Synthesis of N-heterocyclic carbene palladium(II) bis-phosphine complexes and their decomposition in the presence of aryl halides

Abstract

Methylpalladium(II) carbene complexes of the type [Pd(NHC)Me(P–P)]BF4 (NHC = N-heterocyclic carbene, P–P = chelating phosphine) have been synthesised, the complex [Pd(tmiy)Me(dcype)]BF4 (tmiy = 1,3,4,5-tetramethylimidazol-2-ylidene, dcype = 1,2-bis(dicyclohexylphosphino)ethane) being characterised crystallographically. Complexes bearing the tmiy ligands were shown to decompose in an analogous manner to complexes bearing monodentate phosphine ligands, with the rate of decomposition being nominally linked to the size of the chelate ring. The decomposition of these complexes in the presence of aryl halides—expected to yield Pd(Ar)X(P–P)—was studied and shown instead to yield PdX2(P–P) and [Pd(tmiy)X(P–P)]BF4. Additionally, Pd(Me)X(P–P) and Pd(Ar)X(P–P) were observed in some cases. Intermolecular cross-over reactions between the starting complex and Pd(Ar)X(P–P) were found to be the source of these unexpected products.