Synthesis and structural characterization of heteroboroxines with MB 2O3 Core (M = Sb, Bi, Sn)

Article abstract of DOI:10.1021/ic302153s

Reaction of organoantimony and organobismuth oxides (LSbO)₂ and (LBiO)₂ (where L is [2,6-bis(dimethylamino)methyl]phenyl) with four equivalents of the organoboronic acids gave new heteroboroxines LM[(OBR) ₂O] 1a-2c (for M = Sb: R = Ph (1a), 4-CF₃C ₆H₄ (1b), ferrocenyl (1c); for M = Bi: R = Ph (2a), 4-CF₃C₆H₄ (2b), and ferrocenyl (2c)). Analogously, reaction between organotin carbonate L(Ph)Sn(CO₃) and two equivalents of organoboronic acids yielded compounds L(Ph)Sn[(OBR) ₂O] (where R = Ph (3a), 4-CF₃C₆H₄ (3b), and ferrocenyl (3c)). All compounds were characterized by elemental analysis and NMR spectroscopy. Their structure was described both in solution (NMR studies) and in the solid state (X-ray diffraction analyses 1a, 1c, 2b, 3b, and 3c). All compounds contain a central MB₂O₃ core (M = Sb, Bi, Sn), and the bonding situation within these rings and their potential aromaticity was investigated by the help of computational methods.

Full text of DOI:10.1021/ic302153s

Article
pubs.acs.org/IC
Synthesis and Structural Characterization of Heteroboroxines with
MB2O3 Core (M = Sb, Bi, Sn)
†
†
‡
†
§
̌
Barbora Mairychova, Tomas Svoboda, Petr Stepnicka, Ales Ruzicka, Remco W. A. Havenith,
́
́
̌
̌
̌
̌
̊
̌ ̌
Mercedes Alonso,∥ Frank De Proft,*,∥ Roman Jambor,*,† and Libor Dostal*,†
́
†Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska
Pardubice 53210, Czech Republic
́
573,
‡Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, CZ-12840 Prague, Czech
Republic
§Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen Nijenborgh 4, 9747 AG Groningen, The
Netherlands
∥Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium
S
* Supporting Information
ABSTRACT: Reaction of organoantimony and organobismuth oxides
(LSbO)2 and (LBiO)2 (where L is [2,6-bis(dimethylamino)methyl]phenyl)
with four equivalents of the organoboronic acids gave new heteroboroxines
LM[(OBR)2O] 1a−2c (for M = Sb: R = Ph (1a), 4-CF3C6H4 (1b), ferrocenyl
(1c); for M = Bi: R = Ph (2a), 4-CF3C6H4 (2b), and ferrocenyl (2c)).
Analogously, reaction between organotin carbonate L(Ph)Sn(CO3) and two
equivalents of organoboronic acids yielded compounds L(Ph)Sn[(OBR)2O]
(where R = Ph (3a), 4-CF3C6H4 (3b), and ferrocenyl (3c)). All compounds
were characterized by elemental analysis and NMR spectroscopy. Their
structure was described both in solution (NMR studies) and in the solid state
(X-ray diffraction analyses 1a, 1c, 2b, 3b, and 3c). All compounds contain a
central MB2O3 core (M = Sb, Bi, Sn), and the bonding situation within these
rings and their potential aromaticity was investigated by the help of
computational methods.
aluminum(I) compound L′Al or the hydride L′AlH2 and the
corresponding arylboronic acid, represent prominent examples
of such compounds.10 Molecular borasiloxanes with the
Si2B2O4 eight-membered ring were also reported.11 Structurally
characterized organotin(IV) compounds Sn(t-Bu)2[OB(OH)-
Ph]2 , Sn(t-Bu)2 (OH)2 [(t-Bu2 SnO)2 OBC6 H2 Me3 -
2,4,6]2·2MeCN,12 and organophosporus(V) compounds13
stabilized by Salen-type ligand are other rare examples of
compounds bearing a heteroboroxine motif in the structure. In
a continuation of our investigations into main group organo-
metallic oxides,14 we report herein a straightforward synthesis
of hitherto unknown heteroboroxines LM[(OBR)2O] and
L(Ph)Sn[(OBR)2O], where M = Sb and Bi, L = [2,6-
bis(dimethylamino)methyl]phenyl, R = Ph, 4-CF3C6H4, and
ferrocenyl, comprising MB2O3 six-membered rings, from oxides
(LSbO)2 and (LBiO)2 or organotin carbonate L(Ph)Sn(CO3)
and the corresponding organoboronic acids. All compounds
were characterized by elemental analysis and multinuclear
NMR spectroscopy. The bonding situation within the central
INTRODUCTION
■
Boroxines are well-known species, readily accessible by
dehydration of the corresponding organoboronic acids.1
Recently, the chemistry of these compounds featuring six-
membered boroxine rings has undergone a period of
renaissance due to potential applications in material science.2
In 2005, Yaghi and co-workers reported the first crystalline
boroxine covalent organic framework (COF),3 a landmark in
the boroxine chemistry. Since this disclosure, many COF-
related materials have emerged, which significantly expanded
the interest in material properties of boroxines.4 Consequently,
boroxines were studied as high-performance polymer electro-
lytes,5 flame retardants,2 materials for nonlinear optics,6
reactants or catalysts for various organic transformation,7 and
Lewis acids for formation of adducts with N-donor molecules.8
Substitution of the central B3O3 core with ferrocene moieties in
turn allowed isolation of redox-active boroxines.9 Nevertheless,
the chemistry of heteroboroxines, in which one of the boron
atoms is substituted by a heteroatom M to form a MB2O3 six-
membered ring, remains nearly unexplored, as recently pointed
out by Korich and Iovine.2a Compounds L′Al[(OBR)2O]
(where L′ = HC(CMeNAr)2, Ar = 2,6-i-PrC6H3, and R = Ph, 3-
MeC6H4, 3-FC6H4), prepared by the reactions between the
Received: October 3, 2012
Published: January 17, 2013
© 2013 American Chemical Society
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MB2O3 rings as well as their aromaticity has been investigated
by computational methods.
RESULTS AND DISCUSSION
■
Treatment of intramolecularly coordinated organoantimony
and organobismuth oxides (LSbO)2 and (LBiO)2
14a
14b
with
four equivalents of the corresponding organoboronic acids
smoothly provided heteroboroxines 1a−c and 2a−c, respec-
tively (Scheme 1). Complete deprotonation of the starting
boronic acids was confirmed by IR spectra, lacking character-
istic vibrations due to the B−OH moieties.
Scheme 1. Preparation of Compounds 1a−3c
Figure 1. Molecular structure of 1a; hydrogen atoms omitted for
clarity. Selected bond lengths [Angstroms] and angles [degrees]:
Sb(1)−C(1) 2.155(3), Sb(1)−N(1) 2.759(3), Sb(1)−N(2) 2.798(4),
Sb(1)−O(1) 2.042(2), Sb(1)−O(3) 2.0565(19), B(1)−O(1)
1.334(4), B(1)−O(2) 1.387(4), B(2)−O(2) 1.381(4), B(2)−O(3)
1.334(4), O(1)−Sb(1)−O(3) 86.07(8), O(1)−B(1)−O(2) 123.8(3),
O(2)−B(2)−O(3) 124.7(3), Sb(1)−O(1)−B(1) 130.11(19), B(1)−
O(2)−B(2) 125.8(3), B(1)−O(3)−Sb(1) 128.99(19).
1H NMR spectra of 1a−2c revealed the signals of the ligand
L and groups R in a mutual ratio of 1:2 and further showed an
AX pattern for methylene CH2N and two signals for the NMe2
groups in L. The observed patterns suggest the presence of N→
M interactions in 1a−2c with a pseudofacial coordination of
the CH2NMe2 arms to the central atom M. One set of signals
was observed for the substituents R on the heteroboroxine ring
1
(including the ferrocenyl moieties in 1c and 2c) in the H and
13C NMR spectra, indicating their equivalency in solution.
The formulation of 1a−2c was unambiguously corroborated
by X-ray diffraction analysis. Suitable single crystals of 1a, 1c,
and 2b were obtained from saturated toluene solutions at room
temperature. The molecular structures of 1a, 1c, and 2b
together with the relevant structural parameters are depicted in
Figures 1, 2, and 3, respectively.
The central Sb and Bi atoms are stabilized by coordination of
the NCN pincer-type ligand. The bond lengths of the M−N
bonds (M = Sb: 2.621(3)−2.798(4), M = Bi: 2.671(5) Å)
indicate the presence of significant N→M intramolecular
interactions in all compounds. Coordination of the ligand L
may be described as pseudo-facial as indicated by the N−M−N
angles, which fall within a narrow range 118.48(14)−
119.90(8)°. The coordination polyhedron around the antimony
and bismuth atoms in 1a, 1c, and 2b is completed by two
oxygen atoms from the boronic acid residues and is best
described as a strongly distorted tetragonal pyramid with the
ipso-carbon atom C1 located in the apical position. The values
of two M−O bond distances within each central heteroborox-
ine ring are similar (2.042(2) and 2.0565(19) Å for 1a,
2.181(4) Å for 2b, 2.041(2) and 2.034(2) Å for 1c), suggesting
M−O covalent bonding (Σcov(M, O) = 2.03 Å (Sb), 2.14 Å
(Bi)).15
Figure 2. Molecular structure of 2b; hydrogen atoms omitted for
clarity. Selected bond lengths [Angstroms] and angles [degrees]: (2b,
symmetry operator a = x, 1/2 − y, z): Bi(1)−C(1) 2.234(8), Bi(1)−
N(1) 2.671(5), Bi(1)−O(1) 2.181(4), B(1)−O(1) 1.326(7), B(1)−
O(2) 1.378(7), O(1)−Bi(1)−O(1a) 83.76(14), O(1)−B(1)−O(2)
126.1(6), Bi(1)−O(1)−B(1) 127.1(4), B(1)−O(2)−B(2) 126.6(5).
Reactions of organotin carbonate L(Ph)Sn(CO3)14c with two
equivalents of the respective organoboronic acid provided
analogous organotin boroxines 3a−c (Scheme 1). IR spectra of
the crude reaction mixtures showed no vibrations due to B−
OH, thereby proving complete deprotonation of the starting
boronic acids.
1H NMR spectra of 3a−c again confirmed the presence of
the ligand L and groups R in a 1:2 ratio. Besides, the spectra
revealed an AX spin system due to methylene CH2N and one
broad signal for the NMe2 groups of the ligand L, suggesting
the presence of N→Sn interaction with pseudofacial coordina-
tion of CH2NMe2 arms of the ligand L to the central tin atom.
An equivalency of substituents R on the heteroboroxine ring
1
was proven by the H and 13C NMR spectra, where only one
set of signals was observed. 119Sn NMR spectra displayed single
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Figure 3. Molecular structure of 1c; hydrogen atoms omitted for
clarity. Selected bond lengths [Angstroms] and angles [degrees]:
Sb(1)−C(1) 2.164(3), Sb(1)−N(1) 2.670(3), Sb(1)−N(2) 2.621(3),
Sb(1)−O(1) 2.041(2), Sb(1)−O(3) 2.034(2), B(1)−O(1) 1.340(4),
B(1)−O(3) 1.386(4), B(2)−O(2) 1.346(4), B(2)−O(3) 1.383(4),
O(1)−Sb(1)−O(3) 85.84(9), O(1)−B(1)−O(2) 123.7(3), O(2)−
B(2)−O(3) 124.0(3), Sb(1)−O(1)−B(1) 128.2(2), B(1)−O(3)−
B(2) 125.4(3), Sb(1)−O(2)−B(2) 128.7(2).
Figure 5. Molecular structures of 3c; hydrogen atoms omitted for
clarity. Selected bond lengths [Angstroms] and angles [degrees]:
Sn(1)−C(1) 2.115(3), Sn(1)−C(13) 2.123(3), Sn(1)−N(1)
2.727(3), Sn(1)−N(2) 2.574(3), Sn(1)−O(1) 2.032(2), Sn(1)−
O(2) 2.035(2), B(1)−O(1) 1.332(4), B(1)−O(3) 1.394(4), B(2)−
O(2) 1.339(4), B(2)−O(3) 1.390(4), O(1)−Sn(1)−O(2) 88.75(9),
O(1)−B(1)−O(3) 124.5(3), O(2)−B(2)−O(3) 124.4(3), Sn(1)−
O(1)−B(1) 127.2(2), Sn(1)−O(2)−B(2) 126.5(2), B(1)−O(3)−
B(2) 126.3(3).
resonances at δ = −359.1 ppm for 3a, δ = −364.0 ppm for 3b,
and at δ = −358.0 ppm for 3c, which is indeed consistent with
the presence of hexacoordinated tin atoms.16
Single crystals of 3b and 3c were obtained from saturated
toluene solution at room temperature and subjected to X-ray
diffraction analysis. Views of the molecular structures are
presented in Figures 4 (3a) and 5 (3c) along with selected
structural parameters.
In both compounds 3b and 3c, the tin center is coordinated
by two nitrogen atoms (N−Sn distances in the range of
2.574(3)−2.727(3) Å) in a pseudofacial fashion as demon-
strated by the angles N−Sn−N 118.36(13)° for 3b and
118.58(9)° for 3c. The geometry of the central tin atom can be
described as distorted octahedral. The Sn−O bond distances
(2.032(2)−2.046(3) Å) within the heteroboroxine ring again
indicate formation of Sn−O covalent bonds (Σcov(Sn, O) =
2.03).15
It is noteworthy that all structures of 1a, 1c, 2b, 3b, and 3c
contain the central MB2O3 ring system, where both B−O(M)
bonds are slightly shorter (1.326(7)−1.346(4) Å) than the
remaining B−O(B) bonds (1.378(7)−1.394(4) Å) and also
shorter than the B−O distances in the corresponding boroxines
R3B3O3.17,9d All values are, however, still slightly shorter than
the sum of the covalent radii Σcov(B, O) = 1.48 Å.15 The
presence of the heteroatom M leads also to a significant
distortion of the central six-membered ring as demonstrated by
the values of the O−B−O (123.8(3)−127.3(3)° in 1a, 1c, 2b,
3b, and 3c), B−O−B (125.4(3)−126.8(4)°), and M−B−O
angles (126.5(2)−130.11(19)°), which are significantly wider
than the ideal value of 120° observed usually for simple
17
boroxines such as Ph3B3O3 or Fc3B3O3.9d This distortion is
also reflected in acute O−M−O angles (86.07(8)° 1a,
83.76(14)° 2b, 88.39(13)° 3b, 85.84(9)° 1c, 88.39(13)° 3c).
The presence of heteroatom M is further reflected in a
distortion of the MB2O3 rings from planarity. Atoms M are
slightly displaced from the mean plane of the B2O3 moiety,
distances of M from this plane being 0.165 (1a), 0.423 (1c),
0.411 (2b), 0.071 (3b), and 0.253 (3c) Å. The ligand L is
oriented nearly perpendicularly to the MB2O3 ring system in all
structurally characterized compounds.
The most striking feature regarding the substituents on the
heteroboroxine rings is the position of the ferrocenyl moieties
in 1c and 3c. While in the case of 1c the ferrocenyl residues are
placed on the opposite side (anti) of the heteroboroxine ring
with respect to L, those in 3c are directed toward ligand L (i.e.,
in syn fashion). In the case of the triferrocenylboroxine, the
ferrocenyl moieties are also oriented in all-syn fashion.9d
Structures of all possible isomers of 1c, 2c, and 3c (all-syn, all-
anti, and anti-syn) were optimized by DFT methods and
Figure 4. Molecular structure of 3b; hydrogen atoms omitted for
clarity. Selected bond lengths [Angstroms] and angles [degrees]:
Sn(1)−C(1) 2.105(5), Sn(1)−C(13) 2.122(5), Sn(1)−N(1)
2.582(4), Sn(1)−N(2) 2.716(4), Sn(1)−O(1) 2.044(3), Sn(1)−
O(2) 2.046(3), B(1)−O(1) 1.335(6), B(1)−O(3) 1.385(6), B(2)−
O(2) 1.333(6), B(2)−O(3) 1.385(6), O(1)−Sn(1)−O(2) 88.39(13),
O(1)−B(1)−O(3) 124.5(4), O(2)−B(2)−O(3) 124.6(4), Sn(1)−
O(1)−B(1) 127.8(3), Sn(1)−O(2)−B(2) 127.4(3), B(1)−O(3)−
B(2) 126.8(4).
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b
b
Table 1. GIAO-B3LYP/cc-pVDZ //B3LYP/cc-pVDZ Computed NICS for Heteroboroxines Reported in This Work: Model
a
Compounds 1, 2, and 3 and Parent Boroxine H3B3O3
c
c
d
d
d
e
molecule
NICS(0)
NICS(1)
NICS(−1)
NICSzz(0)
NICSzz (1)
NICSzz(−1)
σip
f
1
0.1
1.2
1.1
0.7
−0.6
1.1
1.1
0.8
0.0
1.4
1.4
1.1
0.6
0.8
0.8
0.1
0.3
0.7
0.7
0.3
0.1
0.8
0.8
0.9
−1.6
0.1
0.9
25.7
28.1
27.9
26.8
24.7
27.8
27.7
27.0
25.9
29.1
29.1
28.2
18.9
5.2
9.0
9.0
7.4
5.6
9.2
9.3
8.2
5.0
9.2
9.2
9.8
−1.9
5.0
7.2
7.2
7.7
4.2
7.5
7.5
8.2
5.0
7.9
8.0
6.4
−1.9
−12.7
−12.3
−12.3
−12.3
−13.2
−12.3
−12.2
−12.3
−12.9
−12.5
−12.4
−12.4
−9.8
1a
1b
1c
0.8
0.9
f
2
−0.2
0.9
2a
2b
2c
0.8
1.0
f
3
0.1
3a
1.2
3b
1.2
3c
0.9
H3B3O3
0.2
−1.6
a
b
c
All values are in ppm. cc-pVTZ-PP on Sb, Bi, and Sn. NICS values computed 1 Å away from the geometric center of the heteroboroxine ring on
the same side (1) and opposite side (−1) as the pincer-type ligand L on the Sb or Bi. NICS based on the out-of-plane zz shielding tensor
component. Average of the in-plane components of the shielding tensor. On B3LYP/cc-pVTZ geometries.
d
e
f
Table 2. Relevant Bond Distances (r, Angstroms), Wiberg Bond Indices (WBI), and NPA Atomic Charges for Model
Compounds 1, 2, and 3, Parent Boroxine H3B3O3, and Extended Model Compounds 1′, 2′, and 3′
molecule
rB−O(M)
rB−O(B)
WBIB−O(M)
WBIB−O(B)
WBIM‑O
qM
qO(M)
qO(B)
qB
1
1.358
1.354
1.358
1.355
1.352
1.354
1.378
1.380
1.381
1.387
1.388
1.389
1.000
1.028
0.893
1.004
1.029
1.015
0.904
0.883
0.879
0.825
0.862
0.858
0.859
0.904
0.583
0.570
0.533
0.465
0.44
1.460
1.483
1.675
1.674
1.703
2.090
−1.009
−1.000
−1.018
−1.060
−1.062
−1.070
−0.856
−0.878
−0.883
−0.882
−0.890
−0.898
−0.896
−0.856
0.941
0.934
0.937
1.104
1.082
1.088
0.958
2
3
1′
2′
3′
H3B3O3
0.421
basically have the same energy (see Supporting Information).
Thus, it seems that the orientation of the substituents is not
determined by single-molecule effects but results from the
crystal packing.
B3LYP19/cc-pVDZ20 (cc-pVDZ-PP21 for Sb, Bi, and Sn) level
of theory. The computed geometrical parameters were found to
be in good agreement with the experimental values. The
structures of 1b, 2a, 2c, and 3a were also constructed in silico
and optimized at the same level of theory. Cartesian
coordinates of all optimized geometries are given in the
Supporting Information.
The redox properties of ferrocenylated derivatives 1c, 2c, and
3c were studied by cyclic voltammetry at the Pt disc electrode
in 1,2-dichloroethane containing 0.1 M Bu4N[PF6] as the
supporting electrolyte. Compounds showed only one wave
attributable to the ferrocene/ferrocenium couple (Figure S1,
Supporting Information). The redox change was controlled by
diffusion as indicated by ipa (anodic peak current) increasing
linearly with the square root of the scan rate (ipa ∝ ν1/2).
However, the observed waves were composite resulting from a
convolution of two narrow-spaced one-electron redox waves
(anodic peak potential, Epa = 0.575 V for 1c, 0.540 V for 2c,
and 0.550 V for 3c vs decamethylferrocene/decamethylferro-
cenium;18 peak separations were ca. 140 mV for 1c and 2c and
120 mV for 3c). The composite nature of the redox waves
suggests a weak, probably electrostatic communication between
the ferrocenyl substituents and was confirmed by square-wave
voltammograms (Figure S2, Supporting Information). It is also
noteworthy that the primary electrochemical reaction was not
fully reversible, giving rise to another electrochemically active
compound, which was reduced at a relatively lower potential
(and also oxidized during the second or following scans; see
Figure S1, Supporting Information).
The observed shortening of the B−O bonds within all
structurally characterized heteroboroxines 1a, 1c, 2b, 3b, and
3c may indicate a multiple character of these bonds. In order to
investigate possible aromaticity of the heteroboroxines,22 the
model compounds HSb[(OBH)2O] (1), HBi[(OBH)2O] (2),
and H2Sn[(OBH)2O] (3) were constructed and optimized at
the B3LYP19/cc-pVTZ20 (cc-pVTZ-PP21 on Sb, Bi, or Sn)
level. In all optimized model systems, confirmed to be minima
on the potential energy surface by computation of the
vibrational frequencies, the heteroboroxine ring is planar and
the bond distances of the B−O(M) bonds are slightly shorter
than the B−O(B) bonds, in agreement with experimental
findings. The possible aromaticity of 1−3 through the magnetic
criterion was assessed via computation of the different
variations of the nucleus-independent chemical shifts
(NICS)23 and induced current densities.24 These were
compared with the properties of the parent boroxine
H3B3O3.22 Results are summarized in Table 1 together with
results of the NICS evaluations in the real compounds 1a−c,
2a−c, and 3a−c. On the basis of the comparison of the NICS
values in the center of the ring (NICS(0)), the NICS values 1 Å
above and below the ring (NICS( 1)), and the component of
the shielding tensor perpendicular to the ring plane component
The bonding situation in the heteroboroxines was studied
theoretically using DFT computations. As a first step, the
molecular structures of compounds 1a, 1c, 2b, 3b, and 3c were
optimized from the crystal structure geometries at the
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recorded in the 4000−50 cm−1 region on a Nicolet 6700 FTIR
spectrometer using a single-bounce diamond ATR crystal. Elemental
analyses were performed on an LECO-CHNS-932 analyzer. Starting
compounds (LSbO)2,14a (LBiO)2,14b and L(Ph)Sn(CO3)14c were
prepared according to literature procedures.
of the shielding tensor, these rings can be classified as
nonaromatic, in agreement with the parent boroxine.
This is nicely confirmed by current density plots (computed
using the ipsocentric formalism;24 see Figure S3, Supporting
Information) showing only localized diatropic circulations
around the oxygens and no diatropic or paratropic ring current,
the footprint of an aromatic or nonaromatic ring. The bonding
in the heteroboroxine rings was further investigated on the
model compounds 1, 2, and 3 using natural bond orbitals25 and
Wiberg bond indices.26 The Wiberg bond indices (Table 2) of
the B−O(M) bonds are always higher than those of the B−
O(B) bonds consistent with the shorter distance of the former
as compared to the latter, which also well corresponds with the
experimental results. The Wiberg bond index of the B−O bond
in the parent boroxine H3B3O3 is smaller than the bond order
of the B−O(M) bonds for compounds 1 and 2; in the case of
compound 3, these bond orders are comparable. The boroxine
B−O bond order in H3B3O3 is however larger than the B−
O(B) bond orders in all compounds. For compounds 1 and 2,
NBO analysis reveals the presence of one largely s-type lone
pair on Sb or Bi and two bonds from the metal atoms to the
neighboring oxygen atoms (with bond orders of 0.583 and
0.570, respectively). In the case of compound 3, four bonds
emerge from the Sn atom to its neighbors with Wiberg bond
indices of 0.533 (Sn−O) and 0.876 (Sn−H), respectively.
In order to investigate in more detail the presence of the
pincer-type ligands on the heteroboroxine rings, larger model
systems LSb[(OBMe)2O] (1′), LBi[(OBMe)2O] (2′), and
L(Me)Sn[(OBMe)2O] (3′) were constructed. These contain
the ligands L on the metal atom, while methyl groups are used
for termination of the free valencies. Relevant bond distances of
the heteroboroxine rings remain essentially unaltered in these
larger systems. In addition, NBO analysis is fully consistent
with the bonding situation around the metal atoms in the
simpler model systems 1−3. The only differences are seen in
the bond orders of the metal to oxygen bonds in the boroxine
rings that are somewhat decreased (0.465 for 1′, 0.440 for 2′,
and 0.421 for 3′) and in the atomic charges on the metals that
become more positive, favoring a stabilizing interaction with
the negatively charged nitrogen atoms of the ligand L. Both
facts reflect significant polarization of the M−O bonds in 1′−3′
in comparison with 1−3 induced by the pincer ligand.
Syntheses. Compound LSb[(OBPh)2O] (1a). A solution of
(LSbO)2 (417 mg, 0.63 mmol) in CH2Cl2 (20 mL) was added to a
stirred solution of PhB(OH)2 (309 mg, 2.53 mmol) in CH2Cl2 (15
mL) at room temperature, and the reaction mixture was stirred for an
additional 12 h at this temperature. Then the volume of the solution
was evaporated to ca. 5 mL; hexane (15 mL) was added leading to
formation of a white precipitate. The white solid was collected by
filtration, washed with 5 mL of hexane, and dried in vacuo to give 1a as
a white solid. Yield: 571 mg (84%). Mp: 155−157 °C. 1H NMR
(CDCl3, 25 °C): δ = 2.19 (s(br), 6H, (CH3)2N), 2.84 (s(br), 6H,
(CH3)2N), 3.09 and 4.75 (AX pattern, 4H, CH2N), 7.03 (d, 2H, L-Ph-
H3,5), 7.17 (t, 1H, L-Ph-H4), 7.38 (m, 6H, B-Ph3,4,5), 8.02 (d, 4H,
B-Ph2,6). 13C{1H} NMR (CDCl3, 25 °C): δ = 42.4 ((CH3)2N), 45.0
((CH3)2N), 63.3 (CH2N), 126.3 (L-Ph-C3,5), 127.6 (B-Ph-C3,5),
129.3 (L-Ph-C4), 130.1 (B-Ph-C4), 135.0 (B-Ph-C2,6), 147.4 (L-Ph-
C2,6), 155.5 (L-Ph-C1). 11B{1H} NMR (CDCl3, 25 °C): δ = 26.9.
Anal. Calcd for C24H29N2O3B2Sb (536.86): C, 53.7; H, 5.4. Found: C,
54.0; H, 5.1.
Compound LSb[(OB-4-CF3C6H4)2O] (1b). A solution of (LSbO)2
(205 mg, 0.31 mmol) in CH2Cl2 (20 mL) was added to a stirred
suspension of 4-CF3C6H4B(OH)2 (237 mg, 1.25 mmol) in CH2Cl2
(15 mL) at room temperature, and the reaction mixture was stirred for
additional 12 h at this temperature whereupon a clear solution
resulted. Then the volume of the solution was evaporated to ca. 5 mL;
hexane (15 mL) was added, leading to formation of a white precipitate.
The white solid was collected by filtration, washed with 5 mL of
hexane, and dried in vacuo to give 1b as a white solid. Yield: 334 mg
(80%). Mp: 199−202 °C. 1H NMR (CDCl3, 25 °C): δ = 2.10 (s(br),
6H, (CH3)2N), 2.86 (s(br), 6H, (CH3)2N), 3.12 and 4.68 (AX
pattern, 4H, CH2N), 7.06 (d, 2H, L-Ph-H3,5), 7.21 (t, 1H, L-Ph-H4),
7.63 (d, 4H, B-Ph3,5), 8.08 (d, 4H, B-Ph2,6). 13C{1H} NMR (CDCl3,
25 °C): δ = 42.4 ((CH3)2N), 45.5 ((CH3)2N), 63.2 (CH2N), 124.2
3
1
(q, B-Ph-C3,5, J(C,F) = 4 Hz), 126.3 (q, CF3, J(C,F) = 265 Hz),
126.4 (L-Ph-C3,5), 129.6 (L-Ph-C4), 131.7 (q, B-Ph-C4, 2J(C,F) = 32
Hz), 135.0 (B-Ph-C2,6), 141.4 (B-Ph-C1), 147.2 (L-Ph-C2,6), 155.1
(L-Ph-C1). 11B{1H} NMR (CDCl3, 25 °C): δ = 27.5. 19F NMR
(CDCl3, 25 °C): δ = −62.6. Anal. Calcd for C26H27N2O3F6B2Sb: C,
46.2; H, 4.7. Found: C, 46.5; H, 4.4.
Compound LSb[(OBFc)2O] (1c). A solution of (LSbO)2 (98 mg,
0.15 mmol) in CH2Cl2 (20 mL) was added to a stirred solution of
FcB(OH)2 (138 mg, 0.60 mmol) in CH2Cl2 (15 mL) at room
temperature, and the reaction mixture was stirred for an additional 12
h at this temperature. Then the volume of the solution was evaporated
to ca. 5 mL; hexane (15 mL) was added, leading to formation of a
yellow-orange precipitate. Solid was collected by filtration, washed
with 5 mL of hexane, and dried in vacuo to give 1c as a yellow solid.
CONCLUSION
■
We discovered a new straightforward and high-yielding
synthetic route to novel heteroboroxines possessing Sb, Bi,
and Sn heteroatoms in the central MB2O3 ring. Theoretical
considerations proved nonaromatic character of these hetero-
boroxines and suggest that the presence of the pincer-type
ligand leads to a significant polarization of the MB2O3 moieties.
Results of further reactivity studies and attempts at preparation
of other heteroboroxines bearing various substituents at boron
and heteroatoms M will be reported in due course.
1
Yield: 172 mg (84%). Mp >250 °C. H NMR (CDCl3, 25 °C): δ =
2.15 (s(br), 6H, (CH3)2N), 2.80 (s(br), 6H, (CH3)2N), 3.15 and 4.89
(AX pattern, 4H, CH2N), 3.96 (s, 10H CpFe), 4.33 (s, 4H,
Cp(BO2)Fe), 4.42 (s, 2H, Cp(BO2)Fe) 4.50 (2H, s, Cp(BO2)Fe),
7.10 (d, 2H, L-Ph-H3,5), 7.16 (t, 1H, L-Ph-H4). 13C{1H} NMR
(CDCl3, 25 °C): δ = 42.5 ((CH3)2N), 44.9 ((CH3)2N), 63.4 (CH2N),
68.3 (CpFe), 71.2 (Cp(BO2)Fe), 71.3 (Cp(BO2)Fe), 73.8 (Cp(BO2)-
Fe), 74.5 (Cp(BO2)Fe), 126.5 (L-Ph-C3,5), 129.5 (L-Ph-C4), 147.3
(L-Ph-C2,6), 155.9 (L-Ph-C1). 11B{1H} NMR (CDCl3, 25 °C): δ =
29.3. Anal. Calcd for C32H37N2O3Fe2B2Sb: C, 51.0; H, 4.9. Found: C,
51.3; H, 4.7.
EXPERIMENTAL SECTION
■
General Remarks. 1H, 11B, 13C, and 119Sn NMR spectra were
recorded on a Bruker Avance 500 MHz spectrometer or Bruker
Ultrashield 400 MHz using a 5 mm tunable broad-band probe.
Compound LBi[(OBPh)2O] (2a). A solution of (LBiO)2 (189 mg,
0.23 mmol) in toluene (20 mL) was added to a stirred solution of
PhB(OH)2 (111 mg, 0.91 mmol) in toluene (20 mL) at room
temperature, and the reaction mixture was stirred for an additional 12
h at this temperature. Then the volume of the resulting solution was
evaporated to ca. 5 mL; hexane (10 mL) was added, causing a
separation of a white precipitate. White solid was collected by
filtration, washed with 5 mL of hexane, and dried in vacuo to give 2a as
1
Appropriate chemical shifts in H and 13C NMR spectra were related
to the residual signals of the solvent (CDCl3: δ(1H) = 7.27 ppm and
δ(13C) = 77.23 ppm) or to the external Me4Sn δ(119Sn) = 0.00 ppm in
the case of the 119Sn NMR spectra. 11B NMR spectra were related to
external standard B(OMe)3 (δ(10B) = 18.1 ppm). IR spectra were
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Inorganic Chemistry
Article
Table 3. Crystallographic Data for 1a, 1c, 2b, 3b, and 3c
1a
1c
2b
3b
3c
chemical formula
cryst syst
space group
a [A]
C24H29B2N2O3Sb
monoclinic
P21/c
C32H37B2N2Fe2O3Sb
monoclinic
P21/c
C26H27B2BiF6N2O3
orthorhombic
Pnma
C32H32B2F6N2O3Sn
monoclinic
P21/c
C38H42B2Fe2N2O3Sn
orthorhombic
P212121
11.5029(13)
16.3892(14)
18.9900(13)
90
6.5320(5)
31.944(3)
12.5390(3)
90
20.4090(9)
12.1311(10)
12.4800(14)
90
12.6610(9)
24.1351(10)
9.2950(13)
90
22.3600(6)
13.2700(10)
11.0911(16)
90
b [A]
c [A]
α [deg]
β [deg]
γ [deg]
Z
113.171(5)
90
95.455(5)
90
90
93.940(6)
90
90
90
90
4
4
4
4
4
μ [mm−1
Dx [Mg m−3
]
1.175
1.839
6.277
0.848
1.531
]
1.483
1.625
1.778
1.511
1.534
cryst size [mm]
0.33 × 0.16 × 0.12
1−27.5
0.37 × 0.09 × 0.07
1−27.5
0.26 × 0.17 × 0.10
1−27.5
0.34 × 0.25 × 0.04
1−27.5
0.47 × 0.29 × 0.20
1−27.5
θ range [deg]
Tmin, Tmax
0.835, 0.891
0.779, 0.891
0.425, 0.691
0.864, 0.964
0.636, 0.791
no. of reflns measd
23 671
26 518
20 374
29 517
25 341
a
no. of unique reflns, Rint
5415, 0.039
4339
7004, 0.055
5357
3325, 0.040
2729
7497, 0.066
5245
8054, 0.041
7293
no. of obsd reflns [I > 2σ(I)]
no. of params
289
379
187
415
433
b
S
all data
1.087
1.100
1.098
1.117
1.130
c
final R indices [I > 2σ(I))]
0.033
0.039
0.040
0.058
0.032
c
wR2 indices (all data)
Δρ, max, min [e Å−3
Rint = Σ|Fo − Fo,mean2|/ΣFo . S = [Σ(w(Fo − Fc2)2)/(Ndiffrs − Nparams)]1/2
0.058
0.062
0.085
0.105
0.058
]
0.391, −0.577
0.512, −0.661
1.792, −1.162
1.902, −0.890
0.573, −0.547
a
b
c
2
2
2
2
.
R(F) = Σ∥Fo| − |Fc∥/Σ|Fo|, wR(F2) = [Σ(w(Fo − Fc2)2)/
2
(Σw(Fo )2)]1/2
.
a white solid. Yield: 219 mg (95%). Mp: 170−174 °C. 1H NMR
(CDCl3, 25 °C): δ = 2.17 (s(br), 6H, (CH3)2N), 2.93 (s(br), 6H,
(CH3)2N), 3.29 and 4.76 (AX pattern, 4H, CH2N), 7.25 (t, 1H, L-Ph-
H4), 7.39 (m, 8H, L-Ph-H3,5 and B-Ph3,4,5), 8.04 (d, 4H, B-Ph2,6).
13C{1H} NMR (CDCl3, 25 °C): δ = 42.5 ((CH3)2N), 46.2
((CH3)2N), 65.7 (CH2N), 127.6 (B-Ph-C3,5), 128.5 (L-Ph-C3,5),
128.7 (L-Ph-C4), 129.6 (B-Ph-C4), 135.1 (B-Ph-C2,6), 152.1 (L-Ph-
C2,6), 205.0 (L-Ph-C1). 11B{1H} NMR (CDCl3, 25 °C): δ = 26.7.
Anal. Calcd for C24H29N2O3B2Bi: C, 46.2; H, 4.7. Found: C, 46.3; H,
4.6.
Yield: 156 mg (86%). Mp: >237 °C (dec). 1H NMR (CDCl3, 25 °C):
δ = 2.18 (s(br), 6H, (CH3)2N), 2.67 (s(br), 6H, (CH3)2N), 3.35 and
4.93 (AX pattern, 4H, CH2N), 3.95 (s, 10H CpFe), 4.30 (s, 4H,
Cp(BO2)Fe), 4.40 (s, 2H, Cp(BO2)Fe), 4.91 (2H, s, Cp(BO2)Fe),
7.26 (t, 1H, L-Ph-H4), 7.47 (d, 2H, L-Ph-H3,5). 13C{1H} NMR
(CDCl3, 25 °C): δ = 42.0 ((CH3)2N), 45.7 ((CH3)2N), 65.7 (CH2N),
68.2 (CpFe), 71.0 (Cp(BO2)Fe), 73.6 (Cp(BO2)Fe), 74.7 (Cp(BO2)-
Fe), 128.6 (L-Ph-C3,5), 128.9 (L-Ph-C4), 152.0 (L-Ph-C2,6), (L-Ph-
C1) not found. 11B{1H} NMR (CDCl3, 25 °C): δ = 28.4. Anal. Calcd
for C32H37N2O3Fe2B2Bi: C, 45.6; H, 4.4. Found: C, 45.9; H, 4.1.
Compound LSn(Ph)[(OBPh)2O] (3a). PhB(OH)2 (77.8 mg; 0.64
mmol) was added to a stirred solution of L(Ph)Sn(CO3) (142.6 mg;
0.32 mmol) in CH2Cl2 (20 mL) at room temperature. The reaction
mixture was stirred for an additional 24 h and then evaporated. White
solid residue was washed with hexane (5 mL) and dried in vacuo to
Compound LBi[(OB-4-CF3C6H4)2O] (2b). A solution of (LBiO)2
(100 mg, 0.12 mmol) in toluene (20 mL) was added to a stirred
suspension of 4-CF3C6H4B(OH)2 (91 mg, 0.48 mmol) in the same
solvent (15 mL) at room temperature. The reaction mixture was
stirred for an additional 12 h at this temperature, affording a clear
solution. Then the volume of the solution was evaporated to ca. 5 mL;
hexane (15 mL) was added, leading to formation of a white precipitate.
White solid was collected by filtration, washed with 5 mL of hexane,
and dried in vacuo to give 2b as a white solid. Yield: 182 mg (90%).
1
give 3a as a white solid. Yield: 571 mg (84%). Mp: 119−124 °C. H
NMR (CDCl3, 25°): δ = 2.31 (s, 12H, (CH3)2N), 3.10 and 4.70 (AX
pattern, 4H, CH2N), 7.15 (d, 2H, ArH), 7.31 (t, 1H, ArH), 7.49 (m,
9H, ArH), 7.82 (d, 2H, ArH), 8.20 (d, 4H, ArH). 13C{1H} NMR
(CDCl3, 25°): δ = 44.8 ((CH3)2N), 63.1 (CH2N), 127.2 (Ph-C3,5),
127.4 (B-Ph-C3,5), 128.1 (L-Ph-C3,5), 129.6 (Ph-C4), 129.7 (B-Ph-
C4), 130.1 (L-Ph-C4), 134.6 (Ph-C2,6), 134.9 (B-Ph-C2,6), 138.3 (B-
Ph-C1), 139.3 (Ph-C1), 141.9 (L-Ph-C1), 145.2 (L-Ph-C2,6). 11B{1H}
NMR (CDCl3, 25 °C): δ = 27.1. 119Sn NMR (CDCl3, 25°): δ =
−359.1. Anal. Calcd for C29H32N2O3B2Sn (476.76): C, 58.9; H, 5.6.
Found: C, 58.5; H, 5.3.
1
Mp: 184−187 °C. H NMR (400 MHz, CDCl3, 25 °C): δ = 2.17
(s(br), 6H, (CH3)2N), 2.93 (s(br), 6H, (CH3)2N), 3.35 and 4.70 (AX
pattern, 4H, CH2N), 7.29 (t, 1H, L-Ph-H4), 7.42 (d, 2H, L-Ph-H3,5),
7.61 (d, 4H, B-Ph3,5), 8.08 (d, 4H, B-Ph2,6). 13C{1H} NMR (100.6
MHz, CDCl3, 25 °C): δ = 42.5 ((CH3)2N), 46.3 ((CH3)2N), 65.7
(CH2N), 124.2 (q, B-Ph-C3,5, 3J(C,F) = 4 Hz), 124.2 (q, CF3, 1J(C,F)
= 266 Hz), 128.7 (L-Ph-C3,5), 129.1 (L-Ph-C4), 131.3 (q, B-Ph-C4,
2J(C,F) = 32 Hz), 135.3 (B-Ph-C2,6), 138.1 (B-Ph-C1), 152.2 (L-Ph-
Compound LSn(Ph)[(OB-4-CF3C6H4)2O] (3b). 4-CF3C6H4−B-
(OH)2 (122.6 mg; 0.32 mmol) was added to a stirred solution of
L(Ph)Sn(CO3) (144.3 mg; 0.65 mmol) in CH2Cl2 (20 mL) at room
temperature. This reaction mixture was stirred for an additional 24 h at
this temperature and then evaporated in vacuo. The resulting white
solid was washed with hexane (5 mL) and dried in vacuo to give 3b as
a white solid. Yield: 571 mg (84%). Mp: 189−191 °C. 1H NMR
(CDCl3, 25°): δ = 2.48 (s, 12H, (CH3)2N), 3.19 and 4.71 (AX pattern,
4H, CH2N), 7.23 (d, 2H, ArH), 7.43 (t, 1H, ArH), 7.56 (m, 1H, ArH),
7.78 (d, 6H, ArH), 7.85 (d, 2H, ArH), 8.29 (d, 4H, ArH). 13C{1H}
NMR (CDCl3, 25°): δ = 44.7 ((CH3)2N), 63.1 (CH2N), 123.3 (CF3),
124.1 (B-Ph-C3,5), 126.1 (CF3), 127.4 (Ph-C3,5), 128.8 (L-Ph-C3,5),
C2,6), 205.4 (L-Ph-C1). 11B{1H} NMR (CDCl3, 25 °C): δ = 26.4. 19
F
NMR (CDCl3, 25 °C): δ = −62.6. Anal. Calcd for C26H27N2O3F6B2Bi:
C, 41.1; H, 3.6. Found: C, 41.4; H, 3.4.
Compound LBi[(OBFc)2O] (2c). A solution of (LBiO)2 (100 mg,
0.12 mmol) in toluene (20 mL) was added to a stirred solution of
FcB(OH)2 (110 mg, 0.48 mmol) in toluene (15 mL) at room
temperature. The reaction mixture was stirred for an additional 12 h at
this temperature. Then the volume of the solution was evaporated to
ca. 5 mL; hexane (15 mL) was added, leading to formation of a yellow-
orange precipitate. Solid was collected by filtration, washed with 5 mL
of hexane, and dried in vacuo to give compound 2c as a yellow solid.
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129.8 (L-Ph-C4), 130.4 (Ph-C4), 131.2 (B-Ph-C4), 131.5 (B-Ph-C4),
134.5 (Ph-C2,6), 135.0 (B-Ph−C2,6), 139.0 (Ph-C1), 141.4 (L-Ph-
C1), 142.2 (B-Ph-C1), 145.0 (L-Ph-C2,6). 11B{1H} NMR (CDCl3, 25
°C): δ = 29.3. 19F NMR (CDCl3, 25 °C): δ = −62.5. 119Sn NMR
(CDCl3, 25°): δ = −364.0. Anal. Calcd for C31H36N2O3F6B2Sn
(490.77): C, 51.4; H, 4.3. Found: C, 51.1; H, 4.1.
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fdeprof@vub.ac.be (F.D.P.); roman.jambor@upce.cz
(R.J.); libor.dostal@upce.cz (L.D.).
■
Compound LSn(Ph)[(OBFc)2O] (3c). FcB(OH)2 (78.4 mg; 0.34
mmol) was added to a stirred solution of L(Ph)Sn(CO3) (76.2 mg;
0.17 mmol) in CH2Cl2 (20 mL) at room temperature, and the reaction
mixture was stirred for a further 24 h at this temperature and
evaporated in vacuo. The resulting orange solid was washed with
hexane (5 mL) and dried in vacuo to give 3c as an orange solid. Yield:
156 mg (86%). Mp: 213−215 °C. 1H NMR (CDCl3, 25°): δ = 2.37 (s,
12H, (CH3)2N), 3.11 and 4.73 (AX pattern, 4H, CH2N), 4.05 (s, 10H
CpFe), 4.35 (s, 4H, Cp(BO2)Fe), 4.53 (s, 4H, Cp(BO2)Fe), 7.14 (t,
2H, ArH), 7.31 (d, 1H, ArH), 7.14 (t, 3H, ArH), 7.85 (d, 2H, ArH).
13C{1H} NMR (CDCl3, 25°): δ = 44.7 ((CH3)2N), 63.3 (CH2N),
68.3 (CpFe), 71.1 (Cp(BO2)Fe), 74.2 (Cp(BO2)Fe), 127.3 (L-Ph-
C3,5), 128.7 (Ph-C3,5), 129.7 (Ph-C4), 130.3 (L-Ph-C4), 134.9 (Ph-
C2,6), 139.7 (Ph-C1), 142.2 (L-Ph-C1), 145.3 (L-Ph−C2,6). 11B{1H}
NMR (CDCl3, 25 °C): δ = 28.2. 119Sn NMR (CDCl3, 25°): δ =
−358.0. Anal. Calcd for C37H42N2O3Fe2B2Sn: C, 45.6; H, 4.4. Found:
C, 45.9; H, 4.1.
X-ray Diffraction Analyses. Suitable single crystals of 1a, 1c, 2b,
3b, and 3c were mounted on a glass fiber with an oil and measured on
a four-circle diffractometer KappaCCD with CCD area detector by
monochromatized Mo Kα radiation (λ = 0.71073 Å). Corresponding
crystallographic data are given in Table 3. The numerical27 absorption
correction from the crystal shape was applied for all crystals. Structures
were solved by direct methods (SIR9228) and refined by a full matrix
least-squares procedure based on F2 (SHELXL9729). Hydrogen atoms
were mostly localized on a difference Fourier map; however, to ensure
the uniformity of treatment of the crystal, all hydrogen were
recalculated into idealized positions (riding model) and assigned
temperature factors Hiso(H) = 1.2Ueq(pivot atom) or 1.5Ueq for the
methyl moiety with C−H = 0.96, 0.98, and 0.93 Å for methyl,
methylene, and hydrogen atoms in the aromatic rings, respectively.
The final difference maps displayed no peaks of chemical significance
as the highest peaks and holes are in close vicinity of heavy atoms.
Crystallographic data for structural analysis has been deposited with
the Cambridge Crystallographic Data Centre, CCDC nos. 895166−
895170. Copies of this information may be obtained free of charge
from The Director, CCDC, 12 Union Road, Cambridge CB2 1EY, UK
(Fax +44-1223-336033; e-mail deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
Computational Details. Model compounds 1, 2, and 3, the
parent boroxine H3B3O3, and the “extended” model compounds 1′, 2′,
and 3′ were optimized at the B3LYP level of theory19 using the cc-
pVTZ basis set20 (cc-pVTZ-PP21 basis set for Sb, Bi, and Sn).
Subsequent NBO analysis25 and calculation of Wiberg bond indices26
and nucleus-independent chemical shifts23 (for 1, 2, and 3) were
performed at the same level of theory. In addition, the current density
induced by the external magnetic field, directed perpendicular to the
ring of interest, was calculated for 1, 2, and 3 at the same level of
theory using the ipsocentric CTOCD-DZ method.24 Compounds 1a−
c, 2a−c, and 3a−c were optimized at the B3LYP level of theory18
using the cc-pVDZ basis set (cc-pVDZ-PP basis set for Sb, Bi, and Sn).
Subsequent NBO analysis21 and calculation of Wiberg bond indices25
and nucleus-independent chemical shifts26 for these compounds were
performed at the same level of theory. All calculations were performed
using the Gaussian 09 program,30 except for computation of the
induced current densities, which were performed using the GAMESS-
UK31 and SYSMO programs.32
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Czech Science
Foundation (project no. P207/13-00289S), the Fund for
Scientific Research Flanders (Belgium) FWO, the Vrije
Universiteit Brussel, the Zernike Institute for Advanced
Materials (“Dieptestrategie” program), and the Ministry of
Education, Youth and Sports of the Czech Republic (project
no. MSM0021620857).
DEDICATION
Dedicated to Prof. Jaroslav Holece
birthday.
■
̌
k on the occasion of his 80th
REFERENCES
■
(1) Hall, D. G.; Frye, G. C. In Boronic Acids; Wiley-VCH: Weinheim,
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ASSOCIATED CONTENT
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* Supporting Information
Figures S1−S3, crystallographic data in cif format, and details of
DFT calculations together with ball and stick representation
and Cartesian coordinates (Å) of optimized compounds. This
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