Synthesis of methyl palladium complexes on silica as single site catalysts activating C[sbnd]Cl bonds in heck reactions

Abstract

Supported catalysts with molecularly dispersed, uniform palladium species were synthesized by the method of grafting. New specific synthesis protocols were developed in order to exclude self-reduction and agglomeration of palladium during immobilization. The successful synthesis of isolated mononuclear palladium(II) surface complexes of uniform structure was confirmed by MAS NMR, infrared spectroscopy and elemental analysis. The catalytic potential of these single-site catalysts for the Heck reaction was directly compared to well-characterized supported palladium oxide materials known and reported for their extraordinarily high activity in the conversion of aryl bromides. In contrast to those, the immobilized palladium surface complexes were able to quantitatively convert demanding non-activated aryl chlorides as substrates with conversions up to 62% and high selectivity. Although believed to be only pre-catalysts, the ligand sphere of the surface complexes directly affects their activity. The time-resolved reaction of chlorobenzene with styrene revealed that catalysts with supported amine complexes are able to constantly convert chlorobenzene during several hours of reaction time. It is proposed that the catalysts continuously provide active, dissolved Pd(0) species during the whole course of reaction.