Mono- and Bis-Cyclometalated Palladium Complexes: Synthesis, Characterization, and Catalytic Activity

Cyclometalated palladium complexes have found a variety of applications, above all in homogeneous catalysis. Most complexes include mono-cyclometalated ligands, particularly systems with P- or N-donors. Herein, we report the preparation of a series of mono- and bis-cyclometalated palladium complexes with a silyl-substituted thiophosphinoyl ligand. The complexes have been synthesized via oxidative addition and dehydrohalogenation reactions. Thereby, dehydrohalogenation selectively results in the second cyclometalation and not in the formation of a carbene species. In the formed square-planar palladacycles the ligands exhibit S,C- and S,C,C-coordination modes, respectively. Depending on the silyl moiety, cyclometalation occurs via an aryl or even a methyl group, thus also giving way to unusual silapalladacyclobutanes with an open-book geometry. The complexes have been characterized in solution as well as in the solid state. Preliminary catalytic studies show that both the mono- and bis-cyclometalated complexes can be applied as catalysts in C–C coupling reactions.