Applications of Supramolecular Anion Recognition

Recent advances in luminescent heavy metal complexes for sensing

Lewis acids play a major role in all areas of chemistry. For a long time, toxic, corrosive and oxidizing SbF5 was considered as the strongest Lewis acid known. Lately, species significantly exceeding the Lewis acidity of SbF5 have been realized and were termed Lewis superacids (LSA). Prospective new candidates emerge steadily, which not only outperform SbF5 by their strength, but also in terms of their accessibility and ease of handling. In principle, Lewis superacids allow us to combine the outstanding activity of Bronsted superacids with the excellent selectivity of a common Lewis acid. However, the broad application of Lewis superacids in synthesis is all but popular. The present review deals with strong Lewis acids. First, it critically discusses Lewis acidity scaling methods and suggests an extended definition for Lewis superacidity. It then summarizes the properties and applications of the strongest currently known Lewis acids, indexed by the fluoride ion affinity (FIA). The supporting information contains a comprehensive list of experimentally and theoretically derived FIA data as a guide for the choice of Lewis acidic reagents/catalyst. This contribution shall encourage the search for new Lewis superacids and promote their application in non-specialized laboratories.

Lewis Superacids: Classifications, Candidates, and Applications.

The 9-anthryltriphenylstibonium cation, [1](+), has been synthesized and used as a sensor for the toxic fluoride anion in water. This stibonium cation complexes fluoride ions to afford the corresponding fluorostiborane 1-F. This reaction, which occurs at fluoride concentrations in the parts per million range, is accompanied by a drastic fluorescence turn-on response. It is also highly selective and can be used in plain tap water or bottled water to test fluoridation levels.

Stibonium ions for the fluorescence turn-on sensing of F- in drinking water at parts per million concentrations.

Through click chemistry, triazole and triazolium groups have been explored to recognize anions through C–H···A– hydrogen-bonding complexion. Herein, we demonstrate evidence of fluoride-induced deprotonation of a C–H bond and its application in fluoride detection. The combination of fluorene and triazolium units produced a highly selective fluorescence turn-on prototype sensor for fluoride. The interactions between the C–H bond and F– were studied by fluorescence spectroscopy and 1H NMR titrations. Test papers were prepared to detect fluoride in aqueous media at concentrations down to 1.9 ppm, important for estimating whether the fluoride concentration in drinking water is at a safe level.

Highly Selective Fluorescence Turn-On Sensor for Fluoride Detection

Turn on the lantern! The stibine donor ligand of a cationic palladium complex acts as a Lewis acid and reacts with a fluoride anion to afford the corresponding fluorostiboranyl-palladium species (see scheme). Bindung of the fluoride anion to the antimony center induces a change in denticity of the triphosphine unit and leads to a bright-orange trigonal-bipyramidal d(8) lantern complex.

Sensing of Aqueous Fluoride Anions by Cationic Stibine–Palladium Complexes

BiZness as usual? Not exactly! The bismuth atom of the tridentate diphosphinobismuthine (o-(Ph(2)P)C(6)H(4))(2) BiCl behaves as a Z rather than L ligand when in the coordination sphere of late transition metals such as gold. The σ-acceptor behavior of Bi is supported by its disphenoid coordination geometry and theoretical studies, which show a Au→Bi interaction.

σ‐Accepting Properties of a Chlorobismuthine Ligand

The tetravalent platinum stiboranyl complex [(o-(Ph2P)C6H4)2(o-C6Cl4O2)Sb]PtCl2Ph (2) has been synthesized by reaction of [(o-(Ph2P)C6H4)2SbClPh]PtCl (1) with o-chloranil. In the presence of fluoride anions, the stiboranyl moiety of 2 displays non-innocent behavior and is readily converted into a fluorostiborane unit. This transformation, which is accompanied by elimination of a chloride ligand from the Pt center, results in the formation of [(o-(Ph2P)C6H4)2(o-C6Cl4O2)SbF]PtClPh (3). Structural, spectroscopic, and computational studies show that the conversion of 2 into 3 is accompanied by a cleavage of the covalent PtSb bond present in 2 and formation of a longer and weaker Pt→Sb interaction in 3. These results show that this new Pt–Sb platform supports the fluoride-induced metamorphosis of a stiboranyl X ligand into a stiborane Z ligand.

Anion‐Controlled Switching of an X Ligand into a Z Ligand: Coordination Non‐innocence of a Stiboranyl Ligand

In search for new examples of σ-acceptor ligands, we have investigated the tridentate ligands (o-(iPr2P)C6H4)2SbPh) (LPh) and (o-(iPr2P)C6H4)2SbCl) (LCl) which react with (tht)AuCl (tht = tetrahydrothiophene) to afford LPhAuCl (1) and LClAuCl (2), respectively. As suggested by the structure of these complexes, which confirm complexation of the SbP2 ligands to the gold chloride fragment, and in agreement with the results of the density functional theory (DFT) and natural bond orbital (NBO) calculations, the gold and antimony atom of 1 and 2 are involved in a Au→Sb donor–acceptor interaction. The magnitude of this interaction is higher in complex 2 which possesses a chlorinated and thus more Lewis acidic antimony center. We have also compared the strength of the Au→Sb interaction present in 2 with the Au→Bi interaction observed in the newly prepared bismuth analogue [(o-(iPr2P)C6H4)2BiCl]AuCl (3). This comparison reveals that 2 possesses a stronger Au→Pn bond (Pn = pnictogen), an observation reconciled by i...

Iou-Sheng Ke

Papers

Stibonium ions for the fluorescence turn-on sensing of F- in drinking water at parts per million concentrations.

Sensing of Aqueous Fluoride Anions by Cationic Stibine–Palladium Complexes

σ‐Accepting Properties of a Chlorobismuthine Ligand

Anion‐Controlled Switching of an X Ligand into a Z Ligand: Coordination Non‐innocence of a Stiboranyl Ligand

σ-Donor/acceptor-confused ligands: the case of a chlorostibine.