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ABSTRACT: Hyperconjugative stabilization of positive charge in tertiary carbocations is the textbook explanation for their stability and low frequency νCH bands in their IR spectra have long been taken as confirming evidence. While this is substantiated in the gas phase by the very close match of the IR spectrum of argon-tagged t-butyl cation with that calculated under Cs symmetry, the situation in condensed phases is much less clear. The congruence of νCHmax of t-Bu(+) in superacid media (2830 cm(-1)) with that in the gas phase (2834 cm(-1)) has recently been shown to be accidental. Rather, νCHmax varies considerably as a function of counterion in a manner that reveals the presence of significant C-Hanion hydrogen bonding. This paper addresses the question of the relative importance of hyperconjugation versus H-bonding. We show by assigning IR spectra in the νCH region to specific C-H bonds in t-butyl cation that the low frequency νCHmax band in the IR spectrum of t-butyl cation, long taken as direct evidence for hyperconjugation, appears to be due mostly to H-bonding. The appearance of similar low frequency νCH bands in the IR spectra of secondary alkyl carboranes such as i-Pr(CHB11Cl11), which have predominant sp(3) centres rather than sp(2) centres (and are therefore less supportive of hyperconjugation), also suggests the dominance of H-bonding over hyperconjugation.
Organic & Biomolecular Chemistry 05/2013; · 3.70 Impact Factor
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ABSTRACT: Environmentally sensitive: A combination of C-H anion hydrogen bonding and hyperconjugative charge delocalization explains the sensitivity of the IR spectrum of the tert-butyl cation to its anion (see high-resolution X-ray structure with a CHB(11)Cl(11)(-) counterion). The νCH vibration of the cation scales linearly with the basicity of carborane anions on the νNH scale. The same also holds for the C(6)H(7)(+) benzenium ion.
Angewandte Chemie International Edition 08/2012; 51(36):9149-51. · 13.45 Impact Factor
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ABSTRACT: Oxatriquinanes are tricyclic oxonium ions which are known to possess remarkable solvolytic stability compared to simple alkyl oxonium salts. Their rigid, hemispherical structure presents an oxygen at the apex of three fused five-membered rings. While trivalent oxygen species like these have been well described in the literature, the ability of oxygen to enter into a fourth covalent bonding relationship has been visited in theory and suggested by the outcome of certain reactions conducted in superacidic media, but has never been established by the characterization of a stable, persistent R(3)OH(2+) or R(4)O(2+) ion. In this study, the nucleophilicity of the oxatriquinane oxygen was evaluated first by a series of protonation studies using the Brønsted superacid H(CHB(11)Cl(11)) both in the solid state and in liquid HCl solution. The interaction of the oxatriquinane oxygen with a bridging carbocation was also examined. A strong case could be made for the occurrence of hydrogen bonding between H(CHB(11)Cl(11)) and oxatriquinane using IR spectroscopy. Under the most forcing protonation conditions, the oxatriquinane ring is cleaved to give a bridged, dicationic, protonated tetrahydrofuran-carbenium ion.
Journal of the American Chemical Society 12/2011; 134(1):707-14. · 9.91 Impact Factor
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ABSTRACT: The commonly used triethylsilyl per-fluoro-tetraphenylborate salt, [Et(3)Si(+)][F(20)-BPh(4) (-)], has been misidentified. As prepared, the cation is a hydride-bridged silane adduct [R(3)Si-H-SiR(3) (+)]. Under favorable circumstances it can be an effective source of the triethylsilylium ion Et(3)Si(+) but in the absence of a stabilizing base the potent electrophilicity of Et(3)Si(+) decomposes the "inert" F(20)-BPh(4) (-) counterion.
Organometallics 09/2011; 30(17):4798-4800. · 3.96 Impact Factor
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ABSTRACT: The commonly used triethylsilyl perfluoro-tetraphenylborate salt, [Et3Si+][F20-BPh4–], has been misidentified. As prepared, the cation is a hydride-bridged silane adduct [R3Si–H–SiR3+]. Under favorable circumstances it can be an effective source of the triethylsilylium ion Et3Si+, but in the absence of a stabilizing base the potent electrophilicity of Et3Si+ decomposes the “inert” F20-BPh4– counterion.
08/2011;
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ABSTRACT: Subporphyrin borenium cations with a carborane counterion have been prepared by treatment of B-methoxy subporphyrins with the silylium reagent Et(3)Si(CH(6)B(11)Br(6)). In contrast to the distinctly domed subphthalocyanine borenium cation, a nearly planar structure with sp(2) hybridized boron is found in the X-ray structure of the triphenylsubporphyrin borenium cation. The cations exhibit absorption and fluorescence spectra that are quite similar to those of B-methoxy subporphyrins. B-phenyl subporphyrins were prepared in good yield by reaction of subporphyrin borenium cations with phenyllithium.
Journal of the American Chemical Society 08/2011; 133(31):11956-9. · 9.91 Impact Factor
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ABSTRACT: The protonation of chloroethane by the strongest known solid superacid, the carborane acid H(CHB(11)Cl(11)), has been studied by quantitative IR spectroscopic methods to track mass balance and uncover previously unobserved chemistry. In the first step, an intermediate EtCl·H(CHB(11)Cl(11)) species without full proton transfer to EtCl can be observed when d(5)-deuterated chloroethane is used. It rapidly eliminates HCl (but not DCl) to form ethyl carborane, Et(CHB(11)Cl(11)), which binds a second molecule of chloroethane to form the Et(2)Cl(+) chloronium ion. This undergoes a slower, previously unrecognized HCl elimination reaction to form a butyl carborane, Bu(CHB(11)Cl(11)), beginning an oligomerization process whereby unsymmetrical dialkylchloronium ions decompose to alkyl carboranes of formula Bu(C(2)H(4))(n)(CHB(11)Cl(11)) up to n = 4. Over time, a parallel competing process of de-oligomerization take place in the presence of free carborane acid that finishes with the formation of hexyl or butyl carboranes. Upon heating to 150 C, the final products are all converted to the remarkably stable tert-butyl cation carborane salt.
Journal of the American Chemical Society 06/2011; 133(22):8452-4. · 9.91 Impact Factor
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Angewandte Chemie International Edition 10/2010; 49(41):7519-22. · 13.45 Impact Factor
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ABSTRACT: Unlike the "parent" carborane anion CHB(11)H(11)(-), halogenated carborane anions such as CHB(11)H(5)Br(6)(-) can be readily C-functionalized in high yield and purity, enhancing their utility as weakly coordinating anions.
Inorganic Chemistry 06/2010; 49(11):4726-8. · 4.60 Impact Factor
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ABSTRACT: The carborane acid H(CHB(11)Cl(11)) reacts with chloroalkanes RCl to give isolable dialkylchloronium ion salts, [R(2)Cl][CHB(11)Cl(11)], that are stable at room temperature. X-ray crystal structures have been obtained for R = CH(3) and CH(2)CH(3), revealing bent cation structures with C-Cl-C angles of 101.5 and 105.8 degrees , respectively. The dimethylchloronium ion salt loses CH(3)Cl upon heating and forms sublimable CH(3)(CHB(11)Cl(11)), providing a clean synthetic route to an extremely potent electrophilic methylating agent. IR spectra of all species have been interpreted, including the C-Cl stretch in CH(3)-ClCHB(11)Cl(10).
Journal of the American Chemical Society 03/2010; 132(12):4062-3. · 9.91 Impact Factor
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ABSTRACT: The hydrogen ion in water, H(aq)(+), is a unique H(13)O(6)(+) entity that defines the boundary of positive-charge delocalization. Its central unit is neither a C(3v) H(3)O(+) Eigen-type ion nor a typical H(5)O(2)(+) Zundel-type ion. IR spectroscopy indicates that the H(13)O(6)(+) ion has an unexpectedly long central O...O separation (>2.43 A), showing that in comparison with the gas and solid phases, the environment of liquid water is uniquely proficient in delocalizing positive charge. These results will change the description of H(aq)(+) in textbooks of chemistry, and a more extensive delocalization of positive charge may need to be incorporated into descriptions of mechanisms of aqueous proton transport.
Journal of the American Chemical Society 02/2010; 132(5):1484-5. · 9.91 Impact Factor
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ABSTRACT: The hydrated carborane acid H(CHB(11)I(11)).8H(2)O crystallizes in nanometer-diameter tubes of H(aq)(+) enclosed by walls of carborane anions. Three different types of H(aq)(+) clusters are found in these tubes: a symmetrical H(13)O(6)(+) ion with an unusually elongated Zundel-type H(5)O(2)(+) core, two hydrated H(7)O(3)(+) ions, and an unprecedented H(17)O(8)(+) ion having a nearly square core. All of the H(aq)(+) cations show unexpectedly longer O...O separations than in discrete H(aq)(+) ions, indicating greater delocalization of positive charge. The centrosymmetric H(aq)(+) ions are linked via short H bonds, forming a true one-dimensional proton wire.
Journal of the American Chemical Society 11/2009; 131(48):17540-1. · 9.91 Impact Factor
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Christopher A Reed
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ABSTRACT: For decades, triflic acid, methyl triflate, and trialkylsilyl triflate reagents have served synthetic chemistry well as clean, strong electrophilic sources of H(+), CH(3)(+), and R(3)Si(+), respectively. However, a number of weakly basic substrates are unreactive toward these reagents. In addition, triflate anion can express undesired nucleophilicity toward electrophilically activated substrates. In this Account, we describe methods that replace triflate-based electrophilic reagents with carborane reagents. Using carborane anions of type CHB(11)R(5)X(6)(-) (R = H, Me, X; X = Br, Cl), members of a class of notably inert, weakly nucleophilic anions, significantly increases the electrophilicity of these reagents and shuts down subsequent nucleophilic chemistry of the anion. Thus, H(carborane) acids cleanly protonate benzene, phosphabenzene, C(60), etc., while triflic acid does not. Similarly, CH(3)(carborane) reagents can methylate substrates that are inert to boiling neat methyl triflate, including benzene, phosphabenzenes, phosphazenes, and the pentamethylhydrazinium ion, which forms the dipositive ethane analogue, Me(6)N(2)(2+). Methyl carboranes are also surprisingly effective in abstracting hydride from simple alkanes to give isolable carbocation salts, e.g., t-butyl cation. Trialkylsilyl carborane reagents, R(3)Si(carborane), abstract halides from substrates to produce cations of unprecedented reactivity. For example, fluoride is extracted from freons to form carbocations; chloride is extracted from IrCl(CO)(PPh(3))(2) to form a coordinatively unsaturated iridium cation that undergoes oxidative addition with chlorobenzene at room temperature; and silylation of cyclo-N(3)P(3)Cl(6) produces a catalyst for the polymerization of phosphazenes that functions at room temperature. Although currently too expensive for widespread use, carborane reagents are nevertheless of considerable interest as specialty reagents for making reactive cations and catalysts.
Accounts of Chemical Research 10/2009; 43(1):121-8. · 21.64 Impact Factor
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ABSTRACT: Acid remarks: The anhydrous diprotic boron acids H(2)(B(12)X(12)) (X = Cl, Br; see picture, B orange, X green) are the first examples of diprotic superacids and may be the strongest acids yet isolated. Both protons protonate benzene to give benzenium ion salts that are stable at room temperature. These acids owe their existence to the stability of the icosahedral B(12) cluster with its dinegative charge buried beneath a layer of halide substituents.
Angewandte Chemie International Edition 05/2009; 48(19):3491-3. · 13.45 Impact Factor
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ABSTRACT: Sly silyl caught in the act: Protonation of a mesitylene ring by the strongly acidic arenium carborane [CH(3)C(6)H(6)]- [CHB(11)Me(5)Br(6)] initiates a cascade reaction that results in a stable beta-silyl allyl cation (see picture, H yellow, C blue, silyl allyl group red). Remarkably, the driving force in the reaction suffices to disrupt a stable aromatic ring in favor of a cationic reactive intermediate.
Angewandte Chemie International Edition 05/2009; 48(21):3787-90. · 13.45 Impact Factor
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Angewandte Chemie 04/2009; 121(19):3543 - 3545.
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ABSTRACT: The nature of H(H2O)n(+) cations for n = 3-8 with weakly basic carborane counterions has been studied by IR spectroscopy in benzene and dichloroethane solution. Contrary to general expectation, neither Eigen-type H3O x 3 H2O(+) nor Zundel-type H5O2(+) x 4 H2O ions are present. Rather, the core species is the H7O3(+) ion.
Journal of the American Chemical Society 10/2008; 130(36):12128-38. · 9.91 Impact Factor
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ABSTRACT: The long sought hexamethylhydrazinium(2+) dication, Me(3)N-NMe(3)(2+), calculationally unstable towards "coulombic explosion" because of formal positive charges on adjacent N atoms, can be synthesized and isolated as a CHB(11)Cl(11)(-) carborane salt.
Dalton Transactions 10/2008; · 3.84 Impact Factor
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ABSTRACT: Silylium ion-like reagents, R3Siδ+(carboraneδ−), are successful where silver salts fail in abstracting chloride ion from IrCl(CO)(PPh3)2 in arene solvents. Replacement of chloride by a very weakly coordinating carborane anion such as CHB11H5Cl6− promotes unusually facile oxidative addition of chlorobenzene, giving the coordinatively unsaturated IrCl(CO)(C6H5)(PPh3)2]+ cation. Other arene solvents promote disproportionation with loss of a phosphine ligand to give labile Ir(I) cations of the type [Ir(arene)(CO)(PPh3)]+, having three latent vacant coordination sites.
02/2008;
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Angewandte Chemie International Edition 02/2008; 47(20):3801-4. · 13.45 Impact Factor
