Air- and Moisture-Stable Cyclopalladated Complexes as Efficient Catalysts for Suzuki-Miyaura Coupling Reaction

A series of cylcopalladated complexes containing a six-membered chelating ring with the general formula [Pd(Cl)( k 2 N,C -CH 2 C 6 H 2 (Me) 2 CH NAr)] 2 [where Ar = 2,6-(Me) 2 C 6 H 3 − ( 3a ); 2,6-( i Pr) 2 C 6 H 3 − ( 3b )] and the related phosphine-substituted species [PdCl( k 2 N,C -CH 2 C 6 H 2 (Me) 2 CH NAr)(PR 3 )] [Ar = 2,6-(Me) 2 C 6 H 3 −, R = Cy ( 5a ); Ar = 2,6-( i Pr) 2 C 6 H 3, R = Cy ( 5b ); Ar = 2,6-(Me) 2 C 6 H 3 −, R = Ph ( 5c ); Ar = 2,6-( i Pr) 2 C 6 H 3 −, R = Ph ( 5d )] have been synthesized. In addition, an ortho -metalated complex [PdCl( k 2 N,C -C 6 H 4 CH N(2,6- i Pr 2 C 6 H 3 ))(PCy 3 )] ( 7 ) was prepared by a similar manner. Crystal structures of 3a, 5b, c, and 7 have been determined. The use of these palladium complexes as catalysts for Suzuki−Miyaura coupling reaction of aryl halides with arylboronic acids in ethanol solution was examined. It is found that this series of palladacycles are considerably active under aerobic conditions. Typically, the best activity (TON ≈ 10 6 ) is seen with 3a, b in the coupling reaction of aryl bromide with phenylboronic acid. However, a TEM study showed that the palladium nanoparticles were formed under the reaction conditions, which might be the active species for the catalysis.