Relative Abilities of Fluorine and Chlorine to Stabilize Carbenium Ions:  Crystal Structures of Two Fluoro-Substituted Carbocations and of As2F11-

Christe, Karl O.; Zhang, Xiongzhi; Bau, Robert; Hegge, Joachim; Oláh, George A.; Prakash, G. K. Surya; Sheehy, Jeffrey A.

doi:10.1021/ja9933137

Cited by 86 publications

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“…The cations (m-CF 3 -C 6 H 4 )(C 6 H 5 )CF + and (CH 3 ) 2 CF + were isolated at low temperature from superacid media and characterized as fluoroarsenate salts. 5 Given the ability of carborane counterions to stabilize carbocations such t-butyl cation for X-ray characterization at room temperature, 6 it is of interest to investigate whether carborane anions would confer comparable stability on fluorinated carbocations. Fluoride ion abstraction from a fluorocarbon using a trialkylsilyl carborane agent is a logical extension of successful chloride and triflate ion abstraction reactions with R 3 Si(carborane) in the formation of H(carborane) superacids from HCl and highly electrophilic alkylating reagents, R(carborane), from alkyl triflates, respectively.…”

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confidence: 99%

“…In summary, this work shows that the use of carborane counterions instead of those from traditional superacids (fluoroarsenates and fluorantimonates) allows the intrinsic stability of fluorinated carbocations to be demonstrated. Corrosive media such as AsF 5 or SbF 5 can be avoided and long lived fluorinated carbocations can be produced in common organic solvents. The good crystallizing properties of carborane salts allow single crystals to be readily grown for X-ray characterization and IR analysis.…”

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Isolating fluorinated carbocations

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Isolating fluorinated carbocations

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“…The solid-state structures of several salts of related cations such as Me 2 CF + , [10] C(OH) 3 + , and (H 3 CO)C(H)X + (X = F, Cl) have been structurally characterized.[11] The CX 3 + ions of the heavier homologues (X = Cl-I) were characterized by 13 C NMR spectroscopy [2a,e, 12] as long-lived intermediates at À78 8C in SO 2 ClF solution [Eq. (1)].…”

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Structure and Characterization of CI₃⁺[Al{OC(CF₃)₃}₄]⁻; Lewis Acidities of CX₃⁺and BX₃

et al. 2003

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There has been continuing interest in the elicidation of the properties of reactive small carbon-containing cations [1] such as CX 3 +[2-4] or OCX + [5][6][7][8] (X = F-I), which are frequently observed in fragmentation reactions of gaseous ions in the mass spectrometer [9] and are thought to be reactive intermediates in organic synthesis. The solid-state structures of several salts of related cations such as Me 2 CF + , [10] C(OH) 3 + , and (H 3 CO)C(H)X + (X = F, Cl) have been structurally characterized.[11] The CX 3 + ions of the heavier homologues (X = Cl-I) were characterized by 13 C NMR spectroscopy [2a,e, 12] as long-lived intermediates at À78 8C in SO 2 ClF solution [Eq. (1)].Recent computational work [3,4,12] revealed that, in contrast to earlier [2a] conclusions, cations bearing the heavier halogens should be thermodynamically more stable and less electrophilic than the F and Cl homologues. However, despite the strong s-acceptor effect of the electronegative fluorine atoms there is sufficient p donation to stabilize the CF 3 + ion over the parent methenium ion CH 3 + .[3] By contrast, the heavier halogens Cl, Br, and I in CX 3 + ions were shown to be s and p donors towards the C atom.[3] For gaseous CX 3 + it was suggested [3] that the partial C¼X + double-bond character also remained for X = I. To test the influence of p bonding on the solid-state structure of the CX 3 + ions it was therefore desirable to prepare a stable salt of these species, in particular for the CI 3 + ion which would include an unusual partial 2p-5p double bond. [13] Recently we reported on nonoxidizing weakly coordinating anions of the type Al(OR) 4 À (R = C(CF 3 ) 3 ) [14,15] and their ability to stabilize gas-phase cations such as Ag(h 2 -P 4 ) 2and Ag(h 4 -S 8 ) 2 +[17] as well as the highly electrophilic binary P-X cations PX 4 + , P 2 X 5 + , P 3 I 6 + , and P 5 X 2 + , which arose through halide-ion abstraction from Ag[Al(OR) 4 ] and PX 3 (X = Br, I).[ [19] had to be performed with the complete exclusion of light and with carefully purified diiodine-free CI 4 , since CI 4 decomposes within minutes in solution in light with formation of I 2 and other carbon iodides, and Ag[Al(OR) 4 ] reacts immediately with I 2 to give yet unidentified decomposition products.[20] With these precautions CI 3À (1) was prepared in quantitative yield. [19] According to calculations at the MP2/TZVPP level and estimation of the sublimation and lattice enthalpies in a suitable cycle, we estimate that Equation (2) [12] ) and the anion (d( 13 C) = 121.5 ppm (q); d( 27 Al) = 38.0 ppm). A CH 2 Cl 2 solution of 1 is stable for about 8 to 12 h at ambient temperature and much longer if stored at À30 or À78 8C. Single crystals that appeared yellow as thin plates but black when thicker were grown at À78 8C from cooled concentrated CH 2 Cl 2 solutions of 1. The crystal structure determination [25] of a dark orthorhombic block of 1 showed that it contained isolated trigonal-planar CI 3 + ions with a sum of I-C-I bond angles of 360.08 (range: 118.8-12...

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“…Die Festkörperstrukturen einiger Salze von verwandten Kationen wie Me 2 CF + , [10] C(OH) 3 + und (H 3 CO)C(H)X + (X = F, Cl) konnten bereits aufgeklärt werden. [11] Von den CX 3 + -Ionen (X = F-I) hingegen wurden nur die schwereren Homologen mit X = Cl-I als langlebige Intermediate bei À78 8C in SO 2 ClF-Lösung 13 C-NMR-spektroskopisch charakterisiert (Reaktion (1)).…”

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“…[22] Alle drei Iodatome in 1 bilden drei bis fünf I-F-Kontakte zu den CF 3 -Gruppen des Anions mit Abständen d(I-F) von 328.9 bis 358.2 pm und einem durchschnittlichen Abstand der 13 I-F-Kontakte von 344.5 pm (Summe der Van-der-Waals-Radien I + F: 360 pm). Hingegen ist der kleinste C-F-Abstand mit 326.4 pm größer als die Summe der Van-der-Waals-Radien von C und F (320 pm gen zwei starke C-F-Kontakte zum AsF 6 À -Gegenion (d(C-F) = 266 und 278 pm), [10] was dafür spricht, dass zumindest ein Teil der positiven Ladung am C + -Carbeniumzentrum verbleibt. Die Schwingungsspektren von 1 [27] belegen zudem das Vorhandensein starker C-I-Bindungen: Die Frequenz der C-I-Streckschwingung (e'-Symmetrie) beträgt 739 cm À1 , die Vergleichsmode in 11 …”

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Struktur und Charakterisierung von CI₃⁺[Al{OC(CF₃)₃}₄]⁻; Lewis‐Aciditäten von CX₃⁺und BX₃

et al. 2003

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Fast ausschließlich auf den Iod‐Atomen, und nicht am formalen Carbeniumzentrum A, ist die positive Ladung des CI3+‐Ions in CI3+[Al{OC(CF3)3}4]− lokalisiert (siehe Resonanzstrukturen B–D). Rechnungen und der experimentellen Charakterisierung zufolge ist die Lewis‐Säurestärke der CX3+‐Homologen (X=F–I) gegen Fluoridionen umgekehrt abgestuft als die der isoelektronischen Borhalogenide BX3 und nimmt demzufolge von CF3+ nach CI3+ ab.

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Relative Abilities of Fluorine and Chlorine to Stabilize Carbenium Ions: Crystal Structures of Two Fluoro-Substituted Carbocations and of As₂F₁₁^-

Cited by 86 publications

References 43 publications

Isolating fluorinated carbocations

Isolating fluorinated carbocations

Structure and Characterization of CI₃⁺[Al{OC(CF₃)₃}₄]⁻; Lewis Acidities of CX₃⁺and BX₃

Struktur und Charakterisierung von CI₃⁺[Al{OC(CF₃)₃}₄]⁻; Lewis‐Aciditäten von CX₃⁺und BX₃

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Relative Abilities of Fluorine and Chlorine to Stabilize Carbenium Ions: Crystal Structures of Two Fluoro-Substituted Carbocations and of As2F11-

Cited by 86 publications

References 43 publications

Isolating fluorinated carbocations

Isolating fluorinated carbocations

Structure and Characterization of CI3+[Al{OC(CF3)3}4]−; Lewis Acidities of CX3+and BX3

Struktur und Charakterisierung von CI3+[Al{OC(CF3)3}4]−; Lewis‐Aciditäten von CX3+und BX3

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Relative Abilities of Fluorine and Chlorine to Stabilize Carbenium Ions: Crystal Structures of Two Fluoro-Substituted Carbocations and of As₂F₁₁^-

Structure and Characterization of CI₃⁺[Al{OC(CF₃)₃}₄]⁻; Lewis Acidities of CX₃⁺and BX₃

Struktur und Charakterisierung von CI₃⁺[Al{OC(CF₃)₃}₄]⁻; Lewis‐Aciditäten von CX₃⁺und BX₃