Concave carrier molecules: selective transport of copper(II) and lead(II) nitrates through supported liquid membranes

Abstract

Ion-selective electrodes [1] are used as chemical sensors for many analytical applications. Their selectivity originates from mediated transport of ions through a hydrophobic membrane. The transport selectivity, however, is generally determined by a substrate-specific receptor molecule present in the membrane, where it can act as a carrier between source and receiving aqueous phase. Most of the work with artificial membranes has been focused on carrier-facilitated transport of metal cations by macrocyclic receptors, and many data from transport experiments under various conditions are avail- Abstract. Concave 1,10-phenanthrolines 1 are selective and efficient carrier molecules for membrane transport of copper(II) and lead(II) ions. With competitive membrane transport experiments the selectivity patterns of three concave 1,10-phenanthroline membrane carriers have been determined. Measurements with copper(II) and lead(II) nitrates confirmed the predicted selectivities and revealed high transport rates here membrane transport properties of concave 1,10-phenanthrolines [3], which posses the structural requirements to act as selective carriers for transition metal ions: 1,10-phenanthroline is a well known ligand for binding of transition metal cations, and we expect the concave shielding to keep the metal ion binding reversible and to provide binding selectivity. Results and DiscussionAlthough the rate of cation cotransport for a new carrier molecule can be predicted if only one cation is involved [4] this is certainly difficult for unusual structures, such as concave 1,10-phenanthrolines. To facilitate the testing of transport selectivities of new carrier molecules towards several cations we have recently introduced a new method [5] by which a competitive cation transport is monitored with simultaneous element analysis by inductively coupled plasma optical emission spectroscopy (ICP-OES) [6]. A single experiment of this kind reveals the transport selectivity of a carrier towards several cations. With a set of ten metal nitrates, including alkali, alkali earth and transition metal cations in the source phase, the membrane transport ability for 1a and 1b. While 1b shows a high preference for copper(II) nitrate, 1a transports lead and copper nitrates equally well, but with much higher flux. In bulk liquid membranes, the carrier mediated ion flux remained constant over an extended period, whereas it decreased significantly in reused supported liquid membranes, which indicates rapid loss of carrier from the membrane under these conditions.

Concave 1,10-Phenanthrolines by Ring-Closing Metathesis

“…They are concave bases2 but they are also good ligands due to the two nitrogen atoms of the 1,10‐phenanthroline moiety that can strongly bind metal ions by chelation 2,3. Besides using them as carriers,4 these complexes have been used to enhance selectivities in transition‐metal‐catalyzed Diels−Alder reactions,5 allylations6 and cyclopropanations 7…”