349531-90-4Relevant academic research and scientific papers
Co2+ translocation in a terpyridine-cyclam ditopic receptor
Padilla-Tosta, Miguel E.,Lloris, Jose M.,Martinez-Manez, Ramon,Pardo, Teresa,Sancenon, Felix,Soto, Juan,Marcos, M. Dolores
, p. 1227 - 1234 (2001)
The coordination behaviour of the difopic receptor of 1-[p-(2,2′:6′,2″-terpyrid-4′-yl) tolyl]-1,4,8,11-tetraazacyclo-tetradecane (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 betwee 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.
