Tris(pentafluorophenyl)borane

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Tris(pentafluorophenyl)borane
Tris(pentafluorophenyl)boron-2D-skeletal.png
Tris(pentafluorophenyl)boron-3D-vdW.png
Names
Preferred IUPAC name
Tris(pentafluorophenyl)borane
Other names
Perfluorotriphenylboron
Tris(pentafluorophenyl)boron
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.101.316 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
UNII
  • InChI=1S/C18BF15/c20-4-1(5(21)11(27)16(32)10(4)26)19(2-6(22)12(28)17(33)13(29)7(2)23)3-8(24)14(30)18(34)15(31)9(3)25 Yes check.svgY
    Key: OBAJXDYVZBHCGT-UHFFFAOYSA-N Yes check.svgY
  • B(c1c(c(c(c(c1F)F)F)F)F)(c2c(c(c(c(c2F)F)F)F)F)c3c(c(c(c(c3F)F)F)F)F
Properties
C18F15B
Molar mass 511.98 g/mol
Appearancecolorless solid
Melting point 126 to 131 °C (259 to 268 °F; 399 to 404 K)
forms adduct
Structure
trigonal planar
0 D
Hazards [1]
GHS pictograms GHS-pictogram-exclam.svg
GHS Signal word Danger
H315, H319, H335
P261, P280, P302+P352, P305+P351+P338
Related compounds
Related compounds
Triphenylborane (C6H5)3B
BF3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Tris(pentafluorophenyl)borane, sometimes referred to as "BCF", is the chemical compound (C6F5)3B. It is a white, volatile solid. 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 high thermal stability and the relative inertness of the B-C bonds. Related fluoro-substituted boron compounds, such as those containing B−CF3 groups, decompose with formation of B-F bonds. Tris(pentafluorophenyl)borane is thermally stable at temperatures wide over 200 °C, resistant to oxygen and water-tolerant. [2]

Contents

Preparation

Tris(pentafluorophenyl)borane is prepared using a Grignard reagent derived from bromopentafluorobenzene:

3C6F5MgBr + BCl3 → (C6F5)3B + 3MgBrCl

The synthesis originally employed C6F5Li, but this reagent can detonate with elimination of LiF. [2]

Structure

The structure of tris(pentafluorophenyl)borane (BCF) was determined by gas electron diffraction. [3] It has a propeller-like arrangement of its three pentafluorophenyl groups with a torsional angle of 40.6(3)° for the deviation of these groups from a hypothetically planar arrangement. Compared with a torsional angle of 56.8(4)° for tris(perfluoro-para-tolyl)borane, which is a stronger Lewis acid than BCF, this shows that there is some delocalization of electron density from the para-fluorine atoms to the boron atom that reduces its acidity.

Lewis acidity

The most noteworthy property of this molecule is its strong Lewis acidity. Its Lewis acid strength, as quantified by experimental equilibrium constants, is by 7 orders of magnitude higher than the one of structurally analogous triphenylborane. [4] Experimental equilibrium measurements, its AN value (Gutmann-Beckett method) as well as quantum-chemical calculations all indicate that the Lewis acidity of B(C6F5)3 is slightly lower than that of BF3 and significantly reduced compared to BCl3. B(C6F5)3 forms a strong Lewis adduct with water, [5] which was shown to be a strong Brønsted acid having an acidity comparable to hydrochloric acid (in acetonitrile). [6] In consequence, even traces of moisture are able to deactivate B(C6F5)3 and remaining catalytic activity might only be due to the Brønsted acidity of the water adduct.

Applications in catalysis

In one application (C6F5)3B forms noncoordinating anions by removing anionic ligands from metal centers. [7] Illustrative is a reaction that give rise to alkene polymerization catalysts where tris(pentafluorophenyl)boron is used as an activator or cocatalyst:

(C6F5)3B + (C5H5)2Zr(CH3)2 → [(C5H5)2ZrCH3]+[(C6F5)3BCH3]

In this process, the strongly coordinating methyl group transfers to the boron to expose a reactive site on zirconium. The resulting cationic zirconocene species is stabilised by the non coordinating borane anion. The exposed site on the zirconium allows for coordination of alkenes, whereupon migratory insertion into the remaining carbon-methyl ligand gives rise to a propyl ligand this process continues resulting in the growth of a polymer chain. This reagent has led to the development of immobilised catalyst/activator species; where the catalyst/activator is immobilised on an inert inorganic support such as silica. [8]

Tris(pentafluorophenyl)boraneis also capable of abstracting hydride to give [(C6F5)3BH], and it catalyzes hydrosilylation of aldehydes. Otherwise (C6F5)3B binds to a wide range of Lewis bases, even weak ones. [9] The compound is hygroscopic, forming the trihydrate [(C6F5)3BOH2](H2O)2, wherein one water in coordinated to boron and the other two waters are hydrogen-bonded to the coordinated water.

Related compounds are pentafluorophenylboron halides. [10]

Frustrated Lewis pair

Tris(pentafluorophenyl)borane is a key reagent leading to the concept of frustrated Lewis pairs. The combination of BCF and bulky basic phosphines, such as tricyclohexylphosphine (PCy3) cleaves H2: [11]

(C6F5)3B + PCy3 + H2 → (C6F5)3BH + HPCy3+

Many related phosphines, boranes, and substrates participate in related reactions.

Other reactions

(C6F5)3B was used to prepare a compound containing a Xe-C bond:

(C6F5)3B + XeF2 → [C6F5Xe]+[(C6F5)2BF2]

Upon reaction with pentafluorophenyllithium, the salt of the noncoordinating anion lithium tetrakis(pentafluorophenyl)borate is formed.

(C6F5)3B + C6F5Li → Li[(C6F5)4B]

B(C6F5)3 reacts with di mesitylphosphine to give the zwitterionic phosphonic-boronate (mes = C6H2Me3):

(C6F5)3B + mes2PH → (C6F5)2B(F)−C6F4−P(H)mes2

This zwitterionic salt can be converted to a system that reversibly binds molecular H2:

(C6F5)2B(F)−C6F4−P(H)mes2 + Me2SiHCl → (C6F5)2B(H)−C6F4−P(H)mes2 + Me2SiFCl
(C6F5)2B(H)−C6F4−P(H)mes2 → (C6F5)2B−C6F4−Pmes2 + H2

Related Research Articles

Boranes

Boranes is the name given to compounds with the formula BxHy and related anions. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.

Lewis acids and bases

A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that has a filled orbital containing an electron pair which is not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH3 is a Lewis base, because it can donate its lone pair of electrons. Trimethylborane (Me3B) is a Lewis acid as it is capable of accepting a lone pair. In a Lewis adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, forming a dative bond. In the context of a specific chemical reaction between NH3 and Me3B, the lone pair from NH3 will form a dative bond with the empty orbital of Me3B to form an adduct NH3•BMe3. The terminology refers to the contributions of Gilbert N. Lewis.

Diborane Chemical compound

Diborane(6), generally known as diborane, is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless, pyrophoric gas with a repulsively sweet odor. Synonyms include boroethane, boron hydride, and diboron hexahydride. Diborane is a key boron compound with a variety of applications. It has attracted wide attention for its electronic structure. Its derivatives are useful reagents.

Boron trifluoride is the inorganic 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.

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 alkene 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 non-coordinating anions have been used to catalyze hydrogenation, hydrosilylation, oligomerization, and the living polymerization of alkenes. The popularization of non-coordinating anions has contributed to increased understanding of agostic complexes wherein hydrocarbons and hydrogen serve as ligands. Non-coordinating anions are important components of many superacids, which result from the combination of Brønsted acids and Lewis acids.

Organoboron chemistry

Organoborane or organoboron compounds are chemical compounds of boron and carbon that are organic derivatives of BH3, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compounds.

In chemistry, hydroboration refers to the addition of a hydrogen-boron bond to C-C, C-N, and C-O double bonds, as well as C-C triple bonds. This chemical reaction is useful in the organic synthesis of organic compounds. The development of this technology and the underlying concepts were recognized by the Nobel Prize in Chemistry to Herbert C. Brown. He shared the Nobel prize in chemistry with Georg Wittig in 1979 for his pioneering research on organoboranes as important synthetic intermediates.

Ammonia borane Chemical compound

Ammonia borane (also systematically named amminetrihydridoboron), also called borazane, is the chemical compound with the formula H3NBH3. The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention as a source of hydrogen fuel, but is otherwise primarily of academic interest.

Organoaluminium chemistry

Organoaluminium chemistry is the study of compounds containing bonds between carbon and aluminium. It is one of the major themes within organometallic chemistry. Illustrative organoaluminium compounds are the dimer trimethylaluminium, the monomer triisobutylaluminium, and the titanium-aluminium compound called Tebbe's reagent. The behavior of organoaluminium compounds can be understood in terms of the polarity of the C−Al bond and the high Lewis acidity of the three-coordinated species. Industrially, these compounds are mainly used for the production of polyolefins.

In chemistry, a frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhibit activation.

Triphenylborane Chemical compound

Triphenylborane, often abbreviated to BPh3 where Ph is the phenyl group C6H5-, is a chemical compound with the formula B(C6H5)3. It is a white crystalline solid and is both air and moisture sensitive, slowly forming benzene and triphenylboroxine. It is soluble in aromatic solvents.

Borane dimethylsulfide Chemical compound

Borane dimethylsulfide (BMS) is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility. BMS is commercially available at much higher concentrations than its tetrahydrofuran counterpart and does not require sodium borohydride as a stabilizer, which could result in undesired side reactions. In contrast, borane·THF requires sodium borohydride to inhibit reduction of THF to tributyl borate. BMS is soluble in most aprotic solvents.

Lithium tetrakis(pentafluorophenyl)borate Chemical compound

Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of the weakly coordinating anion (B(C6F5)4). Because of its weakly coordinating abilities, lithium tetrakis(pentafluorophenyl)borate makes it commercially valuable in the salt form in the catalyst composition for olefin polymerization reactions and in electrochemistry. It is a water-soluble compound. Its anion is closely related to the non-coordinating anion known as BARF. The tetrakis(pentafluorophenyl)borates have the advantage of operating on a one-to-one stoichiometric basis with Group IV transition metal polyolefin catalysts, unlike methylaluminoxane (MAO) which may be used in large excess.

Boranylium ions

In chemistry, a boranylium ion is an inorganic cation with the chemical formula BR+
2
, where R represents a non-specific substituent. Being electron-deficient, boranylium ions form adducts with Lewis bases. Boranylium ions have historical names that depend on the number of coordinated ligands:

Dehydrogenation of amine-boranes or dehydrocoupling of amine-boranes is a chemical process in main group and organometallic chemistry wherein dihydrogen is released by the coupling of two or more amine-borane adducts. This process gained some interests due to the potential of using amine-boranes for hydrogen storage.

The Gutmann–Beckett method is an experimental procedure used by chemists to assess the Lewis acidity of molecular species. Triethylphosphine oxide (Et3PO, TEPO) is used as a probe molecule and systems are evaluated by 31P-NMR spectroscopy. In 1975, Viktor Gutmann used 31P-NMR spectroscopy to parameterize Lewis acidity of solvents by acceptor numbers (AN). In 1996, Michael A. Beckett recognised its more generally utility and adapted the procedure so that it could be easily applied to molecular species, when dissolved in weakly Lewis acidic solvents. The term Gutmann–Beckett method was first used in chemical literature in 2007.

Trihydridoboron, also known as borane or borine, is an unstable and highly reactive molecule with the chemical formula BH
3
. The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated the likely existence of the borane molecule. However, the molecular species BH3 is a very strong Lewis acid. Consequently it is highly reactive and can only be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen.

Borinic acid Chemical compound

Borinic acid, also known as boronous acid, is an oxyacid of boron with formula H
2
BOH
. Borinate is the associated anion of borinic acid with formula H
2
BO
; however, being a Lewis acid, the form in basic solution is H
2
B(OH)
2
.

Tris(2,2,2-trifluoroethyl) borate Chemical compound

Tris(2,2,2-trifluoroethyl) borate, also commonly referred to as the Sheppard amidation reagent, is a chemical compound with the formula B(OCH2CF3)3. This borate ester reagent is used in organic synthesis.

Among pnictogen group Lewis acidic compounds, unusual lewis acidity of Lewis acidic antimony compounds have long been exploited as both stable conjugate acids of non-coordinating anions, and strong Lewis acid counterparts of well-known superacids. Also, Lewis-acidic antimony compounds have recently been investigated to extend the chemistry of boron because of the isolobal analogy between the vacant p orbital of borane and σ*(Sb–X) orbitals of stiborane, and the similar electronegativities of antimony (2.05) and boron (2.04).

References

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