Synthesis of boroxine-linked aluminum complexes

Article abstract of DOI:10.1021/ic1022906

The reaction of LAlH₂ (L = HC(CMeNAr)₂, Ar = 2,6-iPr₂C₆H₃) (1) with 3-methylphenylboronic acid and 3-fluorophenylboronic acid resulted in the boroxine-linked aluminum compounds LAl[OB(3-CH₃

Full text of DOI:10.1021/ic1022906

2010 Inorg. Chem. 2011, 50, 2010–2014
DOI: 10.1021/ic1022906
Synthesis of Boroxine-Linked Aluminum Complexes
Xiaoli Ma,† Zhi Yang,*,† Xiujuan Wang,† Herbert W. Roesky,*,‡ Feng Wu,† and Hongping Zhu§
†School of Chemical Engineering and Environment, Beijing Institute of Technology, 100081 Beijing, China,
‡
€
€
Institut fu€r Anorganische Chemie der Georg-August-Universitat Gottingen, Tammannstrasse 4,
§
€
D-37077 Gottingen, Germany, and State Key Laboratory of Physical Chemistry of Solid Surfaces,
National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters,
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, P. R. China
Received November 16, 2010
The reaction of LAlH2 (L = HC(CMeNAr)2, Ar = 2,6-i Pr2C6H3) (1) with 3-methylphenylboronic acid and 3-fluoro-
phenylboronic acid resulted in the boroxine-linked aluminum compounds LAl[OB(3-CH3C6H4)]2(μ-O) (2) and
LAl[OB(3-FC6H4)]2(μ-O) (3), respectively. LAl[OB(2-PhC6H4)(OH)]2 (4) was synthesized by the reaction of 1 with
2-biphenylboronic acid. Compound 4 is the intermediate analogue to those, which we postulated for the formation of 2
and 3. The reaction of 1 with 3-hydroxyphenylboronic acid resulted in the first metal benzoboroxole oxide LAl[OB(o-
CH2O)C6H4]2 (5), which is formed from a compound with B-(OH)2 and C-OH functionalities.
Introduction
boroxines. However, only two examples of aluminum sub-
stituted boroxines were reported due to limited synthetic
methods.5,6
Boroxines are polymeric materials prepared from organo-
boronic acids by dehydration.1 They are not only of funda-
mental academic interest (e.g., structural investigations,
electrochemistry, intermediate products, etc.) but also im-
portant for industrial applications (flame retardants for light
metals, lithium ion battery materials, etc.).2 In 2005, Yaghi
et al. reported the first crystalline arylboroxine-based covalent
organic framework material.3 Furthermore, aluminum hetero-
metallic oxides have attracted much interest due to their
broad applications in chemical processes.4 We reasoned that
aluminum substituted boroxines containing the Al-O-B
moiety might have unusual properties compared to those of
Benzoboroxoles are highly useful synthetic intermediates
for transition metal catalyzed cross-coupling reactions and
are widely utilized in medicinal and materials chemistry.7 In
2006, we reported a rare aluminum spirocyclic hybrid with an
inorganic B2O3 and an organic C3N2 core.6 In that paper, we
explained the annelation mechanism only by DFT calcula-
tion without further experimental proof. The electronic and
sterical effects of bulky β-diketiminato ligands are usually
used to stabilize the metal center to avoid condensation of
molecules and to form unique compounds. It is possible to
change selectively the functionalities at the Al center by using
those ligands. Herein, we report a series of novel structures
bearing the Al-O-B unit, give solid experimental proof of
the annelation mechanism, and discuss a one step route to the
first metal benzoboroxole oxide by the reaction of 3-hydro-
xyphenylboronic acid with aluminum dihydride LAlH2
(1; L = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3) supported by
the bulky β-diketiminato ligand.
*To whom correspondence should be addressed. Fax: (þ86) 10-68911032.
E-mail: zhiyang@bit.edu.cn (Z.Y.); hroesky@gwdg.de (H.W.R.).
(1) Hall, D. G. Boronic Acids, 1st ed.; Wiley-VCH: Weinheim, Germany,
2005.
(2) Korich, A. L.; Iovine, P. M. Dalton Trans. 2010, 1423–1431.
(3) Cote, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.;
Yaghi, O. M. Science 2005, 310, 1166–1170.
(4) (a) Mandal, S. K.; Roesky, H. W. Acc. Chem. Res. 2010, 43, 248–259.
(b) Bai, G. C.; Singh, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G. J. Am.
Chem. Soc. 2005, 127, 3449–3455. (c) Yang, Z.; Zhu, H.; Ma, X.; Chai, J.;
Roesky, H. W.; He, C.; Magull, J.; Schmidt, H.-G.; Noltemeyer, M. Inorg. Chem.
2006, 45, 1823–1827. (d) Nembenna, S.; Roesky, H. W.; Mandal, S. K.; Oswald,
R. B.; Pal, A.; Herbst-Irmer, R.; Noltemeyer, M.; Schmidt, H.-G. J. Am. Chem.
Soc. 2006, 128, 13056–13057. (e) Sarish, S. P.; Nembenna, S.; Roesky, H. W.;
Ott, H.; Pal, A.; Stalke, D.; Dutta, S.; Pati, S. K. Angew. Chem., Int. Ed. 2009, 48,
8740–8742. (f) Roesky, H. W.; Bai, G.; Jancik, V.; Singh, S. U.S. Patent
17,645,716 B2, 2010. (g) Roesky, H. W.; Gurubasavaraj. U.S. Patent 0306227
A1, 2008.
(7) (a) Nicolaou, K. C.; Li, H.; Boddy, C. N. C.; Ramanjulu, J. M.; Yue,
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€
T.-Y.; Natarajan, S.; Chu, X.-J.; Brase, S.; Rubsam, F. Chem. Eur. J. 1999,
;
5, 2584–2601. (b) Nicolaou, K. C.; Natarajan, S.; Li, H.; Jain, N. F.; Hughes, R.;
Solomon, M. E.; Ramanjulu, J. M.; Boddy, C. N. C.; Takayanagi, M. Angew.
Chem., Int. Ed. 1998, 37, 2708–2714. (c) Tan, Y. L.; White, A. J. P.; Widdowson,
D. A.; Wilhelm, R.; Williams, D. J. J. Chem. Soc. Perkin. Trans. 2001, 1, 3269–
3280. (d) Baker, S. J.; Zhang, Y. K.; Akama, T.; Lau, A.; Zhou, H.; Hermandez, Y.;
Mao, W.; Alley, M. R. K.; Sanders, Y.; Plattner, J. J. J. Med. Chem. 2006, 49,
4447–4450. (e) Alexander, C.; Smith, C. R.; Whitcombe, M. J.; Vulfson, E. N.
J. Am. Chem. Soc. 1999, 121, 6640–6651. (f) Robin, B.; Buell, G.; Kiprof, P.;
Nemykin, V. N. Acta Crystallogr. 2008, E64, o314–o315. (g) Sørenson, M. D.;
Martins, R.; Hindsgaul, O. Angew. Chem., Int. Ed. 2007, 46, 2403–2407. (h) Pal,
A.; Berube, M.; Hall, D. G. Angew. Chem., Int. Ed. 2010, 49, 1492–1495.
€
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r
pubs.acs.org/IC
Published on Web 02/14/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 2011
Table 1. Crystallographic Data for Compound 2, 3, 4, and 5
2
3
4
5
formula
fw
C43H55AlB2N2O3
696.49
133(2)
triclinic
P1
11.171(2)
13.209(3)
14.544(3)
76.869(8)
76.866(7)
77.367(8)
2003.4(7)
2
C41H49AlB2F2N2O3 0.5C6H14
747.53
133(2)
orthorhombic
Pbcn
29.861(4)
21.602(3)
12.9186(17)
90
90
90
8333(2)
8
1.180
0.098
3136
C53H61AlB2N2O4
838.64
103(2)
triclinic
P1
12.125(3)
12.502(3)
17.663(4)
104.381(2)
101.364(2)
108.018(2)
2354.9(8)
2
C43H53AlB2N2O4
710.47
93(2)
monoclinic
P21/n
12.749(4)
15.966(4)
20.125(6)
90
94.251(5)
90
4085(2)
3
temp (K)
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
4
1.152
0.092
1512
Fc (Mg/m3)
1.155
0.091
748
1.183
0.090
896
M (mm-1
F(000)
)
Θ range (deg)
index ranges
3.03 - 27.46
-14 e h e 14
-14 e k e 17
-18 e l e 18
20041
9048 (0.0381)
9048/0/472
0.999
3.20 to 26.00
-36 e h e 36
-24 e k e 26
-14 e l e 15
56882
8164 (0.0925)
8164/0/518
1.150
3.19 to 27.49
-15 e h e 15
-16 e k e 16
-22 e l e 20
23537
10633 (0.0340)
10633/0/577
1.001
3.20 to 27.50
-16 e h e 16
-18 e k e 19
-19 e l e 26
32197
9184 (0.0695)
9184/0/479
0.999
no. of reflns collected
no. of indep reflns R(int)
no. of data/restraints/params
GoF/F2
R1,a wR2b (I > 2σ(I))
R1,a wR2b(all data)
0.0581, 0.1341
0.0974, 0.1452
0.0778, 0.1491
0.1050, 0.1609
0.0498, 0.1044
0.0805, 0.1185
0.0768, 0.1564
0.1431, 0.1856
P
Scheme 1. Preparation of Compounds 2 and 3
P
P
P
a R = ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ (Fo2)]1/2
.
Experimental Section
spectrometer. The infrared spectrum was recorded on a Perkin-
Elmer spectrophotometer. Melting points were measured in
sealed glass tubes.
General Procedures. All manipulations were carried out
under a purified nitrogen atmosphere using Schlenk techniques
or inside a Mbraun MB 150-GI glovebox. All solvents were
distilled from Na/benzophenone ketyl prior to use. Commer-
cially available chemicals were purchased from Aldrich, Fluka
Synthesis of LAl[OB(3-CH3C6H4)]2(μ-O) (2). A solution of 1
(0.223 g, 0.5 mmol) in toluene (10 mL) was added drop by drop
to a solution of 3-methylphenylboronic acid (0.136 g, 1 mmol) in
toluene (10 mL) at 0 °C. After the addition was complete, the
reaction mixture was allowed to warm to room temperature,
and stirring was continued overnight. The solvent was removed
in vacuo. The solid was extracted with n-hexane (30 mL), and the
extract was stored at room temperature for 2 days to afford 2 as
colorless crystals. An additional crop of 2 was obtained from the
and used as received. LH8 and LAlH2 were prepared as
9
described in the literature. Elemental analyses were performed
by the Analytical Instrumentation Center of the Peking Uni-
versity. 1H NMR spectra were recorded on Bruker AM 400
(8) Qian, B.; Ward, D. L.; Smith, M. R., III. Organometallics 1998, 17,
3070–3076.
(9) Cui, C.; Roesky, H. W.; Hao, H. J.; Schmidt, H.-G.; Noltemeyer, M.
Angew. Chem., Int. Ed. 2000, 39, 1815–1817.
1
mother liquor. Total yield, 0.296 g (85.1%); mp, 212 °C. H
NMR (399.13 MHz, CDCl3, 25 °C, TMS): δ 7.81-7.12 (m, 14
H, Ar-H), 5.48 (s, 1 H, γ-H), 3.44 (sept, 3JH-H = 6.8 Hz, 4 H,

2012 Inorganic Chemistry, Vol. 50, No. 5, 2011
Scheme 2. Preparation of Compound 4
Ma et al.
Scheme 3. Preparation of Compound 5
CHMe2), 2.39 (s, 6 H, Me-C6H5), 1.60 (s, 6 H, Me), 1.33 (d,
3JH-H = 6.8 Hz, 12 H, CHMe2), 1.24 ppm (d, 3JH-H = 6.8 Hz,
12 H, CHMe2). C43H55AlB2N2O3 (696.51) Calcd: C, 74.15; H,
7.96; N, 4.02. Found: C, 74.01; H, 7.95; N, 4.01%.
glass fiber, and crystal data were collected on the Rigaku AFC10
Saturn724 þ (2 ꢀ 2 bin mode) diffractometer equipped with
˚
graphite-monochromated Mo KR radiation (λ = 0.710747 A).
Empirical absorption correction was applied using the SA-
DABS program.10 The structures were solved by direct
methods11 and refined by full-matrix least-squares on F2 using
the SHELXL-97 program.12 The crystal structure of 3 contains
a seriously disordered solvent molecule, which was assigned to
hexane. Except those of solvent of 3, all non-hydrogen atoms
were refined anisotropically, and the hydrogen atoms were
generated geometrically and treated by an independent and
constrained refinement. The carbon atoms of the solvent mole-
cules in 3 were refined isotropically, and the hydrogen atoms of
the solvent molecules were not included in the refinement. A
summary of the crystal data is given in Table 1.
Synthesis of LAl[OB(3-FC6H4)]2(μ-O) (3). The preparation
of 3 is like that of 2 from 1 (0.223 g, 0.5 mmol) and 3-fluor-
ophenylboronic acid (0.140 g, 1 mmol). Product 3 was isolated
as colorless crystals. Total yield, 0.306 g (87%); mp, 221 °C. 1H
NMR (399.13 MHz, C6D6, 25 °C, TMS): δ 7.88-7.89 (m, 14 H,
3
Ar-H), 4.87 (s, 1 H, γ-H), 3.26 (sept, JH-H = 6.8 Hz, 4 H,
3
CHMe2), 1.34 (s, 6 H, Me), 1.11 (d, JH-H = 6.8 Hz, 12 H,
3
CHMe2), 1.09 ppm (d, JH-H = 6.8 Hz, 12 H, CHMe2). 19F
NMR (399.13 MHz, C6D6, 25 °C, CF3COOH): δ -36.65 ppm.
C41H49AlB2F2N2O3 (704.44) Calcd: C, 69.91; H, 7.01; N 3.98.
Found: C, 69.35; H, 7.01; N, 3.76%.
Synthesis of LAl[OB(2-PhC6H4)(OH)]2 (4). A solution of 1
(0.223 g, 0.5 mmol) in toluene (10 mL) was added drop by drop
to a suspension of 2-biphenylboronic acid (0.198 g, 1 mmol) in
toluene (10 mL) at 25 °C. After the addition was complete, the
reaction mixture was kept stirring for 72 h. The solvent was
removed in vacuo. The solid was extracted with n-hexane (30
mL), and the extract was stored at room temperature for 3 days
to afford 4 as colorless crystals. Total yield, 0.383 g (91.4%); mp,
253 °C. 1H NMR (399.13 MHz, CDCl3, 25 °C, TMS): δ
Results and Discussion
The reaction of 1 with 3-methylphenylboronic acid,
3-fluorophenylboronic acid (Scheme 1), 2-biphenylboronic
acid (Scheme 2), and 3-hydroxyphenylboronic acid (Scheme 3)
in a molar ratio of 1:2 resulted in the products LAl[OB(3-
CH3C6H4)]2(μ-O) (2), LAl[OB(3-FC6H4)]2(μ-O) (3), LAl-
[OB(2-PhC6H4)(OH)]2 (4), and LAl[OB(o-CH2O)C6H4)]2
(5), respectively. During the course of these reactions, hydro-
gen gas evolution was observed. Compounds 2, 3, 4, and 5
were isolated after growing colorless crystals from the con-
centrated n-hexane solutions. They are soluble in toluene,
benzene, and trichloromethane.
7.52-6.99 (m, 24 H, Ar-H), 4.90 (s, 1 H, γ-H), 3.25 (sept, 3JH-H
=
6.8 Hz, 4 H, CHMe2), 1.48 (s, 6 H, Me), 1.19 (d, 3JH-H=6.8 Hz,
3
12 H, CHMe2), 1.08 ppm (d, JH-H=6.8 Hz, 12 H, CHMe2).
IR(KBr, cm-1): νh 3422 (w). C53H61AlB2N2O4 (838.64) Calcd: C,
77.27; H, 7.35; N, 3.34. Found: C, 75.90; H, 7.33; N, 3.34%.
Synthesis of LAl[OB(o-CH2O)C6H4)]2 (5). The preparation
of 5 is like that of 4 from 1 (0.223 g, 0.5 mmol) and 3-hydro-
xyphenylboronic acid (0.152 g, 1 mmol). Product 5 was isolated
as colorless crystals. Total yield, 0.292 g (82.4%); mp, 185 °C. 1H
NMR (399.13 MHz, C6D6, 25 °C, TMS): δ 7.59-6.90 (m, 14 H,
Compounds 2, 3, 4, and 5 were characterized by 1H NMR
investigation in CDCl3 and C6D6 solutions, as well as by
elemental analysis. Compound 3 was additionally character-
ized by 19F NMR. The H NMR spectra of 2, 3, 4, and 5
1
exhibit one set of resonances for the aryl group both on boron
and on the ligand, indicating symmetric molecules. Compound 2
Ar-H), 5.16 (s, 1 H, γ-H), 4.77 (s, 4 H, CH2), 3.69 (sept, 3JH-H
=
6.8 Hz, 4 H, CHMe2), 1.77 (s, 6 H, Me), 1.45 (d, 3JH-H=6.8 Hz,
12 H, CHMe2), 1.23 ppm (d, JH-H=6.8 Hz, 12 H, CHMe2).
3
(10) Sheldrick, G. M. SADABS; University of Goettingen: Goettingen,
Germany, 1997.
(11) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(12) Sheldrick, G. M. SHELXL-97; University of Goettingen: Goettingen,
Germany, 1997.
C43H53AlB2N2O4 (710.50) Calcd: C, 72.69; H, 7.52; N 3.94.
Found: C, 72.49; H, 7.58; N, 3.71%.
Single Crystal X-Ray Structure Determination and Refine-
ment. Single crystals of 2, 3, 4, and 5 were mounted with glue on a

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 2013
Figure 1. Molecular structure of 2. Thermal ellipsoids are drawn at the
50% level. The hydrogen atoms are omitted for clarity.
Figure 4. Molecular structure of 5. Thermal ellipsoids are drawn at the
50% level. The hydrogen atoms are omitted for clarity.
˚
Table 2. Selected Bond Distances (A) and Angles (deg) for Compound 2
Al(1)-N(1)
Al(1)-O(2)
O(2)-B(1)
B(1)-O(1)
O(3)-Al(1)-O(2) 104.00(8)
Al(1)-O(3)-B(2) 121.96(15)
O(2)-B(1)-O(1)
1.872(2)
Al(1)-N(2)
1.862(2)
1.7418(17)
1.350(3)
1.393(3)
125.65(19)
1.7362(17) Al(1)-O(3)
1.348(3)
1.395(3)
O(3)-B(2)
B(2)-O(1)
B(2)-O(1)-B(1)
Al(1)-O(2)-B(1) 121.84(16)
122.5(2)
O(3)-B(2)-O(1)
122.0(2)
˚
Table 3. Selected Bond Distances (A) and Angles (deg) for Compound 3
0.5C6H14
3
Al(1)-N(1)
Al(1)-O(2)
O(2)-B(1)
B(1)-O(1)
O(3)-Al(1)-O(2) 103.52(9)
Al(1)-O(3)-B(2) 123.07(17)
O(2)-B(1)-O(1)
1.865(2)
Al(1)-N(2)
1.870(2)
1.7398(19) Al(1)-O(3)
1.346(4)
1.392(3)
1.7386(19)
1.345(4)
1.397(3)
O(3)-B(2)
B(2)-O(1)
B(2)-O(1)-B(1)
125.7(2)
Al(1)-O(2)-B(1) 122.39(17)
122.6(2)
O(3)-B(2)-O(1)
122.2(2)
˚
Table 4. Selected Bond Distances (A) and Angles (deg) for Compound 4
Figure 2. Molecular structure of 3. Thermal ellipsoids are drawn at the
50% level. The solvent molecule and the hydrogen atoms are omitted for
clarity.
Al(1)-N(1)
Al(1)-O(1)
O(1)-B(1)
B(2)-O(4)
1.8730(15) Al(1)-N(2)
1.7027(12) Al(1)-O(3)
1.8768(15)
1.7094(12)
1.338(2)
1.347(2)
1.381(2)
O(3)-B(2)
B(1)-O(2)
1.350(2)
O(3)-Al(1)-O(1) 118.20(6)
Al(1)-O(3)-B(2) 141.49(12)
Al(1)-O(1)-B(1) 154.47(12)
O(1)-B(1)-O(2) 122.94(16)
O(3)-B(2)-O(4)
117.73(15)
˚
Table 5. Selected Bond Distances (A) and Angles (deg) for Compound 5
Al(1)-N(1)
1.887(3) Al(1)-N(2)
1.713(2) Al(1)-O(4)
1.314(4) O(4)-B(2)
1.420(5) B(2)-O(3)
109.66(11)
1.872(3)
1.707(2)
1.335(4)
1.394(5)
Al(1)-O(2)
O(2)-B(1)
B(1)-O(1)
O(3)-Al(1)-O(1)
C(30)-B(1)-O(1)
C(35)-C(30)-B(1)
C(36)-C(35)-C(30) 110.2(3)
O(1)-C(36)-C(35)
C(36)-O(1)-B(1)
107.3(3)
106.5(3)
C(37)-B(2)-O(3)
C(42)-C(37)-B(2)
C(37)-C(42)-C(43) 110.4(3)
107.0(3)
105.5(3)
105.6(3)
110.2(3)
C(42)-C(43)-O(3)
C(43)-O(3)-B(2)
106.1(3)
110.9(3)
shows the Me-Ph resonances at δ 2.39 ppm in a 6:1 ratio
with the γ-C-H proton. Compound 3 exhibits one resonance
at δ -36.63 ppm in the 19F NMR spectrum. The IR spectrum
of compound 4 shows a strong band at νh 3422 cm-1, which is
attributed to the hydroxyl group attached to the boron atom,
Figure 3. Molecular structure of 4. Thermal ellipsoids are drawn at the
50% level. The hydrogen atoms are omitted for clarity.

2014 Inorganic Chemistry, Vol. 50, No. 5, 2011
Ma et al.
and it is quite close to that (3494 cm-1) reported by Barba
for 2 and 3) are longer than the Al-OH bond distance (av
et al.13
1.705 A) in LAl(OH)2,14 which is quite close to those of 4 (av
˚
˚
˚
For the progress of the two reactions to 2 and 3, we
assumed two similar concerted mechanisms through inter-
mediate A shown in Scheme 1. The formation of 2 and 3
through intermediate A is driven by the exothermic Al-O
bond enthalpy. The increase of the proton acidity of the
intermediate LAl[OB(3-MeC6H4)OH]2 leads to the elimina-
tion of water under AlO3B2 ring formation. Compounds 2
and 3 are rare examples containing a spiro-centered alumi-
num atom, where the inorganic AlO3B2 ring is fused to the
organic C3N2 part. The less acidic protons of the OH groups
of the 3-MeC6H4B(OH)2 are reactive enough to form the
Al-O bonds, while those of the 2-PhC6H4B(OH)2 react only
with one OH group to yield LAl[OB(2-PhC6H4)(OH)]2 (4)
containing two terminal boron hydroxyl groups.
To study the steric effect for the AlO3B2 rings formation,
we selected 2-biphenylboronic acid as a precursor molecule.
The reaction did not proceed under dehydration like those
observed for 2 and 3. The bulky 2-biphenyl groups on each of
the boron atoms repel each other, which makes the two
B-OH groups too far away to react with each other under
elimination of H2O. So one of the two hydroxyl groups on
each boron remained. The formation of 4 strongly supports
the mechanism we postulated for the formation of com-
pounds 2 and 3. The structure of 4 is quite similar to that of
intermediate A, which we assumed in the reaction of produ-
cing compounds 2 and 3.
1.706 A) and 5 (av 1.710 A). The O-Al-O angles (104.00(8)°
for 2 and 103.53(10)° for 3) are smaller than that in LAl(OH)2
(115.38(8)°) and those in 4 (118.20(6)°) and 5 (109.66(11)°).
The difference in bond lengths and bond angles of 2 and 3
from those of 4 and 5 is due to a certain strain within the six-
membered rings of 2 and 3. The two Me groups at the aryl
groups of 2 are in a cis position of the C2 axis, which exhibits a
nonsymmetric molecule, but due to the free rotation of the
C-B σ bond, the molecule is symmetric in solution, which
was shown by 1H NMR at room temperature. The structure
of 4 contains the Al-(O-B-OH)2 moiety with the terminal
boron hydroxyl groups. The sum of the inner angles of the
two C3BO five-membered rings in 5 are 539.95° and 540.05°,
respectively, which is quite close to that of the planar
pentagon of 540°. The most distinct characters of 5 are the
benzoboroxole moieties. Compound 5 is the first benzobor-
oxole compound formed in a one step synthesis. The known
benzoboroxoles have been prepared mostly by a salt elimina-
tion reaction of borate with dianionic species15,16 in two or
three steps. Moreover, compound 5 is the first example of a
metal benzoboroxole oxide.
Conclusion
In summary, by selecting the groups with different substit-
uents on boron, the reactions of LAlH2 with aryl boronic
acids result in cyclic or acyclic boroxines. The latter contains
terminal boron hydroxyl groups. LAlH2 reacts with 3-hydro-
xyphenylboronic acid under the formation of the first metal
benzoboroxole oxide. This benzoboroxole is formed by
an intramolecular reaction of B-(OH)2 and C-OH func-
tionalities.
Furthermore, we were interested in extending these inves-
tigations by using a precursor bearing the B(OH)2 as well as
the C-OH functionality. By the reaction of 3-hydroxyphe-
nylboronic acid with 1, we obtained a novel organoboron
heterocyle, 5, containing two C3OB five-membered rings.
X-ray quality single crystals of 2, 3, 4, and 5 were obtained
from a n-hexane solution at room temperature. Compounds
2 and 5 crystallize in the monoclinic space group P21/n; 3
forms crystals in the orthorhombic space group Pbcn and 4 in
the triclinic P1 space group. The molecular structures are
shown in Figures 1, 2, 3, and 4, respectively, and the selected
bond lengths and angles are shown in Tables 2, 3, 4, and 5.
For the structures of 2 and 3, one aluminum atom, two boron
atoms, and three oxygen atoms form a six-membered planar
AlO3B2 ring. The central aluminum atom is located in the
spirocyclic center of the two fused six-membered rings
Acknowledgment. This work was supported by the
National Nature Science Foundation of China (2100-
1016, 20901009); the Research Fund for the Doctoral
Program of Higher Education of China (2009110112043);
the Program of NCET-10-0050; and the National 973
Program (2009CB220100)
Supporting Information Available: CIF files for compounds 2,
3, 4, and 5 are available free of charge via the Internet at http://
pubs.acs.org.
˚
(C3N2Al and AlO3B2). The Al-O bond lengths (av 1.739 A
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