The reactivity of hydrogen an carbon dioxide mediated by main group compounds

The focus of this thesis has been the design and synthesis of new frustrated Lewis pair (FLP) systems which from structural modifications retain their ability to activate H2/CO2, while displaying differing reactivity modes. Chapter Two describes the first practical synthesis of tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF18). Gutmann-Beckett Lewis acidity measurements reveal that this borane is a more powerful Lewis acid than B(C6F5)3, but it nevertheless is found to bind H2O much more reversibly than B(C6F5)3. The BArF18/2,2,6,6-tetramethylpiperidine (TMP) FLP provides a rare example of H2 activation in Et2O solvent, in which the borohydride salt has been structurally characterised by X-ray crystallography. A novel bridging borohydride [mu-H(BArF18)2]– was revealed, which contrasts to the characteristic terminal borohydrides formed by other borane based mediated FLP systems. Chapter Three details the design of fluorinated trisalkylboranes including B[CH(C6F5)2]3 which has been synthesised for the first time. This borane has been structurally characterised using X-ray crystallography and displays hydrogen bonding interactions between the ortho fluorines on each aryl ring and the adjacent CH proton. Interestingly, and despite this borane showing no Lewis acidity using Gutmann-Beckett and Childs techniques, the B[CH(C6F5)2]3/TMP FLP provides a rare example of H2 activation in THF solvent. Chapter Four details the synthesis of two classical trialkylsilylium-phosphane adducts [R3Si–PtBu3]+[B(C6F5)4]– [R = Et; R = iPr] derived from the sterically unencumbered silylium ions R3Si+ (R = Et, iPr). Both adducts are not found to dissociate at elevated temperature and are appreciably stable towards decomposition. Moreover, adduct formation does not impede archetypal FLP reactivity; admittance of H2 led to heterolysis at elevated temperatures (90-100 ºC), while CO2 activation occurs under ambient conditions. The latent stability of the CO2 adducts has allowed for their crystallographic characterisation. Subsequently, the activation parameters for CO2 uptake were investigated and support computational calculations.Open Acces