Evidence for CH Hydrogen Bonding in Salts of <i>tert</i>‐Butyl Cation

Stoyanov, Evgenii S.; Stoyanova, I. V.; Tham, Fook S.; Reed, Christopher A.

doi:10.1002/ange.201203958

Cited by 9 publications

(19 citation statements)

References 19 publications

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“…In contrast to previously reported structural data on t-Bu + salts, 17,20,22 we obtained a high-resolution X-ray crystal structure of t-Bu + {Cl 11 − }, 18 which allowed us to determine the H atom positions with certainty, sufficient to measure the C−C−H angles and direction of the C−H bond toward the Cl atoms of the ions.…”

Section: Resultsmentioning

confidence: 99%

“…As a consequence, it forms the weaker bifurcated H-bond. 18 Two other C−H bonds of this group form bifurcated and mostly weak trifurcated H-bonds. Thus, the third CH 3 group, which we labeled as CH 3 *, is not equivalent to the other two, undergoes weaker hyperconjugation, and forms weaker H-bonds.…”

Section: Resultsmentioning

confidence: 99%

“…Meanwhile, in solid-phase IR spectra, the bands corresponding to vibrations of "free" C−H bonds almost disappear. 18 Moreover, recently it was shown that the t-Bu + cation forms sufficiently strong H-bonds with counterions in its solid salts, 18 thus confirming that the influence of the environment on the tBu + is significant and may be an additional factor affecting its stabilization. Therefore, the seeming similarity of IR spectra of t-Bu + in vacuum and in condensed phases requires an explanation.…”

Section: Introductionmentioning

confidence: 91%

“…The cations of the t-Bu + Carb − salts are H-bonded to anions that have decreasing C−H stretch frequencies. 18 It is possible to plot νCH max against the anion basicity on the NH basicity scale (in cm −1 ). 35 This scale is based on measurement of the νNH frequency of the contact ion pairs Oct 3 NH + ···Anion − in CCl 4 solutions relative to νNH of the Oct 3 NH + ···CCl 4 solvate in the solvent-separated ion pairs (ΔνNH).…”

Section: Scheme 4 Proton Fluctuation Between Two Minima When It Formentioning

confidence: 99%

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tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

Stoyanov

Gomes

2015

J. Phys. Chem. A

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Despite the seeming similarity of the infrared (IR) spectra between tert-butyl cations (t-Bu(+)) in gaseous and condensed phases, there are important but so far unrecognized differences. The IR spectroscopic investigation of the hydrogen (H)-bonding of t-Bu(+) with the immediate environment together with the X-ray crystallographic data shows that one CH3 group of t-Bu(+) differs from the other two. In the Ar-tagged t-Bu(+) in vacuum, this group is predominantly polarized, showing three C-H stretch vibrations at 2913, 2965, and 3036 cm(-1) whereas the other two methyls are predominantly involved in strong hyperconjugation, yielding an intense triple IR band with a maximum at 2839 cm(-1). In a condensed phase, the bulk solvent effect promoted participation of the polarized CH3 group in additional hyperconjugation, decreasing its νCH3 frequencies by approximately 120 cm(-1), whereas frequencies of the other CH3 groups decreased by only ca. 4-10 cm(-1). This observation indicates that the influence of the condensed phase on t-Bu(+) stabilization is substantial. Thus, enhancement of H-bonding between t-Bu(+) and Anion(-) strengthens hyperconjugation and promotes further cation stabilization.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 91%

Section: Scheme 4 Proton Fluctuation Between Two Minima When It Formentioning

confidence: 99%

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tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

Stoyanov

Gomes

2015

J. Phys. Chem. A

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“…13 Similarly, t -butyl cation (i.e. protonated iso -butene) salts are significantly stabilized by C-H⋯anion H-bonding 13 (possibly trumping hyperconjugation 14 ), again illustrating the propensity of acidic H atoms to become two-coordinate (or higher, when bifurcated CH hydrogen bonding is involved). See Fig.…”

Section: Introductionmentioning

confidence: 99%

Myths about the Proton. The Nature of H⁺ in Condensed Media

2013

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CONSPECTUS Recent research has taught us that most protonated species are decidedly not well represented by a simple proton addition. What is the actual nature of the hydrogen ion (the “proton”) when H+, HA, H2A+ etc. are written in formulae, chemical equations and acid catalyzed reactions? In condensed media, H+ must be solvated and is nearly always di-coordinate – as illustrated by isolable bisdiethyletherate salts having H(OEt2)2+ cations and weakly coordinating anions. Even carbocations such as protonated alkenes have significant C-H---anion hydrogen bonding that gives the active protons two-coordinate character. Hydrogen bonding is everywhere, particularly when acids are involved. In contrast to the normal, asymmetric O-H---O hydrogen bonding found in water, ice and proteins, short, strong, low-barrier (SSLB) H-bonding commonly appears when strong acids are present. Unusually low frequency IR νOHO bands are a good indicator of SSLB H-bonds and curiously, bands associated with group vibrations near H+ in low-barrier H-bonding often disappear from the IR spectrum. Writing H3O+ (the Eigen ion), as often appears in textbooks, might seem more realistic than H+ for an ionized acid in water. However, this, too, is an unrealistic description of H(aq)+. The dihydrated H+ in the H5O2+ cation (the Zundel ion) gets somewhat closer but still fails to rationalize all the experimental and computational data on H(aq)+. Researchers do not understand the broad swath of IR absorption from H(aq)+, known as the “continuous broad absorption” (cba). Theory has not reproduced the cba, but it appears to be the signature of delocalized protons whose motion is faster than the IR timescale. What does this mean for reaction mechanisms involving H(aq)+? For the past decade, the carborane acid H(CHB11Cl11) has been the strongest known Brøsted acid. (It is now surpassed by the fluorinated analogue H(CHB11F11).) Carborane acids are strong enough to protonate alkanes at room temperature, giving H2 and carbocations. They protonate chloroalkanes to give dialkylchloronium ions, which decay to carbocations. By partially protonating an oxonium cation, they get as close to the fabled H4O2+ ion as can be achieved outside of a computer.

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Perhalogenated Carba‐closo‐dodecaborate Anions as Ligand Substituents: Applications in Gold Catalysis

et al. 2013

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The success of modern transition-metal catalysis is largely due to the availability of a diverse range of ligand frameworks. One of the most universal aspects of ligand design is the strategic attachment of bulky substituents to influence the activity of catalysts. Sterically demanding substituents kinetically protect the active metal center and, at the same time, promote substrate exchange and low coordination, two processes necessary for turnover. Bulky ligand substituents also influence the selectivity of the metal center for product formation and substrate consumption. For these purposes, the most common groups appended to ancillary ligands are alkyl and aryl groups, such as adamantyl or 2,6-diisopropylphenyl. Far less common is the use of dicarba-closo-dodecaborane (C 2 B 10 ) clusters [1] as surrogates for alkyl or aryl groups. [2] The three-dimensional aromatic nature of these species and their icosahedral shape lend them properties akin to those of both alkyl and aryl groups. However, owing to the hydridic nature of the BÀH vertices, ligands bearing these substituents tend to undergo undesirable B À H activation reactions, such as cyclometalation. [3] It is this tendency for such intramolecular reactions to occur that has limited the synthetic utility of dicarba-closo-dodecaborane-bearing ligands in the area of catalysis.Interestingly, complexes that contain ligands functionalized with related carba-closo-dodecaborate anions (CB 11 À ) [4] directly bound to the coordinating atom through the carborane cage carbon atom have not been reported. [5] Although similar in size to their neutral "dicarba" cousins (C 2 B 10 ), isoelectronic (CB 11 À ) clusters have significantly different properties. The negative charge is delocalized over all of the 12 cage atoms, and as a result, the anion is very weakly coordinating. This weak coordination ability can be enhanced by the substitution of some or all of the B À H vertices for alkyl or halo groups. In the case of alkyl substitution, the cluster becomes more reactive towards substitution and oxidation. On the other hand, exhaustive halogenation of the boron vertices of the cluster introduces a blanket of electronwithdrawing substituents that enhances the inherent weak coordination ability of the anion and also confers upon these molecules exceptional inertness. It has been demonstrated that perhalogenated carborane counteranions are sufficiently unreactive that they can form isolable salts with potent oxidants, such as C 60 +[6] and CH 3 + . [7] Thus, these clusters can be far more inert than even simple hydrocarbons: CH 3 + abstracts hydrides from n-alkanes with loss of methane at room temperature. It is these properties that have generated increasing interest [8] in the use of perhalogenated carborane anions in silylium catalysis [8i,l] and related processes. [8f] The anion of choice for most applications is the HCB 11 Cl 11 À anion (1), since it is readily accessible [8g] and arguably the most inert carborane anion (Scheme 1).We are interested in using the HCB 1...

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Evidence for CH Hydrogen Bonding in Salts of tert‐Butyl Cation

Cited by 9 publications

References 19 publications

tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

Myths about the Proton. The Nature of H⁺ in Condensed Media

Perhalogenated Carba‐closo‐dodecaborate Anions as Ligand Substituents: Applications in Gold Catalysis

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Evidence for CH Hydrogen Bonding in Salts of tert‐Butyl Cation

Cited by 9 publications

References 19 publications

tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

Myths about the Proton. The Nature of H+ in Condensed Media

Perhalogenated Carba‐closo‐dodecaborate Anions as Ligand Substituents: Applications in Gold Catalysis

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Myths about the Proton. The Nature of H⁺ in Condensed Media