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 Pd-2(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 Pd-2(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.