Stereoelectronic Flexibility of Ammonium-Functionalized Triazole-Derived Carbenes: Palladation and Catalytic Activities in Water

The postmodification approach allows convenient access to charge tagged ammonium-functionalized bis(1,2,4-triazolin-5-ylidene)palladium(II) complexes even when the respective azolium salts are elusive. Bromo-functionalized 1,2,4-triazolium salts were first metalated to form the respective bromo-functionalized bis(NHC) complexes trans-[PdBr2(R-tazy-Br)(2)] (R = Ph, Cy, la,b). Subsequent postcoordinative nucleophilic substitutions converted the bromo into ammonium functions, leading to the water-soluble, charge-tagged complexes trans-{PdBr2(R-tazy-NEt3)(2)]Br-2 (R = Ph, Cy, 2a,b). The catalytic activities of 2a,b in the aqueous Suzuki-Miyaura reaction were compared to those of their analogues 2c,d bearing more bulky mesityl and diisopropylphenyl substituents, and a detailed stereoelectronic profiling of the NHCs using %V-bur,HEP, and DFT calculations was conducted to rationalize their catalytic differences. Although all complexes are active, the more donating and less bulky complexes 2a,b performed significantly better than 2c,d at a very low catalyst loading of 0.001 mol %. 2a was found to be highly active for various aryl and heteroaryl bromides and some aryl chlorides. More importantly, this study discloses that NHCs with N-phenyl and N-cyclohexyl groups exhibit stereoelectronic flexibility, which could be the cause for the greater activities of their complexes.