Homopiperazine and Piperazine Complexes of Zr^IV and Hf^IV and Their Application to the Ring‐Opening Polymerisation of Lactide

Abstract

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‐to‐metal 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 HfIV and ZrIV 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.

Monomeric Ti(IV) homopiperazine complexes and their exploitation for the ring opening polymerisation of rac-lactide

“…Literature preparation methods were utilised to prepare the homopiperazine salan ligands, ( 1 - 6 )H 2 [3,45]. The complexes were prepared by a 1:1 reaction of the salan with Ti(O i Pr) 4 at 80°C, this was carried out under a flow of Ar to facilitate the removal of isopropanol to drive the reaction to the formation of the 1:1 complex.…”

“…As part of our on-going studies into the chemistry of group 4 metals and homo/piperazine derived salan ligands [1-3] in this paper we report the synthesis and characterisation of series of monometallic complexes based on the homopiperazine backbone. This ligand family has also been applied to Fe(III) [4,5], Cu(II) [6], Ni(II) [6] and Mo(VI) [7] metal centres.…”

“…These 7-membered ring ligands are under-represented in the literature compared to their 6-membered brothers or their linear amine bis(phenolate) cousins [8-13]. To re-address this imbalance we have previously reported the formation of Ti 2 (O i Pr) 6 L or monometallic Zr/Hf(O i Pr) 2 L species (containing homopiperazine salan ligands) where, in the monometallic examples, the O i Pr moieties are trans to one another [3]. Utilising the piperazine derived salan ligands with Zr(IV) and Hf(IV) starting materials leads to unpredictable reactions with no rationale control over the product formed [3].…”

“…To re-address this imbalance we have previously reported the formation of Ti 2 (O i Pr) 6 L or monometallic Zr/Hf(O i Pr) 2 L species (containing homopiperazine salan ligands) where, in the monometallic examples, the O i Pr moieties are trans to one another [3]. Utilising the piperazine derived salan ligands with Zr(IV) and Hf(IV) starting materials leads to unpredictable reactions with no rationale control over the product formed [3]. These complexes have been shown to be effective initiators for the ring opening polymerisation (ROP) of cyclic esters [2,3].…”

“…Utilising the piperazine derived salan ligands with Zr(IV) and Hf(IV) starting materials leads to unpredictable reactions with no rationale control over the product formed [3]. These complexes have been shown to be effective initiators for the ring opening polymerisation (ROP) of cyclic esters [2,3]. Moreover, we have prepared Al(III) complexes of homopiperazine salan ligands for co-polymerisations of cyclic esters [1].…”