The Influence of Dynamic Coordination Spheres in Metal-Based Homogeneous Catalysis

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The role of supporting ligands in controlling the catalytic efficacy of metal complexes cannot be overstated. A balance between developing high denticity ligands that are able to effectively stabilise catalytic intermediates, and low coordination numbers that leave vacant coordination sites for substrate binding, can be challenging. We have recently discovered that the catalytic efficacy of main group metals can be increased by employing high denticity ligands by exploiting the inherent lability of these closed shell metal ions.This catalyst design strategy is exemplified by the aluminium-catalysed trimerization of isocyanates with a 5-coordinate ligand that contains a hemi-labile pyridyl donor. The pyridyl is able to de-coordinate, and by so doing disfavours the backwards reaction that would lead to slower reaction kinetics; this catalyst is much more active than its pyridyl-free counterpart. The aim of this project is to probe the scope of this effect with other main group metals, and with other catalytic transformations. These metals are expected to cover Group 13 but could also extend into the s-block and early d block. We also envisage probing the effectiveness for this reaction in polymerisation and co-polymerisation reactions. Polymerisation reactions are expected to focus primarily on epoxide/anhydride co-polymers, which contain ester linkages and are therefore biodegradable / hydrolysable. There are > 100,000 different monomer combinations, which means that they can be prepared with greater property scope than traditional polymers; for example, we have prepared flame-retardant derivatives with no toxic additives. Most co-polymerisation catalysts are based upon salen-type ligands; the lack of extensive studies with alternative ligand environments is an inherent limitation to further exploitation of these co-polymers. We are particularly interested to determine if a dynamic coordination sphere could be exploited to increase efficiency in preparing alternating co-polymers.While polymerisation remains the primary focus, there is scope for exploring the influence of complexes exhibiting dyanamic coordination behaviour in the activation of other small molecules such as H2 and CO which leads into hydrogenation and/or carbonylation catalysis.New ligand leads and complexes will derive from initial studies and continually inform subsequent studies.