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

Abstract

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 N 3 – 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 – , CH 3 COO – , NO 3 – , and H 2 PO 4 – ) 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 [Cu 2 ( 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 [Cu 2 ( 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 , Δ H , and T Δ S ) associated with the considered complex/anion equilibria. Thermodynamic data showed that adducts formed by [Cu 2 ( 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.

Selective synthesis of an elusive C-functional bis-cyclam and study of its inhibition of the CXCR4 chemokine receptor

“…10,11 Their ditopic analogues, bis-cyclams, have also attracted interest in the last decades. Some were not only used for their coordination properties with transition metals and anions [12][13][14][15][16][17] but also for their spectacular anti-human immunodeficiency virus (anti-HIV) activity, which was first discovered in the 1990s. 18,19 In this case, their potency has been attributed by some of us, among others, to their inhibition properties of the CXCR4 chemokine receptor, [20][21][22] a key cofactor in the HIV virus cell-entry process.…”