Homopiperazine and piperazine complexes of Zr(IV) and Hf(IV) and their application to the ring-opening polymerisation of lactide

In this paper we describe the preparation and characterisation, by single-crystal X-ray diffraction, of twelve ZrIV/HfIV complexes based on piperazine or homopiperazine salan ligands. With the piperazine ligands, a mixture of species was observed and various solid-state structures were isolated. However, with homopiperazine salan ligands, 1:1 ligand-tometal complexes were observed both in solution and in the solid state. Interestingly, for the homopiperazine complexes the isopropoxide ligands are trans to one another in the solid state, most likely because of the rigid nature of the homopiperazine backbone. All homopiperazine Hf(IV) and Zr(IV) complexes were tested for the ring-opening polymerisation (ROP) of rac-lactide. The complexes are active and produce polylactide with narrow polydispersity indices. The kinetics and living characteristics of the polymerisation have also been investigated.