Co2+ Translocation in a Terpyridine−Cyclam Ditopic Receptor

Abstract

The coordination behaviour of the ditopic receptor of 1‐[p‐(2,2′:6′,2′′‐terpyrid‐4′‐yl)tolyl]‐1,4,8,11‐tetraazacyclotetradecane (L1) towards Co2+ in acetonitrile/water (70:30, v/v) has been investigated. At acidic pH values, the L1−H+−Co2+ system shows an oxidation wave at 150 mV vs. SCE, characteristic of Co2+ in a bis(terpyridine) environment. Upon addition of OH−, a reduction in the intensity of the wave at 150 mV is observed and a new oxidation wave appears at 1.30 V. This new wave is close to that found for [Co(cyclam)]2+ under similar working conditions. The electrochemical data thus suggest that there is a pH‐controlled translocation of the Co2+ ion from the bis(terpyridine) to the cyclam environment. A similar inference can be made from the results of UV/Vis studies. A thermodynamic characterization of the L1−H+−Co2+ system has also been carried out. Stability constants have been determined in acetonitrile/water (70:30, v/v, containing 0.1 mol dm−3 nBu4NClO4 at 25 °C) using potentiometric techniques. The following species were found: [Co(L1)2H4]6+, [Co(L1)2H3]5+, [Co(L1)H]3+, [Co(L1)]2+, and [Co(L1)(OH)]+. The 2:1 ligand‐to‐metal species [Co(L1)2H4]6+ and [Co(L1)2H3]5+ are assigned to complexes where Co2+ is coordinated by two terpyridine units. The [Co(L1)H]3+, [Co(L1)]2+, and [Co(L1)(OH)]+ species are assigned to complexes where the Co2+ cation is in the cyclam binding domain. The pH‐induced Co2+ jumping between coordination sites is discussed in terms of the different basicities of cyclam and terpyridine. Co2+ jumping between coordination sites has also been studied by the sequential acidification of basic mixtures of L1 and Co2+. Under these conditions, the translocation was observed at a different pH value than when an acid‐to‐basic path was followed. The effect of anions such as phosphate and chloride on the translocation process has also been studied. In the presence of phosphate, translocation occurs at pH = 11 rather than at pH ≈︁ 8. The phosphate−L1 and phosphate−L1−Co2+ systems have been characterized by potentiometry and a remarkably good agreement has been found between the electrochemical and potentiometric data.

Artificial Receptors for the Recognition of Phosphorylated Molecules

“…In preliminary studies, receptor 736 was found to serve as a selective fluorescent sensor for ATP in 7:3 acetonitrile:water at slightly acidic pH values (4-6). 746 No changes in the emission spectrum were observed upon the addition of chloride, sulfate, or inorganic phosphate. Further studies examined the dimetallic complex produced upon coordination of Cu 2+ to the cyclam unit of ligand 736 .…”

“…Further studies examined the dimetallic complex produced upon coordination of Cu 2+ to the cyclam unit of ligand 736 . 747 For example, in acetonitrile:water (7:3), the stability constant between [ 736:Cu )] 4+ and ATP was determined to be log K = 6.10 through potentiometric titrations. CV studies supported the conclusion that non-coordinating anions (ATP, ADP, GMP, sulfate, and inorganic phosphate) interacted with the 736:Cu receptor through electrostatic (Coulombic) interactions rather than metal-anion coordination.…”

Reversible Covalent Bond Toolbox

“…Such interactions have also been used by scientists in the development of binders for anions with impressive results. [54][55][56][57][58][59][60][61][62][63][64] These sensors are composed of a fluorescent or nonfluorescent multivalent ligand, which coordinates with one or two metal cations, such as Zn 2+ , Cu 2+ , and Cd 2+ , to form a ligand-metal complex. Besides, fluorescent lanthanides, such as Tb and Eu have also been used in some sensors, especially those made by the Gunnlaugsson group.…”

Covalent Interactions in Chemosensor Design

“…One area of application of covalent interactions involving metals is organic anion detection because a large number of biomolecules such as peptides, nucleotides, phospholipids, and carbohydrates are anionic compounds [162,163]. There have been some impressive chemosensors [162][163][164][165][166][167][168][169][170][171][172][173]. One interesting strategy is the design of sensors based on dimetallic architectures that utilize a bridging mechanism.…”

“…These receptors bind with histamine in water with high selectivity and binding constants, with the highest being 1.6 × 10 5 /M in pH 8 buffer. Finally, Manez and coworkers developed a sensor (52) using a multicomponent dinuclear system containing a Ru(Tpy) 2+ 2 fluorescent core and a cyclam-Cu 2+ complex [168]. The fluorescence of the Ru(Tpy) 2 2+ core in an aqueous environment (acetonitrile/water, 70 : 30) is quenched by the Cu 2+ cation.…”