Synthesis, characterization, X-ray studies and DFT calculations of fused five-six and seven-six membered ring of new heterobicyclic system of boron compounds

Article abstract of DOI:10.1016/j.molstruc.2013.08.039

Two tridentate ligands derivated from ethanolamines and 2-hydroxyacetophenone and 2-hydroxybenzophenone were reacted with phenylboronic acid and naphthylboronic acid in different proportions to obtain compounds 7a, 8b and 9a which are [4.3.0] and [4.5.0] heterobicyclic systems. Compounds 7a, 8b and 9a were fully characterized and the structures of 7a and 8b have been analyzed by X-ray diffraction, where a series of parameters such as bond distances, bond angles, torsion angles, tetrahedral character at the boron atom, steric effects and deviation of the boron atom from the mean plane have been evaluated and comparated. DFT calculations, PBE/6-31G(d,p), were performed in order to evaluate steric effects and the stability of [4.3.0] and [4.5.0] heterobicyclic system.

Full text of DOI:10.1016/j.molstruc.2013.08.039

Journal of Molecular Structure 1052 (2013) 24–31
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, characterization, X-ray studies and DFT calculations of fused
five–six and seven–six membered ring of new heterobicyclic
system of boron compounds
a
a
b
José María Rivera a, , Raúl Colorado-Peralta , Daniel J. Ramírez , Mario Sanchez ,
⇑
Maria Esther Sanchez-Castro c
a Facultad de Ciencias Químicas, Universidad Veracruzana, Prolongación Oriente 6, No. 1009, Colonia Rafael Alvarado, Apartado Postal 215, C.P. 94340 Orizaba, Veracruz, Mexico
b Centro de Investigación en Materiales Avanzados, S.C. Alianza Norte 202, PIIT, Carretera Monterrey-Aeropuerto Km. 10, C.P. 66600 Apodaca, N.L., Mexico
c Grupo de Sustentabilidad de los Recursos Naturales y Energía, Centro de Investigación y de Estudios Avanzados, Unidad Saltillo, Carretera Saltillo-Monterrey Km. 13, C.P. 25900
Saltillo, Coahuila, Mexico
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Reaction of tridentate ligand 2-[(2-Hydroxy-ethylimino)-phenyl-methyl]-phenol 5b with one and two
equivalents of phenyl and naphthyl boronic acid in toluene during 24 h and 1 h. give place to the
synthesis of the monomeric boronates with fused five–six membered rings [4.3.0] and fused six–seven
membered rings [4.5.0] respectively.
ꢀ
Reaction of tridentate ligands with
phenylboronic acid gave [4.3.0] and
[4.5.0] heterobicyclic systems.
Reaction conditions must be full
controlled to obtain just one product.
DFT calculations, PBE/6-31G(d,p),
were performed in order to evaluate
steric effects in the structures.
ꢀ
ꢀ
HO
OH
B
ꢀ
ꢀ
Seven membered ring shows a chair
conformation but the difference in
energy with boat is minimal.
Suzuki reaction can be achieved
because a tricoordinated boron atom
is found in some of the structures.
N
B
N
B
N
Tol
Δ
-H2O
2
+
O
+
O
HO
B
O
O
OH
O
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2013
Received in revised form 20 August 2013
Accepted 20 August 2013
Available online 27 August 2013
Two tridentate ligands derivated from ethanolamines and 2-hydroxyacetophenone and 2-hydroxybenzo-
phenone were reacted with phenylboronic acid and naphthylboronic acid in different proportions to
obtain compounds 7a, 8b and 9a which are [4.3.0] and [4.5.0] heterobicyclic systems. Compounds 7a,
8b and 9a were fully characterized and the structures of 7a and 8b have been analyzed by X-ray diffrac-
tion, where a series of parameters such as bond distances, bond angles, torsion angles, tetrahedral char-
acter at the boron atom, steric effects and deviation of the boron atom from the mean plane have been
evaluated and comparated. DFT calculations, PBE/6-31G(d,p), were performed in order to evaluate steric
effects and the stability of [4.3.0] and [4.5.0] heterobicyclic system.
Keywords:
Boronates
Tridentate ligands
X-ray studies
DFT calculations
Structural analysis
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
In the last years there has been a great interest in the study of
organoboron compounds, due to a great variety of applications
found, for example in medicinal chemistry, as anticancer agents
⇑
Corresponding author. Tel.: +52 2721010934; fax: +52 2727240120.
E-mail address: chemax7@yahoo.com.mx (J.M. Rivera).
in the technique known as boron neutron capture therapy used
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.08.039

J.M. Rivera et al. / Journal of Molecular Structure 1052 (2013) 24–31
25
for the treatment of certain brain tumors [1–3]; in supramolecular
2. Computational details
chemistry [4,5]; macrocyclic chemistry [6,7]; organometallics [8];
dendrimers [9,10]; they also display applications in organic syn-
thesis [11]; in the Suzuki reaction [12–15]; and as materials with
fluorescence [16]. Some groups have reported several studies in
N ? B coordination bond [17–20] and recently the synthesis and
characterization of boron complexes obtained from the condensa-
tion of 3-amino-phenylboronic acid and 1,3-diketones [21]. In this
context the synthesis and characterization of boron complexes
with fused five-six membered rings, [4.3.0] heterobicyclic systems
[22] and the synthesis, characterization of fused six-seven mem-
bered rings, [4.5.0] heterobicyclic systems derived from optically
active tridentate ligands [23] have been reported. Those studies
indicate that fused five-six membered ring compounds are ob-
tained under strong reaction conditions such as reflux of toluene
for 36 h. and fused six–seven-membered rings compounds are ob-
tained using two equivalents of phenylboronic acid and one equiv-
alent of the tridentate ligand in reflux of toluene for approximately
1 h. (Scheme 1) [23]. The results from X-ray diffraction and compu-
tational analyses indicated that reaction of tridentate ligand 5a
with two eq. of phenylboronic acid or two eq. of naphthylboronic
acid to obtain [4.5.0] heterobicyclic systems cannot occur due to
the steric effect caused in the two methyl groups located in the ali-
phatic carbons (Schemes 2 and 3). The synthesis of polyolefins, sty-
rene and substituted biphenyls compounds by the Suzuki reaction
can occur because of the presence of a trisubstituted boron atom in
structure 8b [12–15]. In this work, we report the synthesis of three
new heterobicyclic compounds, 7a, 8b and 9a. We determined the
geometry of compounds 6a–b, 7a–b, 8b and 9b in gas phase,
describing the most stable conformations and evaluating the influ-
ence of steric factors in the formation of reaction products between
the tridentate ligands 5a and 5b and the corresponding phenyl- or
naphthyl-boronic acid.
The geometry of each closed shell structure was fully optimized
by using the GAUSSIAN 09 software package [24] and their vibra-
tional frequencies were calculated using the density functional
theory (PBE) [25,26] in combination with the 6-31G(d,p) [27,28]
basis set. For the calculations of 11B NMR shielding constants the
GIAO-DFT (gauge-independent atomic orbital) with the 6-
31G(d,p) basis set approach was used. Results were analyzed with
the Chemcraft program v1.6 [29].
3. Experimental
3.1. Instrumentation
NMR spectra were recorded in CDCl3 and DMSO-d6 solutions on
Bruker Avance 300 spectrometer. Chemical shifts (ppm) are rela-
tive to (CH3)4Si for 1H (300.13 MHz) and 13C (75.47 MHz) and BF3-
ꢁOEt2 for 11B (96.29 MHz). Coupling constants are reported in Hz.
Infrared spectra were recorded on a Perkin–Elmer 16F-PC FT-IR
spectrometer. Mass spectra were recorded on a Hewlett-Packard
model 5989A MS engine, coupled to a GC 5890 series II. Melting
points were obtained on a Gallenkamp MFB-595 apparatus and
are uncorrected. Elemental analyses were carried out on a FLASH
(EA) 1112 series, thermo Finnigan apparatus. The X-ray diffraction
study was determined on an Enraf-Nonius-Fr590 Kappa-CCD
(kMoK = 0.71073 Å, graphite monochromator, T = 293 K, CCD rotat-
a
ing images scan mode) and the crystals were mounted on a LINDE-
MANN tube. Absorption correction was not necessary. All
reflections data set were corrected for Lorentz and Polarizations ef-
fects. Structure solution and refinement were performed using the
SHELX-S-97 program and then SHELX-L-97 program was applied
THF, 30 min
Δ
2
2
Δ
Tol, 1 h
1
3
Δ
Tol, 36 h
4
Scheme 1. Monomeric and dimeric boronates obtained using different reactions conditions [23].

26
J.M. Rivera et al. / Journal of Molecular Structure 1052 (2013) 24–31
steric effect
(1) 1eq.
(2) 2eq.
5a
(1) Compound 6a
(2) Structure 6b
steric effect
(1) 1eq.
(2) 2eq.
5a
(1) Compound 7a
(2) Structure 7b
Scheme 2. Synthesis of monomeric boronates 6a [22] and 7a with fused five–six membered rings. Structures of monomeric oligodiboronates 6b and 7b with fused six–seven
membered rings were modeled by PBE/6-31G(d,p).
21
17
8
(1) 1eq.
(2) 2eq.
5b
(1) Compound 8a
(2) Compound 8b
(1) 1eq.
(2) 2eq.
5b
(1) Compound 9a
(2) Structure 9b
Scheme 3. Synthesis of monomeric boronates 8a [22], 8b and 9a with fused five–six and six-seven membered rings. Monomeric oligodiboronate 9b with fused six–seven
membered rings was not obtained but modeled by PBE/6-31G(d,p).
for refinement and output data [30,31]. All software manipulations
were done under the WINGX environment program set [32]. Single
crystal structure validation was done with PLATON [33]. Molecular
perspectives were drawn under ORTEP-3 [34].
3.3. Synthesis
3.3.1. General method for the preparation of tridentate ligands 5a and
5b
In order to prepare the tridentate ligands 5a and 5b, equimolec-
ular quantities of the corresponding aminoalcohol and 2-hydroxy-
acetophenone and 2-hydroxybenzophenone, were refluxed in
ethanol for 1 h. The solvent and water formed during the reaction
were eliminated with a Dean–Stark trap to yield yellow solids that
were washed with methylene chloride and used without further
3.2. Reagents
All reactants and solvents were purchased from Aldrich Chem-
ical Co. and solvents were dried previous to use [35]. Single crystals
were grown using spectroscopic grade solvents.

J.M. Rivera et al. / Journal of Molecular Structure 1052 (2013) 24–31
27
1
1
purification. Tridentate ligands 5a and 5b have already been pub-
6.99 (d, 1H, J
8.1 Hz, H-3), 7.02 (d, 1H, J
6.0 Hz, H-6),
4
4
lished [36–43].
7.21–7.33 (m, 3H, H-11, H-17, H-18), 7.39–7.43 (m, 3H, H-22,
H-23, H-24), 7.42–7.47 (m, 2H, H-21, H-25), 7.49–7.53 (m, 3H,
H-12, H-16, H-19). 13C NMR (75.47 MHz, DMSO-d6) [d, ppm]:
53.2 (C-8), 60.5 (C-9), 119.2 (C-5), 120.5 (C-1), 121.4 (C-3), 124.6
(C-12), 124.7 (C-14), 126.1 (C-19), 126.6 (C-22, C-24), 127.0
(C-16), 127.1 (C-15), 127.9 (C-6), 129.3 (C-21, C-25), 129.7
(C-11), 130.1 (C-17), 131.7 (C-23), 133.2 (C-13), 133.4 (C-18),
136.9 (C-4), 139.1 (C-10), 139.3 (C-20), 160.6 (C2), 168.9 (C7). 11B
NMR (96.29 MHz, CDCl3), [d, ppm]: 4.1. (h1/2 = 270 Hz.). MS
(m/z,%): 377 (M+, 0.2), 250 (100), 234 (1), 209 (31), 181(1),
104.10 (0.2). Anal. Calc. for C25H20BNO2: C, 79.52; H, 5.30; N,
3.71. Found C, 79.22; H, 5.41; N, 3.78%.
3.3.2. General method for the preparation of compounds 7a, 8b and 9a
3.3.2.1.
(2S,4R,5S)-2-(Naphthyl)benzo[j]-4-phenyl-5,7-dimethyl-6-
aza-1,3-dioxa-2-borciclononen-6-ene. 7a. Compound 7a was syn-
thesized from 0.30 g. (1.11 mmol) of 5a and 0.19 g. (1.11 mmol)
of naphthylboronic acid. The reaction was carried out in toluene
under reflux, using a Dean–Stark trap during 24 h. The product
was obtained as a yellow solid 0.33 g. (0.81 mmol), yield, 73.3%,
½
a 2D5
ꢂ
¼ þ127:5 (c 0.102 CHCl3), mp: 214–218 °C. IR tmax (KBr):
3060, 2925, 2860, 1634 (C@N), 1551, 1463, 1438, 1312, 1259,
1169, 1072, 1039, 858, 751, 696, 647, 578, 521, 470 cmꢃ1 1H
.
NMR (300.13 MHz, DMSO-d6) [d, ppm]: 0.99 (d, 3H, J 1/4 6 Hz,
CH3), 2.63 (s,3H, CH3), 4.36 (m, 1H, H-8), 5.50 (d, 1H, J 1/4 5 Hz,
4. Results and discussion
H-9), 6.85 (t, 1H, J 1/4 8.1 Hz, H-5), 7.06 (d, 1H, J 1/4 8.2 Hz, H-
1
3), 7.15–7.29 (m, 4H, H-22, H-24, H-17, H-18), 7.30 (t, 1H, J
4
It is well known that tridentate ligands obtained from alde-
hydes or ketones with different ethanolamines react in mild condi-
tions with electron acceptor molecules to give heterocyclic
systems. In this work tridentate ligands 5a and 5b were allowed
to react with one and two equivalents of phenylboronic acid and
naphthylboronic acid as described [22,23] to obtain compounds
7a, 8b and 9a in yields between 69.0% and 76.2%. Due to the forma-
tion of a new chiral center at the boron atom it is possible to induce
its stereochemistry. Compound 7a is derived from chiral (1S,2R)-
(+)-Norephedrine which induces the attack of the phenyl and
naphthylboronic acid in the less hindered face of the molecule
(Scheme 4). In contrast compound 8b is obtained as a racemic mix-
ture from the achiral ethanolamine, the attack of the phenyl and
naphthylboronic acid can occur in both sides of the tridentate
ligand 5b at the same time.
8.1 Hz, H-4), 7.35–7.39 (m, 3H, H-12, H-16, H-19), 7.40–7.43 (m,
2H, H-21, H-25), 7.48–7.54 (m, 4H, H-6, H-11, H-15, H-23). 13C
NMR (75.47 MHz, DMSO-d6) [d, ppm]: 14.7 (CH3), 18.0 (CH3),
62.9 (C-8), 80.2 (C-9), 119.0 (C-5), 120.6 (C-1), 121.2 (C-3), 124.8
(C12), 124.9 (C-14), 126.2 (C-19), 126.5 (C-22, C-24), 127.4
(C-16), 127.5 (C-15), 128.1 (C-6), 128.2 (C-23), 128.3 (C-21,
C-25), 129.8 (C-11), 130.3 (C-17), 133.0 (C-13), 133.2 (C-18),
136.7 (C-4), 139.6 (C-10), 139.8 (C-20), 160.6 (C-2), 166.9 (C-7)
11B NMR (96.29 MHz, CDCl3), [d, ppm]: 6.1. (h1/2 = 294 Hz.). MS
(m/z,%): 405 (M+, 0.2), 298 (6), 278 (100), 262 (1), 162 (30), 117
(1). Anal. Calc. for C27H24BNO2: C, 80.01; H, 5.97; N, 3.46. Found
C, 79.96; H, 5.92; N, 3.41%.
3.3.2.2. 9-Phenyl-2,4-diphenylbenzo[j]-8-aza-1,3,5,2,4-trioxadibora-
cycloundec-8-ene. 8b. Compound 8b was synthesized from 0.50 g.
(2.07 mmol) of 5b and two equivalents 0.51 g. (4.14 mmol) of
phenylboronic acid. The reaction was carried out under reflux of
toluene for 1 h, using a Dean–Stark trap. The product was obtained
as a yellow solid 0.68 g. (1.58 mmol), yield, 76.2%, mp: 208–212 °C.
IR tmax (KBr): 3010, 2944, 1956, 1608 (C@N), 1549, 1468, 1432,
1273, 1149, 1068, 1027, 993, 949, 835, 744, 700, 639, 560, 504,
1H and 13C chemical shifts were unequivocally assigned by 2D
experiments HETCOR and COSY of the boron compounds 7a, 8b
and 9a and comparing with the spectra of the ligands 5a and 5b
in the same solvent. 1H and 13C NMR spectra of compounds7a,
8b and 9a in DMSO-d6, at room temperature, show no significant
changes in the chemical shift and coupling constants, as compared
with those of the ligands. The proton NMR analysis showed that
the signals corresponding to the aliphatic atoms C8 and C9 in 7a,
8b and 9a are in the range from 3.09 to 4.22 ppm and the fact that
a new chiral center is been created at the boron atom, compounds
8b and 9a were obtained as a racemic mixture. Because compound
7a was obtained from a chiral ethanolamine just one product is ob-
served by NMR. The aromatic protons are in the range from 6.85 to
7.54, 6.70–8.08 and 6.99–7.53 for 7a, 8b and 9a respectively (Ta-
ble 1). The existence of the N ? B bond in compounds 7a, 8b and
9a was established by 11B NMR which shows the signals between
3.8 and 6.1 ppm, characteristic for the tetracoordinated boron
435 cmꢃ1 1H NMR (300.13 MHz, DMSO-d6) [d, ppm]: 3.63 (dd,
.
1H, J = 7.3 Hz, J = 6.5 Hz, H-8a), 3.73 (dd, 1H, J = 8.1 Hz, J = 5.8 Hz,
H-8b), 3.89 (dd, 1H, J = 6.9 Hz, 5.5 Hz, 9a), 4.18 (dd, 1H, J = 5.9 Hz,
7.7 Hz, 9b), 6.70 (t, 1H, J = 8.0 Hz, H-5), 7.11 (d, 1H, J = 8.4 Hz, H-
3), 7.21–7.33 (m, 4H, H-6, H-12, H-14, H-26), 7.35–7.58 (m, 8H,
H-4, H-11, H-13, H-15, H-18, H-20, H-24, H-27), 7.59–7.68 (m,
3H, H-19, H-23, H-25), 8.08 (dd, 2H, J = 1.1 Hz, J = 7.7 Hz, H-17,
H-21). 13C NMR (75.47 MHz, CDCl3) [d, ppm]: 53.3 (C-8), 64.4 (C-
9), 118.5 (C-5), 120.2 (C-3), 120.4 (C-1), 126.8 (C-22), 127.3 (C-6),
127.4 (C-26), 127.5 (C-18, C-20), 127.9 (C-12, C-14), 128.2 (C-19),
129.6 (C-24), 130.4 (C-13), 130.6 (C-25), 131.1 (C-11, C-15),
132.3 (C-27), 135.1 (C-17, C-21), 135.8 (C-23), 137.7 (C-4), 159.9
(C-2), 169.8 (C-7). 11B NMR (96.29 MHz, CDCl3), [d, ppm]: 3.8,
27.0. (h1/2 = 282, 692 Hz.). MS (m/z,%): 431 (M+, 0.1), 354 (41),
250 (100), 174 (32), 167 (0.2). Anal. Calc. for C27H23B2NO3: C,
75.22; H, 5.38; N, 3.25. Found C, 75.15; H, 5.40; N, 3.27%.
Backside attack less hindered
3.3.2.3. (Rac)-2-(Naphthyl)benzo[j]-7-phenyl-6-aza-1,3-dioxa-2-bor-
ciclononen-6-ene. 9a. Compound 9a was synthesized from 0.30 g
(1.24 mmol) of 5b and 0.21 g (1.24 mmol) of naphthylboronic acid.
The reaction was carried out in toluene under reflux, using a Dean–
Stark tramp during 24 h. The product was obtained as a yellow so-
lid 0.31 g (0.85 mmol), yield, 69%, mp: 199–203 °C, IR tmax (KBr):
3059, 2930, 2863, 1629 (C@N), 1549, 1473, 1453, 1316, 1257,
1169, 1074, 1036, 863, 750, 698, 648, 576, 523, 469 cmꢃ1
.
1H
Frontside Attack hindered
1
NMR (300.13 MHz, DMSO-d6) [d, ppm]: 3.09 (dd, 1H, J
3.4 Hz, J
4
1
1
1
8.8 Hz, H-8b), 3.36 (dd, 1H, J
3.2 Hz, J
4.9 Hz, H-8a), 3.81
4
4
4
Scheme 4. Attack of the phenyl and naphthylboronic acid occur from the less
hindered backside of the molecule.
1
(m, 1H, H-9a), 4.22 (m, 1H, H-9b), 6.83 (t, 1H, J
7.0 Hz, H-5),
4

28
J.M. Rivera et al. / Journal of Molecular Structure 1052 (2013) 24–31
Table 1
Table 2
Selected d (ppm, DMSO-d6 at 25 °C) 1H and 13C NMR for 7a, 8b and 9a. Proton signals
Crystallographic data for compounds 7a and 8b.
are displayed in italics.
Compound
7a
8b
7
8
9
Chemical formula
Formula weight
Space group
C
27H24BNO2
C27H23B2NO3
431.08
P c a b
(7a)
(8b)
(9a)
166.9
62.9
4.36 (m)
80.2
5.50 (d)
405.29
P212121
169.8
168.9
53.3
64.4
Crystal system
Orthorhombic Orthorhombic
0.2 ꢄ 0.2 ꢄ 0.1 0.2 ꢄ 0.1 ꢄ 0.1
3.63(dd), 3.73 (dd)
53.2
3.09(dd), 3.36 (dd)
3.89 (dd), 4.18 (dd)
60.5
3.81 (m), 4.22 (m)
Crystal size (mm3)
Unit cell dimensions
a (Å)
b (Å)
c (Å)
9.6762(2)
13.9307(4)
16.2949(5)
90
13.0321(2)
15.9638(3)
21.7660(5)
90
a
(°)
b (°)
(°)
atom and a signal in 27 ppm for the tricoordinated boron atom in
compound 8a, [36–40]. 11B NMR chemical shifts for 7a, 8b and 9a
were calculated using NMR by the method GIAO with isotropic
protection and they are relative to BF3ꢁOEt2, the results are very
close to the experimentally observed, as shown in Fig. 1. The char-
acteristic signal in the 13C NMR spectra for the imine group appears
in 166.9, 169.8 and 168.9 ppm, and it was confirmed by the band
for (C@N) in the infrared experimental spectra in 1634, 1608 and
1629 cmꢃ1for 7a, 8b and 9a respectively, while C@N stretching
vibrations calculated by PBE/6-31G(d,p) appeared in 1619, 1618
and 1616 cmꢃ1 for 7a, 8b and 9a respectively. The mass spectra
give the molecular ions in low abundance, and the base peak cor-
responds to the [M+–C6H5] ion in agreement with previous results,
[22,23].
Compounds 7a and 8b provided crystals suitable for X-ray anal-
yses. The X-ray structures are shown in Fig. 2. The crystallographic
data are summarized in Table 2. The structure of compound 7a cor-
responds to the fused five-six membered rings, [4.3.0] where the
naphthyl group attached to the boron atom prefers the position
in the same face of the other substituents. The phenyl group
bonded to the aliphatic carbon C9 is big enough to induce the con-
figuration at the boron atom. The structure of compound 8b corre-
sponds to the fused six-seven membered rings, [4.5.0]. The N ? B
coordination bond distances for 7a and 8b are 1.589(3) and
1.603(3) Å and they are similar to those observed in boron
90
90
c
90
90
Formula units per cell
4
8
dcalc. (g cmꢃ3
F(000)
)
0.545
384
1.265
1808
T (K)
h (°)
293(2)
3.58–27.48
4858
4858
3337
293(2)
3.46–27.88
10,151
10,719
5385
Reflections collected
Independent reflections
Observed reflections, (Fo)2>4
r
(Fo)2
R =
R
|Fo| ꢃ |Fc||/
R
|Fo|
0.0446
0.0825
0.0499
0.1581
P
P
Rw ¼ ½ wðjFoj ꢃ jFcjÞ = wF2o ꢂ1=2; w ¼ 1=r2
2
(all data)
Goodness-of-fit (GOF)
No. of parameters
1.039
377
0.909
391
Maximun
D
/
)
r
ꢃ0.105
ꢃ0.091
0.102
ꢃ0.029
ꢃ0.128
0.155
D
D
qmin (e Åꢃ3
qmax (e Åꢃ3
)
complexes [36–40]. The bond angles around the tetracoordinated
boron atom B1 for compounds 7a and 8b are in the range between
100.92° and 115.19° (Table 3). The average value for the bond an-
gles in the five and seven membered rings are 104.63° and 120.27°
for 7a and 8b respectively.
The values for the tetrahedral character (THC) [44] for boron
atoms B1 are 69.0% and 89.3% for 7a and 8b respectively, these
Fig. 1. 11B chemical shifts (ppm) obtained by NMR experimental analyses (red), with its NMR calculated values obtained by the method GIAO (black) with isotropic
protection. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. ORTEP representation of compounds 7a and 8b, hydrogens are omitted for clarity and ellipsoids containing 50% probability level.

J.M. Rivera et al. / Journal of Molecular Structure 1052 (2013) 24–31
29
Table 3
Selected bond distances (Å), bond angles (°) and dihedral angles (°) for compounds 7a
and 8b.
Compound
7a
8b
Bond distances (Å)
N(1)AC(8)
N(1)AC(7)
O(1)AC(2)
O(3)AC(9)
O(2)AC(9)
C(8)AC(9)
C(1)AC(7)
C(7)AC(22)
B(1)AO(1)
B(1)AO(2)
B(1)AN(1)
B(1)AC(10)
B(2)AO(2)
B(2)AO(3)
B(2)AC(16)
1.478(2)
1.294(2)
1.347(2)
–
1.423(2)
1.553(3)
1.452(3)
1.498(3)
1.498(2)
1.443(2)
1.589(3)
1.614(3)
–
1.474(3)
1.300(2)
1.330(2)
1.428(3)
–
1.486(4)
1.449(3)
1.491(3)
1.474(3)
1.432(3)
1.603(3)
1.612(3)
1.340(3)
1.358(3)
1.565(3)
Fig. 3. Envelope and chair conformations in compounds 7a and 8b by X-ray
diffraction.
–
–
Bond angles (°)
N(1)AB(1)AO(1)
N(1)AB(1)AO(2)
N(1)AB(1)AC(10)
O(1)AB(1)AC(10)
O(2)AB(1)AC(10)
O(1)AB(1)AO(2)
N(1)AC(7)AC(1)
C(7)AC(1)AC(2)
C(1)AC(2)AO(1)
C(9)AC(8)AN(1)
C(8)AN(1)AB(1)
B(1)AO(1)AC(2)
C(7)AN(1)AB(1)
C(8)AN(1)AC(7)
B(1)AO(2)AC(9)
O(2)AC(9)AC(8)
B(1)AO(2)AB(2)
O(2)AB(2)AO(3)
B(2)AO(3)AC(9)
O(3)AC(9)AC(8)
103.66(14)
100.92(14)
114.07(15)
109.50(15)
115.19(16)
112.75(15)
116.52(16)
117.7(17)
108.8(15)
109.6(17)
107.9(15)
109.9(17)
113.7(17)
106.8(16)
119.6(17)
119.9(17)
121.92(17)
113.8(2)
114.3(16)
124.5(15)
124.1(16)
121.7(17)
–
121.75(16)
98.20(14)
109.48(15)
113.85(14)
121.90(15)
126.71(16)
109.59(14)
104.94(15)
–
Fig. 4. Intermolecular interactions in compound 7a.
138.3(17)
124.4(2)
125.4(19)
116.2(2)
–
–
–
Dihedral angles (°)
O(1)AB(1)AC(10)AC(15)
O(2)AB(1)AC(10)AC(11)
N(1)AB(1)AC(10)AC(15)
N(1)AB(1)AC(10)AC(11)
17.7(2)
72.4(2)
133.29(17)
ꢃ43.7(3)
ꢃ61.4(2)
ꢃ4.7(3)
57.1(2)
ꢃ126.6(19)
Deviation from mean plane (Å)
Plane: C(2)AC(1)AC(7)AN(1)AB(1)AO(1)
B(1)-0.327
O(1)0.068
Fig. 5. Superimposition of the fused rings in compounds 7a and 8b obtained by X-
ray diffraction analyses (gray), with its modeled structure (black).
calculating their vibrational modes. The calculated structures were
compared with the IR and NMR spectroscopic data of the herein syn-
thesized compounds, which were in good agreement. The theoreti-
cal results are validated when superimposed structures obtained by
gas phase calculations (black) and X-ray diffraction in the solid state
(gray) of the compounds 7a (fused five–six membered ring) and 8b
(fused six–seven membered ring), Fig. 5. The structures 6a and 7a
correspond to heterobicyclic systems [4.3.0], whereas the rest of
the molecules modeled correspond to heterobicyclic systems
[4.5.0]. In all structures, there is a coordination bond N1AB1 with
a bond distance between 1.59 and 1.63 Å characteristics for this type
of compounds [17–23]. The bond distances B1AO1 (ꢅ1.48 Å) are
slightly longer than the corresponding B1AO2 (ꢅ1.45 Å) in the se-
ven-membered rings. The angles around tetracoordinated boron
atom are in the range of 106–114° and tricoordinated boron atom
in the range of 116–123°. The boat conformation reduces the steric
effect between the two methyl groups in 6b and 7b in accordance
with the torsion angles of the fragments H3CAC7@N1AC8
(7.11°,6.90°) and C7@N1AC8ACH3 (98.64°,98.33°). It is also noted
that naphthyl groups bonded to the boron atoms in 7b and 9b are
perpendicular avoiding steric repulsion.
values show that the boron atom is more distorted in compound
7a with fused five-six membered ring. The deviations from the
mean
plane
of
the
six
membered
ring
C(2)AC(1)AC(7)AN(1)AB(1)AO(1) in 7a and 8b are ꢃ0.327 (B1)
and 0.068 (O1) Å respectively, showing that the largest deviation
from the mean plane corresponds to (B1) in compound 7a which
presents the less THC. The dihedral angles for the
O1AB1AC10AC15 fragment in compounds 7a and 8b are 17.63°
and ꢃ61.47° which are indicative of the two conformations pre-
ferred around CAB bond, eclipsed and gauche. The conformation
in the five member ring in compound 7a is envelope where C9 is
out the plane formed by C8, N1, B1 and O2; the conformation in
the seven member ring in compound 8b is chair, N1 is above the
plane formed by B1, O2, B2 and C8, C9, O3 Fig. 3. Structure 7a
shows intermolecular hydrogen bond interactions between
O1ꢁ ꢁ ꢁH21 and O2ꢁ ꢁ ꢁH12, with distances of 2.65 Å and 2.69 Å, in
both cases the distances are less than the sum of the van der Walls
radii (2.72 Å) [45] Fig. 4.
The geometries of 6a–b, 7a–b, 8b and 9b were optimized in gas
phase using the density functional theory (PBE) in combination with
the 6-31G(d,p) basis set. All structures were characterized by

30
J.M. Rivera et al. / Journal of Molecular Structure 1052 (2013) 24–31
Fig. 6. Boat conformation calculated for structures 6b and 7b by PBE/6-31G(d,p), the hydrogen are omitted for clarity.
Fig. 7. Boat and chair conformations calculated for structures 8b and 9b by PBE/6-31G(d,p), hydrogens are omitted for clarity.
The synthesis, characterization and crystal structure of com-
which avoid the formation of the systems [4.5.0] in compounds 6b
pound 6a was previously reported by our research group [22],
while the crystal structure of 7a is discussed in this paper, however
it was not possible to obtain experimentally compounds 6b and 7b,
(Scheme 2), but we could determine the minimum energy struc-
tures for compounds 6b and 7b, showing a significant steric repul-
sion between the two methyl groups bonded to carbons C7 and C8,
and 7b. The modeled structures adopt a boat conformation as the
most stable for the seven-membered heterobicyclic boronates,
Fig. 6. The distances N1AB1, B1AO2, O2AB2, B2AO3 are 1.63,
1.45, 1.35, 1.39 Å for 6b and 1.63, 1.45, 1.34, 1.38 Å for 7b.
The seven membered ring in 8b shows a chair conformation,
which was proved by X-ray diffraction; calculations were made

J.M. Rivera et al. / Journal of Molecular Structure 1052 (2013) 24–31
31
to model the boat conformation showing a minimal difference in
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Acknowledgements
The authors acknowledge financial support from Consejo Nac-
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