Before the 1990s, tetrafluoroborate, hexafluorophosphate, and perchlorate were considered weakly coordinating anions. Only by exclusion of conventional solvents were transition metal perchlorate complexes found to exist, for example. It is now appreciated that BF− 4, PF− 6, and ClO− 4 bind to strongly electrophilic metal centers of the type use in some catalytic reactions.[2][3] 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 triflates are considered to be low-coordinating with some cations.
Era of BARF
Structure of the weakly coordinating anion [Al(OC(CF3)3)4]−,[4] illustrating its high symmetry. Color code: green = F, red = O, blue = Al.
A revolution in this area occurred in the 1990s with the introduction of the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ion, B[3,5-(CF 3) 2C 6H 3]− 4, commonly abbreviated as B(ArF)4− and colloquially called "BARF".[5] This anion is far less coordinating than tetrafluoroborate, hexafluorophosphate, and perchlorate, and consequently has enabled the study of still more electrophilic cations.[6] Related tetrahedral anions include tetrakis(pentafluorophenyl)borateB(C 6F 5)− 4, and Al[OC(CF 3) 3]− 4.
Salts of the anion B[3,5-(CF 3) 2C 6H 3]− 4 were first reported by Kobayashi and co-workers. For that reason, it is sometimes referred to as Kobayashi's anion.[7] Kobayashi's method of preparation has been superseded by a safer route.[5]
The crystal structure of the compound [H(Et2O)2][B(C6F5)4],.[8] H atoms are omitted from the image. Color code: red = O, yellow = F, gray = C.
Another large class of non-coordinating anions are derived from carborane anion CB 11H− 12. Using this anion, the first example of a three-coordinate silicon compound, the salt [(mesityl)3Si][HCB11Me5Br6] contains a non-coordinating anion derived from a carborane.[10]
^Honeychuck, R. V.; Hersh, W. H. (1989). "Coordination of "Noncoordinating" Anions: Synthesis, Characterization, and X-ray Crystal Structures of Fluorine-Bridged [SbF6]−, [BF4]−, and [PF6]− Adducts of [R3P(CO)3(NO)W]+. An Unconventional Order of Anion Donor Strength". Inorganic Chemistry. 28 (14): 2869–2886. doi:10.1021/ic00313a034.
^M. Brookhart; B. Grant; A. F. Volpe, Jr. (1992). "[(3,5-(CF3)2C6H3)4B]-[H(OEt2)2]+: a convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexes". Organometallics. 11 (11): 3920–3922. doi:10.1021/om00059a071.
^Jutzi, P.; Müller, C.; Stammler, A.; Stammler, H. G. (2000). "Synthesis, Crystal Structure, and Application of the Oxonium Acid [H(OEt2)2]+[B(C6F5)4]−". Organometallics. 19 (7): 1442. doi:10.1021/om990612w.
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