Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

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The [BArF4] anion with four fluorinated aryl groups distributed tetrahedrally about a central boron atom

Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate is an anion with chemical formula [{3,5-(CF3)2C6H3}4B], which is commonly abbreviated as [BArF4] as the boron atom (B) is surrounded by four fluorinated aryl (ArF) groups. It is sometimes referred to as Kobayashi's anion in honour of Hiroshi Kobayashi who led the team that first synthesised it,[1] but more commonly it is affectionately nicknamed "BARF."[2] BARF has a tetrahedral geometry around the central boron atom but each of the four surrounding aryl groups is aromatic and planar. The motivation for its preparation was the search for an anion which coordinates more weakly than the then-available ions hexafluorophosphate, tetrafluoroborate, or perchlorate.[3] Salts of this anion are known as solids and in both aqueous and non-aqueous solutions. BARF can be used in 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 polyketones.[4]

Synthesis of sodium BARF[edit]

The sodium salt of the BARF ion is abbreviated NaBArF4 (or sometimes as NaBArF24[2] recognising the 24 fluorine atoms in each BARF ion). It was first prepared by a group led by Kobayashi and reported in 1984.[1] A Grignard reagent was prepared by reacting 1-iodo-3,5-bis(trifluoromethyl)benzene with magnesium metal in dry diethyl ether (Et2O) to form 3,5-bis(trifluoromethyl)phenylmagnesium iodide, ArFMgI where ArF = 3,5-(CF3)2C6H3. An ethereal solution of boron trifluoride is then added to this Grignard and the purified salt produced in 84% yield after workup and column chromatography.[1]

BF3.OEt2   +   4 ArFMgI   +   NaF   →   4 MgIF   +   NaBArF4   +   OEt2

The preparation of 3,5-bis(trifluoromethyl)phenylmagnesium halide Grignard reagents by direct insertion of aryl halide into magnesium metal can result in a thermal runaway with explosive consequences. A safer synthesis has since been developed, utilising the magnesium-bromine exchange reaction between 1-bromo-3,5-bis(trifluoromethyl)benzene and isopropylmagnesium chloride to generate the required aryl-Grignard reagent, which is then treated with sodium tetrafluoroborate. A method for assaying the water content of the product was also described.[2]

NaBF4   +   4 ArFMgBr   →     NaBArF4   +   4 MgBrF

Properties[edit]

Non-coordinating anions are anions that interact only weakly with cations, a useful property when studying highly electrophilic cations.[5] In 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.[6][7] Three widely used non-coordinating anions are hexafluorophosphate, tetrafluoroborate BF
4
, and perchlorate ClO
4
; of these, the hexafluorophosphate ion has the least coordinating ability[8] 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.[3] Extremely Lewis acidic metal centers can, however, cleave the carbon-boron bond in BARF.[9]

NaBArF4 can be used in deprotection of acetal or ketal-protected carbonyl compounds.[10][11] For example, deprotection of 2-phenyl-1,3-dioxolane to benzaldehyde can be achieved in water in five minutes at 30 °C.[12]

PhCH(OCH2)2   +   H2O     PhCHO   +   HOCH2CH2OH

Known compounds[edit]

Brookhart's acid is the salt of the BARF anion with the diethyl ether oxonium cation, [(Et2O)2H]BArF4. 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.[3]

NaBArF4   +   HCl(g)   +   2 Et2O   →   [(Et2O)2H]BArF4   +   NaCl(s)

Synthesis of BARF salts with hexa(acetonitrile)metal(II) cations, [M(CH3CN)6]2+, are known for vanadium, chromium, manganese, iron, cobalt, and nickel.[13]

References[edit]

  1. ^ a b c Nishida, H.; Takada, N.; Yoshimura, M.; Sonods, T.; Kobayshi, H. (1984). "Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. Highly Lipophilic Stable Anionic Agent for Solvent-Extraction of Cations". Bull. Chem. Soc. Jpn. 57 (9): 2600–2604. doi:10.1246/bcsj.57.2600.
  2. ^ a b c Yakelis, N. A.; Bergman, R. G. (2005). "Safe Preparation and Purification of Sodium Tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBArF24): Reliable and Sensitive Analysis of Water in Solutions of Fluorinated Tetraarylborates". Organometallics. 24 (14): 3579–3581. doi:10.1021/om0501428. PMC 2600718. PMID 19079785.
  3. ^ a b c Brookhart, M.; Grant, B.; Volpe, A. F. (1992). "[(3,5-(CF3)2C6H3)4B][H(OEt2)2]+: A Convenient Reagent for Generation and Stabilization of Cationic, Highly Electrophilic Organometallic Complexes". Organometallics. 11: 3920–3922. doi:10.1021/om00059a071.
  4. ^ Brookhart, M.; Rix, F. C.; DeSimone, J. M.; Barborak, J. C. (1992). "Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide". J. Am. Chem. Soc. 114 (14): 5894–5895. doi:10.1021/ja00040a082.
  5. ^ Krossing, I.; Raabe, I. (2004). "Noncoordinating Anions - Fact or Fiction? A Survey of Likely Candidates". Angew. Chem. Int. Ed. 43 (16): 2066–2090. doi:10.1002/anie.200300620. PMID 15083452.
  6. ^ Davies, J. A. (1996). Synthetic Coordination Chemistry: Principles and Practice. World Scientific. p. 165. ISBN 9810220847.
  7. ^ Constant, S.; Lacour, J. (2005). Majoral, J.-P. (ed.). New Trends in Hexacoordinated Phosphorus Chemistry. New Aspects in Phosphorus Chemistry. 5. Springer. p. 3. ISBN 354022498X.
  8. ^ Mayfield, H. G.; Bull, W. E. (1971). "Co-ordinating Tendencies of the Hexafluorophosphate Ion". J. Chem. Soc. A (14): 2279–2281. doi:10.1039/J19710002279.
  9. ^ Salem, Hiyam; Shimon, Linda J. W.; Leitus, Gregory; Weiner, Lev; Milstein, David (2008-05-01). "B−C Bond Cleavage of BArF Anion Upon Oxidation of Rhodium(I) with AgBArF. Phosphinite Rhodium(I), Rhodium(II), and Rhodium(III) Pincer Complexes". Organometallics. 27 (10): 2293–2299. doi:10.1021/om800034t. ISSN 0276-7333.
  10. ^ Greene, Theodora W.; Wuts, Peter G. M. (1999). "Dimethyl acetals". Greene's Protective Groups in Organic Synthesis (3rd ed.). Wiley-Interscience. pp. 297–304, 724–727. ISBN 9780471160199. Archived from the original on December 3, 2016. Retrieved June 20, 2017.
  11. ^ Greene, Theodora W.; Wuts, Peter G. M. (1999). "1,3-Dioxanes, 1,3-Dioxolanes". Greene's Protective Groups in Organic Synthesis (3rd ed.). Wiley-Interscience. pp. 308–322, 724–727. ISBN 9780471160199. Archived from the original on December 7, 2016. Retrieved June 20, 2017.
  12. ^ Chang, Chih-Ching; Liao, Bei-Sih; Liu, Shiuh-Tzung (2007). "Deprotection of Acetals and Ketals in a Colloidal Suspension Generated by Sodium Tetrakis(3,5-trifluoromethylphenyl)borate in Water". Synlett. 2007 (2): 283–287. doi:10.1055/s-2007-968009.
  13. ^ Buschman, W. E.; Miller, J. S. (2002). "Synthesis of [MII(NCMe)6]2+ (M = V, Cr, Mn, Fe, Co, Ni) salts of tetra[3,5-bis(trifluoromethyl)phenyl]borate". Inorg. Synth. 33: 83. doi:10.1002/0471224502.ch2.