Bifunctional perfluoroaryl boranes: Synthesis and coordination chemistry with neutral lewis base donors

Article abstract of DOI:10.1021/om050764t

A practical synthetic route for the preparation of the known bifunctional perfluoroaryldiborane C₆F₄-1,2-[B(C₆F ₅)₂]₂,1, and the new diborane C ₆F₄-1,2-[B(C₁₂F₈)]₂, 2, that circumvents the use of toxic mercury reagents has been developed. Key intermediates and 2 have been fully characterized in solution and the solid state. Solution and structural investigations of their MeCN and THF adducts have shown that 2 is a superior Lewis acid to 1, primarily owing to the reduction in steric pressure afforded by the planar 9-borafluorene rings in 2 versus the freely rotating set of pentafluorophenyl rings in 1. The two compounds bind Lewis bases via differing coordination modes, as demonstrated by the solid-state structures of the Lewis base adducts 1·MeCN, 2·THF, and 2·MeCN. Solution studies suggest that bis Lewis base adducts are also accessible; the structures of two examples (1·(MeCN)₂ and 2·(THF)₂) have been determined, and their role in the dynamic solution behavior of these systems is discussed.

Full text of DOI:10.1021/om050764t

Organometallics 2006, 25, 349-357
349
Bifunctional Perfluoroaryl Boranes: Synthesis and Coordination
Chemistry with Neutral Lewis Base Donors
Preston A. Chase,† Lee D. Henderson,† Warren E. Piers,*,† Masood Parvez,†
William Clegg,‡ and Mark R. J. Elsegood‡
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe N.W., Calgary,
Alberta, T2N 1N4 Canada, and Department of Chemistry, UniVersity of Newcastle,
Newcastle upon Tyne NE1 7RU, U.K.
ReceiVed September 2, 2005
A practical synthetic route for the preparation of the known bifunctional perfluoroaryldiborane C6F4-
1,2-[B(C6F5)2]2, 1, and the new diborane C6F4-1,2-[B(C12F8)]2, 2, that circumvents the use of toxic mercury
reagents has been developed. Key intermediates and 2 have been fully characterized in solution and the
solid state. Solution and structural investigations of their MeCN and THF adducts have shown that 2 is
a superior Lewis acid to 1, primarily owing to the reduction in steric pressure afforded by the planar
9-borafluorene rings in 2 versus the freely rotating set of pentafluorophenyl rings in 1. The two compounds
bind Lewis bases via differing coordination modes, as demonstrated by the solid-state structures of the
Lewis base adducts 1‚MeCN, 2‚THF, and 2‚MeCN. Solution studies suggest that bis Lewis base adducts
are also accessible; the structures of two examples (1‚(MeCN)2 and 2‚(THF)2) have been determined,
and their role in the dynamic solution behavior of these systems is discussed.
by the ortho-C6F4 moiety, and in particular the simplest chelating
diborane based on B(C6F5)3, C6F4-1,2-[B(C6F5)2]2, 1.9 This
compound is able to activate simple group 4 metallocene
dialkyls,10 chelate a variety of anions to form new classes of
weakly coordinating anions,11 and initiate cationic polymeri-
zation of isobutylene in hydrocarbon12 and aqueous suspension
media.13 In the latter application in particular, the chelating array
of two Lewis acid centers appears to be crucial for the observed
efficacy of this chemistry, since nonchelating diboranes or
monofunctional boranes do not serve as effective polymerization
initiators under these unusual conditions.
Introduction
Perfluoroaryl boranes are an important class of Lewis acids1
with a variety of applications as catalysts for organic transfor-
mations1,2 polymer synthesis3 and cocatalysts in olefin polym-
erization processes.4 They are generally comparable in terms
of Lewis acid strength to the prototypical haloborane Lewis
acids, but exhibit a high degree of tolerance toward protic Lewis
bases such as alcohols and water. The perfluorination also results
in extensive charge delocalization in borate anions formed from
these Lewis acids, such that perfluoroarylborate anions are
among the most weakly coordinating anions available to
synthetic chemists.5
To extend the chemistry of this class of Lewis acids and
explore the effect of having multiple perfluoroaryl Lewis acid
centers in a molecular framework, we and others6 have prepared
bi- and multifunctional7 perfluoroaryl boranes where the borane
centers are aligned in both chelating and nonchelating arrays.1,8
Our interest has focused mainly on chelating diboranes bridged
Given the several potential applications for 1, we became
interested in both improved synthetic routes to the compound
and modifications to 1 so as to explore structure-activity
relationships for various chelating perfluoroaryl diboranes. Our
original synthesis of 1 utilized the known mercury trimer [1,2-
14
Hg(C6F4)]3 as a o-C6F4 synthon for installing the borane
centers, and while effective, for larger scale syntheses use of
this reagent is not convenient nor desirable from an environ-
mental perspective. We have therefore developed a “mercury-
free” route to 1,15 which can also be used to prepare a new
member of this family of diboranes, 2, which is based on a
bis-9-borafluorene framework. Constraining the boron centers
into a 9-borafluorene heterocycle renders them more sterically
* Corresponding author. E-mail: wpiers@ucalgary.ca. Phone: 403-220-
5746.
† University of Calgary.
‡ University of Newcastle
(1) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1.
(2) (a) Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999, 527. (b)
Chivers, T. J. Fluorine Chem. 2002, 115, 1. (c) Piers, W. E.; Chivers, T.
Chem. Soc. ReV. 1997, 26, 345.
(3) (a) Denis, J.-M.; Forintos, H.; Szelke, H.; Toupet, L.; Pham, T.-N.;
Madec, P.-J.; Gaumont, A.-C. Chem. Commun. 2003, 54. (b) Rubinsztajn,
S.; Cella, J. A. Macromolecules 2005, 38, 1061.
(4) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391.
(5) Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43, 2066.
(6) (a) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks,
T. J. Angew. Chem., Int. Ed. 2000, 39, 1312. (b) Metz, M. V.; Schwartz,
D. J.; Stern, C. L.; Marks, T. J. Organometallics 2002, 21, 4159. (c) Li,
H.; Li, L.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem.
Soc. 2003, 125, 10788.
(9) Williams, V. C.; Piers, W. E.; Clegg, W.; Collins, S.; Marder, T. B.
J. Am. Chem. Soc. 1999, 121, 3244.
(10) Williams, V. C.; Dai, C.; Li, Z.; Collins, S.; Piers, W. E.; Clegg,
W. C.; Elsegood, M. R. J.; Marder, T. B. Angew. Chem., Int. Ed. 1999, 38,
3695.
(11) (a) Williams, C. V.; Irvine, G. J.; Piers, W. E.; Li, Z.; Collins, S.;
Clegg, W.; Elsegood, M. R. J.; Marder, T. B. Organometallics 2000, 19,
1619. (b) Henderson, L. D.; Piers, W. E.; Irvine, G. J.; McDonald, R.
Organometallics 2002, 21, 340.
(12) Lewis, S. P.; Taylor, N. J.; Piers, W. E.; Collins, S. J. Am. Chem.
Soc. 2003, 125, 16486.
(13) Lewis, S. P.; Henderson, L. D.; Chandler, B. D.; Parvez, M.; Piers,
W. E.; Collins, S. J. Am. Chem. Soc. 2004, 127, 46.
(14) Sartori, P.; Golloch, A. Chem. Ber. 1968, 101, 2004.
(15) Piers, W. E.; Chase, P. A.; Henderson, L. D. Can. Pat. Appl.
2,463,045, 2004.
(7) Roesler, R.; Har, B. J. N.; Piers, W. E. Organometallics 2002, 21,
4300.
(8) Piers, W. E.; Williams, V. C.; Irvine, G. J. Eur. J. Inorg. Chem.
2000, 2131.
10.1021/om050764t CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/01/2005

350 Organometallics, Vol. 25, No. 2, 2006
Chase et al.
accessible, while sacrificing little in terms of the inherent Lewis
acid strength of the boron.16 In addition to the synthetic
chemistry described, we compare the coordination chemistry
of 1 and 2 with the neutral Lewis bases acetonitrile and THF.
of an electrophile. Once formed, the dilithio derivative may be
quenched with B(OMe)3, and hydrolytic workup leads to the
crude bis-boronic acid product. Subsequent fractional recrys-
tallization from hot water afforded C6F4-1,2-[B(OH)2]2 as an
off-white powder in greater than 85% yield. Both 19F and 11B
NMR spectroscopy indicated that this material was >95%
pure,20 so it was used without further purification.
A methanol solution of the bis-boronic acid was treated with
an aqueous solution of KHF2 to give a solid comprised of a
mixture of [K]2+[C6F4-1,2-(BF3)2]2-, monoanionic [K]+{C6F4-
1,2-[(BF2)2(µ-F)]}-, and KF in 65% overall yield based on mass
balance. As shown in Scheme 1, the two fluoroborate species
are in equilibrium and the position of the equilibrium is
dependent to some degree on the amount of water present. In
solution, both species are observable and each has a diagnostic
19F NMR spectrum in CD3CN/D2O. [K]2+[C6F4-1,2-(BF3)2]2-
exhibits three signals at -141.2, -162.9, and -146.2 ppm in a
1:1:3 ratio, while [K]+{C6F4-1,2-[(BF2)2(µ-F)]}- shows slightly
shifted signals for the C6F4 fluorines, and two other signals at
-143.4 and -139.7 ppm in a 4:1 ratio for the terminal and
bridging fluorines, respectively. Although C6F5BF3K has been
observed to condense in an intermolecular sense to give
observable amounts of (C6F5)2BF2K and KF,19 only intramo-
Results and Discussion
Synthesis. The syntheses of both 1 and 2 rely on the key
bis-dibromoboryl intermediate C6F4-1,2-(BBr2)2, which was
originally prepared in 51% yield via reaction of [1,2-Hg(C6F4)]3
with BBr3.8 Given that the preparation of the mercury trimer
via thermal decarboxylation of [1,2-Hg(O2CC6F4)]3 occurs in
modest yields, and the fact that organomercury compounds are
toxic,17 we sought an alternative route to this compound. The
protocol that has been developed is outlined in Scheme 1 and
lecular condensation is observed for [K]2+[C6F4-1,2-(BF3)2]2-
.
Pure samples of [K]2+[C6F4-1,2-(BF3)2]2- may be obtained
by crystallization from concentrated solutions of CH3CN, while
crystals of the µ-fluoride were obtained from CH3CN/H2O
solutions. The molecular structures of both compounds have
been determined by X-ray crystallography and are shown in
Figure 1 along with selected metrical parameters.21 While both
compounds exhibit extensive intermolecular contacts in the
extended structure, with many K‚‚‚F contacts under 3.25 Å, from
a synthetic perspective, the structures illustrate conclusively that
the ortho-diboryl groups have been installed successfully. For
[K]2+[C6F4-1,2-(BF3)2]2-, Figure 1a, the network of K‚‚‚F
contacts distorts the metrical parameters within the molecule
somewhat, while in the condensed compound [K]+{C6F4-1,2-
[(BF2)2(µ-F)]}-, Figure 1b, the molecular structure is sym-
metrical since the potassium ions link adjacent molecules in
the extended structure.
Scheme 1
Both of these air- and moisture-stable potassium perfluoro-
aryltrifluoroborate salts can be readily converted to the bis-
difluoroborane, C6F4-1,2-(BF2)2, in 84% yield by treatment of
a CH2Cl2 solution with a BF3(g) atmosphere at -40 °C for 30
min. This compound may be isolated as an air- and moisture-
sensitive oil after removal of the KBF4 byproduct and the solvent
volatiles, taking care not to pump away the product. This
compound, which is an interesting bifunctional diborane in its
own right, unfortunately cannot be converted into compound 1
conveniently using copper,22 zinc,23 or tin24 -C6F5 transfer
agents. However, halogen exchange by reaction with a 5-fold
excess of BBr3 with the bis-difluoroborane at 50 °C for 30 min
cleanly afforded the known dibromo analogue C6F4-1,2-(BBr2)2.
starts from commercially available 1,2-dibromotetrafluoroben-
zene; installation of the dihalo boryl groups takes advantage of
some boron functional group manipulations recently reported
by Vedejs18 and Fro¨hn.19 While the route contains a few more
steps, each step is convenient and high yielding, resulting in a
protocol that, in terms of overall yield, is superior to the mercury
trimer route and avoids the use of organomercurials.
(20) “Impurities” are constituted mainly of anhydrides of this compound
and also participate in subsequent reactions in the sequence.
(21) Molecular structures are depicted using Crystalmaker; ORTEP
diagrams of crystallographically characterized molecules can be found in
the Supporting Information.
(22) (a) Cairncross, A.; Sheppard, W. A.; Wonchoba, E.; Guilford, W.
J.; House, C. B.; Coates, R. M. Org. Synth. 1980, 59, 122. (b) Sundararaman,
A.; Lalancette, R. A.; Zakharov, L. N.; Rheingold, A. L.; Ja¨kle, F.
Organometallics 2003, 22, 3526. (c) Sundararaman, A.; Ja¨kle, F. J.
Organomet. Chem. 2003, 681, 134.
Dilithiation of 1,2-Br2C6F4 is accomplished with 2.4 equiv
of nBuLi at -78 °C in Et2O or THF. WARNING: this solution
should not be warmed aboVe this temperature in the absence
(16) Chase, P. A.; Piers, W. E.; Patrick, B. O. J. Am. Chem. Soc. 2000,
122, 12911.
(17) Tilson, H. A., Sparber, S. B., Eds. Neurotoxicants and Neurobio-
logical Functions-Effects of OrganoheaVy Metals; Wiley: New York, 1987.
(18) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf, M.
R. J. Org. Chem. 1995, 60, 3020.
(23) See: Sun, Y.; Piers, W. E.; Parvez, M. Can. J. Chem. 1998, 76,
513, and references therein.
(24) Chambers, R. D.; Chivers, T. J. Chem. Soc. 1965, 3933.
(19) Fro¨hn, H.-J.; Franke, H.; Fritzen, P.; Bardin, V. V. J. Organomet.
Chem. 2000, 598, 127.

Bifunctional Perfluoroaryl Boranes
Organometallics, Vol. 25, No. 2, 2006 351
Figure 2. Crystalmaker depiction of the molecular structure of 2.
Selected bond distances (Å): B(1)-C(1), 1.563(4); B(1)-C(7),
1.558(4); B(1)-C(18), 1.557(4); B(2)-C(6), 1.567(4); B(2)-C(19),
1.564(4); B(2)-C(30), 1.557(4). Selected bond angles (deg): C(1)-
B(1)-C(7), 129.7(2); C(1)-B(1)-C(18), 126.7(2); C(7)-B(1)-
C(18), 103.6(2); C(6)-B(2)-C(19), 127.5(2); C(6)-B(2)-C(30),
129.1(2); C(19)-B(2)-C(30). Selected dihedral angles (deg):
B(1)-C(1)-C(6)-B(2) 6.48(2).
Scheme 2
Figure 1. (a) Crystalmaker depiction of the molecular structure
of a monomeric unit of [K]+2[C6F5-1,2-(BF3)2]2-. Selected bond
lengths (Å): K(1)‚‚‚F(6), 2.963(14); K(1)‚‚‚F(4), 3.017(17);
K(2)‚‚‚F(4), 3.170(29); K(2)‚‚‚F(1), 2.607(20); C(1)-B(1), 1.631-
(6); C(2)-B(2), 1.634(7); B(1)-F(1), 1.415(3); B(1)-F(2), 1.419-
(8); B(1)-F(3), 1.406(14); B(2)-F(4), 1.419(4); B(2)-F(5), 1.413-
(18); B(2)-F(6), 1.417(4); B(1)‚‚‚B(2), 3.201(11); K(1)‚‚‚K(2),
4.550(9). Selected bond angles (deg): B(2)-C(2)-C(1), 122.1-
(2); B(1)-C(1)-C(2), 122.7(2). Selected dihedral angle (deg):
B(2)-C(2)-C(1)-B(1), 11.07(37). (b) Crystalmaker depiction of
the molecular structure of a monomeric unit of [K]+{C6F4-1,2-
[(BF2)2(µ-F)]}-. Selected bond lengths (Å): K(1)‚‚‚F(1), 2.660-
(2); B(1)-F(1), 1.427(4); B(1)-F(2), 1.405(4); B(1)-F(3), 1.487-
(4); B(1)-C(1), 1.610(4); C(1)-C(1′), 1.409(5). Selected bond
angles (deg): B(1)-F(3)-B(1′), 113.2(3); B(1)-C(1)-C(1′),
109.5(2). Selected dihedral angles (deg): B(1)-C(1)-C(1′)-B(1′),
0.0; B(1)-F(3)-B(1′)-C(1′), -14.1(4).
X-ray quality crystals of 2 were obtained from cold toluene
solutions, and a view of the molecular structure is shown in
Figure 2. The metrical parameters of the two 9-borafluorene
groups are essentially identical to those observed in (F5C6)B-
(C12F8).16 The bond lengths between the ortho-phenylene linker
and the boron atoms (B(1)-C(1) ) 1.563(2) Å, B(2)-C(6) )
1.567(2) Å) are comparable with those in 1.9 As in 1, the C(1)-
C(6) distance of 1.425(1) Å is elongated to a small degree over
the remaining C-C linkages (average C-C ) 1.379(2) Å).6b
There are, however, some striking differences in the structures
of 1 and 2, due to the planarity enforced by the 9-borafluorene
framework. For example, the B(1)-C(1)-C(6)-B(2) torsion
angle in 2 (6.78(1)°) is significantly lower than observed for 1
(19.9(2)°). As a result, the B‚‚‚B distance in 2 is smaller (3.056
Å) than that observed in 1 (3.138 Å).9 Although all boron centers
in both of these systems are essentially planar (∑C-B-C ) 360°),
the angle between the trigonal planes of the individual boron
atoms of 2 is ∼14° (cf. ∼39° for 1). The borafluorene groups
form a dihedral angle of ∼57-61° with respect to the backbone
C6F4 plane. The reduced distortions in 2 as opposed to 1 reflect
the greater steric pressures in 1 since the C6F5 rings are free to
rotate and occupy more volumetric space in the B‚‚‚B region;
the aryl substituents in 2 are constrained into planes that coexist
(Unfortunately, conversion of [K]2+[C6F4-1,2-(BF3)2]2- directly
into C6F4-1,2-(BBr2)2 by treatment with BBr3 was not success-
ful.) Given the relatively mild reaction conditions employed,
condensation to the 9,10-diboraanthracene C12F8-9,10-(µ-BBr)2,
which is a complication in the conversion of the mercury trimer
to C6F4-1,2-(BBr2)2 using BBr3, is not observed. Thus, this key
bis-dibromoboryl compound is obtained in 41% overall yield
from 1,2-Br2C6F4 in four convenient steps.
Perfluoroaryl diboranes 1 and 2 are readily prepared as shown
in Scheme 2. The synthesis of 1 from C6F4-1,2-(BBr2)2 and Zn-
(C6F5)2 has been reported previously.9 The preparation of 2, a
new diborane Lewis acid, utilizes the organotin reagent Me2-
24
SnC12F8 to install the C12F8 unit via tin-boron exchange.16
Although relatively forcing conditions are required, the reaction
is clean and high yielding. The 19F NMR spectrum of 2 shows
a six-line pattern consistent with a molecule having C2V
symmetry. Like its monofunctional counterpart,16 2 is deep
orange in the solid and in dry, noncoordinating solvents due to
the presence of a low-energy π-π transition. The UV-vis
spectrum (λmax (hexanes) ) 425 nm, ꢀ ) 5.9 × 102 L mol-1
cm-1) is essentially identical to that of (F5C6)B(C12F8), indicat-
ing the two chromophores are not coupled and offer similar
electronic environments. The UV-vis properties of boroles have
been used as the basis for sensing fluoride ions.25
(25) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am. Chem.
Soc. 2002, 124, 8816.

352 Organometallics, Vol. 25, No. 2, 2006
Chase et al.
to form stable adducts with the monofunctional analogues. These
bases are of similar strength (as judged by their proton
affinities29) but are sterically somewhat differentiated.
Reactions with MeCN and THF. Methylene chloride
solutions of diborane 1 (∼0.03 M) were treated with 1 equiv
of either THF or MeCN and studied by multinuclear NMR
spectroscopy. In the case of THF, minimal interaction between
1 and THF was observed, as judged by the minor perturbations
to the room-temperature 19F NMR spectrum of 1‚THF versus
1 in the absence of THF. Cooling the sample resulted in some
broadening of the spectrum and shifting of the positions of the
resonances, indicative of reversible formation of a labile adduct.
Dissolution of 1 in THF, followed by removal of the solvent in
vacuo, gave 19F NMR spectra of unligated 1, demonstrating that
the adduct, if present, is quite weak. Using 1 equiv of the more
rod-like Lewis base MeCN did, however, result in an observable,
isolable adduct. The signal for the CH3CN protons shifted to
2.47 from 2.10 ppm, while in the 19F NMR spectrum, all the
signals shift upfield of those seen for free 1. At room temper-
ature, rapid exchange of MeCN between boron centers in 1 is
implied by the retention of the five-line pattern in these spectra;
cooling the sample results in coalescence behavior, and at
-40 °C, the number of signals is reflective of the disruption of
C2V symmetry expected upon ligation of MeCN to one boron
center. As the number of equivalents of acetonitrile is raised to
3, the resonance for the para-fluorines of the C6F5 groups in
the room-temperature spectra gradually shifts upfield from -149
to -152 ppm, indicating a shift in this dynamic system toward
the adduct.30 Two signals, one for neutral, four-coordinate boron
at -7.6 ppm and one for three-coordinate boron at 41.7 ppm,
were observed in the 11B NMR spectrum at -40 °C, consistent
with this picture. Removal of solvent from solutions of 1‚MeCN
and redissolution indicated that the acetonitrile is retained in
the solid state (see below).
The behavior of diborole 2 under similar treatment indicates
it is a significantly stronger Lewis acid than 1 toward these
two Lewis bases. Treatment of 2 with exactly 1 equiv (or a
slight deficiency) of either THF or MeCN results in a lowering
of symmetry from C2V to Cs as demonstrated by the presence
of two sets of four signals for the complexed and uncomplexed
-B(C12F8) rings and four separate signals for the inequivalent
C6F4 backbone fluorines in the room-temperature 19F NMR
spectra. The 11B NMR spectra are also consistent with formation
of 2‚THF and 2‚MeCN; furthermore, solutions of these adducts
retain a light green color, indicative of a base-free 9-borafluorene
function. These observations indicate that, unlike the situation
in 1‚MeCN, exchange of the coordinated Lewis base from one
center to the other (by whatever mechanism) is slow in the
monoadducts involving 2. However, addition of even a slight
(10%) excess of Lewis base to solutions of 2‚LB results in
symmetrization of the 19F NMR spectrum to one reflective of
rapid exchange between two Cs symmetric structures. Thus,
while 2‚LB compounds are nonlabile at room temperature,
exchange of LB from one borole center to the other is catalyzed
by free Lewis base. As the equivalency of Lewis base is
Figure 3. (a) Front and back strain in Lewis base/perfluoroarylbo-
rane adducts. (b) Inner and outer coordination to bifunctional Lewis
acids 1 and 2.
in the ortho positions more easily. As discussed below, this
effect has a significant impact on the relative Lewis acidity of
2 versus 1.
Factors Affecting Lewis Acidity. The Lewis acid strength
of monofunctional perfluoroaryl boranes is influenced by several
factors.1 The fluorination of the aryl groups is a key electronic
influence that serves to increase the Lewis acidity of the boron
center by lowering the energy of the boron-based LUMO as
the extent of fluorination increases.26 The lower degree of
fluorination in the borole function, as opposed to the B(C6F5)2
analogue, is compensated by the antiaromatic nature27 of the
five-membered BC4 ring, resulting in comparable Lewis acid
strength despite the fewer number of electron-withdrawing
fluorines present. Steric factors, which include front strain and
back strain28 engendered upon complexation of a Lewis base
with the boron center, are illustrated in Figure 3a. Both of these
effects are likely alleviated in the 9-borafluorene Lewis acid
(F5C6)B(C12F8) relative to B(C6F5)3 due to the planar nature of
the -B(C12F8) moiety. Back strain is also related to the
preorganization energy required to pyramidalize the boron center
from trigonal planar to pyramidal geometry, a process that
requires less energy in the borole Lewis acids since the C-B-C
angle of the five-membered ring is closer to the ideal tetrahedral
value than the trigonal planar value.
For the bifunctional Lewis acids 1 and 2, consideration of
these factors must take into account the presence of the two
Lewis acid centers, which serves to delineate the two faces of
each borane center, which are likely unable to interconvert via
rotation about the C-B bonds, as shown in Figure 3b.
Coordination of a single Lewis base thus can occur on the outer
face of one of the borons, or by approach on a vector into the
chelation pocket, resulting in coordination to the inner face.
Coordination of one Lewis base molecule, whether to the outer
or inner face, results in a Lewis acid of different structure; so
the ability to coordinate a second Lewis base will be dependent
upon the properties of the monoadducts. It can be appreciated
that, relative to their monofunctional counterparts, these bi-
functional Lewis acids experience increased front strain forces
for approach of a Lewis base to the inner face and more severe
back strain for approach from the outer face. Thus, while
electronically they should be comparable (or even more electron
deficient) to monofunctional systems, they are sterically more
constrained and, in the absence of chelation, are not expected
to be competitive with the monofunctional systems for simple,
nonchelatable Lewis bases. With this discussion as a backdrop,
the following compares the Lewis acid properties of 1 and 2
using the Lewis bases THF and MeCN, both of which are known
(29) They have similar proton affinities: http://webbook.nist.gov/
chemistry/pa-ser.html.
(30) (a) The ∆m,p value has been used as an indication of the coordination
environment at boron in C6F5-substituted boranes.1 With 1 equiv of MeCN,
the value is 13.1 ppm, which reaches a minimum of ∼10.5 ppm, which is
substantially lower than the values of ∼18 ppm observed for base-free,
three-coordinate boron centers, but not as low as those observed for fully
ligated systems such as MeCN‚B(C6F5)3 (∆m,p ) 8.4 ppm).32b Thus, these
observations point toward a solution equilibrium between 1 and 1‚MeCN
at equimolar amounts, with formation of low amounts of 1‚(MeCN)2 at
higher equivalencies of base.
(26) (a) Vanka, K.; Chan, M. S. W.; Pye, C. C.; Ziegler, T. Organome-
tallics 2000, 19, 1841. (b) Rauk, A. Orbital Interaction Theory of Organic
Chemistry, 2nd ed.; Wiley: New York, 2001.
(27) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem. Soc. 1986, 108,
379.
(28) Brown, H. C. J. Chem. Soc. 1956, 1248.

Bifunctional Perfluoroaryl Boranes
Organometallics, Vol. 25, No. 2, 2006 353
Table 1. Selected Metrical Data for 1‚MeCN and 2‚MeCN
1‚MeCN
2‚MeCN
Selected Bond Lengths (Å)
B(1)-C(5)
B(1)-C(11)
B(1)-C(17)
B(1)-N(1)
B(2)-C(10)
B(2)-C(23)
B(2)-C(29)
C(5)-C(10)
N(1)-C(1)
C(1)-C(2)
B(1)‚‚‚B(2)
1.658(7)
1.647(8)
1.656(8)
1.607(6)
1.571(6)
1.583(8)
1.602(7)
1.451(7)
1.155(5)
1.442(7)
3.321
B(1)-C(15)
B(1)-C(3)
B(1)-C(14)
B(1)-N(1)
B(2)-C(20)
B(2)-C(21)
B(2)-C(32)
C(15)-C(20)
N(1)-C(1)
C(1)-C(2)
B(1)‚‚‚B(2)
1.603(9)
1.626(8)
1.619(9)
1.592(8)
1.567(9)
1.573(9)
1.578(9)
1.425(8)
1.138(7)
1.441(9)
3.174
Figure 4. Crystalmaker depiction of the molecular structure of
1‚MeCN.
Selected Bond Angles (deg)
B(1)-N(1)-C(1)
N(1)-C(1)-C(2)
C(11)-B(1)-C(5)a
C(17)-B(1)-C(5)
C(11)-B(1)-C(17)
C(10)-B(2)-C(23)b
C(10)-B(2)-C(29)
C(23)-B(2)-C(29)
B(1)-C(5)-C(10)
B(2)-C(10)-C(5)
170.6(4)
179.2(5)
114.3(4)
112.9(4)
113.6(4)
118.4(4)
121.9(4)
119.2(4)
123.9(4)
126.7(4)
B(1)-N(1)-C(1)
170.1(6)
177.2(6)
97.9(5)
115.0(5)
113.9(5)
102.2(5)
129.3(5)
128.2(5)
120.0(5)
127.1(5)
N(1)-C(1)-C(2)
C(3)-B(1)-C(14)c
C(14)-B(1)-C(15)
C(3)-B(1)-C(15)
C(21)-B(2)-C(32)d
C(20)-B(2)-C(32)
C(20)-B(2)-C(21)
B(1)-C(15)-C(20)
B(2)-C(20)-C(15)
Selected Dihedral Angles (deg)
7.0(1) B(1)-C(15)-C(20)-
B(2)
B(1)-C(5)-C(10)-
B(2)
0.9(1)
b
a Sum of C-B-C angles about B(1) for 1‚MeCN: 340.7°. Sum of
C-B-C angles about B(2) for 1‚MeCN: 359.5°. c Sum of C-B-C angles
d
about B(1) for 2‚MeCN: 326.8°. Sum of C-B-C angles about B(2) for
2‚MeCN: 359.7°.
of B(1) as measured by the lower ∑C-B-C value in 2‚MeCN.
The other obvious difference in these structures is that, in
1‚MeCN, the acetonitrile coordinates to the inner face of B(1),
while in 2‚MeCN, coordination occurs on one of the outer faces.
This is likely due to the significantly lower degree of back strain
encountered upon pyramidalization of the borole boron upon
outer face coordination in 2; it is also likely that, in the event
of coordination of the rod-like MeCN to an inner face of 2,
strong steric interactions with the (more rigid) adjacent bo-
rafluorene ring would result. The flat 9-borafluorene rings are
able to stack and are essentially parallel in 2‚MeCN, spaced
by about 3.2-3.3 Å. In 1‚MeCN, this mode of coordination
would push two C6F5 rings into the crowded B-B pocket, and
so the base opts to coordinate to the more sterically challenging
inner face. To accommodate the coordination of MeCN to the
inner boron face, the (B(1)-C(5)-C(10) (123.9(4)°) and B(2)-
C(10)-C(5) (126.7(4)°) angles have increased relative to the
analogous angles in 1 (122.8(3)° and 121.8(3)°). Although the
face of the planar, noncoordinated boron center B(2) is oriented
toward the coordinated nitrile, there is no indication of any
interaction of this center with the nitrile (B(2)-N(1) ) 2.949
Å, ∑C-B(2)-C ) 359.7°).
Figure 5. Crystalmaker depiction of the molecular structure of
2‚MeCN.
increased to 3.0 equiv, the 19F NMR spectrum sharpens
somewhat, but remains averaged with little change in the
positions of the signals.
Although 2 is clearly a more powerful Lewis acid toward
THF, the greater affinity of 2 for MeCN in comparison to 1
was demonstrated semiquantitatively by a competition experi-
ment. Mixtures of 1, 2, and MeCN (1:1:1) or isolated 1‚MeCN
and 2 (1:1) resulted in solutions that were, within the detection
limits of 19F NMR spectroscopy, exclusively free 1 and
2‚MeCN, placing a lower limit on the Keq for the equilibrium
shown in eq 1 at ∼2500. In the monofunctional analogues of 1
1‚MeCN + 2 h 1 + 2‚MeCN
(1)
and 2 (B(C6F5)3 and (F5C6)B(C12F8), respectively) a competition
experiment with MeCN had a Keq ) 1.30(3) at 25 °C in favor
of the 9-borafluorene adduct.16
A rationale for the greater LA strength of 2 toward THF and
MeCN is afforded by the molecular structures of the monoad-
ducts 1‚MeCN and 2‚MeCN,31 shown in Figures 4 and 5,
respectively; comparative selected metrical data are given in
Table 1. Metrical parameters associated with the coordinated
nitrile moiety are in general agreement with data for mono-
functional B(C6F5)332 and (C6F5)BC12F833 adducts of nitriles and
will not be discussed in detail here. That 2 coordinates MeCN
more strongly than 1 is indicated by the shorter B(1)-N(1)
distance in 2‚MeCN and the greater degree of pyramidalization
The presence of a second, unligated borane center in these
adducts raises the possibility of coordination of a second Lewis
base.34 Indeed, the dynamic character of the 19F NMR spectra
for these adducts as discussed above implies that bis-Lewis base
adducts might be accessible, particularly in the case of 2
(32) (a) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.;
Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581. (b)
Jacobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.; Meyer,
O. Organometallics 1999, 18, 1724. (c) Fraenk, W.; Klapo¨tke, T. M.;
Krumm, B.; Mayer, P.; Piotrowski, H.; Vogt, M. Z. Anorg. Allg. Chem.
2002, 628, 745.
(31) The molecular structure of 2‚THF has also been determined and is
comparable to 2‚MeCN in terms of overall geometry and the metrical
parameters. Full details of this structure are included in the Supporting
Information.
(33) Chase, P. A.; Romero, P. E.; Piers, W. E.; Parvez, M.; Patrick, B.
O. Can. J. Chem., submitted.
(34) Perfluoro-9,10-diboraanthracene readily coordinates 2 equiv of
MeCN.6b

354 Organometallics, Vol. 25, No. 2, 2006
Chase et al.
Table 2. Selected Metrical Data for 1‚(MeCN)2
and 2‚(THF)2
1‚(MeCN)2
2‚(THF)2
Selected Bond Lengths (Å)
B(1)-C(5)
1.645(3)
1.636(4)
1.641(4)
1.630(4)
1.647(4)
1.655(4)
1.639(4)
1.607(3)
1.434(3)
1.135(3)
1.450(7)
1.134(3)
1.455(4)
3.468
B(1)-C(1)
1.631(3)
1.619(3)
1.614(3)
1.611(3)
B(1)-C(11)
B(1)-C(17)
B(1)-N(1)
B(2)-C(10)
B(2)-C(23)
B(2)-C(29)
B(2)-N(2)
C(5)-C(10)
N(1)-C(1)
C(1)-C(2)
N(2)-C(3)
C(3)-C(4)
B(1)‚‚‚B(2)
B(1)-C(4)
B(1)-C(15)
B(1)-O(1)
C(1)-C(1′)
1.440(4)
3.344
Figure 6. Crystalmaker depiction of the molecular structure of
1‚(MeCN)2.
B(1)‚‚‚B(1′)
Selected Bond Angles (deg)
167.8(2)
B(1)-N(1)-C(1)
N(1)-C(1)-C(2)
C(11)-B(1)-C(5)a
C(17)-B(1)-C(5)
C(11)-B(1)-C(17)
B(2)-N(2)-C(3)
N(2)-C(3)-C(4)
C(10)-B(2)-C(23)b
C(10)-B(2)-C(29)
C(23)-B(2)-C(29)
B(1)-C(5)-C(10)
B(2)-C(10)-C(5)
177.5(3)
117.3(2)
112.8(2)
116.1(2)
169.7(2)
179.1(3)
109.7(2)
118.7(2)
112.4(2)
127.0(2)
126.8(2)
C(1)-B(1)-C(4)c
C(1)-B(1)-C(15)
C(4)-B(1)-C(15)
118.7(2)
122.7(2)
98.6(2)
B(1)-C(1)-C(1′)
124.4(1)
Selected Dihedral Angles (deg)
28.1(1) B(1)-C(1)-C(1′)-
B(1′)
B(1)-C(5)-C(10)-
B(2)
27.7(1)
b
a Sum of C-B-C angles about B(1) for 1‚(MeCN)2: 346.2°. Sum of
c
C-B-C angles about B(2) for 1‚(MeCN)2: 340.8°. Sum of C-B-C
angles about B(1) for 2‚(THF)2: 340.0°.
Figure 7. Crystalmaker depiction of the molecular structure of
2‚(THF)2.
Scheme 4
Scheme 3
(Scheme 3). Although their viability in solution is based on
indirect evidence, crystals grown from solutions of 1/excess
MeCN and 2/excess THF demonstrate that bis-adducts are
accessible. Figure 6 shows the molecular structure of 1‚(MeCN)2,
Figure 7 shows the structure of 2‚(THF)2, while Table 2 gives
comparative metrical data.
In the structure of 2‚(THF)2, the two THF bases coordinate
to the outer faces of the Lewis acid centers, giving a structure
with crystallographic 2-fold symmetry. The B(1)-O(1) distance
of 1.611(3) Å is somewhat longer that typical B-O distances
in adducts of B(C6F5)3 with oxygen-based donors;1 the B-O
distance in 2‚THF is 1.581(3) Å.31 The flat borafluorene rings
are essentially parallel and slightly closer together than the rings
in 2‚MeCN, and the B(1)-C(1)-C(1′)-B(1′) dihedral angle
is quite large at 27.7°, indicating a fair degree of steric tension
in this structure. Nonetheless, it lends credence to the involve-
ment of such a species in the rapid symmetrization observed in
adducts 2‚LB in the presence of slight excesses of base via the
process given in Scheme 3.
system. Here, the first Lewis base coordinates to the inner face
(see above), but the second acetonitrile donor approaches the
other boron center from the outer face. The metrical parameters
associated with the inner coordinated MeCN (the boron labeled
B(2) in Figure 6) differ only slightly from that observed in
1‚MeCN, but indications are that the outer coordinated aceto-
nitrile is more weakly bound. The B(1)-N(1) distance is longer,
and the extent of pyramidalization at B(1) (∑C-B-C ) 346.2°)
is less than that at B(2) (∑C-B-C ) 340.8°). Given that the
second acetonitrile coordinates in this fashion, it is unlikely that
this bis-acetonitrile adduct is involved in the mechanism of
process responsible for symmetrization of the 19F atoms in
1‚MeCN (Scheme 4). Dissociation of the inner coordinated
acetonitrile in 1‚(MeCN)2 would lead to an outer coordinated
isomer of 1‚MeCN, which could not revert to the inner
coordinated species observed via simple rotation about the
appropriate B-C bond based on steric arguments. Thus the
symmetrization observed in the 19F NMR spectra of 1‚MeCN
The isolation of crystals of 1‚(MeCN)2 is perhaps more
surprising given the higher steric congestion present in this

Bifunctional Perfluoroaryl Boranes
Organometallics, Vol. 25, No. 2, 2006 355
elemental analyzer by Mrs. Dorothy Fox, Mrs. Roxanna Smith, and
Mrs. Olivera Blagojevic of this department. GC-MS experiments
were run on a Varian Saturn 2000 GC-MS. Column and injector
temperature were initially 50 °C, and the column temperature was
ramped to 250 °C over 20 min and held at that temperature for an
additional 20 min.
is likely due simply to a labile acetonitrile that dissociates from
one borane and recoordinates to the other. While it may be
tempting to invoke a transition state in which the acetonitrile is
simultaneously engaging both borane centers, there is no direct
evidence for chelation of acetonitrile in this instance.
Conclusions. In summary, a new, mercury-free route to the
synthon C6F4-1,2-(BBr2)2 has been developed from 1,2-Br2C6F4
in a four-step synthesis with an overall yield of ∼47%, on par
with the previously reported procedure employing mercury
reagents. This was used to generate the new bifunctional
9-borafluorene Lewis acid C6F4-1,2-[B(C12F8)]2 (2) by reaction
with 2 equiv of Me2SnC12F8 in 81% yield and generate C6F4-
1,2-[B(C6F5)2]2 (1) via existing methodology. Compound 2 is
deep orange, due to the presence of a low-energy π-π transition
in the 9-borafluorene chromophores.
20,38
24
The compounds Zn(C6F5)2
and C12F8SnMe2 were synthe-
sized via literature procedures. Bifunctional Lewis acid 1 was
prepared from C6F4-1,2-(BBr2)2 using the previously described
procedure.9 Reagent quantities of MeOH and B(OMe)3 were dried
over Na, while MeCN was dried over CaH2; all reagents were
freshly distilled prior to use. BBr3 was dried over Cu wire and
vacuum transferred directly into reaction flasks. All other reagents
were purchased from Aldrich Chemicals or Boulder Scientific and
used as received.
Synthesis of C6F4-1,2-[B(OH)2]2. Butyllithium (12.6 mL, 1.6
M in hexanes, 20.16 mmol) was added dropwise to a stirred solution
of 1,2-Br2C6F4 (2.6 g, 8.44 mmol) in ether (50 mL) at -78 °C.
After 2 h this solution was slowly added to a precooled solution
(-78 °C) of B(OMe)3 (2.3 mL, 20.52 mmol) in ether (25 mL) via
cannula. The fluoroaryllithium reagent was kept cool during this
transfer by constantly rubbing the cannula with a cotton swab cooled
in a dry ice/acetone bath. The resulting suspension was stirred at
-78 °C for 2 h, then gradually warmed to room temperature and
hydrolyzed with 20 mL of 10% HCl. Volatiles were removed from
the sample, leaving an oily residue, which was extracted with hot
water (3 × 25 mL). The solution was concentrated (ca. 50 mL)
and left to recrystallize at 5 °C overnight. The resulting solid
(predominantly borate salts) was filtered, and the remaining water
layer was evaporated to dryness, affording a white solid (1.76 g,
88%). 19F NMR (D2O/HCl): δ -136.6 (2F, -C6F4), -156.5 (2F,
-C6F4). 11B{1H} NMR (D2O/HCl): δ 20.7. Anal. Calcd for
C6H4B2F4O4: C, 30.31; H, 1.70. Found: C, 30.55; H, 1.57.
CAUTION: The isolation or warming of solutions containing
fluoroaryllithium compounds has been known to cause explosions.39
Synthesis of [K]+2[C6F4-1,2-(BF3)2]2-. A solution of C6F4-1,2-
{B(OH)2}2 (2.75 g, 11.56 mmol) in methanol (40 mL) was added
to a stirred solution of KHF2 (5.92 g, 75.79 mmol) in water (20
mL) and stirred for 3.5 h. The solution was filtered, and the solid
was washed with a 25% water solution in ether (3 × 10 mL), then
ether (3 × 10 mL), and dried under reduced pressure, affording an
off-white solid (2.51 g, 60%). 19F NMR (CD3CN/D2O): δ -141.2
(2F, -C6F4), -146.2 (q, JF-B ) 56 Hz, 6F, -BF3), -162.9 (2F,
-C6F4). 11B{1H} NMR (CD3CN/D2O): δ 3.9. X-ray quality crystals
were grown from a concentrated acetonitrile solution of the product.
[K]+[C6F4-1,2-(BF2)2(µ-F)]-: 19F NMR (CD3CN/D2O): δ -138.4
(2F, -C6F4), -139.7 (br s, 1F, µ-F), -143.4 (4F, -BF2), -159.8
(2F, -C6F4). X-ray quality crystals were grown from a concentrated
solution of [K]+2[C6F4-1,2-(BF3)2]2- containing ∼10% water.
Synthesis of C6F4-1,2-(BF2)2. An excess of boron trifluoride gas
was introduced to a stirred suspension of [K]+2[C6F4-1,2-(BF3)2]2-
(1.0 g, 2.76 mmol) in dichloromethane (10 mL) in a thick walled
pressure vessel at -40 °C. After 30 min the solution was degassed
under vacuum at -78 °C. The faint red colored solution was filtered,
and the remaining solid (KBF4) was washed with dichloromethane
(2 × 5 mL). A red liquid was obtained after solvent was distilled
from the product under reduced pressure (0.57 g, 84%). 19F NMR
(CD2Cl2): δ -72.0 (4F, -BF2), -126.9 (2F, -C6F4), -146.5 (2F,
-C6F4). 11B{1H} NMR (CD2Cl2): δ 22.6. NOTE: The product is
relatively volatile and will be removed under full vacuum.
Synthesis of C6F4-1,2-(BBr2)2. Boron tribromide (0.76 g, 3.05
mmol) was introduced to a solution of 1,2-(BF2)2C6F4, (0.15 g, 0.61
mmol) in toluene (5 mL) in a thick walled bomb at -78 °C. The
Fundamental aspects of the Lewis acid/base chemistry of
these two diboranes were also investigated. Spectroscopic,
structural, and solution competition studies have shown 2 to be
a significantly stronger Lewis acid than 1 for neutral Lewis bases
MeCN and THF. For 2, strong stable adducts are generated,
whereas a relatively weak MeCN adduct of 1 is formed and
virtually no interaction is observed with THF. This is primarily
ascribed to steric effects due the significant reduction in front-
and back-strain engendered by the planar 9-borafluorene groups.
Structural studies also show significant differences in the
coordination mode of the Lewis bases, especially in terms of
facial discrimination. Exclusive coordination to the sterically
more accessible outer face is observed with 2. However, the
seemingly more congested Lewis acid 1 exhibits MeCN
coordination inside the B-B pocket to the hindered inner face.
Also, the differences in facial coordination gave rise to distinct
pathways for Lewis base exchange between Lewis acid sites.
This difference in facial reactivity could have potential impact
on the use of these systems in catalysis, especially for 2, where
potential chelation of a substrate by both acidic centers is
precluded by preferential outer coordination.
Experimental Section
General Procedures. All manipulations of air- and moisture-
sensitive materials were performed on a double-manifold high-
vacuum line using modified Schlenck techniques or in a glovebox
under an atmosphere of nitrogen.35 Residual oxygen and moisture
were removed from the argon stream by passage through an
Oxisorb-W scrubber from Matheson Gas Products. All solvents
were dried and purified by passing through suitable drying agents
(toluene, hexanes, and tetrahydrofuran, Grubbs/Dow purification
system;36 benzene, d6-benzene, and d8-toluene, sodium/benzophe-
none ketal; diethyl ether, LiAlH4; methylene chloride and d2-
methylene chloride, CaH2), with the exception of d3-chloroform,
d3-acetonitrile, and D2O, which were used as received. Nuclear
magnetic resonance spectroscopy was performed on a AMX-300
MHz (1H, 300.138 MHz; 11B{1H}, 96.293 MHz, 19F, 282.371 MHz)
or Bruker Avance DRX-400 (1H, 400.134 MHz; 11B{1H}, 128.375
MHz) spectrometers. Data are given in ppm relative to the solvent
signals for 1H spectra or relative to external standards for 11B
(BF3‚OEt2, 0 ppm) and 19F (C6F6, -163.0 ppm). Temperature
calibration for NMR experiments was achieved by monitoring the
1H NMR spectrum of pure methanol (below room temperature) and
pure ethylene glycol (above room temperature).37 Elemental
analyses were performed on a Control Equipment Corporation 440
(35) Burger, B. J.; Bercaw, J. E. Experimental Organometallic Chemistry;
American Chemical Society: Washington, D.C., 1987.
(36) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(38) Sartori, P.; Weidenbruch, M. Chem. Ber. 1967, 100, 3016.
(39) (a) Chen, E. Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J.
J. Am. Chem. Soc. 1998, 120, 6287. (b) Spence, R. E. v. H. Chem. Eng.
News 1996, 74, 4.
(37) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46.

356 Organometallics, Vol. 25, No. 2, 2006
Table 3. Summary of Data Collection and Structure Refinement Details for [K]2+[C6F4-1,2-(BF3)2]2-
Chase et al.
,
[K]+{C6F4-1,2-[(BF2)2(µ-F)]}-, 2, 1‚MeCN, 2‚MeCN, 1‚(MeCN)2, and 2‚(THF)2
[K]2+[C6F4-1,2-(BF3)2]-
[K]+{C6F4-1,2-[(BF2)2(µ-F)]}-
2
1‚MeCN
formula
fw
C6B2F10K2
361.88
C8H5B2F9KNO
362.85
C30B2F20‚1.5C7H8
900.12
C32H3B2F24N
878.97
temp, K
cryst syst
space group
a, Å
b, Å
c, Å
173(2)
monoclinic
P21/n
7.046(2)
15.853(6)
9.341(4)
90
104.46(2)
90
173(2)
orthorhombic
Cmcm
8.111(5)
26.229(5)
11.966(5)
90
90
90
2546(3)
8
1.893
160(2)
triclinic
P1h
173(2)
triclinic
P1h
9.9667(7)
11.3162(7)
16.8523(11)
83.699(2)
89.897(2)
71.640(2)
1792.0(2)
2
9.5370(15)
10.7220(7)
17.0989(12)
73.417(5)
82.369(3)
71.803(2)
1591.1(3)
2
R, deg
â, deg
γ, deg
V, Å3
Z
1010.3(6)
4
2.379
d
calc, Mg m-3
1.668
0.169
1.836
0.206
µ, mm-1
1.068
0.526
cryst size, mm
no. of rflns measd
no. of unique rflns
R1/wR2
0.012 × 0.08 × 0.08
0.14 × 0.12 × 0.10
0.75 × 0.50 × 0.50
0.30 × 0.25 × 0.08
4494
4973
14 384
33 552
2310
0.031/0.090
0.97
1609
0.0508/0.1365
1.067
7982
0.0643/0.2118
1.076
7211
0.0736/0.2472
1.001
gof
res density, e/ Å3
0.33 / -0.43
0.85 / -0.50
0.29 / -0.38
2‚MeCN
1‚(MeCN)2
2‚(THF)2
formula
fw
C32H3B2F20N‚C7H8
895.11
C34H6B2F24N2‚CH2Cl2
1004.95
(C19H8BF10O)2‚0.5CH2Cl2
991.05
temp, K
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
160(2)
monoclinic
P21/c
9.8508(6)
12.5539(7)
26.6988(15)
90
95.590(2)
90
173(2)
monoclinic
P21/c
13.8512(10)
13.3129(8)
20.046(2)
173(1)
monoclinic
P2/c
9.0312(3)
12.3044(4)
16.8288(6)
90
99.018(2)
90
105.369(8)
3286.0(3)
4
1.809
3564.3(5)
4
1.873
1846.96(11)
2
1.782
Z
d
calc, Mg m-3
µ, mm-1
0.185
0.342
0.317
cryst size, mm
no. of rflns measd
no. of unique rflns
R1/wR2
0.52 × 0.10 × 0.03
0.30 × 0.15 × 0.10
0.35 × 0.20 × 0.05
16 280
41 462
12 594
4297
0.0730/0.01741
1.168
8008
0.0466/0.1156
1.068
4152
0.052/0.149
1.03
gof
res density, e/ Å3
0.56/-0.46
0.44/-0.59
0.33/-0.54
1
bomb was sealed and heated at 60 °C for 30 min. Volatiles were
removed under reduced pressure to afford a yellow-green oil with
the same spectroscopic properties as those previously reported9 (0.25
g, 85%). 19F NMR (C6D6): δ -125.7 (2F, -C6F4), -147.3 (2F,
-C6F4). 11B{1H} NMR (C6D6): δ 54.0.
white solid (36 mg, 69% yield). H NMR (CD2Cl2): δ 2.47 (s,
3H, MeCN). 19F NMR (CD2Cl2): δ -128.7 (2F, -C6F4), -130.0
(8F, o-B(C6F5)2), -149.7 (4F, p- B(C6F5)2), -156.4 (2F, -C6F4),
-162.1 (8F, m-B(C6F5)2). 11B{1H} NMR (CD2Cl2, 228 K): δ 41.7,
-7.6. 13C{1H} (CD2Cl2): δ 2.95 (MeCN). 1H NMR (CD2Cl2, 228
K): δ 2.50 (s, 3H, MeCN). 19F NMR (CD2Cl2, 228 K): δ -126.3
(4F, o-B(C6F5)2), -126.9 (1F, -C6F4), -130.9 (1F, -C6F4),
-132.9 (4F, o-B(C6F5)2), -141.9 (2F, p-B(C6F5)2, -154.0 (1F,
-C6F4), -154.6 (2F, p-B(C6F5)2), -157.8 (1F, -C6F4), -160.0
(4F, m-B(C6F5)2), -163.1 (4F, m-B(C6F5)2). X-ray quality crystals
of 1‚MeCN were obtained by layering 1 equiv of MeCN (0.320
mL, 0.0487 M in hexanes, 0.016 mmol) on a solution of C6F4-1,2-
[B(C6F5)2]2, 1 (13 mg, 0.016 mmol), in CH2Cl2 (0.3 mL), which
was allowed to stand at -40 °C. X-ray quality crystals of
1‚(MeCN)2 were obtained by layering excess equivalents of MeCN
(1.274 mL, 0.0487 M solution in hexanes, 0.062 mmol) over a
solution of C6F4-1,2-[B(C6F5)2]2 (13 mg, 0.016 mmol) in CH2Cl2
(0.3 mL), which was allowed to stand at -40 °C.
Synthesis of C6F4-1,2-[B(C12F8)]2 (2). Me2Sn(C12F8) (0.786 g,
1.77 mmol) and C6F4(BBr2)2 (0.435 g, 0.84 mmol) were dissolved
in toluene (20 mL), sealed in a glass bomb equipped with a Kontes
valve, and heated to 85 °C for 36 h. The solvent was removed in
vacuo, and the Me2SnBr2 byproduct was removed via sublimation
(30 °C, 0.01 mmHg). The sublimation residues were placed in a
frit assembly, and hexanes (30 mL) was condensed into the flask.
The solution was cooled to -78 °C and stirred for 1 h. The solution
was cold filtered, and a light yellow solid was obtained and dried
in vacuo (0.520 g, 81%). 19F NMR (C6D6): δ -121.1 (4F, C12F8),
-123.6 (2F, C6F4), -128.9 (4F, C12F8), -138.6 (4F, C12F8), -148.6
(2F, C6F4), -151.9 (4F, C12F8). λmax (hexanes): 425 nm ꢀ )
5.9 × 102 L mol-1 cm-1. Anal. Calcd for C30B2F20: C 47.29.
Found: C 47.78. X-ray quality crystals were grown from a
concentrated toluene solution cooled to -40 °C.
Synthesis of C6F4-1,2-[B(C12F8)]2‚MeCN (2‚MeCN). In the
glovebox, C6F4-1,2-[B(C12F8)]2, 2 (0.100 g, 0.13 mmol), was
weighed into a vial and dissolved in CH2Cl2 (8 mL). MeCN (0.006
g, 0.14 mmol) was also weighed into a vial and diluted with CH2-
Cl2 (2 mL). The solutions were combined, stirred for 10 min, and
place in the freezer (-40 °C) overnight. The solution was decanted
and the solid product was dried in vacuo (0.090 g, 86%). 1H NMR
Synthesis of C6F4-1,2-[B(C6F5)2]2‚MeCN (1‚MeCN). Solid
C6F4-1,2-[B(C6F5)2]2, 1 (50 mg, 0.060 mmol), was dissolved in CH2-
Cl2 (3 mL) and stirred while an excess of MeCN (ca. 2 mL) was
added. The reaction mixture was stirred for 1 h before volatiles
were removed under reduced pressure, affording the product as a

Bifunctional Perfluoroaryl Boranes
Organometallics, Vol. 25, No. 2, 2006 357
(CD2Cl2): δ 2.65 (s, 3H, MeCN). 19F NMR (CD2Cl2): δ -124.5
(2F, C12F8), -130.7 (1F, C6F4), -131.4 (2F, C12F8), -131.8 (2F,
C12F8), -132.6 (1F, C6F4), -133.5 (2F, C12F8), -142.2 (2F, C12F8),
-153.5 (4F, 2 overlapping signals, C12F8), -155.1 (2F, C12F8),
-155.3 (1F, C6F4), -158.0 (1F, C6F4). 11B{1H} (CD2Cl2): 49.2
(br), -5.6 (br). X-ray quality crystals of 2‚MeCN were obtained
by layering hexanes onto a CH2Cl2 solution of 2‚MeCN and cooling
to -40 °C.
Synthesis of C6F4-1,2-[B(C12F8)2]‚THF (2‚THF). In the glove-
box, C6F4-1,2-[B(C12F8)]2, 2 (0.100 g, 0.13 mmol), was weighed
into a vial and dissolved in CH2Cl2 (8 mL). THF (0.010 g, 0.14
mmol) was also weighed into a vial and diluted with CH2Cl2 (2
mL). The solutions were combined, stirred for 10 min, and place
in the freezer (-40 °C) overnight. The solution was decanted, and
the solid product was dried in vacuo (0.103 g, 94%). 1H NMR (CD2-
Cl2): δ 4.24 (m, 4H, THF)), 2.13 (m, 4H, THF). 19F NMR (CD2-
Cl2): δ -124.6 (2F, C12F8), -130.7 (2F, C12F8), -131.5 (2F,
C12F8), -132.2 (3F, 2 overlapping signals, C12F8 and C6F4), -132.5
(1F, C6F4), -142.2 (2F, C12F8), -152.4 (2F, C12F8), -153.5 (2F,
C12F8), -154.2 (2F, C12F8), -154.9 (1F, C6F4), -157.2 (1F, C6F4).
11B{1H} NMR (CD2Cl2): δ 39.9, 6.3. X-ray quality crystals of
2‚THF were obtained by layering hexanes onto a CH2Cl2 solution
of 2‚THF and cooling to -40 °C. X-ray quality crystals of
2‚(THF)2 were obtained by layering excess equivalents of THF
(1.0 mL, 0.05 M solution in hexanes, 0.062 mmol) over a solution
of 2 (10 mg, 0.013 mmol) in CH2Cl2 (0.3 mL), which was allowed
to stand at -40 °C.
(1F, C6F4), -133.5 (2F, C12F8), -142.2 (2F, C12F8), -153.5 (4F,
2 overlapping signals, C12F8), -155.1 (2F, C12F8), -155.3 (1F,
C6F4), -158.0 (1F, C6F4). 11B{1H} (CD2Cl2): 49.2 (br), -5.6 (br).
2 + 2.0 equiv MeCN (2‚(MeCN)2): 1H NMR (CD2Cl2, 301 K):
2.40 (s, 3H, MeCN). 19F NMR (CD2Cl2, 301 K): -128.0 (4F,
C12F8), -131.6 (2F, C6F4), -132.6 (4F, C12F8), -147.9 (4F, C12F8),
-154.3 (4F, C12F8), -156.7 (2F, C6F4).
2 + 0.9 equiv THF. 1H NMR (CD2Cl2, 301 K): δ 3.77 (m, 4H,
THF), 1.87 (m, 4H, THF). 19F NMR (CD2Cl2, 301 K): δ -126.5
(4F, C12F8), -131.9 (4F, C12F8), -132.3 (2F, C6F4), -147.5 (4F,
1
C12F8), -153.9 (4F, C12F8), -156.1 (2F, C6F4). H NMR (CD2-
Cl2, 193 K): δ 3.62 (br, 4H, THF), 1.76 (br, 4H, THF). 19F NMR
(CD2Cl2, 193 K): δ -131.2 (2F, C6F4), -131.5 (4F, C12F8), -134.2
(4F, C12F8), -154.8 (4F, C12F8), -155.2 (4F, C12F8), -158.3 (2F,
C6F4). 11B {1H} NMR: 39.9 (br s), 6.3 (br s). 2 + 2.0 equiv THF
(2‚(THF)2): 1H NMR (CD2Cl2, 301 K): δ 3.94 (m, 4H, THF),
-1.97 (m, 4H, THF). 19F NMR (CD2Cl2, 301 K): -126.4 (4F,
C12F8), -131.9 (4F, C12F8), -132.3 (2F, C6F4), -147.4 (4F, C12F8),
-153.9 (4F, C12F8), -156.1 (2F, C6F4).
X-ray Crystallography. Measurements were made on either a
Nonius Kappa CCD or Bruker SMART 1K CCD diffractometer
using graphite-monochromated Mo KR radiation (0.71073 Å). The
structures were solved by direct methods and refined on F2 values
by full-matrix least squares for all unique data. Table 3 gives further
details, and the crystallographic information files are available as
Supporting Information. The CCDC contains the supplementary
crystallographic data for the crystal structures reported in this paper
(CCDC 278193, [K]+2[C6F5-1,2-(BF3)2]2-; CCDC 278389, [K]+-
{C6F4-1,2-[(BF2)2(µ-F)]}-; CCDC 278192, 2; CCDC 279195,
1‚MeCN; CCDC 281017, 2‚MeCN; CCDC 278194, 1‚(MeCN)2;
CCDC 278196, 2‚(THF)2; CCDC 278197, 2‚THF, Supporting
Information only). These data can be obtained, free of charge, via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, U.K. (fax: 44-1223-336033 or e-mail: deposit@
ccdc.cam.ac.uk)).
Spectroscopic Studies of the Addition of LB to 1 and 2. In a
glovebox a 5 mm NMR tube was charged with a known amount
of 1 or 2 (∼0.012 mmol), and CD2Cl2 (0.4 mL) was added. The
sample was capped with a rubber septum and removed from the
glovebox. Varying equivalents of dry and degassed LB (LB )
MeCN, 0.464 M in CD2Cl2; LB ) THF, 0.500 M in CD2Cl2) was
added via gastight syringe, and the NMR tube was placed in the
spectrometer.
1
1 + 1.0 MeCN. H NMR (CD2Cl2, 298 K): δ 2.51 (s, 3H,
MeCN). 19F NMR (CD2Cl2, 298 K): δ -127.5 (10F, -C6F4 and
o-B(C6F5)2), -143.2 (br, 4F, o-B(C6F5)2), -148.7 (br, 2F, -C6F4),
-161.0 (8F, m-B(C6F5)2). 11B{1H} NMR (CD2Cl2. 228 K): δ 41.7,
-8.6. 1 + 2.0 MeCN (1‚(MeCN)2): 1H NMR (CD2Cl2, 298 K):
δ 2.27 (s, 3H, MeCN). 19F NMR (CD2Cl2, 298 K): δ -128.2 (2F,
-C6F4), -130.6 (8F, o-B(C6F5)2), -151.0 (4F, p-B(C6F5)2), -157.1
(2F, -C6F4), -162.3 (8F, m-B(C6F5)2). 1 + 3.0 MeCN: 1H NMR
(CD2Cl2, 298 K): δ 2.15 (s, 3H, MeCN). 19F NMR (CD2Cl2, 298
K): δ -127.8 (2F, -C6F4), -130.9 (8F, o-B(C6F5)2), -152.0 (4F,
p-B(C6F5)2), -157.6 (2F, -C6F4), -162.5 (8F, m-B(C6F5)2). 11B-
{1H} NMR (CD2Cl2): δ -7.0.
Acknowledgment. Funding for this work came from the
Natural Sciences and Engineering Research Council of Canada
in the form of a Discovery Grant (to W.E.P.) and Postgraduate
Fellowship support (to P.A.C. and L.D.H.). W.C. acknowledges
the EPSRC (UK) for equipment funding.
Supporting Information Available: Crystallographic informa-
tion files for [K]2+[C6F4-1,2-(BF3)2]2-, [K]+{C6F4-1,2-[(BF2)2-
(µ-F)]}-, 2, 1‚MeCN, 2‚MeCN, 2‚THF, 1‚(MeCN)2, and 2‚(THF)2,
as well as ORTEP diagrams for each structure and selected 19F
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
2 + 0.8 equiv MeCN. H NMR (CD2Cl2, 301 K): δ 2.65 (s,
3H, MeCN). 19F NMR (CD2Cl2, 301 K): δ -124.5 (2F, C12F8),
-130.7 (1F, C6F4), -131.4 (2F, C12F8), -131.8 (2F, C12F8), -132.6
OM050764T