Examination by F NMR spectroscopy of the admixture of IPr (0.036 M) and 2 (0.018 M) in CD2Cl2 at -80 °C after 1 h (Figure energy of the syn diastereomer of 3g might reflect favorable throughspace non-covalent interactions between adjacent IPr ligand substituents. The predicted F NMR chemical shifts for the monomer (-126.2 ppm), anti-dimer (-127.0 ppm), and syn-dimer (-127.5 ppm) also track closely to the experimental values (-126.0, -127.2, and -127.5 ppm), respectively. Warming the solution of 3g from -80 °C to -20 °C did not demonstrably change the NMR signals, but C-C reductive elimination to form the corresponding 2-arylimidazolium salt (60%) was observed after further warming to room temperature. This observation tracks closely with an established catalyst deactivation pathway of (NHC)Pd catalysts from mechanistic studies of catalytic cross-coupling reactions. 25 Direct observation of this thermal side reaction underscores why mono-NHC OACs have been difficult to synthesize to date, because oxidative addition reactions are generally conducted at or above room temperature leading to competitive decomposition of Pd OAC as soon as it is generated. Ligand exchange, on the other hand, occurs much faster than oxidative addition as exemplified by formation of 3g even at -80 °C.