Mono- and diboronates derived from tridentate ONO ligands and arylboronic acids

Article abstract of DOI:10.1016/j.jorganchem.2005.01.060

The syntheses of eight [4.3.0] heterobicyclic boronates containing a N → B coordinative bond are described. The monomeric compounds were prepared by reaction of arylboronic acids with a tridentate ligand having the ONO donor set of atoms. It was shown that substituents at the para-position of the B-phenyl moiety transmit electronic effects to the CN bond which in turn is polarized by formation of the N → B coordination bond. At the same time, related tridentate ligands were also reacted with 1,4-benzenediboronic acid in order to prepare benzene diboron complexes. The structure of this type of compounds was confirmed by X-ray analysis for one of the derivatives.

Full text of DOI:10.1016/j.jorganchem.2005.01.060

Journal of Organometallic Chemistry 690 (2005) 2351–2357
www.elsevier.com/locate/jorganchem
Mono- and diboronates derived from tridentate ONO ligands
and arylboronic acids
a,
Victor Barba , Julian Vazquez , Fernando Lopez ,
b
b
*
´
´
Rosa Santillan , Norberto Farfan
´
b
b
´
a
´ ´ ´
Centro de Investigaciones Quımicas, Universidad Autonoma del Estado de Morelos, Av. Universidad 1001, C.P. 62210 Cuernavaca, Morelos, Mexico
b
´ ´ ´
Departamento de Quımica, Centro de Investigacion y de Estudios Avanzados del IPN, Mexico D.F. 07000, A.P. 14-740, Mexico
´
Received 19 November 2004; revised 28 January 2005; accepted 28 January 2005
Available online 20 April 2005
Abstract
The syntheses of eight [4.3.0] heterobicyclic boronates containing a N ! B coordinative bond are described. The monomeric
compounds were prepared by reaction of arylboronic acids with a tridentate ligand having the ONO donor set of atoms. It was
shown that substituents at the para-position of the B-phenyl moiety transmit electronic effects to the C@N bond which in turn is
polarized by formation of the N ! B coordination bond. At the same time, related tridentate ligands were also reacted with 1,4-
benzenediboronic acid in order to prepare benzene diboron complexes. The structure of this type of compounds was confirmed
by X-ray analysis for one of the derivatives.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Tridentate ligands; Boron; Schiff base; Hammett equation; 1,4-benzene diboronic acid
1. Introduction
pounds depends on the conformation and structure of
the ligand. For example, the reaction of phenylboronic
acid with the tridentate ligand derived from salicylalde-
hyde and 2-aminophenol, leads to a [4.3.0] bicyclic bor-
on compound (Scheme 1, I) [6] which is stabilized by the
presence of an intramolecular N ! B coordinative
bond. The existence of electronic charge transfer com-
plexes (Scheme 1, II) [7], suggests that the electronic
delocalization present in the free ligand is retained upon
formation of the coordinative bond. Nonetheless, coor-
dination of the nitrogen atom to the boron atom polar-
izes the C@N double bond, as evidenced by the changes
in 1H NMR chemical shifts of the azomethine group and
IR bands [5,6]. This polarization is confirmed also by
the facile, stereoselective addition of acetone in these
compounds (Scheme 1, III) [8] but not in the free ligand.
At present, there is increasing interest in this type of
compounds, because some of them are useful in propyl-
ene polymerization reactions [9] via the formation of a
boron cation.
The use of tridentate ligands in coordination chemis-
try provides a facile means to stabilize transition metals
[1] and main group elements [2] taking advantage of the
chelate effect. Thus, it has been reported that tridentate
ligands having an ONO donor set of atoms react with
main group elements to give stable heterocycles [3]. In
many cases, the stabilization of the complexes has been
increased by the presence of coordinative bonds between
the nitrogen atom and the corresponding main group
element [2c,4].
In the course of our studies on the reaction of triden-
tate ONO ligands, we have established the synthesis of
several monomeric and dimeric boron complexes [5]
and have found that the formation of the different com-
*
Corresponding author. Tel./fax: 52 777 3297997.
E-mail addresses: vbarba@ciq.uaem.mx (V. Barba), jfarfan@
´
cinvestav.mx (N. Farfan).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.01.060

2352
V. Barba et al. / Journal of Organometallic Chemistry 690 (2005) 2351–2357
O
III
I
II
A
H
H
N
N
N
Pull
B
O
D
B
O
B
O
O
Ph
O
O
Ph
Push
Ph
D= Donor
A= Aceptor
Stereoselective
addition to the C=N group
Bicyclic boron
complex
Push-pull effect
Scheme 1. Examples of some monomeric boron compounds. (I) Heterobicycle formed by the presence of an intramolecular N ! B coordination
bond. (II) Electronic charge transfer boron complex with a push–pull effect. (III) The addition of acetone to compound I is stereoselective.
13
12
10
B(OH)2
6
3
7
8
11
5
4
N
B
THF
, ∆
1
N
+
9
O
O
2
- 2 H2O
i
HO
OH
1a
o
R
R
a: F
R
m
e: OMe
p
b: Cl
c: Br
d: Me h : CF3
f : CHO
g : COCH3
R
2a-2h
Scheme 2. Synthesis of the monomeric boron complexes 2a–2h.
In the present work, several new monomeric boron
complexes analogous to I that contain different substit-
uents at the para-position of the B-aryl moiety
(Scheme 2) were synthesized to evaluate the transmis-
sion of electronic effects to the C@N bond as a means
to control and predict, in the future, the reactivity of
the azomethine group. Additionally, the new diboro-
nates described herein allow evaluation of the effect of
alternate conjugated systems on the electronic structure
of the complex.
The formation of monomeric boron compounds was
confirmed by EI-mass spectra. In all eight cases, the
molecular ion [M]+ was observed and the base peak
(m/z = 222) corresponds to the loss of the aryl group at-
tached to boron atom [M ꢀ Ar]+. The C@N stretching
band in the IR spectra is found between 1626 and
1634 cmꢀ1, and is shifted to lower wave numbers with
respect to the free ligand (Dm = 2–12 cmꢀ1) owing to
the formation of the N ! B coordination bond.
The influence that is generated on the C@N group by
the substituents at the para-position of the B-aryl moi-
ety, can be analyzed from the NMR data. As a direct
consequence of the formation of the N ! B bond, the
1H NMR data shows that the signals corresponding to
the azomethine group (H-7), are shifted to lower fre-
quencies in the boron complexes (d = 8.30–8.42 ppm)
when compared to the free ligand (d = 8.97 ppm). The
differences in the shifts can be attributed to the elec-
tronic changes induced by the substituents; the signals
shifted to lower frequencies (d = 8.30, 8.31 ppm) corre-
spond to compounds 2d and 2e which contain electron
donor groups (Me and MeO), while those at higher fre-
quency (d = 8.41, 8.42 ppm) correspond to compounds
2g and 2h, which have an electron withdrawing effect
due to the presence of the –CF3 and –COMe groups.
Fig. 1 shows that there is a linear correlation between
2. Results and discussion
2.1. Preparation and characterization of the monomeric
boron complexes 2a–2h
Addition of different arylboronic acids to tridentate
ligand 1a derived from salicylaldehyde and 2-aminophe-
nol [6] containing the ONO donor set of atoms, lead to
formation of the monomeric compounds 2a–2h. As
mentioned above, the arylboronic acids selected for this
study enclose different substituents at the para-position
of the phenyl moiety (R = F, Cl, Br, Me, OMe, CHO,
COCH3, and CF3), in order to evaluate the influence
of electronic effects on the C@N bond (Scheme 2). The
[4.3.0] heterobicyclic boronates 2a–2h were obtained in
moderate yields and are stable to moisture due to the
presence of two B–O covalent bonds and a N ! B coor-
dinative bond, in addition to the chelate effect.
1
the r Hammett values [10] and the H NMR chemical
shifts (d), giving evidence of the electronic effect pro-
duced by the para-substituents in the B-phenyl moiety.
Substituent effects are also evident in the B–CPh bond

V. Barba et al. / Journal of Organometallic Chemistry 690 (2005) 2351–2357
2353
Table 1
Crystallographic data for 2c and 3e
2c
3e
Empirical formula
Formula weight
Crystal size (mm3)
Crystal system
C19H23BBrNO2
378.20
C38H34B2N2O4 Æ CHCl3
723.52
0.5 · 0.3 · 0.3
Monoclinic
P21/n
0.48 · 0.36 · 0.32
Monoclinic
P21/c
Space group
Unit cell dimensions
˚
a (A)
5.8960(10)
17.342(3)
15.840(3)
93.39(3)
1616.8(5)
4
9.051(2)
19.368(4)
11.796(2)
99.21(3)
2041.2(7)
4
˚
b (A)
˚
c (A)
b (ꢁ)
Volume (A )
3
˚
Z
Dcalc (g/cm3)
Absorption coefficient
(mmꢀ1
1.553
2.553
1.372
0.464
Fig. 1. Correlation between the 1H NMR chemical shifts (d) of the
imine proton versus the r Hammett values for the para-substituents in
the monomeric boron complexes. Equation: r Hammett = 6.45*(d, 1H)
– 54.119, R = 0.9614. The chemical shift value for I was taken from [6].
)
Collected reflections
Independent reflections
Parameters
3189
2826
3792
3558
237
R = 0.0477
244
R = 0.0485
Final R indices
[I > 2sigma(I)]
R indices (all data)
Goodness-of-fit
wR = 0.1407
1.004
wR = 0.1699
1.011
lengths, as has been shown for a series of dimeric boron
complexes [11].
A similar behavior was observed in the 13C NMR data
in relation to the C@N group. In all compounds, the sig-
nal corresponding to the C@N group is shifted to lower
frequency (ca. Dd = 12.0 ppm) compared to that of the
free ligand; particularly complexes containing electron
donor groups showed the lowest frequency values. The
fact that the boron atoms are in a tetrahedral environ-
ment was deduced from the 11B NMR spectra, wherein
the chemical shifts were observed in a range of d =
7–8 ppm, that is typical for this type of complexes [5].
Crystals of compound 2c that were suitable for X-
ray analysis were obtained by slow evaporation of a
concentrated solution of 2c in CHCl3. Crystallographic
data and selected geometrical parameters are listed in
Tables 1 and 2. The molecular structure is shown in
Fig. 2 confirming the formation of a monomeric com-
pound, in which an intramolecular N ! B coordinative
bond forms the [4.3.0] heterobicycle. In the crystal lat-
tice, two enantiomeric molecules of 2c are located at
the same site, as a consequence, the C@N group was
disordered in a statistic probability of 50:50, to refine
this disorder the PART instruction was used [12]. The
N ! B bond lengths were found to be comparable with
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) for 2c and 3e
2ca
3e
˚
Bond distances (A)
B(1)–N(1)
B(1)–O(1)
B(1)–O(2)
B(1)–CPh
N(1)–C(7)
N(1)–C(8)
1.611(8)/1.625(13)
1.470(6)
1.477(6)
1.616(5)
1.467(5)
1.449(5)
1.620(5)
1.290(4)
1.486(5)
1.604(6)
1.285(8)/1.285(15)
1.412(8)
Bond angles (ꢁ)
O(1)–B(1)–N(1)
O(1)–B(1)–O(2)
O(1)–B(1)–CPh
O(2)–B(1)–N(1)
O(2)–B(1)–CPh
N(1)–B(1)–CPh
a
114.3(4)/123.0(5)
111.9(3)
112.0(4)
105.6(2)
113.5(2)
113.9(3)
98.4(2)
94.4(4)/86.8(5)
112.5(3)
110.6(4)/108.0(5)
109.9(2)
114.5(3)
There are two positions for the C(7) and N(1) atoms because of the
presence of two enantiomeric molecules at the same site in the crystal
lattice.
para-position of the B-phenyl moiety, giving additional
evidence for the polarization change in the C@N group.
˚
that of compound I; (1.611(8)/1.625(13) A for 2c and
˚
1.613(5) A for I). The two B–O bond distances in com-
pound 2c are equivalent (dB–O = 1.470(6)/1.477(6)) most
probably because of the disorder, since they are an
2.2. Preparation and characterization of the diboronate
complexes 3a–3e
average of those found in compound I (dB–O
˚
=
1.459(5)/1.503(5) A). The most important difference be-
When the same ligand used for the preparation of
monomeric derivatives was allowed to react with 1,4-
benzenediboronic acid in a 1:1 ratio, the monochelated
complex was not formed but instead the diboronate 3a
was obtained in low yields (ca. 40%). The yield was in-
creased to 89%, when the reaction was carried out in a
tween compounds 2c and I is the C@N bond distance,
˚
which is shorter in I (1.249(4) A) than in compound 2c
˚
(1.285(8)/1.285(15) A). This variation can be attributed
to the electronic effect of the bromine atom at the

2354
V. Barba et al. / Journal of Organometallic Chemistry 690 (2005) 2351–2357
chemical shifts were found for compound 3b, in this
case, the signal for H-7 observed in 1H NMR was
shifted to d = 9.25 ppm, and for C-7 at d = 158.5 ppm
in 13C NMR. The hydrogen and carbon atoms at posi-
tion 15 gave signals at d = 6.87/130.2 and 6.84/
130.6 ppm, for 3a and 3b, respectively. The 11B NMR
spectra showed signals at d = 7.6 and 6.7 ppm for 3a
and 3b, respectively, evidencing the tetrahedral charac-
ter of the atoms.
For comparison, related compounds were synthesized
by reaction of tridentate ligands 1c–1e with 1,4-benzen-
ediborobic acid. Ligands 1c–1e have H, Me and Ph
groups at the imine position and are derived from pro-
panol amine; this type of ligands favor the formation
of complexes with a N ! B coordinative bond contain-
ing two six-membered rings around each boron atom
(Scheme 4). The diboronate compounds 3c–3e were ob-
tained and their structure was confirmed by observation
of the corresponding molecular ion [M]+ in the FAB-
mass spectra.
The spectroscopic data also established the symmetri-
cal nature of these compounds. The IR spectra of com-
pounds 3c–3e showed strong bands for the C@N group
at 1628, 1618 and 1606 cmꢀ1, respectively. The 1H
NMR spectra showed diastereotopic signals in the range
of d = 1.50 and 4.00 ppm, which correspond to the CH2
groups. As a consequence of the inductive effect of the
organic group (H, Me, and Ph) present at the C@N moi-
ety, the hydrogen atoms of the aromatic bridge (H-12)
presented different shifts, d = 7.09, 7.20 and 7.47 ppm
for 3c–3e, respectively, all of them shifted to higher fre-
quency in relation to 3a and 3b whereby the planarity of
the ligand produced a shielding effect. In the case of
compound 3c, the signal corresponding to the hydrogen
atom of the HC@N fragment was observed at
d = 7.09 ppm. The 13C NMR spectra showed signals at
d = 171.9 and 170.3 ppm for 3d and 3e, respectively, cor-
responding to the azomethine group (C-7); while the sig-
nals for C-12 were observed at d = 131.3 and 131.2 ppm
for 3d and 3e. The 13C NMR spectrum for 3c could not
Fig. 2. Molecular structure of compound 2c.
2:1 ratio (Scheme 3). Additionally, ligand 1b was used
for the reaction with diboronic acid to determine if
diboronate [4.4.0] heterobicycles are also formed.
The formation of diboronates 3a and 3b was estab-
lished by FAB-mass spectrometry, which permitted to
observe peaks for the molecular ions [M]+. The NMR
spectra showed only signals for half of the molecule,
which indicate the symmetric nature of the complex
1
(at least in solution). The H NMR spectrum of com-
pound 3a shows that the signal corresponding to the
hydrogen atom of the C@N group, is shifted to high fre-
quency (Dd = 0.17 ppm) with respect to the free ligand
and also in relation to the monomeric boronates (2a–
2h). This observation suggests that the boron atom lo-
cated at the para-position creates a deshielding effect
on the hydrogen of the imine group. These results are
supported by the fact that the signal for the carbon atom
(C-7) in the 13C NMR spectrum, is shifted to high fre-
quency (d = 152.4 ppm) in relation to the chemical shift
observed for the analogous carbon atoms in compounds
2a–2h. Similar results concerning the symmetry and
12
13
11
7
6
3
8
10
5
4
N
B
1
9(CH2)n
B(OH)2
O14
O
2
THF,∆
15
N
+
2
- 4 H2O
(CH2)n
OH
OH
O
O
B(OH)2
(CH2)n
B
N
1a-1b
a: n=0, b: n=1
3a,3b
Scheme 3. Synthesis of the diboronate complexes 3a–3b.

V. Barba et al. / Journal of Organometallic Chemistry 690 (2005) 2351–2357
2355
R
7
6
8
9
5
N
1
B(OH)2
B(OH)2
R
4
B
10
2
O11
O
3
THF,∆
12
N
OH
+
2
- 4 H2O
OH
O
O
B
N
1c-1e
c: R=H, d: R=Me, e: R=Ph
R
3c-3e
Scheme 4. Synthesis of the diboronate complexes 3c–3e derived from aliphatic amino alcohols.
be obtained due to poor solubility of the compound.
Interestingly, the phenyl group attached to the azome-
thine moiety in compound 3e shows two distinct signals
for the atoms at the ortho- and meta-positions in the
proton and carbon NMR spectra, probably due to re-
stricted rotation of the aromatic ring.
gles around the boron atoms are in the range from
104.2ꢁ to 115.1ꢁ which is an indication of the distorted
tetrahedral geometry of the boron atom. The phenyl
group attached to the C@N group is oriented nearly per-
pendicular to the mean plane of the salicylaldehyde, as
evidenced by the C(12)–C(11)–C(7)–C(1) and C(12)–
C(11)–C(7)–N(1) torsion angles which have values of
ꢀ73.52ꢁ and 106.6ꢁ, respectively. In a similar way, the
N ! B coordination bonds have an eclipsed conforma-
tion with respect to the bridging aromatic moiety where-
by the torsion angle C(19)–C(17)–B(1)–N(1) has a value
of only 0.80ꢁ.
In addition to the NMR and mass spectrometry anal-
ysis, the molecular structure of 3e was confirmed by X-
ray crystallography. The most relevant crystallographic
data are summarized in Table 1, selected bond lengths
and angles are listed in Table 2. The structure of 3e con-
tains an inversion center located at the center of the aro-
matic ring that bridges the two boron chelates and has
therefore a Ci punctual group symmetry. The configura-
tions of the boron atoms are R and S. As can be seen
from the molecular structure shown in Fig. 3, compound
3e contains two fused six-membered rings at each side of
the central ‘‘BC6H4B’’ moiety. Each one of these hetero-
bicycles is formed through a N ! B coordinative, which
At the supramolecular level the molecules are orga-
nized so that two adjacent molecules show interactions
involving the aromatic fragments. There is (i) a parallel
displacement through pꢁ ꢁ ꢁp interactions with a distance
˚
of 3.71 A and, (ii) a T-shaped arrangement with an
˚
intermolecular C–Hꢁ ꢁ ꢁp distance of 2.87 A (Fig. 4) [13].
˚
has a bond length of 1.616(5) A that is comparable to
that of compound 2c. The six-membered heterocycle
containing the C@N moiety has an envelop conforma-
tion, with the boron atom having the largest deviation
˚
from the mean plane (D = ꢀ0.671 A). The six-membered
heterocycles containing the aliphatic chains have chair
conformations.
The bond lengths and angles are similar to those of
other boronates [5], for instance, the values for the an-
Fig. 4. Schematic representation showing the pꢁ ꢁ ꢁp and CHꢁ ꢁ ꢁp
aromatic interactions presents in 3e.
Fig. 3. Molecular structure of compound 3e.

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V. Barba et al. / Journal of Organometallic Chemistry 690 (2005) 2351–2357
2.3. Conclusions
3.3. Preparative part
The present study shows that substituents attached
to the phenyl moiety of boronates in the para-position
transmit electronic effects to the C@N bond. Electron
withdrawing groups induce a deshielding effect on the
azomethine group and electron donating groups cause
All reagents were purchased from Aldrich and were
used without further purification. All five ligands used
(1a–1e) [5a,5b,5c,6] as well as 1,4-benzenediboronic acid
[15], were synthesized in accordance to reported
methods.
1
the opposite effect, as evidenced by the H NMR data.
The presence of an additional boronate in the B-phe-
nyl moiety (compounds 3a–3e), enhances the deshiel-
ding effect due to increased electronic delocalization
between the two chelates present in the diboronates.
This possibility to modulate the electrophilicity of
the imine bond could allow, in the near future, to
control nucleophilic attack on azomethine groups in-
volved in boron complexes. Furthermore, in the case
of formation of the diboronate compounds, it was no-
ticed that the presence of aromatic or aliphatic chains
in the ligand has no effect on the course of the
reaction.
3.3.1. General method for the preparation of the
monomeric boron complexes 2a–2h1
Compounds 2a–2h were synthesized by reaction of
equimolecular quantities of ligand 1a and the corre-
sponding arylboronic acid. The reaction mixture was
dissolved in THF and refluxed. After 1 h under stirring,
the water and part of the solvent were removed with a
Dean-Stark trap. The solvent was completely removed
using a vacuum pump and the product was washed with
several portions of hexane.
3.3.1.1. 2-(4-fluorophenyl)-dibenzo-[d,h]-6-aza-1,3-dioxa-
2-boracyclonon-6-ene (2a). 2a was prepared from
0.21 g (1.50 mmol) of 1a and 0.32 g (1.50 mmol) of 4-flu-
orophenylboronic acid. A yellow solid was obtained,
yield 67% (0.32 g, 1.00 mmol), m.p. = 206–208 ꢁC. IR
m (KBr) 1626 (C@N), 1608, 1550, 1468, 1376, 1174,
952, 824, 742 cmꢀ1. EI-MS m/z (%), 317 (M+, 6), 222
([M ꢀ C6H4F]+, 100), 95 (7), 77 (16), 75 (10), 50 (6).
1H NMR (300 MHz, CDCl3) d: 8.35 (1H, s, H-7), 7.57
(1H, ddd, J = 8.5, 7.2, 1.6 Hz, H-4), 7.46 (1H, dd,
J = 7.9, 1.3 Hz, H-10), 7.40 (1H, dd, J = 7.2, 1.6 Hz,
H-6), 7,37 (1H, td, J = 7.9, 1.3 Hz, H-12), 7.33 (2H,
dd, J = 8.9, 2.0 Hz, H-o), 7.21 (1H, d, J = 8.5 Hz, H-
3), 7.11 (1H, dd, J = 7.9, 1.3 Hz, H-13), 6.96 (1H, td,
J = 7.2, 1.6 Hz, H-5), 6.92 (1H, td, J = 7.9, 1.3, Hz, H-
11), 6.84, (2H, dd, J = 8.9, 4.0 Hz, H-m) ppm. 13C
NMR (75 MHz, CDCl3) d: 158.6 (C-2), 157.4 (C-9),
149.0 (C-7), 138.2 (C-4), 133.0 (C-o), 132.6 (C-12, m),
131.6 (C-6), 130.5 (C-8), 120.6 (C-5), 120.3 (C-3),
119.2 (C-11, 1), 115.4 (C-13), 115.2 (C-10), 114.2 (C-
3. Experimental part
3.1. Instrumental
NMR spectra were recorded at room temperature
using a Bruker 300 spectrometer. Chemical shifts are gi-
ven 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 spectrom-
eter operating in the electron impact mode. Melting
points were determined with a Gallenkamp MFB-595
apparatus.
3.2. X-ray crystallography
Crystal structure determination of 2c and 3e. Crystals
suitable for X-ray structure analysis were grown by slow
evaporation of concentrated CHCl3 solutions of the
complexes. Intensity data were collected at 293 K with
an Enraf-Nonius CAD4 diffractometer, Mo Ka-radia-
p) ppm. 11B NMR (96 MHz, CDCl3) d: 7.7 (h1/2
143 Hz) ppm. Anal. Calc. C, 71.92; H, 4.10; N, 4.42.
Found: C, 71.57; H, 4.08; N, 4.36%.
=
˚
tion, k = 0.71073 A, graphite monochromator. Empiri-
3.3.2. General method for the preparation of diboronate
complexes 3a–3e
cal absorption corrections (DIFABS) were applied. The
structures were solved by direct methods (SHELXS-86)
[14] and refined using SHELXL-97 [12]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms
were placed in geometrically calculated positions using
a riding model. Crystallographic data have been depos-
ited at the Cambridge Crystallographic Data Center as
supplementary material Nos. 256294 and 256295 for
2c and 3e, respectively. Copies of the data can be
obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK. E-mail:
deposit@ccdc.cam.ac.uk.
Compounds 3a–3e were synthesized using two equiv-
alents of ligands 1a–1e and one equivalent of 1,4-ben-
zenediboronic acid. The reaction was refluxed under
THF, after 1 h under stirring, the water and part of
the solvent was removed with a Dean-Stark trap. The
solid was precipitated and filtered; finally the product
was washed with several portions of hexane.
1
Electronic Supplementary Information (ESI) available: experimen-
tal and spectroscopic data for complexes 2b–2h and 3b–3e.

V. Barba et al. / Journal of Organometallic Chemistry 690 (2005) 2351–2357
2357
(c) J.U. Mondal, F.A. Schultz, T.D. Brennan, W.R. Scheidt, Inorg.
Chem. 27 (1988) 3950;
3.3.2.1. 1,4-Benzene-bis[1-[2-[1-(2-phenolate-jO)-imi-
nomethyl-jN]-phenolate-jO]-boronate (3a). 3a was
prepared from 0.50 g (2.34 mmol) of ligand 1a and
0.19 g (1.17 mmol) of 1,4-benzenediboronic acid. A yel-
low solid was obtained, yield 89% (0.54 g, 1.04 mmol)
slightly soluble in DMSO-d6, m.p. = 354–356 ꢁC. IR m
(KBr): 3314, 2942, 1608(C@N), 1572, 1442, 1056, 754,
702 cmꢀ1. FAB-MS m/z: 520. 1H NMR (300 MHz,
DMSO-d6): 9.14 (1H, s, H-7), 7.74 (1H, d, J = 7.6 Hz,
H-10), 7.58-7.52 (2H, m, H-12, H-13) 7.30 (1H, t,
J = 7.9 Hz, H-4), 7.04 (1H, d, J = 7.6 Hz, H-6), 6.97
(1H, t, J = 7.6 Hz, H-11), 6.94 (1H, d, J = 7.9 Hz,
H-3), 6.91 (1H, t, J = 7.9 Hz, H-5), 6.87 (1H, s,
H-15) ppm. 13C NMR (75 MHz, DMSO-d6) d: 158.3
(C-2), 157.0 (C-9), 152.4 (C-7), 138.1 (C-4), 132.7
(C-6), 132.3 (C-12), 130.9 (C-8), 130.2 (C-15), 120.7 (C-
5), 119.9 (C-1), 119.8 (C-3), 119.7 (C-11), 117.1
(C-13), 114.3 (C-10) ppm. 11B NMR (96 MHz, DMSO-
d6) d: 7.6 ppm (h1/2 = 960 Hz). Anal. Calc. C, 73.89; H,
4.26; N, 5.38. Found: C, 73.67; H; 4.08; N, 5.21%.
(d) E.Y. Tshuva, M. Versano, I. Goldberg, M. Kol, H. Weitman, Z.
Goldschmidt, Inorg. Chem. Commun. 2 (1999) 371.
[2] (a) C.Y. Wong, R. McDonald, Inorg. Chem. 35 (1996) 325;
(b) M. Caligaris, L. Randaccio, in: G. Wilkinson, R.D. Gillard,
J. McCleverty (Eds.), Comprehensive Coordination Chemistry,
2, Pergamon Press, Elmsford, NY, 1987;
(c) D.A. Atwood, M.J. Harvey, Chem. Rev. 101 (2001) 37.
[3] (a) Y. Li, Y. Liu, W. Bu, J. Guo, Y. Wang, Chem. Commun.
(2000) 1551;
(b) N. Yalc¸in, A. Kenar, C. Arici, O. Atakol, M. Tastekin, Main
Group Met. Chem. 24 (2001) 247;
´
(c) H.I. Beltran, L.S. Zamudio-Rivera, T. Mancilla, R. Santillan,
´
N. Farfan, Chem. Eur. J. 9 (2003) 2291;
´
´
(d) M. Sanchez, O. Sanchez, H. Ho¨pfl, M.E. Ochoa, D. Castillo,
´
N. Farfan, S. Rojas-Lima, J. Organomet. Chem. 689 (2004)
811;
(e) N. Farfa´n, H. Ho¨pfl, V. Barba, M.E. Ochoa, R. Santillan, E.
´ ´
Gomez, A. Gutierrez, J. Organomet. Chem. 581 (1999)
70.
[4] H. Ho¨pfl, J. Organomet. Chem. 581 (1999) 129.
´ ´
[5] (a) H. Ho¨pfl, M. Sanchez, V. Barba, N. Farfan, S. Rojas, R.
Santillan, Inorg. Chem. 37 (1998) 1679;
(b) V. Barba, D. Cuahutle, M.E. Ochoa, R. Santillan, N. Farfan,
´
Inorg. Chim. Acta 303 (2000) 7;
(c) V. Barba, R. Xochipa, R. Santillan, N. Farfan, Eur. J. Inorg.
Chem. (2004) 118.
´
Acknowledgments
´ ´
[6] H. Ho¨pfl, M. Sanchez, N. Farfan, V. Barba, Can. J. Chem. 76
The authors thank the Consejo Nacional de Ciencia y
(1998) 1352.
[7] H. Reyes, B.M. Mun˜oz, N. Farfan, R. Santillan, S. Rojas-Lima,
P.G. Lacroix, K. Nakani, J. Mater. Chem. 12 (2002) 2898.
´
Tecnologıa (CONACyT, Mexico) for financial support
and Q. Ma. Luisa Rodrıguez for NMR spectra.
´
´
´
´
[8] V. Barba, D. Cuahutle, R. Santillan, N. Farfan, Can. J. Chem. 79
(2001) 1229.
[9] P. Wei, D. Atwood, Inorg. Chem. 37 (1998) 4934.
[10] C.D. Johnson, The Hammett Equation, Cambridge University
Press, Cambridge, 1973.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.jorganchem.2005.01.060.
´
[11] V. Barba, R. Luna, D. Castillo, R. Santillan, N. Farfan,
J. Organomet. Chem. 604 (2000) 273.
´
[12] L.M. Glowka, D. Martynowski, K. Kozlowska, J. Mol. Struct.
474 (1999) 81.
[13] G.M. Sheldrick, SHELXS-86, Program for Crystal Structure Solu-
tion, University of Goettingen, Germany, 1986.
[14] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Goettingen, Germany, 1997.
[15] I.G.C. Coutts, H.R. Goldschmid, C. Musgrave, J. Chem. Soc. C
(1970) 488.
References
[1] (a) S. Dutta, A. Chakratvorty, Polyhedron 13 (1994) 1811;
(b) M.J. Clague, N.L. Keder, A. Butler, Inorg. Chem. 32 (1993) 5343;