Peripheral Metal-Ion Binding to Tris(thia−oxo crown) Porphyrazines

• Published in: Inorganic chemistry 2003-02, Vol.42 (3), p.814-820
• Main Authors: Michel, Sarah L. J, Barrett, Anthony G. M, Hoffman, Brian M
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic025639d

We report the preparation and solution properties of metal-free [1b(H 2 )] and cobalt [1c(Co)] porphyrazines (pz's), where three pyrroles are functionalized with a S2O3 crown ether for metal-ion binding and the fourth pyrrole is appended with two long-chain alcohols (bis((11-hydroxyundecyl)thio)) for potential use as surface anchors. Compounds 1b(H 2 ) and 1c(Co) exhibit ion-specific optical changes in the presence of Ag+ and Hg2+. Binding of Ag+ to 1b(H 2 ) is described by a 1:1 binding isotherm, with K D ∼ 147 μM, whereas binding of Hg2+ to 1b(H 2 ) appears more complex. For 1c(Co), binding of Ag+ and Hg2+ also can be fit to a 1:1 isotherm, with K D ∼ 109 and 83 μM, respectively. All four titrations show nonisosbesticity, including those apparently describable by the 1:1 isotherm, which indicates that there are intermediate stages as multiple ions bind. Neither 1b(H 2 ) or 1c(Co) gives optical responses to Ni2+, Zn2+, Pb2+, or Cu2+ or to alkali (Li+, Na+, K+, and Cs+) and alkaline earth (Mg2+, Ca2+, and Ba2+) metal ions. Nonetheless, “hard” ions are sensed electrochemically:  Na+ and Li+ strongly shift the pz/pz- couple of 1b(H 2 ) and CoIII/CoII couple of 1c(Co). For the addition of 4 equiv of Li+ to 1c(Co), the CoIII/CoII shifts +40 mV, and for the addition of 4 equiv of Na+ to 1c(Co), the shift is +155 mV. The shifted redox waves of 1c(Co) all retain their reversibility. In contrast, for the addition of either 4 equiv of Li+ or Na+ to 1b(H 2 ), the shifts of the pz/pz- couple are essentially the same, ∼170 mV, and the shifted redox waves become broadened and less reversible, due to ion-induced aggregation. For 1c(Co) the shape of the titration curves [M]/[1b(H 2 )] versus E 1/2 is concave to the x-axis, implying cooperative binding of multiple M+ ions as part of the redox/binding system of equilibria.