Oxo-Anion Recognition by Mono- and Bisurea Pendant-Arm Macrocyclic Complexes

The novel macrocyclic copper­(II) complexes [<b>2</b>]²⁺ and [<b>3</b>]²⁺, carrying one or two (nitrophenyl)­urea fragments appended to an azacyclam or diazacyclam framework, exploit the hydrogen-bond-forming abilities of the urea subunits, along with the metal–ligand interaction, in the recognition of anionic species. Equilibrium studies in acetonitrile performed on [<b>2</b>]²⁺ and [<b>3</b>]²⁺ show that (nitrophenyl)­urea pendant arms strongly interact with anionic species such as carboxylates and phosphates, which display both coordinating tendencies toward copper­(II) and good affinity toward urea subunits. Stability constants of the adducts are considerably higher than those determined for the interaction of the same anions with a “plain urea” reference compound, confirming the synergistic action of metallomacrocyclic and urea subunits. Complex [<b>2</b>]²⁺ forms 1:1 adducts with acetate, benzoate, hydrogendiphosphate, and dihydrogen phosphate, while complex [<b>3</b>]²⁺ interacts with the same anions according to both 1:1 and 1:2 stoichiometries, with the exception of hydrogendiphosphate, which forms only the 1:1 adduct with a distinctly high association constant (log <i>K</i> > 7). Spectrophotometric investigations suggest that oxoanionic species interact with the complexes according to a “bridged” mode, inducing the macrocyclic systems to adopt a scorpionate-like conformation, as confirmed by crystallographic studies on the [<b>3</b>]²⁺/succinate adduct