Bimacrocyclic Effect in Anion Recognition by a Copper(II) Bicyclam Complex

The dicopper­(II) complex of the bimacrocyclic
ligand α,α′-bis­(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene, 2, interacts with selected anions
in dimethyl sulfoxide solution according to two different modes: (i)
halides (Cl–, Br–, and I–) and N3– coordinate the two metal centers
at the same time between the two macrocyclic subunits that face each
other and (ii) anionic species that do not fit the bridging coordination
mode (e.g., NCO–, SCN–, CH3COO–, NO3–,
and H2PO4–) interact with
copper­(II) ions only at the “external” positions or
their interaction is too weak to be detected. Occurrence of the bridging
interaction is demonstrated by X-ray crystallographic studies performed
on the adduct formed by [Cu2(2)]4+ with azide and by electron paramagnetic resonance investigation,
as the anion coordination between the two copper­(II) centers induces
spin–spin coupling. Isothermal titration calorimetry experiments
performed on [Cu2(2)]4+ and, for
comparison, on [(5,7-dimethyl-6-benzyl-1,4,8,11-tetraazacyclotetradecane)­copper­(II)],
representing the mononuclear analogue, allowed determination of thermodynamic
parameters (log K, ΔH, and TΔS) associated with the considered
complex/anion equilibria. Thermodynamic data showed that adducts formed
by [Cu2(2)]4+ with halides and
azide benefit from an extra stability that can be explained on the
basis of the anion advantage of simultaneously binding the two metal
centers, i.e., in terms of the bimacrocyclic effect.