Investigation of the Catalytic Activity of a 2‐Phenylidenepyridine Palladium(II) Complex Bearing 4,5‐Dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene in the Mizoroki‐Heck Reaction

Abstract

Abstract The phenylidenepyridine ( ppy ) palladacycles [PdCl( ppy )(IMes)] ( 4 ) [IMes = 1,3‐bis(mesityl)imidazol‐2‐ylidene] and [PdCl( ppy ){(CN) 2 IMes}] ( 6 ) [(CN) 2 IMes = 4,5‐dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene] were prepared by facile two step syntheses, starting with the reaction of palladium(II) chloride with 2‐phenylpyridine followed by subsequent addition of the NHC ligand to the precatalyst precursor [PdCl( ppy )] 2 . Suitable crystals for the X‐ray analysis of the complexes 4 and 6 were obtained. It was shown that 6 has a shorter NHC‐palladium bond than the IMes complex 4 . The difference of the palladium carbene bond lengths based on the higher π‐acceptor strength of (CN) 2 IMes in comparison to IMes. Thus, (CN) 2 IMes should stabilize the catalytically active central palladium atom better than IMes. As a measure for the π‐acceptor strength of (CN) 2 IMes compared to IMes, the selone (CN) 2 IMes · Se ( 7 ) was prepared and characterized by 77 Se‐NMR spectroscopy. The π‐acceptor strength of 7 was illuminated by the shift of its 77 Se‐NMR signal. The 77 Se‐NMR signal of 7 was shifted to much higher frequencies than the 77 Se‐NMR signal of IMes · Se. Catalytic experiments using the Mizoroki‐Heck reaction of aryl chlorides with n ‐butyl acrylate showed that 6 is the superior performer in comparison to 4 . Using complex 6 , an extensive substrate screening of 26 different aryl bromides with n ‐butyl acrylate was performed. Complex 6 is a suitable precatalyst for para ‐substituted aryl bromides. The catalytically active species was identified by mercury poisoning experiments to be palladium nanoparticles.