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

Abstract

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, 1a,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]Br2 (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 %Vbur, 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...