Hydrogen bonding and Lewis acid-base interactions in the system bis(pentafluorophenyl)borinic acid/methanol

Article abstract of DOI:10.1002/ejic.200701210

Methanol does not significantly affect the position and the rate of the equilibrium between the monomeric (1_m) and cyclic trimeric (1 _t) forms of bis(pentafluorophenyl)borinic acid, Ar₂BOH (1, Ar = C₆F₅

Full text of DOI:10.1002/ejic.200701210

FULL PAPER
DOI: 10.1002/ejic.200701210
Hydrogen Bonding and Lewis Acid–Base Interactions in the System
Bis(pentafluorophenyl)borinic Acid/Methanol
Daniela Donghi,[a] Daniela Maggioni,[a] Tiziana Beringhelli,[a] Giuseppe D’Alfonso,*[a,b]
Pierluigi Mercandelli,*[b,c] and Angelo Sironi[b,c]
Keywords: Fluoroarylboranes / Lewis acids / Hydrogen bonds / NMR spectroscopy / X-ray diffraction
Methanol does not significantly affect the position and the
rate of the equilibrium between the monomeric (1m) and cy-
clic trimeric (1t) forms of bis(pentafluorophenyl)borinic acid,
Ar2BOH (1, Ar = C6F5), in CD2Cl2 solution. This contrasts
with what was previously observed in the presence of thf and
is mainly due to the stabilization of the 1m·MeOH covalent
adduct through the formation of the hydrogen-bonded dimer
[Ar2B(OH)(MeOH)]2 (7), characterized in solution and by sin-
gle-crystal X-ray analysis. This dimer can be viewed as the
intermediate in the conversion of 1 into its methyl ester 6 by
fast proton transfer along the hydrogen bond, which trans-
forms the 1m·MeOH adduct into the 6·H2O adduct. Extrusion
of H2O by water scavengers drives the equilibrium toward
ester 6. X-ray analysis showed that, at variance with 1, ester
instability of its adducts with MeOH and thf. Water addition
to 6 affords the dimeric adduct 7, in fast equilibrium with
both 1m and 6, even at 183 K, as revealed by 2D EXSY analy-
sis. Borinic acid itself can act as a water scavenger at low
temperature, so that 6 is formed also in the early stages of
the titration of 1 with MeOH. The other main initial product
was the adduct [Ar2B(OH)]3·MeOH (3) containing a MeOH
molecule bound within an octaatomic –B–O(H)–B–O-
(Me)–H···O(H)–B–O(H)– ring. Fast proton transfer along the
strong hydrogen bond was revealed by 2D EXSY analysis. At
variance with thf, the adduct containing MeOH exocyclically
hydrogen-bonded to 1t could be obtained in high concentra-
tion at 183 K only when due allowance for the kinetic
requirements of the slow trimerization equilibrium was
made.
6 in the solid state is a monomer. Hindered rotation [∆H#
38(1) kJmol–1, ∆S# = –35(6) Jmol–1] around the B–OMe bond,
due to O-to-B π-donation, has been observed. This π-do- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
=
nation lowers the Lewis acidity of 6, as shown by the thermal
Germany, 2008)
(toluene)[8] or partial (dichloromethane)[9] dissociation to
the (C6F5)2BOH monomer 1m (hereafter Ar2BOH, Ar =
C6F5) occurs, according to equilibrium (1).
Introduction
The high current interest in the uses of Lewis acids in
organic synthesis and in catalysis[1] has stimulated a great
number of studies on the reactivity of fluoroarylboranes.[2]
Much of the work focused on tris(pentafluorophenyl)-
borane,[3] but related molecules have also been extensively
investigated. Among these, bis(pentafluorophenyl)borinic
acid (C6F5)2BOH (1, hereafter borinic acid)[4,5] has been the
object of a number of fundamental and applicative stud-
ies,[6–11] because of its peculiar reactivity.
cyclo-(Ar2BOH)3 i 3 Ar2BOH
(1)
The structure of the trimer is unprecedented in boron
chemistry, since, unlike the well known cyclo-(RBO)3 borox-
ines, it only contains tetracoordinate boron atoms. Such a
structure rather resembles that of group 14 organometallic
oxides, cyclo-(R2EO)3 (E = Si, Ge, or Sn), formed by the
condensation of the corresponding R2E(OH)2 precursors.
In this case, however, 1t arises by self-association, driven by
the Lewis acidity of boron and the basicity of oxygen, with-
out any elimination reaction.
The novelty of this structure prompted us to investigate
in more detail the oligomerization reaction (in dichloro-
methane solution) and the effects of the presence of mole-
cules that are able to promote intermolecular associations
by Lewis acid–base or hydrogen-bonding interactions.
It was found that Lewis bases, such as water[9] or tetra-
hydrofuran (thf),[11] stabilize the trimeric form (whose con-
centration is usually very low), by formation of the hydro-
gen-bonded adducts 2 shown in Scheme 1 (where the labels
We have previously shown[8] that 1 is a cyclic trimer (1t,
Scheme 1) in the solid state,[12] whilst in solution, complete
[a] Dipartimento di Chimica Inorganica, Metallorganica e Analit-
ica, Facoltà di Farmacia, Università degli Studi di Milano,
Via Venezian 21, 20133 Milano, Italy
Fax: +39-02-50314405
E-mail: giuseppe.dalfonso@unimi.it
[b] Istituto di Scienze e Tecnologie Molecolari, CNR,
Via Golgi 19, 20133 Milano, Italy
[c] Dipartimento di Chimica Strutturale e Stereochimica Inor-
ganica, Università degli Studi di Milano,
Via Venezian 21, 20133 Milano, Italy
Fax: +39-02-50314454
E-mail: pierluigi.mercandelli@unimi.it
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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FULL PAPER
Several differences between H2O and thf have been ob-
served in relation to their interaction with 1m and the nature
of the adducts present in solution at different temperatures.
In this work we have investigated the behavior of borinic
acid 1 in the presence of a Lewis base with properties inter-
mediate between water and thf, namely methanol
(MeOH).[13,14] From the point of view of the hydrogen-
bonding interactions, MeOH, as water, can act both as hy-
drogen-bond acceptor and donor. In both cases, its effec-
tiveness is expected to be comparable to that of water.[15–17]
This dual nature might result in a rich reactivity. Moreover,
with methanol, the occurrence of an esterification equilib-
rium can be anticipated [equation (2)], and this might fur-
ther complicate the scenario or even become the dominant
feature.
Ar2BOH + MeOH i Ar2BOMe + H2O
(2)
Results and Discussion
The reactivity of 1 with MeOH has been investigated
mainly by titrations at high (283 K) and low (183 K) tem-
perature, monitored by 1H, 19F, and 11B NMR spectroscopy
(the latter at 283 K only).[18]
Scheme 1. Monomeric and trimeric borinic acid and their adducts
with RRЈO bases (a: RRЈO = H2O, b: RRЈO = thf, c: RRЈO =
MeOH).
These titrations clearly showed that the base MeOH is
much less effective than thf in stabilizing the trimeric cyclic
structure of 1t. Actually, at 283 K the relative amount of
the trimeric species remained very low in the early titration
steps, and 1t disappeared completely after the addition of
1 equiv. of MeOH (Figure 1). Even at 183 K, the adduct
a, b, and c refer to the adducts with water, thf, and MeOH,
respectively). Moreover, the bases accelerate the attainment
of the equilibrium (which is otherwise very slow, even at
room temperature), by forming the intermediate adducts 4
(Scheme 1), in which the increased (with respect to 1m) nu-
cleophilicity of the BOH oxygen atom favors the oligomer-
ization path depicted in Scheme 2.
Figure 1. para region of the 19F NMR spectra of 1 treated with
increasing amounts of MeOH (the number of equivalents added is
shown for each spectrum) in CD2Cl2 at 283 K. At this temperature,
1m and 1t are in fast exchange with their respective adducts with
methanol, so that mol-fraction-weighted average resonances are
observed. The formation of pentafluorobenzene (labeled as HAr)
indicates some progressive decomposition.
Scheme 2. Oligomerization of 1 in the presence of Lewis bases
RRЈO.
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Interactions in the System Bis(pentafluorophenyl)borinic Acid/Methanol
1
Figure 2. H NMR spectra at 183 K of 1 in CD2Cl2 (#) in the presence of different amounts of methanol: (a) 0 equiv., (b) 0.1 equiv., (c)
0.2 equiv., (d) 0.3 equiv.
between 1t and MeOH (2c in Scheme 1) was a minor species MeOH, as demonstrated by a conductimetric titration at
(Figure 2), while in the presence of stoichiometric thf 273 K. By contrast, under the same conditions, ionization
(0.33 equiv.) quantitative conversion to the trimeric adduct equilibria involving deprotonation of 2b,[11] were observed
2b occurs.[11] Interestingly, 2c became the main species in as soon as more than 0.33 equiv. of thf were added.
solution when a sample of 1, treated with 0.33 equiv. of
Low-temperature NMR spectroscopic titrations revealed
MeOH at room temperature, was very slowly cooled down that under these conditions the interaction of borinic acid
to 183 K. This indicates that kinetic factors, concerning the with MeOH led to the formation of three main products.
slow monomer–trimer equilibrium, affect product distribu- The major species formed at the very beginning of the ti-
tion. In contrast, with thf the trimerization to adduct 2b tration (Figure 2b) was identified as methyl borinate (6 in
was instantaneous even at 183 K.
Scheme 1, spectroscopic data in Table 1).[19]
The very low concentration of 2c likely explains the ab-
The other main initial product detected in the low-tem-
sence of any significant ionization up to 10 equiv. of perature titrations (Figure 2) was 3c (Scheme 1), in which a
1
Table 1. H and 19F chemical shifts (183 K, CD2Cl2) of the species described in this work.
Compound
1H [ppm]
19F [ppm][a]
para
ortho
meta
2c
13.59 (s, OH)
8.53 (t, 2OH)
3.21 (s, CH3)
2.86 (s, OH)
17.03 (s, OHa)
8.41 (t, OHc)
7.87 (t, OHd)
5.24 (d, OHb)
3.54 (s, CH3)
–132.33 (A2), –142.01 (A6)
–135.81 (B2), –141.79 (B6)
–140.53 (C2), –132.75 (C6)
–151.79 (A4)
–152.01 (B4)
–152.98 (C4)
–160.74 (A3), –160.56 (A5)
–160.68 (B3), –161.92 (B5)
–161.05 (C3), –163.55 (C5)
3c[b]
–129.79 (A6), –130.29 (A2)
–130.70 (C6), –134.36 (D2)
–134.70 (F2), –137.72 (B6)
–137.94 (E2), –139.92 (C2)
–140.29 (E6), –140.72 (B2)
–141.49 (F6), –143.84 (D6)
–131.11
–151.37 (A4)
–151.39 (B4)
–151.49 (C4)
–151.91 (D4)
–153.44 (F4)
–154.23 (E4)
–148.05
–159.71 (A3), –160.12 (D3)
–160.41 (B3), –160.65 (C3)
–160.84 (B5), –161.50 (F3)
–161.47 (E3), –162.17 (C5)
–162.40 (E5), –162.43 (F5)
–162.67 (D5), –162.84 (A5)
–161.49
6
7
4.00 (s, CH3)
–131.90
–137.05
–149.70
–154.94
–160.44
–162.35
16.06 (s, OH)
4.81 (s, OH)
3.21 (s, CH3)
8[c]
9
3.80 (s, CH3+2CH2
)
–132.20
–136.34
–150.57
–155.40
–161.61
–162.90
α
β
1.88 (s, 2CH2
16.61 (s, OH)
3.45 (s, 2CH3)
)
[a] See the Experimental Section for the labeling of the 19F resonances. [b] The 1H and 19F resonances are labeled according to Scheme 5.
[c] Data at 203 K.
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MeOH molecule is simultaneously B-bonded and H- 1m and MeOH: Figure 5 shows that, on increasing the
bonded within an octanuclear ring. This species is analo- amount of MeOH, the (averaged) 11B resonance progress-
gous to the species 3a, containing a water molecule instead ively shifts from a position typical of tricoordination (δ =
of MeOH, which spontaneously forms [together with the 40 ppm) toward one typical of tetracoordination (δ =
anhydride Ar2BOBAr2,[5,20] equation (3a)] whenever dichlo- 7 ppm).[3a,22] This behavior is analogous to that observed
romethane solutions of 1 are cooled to temperatures low with water, although a much larger amount of MeOH
enough to make the very unfavorable entropy term in equi- (9 equiv., vs. 2.5 equiv. of H2O) was necessary to shift the
librium 3b (see Figure 2a) less important.
equilibrium to the right completely.
2 Ar2BOH i Ar2BOBAr2 + H2O
(3a)
(3b)
3 Ar2BOH + H2O i [Ar2BOH]3·H2O (isomer 3a)
The spectroscopic data of 3c (Table 1) are consistent with
a structure of C1 symmetry. It is worth mentioning that the
five high-field ortho 19F signals correspond to the fluorine
atoms coupled with the OH protons in a scalar fashion.
The same occurs for the three high-field ortho signals for 2c
and 2b[11] and is in line with what was previously observed
in similar systems for fluorine atoms involved in intramo-
lecular X–H···F–C hydrogen bonding.[21]
1
Figure 4. Selected regions of the spectrum of a H COSY analysis
of a CD2Cl2 solution of 7 (183 K), showing that the Me group has
a scalar cross-peak with Ha only and not with Hb (Scheme 1). The
same experiment does not show a correlation between Ha and Hb.
1H and 19F EXSY experiments [Figure 3 and Figure S3
(Supporting Information), respectively] showed exchange
cross-peaks between 3c and 1m, a behavior similar to that
previously observed for 3a.[9]
Figure 5. Variation of the chemical shifts of the (averaged) 11B
NMR resonances of the monomeric (o) and trimeric (᭡) species,
during a titration of 1 with methanol, at 283 K.
Figure 3. Section of a 1H EXSY/NOESY experiment performed
with a solution of 1 treated with 0.23 equiv. of MeOH (CD2Cl2,
183 K), showing the intramolecular (NOE) and intermolecular (ex-
changes shown by dotted squares and straight lines) correlations of
the hydrogen-bonded OH protons. The methyl region of the same
spectrum shows the exchange between the methoxy group of 6 and
the methyl groups of adducts 7 and 3c.
The hydrogen-bonded dimeric structure 7 in Scheme 1
(which bears some resemblance to the dimeric form of car-
boxylic acids) has been supported by a solid-state single-
crystal X-ray analysis (Figure 6).
Each of the boron atoms in the dimeric species 7 is
bound to a hydroxido ligand, a methanol molecule, and two
pentafluorophenyl groups, arranged in a tetrahedral coordi-
At subsequent stages of the titrations at 183 K, a third nation geometry. The formulation of 7 as a hydroxido/meth-
product appeared in progressively increasing concentration. anol species (instead of an aqua/methoxido tautomer) is
Its formulation as a dimer of adduct 4c (7 in Scheme 1) based on the direct observation of the oxygen-bound hydro-
comes from the following evidence: The spectroscopic data gen atoms and on the value of the boron–oxygen bond
point to an adduct between monomeric borinic acid and lengths (which are much shorter for the negatively charged
MeOH, because they show a 1:1 ratio between Ar2BOH hydroxido ligand relative to the neutral methanol ligand).
and MeOH. The OH resonance at δ = 16.06 ppm indicates The position of the proton could not be easily anticipated
the presence of a strong hydrogen bond. The observation on the basis of the relative acidity of water with respect to
that only this low-field proton has a COSY correlation with methanol, which has been shown to be strongly dependent
the methyl group (Figure 4) rules out the hydrogen-bonded on the environment.[23,24]
adduct 5c in Scheme 1. The 11B NMR spectroscopic data
also support the formation of a covalent adduct between two strong O–H···O asymmetrical hydrogen bonds, leading
Two [Ar2B(OH)(MeOH)] moieties are tied together by
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Interactions in the System Bis(pentafluorophenyl)borinic Acid/Methanol
Indeed, after addition of 0.1 equiv. of MeOH, almost 60%
of the alcohol was converted into the borinate, as shown in
Figure 2b.
This surprising finding can be explained by the “dehy-
drating capability” exhibited at low temperature by the
solutions of 1 itself, due to the stabilization of the released
water molecule within the hydrogen-bonded ring of adduct
3a (equilibrium 3b). In agreement with this, the formation
of the borinate ester under the latter conditions was ac-
companied by an increase in the concentration of the water-
containing adduct 3a (see Figure 2b). When the amount of
free 1 decreased, this reaction (which “consumes” 4 mol of
1 per mol of 6 formed) stopped, and no further increase in
the amount of 6 was observed in the subsequent titration
stages.[30]
Figure 6. ORTEP drawing of the solid-state structure of the dimeric
adduct [Ar2B(OH)(MeOH)]2 (7) with a partial labeling scheme. Se-
lected bond lengths [pm] and angles [°]: B(1)–O(1) 154.1(2), B(1)–
O(2) 147.2(2), B(1)–C(1) 162.8(3), B(1)–C(7) 163.2(3), O(1)–H(1)
96(3), H(1)···O(2Ј) 153(3), O(1)···O(2Ј) 248.18(18); O(1)–B(1)–O(2)
104.54(14), O(1)–B(1)–C(1) 107.11(13), O(1)–B(1)–C(7) 108.57(14),
O(2)–B(1)–C(1) 114.69(15), O(2)–B(1)–C(7) 109.64(14), C(1)–B(1)–
C(7) 111.84(14), O(1)–H(1)···O(2Ј) 175(3). Primes address sym-
metry-equivalent atoms (1 – x, –y, 1 – z).
At low temperature, the two aryl rings became nonequiv-
alent, so two 19F signals are observed in each of the ortho,
para, and meta regions (see Figures S1–S4 in Supporting
Information).[19] This behavior, analogous to that pre-
[8]
viously observed for 1m and other esters of diarylborinic
acids,[28,31] arises from the hindered rotation around the B–
O bond, as further confirmed by the scalar and dipolar
couplings observed, at 183 K, between the hydrogen atoms
of the methoxy group and the ortho fluorine atoms of only
one perfluorinated ring. Band-shape analysis of the para
19F signals in the temperature range 197–281 K provided
the kinetic parameters for the rotation of the OMe group
around the Ar2B–O bond. The kinetic constants turned out
to be about one order of magnitude smaller than those pre-
viously measured for the rotation of the OH group around
the corresponding Ar2B–O bond in 1m (Figure 7). The
analysis of the activation parameters indicates that this is
attributable to the entropy term, rather than to a stronger
oxygen-to-boron π donation in 6: ∆H# = 38(1) kJmol–1,
∆S# = –35(6) Jmol–1 K–1 for 6 vs. ∆H# = 42(1) kJmol–1,
∆S# = 14(3) Jmol–1 K–1 for 1m. The unfavorable activation
entropy for 6 is possibly due to the higher steric require-
ments of the methyl group with respect to a proton, which
reduces the conformational freedom in the transition state.
to a B2H2O4 eight-membered ring with a chair conforma-
tion. A similar dimeric structure was previously found for
a related derivative, namely [(1,5-cyclooctanediyl)B(OMe)-
(MeOH)]2,[25] in which more symmetrical hydrogen-bond
patterns and boron–oxygen distances are observed, as a re-
sult of the identical acidity of the two ligands sharing the
proton (methoxido and methanol).
Upon increasing the amount of MeOH at low tempera-
ture, the relative amount of 7 progressively increased, but
simultaneously a gelatinous white precipitate began to
form: the higher the concentration of 1, the lower the
amount of MeOH needed to induce the precipitation.
Likely, this precipitate, which reversibly dissolves on in-
creasing the temperature, is constituted by hydrogen-
bonded oligomers, made from 1m and its adduct with
MeOH, in different relative amounts according to the com-
position of the solutions.
Synthesis and Characterization of Methyl Borinate 6
The methyl ester of bis(pentafluorophenyl)borinic acid,
ester 6, can be easily and quantitatively obtained by treating
1 with one equivalent of MeOH in the presence of effective
water scavengers, such as activated 3-Å molecular sieves,
which are necessary to drive equilibrium (2) to the right.
Ester 6 has been previously obtained by reaction of MeOH
with Ar2BH or Ar2BCl, or by decomposition of an unusual
ion pair, featuring an OMe group bridging the two boron
centers of the diborane 1,2-C6F4(Ar2B)2.[19] In addition, the
other previously known alkyl or aryl Ar2BOR esters had
been obtained by routes different from the simple reaction
of 1 with alcohols or phenols.[7h,26–29]
As mentioned above, methyl borinate is formed even in
the absence of an external water scavenger, upon treatment
Figure 7. Eyring plot of the kinetic constants for the hindered rota-
of borinic acid with small amounts of MeOH, at 183 K. tion around the B–OR bond in 1 (R = H, ᭿) and 6 (R = Me, ᭹).
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The NMR spectroscopic data did not show for 6 any
evidence of a trimeric form analogous to 1t. Actually, a
methyl group is too bulky to be hosted in the small pockets
between adjacent pentafluorophenyl rings. Steric hindrance
is the key factor determining the extent of dimerization also
in the case of the related Ar2BNR2 derivatives, which may
give rise to monomer–dimer equilibria.[32] X-ray analysis
confirmed that 6 has a monomeric structure also in the so-
lid state.
Scheme 3. Adducts of methyl borinate with Lewis bases.
The boron atom in ester 6 is bound to a methoxido li-
gand and two pentafluorophenyl groups, arranged in a tri-
gonal planar coordination geometry (see Figure 8). The
short boron–oxygen distance found in 6 is in agreement
with the partial double-bond character of this interaction
and is the shortest so far observed within esters of a borinic
acid.[28,33] In addition, an optimal π overlap between the p
orbitals of boron and oxygen is granted by the coplanar
arrangement of the substituents at the two centers, the OMe
group lying on the (Cipso)2BO plane.
(MeOH) (9, NMR spectroscopic data in Table 1). The di-
meric structure depicted in Scheme 3, analogous to that of
7, is suggested by the presence of a strong hydrogen bond,
revealed by the proton resonance at δ = 16.6 ppm. The ob-
servation of a unique methyl resonance is consistent with
a fast (even at 183 K) proton transfer between the OMe
moieties.[34] Also in this case, reversible dissociation of the
adduct occurred at temperatures higher than 233 K (equi-
librium 4), where the observed 1H, 19F, and 11B signals were
averaged by fast exchange between 9, methyl borinate, and
free MeOH.
Ar2BOMe + MeOH i Ar2B(OMe)(MeOH)
(4)
The reaction of methyl borinate 6 with water, monitored
1
at 183 K, afforded a species exhibiting H and 19F signals
identical to those of the above-discussed dimeric adduct 7
(Scheme 1). Actually, a simple proton oscillation along the
O–H···O interaction would transform the dimeric form of
the adduct between methyl borinate and H2O (7Ј in
1
Scheme 3) into tautomer 7 and vice versa. The H COSY
spectrum (Figure 4) suggests that the main species in solu-
tion is really 7, because the hydrogen-bonded proton Ha has
a scalar correlation with the methyl group and not with the
other OH proton, Hb.
Moreover, crystals grown from a solution of methyl bor-
inate treated with water were shown by X-ray analysis to
contain tautomer 7, proving that protonation of the boron-
bound methoxy group by boron-bound water does occur.
Figure 8. ORTEP drawing of the solid-state structure of the ester
[Ar2BOMe] (6) with a partial labeling scheme. Selected bond
lengths [pm] and angles [°]: B(1)–O(1) 132.4(3), B(1)–C(1) 158.6(4),
B(1)–C(7) 157.4(4), O(1)–C(13) 144.8(3); O(1)–B(1)–C(1) 123.5(2),
O(1)–B(1)–C(7) 118.0(2), C(1)–B(1)–C(7) 118.5(2), B(1)–O(1)–
C(13) 123.3(2); C(1)–B(1)–O(1)–C(13) 0.8(4), C(1)–B(1)–O(1)–
C(13) –177.8(2).
The behavior of methyl ester 6 in the presence of oxygen
bases is expected to be much simpler than that of the corre-
sponding acid, 1, because 6 cannot act as a hydrogen-bond
donor and is not involved in a monomer–trimer equilib-
rium, as opposed to 1. In agreement with this expectation,
the reaction of 6 with 1 equiv. of thf at 203 K cleanly pro-
duced the Lewis acid–base adduct Ar2B(OMe)(thf) (8 in
Scheme 3, NMR spectroscopic data in Table 1). The adduct
was stable only at very low temperature: indeed, already
1
at 233 K the chemical shifts of the H and 19F resonances
(averaged by fast exchange between free 6 and its adduct 8)
indicated almost complete dissociation of 8. This proves the
very poor Lewis acidity of boron in methyl borinate.
The addition of one equivalent of MeOH afforded
rapidly and selectively a covalent adduct, Ar2B(OMe)- Scheme 4. The esterification–hydrolysis equilibrium.
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Interactions in the System Bis(pentafluorophenyl)borinic Acid/Methanol
Nevertheless, the (transient) existence of 7Ј must be as- B(C6F5)3 and (C6F5)B(OH)2.[1] The data reported here
sumed to account for the exchange of 7 with both 1m and show that methyl ester 6 shares with 1 a relatively poor
methyl borinate 6 (Scheme 4), observed in 1H and 19F Lewis acidity, due to the oxygen-to-boron π donation. This
EXSY maps at low temperature.
may be of advantage in certain catalytic applications, where
An analogous proton transfer along the hydrogen bond the reversibility of bond activation is crucial: for instance it
of 3c explains the surprising finding that 3c exchanges not has been shown that the related isopropyl ester is a very
only with 1m but also with methyl borinate 6. Different aryl effective catalyst for the polymerization of propylene oxide,
rings are involved in the two exchange processes: a 19F while the much stronger Lewis acid B(C6F5)3 vigorously
EXSY experiment at 183 K (Figure S3 in Supporting Infor- and irreversibly reacts with the substrate itself, hampering
mation) shows cross-peaks between 1m and the rings of 3c the progress of the polymerization.[27]
labeled as E and F in Scheme 5, whilst methyl borinate 6
has cross-peaks with rings A and B.
Moreover, the studies reported here have provided evi-
dence for the importance of hydrogen-bonding interactions,
either in competing with the formation of Lewis acid–base
adducts or in stabilizing some of them. We therefore suggest
that the catalytic role of 1 might be related not only to its
Lewis acidity, but also to its hydrogen-bond-donor/acceptor
capability. Actually, hydrogen bonding is emerging as an
effective tool in metal-free organocatalysis,[35] particularly
for carbonyl derivatives;[36] and therefore synergic Lewis
acid–base and hydrogen-bonding interactions might be in-
volved in the reactions catalyzed by 1, which mostly con-
cern carbonyl species.
Scheme 5. Proton transfer in oligomer 3c and the resulting inter-
change of 1 and 6 as chain terminals.
Experimental Section
Conclusions
General: All manipulations were performed under nitrogen, in
oven-dried Schlenk-type glassware. CD2Cl2 (C.I.L.) and MeOH
(Fluka) were dried on activated molecular sieves. (C6F5)2BOH was
a gift from Basell Polyolefins. NMR spectra were acquired with a
This work has shed further light on the complex specia-
tion equilibria in which bis(pentafluorophenyl)borinic acid
is involved, in dichloromethane solution, in the presence of
molecules that are able to act as Lewis bases and hydrogen- Bruker AVANCE DRX-300 spectrometer, equipped with a 5-mm
TBI probe or with a 5-mm QNP probe, and with 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 solu-
tion.[37] Pentafluorotoluene (C6F5CH3, 1 µL) was added to each
bond acceptors or donors.
Methanol has been found to be unable to mimic the most
impressive features of the interaction of 1 with thf: neither
instantaneous trimerization upon addition of stoichiomet-
ric base at low temperature, nor ionization upon addition
of an excess of the reactant have been observed.
1
sample as internal standard for both H and 19F NMR spectra.
Interaction Between (C6F5)2BOH (1) and MeOH: A typical experi-
ment was performed as follows: The appropriate amount of
(C6F5)2BOH was weighed under nitrogen directly into the NMR
tube and then dissolved in CD2Cl2, affording typically 0.10- solu-
tions. Stepwise additions of MeOH were performed by using a 10-
µL microsyringe. It was checked that the results did not change
upon performing the MeOH addition either at room temperature
or at 183 K. After each addition, the tube was briefly shaken and
The key feature explaining this different reactivity is the
hydrogen-bond-donating capability of MeOH, which allows
the formation of dimeric species 7. This stabilizes the mo-
nomeric form of 1 (competing with the extra stabilization
of the trimer, provided by its hydrogen bonding with the
Lewis base) and at the same time traps the covalent adduct
(hampering the first steps of the oligomerization,
Scheme 2). This explains why MeOH is much less effective rapidly inserted into the NMR probe at 183 K. The titration course
1
(up to 1 equiv.) was monitored by H and 19F NMR spectroscopy
than thf in increasing the trimerization rate.
Another significant feature emerging from our studies is
the high lability of the aggregates formed in these mixtures,
at 183 K. 2D experiments (1H COSY and NOESY, 19F COSY and
NOESY, [19F–1H] HOESY, [19F–1H] COSY) were performed at dif-
ferent titration steps. Some of these experiments are shown in Fig-
as shown by the exchanges, detectable even at 183 K, of
ures S1–S4 in Supporting Information. The δ values of all the ob-
dimeric species 7, or of trimeric adduct 3c, with both acid
served species are reported in Table 1.
1 and ester 6. In both cases this implies proton transfer
along the B–O(H)···H–O(Me)–B hydrogen bonds and fast
reversible dissociation of the covalent adducts.
The results are of interest not only for a better under-
standing of the basic chemical properties of these fluoroar-
ylboranes, but also from the point of view of their possible
applications. Actually, most of the uses of these species are
based on their Lewis acidity, and it has already been ob-
The results of a typical experiment are shown in Figure 2. In the
first steps of the titration (up to ca. 0.1 equiv.) the formation of 6
and 3c was observed. The 19F spectrum of 3c shows 30 partially
overlapping signals (C1 symmetry, Table 1, and Figure S1 in Sup-
1
porting Information), while its H spectrum exhibits one CH3 and
four OH resonances. A {19F}-1H spectrum (Figure S5 in Support-
ing Information) proved that the multiplicities of the three OH sig-
nals at δ = 8.41, 7.87, and 5.24 ppm are due to H–F through-space
served that the acidity of 1 is intermediate between those of coupling with fluorine atoms, which were identified by a [19F–1H]
Eur. J. Inorg. Chem. 2008, 1645–1653
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1651

G. D’Alfonso, P. Mercandelli et al.
FULL PAPER
COSY experiment at 183 K. An AB system results from strong
coupling between the two ortho 19F signals at –139.92 ppm (C2,
according to the ring labels of Scheme 5) and –140.72 ppm (B2),
both coupled with Hc at δ = 8.41 ppm (t, apparent JHF = 19 Hz).
The same occurs for the other two 19F signals at –140.29 ppm (E6)
and –143.84 ppm (D6), coupled with Hd at δ = 7.87 ppm (with the
addition of water to avoid the formation of the gelatinous precipi-
tate) showed the almost quantitative formation of the adduct 7. On
1
raising the temperature to 298 K, the position of the H, 19F, and
11B resonances indicated that, in this diluted solution, adduct 7 was
no longer present.
X-ray Diffraction Structural Analysis: Data for [Ar2BOMe] (6):
1
same JHF). The H signal at δ = 5.24 ppm (Hb) is coupled to one
fluorine atom only (–141.49 ppm, F6); JHF = 12 Hz.
¯
C13H3BF10O, Mr = 375.96, triclinic, P1 (No. 2), a = 7.339(2) Å, b
= 7.455(2) Å, c = 12.291(2) Å, α = 90.99(1)°, β = 97.25(1)°, γ =
99.30(1)°, V = 657.8(3) Å3, T = 110(2) K, Z = 2, F(000) = 368,
λ(Mo-Kα) = 0.71073 Å, µ(Mo-Kα) = 0.214 mm–1, Tmin = 0.901, 1.7°
Յ θ Յ 26.0°, 10113 measured reflections, 2566 independent reflec-
tions (Rint = 0.0315, Rσ = 0.0276), 1953 reflections with IϾ2σ(I),
R[F, IϾ2σ(I)] = 0.0366, wR(F2, all data) = 0.0834, S = 1.088, data/
parameters = 2566/229, ∆ρmax, min = +0.30, –0.28 eÅ–3.
Upon further additions of MeOH, 2c and 7 were formed (Figure 2,
Table 1). Adduct 2c was identified by its four 1H signals (ratio
1:2:3:1) and fifteen 19F resonances, in agreement with C2 symmetry.
The three couples of magnetically nonequivalent rings were arbi-
trarily labeled as follows: A and B were the labels for those on the
hydrogen-bonded Ar2B–O(H)–BAr2 moiety and C for those bound
to the boron atom on the symmetry axis. The two high-field ortho
19F signals at –140.53 and –142.01 ppm, both coupled to the proton
at δ = 8.53 ppm (t, apparent JHF = 18 Hz), are strongly coupled,
giving rise to an AB system. The resonances have been assigned by
2D experiments performed on a sample containing 2c as the main
component, obtained as follows: a CD2Cl2 solution of 1 (0.12 )
treated with MeOH (0.33 equiv.) was slowly cooled down to 183 K,
and 1H and 19F NMR spectra were acquired at several intermediate
temperatures.
Data for [Ar2B(OH)(MeOH)] (½7): C13H5BF10O2, Mr = 393.98,
¯
triclinic, P1 (No. 2), a = 6.014(2) Å, b = 10.101(2) Å, c =
12.425(2) Å, α = 73.02(1)°, β = 77.49(1)°, γ = 73.48(1)°, V =
684.7(3) Å3, T = 110(2) K, Z = 2, F(000) = 388, λ(Mo-Kα) =
0.71073 Å, µ(Mo-Kα) = 0.216 mm–1, Tmin = 0.815, 1.7° Յ θ Յ
26.0°, 8372 measured reflections, 2698 independent reflections (Rint
= 0.0262, Rσ = 0.0237), 2394 reflections with IϾ2σ(I), R[F,
IϾ2σ(I)] = 0.0349, wR(F2, all data) = 0.0792, S = 0.997, data/
parameters = 2698/246, ∆ρmax, min = +0.37, –0.26 eÅ–3.
1
Species 7 has three H resonances (ratio 1:1:3) and only one set of
2
2
2
Residual factors are defined as follows: Rint = Σ|Fo – ͗Fo ͘|/ΣFo
,
19F signals (Table 1). Large colorless crystals of 7 suitable for X-
ray analysis were obtained at 248 K by slow diffusion of pentane
in a CD2Cl2 solution of 6 (0.05 ) treated with water (1 equiv.,
0.35 µL).
Rσ = Σσ(Fo2)/ΣFo2, R(F) = Σ||Fo| – |Fc||/Σ|Fo|, and wR(F2) =
[Σw(Fo – Fc ) /ΣwFo ]
4 1/2. Goodness-of-fit is defined as S =
2
2 2
[Σw(Fo – Fc ) /(nd – np)]1/2, in which nd and np are the number of
2
2 2
data and parameters.
The titrations monitored at 283 K by 1H, 19F, and 11B NMR spec-
troscopy were performed analogously.
CCDC-660635 and -660636 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Preparation of Methyl Borinate 6: MeOH (1 equiv.) was added to
an NMR tube containing a solution of 1 (0.094 ) in CD2Cl2
(0.510 mL) in the presence of freshly activated 3-Å molecular si-
eves. The tube was kept at room temperature, and the progress of
the reaction was followed by 1H and 19F NMR spectroscopy, show-
ing the almost quantitative formation of 6 after 4 h. The NMR
spectroscopic data at 183 K are reported in Table 1, while the δ
values at room temperature (dynamically averaged in the case of
Supporting Information (see footnote on the first page of this arti-
cle): Figures S1–S5 showing details of the NMR spectroscopic
characterization.
Acknowledgments
1
the 19F resonances) are the following: H NMR δ = 3.94 ppm; 19F
NMR: δ = –132.50 (o), –149.73 (p), –161.75 (m) ppm.
The authors are indebted to Basell Polyolefins for a gift of bis(pen-
tafluorophenyl)borinic acid.
Reaction Between Methyl Borinate and thf: A CD2Cl2 solution of 6
(0.16 ) was treated with thf (1 equiv.) at room temperature directly
1
in an NMR tube. H and 19F NMR spectra at 203 K showed the
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1
19F signals, while the H NMR spectrum shows two signals at an
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integration ratio of 7:4, due to accidental overlap of the methyl
signal and the low-field thf signals (Table 1). On increasing the tem-
perature, progressive dissociation of adduct 8 occurred, as shown
by the shift of the 1H, 19F, and 11B signals towards the positions
of 6 and free thf.
Reaction Between Methyl Borinate and MeOH: A CD2Cl2 solution
of 6 (0.06 ) was treated with MeOH (1 equiv.) as described above.
1H and 19F NMR spectra at 203 K showed the quantitative forma-
tion of the dimeric adduct 9 depicted in Scheme 3 (Table 1). 1H
COSY and NOESY spectra recorded at 183 K showed correlation
between the OH proton and the (unique) methyl resonance. At
room temperature, the positions of the 1H, 19F, and 11B resonances
indicated almost complete dissociation.
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Reaction Between Methyl Borinate and H2O: A CD2Cl2 solution of
1
6 (0.05 ) was treated with H2O (1 equiv.) as described above. H
and 19F NMR spectra at 203 K (acquired immediately after the
1652
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1645–1653

Interactions in the System Bis(pentafluorophenyl)borinic Acid/Methanol
[7] Recent patents dealing with the synthesis or uses of 1 are: a)
T. Sell, J. Schottek, N. S. Paczkowski, A. Winter (Novolen
Technology), WO 2006124231, 2006; b) T. Neumann, S.
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H. Bohnen, P. Becker (Targor), WO 0020466, 2000; n) H.
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Bohnen, H. Hahn (Aventis R&T GmbH), WO0017208, 2000.
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[10] G. J. P. Britovsek, J. Ugolotti, P. Hunt, A. J. P. White, Chem.
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[11] T. Beringhelli, G. D’Alfonso, D. Donghi, D. Maggioni, P. Mer-
candelli, A. Sironi, Organometallics 2007, 26, 2088–2095.
[12] The existence of a solid-state structure of 1 was previously
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metallics 2002, 21, 1719–1726.
[13] Ab initio studies on the proton and BF3 affinities have revealed
that MeOH possesses both a Lewis and a Brønsted basicity
intermediate between those of water and thf, the values of the
affinities for BF3 and for the proton (the latter in parenthesis)
being 46 (707), 65 (766), and 82 (832) kJmol–1 for water,
MeOH, and thf, respectively (298 K).[14]
[19]
[20]
[21]
[22]
[23]
[24]
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It is interesting to observe that in a series of pyrazolato-based
dizinc(II) complexes the –(Me)OHO(H)– bridging ligand can
assume either one of the two limiting structures, that is
–(Me)OH···O(H)– or –(Me)O···HO(H)–, when the pyrazolato
moiety is varied: a) B. Bauer-Siebenlist, F. Meyer, E. Farkas,
D. Vidovic, J. A. Cuesta-Seijo, R. Herbst-Irmer, H. Pritzkow,
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Some methyl borinate was formed also by the direct reaction
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Ar2BOBAr2 + MeOH i Ar2BOMe + Ar2BOH.
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[31]
[32]
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Truly symmetric hydrogen bonds are rare, and even in closely
related cases in which the hydrogen-bonded moieties are
[15] Indeed, for MeOH the parameter β2, which measures the
strength of a hydrogen-bond acceptor (or its hydrogen-bond
basicity),[16] has a value (0.41) much closer to water (0.38), than
to thf (0.51). Also the α2 parameter, which measures the hydro-
gen-bond acidity of a solute,[17] is very close to that of water
(0.367 vs. 0.353).
[34]
identical, such as adduct [(1,5-cyclooctanediyl)B(OMe)-
[25]
[16] M. H. Abraham, P. L. Grellier, D. V. Prior, J. J. Morris, P. J.
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(MeOH)]2
or oligomer 3a, an asymmetric location of the
proton has been revealed by X-ray[25] or NMR[9] spectroscopic
data, respectively.
[35]
[36]
[37]
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Received: November 10, 2007
[18] Quadrupolar broadening hampers the obtainment of 11B
NMR spectroscopic data at low temperatures; see for instance:
J. D. Kennedy in Multinuclear NMR (Ed.: J. Mason), Plenum
Press, New York, 1987, pp. 221–258.
Published Online: February 5, 2008
Eur. J. Inorg. Chem. 2008, 1645–1653
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1653