The Inorganic, Bioinorganic and Organometallic Chemistry Program supports the work of Professor Christopher A. Reed of the University of California - Riverside on reactivity and mechanistic studies of the carborane anions. Carborane acids, such as H(CHB11Cl11), are the strongest known pure protic acids, yet they are also the gentlest - because their conjugate base anions have low nucleophilicity and are inert towards redox processes. This work focuses on two questions related to carborane activity: What processes require the extreme inertness of carborane anions above all others? and What mechanisms can be uniquely studied because of the special properties of carborane anions? The knowledge gained from this project has ramifications for commercially important areas such as petroleum refining, polymerization catalysis, batteries and fuel cell technologies. Postdoctoral, graduate and undergraduate students working on the project regularly collaborate with scientists from Russia, Germany and the United Kingdom.
Carborane anions, such as CHB11Cl11- shown in Figure 1, are amongst the most chemically inert and least reactive negatively-charged molecules (i.e. anions) presently known. As such, they are ideal partners for stabilizing highly reactive or unstable cations (i.e., positively-charged molecules). Resulting carborane salts of reactive cations are often stable at room temperature, enabling ready characterization and study that was heretofore unavailable. Two examples from the current grant period involve the isolation of a novel borenium ion (a three-coordinate boron cation) and a reactive allyl cation. Carborane anions are not only chemically inert but they are very weakly basic. This means that their corresponding acids, H(carborane), are extremely strong. They are superacids. Indeed, the conjugate acid prepared with the carborane anion shown in Figure 1 is currently the world's strongest acid. This has been demonstrated in all phases: gas, liquid and solid. Somewhat ironically, it is also one of the gentlest acids known. This is because the carborane anion is so chemically inert that it does not engage in any of the corrosive redox chemistry typical of the anions found in most other acids (e.g., sulfuric and triflic acids). Thus, we have been able to use carborane acids (as well as carborane reagents based on methyl or trialkysilyl cations) to carry out chemistry that could not be performed with commonly available reagents based on triflate or sulfate anions. We discovered that carborane acids are so strong that they protonate chlorocarbon solvents, turning them into hydrocarbons via carbocation intermediates. This could be useful for dealing with toxic chlorocarbon solvent waste. The problem with carborane reagents, however, is their high cost. They are too expensive for common usage. Thus, the challenge is now to find ways of using them in highly efficient catalytic processes. Other scientists have picked up this challenge and are making progress along these lines. A very old problem in chemistry has been to understand the nature of H+ when an acid dissolves in water. Using carborane acids, we have shown (a) that H+ has six water molecules surrounding it, (b) that the structure must be quite symmetrical based on the simplicity of its vibrational spectrum, (c) that there is also a unique "continuous broad absortion" in its infrared spectrum indicative of extremely fast H+ motion, and (d) the O...O distances are unusually long and without precedent. Indeed, the structure appears to be unique to H+ in water. The results will change the way H+ is presented in textbooks. The findings also present a real challenge for theory.
