In search of new platforms that support redox-controlled catalysis, we have investigated the noninnocent behavior of chlorostibine ligands coordinated to gold. The gold chlorostibine complex ...((o-(Ph2P)C6H4)2SbCl)AuCl (1-Cl) undergoes a clean oxidation reaction on treatment with PhICl2. This oxidation reaction affords the corresponding trichlorostiborane complex ((o-(Ph2P)C6H4)2SbCl3)AuCl (2-Cl), which can be converted into the more tractable trifluoride analogue ((o-(Ph2P)C6H4)2SbF3)AuCl (3-Cl) by treatment with a fluoride source. As supported by experimental and computational results, these complexes possess a Au→Sb donor–acceptor interaction which is distinctly stronger in the oxidized complexes 2-Cl and 3-Cl. Both 1-Cl and 3-Cl undergo a clean chloride abstraction reaction to afford the corresponding cationic gold species ((o-(Ph2P)C6H4)2SbCl)Au+ (1+) and ((o-(Ph2P)C6H4)2SbF3)Au+ (3+), which have been isolated as SbF6 – salts. As a result of a stronger Au→Sb interaction, cation 3+ features a more Lewis acidic gold center. It forms an isolable water adduct and also activates terminal alkynes toward hydroamination with arylamines. These results demonstrate that the redox state of noninnocent Z-ligands can be used to control the catalytic activity of the adjoining metal center.
Telluronium cations have long been known to engage their counteranions via secondary interactions. Yet, this property has rarely been exploited for anion binding. Motivated by such an application, we ...have now synthesized a bis-telluronium dication (32+) that was obtained as a tetrafluoroborate salt by reaction of 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid with phenoxatellurine difluoride and BF3·OEt2. As confirmed by the formation of Te-(μ-BF4)-Te bridges in the structure of 3BF42, 32+ functions as a bidentate Lewis acid toward anions. 3BF42 has also been converted into the more exposed 3BArF242 (BArF24− = B(3,5-(CF3)2C6H3)4−). The latter, which readily ionizes Ph3CCl, displays a chloride anion binding constant that exceeds that of a monofunctional model compound by almost 4 orders of magnitude. The unique properties of this new bis-telluronium dication are further highlighted by its ability to activate Ph3PAuCl and cis-(Ph3P)2PtCl2, leading to catalytic systems highly active in the cycloisomerization of propargylamide or enyne substrates.
Stimulated by applications in catalysis, the chemistry of ambiphilic ligands featuring both donor and acceptor functionalities has experienced substantial growth in the past several years. The unique ...opportunities in catalysis offered by ambiphilic ligands stem from the ability of their acceptor functionalities to play key roles via metal–ligand cooperation or modulation of the reactivity of the metal center. Ligands featuring group 13 centers, most notably boranes, as their acceptor functionalities have undoubtedly spearheaded these developments, with remarkable results having been achieved in catalytic hydrogenation and hydrosilylation. Motivated by these developments as well as by our fundamental interest in the chemistry of heavy group 15 elements, we became fascinated by the possibility of employing antimony centers as Lewis acids within ambiphilic ligands. The chemistry of antimony-based ligands, most often encountered as trivalent stibines, has historically been considered to mirror that of their lighter phosphorus-based congeners. There is growing evidence, however, that antimony-based ligands may display unique coordination behavior and reactivity. Additionally, despite the diverse Lewis acid and redox chemistry that antimony exhibits, there have been only limited efforts to explore this chemistry within the coordination sphere of a transition metal. By incorporation of antimony into the framework of polydentate ligands in order to enforce the main group metal–transition metal interaction, the effect of redox and coordination events at the antimony center on the structure, electronics, and reactivity of the metal complex may be investigated. This Account describes our group’s continuing efforts to probe the coordination behavior, reactivity, and application of ambiphilic ligands incorporating antimony centers. Structural and theoretical studies have established that both Sb(III) and Sb(V) centers in polydentate ligands may act as Z-type ligands toward late transition metals. Although coordinated to a metal, the antimony centers in these complexes retain residual Lewis acidity, as evidenced by their ability to participate in anion binding. Anion binding events at the antimony center have been shown by structural, spectroscopic, and theoretical studies to perturb the antimony–transition metal interaction and in some cases to trigger reactivity at the metal center. Coordinated Sb(III) centers in polydentate ligands have also been found to readily undergo two-electron oxidation, generating strongly Lewis acidic Sb(V) centers in the coordination sphere of the metal. Theoretical studies suggest that oxidation of the coordinated antimony center induces an umpolung of the antimony–metal bond, resulting in depletion of electron density at the metal center. In addition to elucidating the fundamental coordination and redox chemistry of antimony-containing ambiphilic ligands, our work has demonstrated that these unusual behaviors show promise for use in a variety of applications. The ability of coordinated antimony centers to bind anions has been exploited for sensing applications, in which anion coordination at antimony leads to a colorimetric response via a change in the geometry about the metal center. In addition, the capacity of antimony Lewis acids to modulate the electron density of coordinated metals has proved to be key in facilitating photochemical activation of M–X bonds as well as antimony-centered redox-controlled catalysis.
The complexation and sensing of fluoride ions via organoboron compounds is discussed. While fluoride in drinking water presents significant dental benefits, exposure to high fluoride levels may be ...harmful. Therefore, research into new methods of sensing and measuring fluoride ions is important.
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).
We describe the synthesis and structures of o-C6F4(TeMes)2 (1) and o-C6F4(TeArF)2 (2, ArF = 3,5-(CF3)2C6H3)), two new bifunctional tellurides featuring an electron-withdrawing backbone. While 2 ...resisted methylation, 1 reacted with Me3O·BF4 in CH2Cl2 to afford o-C6F4(TeMes)(TeMeMes) (3+), a mixed-valent telluride/telluronium cation isolated as a tetrafluoroborate salt. Although attempts to methylate the second telluride have been unsuccessful, 3+ readily catalyzes the hydrogenation of 2-phenyl-quinoline with Hantzsch ester. Comparison with simple telluronium cations including ArF 2TeMe+ and MesArFTeMe+ confirms that the catalytic activity of these compounds originates from the presence of a tetravalent, cationic tellurium center.
With the view of developing self-activating electrophilic catalysts, we are now investigating complexes with a Lewis acidic moiety in the immediate vicinity of the transition metal center. Toward ...this end, we have synthesized a platinum complex in which the metal is connected to a Lewis acidic bis(triflato)stiboranyl ligand. This complex, ((o-(Ph2P)C6H4)2SbOTf2)PtCl (2), which was obtained by treatment of ((o-(Ph2P)C6H4)2SbCl2)PtCl (1) with 2 equiv of AgOTf, is surprisingly air stable. Yet, it promptly reacts with cyclohexylisocyanide to afford the dicationic chlorostibine complex ((o-(Ph2P)C6H4)2SbCl)PtCNCy2+ (32+) as a bis-triflate salt. Formation of 32+ occurs through abstraction of the platinum-bound chloride ligand by the adjacent Lewis acidic antimony center. This halide migration reaction leads to activation of the platinum center. In turn, 2 behaves as a self-activating catalyst in reactions involving alkynes and readily mediates both enyne cyclization and intramolecular hydroarylation reactions, at room temperature, without addition of a chloride abstracting reagent. These results demonstrate that the coordination non-innocence of antimony ligands can be exploited for the purpose of electrophilic catalysis.
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 UF6) 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 main group Lewis acids for fluoride anion complexation and element-fluorine bond activation, we have synthesized the stibonium borate salt Sb(C6F5)4B(C6F5)4 (3). ...The perfluorinated stibonium cation Sb(C6F5)4+ present in this salt is a potent Lewis acid which abstracts a fluoride anion from SbF6− and BF(C6F5)3− indicating that it is a stronger Lewis acid than SbF5 and B(C6F5)3. The unusual Lewis acidic properties of 3 are further reflected by its ability to polymerize THF or to promote the hydrodefluorination of fluoroalkanes in the presence of Et3SiH. While highly reactive in solution, 3 is a perfectly air stable salt, making it a convenient Lewis acidic reagent.
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