Fluoride is often added to drinking water and toothpaste because of its beneficial effects in dental health. It is also administered in the treatment of osteoporosis.1 While the beneficial effects of fluoride are well documented, chronic exposure to high levels of this anion can lead to dental or even skeletal fluorosis.2-4 Taking into account these adverse effects, a great deal of attention has been devoted to the discovery of improved analytical methods for the detection of fluoride, especially in water. This field of research has also been stimulated by the potential use of such methods for the detection of phosphorofluoridate nerve agents such as Sarin or uranium hexafluoride, which release fluoride upon hydrolysis. In addition to these applications, the capture of fluoride, especially in water, is a stimulating academic challenge because of the high hydration enthalpy of this anion (∆H° ) -504 kJ mol-1). * To whom correspondence should be addressed. E-mail: simon.aldridge@ chem.ox.ac.uk (S.A.) and francois@tamu.edu (F.P.G.). † Texas A&M University. ‡ University of Oxford. Casey Wade was born in 1983 in Lynch, Nebraska. He earned a B.S. degree from the University of NebraskasLincoln in 2006. During his time there, he worked as an undergraduate researcher under the supervision of J. A. Belot and participated in a joint research internship at the American Air Liquide Chicago Research Center. In 2007, he joined the group of F. P. Gabbaı̈ at Texas A&M University, where he currently studies anion recognition using main group Lewis acids.

Fluoride Ion Complexation and Sensing Using Organoboron Compounds

Transition metal hydrides play a critical role in stoichiometric and catalytic transformations. Knowledge of free energies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond-breaking events that occur in a catalytic reaction. Thermodynamic hydricity is the free energy required to cleave an M–H bond to generate a hydride ion (H–). Three primary methods have been developed for hydricity determination: the hydride transfer method establishes hydride transfer equilibrium with a hydride donor/acceptor pair of known hydricity, the H2 heterolysis method involves measuring the equilibrium of heterolytic cleavage of H2 in the presence of a base, and the potential–pKa method considers stepwise transfer of a proton and two electrons to give a net hydride transfer. Using these methods, over 100 thermodynamic hydricity values for transition metal hydrides have been determined in acetonitrile or water. In acetonitrile, the hydricity of metal hydrides sp...

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00168

Thermodynamic Hydricity of Transition Metal Hydrides

In the mid-1990s, it was discovered that tris(pentafluorophenyl)borane, B(C6F5)3, was an effective catalyst for hydrosilylation of a variety of carbonyl and imine functions. Mechanistic studies revealed a counterintuitive path in which the function of the borane was to activate the silane rather than the organic substrate. This was the first example of what has come to be known as “frustrated Lewis pair” chemistry utilizing this remarkable class of electrophilic boranes. Subsequent discoveries by the groups of Stephan and Erker showed that this could be extended to the activation of dihydrogen, initiating an intense period of activity in this area in the past 5 years. This article describes the early hydrosilylation chemistry and its subsequent applications to a variety of transformations of importance to organic and inorganic chemists, drawing parallels with the more recent hydrogen activation chemistry. Here, we emphasize the current understanding of the mechanism of this process rather than focusing on...

Mechanistic Aspects of Bond Activation with Perfluoroarylboranes

Cyclopentadienone iron alcohol complexes generated from the reactions of [2,5-(SiMe3)2-3,4-(CH2)4(η5-C4COH)]Fe(CO)2H (3) and aldehydes were characterized by 1H NMR, 13C NMR, and IR spectroscopy. The benzyl alcohol complex [2,5-(SiMe3)2-3,4-(CH2)4(η4-C4C═O)]Fe(CO)2(HOCH2C6H5) (6-H) was also characterized by X-ray crystallography. These alcohol complexes are thermally unstable and prone to dissociate the coordinated alcohols. The alcohol ligand is easily replaced by other ligands such as PhCN, pyridine, and PPh3. Dissociation of the alcohol ligand in the presence of H2 leads to the formation of iron hydride 3. The reduction of aldehydes by 3 was carried out in the presence of both potential intermolecular and intramolecular traps. The reaction of 3 with PhCHO in the presence of 4-methylbenzyl alcohol as a potential intermolecular trapping agent initially produced only iron complex 6-H of the newly formed benzyl alcohol. However, the reaction of 3 with 4-(HOCD2)C6H4CHO, which possesses a potential intramolec...

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Cyclopentadienone Iron Alcohol Complexes: Synthesis, Reactivity, and Implications for the Mechanism of Iron-Catalyzed Hydrogenation of Aldehydes

The further development of the field of catalysis is based on the discovery, understanding, and implementation of novel activation modes that allow unprecedented transformations and open new perspectives in synthetic chemistry. In this context, the recently introduced concept of frustrated Lewis pair (FLP) from the Stephan research group represents a fundamental and novel strategy to develop catalysts based on main-group elements for small-molecule activation. These sterically encumbered Lewis acid–base systems are not able to form a stable donor–acceptor adduct, nevertheless, an intermolecular association of the Lewis acidic (LA) and basic (LB) components to a unique “frustrated complex” was proposed. Our research group has also shown that this encounter pair cleaves hydrogen in a cooperative manner and the steric congestion implies a strain, which can be directly utilized for bond activation. Using steric hindrance as a critical design element, several combinations of bulky Lewis acid–base pairs were effectively probed for heterolytic cleavage of hydrogen. Moreover, this remarkable capacity of FLPs was exploited in metal-free hydrogenation procedures. Additionally, the bifunctional and unquenched nature of the FLPs makes them capable of reacting with alkenes, dienes, acetylenes, and THF. Although this type of reactivity represents a breakthrough in main-group chemistry, its enhanced and non-orthogonal nature obviously limits the synthetic applicability of FLPs. Herein we report an attempt to develop frustrated Lewis pairs with orthogonal reactivity and improved functional-group tolerance for catalytic metal-free hydrogenation. The previously reported FLP-based hydrogen activation relied mostly on tris(pentafluorophenyl)borane (1) as the LA component. Because of the hard-type Lewis acidity of boron in 1 and its inactivation by common oxygenand/or nitrogen-containing molecules, careful substrate design was needed for successful catalytic hydrogenation reactions. This synthetic limitation triggered us to develop FLP catalysts that have a broader range of applications and possible selectivity in reduction processes. Our design concept for increased functional-group tolerance is based on the simple hypothesis that steric hindrance in FLPs is a relative phenomenon (Figure 1): further increase of

Mats Tilset

Papers

An Estimate of the Reduction Potential of B(C6F5)3 from Electrochemical Measurements on Related Mesityl Boranes

Single-step hydride transfer from cpmo(co)2(pph3)h to protonated ketones