Bis(pentafluorophenyl)borinic acid: A cyclic trimer in the solid state and a monomer, with hindered rotation around the B-OH bond, in solution

Article abstract of DOI:10.1021/om0300732

The title molecule in the solid state exists as a cyclic trimer, with B-O(H)-B bridges and a cyclo-hexane-like structure (C₂ twist-boat conformation); dissolution in toluene-d₈ affords the B(C₆F₅)₂OH monomer, in which the low-temperature ¹⁹F NMR data reveal restricted rotation of the OH substituent around the Ar₂B-OH bond (E_a = 39 kJ mol^-1), as a result of the partial double-bond character of this interaction.

Full text of DOI:10.1021/om0300732

1588
Organometallics 2003, 22, 1588-1590
Bis(p en ta flu or op h en yl)bor in ic Acid : a Cyclic Tr im er in
th e Solid Sta te a n d a Mon om er , w ith Hin d er ed Rota tion
a r ou n d th e B-OH Bon d , in Solu tion
Tiziana Beringhelli,† Giuseppe D’Alfonso,*,† Daniela Donghi,† Daniela Maggioni,†
Pierluigi Mercandelli,‡ and Angelo Sironi*,‡
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Via Venezian 21,
20133 Milano, Italy, Dipartimento di Chimica Strutturale e Stereochimica Inorganica,
Via Venezian 21, 20133 Milano, Italy, and CNR-ISTM, via Golgi 19, 20133 Milano, Italy
Received J anuary 30, 2003
Summary: The title molecule in the solid state exists as
a cyclic trimer, with B-O(H)-B bridges and a cyclo-
hexane-like structure (C2 twist-boat conformation); dis-
solution in toluene-d8 affords the B(C6F5)2OH monomer,
in which the low-temperature 19F NMR data reveal
restricted rotation of the OH substituent around the
Ar2B-OH bond (Ea ) 39 kJ mol-1), as a result of the
partial double-bond character of this interaction.
The chemistry of bis(pentafluorophenyl)borinic acid,
(C6F5)2B(OH) (1),1 has been at present actively inves-
tigated, and several applications in catalysis and organic
synthesis have been reported.1-4 This is mainly at-
tributable to some peculiar features of this molecule,
where both Lewis and Brønsted acidic sites are simul-
taneously present, as well as a Lewis basic site (i.e. the
nonbonding electrons on the oxygen substituent). A
variety of intra- and intermolecular interactions could
result from such multiplicity of active sites, and we
report here on the cyclotrimerization of 1 in the solid
state and its deoligomerization in solution, to give a
monomer in which the pπ-pπ boron-oxygen interac-
tions are strong enough to “freeze” the rotation around
the B-OH bond.
F igu r e 1. Two views of 1 highlighting the idealized C2
symmetry (2-fold axis through B1 and O1) and the labeling
scheme.
state experimental conformation, the perfluorophenyl
substituents, being either equatorial or bisectional, are
spread around the ring in a more homogeneous way; in
addition, the three hydroxyl hydrogens are involved in
bifurcated O-H‚‚‚F hydrogen bonds with six (one for
each perfluorophenyl ligand) ortho fluorine atoms (aver-
age O-H, H‚‚‚F, and O‚‚‚F distances 0.79, 2.09, and 2.67
Å, respectively).
An X-ray powder diffraction spectrum of a sample of
B(C6F5)2OH (synthesized according to ref 1c, with a
Single-crystal X-ray analysis5 revealed that, in the
solid state, 1 exists as a cyclic trimer constituted of
three B(C6F5)2OH monomers interconnected through
B-O(H)-B bridges. The resulting structure resembles
that of cyclohexane, but the [(C6F5)2BO(H)]3 hexagon
has a C2 twist-boat conformation (see Figure 1) and not
the archetypal chair conformation. Indeed, a chair
conformation of idealized C3 symmetry, having three
“axial” perfluorophenyls on the same side of the six-
membered ring, would be rather crowded. In the solid-
melting point in agreement with literature data1a
)
confirmed that the bulk is made up of the same phase
of the examined single crystal.
Several boron-oxygen compounds exhibit cyclic
structures, with the boron atoms connected through
oxygen bridges;6 those more closely related to the pre-
sent derivative are [B3(µ-O)3(C6H5)4]- 7 and [B3(µ-O)3-
(5) Crystal data: C36H3B3F30O3, Mr ) 1085.81, monoclinic, space
group P21/c (No. 14), a ) 19.575(3) Å, b ) 13.127(2) Å, c ) 14.823(2)
Å, â ) 98.14(1)°, V ) 3771(1) Å3, Z ) 4, T ) 295 K, graphite-
monochromated Mo KR radiation, λ ) 0.710 73 Å, Fcalcd ) 1.913 Mg
m-3, F(000) ) 2112, white crystal, 0.28 × 0.28 × 0.18 mm, µ(Mo KR)
) 0.220 mm-1, absorption correction with SADABS (Sheldrick, G. M.
SADABS, University of Go¨ttingen, Go¨ttingen, Germany, 1996), SMART
diffractometer, ω scan, frame width 0.3°, maximum time per frame
30 s, θ range 1.9-23.3°, 23 734 reflections collected, 5426 independent
reflections (Rint ) 0.0365), no crystal decay, solution by direct methods
(Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna R. SIR97.
J . Appl. Crystallogr. 1999, 32, 115) and subsequent Fourier syntheses,
† Dipartimento di Chimica Inorganica, Metallorganica e Analitica.
‡ Dipartimento di Chimica Strutturale e Stereochimica Inorganica
and CNR-ISTM.
(1) (a) Chambers, R. D.; Chivers, T. J . Chem. Soc. 1965, 3933. (b)
Ishihara, K.; Kurihara, H.; Yamamoto, H. J . Org. Chem. 1997, 62,
5664. (c) Bohnen, H.; Hahn, U. Patent WO0017208, 2000, to Aventis
R&T GMBH.
(2) Ishihara, K.; Kurihara, H.; Yamamoto, H. Synlett 1997, 597.
(3) Yamamoto, H., Lewis Acids in Organic Synthesis; Wiley-VCH:
Weinheim, Germany, 2000.
(4) Recent patents dealing with synthesis or uses of 1: (a) Ikeno,
I.; Mitsui, H.; Iida, T.; Moriguchi, T. Patent WO0248156, 2002, to
Nippon Shokubai Co. (b) Ikeno, I.; Mitsui, H.; Iida, T.; Moriguchi, T.
Patent WO0244185, 2002, to Nippon Shokubai Co. (c) Schottek, J .;
Fritze, C. Patent DE10009714, 2001, to Targor. (d) Kratzer, R. Patent
DE10059717, 2001, to Basell Polyolefins. (e) Kratzer, R.; Fritze, C.;
Schottek, J . Patent DE19962814, 2001, to Targor. (f) Frances, J . M.;
Deforth, T. Patent WO0130903, 2001, to Rhodia Chimie. (g) Schottek,
J .; Fritze, C.; Bohnen, H.; Becker, P. Patent DE19845240, 2000, to
Targor. (h) Bohnen, H. Patent DE19733017, 1999, to Hoechst AG.
2
full-matrix least squares on Fo (Sheldrick, G. M. SHELXL-97: Pro-
gram for Structure Refinement, 1997), anisotropic thermal factors
assigned to all atoms but the three hydroxyl hydrogen atoms, which
were refined with a common isotropic thermal factor, data/parameters
) 3997/659, GOF(Fo2) ) 0.775, R1 ) 0.0589 and wR2 ) 0.0775 on all
data, R1 ) 0.0332 and wR2 ) 0.0693 for reflections with I > 2σ(I),
weighting scheme w ) 1/[σ2(Fo2) + (0.0416P)2 + 0.3065P], where P )
(Fo + 2Fc2)/3, largest peak and hole 0.19 and -0.20 e Å-3
.
2
(6) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd
ed.; BH, 1998.
10.1021/om0300732 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 03/12/2003

Communications
Organometallics, Vol. 22, No. 8, 2003 1589
(C6F5)5]2-.8 To the best of our knowledge, however, the
title compound is the first trimeric structure based on
tetracoordinated boron atoms only. Donor-acceptor
bonds involving tetracoordinated boron atoms can easily
dissociate, since they afford “stable” closed-shell frag-
ments; accordingly, a few dimeric (C6F5)2B(µ-X)2B(C6F5)2
structures are known in the solid state (X ) H,9 N310),
which in solution undergo complete10 or partial9,11
conversion into the (C6F5)2BX monomer.12 The same
occurs in the present case. The NMR data indicate that
the dissolution of 1 in toluene-d8 at room temperature
is accompanied by disruption of the trimeric structure
1t. Indeed, resonances attributable to the intact oligomer
1t could be detected only when the dissolution of 1 was
performed at low temperature.13 At higher temperatures
these signals convert into a novel set of resonances
(Figure S1; Supporting Information), attributable to the
monomeric species, hereafter called 1m .14 Indeed, the
11B chemical shift of 1m (42.2 ppm, invariant with the
temperature, Figure S2; Supporting Information) falls
in the range typical of three-coordinated boron,16,17 while
the resonance of 1t and that measured in the solid state
(7.2 and 2.4 ppm, respectively) fall in the range diag-
nostic of tetrahedral neutral boron compounds.20
The rate of the 1t f 1m conversion is strongly
dependent upon the amount of water in solution.25 This
can be attributed to nucleophilic attack of water on a
boron atom, causing the cleavage of one of the B-O
bonds in the B3O3 cyclic structure, which in turn results
in the fast dissociation of the oligomer.
F igu r e 2. Variable-temperature 19F NMR spectra of 1m
in toluene-d8.
The 19F signals of 1t are sharp also at low tempera-
ture, down to 188 K. In contrast, when the temperature
is lowered, each of the three 19F resonances of 1m split
into two signals of equal intensity (Figure 2). The
splitting of the para signal indicates magnetic un-
equivalence of the two perfluoroaryl rings, confirmed
by the separated scalar connectivities (ortho f meta f
para) for each of the two sets of three 19F signals (see
the 2D 19F COSY at 188 K in Figure S3; Supporting
Information). The presence of one averaged signal for
the two ortho (and meta) positions of each ring indicates
that free rotation around the B-Cipso bond is maintained
even at the lowest temperatures.
The nonequivalence of the two phenyl rings can be
explained only by assuming freezing of the free rotation
of the OH substituent around the Ar2B-OH bond (see
Scheme 1). In agreement with this, (i) a 1H-19F NOE
experiment showed that the proton of 1m at 180 K has
a strong correlation with the ortho fluorine atoms of one
aryl ring only (Figure S4; Supporting Information) and
(ii) the protonic resonance, which at room temperature
is a quintet (averaged J H-F ) 1.6 Hz), at 180 K became
a triplet (averaged J H-F ) ca. 3 Hz, Figure S5; Sup-
porting Information).
(7) (a) Kliegel, W.; Motzkus, H.-W. Can. J . Chem. 1985, 63, 3516.
(b) Zeller, E.; Beruda, H.; Schmidbaur, H. Chem. Ber. 1993, 126, 2033.
(8) Priego, J . L.; Doerrer, L. H.; Rees, L. H.; Green, M. L. H. Chem.
Commun. 2000, 779.
(9) Parks, D. J .; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492.
(10) Fraenk, W.; Klapo¨tke, T. M.; Krumm, B.; Mayer, P. Chem.
Commun. 2000, 667.
(11) Parks, D. J .; Spence, R. E. von H.; Piers, W. E. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 809.
(12) At the extreme end, the chloride derivative is monomeric also
in the solid state: Piers, W. E.; Spence, R. E. v. H.; MacGillivray, L.
R.; Zaworotko, M. J . Acta Crystallogr. 1995, C51, 1688.
(13) Toluene-d8 was added to a sample of 1 in an NMR tube at 193
K, and the tube was shaken for a short time and introduced into the
NMR probe, at 253 K. 19F NMR (toluene-d8, 283 K): δ -139.3 (ortho),
-150.0 (para), -159.9 (meta) ppm. 1H NMR: δ 7.78. 11B NMR: δ 7.2
ppm. Solid-state 11B MAS NMR: δ 2.4 ppm.
(14) NMR data (toluene-d8, 293 K): 1H, δ 6.32 ppm (q, J ) 1.6 Hz),
lit.15 6.30 (J ) 1.7 Hz); 19F, -133.5 (ortho), -148.6 (para), -161.7 ppm
(meta), lit.1b (CHCl3) -132.8, -147.7, -160.9; 11B, δ 42.2 ppm.
(15) Bradley, D. C.; Harding, I. S.; Keefe, A. D.; Montevalli, M.;
Zheng, D. H. J . Chem. Soc., Dalton Trans. 1996, 3931.
(16) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983; Vol. 2.
(17) Typical values for the 11B chemical shift in three-coordinated
boron derivatives bearing pentafluorophenyl substituents are in the
range 32.5-74.9 ppm.9,10,18,19 Particularly significant is the value of
39.5 ppm observed for the closely related borinate B(C6F5)2(OnBu).9
(18) (a) Vagedes, D.; Fro¨hlich, R.; Erker, G. Angew. Chem., Int. Ed.
1999, 38, 3362. (b) Doerrer, L. H.; Green, M. L. H. J . Chem. Soc., Dalton
Trans. 1999, 4325. (c) Dagorne, S.; Guzei, I. A.; Coles, M. P.; J ordan,
R. F. J . Am. Chem. Soc. 2000, 122, 274.
Such hindered rotation results from the partial double-
bond character of the B-OH interaction, arising from
π-donation from oxygen to the electron-poor B atom.26
These pπ-pπ interactions are a common feature of
(19) Kehr, G.; Fro¨hlich, R.; Wibbeling, B.; Erker, G. Chem. Eur. J .
2000, 6, 258.
(20) The resonances for
a
neutral tetrahedral boron derivative
(25) When 1 was dissolved (at low temperature) in toluene-d8,
“dried” with standard methods (i.e. upon standing for more than 1 day
over activated 4A molecular sieves), a 19F spectrum acquired at 253 K
containing pentafluorophenyl substituents are usually in the range
-20 to +20 ppm.9,10,19,21-24
(21) Sun, Y.; Piers, W. E.; Rettig, J . R. Organometallics 1996, 15,
4110.
within 5 min from dissolution showed that 1m was already the
dominant species (>70%). In contrast, the amount of 1m was almost
negligible (Figure S1a; Supporting Information) when 1 was dissolved
in toluene “dried” as above and containing a small amount (0.15 equiv)
of B(C6F5)3, which is able to remove any adventitious water, owing to
the quantitative formation of the [(C6F5)3B(OH2)] adduct.24 Under these
more anhydrous conditions the rate of the 1t f 1m conversion was
significantly reduced (t1/2 ) 29 min at 283 K, Figure S1b-d).
(22) J acobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.;
Fro¨hlich, R.; Meyer, O. Organometallics 1999, 18, 1724.
(23) Parks, D. J .; Piers, W. E.; Parvez, M.; Atencio, R.; Zaworotko,
M. J . Organometallics 1998, 17, 1369.
(24) See: Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometal-
lics 2001, 20, 4927 and references therein.

1590 Organometallics, Vol. 22, No. 8, 2003
Communications
Sch em e 1
Sch em e 2
trigonal boron compounds bearing substituents with
nonbonding electron pairs.6,10 For BF2OH the planar
configuration was found to be more stable than the
orthogonal one,27 and theoretical computations concern-
ing boron-oxygen compounds have found considerable
π-electron OfB delocalization, even more marked than
in the corresponding halogeno compounds.28
The equalization of the phenyl rings at higher tem-
peratures could result not only from the fast rotation
of the OH group around the B-O bond but also from
several intermolecular processes, such as those depicted
in Scheme 2. However, direct proton exchange between
different molecules of 1 (paths c) and protonation
equilibria caused by the presence of adventitious acids
(path b) have been ruled out, because we verified that
the exchange rate did not increase on increasing the
concentration of 129 or of a strong Brønsted acid, such
as the [(C6F5)3B‚OH2] adduct.30 In contrast, we observed
a marked decrease of the exchange rate upon addition
of a small amount of B(C6F5)3 as water scavenger:29 this
indicated a significant role of adventitious water as
proton shuttle, according to paths a. This contribution
can be removed by removing water. Definitive evidence
of the absence of intermolecular proton transfer under
strictly anhydrous conditions was provided by 1H EXSY
experiments, which showed no exchange cross-peak
between the (sharp) signals of 1 and [(C6F5)3BOH2], at
temperatures (193-212 K) where the exchange rate of
constants obtained from 19F NMR spectra acquired in
the presence of B(C6F5)3 as water scavenger can confi-
dently be ascribed to the intramolecular rotation of the
OH group. The activation barrier estimated from these
constants, Ea ) 39(1) kJ mol-1, is comparable to values
previously experimentally determined or theoretically
evaluated for the barriers to rotation about C-OR bonds
in R′C(dO)OR species (R, R′ ) H, CH3),33 which were
attributed to the contribution of a resonance form
implying double-bond character of the C-OR bond. To
the best of our knowledge, this is the first time that a
restricted rotation around a B-OH bond has been
observed and measured.
Ack n ow led gm en t. We thank Dr. R. Kratzer (Basell
Polyolefins) for a sample of (C6F5)2B(OH), Dr. G. Althoff
(Bruker Biospin Gmbh), for the solid-state 11B MAS
spectrum, and Basell for partial funding of this work.
We also thank the anonymous reviewers who suggested
useful experiments to provide a firmer basis to our
thesis.
the phenyl rings is already >10 s-1 31
. Therefore, the rate
Su p p or tin g In for m a tion Ava ila ble: Figures S1-S7 (19F
NMR monitoring of the 1t f 1m conversion, variable-temper-
ature 11B spectra, 2D 19F COSY45 spectra, 19F-1H NOE
spectra, variable-temperature 1H spectra, 19F spectra at dif-
ferent concentrations of 1, Arrhenius plot) and tables giving
the pertinent X-ray structural information. This material is
available free of charge via the Internet at http://pubs.acs.org.
(26) In fact, OH‚‚‚F interactions involving the ortho F atoms of the
phenyl rings are not the key factor responsible for the observed
restricted rotation, since the aromatic rings freely rotate around the
B-Cipso bond.
(27) Spoliti, M.; Ramondo, F.; Bencivenni, L. Folia Chim. Theor. Lat.
1992, 20, 201.
(28) (a) Armstrong, D. R.; Perkins, P. G. J . Chem. Soc. A 1967, 123.
(b) Armstrong, D. R.; Perkins, P. G. J . Chem. Soc. A 1967, 790. (c)
Theor. Chim. Acta 1966, 5, 222.
(29) Three samples were prepared by dissolving different amounts
of 1 in the same amount of toluene-d8 “dried” over molecular sieves,
and 19F NMR spectra were acquired at 205 K (Figure S6; Supporting
Information). The samples containing the lowest and the highest con-
centrations of 1 were then treated with the same amount of B(C6F5)3,
causing the exchange rate to become slower and equal in the two
samples. The more concentrated sample was then added with the
amount of water necessary to double the concentration of the [(C6F5)3-
BOH2] adduct: this did not cause any variation of the exchange rate.
(30) It has been reported that [(C6F5)3BOH2] in acetonitrile must
be regarded as a strong acid, comparable to HCl: Bergquist, C.;
Bridgwater, B. M.; Harlan, C. J .; Norton, J . R.; Friesner, R. A.; Parkin,
G. J . Am. Chem. Soc. 2000, 122, 10581-19590.
OM0300732
(31) The kinetic constants for the exchange process were obtained
by band-shape analysis on 19F NMR spectra using Bruker WIN-
DYNAMICS software. k, s-1 (T, K): 40 (193), 170 (205), 330 (212), 580
(216), 1100 (224), 2000 (230), 6000 (242). The temperatures were
calibrated with a standard CH3OH/CD3OD solution.32 The Arrhenius
plot is reported in Figure S7 (Supporting Information).
(32) Van Geet, A. L. Anal. Chem. 1970, 42, 679.
(33) Computed barriers (MP3/6-311+G**) in the range 36.07-53.63
kJ mol-1: Wiberg, K. B.; Laidig, K. E. J . Am. Chem. Soc. 1987, 109,
5935.