Cis, Trans, or Both: Steric Bulk Determines Coordination Mode of Dimeric Palladium Complexes with Bridging Phosphine-Pyridyl Ligands.

The coordination mode in a metal complex is critically dependent on the ligands surrounding the metal, on the precursors used, and on the conditions during synthesis. Our goal was to obtain palladium compounds with pyridine-phosphane ligands for catalysis. Therefore, we synthesized ligands 1a-d, which differ in the bulky aryl substituent at the pyridyl moiety. The palladium complexes 6a-d of these ligands are insoluble and have been characterized by various techniques, including solid-state NMR and (for 6a, b, and d) single-crystal X-ray diffraction, showing that bimetallic complexes are formed in which two ligands span two palladium centers. The configuration around these centers is determined by the steric bulk of the ligand. In complexes 6c,d, with the ligands bearing the largest steric groups, both centers have a trans configuration of the methyl and chloride anions. With intermediately sized pyridyl substituents, complex 6b is formed, having one cis surrounded and one trans-surrounded palladium center in the molecule. This kind of complexation has not been observed before. With the least bulky ligand, complex 6a is formed. Depending on the synthetic conditions employed, the methyl and the chloride in this complex can be in a cis configuration at both palladium atoms, or show the unique cis-trans coordination. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)