Substitution reactions of some novel sterically hindered monofunctional Pd(II) complexes

Substitution reactions of the monofunctional complexes [(TLtBu)PdCl](+) and [Pd(tpdm)Cl](+), where TLtBu = 2, 6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine and tpdm = terpyridinedimethane, with nucleophiles such as: thiourea, I , Br , NO2 , pyridine and dimethyl-sulfoxide (DMSO) were studied in 0.1 M NaClO4 aqueous solution in the presence of 10 mM NaCl. The reactions were carried out at three different temperatures (288, 298 and 308 K) using a variable-temperature stopped-flow technique. The substitutions were followed under the pseudo-first-order conditions with a large excess of nucleophiles. Obtained results show that the complex with tpdm ligand reacts faster than the complexes with TLtBu ligand due to the bulkiness of the inert tridentate ligands. Also, the presence of t-Bu groups on the terminal imadazole rings of TLtBu ligand significantly slow down the rate of the substitution. The order of reactivity of used ligands is: thiourea > I > Br > NO2 > pyridine > DMSO. This order is in agreement with their electronic and structural characteristics. The negative values reported for the entropy of activation confirmed the associative substitution mode. These results are discussed in order to find the connection between the structure and reactivity of the complexes with tridentate sterically hindered ligands. (C) 2011 Elsevier B. V. All rights reserved.