Competition between hydrogen bonds and lewis acid-base interactions in the equilibria between bis(pentafluorophenyl)borinic acid and pyridine: Insights from NMR, diffractometric and computational studies

Article abstract of DOI:10.1524/zpch.2013.0379

¹H and ¹⁹F NMR spectroscopy, X-ray diffractometry and DFT computations have been used for investigating the interaction between the Lewis base pyridine and bis(pentafluorophenyl)borinic acid Ar₂BOH (1, Ar=C₆F₅). Previous studies showed that the latter species in solution exists as an equilibrium mixture of the monomer (1_m) and a cyclic trimer (1_t), in variable ratios, and that the trimer is stabilized in the presence of Lewis bases, by the formation of strong 1 _t? L hydrogen bonds. In the present case, upon addition of 0.33 equivalents of pyridine, low-temperature NMR spectra showed the formation of deprotonated 1_t (anion 3), strongly hydrogen-bonded to the pyridinium cation Hpy⁺. The presence of this cation was confirmed by the high value of ¹J_HN (88 Hz). DFT computations confirmed the higher stability of the O^-? H-N⁺ limit form over the neutral O-H?N one. Variable temperature NMR spectra showed that the Hpy⁺·3 ion pair has high conformational freedom. At temperatures higher than 260 K, 3 underwent reversible partial fragmentation to give 1_m and the Lewis acid-base covalent adducts between pyridine and either 1_m (2_py), or its anhydride (5_py). The fragmentation was perfectly reversible on lowering the temperature. The adduct 2_py became the only species in solution in the presence of 1 equivalent of pyridine, showing that the Lewis basicity of pyridine plays the major role, at variance with what previously observed with other bases. Hydrogen bond interactions, however, promote the supramolecular organization of 2 _py in the form of a cyclic tetramer, as revealed by X-ray single crystal diffractometric analysis. The formation of the deprotonated trimer 3 was previously observed in the reaction of 1 with 1,8-bis(dimethylamino)naphtalene (DMAN). However the stepwise dearylation processes, which were dominant in the reactions with DMAN, have a very marginal role in the reaction with pyridine, due to its lower Br?nsted basicity. by Oldenbourg Wissenschaftsverlag, Mu?nchen.

Full text of DOI:10.1524/zpch.2013.0379

Z. Phys. Chem. 227 (2013) 751–773 / DOI 10.1524/zpch.2013.0379
© by Oldenbourg Wissenschaftsverlag, München
Competition between Hydrogen Bonds and Lewis
Acid-Base Interactions in the Equilibria between
Bis(pentafluorophenyl)borinic Acid and Pyridine:
Insights from NMR, Diffractometric and
Computational Studies
By Daniela Maggioni, Tiziana Beringhelli, Giuseppe D’Alfonso,
Maria Carlotta Malatesta, Pierluigi Mercandelli , and Daniela Donghi
∗
∗, #
Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, 20133 Milano, Italy
Dedicated to Prof. Dr. Hans-Heinrich Limbach on the occasion of his 70th birthday
(Received December 17, 2012; accepted February 26, 2013)
(Published online June 10, 2013)
Hydrogen Bond / Lewis Bases / 19F NMR Spectroscopy / DFT Computations /
Fluoroarylboranes
1H and 19F NMR spectroscopy, X-ray diffractometry and DFT computations have been used for
investigating the interaction between the Lewis base pyridine and bis(pentafluorophenyl)borinic
acid Ar2BOH (1, Ar = C6F5). Previous studies showed that the latter species in solution exists
as an equilibrium mixture of the monomer (1m) and a cyclic trimer (1t), in variable ratios, and
that the trimer is stabilized in the presence of Lewis bases, by the formation of strong 1t· · · L
hydrogen bonds. In the present case, upon addition of 0.33 equivalents of pyridine, low-temperature
NMR spectra showed the formation of deprotonated 1t (anion 3), strongly hydrogen-bonded to
the pyridinium cation Hpy+. The presence of this cation was confirmed by the high value of
1JHN (88 Hz). DFT computations confirmed the higher stability of the O−· · · H–N+ limit form
over the neutral O–H· · · N one. Variable temperature NMR spectra showed that the Hpy+·3
ion pair has high conformational freedom. At temperatures higher than 260 K, 3 underwent
reversible partial fragmentation to give 1m and the Lewis acid-base covalent adducts between
pyridine and either 1m (2py), or its anhydride (5py). The fragmentation was perfectly reversible
on lowering the temperature. The adduct 2py became the only species in solution in the presence
of 1 equivalent of pyridine, showing that the Lewis basicity of pyridine plays the major role, at
variance with what previously observed with other bases. Hydrogen bond interactions, however,
promote the supramolecular organization of 2py in the form of a cyclic tetramer, as revealed
by X-ray single crystal diffractometric analysis. The formation of the deprotonated trimer 3 was
previously observed in the reaction of 1 with 1,8-bis(dimethylamino)naphtalene (DMAN). However
* Corresponding authors. E-mail: daniela.donghi@aci.uzh.ch; pierluigi.mercandelli@unimi.it
#
Present address: Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

752
D. Maggioni et al.
Scheme 1. The main species previously observed in dichloromethane solutions of Ar2BOH (1, Ar = C6F5),
alone or in the presence of bases L, with L = water, tetrahydrofuran (THF), methanol (MeOH) or 1,8-
bis(dimethylamino)naphthalene (DMAN).
the stepwise dearylation processes, which were dominant in the reactions with DMAN, have a very
marginal role in the reaction with pyridine, due to its lower Brønsted basicity.
For Supplementary Material see online version. Supporting information available: details of the
NMR spectroscopic characterization (Figs. S1–S4), computed chemical shifts (Table S1), details of
the X-ray diffractometric analysis, and tables of geometrical parameters (Tables S2–S3).
1. Introduction
Bis(pentafluorophenyl)borinic acid Ar2BOH (1, Ar = C6F5), is an intriguing Lewis
acid, which is able to dramatically modify its speciation in solution in response to
external stimuli [1]. These properties have stimulated a number of fundamental inves-
tigations [2–7], as well as applications in synthesis and catalysis [8,9].
The chameleonic nature of this small molecule arises from a peculiar combination
of properties of opposite sign: Lewis and Brønsted acid and base, hydrogen bond donor
and acceptor. The Lewis acidity of boron and the Lewis basicity of oxygen, although
depressed by the partial double-bond character of the boron-oxygen interaction [2],
are strong enough to promote the auto-association equilibrium (1). Therefore solutions
of 1 always contain both the monomeric (1m) and the trimeric (1t) forms depicted in
Scheme 1, in a ratio which depends on temperature, concentration and polarity of the
solvent [3]. The monomer is usually largely dominant, while in the solid state the trimer
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
753
Scheme 2. Trimerization pathway in the presence of L, as suggested by DFT computations [3]. In square
brackets are indicated intermediates undetected in our experimental conditions.
only is present [2].
3Ar2BOH ꢀ Ar6B3(OH)3
(1)
Interestingly, position and rate of equilibrium 1 are strongly affected by the pres-
ence of even small amounts of Lewis bases L, like tetrahydrofuran (THF) [4] or
H2O [3]. In general, in the absence of base, equilibrium (1) is slow and 1 is mainly in its
monomeric form. Nevertheless, the presence of water accelerates the equilibrium [3],
and instantaneous complete trimerization was observed at 173 K upon addition of stoi-
chiometric THF to dichloromethane solutions of 1 (0.33 equivalents of L with respect
to the Ar2BOH units) [4]. DFT computations showed that the presence of L is responsi-
ble for the aggregation pathway depicted on the right side of Scheme 2 [3]. The kinetic
barriers of such pathway are lower than in the absence of L, and this explains the accel-
eration effect of L in the aggregation process. The shift to the right of the equilibrium
arises from the high stability of the hydrogen-bonded trimeric species 1t·L. The forma-
tion of this species is preferred to that of the Lewis acid-base adduct 2L, highlighting the
low Lewis acidity of the boron atom in 1m.
In principle, a Lewis base has two potential way of interaction with 1m: via covalent
bond with the boron atom or via hydrogen bond with the BOH group. This results in
a complex interplay between hydrogen bonding and Lewis acid-base interactions, well
evidenced by the different preferred way of interacting with 1m shown by H2O and THF.
In the first case, covalent Lewis acid-base interaction was preferred, leading to the for-
mation of adduct 2L, with a tetra-coordinated boron atom, whereas in the case of THF
the interaction occurred via hydrogen bond (1m·L in Scheme 1), leaving tri-coordinated
the boron atom.
For Lewis bases able to act also as hydrogen bond donors, such as H2O or methanol
(MeOH) it was found that the two functionalities can work in a cooperative way, afford-
ing very stable species in which the bases are simultaneously B-bonded and H-bonded.
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

754
D. Maggioni et al.
Scheme 3. 1,8-Bis(dimethylamino)naphthalene (DMAN).
This is the case of the trimeric adducts containing one endocyclic hydrogen-bonded
water/methanol molecule, 1t·ROHendo shown in Scheme 1. The stabilization of the
water molecule in this environment was so strong to promote water formation, by spon-
taneous partial dehydration of borinic acid to its anhydride (5 in Scheme 1). In the
case of methanol, the formation of such species, in the presence of 0.33 equivalents of
MeOH, occurred preferentially with respect to the formation of the 1t·MeOH adduct.
The same twofold interaction allowed stabilization by dimerization of the adduct 2MeOH
(shown in Scheme 1), thus inhibiting the fast trimerization pathway operative with THF
and H2O.
A fully different behaviour was observed in the presence of the Brønsted base
1,8-bis(dimethylamino)naphthalene (DMAN, Scheme 3) [6]. In this case, the strongest
accelerating effect in the 1m-1t equilibrium was observed, since catalytic amounts of
base promoted the complete conversion of 1m into 1t. This trimerization reaction most
likely occurs via a pathway that includes first deprotonation of 1m, with consequent
formation of deprotonated 1t (3 in Scheme 1). In addition to deprotonation, fast irre-
versible dearylation reactions took place, leading to the formation of the penta-aryl and
tetra-aryl boroxinate anions Ar5B3(OH)O−2 (4 in Scheme 1) and Ar4B3O3−. Base cat-
alyzed hydrolysis of 1m to its boronate analogue ArB(OH)O−, followed by aggregation
and condensation, is the most likely mechanism leading to the first dearylation process,
which occurs in parallel to the formation of the deprotonated trimer 3 of Scheme 1 [6].
Taken together, all the previous evidence allowed depicting an elaborate but still not
comprehensive picture of the possible interactions between 1 and various bases. Chang-
ing only slightly the characteristics of the interacting base could give rise to unexpected
behaviours. It was therefore of interest to investigate how the title compound reacts in
the presence of a molecule like pyridine. This molecule is able to act both as a Brøn-
sted and as a Lewis base, as well as a hydrogen bond acceptor, but, unlike water and
methanol, it does not have any hydrogen-bond donor ability. The study was addressed
1
both spectroscopically, by use of variable temperature H and 19F NMR spectra, and
theoretically, by using DFT computations.
2. Experimental
All manipulations were performed under inert atmosphere (N2 or Ar) using oven
dried Schlenk-type glassware. CD2Cl2 (C.I.L.) was anhydrified over activated molecu-
lar sieves. Ar2BOH was a gift from Basell Polyolefins. Deuterated pyridine (py-d5, D
100%) was purchased by Aldrich and used as received. All the NMR spectra were ac-
quired with a Bruker AVANCE DRX-300 spectrometer, equipped with a 5-mm TBI
probe or with a 5-mm QNP probe, with a Bruker AVANCE DRX-400 spectrometer,
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
755
equipped with a 5-mm BBI probe, and with a Bruker AC-200 spectrometer, equipped
with a 5 mm BBI probe. 19F NMR spectra were referenced to external CFCl3. 11B NMR
spectra were referenced to external BF3·Et2O. Pentafluorotoluene (C6F5CH3, 1 L) was
added as internal standard (std) for both 1H and 19F NMR spectra. The temperature was
calibrated with a standard CH3OH/CD3OD solution [10].
2.1 Sample preparation and NMR experiments
Different amounts of 1 were weighted directly into the NMR tube under N2 and
dissolved in measured amounts of pre-dried CD2Cl2, affording typically ca. 0.1 M so-
lutions. Stepwise additions of pyridine with a micro syringe were done directly into
the NMR tube, both at low and at room temperature. No significant differences in the
spectra were observed in the two cases. The experiments were usually carried on using
pyridine-d5, to avoid the overlap of the aromatic signals of pyridine with the signals of
the B–OH groups in the proton spectra.
1
After each addition, H and 19F NMR spectra were recorded. Variable tempera-
ture mono and bidimensional experiments were performed on samples containing ca.
0.33 equivalents (hereafter equiv) of pyridine in the range 173–293 K. These experi-
ments were useful to evaluate the speciation of 1 at different temperatures, as well
as to evaluate the inter- and intra-molecular dynamic processes undergone by the
various species in solution. [19F,19F]-EXSY experiments recorded at different tem-
peratures confirmed the nature of the observed exchange processes, as discussed
in the text. In addition, a [1H,19F]-HOESY experiment recorded at low temperature
suggested the presence of hydrogen-bond interactions between the OH resonances
and the ortho fluorine of the aryl rings. A 11B NMR spectrum was recorded at
293 K to confirm the ratio between species containing tetra- and tri-coordinated boron
atoms.
The titration with py-d5 was carried on up to 1 equiv, and the corresponding spectra
(1H, 19F and 11B) recorded at different temperatures.
A sample in which 0.33 equiv of 15N-pyridine (purchased by C.I.L., 15N 98%) were
added to 1 was also prepared, and 1H NMR spectra at low temperature were recorded to
confirm the presence of pyridinium cation (see below).
2.2 Computational details
Ground state geometries were optimized by means of density functional calculations
employing the hybrid functional B3LYP [11] along with the standard valence double-
ζ polarized basis set 6-31G(d,p). All the calculations were done without imposing any
symmetry, however the species 1t·py and Hpy+·3 resulted to possess C2 symmetry.
The nature of all the stationary points was checked by computing vibrational frequen-
cies and all the species were found to be true minima. Single point energy calculations
were done in presence of solvent (dichloromethane, used in the NMR studies) described
by the polarizable continuum model (PCM) by means of the integral equation formal-
ism variant [12]. NMR shielding tensors were computed with the gauge-independent
atomic orbital (GIAO) method [13] at the B3LYP/6-31++G(d,p) level of theory [14].
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

756
D. Maggioni et al.
1
Chemical shift are referenced to TMS and CFCl3 for H and 19F, respectively. All the
calculations were done with Gaussian 09 [15].
2.3 X-ray diffractometric analysis
Colorless crystals of Ar2B(OH)(py) (2py) were obtained by slow diffusion of n-pentane
into a dichloromethane solution of a 1 : 1 Ar2BOH/pyridine mixture, at 248 K. Crystal
data: C17H6BF10NO, Mr = 441.04, triclinic, space group P1 (No. 2), a = 14.166(2) Å,
b = 14.833(2) Å, c = 16.752(2) Å, α = 83.81(2)◦, β = 77.80(2)◦, γ = 85.63(2)◦, V =
3415.3(8) Å3, Z = 8, dcalc = 1.715 g cm−3, T = 295(2) K, crystal size = 0.28 ×0.28 ×
0.16 mm, μ = 0.18 mm−1, λ = 0.71073 Å (Mo Kα). Refinement of 1085 parameters
on 12081 independent reflections out of 38878 measured reflections (Rint = 0.0465,
Rσ = 0.0369, 2θmax = 50.1◦) led to R1 = 0.0397 (I > 2σ(I)), wR2 = 0.1185 (all data),
and S = 1.065, with the largest peak and hole of 0.238 and −0.207 e Å−3. Details of
the diffractometric analysis can be found in the Supporting Information. CCDC-915451
contains the crystallographic data for 2py. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
3. Results and discussion
3.1 The reaction of 1 with 0.33 equivalents of pyridine at low temperature
The 1H and 19F NMR spectra showed that the addition, at 178 K, of ca. 0.33 equiv of
pyridine (with respect to the moles of Ar2BOH present in solution) caused the complete
disappearance of the starting compound, accompanied by the formation of two trimeric
species.
The main reaction product (accounting for ca. 80–90% of the species in solution)
can be formulated as the ion pair between deprotonated 1t and the pyridinium cation
(Hpy+·Ar6B3(OH)2O−, i.e. Hpy+·3, depicted in Scheme 4), on the base of the following
evidence. The trimeric structure agrees both with the chemistry (0.33 equiv of pyridine)
and with the NMR data. The 19F spectrum (Fig. 1), although heavily affected by various
dynamic processes not completely frozen even at very low temperatures (see Sect. 3.3),
at 173 K showed three signals with the same integrated intensity in the para region of
the spectrum (see inset of Fig. 1), two of which were broad and rapidly coalesced on
raising the temperature. At 293 K, the 19F spectrum shows two sets of signals in 1 : 2
ratio in the ortho and para regions, whereas only one signal is present in the meta re-
gion due to accidental overlap. The attributions of these signals to a single species were
confirmed by a [19F,19F]-COSY experiment (Fig. S1 in Supporting Information).
The 1H spectrum at 178 K showed two resonances in the 1 : 2 ratio, at δ 17.01 and
7.86, respectively. The position of the lower field resonance indicates that one proton
is involved in a strong O· · · H· · · N hydrogen bond. The value of the chemical shift can-
not discriminate between the neutral O–H· · · N or charged O−· · · H–N+ limit forms, i.e.
cannot establish if proton transfer (Eq. 2) to the N atom of the Brønsted base pyridine
(with pyridinium formation) occurred or not.
Ar6B3(OH)2O−H· · ·NC5H5 ꢁ Ar6B3(OH)2O−· · ·H−NC5H5+
(2)
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
757
Scheme 4. A summary of the equilibria operative in solutions of 1 upon pyridine addition.
Fig. 1. 19F NMR (top, 178 K, inset at 173 K, 9.4T) and 1H NMR (bottom, 178 K) spectra of 1 treated with
ca. 0.33 equiv of py-d5 (7.1T; std is pentafluorotoluene, used as internal standard; HAr is C6F5H; # indi-
cates the signal due to the CD2Cl2 solvent). 1t·H2O indicates the small amount of the adduct between 1t
and water [3] arising from adventitious water present in the solution.
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

758
D. Maggioni et al.
Fig. 2. Variable temperature 19F NMR spectra of a sample of 1 treated with ca. 0.33 equiv of py-d5
(CD2Cl2, 7.1T). 1t·H2O indicates the adduct between 1t and water [3].
For this reason an experiment with 15NC5H5 was performed: in this case the NH
resonance of 3 (as well as that of 4, described below), appeared as a doublet with
JHN = 88 Hz. Such a high value of JHN is typical of the pyridinium cation [16], so def-
initely confirming the ionic nature of the species present in solution at low temperature.
The signal remained a sharp doublet up to 283 K, then broadened, as the result of the
several intermolecular exchange processes occurring at room temperature. DFT com-
putations support the formulation of the adduct as a double-charged hydrogen-bonded
species. Indeed, even if a geometry optimization of Hpy+·3 and 1t·py shows that both
the species correspond to minima of C2 symmetry, differing almost exclusively in the
position of the hydrogen atom involved in the hydrogen bond [17], the neutral species
is computed to lie 17 kJ mol−1 higher in energy than the charged one.
A second product, accounting for ca. 10% of species in solution, was identified
as the ion pair Hpy+·4 (see Scheme 4). This species is responsible for two resonances
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
759
1
in the ratio 1 : 1 in the H spectrum (one, sharp, at δ 7.45 ppm and another, broad, at
δ 14.60, the latter being attributable to a hydrogen-bonded proton) and for two reso-
nances in the ratio 1 : 4 in each of the ortho, para and meta regions of the 19F spectra.
Such C2v pattern of the 19F signals, which is typical of pentaarylboroxinates, indicates
easy migration of the hydrogen-bonded pyridinium cation between the two deproto-
nated oxygen atoms. A similar pattern was observed for the ion pair HDMAN+·4 [6].
The three sets of 1 : 4 resonances are clearly detected at temperatures higher than 180 K
(see Fig. 2), whereas at lower temperatures partial overlaps occurred with the broad and
more intense 19F signals of the major product 3 (Fig. 1). Their attribution to a unique
species was confirmed by the [19F,19F]-COSY experiment recorded at 293 K (Fig. S1 in
1
Supporting Information). The comparison between the H and 19F intensity ratios (by
use of pentafluorotoluene as internal standard in both the spectra) confirmed that the
most intense signal in each of the ortho, para and meta regions was due to four equiva-
lent aryl rings. Further support to the formulation of this species was provided by the
separation between the meta and para 19F signals ( δm,p). At low temperature δm,p of
10.8 and 6.9 ppm were found for the signals of intensity 1 and 4, respectively. This is
in line with previous findings, that showed that δm,p is smaller for tetra-coordinated
than for tri-coordinated boron compounds, due to the stronger upfield shift of the 19F
para resonances resulting from the increased electron density on the tetra-coordinated
boron atom [1]. A similar pattern was observed for the 19F para and meta resonances of
HDMAN+·4 [6].
1
Even if the general behaviour is similar, the absolute values of H and 19F NMR
chemical shifts for the Hpy+·3 and Hpy+·4 ion pairs (see Table 1) were found to be
quite different with respect to those of the analogous species identified in the reaction
of 1 with DMAN (see for instance, for the para 19F resonances at 178 K: −154.3 and
−155.6 ppm for Hpy+·3 vs. −156.0 and −159.0 ppm for HDMAN+·3; see also −150.5
and −156.6 ppm for Hpy+·4, vs. −154.1 and −158.1 ppm for HDMAN+·4). These dif-
ferences are attributable to the different nature (tight or loose) of the ion pairs in the
two compounds, as discussed in the following Sect. 3.4. Such differences mainly affect
the resonances of the aryl groups closer to the O−· · · Hpy+ site, i.e. the ones of higher
intensity in 3 and the ones of lower intensity in 4.
Two separated resonances are observed for the NH+ protons of the two ion
pairs Hpy+·3 and Hpy+·4 (3 δ 17.01, 4 δ 14.60), unlike what previously observed for
HDMAN+·3 and HDMAN+·4, where only one averaged signal for HDMAN+ was
present, at δ 20.1 ppm. The large sterical hindrance of the HDMAN+ cation prevents
any close interaction with 3 or 4, resulting in an averaged signal, whose position is de-
termined by the strength of the N· · · H· · · N+ intra-molecular hydrogen bond. In the
present case, the two separated resonances for Hpy+·3 and Hpy+·4 agree with the pres-
ence of a hydrogen-bond interaction between anion and cation. In particular, the higher
field value for the proton resonance of 4 suggests that in this species the hydrogen-bond
interaction is weaker than in 3. This is in line with the lower propensity of the deproto-
nated oxygen atoms of 4 to accept hydrogen bond, being involved in partial π-donation
to the tri-coordinated boron atom [6] (as depicted in Scheme 1). Moreover, in 4 the
negative charge is delocalized over the two equivalent oxygen atoms surrounding tri-
coordinated boron, while in 3 it is localized on the unique deprotonated oxygen atom.
Nevertheless, at 220 K the signals of the NH+ protons coalesced in an averaged signal
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

760
D. Maggioni et al.
1
Table 1. H and 19F chemical shift of the pyridine derivatives described in this work (CD2Cl2).
T
[K]
δ1H
[ppm]
δ19F
[ppm]
para
δm,p
[ppm]
NH
OH
ortho
meta
2py 173
4.1
−135.5 (2)
−135.5 (2)
−157.5 (1)
−157.0 (1)
−163.7 (2)
−163.4 (2)
−162.6 (12)
6.2
293
4.4
6.4
3
178
17.01
17.00
7.86
−133.6 (4)
−154.3 (2)
8.3, 7.0
−141.4 (8)
−155.6 (4)
170
7.87
7.45
−133.7 (4)
−136.7 (2)
−154.2 (2)
−154.8 (2)
−161.1 (2)
−162.1 (4)
−162.6 (2)
−163.1 (2)
−165.1 (2)
−140.9 (2, JHF = 80 Hz) −156.3 (2)
−142.0 (2)
−142.1 (2, JHF = 80 Hz)
4
178
14.60
−131.9 (2)
−150.5 (1)
−156.6 (4)
−161.3 (2)
10.8
6.9
−138.1 (8)
−163.5 (8)
5py 293
−132.9 (4)
−133.1 (4)
−151.5 (2)
−162.4 (4)
−163.2 (4)
11
7.6
−155.6 (2)
at δ 16.70, (Fig. 3), indicating exchange of the Hpy+ cation between anions 3 and 4 (see
Sect. 3.2).
The formation of the pentaarylic anion 4 was never detected in the reaction of 1 with
the Lewis bases THF, MeOH or H2O, but only in the reaction with the Brønsted base
DMAN [18]. In the latter case, however, after addition of 0.33 equiv at low temperature,
the anions 3 and 4 were formed in comparable amounts, while with pyridine the amount
of 4 hardly reached 10% of the products present in the mixture.
There is another significant difference between the behaviour of DMAN and pyri-
dine. The latter does not play a catalytic role in the low-temperature trimerisation
reaction, as it occurs with DMAN. The addition of less than 0.33 equiv of pyridine at
193 K does not cause complete trimerisation, as in the case of DMAN, but rather the in-
crease of the trimeric species at expenses of 1m. From this point of view, the behaviour
of 1 with pyridine is comparable to that observed with the Lewis base THF, where
trimerization was complete only at 0.33 equiv of added base.
Moreover, at room temperature the conductivity of the solution obtained upon add-
ition of 0.33 equiv of pyridine to 1 was very low (ca. 40 S in CH2Cl2), much lower
than in the case of the corresponding solution obtained with DMAN (ca. 500 S, in the
same conditions). This might result from two factors. The ion pairs involving Hpy+ are
expected to be much tighter and therefore much less conductor than those formed by
HDMAN+, because in the latter case the proton is trapped by the two N atoms of the
proton sponge cation and cannot establish a strong hydrogen bond interaction with the
negative oxygen atom of 3 (see above). In addition, on increasing the temperature the
O−· · · H–N+ limit form of equilibrium 2 might progressively shifts towards the neutral
O–H· · · N form, on the bases both of the thermodynamics of this kind of proton transfer
equilibria [19] and of the decrease of the dielectric constant of CD2Cl2 at higher tem-
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
761
Fig. 3. Variable temperature 1H NMR spectra of a sample of 1 treated with ca. 0.33 equiv of py-d5
(CD2Cl2, 7.1T). The inset shows the fine structure of the BOH signal of 4. The asterisk marks the average
signal of 1m, 2H and 2py. # marks the solvent signal.
O
2
perature [20]. The neutral form (corresponding to the 1t·py adduct) at high temperature
undergoes fragmentation to the variety of neutral mono and dinuclear species described
in the following paragraph.
3.2 Partial fragmentation of Hpy+·3 at room temperature: the novel species
2py and 5py
On increasing the temperature the 19F spectra dramatically changed (Fig. 2), as the re-
sult of the increase of the rate of the dynamic processes, which will be discussed in the
next Sect. 3.3. Some of these processes involve fast dissociation/reaggregation of the
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

762
D. Maggioni et al.
trimeric units. However, up to ca. 263 K such dissociative processes are kinetically but
not thermodynamically significant, that means that the amount of the dissociation prod-
ucts is small, although sufficient to affect the exchange processes. On the contrary, at
higher temperatures, the fragmentation increased, as revealed by the decrease of the in-
tensity of the 19F resonances of 3 (at 293 K they were reduced to about one half of their
original intensity), and the appearance of the signals of 1m and of the new species 2py
and 5py.
Species 2py contains one molecule of borinic acid (monomer) and one of pyridine.
In these conditions it was in small concentration, and its resonances were barely de-
tectable, being weak and broad, for intermolecular exchange. However its presence was
unquestionably indicated by the cross peaks with 1m and 3 clearly recognizable in the
[19F,19F]-EXSY exchange spectrum shown in Fig. 4. Moreover, 2py became the main
species in solution when the amount of pyridine exceeded 0.33 equiv (see Sect. 3.4 for
details). It has been formulated as the Lewis acid-base adduct between 1m and pyridine,
Ar2B(OH)(py) (2py in Scheme 4), rather than the hydrogen-bonded species 1m· · · py, on
the bases of many pieces of evidence. First, both its 11B signal (δ2.6, Fig. S2 in Sup-
porting Information) and the chemical shift difference between its 19F meta and para
resonances ( δm,p = 6.4 ppm) lie in the range typical for tetra-coordinated boron atoms.
Second, DFT computations indicate a higher stability for this form with respect to the
hydrogen-bonded one, by 7 kJ mol−1. Finally, a X-ray analysis provided the details of
its molecular and supramolecular structure (see Sect. 3.4).
The fragmentation process leading to 2py can therefore be described by equations 3,
where the equilibrium symbols accounts for the exchange processes shown in Fig. 4.
Ar6B3(OH)2O−· · · Hpy+ ꢀ Ar6B3(OH)3· · · py
Ar6B3(OH)3· · ·py ꢀ 2Ar2BOH+Ar2B(OH)(py)
(3a)
(3b)
The other novel species is formulated as the dimeric adduct between pyridine and
the borinic anhydride, Ar2BOBAr2(py) (5py in Scheme 4). This formulation is based
on the 19F spectrum (no proton signal attributable to this species was detected). The
patterns of resonances (three couples of signals of the same intensity in the three or-
tho, para, meta regions, see Fig. 2, top trace) cannot arise either by a trimeric or by
a monomeric species (taking into account that at room temperature the two aromatic
groups on each boron centre are always dynamically equivalent). On the contrary, this
.
pattern agrees with a dimeric structure, in which two couples of aromatic rings are
bonded to two different boron atoms. The presence of a tri-coordinated boron centre
is revealed by the position of one of the para resonances, with lies at very low field
(quite close to the signal of 1m), with a δm,p of 11.0 ppm at 293 K. Strictly comparable
NMR data were previously found for the analogous adduct between borinic anhydride
and THF [4], whose nature had been confirmed by the direct reaction between THF and
the anhydride. The slight upfield shift observed for 5py is in line with the higher donor
power of pyridine with respect to THF.
The formation of the anhydride implies release of a corresponding amount of wa-
ter, which at this temperature is bonded to 1m, to give the well known Ar2B(OH)(OH2)
adduct (2H2O) [3,4]. Therefore the overall process of formation of 5py from Hpy+·3
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
763
Fig. 4. Para region of a [19F,19F]-EXSY experiment on a sample of 1 treated with ca. 0.33 equiv of py-d5
(CD2Cl2, 293 K, 7.1T).
could follow the stoichiometry expressed by Eq. (4).
Ar6B3(OH)2O−· · · Hpy+ ꢀ Ar2BOBAr2(py)+Ar2B(OH)(OH2)
(4)
It is reasonable to assume that the B-bonded water molecule is further stabilized
by hydrogen-bonding interactions, either mutual (as for the dimeric form of 2MeOH de-
picted in Scheme 1) or with pyridine. In the latter case, proton transfer might take place
(equilibrium 5), to give the Hpy+Ar2B(OH)−2 ion pair, due to the Brønsted acidity of
a water molecule bonded to a boron atom, as it occurs for the adduct between water and
B(C6F5)3 [21].
Ar2B(OH)(H2O)+ py ꢀ Ar2B(OH)(OH2)· · · py ꢀ Ar2B(OH)−2 · · · Hpy+
(5)
A detailed analysis of the integration ratios suggest that 2H O does interact with pyri-
dine. However, a deeper insight into the nature of this species2was hampered by its low
concentration and by the fact that equilibrium 4 is shifted to right only at high tem-
perature, where fast intermolecular exchange processes hinder detection of separated
resonances for each species. Indeed, the position and the large bandwidth of the 19F
signals ascribable to the different monomeric forms of 1 (altogether indicated by 1m
in Fig. 2), indicate that they are averaged by fast exchange between 1m and its water
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

764
D. Maggioni et al.
adduct [3]. The 1H spectrum (Fig. 3) shows a broad resonance at δ 4.4 ppm attributable
to the BOH protons of 1m (including its water adduct) and 2py, which at this temperature
are in fast exchange with the protons of water (either free or bonded).
By contrast, the trinuclear anions 3 and 4 exhibit separated BOH resonances, with
respect to such broad averaged resonance (Fig. 3). This can be attributed to the fact that
all the BOH protons of 3 and 4 are buried within a cage of Ar groups and are then in-
volved in strong hydrogen-bond interactions with fluorine atoms. This is supported by
a [1H,19F]-HOESY experiment, that showed cross peaks between the BOH protons and
the ortho fluorine atoms (see Fig. S3 in Supporting Information), thus confirming their
close proximity. The resonances of the Hpy+ counterion are instead averaged, at 293 K,
either by proton exchange or by fast migration of the cation among the two different
anions.
In the case of 4, the close interaction between the BOH proton and the surround-
1
ing ortho fluorine atoms was indicated also by the structure of the H signal (at δ ca.
7.5 ppm), which at 293 K acquired a fine structure (9 lines, JHF 3.3 Hz, as shown in
the inset of Fig. 3), attributable to through-space coupling with the eight ortho fluorine
atoms of the four aromatic rings of the Ar2B(OH)BAr2 fragment, made equivalent by
fast dynamics (as previously observed for HDMAN+·4 [6]). This was not observed for
the BOH protons of 3, because the proton signal is slightly broadened by the exchange
processes described in the next paragraph.
Noteworthy, the partial fragmentation of 3 was perfectly reversible: on lowering
the temperature back to 263 K the adduct 3 was quantitatively restored, as shown in
Fig. 5.
A
11B NMR spectrum at 293 K (Fig. 6) showed two resonances, one in the re-
gion of tetra-coordinated (δ 5.8) and one in the region of tri-coordinated (δ 39.9) boron
atoms [1,22–26]. Their intensity ratio well corresponded to the overall ratio (ca. 2 : 3)
between the tri-coordinated (mainly 1m, plus the tri-coordinated boron centres in 4 and
5py) and tetra-coordinated (all the other ones, including 2H O) species observed in the 19F
2
spectra. In particular, the spectrum confirmed the presence of a large amount of 1m in
the reaction mixture at room temperature, after addition of 0.33 equiv.
3.3 The intra and inter-molecular dynamic processes
The 19F spectra of Fig. 1 (in particular the inset, showing three signals with the same
integrated intensity in the para region) indicates that in the Hpy+·3 ion pair the anion
3 has C2 symmetry, at variance with the Cs symmetry observed for the same anion in
the ion pair with a different cation, i.e. the Ar2B(H2O)+2 species formed in the mixture
1/THF [4]. In that case, four signals, in the ratio 2 : 2 : 1 : 1, had been observed in the
para region. The DFT modelling reported in paragraph 3.1 confirmed the C2 symme-
try of the anion 3 paired to Hpy+ [17]. Therefore, in the limiting 19F spectrum the three
couples of unequivalent aromatic rings of 3 should afford fifteen (six ortho, three para,
six meta) resonances of the same integrated intensities, as found for instance in the
low temperature spectrum of the 1t·THF adduct. However, the spectra resulted heavily
affected by dynamics.
Two kinds of intramolecular processes can occur in the Hpy+·3 ion pair: (i) the
concerted rotation of each aromatic ring around its B–C bond, which leads to the
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
765
Fig. 5. Para region of variable temperature 19F NMR spectra of a sample of 1 treated with ca. 0.33 equiv
of py-d5 (CD2Cl2, 7.1T). The temperature was moved from 263 K (bottom trace) to 293 K (middle trace)
and then back to 263 K (top trace), showing the perfect reversibility of the fragmentation of anion 3.
equalization of the ortho and meta positions within each ring, and (ii) the flopping
of the cycle, consisting in the pseudo-axial-pseudo-equatorial interconversion of the
two aromatic rings on each of two boron atoms connected by the bridging O−· · · H–
py+ group. At a temperature as low as 178 K both these processes were still active
on the NMR time scale, giving rise to very broad and unresolved resonances, partic-
ularly in the ortho and meta regions, as shown in Fig. 1. The limiting spectrum was
attained only by using over-frozen samples (at 170 K), as shown in Fig. 7. A G‡
value of ca. 32 kJ mol−1 at 176 K was estimated for process ii, by the coalescence
temperature of the two resonances at higher field in the para region, not affected by
process i.
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

766
D. Maggioni et al.
Fig. 6. 11B NMR spectrum on a sample of 1 treated with ca. 0.33 equiv of py-d5 (CD2Cl2, 293 K, 7.1T).
Fig. 7. 19F NMR spectrum of 1 treated with ca. 0.33 equiv of py-d5 (CD2Cl2, 170 K, 9.4T). The asterisk
marks the detectable separated resonances of 1t·H2O [27].
On raising the temperature, the increase of the rate of both the processes creates an
apparent mirror, raising the symmetry of the ion pair from C2 to C2v. Accordingly, at
temperatures higher than 220 K the resonances in the ortho and in the para regions pro-
gressively resolved into two signals in the ratio 2 : 1, while accidental overlap occurred
in the meta region, as shown in the variable temperature spectra of Fig. 2.
At higher temperatures a third dynamic process became active on the NMR time
scale. A [19F,19F]-EXSY experiment at 256 K showed exchange cross-peaks between
the two signals of 3 in each of the ortho, para and meta regions (Fig. S4 in Supporting
Information), implying exchange between the singular boron atom and the two boron
atoms bridged by the negatively charged oxygen atom. Such exchange cannot be in-
tramolecular, but rather it must go via bimolecular (either associative or dissociative)
mechanisms. This process results also in the exchange between the pyridinium proton
and the two BOH protons of Hpy+·3 and accounts for the broadening of their reso-
nances observed at the higher temperatures.
The dynamic behavior of Hpy+·3 shows significant differences with respect to what
observed for the neutral 1t·THF adduct [4]. In the latter cases both processes i (the ro-
tation of the aromatic rings around the B–C bonds) and ii (the flopping of the cycle
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
767
conformation) were much more hindered than in the present case, so that the lim-
iting spectrum appeared at higher temperatures (at 183 K it was possible to observe
fifteen sharp resonances with a clearly detectable fine structure). In particular the ki-
netic constant estimated for the process of cycle flopping in the neutral 1t·THF adduct
was as low as 0.9 s−1 at 183 K, whereas the G‡ value estimated for Hpy+·3 at 176 K
corresponds to a much larger kinetic constant value (> 103 s−1). Most likely the de-
creased rigidity of anion 3, with respect to neutral 1t·THF, is due to lack of one of
the OH protons, which relieves two aromatic rings from the constraints arising from
hydrogen-bonding with this hydrogen atom. In agreement with this, the cycle flop-
ping of anion 3 in the HDMAN+·3 ion pair was even faster than in Hpy+·3, due to the
weak cation-anion interaction, and only two 19F signals in the ratio 2 : 1 were present at
a temperature as low as 173 K [6].
By contrast, in the neutral 1t·THF adduct, the equalization of the three boron
centres was much faster than in anion 3, because it required only migration of THF
among the three BOH groups, whereas in Hpy+·3 the equalization requires prelimi-
nary back proton transfer (equilibrium 2), followed by pyridine migration. Therefore
even at room temperature two signals (ratio 1 : 2) are observed for each of the three re-
gions of the 19F spectrum of Hpy+·3, whereas in the case of 1t·THF all the resonances,
in each of the three regions, merged in a single averaged signal, starting already from
240 K [4].
On the other hand, the species Hpy+·4 shows always two signals in a 1 : 4 ratio, in
line with the higher conformational lability expected for an anionic species possessing
five aromatic rings only.
A few dynamic processes involving exchange of Ar2BOH units among different
compounds have also been detected, by using [19F,19F]-EXSY experiments.
The experiment of Fig. S4, at 256 K, showed exchange between Hpy+·3 and 1m.
This implies fragmentation/reaggregation of the trimeric unit, which must be catalysed
by pyridine, since the equilibrium between 1m and 1t is very slow in the absence of
a base (see Introduction). The small volume of the cross-peaks indicates that such ex-
change is very slow, and then it alone cannot be responsible for the above discussed
strong correlations between the signals of 3.
An analogous experiment at 293 K (Fig. 4) showed that at this temperature also the
pyridine adduct 2py is involved in the monomer-trimer exchange (most likely due to mi-
gration of pyridine among different 1m molecules). Such exchange is responsible for the
large bandwidth of these signals, which makes poorly detectable the species with the
lowest concentration (i.e. 2py). Fig. 4 shows that even the Ar2B fragments of the deary-
lated species 4 are exchanging with 1m (and then with 3) at 293 K (although the rate
of the exchange is quite low, on the basis of the cross-peaks intensities). By contrast,
as expected, 5py is not involved in this intermolecular exchange and Fig. 4 shows only
exchange within the species 5py itself, arising from pyridine migration between the two
boron centres.
3.4 Addition of 1 equivalent of pyridine
Further pyridine addition (above 0.33 equiv), at low temperature, caused fragmentation
of the trimeric oligomers, which were the dominant species in the low temperature re-
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

768
D. Maggioni et al.
action mixtures obtained by addition of 0.33 equiv (as described in the Sect. 3.1). The
decrease of the concentration of the oligomers is due to the formation of the covalent
Lewis acid-base adduct between 1m and pyridine, namely 2py in Scheme 4. After add-
ition of 1 equiv of pyridine, 2py was the only boron containing species detectable in
solution, either at low or room temperature: three sharp signals were observed in the 19F
spectrum and one resonance in the 1H spectrum (δ 4.1 ppm) [28]. The latter resonance
is significantly upfield shifted with respect to the usual position of the BOH signals in
these systems (δ 6–8 ppm): DFT computations reproduced such high field shift. Indeed,
the chemical shift predicted by B3LYP-GIAO calculations (see Table S1) for the Lewis
acid-base adduct 2py is 3.92 ppm, to be compared with the value of 7.81 computed
for 1m (very close to the experimental one, 7.41 ppm [3]). Moreover, the computa-
tion further excludes that pyridine interact with 1m via hydrogen bond, since a value
of 14.28 ppm is computed for the hypothetical hydrogen-bonded species 1m· · · py (see
Table S1). As a further evidence, the small difference between the 19F chemical shift of
the para and meta fluorine atoms observed experimentally is well reproduced by cal-
culations on the adduct 2py ( δm,p = 5.4 ppm), while the value of δm,p = 12.0 ppm
predicted for 1m· · · py is more similar to those computed for species containing trigo-
nally coordinated boron atoms (see Table S1) [29].
The molecular structure of 2py was confirmed by a single crystal X-ray analysis.
The asymmetric unit comprises four 2py molecules (labeled A to D in Fig. 8), forming
a hydrogen-bonded tetramer with a folded square (OH)4 core, as depicted in Scheme 5.
The geometric parameters of the four independent molecules are similar, in particu-
lar as far as the coordination of the tetrahedral boron atom is concerned (Table S2).
The molecules however significantly differ in the propeller-like conformation of the
three six-membered ring bound to the boron center (one pyridine and two pentafluo-
rophenyl ligands), as evidenced by the dihedral angles around the B–N and B–C bonds.
The supramolecular tetramer is held together by four O–H· · · O hydrogen bonds and
by face-to-face π-stacking interactions involving couples of pentafluorophenyl ligands
bound to adjacent boron atoms (Fig. 8). All these structural features have been previ-
ously observed in the crystal structure of some bisarylborinic acids [30], which aggre-
gates as hydrogen-bonded tetramers composed of trigonally (instead of tetrahedrally)
coordinated boron species. The four pyridine ligands form (less tight) π-stacking inter-
actions, as can be seen in Fig. 8. The 19F NMR data, measured by dissolving selected
crystals in CD2Cl2 at low temperature, were in good agreement with those measured in
the reaction mixtures containing 1 equiv of pyridine.
The disappearance of 3 upon addition of an amount of base exceeding 0.33 equiv
was previously observed in the titrations of 1 with DMAN. In that case, however, the
reaction product was the trimeric dearylated species 4, whereas in the present case frag-
mentation of the trimer is the main process, as described above. Such different reactivity
can be explained taking into account that DMAN cannot behave as Lewis base, so start-
ing all the cascade of events responsible for the fragmentation observed in the case of
pyridine [31].
Noteworthy, the 19F signals of the ion pair Hpy+·4 in the spectra recorded at in-
termediate titration steps (for instance 0.66 equiv) were strongly upfield shifted with
respect to those observed in the spectra at 0.33 equiv, and were almost identical to those
observed for the ion pair HDMAN+·4. This can be explained by assuming that a frac-
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
769
Fig. 8. (a) Ellipsoid plot of one of the four independent molecules found in the crystal structure of 2py
(molecule A) showing a partial labeling scheme. Top (b) and frontal (c) view of the hydrogen-bonded
tetramer showing the labeling of the molecules.
Scheme 5. The supramolecular aggregation of 2py observed in the crystal structure.
tion of pyridine interacts with the pyridinium cation, to give the homoconjugated pair
py· · · H· · · py+. This interaction breaks the hydrogen bond with the oxygen atom of 4
and leads to a loose ion pair, similar to that formed with HDMAN+·4. In agreement with
this, the separation between the para and meta resonances of the trivalent boron atom
of 4 decreased from 10.8 to 8.9 ppm, on increasing the amount of pyridine from 0.33
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

770
D. Maggioni et al.
to 0.66 equiv. It has been previously remarked that the delocalization of the negative
charge in tight ion pairs shifts downfield the 19F para resonance [1,32].
This effect could not be observed for anion 3, due to its broad signals and its fast
disappearance upon pyridine addition.
4. Conclusions
The data presented in this work provided additional insights into the not trivial be-
haviour of the chameleonic compound bis(pentafluorophenyl)borinic acid (1) in the
presence of different oxygen and nitrogen bases.
Pyridine is a stronger Lewis base than THF [33], when we consider its propensity
to form dative bonds with an archetypal Lewis acid such as BF3 in CH2Cl2. Along the
same line, it can be reasonably considered also stronger than methanol and water, since
the gas phase affinity of the latter two bases for BF3 is lower than that of THF [34].
On the other hand, pyridine is also a better hydrogen-bond acceptor than the other
three bases: according to the pKBHX scale, which measures the equilibrium constants of
the association with 4-fluorophenol, the following values have been reported: pyridine
1.86, THF 1.28, methanol 0.82, water 0.65 [35]. The same order is found in the β2 scale:
pyridine 0.62, THF 0.51, methanol 0.41, water 0.38 [36].
Moreover, pyridine is a much stronger Brønsted base than the other three Lewis
bases. A priori it is therefore impossible to establish if Lewis acid-base interactions
or hydrogen bonds or proton transfer processes would play the dominant role in the
speciation of 1, at different temperatures in the presence of different amounts of
pyridine.
The experimental results have shown that the reaction of 1 with 1 equiv of pyridine
affords quantitatively the Lewis acid-base adduct Ar2B(OH)(py) (2py). Surprisingly, this
is the first time that such apparently trivial result is obtained, since the other three Lewis
bases previously investigated had shown a quite different behaviour. This finding sug-
gests that the Lewis basicity of pyridine plays here the dominant role in defining the
nature of the species. DFT computations have confirmed the higher stability of the
adduct 2py with respect to 1m· · · py. However, at least in the solid state, also hydrogen
bond interactions are operative, leading to the supramolecular organization of the 2py
adduct in the form of the cyclic tetramer shown in Fig. 8.
On the other hand, when 1 is treated with less than 1 equiv of pyridine, the fast
trimerization pathway already observed in the case of other Lewis bases (Scheme 2)
becomes operative. The driving force for this process is provided by the Lewis basic-
ity of pyridine (with the consequent increased Lewis basicity of the oxygen atom in
the adducts 2L, with respect to 1m), but the Brønsted basicity of pyridine determines
the nature of the final product, which was shown, by both spectroscopic and computa-
tional evidence, to be the ion pair between deprotonated trimer and pyridinium cation
(Hpy+·3).
On raising the temperature the trimer underwent entropy-driven fragmentation,
while upon addition of pyridine enthalpy-driven fragmentation was observed. In the lat-
ter case, the Lewis-acidic Ar2BOH units, which in the trimeric oligomer were engaged
in mutual Lewis acid-base Ar2(OH)B-O(H)BAr2 interactions, were progressively cap-
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
771
tured by pyridine, which is a stronger Lewis base than 1, to give the covalent Lewis
acid-base adduct 2py.
The behaviour of pyridine showed also many significant differences with respect to
that of the stronger Brønsted base DMAN (pKa 12.1 vs. 5.25 for pyridine, in water).
First, the interaction of 1m with DMAN afforded the anion Ar2BO−, responsible for the
anionic route followed in the trimerization pathway in that case. With pyridine, on the
contrary, the neutral adduct 2py is initially formed, which is responsible for the ‘neutral’
trimerization pathway observed in this case. Moreover, the pyridinium cation is able to
establish a strong hydrogen bond with the deprotonated trimer (resulting in the tight
Hpy+·3 ion-pair), which was not possible for the HDMAN+ cation, resulting in a much
looser interaction between anion 3 and HDMAN+ cation. 19F chemical shifts proved to
be a reliable probe of the nature of the ion pair [1]. A further difference with respect to
DMAN is provided by the marginal role here played by dearylated species: pyridine is
not a Brønsted base strong enough to efficiently promote the dearylation process.
Worth of mention is also the reversible formation of the adduct between pyridine
and the anhydride of borinic acid. The partial dehydration of 1 to its anhydride had been
previously observed at low temperature [3], and it was driven by the stabilization of the
released water molecule in a cycle in which it was simultaneously covalently bonded
to a boron atom and hydrogen-bonded to an oxygen atom (see the species 1t·H2Oendo
in Scheme 1). Here the formation of the anhydride occurs at high temperature, but the
driving force is analogous, i.e. the two-fold stabilization of the water molecule in the
Ar2B(OH)OH2· · · py adduct, which most likely evolves to give the Ar2B(OH)−2 · · · Hpy+
ion pair.
In conclusion the work here described confirms that the speciation of 1 in the pres-
ence of bases is not straightforwardly predicable, but can be rationalized by the subtle
balance among Lewis and Brønsted acidity-basicity as well as hydrogen bond donor-
acceptor capability of the two reagents.
References
1. T. Beringhelli, D. Donghi, D. Maggioni, and G. D’Alfonso, Coordin. Chem. Rev. 252 (2008)
2292.
2. T. Beringhelli, G. D’Alfonso, D. Donghi, D. Maggioni, P. Mercandelli, and A. Sironi,
Organometallics 22 (2003) 1588.
3. T. Beringhelli, G. D’Alfonso, D. Donghi, D. Maggioni, P. Mercandelli, and A. Sironi,
Organometallics 23 (2004) 5493.
4. T. Beringhelli, G. D’Alfonso, D. Donghi, D. Maggioni, P. Mercandelli, and A. Sironi,
Organometallics 26 (2007) 2088.
5. D. Donghi, D. Maggioni, T. Beringhelli, G. D’Alfonso, P. Mercandelli, and A. Sironi, Eur. J.
Inorg. Chem. (2008) 1645.
6. D. Donghi, D. Maggioni, T. Beringhelli, and G. D’Alfonso, Eur. J. Inorg. Chem. (2008) 3606.
7. G. J. P. Britovsek, J. Ugolotti, P. Hunt, and A. J. P. White, Chem. Commun. (2006) 1295.
8. See for example (a) K. Ishihara, and H. Yamamoto, Eur. J. Org. Chem. (1999) 527; (b)
K. Ishihara, H. Kurihara, and H. Yamamoto, J. Org. Chem. 62 (1997) 5664; (c) K. Ishihara,
H. Kurihara, and H. Yamamoto, Synlett (1997) 597.
9. Recent patents dealing with the synthesis or use of bis(pentafluorophenyl)borinic acid include
(a) T. Sell, J. Schottek, N. S. Paczkowski, and A. Winter, Patent WO 2006124231 (2006) to
Novolen Technology, (b) T. Neumann, S. Herrwerth, T. Reibold, and H.-G. Krohm, Patent DE
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

772
D. Maggioni et al.
102005004676 (2005) to Goldschmidt G.m.b.H., (c) R. Kratzer, Patent WO 04041871 (2004)
to Basell Polyolefins, (d) R. Kratzer, Patent WO 04007570 (2004) to Basell Polyolefins, (e)
R. Kratzer and V. Fraaije, Patent WO 04007569 (2004) to Basell Polyolefins, (f) K. Takei,
K. Mizuta, K. Aoki, and K. Takebe, Patent JP 2004265785 (2004) to Nippon Shokubai.
10. A. L. Van Geet, Anal. Chem. 42 (1970) 679.
11. A. D. Becke, J. Chem. Phys. 98 (1993) 5648.
12. J. Tomasi, B. Mennucci, and R. Cammi, Chem. Rev. 105 (2005) 2999.
13. J. R. Cheeseman, G. W. Trucks, T. A. Keith, and M. J. Frisch, J. Chem. Phys. 104 (1996) 5497.
14. (a) H. Fukaya and T. Ono, J. Comput. Chem. 25 (2004) 51, (b) R. Jain, T. Bally, and
P. R. Rablen, J. Org. Chem. 74 (2009) 4017.
15. Gaussian 09 (revision C.01), Gaussian Inc., Wallingford, CT (2010).
16. See for example N.S Golubev, G. S. Denisov, S. N. Smirnov, D. N. Shchepkin, and H.-H. Lim-
bach, Z. Phys. Chem. 196 (1996) 73 and references therein.
17. For the isolated anion 3 we previously found a Cs structure to be more stable than a cor-
responding C2 one [4]. However, in Hpy+·3 unfavorable steric interactions between the
pentafluorophenyl groups and the pyridinium cation led to a stabilization of the C2 symmetric
conformation, while the corresponding Cs geometry results not to be a minimum. Analogous
considerations apply to the neutral species 1t·py.
18. In the case of the reaction with DMAN, the formation of dearylated anion 4 was accompanied
by the formation of a corresponding amount of pentafluorobenzene [6]. In the present case the
detected amount of pentafluorobenzene seems to be smaller than expected, accounting only
for the 50–60% of the anion 4 formed, although the much smaller percentage of 4 formed
in the reaction with pyridine and the presence of many very broad 19F resonances hamper an
accurate evaluation of the integration ratios. Most likely the formation of Hpy+· 4 involved,
in part, the very little amount of ArB(OH)2 which always contaminates samples of Ar2BOH,
due to slow hydrolysis by traces of water, and which is hardly detectable by NMR for its small
solubility in CD2Cl2.
19. See for example (a) L. Sobczyk, B. Czarnik-Matusewicz, M. Rospenk, and M. Obrzud, Jour-
nal of Atomic, Molecular, and Optical Physics (2012), 217932, (b) V. Schreiber, A. Kulbida,
M. Rospenk, L. Sobczyk, A. Rabold, and G. Zundel, J. Chem. Soc. Faraday T. 92 (1996) 2555.
20. S. Grundemann, S. Ulrich, H.-H. Limbach, N. S. Golubev, G. S. Denisov, L. M. Epstein,
S. Sabo-Etienne, and B. Chaudret, Inorg. Chem. 38 (1999) 2550.
21. (a) T. Berighelli, D. Maggioni, and G. D’Alfonso Organometallics 20 (2001) 4927, (b)
C. Bergquist, B. M. Bridgewater, C. J. Harlan, J. R. Norton, R. A. Friesner, and G. Parkin,
J. Am. Chem. Soc. 122 (2000) 10581.
22. R. G. Kidd, Boron-11, in: NMR of Newly Accessible Nuclei, Vol. 2, Chapt. 3, P. Laszlo (Ed.),
Academic Press, New York (1983), pp. 49–77.
23. 11B chemical shifts for neutral tetrahedral boron derivatives bearing pentafluorophenyl sub-
stituents are normally found between −20 and +20 ppm [24,25], whereas tri-coordinated
boron derivatives show typically 11B chemical shift in the range 32.5–74.9 ppm [24,26].
24. See for example (a) D. J. Parks, W. E. Piers, and G. P. A. Yap, Organometallics 17 (1998)
5492, (b) W. Fraenk, T. M. Klapotke, B. Krumm, and P. Mayer, Chem. Commun. (2000) 667,
(c) G. Kehr, R. Fröhlich, B. Wibbeling, and G. Erker, Chem.-Eur. J. 6 (2000) 258.
25. See for example (a) Y. Sun, W. Piers, and J. R. Rettig, Organometallics 15 (1996) 4110,
(b) D. J. Parks, W. E. Piers, M. Parvez, R. Atencio, and M. J. Zaworotko, Organometallics
17 (1998) 1369, (c) H. Jacobsen, H. Berke, S. Döring, G. Kehr, G. Erker, R. Fröhlich, and
O. Meyer, Organometallics 18 (1999) 1724.
26. See for example (a) D. Vagedes, R. Fröhlich, and G. Erker, Angew. Chem. Int. Edit. 38 (1999)
3362, (b) L. H. Doerrer and M. L. H. Green, J. Chem. Soc. Dalton Trans. (1999) 4325, (c)
S. Dagorne, I. A. Guzei, M. P. Coles, and R. F. Jordan, J. Am. Chem. Soc. 122 (2000) 274.
27. Unpublished results.
28. The positions of the 19F and 1H signals of 2py at intermediate titration stages were downfield
shifted with respected to those above reported, due to fast exchange with some 1m still present
in solution before the attainment of the stoichiometric ratio (see for instance the analogous
progressive upfield shift of the resonances of 1m upon stepwise addition of water [3]).
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM

Interaction between (C6F5)2BOH and Pyridine
773
29. A direct comparison between experimental and computed 19F chemical shift values can be
misleading. Indeed, the excellent agreement between the chemical shift experimentally ob-
served for the ortho and para fluorine atoms of the 2py adduct and those computed for
1m· · · py is fortuitous, since the computed 19F chemical shift are systematically high field
shifted by 2–6 ppm (see the entries for 1m and 1t in Table S1 in Supporting Information).
30. (a) K. J. Weese, R. A. Bartlett, B. D. Murray, M. M. Olmstead, and P. P. Power, Inorg. Chem.
26 (1987) 2409, (b) C. D. Entwistle, A. S. Batsanov, and T. B. Marder, Acta Crystallogr. E63
(2007) o2639.
31. The disappearance, at the end of the titration, of signals attributable to species arising from
the monoarylated boron fragment of 4 (which cannot transform into 2py) can be explained by
formation of insoluble boronic acid ArB(OH)2, by the adventitious water present in solution.
32. A. D. Horton and J. de With, Organometallics 16 (1997) 5424.
33. P.-C. Maria and J.-F. Gal, J. Phys. Chem. 89 (1985) 1296.
34. A. Rauk, I. R. Hunt, and B. A. Keay, J. Org. Chem. 59 (1994) 6808.
35. C. Laurence, K. A. Brameld, J. Graton, J.-Y. Le Questel, and E. Renault, J. Med. Chem. 52
(2009) 4073.
36. M. H. Abraham, P. L. Grellier, D. V. Prior, J. J. Morris, and P. J. Taylor, J. Chem. Soc. Perk. T.
2 (1990) 521.
Brought to you by | Cornell University Library
Authenticated
Download Date | 7/12/17 11:54 AM