Authors: Weixing Gu and Prof. Oleg Ozerov
Journal: Inorganic Chemistry
Affiliation: Department of Chemistry, Texas A&M University
Synthesis of [B12Cl12]2- from [B12H12]2-, each point on the icosahedron represents a Boron
Reactions catalyzed by transition metals are aided by the creation of open coordination sites on the metal, allowing substrates to interact with the metal atom directly. In order to ease this process, inorganic chemists have been engaged in the search for weakly or non coordinating anions with which to make transition metal catalysts. One weakly coordinating anion of recent interest has been a family of molecules derived from closo-dodecaborate dianion, [B12H12]2-. the structure of which is shown above.
As it turns out, [B12H12]2- is still too coordinating for some purposes, so syntheses have been designed to replace the hydrogens with halogen atoms. Unfortunately, these syntheses require extremely high temperatures, pressures, and the use of dangerous gases such as Cl2 and F2. The Ozerov group has been studying the applications of borane clusters in catalysis for a few years now, and they recently discovered a synthesis of halogenated dodecaborate without the use of chlorine gas or bomb reactors. They found that by refluxing closo-dodecaborate in a 1:1 mixture of acetonitrile:sulfuryl chloride for 24 hours, they could completely chlorinate closo-dodecaborate in relatively high yield. Interestingly, refluxing dodecaborate in 100% sulfuryl chloride gave a mess of products.
The Ozerov group developed a system for the catalytic hydrodefluorination of benzyl C-F bonds, which would be important in the development of systems for the disposal of fluorocarbons, which are refrigerants that persist in the environment and are “super greenhouse gases”. The inert dodecachlorododecaborate dianion is able to stabilize the extremely reactive cations needed to perform hydrodefluorination. The starting material is a salt of [B12Cl12]2- with triphenylmethyl (trityl) countercations. With the introduction of a molecule containing benzyl C-F bonds and triethylsilane (Et3SiH), the system replaces the benzyl fluorines with hydrogens. The proposed mechanism begins with the abstraction of a hydride (H–) by a trityl cation, producing a silyl cation, Et3Si+. This silyl cation abstracts an F– from the benzyl position of the fluorocarbon, creating a benzyl cation. The catalytic cycle is completed when the benzyl cation abstracts a H– from another Et3SiH. Using this system, the researchers could transform over 2000 equivalents of 1-(trifluoromethyl)pentafluorobenzene (vs. dodecaborate) to 1-(methyl)pentafluorobenzene in half an hour at nearly quantitatively at room temperature. At 80 oC, 2000 equivalents of p-(trifluormethyl)fluorobenzene were reduced to p-fluorotoluene nearly quantitatively in one hour.
The Ozerov group reported the same reaction with a slightly different, carborane ([HCB11X11]–, X = H, Cl, Br) anion. Dodecaborane is less expensive and easier to synthesize, but aides in hydrodefluorination about as effectively as carboranes. Further work will focus on attempting to perform hydrodefluorination on less activated C-F bonds such as aryl fluorides or alkyl fluorides.