Macrocycles, adducts and monomeric boron compounds derived from 2,6-dimethanolpyridine and arylboronic acids

Article abstract of DOI:10.1016/j.ica.2005.03.014

The reaction of 2,6-dimethanolpyridine with arylboronic acids at room temperature led to the formation of tetrameric compounds 1a-1e in good yields. Since the tetrameric derivatives were insoluble in common organic solvents, their characterization was based on IR, mass spectrometry, as well as ¹³C and ¹¹B NMR, in the solid state. Macrocyclic compounds 1a-1e can be hydrolyzed upon heating in DMSO to give adducts 2a-2e, which are only held by a coordination bond between the nitrogen and boron atoms, as demonstrated by ¹H, ¹³C and ¹¹B NMR, in solution. Moreover, the presence of an additional carbon atom in the aliphatic chain of the ligand, as in the case of 2,6(β-diethanolamine)pyridine, leads exclusively to the formation of the monomeric specie 4, as established by X-ray diffraction analysis.

Full text of DOI:10.1016/j.ica.2005.03.014

Inorganica Chimica Acta 358 (2005) 2996–3002
www.elsevier.com/locate/ica
Macrocycles, adducts and monomeric boron compounds
derived from 2,6-dimethanolpyridine and arylboronic acids
a
Gabriela Vargas , Norberto Farfan , Rosa Santillan , Atilano Gutierrez ,
b
b
c
´
Elizabeth Gomez , Victor Barba
´
d
a,
*
´
a
´ ´
Centro de Investigaciones Quımicas, Universidad Autonoma del Estado de Morelos, Av. Universidad 1001, C.P. 62210 Cuernavaca, Morelos, Mexico
b
´ ´
Departamento de Quımica, Centro de Investigacion y de Estudios Avanzados del IPN, Apdo. Postal. 14-740, Mexico D.F. 07000, Mexico
´
c
´ ´ ´
Laboratorio de Resonancia Magnetica Nuclear, Universidad Autonoma Metropolitana, Unidad Ixtapalapa, Av. Michoacan y la Purısima s/n,
Col. Vicentina, 09340 Ixtapalapa, Mexico
´
Instituto de Quımica-UNAM, Ciudad Universitaria, 04510 Mexico D.F., Mexico
d
Received 2 September 2004; accepted 6 March 2005
Available online 28 April 2005
Abstract
The reaction of 2,6-dimethanolpyridine with arylboronic acids at room temperature led to the formation of tetrameric com-
pounds 1a–1e in good yields. Since the tetrameric derivatives were insoluble in common organic solvents, their characterization
was based on IR, mass spectrometry, as well as 13C and 11B NMR, in the solid state. Macrocyclic compounds 1a–1e can be hydro-
lyzed upon heating in DMSO to give adducts 2a–2e, which are only held by a coordination bond between the nitrogen and boron
atoms, as demonstrated by 1H, 13C and 11B NMR, in solution. Moreover, the presence of an additional carbon atom in the aliphatic
chain of the ligand, as in the case of 2,6(b-diethanolamine)pyridine, leads exclusively to the formation of the monomeric specie 4, as
established by X-ray diffraction analysis.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Boron; Macrocycles; Tridentate ligands; 11B NMR solid state
1. Introduction
The formation of macrocyclic structures using organic
synthesis generally involves several steps, as a conse-
quence, the products are obtained in low yields; this con-
stitutes a major disadvantage over other preparative
methods. Alternatively, coordinative connections are
preferred for the formation of macrocyclic structures
and these processes usually occur through self-assembly.
These mechanisms are frequently kinetically favored;
the main disadvantage is, however, that coordinative
connections are normally thermodynamically weaker
than covalent ones.
In this context, it is well known that boron atoms can
form strong coordinative bonds with nitrogen atoms [3].
With this simple approach, it has been possible to grow
several fragments to give macrocyclic structures via
self-assembly processes using boron derivatives. The
The chemistry of macrocycles has been extensively
studied in the last years, due to possible applications
of these systems in different fields such as chemistry,
biology, physics and materials science [1]. Several syn-
thetic strategies have been applied to the formation of
macrocycles, in addition to the ones used in traditional
organic synthesis [1c]. Thus, a number of organic and
inorganic macrocycles have been prepared using tem-
plate effects, or self-assembly processes via coordinative
connections by reaction of bi-, tri- or oligo-dentate li-
gands with the appropriate metal [2].
*
Corresponding author. Tel.:/fax: +52 777 329 79 97.
E-mail address: vbarba@ciq.uaem.mx (V. Barba).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.03.014

G. Vargas et al. / Inorganica Chimica Acta 358 (2005) 2996–3002
2997
synthesis of boron macrocycles that contain coordina-
tive bonds between the nitrogen and boron atoms has
been achieved using the above method, and dimeric, tri-
meric, tetrameric and pentameric species have been re-
ported [3].
out solid state NMR analyses in order to complete the
characterization.
It is important to remark that a wide variety of solu-
ble boron compounds including boranes, carboranes,
boron nitrides, metalloboranes and metallocarboranes
have been extensively analyzed using 11B NMR spec-
troscopy in the liquid state [6]. In contrast, 11B NMR
studies in the solid state are scarce and, to our knowl-
edge, the only molecules analyzed so far are carboranes,
borides, chalcogenides and carbides [7].
On the other hand, 2,6-dimethanolpyridine (dmpy)
acts as tridentate ligand with transition metals to form
monomeric or dimeric complexes [4]. Nevertheless, in
the case of the reaction of dmpy with arylboronic acids,
two tetrameric species containing a 20-membered ring
cavity have been obtained in good yields (>90%)
[3b,3f] (Scheme 1(d)). These tetrameric compounds are
stable to moisture, however, since they show poor solu-
bility in common organic solvents, extensive spectral
analysis was difficult. Fortunately, an X-ray diffraction
analysis was possible and both structures were analyzed
[3b,3f]. It is noteworthy that only a few tetrameric com-
pounds have been described in the literature where the
presence of N–B coordinative bonds favor self-assembly
of the fragments (Scheme 1) [5].
In the present work, we describe the reaction of 2,6-
dimethanolpyridine with five arylboronic acids. Differ-
ent substituents have been introduced in the aromatic
ring of the boronic acid fragment in order to study elec-
tronic effects along the N–B coordinative bond, and
determine possible changes in the reaction pathway
and solubility, with the aim to use them in host–guest
chemistry. However, due to the fact that these deriva-
tives were insoluble solids, it was necessary to carry
2. Experimental
2.1. Materials and instrumentation
All reagents and solvents were commercial products
from Aldrich, Fluka and Merck and were used without
further purification. The 13C (CP-MAS) and 11B (MAS)
NMR spectra were determined on a Bruker ASX300
1
spectrometer. The H, 13C and 11B NMR, as well as
2D spectra were determined on a Bruker Avance 300
spectrometer, using DMSO-d6 as solvent and (CH3)4Si
and BF3OEt2 as internal references. Chemical shifts
are reported in parts per million (ppm). Infrared spectra
(IR) were obtained on a Perkin–Elmer 16F-PC coupled
to FT-IR spectrometer. FAB mass spectra were deter-
mined with a Jeol JMS-700 spectrometer. Mass spec-
trum (EI) for 4 was determined on a Hewlett–Packard
5989A apparatus coupled to a gas-chromatograph series
5890 II. Melting points were determined with a Gallenk-
amp digital MFB-595 apparatus and have not been cor-
rected. Elemental microanalysis for compound 4 was
performed by Oneida Research Services, Whitesboro,
NY 13492, and for Midwest Microlab, LLC. Indianap-
olis, IN 46250 [8].
R
(a)
(b)
R'
R'
N
R'
B
R'
R'
N
N
R'
R'
B
B
N
N
R'
R'
N
R
R
R
R'
R'
B
R
R
B
The single-crystal X-ray diffraction study of 4 was
determined on an Enraf-Nonius CAD4 diffractometer
R
N
N
N
B
R'
B
˚
N
N
(Mo Ka radiation; k = 0.71073 A) with a graphite
N
B
R'
R'
monochromator, x = 2h in the interval 2 < h < 25. Di-
rect methods SHELXS-86 [9] were used for structure solu-
tion and the SHELXL-97 program [10], for refinement and
final data. Crystallographic data for compound 4 have
been deposited at the Cambridge Crystallographic Data
Center No. 249146.
R'
R'
R
R = H, Br R' = H, Me
R = H, CH3OCH2CH2O- , R' = Et
(c)
(d)
O
N
O
O
O
B
R
N
B
N
O
B
R
2.2. Syntheses of complexes
N
B
O
O
O
B
N
O
B
O
O
2.2.1. General method for the syntheses of tetrameric
compounds (1a–1e)
R
B
R
O
O
N
O
N
B
N
Tetrameric compounds 1a–1e were synthesized from
2,6-dimethanolpyridine and the corresponding arylbo-
ronic acid (1:1 ratio) in 20 ml of THF, the mixture
was stirred for 30 min at room temperature yielding a
white precipitate, which was recovered by filtration
and washed twice with 5 ml of THF.
O
O
R = C6H5, m-NO2-C6H4-
Scheme 1. Examples of tetrameric boron compounds obtained by self-
assembly via the formation of N–B coordinative bonds.

2998
G. Vargas et al. / Inorganica Chimica Acta 358 (2005) 2996–3002
2.2.1.1. Tetrakis[[l-(2,6-pyridinemethanolate-kN1, kO2,
kO6)](4-formylphenyl)boron]. Compound 1a was pre-
pared from 0.20 g (1.44 mmol) of 2,6-dimethanolpyri-
dine and 0.21 g (1.44 mmol) of p-formylphenylboronic
acid, a white solid was obtained, yield 83% (0.30 g,
0.30 mmol), mp(decomp) = 350 °C. FAB-MS (15 eV,
m/z, %): 1011 [M+, 37]; IR (KBr) m: 3420, 2918, 2848,
1694, 1622, 1478, 1212, 1188, 1130, 1112, 1090, 810,
from mixture of the starting materials in CH2Cl2 at
room temperature without stirring. C39H32NBO2.
mp = 191–193 °C. IR (KBr): 3056, 3022, 1482, 1442,
1368, 1352, 1310, 1024, 700 cmꢀ1. EI-MS (15 eV, m/z,
%) 480 [(M+ ꢀ C6H5), 100], 436 (30), 271 (16), 183 (5),
1
105 (41), 77 (60). H NMR (300 MHz, DMSO-d6) d:
6.90–8.10 (28H, m, Harom), 4.23 and 3.23 (4H, AB,
J = 14.9 Hz, CH2). 13C NMR (75 MHz, DMSO-d6) d:
154.3, 151.0, 148.7, 134.9, 131.4, 130.8, 128.5, 128.3,
128.1, 127.7, 127.3, 126.8, 126.2, 125.1 (Carom), 76.1
(C–O), 42.2 (CH2). 11B NMR (96 MHz, DMSO-d6) d:
4.3 (h1/2 = 736 Hz) ppm. Anal. Calc. for C39H32NBO2:
C, 84.02; H, 5.75; N, 2.51. Found: C, 83.93; H, 5.78;
N, 2.26%.
790, 754 cmꢀ1
.
2.2.1.2. Tetrakis[[l-(2,6-pyridinemethanolate-kN1, kO2,
kO6)](3-methoxyphenyl)boron]. Compound 1b was
prepared from 0.20 g (1.44 mmol) of 2,6-dimethanol-
pyridine and 0.19 g (1.44 mmol) of m-methoxyphenylbo-
ronic acid, a white solid was obtained, yield 85% (0.29 g,
0.28 mmol), mp(decomp) = 316 °C. FAB-MS (10 eV, m/z,
%): 1019 [M+, 91]; IR (KBr) m: 3348, 2916, 2848, 1620,
1570, 1478, 1466, 1418, 1280, 1240, 1164, 1090, 1036,
3. Results and discussion
750, 732 cmꢀ1
.
Compounds 1a–1e were synthesized by condensation
of 2,6-dimethanolpyridine with 4-formylphenylboronic,
3-methoxyphenylboronic, 3-methylphenylboronic, 4-
bromophenylboronic and 3-thiopheneboronic acids in
THF at room temperature. After 30 min under stirring,
a white precipitate was obtained in yields between 81%
and 85% (Scheme 2).
2.2.1.3. Tetrakis[[l-(2,6-pyridinemethanolate-kN1, kO2,
kO6)](3-tolyl)boron]. Compound 1c was prepared
from 0.20 g (1.44 mmol) of 2,6-dimethanolpyridine
and 0.19 g (1.44 mmol) of m-tolylboronic acid, a white
solid was obtained, yield 81% (0.28 g, 0.29 mmol),
mp(decomp) = 272 °C. FAB-MS (10 eV, m/z, %): 955
[M+, 66]; IR (KBr) m: 2916, 2848, 1620, 1570, 1478,
1466, 1418, 1280, 1240, 1164, 1090, 1036, 784, 750, 732
The melting points for all five compounds were over
300 °C with decomposition and all of them were insolu-
ble in common organic solvents. Nonetheless, com-
pounds 1a–1e were characterized using IR, FAB mass
spectrometry, 13C and 11B NMR1 in the solid state.
The IR spectra for complexes 1a–1e showed the disap-
pearance of the bands corresponding to the OH groups,
which are present in the starting material (dmpy). FAB
mass spectrometry analysis showed, in all five cases, a
molecular ion that corresponds to the tetrameric com-
pounds. No peaks were observed at higher m/z, exclud-
ing in this way the formation of polymeric products.
Additional evidence for the formation of tetrameric
compounds 1a–1e is also based on the 13C NMR CP-
MAS experiments, which showed a single set of signals
indicating the high symmetry of these molecules, in
agreement with the X-ray diffraction data reported for
related compounds (S4) [3b,3f]. NMR spectral assign-
ment for these compounds was carried out by compari-
son with analogous boron complexes described
previously [3f,3g] and by using substituent chemical shift
effects [11]. The 13C NMR spectra showed the presence of
two different signals for the methylene groups (Table 1),
owing to the loss of symmetry of the dmpy moiety when
the macrocycle containing a five-membered, as well as a
20-membered ring is formed. The signal in the range
cmꢀ1
.
2.2.1.4. Tetrakis[[l-(2,6-pyridinedimethanolate-kN1, kO2,
kO6)](4-bromophenyl)boron]. Compound 1d was pre-
pared from 0.20 g (1.44 mmol) of 2,6-dimethanolpyri-
dine and 0.22 g (1.44 mmol) of p-bromophenylboronic
acid, a white solid was obtained, yield 85% (0.33 g,
0.30 mmol), mp(decomp) = 313 °C. FAB-MS (10 eV,
m/z, %): 1219 [M++2, 38], 1211 (36) [M+]; IR (KBr) m:
3388, 2918, 2848, 1578, 1476, 1376, 1192, 1160, 1136,
1114, 1090, 1012, 930, 804, 790 cmꢀ1
.
2.2.1.5. Tetrakis[[l-(2,6-pyridinedimethanolate-kN1, kO2,
kO6)](3-thiophenyl)boron]. Compound 1e was prepared
from 0.20 g (1.44 mmol) of 2,6-dimethanolpyridine and
0.22 g (0.36 mmol) of 3-thiopheneboronic acid, a white
solid was obtained, yield 81% (0.27 g, 0.29 mmol),
mp(decomp) = 210 °C. FAB-MS (10 eV, m/z, %): 923
[M+, 58]; IR (KBr) m: 3094, 2846, 1700, 1622, 1478,
1442, 1210, 1162, 1152, 1128, 1102, 1090, 1026, 702 cmꢀ1
.
2.2.2. [2,6-Bis-(1,1-diphenyl-(1-ethoxy))]phenylboronate
Compound 4 was prepared from 0.50 g (1.06 mmol)
of ligand 3 and 0.14 g (1.06 mmol) of phenylboronic
acid in 50 ml of THF. The reaction mixture was stirred
under reflux for 4 h and allowed to stand overnight; a
white solid was obtained in 80% yield (0.47 g, 0.84
mmol). Suitable crystals for X-ray analysis were grown
1
Electronic supplementary information (ESI) available: 13C and 11
B
NMR spectra for compound 1c, also a view of the O–Hꢁ ꢁ ꢁO
interactions between complex 4 with one molecule of phenylboronic
acid in the solid state.

G. Vargas et al. / Inorganica Chimica Acta 358 (2005) 2996–3002
2999
4
In our efforts to dissolve the tetrameric compounds
1a–1e, the samples were mixed with DMSO-d6, placed
in the NMR tubes and heated. After 1 h, the compounds
were completely dissolved and the 1H NMR spectra
were determined showing that the signals were very sim-
ilar to those of the starting materials (2,6-dimethanol
pyridine and arylboronic acid). For example, the signals
corresponding to the hydroxyl and methylene groups of
the starting material were observed as triplet and dou-
blet, respectively (see Table 2). In the 13C NMR spectra,
signals for only half of the pyridine moiety were ob-
served, indicating that the 2, 20, 3, 30 and 5, 50 carbons
were equivalents by a C2 symmetry axis (Table 3). The
results above suggest that both oxygen–boron bonds
were cleaved by hydrolysis, because of the presence of
water in the solvent. Nevertheless, the 11B NMR spectra
showed signals with chemical shifts between 8.0 and 8.4
ppm, indicating that the boron atoms are in a tetrahe-
dral environment and the N ! B coordinative bond re-
mains after hydrolysis of compounds 1a–1e. Based on
this evidence, we propose that the formation of adducts
2a–2e must occur after heating the tetrameric com-
pounds in hydrated DMSO (see Scheme 3). In anhy-
drous DMSO, the hydrolysis does not take place, nor
are the compounds soluble. A possible coordination be-
tween DMSO and the boronic acids was excluded based
on the fact that 11B NMR analyses of the boronic acids
in DMSO-d6 gave signals with completely different
chemical shifts, in the range between 25 and 30 ppm.
These results provide additional evidence that N–B
coordination remains even after hydrolysis, due to the
high nucleophilicity of the nitrogen in the pyridine
moiety.
5
3
10
6
O
B
2
7
N1
O
11
N
8O
B
R
9
R
THF r.t.
- 8 H2O
O
N
R-B(OH)2
+
O
B
OH
OH
R
R
O
N
B
O
N
O
1a-1e
12
13
16
a: Y = p-COH-
b: Y = m-CH3O-
c: Y = m-CH3-
d: Y = p-Br-
12
17
16
1a-d: R =
1e: R =
13
15
14
S
14
15
Y
Scheme 2. Synthesis of tetrameric boron derivatives (numbering
scheme used for NMR assignment).
between d = 56.2 and d = 58.8 ppm was assigned to the
CH2 group which is part of the 20-membered heterocy-
clic ring (C-10). The signal from d = 68.2 to d = 70.9
ppm corresponds to the CH2 group, which is part of
the five-membered heterocyclic ring (C-7); The shift of
C-7 to higher frequency is attributed to the shielding ef-
fect of the pyridyl group. For comparison, the starting
material (dmpy) shows only a single signal for both meth-
ylene carbons C-7 and C-10. Also, for tetrameric com-
pounds, carbon atoms 2 and 6, as well as 3 and 5, show
different chemical shifts, indicating that the pyridine moi-
ety is not symmetrical. The signal for the carbon attached
to the boron atom (C-12) was observed between
d = 130.2 and d = 152.5 ppm, the chemical shift for this
carbon is not constant due to the different electronic
effects of the substituent attached to the aromatic ring
(Table 1).
The 11B NMR spectra in the solid state showed sig-
nals between d = 8.0 and d = 9.0 ppm (Table 1), which
are in agreement with the values reported in the litera-
ture [6,7] and are similar to the 11B NMR shifts of sam-
ples determined in solution [3], evidencing that the
boron atoms are tetracoordinate. It is important to re-
mark that in accordance with the literature, chemical
shifts for boron compounds are similar in the solid
and liquid states [6,7].
To evaluate the effect of the chain length, a com-
pound having an additional carbon atom between the
hydroxyl group and the aromatic ring of the pyridine
(3) was synthesized, according to the methodology de-
scribed in the literature [12]. The reaction of ligand 3
with phenylboronic acid was carried out in THF. After
3 h under reflux, the mixture was slowly cooled to room
temperature; a white solid was obtained and collected by
filtration. In this case, the analysis showed that the reac-
tion leads to the formation of a monomeric specie 4
Table 1
13C (75 MHz, CP-MAS) and 11B (96 MHz, MAS) chemical shifts (ppm) for tetrameric compounds 1a–1e
Compound
C2
C-3
C-4
C-5
C-6
C-7
C-10
C-12
C-13
C-14
C-15
C-16
C-17
11B (h1/2, Hz)
1aa
1bb
1cc
1d
156.6
155.0
154.8
156.9
157.2
120.8
120.4
120.4
122.1
121.2
142.2
140.5
140.5
142.5
144.3
120.8
118.3
118.0
120.4
121.2
160.3
159.5
156.7
158.3
159.2
70.7
68.2
68.4
70.4
70.9
58.8
58.5
56.2
58.1
58.6
130.2
149.8
147.2
148.2
152.5
138.2
129.0
134.2
136.7
128.6
127.5
157.2
135.5
132.3
136.5
123.1
128.9
122.1
132.6
127.5
109.8
127.9
132.3
128.6
138.2
127.3
130.9
136.7
9.0 (321)
8.0 (344)
8.0 (358)
9.0 (361)
9.0 (363)
1ed
a
COH d: 197.1 ppm, signals for C-3 and C-5 are overlapped.
OMe d: 56.3 ppm.
Me d: 22.4 ppm.
Signals for C-3 and C-5 are overlapped.
b
c
d

3000
G. Vargas et al. / Inorganica Chimica Acta 358 (2005) 2996–3002
Table 2
1H NMR (300 MHz, DMSO-d6) data for adducts 2a–2e
Compound
H-3
H-4
H-5
–OH
H-7
H-8
H-9
2aa
7.31 (d)
J = 7.3 Hz
7.31 (d)
7.77 (t)
J = 7.3 Hz
7.78 (t)
4.51 (d)
J = 5.9 Hz
4.52 (d)
5.38 (t)
J = 5.9 Hz
5.50 (t)
7.97 (d)
J = 8.1 Hz
7.32 (s)
7.86 (d)
J = 8.1 Hz
2bb
2cc
2d
2e
a
6.94 (d)
J = 7.7 Hz
7.31 (d)
J = 7.7 Hz
7.78 (t)
J = 5.8 Hz
4.52 (d)
J = 5.8 Hz
5.38 (t)
J = 6.8 Hz
7.20 (d)
J = 8.1 Hz
7.59 (s)
J = 7.7 Hz
7.31 (d)
J = 7.7 Hz
7.78 (t)
J = 5.9 Hz
4.52 (d)
J = 5.9 Hz
5.39 (t)
7.95 (d)
7.54 (d)
J = 7.8 Hz
7.31 (d)
J = 7.7 Hz
J = 7.8 Hz
7.78 (t)
J = 7.7 Hz
J = 5.7 Hz
4.51 (d)
J = 5.8 Hz
J = 5.7 Hz
5.44 (t)
J = 5.8 Hz
J = 8.3 Hz
7.95 (dd)
J = 7.9 Hz
J = 8.3 Hz
7.40 (dd)
J = 4.8, 0.8 Hz
7.47 (dd)
J = 4.8, 2.3 Hz
COH d: 10.02 ppm.
b
c
H-10 d: 7.24 ppm (t, J = 6.8 Hz), H-11 d: 7.34 ppm (d, J = 6.8 Hz), OMe d: 3.64 ppm.
H-10 d: 7.21 ppm (t, J = 8.1 Hz), H-11 d: 7.57 ppm (t, J = 8.1 Hz), Me d: 2.29 ppm.
Table 3
13C (75 MHz) and 11B (96 MHz) NMR data in DMSO-d6 for adducts 2a–2e
Compound
C-2
C-3
C-4
C-5
C-7
C-8
C-9
11B (h1/2, Hz)
2,6-dmpy
160.4
161.3
161.5
161.4
161.7
161.6
118.2
118.6
119.1
118.7
118.9
119.0
136.8
137.5
138.1
137.6
137.9
137.9
64.4
64.7
64.9
64.7
65.0
65.0
2aa
2bb
2cc
2d
2e
135.0
116.7
135.3
137.1
133.3
128.8
159.4
136.6
131.2
135.8
137.6
127.2
131.1
125.0
126.0
8.4 (237)
8.1 (321)
8.3 (167)
8.2 (276)
8.0 (485)
a
COH d: 193.9 ppm.
C-10 d: 129.5 ppm, C-11 d: 119.7 ppm, OMe: 55.6 ppm.
C-10 d: 127.8 ppm, C-11 d: 131.7 ppm, Me d: 21.7 ppm. dmpy = 2,6-dimethanol pyridine. (In no case was C-6 observed.)
b
c
4
PhB(OH)2
3
2
3'
2'
R1
R2
THF, ∆
N
N
1
N
- 2 H2O
5'
5
Ph
Ph
Ph
Ph
B
Ph
Ph
Ph
Ph
a: COH
H
O
O
OH HO
b:
c:
H
H
CH3O
CH3
H
OH
OH
Ph
4
B
6
HO
OH
7
3
d: Br
11
Scheme 4. Synthesis of a monomeric boron derivative.
9
8
8
e:
R2
10
6
9
R1
2
S
7
hydroxyl group introduces important steric effects
that restrict the formation of a macrocycle. A similar
system containing an aromatic ring between the hy-
droxyl group and the pyridine ring has been reported
in the literature [13]. In that case, the presence of the
[4.4.0] heterobicycle and an intramolecular N–B coor-
dinative bond has also been observed.
Scheme 3. Adducts obtained by hydrolysis of the tetrameric com-
pounds (numbering scheme used for NMR assignment).
(Scheme 4), rather than tetrameric species, as described
for 2,6-dimethanol pyridine.
1
The H NMR spectrum of compound 4 showed an
The EI mass spectrum showed a peak at m/z = 480
corresponding to the (M ꢀ C6H5)+ ion, which is char-
acteristic for this type of systems [3]. The selective
formation of the [4.4.0] heterobicycle shows that
six-membered ring heterocycles are favored over mac-
rocyclic species, which is a result of the relief of
annular tension in the six membered heterocycles.
Also, the presence of phenyl groups adjacent to the
AB system at d = 4.23 and d = 3.23 ppm, with a cou-
pling constant of J = 14.9 Hz corresponding to the
methylene group, that confirms the ring closure. 1H
NMR variable temperature experiments for compound
4 were carried out to evaluate the stability of the N–B
coordinative bond, showing that, at least up to 150
°C, the N–B coordinative bond remains (as evidenced
by the persistence of the AB system), consequently the

G. Vargas et al. / Inorganica Chimica Acta 358 (2005) 2996–3002
3001
Table 4
Selected bond lengths (A) and bond angles (°) for compound 4
a
˚
Molecule 1
Molecule 2
˚
Bond lengths (A)
B(1)–O(1)
B(1)–O(2)
B(1)–N(1)
B(1)–C(10)
O(1)–C(1)
O(2)–C(9)
C(1)–C(2)
C(2)–C(3)
C(3)–N(1)
N(1)–C(7)
C(7)–C(8)
C(8)–C(9)
1.479(6)
1.459(6)
1.602(6)
1.627(7)
1.435(6)
1.428(5)
1.531(7)
1.493(8)
1.345(7)
1.367(6)
1.487(8)
1.542(7)
1.476(6)
1.466(6)
1.614(6)
1.612(7)
1.429(5)
1.439(5)
1.539(6)
1.485(7)
1.349(6)
1.345(6)
1.486(7)
1.541(7)
Bond angles (°)
O(2)–B(1)–O(1)
O(2)–B(1)–N(1)
105.3(4)
108.2(4)
109.6(4)
114.1(4)
110.4(4)
109.0(4)
122.9(4)
108.5(4)
124.3(4)
108.9(4)
104.3(4)
108.0(4)
107.6(4)
114.1(4)
113.1(4)
109.3(4)
125.9(3)
108.1(4)
123.4(3)
108.2(4)
Fig. 1. Molecular structure for compound 4.
O(1)–B(1)–N(1)
O(2)–B(1)–C(10)
O(1)–B(1)–C(10)
N(1)–B(1)–C(10)
C(1)–O(1)–B(1)
C(2)–C(1)–O(1)
C(9)–O(2)–B(1)
C(8)–C(9)–O(2)
a
activation energy of the coordination bond must be
higher than 82.15 kJ/mol and for that reason compound
4 is highly stable. The 11B NMR for this compound
showed a signal at d = 4.3 ppm, indicative of a boron
atom in a tetracoordinated geometry.
Crystallographic analysis2 of compound 4 showed
that there are two independent molecules in the asym-
metric unit. The unit cell also contains two free mole-
There are two independent molecules in the asymmetric unit.
showing in this way that the tetrahedral geometry for
the boron atom is slightly distorted [3a].
cules of phenylboronic acid and
a molecule of
dichloromethane. The O–Hꢁ ꢁ ꢁO interactions between
the OH groups of the boronic acid and the oxygen of
˚
compound 4 (distance, 2.54 A) show that the additional
4. Conclusions
molecule of phenylboronic acid stabilizes the system
allowing its crystallization.
Tetrameric species 1a–1e were synthesized in good
yields (>80%) using a one-pot procedure. The com-
pounds are highly stable and it was observed that hydro-
lysis of the tetrameric compounds could be carried out
only by heating, leading to the formation of adducts
2a–2e. Derivatives 2a–2e are held together only by a
N–B coordinative bond, which is strong enough based
on the DH value (>82 kJ/mol) obtained from the vari-
able temperature NMR spectra for compound 4. The
fact that a monomeric [4.4.0] bicyclic structure is ob-
tained from 2,6-(di-b-hydroxy) pyridine evidences that
the structure of the ligand is also an important factor
in macrocyclic formation. On the other hand, it has been
shown that boron macrocycles which are insoluble in all
common solvents can be studied using solid-state 13C
and 11B NMR, this powerful technique constitutes an
excellent tool that provides composition and structural
information.
The molecular perspective (Fig. 1) showed a twist
boat conformation for the two six-membered ring het-
erocycles, wherein the C(2), B(1) and C(8) atoms are
in the upper corners of the boat. It can also be observed
that two of the phenyl groups attached to the carbons
are in pseudo-equatorial positions, while the other two
occupy pseudo-axial positions of the six-membered ring;
the phenyl group attached to the boron atom is also
pseudo-axial. Table 4 summarizes the bond lengths
and angles for compound 4. It can be noticed that there
are no significant differences between the angles and
bond lengths of 4 compared with similar boronates de-
scribed in the literature [3]. For example, the N–B bond
˚
length has a value of 1.608(6) A (average), characteristic
for species with a N–B coordinative bond [4a]; the an-
gles around the boron atom are between 104.3(4)° and
114.1(4)° with a Tetrahedral Character value of 85%,
2
˚
Crystal data for complex 4: Triclinic, a = 10.403(2) A,
Acknowledgments
˚
˚
b = 12.451(2) A, c = 16.425(3) A, a = 102.74(3)°, b = 95.43(3)°,
3
ꢀ
˚
c = 110.12(3)°, space group P1, V = 1914.4(6) A , T = 293 K, Z = 2,
l(Mo Ka) = 0.145 mmꢀ1, 7120 reflections measured, 4163 unique, R1
[I > 2rI] = 0.0423, wR2 (all data) = 0.1426.
We thank CONACyT for financial support and G.
Cuellar for Mass spectra. Thanks are given to Consejo

3002
G. Vargas et al. / Inorganica Chimica Acta 358 (2005) 2996–3002
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´