Reactions of hydroxymesitylboranes with metal alkyls: An approach to new sterically hindered (metaloxy)mesitylboranes

Article abstract of DOI:10.1021/ic0112490

Reactions of mesitylboronic acid with alkyl derivatives of aluminum R₃AI (R = Me, Et, Buⁱ), gallium (Me₃Ga), and zinc (Et₂Zn) were investigated. The treatment of mesitylboronic acid, MesB(OH)₂, with trimethylgallium afforded the discrete dimer [μ-(MesB(OH)O)GaMe₂]₂ (1), which is the simple example of a O-metalated boronic acid with no hydrogen bonding in the crystal lattice. In addition, the reaction of dimesitylborinic acid, Mes₂BOH, with diethylzinc produced the low-valent zinc compound [(μ-Mes₂BO)ZnEt]₂ (2), which was also characterized by X-ray diffraction.

Full text of DOI:10.1021/ic0112490

Inorg. Chem. 2002, 41, 2525−2528
Reactions of Hydroxymesitylboranes with Metal Alkyls: An Approach to
New Sterically Hindered (Metaloxy)mesitylboranes
,‡
Romana Anulewicz-Ostrowska,† Sergiusz Lulin´ski,‡ Edyta Pindelska,† and Janusz Serwatowski*
Faculty of Chemistry, UniVersity of Warsaw, Pasteura 1, 02-093 Warsaw, Poland, and Faculty of
Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
Received December 5, 2001
i
Reactions of mesitylboronic acid with alkyl derivatives of aluminum R Al (R ) Me, Et, Bu), gallium (Me3Ga), and
3
zinc (Et2Zn) were investigated. The treatment of mesitylboronic acid, MesB(OH)2, with trimethylgallium afforded the
discrete dimer [µ-(MesB(OH)O)GaMe2]2 (1), which is the simple example of a O-metalated boronic acid with no
hydrogen bonding in the crystal lattice. In addition, the reaction of dimesitylborinic acid, Mes2BOH, with diethylzinc
produced the low-valent zinc compound [(µ-Mes2BO)ZnEt]2 (2), which was also characterized by X-ray diffraction.
Introduction
activity of dialkylaluminum dimesitylboryl oxides, [(µ-Mes2-
BO)AlR2]2, especially their catalytic activity, have been
presented recently by our group5 and independently by
Gibson and co-workers.6 This work is the continuation of
our research on sterically hindered metal organoboryl oxides.
We describe some results concerning mainly gallium and
zinc derivatives of mesitylboronic and dimesitylborinic acids.
It is well-known that the role of steric factors in boron
chemistry is important. It was found earlier that the chemical
properties of sterically hindered boron compounds may vary
dramatically when compared to their nonhindered analogues.
Obviously, it can be expected that the use of bulky
substituents should reduce the reactivity of the boron center.
A good example of such an effect is trimesitylborane, which,
contrary to triphenylborane, is completely resistant against
reactions involving the cleavage of the boron-carbon bond;
e.g., this compound could not be hydrolyzed under normal
conditions and is stable to air.1 However, this compound does
undergo some reactions but the boron environment remains
unchanged.2 Two mesityl groups provide still considerable
protection to boron. This property was found earlier,1 and
then it was exploited by several research groups.3
Experimental Section
General Comments. All reactions were carried out under an
argon atmosphere using the standard Schlenk techniques. Solvents
were dried with sodium benzophenone ketyl, distilled, and stored
under argon. Aluminum alkyls (Aldrich) were used as received.
Trimethylgallium was a gift from Prof. K. Starowieyski from our
university. Boron-containing reagents dimesitylborinic acid, Mes2-
BOH,1 and mesitylboronic acid, MesB(OH)2,7 have been prepared
1
according to the literature descriptions. H and 11B NMR spectra
were recorded at room temperature on a Varian Unity Plus 200
spectrometer using benzene-d6 as the solvent (unless otherwise
noted). Chemical shifts are given in ppm relative to C6D5H (δ )
7.17 ppm) and Et2O‚BF3 in 1H and 11B spectra, respectively.
Elemental analyses were performed using Perkin-Elmer 2400
apparatus.
Our work has focused on metal organoboryl oxides.
Sterically hindered diorganoborinic acids were used suc-
cessfully for the preparation of some systems possessing
B-O-M linkage, and this involves also dimesitylborinic
acid and the closely related compound Trip2BOH (Trip )
2,4,6-Pri3C6H2).4 Interesting results concerning diverse re-
(4) For example, see the following: (a) Beck, G.; Hitchcock, P. B.;
Lappert, M. F.; MacKinnon, I. A. J. Chem. Soc., Chem. Commun.
1989, 1312. (b) Chen, H.; Power, P. P.; Shoner, S. C. Inorg. Chem.
1991, 30, 2884. (c) Weese, K. J.; Bartlett, R. A.; Murray, B. D.;
Olmstead, M. M.; Power, P. P. Inorg. Chem. 1987, 26, 2409.
(5) Anulewicz-Ostrowska, R.; Lulin´ski, S.; Serwatowski, J.; Suwin´ska,
K. Inorg. Chem. 2000, 39, 5763.
(6) Gibson, V. C.; Mastroianni, S.; White, A. J. P.; Williams, D. J. Inorg.
Chem. 2001, 40, 826.
(7) Brown, P.; Mahon, M. F.; Molloy, K. C. J. Chem. Soc., Dalton Trans.
1992, 3503.
* To whom correspondence should be addressed. E-mail: SERWAT@
ch.pw.edu.pl
† University of Warsaw.
‡ Warsaw University of Technology.
(1) Brown, H. C.; Dodson, V. H. J. Am. Chem. Soc. 1957, 79, 2302.
(2) Ramsey, B. G.; Lorne, M. I. J. Org. Chem. 1981, 46, 179.
(3) For example, see the following: (a) Wilson, J. W. J. Organomet. Chem.
1980, 186, 297. (b) Brown, N. M. D.; Davidson, F.; Wilson, J. W. J.
Organomet. Chem. 1980, 185, 277. (c) Pelter, A.; Singaram, B.;
Williams, L.; Nelson, J. W. Tetrahedron Lett. 1983, 24, 623.
10.1021/ic0112490 CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/05/2002
Inorganic Chemistry, Vol. 41, No. 9, 2002 2525

Anulewicz-Ostrowska et al.
Scheme 1
Table 1. Crystal Data and Structure Refinement for Compounds 1 and
2
param
1
2
emp formula
fw
temp, K
λ, Å
cryst syst
space group
a, Å
C22H36B2Ga2O4
262.78
293(2)
C47H54B2O2Zn2
803.26
293(2)
stirring. A white precipitate was formed. It was dissolved by
warming the suspension to ca. 50 °C. The solution was slowly
cooled to room temperature. Colorless needles of compound were
obtained. The supernatant solution was cooled to 0 °C, to give a
further portion of crystals. They were washed with a small amount
of hexane and dried under reduced pressure. The total amount of
the product was 1.15 g (64%), mp 142-146 °C: 1H NMR δ 6.76
(s, 4H, arom), 2.39 (s, 12H, o-Me), 2.14 (s, 6H, p-Me), 0.69 (t,
3H, ZnCH2CH3), 0.06 (q, ZnCH2CH3); 13C{1H} NMR δ 140.97,
140.49, 137.96, 129.13 (arom), 23.39 (o-Me), 21.61 (p-Me), 11.51
(ZnCH2CH3), 1.05 (ZnCH2CH3); 11B NMR δ 51.0. Anal. Calcd
for C20H27BOZn: C, 66.80; H, 7.57. Found: C, 66.16; H, 7.28.
(Mes2BO)ZnEt‚2,2′-bipy (3) was prepared by the treatment of
[(µ-Mes2BO)ZnEt]2 (0.90 g, 1.25 mmol), freshly prepared in
tetrahydrofuran (7 mL), with 2,2′-bipyridine (0.42 g, 2.7 mmol).
The resultant suspension was warmed to 50 °C to give the clear
orange solution. Slow cooling to the room temperature afforded
yellow needles of the complex 3. Crystals were washed with a little
cold THF (2 × 2 mL) and dried under reduced pressure. The yield
of 3 was 0.81 g (63%), mp 165-168 °C: 1H NMR (THF-d8) δ
8.46 (d, 4H, bipy), 7.93 (t, 2H, bipy), 7.39 (t, 2H, bipy), 6.52 (s,
4H, arom), 2.19 (s, 6H, p-Me), 1.95 (s, 12H, o-Me), 1.02 (t, 3H,
ZnCH2CH3), 0.16 (q, ZnCH2CH3); 13C{1H} NMR δ 149.84, 140.66,
138.09, 135.22, 128.80, 128.05, 124.95, 121.62 (bipy, arom), 22.60
1.541 78
monoclinic
C2/c
10.864(2)
15.785(3)
16.703(3)
108.78(3)
2711.9(9)
4
0.710 73
monoclinic
P21/n
15.020(3)
10.740(2)
15.650(3)
117.83(3)
2232.6(8)
2
b, Å
c, Å
â, deg
V, Å3
Z
d(calcd), Mg/m3
µ, mm-1
F(000)
1.287
2.608
1088
1.195
1.108
844
cryst size, mm
reflcns collcd
R [I > 2σ(I)]
largest diff. peak and
hole, e Å-3
GOF
0.32 × 0.24 × 0.20
0.30 × 0.24 × 0.10
2531
23 919
0.0731
0.683 and 1.758
0.0901
2.549 and 0.591
1.191
1.162
X-ray Diffraction Studies. Crystal data regarding structures of
1 and 2 are given in Table 1 together with refinement details. X-ray
measurements were performed on a Kuma KM-4 and KM4CCD
κ-axis diffractometers with graphite-monochromated Cu KR (1) or
Mo KR (2) radiation, respectively. The data were corrected for
Lorentz and polarization effects. No absorption correction was
applied. Data reduction and analysis were carried out with the Kuma
diffraction programs. The structures were solved by direct methods8
and refined using SHELXL.9 The refinement was based on F2 for
all reflections except those with very negative F2. Weighted R
factors, wR, and all goodness-of-fit S values are based on F2.
Conventional R factors are based on F with F set to zero for
(o-Me), 21.20 (p-Me), 13.54 (ZnCH2CH3), -1.55 (ZnCH2CH3); 11
B
NMR δ 48.5. Anal. Calcd for C30H35BN2OZn: C, 69.86; H, 6.84;
N, 5.43. Found: C, 69.95; H, 6.89; N, 5.32.
Results and Discussion
negative F2. The Fo > 2σ(Fo ) criterion was used only for
calculating R factors and is not relevant to the choice of reflections
for the refinement. All hydrogen atoms were located from a
differential map and refined isotropically. Scattering factors were
taken from ref 10 Molecular diagrams were drawn using ORTEP.11
Preparation of [(µ-MesB(OH)O)GaMe2]2 (1). A solution of
mesitylboronic acid (0.27 g, 1.65 mmol) in tetrahydrofuran (3 mL)
was added dropwise during 2-3 min to a stirred solution of
trimethylgallium (0.19 g, 1.65 mmol) in toluene (2 mL) at -70
°C. The resultant solution was allowed to warm slowly to room
temperature. Solvents were evaporated under reduced pressure. A
solid residue was washed with hexane (3 × 2 mL) and dried under
reduced pressure. Compound 1 was obtained as a white powder in
0.27 g yield (62%), mp 152-154 °C (dec): 1H NMR δ 6.67 (s,
2H, arom), 3.58 (s, 1H, OH), 2.26 (s, 6H, o-Me), 2.11 (s, 3H, p-Me),
0.07 (s, 6H, GaMe); 13C{1H} NMR δ 138.97, 138.46, 128.69,
127.63 (arom), 22.66 (o-Me), 21.64 (p-Me), -4.19 (GaMe); 11B
NMR δ 31.3. Anal. Calcd for C11H18BGaO2: C, 50.28; H, 6.90.
Found: C, 50.09; H, 6.69.
2
2
Synthetic Details. The reaction of mesitylboronic acid
with trimethylgallium proceeds cleanly as the selective
protonolysis of one Ga-C bond occurs to yield the crystalline
compound 1. This is shown in Scheme 1. In this case the
boron alkylation resulting from the transfer of methyl groups
from gallium to boron does not proceed at all. The product
is reasonably stable. The decomposition with gas evolution
was observed only during melting (ca. 150 °C). The reaction
of unsubstituted phenylboronic acid with GaMe3 proceeded
similarly but led to the formation of amorphous material
whose structure was not determined.12
The reaction of mesitylboronic acid with diethylzinc
proceeded with gas (ethane) evolution and afforded insoluble
amorphous product. Similarly, reactions of mesitylboronic
acid with aluminum alkyls do not yield well-defined
products. In the case of trimethyl- and triethylaluminum
insoluble amorphous materials were obtained. We suppose
that oligomeric or polymeric B-O-Al type species are
formed in these reactions.13 However, the 11B NMR analysis
of the reaction mixture of MesB(OH)2 and AlR3, R ) Me
and Et (molar ratio 1:2, respectively), showed the presence
Preparation of [(µ-Mes2BO)ZnEt]2 (2). Diethylzinc (0.7 g, 5.5
mmol) was added during 2-3 min to the stirred solution of Mes2-
BOH (1.33 g, 5.0 mmol) in toluene (10 mL) at 0 °C. The resultant
solution was allowed to warm slowly to room temperature with
(8) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, A46, 467.
(9) Sheldrick, G. M. SHELXL93, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(10) International Tables for Crystallography; Wilson, A. J. C., Ed.;
Kluwer: Dordrecht, Holland, 1992; Vol. C.
(11) Burnett, M. N.; Johnson, C. K. ORTEP-III; ORNL-Report 6895; Oak
Ridge National Laboratory: Oak Ridge, TN, 1996.
(12) Lulin´ski, S.; Serwatowski, J. Contemporary Boron Chemistry. Pro-
ceedings of the 10th International Conference on Boron Chemistry;
University of Durham: Durham, England, 1999.
(13) Very recently, the synthesis and characterization of related compounds
obtained from the reactions of (2,6-diisopropylphenyl)boronic acid,
2,6-Pri2C6H3B(OH)2, with But3Al were reported: Richter, B.; Meetsma,
A.; Hessen, B.; Teuben, J. H. Chem. Commun. 2001, 1286.
2526 Inorganic Chemistry, Vol. 41, No. 9, 2002

Sterically Hindered (Metaloxy)mesitylboranes
Figure 1. Hypothetical molecular structure of [(µ-MesB(OH)O)GaMe2]2
(1).
Scheme 2
Scheme 3
Figure 2. Molecular structure of of [(µ-MesB(OH)O)GaMe2]2 (1).
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Compounds 1 and 2
Compound 1
Ga-C(11)
Ga-C(10)
Ga-O(1)
Ga-O(1)
1.928(9)
1.941(8)
1.972(4)
1.972(5)
B-O(1)
B-O(2)
B-C(1)
1.346(8)
1.352(10)
1.585(9)
of corresponding soluble mesityldialkylboranes MesBR2, too.
This is the evidence that the boron alkylation proceeds in
these systems to a significant extent. Even the replacement
of the mesityl group with the alkyl group was observed as
the presence of trialkylboron in the reaction mixture was also
established by the 11B NMR measurement. Clearly, the
redistribution reaction between MesBR2 and AlR3 is possible.
Interesting results obtained previously for alkylaluminum
dimesitylboryl oxides, [(µ-Mes2BO)AlR2]2,5,6 have prompted
us to investigate their zinc analogues. In fact the treatment
of Mes2BOH with diethylzinc led to gas (ethane) evolution
and the high-yield formation of the soluble dimeric ethylzinc
dimesitylboryl oxide, [(µ-Mes2BO)ZnEt]2 (2) (Scheme 2).
Ethane was not evolved when dimesitylborinic acid was
added to the toluene solution of 2 at room temperature. In
addition, the catalytic decomposition of Mes2BOH which is
known to proceed in the presence of [(µ-Mes2BO)AlR2]2
(R ) Me, Et)5,6 is not the case, apparently because of the
weaker Lewis acidity of zinc with respect to aluminum.
However, compound 1 reacts easily with 2,2′-bipyridine at
room temperature to form the 1:1 addition complex Mes2-
BOZnEt‚bipy (3) as yellow crystals (Scheme 3). The steric
hindrance at zinc was apparently not sufficient to prevent
the donor coordination. Unfortunately, the quality of crystals
was too low to perform an X-ray diffraction study of this
interesting compound.
C(11)-Ga-C(10)
O(1)-Ga-O(1)
Ga-O(1)-Ga
130.2(5)
79.0(2)
101.0(2)
B-O(1)-Ga
B-O(1)-Ga
O(1)-B-O(2)
124.7(5)
134.0(4)
115.5(6)
Compound 2
1.972(4)
1.969(5)
1.941(8)
3.0047(18)
Zn(1)-O(1)
Zn(1)-O(1)
Zn(1)-C(20)
Zn(1)‚‚‚Zn(1)
B(1)-O(1)
B(1)-C(13)
B(1)-C(6)
1.345(10)
1.572(12)
1.574(11)
O(1)-Zn(1)-O(1)
Zn(1)-O(1)-Zn(1)
80.6(2)
99.4(2)
O(1)-B(1)-C(13) 117.1(7)
O(1)-B(1)-C(6) 118.4(7)
C(13)-B(1)-C(6) 124.5(7)
C(20)-Zn(1)-O(1) 139.6(3)
B(1)-O(1)-Zn(1)
130.1(5)
are very similar to those found for related gallium boryl oxide
[(µ-9-BBN-9-O)GaMe2]2.14 The Ga‚‚‚O(2) distance in 1 is
large (3.094 Å), and there is no deviation from the tetrahedral
environment typical for gallium atoms in related compounds.
Hence, hydroxyl oxygens are not engaged in any interaction
with gallium. The Ga-C bond distances as well as the
C-Ga-C angle are also typical. The B-O bond distances
are almost equal to one another (1.346(8) Å for boryl oxide
oxygen and 1.351(10) Å for hydroxyl oxygen) and cor-
respond well to the value found in [(µ-9-BBN-9-O)GaMe2]2
(1.356(7) Å). The O(1)-B-O(2) angle is 115.6(6)°. It should
be noted that the X-ray diffraction determination of the
structure of the dibutyltin derivative of phenylboronic acid
[Ph(OH)O]2SnBut2 showed the very large variation of B-O
bond distances as well as O-B-O angles.7 On the other
hand, the quality of the refinement suggests some care in
using those results.
Structural Description. Initially, we considered the
structure of the dimer 1 in which gallium atoms are donated
by the hydroxyl oxygens. This would lead to the formation
of the eight-membered BOGaOBOGaO ring as shown in
Figure 1.
However, the X-ray analysis of 1 revealed that the central
four-membered planar Ga2O2 ring is formed. Gallium atoms
are bridged by the boryl oxide oxygen atoms as shown in
Figure 2. Selected bond distances and angles are presented
in Table 2. The Ga-O(1) bond distances are 1.972(5) Å.
The Ga-O-Ga and O-Ga-O angles are 101.0(2) and
79.0(2)°, respectively. The metric features of the Ga2O2 core
The C2BO plane in 1 is slightly twisted (by ca. 10°) with
respect to the Ga2O2 plane. An interesting structural feature
is that B-O(1)-Ga and B-O(1)-Ga′ angles vary signifi-
cantly (124.7(4) and 134.0(4)°, respectively). This effect can
(14) Anulewicz-Ostrowska, R.; Lulinski, S.; Serwatowski, J. Inorg. Chem.
1999, 38, 3796.
Inorganic Chemistry, Vol. 41, No. 9, 2002 2527

Anulewicz-Ostrowska et al.
Figure 3. Molecular structure of [(µ-Mes2BO)ZnEt]2 (2). The disordered molecule of toluene has also been shown.
be explained in terms of steric interaction between mesityl
groups and methyl groups at gallium. Alternatively, an
interaction between hydroxyl oxygen and gallium could also
be considered as a reason for the mentioned difference.
However, as was mentioned above, this is not the case since
such an interaction should result in a more acute O(1)-B-
O(2) angle and/or significantly shorter Ga‚‚‚O(2) distance.
It should be stressed that the hydroxyl group does not form
any intra- and/or intermolecular hydrogen bonds. This
observation was confirmed by the IR spectrum showing the
sharp band at 3630 cm-1, i.e., in the range typical for the
nonassociated hydroxyl group. Apparently the steric hin-
drance provided by the bulky mesityl groups together with
dimethylgallium moieties results in the lack of association.
In fact, the orientation of mesityl rings is specific as they
are located almost perpendicular to the Ga2O2 plane. The
lack of association in 1 is again in contrast to the existence
of extensive hydrogen-bonding network in [Ph(OH)O]2SnBut2.7
Molecular structure of 2 is depicted in Figure 3. Compound
2 crystallizes with one molecule of toluene. Selected bond
distances and angles are given in Table 2. The central Zn2O2
is planar. The Zn-O bond distance are 1.969(5) and
1.972(4) Å; i.e., they are significantly shorter than those
found in the tetrameric zinc boryl oxide [(µ3-9-BBN-9-O)-
ZnEt]4,15 where the Zn-O distances vary in the range
2.07-2.14 Å. However, they are comparable with the values
measured in related dimeric alkoxides [(µ-2,6-Pri2C6H3O)-
ZnCH2SiMe3]2 and [(µ-2,4,6-But3C6H2O)ZnCH2SiMe3]2.16
The Zn-O-Zn (99.4(2)°) and O-Zn-O (80.6(2)°) angles
in 2 are also similar to those found in the mentioned dimers.
The shortening of Zn-O distances in 2 is due to the
decreasing the coordination number of zinc from 4 to 3. The
B-O bond distance is 1.345(10) Å and is also shorter (but
only slightly) than the mean value in [(µ3-9-BBN-9-O)ZnEt]4.
It should be noted that the torsion angle between the C2BO
and Zn2O2 planes is about 15°. This nonplanar conformation
probably reflects steric requirements of mesityl groups. In
the related aluminum compound [(µ-Mes2BO)AlMe2]2,5 the
analogous torsion angle is even larger (ca. 30°), but this is
in accord with the smaller size of the Al2O2 core vs Zn2O2
core.
The dimeric structure of 2 is not typical for alkylzinc
alkoxides and also boryl oxides. It is clear that the presence
of bulky mesityl groups prevents the formation of a tetramer.
Obviously, the boryl oxide ligand is expected to be a less
effective π-donor with respect to typical alkoxide ligands.
However, the tetrameric structure of other alkylzinc boryl
oxides12 lends strong support to the conclusion that in the
case of 2 steric factors are crucial for the stabilization of the
dimeric structure.
In conclusion, mesitylboronic acid was used succsessfully
in synthesis of a new gallium boryl oxide. However,
generally dimesitylborinic acid seems to be a more versatile
reagent in the synthesis of metal boryl oxides.
Acknowledgment. We gratefully acknowledge the sup-
port by Aldrich Chemical Co., Inc., Milwaukee, WI, through
continuous donation of chemicals and equipment. We also
thank the State Committee for Scientific Research for
financial support.
Supporting Information Available: Listings of crystal and
refinement data, atomic coordinates, bond distances and angles, and
anisotropic displacement parameters for all non-hydrogen atoms
as well as hydrogen atom coordinates with isotropic displacement
parameters and an X-ray crystallographic file, in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(15) Lulin´ski, S.; Madura, I.; Serwatowski, J.; Zachara, J. Inorg. Chem.
1999, 38, 4937.
(16) Olmstead, M. M.; Power, P. P.; Shoner, S. C. J. Am. Chem. Soc. 1991,
113, 3379.
IC0112490
2528 Inorganic Chemistry, Vol. 41, No. 9, 2002