Formation and Rearrangement of Reduced Diiminepyridine Complexes of Zr and Hf

Reduction of seven-coordinate [(EtDIP)MCl4] {EtDIP = 2,6-(2,6-Et2–C6H3N=CMe)2C5H3N; M = Zr or Hf} produces formally divalent complexes [(EtDIP)MCl2] containing in reality a doubly reduced DIP ligand. The Hf complex is five-coordinate, but the Zr analog crystallizes as a chloride-bridged dimer with an unusual coordination geometry; the monomer-dimer equi- librium is fast at room temperature. Reaction of [(EtDIP)HfCl2] with MeMgBr or LiCH2SiMe3 produced the corresponding dialkyls [(EtDIP)HfR2]. An unexpected ligand isomerization to an amido/enamido form was observed for [(EtDIP)ZrCl2], [(EtDIP)- HfMe2] and [(EtDIP)TiMe2], and a mechanism is proposed based on DFT calculations.