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Brookhart's acid is the salt of the
diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...
oxonium ion In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge. The simplest oxonium ion is the hydronium ion (). Alkyloxonium Hydronium is one of a series of oxonium ions with the formula R''n' ...
and tetrakis ,5-bis(trifluoromethyl)phenylorate (BAr′4). It is a colorless solid, used as a strong acid. The compound was first reported by Volpe, Grant, and Brookhart in 1992.


Preparation

This compound is prepared by treatment of NaBAr′4 in
diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...
(Et2O) with
hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
: : NaBAr′4 + HCl + 2 Et2O → (OEt2)2sup>+ + NaCl NaBAr′4 is soluble in diethyl ether, whereas sodium chloride is not. Precipitation of sodium chloride thus drives the formation of the oxonium acid compound, which is isolable as a solid.


Structure and properties

The acid crystallizes as a white, hygroscopic crystalline solid.
NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
and
elemental analysis Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals, chemical compounds) is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualita ...
showed that the crystal contains two equivalents of diethyl ether. In solution, the compound slowly degrades to ''m''-C6H3(CF3)2 and BAr′3. (OEt2)2B(C6F5)4] is a related compound with a slightly different weakly coordinating anion; it was first reported in 2000. An X-ray crystal structure of that compound was obtained, showing the acidic proton coordinated by both ethereal oxygen centers, although the crystal was not good enough to determine whether the proton is located symmetrically or unsymmetrically between the two.Jutzi, P.; Müller, C.; Stammler, A.; Stammler, H. G. (2000). "Synthesis, Crystal Structure, and Application of the Oxonium Acid (OEt2)2sup>+B(C6F5)4]". Organometallics vol. 19, p. 1442.


Uses

Traditional weakly coordinating anions, such as
perchlorate A perchlorate is a chemical compound containing the perchlorate ion, . The majority of perchlorates are commercially produced salts. They are mainly used as oxidizers for pyrotechnic devices and to control static electricity in food packaging. Per ...
,
tetrafluoroborate Tetrafluoroborate is the anion . This tetrahedral species is isoelectronic with tetrafluoroberyllate (), tetrafluoromethane (CF4), and tetrafluoroammonium () and is valence isoelectronic with many stable and important species including the perchlo ...
, and
hexafluorophosphate Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this an ...
, will nonetheless coordinate to very electrophilic cations, making these counterions unsuitable for some complexes. The highly reactive species p2Zr(CH3)sup>+, for example, has been reported to abstract F from PF6. Starting in the 1980s, new types of weakly coordinating anions began to be developed. BAr′4 anions are used as counterions for highly electrophilic, cationic transition metal species, as they are very weakly coordinating and unreactive towards electrophilic attack. One common method of generating these cationic species is via
protonolysis Protonolysis is the cleavage of a chemical bond by acids. Many examples are found in organometallic chemistry since the reaction requires polar Mδ+-Rδ- bonds, where δ+ and δ- signify partial positive and negative charges associated with the bon ...
of a dialkyl complexes or an olefin complex. For example, an electrophilic
palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
catalyst, CH3CN).html" ;"title="acetonitrile.html" ;"title="2,2′-bipyridine)Pd(CH3)(acetonitrile">CH3CN)">acetonitrile.html" ;"title="2,2′-bipyridine)Pd(CH3)(acetonitrile">CH3CN)BAr′4], is prepared by protonating the dimethyl complex with Brookhart's acid. This electrophilic, cationic palladium species is used for the polymerization of olefins with
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
to polyketones in aprotic solvents.


Potential application

Polyketone Polyketones are a family of high-performance thermoplastic polymers. The polar ketone groups in the polymer backbone of these materials gives rise to a strong attraction between polymer chains, which increases the material's melting point (255&nbs ...
s,
thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s, are formed by the copolymerisation of
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
and one or more
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s (typically ethylene with
propylene Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
). The process utilises a
palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
(II)
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
with a bidentate ligand like 2,2′-bipyridine or 1,10-phenanthroline (phen) with a non-coordinating BARF counterion, such as phen)Pd(CH3)(CO)ArF4. The preparation of the catalyst involves the reaction of a dimethyl palladium complex with Brookhart's acid in acetonitrile with loss of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
and the catalytic species is formed by uptake of carbon monoxide to displace acetonitrile. : Et2O)2HArF4   +   phen)Pd(CH3)2  +   MeCN   →   phen)Pd(CH3)(MeCN)ArF4   +   2 Et2O   +   CH4 : phen)Pd(CH3)(MeCN)ArF4   +   CO   → phen)Pd(CH3)(CO)ArF4   +   MeCN The mechanism involves
migratory insertion In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanis ...
whereby the polymer chain is bound to the catalytic centre and grows by the sequential insertion of carbon monoxide and the alkene between the palladium atom and the existing chain. Defects occur when insertions do not alternate – that is, a carbon monoxide insertion follows a carbon monoxide insertion or an alkene insertion follows an alkene insertion – these are highlighted in red in the figure below. This catalyst produces a very low rate of defects due to the difference in Gibbs energy of activation of each insertion – the
energy barrier In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...
to inserting an alkene immediately following an alkene insertion is ~12 kJ mol−1 higher than barrier to carbon monoxide insertion. Use of monodentate
phosphine Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
ligands also leads to undesirable side-products but bidentate phosphine ligands like 1,3-bis(diphenylphosphino)propane have been used industrially.


References

{{Reflist Acids Non-coordinating anions Trifluoromethyl compounds Oxonium compounds