Manas K. Saha

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

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.

Helical supramolecular host with aquapores anchoring alternate molecules of helical water chains

Unprecedented 1D helical chains of hydrogen bonded water molecules, showing both handedness and anchored onto a helical supramolecular host formed from the self assembly of a dicopper(II) complex (1) containing pentadentate Schiff base (L) and p-hydroxycinnamate in 1.2H(2)O, propagate along the crystallographic 2(1)-screw axis and the compound shows two endotherms due to loss of water molecules at 61.5 and 88.5 degrees C in the differential scanning calorimetry giving an overall change of enthalpy value of approximately 36 kJ mol(-1) per water molecule.