Palladium(II) Pyrazolin-4-ylidenes: Substituent Effects on the Formation and Catalytic Activity of Pyrazole-Based Remote NHC Complexes

Three 4-iodopyrazolium salts with 3,5-dimethyl (3a), 3,5-diphenyl (3b), and 3,5-diisopropyl (3c) substituents, respectively, were synthesized using a modular approach. The oxidative addition of 3a−c to Pd2(dba)3/PPh3 afforded products (trans-4a, cis-/trans-4b and 4c) of different geometries or connectivities, indicating a dramatic substituent effect on the formation of pyrazolin-4-ylidene complexes. In addition, the reactions of 3a−c with Pd2(dba)3 in the presence of pyridine yielded new mixed pyrazolin-4-ylidene/pyridine complexes (5a−c). All complexes have been fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry, and X-ray diffraction analyses. Furthermore, an initial catalytic study on Suzuki−Miyaura and Mizoroki−Heck cross-coupling reactions also reveals a significant substituent effect on catalytic activities.