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

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Dicopper(II) complexes, namely [Cu2L(O2C–CHCH–C6H4-p-OH)]·2H2O (1·2H2O), [Cu2L(O2C–CH2–C6H4-p-OH)]·2H2O (2·2H2O) and [Cu2L(O2C–CH2CH2–C6H4-p-OH)]·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 –CHCH–, –CH2– and –CH2CH2– spacers of the carboxylate ligands on the formation of different supramolecular structures is observed. Complex [Cu2L(O2C–CHCH–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 21 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.