Double and Reversible Alkyl Transfer from ZrBn 4 /HfBn 4 to a Diiminepyridine Ligand

Reaction of MBn4 (M = Zr, Hf ) with diiminepyridine ligands MeDIP, EtDIP and iPrDIP [RDIP = 2,6-(2,6-R2C6H3N=CMe)2- C5H3N] results in the transfer of two benzyl groups to the DIP imine carbon atoms, generating mixtures of rac and meso isomers of [RDIP + 2Bn]MBn2. The diastereomers could in several cases be separated by fractional crystallization, and the X-ray structures of rac and meso [EtDIP + 2Bn]ZrBn2, meso- [MeDIP + 2Bn]ZrBn2 and meso-[EtDIP + 2Bn]HfBn2 are reported. On heating either diastereomer of [EtDIP + 2Bn]ZrBn2 in solution, the other isomer grows back in, demonstrating reversibility of the alkyl transfer. Transfer to other positions of the DIP skeleton was not observed. The reaction of [EtDIP + 2Bn]ZrBn2 with two equivalents of TEMPO produced structurally characterized [EtDIP + 2Bn]Zr(Bn)(TEMPO) which shows no tendency to isomerize.