Superacidity of Boron Acids H₂(B₁₂X₁₂) (X=Cl, Br)

Abstract

Keywords arenium ions; carboranes; IR spectroscopy; superacidic systems; weakly coordinating anions The finding that monoprotic carborane acids of the type H(CHB 11 X 11 ) (X=Cl, Br) are the strongest pure acids isolated to date [1,2] suggests that the analogous diprotic boron acids H 2 (B 12 X 12 ) may have comparable or even higher acidity. The hydrated acids [H(H 2 O) n + ] 2 [B 12 X 12 2− ] were prepared many years ago from aqueous solutions of their alkali metal salts using an ion-exchange resin in acid form [3] and were shown to be slightly stronger acids than H 2 SO 4 in aqueous solution. However, the anhydrous acids are unknown. Can they be synthesized? Are they superacids? Of further interest is that the B 12 H 12 2− ion starting material is considerably cheaper than the isoelectronic CB 11 H 12 − carborane ion, so there is potential for cost savings relative to carborane acids, which are too expensive for widespread use.An indication that the all-boron acids H 2 (B 12 X 12 ) should show comparable acid strengths to their analogous carborane acids H(CHB 11 X 11 ) comes from the position of their anions on the ν(NH) basicity scale. In this ranking, the N-H stretching frequencies of contact-ion-pair trioctylammonium salts Oct 3 N + -H⋯anion − are compared in CCl 4 solution. [2,4] The higher the ν(NH) frequency, the lower the basicity of the anion. As shown in Table 1, the B 12 Cl 12 2− salt has almost the same ν(NH) frequency as the CHB 11 Cl 11 − salt. This result is surprising inasmuch as the dinegative charge on the B 12 Cl 12 2− ion might have been expected to render it more basic than the uninegative CHB 11 Cl 11 − ion. It suggests that chloride substituents on both anions form an effective screen for negative charge that is delocalized and buried within the icosahedral cage. Similar conclusions can be drawn from the data on the B 12 Br 12 2− ion, although low solubility of its trioctylammonium salt in CCl 4 allow comparisons to be made only for crystalline salts. It is also clear from the data in Table 1 that the boron anions are much less basic than the (HSO 4 ) 2 2− ion, so their conjugate acids are expected to be much stronger than 100% H 2 SO 4 , whose acidity (H 0 =−12 on the Hammett scale) defines the onset of superacidity. ** We thank Prof. Dr. E. S. Stoyanov for invaluable assistance with the IR measurements and their interpretation and Dr. Bruno Donnadieu for X-ray crystallographic support. This work was supported by the National Science Foundation (CHE-039878 to C.A.R. and an NSF Graduate Fellowship to A.A.).

Synthesis and Applications of Perfunctionalized Boron Clusters

“…Muetterties and co-workers noted the extreme acidity of B 12 Cl 12 and B 12 Br 12 in the mid-1960s in which the authors disclosed their superior acidity to sulfuric acid. 53 More recently, experimental 106,107 and theoretical 107,108 efforts have been made to further quantify the (potential) acidity of such derivatives. In particular, Reed and co-workers showed 106 that B 12 Cl 12 and B 12 Br 12 exhibit comparable acidity to carborane acids and are sufficiently acidic to protonate benzene, which even trifluoromethanesulfonic acid and fluorosulfonic acid cannot do.…”

Metalltriflimidate sind bessere Katalysatoren für die organische Synthese als Metalltriflate – der Effekt eines stark delokalisierten Gegenions

“…[237] Es ist vorstellbar, dass die konjugierten Basen dieser neuen Supersäuren bald als Gegenionen von stark elektrophilen Metallkatalysatoren verwendet werden. Neben Perfluorsulfonylund Perfluorsulfonylimingruppen werden auf diesem Gebiet vermutlich stark delokalisierte Borspezies wie Natrium-tet- [238] oder Derivate der diprotischen Säure H 2 (B 12 X 12 ) [239] zukünftig von großem Interesse sein.…”

Synthesis and Properties of the Weakly Coordinating Anion [Me3NB12Cl11]−

“…Among the weakly coordinating anions, the boron-clusterbased 1-carba-closo-dodecaborate anions [1-R-CB 11 Y 5 X 6 ] À (R = H, alkyl; Y = H, Me, Hal; X = H, Me, Hal) and the closo-dodecaborate anions [B 12 X 12 ] 2À (X = H, Hal) exhibit exceptional chemical, electrochemical, and thermal stability. [1] They stabilize highly reactive cations (for example, [R 3 Si] + , [2] [R 2 Al] + , [3] [C 60 ] + [4] ), are part of the Brønsted superacids H(R-CB 11 X 5 Y 6 ) [4] or H 2 (B 12 X 12 ), [5] respectively, and form the neutral compounds 1-R-CB 11 X 5 Y 6 (for example, for R, X, Y = Me; [6] R = H, X = F or CF 3 , Y = CF 3 or F [7] ) or B 12 X 12 , [8] which are themselves very strong oxidizing agents. Because of their superior chemical and physical properties the halogenated carborates belong to the most weakly coordinating anions.…”