A R T I C L E S
Peryshkov et al.
Scheme 1
achieved in 1992, when Solntsev and co-workers reported that
the cesium salt could be prepared in 38% yield by heating
Cs2B12H12 in supercritical HF to 550 °C for 5 h.21 The structure
of Cs2(H2O)B12F12 was also reported at that time.21
In 2003 we, in collaboration with Solntsev, reported an
improved synthesis of K2B12F12 (72% recrystallized yield) in a
one-pot reaction that required first heating K2B12H12 in anhy-
drous HF to only 70 °C and then treating the reaction mixture
with 20/80 F2/N2 at 25 °C (see Scheme 1).23,24 However, the
scale of the reaction was limited to ca. 1 g of K2B12H12 starting
material in order to obtain the highest yield with the fewest
byproducts. In addition, a Monel reaction vessel was required
because the pressure of F2/N2 typically used was 50 psi or
higher, too high for routine use of large-volume fluoropolymer
reaction vessels. Furthermore, the fluorination took nearly a
week to complete (nominally 5-6 days, plus an additional day
for workup and recrystallization of the product). In 2004, Casteel
Figure 2. Full-scale negative-ion electrospray-ionization mass spectrum
of a sample of the reaction mixture in Expt 3 taken after 7 h of fluorination.
F2,23–25 the fluorination of B12H12 almost certainly involves
2-
electrophilic attack on the icosahedral borane dianion.28 The
DFT-predicted highest occupied molecular orbital (HOMO) of
B12H122- is shown in Figure 1. The electrophile probably forms
a Lewis acid-base complex with one of the triangular B3 faces
of the B12H12-xFx2- icosahedron, in much the same way that
the 11th H atom (i.e., H+) in B10H11- caps a triangular B3 face
29
-
of the B10H102- anion or the 13th H atom (i.e., H+) in B12H13
2-
and Ivanov reported in a patent that B12H12 salts could be
is predicted to cap a triangular face of the B12H12 anion.30
2-
polyfluorinated with F2 not only in anhydrous HF but also in
aqueous HF, formic acid, and acetic acid.25 However, complete
In order to optimize the conditions for F2 fluorination, we
needed a way to reliably measure the mol % of each
B12H12-xFx2- anion in the reaction mixture over time. As in the
past,31-35 we used negative-ion electrospray-ionization mass
spectrometry (NI-ESI-MS) for this purpose. [The virtues of NI-
ESI-MS for analyzing borane and carborane cluster anions in
general have been discussed by Hop et al.36,37] A typical
intermediate-fluorination-stage NI-ESI mass spectrum is shown
in Figure 2. A control experiment using a 50:50 mol % mixture
of K2B12H12 and K2B12F12 as the sample revealed that the
2-
2-
conversion of B12H12 to B12F12 was not reported for any
solvent other than anhydrous HF, and neither reaction times
nor isolated yields of purified B12F122- salts were reported. Note
that all previous preparations of K2B12F12 or attempts to prepare
2-
B12F12 salts were performed in acidic media.
We now report a procedure for preparing 99.5+ mol %
K2B12F12 in 74% recrystallized yield or 99.5+ mol % Cs2B12F12
in 76% recrystallized yield using up to 10 g of K2B12H12 as the
starting material, acetonitrile as the solvent, and an ordinary
Pyrex flask with standard-taper joints as the reaction vessel.
When a mixture of the partially fluorinated salts K2B12H8F4 and
K2B12H7F5 was used instead of K2B12H12 as the starting material,
the yield of K2B12F12 was 92%. The development of this
procedure was carried out in parallel with DFT calculations,
which helped to explain why rigorous exclusion of any protic
acid was the key to an efficient F2 perfluorination of K2B12H12.
We also report that the anhydrous salt Cs2B12F12 is stable to
600 °C under a helium atmosphere.
2-
sensitivity coefficient of B12F12 is 3.1 times as large as that
of B12H122-. Therefore, all mass-spectral intensities were scaled
according to their relative sensitivity coefficients, assuming that
the relative change in sensitivity coefficient per unit increment
in x is constant. For simplicity, we used the sum of the scaled
intensities of the most abundant isotopomers for a given
B12H12-xFx2-/KB12H12-xFx- pair as an index of the relative
amount of B12H12-xFx2- present for each observed value of x
(27) Ivanov, S. V.; Lupinetti, A. J.; Solntsev, K. A.; Strauss, S. H. J.
Fluorine Chem. 1998, 89, 65–72.
Results and Discussion
(28) Sivaev, I. B.; Bregadze, V. I.; Sjo¨berg, S. Collect. Czech. Chem.
Commun. 2002, 67, 679–727.
2-
General Comments about B12H12 Fluorination and the
2-
Determination of B12H12-xFx Mol % Values. Whether the
(29) Shore, S. G.; Hamilton, E. J. M.; Bridges, A. N.; Bausch, J.; Krause-
Bauer, J. A.; Dou, D.; Liu, J.; Liu, S.; Du, B.; Hall, H.; Meyers, E. A.;
Vermillion, K. E. Inorg. Chem. 2003, 42, 1175–1186.
(30) Mebel, A. M.; Charkin, O. P.; Solntsev, K. A.; Kuznetsov, N. T. Zh.
Neorg. Khim. 1989, 34, 1444–1448.
fluorinating agent is anhydrous HF,21,23 F-TEDA(BF4)2,27 or
(19) Nieuwenhuyzen, M.; Seddon, K. R.; Teixidor, F.; Puga, A. V. Inorg.
Chem. 2009, 48, 889–901.
(31) Ivanov, S. V.; Rockwell, J. J.; Miller, S. M.; Anderson, O. P.; Solntsev,
K. A.; Strauss, S. H. Inorg. Chem. 1996, 35, 7882–7891.
(32) Ivanov, S. V.; Ivanova, S. M.; Miller, S. M.; Anderson, O. P.; Solntsev,
K. A.; Strauss, S. H. Inorg. Chem. 1996, 35, 6914–6915.
(33) Kobayashi, Y.; Ivanov, S. V.; Popov, A. A.; Miller, S. M.; Anderson,
O. P.; Solntsev, K. A.; Strauss, S. H. Heteroatom Chem. 2006, 17,
181–187.
(20) Knapp, C.; Schulz, C. Chem. Commun. 2009, 4991–4993.
(21) Solntsev, K. A.; Mebel, A. M.; Votinova, N. A.; Kuznetsov, N. T.;
Charkin, O. P. Koord. Khim. 1992, 18, 340–364.
(22) Solntsev, K. A.; Ivanov, S. V.; Sakharov, S. G.; Katser, S. B.;
Chernayavskii, A. S.; Votinova, N. A.; Klyuchishche; E. A.; Kuz-
netsov, N. T. Russ. J. Coord. Chem. 1997, 23, 369–376.
(23) Ivanov, S. V.; Miller, S. M.; Anderson, O. P.; Solntsev, K. A.; Strauss,
S. H. J. Am. Chem. Soc. 2003, 125, 4694–4695.
(34) Ivanov, S. V.; Davis, J. A.; Miller, S. M.; Anderson, O. P.; Strauss,
S. H. Inorg. Chem. 2003, 42, 4489–4491.
(24) Strauss, S. H.; Ivanov, S. V. Fluoroborate Salts Comprising a Reactive
Cation and Uses Thereof. U.S. Patent 6,448,447 B1, 2002.
(25) Casteel, W. J., Jr.; Ivanov, S. V. Process for the Fluorination of Boron
Hydrides. U.S. Patent 6781005 B1, 2004.
(26) Knoth, W. H.; Miller, H. C.; England, D. C.; Parshall, G. W.;
Muetterties, E. L. J. Am. Chem. Soc. 1962, 84, 1056–1057.
(35) Kobayashi, Y.; Popov, A. A.; Miller, S. M.; Anderson, O. P.; Strauss,
S. H. Inorg. Chem. 2007, 46, 8505–8507.
(36) Hop, C. E. C. A.; Saulys, D. A.; Gaines, D. F. Inorg. Chem. 1995,
34, 1977–1978.
(37) Hop, C. E. C. A.; Saulys, D. A.; Bridges, A. N.; Gaines, D. F.;
Bakhtiar, R. Main Group Metal Chem. 1996, 19, 743–751.
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