Hetero-Tetranuclear Cu2+/Ca2+/Ca2+/Cu2+ Architectures Based On Malten Ligand: Scaffold for Anion Binding

The hetero-tetranuclear Cu2+/Ca2+/Ca2+/Cu2+ complex obtained with the N,N’-bis((3-hydroxy-4-pyron-2-yl)methyl)-N,N’-dimethylethylendiamine (Malten) ligand has been studied in solid and solution states as scaffold to bind anions. Three crystal structures showing the same metal ions sequence have been examined; they display a tetracharged complex cation neutralized by four monocharged anions. The anions play two different roles: as coordinated (two ClO4−, Cl− or NO3−) or ancillary (two ClO4−) guests. The tetranuclear scaffold hosts two anions also in aqueous and ethanol solutions. Spectrophotometric studies in ethanol allowed to determine the addition constant values for Cl− and Br− (Log K1-2=4.43(4), 4.39(3) for Cl−, 3.80(3), 3.54(2) for Br−) while the others, although bound, showed lower affinity for the scaffold. Both the crystals and the solutions change their color depending on the added anion, namely pink, dark green or blue in the presence of ClO4−, Cl− or NO3−, respectively, thus the presence of the different anions is visible to the naked eye. The hetero-tetranuclear Cu2+/Ca2+/Ca2+/Cu2+ complex is a versatile architecture to be used as scaffold for anion binding.