Benzoxaborolate ligands in group 13 metal complexes

Article abstract of DOI:10.1016/j.jorganchem.2013.01.023

A series of group 13 metal benzoxaborolates [R₂MOB(o-CH ₂O)(C₆H₄)]₂ [R = ^tBu, M = Al (2), R = Me, M = Ga (3), R = ^tBu, M = Ga (4), R = ^tBu, M = In (5)] were synthesize

Full text of DOI:10.1016/j.jorganchem.2013.01.023

Journal of Organometallic Chemistry 732 (2013) 8e14
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Benzoxaborolate ligands in group 13 metal complexes
a
b
c
d
ꢀ
ꢀ
ꢀ
Eliza Jaskowska , Iwona Justyniak , Micha1 K. Cyranski , Agnieszka Adamczyk-Wozniak ,
d
a
a,
*
ꢀ
Andrzej Sporzynski , Ewa Zygad1o-Monikowska , Wanda Ziemkowska
a Warsaw University of Technology, Faculty of Chemistry, Koszykowa 75, 00-662 Warsaw, Poland
b Polish Academy of Sciences, Institute of Physical Chemistry, Kasprzaka 44/52, 01-224 Warsaw, Poland
c Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
d Warsaw University of Technology, Faculty of Chemistry, Department of Physical Chemistry, Noakowskiego3, 00-664 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
A series of group 13 metal benzoxaborolates [R2MOB(o-CH2O)(C6H4)]2 [R ¼ tBu, M ¼ Al (2), R ¼ Me,
M ¼ Ga (3), R ¼ tBu, M ¼ Ga (4), R ¼ tBu, M ¼ In (5)] were synthesized in reactions of 1,3-dihydro-1-
hydroxy-2,1-benzoxaborole (1) with group 13 metal trialkyls. The compounds were characterized by
elemental analysis, melting point measurements, 1H, 13C NMR, and IR spectroscopy. The molecular
structure of 2e5 was determined by single-crystal X-ray diffraction. The structure of the compounds
depends on the kind of metallic center. Reaction of tBu3Al with the benzoxaborole 1 leads to the alu-
minum derivative 2, similar to typical dialkylaluminum carboxylates, with a central eight-membered
B2Al2O4 ring, in which two oxygen atoms of the benzoxaborolate unit are bonded to aluminum atoms.
In the presence of gallium and indium trialkyls, the benzoxaborole 1 acts as an alcohol yielding com-
plexes 3e5, similar to dialkylmetal alkoxides, with a central M2O2 (where M ¼ Ga, In) ring. Thermal
decomposition of compounds 2 and 4 revealed the liberation of a mixture of organoboron compounds.
Ó 2013 Elsevier B.V. All rights reserved.
Article history:
Received 23 October 2012
Received in revised form
10 January 2013
Accepted 14 January 2013
Keywords:
Aluminum
Gallium
Indium
Benzoxaborole
Thermogravimetric analysis
1. Introduction
Until now, only several examples of aluminum-substituted bor-
oxines have been reported [6,7]. Known complexes of aluminum
Boronic acids, RB(OH)2, have been attracting attention owing to
their usefulness as organic reagents, catalysts, building blocks in
crystal and material engineering, and molecular receptors [1].
Recently, they have been used in medicine as antifungal drugs and
tested for the treatment of numerous diseases [2]. Their internal
hemiesters, benzoxaboroles, have lately been reported as excellent
potent antifungal, anti-inflammatory, and antibacterial drugs [3].
Crystal structure of the fluorinated benzoxaborole active against
onychomycosis (AN-2690) has been recently reported [4]. The
structure of benzoxaboroles, their chemical properties, applica-
tions, and methods of preparation have recently been reviewed by
containing OBR2 units are limited to just a few examples [7,8].
Very recently, the first aluminum complex containing benzox-
aborole moieties has been obtained in the reaction of
3-hydroxyphenylboronic acid with aluminum dihydride LAlH2
(L ¼ HC(CMeNAr)2, Ar ¼ 2,6-iPr2C6H3) [7].
Although numerous inorganic compounds containing boron,
gallium, and indium atoms, such as gallium and indium borates, are
useful in materials engineering [9], the products of the reaction of
benzoxaboroles and boronic acids with gallium and indium com-
pounds are unknown.
For benzoxaborolate anions coordinated to metal atoms,
there are several coordination modes possible, because of the
presence of two potentially coordinating oxygen atoms in the
benzoxaborolate units (Scheme 1). The aim of the present work
was to study the benzoxaborolate complexes obtained in re-
actions of 1,3-dihydro-1-hydroxy-2,1-benzoxaborole (1) with
aluminum, gallium, and indium trialkyls. Here, we demonstrate
benzoxaborolate anions as difunctional and monofunctional li-
gands in the group 13 metal complexes depending on the kind
of metal centers. Our study showed that, among the coordina-
tion modes in Scheme 1, structures with modes A and B were
formed.
ꢀ
Sporzynski and coworkers [5].
Although benzoxaboroles can be regarded as electron
donating ligands in complexes with aluminum, gallium, and
indium alkyls due to the presence of two oxygen atoms in the
benzoxaborole molecule, these benzoxaborolate complexes are
unknown. Even reactions of group 13 metal compounds with
other oxygen boron compounds have been largely unexplored.
* Corresponding author. Tel.: þ48 22 2347316.
E-mail address: ziemk@ch.pw.edu.pl (W. Ziemkowska).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.01.023

ꢀ
E. Jaskowska et al. / Journal of Organometallic Chemistry 732 (2013) 8e14
9
R
R
M
R
R
O
O
M
O
B
B
O
B
B
O
M
O
O
O
M
R
R
R
R
B
A
MR
O
3
MR
O
R
R
2
O
M
B
O
B
B
O
M
O
R
R
MR
3
C
D
M = Al, Ga, In
Scheme 1. Possible coordination modes for benzoxaborolate anions coordinated to metal atoms.
2. Experimental
2.3. [tBu2AlOB(o-CH2O)(C6H4)]2 (2)
2.1. Materials and instrumentation
Yield 85%. 1H NMR (CDCl3)
CH2O), 7.38 (2H, m, Haromat), 7.44 (2H, m, Haromat), 7.59 (2H, m,
Haromat), 7.88 (2H, m, Haromat). 13C NMR (CDCl3)
: 14.86 [AlC(CH3)3],
30.31 [AlC(CH3)3], 71.74 (CH2O), 121.08, 128.34, 132.12, 132.26,
147.73 (Caromat) ppm.
d: 1.00 [36H, s, AlC(CH3)3], 5.41 (4H, s,
All manipulations were carried out using standard Schlenk
techniques under an inert gas atmosphere. Solvents were distilled
over the blue benzophenone-K complex. 1,3-Dihydro-1-hydroxy-
2,1-benzoxaborole (1) was synthesized as described in the liter-
ature [10]. 1H and 13C NMR spectra were obtained on a Mercury-
400BB Varian spectrometer. Chemical shifts were referenced to
the residual proton signals of CDCl3 (7.26 ppm). 13C NMR spectra
were acquired at 100.60 MHz (standard: chloroform 13CDCl3,
77.20 ppm). IR spectra were obtained using a Nicolet 6700 FT-IR
(ATR) infrared spectrometer. Thermal stability and decomposition
of compounds 2 and 4 were determined by an SDT Q600 V20.9
Build 20 derivatograph, recording TG, DTG, and DSC curves. The
measurements were made at a heating rate of 2 K minꢀ1 with full
scale recording. The samples (14.91e15.82 mg) were heated in
ceramic crucibles in argon. Hydrolyzable alkyl groups and the
aluminum content for compound 2 were determined according to
the literature [11]. Elemental analyses of compounds 3e5 were
obtained on a PerkineElmer 2400 analyzer.
d
X-ray-quality crystals were obtained from the n-C6H14eCH2Cl2
solution at 10 ꢁC. Decomposition (on the basis of DSC measure-
ments): 247 ꢁC. FTIR (nujol):
n
¼ 1625.7 (m), 1612.2 (m), 1562.1 (s),
1546.6 (s), 14.52.1 (m), 1353.8 (m), 1272.8 (m), 1168.7 (m), 966.2 (s),
946.9 (s), 811.9 (s), 794.5 (m), 761.7 (m), 723.2 (s), 709.7 (m), 632.5
(s), 599.8 (m), 568.9 (m), 491.8 (m) cmꢀ1. Elemental anal. Found: Al,
t
9.50; hydrolyzable Bu groups, 39.21; Calcd for C30H48Al2B2O4: Al,
9.85; tBu, 39.40 wt.%.
2.4. [Me2GaOB(o-CH2O)(C6H4)]2 (3)
Yield 81%. 1H NMR (CDCl3)
CH2O), 7.38 (4H, m, Haromat), 7.50 (2H, m, Haromat), 7.62 (2H, m,
Haromat). 13C NMR (CDCl3)
d: 0.07 (12H, s, GaCH3), 5.08 (4H, s,
d
: ꢀ4.10 (GaCH3), 70.70 (CH2O), 121.40,
127.10, 130.22, 130.80, 154.50 (Caromat) ppm.
X-ray-quality crystals were obtained from the CH2Cl2 solution at
10 ꢁC. Mp.: 191 ꢁC. FTIR (nujol):
n
¼ 1608.3 (m), 1452.1 (m), 1365.4
2.2. Synthesis of [R2MOB(o-CH2O)(C6H4)]2 [R ¼ tBu, M ¼ Al (2),
R ¼ Me, M ¼ Ga (3), R ¼ tBu, M ¼ Ga (4), R ¼ tBu, M ¼ In (5)]:
general procedure
(s), 1286.3 (s), 1203.4 (m), 1182.2 (s), 1105.0 (m), 993.2 (m), 746.3
(m), 719.3 (s), 675.0 (m), 597.8 (s), 541.9 (s), 503.3 (s), 462.8 (s)
cmꢀ1. Elemental anal.: Calcd for C18H24B2Ga2O4: C, 46.41; H, 5.16.
Found: C, 46.04; H, 5.21 wt.%.
A solution of R3M (1 mmol) in 5 cm3 of CH2Cl2 was injected into
a stirred solution of 1 (1.34 g, 1 mmol) in CH2Cl2 (20 cm3) at ꢀ76 ꢁC.
The reaction mixture was allowed to warm slowly to room tem-
perature. A white solid appeared on the surface of the post-reaction
solution. The solution was separated from the solid by an injector.
The solvent was removed in vacuo from the post-reaction solution
and a white product (2e5) was obtained.
2.5. [tBu2GaOB(o-CH2O)(C6H4)]2 (4)
Yield 86%. 1H NMR (CDCl3)
s, CH2O), 7.38 (4H, m, Haromat), 7.48 (2H, m, Haromat), 7.76 (2H, m,
Haromat). 13C NMR (CDCl3)
: 26.30 [GaC(CH3)3], 31.47 [GaC(CH3)3],
69.66 (CH2O), 121.45, 126.83, 130.63, 131.16, 155.30 (Caromat) ppm.
d: 1.22 [36H, s, GaC(CH3)3], 5.04 (4H,
d

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E. Jaskowska et al. / Journal of Organometallic Chemistry 732 (2013) 8e14
tBu
tBu
Al
OH
O
O
O
+ 2 tBu Al
B
2
B
3
B
- 2 tBuH
O
O
Al
tBu
tBu
1
2
Scheme 2. Reaction of benzoxaborole 1 with one equivalent of tBu3Al.
Crystals were obtained from the post-reaction solution at 10 ꢁC.
Mp.: 232e236 ꢁC. FTIR (nujol):
(m), 1365.4 (s), 1355.7 (s), 1282.4 (m), 1176.4 (m), 1101.2 (m), 1000.9
(s), 813.8 (m), 757.9 (m), 738.6 (m), 725.1 (s), 678.8 (m), 653.8 (m),
636.4 (s), 594.0 (s), 540.0 (m), 493.7 (s), 466.7 (s) cmꢀ1. Elemental
anal.: Calcd for C46H42B2Ga2N2O4: C, 67.33; H, 5.13. Found: C, 66.98;
H, 5.34 wt.%.
from a difference map and refined isotropically. The atomic scat-
tering factors were taken from the International Tables [15].
The data for compounds 3e5 were collected at 100(2) K on
a Nonius Kappa CCD diffractometer [16] using graphite mono-
n
¼ 1608.3 (w), 1469.5 (m), 1452.1
chromated MoK
a
radiation (
l
¼ 0.71073 Å). The unit cell parame-
ters were determined from ten frames, then refined on all data. The
data were processed with DENZO and SCALEPACK (HKL2000
package) [17]. The structure was solved by direct methods using the
SHELXS97 [13] program and refined by full matrix least-squares on
F2 using the program SHELXL97 [18]. All non-hydrogen atoms were
refined with anisotropic displacement parameters. The hydrogen
atoms were introduced at geometrically idealized coordinates with
a fixed isotropic displacement parameter equal to 1.2. To improve
2.6. [tBu2InOB(o-CH2O)(C6H4)]2 (5)
Yield 76%. 1H NMR (CDCl3)
CH2O), 7.38 (4H, m, Haromat), 7.46 (2H, m, Haromat), 7.62 (2H, m,
Haromat). 13C NMR (CDCl3)
: 32.37 [InC(CH3)3], 36.02 [InC(CH3)3],
d: 1.30 [36H, s, InC(CH3)3], 5.01 (4H, s
d
69.37 (CH2), 121.53, 126.94, 130.04, 130.39, 154.92 (Caromat) ppm.
X-ray-quality crystals were obtained from the CH2Cl2 solution
at ꢀ15 ꢁC. Decomposition at 205 ꢁC. FTIR (nujol):
n
¼ 1606.4 (m),
1479.1 (m), 1461.8 (m), 1454.1 (m), 1390.4 (s), 1357.6 (s), 1278.6 (m),
1160.9 (m), 1093.4 (m), 1070.3 (m), 997.0 (s), 810.0 (m), 756.0 (m),
723.2 (s), 638.3 (s), 582.4 (m), 464.8 (s) cmꢀ1. Elemental anal.: Calcd
for C30H48B2In2O4: C, 49.73; H, 6.63. Found: C, 49.37; H, 6.81 wt.%.
2.7. Thermal decomposition of [tBu2GaOB(o-CH2O)(C6H4)]2 (4)
For 2 h, a 0.50 g sample of compound 4 was heated without any
solvent at ca. 250e280 ꢁC, followed by distillation under high
vacuum at ca. 250 ꢁC. A yellow liquid (0.21 g) obtained as the dis-
tillate was studied by NMR spectroscopy. 1H NMR (CDCl3)
d
: 1.23,
1.06, 1.04 [(CH3)3C, three singlets], 1.40e0.90 (numerous small
signals), 5.30, 5.28, 5.25 (CH2O, three singlets), 8.30e7.10 (Haromat
m). 11B NMR (CDCl3)
: ꢀ23.7, 33.1, 50.8.
,
d
2.8. Crystallographic data
The X-ray measurement of compound 2 was performed at
100(2) K on a KUMA CCD k-axis diffractometer with graphite-
monochromated MoKa radiation (0.71073 Å). The crystal was
positioned at 61.2 mm from the KM4CCD camera; 703 frames were
measured at 0.7ꢁ intervals on a counting time of 25 s. Data reduc-
tion and analysis were carried out with the Kuma Diffraction pro-
grams. The data were corrected for Lorentz and polarization effects
and multi-scan absorption correction [12] was applied. The struc-
ture was solved by direct methods [13] and refined by using SHELXL
[14]. The refinement was based on F2 for all reflections except for
those with very negative F2. The weighted R factor, wR, and all
goodness-of-fit S values are based on F2. The non-hydrogen atoms
were refined anisotropically. The hydrogen atoms were located
Fig. 1. (top) Molecular structure of (2). Displacement ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (ꢁ): Al(1)eO(2) 1.727(1), Al(1)eO(1a) 1.934(1), O(1)eB(1) 1.444(2),
O(2)eB(1) 1.298(2), O(2)eAl(1)eO(1a) 102.18(6), B(1)eO(2)eAl(1) 169.7(1), O(2)e
B(1)eO(1) 121.0(2), B(1)eO(1)eAl(1a) 129.5(1), O(2)eB(1)eC(7) 133.4(2), O(1)eB(1)e
C(7) 105.6(2). (bottom) View of a molecule of 2 showing that the central B2Al2O4 ring
adapts a flattened chair conformation.

ꢀ
E. Jaskowska et al. / Journal of Organometallic Chemistry 732 (2013) 8e14
11
the geometrical parameters, ultimately restrained instructions
been reported as dimeric complexes of the formula [R2M(m-
DFIX and SIMU were applied in the refinement.
O2CR’)]2 (where M ¼ Al, Ga) that consist of central eight-membered
Crystallographic data for the structures reported in this paper
have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 901057 (2), CCDC
901058 (3), CCDC 902032 (4), CCDC 901059 (5). Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, U.K. (fax: (þ44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
rings M2C2O4 with bridging carboxylate anions [19].
The significant difference between B(1)eO(2) and B(1)eO(1)
bond lengths (0.146 Å) is indicative of the lack of electron deloc-
alization within the OBO group in compound 2, whereas the dif-
ference between CeO bond lengths in the group 13 metal
carboxylates is situated in the region of 0.01e0.03 Å, showing an
appreciable degree of delocalization within the carboxylate groups
[19b-f, 20]. We cannot exclude, that according to the Bent’s rule, the
electron withdrawing effect caused by aluminum atom results in
increasing of p-character of the BeO bond and elongation of B(1)e
O(1) bond in the compound 2.
3. Results and discussion
Treatment of the benzoxaborole 1 with one equivalent of tBu3Al
in methylene dichloride, followed by crystallization from CH2Cl2-n-
C6H12 solution at 10 ꢁC, afforded a white crystalline compound,
whose characterizing data are consistent with the dimeric species
[tBu2AlOB(o-CH2O)(C6H4)]2 (2) (Scheme 2).
The compound was characterized by NMR and IR spectros-
copies, elemental analysis, and thermogravimetric analysis. The
molecular structure was determined on the basis of an X-ray dif-
fraction study and is shown in Fig. 1 (top). Data collection and
structure analyses are listed in Table 1. Crystals of 2 suitable for X-
ray structure determination were grown from a n-C6H14eCH2Cl2
solution at 10 ꢁC. The molecular studies showed that the cen-
trosymmetric molecule of 2 consists of a central eight-membered
B2Al2O4 ring adapting a flattened chair conformation (Fig. 1
bottom).
The sum of the angles about the boron atom S[B(1)], 360.0ꢁ,
indicates a trigonal planar boron. The same boron coordination was
observed in compound 1 and other benzoxaboroles, and in phe-
nylboronic acids [5,10,21,22]. The Al(1)eO(2) [1.727(1) Å] bond
length is comparable to that in aluminum alkoxide derivatives.
In contrast to the reaction of compound 1 with tBu3Al, the
treatment of 1 with gallium and indium trialkyls leads to the for-
mation of dimeric compounds [R2MOB(o-CH2O)(C6H4)]2 [M ¼ Ga,
R ¼ Me (3), M ¼ Ga, R ¼ tBu (4), M ¼ In, R ¼ tBu (5)], analogs of
group 13 metal alkoxides (Scheme 3).
The molecular structures of compounds 3 and 5 are shown in
Figs. 2e4. Data collection and structure analyses of 3e5 are listed in
Table 1. The crystal structure of 4 was insufficiently solved
[R1 ¼ 0.0711, wR2 ¼ 0.1478 for 4904 reflections with Io > 2
s(Io)]
because of a disorder and high thermal motion associated with tBu
groups bonded to gallium atoms. The crystallographic data support
the isostructurality of the gallium compound 4 to the indium
compound 5. Crystals of 4 and 5 contain two kinds of molecules in
a cell, differing in bond lengths and angles. In contrast to the
structure of 2, the molecules of 3e5 consist of a central four-
The molecule is composed of two monoanion benzoxaborolate
ligands, two four-coordinate aluminum atoms and four tBu groups
bonded to the aluminum atoms. The structure of aluminum ben-
zoxaborolate 2 is similar to the structure of typical carboxylates of
group 13 metal alkyls obtained in reaction of trialkylaluminum and
trialkylgallium with monocarboxylic acids. These carboxylates have
Table 1
Crystal data and data collection parameters for 2e5.
2
3
4
5
Empirical formula
Formula weight
Temperature (K)
Wavelength Å
Crystal system
Space group
a(Å)
C
30H48Al2B2O4
C18H24B2Ga2O4
465.43
100(2)
0.71073
Triclinic
P-1
7.4330(4)
8.2430(3)
10.0220(5)
111.377(3)
95.082(2)
109.238(3)
524.45(4)
1
C30H48B2Ga2O4
633.74
100(2)
0.71073
Monoclinic
P 2(1)/c
14.7901(5)
12.6179(3)
21.0951(5)
90
125.951(2)
90
3186.9(2)
4
1.321
1.722
C30H48B2In2O4
723.94
100(2)
0.71073
Monoclinic
P2(1)/c
15.0815(2)
12.7725(2)
21.5260(3)
90
126.134(1)
90
3348.90(9)
4
1.436
1.407
548.26
100(2)
0.71073
Monoclinic
P2(1)/c
9.1147(8)
16.331(1)
11.1717(8)
90
105.440(9)
90
1602.9(2)
2
b(Å)
c(Å)
a
(ꢁ)
(ꢁ)
b
g
(ꢁ)
V(Å3)
Z
Dcalc(g cmꢀ3
Absorption
)
1.136
0.122
1.474
2.587
coefficient (mmꢀ1
)
F(000)
592
236
1328
1472
Crystal size (mm)
 range for
0.20 ꢂ 0.10 ꢂ 0.10
0.46 ꢂ 0.36 ꢂ 0.28
0.58 ꢂ 0.44 ꢂ 0.26
0.36 ꢂ 0.20 ꢂ 0.14
2.86e25.50
0.998e27.485
0.998e27.485
0.998e27.485
data collection (ꢁ)
Index ranges
ꢀ11 ꢃ h ꢃ 8, ꢀ19 ꢃ k ꢃ 19,
ꢀ13 ꢃ l ꢃ 13
ꢀ9 ꢃ h ꢃ 9, ꢀ10 ꢃ k ꢃ 10,
ꢀ13 ꢃ l ꢃ 12
ꢀ17 ꢃ h ꢃ 18, ꢀ15 ꢃ k ꢃ 15,
ꢀ25 ꢃ l ꢃ 25
ꢀ17 ꢃ h ꢃ 17, ꢀ15 ꢃ k ꢃ 14,
ꢀ25 ꢃ l ꢃ 25
Reflections collected
10,974
4163
11,145
9978
Independent reflections
Refinement method
2955 [Rint ¼ 0.0143]
Full matrix least-squares on F2
2955/0/178
2375 [Rint ¼ 0.0313]
Full matrix least-squares on F2
2375/0/121
5982 [Rint ¼ 0.0441]
Full matrix least-squares on F2
5982/60/405
5650 [Rint ¼ 0.0235]
Full matrix least-squares on F2
5650/0/355
Data/restraints/parameters
Goodness-of-fit on F2
1.129
1.082
1.168
1.271
Final R indices [I > 2
R indices (all data)
Max/Min of residual
electron density
s(I)]
R1 ¼ 0.0453, wR2 ¼ 0.1398
R1 ¼ 0.0522, wR2 ¼ 0.1449
0.841 and ꢀ0.221
R1 ¼ 0.0427, wR2 ¼ 0.1090
R1 ¼ 0.0475, wR2 ¼ 0.1124
0.622 and ꢀ0.607
R1 ¼ 0.0711, wR2 ¼ 0.1478
R1 ¼ 0.1216, wR2 ¼ 0.1798
0.83 and ꢀ0.74
R1 ¼ 0.0563 wR2 ¼ 0.1427
R1 ¼ 0.0580, wR2 ¼ 0.1432
0.749 and ꢀ0.629

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E. Jaskowska et al. / Journal of Organometallic Chemistry 732 (2013) 8e14
12
R
R
OH
+ 2 R
M
M
O
O
B
B
B
O
2
M
3
- 2 RH
O
O
R
R
M = Ga, R = Me (3)
M = Ga, R = tBu (4)
M = In, R = tBu (5)
Scheme 3. Reactions of benzoxaborole 1 with gallium- and indium trialkyls.
membered ring M2O2 (M ¼ Ga, In) and two benzoxaborolate moi-
eties. In spite of the similarity between the chemistry of benzox-
aboroles and that of organoboronic acids [23], the structures of 3e5
indicate that the benzoxaborole 1 acts with gallium and indium
trialkyls as alcohol involving a reaction of the OH group only and
yielding products similar to typical alkylmetal alkoxides. The MeO
(M ¼ Ga, In; O ¼ oxygen atom of the oxaborole ring) distances are
about 3.1 Å, indicative of the lack of an MeO coordinative bond.
Boron atoms in all molecules show a planar trigonal coordination
(the sum of the angles around boron is 360ꢁ).
The 1H NMR spectra of compounds 2e5 reveal the following
signals: multiplets of aromatic protons at approximately 7.30e
7.90 ppm, one singlet of CH2O protons at approximately 5.00e
5.50 ppm and one singlet of the protons of alkyl groups bonded
to metal atoms (at approximately 1.00e1.30 ppm in 2, 4, and 5; at
0.07 ppm in 3). The appearance of only one signal of protons of alkyl
groups bonded to metal atoms indicates an equivalence of these
groups in accordance with the crystal structures. On the basis of the
integration ratio of the CH2O protons and the alkyl group protons,
we found that in the molecules there are two alkyl groups bonded
to metal atoms per one benzoxaborolate unit.
Complexes 2 and 4 were characterized by thermogravimetry
(TG), differential thermal analysis (DTA), and differential scanning
calorimetry (DSC) in argon atmosphere. Thermoanalytical curves
are presented in Figs. 5 and 6. The beginning of the compound’s 2
thermal decomposition is observed at ca. 200 ꢁC, and in the range
up to ca. 260 ꢁC, an exothermal transformation occurs, which is
accompanied by an approximately 55% mass loss of the sample on
the TGA diagram. In the thermogram, a sharp peak with a max-
imum at 247 ꢁC appears. Upon increasing the temperature, in-
termediates undergo further decomposition with a total mass loss
of 76.36%. Measurements of the melting point of compound 2 in
Fig. 3. Molecular structure of (4). Displacement ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (ꢁ) for one molecule in a cell: O(1)eGa(1a) 1.997(6), O(1)eGa(1)
2.002(5), O1eB(1) 1.36(1), O(1)eGa(1)eO(1a) 77.3(2), Ga(1a)eO(1)eGa(1) 102.7(2),
B(1)eO(1)eGa(1a) 122.2(6), B(1)eO(1)eGa(1) 135.1(6).
Fig. 2. (top) Molecular structure of (3). Displacement ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (ꢁ): O(1)eB(1) 1.374(4), O(2)eB(1) 1.347(4), O(2)eGa(1) 1.962(2),
O(2a)eGa(1) 1.971(2), B(1)eO(2)eGa(1) 125.0(2), B(1)eO(2)eGa(1a) 134.7(2), Ga(1)e
O(2)eGa(1a) 100.3(1), O(2)eGa(1)eO(2a) 79.7(1). (bottom) View of a molecule of 3
showing that two benzoxaborolate units and the central Ga2O2 ring are coplanar.
Fig. 4. Molecular structure of (5). Displacement ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (ꢁ) for one molecule in a cell: O(1)eIn(1) 1.206(5), In(1)eO(1a)
1.199(5), B(1)eO(1) 1.325(11), B(1)eO(2) 1.395(10), O(1)eIn(1)eO(1a) 74.8(2), In(1)e
O(1)eIn(1a) 105.2(2), B(1)eO(1)eIn(1a) 139.9(5), B(1)eO(1)eIn(1) 114.7(5), O(1)e
B(1)eO(2) 118.4(7), O(1)eB(1)eC(9) 134.0(8), O(2)eB(1)eC(9) 107.6(7).

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E. Jaskowska et al. / Journal of Organometallic Chemistry 732 (2013) 8e14
13
O
B
E
Scheme 4. Structure of the compound E formed as a result of a transition of tBu groups
from gallium to boron.
and alkyl groups of organic compounds. Three intensive singlets at
1.23, 1.06, and 1.04 ppm can be assigned to the protons of tBu
groups bonded to metal atoms, whereas three singlets at 5.30, 5.28,
and 5.25 ppm of CH2O protons indicate compounds similar to the
benzoxaborole 1. The mixture of the decomposition products con-
sists of boronic compounds, which was evidenced by the green
color of the flame during combustion and the presence of boron
signals in the 11B NMR spectrum. Based on the literature data [23],
the signal at 33.1 ppm originates from the boron atom of the
benzoxaborole 1, whereas the signal at 50.8 ppm is typical for bo-
ron bonded to two carbon atoms and one oxygen atom [24]. The
signal is assigned to the boron atom of a compound E (Scheme 4),
Fig. 5. Thermogravimetric analysis traces showing the decomposition of compound 2
in an inert (Ar) atmosphere.
a capillary tube show decomposition in the region 240e250 ꢁC
without melting, which is in agreement with the thermoanalysis.
The decomposition of compound 4 begins at ca. 110 ꢁC. The DTA
curve shows one sharp endothermic peak at 232.4 ꢁC, which is
attributed to melting followed by decomposition. The melting
point found on the DTA curve confirms the melting point deter-
mined by a capillary tube system under argon (232e236 ꢁC). Be-
sides the endothermic peak at 232.4 ꢁC, the first weight loss step is
accompanied by a sharp exothermic peak at 272.8 ꢁC, whereas the
remainder of the decomposition process in the range 280e500 ꢁC is
characterized by a broad endothermic feature with a total mass loss
of 71.27%.
On the basis of the thermogravimetric studies, it is difficult to
link different areas of mass loss to a specific way of the decom-
position of the compounds. To gain a better understanding of the
decomposition process, the structure of the volatile decomposition
products was studied. These products were obtained by heating
compound 4 in the region 250e280 ꢁC under argon. The decom-
position products were isolated by distillation under vacuum as
a yellow liquid. The NMR analysis shows a mixture of products. In
the 1H NMR spectrum, the presence of signals of aromatic protons
(8.30e7.10 ppm) and signals at 1.40e0.90 ppm indicates phenyl
t
which is formed as a result of a transition of Bu groups from gal-
lium to boron. Additional signal at ꢀ23.7 ppm can be attributed to
the borate ion [25], but it is difficult to determine its structure on
the basis of the presented results.
Summarizing, the study has revealed the nature of benzox-
aborolate ligand in group 13 metal complexes. In the reaction with
the trialkylaluminum compound, the benzoxaborole 1 acts as an
acid yielding a product similar to dimeric dialkylaluminum car-
boxylates, in which two oxygen atoms of the benzoxaborolate
moiety are bonded to aluminum atoms. Gallium and indium tri-
alkyls react with the OH group of 1 only to form complexes similar
to alkoxides. Coordination of an oxygen atom of the oxaborole ring
to the gallium and indium centers was not observed, which in-
dicates lower coordination abilities of these metal centers in
comparison with an aluminum center. Thermogravimetric studies
revealed that group 13 metal benzoxaborolates undergo thermal
decomposition with liberation of organoboron compounds mainly
and probably a small amount of organic compounds is formed.
Acknowledgments
We thank Warsaw University of Technology for financial sup-
port. The X-ray structure was determined at the Crystallographic
Unit of the Physical Chemistry Laboratory, Chemistry Department,
University of Warsaw and at the Institute of Physical Chemistry,
Polish Academy of Sciences in Warsaw.
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