View source for Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

{{short description|Chemical compound}}
{{correct title|reason=bracket|Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate}}
[[Image:BARF.png|200px|right|thumb|The [BAr<sup>F</sup><sub>4</sub>]<sup>−</sup> [[anion]] with four fluorinated [[aryl]] groups distributed [[Tetrahedral molecular geometry|tetrahedrally]] about a central [[boron]] atom]]
'''Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate''' is an [[anion]] with chemical formula [{3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}<sub>4</sub>B]<sup>−</sup>, which is commonly abbreviated as [BAr<sup>F</sup><sub>4</sub>]<sup>−</sup>, indicating the presence of fluorinated [[aryl]] (Ar<sup>F</sup>) groups.  It is sometimes referred to as ''Kobayashi's anion'' in honour of Hiroshi Kobayashi who led the team that first synthesised it.<ref name = Kobayashi>{{Cite journal|last1 = Nishida|first1 = H.|first2 = N.|last2 = Takada|first3 = M.|last3 = Yoshimura|first4 = T.|last4 = Sonods|first5 = H.|last5 = Kobayshi|title = Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. Highly Lipophilic Stable Anionic Agent for Solvent-Extraction of Cations|journal = [[Bulletin of the Chemical Society of Japan|Bull. Chem. Soc. Jpn.]]|year = 1984|volume = 57|issue = 9|pages = 2600–2604|doi = 10.1246/bcsj.57.2600|doi-access = free}}</ref> More commonly it is affectionately [[nickname]]d "BARF."<ref name = Bergman>{{Cite journal|last1 = Yakelis|first1 = N. A.|last2 = Bergman|first2 = R. G.|title = Safe Preparation and Purification of Sodium Tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBArF<sub>24</sub>): Reliable and Sensitive Analysis of Water in Solutions of Fluorinated Tetraarylborates|journal = [[Organometallics]]|year = 2005|volume = 24|pages = 3579–3581|doi = 10.1021/om0501428|issue = 14|pmid=19079785|pmc=2600718}}</ref> The BARF ion is also abbreviated BArF<sub>24</sub><sup>−</sup>,<ref name = Bergman /> to distinguish it from the closely related {{chem|BArF|20|−}}, [(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>B]<sup>−</sup>. However, for a small group of chemists, the anion is abbreviated as TFPB otherwise, short for '''T'''etrakis[3,5-bis(tri'''F'''luoromethyl)'''P'''henyl]'''B'''orate.<ref>{{Cite journal |last=Lin |first=You‐Han |last2=Lai |first2=Chien‐Chen |last3=Liu |first3=Yi‐Hung |last4=Peng |first4=Shie‐Ming |last5=Chiu |first5=Sheng‐Hsien |date=2013-09-23 |title=Sodium Ions Template the Formation of Rotaxanes from BPX26C6 and Nonconjugated Amide and Urea Functionalities |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201304636 |journal=Angewandte Chemie International Edition |language=en |volume=52 |issue=39 |pages=10231–10236 |doi=10.1002/anie.201304636 |issn=1433-7851}}</ref>

BARF has a [[Tetrahedral molecular geometry|tetrahedral]] geometry around the central boron atom but each of the four surrounding aryl groups is [[aromaticity|aromatic]] and planar.  The motivation for its preparation was the search for an anion that [[non-coordinating anion|coordinates more weakly]] than the then-available ions [[hexafluorophosphate]], [[tetrafluoroborate]], or [[perchlorate]].<ref name = Brookhart>{{Cite journal|first1 = M.|last1 = Brookhart|first2 = B.|last2 = Grant|first3 = A. F.|last3 = Volpe|title = [(3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>4</sub>B]<sup>−</sup>[H(OEt<sub>2</sub>)<sub>2</sub>]<sup>+</sup>: A Convenient Reagent for Generation and Stabilization of Cationic, Highly Electrophilic Organometallic Complexes|journal = [[Organometallics]]|year = 1992|volume = 11|issue = 11|pages = 3920–3922|doi = 10.1021/om00059a071}}</ref> [[Salt (chemistry)|Salts]] of this anion are known as solids and in both aqueous and non-aqueous solutions.  BARF can be used in [[catalyst|catalytic systems]] where the active site requires an anion which will not coordinate to the metal centre and interfere with the [[catalytic cycle]], such as in the preparation of [[polyketone]]s.<ref name = catalyst>{{cite journal|title = Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide|first1 = M.|last1 = Brookhart|authorlink1 = Maurice Brookhart|first2 = F. C.|last2 = Rix|first3 = J. M.|last3 = DeSimone|first4 = J. C.|last4 = Barborak|journal = [[J. Am. Chem. Soc.]]|year = 1992|volume = 114|issue = 14|pages = 5894–5895|doi = 10.1021/ja00040a082}}</ref>
[[Image:Lithium-tetrakis(pentafluorophenyl)borate-2D-skeletal.png|thumb|244px|[[Lithium tetrakis(pentafluorophenyl)borate]], containing the BARF-20 anion, is used similarly to the BARF-24 anion.]]

==Synthesis of BARF compounds==
The sodium [[salt (chemistry)|salt]] is the starting point for most BARF derivatives.  It is prepared by treating Grignard reagents derived from XC<sub>6</sub>H<sub>3</sub>-3,5-(CF<sub>3</sub>)<sub>2</sub> (X = Br, I) with NaBF<sub>4</sub>.  A popular method is summarized in the following equation:<ref>{{cite journal|author=Smith, C. R.; Zhang, A.; Mans, D. J.; RajanBabu, T. V.|title=(R)-3-Methyl-3-Phenyl-1-Pentene via Catalytic Asymmetric Hydrovinylation|journal=Org. Synth.|year=2008|volume=85|pages=248–266|doi=10.15227/orgsyn.085.0248|pmid=19672483|pmc=2723857|doi-access=free}}</ref>

:NaBF<sub>4</sub> +  4&nbsp;Ar<sup>F</sup>MgBr → 4 MgBrF +  NaBAr<sup>F</sup><sub>4</sub>

[[Brookhart's acid]] is the salt of the BARF anion with the diethyl ether [[oxonium ion|oxonium]] cation, [(Et<sub>2</sub>O)<sub>2</sub>H]BAr<sup>F</sup><sub>4</sub>.  It can be formed from the sodium salt in diethyl ether in the presence of [[hydrogen chloride]] as [[sodium chloride]] is insoluble in diethyl ether, facilitating cation exchange.<ref name = Brookhart />

: NaBAr<sup>F</sup><sub>4</sub> &nbsp; + &nbsp; HCl &nbsp; + &nbsp; 2 Et<sub>2</sub>O &nbsp; → &nbsp; [(Et<sub>2</sub>O)<sub>2</sub>H]BAr<sup>F</sup><sub>4</sub> &nbsp; + &nbsp; NaCl

BARF salts with hexakis(acetonitrile)metal(II) cations, [M(CH<sub>3</sub>CN)<sub>6</sub>]<sup>2+</sup>, are known for [[vanadium]], [[chromium]], [[manganese]], [[iron]], [[cobalt]], and [[nickel]].  They are produced by salt metathesis reactions.<ref>{{cite journal|journal = [[Inorg. Synth.]]|year = 2002|page = 83|doi = 10.1002/0471224502.ch2|title = Synthesis of [M<sup>II</sup>(NCMe)<sub>6</sub>]<sup>2+</sup> (M = V, Cr, Mn, Fe, Co, Ni) salts of tetra[3,5-bis(trifluoromethyl)phenyl]borate|first1 = W. E.|last1 = Buschman|first2 = J. S.|last2 = Miller|volume = 33}}</ref>

==Properties==
[[Non-coordinating anion]]s are anions that interact only weakly with cations, a useful property when studying highly [[electrophilic]] cations.<ref>{{cite journal | last1 = Krossing | first1 = I. | last2 = Raabe | first2 = I. | title = Noncoordinating Anions - Fact or Fiction? A Survey of Likely Candidates | year = 2004 | journal = [[Angewandte Chemie|Angew. Chem. Int. Ed.]] | volume = 43 | issue = 16 | pages = 2066–2090 | doi = 10.1002/anie.200300620 | pmid=15083452}}</ref>  In [[coordination complex|coordination chemistry]], the term can also be used to refer to anions which are unlikely to bind directly to the metal centre of a complex.  Hexafluorophosphate is a non-coordinating anion in both senses of the term.<ref name = "Davies_1996">{{cite book| title = Synthetic Coordination Chemistry: Principles and Practice| last1 = Davies| first1 = J. A.| publisher = World Scientific| year = 1996| isbn = 9810220847| page = 165}}</ref><ref name = "Constant_2005">{{cite book| series = New Aspects in Phosphorus Chemistry| title = New Trends in Hexacoordinated Phosphorus Chemistry| volume = 5| last1 = Constant| first1 = S.| last2 = Lacour| first2 = J.| editor = Majoral, J.-P.| publisher = Springer| year = 2005| isbn = 354022498X| page = 3}}</ref>  Three widely used non-coordinating anions are hexafluorophosphate, [[tetrafluoroborate]] {{chem|BF|4|−}}, and [[perchlorate]] {{chem|ClO|4|−}}; of these, the hexafluorophosphate ion has the least coordinating ability<ref>{{cite journal | last1 = Mayfield | first1 = H. G. | last2 = Bull | first2 = W. E. | title = Co-ordinating Tendencies of the Hexafluorophosphate Ion | year = 1971 | journal = [[Journal of the Chemical Society|J. Chem. Soc. A]] | issue = 14 | pages = 2279–2281 | doi = 10.1039/J19710002279}}</ref> and it is deliberately used for this property.  BARF was developed as a new non-coordinating anion in the 1990s, and is far less coordinating than even the hexafluorophosphate anion.<ref name = Brookhart />  Extremely Lewis acidic metal centers can, however, cleave the carbon-boron bond in BARF.<ref>{{Cite journal|last1=Salem|first1=Hiyam|last2=Shimon|first2=Linda J. W.|last3=Leitus|first3=Gregory|last4=Weiner|first4=Lev|last5=Milstein|first5=David|date=2008-05-01|title=B−C Bond Cleavage of BArF Anion Upon Oxidation of Rhodium(I) with AgBArF. Phosphinite Rhodium(I), Rhodium(II), and Rhodium(III) Pincer Complexes|journal=Organometallics|volume=27|issue=10|pages=2293–2299|doi=10.1021/om800034t|issn=0276-7333}}</ref>

NaBAr<sup>F</sup><sub>4</sub> can be used in [[protecting group|deprotection]] of [[acetal|acetal or ketal]]-protected [[carbonyl compound]]s.<ref>{{cite book|chapter = Dimethyl acetals|first1 = Theodora W.|last1 = Greene|first2 = Peter G. M.|last2 = Wuts|title = Greene's Protective Groups in Organic Synthesis|edition = 3rd|publisher = [[Wiley-Interscience]]|year = 1999|pages = 297–304, 724–727|chapter-url = https://www.organic-chemistry.org/protectivegroups/carbonyl/dimethylacetals.htm|accessdate = June 20, 2017|isbn = 9780471160199|archive-date = December 3, 2016|url-status = live|archive-url = https://web.archive.org/web/20161203200424/http://www.organic-chemistry.org/protectivegroups/carbonyl/dimethylacetals.htm}}</ref><ref>{{cite book|chapter = 1,3-Dioxanes, 1,3-Dioxolanes|first1 = Theodora W.|last1 = Greene|first2 = Peter G. M.|last2 = Wuts|title = Greene's Protective Groups in Organic Synthesis|edition = 3rd|publisher = [[Wiley-Interscience]]|year = 1999|pages = 308–322, 724–727|chapter-url = https://www.organic-chemistry.org/protectivegroups/carbonyl/dioxanes-dioxolanes.htm|accessdate = June 20, 2017|isbn = 9780471160199|archive-date = December 7, 2016|url-status = live|archive-url = https://web.archive.org/web/20161207144346/http://www.organic-chemistry.org/protectivegroups/carbonyl/dioxanes-dioxolanes.htm}}</ref>  For example, deprotection of 2-phenyl-[[1,3-dioxolane]] to [[benzaldehyde]] can be achieved in water in five minutes at 30&nbsp;°C.<ref>{{cite journal|title = Deprotection of Acetals and Ketals in a Colloidal Suspension Generated by Sodium Tetrakis(3,5-trifluoromethylphenyl)borate in Water|first1 = Chih-Ching|last1 = Chang|first2 = Bei-Sih|last2 = Liao|first3 = Shiuh-Tzung|last3 = Liu|journal = [[Synlett]]|year = 2007|volume = 2007|issue = 2|pages = 283–287|doi = 10.1055/s-2007-968009}}</ref>

::PhCH(OCH<sub>2</sub>)<sub>2</sub> &nbsp; + &nbsp; [[water|H<sub>2</sub>O]] &nbsp; <chem>->[\ce{NaBAr4}][\text{30 °C / 5 min}]</chem> &nbsp; [[benzaldehyde|PhCHO]] &nbsp; + &nbsp; [[ethylene glycol|HOCH<sub>2</sub>CH<sub>2</sub>OH]]

==References==
{{Reflist|30em}}

[[Category:Anions]]
[[Category:Trifluoromethyl compounds]]
[[Category:Non-coordinating anions]]
[[Category:Tetraphenylborates]]