Mono- and multi-metallic main group salen complexes: applications in lactide ring-opening polymerisation

Abstract

Ligand-metal complexes have demonstrated high potential for catalytic control over the ring-opening polymerisation (ROP) of rac-lactide. Catalyst performance and control over the resultant polymer structure can be improved by altering the sterics and electronics of the catalyst system, through modifying the ligand substituents and/or incorporating a second (hetero)metal. The development of bimetallic catalysts through the incorporation of a secondary metal into homogenous catalyst ligand architectures is a rapidly developing field within inorganic chemistry. The work in this thesis aims to adapt the electronics of metal complex catalysts derived from salen-ligands, by modification of the phenolic substituents and through the formation of cooperative bimetallic catalysts active toward rac-lactide ROP. Chapter 2 reports four new AlCl-salen complexes featuring NEt2-substituents, which display high catalyst activities towards rac-lactide ROP (kobs < 1.9 x 10-3 s -1 ), with moderate isoselectivity (Pi = 0.7) via in situ activation with a single equivalent of propylene oxide. The inclusion of the Al-Cl bond within the catalysts rather than the more commonly utilised Al-alkoxide moiety serves to expand the scope of these complexes as well as to potentially increase catalyst stability. Incorporating Lewis basic NEt2 groups into the ligand scaffold not only improves the initiation efficiency but also avoids the need for a Lewis basic co-catalyst and excess epoxide. Notably, studies of the amino-substituted catalysts reveal that the formation of a hexacoordinate aluminate species may hinder rather than enhance catalytic activity. The effect of various ammonium salts have been investigated upon the polymerisation and are proposed to act as chain transfer agents, resulting in shorter chain lengths and reduced polymerisation rates. Chapter 3 describes the synthesis, characterisation and catalytic activity of a series of heterobimetallic complexes for rac-lactide ROP. Derived from an ortho-carboxylic acid modified salen ligand scaffold, these complexes combine aluminium with alkali or alkaline earth metals (lithium, magnesium, zinc or calcium). The asymmetric nature of the salen ligand allows for selective metal site binding, with different metals displaying preferences for different binding pockets and thus avoiding metal redistribution. Through the addition of secondary metals, catalytic efficiencies can be enhanced by a factor of up to 11 compared to the monometallic analogues. Mechanistic studies, in tandem with DFT (density functional theory) and AIMD (ab initio molecular dynamics) calculations, suggest that activity enhancements occur through mixed-metal cooperativity, incorporation of an additional monomer coordination site, increased complex rigidity and optimised geometry around the metal centres. These trends contradict some of the catalyst design principles that are widely accepted for monometallic catalysts, emphasizing the importance of understanding heterometallic cooperativity to exploit the full potential of heterometallic catalysts in cyclic ester ROP. While magnesium and zinc enhance the catalyst activity, lithium and calcium both reduce the polymerisation rate, highlighting the importance of the heterometal nature in achieving cooperative catalysis. Chapter 4 builds upon the work in Chapter 3, modifying the tetra-anionic ortho-carboxylic acid salen ligand into a di-anionic ortho-methyl ester salen scaffold. Mono-, homobi- and heterobi-metallic complexes containing lithium, aluminium and zinc have been structurally characterised and tested toward rac-lactide ROP. The activity of the homobimetallic zinc complex was 47 times higher than the monometallic analogue. X-ray crystallographic structures and DFT calculations highlight the importance of the secondary (hetero)metal electropositivity on the Zn-Cl bond length, which correlates to the initiation efficiency in both homo- and heterobimetallic complexes. This suggests that longer (and weaker) Zn-Cl bonds lead to faster initiation through ring-opening propylene oxide to generate an active metal-alkoxide species in situ. Notably, monometallic aluminium and zinc complexes can also catalyse rac-lactide ROP in the absence of an epoxide, which circumnavigates the need for the toxic propylene oxide co-catalyst.