Two new complexes of phenylboronic acid with N-salicylidene-4-aminobutanol

Article abstract of DOI:10.1016/S0020-1693(99)00477-6

The tridentate ligand N-salicylidene-4-aminobutanol (6) was obtained by condensation of salicylaldehyde with 4-aminobutanol and reacted with phenylboronic acid to afford either the dimeric structure, bis[μ-[2-[[(4-hydroxy-κO)butyl]imino-κN]methyl-phenolato(2-)-κO]]diphenyl diboro (7) or the monomeric derivative, 2-phenylbenzo[j]-8-aza-1,3-dioxa-2-boracycloundeca-8-ene (8), depending on the reaction conditions. The monomeric structure is a [5.4.0] heterobicycle boronate and has been analyzed by X-ray crystallography. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00477-6

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 303 (2000) 7–11
Two new complexes of phenylboronic acid with
N-salicylidene-4-aminobutanol
Victor Barba, Damia´n Cuahutle, M. Eugenia Ochoa, Rosa Santillan,
Norberto Farfa´n *
Departamento de Qu´ımica, Centro de In6estigacio´n y de Estudios A6anzados del IPN, Apdo. Postal 14-740, 07000 Mexico, D.F, Mexico
Received 29 July 1999; accepted 19 October 1999
Abstract
The tridentate ligand N-salicylidene-4-aminobutanol (6) was obtained by condensation of salicylaldehyde with 4-aminobutanol
and reacted with phenylboronic acid to afford either the dimeric structure, bis[m-[2-[[(4-hydroxy-kO)butyl]imino-kN]methyl-pheno-
lato(2−)-kO]]diphenyldiboro (7) or the monomeric derivative, 2-phenylbenzo[j]-8-aza-1,3-dioxa-2-boracycloundeca-8-ene (8),
depending on the reaction conditions. The monomeric structure is a [5.4.0] heterobicycle boronate and has been analyzed by X-ray
crystallography. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Boronate complexes; Tridentate ligand complexes
1. Introduction
radius. However, when the imino alcohol is substituted
with three methylene groups, a monomeric species is
formed because the tridentate ligand permits the forma-
tion of a stable [4.4.0] heterobicyclic system (5) [3].
In the present work, we describe the synthesis of an
imino alcohol containing four methylene units between
the hydroxyl and imino groups (6) and its reaction with
phenylboronic acid which leads to both the dimeric (7)
and monomeric (8) boronates.
In previous studies it has been shown that the addi-
tion of phenylboronic acid to salicylidenimino alco-
hols produces the corresponding dimeric (1–4) or
monomeric (5) boronates [1–3] which are stable in air
and are obtained in high yields in a one step synthesis
(Scheme 1). These boronate units are connected by
covalent B–O and coordinative N“B bonds which are
responsible for the hydrolytic stability of these
molecules [4–6].
Dimeric complexes have been obtained when the
ligands have zero (1), two (2) five (3) or six (4)
methylene units between the imino and hydroxyl
groups, and the molecular structures of compounds 1
and 2 were established by X-ray crystallography [1,2].
The studies indicate that formation of the dimeric
compounds is favored by the fact that the monomeric
systems would be too strained owing to the tetrahedral
geometry at the boron atom as well as its small atomic
* Corresponding author. Tel.: +52-5-747 3800, ext. 4031; fax:
+52-5-747 7113.
E-mail address: jfarfan@mail.cinvestav.mx (N. Farfa´n)
Scheme 1. Monomeric and dimeric boronates obtained with different
methylene units in the iminodialcohols.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00477-6

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V. Barba et al. / Inorganica Chimica Acta 303 (2000) 7–11
Scheme 2. Synthesis of compounds 7 and 8.
2. Experimental
used without further purification, but single crystals
were grown from spectrophotometric grade solvents.
2.1. Instrumentation
2.3. Preparation of N-salicylidene-4-aminobutanol (6)
NMR spectra were recorded in CDCl3 and DMSO-d6
solutions on Bruker Advance DPX 300 and Jeol Eclipse
+400 spectrometers. Chemical shifts (ppm) are relative
to (CH3)4Si for 1H and 13C and BF3·OEt2 for 11B.
Coupling constants are quoted in Hz. IR spectra were
recorded on a Perkin–Elmer 16F-PC FT-IR spectro-
photometer. Mass spectra were recorded on a Hewlett–
Packard 5989A spectrometer. Melting points were ob-
tained on a Gallenkamp MFB-595 apparatus and are
uncorrected.
Compound 6 was prepared from 0.33 g (3.73 mmol)
of 4-aminobutanol and 0.46 g (3.73 mmol) of salicyl-
aldehyde; the mixture was refluxed in 40 ml of ethyl
Table 1
Crystallographic data for compound 8
Chemical formula
Formula weight
Space group
C17H18BNO2
279.13
P21/c
The X-ray diffraction study was determined on an
Enraf-Nonius CAD4 diffractometer (uMo Ka=0.71073
Crystal size (mm)
0.3×0.4×0.5
12.021(2)
6.425(1)
19.134(4)
99.97(3)
1455.5(5)
4
,
a (A)
,
A, monochromator: graphite, T=293 K, ꢀ–2q scan)
,
b (A)
,
and the crystal was mounted in a LINDEMAN tube.
Absorption correction was not necessary, corrections
were made for Lorentz and polarization effects. Solu-
tion and refinement: direct methods (SHELXS-86) for
structure solution and the SHELXS (version 1.8, 1993)
software package for refinement and data output. Hy-
drogen atoms were determined by difference Fourier
maps and their positions as well as one overall isotropic
thermal parameter were refined. I\4|(I), R=
S(ꢀꢀFoꢀ−ꢀFcꢀꢀ)/SꢀFoꢀ, Rw=[Sw(ꢀFoꢀ–ꢀFcꢀ)2/SwFo2]1/2. Ele-
mental microanalyses were performed by Oneida
Research Services, Whitesboro, NY 13492.
c (A)
i (°)
3
,
V (A )
Formula units per cell Z
zcalc (g cm−3
F(000)
)
1.274
592
Temperature of measurement (K)
q Limits (°)
293
0–25
No. of reflections collected
No. of independent reflections
No. of observed reflections, (Fo)2\4| (Fo)2
R=SꢀꢀFoꢀ−ꢀFcꢀꢀ)/SꢀFoꢀ
2689
2559
1477
0.044
0.124
1.011
Rw=[Sw(ꢀFoꢀ−ꢀFcꢀ)2/SwFo2]1/2, w=1/|2
Goodness-of-fit |
No. of parameters
191
Maximum D/|
−0.001
−0.214
0.181
2.2. Reagents
−3
,
Dzmin (e A
)
−3
,
Dzmax (e A
)
All starting materials were commercial. Solvents were

V. Barba et al. / Inorganica Chimica Acta 303 (2000) 7–11
9
(1H, td, J=7.8, 1.5 Hz, H-4), 7.17 (1H, dd, J=7.7, 1.5
Hz, H-6), 6.91 (1H, d, J=7.8 Hz, H-3), 6.80 (1H, t,
J=7.7 Hz, H-5), 3.59 (2H, t, J=6.6 Hz, H-11), 3.53
(2H, t, J=6.6 Hz, H-8), 1.68 (2H, q, J=6.6 Hz, H-10),
1.58 (2H, q, J=6.2 Hz, H-9) ppm; 13C NMR (100
MHz, CDCl3) l: 164.8 (C-7), 161.8 (C-2), 132.3 (C-4),
131.3 (C-6), 118.6 (C-1), 118.4 (C-5), 117.2 (C-3), 62.0
(C-11), 58.8 (C-8), 30.2 (C-10), 27.2 (C-9) ppm.
2.4. Preparation of bis[v-[2-[[(4-hydroxy-sO)butyl]-
imino-sN]methyl-phenolato(2−)-sO]]diphenyldi-
boro (7)
A solution of phenylboronic acid (0.32 g, 2.58 mmol)
in 20 ml of benzene was added to a solution of N-sali-
cylidene-4-aminobutanol (0.5 g, 2.58 mmol) in 20 ml of
benzene. The mixture was refluxed for 30 min using a
Dean Stark trap until a yellow precipitate was formed.
The solid was collected by filtration and dried to give a
product insoluble in all common solvents. Yield: 0.54 g,
3.85 mmol, 75%. 180°C (decomp.). IR (KBr): 2924 (m),
2914 (m), 2840 (m), 1644 (CꢀN, s), 1608 (m), 1560 (s),
1478 (s), 1312 (s), 1234 (s), 1188 (s), 1152 (s), 1130 (s),
1116 (s), 1108 (s), 940 (s), 754 (s) cm−1; MS (m/z, 70
eV, %): 481 (M+–C6H6, 3), 296 (6), 268 (2), 220 (3) 202
(100), 148 (13), 132 (7), 77 (8), 55 (10), 44 (10). Anal.
Calc. for C34H36B2N2O4: C, 73.14; H, 6.51; N, 5.02.
Found: C, 72.96; H, 6.39; N, 4.89%.
Fig. 1. Molecular structure for compound 8.
Table 2
,
Selected bond distances (A) and bond angles (°) for compound 8
Bond distances
O(1)–B(1)
O(2)–B(1)
N(1)–B(1)
N(1)–C(8)
1.505(3)
1.425(3)
1.610(3)
1.476(3)
O(1)–C(1)
O(2)–C(11)
N(1)–C(7)
B(1)–C(12)
1.336(3)
1.429(3)
1.282(3)
1.611(4)
Bond angles
B(1)–O(1)–C(1)
B(1)–N(1)–C(7)
C(7)–N(1)–C(8)
O(1)–B(1)–N(1)
O(1)–B(1)–C(12)
N(1)–B(1)–C(12)
C(1)–C(6)–C(7)
119.8(2)
120.7(2)
119.6(2)
105.3(2)
111.4(2)
109.5(2)
118.9(3)
B(1)–O(2)–C(11)
B(1)–N(1)–C(8)
O(1)–B(1)–O(2)
O(2)–B(1)–N(1)
O(2)–B(1)–C(12)
O(1)–C(1)–C(6)
N(1)–C(7)–C(6)
119.6(2)
119.6(2)
109.3(2)
110.9(2)
110.7(2)
120.7(2)
121.7(2)
2.5. Preparation of 2-phenylbenzo[ j]-
8-aza-1,3-dioxa-2-boracycloundeca-8-ene (8)
A solution of phenylboronic acid (0.32 g, 2.58 mmol)
in 20 ml of THF was added to a solution of N-salicyli-
dene-4-aminobutanol (0.5 g, 2.58 mmol) in 20 ml of
THF. The mixture was refluxed for 6 h using a Dean
Stark trap, cooled to room temperature (r.t.) and 10 ml
of hexane were added until a precipitate was formed.
The solid was collected by filtration, and dried. The
product obtained is a yellow solid that is slightly solu-
ble in chloroform, methanol and DMSO. Crystals suit-
able for X-ray diffraction were obtained when the
reaction was performed in a CH2Cl2/hexane mixture at
r.t. without stirring. Yield: 0.56 g, 2.0 mmol, 78%. M.p.
197–198°C. IR (KBr): 2942 (m), 2922 (m), 2888 (s),
2852 (s), 1636 (CꢀN, s), 1608 (m), 1558 (m), 1478 (m),
1234 (m), 1204 (m), 1182 (m), 1136 (s), 1104 (s), 994 (s),
946 (s), 904 (s), 746 (s), 700 (s) cm−1; MS (m/z, 70 eV,
%): 279 (M+, 6), 220 (4), 202 (100), 172 (2), 148 (18),
Scheme 3. Structures of compounds 9 and 10.
acetate for 30 min. The solvent and the water formed
during the reaction were removed by a Dean Stark trap
to yield a yellow oil (0.69 g, 3.6 mmol, 97.4%). IR
(NaCl): 3356 (s), 2934 (s), 2864 (m), 1632 (CꢀN, s),
1498 (m), 1462 (m), 1280 (m), 1050 (m) cm−1; MS
(m/z, 70 eV, %): 193 (M+, 51), 175 (19), 148 (48), 134
1
103 (3), 77 (10), 51 (8), 39 (4); H NMR (300 MHz,
DMSO-d6) l: 8.76 (1H, s, H-7), 7.47 (1H, t, J=7.8 Hz,
H-4), 7.45 (1H, d, J=7.7 Hz, H-6), 7.28 (2H, d, J=6.7
Hz, H-o), 7.09 (3H, d, J=6.7 Hz, H-m, p), 6.86 (1H, d,
J=7.8 Hz, H-3), 6.85 (1H, t, J=7.7 Hz, H-5), 3.66
(4H, m, H-8, 11), 1.48–1.57 (4H, m, H-9, 10) ppm; 13C
NMR (75 MHz, DMSO-d6) l: 165.5 (C-7), 161.8 (C-2),
1
(46), 120 (44), 107 (100), 77 (41), 39 (28), 31 (43); H
NMR (400 MHz, CDCl3) l: 8.24 (1H, s, H-7), 7.23

10
V. Barba et al. / Inorganica Chimica Acta 303 (2000) 7–11
Fig. 2. Molecular packing for compound 8.
138.0 (C-4), 132.9 (C-6), 131.4 (C-o), 127.7 (C-m), 127.4
(C-p), 119.1 (C-3), 119.0 (C-5), 117.7 (C-1), 62.7 (C-8),
55.5 (C-11), 31.8 (C-9), 31.7 (C-10) ppm; 11B NMR (96
MHz, DMSO-d6) l: +7.0 (h1/2=221 Hz) ppm. Anal.
Calc. for C17H18BNO2: C, 73.14; H, 6.51; N, 5.02.
Found: C, 73.07; H, 6.65; N, 5.02%.
When the same reaction was performed in THF
using 6 h of reaction, the monomeric compound 2-
phenylbenzo[j] - 8 - aza - 1,3 - dioxa - 2 - boracycloundeca-
8-ene (8) was obtained in good yield (Scheme 2). The
monomeric composition of compound 8 can also be
concluded from mass spectrometry, where the molecu-
lar ion is detected and the base peak corresponds to the
[M+–C6H5] fragment ion. In this case longer reaction
times produced contraction of the ring size from 14- to
seven-members, indicating that the monomeric com-
pound is the thermodynamic product.
3. Results and discussion
Reacting 4-aminobutanol with salicylaldehyde in
ethanol gives N-salicylidene-4-aminobutanol (6) in high
yields. The reaction of 6 with phenylboronic acid was
carried out in benzene under reflux yielding, after 30
min, bis[m-[2-[[(4-hydroxy-kO)butyl]imino-kN]methyl-
phenolato(2−)-kO]]diphenyldiboro (7), which was a
solid insoluble in all common solvents (Scheme 2). The
dimeric structure of this compound was proved by mass
spectrometry which showed the fragment ion character-
istic for the dimeric compounds [3,7]. Thus, the molecu-
lar ion is not observed and instead the [M+–C6H5] ion
is detected owing to the easy loss to a phenyl radical.
Also the CꢀN stretching band in the IR spectrum is
shifted to higher wavenumbers after complex formation
(1632 cm−1 for 6 and 1644 cm−1 for 7).
1
In the H NMR spectrum the azomethine hydrogen
atom is shifted to lower fields for 8 than the corre-
sponding hydrogen in 6 (Dl: 0.52 ppm) owing to the
coordination of the nitrogen to the boron atom. The
same result is found in the 13C NMR spectrum where
the azomethine carbon atom (C-7) is slightly shifted to
lower fields for compound 8 than in 6 (Dl: 0.70 ppm).
The 11B NMR spectrum of 8 showed a signal at +7.0
ppm, which confirms the N“B bond coordination.
The X-ray analysis establishes the molecular struc-
ture of 8 (Table 1), where a [5.4.0] heterobicycle is
formed (Fig. 1). The selected bond lengths and angles
are summarized in Table 2. The N“B bond distance
,
for 8 is 1.610(3) A, similar to that observed in the

V. Barba et al. / Inorganica Chimica Acta 303 (2000) 7–11
11
,
boron complex 5 (1.601(9) A) [3] and shorter than in
can lead to the monomeric (8) or dimeric boronate (7)
depending on the reaction conditions. The dimeric com-
plex is the kinetic product while the monomeric com-
plex is the thermodynamic product and was obtained
because the formation of a slightly strained [5.4.0]
heterobicyclic boronate is favored.
,
[5.4.0] heterobicycles 9 (1.635(4) A) [8] and 10 (1.629(5)
,
A) [3] (Scheme 3).
Moreover, significant differences are found in the
B(1)–O(1) bond distance, where compound 8 shows the
longest value of 1.505(3) A, while the distances for the
same bond in 5, 9 and 10 are significantly shorter
,
,
(1.480(4), 1.480(10) and 1.470(6) A, respectively). A
comparison of the B(1)–C(12) bond lengths for com-
5. Supplementary material
,
pounds 8, 9 and 10 (1.611(4), 1.609(4) and 1.600(6) A,
respectively) shows that they are very similar.
Tables of crystallographic parameters, atomic coordi-
nates, anisotropic thermal parameters, bond distances,
bond angles as well as a listing of structure factors have
been deposited with the Cambridge Crystallographic
Data Centre (CCDC no. 138687 for 8). Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
The bond angles around the boron atom for 8 are in
the range of 105.3–111.1°, and are very similar to the
value observed for compound 9. The average value for
the bond angles of the seven-membered ring is 114.6(2)°
which is shorter than the value observed for similar
heterocycles such as 9 (117.1(3) and 10 (118.8(3)°) and
indicates a lower ring strain for compound 8. The
seven-membered ring in the boronate compound 8 pre-
sents a chair conformation, while the same ring in 9
and 10 presents a twisted boat conformation. This
observation gives additional evidence for the lower ring
strain in the seven-membered heterocycle in 8, and is in
accordance with the tetrahedral character value (THC,
91% for 8) which is higher than for 9 (81%) and very
similar to that of 10 (90%) [9]. The deviation of the
boron atom from the boronate mean plane in 8 (0.550
Acknowledgements
Financial
support
from
CONACYT
is
acknowledged.
References
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,
A) is similar to that in the boronate 9 (0.585 A) and
,
lower than that in 10 (0.714 A).
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,
,
C–H···p distance of 2.794 A [10].
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4. Conclusions
(
[9] S. Toyota, M. Oki, Bull. Chem. Soc. Jpn. 65 (1992) 1832.
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.