Benzimidazolium Hydrogen Carbonate Salts–Investigation of Thermal Properties in the Context of Small Molecule Inhibitors

The thermal properties of N-heterocyclic carbenes (NHCs) are of interest as these species are attracting attention for use as small molecule inhibitors (SMIs) in area-selective atomic layer deposition (AS-ALD). Free carbenes are known to be generated from the thermolysis of their bicarbonate salts along with one stoichiometric equivalent of water and carbon dioxide byproduct. The liberated NHC volatilizes into the gas phase during heating and is delivered to a surface. Thermal analysis of 19 different NHC derivatives demonstrates the influence of the steric bulk of substituents at nitrogen on the activation temperature and volatility of NHCs. Increasing molecular weight leads to decreased volatility, while increasing the steric bulk of the N-alkyl substituents affects precursor activation temperatures. Density functional theory (DFT) calculations support a stepwise thermolysis mechanism and the correlation between steric bulk and activation barriers for CO2 and H2O elimination. The thermal window measured for these NHC derivatives makes them readily amenable to vapor deposition processes. The data underscore that NHCs are versatile and stable species with significant potential applications in surface modification and as next-generation SMIs.