AbstractLewis acidic ionic liquids, in particular chloroaluminate systems, have been found to be very effective catalysts for reactions such as Diels-Alder and Friedel-Crafts, and made their way to pilot and industrial processes, including oligomerisation of olefins (DifasolTM by IFP) and refinery alkylations (IonikylationTM by PetroChina and ISOALKYTM by Chevron). 1 Despite their popularity, this archetypal group of catalysts faces two key challenges: the use of an expensive organic cation which plays no role in the catalytic process and placing the Lewis acidic function on the chloroaluminate anion, which does not allow for further modification of catalytic activity.
The first of the two shortcomings has been addressed in the Swadźba-Kwaśny group through the development of liquid coordination complexes (LCCs),2 eutectic mixtures formed of an organic donor and a metal halide. The work reported here begins with the development of LCCs containing a breadth of halometallate species, synthesised by combining AlCl3, GaCl3, InCl3, SbCl3, SnCl4, SnCl2, ZnCl2 or TiCl4 with either trioctylphosphine (P888) or trioctylphosphine oxide (P888O), through study of their speciation and quantification of their acidity using Gutmann acceptor numbers (Chapter 2).
The second drawback of chloroaluminate ionic liquids (Lewis acidity placed in the chloroaluminate anion) was addressed by developing borenium ionic liquids, with Lewis superacidic borenium cation. 4 These first generation borenium ionic liquids maintained the chlorometallate anion combined with borenium cation. This work takes the borenium ionic liquids further, delivering metal-free systems with one, well-defined acidity centre in the cations (Chapter 3). These metal-free borenium ionic liquids were subsequently combined with sterically-hindered phosphines, such as tris(tert-butyl)phosphine, and some had the capability for H2 splitting, acting as the first reported ionic liquids frustrated Lewis pairs (ILFLPs). This was achieved by the combination of a catechol ligand, adding steric bulk to the borenium cation, the introduction of long-chain ligands to lower the melting point, and the use of non-coordinating anions to prevent their involvement in the reaction.
Another strand of the FLP work explored the potential for “traditional” ionic liquids as solvents for FLP reactions (Chapter 5). A classic hydrogen activating FLP, tris(pentafluoro)phenyl borane (BCF) and tris(tert-butyl) phosphine (Pt Bu3), was dissolved in a non-coordinating ionic liquid [C10mim][NTf2] and the addition of H2 showed that the FLP in ionic liquid was capable of activating H2. Further to this, the movement of the FLP components in ionic liquid was found to be restricted in comparison to in molecular solvents. This promoted the lifetime of the reactive precursor, the encounter complex. A fundamental study into a weakly-bound BCF/phosphine encounter complexes in both benzene and [C10mim][NTf2] by neutron scattering was carried out and this is described in Chapter 4. 5 The results of the neutron scattering suggested an interatomic P-B distance of ~8 Å which agrees with molecular dynamics simulations in the literature. A pre-requisite for this study was the synthesis of deuterated phosphines via a Grignard synthesis, also described in Chapter 4.
|Date of Award||Jul 2020|
|Sponsors||Engineering & Physical Sciences Research Council|
|Supervisor||John Holbrey (Supervisor) & Gosia Swadzba-Kwasny (Supervisor)|
- main group catalysis
- ionic liquids
- frustrated Lewis pairs