Synthesis and comparative study of throe monomeric boronates by spectroscopic methods and X-ray crystallography

Article abstract of DOI:10.1139/v98-181

Three boronates have been synthesized by reaction of tridentate azomethine ligands derived from salicylaldehyde with phenylboronic acid. The [5.4.0], [4.4.0], and [4.3.0]heterobicyclic structures obtained have been characterized by NMR spectroscopy and X-

Full text of DOI:10.1139/v98-181

1352
Synthesis and comparative study of three
monomeric boronates by spectroscopic methods
and X-ray crystallography1
Herbert Höpfl, Mario Sánchez, Norberto Farfán, and Victor Barba
Abstract: Three boronates have been synthesized by reaction of tridentate azomethine ligands derived from
salicylaldehyde with phenylboronic acid. The [5.4.0], [4.4.0], and [4.3.0]heterobicyclic structures obtained have been
characterized by NMR spectroscopy and X-ray crystallography, whereby it could be proved that the
[4.4.0]heterobicycle is the most stable one. Unexpectedly the salicylideneaminoethanol derivative does not lead to a
macrocyclic structure, although molecular modeling reveals high ring strain for the monomeric product that is actually
obtained.
Key words: boronates, boron complexes, tridentate azomethine ligands, phenylboronic acid, X-ray crystallography.
Résumé : On a synthétisé trois boronates par réaction de l’acide phénylboronique avec des ligands azométhines
tridentates dérivés du salicylaldéhyde. On a caractérisé les structures hétérobicycliques [5.4.0], [4.4.0], et [4.3.0] par
spectroscopie RMN et par diffraction des rayons X; on a ainsi pu prouver que l’hétérobicycle [4.4.0] est le plus stable.
D’une manière inattendue, le dérivé salicylidèneaminoéthanol ne conduit pas à une structure macrocyclique, même si la
modélisation moléculaire indique l’existence d’une forte tension de cycle pour le produit monomère qui est en fait
obtenu.
Mots clés : boronates, complexes du bore, ligands azométhines tridentates, acide phénylboronique, diffraction des
rayons X.
[Traduit par la Rédaction] Höpfl et al. 1360
fluence the formation of monomeric or dimeric boronates
with phenylboronic acid was also reported (12). Thereby, it
During our studies of boron complexes with a coordina-
tive N→B bond (1–10), we recently discovered the synthesis
of two new macrocyclic structures that consist of a dimeric
and a tetrameric boronate ring system of 20 and 10 mem-
bers, respectively (1 and 2a in Fig. 1) (11). Both molecules
are stable in the air and are obtained in high yields in one-
step syntheses between the corresponding tridentate ligands
and phenylboronic acid. The molecular structures of both
macrocycles have been studied in detail by X-ray crystallog-
raphy (11), and it could be shown that monomeric systems
would be more strained due to the tetrahedral geometry at
the boron atom and its small atomic radius. A detailed study
of how different substituents in salicylideneaminoethanol in-
was shown that the dimeric structure type 2 is preferred over
the [4.3.0]heterobicyclic type 3 in the absence of steric ef-
fects that preclude formation of the macrocycle (Fig. 1).
This is the case in compounds 3a and 3b, where two methyl
groups on the carbon atom adjacent to the nitrogen inhibit
the dimer formation (compare also to 2d).
In search of new tridentate ligands that could be probable
promoters of macrocycle formation with phenylboronic acid
it was considered promising to fix the carbon–carbon bond
of the 2-ethanolamine group in the ligand by a double bond
using 2-aminophenol.
Received April 1, 1998.
Instrumentation
H. Höpfl2 and M. Sánchez. Universidad Autónoma del
Estado de Morelos, Centro de Investigaciones Químicas, Av.
Universidad 1001, C.P. 62210 Cuernavaca, Morelos, México.
N. Farfán2 and V. Barba. Departamento de Quimica, Centro
de Investigación y de Estudios Avanzados del IPN, Apdo
Postal 14–740, C.P. 07000 México D.F., México.
The NMR studies were performed with the following
spectrometers: Jeol GSX 270 and Eclipse +400. Special
techniques (COSY, HETCOR) were applied when necessary
to assign the spectra adequately. Standards were TMS (1H,
13C) and BF3·OEt2 (11B). Chemical shifts are stated in ppm;
they are positive, when the signal is shifted to higher
frequencies than the standard. The IR spectra have been
recorded with a Perkin Elmer 16F-PC FT-IR spectrophoto-
meter. Mass spectra were obtained with an HP 5989 A
1Dedicated to the memory of Dr. Rigoberto García-Cantú.
2Authors to whom correspondence may be addressed. HH:
Telephone: +52 73 29 7020. Fax: +52 73 29 7040. E-mail:
hhopfl@servm.fc.uaem.mx; NF: Telephone: +52 747 3800,
ext. 4031. Fax: +52 5 747 7113.
equipment. Melting points were determined with
a
E-mail: jfarfan@mvax1.red.cinvestav.mx
Gallenkamp MFB-595 apparatus and have not been corrected.
Can. J. Chem. 76: 1352–1360 (1998)
© 1998 NRC Canada

Höpfl et al.
1353
Fig. 1. Tetrameric, dimeric, and monomeric boronates obtained with different substituents.
O
R1
O
R1 R2
N
B
R3
O
R2
R2
N
O
Ph
N
Ph
N
B
O
B
O
B
B
N
O
B
O
O
N
O
O
O
Ph
Ph
O
N
R3
R2
B
R1
O
1
R1
H
H
H
H
Me
R2
H
H
H
Me
H
R3
H
Me
Ph
Ph
H
R1 R2
2a
2b
2c
2d
2e
3a
3b
H
Me
Me Me
X-ray diffraction studies of monocrystals were determined
on an Enraf–Nonius CAD4 diffractometer (λMoK = 0.71069
Å, monochromator: graphite, T = 293 K for 5a αand 5c, T =
193 K for 5b, ω–2θ scan). Crystals were mounted in LINDE-
MAN tubes. Cell parameters were determined by least-
squares refinement on diffractometer angles for 24 automati-
cally centered reflections in the θ range of 10–11°. Crystals
of compound 5a diffracted only weakly so that reflections
were only collected up to θ = 24°. Absorption correction was
not necessary, corrections were made for Lorentz and polar-
ization effects. Solution and refinement: direct methods
(SHELXS-86) for structure solution and the CRYSTALS
(version 9, 1994) software package for refinement and data
output. Non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were determined by difference Fourier
maps, and their positions as well as one overall isotropic
thermal parameter were refined; I > 3σ(I); (R = Σ(||Fo| –
|Fc||)/Σ|Fo|, Rw = [Σw(|Fo| – |Fc|)2/ΣwFo2]1/2. In all cases only
independent reflections on the basis of Friedel’s law have
been collected and a reflection–parameter ratio >5 has been
considered sufficient for the type of structural studies per-
formed here.
2-[[(2-Hydroxyphenyl)methylene]imino]benzenemethanol
(4b)
Compound 4b was prepared from 1.00 g (8.13 mmol) of
2-aminobenzyl alcohol and 1.00 g (8.19 mmol) of salicylal-
dehyde. The product obtained is an orange solid (1.63 g,
7.65 mmol); mp 138–140°C; yield 76%. 1H NMR
(270 MHz, CDCl3), δ (ppm): 4.82 (2H, AB, OCH2), 6.91–
7.51 (8H, m, aromat.), 8.57 (1H, s, H-7); 13C NMR
(67.8 MHz, CDCl3), δ (ppm): 61.8 (OCH2), 117.3, 118.0,
119.2 (C-3, C-5, C-6′), 118.1 (C-1), 127.1, 128.3, 128.8 (C-
3′, C-4′, C-5′), 132.4, 133.4 (C-4, C-6), 134.5 (C-2′), 146.6
(C-1′), 161.0 (C-2), 163.1 (C-7). Infrared (KBr): 3260
(br, m), 2912 (m), 2862 (m), 1618 (s), 1594 (w), 1576 (m),
1570 (m), 1560 (m), 1505 (w), 1498 (w), 1432 (m), 1456
(m), 1368 (w), 1280 (m), 1230 (w), 1202 (m), 1178 (m),
1162 (w), 1150 (w), 1114 (w), 1106 (w), 1032 (s) cm–1.
2-[[(2-Hydroxyphenyl)methylene]amino]benzeneethanol
(4c)
Compound 4c was prepared from 0.90 g (7.32 mmol) of
2-aminophenethyl alcohol and 0.90 g (7.32 mmol) of
salicylaldehyde. The product obtained is an orange oil
(1.64 g, 6.81 mmol); yield 93%. 1H NMR (270 MHz,
CDCl3), δ (ppm): 2.95 (2H, t, J = 7 Hz, OCH2CH2), 3.73
(2H, t, J = 7 Hz, OCH2), 6.83–7.30 (8H, m, aromat.), 8.42
(1H, s, H-7); 13C NMR (67.8 MHz, CDCl3), δ (ppm): 35.3
(OCH2CH2), 62.3 (OCH2), 117.0, 117.9, 118.9 (C-3, C-5, C-
6′), 119.1 (C-1), 126.8, 127.5, 130.6 (C-3′, C-4′, C-5′), 132.6
(C-2′), 132.1, 133.0 (C-4, C-6), 147.0 (C-1′), 160.9 (C-2),
162.5 (C-7). Infrared (NaCl): 3396 (br, s), 3060 (w), 3024
(w), 2932 (w), 2878 (w), 1616 (s), 1594 (m), 1570 (m), 1498
(w), 1482 (m), 1456 (w), 1404 (w), 1364 (w), 1278 (m),
1200 (w), 1178 (m), 1150 (m), 1106 (w), 1046 (w), 910
(m) cm–1. EI mass spectrum (70 eV), m/z: 241 (100%, M),
222 (47%), 210 (39%), 130 (26%), 117 (35%), 91 (43%), 77
(26%), 65 (44%), 31 (25%).
Reagents
All starting materials were commercial. Solvents were
used without further purification, but monocrystals were
grown from spectrophotometric grade solvents.
General procedure for the preparation of ligands 4a–4c
To prepare the tridentate ligands 4a–4c, equimolar quanti-
ties of the corresponding aminophenol or aminoalcohol and
salicylaldehyde were refluxed in benzene for 30 min. The
solvent and the water formed during the reaction were then
removed by a Dean Stark trap to yield an oil or a solid that
was washed with chloroform and used without further puri-
fication. The preparation and spectroscopic data of 4a have
already been published (13–17).
General method for the preparation of compounds 5a–
5c
An equimolar amount of phenylboronic acid was added to
a solution of the tridentate ligand in benzene (usually about
10 mL for 2–3 mmol). The mixture was refluxed for 30 min
and quantified by water and solvent separation with a Dean
2-[[(2-Hydroxyphenyl)imino]methyl]phenol (4a)
Compound 4a was prepared from 0.90 g (8.19 mmol) of
2-aminophenol and 1.00 g (8.19 mmol) of salicylaldehyde.
The product obtained is a yellow solid (1.63 g, 7.65 mmol);
mp 189–191°C; yield 93%.
© 1998 NRC Canada

1354
Can. J. Chem. Vol. 76, 1998
Scheme 1. Synthesis of compounds 5a–5c.
Stark trap. The solid obtained was collected by filtration un-
der vacuum and washed with small amounts of benzene and
hexane.
BC6H5), 7.28 (3H, m, H-4′, o-BC6H5), 7.32 (1H, t, J =
10 Hz, H-5′), 7.36 (2H, m, H-6, H-3′), 7.47 (1H, t, J = 8 Hz,
H-4), 7.62 (1H, d, J = 8 Hz, H-6′), 8.59 (1H, s, H-7); 13C
NMR (100.5 MHz, CDCl3), δ (ppm): 63.9 (OCH2), 118.1
(C-6′), 118.2 (C-1), 120.2 (C-5), 120.9 (C-3), 127.5 (p-
BC6H5), 127.9 (C-4′), 128.2 (m-BC6H5), 128.8 (C-3′), 129.6
(C-5′), 132.1 (o-BC6H5), 132.9 (C-6), 136.5 (C-2′), 139.1 (C-
1′), 139.4 (C-4), 156.7 (C-2, C-7); 11B NMR (128.3 MHz,
CDCl3), δ (ppm): 5 (h1/2 = 140 Hz). Infrared (KBr): 3062
(s), 3008 (s), 2928 (s), 2886 (s), 2846 (s), 2828 (s), 1624 (s),
1584 (w), 1552 (s), 1534 (w), 1474 (m), 1456 (m), 1430 (w),
1390 (w), 1376 (w), 1306 (m), 1246 (w), 1196 (s), 1176 (s),
1152 (m), 1124 (w), 1102 (m), 1048 (w), 1032 (w), 1008
(m), 960 (m), 928 (w) cm–1. EI mass spectrum (70 eV, DIP),
m/z: 313 (5%, M), 236 (100%, M – C6H5), 206 (2%), 165
2-Phenyldibenzo[d,h]-6-aza-1,3,2-dioxaboracyclonon-6-ene
(5a)
Compound 5a was prepared from 0.50 g (2.34 mmol) of
4a and 0.28 g (2.34 mmol) of phenylboronic acid. The prod-
uct obtained is a yellow solid (0.61 g, 2.04 mmol) that is
slightly soluble in chloroform, dichloromethane, methanol,
and DMSO. Crystals suitable for X-ray diffraction were ob-
tained, when the reaction was performed in dichloromethane
at room temperature without stirring; mp 229–231°C; yield
87%. 1H NMR (270 MHz, CDCl3), δ (ppm): 6.90, (1H, t, J =
8 Hz, H-4′), 6.94 (1H, t, J = 8 Hz, H-5), 7.10 (1H, d, J =
8 Hz, H-6′), 7.12 (1H, t, J = 8 Hz, p-BC6H5), 7.15 (2H, m,
m-BC6H5), 7.21 (1H, d, J = 8 Hz, H-3), 7.31 (2H, d, J =
7 Hz, o-BC6H5), 7.38 (3H, m, H-6, H-3′, H-5′), 7.54 (1H, t, J
= 7 Hz, H-4), 8.32 (1H, s, H-7); 13C NMR (67.8 MHz,
CDCl3), δ (ppm): 115.0 (C-3′), 115.2 (C-6′), 119.3 (C-1, C-
4′), 120.3 (C-3, C-5), 127.4 (m-BC6H5), 127.8 (p-BC6H5),
131.2 (C-1′, o-BC6H5), 131.3, 132.5 (C-6, C-5′), 137.9 (C-4),
148.7 (C-7), 157.5, 158.7 (C-2, C-2′); 11B NMR (86.6 MHz,
CDCl3), δ (ppm): 8 (h1/2 = 120 Hz). Infrared (KBr): 3066
(w), 3040 (w), 2930 (w), 1628 (s), 1608 (m), 1576 (w), 1570
(w), 1558 (m), 1550 (m), 1540 (m), 1534 (m), 1522 (w),
1506 (w), 1472 (m), 1466 (s), 1458 (m), 1452 (w), 1436 (w),
1430 (w), 1380 (m), 1326 (m), 1314 (m), 1284 (m), 1256
(w), 1240 (w), 1226 (m), 1190 (s), 1182 (s), 1154 (s), 1122
(m), 1044 (w), 1032 (w), 1012 (s), 954 (s) cm–1. EI mass
spectrum (70 eV, DIP), m/z: 299 (14%, M), 222 (100%, M –
+
(1%), 91 (4%), 77 (10%, C6H5 ), 65 (4%), 51 (6%). Anal.
calcd: C 76.71, H 5.14, N 4.47; found: C 75.37, H 5.10, N
4.31.
2-Phenyldibenzo[f,j]-8-aza-1,3,2-dioxaboracycloundec-8-
ene (5c)
Compound 5c was prepared from 0.50 g (2.07 mmol) of
4c and 0.25 g (2.07 mmol) of phenylboronic acid. The prod-
uct obtained is a yellow solid (0.58 g, 1.77 mmol) that is
slightly soluble in chloroform. Crystals suitable for X-ray
diffraction were obtained, when the reaction was performed
in benzene at room temperature without stirring; mp 206–
1
208°C; yield 85%. H NMR (400 MHz, CDCl3), δ (ppm):
2.94–3.24 (2H, m, OCH2CH2), 4.02–4.36 (2H, m, OCH2),
6.88 (1H, t, J = 7 Hz, H-5), 7.10 (5H, m, H-3, H-6′, m-
BC6H5, p-BC6H5), 7.25 (6H, m, H-6, H-3′, H-4′, H-5′, o-
BC6H5), 7.55 (1H, t, J = 8 Hz, H-4), 8.16 (1H, s, H-7); 13C
NMR (100.5 MHz, CDCl3), δ (ppm): 35.9 (OCH2CH2), 59.6
(OCH2), 117.8 (C-1), 120.0 (C-5), 120.9 (C-3), 124.4 (C-6′),
127.7 (p-BC6H5), 127.9 (m-BC6H5), 128.5 (C-4′), 129.7
(C-5′), 131.6 (C-6), 132.4 (o-BC6H5), 133.1 (C-3′), 135.4
(C-2′), 139.5 (C-4), 145.1 (C-1′), 163.2 (C-2), 164.9 (C-7);
11B NMR (86.6 MHz, CDCl3), δ (ppm): 6 (h1/2 = 170 Hz).
Infrared (KBr): 3020 (w), 2952 (w), 2920 (w), 2862 (w),
1622 (s), 1600 (w), 1552 (m), 1476 (m), 1454 (m), 1370
(w), 1300 (w), 1212 (m), 1196 (m), 1182 (w), 1154 (w),
1130 (m), 1098 (w), 1070 (w), 1056 (w), 1040 (w), 1030
(w), 1016 (w), 942 (m) cm–1. EI mass spectrum (70 eV,
DIP), m/z: 327 (3%, M), 250 (100%, M – C6H5), 235 (6%),
+
C6H5), 193 (2%), 151 (1%), 102 (2%), 77 (26%, C6H5 ), 51
(15%), 39 (3%). Anal. calcd: C 76.30, H 4.71, N 4.68;
found: C 76.31, H 4.62, N 4.63.
2-Phenyldibenzo[e,i]-7-aza-1,3,2-dioxaboracyclodec-7-ene
(5b)
Compound 5b was prepared from 0.50 g (2.20 mmol) of
4b and 0.27 g (2.20 mmol) of phenylboronic acid. The prod-
uct obtained is a yellow solid (0.54 g, 1.72 mmol) that is
slightly soluble in benzene, chloroform, dichloromethane,
methanol, ethanol, and DMSO. Crystals suitable for X-ray
diffraction were obtained, when the reaction was performed
in dichloromethane at room temperature without stirring; mp
224–226°C; yield 79%. 1H NMR (400 MHz, CDCl3), δ
(ppm): 4.98 (2H, m, OCH2), 6.84 (1H, t, J = 7 Hz, H-5),
7.03 (1H, d, J = 8 Hz, H-3), 7.13 (3H, m, m-BC6H5, p-
+
220 (13%), 165 (1%), 103 (1%), 77 (7%, C6H5 ), 51 (5%).
Anal. calcd: C 76.89, H 5.52, N 4.26; found: C 76.48, H
5.67, N 4.33.
© 1998 NRC Canada

Höpfl et al.
1355
1
Table 1. The H and 11B NMR data (270 and 86.6 MHz) for compounds 4a–4c and 5a–5c in CDCl3 (ppm).
B-C6H5
11B NMR
OCH2
Comp.
H-3
H-4
H-5
H-6
H-7
H-3′
H-4′
H-5′
H-6′
o
m
p
(h1/2 (Hz))
4aa
4b
6.86–7.63
6.91–7.51
6.83–7.30
8.97
8.57
8.42
8.32
8.59
8.16
6.86–7.63
6.91–7.51
6.83–7.30
—
4.82
3.73
—
4.98
4.19
—
—
—
7.31
7.28
7.25
—
—
—
7.15
7.13
7.10
—
—
—
7.12
7.13
7.10
—
—
—
4cb
5a
7.21
7.03
7.10
7.54
7.47
7.55
6.94
6.84
6.88
7.38
7.36
7.25
7.38
7.36
7.25
6.90
7.28
7.25
7.38
7.32
7.25
7.10
7.62
7.10
+8 (120)
+5 (140)
+6 (170)
5bc
5cc,d
aIn DMSO-d6.
bOCH2CH2, δ (ppm): 2.95 ppm.
c1H NMR: 400 MHz; 11B NMR: 128.3 MHz.
dOCH2CH2, δ (ppm): 3.09.
Table 2. The 13C NMR data (67.8 MHz) for compounds 4a –4c and 5a –5c in CDCl3 (ppm).
Comp. C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-1′ C-2′ C-3′
C-4′
C-5′
C-6′
116.6b
OCH2
4aa
4b
119.4 160.6
118.1 161.0
119.1 160.9
116.4b 132.7c 118.6b 132.2c 161.6 134.9 151.0
118.0b 133.4c 119.2b 132.4c 163.1 146.6 134.5
117.9b 133.0c 118.9b 132.1c 162.5 147.0 132.6
119.5b 119.5b 127.9
127.1
126.8
—
128.8d 128.3d
127.5d 130.6d
119.3
127.9
128.5
117.3b 61.8
117.0b 62.3
4ce
5af
5bg,h
5cg,i,j
119.3 158.7b 120.3
137.9
139.4
139.5
120.3
120.2
120.0
131.3c 148.7 131.2 157.5b 115.0
132.5c
129.6
129.7
115.2
118.1
124.4
—
63.9
59.6
118.2 156.7
117.8 163.2
120.9
120.9
132.9
131.6
156.7 139.1 136.5
164.9 145.1 135.4
128.8
133.1
aIn DMSO-d6.
b, c, dSignals may be interchanged.
eOCH2CH2, δ (ppm): 35.3.
fB-C6H5, δ (ppm): 127.4 (m), 127.8 (p), 131.2 (o).
g100.5 MHz.
hB-C6H5, δ (ppm): 127.5 (p), 128.2 (m), 132.1 (o).
iOCH2CH2, δ (ppm): 35.9.
jB-C6H5, δ (ppm): 127.7 (p), 127.9 (m), 132.4 (o).
Fig. 2. Structures of compounds 6–9.
R
N
B
N
Although a high ring strain can be predicted from molecu-
lar modeling for the monomeric structure, the reaction be-
tween ligand 4a and phenylboronic acid leads to compound
5a (Scheme 1). For comparison compounds 5b and 5c have
been synthesized, too. The formula of the three heterobi-
cyclic compounds are proved by mass spectrometry. In all
molecules the molecular ion is detected. Due to the easy loss
of a phenyl radical the base peak corresponds to the [M –
C6H5]+ ion (18–21), where the boron atom is bound to two
oxygens and a nitrogen atom.
B
O
O
O
R = alkyl,
aryl
6
7
HO
The CϭN stretching band in the IR spectra is shifted to
higher wave numbers after the complex formation in the
case of 5b (1618 cm–1 for 4b and 1624 cm–1 for 5b) and 5c
(1616 cm–1 for 4c and 1622 cm–1 for 5c), as it has been ob-
served already for other boron chelates (22–23), while it is
shifted to lower wave numbers in the case of 5a (1636 cm–1
for 4a and 1628 cm–1 for 5a).
Me
O
Me
O
N
B
N
B
O
O
B
1
The H, 13C, and 11B NMR data of the monomeric boro-
nates 5a–5c and their corresponding ligands 4a–4c are listed
in Tables 1 and 2. A comparison of the H NMR spectra
1
8
9
shows that the azomethine hydrogen atom H-7 is shifted to
higher field for 5a and 5c with N→B coordination (∆δ =
0.65 and 0.26 ppm), while it is not significantly displaced in
5b (∆δ = 0.02 ppm). The resonances of the OCH2 groups in
5b and 5c are detected downfield as diastereotopic signals
(∆δ = 0.10 ppm and 0.34 ppm). The OCH2CH2 methylene
hydrogens in the seven-membered heterocycle of compound
5c are also diastereotopic (∆δ = 0.30 ppm), indicating a
fixed configuration of the complex in solution.
From the 13C NMR spectra the most interesting observa-
tion is the extreme high field shift of the azomethine carbon
atom in compound 5a after the chelate formation (δ =
161.6 ppm ↔ δ = 148.7 ppm). The correct assignment has
© 1998 NRC Canada

1356
Can. J. Chem. Vol. 76, 1998
Table 3. Crystallographic data for compounds 5a–5c.a
Compound
5a
5b
5c
Formula
Crystal size, mm
fw, g mol–1
C19H14BNO2
0.12 × 0.39 × 0.54
299.14
C20H16BNO2
0.4 × 0.4 × 0.4
313.16
C21H18BNO2
0.39 × 0.45 × 0.48
327.19
Space group
Cell parameters
P 21/c
P 21/a
P 21/c
a, Å
b, Å
c, Å
β°
V, Å3
Z
µ, mm–1
ρcalcd, g cm–3
Data collection
10.038 (2)
18.230 (4)
16.945 (4)
95.18 (2)
3042 (6)
8
10.485 (8)
11.081 (6)
13.736 (6)
100.56 (7)
1569 (2)
4
9.0576 (6)
13.337 (1)
14.835 (1)
106.026 (6)
1722.5 (2)
4
0.078
1.29
0.079
1.33
0.075
1.26
Scan range, deg in ω
Data collected
2θ max, deg
0.42 + 0.76 tan θ
±h, –k, +l
48
1.09 + 1.20 tan θ
±h, –k, –l
52
0.47 + 0.52 tan θ
±h, +k, +l
56
Total reflections
Total unique refl.
Rmerge
5123
4761
0.01
3370
3066
0.01
4488
4152
0.01
Refl. with I > 3σ(I)
Intensity variation (σ)
Refinement
2688
<3
1448
<3
1750
<3
R
Rw
0.043
0.036
501
2.32
0.04
0.067
0.049
266
4.67
0.07
0.040
0.035
282
2.52
0.04
No. of variables
gof
Max. ∆/σ (final
cycle)
∆ρmin, e Å–3
∆ρmax, e Å–3
Extinction parameter
–0.23
0.54
84 (11)
–0.25
0.58
—
–0.13
0.12
42 (7)
aTemperature 293 K for 5a and 5c, 193 K for 5b, Enraf Nonius CAD4 diffractometer, MoKα (λ =
2
0.71069) radiation, graphite monochromator, R = Σ(||Fo| – |Fc||)/Σ|Fo|, Rw = [Σw(|Fo| – |Fc|)2/ΣwFo ]1/2, where
w = 1/σ2. Values given for R, Rw, and gof are based on those reflections with I > 3σ(I).
been evidenced by a HETCOR experiment. This difference
diminishes in compound 5b (δ = 163.1 ppm ↔ δ =
156.7 ppm) and changes its sign in 5c (δ = 162.5 ppm ↔ δ =
164.9 ppm). In the similar six-membered diphenylboron
chelate 6 (Fig. 2) only a small high field shift could be de-
tected (17). The downfield shift of 5–6 ppm for the C-4 car-
bon atom is characteristic for the formation of a boron
complex and has been already reported for other boronates
with a similar structure (12). It seems therefore to be a typi-
cal indicator for the N→B bond coordination in salicylid-
eneamino chelates. The OCH2 carbon atom in the boronates
is found at lower field for 5b (∆δ = 2.1 ppm) and at higher
field for 5c (∆δ = 2.7 ppm).
5a (δ = 8 ppm) and the strongest coordination for the
[4.4.0]heterobicycle 5b (δ = 5 ppm), although the difference
is not very striking. The 11B NMR signal for the
[5.4.0]heterobicycle 5c is detected at δ = 6 ppm.
The 13C NMR spectroscopic data indicate effects in the
structures of 5a–5c, that vary the chemical shifts of the
azomethine group so distinctly. To obtain more structural in-
formation an X-ray crystallographic study of the three
boronates 5a–5c was undertaken (Tables 3–6). Crystal data,
atomic coordinates, anisotropic thermal parameters, bond
angles and distances, and torsion angles have been depos-
ited.3 The molecular structures of 5a–5c are shown in
Figs. 3–5.
The 11B NMR spectra of the boron complexes 5a–5c spec-
ify tetracoordinated boron atoms (24). The chemical shifts in-
dicate the weakest coordination for the [4.3.0]heterobicycle
Table 3 shows that the asymmetric unit of 5a contains two
independent molecules.4 The structures are not significantly
different with respect to bond lengths and bond angles, but
3 Supplementary material may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research
Council of Canada, Ottawa, Canada, K1A 0S2. With the exception of anisotropic thermal parameters and torsion angles, these have also
been deposited with the Cambridge Crystallographic Data Centre and can be obtained on request from: The Director, Cambridge Crystallo-
graphic Data Centre, University Chemical Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.
4 The independency has been confirmed on basis of the correlation coefficients, which do not indicate a systematic correlation between the
two molecules.
© 1998 NRC Canada

Höpfl et al.
1357
Fig. 3. Molecular structure of compound 5a. The thermal ellipsoid probability level is 25%.
Table 5. Deviations from the boronate mean plane for
compounds 5a–5c.
Table 4. Selected bond lengths and bond angles of compounds
5a–5c.
5ab
5bc
5cd
5aa
5b
5c
Mean planea
∆ (Å)
∆ (Å)
∆ (Å)
Bond lengths (Å)
C(5)
C(6)
C(7)
C(8)
0.001/0.013
0.011/–0.017
–0.015/0.007
0.007/0.009
0.005/–0.013
–0.009/0.002
0.014
0.000
–0.013
0.010
0.005
–0.017
0.001
–0.010
0.010
0.001
–0.011
0.010
O(1)—B(2)
O(1)—C(6)
B(2)—N(3)
1.459 (5)
1.371 (4)
1.613 (5)
1.503 (5)
1.607 (5)
1.249 (4)
1.445 (4)
1.365 (4)
1.414 (4)
—
1.472 (7)
1.402 (6)
1.615 (7)
1.435 (6)
1.602 (7)
1.235 (6)
1.473 (6)
1.404 (7)
1.474 (8)
1.396 (7)
—
1.480 (4)
1.331 (3)
1.635 (4)
1.416 (3)
1.609 (4)
1.292 (3)
1.433 (3)
1.420 (4)
1.519 (5)
1.496 (4)
1.377 (4)
B(2)—O(11)
B(2)—C(18)
N(3)—C(4)
N(3)—C(13,14,15)
O(11)—C(12)
C(12)—C(13)
C(13)—C(14)
C(14)—C(15)
Bond angles (°)
C(9)
C(10)
Deviations
O(1)
B(2)
N(3)
–0.018/0.017
0.551/–0.487
–0.018/0.174
–0.165/0.286
–0.087
0.284
0.028
0.026
–0.585
0.084
C(4)
–0.057
0.206
—
aThe mean plane is calculated for C(5) to C(10) and additionally for
C(55) to C(60) in the case of 5a.
O(1)-B(2)-N(3)
O(1)-B(2)-O(11)
B(2)-N(3)-C(4)
B(2)-N(3)-C(13,14,15)
B(2)-O(1)-C(6)
B(2)-O(11)-C(12)
N(3)-B(2) -O(11)
N(3)-C(4)-C(5)
N(3)-C(13)-C(12,13,14)
C(4)-N(3)-C(13,14,15)
C(4)-C(5)-C(6)
O(11)-C(12)-C(13)
C(12)-C(13)-C(14)
C(13)-C(14)-C(15)
108.1 (3)
111.3 (3)
120.5 (4)
107.9 (3)
114.3 (3)
109.2 (3)
99.1 (3)
118.5 (4)
105.3 (3)
131.4 (4)
119.4 (3)
114.8 (3)
—
109.6 (4)
105.2 (4)
122.3 (5)
116.7 (4)
119.1 (4)
118.7 (5)
104.7 (4)
122.2 (6)
114.9 (5)
121.0 (5)
120.0 (5)
111.8 (6)
126.4 (6)
—
103.7 (2)
112.3 (2)
117.9 (2)
121.0 (2)
117.5 (2)
122.1 (3)
110.4 (2)
120.8 (3)
117.6 (3)
121.0 (2)
117.8 (3)
114.4 (3)
114.1 (3)
119.8 (3)
bEquations of the planes: –3.99642X – 16.62096Y + 2.31592Z = –3.807;
–8.19089X – 0.86122Y + 10.97109Z = –7.409.
cEquation of the plane: –6.53926X + 6.24804Y – 5.74052Z = –1.148.
dEquation of the plane: –5.60300X + 6.35883Y – 6.37166Z = –2.668.
X, Y, Z are orthogonal coordinates w.r.t. axes a, b, c*; ∆ is the deviation
from the mean plane.
vary in the deviation of the boron atom from the boronate
mean plane (Table 5) and the orientation of the B-phenyl
ring. This differences are probably due to crystal packing ef-
fects.
The N→B bond lengths are 1.613 (5) Å for 5a, 1.615 (7)
Å for 5b, and 1.635 (4) Å for 5c (Table 4), somewhat longer
than for the monomeric structures 3b and 7 (Figs. 1 and 2),
where a value of 1.601 Å has been determined for both mol-
ecules (12).
—
aThere are two independent molecules in the asymmetric unit, their
mean values being indicated in this table.
© 1998 NRC Canada

1358
Can. J. Chem. Vol. 76, 1998
Fig. 4. Molecular structure of compound 5b. The thermal ellipsoid probability level is 25%.
Fig. 5. Molecular structure of compound 5c. The thermal ellipsoid probability level is 25%.
One of the interests of the crystallographic study was to
investigate which one of the three heterobicycles is the most
stable one. From molecular modeling it is expected that it is
the [4.4.0]heterobicycle 5b, but the N→B bond length seems
to favor also the [4.3.0]heterobicycle 5a. However, the devi-
ation of the boron atom from the boronate mean plane (Ta-
ble 5) is lowest for compound 5b (∆ = 0.284 Å), while for
5a and 5c values of ∆ = 0.520 and 0.585 Å are calculated.
Additional evidence for the proposed relative stabilities of
5a–5c is provided by the sum of bond lengths at the boron
atom. This sum tends to be constant for a given set of
bonded atoms (O2NC) for 5a–5c, and it has been shown that
© 1998 NRC Canada

Höpfl et al.
1359
Table 6. Selected torsion angles and sum of bond angles in the heterocycles for compounds 5a–5c.
5a 5b 5c
Torsion angles (°)a
O(11)-B(2)-N(3)-C(13,14,15)
CPh-B(2)-N(3)-C(13,14,15)
C(4)-N(3)-C(13)-C(14)
Sum of bond angles in the
heterocycles (°)
18.4
–100.3
–16.2
45.7
–79.1
–23.2b
19.0
–101.2
–55.7b
Six-membered ring
Five-, six-, and seven-membered ring
702.0 = 6 × 117.0 (3)
536.2 = 5 × 107.2 (3)
713.9 = 6 × 119.0 (5)
693.2 = 6 × 115.5 (5)
698.7 = 6 × 116.5 (3)
819.4 = 7 × 117.1 (3)
aA positive rotation is counter-clockwise from atom 1 when viewed from atom 3 to atom 2.
bC(4)-N(3)-C(14)-C(15) in the case of 5b and C(4)-N(3)-C(15)-C(16) in the case of 5c.
Fig. 6. Correlation between the 13C NMR chemical shift of the
Fig. 7. Twisted boat conformation of compound 5c.
azomethine carbon atom and the CNCC torsion angle for
compounds 5a–5c.
field shift of the azomethine signals in the 13C NMR spectra
of 5a and 5b and as can be seen from Fig. 6 there exists a
correlation between the 13C NMR shifts of C-7 and the cor-
responding torsion angles. It should be noted that the pheno-
lic B(2)—O(11) bond in compound 5a is significantly
longer than the alkanolic B(2)—O(11) bond in compounds
5b and 5c (1.503 (5), 1.435 (6), and 1.416 (3) Å, respec-
tively).
The N(3)—C(4) bond length is significantly shorter in 5a
and 5b when compared to 5c (1.249 (4), 1.235 (6), and
1.292 (3) Å, respectively). A longer bond length (1.297 (5)
Å) has been reported also for the molecular structure of the
diphenylboron chelate 8 (Fig. 2), where the phenol group is
also twisted out of the mean plane (16).
The seven-membered heterocycle in the boronate 5c pres-
ents a twisted boat conformation (26) with N(3) and C(13)
above the plane (Fig. 7). In comparison to the
[5.4.0]heterobicycle 9 (Fig. 2) with a planar B–O–B moiety
the geometry is rather different (12). Nonetheless, the aver-
age values of the bond angles in the seven-membered ring
are practically identical (117.1 (3) and 117.3 (3)° for 5c and
9, respectively). The same is true for the N→B bond lengths
(1.635 (4) and 1.629 (5) Å for 5c and 9, respectively). But,
the deviation of the boron atom from the boronate mean
plane is lower in 5c (0.585 for 5c ↔ 0.714 Å for 9). The
resonances in the 11B NMR spectra are identical for both
compounds (δ = 6 ppm).
the strain at the boron center is related to the magnitude of
this sum (25). The sums are 6.182, 6.124, and 6.140 Å for
5a–5c, respectively.
The mean value of bond angles in the heterocycles
(Table 6) can be added as further proof that the stability of
the bicyclic boronates decreases in the following order:
[4.4.0] >> [5.4.0], [4.3.0].
For 5b mean values of 119.0(5) and 115.5(5)° for the six-
and five-membered rings are calculated, while the corre-
sponding values for 5a are 117.0(3) and 107.2(3)° and for 5c
are 116.5(3) and 117.1(1)°. The ring strains of the three
compounds are perhaps best expressed by the torsion angles
summarized in Table 6. The ideal values for the O(11)-B(2)-
N(3)-C(13, 14, 15) and CPh-B(2)-N(3)-C(13, 14, 15) dihed-
ral angles are 60°, and it can be seen that both compounds
5a and 5c are far away from it. A zero degree value of the
C(4)-N(3)-C(13)-C(14) torsion angle would correspond to a
perfect π-delocalization between the two aromatic rings that
are connected by the imino group. This dihedral angle is in-
creasing from compound 5a to 5c (16.2 → 23.2 → 55.7°), so
that the π-delocalization is strongest in 5a, while it is practi-
cally lost in 5c in favor of one gauche interaction less. This
observation may be the explanation for the unusual high
From the present study it can be concluded that the
tridentate azomethine ligand 4a does not lead to the forma-
tion of a macrocyclic structure, although the monomeric spe-
cies 5a is highly strained. There exist various possible
explanations for this observation. From molecular modeling
it can be seen that one may be the loss of coplanarity be-
tween the two aromatic systems and the imino group upon
© 1998 NRC Canada

1360
Can. J. Chem. Vol. 76, 1998
6. T. Mancilla, H. Höpfl, G. Bravo, and L. Carrillo. Main Group
Met. Chem. 20, 31 (1997).
7. N. Farfán and R. Contreras. Heterocycles, 23, 2989 (1985).
8. N. Farfán, T. Mancilla, D. Castillo, G. Uribe, L. Carillo, P. Jo-
seph-Nathan, and R. Contreras. J. Organomet. Chem. 381, 1
(1990).
9. N. Farfán, P. Joseph-Nathan, L.M. Chiquete, and R. Contreras.
J. Organomet. Chem. 348, 149 (1988).
10. N. Farfán, D. Silva, and R. Santillan. Heteroatom Chem. 4,
533 (1993).
11. H. Höpfl and N. Farfán. J. Organomet. Chem. 547, 71 (1997).
12. H. Höpfl, M. Sánchez, V. Barba, N. Farfán, S. Rojas, and R.
Santillan. Inorg. Chem. 37, 1679 (1998).
13. E. Haegele. Ber. Dtsch. Chem. Ges. 25, 2753 (1892).
14. E. Haegele. Ber. Dtsch. Chem. Ges. 26, 394 (1893).
15. S.M. Tripathi and J.P. Tandon. J. Prakt. Chem. 320, 685
(1978).
dimer formation, and another explanation may be the intro-
duction of additional strain in a dimeric structure due to the
planarity of the 2-aminophenoxy group. It has also to be
considered that 4a is a planar ligand that favors the forma-
tion of a monomeric structure.
The comparative study of 5a–5c has shown that in the
present case the boronates with a [4.4.0] heterobicyclic
structure are less strained than the corresponding [4.3.0] and
[5.4.0] heterobicycles. The conjugation between the two aro-
matic rings and the imino group is lost, when the number of
members in the second heterocyclic ring is increased. The
loss of conjugation can be detected by the 13C NMR chemi-
cal shifts of the carbon atoms that are in correlation with the
corresponding CNCC torsion angles.
16. R. Allmann, E. Hohaus, and S. Olejnik. Z. Naturforsch. B:
Anorg. Chem. Org. Chem. 37B, 1450 (1982).
17. E. Hohaus. Z. Anorg. Allg. Chem. 506, 185 (1983).
18. E. Hohaus, W. Riepe, and H.F. Gruetzmacher. Org. Mass
Spectrom. 18, 359 (1983).
We thank Prof. Dr. H. Nöth for helpful discussions. Ing.
Guillermo Uribe and Q.I. Victor Gonzalez are thanked for
NMR spectra. N.F. thanks CONACYT and H.H. UAEM
(Universidad Autónoma del Estado de Morelos) for financial
support.
19. E. Hohaus and W. Riepe. Z. Naturforsch. B: Anorg. Chem.
Org. Chem. 28B, 440 (1973).
20. E. Hohaus, K.D. Klöppel, B. Paschold, and H.R. Schulten. Z.
Anorg. Allg. Chem. 493, 41 (1982).
21. E. Hohaus and W. Riepe. Z. Naturforsch. B: Anorg. Chem.
Org. Chem. 29B, 663 (1974).
1. H. Höpfl, N. Farfán, D. Castillo, R. Santillan, R. Contreras, F.
Martínez-Martínez, M. Galván, R. Alvarez, L. Fernández, S.
Halut, and J.-C. Daran. J. Organomet. Chem. 544, 175 (1997).
2. H. Höpfl, M. Galván, N. Farfán, and R. Santillan. J. Mol.
Struct. (THEOCHEM), 427, 1 (1998).
3. N. Farfán, D. Castillo, P. Joseph-Nathan, R. Contreras, and
L.v. Szentpály. J. Chem. Soc. Perkin Trans. 2, 527 (1992).
4. N. Farfán and R. Contreras. Nouv. J. Chim. 6, 269 (1982).
5. N. Farfán and R. Contreras. J. Chem. Soc. Perkin Trans. 2,
1787 (1988).
22. E. Hohaus. Monatsh. Chem. 111, 863 (1980).
23. H.B. Singh and J.P. Tandon. J. Indian Chem. Soc. 56, 768
(1979).
24. H. Nöth and B. Wrackmeyer. In NMR-basic principles and
progress. Vol. 14. Edited by P. Diehl, E. Fluck, and R. Kosfeld.
Springer Verlag, Berlin. 1978.
25. H. Amt, W. Kliegel, S.J. Rettig, and J. Trotter. Can. J. Chem.
66, 1117 (1988).
26. D.F. Bocian, H.M. Pickett, T.C. Rounds, and H.L. Strauss. J.
Am. Chem. Soc. 97, 687 (1974).
© 1998 NRC Canada