Boron complexes derived from the condensation reaction of 3-aminophenylboronic acid and 1,3-diketones

Article abstract of DOI:10.1016/j.ica.2010.05.008

The structural analogy of 1,3-dicarbonyl compounds with 2-hydroxybenzencarbonyl compounds allowed to do an analysis towards the reactivity with 3-aminophenylboronic acid, in order to evaluate the synthesis of macrocyclic boron compounds having calixarene

Full text of DOI:10.1016/j.ica.2010.05.008

Inorganica Chimica Acta 363 (2010) 4112–4116
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Inorganica Chimica Acta
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Note
Boron complexes derived from the condensation reaction
of 3-aminophenylboronic acid and 1,3-diketones
a
b
c
Victor Barba a, , Refugio Hernández , Rosa Santillan , Norberto Farfán
*
a Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, C.P. 62209 Cuernavaca, Morelos, 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, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, México D.F. 04510, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The structural analogy of 1,3-dicarbonyl compounds with 2-hydroxybenzencarbonyl compounds allowed
to do an analysis towards the reactivity with 3-aminophenylboronic acid, in order to evaluate the synthe-
sis of macrocyclic boron compounds having calixarene like structures. The results indicate that the
chelate form is preferred over the reaction of the amino group with carbonyl groups. Thus the reaction
of 1,3-diketones (1,3-diphenyl-1,3-propanedione, 1-phenyl-1,3-butanedione and 2,4-pentanedione) with
3-aminophenylboronic acid using methanol or propanol as solvent medium, afforded the six-membered
boron chelates as the only product.
Received 23 October 2009
Received in revised form 26 February 2010
Accepted 5 May 2010
Available online 12 May 2010
Keywords:
Boron
Chelates
Ó 2010 Elsevier B.V. All rights reserved.
Crystal structures
Boronic acid
Diketones
1. Introduction
dinated to the nitrogen atom. The synthesis of these complexes in-
volves the formation of both, N–B coordination and B–O covalent
In the last decade, boronic acids have been employed for the
construction of macrocyclic boron compounds by means of self-
assembly reactions [1]. It is well known that, using electron do-
nor–acceptor interactions the synthesis of macrocycles containing
a large cavity has been possible [2]. In this context, it has been de-
scribed that molecules containing nitrogen atoms as Lewis bases
reacts with compounds having boron atoms as Lewis acids forming
strong N–B coordination bonds [1–3]. In fact, N–B coordination
bonds have been used to build a large number of macrocycles like
dimeric [4], trimeric [5], tetrameric [6] or pentameric [7] com-
pounds. Furthermore, the OH moiety of boronic acids is able to re-
act with alcohols to give boron esters through the formation of
covalent B–O bonds [8]. The high thermodynamic stability of the
boron esters promotes the formation of cyclo-oligomeric and poly-
meric boron compounds [9].
Recently, the formation of calixarenes [10] and hemicarcerands
[11] boron compounds by reaction of 3-aminophenylboronic acid
with salicylaldehyde derivatives was described (Scheme 1). In this
type of compounds, the first step is the reaction between carbonyl
and amine groups to give an imine moiety. In the second step, the
phenolic OH group reacts with the OH attached to the boron atom
to give a B–O covalent bond and finally, the nitrogen atom is coor-
bonds that enhance the macrocyclic stability. The simple formation
of these compounds using one-pot reactions increases the versatil-
ity for possible host–guest chemistry applications. In fact, calix-like
compounds have shown to be useful for the inclusion of small or-
ganic molecules such as benzene and THF [10a], as well as amines
and ammonium salts [10b]. In this work, 1,3-dicarbonyl com-
pounds were allowed to react with 3-aminophenylboronic acid in
order to analyze the reactivity towards the formation of boron
macrocycles.
2. Results and discussion
It is well known that 1,3-diketones are under keto-enol equilib-
rium, this characteristic has been used to analyze their reactivity
towards primary amines, wherein one of the carbonyl groups re-
acts to give an imine while the other one remains in the enol form
[12]. In this work, 1,3-diketone derivatives were used as synthetic
equivalent of 2-hydroxybenzencarbonylic compounds (Scheme 2)
in order to compare their reactivity towards 3-aminophenylboron-
ic acid. Thus, 2,4-pentanedione, 1,3-diphenyl-1,3-propanedione
and 1-phenyl-1,3-butanedione were allowed to react with 3-amin-
ophenylboronic acid. The reaction conditions used were similar to
those reported previously for the synthesis of trimeric compounds
[5,10]. The reactions were carried out under reflux of methanol for
compounds 1a–1c and propanol for compound 1d. In all four cases
* Corresponding author.
E-mail address: vbarba@ciq.uaem.mx (V. Barba).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.05.008

V. Barba et al. / Inorganica Chimica Acta 363 (2010) 4112–4116
4113
O
N
ROH
1/3
+
B
OH
H2N
B(OH)2
R
O
R
OR
3
Scheme 1. Calix-like compounds formed by reaction of salicylaldehyde with 3-aminophenylboronic acid.
R
The mass spectra showed the molecular ion corresponding to
the OCCCOB chelate for the complexes 1a–1d, in all four cases,
the corresponding ion indicating the loss of one RO-group was also
observed. These types of fragments, wherein the ligand remains
coordinated to the metal center through both oxygen atoms, are
in accordance with previous studies even with representative
[13] or transition metals [14].
R
R
O
O
OH
OH
The 1H NMR analysis showed that the signal corresponding to
the hydrogen located between the carbonyl groups (H-2) is shifted
to lower fields when phenyl groups are present (d = 7.35 and
7.34 ppm, for 1a and 1d, respectively). Nonetheless, in the pres-
ence of methyl groups, the signal is shifted to higher fields at
6.07 and 5.23 ppm in 1b and 1c, respectively. The electronic delo-
calization present in the six-membered rings is suggested by the
13C NMR spectra. For instance, the compounds derived from sym-
metric diketones, showed only one carbonyl base signal
(d = 185.8, 160.8 and 184.8 for 1a, 1c and 1d, respectively), the
more shifted signals correspond to compounds having phenyl moi-
eties (1a and 1d). The signal corresponding to the carbon atom
adjacent to both carbonyl groups (C-2) remains without significant
changes for 1a, 1b and 1d compounds, it appears around 93 ppm.
Nonetheless, the signal for the same carbon in 1c is shifted to lower
field (98.2 ppm). The 11B NMR analysis evidenced the tetrahedral
character of the boron atoms, in all four cases the chemical shift
was observed in the range from 3 to 4 ppm, in accordance with
previous reports [10–12].
Scheme 2. Structural comparison between the enol form of 1,3-diketones and
2-hydroxybenzencarbonyl compounds.
8
R'
9
B(OH)2
R'
7
1
O
O
O
O
ROH
4
6
2
B
NH2
+
5
OR
3
NH2
R"
R"
R
R’
R”
Ph
Me
Me
Ph
1a
1b
1c
1d
Me
Me
Me
Pr
Ph
Ph
Me
Ph
Scheme 3. Boron chelate compounds formation from 1,3-dicarbonyl compounds
and 3-aminophenylboronic acid.
Suitable crystals for X-ray diffraction analysis for 1a, 1b and 1d
were grown by slow diffusion of dichloromethane into a methanol
solution. The three structures are shown in Fig. 1 and the crystal-
lographic data are depicted in Table 1. Selected bond lengths and
bond angles are listed in Table 2. From the data, only small differ-
ences in the bond distances around the six-membered heterocycles
were observed, giving evidence of the delocalization present in this
fragment, as noticed previously for analogous compounds [13]. For
example, the C–O bond distances are the same in the three com-
pounds ca. 1.29 Å, while the B–O distances are quite different
colored solid products (yellow-red) were obtained in moderate
yields (Scheme 3). The spectroscopic analysis evidenced that the
formation of boron chelate complexes is preferred over the macro-
cyclic compounds. Herein, both oxygen atoms of the diketone moi-
ety are attached to the boron atom while the amino group remains
unreacted. This behavior has been observed before for 1,3-dicar-
bonyl compounds reacting with arylboronic acids wherein six-
membered rings are obtained [13].
Fig. 1. Molecular structure for compounds 1a, 1b and 1d.

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V. Barba et al. / Inorganica Chimica Acta 363 (2010) 4112–4116
Table 1
Crystallographic data for compounds 1a, 1b and 1d.
Identification code
1a
1b
1d
Empirical formula
Formula weight
Crystal size (mm3)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
22H20B1N1O3
C17H18B1N1O3
295.13
C24H24B1N1O3
385.25
357.20
0.42 Â 0.32 Â 0.18
0.26 Â 0.18 Â 0.14
0.48 Â 0.26 Â 0.22
Triclinic
Triclinic
Triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
8.447(2)
11.049(2)
11.301(2)
94.27(3)
110.43(3)
106.93(3)
927.1(3)
2
8.2247(12)
9.6337(14)
10.7003 (15)
115.999(2)
92.452(3)
96.986(2)
752.21 (19)
2
8.499(2)
11.175(3)
11.667(3)
78.623(4)
75.851(5)
76.390(5)
1032.8(5)
2
b (Å)
c (Å)
a
(Å)
b (°)
(Å)
c
Volume (Å3)
Z
qcalc (g cmÀ3
)
1.280
0.084
1.303
0.088
1.239
0.080
l
(mmÀ1
)
Collected reflections
Independent reflections (Rint
Parameters
3426
3241(0.0141)
245
3660
2447(0.0146)
271
6070
3583 (0.0299)
331
)
R1 [I > 2
wR2 (all data)
Goodness of fit (GOF)
r
(I)]
0.0493
0.1636
1.082
0.0465
0.1192
1.028
0.0784
0.1952
1.061
(
D
d = 0.017, 0.014 and 0.034 for 1a, 1b and 1d, respectively). A
comparison of the C1–C2 and C2–C3 bond distances reveals that
only compound 1a shows a clear difference ( d = 0.072 Å) indicat-
arrangements. The formation of dimeric molecules results from
intermolecular interactions from a bifurcate hydrogen bond (N1–
HÁ Á ÁO1 and N1–HÁ Á ÁO3). As depicted in Fig. 2, in all three cases
one N(1)H moiety interacts with two oxygen atoms of the neigh-
boring molecule with distances of 2.894 (O3) to 2.171 (O1) Å. Pre-
vious reports have shown the dimeric arrangement involving N–
HÁ Á ÁO motives in boron complexes [15]. Also in all three cases,
the oxygen atom of the alkyl group (O3) participate in two hydro-
gen bonds, the first one was described previously forming the di-
meric structure while the second one forms an interaction with
the other NH moiety of a third molecule giving rise to the forma-
tion of 1D polymeric chains (Fig. 3). The polymeric intermolecular
N1–HÁ Á ÁO3 hydrogen bond distances are 2.243, 2.048 and 2.472 Å
for 1a, 1b and 1c, respectively. It is important to remark that, the
molecular arrangement at the unit cell was not affected by the
presence of the different substituents present in the 1,3-diketones.
D
ing less delocalization in this fragment. The bond angles around
the boron atom remain close to the tetrahedral values. Neverthe-
less, in compound 1a there are two values that are slightly differ-
ent from sp3 hybridization, the O3–B1–C16 angle has the largest
value (120.1°), at the same time, the O2–B1–O3 angle decreases
(97.6°). Interestingly, the O–B–O angle inside the heterocycle is
similar (ca. 106.3°) in all three compounds; nonetheless it is the
shorter angle of the heterocycle. The other angles into the six-
membered heterocycle have values closer to sp2 hybridization
being in the range from 118.8° to 126.6°.
At the unit cell, hydrogen bonding interactions owing to the
existence of NH2 groups together with the presence of several oxy-
gen atoms in the molecules give rise to dimeric and polymeric
Table 2
Selected bond distances (Å) and angles (°) for compounds 1a, 1b and 1d.
1a
1ba
1db
O1–B1
O1–C1
B1–O2
B1–O3
B1–C16
C1–C2
C2–C3
C3–O2
1.490(3)
1.294(2)
1.473(3)
1.512(3)
1.648(3)
1.379(3)
1.307(3)
1.289(2)
1.5352(19)
1.2956(19)
1.521(2)
1.426(2)
1.593(3)
1.394(2)
1.383(2)
1.2971(19)
1.508(5)
1.293(4)
1.540(4)
1.417(5)
1.602(5)
1.385(5)
1.364(5)
1.292(4)
O1–B1–O2
O1–B1–O3
O1–B1–C16
O2–B1–O3
O2–B1–C16
O3–B1–C16
B1–O1–C1
O1–C1–C2
C1–C2–C3
C2–C3–O2
C3–O2–B1
106.4(2)
115.5(2)
101.0(2)
97.6(2)
116.3(2)
120.1(2)
120.1(2)
124.0(2)
118.8(2)
119.7(2)
126.6(2)
106.30(12)
108.85(13)
109.77(13)
104.61(13)
109.57(14)
117.13(14)
119.90(12)
120.91(15)
119.00(15)
121.65(15)
119.26(12)
106.9(3)
104.7(3)
109.3(3)
107.8(3)
109.3(3)
118.2(3)
123.5(3)
119.9(3)
121.5(3)
120.5(3)
122.7(3)
a
For compound 1b the C16 = C11.
For compound 1d O1 = O2 and O2 = O1.
Fig. 2. Dimeric molecular structure formed by N–HÁ Á ÁO interactions (compound 1a
b
is showed).

V. Barba et al. / Inorganica Chimica Acta 363 (2010) 4112–4116
4115
Fig. 3. Polymeric structure formed by N–HÁ Á ÁO interactions (compound 1d is showed). N–HÁ Á ÁO interaction distances (Å): 2.243 (1a), 2.048 (1b) and 2.472 (1d).
147.9 (C-6), 133.6 (C-o), 129.6 (C-p), 129.1 (C-i), 128.3 (C-8),
127.9 (C-m), 121.8 (9), 120.6 (C-5), 116.4 (C-7), 93.8 (C-2), 49.9
(Me). 11B NMR (64 MHz, CDCl3) d: 4.2 ppm (h1/2 = 1420 Hz). IR
3. Experimental
3.1. Materials
(KBr)
v
(cmÀ1) = 3346, 2918, 2850, 1618, 1598, 1576, 1560, 1540,
1508, 1488, 1456, 1448, 1362, 1302, 1268, 1230, 756, 716, 708,
682, 668. EI-MS m/z (%): 357 (M+, 12), 224 (75), 223 (90), 207
(5), 178 (5), 165 (5), 147 (44), 105 (100), 93 (7), 77 (86), 51 (30).
Elem. Anal. Calc. for C22H20B1N1O3: C, 73.97; H, 5.64; N, 3.92.
Found: C, 73.66; H, 5.56; N, 3.91%.
All reagents and solvents used were obtained from commercial
suppliers and used without further purification.
3.2. Instrumentation
Compound 1b was prepared from 0.30 g (1.93 mmol) of 3-
aminophenylboronic acid and 0.31 g (1.93 mmol) of 1-phenyl-
1,3-butanedione. The product was obtained as an orange powder.
Yield: 0.30 g (53%); mp = 210–213 °C. 1H NMR (400 MHz, CDCl3)
d: 8.2 (2H, S, NH2), 7.92 (2H, dd, J = 7.7, 2.2, H-o), 7.77 (1H, s,
H5), 7.63 (1H, t, J = 7.7, H-p), 7.45 (1H, t, J = 6.4, H8), 7.35 (1H, t,
J = 7.7, H-m), 6.95 (2H, d, J = 6.4, H7, H9), 6.07 (1H, s, H2), 2.5
(3H, s, OMe), 2.19 (3H, s, CH3) ppm. 13C NMR (100 MHz, CDCl3)
d: 186.5 (C-1), 162.2 (C-3), 147.2 (C-6), 135.0 (C-4), 131.0 (C-p),
129.1 (C-i), 128.1 (C-m), 127.5 (C-8), 126.5 (C-o), 121.6 (C-9),
119.7 (C-5), 115.5 (C-7), 93.3 (C-2), 39.0 (OMe), 20.1 (Me) ppm.
The 1H, 13C and 11B NMR spectra were recorded at room tem-
perature using a Varian VXR 400 spectrophotometer. As standard
references were used TMS (internal, 1H, d = 0.00 ppm, 13C,
d = 0.0 ppm) and BF3ÁOEt2 (external, 11B, d = 0.0 ppm). The 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 MStation Jeol JMS 700
equipment. Melting points were determined with a Büchi B-540
digital apparatus.
11B NMR (64 MHz, CDCl3) d: 3.1 ppm (h1/2 = 3848 Hz). IR (KBr)
v
(cmÀ1) = 3391, 2373, 1601, 1576, 1522, 1488, 1440, 1373, 1325,
1286, 1068, 1027, 752, 707, 555. EI-MS m/z (%): 295 (M+, 40),
280 ([MÀCH3]+, 20), 236 (100), 218 (40), 130 (15), 93 (100), 84
(40). Elem. Anal. Calc. for C17H18B1N1O3: C, 69.18; H, 6.14; N,
4.74. Found: C, 68.67; H, 5.92; N, 4.61%.
3.3. X-ray crystallography
X-ray diffraction studies were performed on a Bruker-APEX dif-
fractometer with
a
CCD area detector, Mo
Ka-radiation,
k = 0.71073 Å, graphite monochromator. Frames were collected at
T = 293 K by
x-rotation (D/x = O.3°) at 10 s per frame. The mea-
Compound 1c was prepared from 0.30 g (1.93 mmol) of 3-amin-
ophenylboronic acid and 0.19 g (1.93 mmol) of 2,4-pentanedione.
The product was obtained as a yellow powder. Yield: 0.18 g
(41%); mp = 140–143 °C. 1H NMR (400 MHz, CDCl3) d: 8.17 (2H, s,
NH2), 7.61 (1H, d, J = 7.6, H9), 7.55 (1H, s, H5), 7.35 (1H, t, J = 7.6,
H8), 7.25 (1H, d, J = 7.6, H7), 5.23 (1H, s, H2), 3.37 (3H, s, OMe),
2.01 (3H, s, CH3–C1), 1.99 (3H, s, CH3–C3) ppm. 13C NMR
(100 MHz, CDCl3) d: 160.8 (C-1,3), 138.5 (C-6), 131.8 (C-8), 130.4
(C-9), 129.2 (C-5), 126.9 (C-7), 98.2 (C-2), 39.7 (OMe), 20.3 (CH3–
C1,3) ppm. 11B NMR (64 MHz, CDCl3) d: 3.2 ppm (h1/2 = 4104 Hz).
sured intensities were reduced to F2. Structure solution, refinement
and data output were carried out with the SHELXTL program package
[16]. All non-hydrogen atoms were refined anisotropically. Hydro-
gen atoms were placed in geometrically calculated positions using
a riding model.
3.4. General method for the preparation of boron complexes 1a–1d
Compounds 1a–1d were synthesized by reacting equimolecular
quantities of 3-aminophenylboronic acid monohydrated and the
corresponding 1,3-dicarbonyl compound using 20 ml of MeOH as
solvent, in the case of 1d acetonitrile was used as solvent adding
2 ml of n-propanol. The reaction mixtures were refluxed 1 h under
stirring. After that, the solvent was completely removed using a
vacuum pump. The products were purified by recrystallization in
solvent mixtures MeOH/CH2Cl2 (1:3 ratio).
Compound 1a was prepared from 0.30 g (1.93 mmol) of 3-amin-
ophenylboronic acid monohydrated and 0.43 g (1.93 mmol) of 1,3-
diphenyl-1,3-propanedione. The product was obtained as an or-
ange powder. Yield: 0.52 g (75%); mp = 187–191 °C. 1H NMR
(400 MHz, CDCl3) d: 8.17 (4H, d, J = 7.3 Hz, H-o), 7.81 (1H, s, H-5),
7.60–7.80 (10H, m, H-m, p), 7.55 (1H, t, J = 7.7 Hz, H-8), 7.35 (1H,
s, H-2), 7.01 (1H, d, J = 7.7 Hz, H-9), 6.97 (1H, d, J = 7.7 Hz, H-7),
3.58 (3H, s, H-Me). 13C NMR (100 MHz, CDCl3) d: 185.8 (C-1,3),
IR (KBr)
v
(cmÀ1) = 3379, 1617, 1590, 1551, 1433, 1344, 1313,
1157, 750, 711. EI-MS m/z (%): 218 ([MÀCH3]+, 10), 204 (5), 175
(25), 160 (70), 132 (30), 117 (15), 101 (100). Elem. Anal. Calc. for
C12H16B1N1O3: C, 61.83; H, 6.91; N, 6.00. Found: C, 61.45; H,
6.73; N, 5.91%.
Compound 1d was prepared from 0.30 g (1.93 mmol) of 3-
aminophenylboronic acid and 0.43 g (1.93 mmol) of 1,3-diphe-
nyl-1,3-propanodione. The product was obtained as a red powder.
Yield: 0.53 g (72%); mp = 187–190 °C. 1H NMR (400 MHz, CDCl3) d:
8.16 (4H, d, J = 7.3, H-o), 7.70 (1H, s, H5), 7.70–7.40 (7H, m, H-m, p,
8), 7.34 (1H, s, H2), 6.95 (1H, dt, J = 8.70, 2.90, H9), 6.58 (1H, dt,
J = 6.3, 2.9, H7), 4.39 (2H, t, J = 5, H10), 1.43 (2H, sex, J = 7.2,
H11), 0.83 (3H, t, J = 7.2, H12) ppm. 13C NMR (100 MHz, CDCl3) d:
184.8 (C-1,3), 147.2 (C-6), 134.0 (C-4), 132.8 (C-p), 128.6 (C-m),
128.0 (C-i), 127.5 (C-8), 127.1 (C-o), 121.6 (C-9), 119.7 (5), 115.5

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V. Barba et al. / Inorganica Chimica Acta 363 (2010) 4112–4116
(7), 93.1 (C-2), 62.4 (C-10), 25.7 (C-11), 10.5 (C-12) ppm. 11B NMR
[2] (a) B.F. Abrahams, D.J. Price, R. Robson, Angew. Chem., Int. Ed. 45 (2006) 806;
(b) B.F. Abrahams, B.A. Boughton, H. Choy, O. Clarke, M.J. Grannas, D.J. Price, R.
Robson, Inorg. Chem. 47 (2008) 9797;
(c) N. Iwasaba, H. Takahagi, J. Am. Chem. Soc. 129 (2007) 7754.
[3] (a) N. Farfán, R. Santillan, H. Höpfl, Main. Group Chem. News 7 (2000) 3;
(b) N. Christinat, R. Scopelli, K. Severin, Chem. Commun. (2004) 1158.
[4] (a) D.W. Norman, J.P. Edwards, C.M. Vogels, A. Decken, S.A. Westcott, Can. J.
Chem. 80 (2002) 31;
(64 MHz, CDCl3) d: 3.1 ppm (h1/2 = 4372 Hz). IR (KBr)
v
(cmÀ1) = 3007, 2969, 1717, 1630, 1436, 1365, 1224, 1093, 902,
530. EI-MS m/z (%): 385 (M+, 5), 350 (50), 294 (40), 224 (100),
146 (70), 104 (95), 77 (55). Elem. Anal. Calc. for C24H24B1N1O3: C,
74.82; H, 6.27; N, 3.63. Found: C, 74.77; H, 6.02; N, 3.54%.
(b) N. Farfán, H. Höpfl, V. Barba, M.E. Ochoa, R. Santillan, E. Gómez, A. Gutierrez,
J. Organomet. Chem. 581 (1999) 70;
(c) H. Höpfl, M. Sanchez, V. Barba, N. Farfán, S. Rojas, R. Santillan, Inorg. Chem.
37 (1998) 1679.
4. Conclusions
[5] (a) V. Barba, E. Gallegos, R. Santillan, N. Farfán, J. Organomet. Chem. 622 (2001)
259;
(b) C. Ikeda, T. Nabeshima, Chem. Commun. (2008) 721.
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In all four cases, reacting 1,3-diketones with 3-aminophenylbo-
ronic acid leads to the six-membered boron compounds. So, the
chelate form is preferred over the reaction of the amino group with
carbonyl groups to get the iminic moieties. A complete delocaliza-
tion in these chelates was not observed as confirmed from the
measure of the C–C internal distances of the heterocycle which
are slightly different. Although discrete molecules are obtained,
in the solid state dimeric and polymeric structures are observed
as a result of hydrogen bonding interactions.
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Acknowledgements
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The authors acknowledge financial support from Consejo Nac-
ional de Ciencia y Tecnología (CONACyT).
(c) K. Kataoka, T.D. James, Y. Kubo, J. Am. Chem. Soc. 129 (2007) 15126.
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Appendix A. Supplementary material
CCDC 752003, 752004 and 752005 contain the supplementary
crystallographic data for 1a, 1b and 1d. 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.ica.2010.05.008.
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