The surprising structures of B8F12 and B10F12

Article abstract of DOI:10.1002/anie.200390164

Unstable, reactive, and yellow: Compound B₈F₁₂, assumed for 30 years to have a diborane-like structure B₂(BF₂)₆, has now been shown also to contain a short, central B-B bond. Across the B-B bond are two bridging BF₂ groups, which are bent out of the plane by long-range, intramolecular F···B interactions. The B-(μ-BF₂) bonds are long and, surprisingly, asymmetric, but calculations confirm that this structure is intrinsic to the molecule.

Full text of DOI:10.1002/anie.200390164

Angewandte
Chemie
Structures of Polyboron Fluorides
The Surprising Structures of B8F12 and B10F12**
Jennifer A. J. Pardoe, Nicholas C. Norman,
Peter L. Timms,* Simon Parsons,* Iain Mackie,
Colin R. Pulham, and David W. H. Rankin
Much is known about the structure of polyboron chlorides,
bromides, and iodides[1,2] and of cluster compounds of other
5]
Group 13 elements derived from their monohalides.[3
However, with the exception of B2F4,[6] experimental
structures of the polyboron fluorides are unknown. It is
notable that no report of a theoretically predicted structure of
B8F12 has appeared since the molecule was first postulated to
adopt a borane-like structure, B2(BF2)6, in 1972.[7] Herein we
describe the experimentally determined structures of B8F12
and of a completely new boron subhalide B10F12, both
obtained by means of X-ray crystallography at low temper-
atures.
Figure 1. Structure of B8F12 in the crystal. The numbering scheme
shown was used for all molecules of B8F12 observed in this study. In
the supplemental data,[10b] atoms in molecule 1 are labeled B11, B21
etc. Molecules 1 4 occur in B8F12, 5 6 in B8F12¥0.5BF3. Average distan-
ces [in ä]: B1-F1 1.303(3), B1-F2 1.301(5), B3-F3 1.335(4), B3-F4
1.316(4), B5-F5 1.311(5), B5-F6 1.313(10), B6-B7 1.328(6), B6-F8
1.315(6), B7-F9 1.309(7), B7-F10 1.312(9), B8-F11 1.316(7), B8-F12
1.311(9), B1-B4 2.11(4), B1-B2 1.86(2), B2-B3 1.792(18), B3-B4
1.92(4), B2-B5 1.745(6), B2-B6 1.714(6), B4-B7 1.736(5), B4-B8
1.717(8), B2-B4 1.667(6). The errors quoted are unweighted sample
standard deviations based on the dimensions of the six independent
molecules observed crystallographically in B8F12 and B8F12¥0.5BF3; indi-
The yellow liquid B8F12 was prepared, as earlier reported,
by low-temperature decomposition of (BF2)2BF, which had
been made by condensing gaseous BF with the vapor of B2F4
at 77 K.[8] The colorless compound B10F12 was discovered by
X-ray crystallography as a component of the mixture of the
less volatile boron fluorides formed by the condensation of
BF. The thermal stability of B10F12 seems comparable to or
higher than that of B8F12, but both compounds decompose
quite rapidly at 273 K. Samples of B8F12, and the fraction
containing B10F12 were sealed in glass capillaries and crystals
were grown at low temperatures on a diffractometer by using
the laser technique employed by Boese and Nussbaumer.[9]
Structures were determined by X-ray crystallography for pure
ꢀ
ꢀ
vidual B Band B F distances in these structures had standard uncer-
tainties of around 0.002 ä and 0.003 ä, respectively. Thermal ellipsoids
are drawn at the 30% probability level.
ꢀ
bonds and a short central B B bond, that is, it combines all
the structural elements once considered diborane. Unlike the
equivalent B(m-H)2B unit of diborane, however, the core B(m-
BF2)2B unit of B8F12 is both nonplanar, with average angles
between the B1-B2-B4 and B2-B3-B4 planes of 123.18 (range
B8F12,[10] B8F12¥0.5BF3, and B10F12 Molecular structures of
[11]
ꢀ
B8F12 and of B10F12 are shown in Figure 1 and Figure 2,
respectively.
121.6 125.78), and markedly asymmetric, with the four B B
bridge bonds varying in length in all the six crystallograph-
There are two crystallographically independent molecules
of B8F12 in the structure of B8F12¥0.5BF3, and four in that of
pure B8F12. However, the structures of all six of these
molecules are essentially the same. It is interesting to view
the structure of B8F12 in relation to earlier attempts to
understand the structure of diborane.[12] It has both bridge
ically independent molecules in the range of 1.79(2) to
ꢀ
ꢀ
ꢀ
ꢀ
2.11(4) ä in the sequence B2 B3 < B2 B1 < B3 B4 ! B1
B4. Overall, these bond lengths are consistent with there
being less electron density available for bridge bonding than
in the classic three-center, two-electron bridge bonds of
diborane, and with substantial electron density associated
ꢀ
with the short, central B2 B4 bond (av 1.667(6) ä). The bond
lengths and fold angles within the core B4 unit might be
expected to be easily deformed within a crystal, but all six
independent molecules have very similar structures. The
results of molecular orbital calculations at the MP2/6-31G*
[*] Dr. P. L. Timms, J. A. J. Pardoe, Dr. N. C. Norman
School of Chemistry
University of Bristol
Bristol, BS8 1TS (UK)
15]
level[13 show that this distorted and folded structure does
Fax: (þ44)117-925-1295
E-mail: peter.timms@bris.ac.uk
indeed lie in an energy minimum in the gas phase. The
Dr. S. Parsons, I. Mackie, Dr. C. R. Pulham, Prof. Dr. D. W. H. Rankin
Department of Chemistry
University of Edinburgh
optimized C2v (mm2) structure with a nonplanar B4 unit, in
ꢀ
which the core B B bond lengths are constrained to be equal,
is not a potential energy minimum and is 8.77 kJmolꢀ1 higher
in energy than the distorted structure.
King©s Building
West Main Road, Edinburgh EH93JJ (UK)
Fax: (þ44)131-650-4743
More detailed and extensive calculations (which will be
the subject of a subsequent full paper) are needed to fully
understand the origin of the observed unsymmetrical struc-
ture, but preliminary results indicate that intramolecular F¥¥¥B
interactions play a critical role. For example, for the
E-mail: s.parsons@ed.ac.uk
[**] This research was supported by the Engineering and Physical
Sciences Research Council, and help from the UK Computational
Facility is gratefully acknowledged.
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571

Communications
compound B8F12 a long bifurcated interaction between F3 and
the boron atoms B5and B7 in two of the terminal BF 2 groups
pulls the B3 atom out of the plane of B1, B2, and B4 (Figure 1;
observed experimental ranges 2.603(2) 2.675(2) ä for F3¥¥¥B5
and 2.620(2) 2.693(2) ä for F3¥¥¥B7; calculated ab initio
values 2.572 and 2.587 ä, respectively). These interactions
ꢀ
also affect the B3 F3 bond which, at an average of 1.335(4) ä
(calcd 1.361 ä), is longer than other such bonds in the
molecule. Furthermore, the atoms B2, B4, B5, B7, and B8 are
coplanar, but B6 is displaced from this plane by an average of
0.43(7) ä to accommodate an interaction between F7 and B8.
The longest B¥¥¥F interaction (obs. 2.733(2) 2.942(2) ä; calcd
2.855 ä) is formed between F5 and B1. This contact may be
ꢀ
responsible for the lengthening of the B1 B4 bridge bond.
While the results of calculations at the MP2/6-31G* level
ꢀ
point to some polarity in the B2 B4 bond (Mulliken charges;
B2 ꢀ0.95, B4 ꢀ0.41), the very high volatility of B8F12 indicates
Figure 2. Structure of B10F12 in the crystal. Bond lengths [ä]: B1-B1A
1.7583(14), B1-B1B 1.6053(18), B1-B2 1.8070(15), B1-B3 1.7037(13),
B2-F1 1.3266(9), B3-F2 1.3052(11), B3-F3 1.3191(11). Thermal ellip-
soids are drawn at the 30% probability level.
a compound with negligible intermolecular dipole dipole
interactions.
15]
Calculations[13
on the model compound B8H4F8, in
which the bridging BF2 groups are replaced by bridging BH2
groups, reveal a symmetrically bridged structure with a planar
central B4 unit and overall D2 (222) symmetry. The calcu-
lations also indicate a twisting of the planes of the BH2 groups
with respect to the central B4 unit, which can be attributed to
intramolecular interactions between the highly electron-
deficient bridging boron atoms and the fluorine atoms of
terminal BF2 groups. The p-donating ability of fluorine atoms
means that BF2 is less electron deficient than BH2. However,
in B8F12 the fluorine atoms of both the bridging and terminal
BF2 groups seem to be involved in long-range interactions
with the boron atoms of other BF2 groups. Clearly both the
observed intramolecular interactions and the inherent elec-
tronic properties are critical in determining the precise
structures adopted.
As pointed out by a referee, unsymmetrical bridging is
well-known in carbocation chemistry, where there is a
continuum of hyperconjugation, asymmetric bridging, and
symmetric bridging, depending on the strength of interaction
between a s donor and a center of electron deficiency. To
quote the referee, ™B8F12 would seem to belong to the world
between pure classical and pure nonclassical structures∫. The
detailed calculations referred to above should shed light on
these structural issues (as well as the reasons for the intense
yellow color of the compound) and we are currently trying to
refine gas-phase electron-diffraction data for the molecule.
The structure of B10F12 (Figure 2) has crystallographic S4
symmetry and is based on a central distorted tetrahedron of
boron atoms (B1 and its symmetry equivalents B1A, B1B, and
B1C) each with a terminal BF2 substituent, similar to the
known B4X4 tetrahedra,[1] but with BF2 bridges across the B1
B1B and B1A B1C edges. If each BF2 group is replaced by an
H atom, this structure equates to B4H4(m-H)2. Organic
derivatives of B4H4(m-H)2 of type B4R4(m-H)2 have been
reported[16,17] and while the structures were not established by
X-ray crystallography, there seems little doubt that they have
a similar B4 core to B10F12 but with bridging H atoms across
opposite edges of the tetrahedron. Nevertheless, such organo-
boron compounds and related derivatives of tetrabor-
ane (6)[18,19] are much more electron rich and thus more
ꢀ
stable than B10F12, which depends on relatively weak F B p-
bonding to compensate for its electron deficiency.
Exchange reactions of BCl3 with B8F12 and B10F12 were
studied, and are a route to the formation of new polyboron
chlorides.[20] Polyboron fluorides less volatile than B10F12 are
also formed by the condensation of BF, and efforts are being
made to determine their molecular structures by X-ray
crystallography.
Experimental Section
The volatile products, obtained from the condensation of gaseous BF
(1.5g, 50 mmol) with excess gaseous B 2F4 at 77 K, were fractionated
on a low-temperature distillation column[21] to yield (F2B)2BF (ca.
0.35g, 2.7 mmol), which came off the column at about 200 K, and a
fraction (ca. 0.2 g) that was partly liquid, partly crystalline on the
column, which came off slowly at about 235K. The (F 2B)2BF was held
at 250 K to allow it to disproportionate to B8F12 (ca. 0.17 g,
0.54 mmol) and B2F4, which were easily separated. Both B8F12 and
the 235K fraction were sealed in Pyrex capillaries (ca. 0.3 mm bore)
on a vacuum line. Crystals of the compounds were grown in these
capillaries, which were mounted on a Bruker AXS Smart Apex
diffractometer equipped with an Oxford Cryosystems Cryostream
and an OHCD IR laser-assisted crystallization device. Crystal growth
of B8F12 was hampered by glass formation, but crystals were grown
from a sample that had been annealed into a polycrystalline powder
at 160 K, by moving the point of focus along the capillary over a
period of 13 h, then ramping down the laser power from 1.9 W to zero
over 1 h. Prior to this study, crystallization of B8F12 had never been
observed to occur; when it was handled at low temperature on a
vacuum line, it always formed a glassy solid.
A crystal of B10F12 was grown without difficulty by establishing a
stable solid liquid equilibrium at 200 K with a laser power of 2.5W at
one end of the sample. The position of the phase boundary was moved
along the length of the sample over the course of 5min, then the laser
power was ramped down to zero over another 5min.
Received: July 16, 2002
Revised: November 5, 2002 [Z19745]
572
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Angewandte
Chemie
to determine how removing fluorine atoms from the bridging
[1] J. A. Morrison, Chem. Rev. 1991, 91, 35 .
groups affects the structural symmetry in B8F12. Frequency
calculations allowed the nature of any stationary points to be
determined, which confirms the structures as either local
minima, transition states, or saddle points on the potential
energy surfaces.
[2] W. Hˆnle, Y. Grin, A. Burkhardt, U. Wedig, M. Schultheiss,
H. G. von Schnering, J. Solid State Chem. 1997, 133, 5 9.
[3] C. Dohmeier, D. Loos, H. Schnˆckel, Angew. Chem. 1996, 108,
141; Angew. Chem. Int. Ed. Engl. 1996, 35, 129.
[4] A. Rodig, G. Linti, Angew. Chem. 2000, 112, 3076; Angew.
Chem. Int. Ed. 2000, 39, 2952.
[5] A. Schnepf, H. Schnˆckel, Angew. Chem. 2001, 113, 734; Angew.
Chem. Int. Ed. 2001, 40, 712.
[6] L. Trefonas, W. N. Lipsomb, J. Chem. Phys. 1958, 28, 5 4.
[7] R. W. Kirk, D. L. Smith, W. Airey, P. L. Timms, J. Chem. Soc.
Dalton Trans. 1972, 1392.
[14] GAUSSIAN98 (RevisionA.7), M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S.
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain,
O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J.
V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W.
Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A.
Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
[8] P. L. Timms, J. Am. Chem. Soc. 1967, 89, 1629.
[9] R. Boese, M. Nussbaumer in Correlations, Transformations and
Interactions in Organic Crystal Chemistry, IUCr Crystallograph-
ic Symposia, Vol. 7 (Eds.: D. W. Jones, A. Katrusiak), Oxford
University Press, Oxford, UK, 1994, pp. 20 37.
[10] a) Crystal structure analysis of B8F12: Mr ¼ 1257.86, monoclinic
P2/c, a ¼ 24.577(3), b ¼ 7.3341(8), c ¼ 24.493(3) ä, b ¼
106.708(2)8, V¼ 4401.1(9) ä3, T¼ 120 K, 1cald ¼ 1.898 gcmꢀ3
,
[15] C. M˘ller, M. S. Plesset, Phys. Rev. 1934, 46, 618; P. Hohenburg,
W. Kohn, Phys. Rev. B 1964, 136, 864.
l ¼ 0.71073 ä. Yellow cylinder, 0.30 î 0.30 î 1.0 mm, m(MoKa) ¼
0.244 mmꢀ1. An absorption correction was performed by the
multiscan method using the program SADABS (0.76 < T< 0.93).
Data were collected to 2qmax ¼ 57.68 comprising 39515 measured
and 10721 unique data, of which 7462 with F > 4s(F) were used
for refinement. The structure was solved by direct methods using
SHELXTL[22] and refined by full-matrix least-squares against F,
with anisotropic displacement parameters on all atoms and a
Chebychev three-term polynomial weighting scheme (CRYS-
TALS).[23] The final R factor was 0.0298, Rw ¼ 0.0334, for
721 parameters. The final difference map max. and min were
þ 0.36 and ꢀ0.19 eäꢀ3, respectively. Further analyses were
performed with PLATON.[24] b) Further details on the crystal
structure investigations may be obtained from the Fachinforma-
tionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Ger-
many (fax: (þ 49)7247-808-666; email: crysdata@fiz-karlsruhe.
de), on quoting the depository numbers CSD-412616 (B8F12),
CSD-412617 (B17F27), and CSD-412618 (B10F12).
[16] A. Neu, T. Mannekes, U. Englert, P. Paetzold, M. Hoffman, P.
von Schleyer, Angew. Chem. 1997, 109, 2211; Angew. Chem. Int.
Ed. Engl. 1997, 36, 2117.
[17] A. Neu, T. Mannekes, U. Englert, P. Paetzold, M. Hoffman, P.
von Schleyer, Inorg. Chim. Acta 1999, 289, 5 8.
[18] C. Pr‰sang, M. Hofmann, G. Geiseler, W. Massa, A. Berndt,
Angew. Chem. 2002, 114, 1597; Angew. Chem. Int. Ed. 2002, 41,
1526.
[19] A. Maier, M. Hofmann, H. Pritzkow, W. Siebert, Angew. Chem.
2002, 114, 1600; Angew. Chem. Int. Ed. 2002, 41, 1529.
[20] J. A. J. Pardoe, N. C. Norman, P. L. Timms, Polyhedron 2002, 21,
543.
[21] See: D. F. Shriver, Manipulation of Air Sensitive Compounds,
McGraw-Hill, New York, 1969, p. 91.
[22] G. M. Sheldrick, SHELXTL version 5, Bruker-AXS, Madison,
Wisconsin, 1995.
[23] D. J. Watkin, C. K. Prout, J. R. Carruthers, P. W. Betteridge, R. I.
Cooper, CRYSTALS Issue 11. Chemical Crystallography Labo-
ratory, Oxford, UK, 2001.
[24] A. L. Spek, PLATON, a Multipurpose Crystallographic Tool,
Utrecht University, Utrecht, The Netherlands, 2002. PC version
part of the WINGX suite: L. J. Farrugia, J. Appl. Crystallogr.
1999, 32, 837.
[11] Crystal structure analysis of B10F12: M ¼ 336.08, tetragonal I41/a,
a ¼ 6.4118(8), c ¼ 25.551(5) ä, V¼ 1132.6(3) ä3, T¼ 150 K,
1cald ¼ 1.971 gcmꢀ3
,
l ¼ 0.71073 ä. Colorless cylinder, o. d.
0.36 mm, m(MoKa) ¼ 0.242 mmꢀ1. An absorption correction was
performed by the multiscan method using the program SA-
DABS (0.762 < T< 1). Data were collected to 2qmax ¼ 588
comprising 3613 measured and 728 unique data, of which
685with F > 4s(F) were used for refinement. The structure
was solved by direct methods and refined as a merohedral twin
using a twofold axis about [110] as the twin law by full-matrix
least-squares against F, with anisotropic displacement parame-
ters on all atoms and a Chebychev three-term polynomial
weighting scheme. The final R factor was 0.0219, Rw ¼ 0.0252, for
52 parameters. The final difference map max. and min. were
þ 0.22 and ꢀ0.15eä ꢀ3, respectively, and the twin-component
scale factors were 0.5841 and 0.4186(16).[10b] The programs used
were the same as those in reference [10a].
[12] P. Laszlo, Angew. Chem. 2000, 112, 2151; Angew. Chem. Int. Ed.
2000, 39, 2071.
[13] All calculations were performed by using the GAUSSIAN98
computer program[14] using resources of the UK Computational
Chemistry Facility, on a DEC 8400 superscalar cluster equipped
with 10 fast processors, 6 GB of memory, and 150 GB disc.
Calculations were performed at the MP2/6-31G* level of
theory.[14] Calculations were carried out starting with the crystal
coordinates of B8F12, and a similar structure, whereby the central
ꢀ
four B B distances were constrained to be equal. Calculations
were also carried out on the structure of (F2B)2B(m-BH2)2(BF2)2
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