Zirconium
and Hafnium Complexes Containing <i>N</i>‑Alkyl-Substituted
Amine Biphenolate Ligands: Unexpected
Ligand Degradation and Divergent Complex Constitutions Governed by <i>N</i>‑Alkyls
- Publication date
- Publisher
Abstract
The reactivity and thermal stability
of zirconium and hafnium complexes
containing the <i>N</i>-alkyl-substituted amine biphenolate
ligands of the type [RN(CH<sub>2</sub>-2-O-3,5-C<sub>6</sub>H<sub>2</sub>(<i>t</i>Bu)<sub>2</sub>)<sub>2</sub>]<sup>2–</sup> ([R-ONO]<sup>2–</sup>; R = <i>t</i>Bu (<b>1a</b>), <i>i</i>Pr (<b>1b</b>), or <i>n</i>Pr
(<b>1c</b>)) were investigated. The reactions of either [<b>1a</b>]M(O<i>i</i>Pr)<sub>2</sub> (M = Zr or Hf) with
equimolar H<sub>2</sub>[<b>1a</b>] or M(O<i>i</i>Pr)<sub>4</sub>(HO<i>i</i>Pr) (M = Zr or Hf) with 2 equiv of H<sub>2</sub>[<b>1a</b>] at 25 °C in diethyl ether or 80 °C
in toluene afford moderate yields of colorless crystals of M[<b>1a</b>](O<i>i</i>Pr)(<i>i</i>PrOCH<sub>2</sub>-2-O-3,5-C<sub>6</sub>H<sub>2</sub>(<i>t</i>Bu)<sub>2</sub>) (M = Zr (<b>4a</b>) or Hf (<b>5a</b>)). Controlled
experiments revealed that the production of <b>4a</b> and <b>5a</b> proceeds via unexpected thermal degradation of H<sub>2</sub>[<b>1a</b>] that produces a highly reactive, transient <i>ortho</i>-quinone methide intermediate. Similar reactions employing
H<sub>2</sub>[<b>1b</b>] and H<sub>2</sub>[<b>1c</b>],
however, led to the formation of homoleptic bis-ligand complexes Zr[<b>1b</b>]<sub>2</sub> (<b>8b</b>) and M[<b>1c</b>]<sub>2</sub> (M = Zr (<b>8c</b>) or Hf (<b>9c</b>)) as colorless
crystals. Decisive factors governing these divergent reaction pathways
and complex constitutions are discussed. The X-ray structures of <b>4a</b>, <b>5a</b>, <b>8b</b>, <b>8c</b>, and <b>9c</b> are presented
