Effect of carboxylate spacers on the supramolecular self-assembly of dicopper(II) Schiff base complexes stabilizing water assemblies of different conformations

Abstract

Dicopper(II) complexes, namely [Cu₂L(O₂C-CHCH-C₆H₄-p-OH)]1·2H₂O ( 1·2H₂O), [Cu₂L(O₂C-CH₂-C₆H₄-p-OH)]2·2H₂O ( 2·2H₂O) and [Cu₂L(O₂C-CH₂CH₂-C₆H₄-p-OH)]3·0.5H₂O ( 3·0.5H₂O), having different carboxylate ligands with a p-hydroxyphenyl moiety and the pentadentate Schiff base N,N'-1,3-diylbis(salicylaldimino)propan-2-ol (H₃L) in its trianionic form, were prepared and structurally characterized by X-ray crystallography. The complexes have a dicopper(II) unit with an alkoxo bridge from the Schiff base and the carboxylate, showing a three-atom bridging mode. The metal centres in a square planar CuNO₃ coordination geometry are antiferromagnetically coupled in the asymmetrically double-bridged dicopper(II) core. A significant effect of the -CH=CH-, -CH₂- and -CH₂CH₂- spacers of the carboxylate ligands on the formation of different supramolecular structures is observed. Complex [Cu₂L(O₂C-CH=CH-C₆H₄-p-OH)], 1, forms a helical supramolecular structure due to hydrogen-bonding interactions involving the p-hydroxy group of the phenol from the carboxylate and one phenoxo oxygen atom from the Schiff base. The lattice waters form a helical one-dimensional chain, in which alternate water molecules are anchored to the supramolecular host and the chain propagates along the crystallographic 2, screw axis. Complex 2 forms water aggregates of quasi-linear and pseudo-hexameric cyclic chair conformations involving lattice water molecules, and the previously mentioned para OH group phenoxo oxygen atom. Complex 3·0.5H₂O shows the formation of a supramolecular one-dimensional chain structure due to hydrogen-bonding interactions between the p-OH group and the phenoxo oxygen atom. Two such supramolecular structures are linked by hydrogen-bonding interactions involving the lattice water. Differential scanning calorimetry (DSC) of 1·2H₂O gives two endotherms at 61.5 and 88.5 °C for the loss of the free and the anchored water molecules, respectively. The overall change of enthalpy per water molecule is ~36 kJ mol^-1. Complex 2·2H₂O shows an endotherm at 131 °C with a shoulder at ~126 °C. The enthalpy change per water molecule is ~26 kJ mol^-1. The reversibility in loss or addition of lattice water molecules and the corresponding effect on the overall structure is probed by X-ray powder diffraction studies.