Reversible formation of organyl(oxo)boranes (RBO) (R = C6H 5 or CH3) from boroxins ((RBO)3): A matrix isolation study

Article abstract of DOI:10.1021/om061057i

Flash vacuum pyrolysis of triphenyl- and trimethylboroxin and subsequent trapping of the gas phase products in a large excess of argon at T < 20 K allows matrix isolation and characterization by IR spectroscopy of phenyl(oxo)borane and methyl(oxo)borane. The nature of the matrix isolated species, monomelic CH₃BO, 1:1 dimer complex of CH₃BO, (CH₃BO)₂, and (CH₃BO)₃, depends strongly on the trapping temperature and matrix host gas (Ar vs Xe) with the boroxin dominating at 30 K (Ar) or 55 K (Xe). An ab initio investigation (second-order Moller - Plesset perturbation theory) of the potential energy surface for trimerization of CH₃BO is in agreement with the experimental observations.

Full text of DOI:10.1021/om061057i

Organometallics 2007, 26, 6263-6267
6263
Reversible Formation of Organyl(oxo)boranes (RBO) (R ) C H or
6
5
CH ) from Boroxins ((RBO) ): A Matrix Isolation Study
3
3
Holger F. Bettinger*
Lehrstuhl f u¨ r Organische Chemie II, Ruhr-UniVersit a¨ t Bochum, UniVersit a¨ tsstrasse 150,
4780 Bochum, Germany
4
ReceiVed NoVember 17, 2006
Flash vacuum pyrolysis of triphenyl- and trimethylboroxin and subsequent trapping of the gas phase
products in a large excess of argon at T < 20 K allows matrix isolation and characterization by IR
spectroscopy of phenyl(oxo)borane and methyl(oxo)borane. The nature of the matrix isolated species,
monomeric CH
3 3 3 2 3 3
BO, 1:1 dimer complex of CH BO, (CH BO) , and (CH BO) , depends strongly on the
trapping temperature and matrix host gas (Ar vs Xe) with the boroxin dominating at 30 K (Ar) or 55 K
(
Xe). An ab initio investigation (second-order Møller-Plesset perturbation theory) of the potential energy
surface for trimerization of CH
3
BO is in agreement with the experimental observations.
Introduction
due to the kinetic and thermodynamic instability of 1 toward
1
6,17
trimerization (Scheme 1).
Oxoboranes XBO are fundamental yet highly elusive boron
compounds. Inorganic derivatives of XBO, X ) H, N, O, F,
Organoboroxins have been structurally characterized18-20 and
1
are rather stable compounds under anhydrous conditions (e.g.,
Cl, Br, or I, have been obtained previously by reaction of either
the boron halides with B2O3 at temperatures exceeding 1000 K
or by co-condensation of boron atoms with small molecules
they form 1:1 complexes with nitrogen bases without degrada-
21,22
tion of the six-membered heterocyclic ring),
and their
thermal decomposition is reported to go along with the
(
H2O, O2, or NO).2-9 Such techniques are clearly not suitable
23
degradation of the organic ligands. Consequently, other routes
for the generation of organyl(oxo)boranes (RBO) (1; R )
organyl), the monomeric anhydrides of the boronic acids 2.
These are invaluable reagents in modern organic synthesis (e.g.,
in the Suzuki10 cross-coupling reactions) and are currently
receiving increased interest as molecular building blocks in
to 1 were sought, and the methyl-1,3,2-diborolane-4,5-dione (4)
proved useful. After the mass spectrometric detection of 1b (R
16
)
Me) in 1977, the first direct spectroscopic characterization
of an oxoborane 1b in the pyrolysis mixture of 4 was achieved
24
by Bock et al. using photoelectron spectroscopy (Scheme 2).
crystal engineering as they combine both hydrogen bonding and
As organic groups larger than a methyl group resulted in
insoluble derivatives of 4 which lack the necessary volatility
for gas phase experiments, no organyl(oxo)boranes other than
Lewis acidic motifs.1
1-13
As compared to boronic acids, the
dicoordinated boron center in 1 is expected to show increased
Lewis acidity, while the oxygen atom should still be able to
act as a hydrogen bond acceptor. Such aspects of the chemistry
of 1 have never been investigated due to the lack of generally
applicable access to these systems. A straightforward route,
1b have been studied by direct spectroscopic techniques to date.
Lanzisera and Andrews characterized 1b by IR spectroscopy
as one of the products of the reaction of boron atoms with
25
methanol under the conditions of matrix isolation. A number
14,15
dehydration of 2, yields the trimeric boroxins 3 instead of 1,
of trapping experiments have furnished additional evidence for
2
6,27
the existence of RBO (R ) 2,4,6-tri-t-butylphenyl,
2,4,6-
28
29
*
Corresponding
holger.bettinger@ruhr-uni-bochum.de.
1) K o¨ ster, R. In Houben-Weyl, OrganoborVerbindungen I; K o¨ ster, R.,
Ed.; Georg Thieme Verlag: Stuttgart, 1982; Vol. 13/3a, p 6.
author.
Tel.:
49-234-32-14353;
e-mail:
tris[bis(trimethylsilyl)methyl]phenyl, or C(SiMe3)3 ) as reac-
tive intermediates since the mid-1980s, while the detection of
C2BO and C4BO from (CH3)3Si-(CtC)n-B(O-iPr)2 (n ) 1 or
(
(
(
(
(
2) Snelson, A. High Temp. Sci. 1972, 4, 141.
3) Snelson, A. High Temp. Sci. 1972, 4, 318.
4) Andrews, L.; Burkholder, T. R. J. Phys. Chem. 1991, 95, 8554.
5) Zhou, M.; Tsumori, N.; Xu, Q.; Kushto, G. P.; Andrews, L. J. Am.
(16) Paetzold, P. I.; Scheibitz, W.; Scholl, E. Z. Naturforsch., B: Chem.
Sci. 1971, 26, 646.
(17) Paetzold, P.; Bohm, P.; Richter, A.; Scholl, E. Z. Naturforsch., B:
Chem. Sci. 1976, 31, 754.
Chem. Soc. 2003, 125, 11371.
(
(
81.
6) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697.
7) Kawaguchi, K.; Endo, Y.; Hirota, E. J. Mol. Spectrosc. 1982, 93,
(18) Bauer, S. H.; Beach, J. Y. J. Am. Chem. Soc. 1941, 63, 1394.
(19) Boese, R.; Polk, M.; Bl a¨ ser, D. Angew. Chem. 1987, 99, 239; Angew.
Chem., Int. Ed. Engl. 1987, 26, 245.
3
1
1
(
8) Kawashima, Y.; Kawaguchi, K.; Endo, Y.; Hirota, E. J. Chem. Phys.
(20) Brock, C. P.; Minton, R. P.; Niedenzu, K. Acta Crystallogr., Sect.
C: Cryst. Struct. Commun. 1987, 43, 1775.
987, 87, 2006.
(9) Hunt, N. T.; R o¨ pcke, J.; Davies, P. B. J. Mol. Spectrosc. 2000, 204,
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(22) Yalpani, M.; K o¨ ster, R. Chem. Ber. 1988, 121, 1553.
(23) Nencetti, G.; Nardini, G.; Zanelli, S. Energia Nucl. 1964, 11, 248.
(24) Bock, H.; Cederbaum, L. S.; von Niessen, W.; Paetzold, P.; Rosmus,
P.; Solouki, B. Angew. Chem. 1989, 101, 77; Angew. Chem., Int. Ed. Engl.
1989, 28, 88.
(25) Lanzisera, D. V.; Andrews, L. J. Phys. Chem. A 1997, 101, 1482.
(26) Pachaly, B.; West, R. J. Am. Chem. Soc. 1985, 107, 2987.
(27) Groteklaes, M.; Paetzold, P. Chem. Ber. 1988, 121, 809.
(28) Ito, M.; Tokitoh, N.; Okazaki, R. Tetrahedron Lett. 1997, 38, 4451.
(29) Paetzold, P.; Neyses, S.; Geret, L. Z. Anorg. Allg. Chem. 1995, 621,
732.
20–124.
(
(
10) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
11) Fournier, J.-H.; Maris, T.; Wuest, J. D.; Guo, W.; Galoppini, E. J.
Am. Chem. Soc. 2003, 125, 1002.
12) Maly, K. E.; Malek, N.; Fournier, J.-H.; Rodriguez-Cuamatzi, P.;
Maris, T.; Wuest, J. D. Pure Appl. Chem. 2006, 78, 1305.
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Tetrahedron Lett. 2006, 47, 1389.
(
(
(
(
14) Kinney, R. C.; Pontz, D. F. J. Am. Chem. Soc. 1936, 58, 197.
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3
1.
1
0.1021/om061057i CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/08/2007

6
264 Organometallics, Vol. 26, No. 25, 2007
Bettinger
Table 1. Vibrational Frequencies (in cm-1) and Infrared
Intensities of Phenyl(oxo)borane 1a from Experiment (Ar
Matrix, 10 K) and Theorya
-
symmetry
species
rel
ω
theor (cm 1)
-
-
Number
υ
exp (cm 1)
intensity (I (km mol 1))
3
3
3
3
2
2
2
2
2
2
2
2
2
3
2
1
0
9
8
8
7
6
5
4
3
2
A
1
3240.1 (5.6)
B
2
3053
0.02
3232.6 (10.7)
3221.9 (2.7)
3214.9 (0.1)
3208.8 (0.1)
2054.5 (248.6)
1989.2 (232.1)
1645.9 (0.6)
1617.1 (0.7)
1518.7 (4.9)
1475.2 (8.2)
1439.4 (2.3)
1339.0 (1.0)
1210.3 (0.7)
1185.1 (0.0)
1182.1 (6.6)
1100.8 (1.2)
1051.6 (2.5)
1010.7 (0.9)
984.7 (0.0)
974.7 (0.0)
930.0 (1.2)
869.9 (0.0)
754.9 (46.7)
733.6 (0.0)
697.3 (20.8)
624.6 (1.0)
554.6 (39.9)
543.9 (36.3)
535.0 (14.0)
426.8 (0.6)
395.3 (0.0)
379.3 (5.5)
127.5 (5.4)
125.2 (1.9)
A
1
B
2
A
1
A
1
A
1
A
1
b
2040, 2038, 2035
1977, 1975,1972, 1971
0.21
1.00
B
2
A
1
B
2
B
2
B
2
1438
1168
0.06
0.02
Figure 1. IR spectra of the triphenylboroxin pyrolysis products
trapped in Ar at 10 K (bottom trace) and spectrum computed for
C H BO (1a) at the MP2/cc-pVTZ level of theory (top trace). Bands
6 5
due to CO, HBO, and water (w) are labeled. Stars identify signals
21
A
1
B
2
20
19
18
A
1
B
2
17
A1
A
1
16
15
14
13
12
11
10
9
8
7
7
6
5
4
3
2
1
due to triphenylboroxin 3a.
B
1
A
2
Scheme 1. Dehydration of Boronic Acids 2 Generally Yields
Trimeric Anhydrides, Boroxins 3
B
1
A
2
B
1
749, 747
692
0.13
0.18
A
1
B
1
B
2
B
1
B
1
B
2
b
551, 550
540, 539
535
0.03
0.17
0.05
A
1
A
2
B
B
1
2
Scheme 2. Synthesis of Methyl(oxo)borane (1b) by
Thermolysis of Methyl-1,3,2-diborolane-4,5-dione (4) as
Reported by Bock et al.24
B1
a
MP2/cc-pVTZ, unscaled; intensities in km mol-1 given in parentheses.
b 10
B isotopomer, natural abundance 19.2%.
bands can be assigned to 1a based on comparison with ab initio
data (MP2/cc-pVTZ, see Table 1).
The signal pairs at 2037/1975 and 550/540 cm- have a 1:4
1
intensity typical for boron compounds with a natural isotope
1
0
11
composition ( B/ B ) 1:4.1) and are due to the B-O stretching
2
) by chemical ionization in a mass spectrometer has been
30
(a1) and bending (b1) vibrations, respectively. These bands, along
reported more recently. A stabilized oxoborane has also been
obtained recently.
-
1
3
1
with the CH wagging mode at around 750 cm , show a
pronounced fine structure, probably due to matrix site effects.
In an attempt to investigate the interaction of 1a with CO,
pyrolysis products were trapped with Ar/CO (1000:1) mixtures.
We report here that the organyl(oxo)boranes 1a (R ) C6H5)
and 1b (R ) CH3) can be generated thermally form the
corresponding boroxins 3 by pyrolysis in the gas phase, isolated
in cryogenic noble gas matrices, and studied by FTIR spec-
troscopy (Scheme 1). This allows, for the first time, for the
synthesis of phenyl(oxo)borane 1a and the observation of the
thermally initiated aggregation of 1b in argon and xenon
matrices.
-1
Additional signals at 2145.9 and 1967.5 cm result, which upon
-1
annealing to 38 K increase, while signals due to CO (2139 cm )
and 1a decrease. We assign these two signals to the ν(CO) and
ν(BO) vibrations of a C6H5-BO‚‚‚CO complex, which is bound
by only 1 kcal mol (CCSD(T)/cc-pVTZ//MP2/cc-pVTZ). The
shifts of the CO (7 cm blue) and BO (7 cm red) stretching
vibrations are in reasonable agreement with ab initio (MP2/cc-
-
1
-
1
-1
Results and Discussion
-1
pVDZ) data (6 and 4 cm , respectively).
The thermal fragmentation of boroxins is not limited to the
phenyl system 3a. Carrier gas pyrolysis of 3b (0.01% in Ar)
yields 1b, whose IR spectral data in solid argon are in excellent
agreement with those reported previously by Lanzisera and
Andrews (see Table 2). The trapping temperature proved to
be extremely important for the detection of 1b. Increasing the
temperature of the spectroscopic window from 18 to 30 K and
above does not yield any 1b under otherwise identical experi-
mental conditions. Rather, 3b is observed as the only product
demonstrating the high kinetic instability of 1b.
Fragmentation of Boroxins and Recyclization. Subjecting
the boroxin 3a to flash vacuum pyrolysis (FVP; T ≈ 650 °C,
quartz tube) and freezing the gaseous products in a large excess
of Ar at T < 20 K, IR signals (Figure 1) due to the usual
contaminants in FVP experiments (CO2, CO, and H2O), 3a, and
2
5
4
traces of HBO are observed. The yield of the latter species
increases at higher thermolysis temperatures. The remaining IR
(30) (a) McAnoy, A. M.; Dua, S.; Schr o¨ der, D.; Bowie, J. H.; Schwarz,
H. J. Phys. Chem. A 2003, 107, 1181. (b) McAnoy, A. M.; Dua, S.;
Schr o¨ der, D.; Bowie, J. H.; Schwarz, H. J. Phys. Chem. A 2004, 108, 2426.
The composition of the trapped thermolysis mixture does not
only depend on the temperature of the cold window but also
(31) Vidovic, D.; Moore, J. A.; Jones, J. N.; Cowley, A. H. J. Am. Chem.
Soc. 2005, 127, 4566.

ReVersible Formation of RBO (R ) C
H
6 5
or CH
3
) from (RBO)
3
Organometallics, Vol. 26, No. 25, 2007 6265
Table 2. Vibrational Frequencies (in cm-1) and Infrared
Intensities of Methyl(oxo)borane 1b from Experiment (Ar
Matrix, 10 K) and Theorya
is observed. Under various decomposition conditions, a set of
three very weak signals (1210, 1271, and a shoulder at 1343
cm ) is reproducibly observed, and these are tentatively
assigned to 7b. The highest yield of 7b is obtained (750 °C;
0.2% mixture of 3b in Xe) at a trapping temperature of 30 K,
which gives 6b as the most prevalent species. Trapping at lower
-1
υexp (cm-1)
ωtheor (cm 1)
-
-
1
(
I (km mol )),
present
literatureb
symmetry species
assignment
(18 K) or higher temperatures (50 K) gives 1b or 3b,
1
1
3
3
186 (0.1), E
092 (0.2), A1
[ B]-1b, ν(CH)
respectively, as the dominating compounds. The tentative
assignment of the weak bands to 7b is supported by further
experiments and by theory. Two-photon absorption during
irradiation with light of a KrF excimer laser (248 nm) allows
thermal energy transfer from the solid xenon matrix to the
1
1
[ B]-1b, ν(CH)
10
2
2
038.8
030.2
2038.5
2029.9
2040 (148.1), A1
combination in
Fermi resonance
[ B]-1b, ν(BO)
10 b
[ B]-1b
1
0
b
with ν( BO)
1
0
11
32
2
1
1
1
021.2
972.8
962.2
957.7
2027 (180.1), A′
1975 (138.9), A1
1966 (306.6), Bu
1962 (136.3), A′
[ B, B]-6b, ν(BO)
entrapped molecules. This technique, successfully employed
1
1
1972.4
1305.0
[ B]-1b, ν(BO)
32-34
by Maier and co-workers for the generation of radicals,
1
1
[ B2]-6b, ν(BO)
1
0
11
provides a means for thermal excitation of molecules, which
themselves do not absorb at 248 nm, a condition fulfilled by
the molecules investigated here. Such treatment (10 Hz pulse
repetition) of the xenon matrix described previously results in
[ B, B]-6b, ν(BO)
1
1
1
479 (10.7), E
[ B]-1b, δ(CH)
11
1
1
9
8
305.2
299.7
1344 (11.0), A1
1336 (37.9), Bu
[ B]-1b
1
1
[ B2]-6b
1
0
04.7, 903.5 904.5, 903.3 929 (45.1), E
97.2, 895.8 896.8, 895.6 922 (42.1), E
[ B]-1b
-
1
-1
-1
an increase of the 1210 cm , 1271 cm , and 1343 cm
1
1
[ B]-1b
1
1
absorptions (see Supporting Information). However, not only
the signals of 6b but also those of 1b and 3b reduce in intensity
under these conditions, although 6b decreases slightly more than
1b. The loss of almost all species, except for that responsible
for the three new signals, is most likely due to laser ablation of
the xenon matrix, whose low optical quality (it is opaque rather
than transparent) is a consequence of the necessary deposition
conditions (FVP; CsI window at 30 K). The tentative assignment
of the signals to 7b is also supported by computational data
8
3
21 (0.0), A1
56 (23.3), E
[ B]-1b
1
1
[ B]-1b
a
-1
MP2/aug-cc-pVTZ, unscaled; intensities in km mol are given in
b
parentheses. Ref 25.
(Table 3).
Finally, as expected, the IR spectra of 1b and 3b are only
slightly shifted in Xe as compared to the data obtained in Ar.
The most pronounced difference is that the splitting of the ν-
1
0
(
BO) signal in 1b, previously ascribed to a Fermi resonance
2
5
with a combination band, is not observed in solid Xe. The
observed vibrational frequencies, 2038.6 ν( BO), 1974.8 ν( -
BO), 1300.1, 901.9/899.5, and 894.2/891.9 cm , show boron
isotopic ratios of 1.03231 (exptl) in good agreement with theory
1
0
11
-
1
(1.03307).
Computational Analysis. A computational investigation of
the cyclotrimerization of 1b at the MP2/aug-cc-pVTZ level of
theory is in agreement with the experimental observations (see
Figure 3). The formation of 3b from 1b is a strongly exothermic
-1
process (∆HR (0 K) ) -99 kcal mol ) involving the complexes
b and 8b as well as the 1,3-dioxa-2,4-diboretane 7b as
intermediates, which are all lower in energy than three separated
Figure 2. IR spectra of the pyrolysate of trimethylboroxin (3b) in
the BO stretching region. (A) 0.05% in Ar; (B) difference
spectrum: bands pointing downward decrease in intensity upon
annealing the matrix in trace A to 38 K; and (C) 0.01% in Ar.
6
molecules of 1b.
The methyl derivative 1b has been the subject of a number
of computational investigations aimed at understanding its
electronic structure as well as at deriving experimentally
on the flow rate through the pyrolysis tube and on the boroxin
concentration in the gas mixture. In addition to the signals due
to 1b, we observe weak signals at 2021.2, 1962.2, 1957.7, and
3
5-39
inaccessible data.
The similarity of the photoelectron
-1
spectrum of 1b with that of the isoelectronic CH3CN molecule
1299.7 cm when using a 0.05% mixture of 3b in Ar for carrier
led Bock et al. to conclude that the BO bond in this oxoborane
gas pyrolysis. The intensities of these signals increase upon
subsequent annealing of the matrix to 35-38 K, while they are
not observed if the concentration of 3b in the pyrolysis gas
mixture is lowered to 0.01% (Figure 2). We hence assign these
absorptions to the dimeric complex 6b of 1b (Scheme 3). More
extended annealing in argon does not give any convincing
evidence for the cyclization of 6b to the 1,3-dioxa-2,4-diboretane
2
4
is a triple bond. This interpretation of the bonding in 1b is in
(32) Maier, G.; Senger, S. Liebigs Ann. 1996, 45.
(
33) Maier, G.; J u¨ rgen, D.; Tross, R.; Reisenauer, H. P.; Hess, B. A.;
Schaad, L. J. Chem. Phys. 1994, 189, 383.
34) Maier, G.; Lautz, C.; Senger, S. Chem.sEur. J. 2000, 6, 1467.
(
(35) Raabe, G.; Schleker, W.; Strassburger, W.; Heyne, E.; Fleischhauer,
J.; Bachler, V. Z. Naturforsch., A: Phys. Sci. 1987, 42, 1027.
7
b.
(
36) (a) Nguyen, M. T.; Groarke, P. J.; Ha, T.-K. Mol. Phys. 1992, 75,
105. (b) Nguyen, M. T.; Vanquickenborne, L. G.; Sana, M.; Leroy, G. J.
Phys. Chem. 1993, 97, 5224.
There is, however, some indication for the formation of 7b
1
during freezing of the pyrolysate mixture onto the cold
spectroscopic window, in particular if xenon is used as the host
gas. As with argon, a strong dependence of the composition of
the matrix on the trapping temperature and boroxin concentration
(
(
(
37) So, S. P. J. Phys. Chem. A 2002, 106, 3181.
38) Sana, M.; Leroy, G.; Wilante, C. Organometallics 1992, 11, 781.
39) Ibrahim, M. R.; B u¨ hl, M.; Knab, R.; Schleyer, P. v. R. J. Comput.
Chem. 1992, 13, 423.

6
266 Organometallics, Vol. 26, No. 25, 2007
Bettinger
Figure 3. Geometries of species involved in the cyclotrimerization of methyl(oxo)borane 1b as computed at the MP2/aug-cc-pVTZ level
-
1
of theory. Energies (in kcal mol ) given for the dimeric (6b, TS1, and 7b) and trimeric (8b, TS2, and 3b) species are relative to two and
three monomers 1b, respectively.
Scheme 3. Dimerization of Methyl(oxo)borane 1b, 1:1
Dimer Complex 6b, and 1,3-Dioxa-2,4-diboretane 7b
Scheme 4. Stepwise Trimerization of Methyl(oxo)borane 1b
To Give Trimethylboroxin 3b
Table 3. Vibrational Frequencies (in cm-1) of
,3-Dioxa-2,4-dimethylboretane (7b) from Experiment (Xe
Matrix, 30 K) and Theorya
formation of 3b from 8b is significantly more exothermic than
the formation of 7b from 6b, and consequently, the barrier (TS2)
for reaction is significantly smaller (4 kcal mol ). In the
transition state TS2, there is a pronounced asymmetry in the
newly forming B-O bonds (1.91 and 2.34 Å), while the BO
bond of the attacking monomer is much less stretched than in
TS1. The fact that the barrier of formation of 7b is higher than
that for its disappearance by reaction with another molecule of
1
-1
-
symmetry
species
ωtheor (cm 1)
νexp (cm-1)
(I (km mol 1))
-
Number
19
17
16
13
9
Bu
Au
Bu
Bu
Bu
1343
1270
1211
1382.9 (550.4)
1302.9 (309.6)
1248.8 (344.5)
927.3 (65.5)
1
b explains why 7b could only be detected in low concentra-
737.6 (35.1)
tions. The rather low barriers and their relative heights are
responsible for the facile de novo generation of 3b at slightly
elevated trapping temperatures and/or 1b concentration.
Comparison with Iminoboranes. The trimerization of ox-
oboranes RBO (Scheme 4) appears to be analogous to that of
iminoboranes RBNR, which is known to involve cyclic dimers
a
MP2/aug-cc-pVTZ, unscaled; intensities in km mol-1 given in paren-
-
1
theses. Only bands with an absolute intensity larger than 10 km mol are
given. Experimental intensities could not be obtained due to overlap with
bands of 1b.
agreement with a computational analysis of HBO, which was
concluded to have “essentially a dative triple character”.
4
0,41
(
RBNR)2.
These (RBNR)2 dimers can be isolated in the presence of
bulky substituents R, in analogy to the known 1,3-dioxa-2,4-
3
6
Our MP2 computations using the large aug-cc-pVTZ basis
set give a dipole moment of 3.60 D for 1b. This number is
somewhat larger than the 3.4 D reported earlier at the MP2/
DZP level of theory.36 The strong association (-6 kcal mol )
of the two molecules of 1b to yield the 1:1 complex 6b is driven
by the annihilation of this large dipole moment. The long
intermolecular B‚‚‚O distances of 3.05 Å in 6b and a minor
change in the BO bond lengths in the individual monomers are
clearly incompatible with dative boron‚‚‚oxygen interactions.
Thus, 6b is best considered a dipole-dipole complex.
The barrier through TS1 for the exothermic formation (∆HR-
0 K) ) -30 kcal mol ) of the 1,3-dioxa-2,4-diboretane (7b)
from this complex is 8.1 kcal mol (Figure 3). In the transition
state TS1, the intermolecular BO distances are much shorter,
.08 Å, while the BO bonds have stretched significantly to 1.25
Å. The relatively high barrier explains why annealing experi-
ments in argon yield 6b, while 7b could not be observed. The
association of 7b with another molecule of 1b to give the adduct
2
6,42,43
diboretanes 7 bearing large groups.
The presence of trimers
resembling Dewar benzene, established in the RBNR series,4
is not observed here. Indeed, the attempted geometry optimiza-
tion of a Dewar boroxole resulted in 3b instead.
4,45
-1
Conclusion
In summary, we have shown that the hitherto unknown
phenyl(oxo)borane 1a, the monomeric anhydride of the syn-
-
1
(
(40) Paetzold, P. AdV. Inorg. Chem. 1987, 31, 123.
-1
(41) Paetzold, P. Phosphorus, Sulfur Silicon Relat. Elem. 1994, 93-94,
9.
3
(42) M a¨ nnig, D.; Narula, C. K.; N o¨ th, H.; Wietelmann, U. Chem. Ber.
2
1985, 118, 3748.
(43) Hanecker, E.; N o¨ th, H.; Wietelmann, U. Chem. Ber. 1986, 119, 1904.
(
44) Paetzold, P.; von Plotho, C.; Schmid, G.; Boese, R. Z. Naturforsch.,
B: Chem. Sci. 1984, 39, 1069.
45) Steuer, H. A.; Meller, A.; Elter, G. J. Organomet. Chem. 1985, 295,
(
-
1
8
b is also exothermic (∆HR(0 K) ) -5 kcal mol ). The
1.

ReVersible Formation of RBO (R ) C
H
6 5
or CH
3
) from (RBO)
3
Organometallics, Vol. 26, No. 25, 2007 6267
thetically important benzeneboronic acid 2a, can be synthesized
by FVP of its boroxin trimer 3a and trapped in an argon matrix
at cryogenic temperatures. This approach to monomeric anhy-
drides of organoboronic acids allows for the study of methyl-
temperature by resistive heating using an Oxford ITC 503 temper-
-1
ature controller. IR spectra were measured between 4000-400 cm
on Bruker IFS 66 and IFS 66/S spectrometers using a resolution
-
1
of 0.5 cm
.
The computational investigations used second-order Møller-
Plesset perturbation theory (MP2) in conjunction with the cc-pVTZ
and aug-cc-pVTZ correlation consistent basis sets.5 Geometries
of stationary points were optimized, and harmonic vibrational
frequencies were computed at the MP2/cc-pVTZ level of theory
for all compounds. In addition, harmonic vibrational frequencies
were also obtained at the MP2/aug-cc-pVTZ level of theory for
(
3
oxo)borane 1b and its facile reaction back to trimethylboroxin
b under the conditions of matrix isolation. We find that 1,3-
0
dioxa-2,4-diboretane 7b is formed only in low concentrations
at elevated trapping temperatures using xenon as a matrix host
gas, indicating that the trimerization of 1b is stepwise and
involves 7b as an intermediate. As the chemistry of boron oxides
RBO has received very limited attention until now, we are
currently studying complex formation and possible reactions
with Lewis bases.
1b. The geometries of species involved in the cyclotrimerization
of 1b (i.e., 3b, 6b, 7b, TS1, and TS2) were further optimized using
the aug-cc-pVTZ basis set, but the corrections for zero-point
vibrational energies obtained at MP2/cc-pVTZ were used for
deriving the relative energies given in Figure 4. The interaction
energy of the binary complex of 1a with CO was corrected for
basis set superposition errors by the counterpoise method a
posteriori.51 All computations employed the frozen-core approxima-
tion and were performed with Gaussian 03.52
Experimental and Computational Procedures
Triphenylboroxin (3a) was obtained by dehydration of ben-
zeneboronic acid (Aldrich, 97%), which was recrystallized twice
from water and washed with petrol ether,46 by heating in vacuo
47
2
or by reaction with SOCl in diethylether as described by Frohn et
48
al. for pentafluorophenylboronic acid. Trimethylboroxin was
purchased from Aldrich (99%) and was used without further
purification. Matrix experiments were carried out according to
standard techniques49 with APD CSW-20 displex closed-cycle
helium cryostats. Pyrolyses were performed with a quartz tube (10
mm i.d., 200 mm length), which was resistively heated by Ta wire
coiled around it. Temperatures were measured with Ni-Cr/Ni
thermocouples (Thermocoax). The boroxin 3a was sublimed out
of a glass flask by heating it between 90 and 120 °C (B u¨ chi GKR-
Acknowledgment. Financial support was received from the
Fonds der Chemischen Industrie, Deutsche Forschungsgemein-
schaft, and Ruhr-Universit a¨ t Bochum. I am very grateful to Prof.
W. Sander for access to the matrix isolation equipment and
helpful discussions.
Supporting Information Available: Cartesian coordinates of
all stationary points. This material is available free of charge via
the Internet at http://pubs.acs.org.
5
1), and all gaseous materials leaving the pyrolysis tube were
condensed onto a cold CsI window with a large excess of argon
Messer Griesheim, 99.9999%), which was dosed to 2.0 sccm by
OM061057I
(
(50) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007.
a mass flow controller (MKS Mass Flow Type 247 four-channel
read out). The more volatile 3b was premixed with argon (1:10 000
to 2:1000) by standard manometric techniques, and these mixtures
were bled into the vacuum system at 2.0 sccm by a mass flow
controller. The experiments with 3b also employed xenon gas (Air
Liquide, 4.0). The CsI window was either kept at the lowest possible
temperature (18-20 K, rather than the usual 10 K, due to radiative
heating from the pyrolysis oven) or was warmed to a defined
(51) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553.
(52) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
revision B.4; Gaussian, Inc.: Pittsburgh, PA, 2003.
(46) Washburn, R. M.; Levens, E.; Albright, C. F.; Billig, F. A. Org.
Synth. 1963, 4, 68.
(
(
47) Morgan, J.; Pinhey, J. T. J. Chem. Soc., Perkin Trans. 1 1990, 715.
48) Frohn, H.-J.; Adonin, N. Y.; Bardin, V. V.; Starichenko, V. F. Z.
Anorg. Allg. Chem. 2002, 628, 2827.
49) Dunkin, I. R. Matrix-Isolation Techniques; Oxford University
Press: New York, 1998.
(