Aggregation and ionization equilibria of bis(pentafluorophenyl)borinic acid driven by hydrogen-bonding with tetrahydrofuran

Article abstract of DOI:10.1021/om061167p

Bis(pentafluophenyl)borinic acid, Ar₂BOH (1, Ar = C ₆F₅), in dichloromethane solution is present as an equilibrium mixture of monomeric (1_m) and trimeric (1_t) forms. Previous studies showed that water affects both the position and the rate of this equilibrium. Here, the behavior of 1 in the presence of tetrahydrofuran (THF), a nucleophile able to behave as a Lewis base and H-bond acceptor only, has been studied, by monitoring with ¹H and ¹⁹F NMR the course of titrations performed directly into NMR tubes. The addition, at 183 K, of 0.33 equiv of THF caused the instantaneous and quantitative formation of the hydrogen-bonded adduct between the trimer 1_t and one molecule of THF. Homo- and heteronuclear 2D NMR correlation experiments led to a solution structure consistent with the C₂-optimized geometry obtained by PM3 computations. The H-bonding of the THF molecule causes major deformations of the molecular geometry of the trimer, so that only one molecule of THF can interact with the trimer, in spite of its three OH groups. Intra- and intermolecular exchange processes involving this adduct have been investigated by 2D EXSY experiments, showing flopping of the cycle conformation, rotation of the aromatic rings around their B-C bonds, and exchange of THF among the three OH groups, in addition to the exchange between free 1_t and the adduct. When the amount of added THF was higher than 0.33 equiv, an unexpected ionization process occurred, leading to the cation [Ar₂B(OH ₂)₂]⁺ and to deprotonated 1_t, i.e., to the anion [Ar₆B₃O₃H₂]^- of C_s symmetry. On increasing the temperature, progressive partial fragmentation of the trimeric species was observed. Both ¹¹B NMR evidence and PM3 computations indicated that, at variance with what is observed in the interaction with H₂O, the interaction between THF and 1 _m occurs preferentially via an H-bonded adduct, Ar ₂BO-H...THF, rather than a Lewis acid-base complex, Ar ₂B(OH)(THF). This confirms the poor Lewis acidity of the boron atom of 1_m.

Full text of DOI:10.1021/om061167p

2088
Organometallics 2007, 26, 2088-2095
Aggregation and Ionization Equilibria of
Bis(pentafluorophenyl)borinic Acid Driven by Hydrogen-Bonding
with Tetrahydrofuran
Tiziana Beringhelli,§ Giuseppe D’Alfonso,*,§ Daniela Donghi,§ Daniela Maggioni,§
Pierluigi Mercandelli,*,† and Angelo Sironi†
Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Dipartimento di Chimica Strutturale
e Stereochimica Inorganica, UniVersita di Milano, Via Venezian 21, 20133 Milano, Italy
ReceiVed December 21, 2006
Bis(pentafluophenyl)borinic acid, Ar2BOH (1, Ar ) C6F5), in dichloromethane solution is present as
an equilibrium mixture of monomeric (1m) and trimeric (1t) forms. Previous studies showed that water
affects both the position and the rate of this equilibrium. Here, the behavior of 1 in the presence of
tetrahydrofuran (THF), a nucleophile able to behave as a Lewis base and H-bond acceptor only, has
been studied, by monitoring with 1H and 19F NMR the course of titrations performed directly into NMR
tubes. The addition, at 183 K, of 0.33 equiv of THF caused the instantaneous and quantitative formation
of the hydrogen-bonded adduct between the trimer 1t and one molecule of THF. Homo- and heteronuclear
2D NMR correlation experiments led to a solution structure consistent with the C2-optimized geometry
obtained by PM3 computations. The H-bonding of the THF molecule causes major deformations
of the molecular geometry of the trimer, so that only one molecule of THF can interact with the trimer,
in spite of its three OH groups. Intra- and intermolecular exchange processes involving this adduct
have been investigated by 2D EXSY experiments, showing flopping of the cycle conformation,
rotation of the aromatic rings around their B-C bonds, and exchange of THF among the three OH
groups, in addition to the exchange between free 1t and the adduct. When the amount of added THF was
higher than 0.33 equiv, an unexpected ionization process occurred, leading to the cation [Ar2B(OH2)2]+
and to deprotonated 1t, i.e., to the anion [Ar6B3O3H2]- of Cs symmetry. On increasing the temperature,
progressive partial fragmentation of the trimeric species was observed. Both 11B NMR evidence and
PM3 computations indicated that, at variance with what is observed in the interaction with H2O, the
interaction between THF and 1m occurs preferentially via an H-bonded adduct, Ar2BO-H‚‚‚THF, rather
than a Lewis acid-base complex, Ar2B(OH)(THF). This confirms the poor Lewis acidity of the boron
atom of 1m.
interactions, being able to behave as a Lewis acid or a Lewis
base, as a Brønsted acid, and as a hydrogen-bond donor or
acceptor.
This is clearly shown in the monomer-trimer equilibrium.
We have recently ascertained that in the solid state 1 exists as
a self-associated trimeric species 1t (Chart 1), which in toluene
solution dissociates to give the (C6F5)2BOH monomer 1m
(hereafter Ar2BOH, Ar ) C6F5).4 At variance, in dichlo-
Introduction
The organometallic chemistry of boron is a subject of high
current interest,1 due to the variety of applications that organo-
boron derivatives have found in synthesis and catalysis. The
behavior of these molecules still shows many intriguing aspects
and calls for detailed basic studies on their reactivity and
structural features. The title compound, bis(pentafluorophenyl)-
borinic acid, (C6F5)2BOH (1, hereafter simply borinic acid), is
paradigmatic from this point of view. Recently, it has been the
object of intense investigations, both from academia and
industry.2-6 This is attributable to the fact that, in spite of its
simplicity, 1 can be involved in a variety of intermolecular
(3) Recent patents dealing with synthesis or uses of 1: (a) Kratzer, R.
(Basell Polyolefins) Patent WO 04041871, 2004. (b) Kratzer, R. (Basell
Polyolefins) Patent WO 04007570, 2004. (c) Kratzer, R.; Fraaije, V. (Basell
Polyolefins) Patent WO 04007569, 2004. (d) Takei, K.; Mizuta, K.; Aoki
M.; Takebe, K. (Nippon Shokubai Co.) Patent JP 2004265785, 2004. (e)
Ikeno, I.; Mitsui, H.; Iida, T.; Moriguchi, T. (Nippon Shokubai Co.) Patent
WO 0248156, 2002. (f) Ikeno, I.; Mitsui, H.; Iida, T.; Moriguchi, T. (Nippon
Shokubai Co.) Patent WO 0244185, 2002. (g) Schottek, J.; Fritze, C.
(Targor) Patent DE 10009714, 2001. (h) Kratzer, R. (Basell Polyolefins)
Patent DE 10059717, 2001. (i) Kratzer, R.; Fritze, C.; Schottek, J. (Targor)
Patent DE 19962814, 2001. (j) Frances, J. M.; Deforth, T. (Rhodia Chimie)
Patent WO 0130903, 2001. (k) Schottek, J.; Fritze, C.; Bohnen, H.; Becker,
P. (Targor) WO 0020466, 2000. (l) Bohnen, H.; Hahn, U. (Aventis R&T
GMBH) Patent DE 19843055, 2000. (m) Bohnen, H. (Hoechst A.-G.) Patent
DE 19733017, 1999.
(4) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercan-
delli, P.; Sironi, A. Organometallics 2003, 22, 1588.
(5) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercan-
delli, P.; Sironi, A. Organometallics 2004, 23, 5493.
(6) Britovsek, G, J, P.; Ugolotti, J.; Hunt, P.; White, A, J. P. Chem.
Commun. 2006, 1295.
* Corresponding authors. E-mail:
pierluigi.mercandelli@unimi.it.
giuseppe.dalfonso@unimi.it;
§ Dipartimento di Chimica Inorganica, Metallorganica e Analitica.
† Dipartimento di Chimica Strutturale e Stereochimica Inorganica.
(1) (a) Burkhardt, E. R.; Matos, K. Chem. ReV. 2006, 106, 2617. (b)
Hall, D. G. Boronic Acids; Wiley-VCH: Weinheim, 2005. (c) Smith K.
Organoboron Chemistry. In Organometallics in Synthesis; Manual, A.,
Schlosser, M., Eds.; Wiley: Chichester, 2002. (d) King, R. B., Ed. Boron
Chemistry at the Millennium. J. Organomet. Chem. 1999, 581, 1. (e) Piers,
W. E.; Chivers, T. Chem. Soc. ReV. 1997, 26, 345. (f) Suzuki, A. Pure
Appl. Chem. 1994, 66, 213.
(2) (a) Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999, 527. (b)
Ishihara, K.; Kurihara, H.; Yamamoto, H. J. Org. Chem. 1997, 62, 5664.
(c) Ishihara, K.; Kurihara, H.; Yamamoto, H. Synlett 1997, 597.
10.1021/om061167p CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/17/2007

Aggregation of Bis(pentafluorophenyl)borinic Acid
Organometallics, Vol. 26, No. 8, 2007 2089
Chart 1
romethane solution sizable amounts of the trimeric form of Ar2-
BOH have been detected, in equilibrium with the dominant
monomeric form.5 The trimeric form is unique, in that no other
B3O3 cycle is known containing tetravalent boron atoms only.
3 Ar2BOH a [Ar2BOH]3
(1)
On decreasing the temperature, the equilibrium shifts to the
right, but its attainment is very slow. This can be ascribed to
intramolecular oxygen-to-boron π-donation, which imparts a
partial double-bond character to the B-O interaction4 and
reduces both the nucleophilicity of the oxygen atom and the
Lewis acidity of boron.
The presence of water strongly affects both the kinetics and
the thermodynamics of this equilibrium.5 Semiempirical com-
putations have depicted a trimerization pathway with a lower
kinetic barrier in the presence of water than under anhydrous
conditions and have also confirmed the stabilization of the
trimeric form in the presence of water, due to the formation of
a stable hydrogen-bonded adduct (2 in Chart 1), whose
properties are still under investigation. An isomer of 2 was
detected,5 namely, the adduct 3 of Chart 1, containing a water
molecule so strongly stabilized that it drives the partial
spontaneous dehydration of 1 to its anhydride Ar2BOBAr2, 4
(Chart 1), as shown in the bottom trace of Figure 1.
The peculiar role of water in the association equilibria of 1
results from the multiple roles, complementary to those of 1,
that water is able to play, as Lewis base and as hydrogen-bond
donor and acceptor. In particular, its kinetic and thermodynamic
effects depend on its nucleophilicity, which allows the formation
of adducts 2 and 5 of Chart 1. Other Lewis bases might therefore
be as effective. Here we have investigated the interaction of 1
with tetrahydrofuran (THF), i.e., with a very simple organic
molecule that can act as a Lewis base and hydrogen-bond
acceptor only. In particular, for THF the parameter â2, which
measures the H-bond basicity,7 is even higher than for water
(0.51 vs 0.38).
Figure 1. 19F NMR monitoring (para region) of a titration of a
CD2Cl2 solution of 1 with THF, up to 0.33 equiv, at 183 K. The
bottom trace, corresponding to the solution before THF addition,
shows the mixture of 1m, 1t, 3, and 4 discussed in the Introduction.
involving water,5 i.e., variable-temperature titrations of 1 with
the nucleophile, performed directly into NMR tubes and
1
monitored by H and 19F NMR.
Interaction between Ar2BOH (1) and THF at 283 K. At
the very beginning of the titration (up to ca. 0.3 equiv) THF
interacted preferentially with 1t, as clearly revealed by the larger
shift of the 19F signals of 1t with respect to those of 1m (Figures
2 and 3a). The formation of the 1t‚THF adduct shifted
equilibrium 1 toward the trimeric form (Figure 3b).8 However,
in the successive titration steps THF interacted with 1m also
(Figures 2, 3a, and S1), and this drove back equilibrium 1,
resulting in the increase of the relative amount of the monomeric
species (Figure 3b).
These findings are similar to those observed in the analogous
titration with water. However, an unexpected difference was
revealed by the 11B NMR monitoring,9 which showed that even
in the presence of 3 equiv of THF the most intense signal was
in the region of tricoordinated boron (ca. 40 ppm). With water,
on the contrary, the 11B signal of the monomer shifted from
the region of tri- to that of tetracoordination as soon as water
uptake by 1m became significant, eventually resulting in the
disappearance of tricoordinated boron.
The capability of THF, as a Lewis base, to stabilize the
trimeric form has been confirmed and found to be even stronger
than expected. The interaction of 1m with THF has been shown
to occur via a H-bond, further supporting its poor Lewis acidity.
The unexpected occurrence of ionization equilibria has been
discovered.
(8) These findings were supported also by the 1H NMR spectra (shown
in Figure S1 of the Supporting Information).
(9) This spectroscopy allows to discriminate between tri- and tetraco-
ordinated boron species: (a) Kidd, R. J. In NMR of Newly Accessible Nuclei;
Laszlo, P., Ed.; Academic Press: New York, 1983; Vol. 2; Chapter 3. See
also for instance: (b) Dagorne, S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F.
J. Am. Chem. Soc. 2000, 122, 274. (c) Fraenk, W.; Klapo¨tke, T. M.; Krumm
B.; Mayer, P. Chem. Commun. 2000, 667. (d) Kehr, G.; Fro¨hlich, R.;
Wibbeling, B.; Erker, G. Chem.-Eur. J. 2000, 6, 258. (e) Jacobsen, H.;
Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.; Meyer, O.
Organometallics 1999, 18, 1724. (f) Vagedes, D.; Fro¨hlich, R.; Erker, G.
Angew. Chem., Int. Ed. 1999, 38, 3362. (g) Parks, D. J.; Piers, W. E.; Parvez,
M.; Atencio, R.; Zaworotko, M. J. Organometallics 1998, 17, 1369. (h)
Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17, 5492.
(i) Sun, Y.; Piers, W. E.; Rettig, J. R. Organometallics 1996, 15, 4110.
Results and Discussion
To study the reactivity between 1 and THF, we have used
the same approach used to investigate the association equilibria
(7) Abraham, M. H., Grellier P. L., Prior D. V., Morris J. J., Taylor, P.
J. J. Chem. Soc., Perkin Trans. 2 1990, 521.

2090 Organometallics, Vol. 26, No. 8, 2007
Beringhelli et al.
Figure 2. Para region of 19F NMR spectra of 1 treated with
increasing amounts of THF (CD2Cl2, 283 K). At this temperature
1m and 1t are in fast exchange with their respective adducts with
THF, so that molar fraction weighted averaged resonances are
observed. The resonance marked with HAr is due to pentafluo-
robenzene.
Figure 3. (a) Variation of the chemical shifts of the 19F para
resonances of the monomeric ((, averaged signals for 1m and 6)
and of the trimeric species (2, averaged signals for 1t and 7) during
the titration of 1 with THF, at 283 K. (b) Simultaneous variation
of the corresponding integrated intensities (their overall intensity
being set to 100).
Therefore the 1m-THF interaction occurs mainly through a
hydrogen bond, to give the adduct 6b of Chart 2, rather than
the Lewis acid-base complex 6a of the same chart.
This finds further support in a PM3 calculation (see Experi-
mental Section) of the relative stability of the two complexes.
Indeed, we found that, in the case of THF, the H-bonded
complex is more stable than the acid-base complex by 12 kcal
mol-1, while in the case of water the opposite holds, with a
preference for the acid-base complex by 6 kcal mol-1. This
preference can be attributed to the stronger H-bond basicity of
THF with respect to water. Moreover, as can be seen by
comparing the two optimized structures of the acid-base
adducts 5 and 6a shown in Figure S2, the coordination of
water to a borinic acid molecule is strengthened by the formation
of an intramolecular interaction between one hydrogen atom
of water and one of the ortho fluorine atoms of a perfluorophe-
nyl ring, whereas the coordination of THF, lacking acidic
hydrogen atoms, results in a crowded ligand arrangement around
the boron atom, since the interaction of the incoming THF
molecule with the bulky perfluorophenyl rings can be repulsive
only.
Chart 2
Titration at 183 K up to 0.33 equiv of THF: Quantitative
Formation of the Adduct [Ar2BOH]3‚OC4H8 (7). Low-
temperature (183 K) titrations showed the progressive formation,
complete at 0.33 equiv (Figure 1), of the hydrogen-bonded
adduct between 1t and one molecule of THF (7 in Chart 2).
The minor species 8 that appeared at the initial stages of the
titration has been characterized as the adduct between borinic
anhydride 4 and THF, shown in Chart 2 (see Supporting
Information).
The adduct 7 exhibits 15 19F signals (Figure S3, Table 1)
and two OH resonances (ratio 1:2, Figure S4). The one at lower
field (δ 14.5) is due to Ha, hydrogen-bonded to THF.
[19F-1H] HOESY, [19F-1H] COSY, and 19F COSY experi-
ments (see Figures S5-S7 and Table S1 in the Supporting
Information) led to a solution structure of 7 that is consistent

Aggregation of Bis(pentafluorophenyl)borinic Acid
Organometallics, Vol. 26, No. 8, 2007 2091
Table 1. 1H and 19F Chemical Shifts (183 K, CD2Cl2) of the Novel Species Described in this Work, Labeled as Indicated in the
Corresponding Charts
19F (ppm)
compound
1H (ppm)
ortho
para
meta
7
14.41 (s, Ha)
8.45 (t, 2 Hb)
3.65 (s, 2 HR)
3.03 (s, 2 HR)
1.78 (m, 4 Hâ)
-132.26 (A2), -141.17 (A6),
-132.85 (B2), -140.42 (B6),
-135.86 (C2), -141.92 (C6)
-151.71 (A4),
-153.58 (B4),
-152.46 (C4)
-160.66 (A5), -161.11 (A3),
-161.29 (B5), -163.85 (B3),
-160.93 (C3), -162.27 (C5)
8
4.30 (m, 4 HR)
2.13 (m, 4 Hâ)
-132.97
-133.73
-150.06
-153.90
-161.78
-162.78
9
10a
13.37 (s, 4 H)
8.88 (s, 2H)
-136.17
-153.23
-161.95
-134.90 (A6), -140.04 (A2),
-132.40, -133.80 -135.90,
-142.20
-160.78 (A4),
-159.80 (B4),
-155.59 (C4),
-159.53 (D4)
-165.54 (A3, A5)
-165.05 (B3),
-163.20 (C3, C5),
-166.73 (B5, D3, D5)
a The incomplete assignment of the ortho signals is due to accidental overlap and broadening arising from ring dynamics. The assignments of the para
and meta resonances to C or D rings is arbitrary.
Figure 4. Stick and space-filling models (PM3 C2-optimized geometries) for the trimeric borinic acid 1t and its adduct with THF, 7.
with the C2 symmetric optimized geometry depicted in Figure
4.10 Short intramolecular H‚‚‚F contacts are present between
hydrogen atom Hb and two ortho fluoro substituents on rings A
and C (1.79 and 1.82 Å with A6 and C6, respectively), in
accordance with the observed strong scalar and dipolar coupling
between these nuclei, resulting in particular in an AB system
in the 19F spectrum (Figures S3 and S4). Similar short H‚‚‚F
contacts (1.83 and 1.82 Å) are present in the optimized geometry
of the parent trimeric acid 1t, shown in Figure 4. However, the
highly fluxional behavior of 1t hampers the direct observation
of the dipolar or scalar couplings associated with these
contacts.11-13
The insertion of the THF molecule in the pocket between
the aromatic rings brings its protons close to the ortho fluorine
atoms of rings A and B, as clearly shown in the [19F-1H]
HOESY map reported in Figure S5 and confirmed by PM3
(10) The optimized structure of 7 only marginally deviates from an
idealized C2 symmetry, because of the position of the THF molecule with
respect to the pseudo-2-fold axis. For the sake of clarity a symmetric C2
structure, lying 0.1 kcal mol-1 above the minimum and corresponding to a
first-order saddle point, is reported in Figure 4.
(11) Also the strong coupling between the two fluorine atoms A6 and
C6 found in 7 is attributable to their proximity12 (2.48 Å, much shorter
than that measured in the crystals of 1t, 2.91 Å) and is not to be thought as
mediated by the hydrogen atom, as is the case of FHF scalar coupling
observed in the anions (HF)nF-.13

2092 Organometallics, Vol. 26, No. 8, 2007
Beringhelli et al.
Figure 6. Dependence of the rate constant k7f1t on the concentra-
tion of 1t (estimated from the integrated intensities of the 19F signals)
at 183 K. The plotted values are averages and standard deviations
of the three constants obtained from the volumes of the exchange
cross-peaks of the three para signals of 7, in 2D [19F-19F] EXSY
experiments. The intercept and the slope provide the contributions
of a dissociative (k1 ) 0.42(6) s-1) and a bimolecular (k2 ) 94(11)
M-1 s-1) mechanism.
tion of 1t, with a nonzero intercept (Figure 6), suggesting that
both dissociative and associative mechanisms operate in attain-
ing equilibrium 2.
(Ar2BOH)3‚THF / (Ar2BOH)3 + THF
(2)
Titration at Low Temperature above 0.33 equiv of THF:
Formation of Ionic Species. Above 0.33 equiv, at 183 K,
several new compounds were formed, although 7 remained the
main solution species up to more than 2 equiv of THF.
Conductivity measurements revealed the unexpected formation
of ionic species. The conductivity value measured after the
addition of 5 equiv of THF (at 198 K) corresponded to ca. 25%
of that measured in the same conditions for an equimolar
solution of a true ionic compound (NBu4PF6), showing that the
ionization process involved a minor but significant part of the
solution species.
A detailed analysis of the NMR spectra (see Supporting
Information, Figures S8 and S9) allowed the identification of
the main ionic species as the cation [Ar2B(OH2)2]+ (9) and the
anion [Ar6B3O3H2]-, i.e., deprotonated 1t (10 in Chart 2).
The cation 9 is stabilized by hydrogen bonds with THF, as
indicated by the chemical shift of the protonic resonance of 9
(δ 13.4)14 and by the close proximity between THF and 9,
revealed by homonuclear [1H-1H] (Figure S10) and hetero-
nuclear [19F-1H] 2D NOE correlation experiments.
Figure 5. Para region of a [19F-19F] EXSY experiment performed
on a solution of 1 treated with (a) 0.18 and (b) 0.33 equiv of THF
(183 K, CD2Cl2, τm ) 0.15 s).
calculations. As can be seen comparing the space-filling models
of 1t and 7 (Figure 4), the hydrogen-bonding of a THF molecule
determines major deformations in the molecular geometry of
the borinic acid trimer. In particular, the hydrogen atom Ha,
which in 1t is completely screened by the perfluorophenyl rings,
in 7 becomes accessible to the THF molecule by opening a
pocket between the phenyl groups labeled A and A′ in Figure
4. This is easily possible on one of the three B-O-B edges
only, since the conformation assumed by the fluorinated rings
adjacent to the edge occupied by a H-bonded THF molecule
leads to a more stiff conformation of the molecule, associated
with a limited conformational freedom of the other aryl groups.
The adduct 7 exchanged with 1t, according to equilibrium 2,
as shown by 2D EXSY experiments (Figure 5a). The rate
constants varied in the titration course: in particular, the values
of k7f1t increased roughly linearly with respect to the concentra-
The low-temperature 19F NMR data of 10 (four para
resonances in the ratio 1:1:2:2) indicated that the anion has Cs
symmetry, at variance with 1t or its adduct 7, which show a C2
structure, in the solid state or in solution at low temperature,
respectively. Geometry optimization for 1t and 10 structures
correctly reproduced these experimental findings, although for
both these species both the Cs and C2 conformers were true
minima on the potential energy surface, with energy differences
less than 1 kcal mol-1
.
Ions 9 and 10, even at low temperature in CD2Cl2, do not
constitute a tight ion pair. Indeed, no correlation was detected
(in homo- and heteronuclear NOE correlation experiments)
(12) (a) Mallory, F. B.; Mallory, C. W.; Butler, K. E.; Lewis, M. B.;
Qian Xia, A.; Luzik, E. D., Jr.; Fredenburgh, L. E.; Ramanjulu, M. M.;
Van, Q. N.; Francl, M. M.; Freed, D. A.; Wray, C. C.; Hann. C.; Nerz-
Stormes, M.; Carrol, P. J.; Chirlian, L. E. J. Am. Chem. Soc. 2000, 122,
4108. (b) Peralta, J. E.; Barone, V.; Contreras, R. H.; Zaccari, D. G.; Snyder,
J. P. J. Am. Chem. Soc. 2001, 123, 9162.
(13) Shenderovich, I. G.; Limbach, H-H.; Smirnov, S. N.; Tolstoy, P.
M.; Denisov, G. S.; Golu bev, N. S. Phys. Chem. Chem. Phys. 2002, 4,
5488.
1
between the H or 19F signals of 9 and the 19F resonances of
10,15,16 while a [1H-1H] correlation between 9 and THF was
(14) In the absence of H-bonding, the protons of B-bonded water
molecules resonate at much higher fields: Beringhelli, T.; Maggioni, D.;
D’Alfonso, G. Organometallics 2001, 20, 4927.
(15) In related systems, involving tight cation-borate interactions, inter-
ion [19F-1H] NOE correlations have been clearly detected.16

Aggregation of Bis(pentafluorophenyl)borinic Acid
Organometallics, Vol. 26, No. 8, 2007 2093
(Ar2BOH)3‚THF + THF / [Ar6B3O3H2]- +
[(THF)H(THF)]+ (6)
[(THF)H(THF)]+ + Ar2B(OH)(OH2) /
clearly recognizable (see above). Taking also into account the
conductive properties of the solution, it should be concluded
that the 9.10 ion pair is very labile. This is likely related to the
high delocalization of the negative charge in the anion 10,17,18
as well as to steric factors, preventing a very close O-H‚‚‚O
interaction in the 9.10 pair, and to the high dielectric constant
of CH2Cl2 at low temperature,19 which favors ion separation.
The fate of the ions at higher temperatures could not be clearly
ascertained, because on increasing the temperature all the
resonances broadened and decreased in intensity, leading
eventually at 273 K to one protonic signal and only one set of
19F resonances. However, conductivity measurements at 273 K
showed that ionic species are present even at higher tempera-
tures, although in a smaller relative amount.20
The occurrence of ionization above 0.33 equiv of THF can
be rationalized by the following considerations. The excess of
THF causes a partial cleavage of the trimeric structure of 7,
because at this point THF can interact (by H-bonding) only with
the small amount of 1m in equilibrium with 7 (the binding of a
second THF molecule to the trimer is unfavored for steric
reasons, as discussed above). This drives to the left the
monomer-trimer equilibrium 1, affording the 1m‚THF adduct-
(s) 6 (according to the overall equilibrium 3).
[Ar2B(OH2)2]+‚THF + THF (7)
Dynamics of 7. The rarity of six-membered B3O3 rings
containing tetravalent boron only prompted us to investigate in
detail the solution behavior of 7. Indeed in the case of 1t (which
has C2 symmetry in the solid state) this study was prevented
by its fast dynamics, which afforded an apparent D3h symmetry
even at the lowest temperatures. The low-temperature spectra
of 7, on the contrary, agree with the C2 symmetry expected for
this species (Figures S3 and S4). On increasing the temperature,
the 1H and 19F resonances of 7 broadened and coalesced (Figures
7 and 8), eventually affording one set of averaged 19F signals,
as well as one averaged BOH resonance, indicative of an
apparent D3h symmetry. This implies the onset of at least three
different dynamic processes: (i) the flopping of the cycle
conformation, leading to equalization of the A/B rings of Chart
2; (ii) the rotation of the aromatic rings around their B-C bonds,
equalizing the 2/6 and 3/5 positions within each ring; and then
(iii) the exchange of THF among the three OH groups.
The details of the exchange processes were provided by 2D
EXSY experiments, performed at 183 K. Reliable rate constants
could be evaluated from the cross-peak volumes21 between the
para resonances only, since these did not show the accidental
overlap observed in the meta region; neither were they affected
by NOE contributions, as occurred for the ortho resonances.
Due to these problems, the above ii exchanges could not be
investigated.
(Ar2BOH)3‚THF + 2 THF / 3 Ar2B(OH)‚THF (3)
From this point of view, the role of THF is similar to that
previously observed for water,5 except for the different mode
of preferred interaction with 1m (H-bond for THF, coordination
for water).
The traces of water present in solution can easily react with
the 1m‚THF adducts (eq 4), affording the tetracoordinated Ar2B-
(OH)(OH2) adduct (5), stabilized by H-bonding to THF.
The rate of the cycle flopping (exchange of rings A and B of
Chart 2) remained substantially constant in the titration course
(kAfB 0.78-0.90 s-1). On the contrary, the rate at which C
exchanged with both rings A and B increased with the amount
of added THF (Figures 5a and 5b),22 showing that THF
migration occurs by a bimolecular process. Upon addition of
an excess of THF, the exchange became so fast to average the
Ar2B(OH)‚THF + H2O / Ar2B(OH)(OH2) + THF (4)
Water coordination increases the basicity of the oxygen atom
of borinic acid, driving to the right equilibrium 5.
1
resonances of 7, both in the H and in the 19F spectra, even at
(Ar2BOH)3‚THF + Ar2B(OH)(OH2) / [Ar6B3O3H2]- +
[Ar2B(OH2)2]+‚THF (5)
183 K (Figures S8 and S9).
The higher fluxionality of 1t with respect to 7 might be
ascribed to the increased rigidity imparted to the trimeric cyclic
structure by the coordination of a THF molecule, as previously
stated. However, a low-temperature structure of reduced sym-
metry (Cs in this case) has been observed for the trimeric anion
10 as well. This species can hardly be considered more rigid
than 1t, because the lack of the H‚‚‚F interactions involving
one of the hydroxo groups is expected to promote phenyl ring
mobility in 10.
The above proton transfer is likely mediated by the formation
of the homoconjugate [(THF)H(THF)]+ pair, which drives to
the right the ionization equilibrium 6. Actually, in the absence
of a THF excess, the adduct between 1t and THF is in its neutral
BO-H‚‚‚THF form 7, as confirmed by the absence of significant
conductivity up to 0.33 equiv of THF. The excess of THF would
therefore act as a proton shuttle, according to eqs 6 and 7.
To cast light on these differences, PM3 computations have
been performed on 1t, 7, and 10. The computations showed that
the flopping of the cycle conformation can take place in both
the C2 and the Cs conformers of 7 and 10, leading to the
exchange of the A and B rings and, for the Cs anion, to the
exchange of the rings bound to the boron atom lying on the
(16) Di Saverio, A.; Focante, F.; Camurati, I.; Resconi, L.; Beringhelli,
T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.; Sironi, A.
Inorg. Chem. 2005, 44, 5030.
(17) Electrostatic potential maps computed for a series of perfluorophenyl
borate anions showed a considerable reduction of the negative potential on
the oxygen atom and on the whole accessible surface of the anion on
increasing the number of perfluorophenyl rings.16
(18) Schott, D.; Pregosin, P. S.; Jacques, B.; Chavarot, M.; Rose-Munch,
F.; Rose, E. Inorg. Chem. 2005, 44, 5941.
(19) Gru¨ndemann, S.; Ulrich, S.; Limbach, H. H.; Golubev, N. S.;
Denisov, G. S.; Epstein, L. M.; Sabo-Etienne, S.; Chaudret, B. Inorg. Chem.
1999, 38, 2550.
(20) The conductivity at 273 K, although higher, corresponded to only
11% of that of NBu4PF6 in the same conditions, indicating that the relative
amount of ionized species decreases on increasing the temperature, likely
due to the parallel significant decrease of the dielectric constant of CD2Cl2
(from 17 at 170 K to 9 at room temperature).19
(21) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935.
(22) At the beginning of the titration the k values were too small to be
measured, and successively they increased with THF concentration: kCfA
) kCfB ) 0.13 s-1 (0.10 equiv), 0.23 s-1 (0.18 equiv), 1.26 s-1 (0.30 equiv),
1.70 s-1 (0.33 equiv). It must be considered that when the amount of added
THF is close to the stoichiometric ratio, the A/B cross-peaks account not
only for their direct exchange but also for their indirect exchange, through
successive A f C and C f B steps, and therefore the A/B exchange rate
increases

2094 Organometallics, Vol. 26, No. 8, 2007
Beringhelli et al.
Scheme 1. Role of the Nucleophiles OR2 (H2O or THF) in
the Oligomerization Equilibria of 1
1
Figure 7. Variable-temperature H NMR spectra of 7.
which Hb (and not Ha) lies on the molecular symmetry plane.
This process exchanges rings C with both A and B and leads to
a D3h dynamic structure with an activation energy of less than
1 kcal mol-1. In the case of 7 and 10, this pseudorotation process
is not allowed, unless it is accompanied by the (formal) transfer
of a THF molecule (for 7) or a proton (for 10) from one point
to the other of the molecule.
Thermal Stability of the Trimeric Cycle of 7. The variable-
temperature spectra of Figure 8 revealed also the expected
progressive fragmentation of the trimeric structure of 7 on
increasing the temperature. Indeed at T > 240 K a novel set of
19F resonances appeared, at chemical shifts that can be
confidently viewed as a molar fraction weighted average
between those of 1m and of the 1m‚THF adduct(s) 6.23
The ratio R of the integrated intensities of the novel signals
with respect to those of 7 strongly increased with the temperature
(R ) 0.08 at 243 K, 0.34 at 268 K, 2.3 at 293 K), so that at
room temperature one-third of borinic acid was present in the
trimeric form. This confirms that the stabilization of the trimeric
form upon H-bonding with THF is effective even at room
temperature. Indeed, in the absence of THF, the amount of 1t
at 298 K is negligible.
The fragmentation process was completely reversible, and
on lowering back the temperature quantitative re-aggregation
to the trimeric adduct 7 was observed.
Figure 8. Variable-temperature 19F NMR spectra of 7 (para
region).
symmetry plane of the molecule (C and D in Chart 2). For 7
the Cs conformer, lying 3 kcal mol-1 higher than the C2 one,
appears to be the transition structure of the process at study,
whereas for 10 both structures are minima and the (asymmetric)
transition structure connecting them lies at 2.4 kcal mol-1. This
process of ring inversion, when associated with the rotation of
the phenyl rings around their boron-Cipso bonds, leads to a
dynamic C2V symmetry for both species.
A similar interconversion between the C2 and Cs conformers
can be computed for the parent trimeric acid 1t as well, with an
energy barrier (2.6 kcal mol-1) comparable to that computed
for ring inversion in 7 and 10. However, for 1t another different
dynamic process can be envisaged, i.e., the pseudorotation of
the six-membered B3O3 ring implying the exchange between
the C2 conformer depicted in Figure 4 and a Cs conformer in
Conclusions
The title compound in the solid state offers the first example
of a trimeric structure based on only tetracoordinated boron
atoms,4 and in dichloromethane solution it gives rise to an
unprecedented monomer-trimer equilibrium.5 The study of the
behavior of 1 in the presence of THF has now provided more
insight into this trimerization equilibrium and into structure,
dynamics, and stability of these unusual B3O3 rings.
(23) Moreover, the 19F T1 values of the novel signals, measured at 268
K, were significantly longer than those of 7, in line with a smaller
nuclearity: 1.18(3) s and 1.40(8) s, for the novel ortho and para signals,
vs 0.42(1) s and 0.51(1) s for the corresponding signals of 7 (in parentheses
the uncertainties on the last digit).

Aggregation of Bis(pentafluorophenyl)borinic Acid
Organometallics, Vol. 26, No. 8, 2007 2095
Interaction between 1 and THF. A typical sample was prepared
as follows. The appropriate amount of (C6F5)2BOH was weighted
directly into the NMR tube and then dissolved in CD2Cl2, affording
typically 0.10 M solutions. THF was then added stepwise with a
microsyringe. It has been checked that the results did not change
upon performing THF addition at room temperature or at 193 K.
The spectra acquired at 193 K showed the progressive formation
(up to 0.33 equiv of THF) of the adduct 7, which exhibits two OH
resonances (δ 14.5 and 8.45, ratio 1:2, Figure S4) and 15 19F signals
The addition of a Lewis base to a trivalent boron compound
should be expected to lead to the formation of a covalent acid-
base adduct, and the story should stop there. In the present case,
on the contrary, this is only the beginning of a number of
interlaced fast aggregations equilibria, which deeply modify the
nature of the solution species.
Indeed, it is enough to add 0.33 equiv of THF, at a
temperature as low as 183 K, to instantaneously (and quanti-
tatively) convert the monomer into the trimer (while in the
absence of the nucleophile the attainment of the equilibrium
required more than 10 h at 283 K).5 As previously shown by
PM3 calculations in the case of H2O,5 the impressive effect of
Lewis bases is attributable to the formation of adducts such as
5 or 6a (the latter present in small but kinetically significant
concentration). The stronger nucleophilicity of the oxygen atom
of 1m in these adducts favors the formation of the di- and
trinuclear oligomers of Scheme 1; then the displacement of the
nucleophile by a BO(H) group leads to the formation of the
trimeric cycle of 1t stabilized by the exo-cyclic H-bond.
THF affects also the thermodynamics of the trimerization
equilibrium 1, as shown by the observed increase of the relative
amount of the trimer in the presence of THF: indeed the H-bond
with the trimer (in 7) is stronger than that with the monomer
(in 6b), in agreement with the higher effective electronegativity
of an oxygen atom bound to two acidic boron centers.5
The crucial role played by the establishment of hydrogen
bonds is a recurring feature in the reactivity of 1: the propensity
to act as a H-bond acceptor, together with the poor Lewis acidity
of its B atom, explains the surprising finding that THF
preferentially interacts with a trivalent boron derivative such
as 1m by H-bonding (giving 6b) rather then by formation of
the acid-base adduct.
Another very unexpected feature revealed by this work is
the occurrence of ionization equilibria involving the title
compound in the presence of even a small excess of THF.
Interestingly, in this process a proton is transferred from a
trimeric to a monomeric form of borinic acid: more precisely
the deprotonation of the acid 1t is accompanied by protonation
of the 1m‚H2O adduct 5. This shows that in suitable conditions
(i.e., upon activation by coordination of a nucleophile) bis-
(pentafluorophenyl)borinic acid can exhibit Brønsted base
properties, in addition to its more obvious properties as a
Brønsted acid and Lewis acid and base.
1
(six ortho and meta, three para, Figure S3). All the H and 19F
data are reported in Table 1. [19F-1H] HOESY, [19F-1H] COSY,
and 19F COSY experiments (shown in Figures S5-S7 in the
Supporting Information) as well as 19F NOESY experiments (not
shown) allowed the assignments of the fluorine resonances reported
in Table 1 and the determination of the H-F and F-F correlations
reported in Table S1 of the Supporting Information. Such correla-
tions are consistent with the optimized structure of 7 reported in
Figure 4.
Another titration experiment (performed as above, by treating a
0.164 M CD2Cl2 solution of 1 with THF, up to 3 equiv) was
1
monitored at 283 K. In this case, besides H and 19F spectra, also
11B data were acquired (quadrupolar broadening hampering the
acquisition of these data in the titrations monitored at 183 K).
Conductometric Measurements. A conductometric cell (AMEL
160, cell constant ) 0.90 cm) was introduced under N2 in a Schlenk-
like vessel containing 3 mL of a 0.055 M solution of 1 in CH2Cl2,
in a thermostatic bath (at 193 or 273 K, in two different
experiments). The conductivity was measured in the presence of
different amounts of THF, added under N2 atmosphere, up to 5
equiv. The following values of conductivity (µS cm-1) were
measured (in parentheses the equivalents of added THF): 193 K:
0.29 (0), 0.46 (0.33), 4.07 (1), 12.4 (2), 17.1 (3), 18.6 (4), 19.5
(5); 273 K: 1.00 (0), 3.53 (0.33), 11.0 (1), 18.5 (2), 24.8 (3), 27.1
(4), 28.1 (5). The conductivity of NBu4PF6 in the same conditions
was 85 µS cm-1 at 193 K and 249 µS cm-1 at 273 K.
Computational Studies. Geometry optimizations were per-
formed at the PM3 semiempirical level with SPARTAN 02.25 No
symmetry was imposed; however, most of the structures possess
Cs or C2 symmetry (see the text). Optimizations were done
employing the Hessian matrix computed at the PM3 level; the
convergence criteria in geometry optimization for the maximum
gradient component and the maximum change in a bond length
were set to 10-5 au and 10-4 Å. The nature (true minima or
transition structures) of all the stationary points found was
determined by computing the Hessian matrix. To ascertain that the
transition structures link the two expected minima, the eigenvector
associated with the negative eigenvalue was followed in both
directions (by minimizing two slightly distorted structures). Single-
point energy computations at the B3LYP/6-31G(d,p) level were
done with GAUSSIAN 03.26
In conclusion, this work shows that much care should be taken
when one is working with this type of deceptively simple
molecules, because the nature of the species present in solution
can be very different from that expected.
Experimental Section
Acknowledgment. The authors thank Basell Polyolefins for
providing samples of bis(pentafluophenyl)borinic acid. P.M. and
A.S. acknowledge Prof. Maurizio Sironi for the use of SPAR-
TAN 02. Thanks are also due to Italian CNR (ISTM) for
providing facilities for inert atmosphere and low-temperature
experiments.
All the manipulations were performed under N2 using oven-dried
Schlenk-type glassware. CD2Cl2 (C.I.L.) was dried on activated
molecular sieves. THF was distilled from Na/Ph2CO. B(C6F5)2OH
1
was a gift from Basell Polyolefins. Its purity was checked by H
and 19F NMR and always resulted higher than 99%. For some
experiments, the samples were further sublimed (393 K, 10-5 mbar).
NMR spectra were acquired on a Bruker AVANCE DRX-300
spectrometer, equipped with 5 mm TBI or QNP probes, and on a
Bruker AVANCE DRX-400 spectrometer, equipped with a 5 mm
BBI probe. 19F NMR spectra were referenced to external CFCl3.
The temperature was calibrated with a standard CH3OH/CD3OD
solution.24 Pentafluorotoluene (C6F5CH3, 1 µL) was added to each
Supporting Information Available: Details on the NMR
characterization, one table, and 13 figures. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM061167P
1
sample, as an internal standard for both H and 19F spectra.
(25) SPARTAN 02; Wavefunction Inc.: Irvine, CA, 2002.
(26) GAUSSIAN 03 (revision C.02); Gaussian Inc.: Wallingford, CT,
2004.
(24) Van Geet, A. L. Anal. Chem. 1970, 42, 67.