Our interests in the chemistry of atypical main group Lewis acids have led us to devise strategies that augment the affinity of chalcogen-bond donors for anionic guests. In this study, we describe ...the oxidative methylation of diaryltellurides as one such strategy along with its application to the synthesis of Mes(C
6
F
5
)TeMe
+
and (C
6
F
5
)
2
TeMe
+
starting from Mes(C
6
F
5
)Te and (C
6
F
5
)
2
Te, respectively. These new telluronium cations have been evaluated for their ability to complex and transport chloride anions across phospholipid bilayers. These studies show that, when compared to their neutral Te(
ii
) precursors, these Te(
iv
) cations display both higher Lewis acidity and transport activity. The positive attributes of these telluronium cations, which originate from a lowering of the tellurium-centered σ* orbitals and a deepening of the associated σ-holes, demonstrate that the redox state of the main group element provides a convenient handle over its chalcogen-bonding properties.
The oxidative alkylation of diorganotellurides enhances the chalcogen-bond donor properties of the tellurium center, an effect manifested in the enhanced chloride anion affinity and transport properties of the resulting telluronium cations.
The concomitant activation of carbonyl substrates by two Lewis acids has been investigated by using 1,2‐(Ph2MeSb)2C6H42+ (12+), an antimony‐based bidentate Lewis acid obtained by methylation of the ...corresponding distibine. Unlike the simple stibonium cation Ph3MeSb+, dication 12+ efficiently catalyzes the hydrosilylation of benzaldehyde under mild conditions. The catalytic activity of this dication is correlated to its ability to doubly activate the carbonyl functionality of the organic substrate. This view is supported by the isolation of 1‐μ2‐DMFOTf2, an adduct, in which the DMF oxygen atom bridges the two antimony centers.
Two are better than one! A bifunctional Lewis acid containing two stibonium cations has been used to catalyze the hydrosilylation of benzaldehyde. Comparison with simple monofunctional stibonium salts suggests that the catalytic activity of the dication originates from the double activation of the carbonyl functionality of the organic substrate (see scheme).
Because of the ubiquity of fluoride ions and their potential toxicity at high doses, researchers would like to design receptors that selectively detect this anion. Fluoride is found in drinking ...water, toothpaste, and osteoporosis drugs. In addition, fluoride ions also can be detected as an indicator of uranium enrichment (via hydrolysis of UF(6)) or of the chemical warfare agent sarin, which releases the ion upon hydrolysis. However, because of its high hydration enthalpy, the fluoride anion is one of the most challenging targets for anion recognition. Among the various recognition strategies that are available, researchers have focused a great deal of attention on Lewis acidic boron compounds. These molecules typically interact with fluoride anions to form the corresponding fluoroborate species. In the case of simple triarylboranes, the fluoroborates are formed in organic solvents but not in water. To overcome this limitation, this Account examines various methods we have pursued to increase the fluoride-binding properties of boron-based receptors. We first considered the use of bifunctional boranes, which chelate the fluoride anion, such as 1,8-diborylnaphthalenes or heteronuclear 1-boryl-8-mercurio-naphthalenes. In these molecules, the neighboring Lewis acidic atoms can cooperatively interact with the anionic guest. Although the fluoride binding constants of the bifunctional compounds exceed those of neutral monofunctional boranes by several orders of magnitude, the incompatibility of these systems with aqueous media limits their utility. More recently, we have examined simple triarylboranes whose ligands are decorated by cationic ammonium or phosphonium groups. These cationic groups increase the electrophilic character of these boranes, and unlike their neutral analogs, they are able to complex fluoride in aqueous media. We have also considered cationic boranes, which form chelate complexes with fluoride anions. Our work demonstrates that Coulombic and chelate effects are additive and can be combined to boost the anion affinity of Lewis acidic hosts. The boron compounds that we have investigated present a set of photophysical and electrochemical properties that can serve to signal the fluoride-binding event. We can also apply this approach to cyanide complexation and are continuing our investigations in that area.
As part of our ongoing interest in the design of boron-based cyanide anion receptors, we have synthesized a triaryl borane decorated by a cationic Ru(II) complex and have investigated its anion ...binding properties. This new borane, (2,2'-bpy)Ru(kappa-C,N-2-(dimesitylborylphenyl)pyridinato)OTf (2OTf), binds both fluoride and cyanide anions in organic solvents to afford 2-F and 2-CN whose crystal structures have been determined. UV-vis titrations in 9/1 CHCl(3)/DMF (vol.) afforded K((F(-))) = 1.1(+/-0.1) x 10(4) M(-1) and K((CN(-))) = 3.0(+/-1.0) x 10(6) M(-1) indicating that 2(+) has a higher affinity for cyanide than for fluoride in this solvent mixture. These elevated binding constants show that the cationic Ru(II) complex increases the anion affinity of these complexes via Coulombic and inductive effects. The UV-vis spectral changes which accompany either fluoride or cyanide binding to the boron center are similar and include a 30 nm bathochromic shift of the metal-to-ligand charge transfer band. This shift is attributed to an increase in the donor ability of the boron-substituted phenylpyridine ligand upon anion binding to the boron center. Accordingly, cyclic voltammetry revealed that the Ru(II/III) redox couple of 2OTf (E(1/2) = +0.051 V vs Fc/Fc(+)) undergoes a cathodic shift upon F(-) (DeltaE(1/2) = -0.242 V vs Fc/Fc(+)) or CN(-) (DeltaE(1/2) = -0.198 V vs Fc/Fc(+)) binding.
Going fishing! The sulfonium borane 1⁺ complexes cyanide in pure water at the maximum allowable concentration of 50 ppb recommended by the European Union. The high cyanide ion affinity displayed by ...this compound arises from favorable Coulombic effects augmented by a direct bonding interaction between the cyano and sulfonio groups.
Because of hydration, fluoride ions in water typically elude complexation by neutral Lewis acids. Here, we show how this limitation can be overcome with a bidentate Lewis acid containing two ...antimony(V) centers. This derivative (2) is obtained by the simple reaction of 4,5‐bis(diphenylstibino)‐9,9‐dimethylxanthene (1) with two equivalents of 3,4,5,6‐tetrachlorobenzoquinone (o‐chloranil). It features two square‐pyramidal stiborane units oriented in a face‐to‐face fashion. Titration experiments show that this new bidentate Lewis acid binds fluoride in aqueous solutions containing 95 % water with a binding constant (K) of 700±30 M−1. The structure of the fluoride adduct confirms fluoride anion chelation between the two antimony centers.
Fluoride sponge: Due to hydration, fluoride anions are weakly basic and reluctant to bind with neutral Lewis acids in water. A bifunctional Lewis acid containing two stiboranes as Lewis acidic units forms an anionic fluoride complex stabilized by a Sb‐F‐Sb chelate motif. A monofunctional analogue shows that the bidentate Sb2 system is more acidic by at least two orders of magnitude.
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