Coordination chemistry of SCS Pd-II pincer systems

We have studied the coordination of substituted pyridines, and phosphorus- and sulfur-containing ligands to an SCS Pd-II pincer system. These ligands coordinate to Pd-II (trans to the cyclopalladated aryl group) by quantitati ve substitution of the labile acetonitrile ligand in complex 1. Competition experiments showed that both electronic and steric effects influence the s trength of coordination to the Pd-II 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 Pd-I I motif, as evidenced by NMR spectroscopy and single-crystal X-ray diffract ion studies. They are much stronger Ligands than the pyridines. The coordin ation strength of the thioureas falls in between those of the pyridines and phosphanes/phosphites. Our results lead therefore to the following order o f ligand strength towards Pd-II in SCS Pd-II pincers: PR3 > P(OR)(3) > N,N' -disubstituted thiourea > (substituted) pyridines > MeCN.