Coordination Chemistry of SCS PdII Pincer Systems

We have studied the coordination of substituted pyridines, and phosphorus- and sulfur-containing ligands to an SCS PdII pincer system. These ligands coordinate to PdII (trans to the cyclopalladated aryl group) by quantitative substitution of the labile acetonitrile ligand in complex 1. Competition experiments showed that both electronic and steric effects influence the strength of coordination to the PdII pincer of the substituted pyridines. A quantitative analysis of the substituent effect was achieved by a Hammett correlation. Phosphorus-containing ligands also coordinate to this SCS PdII motif, as evidenced by NMR spectroscopy and single-crystal X-ray diffraction studies. They are much stronger ligands than the pyridines. The coordination strength of the thioureas falls in between those of the pyridines and phosphanes/phosphites. Our results lead therefore to the following order of ligand strength towards PdII in SCS PdII pincers: PR3 > P(OR)3 > N,N-disubstituted thiourea > (substituted) pyridines > MeCN.