Non-coordinating anion
Encyclopedia
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is 'weakly coordinating anion'. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes with an unsaturated coordination sphere
. These special anions are essential components of homogeneous
olefin polymerisation catalysts
, where the active catalyst is a coordinatively unsaturated, cationic transition metal
complex. For example, they are employed as counterions for the 14 valence electron
cations [(C5H5)2ZrR]+ (R = methyl or a growing polyethylene chain). Complexes derived from noncoordinating anions have been used to catalyze hydrogenation
, hydrosilylation
, oligomerization, and the living polymerization
of olefins. The popularization of non-coordinating anions has contributed to increased understanding of agostic complexes wherein hydrocarbons and hydrogen serve as ligands. Noncoordinating anions are important components of many superacid
s, which result from the combination of Bronsted acids and Lewis acid
s.
, [PF6]−
, and [ClO4]−
were considered weakly coordinating anions. These species are now known to bind well to electrophilic metal centers. Tetrafluoroborate and hexafluorophosphate anions are coordinating toward highly electrophilic metal ions, such as cations containing Zr(IV) centers, which can abstract fluoride
from these anions. Other anions, such as triflate
s were considered to be low-coordinating with some cations.
BF4−, hexafluorophosphate
PF6−, and perchlorate
ClO4−, and consequently has enabled the study of still more electrophilic cations. Related tetrahedral anions include tetrakis(pentafluorophenyl)borate
, B(C6F5)4- and Al(OC(CF3)3)4-.
In the bulky borates and aluminates, the negative charge
is symmetrically distributed over many electronegative atoms. Related anions are derived from tris(pentafluorophenyl)boron
B(C6F5)3. Another advantage of these anions is that their salts are more soluble in non-polar organic solvents such as dichloromethane
, toluene
, and, in some cases, even alkane
s. Polar solvents, such as acetonitrile
, THF
, and water
tend to bind to electrophilic centers, in which cases, the use of a noncoordinating anion is pointless.
Salts of the anion B[3,5-(CF3)2C6H3]4− were first reported by Kobayashi and co-workers. For that reason, it is sometimes referred to as Kobayashi's anion. Kobayashi's method of preparation has been superseded by a safer route.
The neutral molecules that represent the parents to the non-coordinating anions are strong Lewis acids, e.g. boron trifluoride
, BF3 and phosphorus pentafluoride
, PF5. A notable Lewis acid of this genre is tris(pentafluorophenyl)boron
, B(C6F5)3, which abstracts alkyl ligand
s:2Zr(CH3)2 + B(C6F5)3 → [(C5H5)2Zr(CH3)]+[(CH3)B(C6F5)3]-
anion CB11H12-. Using this anion, the first example of a three-coordinate silicon compound, the salt [(mesityl)3Si][HCB11Me5Br6] contains a noncoordinating anion derived from a carborane
.
18-Electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...
. These special anions are essential components of homogeneous
Homogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid catalysis:...
olefin polymerisation catalysts
Ziegler-Natta catalyst
A Ziegler–Natta catalyst is a catalyst used in the synthesis of polymers of 1-alkenes . Three types of Ziegler–Natta catalysts are currently employed:* Solid and supported catalysts based on titanium compounds...
, where the active catalyst is a coordinatively unsaturated, cationic transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
complex. For example, they are employed as counterions for the 14 valence electron
Electron counting
Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many rules in chemistry rely on electron-counting:...
cations [(C5H5)2ZrR]+ (R = methyl or a growing polyethylene chain). Complexes derived from noncoordinating anions have been used to catalyze hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
, hydrosilylation
Hydrosilylation
Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds. Ordinarily the reaction is conducted catalytically and usually the substrates are unsaturated organic compounds. Alkenes and alkynes give alkyl and vinyl silanes; aldehydes and...
, oligomerization, and the living polymerization
Living polymerization
In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is...
of olefins. The popularization of non-coordinating anions has contributed to increased understanding of agostic complexes wherein hydrocarbons and hydrogen serve as ligands. Noncoordinating anions are important components of many superacid
Superacid
According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...
s, which result from the combination of Bronsted acids and Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
s.
Pre-"BARF" era
Prior to the 1990s, [BF4]−Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...
, [PF6]−
Hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . This octahedral species is isoelectronic with sulfur hexafluoride, SF6, and is valence isoelectronic with the highly stable superacid anion fluoroantimonate . As a non-coordinating anion, it is a poor nucleophile...
, and [ClO4]−
Perchlorate
Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...
were considered weakly coordinating anions. These species are now known to bind well to electrophilic metal centers. Tetrafluoroborate and hexafluorophosphate anions are coordinating toward highly electrophilic metal ions, such as cations containing Zr(IV) centers, which can abstract fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
from these anions. Other anions, such as triflate
Triflate
Trifluoromethanesulfonate, also known by the trivial name triflate, is a functional group with the formula CF3SO3-. The triflate group is often represented by -OTf, as opposed to -Tf...
s were considered to be low-coordinating with some cations.
Era of BARF
A revolution in this area occurred in the 1990s with the introduction of the [B[3,5-(CF3)2C6H3]4]− anion, commonly abbreviated as [BArF4]− and affectionately called "BARF." This anion is far less coordinating than the tetrafluoroborateTetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...
BF4−, hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . This octahedral species is isoelectronic with sulfur hexafluoride, SF6, and is valence isoelectronic with the highly stable superacid anion fluoroantimonate . As a non-coordinating anion, it is a poor nucleophile...
PF6−, and perchlorate
Perchlorate
Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...
ClO4−, and consequently has enabled the study of still more electrophilic cations. Related tetrahedral anions include tetrakis(pentafluorophenyl)borate
Lithium tetrakis(pentafluorophenyl)borate
Lithium tetrakisborate is the lithium salt of the weakly coordinating anion -. Because of its weakly coordinating abilities, lithium tetrakisborate makes it commercially valuable in the salt form for the starting material in polymerization reactions and electrochemistry. It is a water-soluble...
, B(C6F5)4- and Al(OC(CF3)3)4-.
In the bulky borates and aluminates, the negative charge
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
is symmetrically distributed over many electronegative atoms. Related anions are derived from tris(pentafluorophenyl)boron
Tris(pentafluorophenyl)boron
Trisboron is the chemical compound 3B. The molecule consists of three pentafluorophenyl groups attached in a "paddle-wheel" manner to a central boron atom; the BC3 core is planar. It has been described as the “ideal Lewis acid” because of its versatility and the relative inertness of the B-C bonds...
B(C6F5)3. Another advantage of these anions is that their salts are more soluble in non-polar organic solvents such as dichloromethane
Dichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
, toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
, and, in some cases, even alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
s. Polar solvents, such as acetonitrile
Acetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
, THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
, and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
tend to bind to electrophilic centers, in which cases, the use of a noncoordinating anion is pointless.
Salts of the anion B[3,5-(CF3)2C6H3]4− were first reported by Kobayashi and co-workers. For that reason, it is sometimes referred to as Kobayashi's anion. Kobayashi's method of preparation has been superseded by a safer route.
The neutral molecules that represent the parents to the non-coordinating anions are strong Lewis acids, e.g. boron trifluoride
Boron trifluoride
Boron trifluoride is the chemical compound with the formula BF3. This pungent colourless toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds.-Structure and bonding:...
, BF3 and phosphorus pentafluoride
Phosphorus pentafluoride
Phosphorus pentafluoride, PF5, is a phosphorus halide. It's a colourless gas at room temperature and pressure.-Structure:Single-crystal X-ray studies indicate PF5 molecule has two distinct P−F bonds : P−Fax = 158.0 pm and P−Feq = 152.2 pm...
, PF5. A notable Lewis acid of this genre is tris(pentafluorophenyl)boron
Tris(pentafluorophenyl)boron
Trisboron is the chemical compound 3B. The molecule consists of three pentafluorophenyl groups attached in a "paddle-wheel" manner to a central boron atom; the BC3 core is planar. It has been described as the “ideal Lewis acid” because of its versatility and the relative inertness of the B-C bonds...
, B(C6F5)3, which abstracts alkyl ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s:2Zr(CH3)2 + B(C6F5)3 → [(C5H5)2Zr(CH3)]+[(CH3)B(C6F5)3]-
Other types of non-coordinating anions
Another large class of non-coordinating anions are derived from carboraneCarborane
A carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc...
anion CB11H12-. Using this anion, the first example of a three-coordinate silicon compound, the salt [(mesityl)3Si][HCB11Me5Br6] contains a noncoordinating anion derived from a carborane
Carborane
A carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc...
.