Zirconium and Hafnium Complexes Containing N‑Alkyl-Substituted Amine Biphenolate Ligands: Unexpected Ligand Degradation and Divergent Complex Constitutions Governed by N‑Alkyls

The reactivity and thermal stability of zirconium and hafnium complexes containing the N-alkyl-substituted amine biphenolate ligands of the type [RN­(CH2-2-O-3,5-C6H2(tBu)2)2]2– ([R-ONO]2–; R = tBu (1a), iPr (1b), or nPr (1c)) were investigated. The reactions of either [1a]­M­(OiPr)2 (M = Zr or Hf) with equimolar H2[1a] or M­(OiPr)4(HOiPr) (M = Zr or Hf) with 2 equiv of H2[1a] at 25 °C in diethyl ether or 80 °C in toluene afford moderate yields of colorless crystals of M­[1a]­(OiPr)­(iPrOCH2-2-O-3,5-C6H2(tBu)2) (M = Zr (4a) or Hf (5a)). Controlled experiments revealed that the production of 4a and 5a proceeds via unexpected thermal degradation of H2[1a] that produces a highly reactive, transient ortho-quinone methide intermediate. Similar reactions employing H2[1b] and H2[1c], however, led to the formation of homoleptic bis-ligand complexes Zr­[1b]2 (8b) and M­[1c]2 (M = Zr (8c) or Hf (9c)) as colorless crystals. Decisive factors governing these divergent reaction pathways and complex constitutions are discussed. The X-ray structures of 4a, 5a, 8b, 8c, and 9c are presented.