Cobalt(II)-mediated synthesis of 2,6-bis[5,7-di-tert-butyl-1,3-benzoxazol- 2-yl]-pyridine: Structural analysis and coordination behavior

Article abstract of DOI:10.1016/j.molstruc.2012.09.078

The oxidative cyclization of 2,6-bis[2,4-di-tert-butyl-6- (methylidenylamino)phenol]-pyridine (L1) in acetonitrile, through the cobalt(II) coordination compound of L1, has resulted in a convenient route for the preparation of 2,6-bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine (L3). The X-ray diffraction analysis of L3 shows a planar molecule, with the oxygen atoms from the benzoxazole rings oriented to the pyridine nitrogen atom (conformer L3a). Ab initio calculations indicate that from the three possible planar conformers of L3, the more stable is L3a. The solid state conformation of the free ligand L3 and the relative energy of the three calculated conformers indicated stabilizing N → O interactions. Calculations of the protonated derivative of L3, compound 7, indicated that the most stable conformer has the benzoxazole nitrogen atoms pointing to the protonated pyridine NH (7c). The X-ray crystal analysis of ligand L3 coordinated to cobalt(II) nitrate, compound 4 is presented and conformer L3c is found in this compound. Two acid:base complexes [Zn(NO₃)₂(H₂O)₂][L3c] ₂, compound 5, and [NEt₂H₂Cl][L3c], compound 6, have also been investigated. Complex 5 crystallized and its X-ray diffraction analysis is reported, whereas compound 6 was studied in solution by NMR, mass spectrometry and ab initio calculations. Both complexes show that conformer L3c can form stable hydrogen bonding associations, with molecules having the motif YH₂ (Y = N or O), that are of interest for building up supramolecular associations.

Full text of DOI:10.1016/j.molstruc.2012.09.078

Journal of Molecular Structure 1032 (2013) 265–274
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Cobalt(II)-mediated synthesis of
2,6-bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine:
Structural analysis and coordination behavior
Ariadna Garza-Ortíz a,c, Pablo A. Martínez a, Angelica M. Duarte-Hernández b, Edgar Mijangos b,
Marcos Flores-Álamo a, Carol Pérez-Casas a, Carlos Camacho-Camacho c, Rosalinda Contreras b,
Angelina Flores-Parra b, Jan Reedijk d,e, Norah Barba-Behrens a,
⇑
a Departamento de Química Inorgánica, Facultad de Química, Universidad Nacional Autónoma de México. C.U. Coyoacán, México D.F. 04510, Mexico
b Departamento de Química, Cinvestav México, AP 14-740, CP 07000, México D.F., Mexico
c Departamento de Sistemas Biológicos, Universidad Autónoma Metropolitana, Unidad Xochimilco, Calzada de Hueso 1100, Colonia Villa Quietud, 04960 Coyoacán, Mexico
d Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
e Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
h i g h l i g h t s
"
"
"
"
"
Benzoxazole derivative obtained from a 2,6-bis(arylimino)pyridine derivative.
2,6-bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine. Structural properties.
Formation of a 7-coordinate cobalt(II) intermediate is proposed.
Cation:receptor properties of 2,6-bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine.
Structure of zinc(II) nitrate with 2,6-bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine.
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidative cyclization of 2,6-bis[2,4-di-tert-butyl-6-(methylidenylamino)phenol]-pyridine (L1) in ace-
tonitrile, through the cobalt(II) coordination compound of L1, has resulted in a convenient route for the
preparation of 2,6-bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine (L3). The X-ray diffraction analysis
of L3 shows a planar molecule, with the oxygen atoms from the benzoxazole rings oriented to the pyr-
idine nitrogen atom (conformer L3a). Ab initio calculations indicate that from the three possible planar
conformers of L3, the more stable is L3a. The solid state conformation of the free ligand L3 and the rel-
ative energy of the three calculated conformers indicated stabilizing N ? O interactions. Calculations of
the protonated derivative of L3, compound 7, indicated that the most stable conformer has the benzox-
azole nitrogen atoms pointing to the protonated pyridine NH (7c). The X-ray crystal analysis of ligand L3
coordinated to cobalt(II) nitrate, compound 4 is presented and conformer L3c is found in this compound.
Two acid:base complexes [Zn(NO3)2(H2O)2][L3c]2, compound 5, and [NEt2H2Cl][L3c], compound 6,
have also been investigated. Complex 5 crystallized and its X-ray diffraction analysis is reported, whereas
compound 6 was studied in solution by NMR, mass spectrometry and ab initio calculations. Both com-
plexes show that conformer L3c can form stable hydrogen bonding associations, with molecules having
the motif YH2 (Y = N or O), that are of interest for building up supramolecular associations.
Ó 2012 Elsevier B.V. All rights reserved.
Received 28 May 2012
Received in revised form 27 September
2012
Accepted 28 September 2012
Available online 13 October 2012
Keywords:
2,6-Benzoxazolyl-pyridine
Conformational analysis
Oxidative cyclization
Cobalt(II) coordination
1. Introduction
have been fully described along with their chemical interaction
with metal ions and their catalytic properties; some examples
The 2,6-bis(arylimino)pyridine family of Schiff base ligands are
known for more than 60 years [1]. Since then, several studies re-
lated to the coordination properties of these versatile compounds
are given in Scheme 1 [2–10].
Benzoxazoles are present in several natural products and are
relevant targets in drug synthesis. The synthetic routes described
in the literature involve in many cases cyclization by dehydration
using strong acids and high temperatures, microwave irradiation
[11,12], mesoporous material catalysis [13] or by metal-based
catalysis [14–20].
⇑
Corresponding author. Tel./fax: +52 555622 3810.
E-mail addresses: norah@servidor.unam.mx, norah@unam.mx (N. Barba-Beh-
rens).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.09.078

266
A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
When the X-ray crystal diffraction analysis was carried out on
compound L3 an unexpected conformation was observed, where
the two oxygen atoms were pointing to the pyridine nitrogen
atom, whereas the cobalt(II) coordination compound 4 showed a
different conformation (see later). These results motivated us to
calculate the three different possible planar conformations of L3
by HF/6-31G(d), in order to investigate which is the most stable
and compared them with that found in the crystal.
Molecular recognition of organic species with labile protons is
of interest on construction of supramolecular assemblies. Due to
the planar conformation of compound L3 and the location of its
sp2 lone pairs, we found relevant to investigate the formation of
its acid–base adducts.
R
R
N
R
R
N
N
M
X
X
M=Fe, Co
Scheme 1. Co- and Fe-coordination compounds of 2,6-bis(arylimino)pyridine
derivatives.
We have been developing new coordination compounds from
transition metals like Ru and Au and 2,6-bis(arylimino)pyridine
derivatives [21] with potential anticancer activity. Due to the elec-
tron releasing character of tert-butyl groups, these were intro-
duced in 2,6-bis[2,4-di-tert-butyl-6-(methylidenylamino)phenol]-
pyridine (L1), with the aim to synthesize coordination compounds
with increased cytotoxic effect. Therefore, we have been studying
the reactivity of first-row transition metal complexes with L1. Dur-
ing the synthesis of the cobalt(II) derivative in acetonitrile, de-
scribed herein, we have found an unexpected oxidative
cyclization of the ligand L1, resulting in a new bis-benzoxazolyl-
pyridine 3, Scheme 2. This finding appears to be in accord with
the known role of cobalt(II) salts in acetonitrile as oxidation agents
[22,23]. It has been reported that cobalt(II) complexes catalyze the
synthesis of benzoxazole derivatives by elimination of hydrogen
halides by K2CO3 and cyclization [24].
Chemical evidence indicated that, prior to the cyclization, a se-
ven-coordinated cobalt(II) complex 2 is formed, which was isolated
and spectroscopically characterized, see Scheme 2. Complex 2,
upon standing in acetonitrile, resulted in the formation of 2,6-
bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine L3. The unex-
pected one-pot high yield cyclization of L1 appears to be of interest
in heterocyclic synthesis. The structure of L3 was determined by
spectroscopic and X-ray diffraction analyses. When a similar reac-
tion of compound L1 with cobalt(II) nitrate in acetonitrile was per-
formed, colorless crystals of L3 were obtained together with few
brownish-red crystals of complex 4, which corresponded to the co-
balt(II) coordination compound with ligand L3.
The reaction of compound L1 with zinc(II) nitrate in acetonitrile
was carried out and single crystals of complex 5 were isolated.
Their X-ray diffraction analysis showed that compound L3 was
associated by hydrogen bonds to [Zn(NO3)2(H2O)2], and the struc-
ture will be described below. Another complex 6, from reaction of
L3 with diethyl ammonium chloride, which was likewise prepared
and studied by NMR, mass spectrometry and ab initio calculations
[25–27], is also described.
2. Experimental
2.1. Materials and physical measurements
All of the chemicals and analytical grade solvents were pur-
chased from various commercial sources and were used without
further purification unless otherwise stated. Co(CH3COO)2ꢀ4H2O
was purchased from J.T. Baker and 5-di-tert-butyl-1,2-benzoqui-
none and 2,6-pyridinedimethanol from Aldrich. Dry solvents, when
required, were prepared using standard procedures. All reactions
were performed with freshly distilled and dried solvents.
Melting points were obtained on a Fisher–Johns apparatus and
are uncorrected. IR spectra were recorded on a Perkin Elmer Spec-
trum 400 spectrophotometer (ATR reflectance mode) in the 4000–
400 cmꢁ1 range. UV–Vis–NIR spectra (diffuse reflectance, 40,000–
5000 cmꢁ1) were obtained using a Cary 5000 (Varian) spectropho-
tometer. 1H and 13C NMR spectra were recorded using a 300 MHz
spectrometer (Bruker). The chemical shifts were referenced to
O
H2
N
N
tBu
t
Bu
tBu
N
N
N
O
N
tBu
.
Co(MeCO2)2 4H2O
Co
MeCN, reflux, 4h
H
O
H
H
O
O
2O
t
Bu
tBu
tBu
tBu
L1
2
3
2
1
N
4
N
N
6
7
O
O
5
t
Bu
8
tBu
10
9
t
Bu
tBu
L3
Scheme 2. Compounds L1, 2 and L3.

A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
267
TMS. Electrospray mass spectra were recorded on an Agilent 6410
triple quadrupole instrument using an electrospray ionization
technique (ESI-MS). Stock solutions of 3 (0.021 M) were prepared
in methanol and diluted with methanol:formic acid 0.1% (85:15
v/v) to reduce the concentration to 50%.
IR for compounds L1, complex 2 and L3, NMR spectra for com-
pounds L1, L3 and complex 6, as the mass spectra for 6 are in-
cluded on the Supplementary data (SI).
ꢀ0.5H2O: Calcd. (%): C, 76.89; N, 7.69; H, 8.11. Found: C, 76.62; N,
7.50; H, 7.98.
2.2.4. [Co(L3c)(H2O)3](NO3)2ꢀH2Oꢀ2CH3CN (4)
Co(NO3)2ꢀ6H2O (58.2 mg, 0.2 mmol) was dissolved in acetoni-
trile (30 mL) and 108.2 mg (0.2 mmol) of L1, previously dissolved
in acetonitrile (30 mL), were added to the solution. The reaction
mixture was refluxed for 4 h. The solution was let to stand under
vacuum for 2 h. A microcrystalline solid was obtained and it was
recrystallized on acetonitrile. After a week, red-brownish crystals
suitable for X-ray diffraction studies were obtained (78.5 mg,
2.2. Synthesis
70%). IR,
m
(cmꢁ1): 3262–2871, 1762, 1631, 1562, 1530, 1483–
2.2.1. 2,6-Bis[2,4-di-tert-butyl-6-(methylidenylamino)phenol]-
pyridine (L1)
1365, 1303, 1200, 1161, 1077, 872, 703 and 660. Elemental analy-
sis for [Co(L3c)(H2O)3](NO3)2ꢀH2Oꢀ2CH3CN: Calcd. (%): C, 53.54; N,
11.21; H, 6.57. Found: C, 52.72; N, 10.74; H, 6.08.
The synthetic procedure was previously reported [21] and was
followed with minor modifications, as described below: To a solu-
tion of 2,6-pyridinedicarboxaldehyde (560 mg, 4 mmol) in abso-
lute ethanol (20 mL) was successively added 2-amino-4,6-di-tert-
butylphenol (2.23 g, 8 mmol) [28], and the resulting mixture was
refluxed for 6 h over molecular sieves (4 Å). 2,6-Pyridinedicarbox-
aldehyde was synthesized according the reported procedures [29].
The reaction mixture was filtered while hot. Upon cooling, a yellow
crystalline solid (L1) was obtained in high yield (2.13 g, 95%). Mp
2.2.5. [Zn(NO3)2(H2O)2][L3c]2 (5)
Zn(NO3)2ꢀ6H2O (43.9 mg, 0.15 mmol) was dissolved in acetoni-
trile (30 mL) and 80.0 mg (0.15 mmol) of L1, previously dissolved
in acetonitrile (30 mL), were added to the solution. The reaction
mixture was refluxed for 4 h. The solution was let to stand under
vacuum for 2 h. Pale yellow crystals suitable for X-ray diffraction
m
(cmꢁ1): 3380–3215, 2953–2867, 1586–1560, 1480–
studies were obtained (60.1 mg, 31%). IR,
m
(cmꢁ1): 3086–2870,
236 °C. IR,
1771, 1623, 1562, 1540, 1483–1445, 1364, 1336, 1292, 1157,
873.3 and 660.5. Elemental analysis for [Zn(NO3)2(H2O)2][L3c]2:
Calcd. (%): C, 64.63; N, 8.61; H, 6.97. Found: C, 64.42; N, 8.70; H,
7.01.
1455, 1412–1361, 1294, 1248–1201, 964, 953, 863, 762, 643 and
593. NMR (300 MHz, CDCl3, 21 °C), d (ppm), 1H: 8.94 (s, 2H, H4),
8.30 (d, 2H, H2), 7.94 (t, 1H, H3), 7.74 (d, 2H, H7), 7.37 (s, 2H,
H9), 7.34 (s, 2H, OH), 1.47 (s, 18H, CH3-10), and 1.34 (s, 18H,
CH3-8). 13C: 154.6 (4C, C4, C5), 149.4 (2C, C1), 141.8 (1C, C3),
141.7 (2C, C10), 137.4 (2C, C6), 135.7 (2C, C8), 124.9 (2C, C9),
122.5 (2C, C2), 110.3 (2C, C7), tBu groups: 35.0, 34.6 (2C, Cq),
31.6, 29.4 (2C, CH3). ESI-MS, m/z: 543.02, [(C35H48N3O2)]+. Elemen-
tal analysis for C35H47N3O2: Calcd. (%): C, 77.59; N, 7.76; H, 8.74.
Found: C, 77.25; N, 7.75; H, 8.60.
2.2.6. Acid:base complex (6) of bis[benzoxazol-2-yl]-pyridine (L3c)
and diethyl ammonium chloride. 1H NMR titration
Equimolar ratio of diethyl ammonium chloride and compound
L3 was mixed in CD3CN–CDCl3 (2:1). 1H NMR spectra were re-
corded and its ESI MS spectrum was performed m/z: 612, [C39H54-
DN3O2]+ 612.6.
2.2.2. [Co(L1)(H2O)2] (2)
2.3. X-ray crystallography
Co(CH3COO)2ꢀ4H2O (25 mg, 0.1 mmol) was dissolved in acetoni-
trile (10 mL) and 54 mg (0.1 mmol) of compound L1, previously
dissolved in acetonitrile (10 mL), were added to the solution. The
reaction mixture was refluxed for 4 h. The solvent was immedi-
ately removed, by reduced pressure in a rotary evaporator. A
dark-purple solid was collected by filtration, washed with plenty
of cold acetonitrile (10 mL), cold water (10 mL) and finally dried
with diethyl ether under vacuum (27 mg, 42%). Mp 290 °C dec.
Single crystals of compounds L3, and 4 were mounted on a glass
fiber. X-ray diffraction data were measured using standard proce-
dures [30] on a Bruker Kappa CCD area detector diffractometer
using Mo Ka (k = 0.71073 Å) radiation at 293(2) K and 173 K. The
samples were mounted in MicroMounts (MiTeGen company) [31]
with paratone-N oil. Data collection, determination of unit cell
and integration of frames were carried out using the suite Collect
IR,
m
(cmꢁ1): 3050–2860, 1595.5, 1476–1440, 1377, 1334, 858,
software [32]. Intensities were measured using
u + x scans. A
606 and 452. Elemental analysis for Co(C35H45N3O2)(H2O)2: Calcd.
(%): C, 66.23; N, 6.62; H, 7.78. Found: C, 65.98; N, 6.42; H, 7.89.
semi-empirical absorption correction method (multi-scan SADABS)
[33] was applied. Data were solved by use of SHELXS and refined
against F2 on all data by full-matrix least-squares with SHELXL-
97 [33]. All crystallographic programs were used under WinGX
program [34]. Anisotropic displacement parameters were deter-
mined for all non-hydrogen atoms. H atoms bonded to C were posi-
tioned geometrically and refined using a riding model, with
Uiso(H) = 1.2Ueq(aromatic C) and 1.5Ueq(methyl C). The H atoms
bonded to the water O atom were located in a difference Fourier
map and their position were refined freely with Uiso(H) = 1.5Ueq(O).
The crystallographic details are summarized in Table 1. Selected
bond lengths and angles are listed in Table 2.
2.2.3. 2,6-Bis[5,7-di-tert-butyl-1,3-benzoxazol-2-yl]-pyridine (L3)
Co(CH3COO)2ꢀ4H2O (25 mg, 0.1 mmol) was dissolved in acetoni-
trile (10 mL) and 54 mg (0.1 mmol) of L1, previously dissolved in
acetonitrile (10 mL), were added to the solution. The reaction mix-
ture was refluxed for 4 h. Then, the solution was let aside at rt. A
visible reduction in color was observed after several hours. The
dark purple acetonitrile solution turned light brownish. After
3 days colorless cubic prisms, suitable for X-ray diffraction studies,
were obtained, (51 mg, 93%). Mp. 237–239 °C. IR,
m
(cmꢁ1): 3063–
2869, 1640, 1586, 1564, 1456, 1371, 1337, 1188, 874, 854, 757 and
719. NMR (300 MHz, CD3CN–CDCl3 (4:2), 21 °C), d (ppm), 1H: 8.46
(2H, d, J = 6 Hz, H2), 4 8.15 (1H, t, J = 9 Hz, H3), 7.70 (2H, d, J = 3 Hz,
H7), 7.45 (2H, d, J = 3 Hz, H9), tBu groups: 1.63 (18H, s, C10ACH3),
1.44 (18H, s, C8ACH3). 13C: 159.2 (1C, C4), 157.6 (2C, C6), 153.8
(2C, C5), 145.1 (2C, C1), 142.3 (1C, C3), 134.1 (2C, C8), 130.3 (2C,
C10), 122.3 (2C, C2), 116.2 (2C, C9), 108.8 (2C, C7), 42.1 (2C,
C10ACq), 35.3 (2C, C8ACq), 31.8 (2C, C10ACH3), 30.4 (2C,
C8ACH3). ESI-MS, m/z: 538. Elemental analysis for C35H43N3O2-
Diffraction data for complex 5 were collected on an Oxford Dif-
fraction Gemini ‘‘A’’ diffractometer with a CCD area detector (kMo
= 0.71073 Å, monochromator: graphite) source, equipped with
Ka
a sealed tube X-ray source at 298 °C. Unit cell constants were
determined with a set of 15/3 narrow frame/runs (1° in ) scans.
Data sets consisted of 214 frames of intensity data collected with
a frame width of 1° in , a counting time of 1.0–42.5 s/frame,
x
x
and a crystal-to-detector distance of 55.00 mm. The double pass
method of scanning was used to exclude any noise. The collected

268
A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
Table 1
Full-matrix least-squares refinement was carried out by mini-
mizing (Fo2 ꢁ Fc2)2. All non-hydrogen atoms were refined aniso-
tropically. The H atoms of the water group (HAO) were located
in a difference map and refined isotropically with Uiso(H) = 1.5Ueq
for (O). H atoms attached to C atoms were placed in geometrically
idealized positions and refined as riding on their parent atoms,
with CAH = 0.93 and 0.96 Å with Uiso(H) = 1.2Ueq(C) and Uiso(-
H) = 1.5Ueq(C) for methyne and aromatic groups respectively. Crys-
tal data and experimental details of the structure determination
are listed in Table 1.
Crystallographic data of compounds L3 and 5.
Compound
L3
5
Chemical formula
Formula weight (g molꢁ1
Crystal size (mm)
Crystal color
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
C
35H43N3O2ꢀ0.5 (H2O)
C70H90N8O12Zn
1300.87
)
546.73
0.25 ꢂ 0.25 ꢂ 0.08
Colorless
Monoclinic
P21/c
0.37 ꢂ 0.20 ꢂ 0.20
Pale yellow
Orthorhombic
P 21 21
2
12.4448(3)
14.3029(3)
18.0372(3)
90.0
18.9806(12)
16.8840(9)
11.1605(6)
90
c (Å)
2.4. Ab initio calculations
a
(°) = c (°)
b (°)
90.585(1)
3210.4(1)
4
1.131
0.07
90
3576.6(4)
2
1.208
0.406
V (Å3)
Calculations were performed in order to obtain the molecular
geometries of compounds L3, 6 and the protonated structure (7)
of compound L3 using the Gaussian 98 package [37]. Geometries
were checked to be the minimal by means of the frequency
analysis.
Formula units Z
Dcalc. (g/cm3)
l
(mmꢁ1
)
F (000)
Temp. (K)
h Range (°)
Index range
1180
173 (2)
1384
298 (2)
3.44–26.05°
ꢁ20 6 h 6 23
ꢁ16 6 k 6 20
ꢁ13 6 l 6 13
17,826
7047
3.1–27.5
ꢁ16 6 h 6 16
ꢁ18 6 k 6 18
ꢁ23 6 l 6 23
42,747
3. Results and discussion
Reflections collected
Independent reflections
3.1. Compound L1 and its cobalt(II) complex 2
7301
Reflections
Rint
Parameters
5358 [I > 3
0.051
388
r
(I)]
7047
0.0551
429
The ligand L1 was prepared with high yield, in one step from
the condensation of two equivalents of 2-amino-4,6-di-tert-butyl-
phenol and one of 2,6-pyridinedicarboxaldehyde. Compound L1
was characterized by 1H and 13C NMR, IR (C@N [1619–1569],
C@C [1480–1455] and OAH [3380–3215] cmꢁ1) and elemental
analysis.
R
Rw
S
Maximum
D
D
0.053
0.143
1.02
0.001
0.25
0.0645
0.1174
1.050
0.001
0.327
ꢁ0.687
P
D
/
r
q
q
maximum (e/Å3)
minimum (e/Å3)
ꢁ0.29
The cobalt(II) compound was prepared by treating stoichiome-
tric amounts of Co(CH3COO)2ꢀ4H2O with L1 in acetonitrile under
reflux for 4 h, Scheme 2. Immediately after, the solid was recovered
under reduced pressure, washed and dried under vacuum. It was
characterized by IR spectroscopy and elemental analysis. The li-
gand looses protons after coordination as was confirmed in the
IR spectrum. Typical C@N and C@C stretching frequencies [21]
are shifted to lower values in the cobalt(II) compound (1585–
1516 and 1475–1410 cmꢁ1) compared to the free ligand L1. CoAO
stretching vibration is observed at 590 cmꢁ1. By comparison it is
deduced that the cobalt(II) compound 2 has a similar structure to
that observed by X-ray diffraction analysis in a coordination com-
pound derived from a related ligand, 2,6-bis(2-hydroxyphenylimi-
nomethyl)-pyridine, [38]. Elemental analysis indicated that two
water molecules were present in compound 2.
P
P
P
P
2
Rint ¼ jF2o ꢁ hFo2ij= Fo2; R1
¼
jjFoj ꢁ jFcjj= jFoj; wR2 ¼ ½ wðF2o ꢁ Fc2Þ =
P
2
1=2
wðF2o Þ ꢃ
.
Table 2
Selected bond lengths (Å), bond angles (°) and torsion angles (°) for compound L3.
Bond
Length (Å)
Bond
Angles (°)
C2AN1
C2AC8
C6AN1
C6AC25
C8AN9
1.338(2)
1.468(2)
1.337(2)
1.467(2)
1.295(2)
1.369(2)
1.378(2)
1.403(2)
1.295(2)
1.370(2)
1.382(2)
1.401(2)
C6AN1AC2
C8AN9AC15
C8AO7AC14
N1AC2AC8
N1AC6AC25
N9AC8AC2
N26AC25AC6
O7AC8AC2
O24AC25AC6
Torsion angles (°)
N1AC2AC8AO7
N1AC6AC25AO24
N1AC2AC8AN9
N1AC6AC25AN26
C5AC6AC25AN26
C3AC2AC8AN9
117.0(1)
103.8(1)
103.7(1)
116.1(1)
116.1(1)
127.7(1)
127.2(1)
116.3(1)
117.3(1)
C8AO7
C14AO7
C15AN9
C25AN26
C25AO24
C31AO24
C32AN26
3.2. Compound L3
4.0(2)
2.9(2)
ꢁ176.9(1)
ꢁ177.9(1)
2.2(2)
When working in the slow crystallization of the compound 2 in
acetonitrile solutions, a visible reduction in color was observed
after several hours. The dark purple solution turned to light brown-
ish and after 3 days small colorless square prisms of L3, were ob-
tained, Scheme 2. Hydrate cobalt(II) acetate as a pink powder
was also isolated after complete evaporation of mother liquor
and characterized by IR spectroscopy.
Transformation of L1 into L3 was evident from the IR spectrum
of the crystals. The broad band of the OAH stretching vibration of
the phenol (3380–3215 cmꢁ1) was absent and a strong band
(1643 cmꢁ1) [39] was assigned to CAO stretching vibration of an
oxazole ring. The C@N (oxazole), C@C and C@N (pyridine) stretch-
ing modes were observed in the expected range. 1H and 13C NMR
spectra were in agreement with the dibenzoxazole pyridine struc-
ture of L3.
3.4(2)
frames were integrated by using an orientation matrix determined
from the narrow frame scans. CrysAlisPro and CrysAlis RED soft-
ware packages [35] were used for data collection and data integra-
tion. Analysis of the integrated data did not reveal any decay. Final
cell constants were determined by a global refinement of 7047
reflections (h < 26.05°). Collected data were corrected for absor-
bance by using Analytical numeric absorption correction [36] using
a multifaceted crystal model based on expressions upon the Laue
symmetry using equivalent reflections. Structure solution and
refinement were carried out with the program SHELXL-97 [33].
All crystallographic programs were used under WinGX program
[34].
Compound L3 displays a molecular ion peak of high to medium
intensity in their positive ESI mass spectrum, confirming the struc-
ture (m/z = 538). Characteristic fragmentation patterns in the spec-

A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
269
S
S
M
N
CH3
N
H
H
-
H
O
N
H
N
tBu
N
N
M
N
M
O
O
tBu
O
tBu
tBu
S
t
Bu
tBu
tBu
tBu
S
S
S
S
t
Bu
O
H
N
O
tBu
O
N
tBu
N
N
M
N
N
M
O
t
Bu
tBu
tBu
S
S
tBu
tBu
S
S
Scheme 3. Reaction path for the oxidative cyclization of compound 2.
trum and fragmentation ions interacting with solvent molecules
were observed. All peaks exhibit the correct isotopic distribution.
The reaction of compound L1 with cobalt(II) nitrate in acetoni-
trile afforded also compound L3. From the reaction product few
brownish-red crystals of complex 4 were obtained, which corre-
sponded to the cobalt(II) coordination compound with ligand L3.
The syntheses of benzoxazole from phenol amines occur by
dehydration and/or dehydrohalogenation promoted by diverse
reaction conditions [12,13,19,20,24]. The relevance of the forma-
tion of compound L3 is that it occurs by an oxidative closure of li-
gand L1, via a cobalt(II) acetate or nitrate coordination compound.
The ligand closure is an oxidation reaction where each imine loses
a hydride. A possible pathway for the ring closure could be that the
solvent coordination to Co(II) promotes the breaking of the OACo
bond. Rotation of the phenyl ring CAN bond (90°) lets approach
the phenolate to the imine carbon, followed by formation of a
CAO bond and elimination of a hydride. Both steps are activated
by the metal coordination which favors the phenolate anion attack
to the imine and the most probable acetonitrile reduction, because
the reaction do not proceed in methanol or ethanol, see Scheme 3.
trile solution. The ORTEP representation of L3 is depicted in Fig. 1
and the crystallographic data are presented in Tables 1 and 2. The
relevance of this X-ray crystal structure is the molecule conforma-
tion of the oxygen atoms pointing towards the pyridine nitrogen,
usually this type of molecules is represented with the heterocyclic
nitrogen atoms pointing towards the pyridine nitrogen [12,25–27].
This representation may come from the X-ray diffraction struc-
tures of coordination compounds where the metal atoms are
bound to the three nitrogen atoms, and this conformation was
careless adopted for the free representation of the ligands. There-
fore the solid state conformation of compound L3 motivated us
to calculate the energy of the three planar conformers, to investi-
gate if the solid state conformation of L3 was the result of the crys-
tal packing or the free enthalpy energy of the conformers.
Examination of the data from the Cambridge Crystallographic Data
Center showed that there is only another X-diffraction structure of
a free benzoxazole-pyridine (2-benzoxazolepyridine) [40], in spite
of several examples of coordination compounds with benzoxazole
pyridine derivatives [41]. It is important to note that the conforma-
tion of this benzoxazole has the oxygen atom pointing towards the
pyridine nitrogen. Even more, a sulfur analogue, 2,6-bis-(ben-
zothiazol-2-yl)-pyridine, presents a similar conformation with
the sulfur atoms close to the pyridine nitrogen [42,43].
3.3. Crystal structure of compound L3
Compound L3 has a central pyridine with two benzoxazole
groups in C2 and C6. The molecule has tBu groups at the C11,
Single crystals (P21/c space group) of compound L3 suitable for
X-ray diffraction studies were obtained after 3 days in the acetoni-
Fig. 1. ORTEP diagram of the hydrate of compound L3 with ellipsoids presented at the 50% probability level. Hydrogen atoms were omitted for clarity.

270
A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
C13, C28 and C30 positions of the phenylene groups. It was ex-
pected that this planar molecule could be present in three different
conformations. One with oxygen atoms on the same side of the
pyridinic nitrogen (conformer L3a), a second one, with a nitrogen
and an oxygen close to the pyridinic nitrogen (conformer L3b)
and a third one with two nitrogen atoms pointing to the central
nitrogen (conformer L3c). The X-ray diffraction analysis of solid
L3 showed that the crystals contain conformer L3a. The tBu groups
in conformer L3a produce a steric effect which could give a high
energy structure, as is shown in the space-filling representation
of the solid state structure of compound L3a, Fig. 2. The intermo-
lecular interactions in the crystal packing were examined and
there are not interactions with the benzoxazole nitrogen atoms
that could be responsible for the stabilization of the solid state con-
formation. Ab initio calculations were therefore performed in order
to analyze the energy of the different conformations.
The electron delocalization in compound L3 is denoted by its
planarity and the bond lengths [40,41]. The double bond nature
of the oxazole C8AN9 bond is confirmed by the bond length
[1.295(2) Å], while C8AO7 has a slightly longer bond distance
[1.369(2) Å], is typical of a single bond between sp2 atoms.
A dimer of compound L3 is formed in the crystal through inter-
actions with lattice water molecules, Fig. 3. Hydrogen bonding
[CAHꢀ ꢀ ꢀO (2.62 Å) and OAHꢀ ꢀ ꢀN (1.96 Å)] and OAHꢀ ꢀ ꢀ
p interac-
tions (N1ꢀ ꢀ ꢀH 2.38, C2ꢀ ꢀ ꢀH 2.89, C8ꢀ ꢀ ꢀH 2.88, O7ꢀ ꢀ ꢀH 2.43 Å) of
the water molecules to 3 give place to the dimeric structure, which
is also stabilized by
p
-stacking [C25ꢀ ꢀ ꢀC6 (3.49 Å)], Fig. 4. The
hydrogen bond between the water molecule and the oxazole-nitro-
gen is in agreement with reported data [OAHꢀ ꢀ ꢀN (1.85 Å)] about
preferred protonation at this donor site in a related thio-aromatic
system [42].
3.4. Preliminary structure of cobalt(II) coordination compound 4
Red-brownish crystals of compound 4 collected from the reac-
tion of ligand L1 and cobalt(II) nitrate in acetonitrile were solved
by X-ray diffraction analysis (Supplementary data, SI 9–10). Even
though the quality of the crystals did not allow to obtain a good
refinement of the structure, it was possible to observe the ligand
conformation and its coordination towards the octahedral cobal-
t(II) atom. Anions and solvent molecules could not be refined.
The ligand is planar and is strongly bound to the cobalt(II) atoms
Fig. 2. Space-filling representation of the compound L3 showing the crowded
cavity in front of the pyridine nitrogen atom.
Fig. 3. Dimer of compound L3 in the crystal. Water molecules stabilize two molecules of L3 through CAHꢀ ꢀ ꢀO (2.62 Å) and OAHꢀ ꢀ ꢀN (1.96 Å) hydrogen bonds and OAHꢀ ꢀ ꢀ
interactions (N1ꢀ ꢀ ꢀH 2.38, C2ꢀ ꢀ ꢀH 2.89, C8ꢀꢀꢀH 2.88, O7ꢀꢀꢀH 2.43 Å).
p
Fig. 4.
p-Stacking in compound L3. The distance between the two aromatic planes, (C25ꢀ ꢀ ꢀC6) is 3.49 Å.

A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
271
Fig. 5. Schematic representation of the X-ray crystal structure cationic part of the cobalt(II) coordination compound [Co(L3c)(H2O)3](NO3)2ꢀH2Oꢀ2CH3CN (4).
by the three nitrogen atoms. This structure confirms the stabiliza-
tion of the conformer with the three nitrogen atoms linked to the
metal atom, and the coordination completed by three water mole-
cules, Fig. 5.
3.5. Ab initio calculations
In order to analyze the energy of the three possible planar con-
formers as to have insight of the solid state conformation of L3, we
calculated the minimum energy structure of each of them. The cal-
culated conformers and some atomic distances are presented in
Figs. 6–8. The validation of the calculations was performed by com-
parison with the X-ray data of conformer L3a. The calculation
reproduces accurately the structure. The most stable conformer
Fig. 8. Conformer L3c with some atom distances (Å) and bond angles (°).
is L3a, followed by L3b which is less stable than L3a by 8.2 kJ/mole.
Compound L3c being less stable than L3a by 17.7 kJ/mole. The dif-
ference in energy is related to the dipolar moment [L3a (0.32 D),
L3b (2.06), L3c (3.16 D)]. The repulsion between the N lone pair
electrons for conformer L3c can be appreciated by comparison of
the angles N9AC8AC2, with those of conformer L3a O7AC8AC2.
The sum of the van der Waals radii for nitrogen and oxygen is
3.15 Å, however, the distance between oxygen and the pyridinic
nitrogen atom in conformer L3a is 2.76 Å, while in the crystal it
is 2.66 Å; therefore a stabilizing interaction N ? O appears possi-
ble. This weak interaction has been reported for distances NAO
of 2.85 Å [43,44], 2.542(2) Å and 2.74(2)–2.87(3) Å [45]. Another
point to note is that the distance between the two oxygen atoms
in conformer L3a is 4.68 Å (Fig. 6), whereas in conformer L3c the
distance between the nitrogen atoms is 5.10 Å (Fig. 8).
In order to investigate the conformational behavior of com-
pound L3 upon protonation, we have calculated the three conform-
ers of the monoprotonated molecule 7 at pyridine (Figs. 9–11),
indicating that 7c, with two nitrogen atoms pointing to the pyri-
dine NH is more stable than 7a by 9.0 kJ/mole and then 7b by
3.3 kJ/mole. The difference between the more stable conformers
L3a and 7c could be explained by the fact that in L3a N ? O inter-
Fig. 6. Conformer L3a with some atom distances (Å) and bond angles (°).
Fig. 9. Conformer of protonated compound 7a with some distances between atoms
(Å), bond angles (°) and hydrogen bonding.
Fig. 7. Conformer L3b with some atom distances (Å) and bond angles (°).

272
A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
3.6. Acid:base complexes of bis[benzoxazol-2-yl]-pyridine (L3c)
From the solution of the reaction mixture of ligand L1 with zin-
c(II) nitrate in acetonitrile few crystals were isolated. The X-ray dif-
fraction analysis of compound 5 was performed, crystals were
composed by the acid–base complex of the L3 formulated as
[Zn(NO3)2(H2O)2][L3c]2 Fig. 13. Crystallographic data are included
in Tables 1 and 3. The hexacoordinated ZnII ion is bound to two
chelating nitrates and two water molecules. The ligand L3c is asso-
ciated to the zinc(II) compound using two strong bifurcated hydro-
gen bonds from the coordinated water molecules to the bis-
benzoxazole pyridine cavity. The pyridine nitrogen is linked by
two hydrogen atoms from a water molecule and each OAH proton
is interacting in a bifurcated way with two nitrogen atoms.
Each nitrate group of the ZnII compound is hydrogen bonded to
the external part of a ligand through the nitrate oxygen atoms and
the C3AH from the pyridine and a neighboring tert-butyl CAH, as
shown in Fig. 14. The intermolecular interactions give place to a
2D supramolecular arrangement. Each ZnII compound is interact-
ing with four ligand molecules, Fig. 15.
Fig. 10. Distances between O and N atoms and hydrogen bonds for conformer 7b.
The formation of the zinc(II) complex 5 with the ligand L3 moti-
vated us to also study the acid:base interactions of L3 with diethyl
ammonium chloride in solution. The molecular recognition of cat-
ionic species by families of benzoxazole derivatives have been
studied by other authors [25–27]. 2,6-Bis(2-oxazolyl)pyridine (py-
box) derivatives have been used as building blocks for supramolec-
ular assemblies in combination of secondary dialkyl ammonium
cations [25]. In those systems the 2-oxazoline rings probed to be
Fig. 11. Some distances between nitrogen atoms, bond angles (°) and hydrogen
bonds for conformer 7c.
actions stabilize the configuration, while in 7c the proton on the
pyridine favor NꢀꢀꢀH hydrogen bonds and neutralizes the two nitro-
gen lone pairs repulsion. Conformer L3c has a distance of 5.10 Å
between the two benzoxazolic nitrogen atoms (Fig. 8), whereas
in the protonated molecule 7c is of 4.71 Å, due to the fact that
the two NꢀꢀꢀH hydrogen bonds (2.4 Å) approach the benzoxazolic
nitrogen atoms, Fig. 11.
Another interesting point is that conformer L3a has a small cav-
ity with room for only one proton (Fig. 2), while in conformer L3c
there is enough space for a metal ion as it is shown in the space-
filling model of 7c, Fig. 12. Therefore, it is expected that in coordi-
nation compounds, the ligand would acquire conformation L3c, as
is indeed observed in cobalt(II) compound 4, Fig. 5.
Fig. 13. Asymmetric unit of the zinc(II) complex 5 showing the hydrogen bonds
between one coordinated water molecule and ligand L3.
Table 3
Selected bond lengths (Å), bond angles (°) and torsion angles (°) for compound 5.
Bond
Length (Å)
Bond
Angles (°)
C5AN1
C5AC6
C1AN1
C1AC13
C6AN4
C6AO2
C7AO2
C12AN4
C13AN3
C13AO1
C14AO1AC19AN3
1.329(5)
1.478(6)
1.338(5)
1.473(6)
1.280(5)
1.367(5)
1.393(5)
1.404(5)
1.274(5)
1.374(5)
1.393(5)
1.407(5)
C5AN1AC1
C6AN4AC12
C6AO2AC7
N1AC5AC6
N1AC1AC13
N4AC6AC5
N3AC13AC1
O2AC6AC5
117.9(4)
103.8(4)
103.3(3)
113.6(4)
113.5(4)
126.3(4)
127.4(4)
117.4(4)
116.5(4)
O1AC13AC1
Torsion angles (°)
N1AC5AC6AO2
N1AC1AC13AO1
N1AC5AC5AN4
N1AC1AC13AN3
C2AC1AC13AN3
C4AC5AC6AN4
ꢁ173.4(4)
168.9(4)
4.0(6)
ꢁ6.2(7)
176.1(5)
ꢁ177.5(5)
Fig. 12. Space-filling model for the protonated conformer 7c.

A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
273
Fig. 14. Hydrogen bonding between coordinated oxygen nitrate groups and the pyridinic C3H and a CH from a tert-butyl group in complex 5.
Fig. 15. 2D supramolecular aggregate in complex 5.
unstable under acidic conditions, producing N-(2-hydroxy-ethyl)a-
mide or polymers through hydrolysis and ring opening. However,
2-benzoxazole pyridine has the advantage to resist acidic condi-
tions [27].
Therefore, we investigated whether or not an acid:base complex
can be formed when an equimolar amount of compound L3 and
diethyl ammonium chloride are put together in solution, see
Scheme 4.
A
lower frequency shift of the signal of C7AH protons
(Dd = 0.15 ppm) is observed, while the chemical shift of the other
protons is slightly changed. This could be interpreted as that coor-
dination with the benzoxazole nitrogen atoms is involved in the
complex formation.
The presence of the acid–base complex 6 in the solution as seen
from the NMR analysis was further confirmed by ESI-MS spectrum
The 1H NMR spectrum of the ammonium salt (CD3CN–CDCl3,
2:1 ratio) shows the corresponding CH3A and ACH2A resonance
peaks. 1H NMR titration of the ammonium salt with L3 caused
slight chemical shift of the CH3A protons belonging to the ammo-
nium and negligible changes in the protons of 3. Higher frequency
shift of the broad NH-signal from 7.71 to 8.42 ppm, was observed
due to hydrogen bonding with the benzoxazole nitrogen atoms.
tBu
tBu
7
7
tBu
tBu
Scheme 4. Cation of complex 6.
Fig. 16. Calculated cation of the complex 6 between compound 3 and [NEt2H2]+.

274
A. Garza-Ortíz et al. / Journal of Molecular Structure 1032 (2013) 265–274
shows a peak at m/z = 612.6, which was assigned to the cation [C39-
H54DN4O2]+ (Supplementary data, S.8 spectrum).
at http://dx.doi.org/10.1016/j.molstruc.2012.09.078. These data in-
clude MOL files and InChiKeys of the most important compounds
described in this article.
The 1:1 complex 6 between the NEt2H2 cation and compound
L3 was also calculated by ab initio methods. The structure shows
a perfect fit between compound L3c and the cation. The complex
has two very strong NAHAN hydrogen bonds with distances be-
tween the two nitrogen atoms of 2.4 and 2.5 Å. The angles NAHAN
are 174 and 178°. Fig. 16 shows the calculated cation of the com-
plex between compound 3 and [NEt2H2]+. No interaction between
an NAH and the pyridine nitrogen was observed.
The study of the protonated molecule 7c, and complexes 5 and
6, show that interactions with species with labile protons can occur
with the conformer L3c, as this is an appropriate receptor of YH2
species, enabling stable intermolecular and supramolecular
structures.
References
[1] F. Lions, K.V. Martin, J. Am. Chem. Soc. 79 (1957) 2733.
[2] C. Bianchini, G. Giambastiani, I.G. Rios, G. Mantovani, A. Meli, A.M. Segarra,
Coord. Chem. Rev. 250 (2006) 1391.
[3] G.G. Mohamed, Spectrochim. Acta Part A – Mol. Biomol. Spectrosc. 64 (2006)
188.
[4] G.J.P. Britovsek, J. England, S.K. Spitzmesser, A.J.P. White, D.J. Williams, Dalton
Trans. (2005) 945.
[5] G.J.P. Britovsek, V.C. Gibson, O.D. Hoarau, S.K. Spitzmesser, A.J.P. White, D.J.
Williams, Inorg. Chem. 42 (2003) 3454.
[6] B. Cetinkaya, E. Cetinkaya, M. Brookhart, P.S. White, J. Mol. Catal. A: Chem. 142
(1999) 101.
[7] V.C. Gibson, C. Redshaw, G.A. Solan, Chem. Rev. 107 (2007) 1745.
[8] B.L. Small, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 7143.
[9] B.L. Small, M. Brookhart, A.M.A. Bennett, J. Am. Chem. Soc. 120 (1998) 4049.
[10] R. Gao, K. Wang, Y. Li, F. Wang, W.H. Sun, C. Redshaw, M. Bochmann, J. Mol.
Catal. A: Chem. 309 (2009) 166.
4. Conclusion Remarks
[11] A.R. Katritzky, D.L. Ostercamp, T.I. Yousaf, Tetrahedron 43 (1987) 5171.
[12] L. Feng, Z. Chen, D. Wang, Spectrochim. Acta A 66 (2007) 599.
[13] R.H. Shoar, M. Heidary, M. Farzaneh, R. Malakouti, Synth. Commun. 39 (2009)
1742.
[14] J.A. Seijas, A.P. Vazquez-Tato, A.R. Carballido-Reboredo, J. Crecente-Campo, L.
Romar-Lopez, Synlett (2007) 313.
[15] J.H. Boyer, Chem. Rev. 80 (1990) 495.
[16] A.J. Blacker, M.M. Farah, M.I. Hall, S.P. Marsden, O. Saidi, J.M.J. Williams, Org.
Lett. 11 (2009) 2039.
[17] A.J. Blacker, M.M. Farah, S.P. Marsden, O. Saidi, J.M.J. Williams, Tetrahedron
Lett. 50 (2009) 6106.
[18] J. Bonnamour, C. Bolm, Org. Lett. 10 (2008) 2665.
[19] W. Bo, Z. Yicheng, L. Pinhua, W. Lei, Chin. J. Chem. 28 (2010) 1697.
[20] I. Mohammadpoor-Baltork, A.R. Khosropour, S.F. Hojati, Monatsh. Chem. 138
(2007) 663.
[21] A. Garza-Ortiz, P.U. Maheswari, M. Siegler, A.L. Spek, J. Reedijk, Inorg. Chem. 47
(2008) 6964.
[22] M. Anniyappan, D. Muralidharan, P.T. Perumal, Tetrahedron 58 (2002) 5069.
[23] M.J. da Silva, P. Robles-Dutenhefner, L. Menini, V. Gusevskaya, J. Mol. Catal. A:
Chem. 201 (2003) 71.
[24] P. Saha, M.A. Ali, P. Ghosh, T. Punniyamurthy, Org. Biomol. Chem. 8 (2010)
5692.
[25] K. Sada, T. Sugimoto, T. Tani, Y. Tateishi, T. Yi, S. Shinkai, H. Maeda, N. Tohnai,
M. Miyata, Chem. Lett. 32 (2003) 758.
[26] K. Sada, Y. Tateishi, S. Shinkai, Chem. Lett. 33 (2004) 582.
[27] T. Suzuki, K. Sada, Y. Tateishi, T. Tani, S. Shinkai, Tetrahedron Lett. 45 (2004)
8161.
[28] V.M. Jímenez-Pérez, C. Camacho-Camacho, M. Güizado-Rodrıguez, H. Nöth, R.
Contreras, J. Organomet. Chem. 614–615 (2000) 283.
It has been found that cobalt(II) in acetonitrile promotes an oxi-
dative cyclization process that could be of interest in heterocyclic
chemistry and in the development of new catalytic processes.
Herein we report the successful conversion of 2,6-bis(2-hydroxy-
3,5-tert-butylphenyliminomethyl) pyridine L1 into the cyclization
benzoxazole product L3 and the X-ray diffraction analysis of the
free ligand. The crystallographic data of compound L3 are of partic-
ularly value, due to the reduced number of crystalline structures of
related systems found in the literature.
A relevant fact is that crystal structure of compound L3 reveals
an unusual conformation with the oxygen atoms pointing towards
the nitrogen lone pair electrons, where stabilizing N ? O interac-
tions could be invoked.
Calculations of the three possible conformers of the free ligand
L3 indicated that the conformer L3a, with the two oxygen atoms
pointing to the pyridine nitrogen, is the most stable one. Whereas,
in the calculated monoprotonated molecule 7 the most stable con-
former is 7c, with the two benzoxazole nitrogen atoms pointing to
the cavity.
The preliminary structure of the cobalt(II) in compound 4 indi-
cates that the proposed reaction pathway with this intermediate
for the formation of L3 could occur. This structure also shows that
metal coordination is only possible with conformer L3c.
Two acid:base complexes 5 (in the solid) and 6 (in solution) are
reported. Both showed that ligand L3 forms stable hydrogen bond-
ing associations through conformer L3c, with molecules having the
motif YH2 (Y = N or O), that are of interest for the supramolecular
domain.
[29] A.L. Vance, N.W. Alcock, D.H. Busch, J.A. Heppert, Inorg. Chem. 36 (1997) 5132.
[30] Siemens, XSCAnS (release 2.21) User´s Manual, Siemens Analytical X-ray
Instruments Inc., Madison, WI, USA, 1996.
[31] www.mitegen.com.
[32] Nonius, COLLECT. Nonius BV, Delft, The Netherlands, 2001.
[33] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112.
[34] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[35] Oxford Diffraction, CrysAlis CCD and CrysAlis RED, Oxford Diffraction Ltd.,
Abingdon, England, 2007.
[36] R.C. Clark, J.S. Reid, R.C. Clark, J.S. Reid, Acta Crystallogr. A51 (1995) 887–897.
[37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich,
J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J.
Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D.
Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul,
B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C.
Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L.
Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 98,
Revision A.7, Gaussian, Inc., Pittsburgh PA, 1998.
Acknowledgements
We are grateful to DGAPA for the financial support given to this
research work (Project IN 212210). CPC thanks ICyTDF for postdoc-
toral fellowship and EM thanks the economic support provided by
the CONACyT Project 60894-CB. AGO expresses gratitude to CONA-
CyT for postdoctoral fellowships. We thank Sonia A. Sánchez Ruíz
for ab initio calculations.
[38] Work in progress in our laboratory.
[39] H. Arslan, Ö. Algül, T. Önkol, Spectrochim. Acta Part A 70 (2008) 606.
Appendix A. Supplementary material
ˇ
[40] J. Vinšová, J. Marek, J. Vanco, J. Csöllei, Acta Crystallogr. E Struct. Rep. E63
(2007) o2802.
Crystallographic data of compounds L3 and 5 in CIF format
(CCDC 880828 and 896804) and details of (IR, NMR) characteriza-
tion. This material is available free of charge via the Internet at
http://www.ccdc.cam.ac.uk/conts/retrieving.html. Supplementary
data associated with this article can be found, in the online version,
[41] A. Singh, G. Das, B. Mondal, Polyhedron 27 (2008) 2563.
[42] M. Boca˘, R. Boca˘, G. Kickelbick, W. Linert, I. Svoboda, H. Fuess, Inorg. Chim. Acta
338 (2002) 36.
[43] M. Boca˘, R.F. Jameson, W. Linert, Coord. Chem. Rev. 255 (2011) 290.
[44] J.A. Platts, S.T. Howard, K. Wozniak, Chem. Phys. Lett. 232 (1995) 479.
´
[45] J. O’Leary, J.D. Wallis, Cryst. Eng. Commun. 9 (2007) 941.