3-Aminophenylboronic acid as building block for the construction of calix- and cage-shaped boron complexes

Article abstract of DOI:10.1016/j.jorganchem.2009.02.025

The reactivity of 3-aminophenylboronic acid toward 3,5-disubstituted salicylaldehyde derivatives (R = tBu, MeO, I, Br, NO₂) was analyzed with the aim to synthesize macrocyclic boron compounds having a calix-like structure. The three-component c

Full text of DOI:10.1016/j.jorganchem.2009.02.025

Journal of Organometallic Chemistry 694 (2009) 2127–2133
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
3-Aminophenylboronic acid as building block for the construction
of calix- and cage-shaped boron complexes
a
a
b
c
Victor Barba a, , Refugio Hernández , Herbert Höpfl , Rosa Santillan , Norberto Farfán
*
a Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, 62209 Cuernavaca, Mexico
b Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, México D.F. 07000, A.P. 14-740, Mexico
c Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactivity of 3-aminophenylboronic acid toward 3,5-disubstituted salicylaldehyde derivatives
(R = tBu, MeO, I, Br, NO2) was analyzed with the aim to synthesize macrocyclic boron compounds having
a calix-like structure. The three-component condensation was carried out using different aliphatic alco-
hols (ROH, R = Me, Et, nPr, nBu, nPn, nHex) in order to replace the free B–OH groups by B–OR moieties and
their effect on the structural and physicochemical properties of the resulting compounds was analyzed.
When the reaction was carried out under reflux conditions, trimeric calix-like compounds are formed for
the 3,5-tButyl salicylaldehyde derivatives. The absence of alcohol molecules during the reaction lead to
the condensation of two calixarenes through two of the three B(OH) groups to form two B-O-B moieties,
giving place to the formation of a hexanuclear cage. Using the salicylaldehyde derivatives having I, Br and
NO2 groups, the Schiff bases resulting from the condensation reaction of the aldehyde and the boronic
acid were isolated providing thus evidence for the previously proposed reaction sequence for the forma-
tion of the calix-shaped compounds.
Received 19 December 2008
Received in revised form 16 February 2009
Accepted 17 February 2009
Available online 5 March 2009
Keywords:
Self-assembly
Boron
Calixarenes
Macrocycles
Cages
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
On the other hand, the B(OH) groups present in boronic acids
can form hydrogen bonds and thus give rise to strong intermolec-
Over the past two decades the interest in the preparation of
macrocycles containing boron atoms has increased considerably
[1]. This can be attributed to the – importance of macrocyclic com-
pounds in supramolecular chemistry as molecular receptors [2]. A
large variety of synthetic techniques for the construction of boron
macrocycles compounds have been reported, involving frequently
a direct one-pot synthesis sequence [3]. The ease of boron macro-
cyclic synthesis is due to the versatility of boron atoms to react
with donor atoms (e.g. N and O), thus favoring self-assembly pro-
cesses; this method has been applied for the construction of mol-
ecules with large cavities that might have applications in several
fields of chemistry and materials science [4]. Usually, oligomeric
compounds containing boron atoms are built through donor–
acceptor intermolecular interactions. So far, dimeric, trimeric, tet-
rameric and pentameric species have been described [5]. Further-
more boronic acids have been used to construct molecular boxes
[6] and cages [7] by coupling with transition metals. Recently, a
synthetic strategy for the construction of boron-based dendrimers
has been described employing a multi-component assembly reac-
tion [8].
ular interactions [9]. It is well known that hydrogen-bonding inter-
actions permit the control of molecular recognition, and naturally,
self-assembly processes from molecular components to super-
structures in a well organized manner [10]. For that reason, the
boronic acids are considered important building blocks for molec-
ular crystal engineering [11].
Previously, we have used a the three-component reaction strat-
egy to create trimeric calixarenes and hemicarcerands via conden-
sation
reactions
of
3-aminophenylboronic
acid
with
salicylaldehyde derivatives using methanol as solvent [12]. UV–
Vis, 1H NMR and X-ray crystallographic studies have shown that
these compounds have cone conformations and are capable to in-
clude small organic guest molecules in their interior [13].
Continuing with the study of this type of macrocycles, we de-
scribe herein the reaction of 3,5-di-tbutyl salicylaldehyde with 3-
aminophenylboronic acid using different alcohols as boronate ester
forming reagents, in order to establish the influence that groups of
variable size attached to the boron atoms have on the macrocyclic
compounds formation (1b–1f). This study allowed us also to ana-
lyze their effect on the physicochemical and structural characteris-
tics such as solubility and conformation. Furthermore,
salicylaldehyde derivatives containing MeO, I, Br and NO2 substit-
uents were reacted with 3-aminophenylboronic acid in order to
analyze their effect on the calix-shape structure formation.
* Corresponding author. Tel./fax: +52 777 3297997.
E-mail address: vbarba@ciq.uaem.mx (V. Barba).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.02.025

2128
V. Barba et al. / Journal of Organometallic Chemistry 694 (2009) 2127–2133
(Fig. S1). The 13C NMR spectra showed that the signals correspond-
2. Results and discussion
ing to imine carbon atoms are shifted downfield, appearing around
d = 161 ppm, in accordance with the chemical shifts observed for
related compounds [12,13]. 11B NMR spectroscopy provided evi-
dence for the tetrahedral environment of the boron atoms. In all
cases, the chemical shifts are observed as broad signals in the re-
gion between d = 2.5 and 4.2 ppm, which are in the expected region
for tetracoordinate boronates [12,13].
Compounds 1b–1f were synthesized through three-component
condensation reactions as described previously for compound 1a
(Scheme 1) [12a]. 3,5-Di-tbutylsalicylaldehyde was dissolved in
acetonitrile whereupon 3-aminophenylbornic acid was added.
After 10 min 2 mL of the corresponding alcohol were added under
stirring in order to replace the OH groups attached to the boron
atoms. The reaction mixtures were stirred for 1 h under reflux to
For compound 1c suitable crystals for X-ray diffraction were ob-
tained during the recrystallization process. The crystals diffracted
very poorly, as a result of the relatively large quantity of atoms
and the fact that the heaviest atom in the molecule is oxygen. As
a consequence, the R values are somewhat elevated. Nonetheless,
the atomic positions of the non-hydrogen atoms and the overall
molecules conformation were clearly defined. The molecular struc-
ture is shown in Fig. 1, and the most relevant crystallographic data
are described in Table 1. The molecule has double-cone conforma-
tion which has been observed for analogous compounds [12a,13].
The –OPr groups attached to the boron atoms in 1c are oriented
to the outer sphere, leaving a hydrophobic interior environment.
It is interesting to notice that in the crystal lattice par of a tButyl
group of a neighboring molecules is included part inside the cavity
(Fig. S2), leading in this way to the formation of supramolecular
dimers. It is important to remark that for compound 1a the inclu-
sion of different solvent molecules has been observed, instead of
the self-assembly of two calix-shape molecules. Therefore, in the
case of 1c, as a consequence of the self-assembly process of two tri-
meric units, the exclusion of solvent molecules from its interior is
not surprising. The 1H NMR analysis recorded at different concen-
tration of 1c did not show significant changes, indicating that the
dimerization process is just a packing effect in the solid state.
On the other hand, when the condensation reaction was real-
ized in the absence of alcohol solvent molecules and under reflux,
two calix-shaped molecules were connected to a spherical hexa-
meric boron cage by condensation of four B(OH) groups. It is well
known that B–OH groups can condense to give B–O–B moieties
[14]. Thus, the reaction of 4-methoxysalicylaldehyde with 3-amin-
ophenylboronic acid using a solvent mixture of DMF/CH2Cl2 (1:1
ratio) lead to the formation of cage 2 (Scheme 2). After three days,
yellow crystals had formed, which were suitable for X-ray diffrac-
give yellow solids. Recrystallization in
a solvent mixture of
MeOH/CH2Cl2, (1:3 ratio) lead to the formation of yellow powders
which were recovered by filtration. In the case of compound 1c, the
yellow crystals formed during the recrystallization process were
suitable for X-ray diffraction analysis. Despite of the presence of
different boron ester functions (Me, Et, nPr, nBu, nPn, nHex), the
yields were almost constant (41–56%). Nonetheless, some interest-
ing changes are observed when comparing the different com-
pounds, for instance, the melting point decreases as the chain
size of the B–OR functions increases (298, 297, 247, 229, 213 and
190 °C for 1a–1f, respectively). This observation can be explained
taking into account that in the solid state large chains separate
the macrocyclic molecules more than small ones, diminishing the
intermolecular forces and thus the melting point. Additionally,
the solubility of compounds 1b–1f in common organic solvents
was higher when compared to compound 1a. This behavior is rea-
sonable considering that large aliphatic chains allow for a better
dispersion of the molecules in the solvent.
The IR data provide the first evidence for the formation of com-
pounds 1b–1f. The vibration of the C@N stretching band occurs at
1624–1625 cmÀ1, as observed for compound 1a and other related
compounds [12,13]. Additional evidence was obtained from mass
spectrometry using the FAB+ technique. In all cases, peaks for the
molecular and the [MÀOR]+ ion, which result from the loss of an
alcoxy group, were observed.
The NMR analysis illustrates some interesting observations. In
all cases, as a result of their C3 molecular symmetry, the NMR spec-
tra showed signals for only one third part of the structure, suggest-
ing that the molecules have double cone conformation. The number
of signals would be enhanced for the case that another conforma-
tion was present, according to the loss of the C3 symmetry. In the
1H NMR spectrum a single signal was observed at d = 8.0 ppm, cor-
responding to the imine hydrogen. The signals for the aromatic
moieties appeared in the region of d = 6.93–7.63 ppm without sig-
nificant differences between the different compounds. It is impor-
tant to remark that the hydrogen atoms of the –CH2OB– group
gave diasterotopic signals due the presence of chiral boron atoms
in all compounds. This behavior can be enhanced by existence of
hydrogen bonding interactions as observed in the solid state be-
tween this methylene group and the endocyclic oxygen atom
tion analysis. The FAB+ mass spectrum showed
a peak at
m/z = 1481 corresponding to the molecular ion of the proposed
hexanuclear boron structure 2, indicating the condensation of four
B–OH moieties giving two –B–O–B– bridges giving the double
calix-like structure.
The IR spectrum shows only one broad band at 1621 cmÀ1 cor-
responding to the stretching band of the C@N group. The 1H NMR
analysis was done using DMSO-d6 as solvent and it was necessary
to heat the NMR tube containing the solution up to 45 °C in order
to increase the solubility of 2. The 1H NMR spectrum showed only a
single set of signals indicating that the compound has symmetry
11
12
13
10 O
B
N
B
B(OH)2
6
7
Acetonitile/
ROH
RO
9
8
N
B
O
1
5
3
OR
3
+
4
Δ, 1 h
RO
N
OH
O
NH2
2
3
O
R
1a :
1b : Et
Me
1c :
nPr
1d : nBu
1e : nPn
1f :
nHex
Scheme 1. Three-component condensation reaction for the formation of calix-
shaped boron compounds 1a–1f.
Fig. 1. Lateral view of the molecular structure of compound 1c. Hydrogen atoms are
omitted for clarity.

V. Barba et al. / Journal of Organometallic Chemistry 694 (2009) 2127–2133
2129
Table 1
Crystallographic data for compounds 1c, 2 and 3a.
Identification code
1c
2
3a
Empirical formula
Formula weight
Crystal size (mm3)
Crystal system
Space group
Unit cell dimensions
a (Å)
C149H203B6N7O15
2397.04
C49H47B3N5.5O10.5
913.35
C17H14B1I2N3O3
572.92
0.40 Â 0.28 Â 0.20
Monoclinic
P21/n
0.36 Â 0.32 Â 0.28
0.40 Â 0.26 Â 0.20
Triclinic
Triclinic
ꢀ
ꢀ
P1
P1
15.063(4)
19.358(5)
27.532(7)
86.444(5)
80.791(4)
68.649(4)
7380(3)
2
13.262(3)
13.882(3)
14.999(3)
115.145(3)
95.986(3)
110.501(3)
2234.6(7)
2
4.2801(6)
21.944(3)
20.376(3)
90
94.934(2)
90
1906.7(4)
4
1.996
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
qcalcd (g/cm3)
1.079
0.068
1.357
0.095
l
(mmÀ1
)
3.320
Collected reflections
Independent reflections [R(int)
Parameters
29969
19472 (0.0725)
1639
19125
6614 (0.0636)
624
9026
3275(0.0542)
291
]
R1[I > 2r(I)]
0.1209
0.0906
0.048
wR2 (all data)
GOF
0.3839
1.068
0.2424
1.078
0.1012
1.076
OMe
11
12
13
O
B
B(OH)2
10
DMF/
CH2Cl2
N
B
O
6
7
6
HO
9
8
6
+
5
4
N
B
MeO
OH
1
Δ, 1 h
NH2
O
MeO
O
2
3
O
N
2
OMe
2
Scheme 2. Formation of a cage-shape compound.
(similar to that observed for compounds 1a–1f). This might indi-
cate that the hexameric compound was hydrolyzed to give a tri-
meric compound containing OH groups attached to the boron
atoms. The 1H NMR spectrum showed
a single signal at
d = 8.8 ppm corresponding to the proton of the iminic group. In
addition, a broad signal shifted to low fields (d = 10 ppm) was as-
signed to the B–OH groups. In spite of the presence of two boron
atoms (in the hexameric compound) with different chemical envi-
ronments in the molecule, the 11B NMR spectrum showed only one
broad signal at d = 2.7 ppm indicating a tetrahedral environment
for the boron atoms. The poor solubility of compound 2 in common
organic solvents precluded the 13C NMR analysis.
The X-ray crystallographic study allowed for a detailed analysis
of the cage-like boronate (Fig. 2). The crystal quality was low, but
the connection and overall geometry could be clearly established.
The molecule possesses a Ci symmetry in the solid state, having a
bis-double cone conformation. As a result of the intermolecular
condensation of four B–OH moieties, two BOB moieties act as
bridges connecting two calix-like units. Two B–OH groups remain
intact and are located at the outer sphere of the molecule. As de-
scribed above, there are three boron atoms in each calix-like moi-
ety (six in total), of which the boron atoms involved in the BOB
bridges have RR or SS configuration while the boron atoms of the
B–OH fragment have the opposite configuration (S or R). Whereas
the BOB links are located in the interior of the spherical cavity, the
B–OH groups are located at the periphery of the molecule. In con-
trast, in a double calix-shaped compound was reported previously
Fig. 2. Molecular structure of compound 2 (side view (left) and top view (right)).
by us, the six boron atoms have the same configuration giving a
hemicarcerand structure. In the case of the related compounds
1a–1f, all three boron atoms present in the molecule have the same
configuration. The bridged B–O–B bond angles (127.45°) and B–O
bond distances (1.410/1.412 Å) in compound 2 are very similar to
those found in related systems [14].
If the condensation reactions between 3-aminophenylboronic
acid and salicylaldehyde derivatives are carried out at room tem-
perature, it is possible to isolate the intermediate Schiff bases, giv-
ing evidence that the first step in the reaction mechanism is the
imine moiety formation as suggested before [3c]. Three Schiff
bases were isolated in the course of the present contribution. The
combination of the 3,5-disubstituted salicylaldehyde derivatives
(R = I 3a, R = Br 3b and R = NO2 3c) with 3-aminophenylboronic
acid in a solvent mixture of acetonitrile and methanol at room
temperature, lead to the formation of 3a–3c in form of yellow

2130
V. Barba et al. / Journal of Organometallic Chemistry 694 (2009) 2127–2133
12
13
11
R
6
7
acetonitrile/
metanol
O
R
8
10
+
N
B(OH)2
1
5
4
9
OH
r.t.
H2N
B(OH)2
2
OH
R
3
R
3a: R = I, 3b: R = Br, 3c: R = NO2
Scheme 3. Formation of the Schiff base compounds 3a–3c.
solids (Scheme 3). In the mass spectra for 3a–3c, peaks were ob-
served that correspond to the [M+H]+ ions. The IR spectra showed
bands for the C@N stretching at 1617, 1622 and 1622 cmÀ1, for 3a,
3b and 3c, respectively, evidencing the imine formation. Interest-
ingly, neither of the two boronate-ester reactions took place under
these conditions, thus allowing the isolation of the macrocycle pre-
cursor. It should be noticed that the precursor of the trimeric com-
pounds can be isolated only, if electron-withdrawing substituents
are present in the 3,5-positions of the salicylaldehyde. This is rea-
sonable considering that these groups have negative inductive ef-
fects and reduce the donor character of the nitrogen and oxygen
atoms within the salicylaldehyde moiety thus reducing its ability
to coordinate to the boron atom. This statement is supported by
the fact that compounds 3b and 3c could not be transformed to
the corresponding macrocyclic derivatives when using the same
reaction conditions as for compounds 1a–1f, even if the reaction
times were increased up to 24 h. The macrocyclic compound de-
rived from compound 3a has been reported previously [13]. A
plausible explanation for its formation is that the iodine atoms
provide less negative inductive effects on the nitrogen atom. In
contrast, the corresponding 3,5-di-tbutylsalicylaldehyde Schiff
base derivative (analogous to 3a–c) could not be isolated under
these reaction conditions.
Fig. 3. Molecular structure of compound 3a. The picture shows hydrogen bonding
interactions between two molecules of 3a and two pyrazine molecules giving a
dimeric supramolecular arrangement. Lateral view of molecular structure of
compound 1c. Hydrogen atoms are omitted for clarity.
ionic specie. This is again in agreement with the observation that
the electron-donor character of the imine nitrogen atom is
reduced. Hydrogen bonding interactions are also found for the
B–OH groups, thus giving a dimeric assembly, in which two boro-
nic acids are linked through two BOÁ Á ÁO(H)–B hydrogen bonds,
forming an eight member ring (O(2)–HÁ Á ÁO(3), 2.736 Å, 163.32°).
The two OH groups of each boronic acid moiety are in a syn-anti
disposition (Fig. 3). In the crystal lattice, the molecular dimmers
are organized in columns, with a
the planes of 3.556 Å.
p–p distance between molecular
The 1H NMR spectra gave additional evidence for the Schiff base
formation. A single signal was observed at d = 8.94, 8.98 and
9.07 ppm for 3a–c, respectively. In agreement with the observation
made above, the chemical shift increases according to the electron-
withdrawing effect of the substituent. The signals are shifted to
3. Conclusions
For the formation of calix-shaped compounds using 3-amin-
ophenylboronic acid and salicylaldehyde derivatives, different
reaction conditions have been examined in order to investigate
the factors that influence the macrocyclization process. It was
found that the reaction of 3,5-di-tbutyl-salicylaldehyde with 3-
aminophenyl boronic acid using different alcohols leads to the for-
mation of calix-shape structures. Therefore the presence of large
alkyl groups attached to the boron atoms does not have a signifi-
cant effect over the macrocyclization process aside from increasing
the solubility of the system. Additionally, calix compound 1c gave
rise to a self complementary supramolecular structure in the solid
state consisting in the formation of a dimeric assembly. If solvents
different from alcohols are used during the assembly process, the
B–OH groups condense to B–O–B moieties and, therefore, dimeric
hexanuclear spherical structures are obtained. Interestingly, for
the condensation reaction between two boron-calixarenes a partial
inversion of the boron configuration is required which evidences
the reversibility of the boronate condensation reactions that is
indispensable for the assembly process. At room temperature, the
self assembly process can be stopped after the first step (imine for-
mation), even in the presence of alcohol solvent molecules.
lower fields (
D
d ꢀ 0.9 ppm) when compared to those observed in
compounds 1a–1e. Nonetheless, the 13C NMR chemical shifts ob-
served for the C@N groups (d = 161.2, 162.0 and 161.5 for 3a–c,
respectively) are very similar to those of compounds 1a–1e and
no significant changes are observed. The tri-coordinate character
of the boron atoms was evidenced by 11B NMR spectroscopy,
wherein the signals are observed at d = 27, 28 and 30 ppm for
3a–c, respectively.
Suitable crystals for X-ray diffraction analysis were grown for
compound 3a from a mixture of methanol:pyrazine (1:1 ratio),
showing that in the solid state compound 3a cocrystallized with
a pyrazine molecule. As shown in Fig. 3, the molecular structure
of compound 3a is almost planar. The B–O bond distances are
1.340(8) and 1.373(11) Å, respectively, and are shorter in compar-
ison to those observed for the calix-like complexes, which are in
the range of 1.412(10) to 1.500(7) Å [12]. The difference can be
attributed to the higher B–O retrodonation character in the free
boronic acid group. Also, since there is no N–B coordination bond,
the C@N bond distance is shorter (1.273(9) Å) when compared to
the corresponding bond in the calix-like compounds
(ꢀ1.305(9) Å). The OH group of the phenolic part shows an intra-
molecular hydrogen bonding interaction with the imine moiety
(O–(H)Á Á ÁN, 2.555(6) Å, 154.5°): It is important to remark that the
hydrogen atom remains attached to the oxygen atom (O–H,
0.960(8) Å), since it has been reported that in aromatic imines
without the B(OH)2 group [15], the hydrogen atom is located at
the nitrogen atom (dist. N–H ꢀ 0.97 Å), giving place to a zwitter-
4. Experimental
4.1. Materials
All reagents and solvents were acquired from commercial sup-
pliers and used without further purification.

V. Barba et al. / Journal of Organometallic Chemistry 694 (2009) 2127–2133
2131
4.2. Instrumentation
(49%); m.p. = 247–250 °C. 1H NMR (400 MHz, CDCl3) d: 8.12 (3H,
s, H7), 7.63 (3H, d, J = 7.5, H11), 7.59 (3H, d, J = 2.3, H4), 7.35 (3H,
d, J = 7.55, H13), 6.98 (3H, t, J = 7.5, H12), 6.96 (3H, d, J = 2.3, H6),
6.51 (3H, s, H9), 3.35 (3H, dt, J = 4.8, 9.4, H14a), 3.22 (3H, dt,
J = 4.8, 9.4, H14b), 1.65 (6H, sex, J = 4.8, H15), 1.45(27H, s, Me3-
C5), 1.30(27H, s, Me3-C3), 0.94(9H, t, J = 4.8, H16) ppm. 13C NMR
(100 MHz CDCl3) d: 161.4 (C-7), 158.6 (C-2), 143.7 (C-8), 140.6(C-
3), 138.4 (C-5), 132.9 (C-4), 132.7 (C-11), 127.5 (C-9), 126.6 (C-
12), 125.8 (C-6), 122.8 (C-13), 115.8 (C-1), 63.5(C-14), 35.4 (Me3-
C-C5), 34.3 (Me3-C-C3), 31.4 (Me3-C5), 29.4 (Me3-C3), 25.6.8 (C-
1H, 13C and 11B NMR spectra were recorded at room tempera-
ture using a Varian Gemini 200 spectrophotometer. As standard
references were used TMS (internal, 1H, d = 0.00 ppm, 13C,
d = 0.0 ppm) and BF3 Á OEt2 (external, 11B, d = 0.0 ppm). 2D COSY
and HECTOR experiments have been carried out for the unambig-
uous assignment of the 1H and 13C NMR spectra. Infrared spectra
have been recorded on a Bruker Vector 22 FT-IR spectrophotome-
ter. Mass spectra were obtained with a Jeol JMS 700 equipment.
Melting points were determined with a Büchi B-540 digital
apparatus.
15), 11.3 (C-16) ppm. 11B NMR (64 MHz, CDCl3) d: 3.9 ppm (h1/2
=
608 Hz). IR (KBr) m
(cmÀ1) = 3445, 2959, 1624 (C@N), 1559, 1468,
1207, 1119. FAB-MS m/z (%): 1132 ([M+H]+, 45), 1072 ([MÀOPr]+,
100), 1031 (50), 656 (20), 756 (25), 404 (60), 378 (100), 362 (85),
320 (65).
4.3. X-ray crystal-structure determination
X-ray diffraction studies were performed on a Bruker-APEX dif-
fractometer with a CCD area detector, using Mo K
a-radiation,
4.4.3. Compound 1d
(k = 0.71073 Å) and a graphite monochromator. Frames were col-
lected at T = 100 K for compounds 1c and 2 and at 293 K for 3a,
Compound 1d was prepared from 0.20 g (1.29 mmol) of 3-
aminophenylboronic acid monohydrate and 0.30 g (1.29 mmol) of
3,5-di-tbutyl salicylaldehyde in acetonitrile using 2 mL of n-buta-
nol. The product was obtained as a yellow powder. Yield: 0.26 g
(52%); m.p. = 229–231 °C. 1H NMR (400 MHz, CDCl3) d: 8.07 (3H,
s, H7), 7.57 (3H, d, J = 7.8, H11), 7.55 (3H, d, J = 2.2, H4), 7.29–
7.34 (3H, m, H13), 6.94 (3H, t, J = 7.8, H12), 6.95 (3H, d, J = 2.2,
H6), 6.46 (3H, s, H9), 3.40–3.20 (3H, m, H14), 1.60–1.30 (12H, m,
H15, H16), 1.42 (27H, s, 1.42 (27H, s, Me3-C5), 1.27 (27H, s, Me3-
C3), 0.93 (9H, t, J = 7.3, H17) ppm. 13C NMR (100 MHz CDCl3) d:
160.7 (C-7), 158.1 (C-2), 143.3 (C-8), 140.1 (C-3), 137.9 (C-5),
132.5 (C-4), 132.3 (C-11), 127.1 (C-9), 126.1 (C-12), 125.4 (C-6),
122.4 (C-13), 115.4 (C-1), 62.8 (C-14), 61.6 (C-15), 35.3 (Me3-C-
C5), 34.2 (Me3-C-C3), 31.4 (Me3-C5), 29.4 (Me3-C3), 19.8 (C-16),
by
x//-rotation (D/x = 0.3°) at 10 s per frame. The measured
intensities were reduced to F2 [16]. Structure solution, refinement
and data output were carried out with the SHELXTL-NT program pack-
age [17]. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed in geometrically calculated positions
using a riding model. For compounds 1c and 2 somewhat elevated
R and wR2 values are observed owing to several factors: the disor-
der of solvent molecules, the relatively large quantity of atoms in
the unit cell and the fact that the heaviest atom is oxygen.
4.4. General method for the preparation of boron complexes 1b–1f
Compounds 1b–1f were synthesized from the equimolecular
reaction of 3,5-di-tbutylsalicylaldehyde with 3-aminophenylbo-
ronic acid monohydrate using 20 mL of acetonitrile as solvent
and 2 mL of the corresponding alcohol. The reaction mixtures were
stirred for 1 h under reflux. After that, part of the solvent and the
water formed through the triple condensation reaction were re-
moved using a Dean-Star k trap. The final products were recovered
by filtration and purified by recrystallization in a solvent mixture
MeOH/CH2Cl2 (1:3 ratio).
14.0 (C-17) ppm. 11B NMR (64 MHz, CDCl3) d: 3.3 ppm (h1/2
=
896 Hz). IR (KBr) m
(cmÀ1) = 2959, 1624 (C@N), 1561, 1468, 1207,
1181, 1112, 971, 900, 705. FAB-MS m/z (%):1174 ([M + H]+, 20),
1101 ([MÀOBu]+, 65), 1045 (40), 972 (20), 653 (25), 468 (100),
362 (95), 346 (95), 219 (80).
4.4.4. Compound 1e
Compound 1e was prepared from 0.20 g (1.29 mmol) of 3-amin-
ophenylboronic acid monohydrate and 0.30 g (1.29 mmol) of 3,5-
di-tbutyl salicylaldehyde in acetonitrile using 2 mL of n-pentanol.
The product was obtained as a yellow powder. Yield: 0.24 g
(46%); m.p. = 213–216 °C. 1H NMR (400 MHz, CDCl3) d: 8.01(3H,
s, H7), 7.61 (3H, d, J = 7.2, H11), 7.57 (3H, s, H4), 7.33 (3H, d,
J = 7.7, H13), 6.96 (3H, t, J = 7.4, H12), 6.93 (3H, s, H6), 6.47 (3H,
s, H9), 3.32 (3H, dt, J = 2.5, 9.4, H14a), 3.22 (3H, dt, J = 2.5, 9.4,
H14b), 1.7–1.22 (18H, m, H15, H16,H17), 1.42 (27H, s, Me3-C5),
1.26 (27H, s, Me3-C3), 0.82 (9H, t, J = 6.4, H18) ppm. 13C NMR
(100 MHz CDCl3) d: 161.1 (C-7) 158.5 (C-2), 143.5 (C-8),140.3 (C-
3), 138.2 (C-5) 132.7 (C-4), 132.5 (C-11), 127.3 (C-9), 126.4 (C-
12), 125.6 (C-6), 122.7 (C-13), 115.6 (C-1), 61.8 (C-14), 35.2
(Me3-C-C5), 34.1 (Me3-C-C3), 31.3 (Me3-C5), 29.3 (Me3-C3),
28.8(C-16), 22.7 (C-17), 14.3 (C-18) ppm. 11B NMR (64 MHz, CDCl3)
4.4.1. Compound 1b
Compound 1b was prepared from 0.20 g (1.29 mmol) of 3-
aminophenylboronic acid monohydrate and 0.30 g (1.29 mmol) of
3,5-di-tbutyl salicylaldehyde in acetonitrile using 2 mL of ethanol.
The product was obtained as a yellow powder. Yield: 0.26 g (55%);
m.p. = 297–300 °C. 1H NMR (400 MHz, CDCl3) d: 8.10 (3H, s, H7),
7.62 (3H, d, J = 2.0, H4), 7.34 (3H, d, J = 7.7, H11), 7.24 (3H, d,
J = 7.7, H13), 6.99 (3H, t, J = 7.7, H12), 6.96 (3H, d, J = 2.0, H6),
6.48 (3H, s, H9), 3.45 (3H, dt, J = 2.3, 9.4, H14a), 3.33 (3H, dt,
J = 2.3, 9.4, H14b), 1.42 (27H, s, Me3-C5), 1.27 (27H, s, Me3-C3),
1.26 (9H, t, J = 7.1 H15) ppm. 13C NMR (50 MHz CDCl3) d: 161.3
(C-7), 158.3 (C-2), 145.5 (C-8), 140.5 (C-3), 138.3(C-5), 132.7 (C-
4), 131.9 (C-11), 127.4 (C-9), 126.5(C-12), 125.6 (C-6), 122.7(C-
13), 115.6(C-1), 57.3(C-14), 35.0 (Me3-C-C5), 34.1 (Me3-C-C3),
31.3 (Me3-C5), 29.3 (Me3-C3), 17.9 (C-15) ppm. 11B NMR
(64 MHz, CDCl3) d: 2.7 ppm (h1/2 = 1560 Hz). IR (KBr)
d: 2.5 ppm (h1/2 = 1733 Hz). IR (KBr) m
(cmÀ1) = 3380, 2956, 1624
(C@N), 1558, 1468, 1362,1255, 1208, 1181, 1167, 972, 904, 773.
FAB-MS m/z (%):1216(M+H]+, 45), 1129 ([MÀOPn]+, 100), 1059
(50), 1042 ([MÀ2(OPn)]+, 40), 956 ([MÀ3(OPn)]+, 30), 812 (30),
724 (35), 561 (35), 361 (80), 346 (75).
m
(cmÀ1) = 3433, 2961, 1625 (C@N), 1560, 1469, 1207, 1117, 972,
906. FAB-MS m/z (%): 1089 (M+, 25), 1060 ([MÀEt]+, 15), 1044
([MÀOEt]+, 70), 1028(15), 1016(35), 1001 (15), 988(15).
4.4.5. Compound 1f
Compound 1f was prepared from 0.20 g (1.29 mmol) of 3-amin-
ophenylboronic acid monohydrate and 0.30 g (1.29 mmol) of 3,5-
di-tbutyl salicylaldehyde in acetonitrile using 2 mL of n-hexanol.
The product was obtained as a yellow powder. Yield: 0.22 g
(41%); m.p. = 190–193 °C. 1H NMR (400 MHz, CDCl3) d: 8.10 (3H,
s, H7), 7.56 (3H, d, J = 2.2, H4), 7.55 (3H, d, J = 7.2, H11), 7.35 (3H,
4.4.2. Compound 1c
Compound 1c was prepared from 0.20 g (1.29 mmol) of 3-amin-
ophenylboronic acid monohydrate and 0.30 g (1.29 mmol) of 3,5-
di-tbutyl salicylaldehyde in acetonitrile using 2 mL of n-propanol.
The product was obtained as a yellow powder. Yield: 0.24 g

2132
V. Barba et al. / Journal of Organometallic Chemistry 694 (2009) 2127–2133
d, J = 7.2, H13), 6.98 (3H, t, J = 7.2), 6.96 (3H, d, J = 2.2, H6), 6.48 (3H,
d, J = 2.0, H9), 3.33 (3H, dt, J = 2.8, 9.4, H14a), 3.23 (3H, dt, J = 2.8,
9.6, H14b), 1.62–1.54 (24H, m, H15, H16, H17, H18), 1.43 (27H, s,
Me3-C5), 1.26 (27H, s, Me3-C3), 0.84 (9H, t, J = 0.84, H19) ppm;
13C NMR (100 MHz CDCl3) d: 160.9 (C-7), 158.3 (C-2), 146.3 (C-
8), 140.3 (C-3), 138.2 (C-5), 133 (C-10), 132.7 (C-4), 132.5 (C-11),
127.3 (C-9), 126.4 (C-12), 125.6 (C-6), 122.7 (C-13), 115.6 (C-1),
62 (C-14), 35.5 (Me3-C-C5), 34.4 (Me3-C-C3), 32.6 (C-15), 32.2
(C17), 31.6 (Me3-C5), 29.6 (Me3-C3), 26.5 (C-16), 23.1 (C-18), 14.5
65% (0.51 g), m.p. = 294 °C. IR (KBr) m
(cmÀ1) = 3376, 3304, 3048,
2960, 1622 (C@N), 1606, 1512, 1470, 1282, 1268, 1226, 1216,
898, 764, 748, 742. EI-MS m/z = 397. 1H NMR (300 MHz, CDCl3), d
8.98 (1H, s, H-7), 7.89 (1H, d, J = 2.3 Hz, H-4), 7.86 (1H, d,
J = 2.3 Hz, H-6), 7.82 (1H, s, H-9), 7.75 (1H, d, J = 7.5 Hz, H-11),
7.52 (1H, d, J = 7.5 Hz, H-13), 7.46 (1H, t, J = 7.5 Hz, H-12). 13C
NMR (75 MHz, CDCl3) d: 162.0 (C-7), 158.3 (C-2), 145.0 (C-8),
137.9 (C-6), 134.7 (C-4), 134.1 (C-11), 129.3 (C-12), 127.5 (C-13),
123.3 (C-9), 121.2 (C-1), 112.2 (C-5), 109.6 (C-3). 11B NMR
(96 MHz, CDCl3) d 28.0 (h1/2 = 706 Hz) ppm.
(C-19) ppm. 11B NMR (64 MHz, CDCl3) d: 4.2 ppm (h1/2
=
1603 Hz). IR (KBr)
m
(cmÀ1) = 2957, 2864, 1624 (C@N), 1559,
1466, 1443, 1254, 1206, 1181. FAB-MS m/z (%): 1257 (M+, 20),
1156 ([MÀOhex]+, 100), 1055 ([MÀ2(Ohex)]+, 21), 954
([MÀ3(Ohex)]+, 15), 446 (35), 362 (100), 346 (90), 307 (65), 260
(50).
4.6.3. Compound 3c
Compound 3c was prepared from 0.21 g (1.35 mmol) of 3-amin-
ophenylboronic acid monohydrate and 0.29 g (1.35 mmol) of 3,5-
dinitrosalicylaldehyde. A yellow powder was obtained. Yield: 74%
(0.15 g), m.p. = 282 °C. IR (KBr) m
(cmÀ1) = 3558, 3384, 2916, 1622
4.5. Synthesis of compound 2
(C@N), 1560, 1426, 1368, 1332, 1316, 1236, 704; EI-MS m/z 331.
1H NMR (300 MHz, CDCl3), d 9.07 (1H, s, H-7), 8.80 (1H, d,
J = 2.0 Hz, H-6), 8.26 (1H, d, J = 2.0 Hz, H-4), 8.10 (1H, s, H-9),
7.90 (1H, d, J = 7.6 Hz, H-11), 7.64 (1H, d, J = 7.6 Hz, H-13), 7.61
(1H, t, J = 7.6 Hz, H-12). 13C NMR (75 MHz, CDCl3) d: 169.8 (C-2),
161.5 (C-7), 141.3 (C-5), 137.9 (C-8), 135.6 (C-3), 133.1 (C-11),
129.0 (C-4), 129.0 (C-6), 127.1 (C-12), 126.2 (C-9), 122.6 (C-13),
118.1 (C-1). 11B NMR (96 MHz, CDCl3) d: 30.6 (h1/2 = 1680 Hz) ppm.
Compound 2 was synthesized from 0.20 g (1.29 mmol) of 3-
aminophenylboronic acid monohydrate and 0.20 g (1.29 mmol) of
2-hydroxy-4-methoxybenzaldehyde using 20 ml of a solvent mix-
ture of DMF/CH2Cl2 (1:1 ratio). The reaction mixture was stirred for
1 h under reflux. After three days, yellow crystals had formed
which were suitable for X-ray diffraction. Yield: 0.14 g (42%);
m.p. = >350 °C. 1H NMR (400 MHz, DMSO-d6) d: 10.0 (2H, s, B-
OH), 8.88 (6H, s, H-7), 8.23 (6H, d, J = 8.0 Hz, H-11), 7.95 (6H, s,
H-9), 7.68 (6H, dt, J = 8.0, 2.8 Hz, H-13), 7.55 (6H, d, J = 8.4 Hz, H-
6), 7.41 (6H, t, J = 8.0 Hz, H-12), 6.55 (6H, dd, J = 8.4, 2.4 Hz, H-5),
6.49 (6H, d, J = 2.4 Hz, H-3), 3.8 (18H, s, Ar-OMe). 11B NMR
(64 MHz, DMSO-d6) d: 2.7 ppm (h1/2 = 3328 Hz). IR (KBr)
Acknowledgment
The authors acknowledge financial support from Consejo Nac-
ional de Ciencia y Tecnología (CONACyT).
m
(cmÀ1) = 3450 (s), 1621 (C@N) (s), 1543 (m), 1498 (w), 1441
(w), 1375 (w), 1308 (w), 1258 (m), 1215 (m), 1169 (w), 1119
(w), 1023 (w), 983 (w), 895 (w), 843 (w), 796 (w), 704 (w), 521
(w). FAB-MS m/z (%): 1481 ([MÀ1]+, 7), 1465 (18), 1255 (2.8),
1012 (5), 877 (3), 710 (3.2), 563 (4), 489 (12), 328 (28), 307
(100), 254 (12).
Appendix A. Supplementary material
CCDC 706335, 706336 and 706337 contain the supplementary
crystallographic data for 1c, 2 and 3a. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2009.02.025.
4.6. General method for the preparation of imines 3a–3c
Compounds 3a–3c were synthesized from the equimolecular
reaction of the corresponding 3,5-disubstituted salicylaldehyde
derivative (R = I, Br, NO2) with 3-aminophenylboronic acid mono-
hydrate using 20 mL of acetonitrile and 2 mL of methanol as sol-
vents. The reaction mixtures were stirred for 1 h at room
temperature. After 12 h, yellow powders had precipitated which
were filtered and dried under high vacuum.
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