Supramolecular assemblies from ditopic ligands and transition metal salts

Abstract

Tetradentate salicylaldimine ligands of the H₂salen-type bearing ortho-N-morpholinomethyl substituents function as ditopic ligands, bonding to Ni^II or Cu^II with the [N₂O₂]²⁻ donor set of the salen unit and a sulfate or two nitrate anions with the protonated morpholine units. The binding of the metal salt by the zwitterionic form of the ligand provides a novel approach to the transport of metal sulfates in metal recovery processes. A comparison of the solid state structures of a “free” ligand with a series of nickel(II) complexes demonstrates that the metal ion templates the ligand system, orientating the pendant morpholinium groups to form electrostatic and bifurcated hydrogen bonds in the sulfate complex to the dianion creating a neutral 1∶1∶1 [LM²⁺X²⁻] complex suitable for extraction into water immiscible solvents. Other binding modes involving bridging of metal complex units by anion binding to the pendant morpholine groups suggest that these ditopic ligands could also be used to assemble unusual three-dimensional arrays of metal complexes in the solid state.