Frustrated Lewis pairs derived from xanthene and related backbones
Keywords: intramolecular FLPs
Frustrated Lewis pairs
Lewis base
small molecules activation
activation of hydrogen gas
main group
Lewis acid

This thesis examines the synthesis and the reactivity of both novel and previously reported intramolecular dimethylxanthene, dimethylthioxanthene and acridan derived frustrated Lewis pairs (FLPs). Moreover, it also reports on novel acridan-based Lewis bases and acids, and the subsequent reactivity of such species within intermolecular FLPs
Chapter 1 describes the background of FLPs including the history of FLPs, and the literature studies on the reactivity of FLPs in small molecules activation/captures and catalysis. Chapter 2 reports all the experimental methods used in this thesis and the preparation of dimethylthioxanthene and acridan backbone precursors. Chapter 3 introduces an improved method for the preparation of intramolecular dimethylxanthene derived FLPs featuring Lewis acid/base pairs of -B(C6F5)2/-PPh2 and -B(C6F5)2/-PiPr2. This synthesis proceeds through isolated lithium complexes and not only gives higher overall yields than the literature method, but also avoids the use of large excesses of Br2, pyrophoric sBuLi, and column chromatography techniques for purification. In addition, this chapter introduces the synthesis of novel intramolecular derived ditert-butyldimethylxanthene and dimethylthioxanthene FLPs with wider P···B separations.
In chapter 4, the preparation of novel species derived from an acridan backbone are discussed. This scaffold was used for the synthesis of a range of disubstituted phosphine (PNP) Lewis bases. The large steric bulk of tert-butyl, iso-propyl and mesityl substituents was shown to prevent clean disubstitution of the phosphine moiety onto the acridan backbone. Phenyl and para-tolyl substituents, by contrast, gave the desired disubstituted acridan derivatives, and these species have been characterised spectroscopically and crystallographically. The reactivity of these Lewis bases with the Lewis acid B(C6F5)3 has been investigated. Neither donor/acceptor coordination nor frustrated Lewis pair behaviour is observed; unexpected C-H activation of the N-Me group of the acridan backbone is proposed based on spectroscopic data, and the structural authentication of a related derivative. This represents the first example of FLP-mediated sp3 C-H bond activation. Additionally, the synthesis of a novel intramolecular acridan-derived FLP featuring -PPh2/-B(C6F5)2 as the Lewis base/acid pair has been successfully achieved. This acridan FLP features a P···B separation (4.451(9) Å), which is slightly wider that the analogous dimethylxanthene FLP. Interestingly, it reacts with H2 at room temperature in a manner utilizing the proximal N-CH3 moiety as the Lewis base instead of the PPh2 group. This result led to related synthesis and reactivity studies of the bis-borane Lewis acid (BNB) featuring two pendant B(C6F5)2 groups, which also reacts with H2 at room temperature (1 atm) in similar fashion utilizing B/N cooperativity. Furthermore, this compound also shows unusual C-N bond activation behaviour accompanying the uptake of CO2 and CO in the presence of tBu3P.
Chapter 5 reports on further investigations into the reactivity of previously reported dimethylxanthene-based phosphine/borane FLPs towards alkynes. The –PPh2 and –PMes2 containing systems (featuring B(C6F5)2 as Lewis acid component) were found to C-H activate the terminal alkyne PhCCH; the weaker -PPh2 donor system was found to give rise to an equilibrium mixture of the free FLP and phosphonium acetylide in CD2Cl2 solution at room temperature. This system was also found to react with the B-H bonds of HBpin and HBcat via C-B/B-H metathesis reactions that lead to the replacement of the -B(C6F5)2 moiety by -Bpin/-Bcat, with accompanying transfer of HB(C6F5)2 to the phosphine Lewis base. This transformation underpins the ability of this system to act as a pre-catalyst for the hydroboration of terminal (PhCCH) and internal alkynes (PhCCPh and PhCCMe) with HBpin. The active species is derived from the HB(C6F5)2 fragment generated in the borane metathesis reaction.
Chapter 6 presents further reactivity studies of dimethylxanthene FLPs featuring - PiPr2 and -PPh2 Lewis bases (with -B(C6F5)2 as the Lewis acid) in the activation of Si-H and Sn-H bonds. It was found that the more strongly Lewis basic -PiPr2 FLP could activate the Si-H bonds in PhSiH3, Ph2SiH2 and SiH4; further reaction leads to Si/B metathesis yielding ‘migrated’ species in a similar fashion to that occurring with HBpin and HBcat. The more weakly Lewis basic -PPh2 FLP however, was found to be unreactive towards Ph2SiH2, Et3SiH and Ph3SiH, while the reaction with PhSiH3 leads to the formation of a similar ‘migrated’ species. The -PiPr2-containing FLP was also found to be reactive towards heavier group 14 hydrides, activating the Sn-H bonds of nBu3SnH and Ph3SnH to form zwitterionic phosphonium stannylium hydrides.