Mono- and multi-metallic main group salen complexes: applications in lactide ring-opening polymerisation
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Date
27/11/2021Item status
Restricted AccessEmbargo end date
27/11/2022Author
Gaston, Anand Jay
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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.