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

Abstract

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-κ²C)] (1) and cis-[PdBr₂(C-κ²C)] (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-κ³CSC)]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.