This thesis reports the synthesis of some trifluoromethyl-containing N-heterocyclic carbene salts (NHCs) and some of their spectroscopy, properties, structures and chemistry. A review of the existing literature demonstrates that although there is an extensive number of NHCs, the number of fluorine containing systems is very much smaller. Recently, several studies have shown that NHC ligands featuring electron-withdrawing group such as fluorine lead to modified ligand properties and improved behaviours for some of their metal complexes when used as catalysts. However, there are very few examples of NHCs containing trifluoromethyl-substituted aromatic systems. A series of methane diamine compounds of the type Ar-NHCH2NH-Ar have been prepared and characterised where Ar = 2-(CF3) C6H4, 3-(CF3)C6H4, 4-(CF3)C6H4 and 3,5-(CF3)2C6H3. The single crystal X-ray structures when Ar = 3-(CF3)C6H4 and 4-(CF3)C6H4 are reported, as is the cyclised product 1,3,5-tris(3,5-bis(trifluoromethyl)phenyl)-1,3,5-triazinane, formed when Ar = 3,5-(CF3)2C6H3. By reaction of the methane diamine compounds with glyoxal in the presence of HBF4 the NHC salts were formed in good yields and three of these salts have been structurally characterised: Ar = 2-(CF3)C6H4, 3-(CF3)C6H4 and 4-(CF3)C6H4. The electronic properties of these systems were investigated based on the value of the 1J(CH) coupling constants. By direct reaction of the NHC salts with [Rh(cod)Cl]2 complexes in the presence of base of the type [Rh(cod)Cl(NHC)] were formed for all of the NHC precursors. By comparing the 1JC-Rh coupling constants obtained for the new rhodium complexes with data published for analogues containing mono and di- substituted fluorinated aromatic rings, it is found that all the new complexes have larger C-Rh coupling constants than the fluorinated analogues. Previous work has correlated this parameter with increased catalytic activity of the rhodium complexes in transfer hydrogenation reactions.
|Date of Award||31 Dec 2021|
- The University of Manchester
|Supervisor||Alan Brisdon (Supervisor) & David Mills (Supervisor)|
- Fluorinated NHCs
- Trifluoromethyl imidazolium tetrafluoroborate