Bisoxazaborolidines and boron complexes derived from tetradentate ligands: Synthesis and spectroscopic studies

Article abstract of DOI:10.1016/j.ica.2004.02.018

A series of tetradentate ligands were synthesized and their reaction with arylboronic acids was studied. Two classes of tetradentate ligands are discussed, which are mainly based on ONNO and ONOO donor sets and involve 2-aminophenol derivatives and Schiff bases. The reaction of phenylboronic acid with tetradentate ligands derived from 2-aminophenol leads to bisoxazaborolidines containing five-membered rings formed by CCNBO atoms, where the boron atoms have a trigonal geometry. In contrast, tetradentate ligands derived from Schiff bases lead to monomeric and dimeric boron complexes, in which the boron atoms have a tetrahedral geometry stabilized by an intramolecular N-B dative bond. The structures of two boron compounds were established by X-ray diffraction analysis.

Full text of DOI:10.1016/j.ica.2004.02.018

Inorganica Chimica Acta 357 (2004) 2593–2601
www.elsevier.com/locate/ica
Bisoxazaborolidines and boron complexes derived from
tetradentate ligands: synthesis and spectroscopic studies
a,
Victor Barba , Alejandro Rodrıguez , Ma. Eugenia Ochoa , Rosa Santillan ,
b
b
b
*
ꢀ
Norberto Farfan
b
ꢀ
a
ꢀ
ꢀ
ꢀ
Centro de Investigaciones Quımicas, Universidad Autonoma del Estado de Morelos, Av. Universidad 1001, C.P. 62210 Cuernavaca, Morelos, Mexico
b
Departamento de Quꢀımica, Centro de Investigaciꢀon y de Estudios Avanzados del IPN, Mꢀexico D.F. 07000, A.P. 14-740, Mꢀexico
Received 13 November 2003; accepted 22 February 2004
Available online 28 March 2004
Abstract
A series of tetradentate ligands were synthesized and their reaction with arylboronic acids was studied. Two classes of tetrad-
entate ligands are discussed, which are mainly based on ONNO and ONOO donor sets and involve 2-aminophenol derivatives and
Schiff bases. The reaction of phenylboronic acid with tetradentate ligands derived from 2-aminophenol leads to bisoxazaborolidines
containing five-membered rings formed by CCNBO atoms, where the boron atoms have a trigonal geometry. In contrast, tetrad-
entate ligands derived from Schiff bases lead to monomeric and dimeric boron complexes, in which the boron atoms have a tet-
rahedral geometry stabilized by an intramolecular N–B dative bond. The structures of two boron compounds were established by
X-ray diffraction analysis.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Tetradentate ligand; Aminophenol; Oxazaborolidine; Boron; Schiff base
1. Introduction
tivity of tetradentate N2O2 ligands of the salen type
containing sp2 nitrogen atoms is quite different to that of
ligands having sp3 nitrogen atoms and phenolic oxy-
gens. In recent years, dimetallic boron compounds with
salen ligands have been used as Lewis acid activators or
as dinuclear catalysts in dealkylation reactions of a wide
range of phosphate esters [16,17].
In continuation of our studies on bi- and tridentate
ligands with different boron reagents [18–20], we set to
investigate the reactivity of tetradentate ligands with
arylboronic acids. The boron complexes described
herein contain tetradentate ligands with ONNO and
ONOO donor moieties. Complexes 2a–2g have boron
atoms with trigonal geometry and a C2 fold axis. The
advantages associated with the use of ligands with C2
symmetry for enantioselective catalysis have been high-
lighted in the literature [21].
Tetradentate ligands of the N2O2 type have been
extensively studied, including their reaction with main
group elements and transition metals [1–6]. The versa-
tility of these ligands permits the isolation with transi-
tion metals of both mononuclear and dinuclear
complexes. However, for group 13 elements, mainly di-
nuclear complexes with boron [7–9], aluminum [10] and
gallium [10] are known.
Compared to the salen derivatives which are readily
available by condensation of primary amines with
carbonyl compounds, such as salicylaldehyde and
2-hydroxyphenyl ketones [12,13]; tetradentate ligands
derived from aminophenol have been less explored
mainly due to the fact that their synthesis requires sev-
eral steps [13–15]. It has been observed that the reac-
Moreover, compounds having two Lewis acid sites
have great potential for application in both catalysis [22]
and synthesis [23] and there are a number of biologically
important reactions that are mediated by two Lewis
* Corresponding author. Tel.: +52-777-3297997; fax: +52-777-
3297997.
E-mail address: vbarba@ciq.uaem.mx (V. Barba).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.02.018

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V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
acidic metals in close proximity [24]. Also, derivatives
from tetradentate ligands such as phenols and transition
metals are useful model for the simulation of enzymatic
systems [25].
[26] and refined using SHELXL-97 [27]. All non-hydro-
gen atoms were refined anisotropically. Hydrogen atoms
were placed in geometrically calculated positions using a
riding model. Crystallographic data for the structures
have been deposited at the Cambridge Crystallographic
Data Center as supplementary material Nos. 223333
and 223334 for complexes 2e and 5b, respectively.
Copies of data can be obtained free of charge on ap-
plication to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK. E-mail: deposit@ccdc.cam.ac.uk.
2. Experimental
2.1. Materials and instrumentation
All reagents were purchased from Aldrich and used
without further purification. NMR spectra were re-
corded at room temperature using the following spec-
trometers: Jeol GSX 270, Bruker 300 and Jeol
eclipse + 400. Chemical shifts are given in ppm. Infrared
spectra have been recorded on a Perkin Elmer 16F-PC
FT-IR spectrophotometer. Mass spectra were obtained
with a HP 5989-A mass spectrometer operating in the
electron impact mode. Melting points were determined
with a Gallenkamp MFB-595 apparatus. Elemental
microanalyses were performed by Oneida Research
Services (Whitesboro, NY. 13492).
2.2. Synthesis of the complexes
Ligands 1a–1d were prepared by methods previously
described in the literature [13].
2.2.1. General procedure for the preparation of ligands
1e–1g
A mixture of 1,3-dibromopropane and 2 equivalents
of o-aminophenol was placed into a sealed tube and
heated in an oil bath to 120–125 °C. After 14 h of
heating, a 10 M NaOH solution was added with stirring.
The organic phase was extracted by using several por-
tions of dichloromethane. Finally, the solvent was re-
moved under vacuum and the product washed twice
with 10 ml of hexane.
Crystal structure determination: single crystals of 2e
and 5b suitable for X-ray structure analysis were grown
by slow evaporation of a THF solution of the complex.
Crystal data and details of the structure determinations
are listed in Table 1. Intensity data were collected at
293 K with an Enraf-Nonius CAD4 diffractometer for
compound 2e and a Bruker Smart 6000 diffractometer
with a CCD area detector for 5b, Mo Ka-radiation,
2.2.1.1. 2, 20-(1,3-Propanediamine)bisphenol 1e. Com-
pound 1e was prepared from 2.0 g (18.3 mmol) of
o-aminophenol and 0.93 ml (9.2 mmol) of 1,3-dib-
romopropane. A white solid was obtained with yield
of 32 % (0.76 g, 2.90 mmol). M.p. ¼ 154–156 °C. IR
(KBr)m ¼ 3358, 3318, 3058, 2916, 2848, 1596, 1512,
ꢁ
k ¼ 0:71073 A, graphite monochromator. Empirical
absorption corrections (DIFABS) were applied. The
structures were solved by direct methods (SHELXS-86)
Table 1
Crystallographic data for 2e and 5b
Identification code
2e
5b
Empirical formula
Formula weight
Crystal size (mm3)
Temperature (K)
Crystal system
C
430.10
27H24B2N2O2
C20H22B1N1O4
351.20
0.51 ꢀ 0.43 ꢀ 0.38
273
0.40 ꢀ 0.30 ꢀ 0.25
293
monoclinic
P21=n
monoclinic
P21=c
Space group
Unit cell dimensions
ꢁ
a (A)
14.550(3)
9.240(2)
17.210 (3)
90.92(3)
2313.4(8)
4
8.1627(3)
15.1214(6)
14.5944(5)
105.149(1)
1738.81(11)
4
ꢁ
b (A)
ꢁ
c (A)
b (°)
V (A )
3
ꢁ
Z
Dcalc (g/cm3)
1.235
0.077
1.342
0.386
Absorption coefficient (mmꢁ1
Collected reflections
Independent reflections
Parameters
)
4887
4704
11339
3415
238
299
Final R indices [I > 2sigmaðIÞ]
R indices (all data)
Goodness-of-fit
R ¼ 0:0682
wR ¼ 0:1827
1.038
R ¼ 0:0563
wR ¼ 0:1579
1.053
3
ꢁ
Largest differential peak and hole (e/A )
0.009/)1.954
0.044/0.001

V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
2595
1490, 1440, 1378, 1276, 1254, 1198, 748 cmꢁ1. MS (EI,
70 eV, DIP) m=z (%): 258 [Mþ, 63], 149 (40), 132 (35),
120 (100), 109 (42), 77 (35), 53 (18). 1H NMR (400 MHz,
DMSO-d6) d: 9.17 (1H, s, NH), 6.65 (1H, d, J ¼ 7:7 Hz,
H-6), 6.62 (1H, ddd, J ¼ 7:7, 6.9, 1.1 Hz, H-4), 6.49 (1H,
d, J ¼ 6:9 Hz, H-3), 6.40 (1H, td, J ¼ 7:7, 1.1 Hz, H-5),
3.35 (1H, s, OH), 3.13 (2H, t, J ¼ 6:8 Hz, H-7), 1.84
(2H, qn, J ¼ 6:8 Hz, H-8) ppm. 13C NMR (100 MHz,
DMSO-d6) d: 144.6 (C-1), 137.9 (C-2), 120.2 (C-4),
116.1(C-5), 113.9 (C-6), 110.2 (C-3), 40.2 (C-7), 28.9
(C-8) ppm.
were dissolved in chloroform, precipitated with hexane
and washed with a mixture of hexane:chloroform (9:1).
2.2.2.1. N,N0-[1,2-ethane-bis[2-phenyl-(benzoxazabor-
olidine)]] (2a). Compound 2a was prepared from
0.50 g (2.05 mmol) of 1a and 0.50 g (4.10 mmol) phen-
ylboronic acid. A pale gray solid was obtained with yield
of 34% (0.30 g, 0.72 mmol). M.p. ¼ 195–197 °C. IR
(KBr)m ¼ 3264, 3070, 2370, 1604, 1498, 1458, 1364,
1350, 1192, 1090, 1008, 698, 638, 580 cmꢁ1. MS (EI, 15
eV, DIP) m=z (%): 416 (Mþ, 100), 368 (11), 338 (27), 330
(41), 252 (29), 208 (92), 180 (36), 122 (71). 1H NMR (270
MHz, CDCl3) d: 7.77 (2H, dd, J ¼ 7:7, 1.5 Hz, H-o),
7.49 (1H, dt, J ¼ 7:7, 1.5 Hz, H-p), 7.42 (2H, dt, J ¼ 7:7,
1.5 Hz, H-m), 7.19 (1H, dd, J ¼ 7:0, 1.3 Hz, H-6), 7.11
(1H, dt, J ¼ 7:5, 1.5 Hz, J ¼ H-4), 7.07 (1H, dt, J ¼ 7:5,
1.5 Hz, H-5), 7.05 (1H, d, J ¼ 7:7, 1.3 Hz, H-3), 4.26
(2H, s, CH2) ppm. 13C NMR (68 MHz, CDCl3) d: 149.3
(C-1), 138.0 (C-2), 133.9 (C-o), 130.7 (C-p), 128.4 (C-m),
121.8 (C-4), 120.6 (C-5), 112.6 (C-6) 108.7 (C-3), 42.8
(CH2) ppm. 11B NMR (86 MHz, CDCl3) d: 32.5 ppm
(h1=2 ¼ 283 Hz). Elemental Anal. Calc.: C, 75.0; H, 5.30;
N, 6.70%. Found: C, 74.56; H, 5.54; N, 6.88%.
2.2.1.2. 2,20-(1,3-Propanediamine)-4,40-dimethylbisphe-
nol (1f). Compound 1f was prepared from 2.0 g (16.2
mmol) of 2-amino-4-methylphenol and 0.86 ml (8.1
mmol) of 1,3-dibromopropane. A pale pink solid was
obtained with yield of 45% (1.04 g, 3.63 mmol).
M.p. ¼ 169–170 °C. IR (KBr)m ¼ 3332, 3118, 2962,
2920, 1522, 1508, 1478, 1438, 1374, 1306, 1278, 1254,
1214, 1124, 808, 770 cmꢁ1. MS (EI, 70 eV, DIP) m=z (%):
286 [Mþ, 67], 266 (27), 213 (42), 185 (28), 134 (100), 123
(43), 77 (34), 41 (20). 1H NMR (400 MHz, DMSO-d6) d:
8.99 (1H, s, NH), 6.50 (1H, d, J ¼ 7 Hz, H-6), 6.29 (1H,
s, H-3), 6.17 (1H, d, J ¼ 7:0 Hz, H-5), 3.63 (1H, s, OH),
3.10 (2H. t, J ¼ 8:3 Hz, H-7), 2.11 (3H, s, CH3) 1.82
(2H, qn, J ¼ 8:3 Hz, H-8) ppm. 13C NMR (100 MHz,
DMSO-d6) d: 142.3 (C-1), 137.7 (C-2), 128.5 (C-4), 116.2
(C-5), 113.7 (C-6), 111.1 (C-3), 40.2 (C-7), 28.9 (C-8),
21.4 (CH3) ppm.
2.2.2.2. N,N0-[1,2-ethane-bis[2-phenyl-(8-methylbenzox-
azaborolidine)]] (2b). Compound 2b was prepared
from 0.5 g (1.84 mmol) of 1b and 0.45 g (3.68 mmol) of
phenylboronic acid. A pale gray solid was obtained with
yield of 48% (0.41 g, 0.92 mmol). M.p. ¼ 200–201 °C. IR
(KBr)m ¼ 3422, 3048, 3018, 2850, 1616, 1494, 1468,
1348, 1324, 1302, 1274, 1232, 1194, 1114, 1064, 1022,
800, 742, 692, 648, cmꢁ1. MS (EI, 15 eV, DIP) m=z (%):
444 (100), 366 (11), 352 (45), 280 (39), 222 (95), 91 (51).
1H NMR (300 MHz, CDCl3) d: 7.64 (2H, dd, J ¼ 7:9,
1.2 Hz, H-o), 7.44 (1H, dt, J ¼ 7:9, 1.2 Hz, H-p), 7.32
(2H, dt, J ¼ 7:9, 1.2 Hz, H-m), 7.11 (1H, d, J ¼ 8:0 Hz,
H-6), 6.84 (1H, d, J ¼ 8:0 Hz H-5), 6.75 (1H, s, H-3),
4.20 (2H, s, CH2), 2.39 (3H, s, CH3) ppm. 13C NMR (75
MHz, CDCl3) d: 147.4 (C-1), 138.2 (C-2), 133.7 (C-o),
131.6 (C-4), 130.5 (C-p), 128.2 (C-m), 121.1 (C-5), 112.1
(C-6), 109.7 (C-3), 42.7 (CH2), 21.8 (CH3) ppm. 11B
NMR (96 MHz, CDCl3) d: 34.4 ppm (h1=2 ¼ 584 Hz).
Elemental Anal. Calc.: C, 75.70; H, 5.85; N, 6.31%.
Found: C, 75.36; H, 5.94; N, 6.47%.
2.2.1.3. 2,20-(1,3-Propanediamine)-5,50-dimethylbisphe-
nol (1g). About 1g was prepared from 2.0 g (16.2
mmoles) of 2-amino-5-methylphenol with 0.86 ml (8.1
mmol) of 1,3-dibromopropane. A pale yellow solid was
obtained with yield of 24% (0.55 g, 1.92 mmol).
M.p. ¼ 166–168 °C. IR (KBr)m ¼ 3326, 3084, 3032,
2960, 2916, 2802, 1530, 1468, 1420, 1288, 1272, 1248,
1212, 1136, 798, 584 cm)1. MS (EI, 70 eV, DIP) m=z (%)
286 [Mþ, 87], 266 (1), 242 (2), 164 (2), 136 (69), 134
(100), 91 (25), 77 (26), 28 (7). 1H NMR (400 MHz,
DMSO-d6) d: 9.08 (1H, s, NH), 6.48 (1H, s, H-6), 6.43
(1H, d, J ¼ 7:3 Hz, H-4), 6.37 (1H, d, J ¼ 7:3 Hz, H-3),
4.47 (1H, s, OH), 3.07 (2H, t, J ¼ 6:2 Hz, H-7), 2.02
(3H, s, CH3), 1.80 (2H, qn, J ¼ 6:2 Hz, H-8) ppm. 13C
NMR (100 MHz, DMSO-d6) d: 144.0 (C-1), 135.5 (C-2),
124.9 (C-5), 120.3 (C-4), 115.0 (C-6), 110.5 (C-3), 41.7
(C-7), 29.0 (C-8), 20.8 (CH3) ppm.
2.2.2.3. N,N0-[1,2-ethane-bis[2-phenyl-(7-methylbenzox-
azaborolidine)]] (2c). Compound 2c was prepared
from 0.50 g (1.84 mmol) of 1c and 0.45 g (3.68 mmol) of
phenylboronic acid. A white solid was obtained with
yield of 65% (0.55 g, 1.23 mmol). M.p. ¼ 195–197 °C. IR
(KBr)m ¼ 3220, 3086, 2920, 2370, 2346, 1654, 1496,
1490, 1458, 1420, 1410, 1340, 1302, 1276, 1170, 1118,
790 cmꢁ1. MS (EI, 15 eV, DIP) m=z (%): 444 (72), 368
(13), 306 (26), 264 (6), 223 (18), 222 (100), 171 (37), 130
2.2.2. General procedure for the preparation of oxazabor-
olidines 2a–2g
An equivalent of ligands 1a–1g was added to two
equivalents of phenylboronic acid in 30 ml of xylene.
The solution was refluxed for 6 h using a Dean-Stark
trap to remove the water formed during the reaction and
part of the solvent. The solution was evaporated to
dryness to give compounds 2a–2g as gray solids, which
1
(41), 78 (19). H NMR (300 MHz, CDCl3) d: 7.60 (2H,

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V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
dd, J ¼ 7:2, 1.5 Hz, H-o), 7.41 (1H, dt, J ¼ 7:2, 1.5 Hz,
H-p), 7.27 (2H, dt, J ¼ 7:2, 1.5 Hz, H-m), 7.03 (1H, s, H-
6), 6.92 (1H, d, J ¼ 7:9, 1.5 Hz, H-4), 6.85 (1H, d,
J ¼ 7:9, H-3), 4.21 (2H, s, CH2), 2.43 (3H, s, CH3) ppm.
13C NMR (75 MHz, CDCl3) d: 149.8 (C-1), 136.0 (C-2),
133.8 (C-o), 130.7 (C-5), 130.5 (C-p), 128.3 (C-m), 122.6
(C-4), 113.7 (C-6), 108.6 (C-3), 42.9 (CH2), 21.7 (CH3)
ppm. 11B NMR (96 MHz, CDCl3) d: 33.7 ppm
(h1=2 ¼ 546 Hz). Elemental Anal. Calc.: C, 75.72; H,
5.90; N, 6.31%. Found: C, 75.46; N, 5.94; N, 6.57%.
2.2.2.6. N,N0-[1,3-propane-bis[2-phenyl-(8-methylbenz-
oxazaborolidine)]] (2f). Compound 2f was prepared
from 0.50 g (1.75 mmol) of 1f and 0.43 g (3.50 mmol) of
phenylboronic acid. A pale gray solid was obtained with
yield of 72% (0.60 g, 1.31 mmol). M.p. ¼ 136–137 °C. IR
(KBr)m ¼ 3048, 2918, 1602, 1494, 1476, 1448, 1410,
1380, 1352, 1 1296, 1244, 1192, 1022, 802, 692 cmꢁ1. MS
(EI, 15 eV, DIP) m=z (%): 458 (100), 366 (10), 352 (44),
280 (29), 222 (95), 91 (51). 1H NMR (300 MHz, CDCl3)
d: 7.83 (2H, dd, J ¼ 7:9, 1.5 Hz, H-o), 7.48 (1H, dt,
J ¼ 7:9, 1.5 Hz, H-p), 7.41 (2H, dt, J ¼ 7:9, 1.5 Hz, H-
m), 7.22 (1H, d, J ¼ 8:0 Hz, H-6), 6.87 (1H, dd, J ¼ 8:0,
1.2 Hz, H-5), 6.78 (1H, s, H-3), 4.02 (2H, t, J ¼ 8:0 Hz,
NCH2), 2.41 (2H, qn, J ¼ 8:0 Hz, NCH2CH2), 2.38 (3H,
s, CH3) ppm. 13C NMR (75 MHz, CDCl3): 147.8 (C-1),
138.5 (C-2), 134.2 (C-o), 131.9 (C-4), 130.9 (C-p), 128.7
(C-m), 121.2 (C-5), 112.3 (C-6), 110.4 (C-3), 40.9
(NCH2), 30.5 (NCH2CH2), 21.9 (CH3) ppm. 11B NMR
(96 MHz, CDCl3) d: 34.2 (h1=2 ¼ 1031 Hz) ppm. Ele-
mental Anal. Calc.: C, 73.11; H, 6.30; N, 5.88%. Found:
C, 73.70; H, 6.15; N, 6.32%.
2.2.2.4. N,N0-[1,2-ethane-bis[2-phenyl-(8-tertbutylbenz-
oxazaborolidine)]] (2d). Compound 2d was prepared
from 0.50 g (1.53 mmol) of 1d and 0.37 g (1.85 mmol) of
phenylboronic acid. A white solid was obtained with
yield of 55% (0.46g, 0.87 mmol). M.p. ¼ 138–140 °C. IR
(KBr)m ¼ 3082, 2956, 2904, 2864, 1612, 1600, 1490,
1420, 1406, 1342, 1306, 1290, 1236, 1068, 1054, 1024,
756, 696, 692 cmꢁ1. MS (EI, 15 eV, DIP) m=z (%): 528
(29), 368 (56), 312 (38), 264 (100), 208 (66), 152 (43), 96
1
(44), 44 (25). H NMR (300 MHz, CDCl3) d: 7.58 (2H,
dd, J ¼ 7:2, 1.3 Hz, H-o), 7.41 (1H, dt, J ¼ 7:2, 1.3 Hz,
H-p), 7.28 (2H, dt, J ¼ 7:2, 1.3 Hz, H-m), 7.12 (1H, d,
J ¼ 8:4 Hz, H-6), 7.07 (1H, dd, J ¼ 8:4, 1.3 Hz, H-5),
6.97 (1H, s, H-3), 4.34 (2H, s, CH2), 2.43 (9H, s, CH3)
ppm. 13C NMR (75 MHz, CDCl3) d: 147.4(C-1), 145.7
(C-4), 138.0 (C-2), 133.8 (C-o), 130.6 (C-p), 128.4 (C-m),
117.9 (C-5), 112.1 (C-6), 106.1 (C-3), 43.0 (CH2), 35.1
(C(CH3)3), 32.3 (CH3) ppm. 11B NMR (96 MHz,
CDCl3) d: 32.0 ppm (h1=2 ¼ 1014 Hz). Elemental Anal.
Calc: C, 77.27; H, 7.19; N, 5.30%. Found: C, 77.16; H,
7.22; N, 5.49%.
2.2.2.7. N,N0-[1,3-propane-bis[2-phenyl-(7-methylbenz-
oxazaborolidine)]] (2g). Compound 2g was prepared
from 0.50 g (1.75 mmol) of 1g and 0.43 g (3.50 mmol) of
phenylboronic acid. A white solid was obtained with
yield of 72% (0.60 g, 1.31 mmol). M.p. ¼ 134–136 °C. IR
(KBr)m ¼ 3422, 2850, 1654, 1648, 1636, 1540, 1508,
1448, 1400, 1356, 1276, 1256, 1132, 1118, 1092, 936, 792
cmꢁ1. MS (EI, 15 eV, DIP) m=z (%): 458 (100), 381 (63),
368 (73), 294 (26), 248 (11), 222 (93), 185 (11), 129 (25),
1
73 (16), 60 (12). H NMR (300 MHz, CDCl3) d: 7.79
(2H, dd, J ¼ 7:7, 1.4 Hz, H-o), 7.47 (1H, dt, J ¼ 7:7, 1.4
Hz, H-p), 7.39 (2H, dt, J ¼ 7:7, 1.4 Hz, H-m), 7.20 (1H,
s, H-6), 6.92 (1H, d, J ¼ 7:2 Hz, H-4), 6.84 (1H, d,
J ¼ 7:2 Hz, H-3), 4.00 (2H, t, J ¼ 7:4 Hz, NCH2), 2.43
(3H, s, CH3), 2.37 (2H, qn, J ¼ 7:4 Hz, NCH2CH2)
ppm. 13C NMR (75 MHz, CDCl3) d: 149.7(C-1), 136.0
(C-2), 134.0 (C-o), 130.6 (C-p), 130.4(C-5), 128.4 (C-m),
122.5 (C-4), 113.4 (C-6), 108.9(C-3), 40.8 (NCH2), 30.7
(NCH2CH2), 21.7 (CH3) ppm. 11B NMR (96 MHz,
CDCl3) d: 34.11 ppm (h1=2 ¼ 608 Hz). Elemental Anal.
Calc.: C, 73.11; H, 6.30; N, 5.88%. Found: C, 73.14; H,
6.10; N, 6.09%.
2.2.2.5. N,N0-[1,3-propane-bis-[2-phenyl-(benzoxazabor-
olidine)]] (2e). Compound 2e was prepared from 0.50 g
(1.94 mmol) of 1e and 0.47 g (3.86 mmol) of phen-
ylboronic acid. A pale gray solid was obtained with yield
of 45% (0.39 g, 0.91 mmol). M.p. ¼ 93–94 °C. IR
(KBr)m ¼ 3390, 2918, 2888, 1480, 1468, 1420, 1404,
1364, 1324, 1292, 1242, 1022, 736, 692 cmꢁ1. MS (EI, 15
eV, DIP) m=z (%): 430 (100), 401 (33), 353 (16), 325 (14),
234 (23), 208 (56), 195 (22), 158 (21), 105 (31). 1H NMR
(300 MHz, CDCl3) d: 7.82 (2H, dd, J ¼ 6:8, 1.3 Hz, H-
o), 7.48 (1H, dt, J ¼ 6:8, 1.3 Hz, H-p), 7.41 (2H, dt,
J ¼ 6:8, 1.3 Hz, H-m), 7.36 (1H, dd, J ¼ 7:4, 1.3 Hz,
H-6), 7.14 (1H, dt, J ¼ 7:4, 1.3 Hz, H-4), 7.10 (1H, dt,
J ¼ 7:4, 1.3 Hz, H-5), 7.02 (1H, dd, J ¼ 7:4, 1.3 Hz,
H-3), 4.05 (2H, t, J ¼ 7:4 Hz, NCH2), 2.42 (2H, qn,
J ¼ 7:4, NCH2CH2) ppm. 13C NMR (75 MHz, CDCl3)
d: 149.7 (C-1), 138.5 (C-2), 134.2 (C-o), 130.9 (C-p),
128.6 (C-m), 122.3 (C-4), 120.8 (C-5), 112.8 (C-6), 109.6
(C-3), 41.0 (NCH2), 30.8 (NCH2CH2) ppm. 11B NMR
(96 MHz, CDCl3) d: 34.1 ppm (h1=2 ¼ 248 Hz). Ele-
mental Anal. Calc: C, 75.35; H, 5.58; N, 6.51%. Found:
C, 75.05; H, 5.60; N, 6.62%.
2.2.2.8. 2-[(1-((3-(5-Hydroxy)aza)pentyl)imine)ethyl]
phenol (3a). Compound 3a was prepared from 0.50 g
(3.67 mmol) of 2-hydroxyacetophenone and 0.38 g (3.67
mmol) of 2-(2-aminoethyl) aminoethanol in 20 ml of
ethanol. After 30 min under reflux, the solvent was re-
moved under high vacuum. A yellow solid was obtained
and washed with hexane with yield of 98% (0.80 g, 3.60
mmol). M.p. ¼ 102–104 °C, IR (KBr)m ¼ 3510 (OH),
3248, 2924, 2860, 1614 (C@N), 1500, 1448, 1058, 772,
742 cmꢁ1. MS (EI, 15 eV, DIP) m=z (%): 222 (Mþ, 100),
1
204 (48), 177 (56), 134 (43), 120 (17), 77 (32). H NMR

V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
2597
(400 MHz, CDCl3) d: 7.49 (1H, dd, J ¼ 7:5, 1.0 Hz, H-
3), 7.27 (1H, ddd, J ¼ 8:4, 7.5, 1.0 Hz, H-5), 6.91 (1H, d,
J ¼ 8:4 Hz, H-6), 6.74 (1H, td, J ¼ 7:5, 1.0 Hz, H-4),
3.65–3.70 (4H, m, H-8, H-11), 3.03 (2H, t, J ¼ 5:1 Hz,
H-10), 2.84 (2H, t, J ¼ 5:1 Hz, H-9), 2.36 (3H, s, H–Me)
ppm. 13C NMR (100 MHz, CDCl3) d: 172.6 (C-7), 164.7
(C-1), 132.8 (C-5), 128.0 (C-3), 119.2 (C-6), 118.9 (C-2),
116.9 (C-4), 60.9 (C-11), 51.1 (C-9), 49.1 (C-8, C-10),
14.6 (Me) ppm.
H-11a), 3.76 (1H, ddd, J ¼ 14:3, 10.8, 4.3 Hz, H-11b),
3.66 (1H, dt, J ¼ 14:3, 6.0, 3.1 Hz, H-8b), 3.27 (1H, dt,
J ¼ 12:5, 7.3, 2.9 Hz, H-10a), 3.05 (1H, ddd, J ¼ 14:3,
10.8, 4.3 Hz, H-10b), 2.98 (1H, ddd, J ¼ 14:3, 7.6, 4.0
Hz, H-9a), 2.82 (1H, ddd, J ¼ 14:3, 6.0, 3.2 Hz, H-9b),
2.54 (3H, s, H-Me) ppm. 13C NMR (100 MHz, CDCl3)
d: 170.6 (C-7), 160.5 (C-1), 130.9 (C-5), 128.2 (C-3),
127.5 (C-o), 127.3 (C-m), 126.9 (C-p), 120.5 (C-6),118.9
(C-2), 118.4 (C-4), 67.9 (C-11), 62.8 (C-9), 51.0 (C-8),
49.6 (C-10), 16.5 (C–Me) ppm. 11B NMR (128 MHz,
CDCl3) d: +7.9 ppm (h1=2 ¼ 1032 Hz).
2.2.2.9. 2-[(1-((3-(5-Hydroxy)oxa)pentyl)imine)ethyl]
phenol (3b). Compound 3b was prepared using the
procedure described for 3a, from 0.50 g (3.67 mmol) of
2-hydroxyacetophenone and 0.38 g (3.67 mmol) of 2-(2-
aminoethoxy) ethanol. The solution was allowed to
stand overnight to give a yellow solid with yield of 93%
(0.76 g, 3.40 mmol). M.p. ¼ 45–46 °C. MS (EI, 15 eV,
DIP) m=z (%): 223 (Mþ, 100), 205 (58), 178 (45), 134
(38), 120 (26), 77 (31). IR (KBr)m ¼ 3382 (OH), 2870,
2.2.3.2. Bis[l-[2-[[[3-(5-hydroxy-kO)oxa] pentyl] im-
ine-kN] ethylphenolate(2-)-kO]] diboron (4b). Com-
pound 4b was prepared from 0.20 g (0.89 mmol) of 3b
and 0.11 g (0.89 mmol) of phenylboronic acid. A yellow
solid was obtained with yield of 89% (0.24 g, 0.38
mmol). M.p.
m=z (%): 541 (Mþ–C6H5, 34), 309 (78), 232(100), 77 (27).
IR (KBr)m ¼ 3004, 2918, 1622 (C@N), 1558, 1456, 1380,
¼ 110–115 °C. MS (EI, 15 eV, DIP)
ðdecompÞ
1
1620 (C@N), 1614, 1580, 1454, 1128, 756, 486 cmꢁ1. H
1
NMR (300 MHz, CDCl3) d: 7.48 (1H, dd, J ¼ 8:0, 1.3
Hz, H-3), 7.27 (1H, ddd, J ¼ 8:4, 7.1, 1.3 Hz, H-5), 6.90
(1H, dd, J ¼ 8:4, 1.2 Hz, H-6), 6.72 (1H, ddd, J ¼ 8:0,
7.1, 1.3 Hz, H-4), 3.81 (2H, t, J ¼ 4:7 Hz, H-11), 3.76
(2H, t, J ¼ 4:7 Hz, H-10), 3.73 (2H, t, J ¼ 4:8 Hz, H-8),
1350, 1278, 1188, 1140, 750, 704 cmꢁ1. H NMR (400
MHz, CDCl3) d: 7.50 (3H, d, J ¼ 7:7 Hz, H-3, H-o),
7.44 (1H, t, J ¼ 7:7 Hz, H-5), 7.20–7.26 (3H, m, H-m,
-p), 7.04 (1H, d, J ¼ 7:7 Hz, H-6), 6.84 (1H, t, J ¼ 7:7
Hz, H-4), 4.29 (1H, ddd, J ¼ 14:4, 8.7, 4.6 Hz, H-8a),
4.09 (1H, ddd, J ¼ 14:3, 9.7, 4.8 Hz, H-10a), 4.00 (1H,
ddd, J ¼ 14:3, 9.7, 4.8 Hz, H-11a), 3.76–3.88 (3H, m, H-
9b, H-10b, H-11b), 3.63 (1H, ddd, J ¼ 14:4, 8.7, 4.6 Hz,
H-9a), 3.57 (1H, ddd, J ¼ 14:4, 9.7, 4.6 Hz, H-8b), 2.47
(3H, s, H-Me) ppm. 13C NMR (100 MHz, CDCl3) d:
171.3 (C-7), 160.6 (C-1), 136.6 (C-5), 131.2 (C-o), 128.4
(C-3), 127.5 (C-m), 126.9 (C-p), 120.3 (C-6), 118.4 (C-4),
117.7 (C-2), 74.0 (C-9), 68.5(C-11), 62.8 (C-10), 48.1 (C-
8), 16.6 (Me) ppm. 11B NMR (128 MHz, CDCl3) d: +7.7
ppm (h1=2 ¼ 254 Hz).
3.64 (2H, t, J ¼ 4:8 Hz, H-9), 2.34 (3H, s, Me) ppm. 13
C
NMR (75 MHz, CDCl3) d: 173.0 (C-7), 165.9 (C-1),
133.4 (C-5), 128.5 (C-3), 119.9 (C-6), 119.0 (C-2), 116.9
(C-4), 73.2 (C-9), 70.3 (C-11), 62.1 (C-10), 48.7 (C-8),
14.9 (Me) ppm.
2.2.3. Synthesis of compounds 4a, 4b, 5a and 5b
Compounds 4a and 4b were synthesized from equi-
molar quantities of 3a and 3b with phenylboronic acid in
30 ml of THF. The mixture was refluxed for 30 min,
then the water formed during the reaction and part of
the solvent were removed using a Dean-Stark trap and,
finally the product was dried under high vacuum.
Compounds 5a and 5b were prepared by the same
method refluxing for 6 h instead of 30 min.
2.2.3.3. 2-Phenylbenzo[k]-9-aza-1,3,6-trioxa-2-boracy-
clodoceca-9-ene (5a). Compound 5a was prepared
from 0.50 g (2.22 mmol) of 3b and 0.27 g (2.22 mmol) of
phenylboronic acid. A yellow solid was obtained with
yield of 61% (0.42 g, 1.36 mmol). M.p.
¼ 250–252
ðdecompÞ
2.2.3.1. Bis[l-[2-[[[3-(5-hydroxy-kO)aza] pentyl] im-
ine-kN] ethylphenolate (2-)-kO]] diboron (4a). Com-
pound 4a was prepared from 0.20 g (0.89 mmol) of 3a
and 0.10 g (0.89 mmol) of phenylboronic acid. A yellow
solid was obtained with yield of 75% (0.21 g, 0.34
°C, IR (KBr)m ¼ 3408, 3172, 2918, 1622 (C@N), 1526,
1136, 1128, 1066, 800, 568 cm)1. MS (EI, 15 eV, DIP)
m=z (%): 232 ([M–C6H5]þ, 20), 188 (6), 129 (8), 114 (15),
110 (10), 97 (15), 73(12), 44 (22), 30 (100). 1H NMR (300
MHz, CDCl3) d: 7.54 (1H, d, J ¼ 8:1, 1.2 Hz, H-3), 7.45
(1H, td, J ¼ 8:1, 1.2 Hz, H-5), 7.40 (2H, dd, J ¼ 7:3, 1.7
Hz, H-o), 7.20 (3H, m, H-m, -p), 7.01 (1H, dd, J ¼ 8:1,
1.2 Hz, H-6), 6.84 (1H, td, J ¼ 8:1, 1.2 Hz, H-4), 4.31
(1H, ddd, J ¼ 9:9, 5.8, 4.0 Hz, H-8a), 4.07 (1H, ddd,
J ¼ 10:2, 5.8, 2.8 Hz, H-10a), 4.05 (1H, ddd, J ¼ 9:9,
5.8, 4.0 Hz, H-9a), 3.90 (1H, ddd, J ¼ 13:2, 9.9, 4.0 Hz,
H-9b), 3.87(1H, ddd, J ¼ 15:1, 10.2, 2.8 Hz, H-11a),
3.78(1H, ddd, J ¼ 15:1, 10.2, 5.8 Hz, H-10b), 3.66 (1H,
ddd, J ¼ 10:2, 5.8, 2.8 Hz, H-11b), 3.62 (1H, ddd,
J ¼ 13:2, 9.9, 5.8 Hz, H-8b), 2.58 (3H, s, H–Me) ppm.
mmol). M.p.
¼ 120–123 °C. IR (KBr)m ¼ 3046,
ðdecompÞ
3006, 1616 (C@N), 1456, 1276, 750, 706 cmꢁ1. MS (EI,
15 eV, DIP) m=z (%): 539 (Mþ–C6H5, 34), 435 (50), 407
(35), 313 (24), 308 (81), 256 (15), 257 (17), 231 (100), 213
1
(11), 172 (21), 160 (42), 77 (35). H NMR(400 MHz,
CDCl3) d: 7.52 (1H, dd, J ¼ 7:7, 1.5 Hz, H-3), 7.43 (1H,
dt, J ¼ 7:7, 1.5 Hz, H-5), 7.36–7.42 (2H, m, H-o), 7.16–
7.22 (2H, m, H-m, H-p), 7.01 (1H, d, J ¼ 7:7 Hz, H-6),
6.83 (1H, t, J ¼ 7:7 Hz, H-4), 4.11 (1H, ddd, J ¼ 14:3,
7.6, 4.0 Hz, H-8a), 3.95 (1H, ddd, J ¼ 12:5, 7.3, 2.9 Hz,

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V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
13C NMR (75 MHz, CDCl3) d: 171.6 (C-7), 160.9 (C-1),
137.0 (C-5), 131.4 (C-o), 128.7 (C-6), 127.7 (C-m), 127.3
(C-p), 120.7 (C-6), 118.7 (C-4), 118.0 (C-2), 74.4 (C-9),
68.8(C-11), 63.1 (C-10), 48.1 (C-8), 17.0 (Me) ppm. 11B
NMR (96 MHz, DMSO-d6) d: +7.5 ppm (h1=2 ¼ 192
Hz). Elemental Anal. Calc.: C, 69.90; H, 6.47; N, 4.53%.
Found: C, 69.66; H, 6.54; N, 4.38%.
Elemental Anal. Calc.: C, 68.38; H, 6.27; N, 3.99%.
Found: C, 68.66; H, 6.12; N, 4.08%.
3. Results and discussion
Ligands 1a–1d were prepared by reaction of o-ami-
nophenol with glyoxal derivatives using the methodol-
ogy described in the literature [11]. Compounds 1e–1g
were synthesized from one equivalent of 1,3-dibromo-
propane and two equivalents of the corresponding or-
tho-aminophenol as described in detail in the
experimental part. The reactions of ligands 1a–1g with
two equivalents of phenylboronic acid were carried out
under reflux in xylene. After 6 h, the reaction mixture
was cooled down to room temperature and the solvent
was removed under high vacuum. The product was
washed with several portions of a hexane:chloroform
(9:1) mixture, to give the oxazaborolidines 2a–2g as gray
or white solids, which were fully characterized by spec-
troscopic methods (Scheme 1).
The formation of this type of compounds requires the
use of a solvent with a high boiling point (e.g. xylene), in
order to replace the amine hydrogen atom during the
reaction of the ligand with phenylboronic acid, and to
form a covalent boron–nitrogen bond after water elim-
ination [28,29].
The structures of oxazaborolidines 2a–2g were first
established by mass spectrometry, which in all seven
cases showed the molecular ion as the base peak. The
NMR spectra showed the symmetric nature of these
compounds, since only one set of signals was observed,
due to the presence of a C2 fold axis. The 11B NMR
spectra of compounds 2a–2g showed a single broad
2.2.3.4. 2-(4-Acetyl)phenyl-10-methylbenzo[k]-9-aza-
1,3,6-trioxa-2-boracyclododeca-9-ene (5b). Compound
5b was prepared from 0.50 g (2.22 mmol) of 3b and 0.36
g (2.22 mmol) of 4-acetylphenylboronic acid. A pale
yellow solid was obtained with yield of 71% (0.56 g, 1.59
mmol). M.p. ¼ 211–213 °C. IR (KBr)m ¼ 3568, 2944,
2860, 1674 (C@O), 1624 (C@N), 1560, 1290, 1268, 1178,
1152, 1134, 1114, 1004, 952, 804, 762. MS (EI, 15 eV,
DIP) m=z (%): 351 (Mþ, 6), 308 (10), 232 ([M–
C6H4COCH3]þ, 100), 202 (25), 188 (70), 175 (15), 146
(11). 1H NMR (300 MHz, CDCl3) d: 7.78 (2H, d,
J ¼ 8:1 Hz, H-m), 7.52 (3H, d, J ¼ 8:1 Hz, H-3, H-o),
7.44 (1H, td, J ¼ 8:1, 1.5 Hz, H-5), 7.00 (1H, dd,
J ¼ 8:1, 1.5 Hz, H-6), 6.84 (1H, td, J ¼ 8:1, 1.5 Hz, H-
4), 4.24 (1H, ddd, J ¼ 13:1, 10.1, 5.3 Hz, H-8a), 4.00–
4.10 (2H, m, H-10a, 11a), 3.80 (1H, ddd, J ¼ 13:1, 10.1,
2.8 Hz, H-9a), 3.67 (1H, ddd, J ¼ 13:1, 5.3, 2.8 Hz, H-
8b), 3.62 (1H, dd, J ¼ 13:1, 5.3 Hz, H-9b), 3.20– 3.35
(2H, m, H-10b, 11b), 2.59 (3H, s, Me–C@N), 2.52 (3H,
s, Me–C@O) ppm. 13C NMR (75 MHz, CDCl3) d: 198.0
(CO), 171.8 (C-7), 160.4 (C-1), 137.0 (C-5), 135.9 (C-p),
131.4 (C-m), 128.5 (C-3), 127.4 (C-o), 120.4 (C-6), 118.8
(C-4), 117.7 (C-2), 74.1 (C-9), 68.5 (C-11), 62.9 (C-10),
48.1 (C-8), 26.7 (Me–CO), 17.0 (Me–C@N) ppm. 11B
NMR (96 MHz, CDCl3) d: +7.2 ppm (h1=2 ¼ 192 Hz).
1
R
2
R
4
5
(CH2)n
3
1
R1
6
HN
HO
R
R
NH
OH
B
PhB(OH)2
O
2
1
N
N
(CH2)n
O
2
B
R2
Xylene, ∆ 6h
o
2
R
1
R
m
p
R1
H
Me
H
R2
H
H
Me
H
H
n
2
2
2
2
3
3
3
1a-1g
2a-2g
a:
b:
c:
d:
e:
f:
t
-Bu
H
Me
H
H
Me
g:
Scheme 1. Synthesis of the bisoxazaborolidines 2a–2g.

V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
2599
ꢁ
signal in the range from 32.0 to 34.4 ppm, characteristic
for tricoordinated species having two donor atoms
coupled to an aromatic system [30].
tances are 1.397(6) and 1.415(6) A, and the B–O dis-
ꢁ
tances are 1.400(5) and 1.394(6) A. Both are shorter
than the corresponding distances in aliphatic derivatives
[28,31], due to B–N and B–O p interactions. Based on
the trigonal environment of the boron atom, one would
expect angles close to 120°, however, due to the high
annular tension in the five-membered heterocycle, the
O(1)–B(1)–N(1) and O(2)–B(2)–N(2) angles are smaller,
108.3(4)° and 109.2(4)°, respectively, while the N(1)–
B(1)–C(16) and N(2)–B(2)–C(22) angles are larger,
131.2° and 132.5°, respectively.
1
The H NMR spectra of compounds 2a–2d show a
singlet in the range between 4.20 and 4.34 ppm. Meth-
ylene groups attached to the nitrogen atom (NCH2) in
compounds 2e–2g were observed as triplets between 4.00
and 4.05 ppm, while the adjacent NCH2CH2 groups
appear as a quintuplet at 2.37–2.42 ppm (H-11). The
aromatic protons appear in the range of 6.75–7.36 ppm
in all compounds. In general, the signals are shifted to
higher field in comparison to those of the free ligands
due to the presence of a boron atom.
The 13C NMR data also support this trend and 2D
NMR experiments were carried out to obtain the com-
plete assignment of the compounds. The signals for the
boron derivatives are shifted to higher field with respect
to the starting materials.
The structure of 2e was established by X-ray crys-
tallography. From the structure shown in Fig. 1, it can
be noticed that the symmetry of the molecule observed
in solution is not present in the solid state. The phenyl
groups attached to the boron atoms are in a trans
disposition with respect to each other to give a pseudo-
C2-symmetry. The average values for the O(1)–B(1)–
C(16)–C(17) and N(1)–B(1)–C(16)–C(21) torsion angles
are 30.15° and 34.20°, respectively, indicating that the
phenyl groups are not coplanar to the five-membered
heterocycle, most probably because of a steric effect
between the hydrogen atoms of the phenyl group and
those of the CH2 group from the backbone. The five-
membered ring is almost planar. The maximum devia-
The tetradentate ligands 3a and 3b were also prepared
for comparison and their reaction with phenylboronic
acid was carried out, since it was anticipated that, in this
case, the iminic group would lead to a different struc-
ture. The reaction mixture was refluxed for 30 min under
THF (Scheme 2). Then, the solvent was removed under
high vacuum to give a yellow solid, which corresponded
to the dimeric compounds 4a–4b, as confirmed by mass
spectrometry. In both cases a peak corresponding to the
loss of a phenyl group from the molecular ion was ob-
served, in analogy to dimeric compounds previously
described [32]. The compounds are hygroscopic and
decompose at room temperature to give a highly viscous
liquid (polymeric compound). Nevertheless, it can be
stored at )20° without apparent decomposition.
1
The H NMR spectra allowed to establish the sym-
metry (Ci) of the molecule, since only one set of signals
was observed, in agreement with similar dimeric struc-
tures [20,32]. The methylene protons are diastereotopic
owing to ring closure and a change in hybridization of the
boron atom from sp2 to sp3, which leads to a new stere-
ogenic center in the molecule. The 13C NMR spectra also
confirm the symmetric nature of the molecules showing
signals for half of the molecule. Two dimensional NMR
techniques allowed full characterization of the complexes.
The carbon signals for the C@N groups are shifted to
higher field with respect to the ligands. The 11B NMR
spectra confirm the tetrahedral geometry of the boron
atoms, giving signals at 7.9 and 7.7 ppm for 4a and 4b,
respectively. The large cavity (16 membered central ring)
of the dimeric compounds makes them interesting can-
didates for application in host-guest chemistry, unfortu-
nately we were unable to grow crystals suitable for X-ray
analysis, so the dimensions of this cavity are unknown.
When the reaction time between ligand 3b and phen-
ylboronic acid was enhanced from 30 min to 6 h, the
monomeric structure 5a was isolated in good yields ra-
ther than compound 4b (Scheme 3). This compound was
fully characterized by spectroscopic methods and it is
stable at room temperature. Also, the monomeric com-
pound 5b was obtained from 4-acetylboronic acid and
the ligand 3b (Scheme 3), using the same reaction con-
ditions as in the case of 4b formation, the product is a
yellow solid stable at room temperature. In the last case,
the dimeric compound was not observed which seems to
ꢁ
tion from the mean plane is 0.051 A for boron atom.
This is in accordance with a delocalization of the p
electron density within the heterocycle. The B–N dis-
ꢁ
Fig. 1. Crystal structure view of 2e. Selected bond lengths (A) and angles
(°): B(1)–N(1) 1.415(6), B(1)–O(1) 1.400(5), B(1)–C(16) 1.530(7), O(1)–
C(2) 1.374(5), N(1)–C(1) 1.410(5), N(1)–C(7) 1.465(5), O(1)–B(1)–C(16)
120.5(4), O(1)–B(1)–N(1) 108.3(4), N(1)–B(1)–C(16) 131.2(4).

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V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
3
8
11
7
10
X
9
X
O
X
4
5
PhB(OH)2
O
B
N
N
B
2
OH
N
O
THF, ∆ 1/2 h
1
O
o
OH
6
m
p
3a X = NH, 3b X = O
4a X = NH, 4b X = O
Scheme 2. Synthesis of the dimeric boron complexes 4a and 4b.
be the kinetic product, while the monomeric specie is the
thermodynamic favorable specie. A similar observation
was made previously for a related boronate derivative
[33]. The reaction of ligand 3a under the same conditions
failed to provide a monomeric structure, even when
different reaction conditions were used.
C@N group in 4a (1616 cmꢁ1) is also shifted to higher
wavenumbers compared with the ligand 3a (1614 cmꢁ1).
An X-ray crystallographic analysis confirmed the
molecular structure of 5b (Fig. 2). Crystallographic data
are listed in Table 1. The structure has one six-membered
and one eight-membered heterocyclic ring. Both het-
erocycles are linked by a dative N–B bond. The boron
atom is deviated from the mean plane of the six-mem-
The base peak in the mass spectrum of 5a corresponds
to the [Mþ–C6H5] ion and confirms the monomeric
structure for this compound. For compound 5b, the
molecular ion was observed at m=z ¼ 351 and the base
peak was detected also at m=z ¼ 232, indicating again the
loss of one of the aryl groups attached to the boron at-
oms. The NMR data for 5a and 5b are quite similar to 4b,
proton signals were unambiguously assigned by using 2D
1H NMR techniques. In the 1H NMR spectrum, the
signals for the CH2 groups are diastereotopic due to the
formation of the eight-membered ring. The 11B NMR
shifts (d ¼ 7:5 and 7.2 ppm) are quite similar to those
observed in analogous dimeric compounds [32,33].
The IR spectra of compounds 4b and 5a show a
strong band at 1622 cmꢁ1, attributable to the stretching
band for the C@N groꢁu1p. This band is shifted to higher
wavenumbers by 2 cm compared to the same band in
the ligand 3b (1620 cmꢁ1) due to the coordination of the
nitrogen atom to boron. In compound 5b, this band was
shifted to higher wavenumbers (1624 cmꢁ1) due to the
presence of an electron withdrawing group (COMe) in
para position. In analogy, the band observed for the
ꢁ
bered ring by 0.54 A, therefore the ring is not planar, and
an envelope conformation was observed. In the eight-
membered ring, a twist boat conformation was observed.
A distorted tetrahedral geometry of the boron atoms is
evident from the bond angles around the boron atom,
which are in the range from 105.76(14)° to 111.99(15)°.
ꢁ
The B–N distance is 1.633(2) A and is quite similar to
analogous systems [18–20,32,33]. The C–B distance
ꢁ
(1.625(3) A) is slightly longer compared to related
compounds that have an aryl moiety attached to the
boron atom. This is attributed to the presence of an
electron withdrawing group (COMe) [20]. The two B–O
bonds are different, the one in the six-membered ring has
ꢁ
a distance of 1.475(2) A, while the one in the eight-
ꢁ
membered ring is 1.431(2) A.
3.1. Conclusions
The reactivity of different tetradentate ligands with
arylboronic acids has been described. It has been noticed
8
9
3
6
7
4
5
ArB(OH)2
O
O
N
B
2
OH
N
10
THF, ∆ 6 h
1
O
O
11
OH
o
m
R
5a R = H, 5b R = MeCO
Scheme 3. Synthesis of the monomeric boron complexes 5a and 5b.

V. Barba et al. / Inorganica Chimica Acta 357 (2004) 2593–2601
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angles (°): B(2)–O(9) 1.475(2), B(2)–O(1) 1.431(2), B(2)–N(3) 1.633(2),
B(2)–C(18) 1.625(3), O(9)–C(10) 1.341(2), O(1)–C(8) 1.425(2), N(3)–
C(16) 1.295(2), N(3)–C(4) 1.470(2), O(9)–B(2)–O(1) 111.13(16), O(9)–
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110.61(15), O(9)–B(2)–C(18) 111.99(15), N(3)–B(2)–C(18) 107.18(14),
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deprotonated to form an N–B covalent bond. Never-
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coordinative bond is formed to give tetrahedral boron
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lead to both monomeric and dimeric compounds. Mo-
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Acknowledgements
A
Structure Refinement, University of Goettingen, Germany,
1997.
This work was supported by Consejo Nacional de
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ꢀ
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