Superacidity of boron acids H2(B12X12) (X = Cl, Br).
Superacidity of boron acids H2(B12X12) (X = Cl, Br).
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DOI：
10.1002/anie.200900214
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发表时间：
2009
影响因子：
16.6
通讯作者：
Reed, Christopher A.
中科院分区：
文献类型：
--
作者：
Avelar, Amy;Tham, Fook S.;Reed, Christopher A.
H(CHB11X11)(X=Cl，Br1)是迄今为止分离出的最强的纯酸，这一发现表明类似的二质子硼酸H2(B12X12)的酸度可能与之相当，甚至更高。水合酸[H(H2O)n+]2[B12X12 2]是多年前用酸形式的离子交换树脂从它们的碱金属盐水溶液中制备的，在水溶液中的酸比H_2SO_4略强。然而，无水酸是未知的。它们能被合成吗？它们是超强酸吗？更令人感兴趣的是，B12H12 2离子的起始材料比等电子的CB11H12碳硼离子便宜得多，因此相对于碳硼酸来说，有可能节省成本，因为碳硼酸太贵了，不能广泛使用。全硼酸H2(B12X12)应表现出与其类似碳硼酸H(CHB11X11)相当的酸强度的指示来自于它们的阴离子在ν(NH)碱性标度上的位置。在这个排名中，比较了接触离子对三辛基铵盐OCT3N+H····阴离子在四氯化碳溶液中的N-H伸缩频率。[2，4]ν(NH)频率越高，阴离子的碱性越低。如表1所示，B12Cl12+盐与CHB11Cl11+盐具有几乎相同的ν(NH)频率。这一结果令人惊讶，因为B12Cl12 2离子上的负电荷可能会使它比非负离子CHB11Cl11离子更具碱性。这表明，两个阴离子上的氯取代基对负电荷形成了有效的屏蔽，负电荷被离域并掩埋在二十面体笼子中。从B12Br12-2离子的数据中也可以得出类似的结论，尽管它的三辛基铵盐在CCl4中的低溶解度使得只能对结晶盐进行比较。从表1中的数据还可以清楚地看到，硼阴离子的碱性比(HSO4)2 2-离子弱得多，因此它们的共轭酸预计会比100%的H_2SO_4强得多，后者的酸度(H0=0.12在Hammett标度上)定义了超酸的开始。无水二元酸的合成途径采用了类似于制备H(CHB11Cl11)的多步歧化反应。[1]从B12Br12-2和B12Cl12-2的银盐开始，合成了它们各自的三丁基(C-(C6H5)3+)盐[式(1)]和三乙基硅化合物[式(2)]，并用X-射线结晶学进行了表征。
The finding that monoprotic carborane acids of the type H (CHB11X11)(X= Cl, Br) are the strongest pure acids isolated to date [1, 2] suggests that the analogous diprotic boron acids H2 (B12X12) may have comparable or even higher acidity. The hydrated acids [H (H2O) n+] 2 [B12X12 2Ā] 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 H2SO4 in aqueous solution. However, the anhydrous acids are unknown. Can they be synthesized? Are they superacids? Of further interest is that the B12H12 2Ā ion starting material is considerably cheaper than the isoelectronic CB11H12 Ā 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 H2 (B12X12) should show comparable acid strengths to their analogous carborane acids H (CHB11X11) comes from the position of their anions on the ν (NH) basicity scale. In this ranking, the NĀH stretching frequencies of contact-ion-pair trioctylammonium salts Oct3N+ĀH··· anionĀ are compared in CCl4 solution.[2, 4] The higher the ν (NH) frequency, the lower the basicity of the anion. As shown in Table 1, the B12Cl12 2Ā salt has almost the same ν (NH) frequency as the CHB11Cl11 Ā salt. This result is surprising inasmuch as the dinegative charge on the B12Cl12 2Ā ion might have been expected to render it more basic than the uninegative CHB11Cl11 Ā 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 B12Br12 2Ā ion, although low solubility of its trioctylammonium salt in CCl4 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 (HSO4) 2 2Ā ion, so their conjugate acids are expected to be much stronger than 100% H2SO4, whose acidity (H0= Ā12 on the Hammett scale) defines the onset of superacidity. The synthetic pathway to the anhydrous diprotic acids exploits multistep metathesis reactions similar to those used in the preparation of H (CHB11Cl11).[1] Starting with the silver salts of B12Br12 2Ā and B12Cl12 2Ā, their respective trityl (C-(C6H5) 3+) salts [Eq.(1)] and triethylsilylium compounds [Eq.(2)] have been prepared and characterized by X-ray crystallography.[5]
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