Isolation of Protonated Arenes (Wheland Intermediates) with BAr<sup>F</sup> and Carborane Anions. A Novel Crystalline Superacid

Reed, C. A.; Fackler, Nlp; Kc, Kim; Stasko, Daniel; Evans, Mark D.; Boyd, P. D. W.; Rickard, Clifton E. F.

doi:10.1021/ja981861z

Cited by 151 publications

(129 citation statements)

References 19 publications

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“…This can be deduced from the stability of the silylium ion-like species with the perfluoro-tetraphenylborate anion replacing the carborane in R 3 Si(F 20 -BPh 4 ). 29 The F 20 -BPh 4 2 ion is unstable with respect to boron-aryl bond cleavage at acidities just above that of the mesitylenium ion, 25 which is ca. 10 9 less acidic than the benzenium ion.…”

Section: Silylium Ions (R 3 Simentioning

confidence: 99%

“…organofluoro anions such as B(C 6 F 5 ) 4 2 , 24 but their conjugate acids cannot be prepared because the anion decomposes at high acidity. 25 This is mirrored in the fact that traditional fluoroacids, commonly written as HBF 4 , HPF 6 , HSbF 6 etc., are also non-existent compounds. They are unstable with respect to formation of HF and the corresponding neutral Lewis acid.…”

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confidence: 99%

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Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

2005

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Icosahedral carborane anions such as CHB 11 Cl 11 2 are amongst the least coordinating, most chemically inert anions known. They are also amongst the least basic, so their conjugate acids, H(carborane), are superacids (i.e. stronger than 100% H 2 SO 4 ). Acidity scale measurements indicate that H(CHB 11 Cl 11 ) is the strongest pure Brønsted acid presently known, surpassing triflic and fluorosulfuric acid. Nevertheless, it is also an extremely gentle acid-because its conjugate base engages in so little chemistry. Carborane acids separate protic acidity from anion nucleophilicity and destructive oxidative capacity in the conjugate base, to a degree not previously achieved. As a result, many long-sought, highly acidic, reactive cations such as protonated benzene (C 6 H 7 + ), protonated C 60 (HC 60 +), tertiary carbocations (R 3 C + ), vinyl cations (R 2 CLC + -R), silylium ions (R 3 Si + ) and discrete hydronium ions (H 3 O + , H 5 O 2 + etc.) can be readily isolated as carborane salts and characterized at room temperature by X-ray crystallography.

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Section: Silylium Ions (R 3 Simentioning

confidence: 99%

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confidence: 99%

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

2005

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“…Anhydrous acids ideally suited for protonating easily cleaved oxo bridges have recently been developed in the form of protonated arene salts. 29 The 1 H NMR spectrum of the isolated product in dry dichloromethane-d 2 is shown in Figure 6. The identifying -pyrrole shift appears at +28 ppm.…”

Section: Hydroxo Complex Fe(oh)(tpp) 3 Further Inspection Ofmentioning

confidence: 99%

Reversal of H₂O and OH^- Ligand Field Strength on the Magnetochemical Series Relative to the Spectrochemical Series. Novel 1-equiv Water Chemistry of Iron(III) Tetraphenylporphyrin Complexes

2000

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Contrary to expectations based on the spectrochemical series, H 2 O is found to be a significantly weaker field ligand than OH -in the magnetochemical series ranking of ligand field strengths based on the spin states of iron(III) tetraphenylporphyrin complexes. The preparation and characterization of the [Fe(H 2 O)-(TPP)] + ion and the spectroscopic identification of Fe(OH)(TPP) have made this assessment possible. These two species were previously thought to be unattainable because of the facile formation of the well-known µ-oxo dimer, (TPP)Fe-O-Fe(TPP). However, the special characteristics of single equivalents of water under high acidity, relevant to metalloenzyme active sites and superacidity, make them accessible in benzene solution. Their 1 H NMR -pyrrole chemical shifts at -43 and +82 ppm indicate admixed-intermediate S ) 3 / 2 , 5 / 2 and high S ) 5 / 2 spin states for the aqua and hydroxo species, respectively. The X-ray crystal structure of the aqua complex has been determined for [Fe(H 2 O)(TPP)][CB 11 H 6 Cl 6 ] and is consistent with the high degree of S ) 3 / 2 character indicated by the NMR measurement, Mössbauer spectroscopy (∆E q ) 3.24 mm‚s -1 ), and magnetic susceptibility (µ eff ) 4.1 µ B ). The anhydrous precursor to these species is the "nearly bare" iron(III) porphyrin complex Fe(CB 11 H 6 Br 6 )(TPP). Judged by its magnetic parameters (δ pyrrole ) -62 ppm, ∆E q ) 3.68 mm‚s -1 , µ eff ) 4.0 µ B ) it attains the long sought essentially "pure" S ) 3 / 2 spin state. The magnetochemical ranking of ligand field strengths in five-coordinate high-spin and admixed-intermediate-spin iron(III) porphyrins is useful because it more closely reflects the intuitive field strengths of crystal field theory than does the usual spectrochemical ranking, which is controlled largely by π effects in octahedral low-spin d π 6 complexes.

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“…Conferring this measure of stability on an arenium ion, where traditional anions such as SbF 6 -fail to stabilize them much above dry ice temperatures, illustrates the powerful effect of choosing a carborane as counterion. 14 Oxidative stability is also noted. Whereas the dodecamethylated carborane is susceptible to oxidation at potentials greater than 1.2 V (vs Fc/Fc + ), 24 the pentamethyl hexahalo carboranes show no detectable CV redox processes within the operational window of dry dichloromethane ((2.0 V vs Fc/Fc + ).…”

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Optimizing the Least Nucleophilic Anion. A New, Strong Methyl⁺ Reagent

2002

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The icosahedral carborane anions H-CB11X6H5 - (X = Cl, Br, I) are among the most inert, least coordinating, and least basic anions known. These properties are enhanced by 2,3,4,5,6-pentamethylation with methyl triflate. The resulting anions, H-CB11X6Me5 -, are more inert than their unmethylated precursors, have improved NMR handles, and their salts have higher solubility in low dielectric media. They sustain superacidity in H(H-CB11X6Me5). Protonated benzene has been isolated and characterized by X-ray crystallography, moving Wheland intermediates from the status of spectroscopically observable transients to weighable reagents. The new anions sustain extreme Lewis acidity in silylium ion-like R3Si(H-CB11X6Me5) species. Treatment of Et3Si(H-CB11Br6Me5) with methyl triflate leads to a new methyl+ reagent CH3(H-CB11Br6Me5) that is more potent than methyl triflate. It methylates benzene without heating or acid catalysis to give the toluenium ion. The H-CB11X6Me5 - anions come as close as any to the concept of a univeral weakly coordinating anion and, with cheaper starting materials now available, promise to become specialty chemicals of wide usage.

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Isolation of Protonated Arenes (Wheland Intermediates) with BAr^F and Carborane Anions. A Novel Crystalline Superacid

Cited by 151 publications

References 19 publications

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Reversal of H₂O and OH^- Ligand Field Strength on the Magnetochemical Series Relative to the Spectrochemical Series. Novel 1-equiv Water Chemistry of Iron(III) Tetraphenylporphyrin Complexes

Optimizing the Least Nucleophilic Anion. A New, Strong Methyl⁺ Reagent

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Isolation of Protonated Arenes (Wheland Intermediates) with BArF and Carborane Anions. A Novel Crystalline Superacid

Cited by 151 publications

References 19 publications

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Reversal of H2O and OH- Ligand Field Strength on the Magnetochemical Series Relative to the Spectrochemical Series. Novel 1-equiv Water Chemistry of Iron(III) Tetraphenylporphyrin Complexes

Optimizing the Least Nucleophilic Anion. A New, Strong Methyl+ Reagent

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Isolation of Protonated Arenes (Wheland Intermediates) with BAr^F and Carborane Anions. A Novel Crystalline Superacid

Reversal of H₂O and OH^- Ligand Field Strength on the Magnetochemical Series Relative to the Spectrochemical Series. Novel 1-equiv Water Chemistry of Iron(III) Tetraphenylporphyrin Complexes

Optimizing the Least Nucleophilic Anion. A New, Strong Methyl⁺ Reagent