Palladium(ii) complexes based on 1,8-naphthyridine functionalized N-heterocyclic carbenes (NHC) and their catalytic activity

Abstract

Palladium complexes containing 2,7-bis(mesitylimidazolylidenyl)naphthyridine (NHC-NP) have been synthesized and characterized. Reaction of [{Ag₃(NHC-NP)₂}(PF₆)₃] with [Pd(PhCN)₂Cl₂] provided an unusual dipalladium complex bridged by two NHC-NP units, forming a 20-membered dinuclear metallacycle [{Pd₂(NHC-NP)₂Cl₂}(PF₆)] (2) in high yield. Treatment of 2 with KI in acetone yielded a neutral species [Pd₂(NHC-NP)I₄] (3). Meanwhile, the pyridinylN-heterocyclic carbene (NHC-Py) precursor, 1-(2-pyridinyl)-3-mesitylimidazolium chloride, reacted with Pd₂(dba)₃ directly to form the mononuclear palladium complex [Pd(NHC-Py)Cl₂] (4). These complexes were characterized by elemental analyses as well as NMR spectroscopy, and the structures of 3 and 4 were further identified by X-ray diffraction analysis. The use of these palladium complexes for Suzuki–Miyaura and Kumada–Corriu coupling reactions has been examined. There is no significant difference in catalytic activities between 2 and 4 in Suzuki–Miyaura coupling reactions. However, the catalytic activity of 2 in the Kumada–Corriu coupling of ArBr with cyclohexylmagnesium bromide is quite different from that of 4. Thus complex 2 is active for the cross coupling, but complex 4 is active for the reduction of aryl halides.