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

The novel macrocyclic copper­(II)
complexes [2]2+ and [3]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 [2]2+ and [3]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 [2]2+ forms 1:1 adducts with acetate,
benzoate, hydrogendiphosphate, and dihydrogen phosphate, while complex
[3]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 K > 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 [3]2+/succinate adduct.