Ru(ii) pnx (x = o,n,s) complexes in homogeneous catalysis for sustainable development

Ellison, Elizabeth 2025. Ru(ii) pnx (x = o,n,s) complexes in homogeneous catalysis for sustainable development. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Ruthenium pincer complexes have shown substantial promise as homogeneous catalysts in reactions involving hydrogen transfer processes. Designer ligands provide excellent control over electronic and steric environment, allowing tuning of catalysts to control reactivity. Several authors have demonstrated their activity in useful transformations such as small molecule activation, asymmetric hydrogenations, hydrogenative cleavages, dehydrogenative couplings and activation of alcohols. However, reported pincer ligands are frequently difficult to make and highly air intolerant. The design of practical yet effective complexes for such reactions continues to be relevant. In this work, a series of bis-ligated Ru(II) complexes are prepared using imine-based PNX (X = O, N, S) ligands readily prepared from commercially available starting materials, producing a family of complexes of the structure [RuCl2(PNX)2]. These complexes contained fixed PN moieties and featured a variable pendant arm to demonstrate variable binding modes and showed hemilability in solution. Ligand structures were modified by pendant arm length, bulk, and donor atom. Several complexes were produced readily in good yields as single products, with single crystal X-ray crystallography confirming ligand binding mode. PNO alcohol ligands predominantly demonstrate a bis-tridentate binding mode. Extension of alcohol chain or substitution at the oxygen donor changes binding mode to bis-tridentate. Use of substituted N or S donors gives mixed binding modes within the same complex. These complexes are applied in the Guerbet coupling of ethanol with either ethanol or methanol to produce different isomers of butanol. This reaction is of industrial interest for application in biofuel upgrading and operates through a hydrogen borrowing mechanism. The family of [RuCl2(PNX)2] are found effective in the production of isobutanol, with the best performing PNS complex [RuCl(L14)]Cl yielding 70 % isobutanol at 96 % selectivity. The system shows good tolerance to reduced base loading and use of alternative cheaper base NaOH. The activity scope of these species is expanded in the hydrogenative cleavage of esters for future application in plastics degradation. The novel species are found active towards hydrogenative cleavage but also catalyse competitive transesterification reactions. The most effective systems feature PNN ligands. Further investigation of complexes demonstrates that the activity of symmetrical PNP, SNS and NNN ligands is dependent on the presence of a Ru-N-H moiety.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Chemistry
Date of First Compliant Deposit:	15 May 2025
Last Modified:	15 May 2025 08:45
URI:	https://orca.cardiff.ac.uk/id/eprint/178292

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