A theoretical study on Pd^II complexes containing hemilabile pyrazole-derived ligands

The properties of PdII complexes containing hemilabile pyrazole-derived ligands (L) of the form BPz-(CH2) x-A(CH2)yA(CH2)x-PzB, with A being a donor atom and B a substituent group at the pyrazole ring, have been investigated through quantum-chemical calculations. The geometries of the [PdLCl2] and [PdL]2+ complexes have been optimized and the reaction free energy of [PdLCl2] → [PdL]2+ + 2 Cl- computed for 32 different ligands, using the hybrid B3LYP density functional method. The formation of the tetra-coordinate [PdL]2+ complexes is more favorable for the longer (eight- and nine-membered) chains than for the shorter (six- and seven-membered) ones. Moreover, results show the nature of the donor atom A influences significantly the formation of [PdL] 2+, the process becoming more favorable according to the order PH > NH > S > O. Finally, electron-donor substituents at the pyrazole ring enhance the formation of [PdL]2+, whereas electron-acceptor groups hinder the process. © Wiley-VCH Verlag GmbH & Co. KGaA, 2006.