A self-assembled, redox-responsive receptor for the selective extraction of LiCl from water

Abstract

There is considerable interest in synthetic ionophores with high affinity and selectivity for Li ؉ . But so far, compounds that selectively bind Li ؉ in the presence of other alkali and alkaline earth metal ions are rare and current approaches toward this goal are often accompanied with substantial synthetic efforts. Here we describe a trinuclear ruthenium metallamacrocyclic complex (1) that was obtained by self-assembly of ruthenium halfsandwich complexes and 3-hydroxy-2-pyridone ligands. This complex was shown to be an extremely potent receptor for LiCl with an affinity high enough to extract LiCl from water. The selectivity of this receptor is exceptional: even in the presence of a large excess of Na ؉ , K ؉ , Cs ؉ , Ca 2؉ , and Mg 2؉ , Li ؉ was extracted exclusively. The Li ؉ ͞Na ؉ selectivity ratio was determined to be higher than 1,000:1. Compared with other synthetic ionophores, the receptor 1 offers two additional advantages: (i) the synthesis can be accomplished in one step by using simple starting materials; and (ii) the presence of lithium ions can be detected electrochemically. Complex 1 is therefore a very attractive candidate for the construction of a Li ؉ -specific chemosensor.L ithium salts have found various applications in technology and medicine. Methods to selectively sequester and detect Li ϩ are therefore of importance. Li 2 CO 3 , for example, is one of the most important drugs for the treatment of manic depression (1, 2). Because of its narrow therapeutic range, the Li ϩ concentration in the blood of the patients needs to be controlled during the treatment. Thus, a small amount Li ϩ has to be detected in the presence of other metal ions such as Na ϩ , K ϩ , Ca 2ϩ , and Mg 2ϩ (3).To selectively sequester Li ϩ , ionophores have been considered early on. But the design and the synthesis of molecules that display a high affinity and selectivity for Li ϩ is a challenging task (4-9). Although considerable progress has been made in this field, the synthesis of such ionophores often requires substantial synthetic efforts. This difficulty is nicely illustrated by probably the best Li ϩ receptor known so far, a spherand developed by Cram and his colleagues (10). Because of the perfect preorganization of six oxygen donor atoms, a very high affinity combined with an excellent selectivity for Li ϩ is observed. The synthesis, however, requires several steps and gives less than 7% overall yield.Here we describe a redox-responsive metallamacrocycle (1; see Scheme 1) that was obtained by self-assembly (for selfassembled metallacrowns see ref. 11). With this receptor we were able to selectively extract LiCl from an aqueous solution containing an excess of NaCl, KCl, CsCl, CaCl 2 , and MgCl 2 . The presence of LiCl can subsequently be detected by electrochemical means. Materials and MethodsGeneral. The synthesis of all complexes was performed under an atmosphere of dry dinitrogen, by using standard Schlenk techniques. The 1 H, 13 C, and 7 Li spectra were recorded on a JEOL EX 400 or a GSX 270 spectrometer u...

Modulating the electron-transfer properties of a mixed-valence system through host–guest chemistry

“…Studies by the Severin group on kinetically labile [Ru(arene)]-based neutral macrocycles have resulted in analogues of 12-crown-3, that bind alkali metal ions in non-aqueous solvents with micromolar affinities. 40 , 41 Moreover, Bedford and Tucker 42 have shown that when the [9]aneS 3 ligand is coordinated to a cationic metal center it can recognize anions through C–H···X hydrogen bonding interactions. Given that 1 3+ is cationic and possesses an array of thiacrown-based hydrogen-bonding donor groups, we reasoned that it would be a receptor for anionic guests.…”

Development of organometallic (organo-transition metal) pharmaceuticals

“…For example, Severin and co-workers 71,72 have recently shown that trinuclear metallamacrocycles based on the ruthenium(II)-arene unit, as well as other related systems, display an extremely high affinity and selectivity for lithium and sodium ions. Since lithium salts such as Li 2 CO 3 are among the most frequently used drugs for the treatment of manic depression, and owing to its narrow therapeutic range, the Li + concentration in the blood of the patients needs to be carefully controlled, and compounds like [(C 6 H 5 CO 2 Et)Ru(L)] 3 (L = 3-oxo-pyridonate ligand) selectively extract LiCl from an aqueous solution containing a large excess of alkali and alkaline earth metal salts that could prove to be of therapeutic use.…”

Bioorganometallic chemistry—from teaching paradigms to medicinal applications

“…For example, research on organometallic sensors of pharmacological relevance, largely pioneered by Kay Severin, display considerable promise, notably for Li ion detection and quantification. 19 In addition, other important topics include conjugates of peptides, proteins and PNA organometallic derivatives, 20 bioprobes, 12 tracers in non-isotopic immunoassays, 12 and synthetic modelling of the active sites of organometalloenzymes, 12,21 as well as the development of artificial organometallic containing proteins. 22…”