Preparation of seven- and eight-membered boron heterocycles from different salen ligands and arylboronic acids

Article abstract of DOI:10.1021/ic010580g

Arylboronic acids and different salen ligands have been brought to reaction in a 2:1 stoichiometry in ethanol, toluene, and acetonitrile. In all cases bimetallic boronates with chiral boron atoms could be isolated with the difference that in ethanol mostly open bimetallic boronic esters are obtained, while in toluene or acetonitrile closed bimetallic complexes with a central seven- or eight-membered heterocyclic ring are formed. Both structural types have been characterized by spectroscopic techniques and X-ray crystallography showing that the reactions are diastereoselective in the case of the bimetallic heterocyclic structures. The configurations and conformations of the seven- and eight-membered rings are different, and this may depend on steric effects and/or repulsive intramolecular π-π interactions between the two salicylidene moieties attached to the central ring.

Full text of DOI:10.1021/ic010580g

Inorg. Chem. 2001, 40, 6405-6412
6405
Preparation of Seven- and Eight-Membered Boron Heterocycles from Different Salen
Ligands and Arylboronic Acids
Mario Sa´nchez,† Herbert Ho1pfl,*,† Maria-Eugenia Ochoa,‡ Norberto Farfa´n,‡
Rosa Santillan,‡ and Susana Rojas§
Centro de Investigaciones Qu´ımicas, Universidad Auto´noma del Estado de Morelos, Av. Universidad
1001, C.P. 62210 Cuernavaca, Morelos, Me´xico, Departamento de Qu´ımica, Centro de Investigacio´n y
de Estudios Avanzados del IPN, Apdo Postal 14-740, C.P. 07000 Me´xico D.F., Me´xico, and Centro de
Investigaciones Qu´ımicas, Universidad Auto´noma del Estado de Hidalgo, Carretera Pachuca-Tulancingo
km 4.5, Ciudad Universitaria, C.P. 42076 Pachuca de Soto, Hidalgo, Me´xico
ReceiVed June 4, 2001
Arylboronic acids and different salen ligands have been brought to reaction in a 2:1 stoichiometry in ethanol,
toluene, and acetonitrile. In all cases bimetallic boronates with chiral boron atoms could be isolated with the
difference that in ethanol mostly open bimetallic boronic esters are obtained, while in toluene or acetonitrile
closed bimetallic complexes with a central seven- or eight-membered heterocyclic ring are formed. Both structural
types have been characterized by spectroscopic techniques and X-ray crystallography showing that the reactions
are diastereoselective in the case of the bimetallic heterocyclic structures. The configurations and conformations
of the seven- and eight-membered rings are different, and this may depend on steric effects and/or repulsive
intramolecular π-π interactions between the two salicylidene moieties attached to the central ring.
Introduction
Scheme 1. Schematic Representation of Bimetallic
Complexes between Salen Ligands and Group 13 Elementsa
The coordination chemistry of salen ligands with transition
metal atoms has been studied extensively for many years1 and
some of the complexes already have some interesting applica-
tions, e.g., in catalytic oxidation reactions2 and simulation of
metalloenzyme-mediated catalysis.3 Therefore, it is somewhat
surprising that the coordination behavior of salen ligands with
main group elements has not received much attention until
recently.4-10 An explanation might be the observation that much
of the chemistry, especially with the heavier main group metals,
parallels the observations with the transition metal series.
Nevertheless, the coordination chemistry of some main group
elements can be quite different, e.g., the group 13 elements
boron, aluminum, gallium, and indium form readily bimetallic
a More detailed representations can be seen in ref 11.
complexes with two of these elements ligated by only one ligand
(Scheme 1).6-7,11
(6) Bimetallic complexes with group 13 elements except boron: (a) Chong,
K. S.; Rettig, S. J.; Storr, A.; Trotter, J. Can. J. Chem. 1977, 55, 2540.
(b) Atwood, D. A.; Jegier, J. A.; Rutherford, D. Bull. Chem. Soc. Jpn.
1997, 70, 2093. (c) Hill, M. S.; Wei, P.; Atwood, D. A. Polyhedron
1998, 17, 811. (d) Van Aelstyn, M. A.; Keizer, T. S.; Klopotek, D.
L.; Liu, S.; Mun˜oz-Herna´ndez, M.-A.; Wei, P.; Atwood, D. A.;
Organometallics 2000, 19, 1796.
(7) Bimetallic complexes with boron: (a) Hohaus, E. Fresenius Z. Anal.
Chem. 1983, 315, 696. (b) Atwood, D. A.; Jegier, J. A.; Remington,
M. J.; Rutherford, D. Aust. J. Chem. 1996, 49, 1333. (c) Wei, P.
Atwood, D. A. Inorg. Chem. 1997, 36, 4060. (d) Wei, P., Atwood, D.
Chem. Commun. 1997, 1427. (e) Wei, P.; Keizer, T.; Atwood, D. A.
Inorg. Chem. 1999, 38, 3914.
† Universidad Auto´noma del Estado de Morelos.
‡ Centro de Investigacio´n y de Estudios Avanzados del IPN.
§ Universidad Auto´noma del Estado de Hidalgo.
* Corresponding author. E-mail: hhopfl@buzon.uaem.mx.
(1) (a) Holm, R. H.; Everett, G. W., Jr.; Chakravorty, A. Prog. Inorg.
Chem. 1966, 7, 83. (b) Hobday, M. D.; Smith, T. D. Coord. Chem.
ReV. 1972, 9, 311.
(2) (a) Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N. J. Am.
Chem. Soc. 1990, 112, 2801. (b) Jacobsen, E. N.; Zhang, W.; Muci,
A. R.; Ecker, J. R.; Deng, L. J. Am. Chem. Soc. 1991, 113, 7063. (c)
Konsler, R. G.; Karl, J.; Jacobsen, E. N. J. Am. Chem. Soc. 1998,
120, 10780.
(3) Berkessel, A.; Bolte, M.; Griesinger, C.; Huttner, G.; Neumann, T.;
Schiemenz, B.; Schwalbe, H.; Schwenkreis, T. Angew. Chem., Int.
Ed. Engl. 1993, 32, 1777.
(8) Monometallic complexes with aluminium: (a) Gurian, P. L.; Cheatham,
L. K.; Ziller, J. W.; Barron, A. R. J. Chem. Soc., Dalton Trans. 1991,
1449. (b) Rutherford, D.; Atwood, D. A. Organometallics 1996, 15,
4417. (c) Mun˜oz-Herna´ndez, M.-A.; Sannigrahi, B.; Atwood, D. A.
J. Am. Chem. Soc. 1999, 121, 6747. (d) Mun˜oz-Herna´ndez, M. A.;
Keizer, T. S.; Parkin, S.; Patrick, B.; Atwood, D. A. Organometallics
2000, 19, 4416.
(9) Monometallic complexes with gallium: (a) Chong, K. S.; Rettig, S.
J.; Storr, A.; Trotter, J. Can. J. Chem. 1981, 59, 94. (b) Hill, M. S.;
Atwood, D. A. Eur. J. Inorg. Chem. 1998, 67. (c) Shen, Y.-Z.; Pan,
Y.; Wang, L.-Y.; Dong, G.; Jin, X.-P.; Huang, X.-Y.; Hu, H. J.
Organomet. Chem. 1999, 590, 242 and ref 6a.
(4) Complexes with group 14 elements: (a) Calligaris, M.; Nardin, G.;
Randaccio, L. J. Chem. Soc., Daltons Trans. 1972, 2003. (b) Parr, J.;
Ross, A. T.; Slawin, A. M. Z. J. Chem. Soc., Dalton Trans. 1996,
1509. (c) Teoh, S.-G.; Yeap, G.-Y.; Loh, C.-C.; Goong, L.-W.; Teo,
S.-B.; Fun, H.-K. Polyhedron 1997, 16, 2213. (d) Agustin, D.; Rima,
G.; Gornitzka, H.; Barrau, J. J. Organomet. Chem. 1999, 592, 1. (e)
Mucha, F.; Haberecht, J.; Bo¨hme, U.; Roewer, G. Monatshefte. Chem.
1999, 130, 117. (f) Singh, M. S.; Singh, P. K. Main Group Met. Chem.
2000, 23, 183.
(5) Complexes with group 15 elements: Agustin, D.; Rima, G.; Gornitzka,
H.; Barrau, J. Organometallics 2000, 19, 4276.
(10) Monometallic complexes with indium: Hill, M. S.; Atwood, D. A.
Main Group Chem. 1998, 2, 191.
10.1021/ic010580g CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/26/2001

6406 Inorganic Chemistry, Vol. 40, No. 25, 2001
Sa´nchez et al.
Scheme 3. Preparation of the Boron Complex 3b from
Scheme 2. Preparation of the Boron Complexes 1a-c and
2a-c from SalenH2 and AcenH2
SalphenH2
Scheme 4. Preparation of the Boron Complex 4b from
SalcenH2
While in the case of the boron atom so far only bimetallic
complexes are known,7 the heavier elements aluminum,6,8
gallium,6,9 and indium6,10 form also monometallic complexes.
Bimetallic compounds could be interesting for applications in
catalysis6d and might be used for further reactions with other
ligands or other metal complexes. However, with exception of
the preparation of a macrocyclic derivative,7e this chemistry has
not been explored so far.
Continuing with our studies on the preparation of air-stable
boron compounds with a coordinative NfB bond,12 in this
contribution we report for the first time on the synthesis as well
as spectroscopic, structural, and conformational characterization
of several seven- and eight-membered boron heterocyclic
compounds that have been prepared from different salen ligands
and arylboronic acids.
Scheme 5. Preparation of the Boron Complex 5b from
SalpenH2
Results and Discussion
Synthesis and Spectroscopic Characterization. The prepa-
ration of the bimetallic salen complexes described here is based
on condensation reactions between the corresponding ligand and
an arylboronic acid. The syntheses of the boron complexes are
shown in Schemes 2-6.
Two different products can be obtained, depending on the
ligand and solvent used for the reaction. In ethanol, bimetallic
(11) Atwood, D. A.; Harvey, M. J. Chem. ReV. 2001, 101, 37.
(12) See for example: (a) Ho¨pfl, H.; Farfa´n, N.; Castillo, D.; Santillan,
R.; Contreras, R.; Mart´ınez-Mart´ınez, F.; Galva´n, M.; Alvarez, R.;
Ferna´ndez, L.; Halut, S.; Daran, J.-C. J. Organomet. Chem. 1997, 544,
175. (b) Ho¨pfl, H.; Farfa´n, N. J. Organomet. Chem. 1997, 547, 71.
(c) Ho¨pfl, H., Galva´n, Farfa´n, N.; Santillan, R. J. Mol. Struct.
THEOCHEM 1998, 427, 1. (d) Ho¨pfl, H.; Sa´nchez, M.; Farfa´n, N.
Can. J. Chem. 1998, 76, 1352. (e) Ho¨pfl, H.; Farfa´n, N. Can. J. Chem.
1998, 76, 1853. (f) Ho¨pfl, H.; Sa´nchez, M.; Barba, V.; Farfa´n, N.,
Rojas, S.; Santillan, R. Inorg. Chem. 1998, 37, 1679. (g) Ho¨pfl, H.;
Farfa´n, N.; Castillo, D.; Santillan, R.; Gutierrez, A.; Daran, J.-C. J.
Organomet. Chem. 1998, 553, 221. (h) Trujillo, J.; Ho¨pfl, H.; Castillo,
D.; Santillan, R.; Farfa´n, N. J. Organomet. Chem. 1998, 571, 21. (i)
Farfa´n, N.; Ho¨pfl, H.; Barba, V.; Ochoa, M. E.; Santillan, R.; Go´mez,
E.; Gutie´rrez, A. J. Organomet. Chem. 1999, 581, 70. (j) Farfa´n, N.;
Santillan, R.; Ho¨pfl, H. Main Group Chem. News 1999, 7, 3. (k) Barba,
V.; Cuahutle, D.; Ochoa, M. E.; Santillan, R.; Farfa´n, N. Inorg. Chim.
Acta 2000, 303, 7. (l) Barba, V.; Luna, R.; Castillo, D.; Santillan, R.;
Farfa´n, N. J. Organomet. Chem. 2000, 604, 273.
complexes similar to the ones reported by Hohaus,7a Atwood,7b-e
and Barrau5 are formed with salenH2, acenH2 (Scheme 2), and
acpenH2 (Scheme 6), while in toluene or acetonitrile hetero-
cycles with a central seven- or eight-membered CnB2N2O (n )

Boron Heterocycles from Different Salen Ligands
Inorganic Chemistry, Vol. 40, No. 25, 2001 6407
Scheme 6. Preparation of the Boron Complexes 6a-6b
formed in all cases. Nevertheless, this happens only for the open
bimetallic complexes 1a, 2a, and 6a, while in the case of the
seven- and eight-membered heterocycles 1b, 1c, 2b, 2c, 3b, 4b,
5b, and 6b only one diastereomer could be isolated. The
formation of diastereomers can be easily deduced from 1H and
13C NMR spectroscopic data, since two methylene groups, two
imino groups, and two B-ethoxy groups can be distinguished
in the spectra of 1a and 2a. In the first case, one diastereomer
is produced in higher yields than the other one (approximately
1.5:1), while in the latter case the stoichiometric proportion is
about 1:1. For compound 6a, the shift differences for the two
diastereomers are smaller and almost undistinguishable due to
the presence of three methylene groups between the imine
functions. As already mentioned, the closed bimetallic hetero-
cycles 1b, 1c, 2b, 2c, 3b, 4b, 5b, and 6b have been synthesized
diastereoselectively, since in each case only signals for one
diastereomer could be detected. However, the spectroscopic data
do not permit prediction of which of the two possible diaster-
eomers (cis or trans with respect to the B-phenyl group) has
been formed. One reason is that both have molecular symmetry.
The SS/RR enantiomeric pairs with trans-configuration of the
B-aryl groups have C2 symmetry, while the RS/SR mesomeric
form with cis-configuration has a molecular plane. The only
exception from this consideration is compound 4b, with two
additional stereogenic centers. For this molecule there should
exist finally three diastereomers with their corresponding
enantiomers as outlined in Scheme 7. The RR/SS-trans isomers
have C2-symmetry, while the RS/SR-cis isomers are completely
unsymmetrical. Therefore, on the basis of NMR data it is here
possible to differentiate between these two classes of diaster-
eomers. Both in the 1H and 13C NMR spectrum, signals for an
unsymmetrical molecule are observed, so that the configuration
of the B-aryl groups in compound 4b must be cis.
from AcpenH2
2, 3) ring are isolated. However, the seven-membered hetero-
cycles 1b and 2b outlined in Scheme 2 can be transformed to
the open bimetallic boron esters 1a and 2a, respectively, by
heating them in ethanol. With salphenH2 3, salcenH2 4, and
salpenH2 5, this alcoholic hydrolysis is not possible and only
closed heterocycles can be isolated, even if the boron complex-
ation is realized in ethanol (Schemes 3-5).
All complexes obtained are stable under normal air conditions
but decompose in solution partially to polymeric material. In
general, the closed complexes seem to be more stable in solution,
especially the eight-membered heterocycles (vide infra), since
smaller quantities of decomposition products are observed in
the NMR spectra.
Structural Characterization. To determinate which diaste-
reomer is preferred in the case of the other seven- and eight-
membered heterocycles, an X-ray crystallographic study has
been undertaken for compounds 1c, 2c, and 5b. Furthermore,
the molecular structures of compounds 1a and 6a have been
determined. Crystallographic data as well as selected bond
lengths, bond angles, and torsion angles have been summarized
in Tables 1 and 2.
Figure 1 shows the molecular structure of the open bimetallic
complex 1a. Surprisingly, the molecule has a gauche-conforma-
tion in the solid state with a NCCN torsion angle of 67.9°. The
preference of this conformation may be explained by the
formation of intramolecular hydrogen bonds between the imino
hydrogens and the oxygen atoms of the corresponding adjacent
BOEt functional groups (2.33 and 2.41 Å), an interaction that
would not be possible in the anti-conformation. A similar
interaction has been also reported by Atwood.7c As observed
already in related structure types,12f the mean B-Oph bond
length is significantly longer than the mean B-OEt bond length
(1.495(6) Å T 1.436(6) Å). The NfB bond with a length of
1.616(6) Å is relatively strong.14
In the case of acphenH2 and accenH2 with a methyl group at
the imine function, boron complexation is inhibited most
probably for steric reasons, since no reaction occurred with these
ligands. This destabilizing effect can be also seen, if the boron
complexation with salpenH2 5 and acpenH2 6 is compared.
While in the former case only the closed heterocyclic derivative
5b is obtained in either ethanol, toluene, or acetonitrile (Scheme
5), in the latter case both the open and closed bimetallic complex
(6a, 6b) can be prepared (Scheme 6).
Although the formulas of the open and corresponding closed
bimetallic boron complexes are similar, their physical properties
are quite different. The open compounds have lower melting
points and are quite soluble in organic solvents such as toluene,
chloroform, ethers, and alcohols. In contrast, the closed com-
plexes have high melting points (often more than 300 °C) and
are quite insoluble.
All boron complexes obtained in this study have been
characterized as far as possible by spectroscopic techniques (IR
and 1H, 13C, 11B NMR) and mass spectrometry. The formation
of a coordinative NfB boron bond could be confirmed by the
characteristic shift displacement of the imine hydrogen or imine
methyl group12d,e as well as by the appearance of diastereotopic
NCH2 hydrogens due to the chirality of the tetracoordinated
boron atoms. The bidentate complexation was confirmed in
some cases by 11B NMR spectroscopy that is sensitive to the
coordination sphere of the boron atom.13
Figures 2 and 3 show the molecular structures of the closed
bimetallic seven-membered heterocycles 1c and 2c, and appar-
ently in both cases the trans-configurated heterocycle is formed
preferentially. The central seven-membered heterocycle has a
conformation in which the imino groups are displaced signifi-
cantly out of the mean plane formed by the other five atoms
(0.720 Å for 1c and 0.760 Å for 2c). In compound 1c, the
maximum deviation of this mean plane is only 0.015 Å (for
Due to the complexation of two chiral boron atoms, two
diastereomers with their corresponding enantiomers should be
(13) No¨th, H.; Wrackmeyer, B. NMR Basic Principles and Progress; Diehl,
P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin 1978; Vol.
14.
(14) Ho¨pfl, H. J. Organomet. Chem. 1999, 581, 129.

6408 Inorganic Chemistry, Vol. 40, No. 25, 2001
Scheme 7. Possible Stereoisomers for Compound 4ba
Sa´nchez et al.
Figure 1. Perspective view of the molecular structure of compound
1a.
a According to the presence of four stereogenic centers there should
exist 24/2 stereoisomers, because only the 1,2-trans-cyclohexylene
derivative was used. However, there are only three different diastere-
omers with their corresponding enantiomers, because two of the possible
four diastereomers are identical. Two of the three diastereomers have
C2-symmetry, while the third is completely asymmetrical. NMR studies
proved that only this diastereomer (RS/SR-cis-isomer) is obtained.
atom O1), while in compound 2c this plane is completely planar
due to the fact that the molecules are located on crystallographic
C2-axes in the crystal lattice. As in the case of compound 1b,
the two types of B-O bond lengths are significantly different.
The mean B-O bond lengths with the bridging oxygen atoms
are 1.415(2) and 1.411(6) Å for 1c and 2c, respectively, and
the mean B-OPh bond lengths are 1.499(2) and 1.490(5) Å,
respectively. The corresponding NfB bond lengths are 1.612(2)
and 1.626(6) Å. The B-O-B bond angles are 135.4(1)° and
137.8(5)° for 1c and 2c, respectively; however, such large
B-O-B bond angles are not uncommon, and similar values
have been measured for a series of other boron compounds.15
If the tetrahedral geometry of the boron atoms is compared
between 1a and 1c, it is interesting to notice that it is more
perfect in the closed bimetallic complex than in the open one.
The values of the mean tetrahedral characters14 are 74.2% and
Figure 2. Perspective view of the molecular structure of compound
1c.
87.1%, respectively. The main contribution to this observation
results from a different N-B-OB/Et bond angle with a mean
value of 102.6(4)° for 1a in contrast to a mean value of 110.0(1)°
for 1c. The mean displacement of the boron atoms from the
mean plane of the six-membered salicylidene heterocycles
(aromatic part) is larger for 1a (0.634 Å) than for 1c (0.239 Å).
The mean torsion angles in the seven-membered heterocycles
are quite different and vary in the case of 1c from 28.6° for the
NBOB bonds to 75.9° for the B-N-CH2-CH2 bonds. The
corresponding values for compound 2c are between 28.1° and
80.0°.
Figures 4 and 5 show the molecular structures of the eight-
membered C3B2N2O heterocycle 5b and the related open
bimetallic complex 6a. The X-ray study demonstrates that in
the case of the eight-membered heterocycle the cis-configuration
is preferred over the trans-configuration. Therefore, in this case
(15) Ho¨pfl, H.; Barba, V.; Vargas, G.; Farfa´n, N.; Santillan, R.; Castillo,
D. Chem. Heterocycl. Compounds 1999, 386, 1041, and references
therein.

Boron Heterocycles from Different Salen Ligands
Inorganic Chemistry, Vol. 40, No. 25, 2001 6409
Table 1. Crystallographic Data for Compounds 1a, 1c, 2c, 5b, and 6a
crystal data
formula
1aa
1cb
2cb
5bc
6ab
C32H34B2N2O4
0.3 × 0.3 × 0.3
532.26
C28H20B2F4N2O3
0.3 × 0.3 × 0.4
530.08
C30H24B2F4N2O3
0.3 × 0.4 × 0.4
558.15
C29H26B2N2O3
0.2 × 0.3 × 0.3
472.16
C35H40B2N2O4
0.1 × 0.3 × 0.5
574.31
crystal size (mm)
MW (g mol-1
space group
)
P21/c
P21/c
C2/c
P21/c
Pna21
Cell Parameters
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
13.985(1)
13.831(1)
15.298(1)
90
97.573(9)
90
2933.3
4
0.073
9.7350(4)
21.4186(9)
12.1635(5)
90
100.936(1)
90
2490.2
4
0.111
28.597(2)
13.0190(7)
20.315(1)
90
132.896(1)
90
5540.8
8
0.103
10.150(3)
19.230(5)
12.250(3)
90
95.600(13)
90
2379.6
4
0.084
17.784(4)
7.965(2)
23.028(5)
90
90
90
3261.8
4
0.075
1.17
µ (mm-1
calcd (g cm-3
)
F
)
1.21
1.41
1.34
1.32
Data Collection
θ limits (deg)
hkl limits
no. collected refl
no. ind. refl. (Rint)
no. observed refl
2 < θ < 24
15, 0; 0, 15; 17, 17
5427
2 < θ < 26
11, 12; 24, 26; 15, 13
16431
2 < θ < 25
35, 27; 16, 16; 24, 25
18378
2 < θ < 25
12, 12; 23, 24; 15, 14
5213
2 < θ < 25
21, 18; 7, 9; 27, 25
19159
3033 (0.02)
4888 (0.04)
5429 (0.02)
5047 (0.03)
5398 (0.09)
1882d
2722e
2159d
2702e
1820e
Refinement
R
Rw
w
0.045f
0.041h
1/σ2
0.040g
0.044f
0.040h
1/σ2
0.056f
0.060h
1/σ2
0.046g
0.098i
0.106i
j
k
no. of variables
GOOF
362
352
373
402
369
0.80
0.18
0.17
0.85
0.18
0.14
1.15
0.13
0.17
3.71
0.21
0.24
0.75
0.10
0.11
∆Fmin (e Å-3
)
)
∆Fmax (e Å-3
a Data collection on an Enraf Nonius CAD4 diffractometer. b Data collection on a Bruker Smart 6000 diffractometer. c Data collection on a
2
2
Rigaku Mercury diffractometer. d I > 3σ(I). e Fo > 4σ(Fo). f R ) ∑(||Fo| - |Fc||)/∑|Fo|). g R ) ∑(Fo - Fc2)/∑Fo . h Rw ) [∑w(|Fo| - |Fc|)2/
∑wFo ]1/2
.
i Rw ) [∑w(Fo - Fc2)2/∑w(Fo )2]1/2 j w-1 ) σ2Fo + (0.0511P)2 + 0.00P; P ) (Fo + 2Fc2)/3. k w-1 ) σ2Fo + (0.0358P)2 + 0.00P;
.
2
2
2
2
2
2
P ) (Fo + 2Fc2)/3.
2
Table 2. Selected Bond Lengths, Bond Angles, and Torsion Angles
for Compounds 1a, 1c, 2c, 5b, and 6aa
1a
1c
2c
5b
6a
Bond Lengths (Å)
N-B
1.616(6) 1.612(2) 1.626(6) 1.636(3) 1.611(7)
1.495(6) 1.499(2) 1.490(5) 1.502(3) 1.459(8)
1.436(6) 1.415(2) 1.411(6) 1.420(3) 1.444(6)
1.595(7) 1.623(2) 1.627(6) 1.607(4) 1.610(7)
1.290(5) 1.287(2) 1.303(5) 1.290(3) 1.286(6)
1.473(5) 1.466(2) 1.478(5) 1.477(3) 1.467(5)
1.529(6) 1.507(2) 1.528(8) 1.523(4) 1.527(6)
B-OPh
B-OEt/B-OB
B-C
CdN
N-CH2
CH2-CH2
O-CH2
OCH2-CH3
1.417(6)
1.415(7)
1.428(6)
1.414(8)
Bond Angles (deg)
N-B-OPh
O-B-O
N-B-C
105.1(4) 107.2(1) 107.3(4) 106.7(2) 107.3(5)
112.5(4) 109.4(2) 110.2(4) 112.1(2) 112.6(5)
111.0(4) 109.1(1) 109.5(4) 108.3(2) 112.3(5)
N-B-OEt/N-B-OB 102.6(4) 110.0(1) 110.0(4) 109.4(2) 102.5(4)
O
Ph-B-C
108.9(4) 107.3(1) 105.6(4) 108.7(2) 108.5(5)
O
Et-B-C/OB-B-C 116.3(5) 113.8(1) 114.0(4) 111.7(2) 113.6(5)
B-NdC
119.3(4) 122.2(2) 123.2(4) 122.7(2) 123.1(5)
121.8(4) 118.8(4) 115.1(4) 118.9(2) 115.7(5)
117.7(4) 124.0(2) 122.5(4) 126.0(2) 122.4(5)
110.1(4) 113.0(2) 111.3(3) 114.1(2) 114.6(4)
B-N-CH2
B-O-CPh
N-CH2-CH2
B-O-CH2/B-O-B 117.9(4) 135.4(1) 137.8(5) 134.6(2) 117.6(4)
CH2-CH2-CH2
115.9(2) 107.8(3)
Torsion Angles (deg)
N-C-C-N
67.9
63.0
67.8
N-CH2-CH2-CH2
B-N-CH2-CH2
59.1
54.4
63.3
51.1
58.2
178.8
101.9
74.5
75.9
74.5
28.6
51.0
80.0
73.1
28.6
52.9
O
B-B-N-CH2
B-O-B-N
C-B-N-CH2
31.2
46.7
Figure 3. Perspective view of the molecular structure of compound
2c.
a Mean values in all cases.
a boat conformation is formed, again with the two imino-
nitrogen atoms at the bow/stern positions. There may be an
intramolecular C-H‚‚‚O interaction, since the C18-H‚‚‚O2
distance is only 2.39 Å (C18-H‚‚‚O2, 110.4°), a value that is
significantly smaller than the sum of the van der Waals radii of
oxygen and hydrogen (2.70 Å).16

6410 Inorganic Chemistry, Vol. 40, No. 25, 2001
Sa´nchez et al.
CH2 bonds. This observation may indicate a higher thermody-
namic stability of the eight-membered heterocycle when com-
pared to the seven-membered one and is confirmed by the fact
that the latter one can be hydrolyzed with ethanol, while the
former one cannot (Schemes 2 and 5). The mean deviation of
the boron atoms from the aromatic mean planes of the
salicylidene moieties is 0.149 Å.
The conformation of the open derivative 6a is anti. Surpris-
ingly, in this case there is no significant length difference
between the two types of B-O bonds. The mean values for the
B-OPh and B-OEt bond lengths are 1.459(8) and 1.444(6) Å.
As in the case of the eight-membered heterocycle, the NfB
bond lengths are significantly different, 1.589(6) and 1.632(7)
Å. The mean tetrahedral character for the boron atoms is 77.5%,
whereby the bond angle with the largest deviation from the ideal
tetrahedral angle is the NBOB/Et bond angle with a mean value
of 102.5 (4)°, as was the case for compound 1a. For comparison,
the mean NBOB/Et bond angle in compound 5b is 109.4(2)° and
the tetrahedral character is 88%. The mean deviation of the
boron atom from the aromatic mean planes of the salicylidene
moieties is 0.390 Å.
Unfortunately, we were not able to obtain single crystals for
the bimetallic salphenH2 and acpenH2 derivatives 3b and 6b in
order to establish their preferred configuration. According to
molecular modeling, both configurations may be possible in
either case, but our NMR data indicate only the formation of
one diastereomer.
Figure 4. Perspective view of the molecular structure of compound
5b.
Conclusions
This contribution reports to the best of our knowledge for
the first time on the formation of seven- and eight-membered
bimetallic heterocycles from ligands of the salen type. It could
be shown that both types of heterocycles are held together by
two strong NfB bonds that induce a fixed conformation. For
the seven-membered heterocycle with an ethylene bridge
between the two imino groups, the preferred configuration is
trans and the conformation is a practically undistorted chair
with five atoms forming the central plane. If the ethylene bridge
is substituted by a trans-1,2-cyclohexenyl bridge, the cis-
configuration is preferred. In the case of a 1,2-phenylene unit,
it is difficult to make a prediction without a crystallographic
study. In boron complexes with an eight-membered heterocycle,
the cis-configuration and boat conformation seem to be favored.
As shown by the X-ray crystallographic study, they are probably
more stable than the seven-membered derivatives. The reasons
that different configurations and conformations are obtained for
the two structure types are not clear; however, it can be shown
by molecular modeling that in a cis-configured, seven-membered
heterocycle the proximity between the two salicylidene moieties
would be repulsive, due to π-π interactions.
Figure 5. Perspective view of the molecular structure of compound
6a.
Due to the cis-configuration of the eight-membered hetero-
cycle 5b, the two salicylidene groups are orientated nearly
parallel to each other, but it is important to notice that they are
not superposable as it would be the case for a molecule with
mirror symmetry. The distance between the centroids of the
aromatic rings is 3.90 Å, a distance that is longer than the sum
of the van der Waals radii of two carbon atoms (3.4-3.6 Å).16
Therefore, there should be no transannular π-π interaction.17
Again, in this case the mean B-O bond length with the phenolic
oxygen atoms is much larger (1.502(3) Å) than the mean B-O
bond length with the bridging oxygen (1.420(3) Å). The two
NfB bonds are significantly different, 1.621(3) and 1.650(3)
Å. The B-O-B bond angle with a value of 134.6(2)° is
comparable to the ones found for the seven-membered hetero-
cycles 1c and 2c. Furthermore, it is noteworthy that the C-C-C
bond angle that is located opposite to the BOB bond angle in
the heterocycle has an unusually large value of 115.9(2)°. The
torsion angles in the eight-membered heterocycle are within a
smaller range than in the former case and have values from
51.1° for the B-O-B-N bonds to 63.3° for the O-B-N-
Experimental Section
Instrumental. NMR studies were carried out with Bruker 300, JEOL
270, and JEOL Eclipse+400 instruments. Standards were TMS (1H,
13C) and BF3‚OEt2 (11B). Chemical shifts are stated in parts per million;
they are positive, when the signal is shifted to higher frequencies than
the standard. COSY, HMQC, and NOESY experiments have been
1
carried out in order to assign the H and 13C spectra completely. IR
spectra were recorded on a Bruker Vector 22 FT spectrophotometer.
Mass spectra were obtained on a HP 5989 A equipment. Elemental
analyses have been carried out on a Perkin-Elmer Series II 2400
instrument. It should be mentioned that elemental analyses of boronic
(16) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(17) Glo´wka, M. L.; Martynowski, D.; Kozlowska, K. J. Mol. Struct. 1999,
474, 81, and references therein.

Boron Heterocycles from Different Salen Ligands
Inorganic Chemistry, Vol. 40, No. 25, 2001 6411
1
acid derivatives are complicated by incombustible residues (boron
carbide) and therefore not always in the established limits of exacti-
tude.18
Yield: 94%. Mp: >300 °C. H NMR (300 MHz, DMSO-d6, 25 °C,
TMS): δ ) 3.50 and 3.79 (AB, 4 H, N-CH2), 6.92 (m, 10 H, H-3,
H-5, m-BC6H5, p-BC6H5), 7.19 (dd, 4 H, o-BC6H5), 7.42 (dd, 2 H, H-6),
7.57 (dt, 2 H, H-4), 8.59 (s, 2 H, NdCH) ppm. 13C NMR (75 MHz,
DMSO-d6, 25 °C, TMS): δ ) 56.2 (NCH2), 115.9 (C-1), 117.6 (C-5),
118.4 (C-3), 126.5 (m-BC6H5), 126.8 (p-BC6H5), 131.8 (o-BC6H5),
132.1 (C-6), 137.5 (C-4), 160.2 (C-2), 164.5 (NdCH) ppm. 11B NMR
(96.3 MHz, DMSO-d6, 25 °C, BF3‚OEt2): δ ) 10 (h1/2 ) 900 Hz)
ppm. IR (KBr): ν˜ ) 1638 (s), 1608 (m), 1557 (m), 1479 (m), 1462
(m), 1431 (w), 1400 (w), 1317 (m), 1233 (m), 1198 (s), 1152 (m),
1138 (m), 1096 (w), 1028 (w) cm-1. MS (70 eV, EI), m/z (%): 458
(0.2) [M]+, 457 (0.9) [M - H]+, 381 (100) [M - C6H5]+, 305 (19),
248 (15), 234 (27), 152 (18), 77 (19). Anal. Calcd (%) for C28H24B2N2O3
(458.14): C, 73.41; H, 5.28; N, 6.12. Found: C, 73.69; H, 5.39; N,
6.09.
X-ray Crystallography. X-ray diffraction studies of single crystals
were realized on Enraf Nonius CAD4, Bruker Smart 6000 and Rigaku
Mercury diffractometers (λMo KR ) 0.71069 Å, monochromator: graph-
ite, T ) 293 K). Cell parameters were determined by least squares
refinements on diffractometer angles for 24 automatically centered
reflections or using reflections collected on three sets of 20 frames
each. Absorption correction was not necessary; corrections were made
for Lorentz and polarization effects. Solution and refinement: direct
methods (SHELXS-86)19 for structure solution and the SHELXTL20-22
(1c and 6a) or CRYSTALS23-25 (1a, 2c and 5b) software package for
refinement and data output. Non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were calculated in all cases with the
exception of 5b, and one overall isotropic thermal parameter was
refined. In the case of 5b, the hydrogen atoms were determined with
difference Fourier maps and their coordinates were refined. In general,
a reflection-parameter ratio of 5 has been considered sufficient for
the type of structural studies performed here. The most important
crystallographic data have been summarized in Table 1. In the case of
compound 2c, the molecules are located at special positions (C2-axis)
in the crystal lattice and there are two independent molecule halves in
the asymmetric unit. The B-phenyl groups in 6a are slightly disordered,
so that their bond lengths and bond angles were restrained to 1.390 Å
and 120.0°.
Compound 1c was prepared from salenH2 1 (0.50 g, 1.86 mmol)
and 2,4-difluorophenylboronic acid (0.56 g, 3.72 mmol) in acetonitrile
(20 mL). After 1 h of reflux the solution was concentrated with a Dean-
Stark trap. A white precipitate formed that was filtered under vacuum
and dried. Yield: 93%. Crystals were obtained from acetonitrile. Mp:
1
>300 °C. H NMR (270 MHz, DMSO-d6, 25 °C, TMS): δ ) 3.38
and 3.90 (AB, 4 H, NCH2), 6.38 (ddd, J ) 9 and 2 Hz, 2 H,
m′-BC6H3F2), 6.77 (ddd, J ) 8 and 2 Hz, 2 H, m-BC6H3F2), 6.90 (m,
4 H, H-3, H-5), 7.24 (dd, J ) 16 and 8 Hz, 2 H, o′-BC6H3F2), 7.44
(dd, 2 H, H-6), 7.60 (dt, 2 H, H-4), 8.61 (s, 2 H, NdCH) ppm. 13C
NMR (67.5 MHz, DMSO-d6, 25 °C, TMS): δ ) 56.6 (N-CH2), 102.9
(dd, m-BC6H3F2), 109.7 (dd, m′-BC6H3F2), 116.5 (C-1), 118.6, 118.9
(C-3, C-5), 132.7 (C-6), 136.0 (dd, o′-BC6H3F2), 138.3 (C-4), 160.3
(C-2), 162.4, 166.1 (dd, J ) 240 and 11 Hz, o-BC6H5, p-BC6H5), 164.9
(NdCH) ppm. 11B NMR (128.3 MHz, DMSO-d6, 25 °C, BF3‚OEt2):
δ ) 1.7 (h1/2 ) 500 Hz) ppm. IR (KBr): ν˜ ) 3063 (w), 2957 (w),
1642 (s), 1603 (s), 1558 (s), 1484 (s), 1464 (s), 1448 (s), 1402 (s),
1358 (m), 1316 (s), 1284 (m), 1260 (m), 1237 (s), 1225 (s), 1171 (s),
1155 (s), 1138 (s), 1128 (s), 1113 (s), 1096 (m), 1077 (s), 1028 (m),
1012 (w), 1005 (w) cm-1. MS (70 eV, EI), m/z (%): 530 (0.3) [M]+,
417 (100) [M - C6H3F2]+, 415 (16), 323 (30), 304 (9), 277 (5), 270
(9). Anal. Calcd (%) for C28H20B2F4N2O3 (530.11): C, 63.44; H, 3.80;
N, 5.28. Found: C, 63.96; H, 3.85; N, 4.82.
Preparative Part. Commercial starting materials and solvents have
been used. Ligands 1-6 are known Schiff bases and have been prepared
according to refs 12b and 12f.
Preparation of Boronates 1a-c. Compound 1a was prepared from
salenH2 1 (0.50 g, 1.86 mmol) and phenylboronic acid (0.45 g, 3.72
mmol) in ethanol (30 mL). After 30 min of reflux the solution was
concentrated with a Dean-Stark trap. A yellow precipitate formed that
was filtered under vacuum and dried. Crystals were obtained after
recrystallization from ethanol. Two diastereomers are obtained in a ratio
of approximately 1.5:1. The product decomposes more or less rapidly
in other organic solvents to polymeric material. Yield: 89%. Mp: 180-
182 °C. 1H NMR (400 MHz, C6D6, 25 °C, TMS) of the mayor
product: δ ) 1.14 (t, 6 H, CH3), 3.18 and 3.52 (AB, 4 H, NCH2), 3.76
(m, 4 H, OCH2), 6.41 (m, 2 H, H-5), 6.80 (m, 2 H, H-3), 7.10 (m, 4 H,
H-4, H-6), 7.34 (m, 2 H, p-BC6H5), 7.45 (m, 4 H, m-BC6H5), 7.80 (m,
4 H, o-BC6H5), 8.34 (s, 2 H, NdCH) ppm. 13C NMR (100 MHz, C6D6,
25 °C, TMS): δ ) 18.6 (CH3), 54.6 (N-CH2), 57.3 (OCH2), 117.0,
118.5 (C-1, C-5), 119.4 (C-3), 127.7 (m-BC6H5, p-BC6H5), 131.5 (C-
6), 133.3 (o-BC6H5), 137.2 (C-4), 161.0 (C-2), 165.4 (NdCH) ppm.
IR (KBr): ν˜ ) 3046 (w), 3005 (w), 2958 (w), 2864 (w), 1636 (s),
1609 (m), 1558 (m), 1479 (m), 1461 (m), 1433 (w), 1406 (w), 1315
(m), 1234 (m), 1197 (w), 1151 (s), 1136 (m), 1101 (m), 1063 (w),
1026 (w) cm-1. MS (70 eV, EI), m/z (%): 487 (6) [M - OEt]+, 455
(100) [M - C6H5]+, 411 (31), 277 (19), 204 (17), 189 (15), 174 (14),
152 (26), 147 (14), 105 (25), 77 (11). Anal. Calcd (%) for C32H34B2N2O4
(532.25): C, 72.21; H, 6.43; N, 5.26. Found: C, 71.01; H, 6.45; N,
5.47.
Preparation of Boronates 2a-c. Compound 2a was prepared from
acenH2 2 (0.50 g, 1.69 mmol) and phenylboronic acid (0.41 g, 3.38
mmol) in ethanol (30 mL). After 30 min of reflux the solution was
concentrated with a Dean-Stark trap. A yellow precipitate formed that
was filtered under vacuum and dried. Two diastereomers are obtained
in a ratio of approximately 1:1. The product decomposes more or less
rapidly in other organic solvents to polymeric material. Yield: 77%.
1
Mp: 260-262 °C. H NMR (400 MHz, DMSO-d6, 25 °C, TMS) for
the mixture of the two diastereomers: δ ) 1.06 (m, 6 H, OCH2CH3),
2.34, 2.58 (s, 6 H, NdCCH3), 3.17 and 3.33 (m, 4 H, OCH2), 3.36-
3.78 (m, 4 H, NCH2), 6.83 (m, 4 H, H-3, H-5), 7.14 (m, 6 H, m-BC6H5,
p-BC6H5), 7.36 (m, 4 H, o-BC6H5), 7.47 (m, 2 H, H-4), 7.74 (m, 2 H,
H-6) ppm. IR (KBr): ν˜ ) 3066 (w), 3043 (w), 3005 (w), 2968 (m),
2922 (w), 2862 (m), 1616 (s), 1555 (s), 1478 (m), 1458 (m), 1432
(m), 1384 (m), 1358 (m), 1334 (m), 1281 (m), 1235 (w), 1189 (m),
1172 (m), 1144 (m), 1107 (s), 1069 (m), 1022 (m) cm-1; MS (70 eV,
EI), m/z (%): 515 (1) [M - OEt]+, 483 (14) [M - C6H5]+, 437 (100),
391 (10), 305 (6), 248 (16), 216 (7), 203 (8), 188 (11), 166 (11), 146
(6), 105 (6). Anal. Calcd (%) for C34H38B2F4N2O4 (560.28): C, 72.88;
H, 6.84; N, 5.00. Found: C, 72.82; H, 6.85; N, 5.02.
Compound 1b was prepared from salenH2 1 (0.50 g, 1.86 mmol)
and phenylboronic acid (0.45 g, 3.72 mmol) in toluene (10 mL). After
1 h of reflux, the solution was concentrated with a Dean-Stark trap.
A yellow precipitate formed that was filtered under vacuum and dried.
Compound 2b was prepared from acenH2 2 (0.50 g, 1.69 mmol)
and phenylboronic acid (0.41 g, 3.38 mmol) in acetonitrile (20 mL).
After 1 h of reflux the solution was concentrated with a Dean-Stark
trap. A white precipitate formed that was filtered under vacuum and
(18) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1910, and references therein.
(19) Sheldrick, G. M. SHELX86, Program for Crystal Structure Solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(20) Bruker Analytical X-ray Systems. SMART: Bruker Molecular Analysis
Research Tool V. 5.057 c, 1997-98.
1
dried. Yield: 95%. Mp: 277-280 °C. H NMR (270 MHz, DMSO-
d6, 25 °C, TMS): δ ) 2.47 (s, NdCCH3), 3.84 and 4.16 (AB, 4 H,
NCH2), 6.56 (d, 2 H, H-3), 6.71 (dd, 2 H, H-5), 7.11 (m, 6 H, m-BC6H5,
p-BC6H5), 7.35 (dd, 2 H, H-4), 7.42 (d, 4 H, o-BC6H5), 7.56 (dd, 2 H,
H-6) ppm. 13C NMR (67.5 MHz, DMSO-d6, 25 °C, TMS): δ ) 16.7
(CH3), 46.4 (NCH2), 117.8 (C-1), 118.9, 119.9 (C-3, C-5), 126.5 (p-
BC6H5), 127.3 (m-BC6H5), 129.8 (C-6), 131.9 (o-BC6H5), 136.3
(C-4), 160.9 (C-2), 171.9 (N)CR) ppm. IR (KBr): ν˜ ) 3060 (w),
3040 (w), 3000 (w), 1628 (s), 1616 (s), 1557 (s), 1479 (m), 1456 (m),
(21) Bruker Analytical X-ray Systems. SAINT + NT Version 6.01, 1999.
(22) Bruker Analytical X-ray Systems. SHELXTL-NT Version 5.10, 1999.
(23) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.; Cooper,
T. I. CRYSTALS, Chemical Crystallography Laboratory Oxford:
Oxford, 2000; Issue 11.
(24) Watkin, D. J.; Prout, C. K.; Pearce, L. J. CAMERON; Chemical
Crystallography Laboratory Oxford, Oxford, 1996.
(25) Watkin, D. J.; Prout, C. K.; deQ. Lilley, P. M. RC93; Chemical
Crystallography Laboratory Oxford: Oxford, 1994.

6412 Inorganic Chemistry, Vol. 40, No. 25, 2001
Sa´nchez et al.
1431 (m), 1378 (w), 1347 (m), 1277 (s), 1197 (s), 1168 (m), 1141
(m), 1127 (m), 1089 (w), 1034 (m), 1025 (m) cm-1. MS (70 eV, EI),
m/z (%): 487 (0.1) [M + 1]+, 409 (100) [M - C6H5]+, 331 (15), 305
(27), 187 (17), 166 (18), 77 (10). Anal. Calcd (%) for C30H28B2N2O3
(486.16): C, 74.11; H, 5.81; N, 5.76. Found: C, 74.29; H, 5.94; N,
5.69.
7.56 (d, 4 H, o-BC6H5), 8.54 (s, 2 H, NdCH) ppm. 13C NMR (75 MHz,
DMSO-d6, 25 °C, TMS): δ ) 31.5 (C-9), 53.0 (NCH2), 116.0 (C-1),
117.2 (C-5), 118.4 (C-3), 126.5 (p-BC6H5), 127.2 (m-BC6H5), 131.6
(C-6), 131.8 (o-BC6H5), 137.0 (C-4), 161.1 (C-2), 164.1 (NdCH) ppm.
11B NMR (96.3 MHz, DMSO-d6, 25 °C, BF3‚OEt2): δ ) 8 (h1/2
)
320 Hz) ppm. IR (KBr): ν˜ ) 3065 (w), 3047 (w), 2930 (w), 1646 (s),
1610 (m), 1560 (m), 1482 (m), 1459 (m), 1446 (w), 1432 (w), 1415
(w), 1364 (w), 1321 (m), 1228 (m), 1192 (s), 1151 (m), 1117 (s), 1080
(m), 1066 (m), 1029 (w), 1000 (w) cm-1. MS (70 eV, EI), m/z (%):
471 (0.1) [M - H]+, 395 (100) [M - C6H5]+, 291 (39), 159 (24), 77
(4). Anal. Calcd (%) for C29H26B2N2O3 (472.38): C, 73.77; H, 5.55;
N, 5.93. Found: C, 73.67; H, 5.64; N, 5.89.
Compound 2c was prepared from acenH2 2 (0.50 g, 1.69 mmol)
and 2,4-difluorophenylboronic acid (0.53 g, 3.38 mmol) in acetonitrile
(20 mL). After 1 h of reflux the solution was concentrated with a Dean-
Stark trap. A white precipitate formed that was filtered under vacuum
and dried. Yield: 93%. Mp: 310-312 °C. 1H NMR (300 MHz, DMSO-
d6, 25 °C, TMS): δ ) 2.42 (s, NdCCH3), 3.40 and 4.21 (AB, 4 H,
NCH2), 6.37 (dt, J ) 8 and 2 Hz, 2 H, m′-BC6H3F2), 6.70 (dt, J ) 10
and 2 Hz, 2 H, m- BC6H3F2), 6.81 (dd, 2 H, H-3), 6.90 (dt, 2 H, H-5),
7.07 (dd, J ) 14 and 8 Hz, 2 H, o′-BC6H3F2), 7.52 (dt, 2 H, H-4), 7.75
(dd, 2 H, H-6) ppm. 13C NMR (75 MHz, DMSO-d6, 25 °C, TMS): δ
) 15.4 (CH3), 48.3 (NCH2), 102.2 (m-BC6H3F2), 108.8 (m′-BC6H3F2),
117.5 (C-1), 117.8 (C-5), 119.1 (C-3), 129.3 (C-6), 135.2 (o′-BC6H3F2),
136.2 (C-4), 158.8 (C-2), 170.6 (N)CR) ppm. 11B NMR (96.3 MHz,
DMSO-d6, 25 °C, BF3‚OEt2): δ ) 6 (h1/2 ) 620 Hz) ppm. IR (KBr):
ν˜ ) 1616 (s), 1555 (m), 1487 (m), 1458 (m), 1402 (m), 1357 (m),
1281 (m), 1260 (m), 1226 (w), 1169 (m), 1145 (m), 1127 (m), 1116
(m), 1100 (w), 1077 (m) cm-1. MS (70 eV, EI), m/z (%): 445 (100)
[M - C6H3F2]+, 331 (18), 305 (14), 187 (12), 166 (23), 146 (6), 114
(4), 77 (3). Anal. Calcd (%) for C30H24B2F4N2O3 (558.16): C, 64.56;
H, 4.33; N, 5.02. Found: C, 65.11; H, 4.38; N, 4.53.
Preparation of Boronates 6a,b. Compound 6a was prepared from
acpenH2 6 (0.50 g, 1.61 mmol) and phenylboronic acid (0.391 g, 3.22
mmol) in ethanol (30 mL). After 30 min of reflux the solution was
concentrated with a Dean-Stark trap. A yellow precipitate formed that
was filtered under vacuum and dried. The product decomposes more
or less rapidly in other organic solvents to polymeric material, so the
NMR spectra could not be obtained pure. Data are therefore only
reported for the major compound 6a. Yield: 77%. 1H NMR (270 MHz,
CDCl3, 25 °C, TMS: δ ) 0.74 (m, 2 H, H-9), 1.08 (q, 6 H, OCH2CH3),
1.88 (s, 6 H, NdCCH3), 3.20 and 3.40 (AB, 4 H, OCH2), 2.93 and
3.49 (m, 4 H, NCH2), 6.82 (m, 2 H, H-5), 6.99 (m, 2 H, H-3), 7.23 (m,
6 H, m-BC6H5, p-BC6H5), 7.43 (m, 4 H, H-4, H-6), 7.55 (m, 4 H,
o-BC6H5) ppm. 13C NMR (67.5 MHz, CDCl3, 25 °C, TMS): δ ) 15.9
(NdCCH3), 18.1 (OCH2CH3), 27.9 (C-9), 46.1 (NCH2), 56.9 (OCH2),
117.2 (C-1), 118.3 (C-5), 120.2 (C-3), 127.0 (p-BC6H5, 127.4 (m-
BC6H5), 128.2 (C-6), 133.1 (o-BC6H5), 136.4 (C-4), 160.1 (C-2), 170.4
(NdCR) ppm. 11B NMR (128.3 MHz, CDCl3, 25 °C, BF3‚OEt2): δ )
4 (h1/2 ) 360 Hz) ppm. IR (KBr): ν˜ ) 3066 (w), 3043 (w), 3004 (w),
2963 (m), 2923 (m), 2870 (m), 1619 (s), 1556 (s), 1483 (m), 1457
(m), 1433 (m), 1386 (m), 1358 (m), 1340 (m), 1272 (m), 1185 (m),
1169 (m), 1145 (m), 1105 (m), 1081 (m), 1000 (m); MS (70 eV, EI),
m/z (%): 497 (4) [M - C6H5]+, 482 (11), 451 (100), 423 (9), 396
(10), 319 (98), 262 (39), 246 (46), 234 (74), 206 (19), 185 (19), 173
(10), 161 (6), 146 (11), 105 (14), 77 (26), 51 (12). Anal. Calcd (%) for
C35H40B2N2O4 (574.36): C, 73.19; H, 7.01. Found: C, 72.81; H, 7.35.
Preparation of Boronate 3b. Compound 3b was prepared from
salphenH2 3 (0.50 g, 1.58 mmol) and phenylboronic acid (0.38 g, 3.16
mmol) in acetonitrile (10 mL). After 4 h of reflux the solution was
concentrated with a Dean-Stark trap. A yellow precipitate formed that
was filtered under vacuum and dried. Yield: 86%. Mp: 250-253 °C.
1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ ) 6.94 (m, 6 H, H-5,
m-BC6H5), 6.97 (m, 2 H, p-BC6H5), 7.09 (m, 6 H, H-3, o-BC6H5), 7.14
(dd, 2 H, H-9), 7.20 (dd, 2 H, H-6), 7.43 (dd, 2 H, H-10), 7.56 (s, 2 H,
NdCH), 7.59 (dd, 2 H, H-4) ppm. 13C NMR (75 MHz, CDCl3, 25 °C,
TMS): δ ) 118.3 (C-1), 119.5 (C-5), 121.6 (C-3), 124.2 (C-9), 126.8
(p-BC6H5), 127.1 (m-BC6H5), 130.2 (C-10), 132.8 (o-BC6H5, C-6),
139.3 (C-4, C-8), 161.6 (C-2), 165.2 (NdCR) ppm. 11B NMR (96.3
MHz, CDCl3, 25 °C, BF3‚OEt2): δ ) 9 (h1/2 ) 400 Hz) ppm. IR
(KBr): ν˜ ) 3065 (w), 1629 (s), 1595 (w), 1552 (m), 1475 (m), 1457
Compound 6b was prepared from acpenH2 6 (0.50 g, 1.61 mmol)
and phenylboronic acid (0.39 g, 3.22 mmol) in acetonitrile (15 mL).
After 1 h of reflux the solution was concentrated with a Dean-Stark
trap. A white precipitate formed that was filtered under vacuum and
(w), 1312 (w), 1218 (s), 1152 (w), 1127 (m), 1088 (w), 1049 (w) cm-1
.
MS (70 eV, EI), m/z (%): 506 (0.8) [M]+, 429 (100) [M - C6H5]+,
325 (59), 221 (11), 176 (20), 77 (10). Anal. Calcd (%) for C32H24B2N2O3
(506.16): C, 75.93; H, 4.78; N, 5.53. Found: C, 75.49; H, 4.99; N,
5.30.
1
dried. Yield: 78%. Mp: 294-296 °C. H NMR (400 MHz, CDCl3,
25 °C, TMS): δ ) 2.02 (m, 2 H, H-9), 2.42 (s, 6 H, NdCCH3), 3.53
and 4.09 (m, 4 H, N-CH2), 6.45 (dt, 2 H, H-5), 6.62 (dd, 2 H, H-3),
7.13 (m, 4 H, H-4, H-6), 7.24 (m, 6 H, m-BC6H5, p-BC6H5), 7.72 (dd,
4 H, o-BC6H5) ppm. 13C NMR (100 MHz, CDCl3, 25 °C, TMS): δ )
16.0 (CH3), 29.3 (C-9), 48.9 (NCH2), 116.7 (C-5), 120.7 (C-3), 126.4,
126.8 (C-6, p-BC6H5), 127.0 (m-BC6H5), 131.8 (o-BC6H5), 136.0 (C-
4), 169.4 (N)CR) ppm. 11B NMR (128.3 MHz, CDCl3, 25 °C, BF3‚
OEt2): δ ) 3 ppm; IR (KBr): ν˜ ) 3065 (m), 3042 (m), 3002 (m),
2915 (m), 1623 (s), 1557 (s), 1481 (s), 1459 (s), 1431 (m), 1382 (m),
1365 (m), 1341 (m), 1296 (m), 1272 (s), 1234 (m), 1192 (s), 1143
(m), 1115 (s), 1087 (m), 1061 (m), 1028 (m) cm-1. MS (70 eV, EI),
m/z (%): 423 (100) [M - C6H5]+, 345 (18), 319 (33), 262 (6), 234
(4), 201 (7), 173 (19), 146 (4), 77 (6). Anal. Calcd (%) for
C31H30B2N2O3 (500.19): C, 74.44; H, 6.04; N, 5.60. Found: C, 74.60;
H, 6.14; N, 5.74.
Preparation of Boronate 4b. Compound 4b was prepared from
salcenH2 4 (0.50 g, 1.55 mmol) and phenylboronic acid (0.37 g, 3.10
mmol) in toluene (10 mL). After 2 h of reflux the solution was
concentrated with a Dean-Stark trap. A white precipitate formed that
was filtered under vacuum and dried. Yield: 38%. Mp: 304-306 °C.
1H NMR (300 MHz, DMSO-d6, 25 °C, TMS): δ ) 0.71-1.97 (m, 8
H, H-9, H-9′, H-10, H-10′), 3.51 (m, 1 H, H-8), 4.41 (m, 1 H, H-8′),
6.61, 6.77, 7.14 (m, 10 H, H-3, H-3′, H-5, H-5′, m-BC6H5, m′-BC6H5,
p-BC6H5, p′-BC6H5), 7.34, 7.48 (m, 2 H, H-6, H-6′), 8.53 (s, 1 H, H-7),
8.66 (s, 1 H, H-7′) ppm. IR (KBr): ν˜ ) 3063 (w), 3043 (w), 3001 (w),
2941 (w), 2863 (w), 1637 (s), 1608 (m), 1558 (s), 1522 (w), 1508 (w),
1480 (m), 1461 (m), 1431 (m), 1405 (w), 1364 (w), 1322 (s), 1258
(m), 1198 (s), 1151 (s), 1130 (s), 1085 (m), 1027 (m) cm-1. MS (70
eV, EI), m/z (%): 435 (100) [M - C6H5]+, 331 (25), 277 (15), 236
(11), 179 (18), 77 (20), 51 (14). Anal. Calcd (%) for C32H30B2N2O3
(512.20): C, 75.04; H, 5.90; N, 5.47. Found: C, 75.47; H, 6.04; N,
5.36.
Preparation of Boronate 5b. Compound 5b was prepared from
salpenH2 5 (0.50 g, 1.77 mmol) and phenylboronic acid (0.43 g, 3.54
mmol) in ethanol (30 mL). After 1 h of reflux the solution was
concentrated with a Dean-Stark trap. A yellow precipitate formed that
was filtered under vacuum and dried. Yield: 97%. Mp: >300 °C. 1H
NMR (400 MHz, DMSO-d6, 25 °C, TMS): δ ) 1.98 (m, 2 H, H-9),
3.49 (m, 4 H, NCH2), 6.35 (d, 2 H, H-3), 6.47 (dd, 2 H, H-5), 7.04 (d,
2 H, H-6), 7.14 (m, 4 H, H-4, p-BC6H5), 7.19 (m, 4 H, m-BC6H5),
Acknowledgment. The authors thank Dr. Catherine Klein
from Molecular Structure Corporation for data collection for
compound 5b. Financial support from CONACyT is acknowl-
edged.
Supporting Information Available: CIF files, tables of crystal-
lographic details, atomic coordinates and U values, distances and angles,
anisotropic displacement parameters, torsion angles, and mean planes
and deviations. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC010580G