Unsymmetrically substituted group 13 and 15 compounds in intramolecular frustrated Lewis pair synthesis

Thesis

This thesis explores the viability of potential routes towards surface supported xanthene based Frustrated Lewis Pairs (FLPs). This is carried out primarily through the synthesis of new Group 13 and 15 Lewis acids and bases, and their addition to xanthene precursors.The dihydrogen activation properties of the FLPs obtained is also examined. An introduction to relevant literature is made in Chapter 1, and experimental/characterisation methods are defined in Chapter 2. Chapter 3 details the preparation of a series of heteroleptic tris aryl boranes. A robust synthetic route towards bis aryl methoxyboranes is introduced, and these compounds are used to prepare a selection of corresponding bromoboranes which are precursors to a series of triaryl boranes. Successful synthesis of desired tris as opposed to tetra substituted aryl boranes is highly dependent on the steric profile of the final substituent introduced. Chapter 4 employs the bromoborane reagents prepared in the previous chapter for the synthesis of a series of FLPs varied at the Lewis acid moiety. Three of these systems are capable of reversible H2 activation, two to a sufficient extent such that this equilibrium can be studied by variable temperature NMR spectroscopy. In contrast to previous reports of related species, the thermodynamic parameters underpinning hydrogenation cannot be described by a single linear van’t Hoff plot. Instead, two disparate linear regimes are observed. A ‘high temperature’ series follows the expected trend of equilibrium moving towards the reaction product as temperature decreases. Below an inflection point, the opposite trend is seen – implying reversal of the reaction with temperature decrease. This unexpected reactivity was probed by quantum chemical calculations (carried out by Agamemnon Crumpton). These calculations suggest that below a certain temperature, intermolecular aggregation of the FLPs is energetically preferred over hydrogenation, preventing facile hydrogen activation. The final results section, Chapter 5, looks at the construction of a range of xanthene-phosphines featuring substituents intended to be amenable to surface attachment. Following the preparation of these reagents, they are examined for their potential as precursors to model surface supported FLPs. It is found that placing a methoxy aryl group on a xanthene-phosphine inhibits subsequent functionalisation of the backbone to attain an intramolecular FLP. In contrast, introduction of a dimethylamino aryl lends the system to facile synthesis of FLPs furnished with three different functional groups on phosphorus. Whilst the pairs synthesised in this chapter did not prove suitable for attachment to a surface, the design principles gained from this work provide a clear direction for future synthetic strategies. The outlook is discussed in Chapter 6.

Authors: Roper, AF

• DOI: 10.5287/ora-z5bxb94bp
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Main group chemistry, organometallic chemistry