Synthesis and catalytic applications of titanium amine bis(phenolate) complexes

Abstract

Many catalytic processes rely on the use of transition metal complexes, frequently utilising precious metals, such as ruthenium, iridium, palladium or platinum. While these metals exhibit excellent performances in well-established processes, they also bear the inherent disadvantages of high cost and low abundance. It is therefore desirable to move towards more sustainable alternatives. In this work, molecular titanium complexes were investigated as potential replacements for noble metals in photocatalysis and as polymerisation catalysts. Titanium is an abundant, inexpensive metal with low toxicity. Consequently, TiO2 is used widely in ‘low cost’ applications, for example as white pigments and as a photocatalyst, although it only absorbs in the ultraviolet region of the electromagnetic spectrum. Ideally, a photocatalyst should be able to utilise visible light (i.e. sunlight) to drive chemical reactions, therefore the application of molecular, visible-light-absorbing titanium complexes as photosensitisers was explored here. A series of mono- and bimetallic complexes with amine bis(phenol) ligands was synthesised, characterised and subsequently trialled in the generation of singlet oxygen under visible light irradiation (420–450 nm). The active, singlet oxygen species was trapped using - terpinene, cyclopentadiene, 1,3-cyclohexadiene and anthracene. All complexes were found to be active photosensitisers, however, the presence of a ‘TiO2-like’ titanium-oxo moiety was found to be crucial for the activity of the complexes. These experiments were conducted both under continuous flow conditions and as batch reactions. Structure-activity relationships were investigated, giving rise to a better understanding of the key features of these complexes and targets for further development of the photosensitisers. Moreover, the synthesised complexes were utilised as polymerisation catalysts in the ring opening polymerisation of the cyclic ester ɛ-caprolactone. The polymeric form, poly(ɛ-caprolactone), is biodegradable and of interest in food packaging and medical applications. Using benzyl alcohol initiator, the complexes afforded polymers, with molecular weights of up to 21,000 g mol-1 and reasonably narrow dispersities (1.09–1.73), suggesting moderate to good control over the polymerisations. Two vanadium complexes and two manganese complexes were synthesised for comparison, although it was found that the titanium complexes typically had a more favourable balance between catalytic activity and properties of the resulting polymer. Mechanistic aspects of these polymerisation reactions were explored and a first indication of potential reaction mechanisms was uncovered.