Syntheses and catalytic activities of pseudo-pincer and CSC pincer-type Pd(II) complexes derived from benzannulated N-heterocyclic carbenes

Three new dibenzimidazolium salts bearing thioether (B.2HBr, B.2HNO3) and sulfoxide (C.2HBr) containing bridges have been synthesized as sulfur-functionalized dicarbene precursors. Palladation of B.2HBr and C.2HBr afforded two new pseudo-pincer complexes cis-[PdBr2(B-kappa2C)] (1) and cis-[PdBr2(C-kappa2C)] (2), in which the sulfur-donor remains pendant. Reaction of precursor B.2HNO3 in the presence of 1 equiv of KBr, on the other hand, yields the first CSC-Pd(II) pincer complex [PdBr(B-kappa3CSC)]NO3 (3) bearing two carbene moieties. All three complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Their catalytic activities in the Mizoroki-Heck reaction have been evaluated as well.