pronounced C<sub>2</sub> twisting of the SNS ligand. 1D and 2D NMR experiments show that (SNS)Ta(NMe<sub>2</sub>)<sub>3</sub> is fluxional with rotation about the Ta N(amide) bonds occurring on the NMR timescale. The fluxional behavior of (SNS)TaCl(NEt<sub>2</sub>)<sub>2</sub> in solution was also studied by variable temperature <sup>1</sup>H NMR. Observation of separate signals for the diastereotopic protons of the methylene unit of the diethylamide indicates that the complex remains locked on the NMR timescale in one diastereomeric conformation at temperatures below -50 °C.</p>
<p>Reduction of Zr(IV) metallocenium cations with sodium amalgam (NaHg) produces EPR signals assignable to Zr(III) metallocene complexes. Thus, chloro-bridged heterobinuclear ansa-zirconocenium cation [((SBI))Zr(μ-Cl)<sub>2</sub>AlMe<sub>2</sub>]<sup>+</sup>B(C<sub>6</sub>F<sub>5</sub>)<sub>4¯</sub> (SBI = rac-dimethylsilylbis(1-indenyl)), gives rise to an EPR signal assignable to the complex (SBI)Zr<sup>III</sup>(μ-Cl)<sub>2</sub>AlMe<sub>2</sub>, while (SBI)Zr<sup>III</sup>-Me and (SBI)Zr<sup>III</sup>(-H)2Al<sup>i</sup>Bu<sub>2</sub> are formed by reduction of [(SBI)Zr(μ-Me)<sub>2</sub>AlMe<sub>2</sub>]<sup>+</sup>B(C<sub>6</sub>F<sub>5</sub>)<sub>4¯</sub> and [(SBI)Zr(μ-H)<sub>3</sub>(Al<sup>i</sup>Bu<sub>2</sub>)<sub>2</sub>]<sup>+</sup>B(C<sub>6</sub>F<sub>5</sub>)4¯, respectively. These products are also formed, along with (SBI)Zr<sup>III</sup>-<sup>i</sup>Bu and [(SBI)Zr<sup>III</sup>]<sup>+</sup> AlR4¯ when (SBI)ZrMe<sub>2</sub> reacts with HAl<sup>i</sup>Bu<sub>2</sub>, eliminating isobutane en route to the Zr(III) complex. Studies concerning the interconversion reactions between these and other (SBI)Zr(III) complexes and reaction mechanisms involved in their formation are also reported.</p>
<p>The addition of HAl<sup>i</sup>Bu<sub>2</sub> to precatalyst [(SBI)Zr(µ-H)<sub>3</sub>(Al<sup>i</sup>Bu<sub>2</sub>)<sub>2</sub>]<sup>+</sup> significantly slows the polymerization of propylene and changes the kinetics of polymerization from 1st to 2nd order with respect to propylene. This is likely due to competitive inhibition by HAl<sup>i</sup>Bu<sub>2</sub>. When the same reaction is investigated using [(<sup>n</sup>BuCp)<sub>2</sub>Zr(μ-H)<sub>3</sub>(Al<sup>i</sup>Bu<sub>2</sub>)<sub>2</sub>]<sup>+</sup>, hydroalumination between propylene and HAl<sup>i</sup>Bu<sub>2</sub> is observed instead of propylene polymerization.</p>
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