Intermolecular interactions in dictating the self-assembly of halogen derivatives of bis-(N-substituted oxamato)palladate(ii) complexes

Abstract

Three palladium(II) complexes of formula (n-Bu₄N)₂[Pd(4-Fpma)₂] (1), (n-Bu₄N)₂[Pd(4-Clpma)₂]·4H₂O (2) and (n-Bu₄N)₂[Pd(4-Brpma)₂]·4H₂O (3) [n-Bu₄N⁺ = tetra-n-butylammonium cation, 4-Fpma = N-4-fluorophenyloxamate, 4-Clpma = N-4-chlorophenyloxamate and 4-Brpma = N-4-bromophenyloxamate] have been prepared and their structures determined by single crystal X-ray diffraction. Each palladium(II) ion in 1–3 is four-coordinate with two oxygen and two nitrogen atoms from two fully deprotonated oxamate ligands building PdO₂N₂ square planar surroundings, the oxamate ligands exhibiting trans (1 and 2) and cis (3) dispositions. The fluoro substituent and the organic counterion in 1 are involved in C–H⋯F type interactions whereas the anionic complex assembly is driven by C–H⋯O_oxamate interactions. The structures of 2 and 3 shed light on the effects of the different chemical nature of chloro and bromo substituents on the C–H⋯X interactions joined to hydrogen bonds involving oxamate oxygen atoms and crystallization water molecules. The complex anions in 2 are interlinked by water chains to afford a supramolecular two-dimensional network where O_w⋯O_w, O_w⋯O_oxamate and O_w⋯Cl hydrogen bonds are involved. The reduction of the polarity of C–Br bonds with respect to C–X (X = F, Cl) is very likely to be one of the key reasons allowing for the cis-arrangement of the two 4-Brpma ligands around each palladium(II) ion of the anionic complex in 3 in water. These units are interconnected by hydrogen bonds between the peripheral oxamato-oxygens and a four-membered ring of water molecules leading to supramolecular cationic chains extending along the crystallographic a axis.