Synthesis, characterization and structural studies of palladium(II) complexes with N-(aroyl)-N′-(2,4-dimethoxybenzylidene)hydrazines

Reactions of Li-2[PdCl4] with N-(aroyl)-N'-(2,4-dimethoxybenzylidene)hydrazines (H2L = (4-R-C6H4)C(=O)NHN=CH(2,4-(CH3O)(2)C6H3)) in presence of PPh3 have provided cyclopalladated complexes having the general formula [PdL(PPh3)] (1, 2 and 3 where R = OCH3, CH3 and Cl, respectively) and a coordination complex trans-[Pd(HL)(PPh3)(2)Cl] (4 where R = NO2). The complexes have been characterized by elemental analysis, infrared, H-1 NMR and electronic absorption spectroscopy. X-ray structures of all the complexes have been determined. In 1, 2 and 3, the C,N,O-donor dianionic ligand (L2-) forms two fused five-membered chelate rings at the metal centre. On the other hand, the monoanionic ligand (HL-) acts as deprotonated amide N-donor in 4. The strong electron withdrawing effect of the nitro group on the aroyl fragment is possibly responsible for the monodentate amide N-coordinating behavior of HL- in 4. The orientation of the 4-methoxy group in I is different than that in 2 and 3 due to intramolecular C-H...pi interaction in the last two complexes. The structure of 4 shows an apical C-H...Pd interaction involving the azomethine (-CH=N-) group of HL-. In the crystal lattice of all the structures, various types of intermolecular non-covalent interactions are present. The self-assembly of the molecules of 1, 2 and 3 leads to two-dimensional networks. The same network observed for 2 and 3 reflects the interchangeability of the chloro and methyl groups due to their similar volumes. In the case of 4, the complex and the water molecules present in the crystal lattice form parallel homo-chiral helices and finally interhelical interactions lead to a three-dimensional network. (c) 2006 Elsevier B.V. All rights reserved.