Palladium(0) complexes of N-heterocyclic carbene ligands with dangling NMeC[dbnd]O functionalities: Synthesis, reactivity and application in Mizoroki–Heck reactions

Abstract

A series of new Pd(0) complexes of N-heterocyclic carbene (NHC) ligands with dangling NMe(CO) functionalities (LMe) is reported, with yields in the range 56–73%. The reactivity of one of the Pd(0) complexes, bearing bulky N-mesityl groups, towards the oxidative addition of pentafluoroiodobenzene was studied, revealing the formation of decafluorobiphenyl, a diiodo Pd(II) NHC complex and an intriguing Pd(II) complex with four carbon-donor ligands coordinated to the metal center. The latter complex contains two new NHC ligands derived from LMe. One of the NHC ligands is a bidentate ligand, resulting from an addition reaction between LMe and the coordinated maleic anhydride. The other NHC ligand is derived from the N-protonation of LMe. A pentafluorophenyl ligand completes the fourth coordination site. A mechanism for the formation of this intriguing Pd(II) complex is proposed. The structures of these two new Pd(II) complexes were established by single-crystal X-ray diffraction studies. The catalytic activities of the new Pd(0) complex with LMe in the Mizoroki–Heck coupling reaction between 4-chloroacetophenone and styrene were compared and they contrast with those reported for the Pd(0) NHC complex with dangling NH(CO) groups (LH).