A Highly Selective Compound for Lead:  Complexation Studies of Decamethylcucurbit[5]uril with Metal Ions

Abstract

Calorimetric and potentiometric studies indicate that a cage-type compound, decamethylcucurbit-[5]uril (MC5), shows very high selectivity for Pb 2+ over alkali, alkaline-earth, NH 4 + , and Cd 2+ cations. The selectivity factor is >10 5.5 . Thermodynamic quantities (log K, ∆H, and ∆S values) for the interactions of MC5 with metal ions were determined at 25 °C by calorimetric titration in 50% formic acid/water (v/v) and by potentiometric titration in an aqueous solution. The log K(H 2 O) value for protonation of MC5 was determined to be 9.56. Pb 2+ forms both 1:1 and 2:1 (Pb 2+ /MC5) complexes in 50% formic acid/water and aqueous solutions, while all other cations studied form only 1:1 complexes with MC5 in the 50% formic acid/water solution. However, both 1:1 and 2:1 complexes have been detected in an aqueous solution for Sr 2+ , indicating that the stoichiometry of MC5 complexes in 50% formic acid and in aqueous solutions could be different.

The Cucurbit[n]uril Family

“…[6] Given that CB [6] binds H + at the ureido carbonyl groups of the portals, it is perhaps unsurprising that CB [6] also binds alkali-metal, alkaline-earth, transition-metal, and lanthanide cations in homogenous solution (Scheme 2, blue equilibria). [4,63,66,67,[76][77][78][79] Table 3 shows the binding constants determined by Buschmann et al for CB [6] with a variety of monovalent and divalent cations. The selectivity between the different cations is less than tenfold.…”

“…Consequently, much of the supramolecular chemistry of CB [5] has been limited to the binding of protons as well as metal and ammonium ions at their portals. [67,68,76,77,79,91] Bradshaw, Izatt, and co-workers studied the ability of Me 10 CB [5] to bind monovalent and divalent cations in HCO 2 H/H 2 O (1:1) and found a remarkably high selectivity toward Pb 2+ ions (> 10 5.5 relative to alkali cations). [79] Somewhat surprisingly, CB [5] itself does not display a similar selectivity for Pb 2+ ions.…”

Cucurbiturilas

“…Foram relados na literatura alguns complexos de CBs com metais de transição d, mas ainda assim de maneira muito incipiente. Buschmann, Kim e Schollmeyer demonstraram a formação de complexos estáveis de CBs com lantanídeos, alcalinos e alcalino-terrosos 29,40,107,140,141,143,144;170,171 juntamente com Izatt, que estudou a formação de complexos de decametil-CB [5] com chumbo e alcalinos 145 . Outros complexos com Bi III 169 e Cu II 109 e compostos de inclusão com ferrocenos e cobaltocenos 172 , metal-ciclen e metal-ciclam 173 além de outras espécies inorgânicas também foram obtidos, mostrando que é possível inserir e sintetizar complexos dentro das cavidades dos cucurbitandos 174 .…”

Cucurbit[n]uril Analogues.

“…Consequently, much of the supramolecular chemistry of CB [5] has been limited to proton, metal, and ammonium binding at their portals. 64,65,73,74,76,87 Bradshaw, Izatt, and co-workers studied the ability of Me 10 CB [5] with -, -, and -cyclodextrins (K a 10). 58 Recently, Tao and co-workers disclosed that hexamethylene tetramine is capable of lidding CB [5].…”