Phenylborylene: Direct spectroscopic characterization in inert gas matrices

Article abstract of DOI:10.1021/ja0548642

The photolysis of bisazidophenylborane isolated in cryogenic matrices results in phenylborylene, a subvalent boron(I) species with a singlet ground state. Broad band irradiation of phenylborylene causes formation of benzoborirene by insertion into an ortho-CH bond. Copyright

Full text of DOI:10.1021/ja0548642

Published on Web 02/04/2006
Phenylborylene: Direct Spectroscopic Characterization in Inert Gas Matrices
Holger F. Bettinger
Lehrstuhl f u¨ r Organische Chemie 2, Ruhr-UniVersit a¨ t Bochum, UniVersit a¨ tsstr. 150, 44780 Bochum, Germany
Received July 20, 2005; E-mail: holger.bettinger@rub.de
1
Borylenes 1, the boron analogues of carbenes (2) and nitrenes
3),2 also termed borenes or boranediyls,3 are highly reactive
Scheme 1
(
intermediates. While the chemistry of 2 and 3 is well developed,
chemical trapping of the subvalent boron(I) species 1 has only been
4
reported occasionally. Direct IR spectroscopic characterizations
of organoborylenes are limited to ethynylborylene HC
2
B (1a, R )
C
2
H), which formed during the co-deposition of boron atoms and
5
acetylene.
Table 1. IR Spectroscopic Data of 7 Measured in N2 at 10 K and
Computed at the UB3LYP/6-311+G** Level of Theory
The electronic structure of 1 is related to that of nitrenes, which
have a nonbonding electron pair and two unpaired electrons
described by two p-orbitals and thus generally prefer triplet
electronic ground states. The two p-orbitals are unoccupied in
borylenes, and consequently, singlet ground states result with a
nonbonding electron pair described by a sp-type orbital. Borylenes,
like carbenes, can be stabilized by coordination to metal centers,
and significant progress has been made in the field of transition
metal-borylene complexes in recent years.7
experiment
B3LYP/6-311+G**
ν
exp
ν
calcd
[cm-1]
[cm-1]
mode
I
rel
I
rel
1
0
3
3
3
26
20
1
1
1
1 [ B]
1715
1679
1560
1314
1024
1009
972
0.24
1.00
0.38
0.13
0.02
0.03
0.04
0.03
1807.1
1765.1
1596.3
1356.3
1041.3
1022.01
987.9
0.24
1.00
0.29
0.12
0.03
0.03
0.11
0.07
6
11
1 [ B]
0
9
7
3
Phenylborylene (1b) has been sought for some time, but has
remained elusive to date.3 Nonetheless, computational investiga-
tion reveals that 1b should be stable under suitable conditions, as
barriers for rearrangements to benzoborirene 4 and didehydrobore-
a,8
760
775.6
-1
pine 5 are in excess of 27 kcal mol (see Supporting Information).
We here show that UV photolysis of diazidophenylborane (6), first
synthesized and characterized by Mennekes and Paetzold in 1995,9
provides an access to the desired 1b as well as a nitreno-N-
phenyliminoborane 7 (Scheme 1). The related chlorine compound,
ClBN
BCl upon irradiation in an argon matrix at 10 K.
The IR spectra of the phenyl compound 6 isolated in solid inert
gas matrices (Ar, Ne, N ) are characterized by an intense group of
6 2
, is known to decompose into a ClBN compound and into
10
2
-1
signals centered at 2153 cm . Irradiation (λ ) 254 nm) of matrices
containing 6 results in very rapid and complete disappearance of
its IR signals.11
From a number of experiments, a set of IR bands (Table 1, Figure
1
), the most prominent at 1715, 1679, and 1560 cm-1, is identified
which shows an identical growth behavior upon photolysis. These
signals are assigned to 7 based on comparison with B3LYP/
Figure 1. IR spectra obtained at 10 K in N2 after irradiation (λ ) 254 nm)
of 6 (bottom). The open circle marks HN3, while the bold squares are signals
due to 1b (see also Figure 2). The computed spectrum was obtained at the
6
-311+G** computations.
Characteristic of 7 is a pair of signals at 1715 and 1679 cm
3
UB3LYP/6-311+G** level of theory for the A2 state of 7.
-
1
with 1:4 intensities corresponding to the natural 10B/ B composi-
tion. Both of these signals are split considerably and show
pronounced fine structure, more so in argon and least in neon,
11
splitting parameters, |D/hc| ) 1.240 cm and |E/hc| ) 0.0021
-1
-
1 13
cm
.
In nitrogen and, to a lesser extend, in neon matrices, another set
of bands (Table 2) shows a growth behavior different from that of
7. These bands are assigned to the borylene 1b based on comparison
with computations and deuterium labeling (see Supporting Informa-
possibly due to the two molecules of N
2
lying within the same
matrix cage. The nitrene 7 prefers a triplet electronic ground state,
3
12
A
2
, according to computations. This is confirmed by electron
spin resonance (ESR) measurements. Irradiation (λ ) 254 nm) of
isolated in argon at 10 K results in an ESR signal with zero-field
-1
tion). The 1225/1215 cm pair has the 1:4 intensity pattern typical
for natural boron isotope composition, and according to the B3LYP
6
2534
9
J. AM. CHEM. SOC. 2006, 128, 2534-2535
10.1021/ja0548642 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
9
Table 2. IR Spectroscopic Data of 1b Measured in N2 at 10 K
and Computed at the B3LYP/6-311+G** Level of Theory
In conclusion, this work demonstrates that Paetzold’s bisazi-
dophenylborane is a precursor for opening the chemistries of
Experiment
B3LYP/6-311
+G**
6 5 2 6 5
hitherto unknown reactive intermediates of C H BN and C H B
stoichiometries. While the rearrangement to an iminoborane deriva-
tive is competing with phenylborylene formation, this system will
ν
exp
ν
calcd
[cm-1]
[cm-1]
I
I
16
mode
rel
rel
nonetheless allow future reactivity studies of 1b.
2
1
1
1
9
8
5
1597
1225
1215
1186
798
0.60
0.24
1.00
0.06
0.07
0.20
1630.6
1250.5
1239.5
1202.8
807.9
0.32
0.24
1.00
0.14
0.08
0.26
1
0
9 [ B]
9 [ B]
8
1
1
Acknowledgment. This work was supported by the DFG and
by the “Fonds der Chemischen Industrie.” I sincerely thank Prof.
W. Sander for access to the matrix isolation equipment, and D.
Grote and K. Gomann for measurement of the ESR spectra.
743
755.2
Supporting Information Available: Technical details, complete
tables of vibrational data, enlarged IR spectra, ESR spectra, and
Cartesian coordinates and energies of stationary points. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
(
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1
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(
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Figure 2. Difference spectra (middle) measured at 10 K after photolysis
350-450 nm) of the matrix depicted in Figure 2. Bands pointing down
disappear, bands pointing upward increase upon irradiation. The open circle
marks HN3. The IR spectra for 4 (top) and 1b (bottom) were computed at
the B3LYP/6-311+G** level of theory.
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(
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trapping of borylenes have since been refuted; see: (g) Meller, A.; Seebold,
U.; Maringgele, W.; Noltemeyer, M.; Sheldrick, G. M. J. Am. Chem. Soc.
Table 3. IR Spectroscopic Data of 4 Measured in N2 at 10 K and
Computed at the B3LYP/6-311+G** Level of Theory
1
989, 111, 8299. (h) ref 3a. (i) Calhoun, G. C.; Schuster, G. B. J. Org.
Chem. 1984, 49, 1925. (j) Schl o¨ gl, R.; Wrackmeyer, B. Polyhedron 1985,
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Experiment
B3LYP/6-311+G**
4
ν
exp
ν
calcd
[cm-1]
(
5) (a) Andrews, L.; Hassanzadeh, P.; Martin, J. M. L.; Taylor, P. R. J. Phys.
Chem. 1993, 97, 5839. (b) Andrews, L.; Lanzisera, D. V.; Hassanzadeh,
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condensation of boron atoms with halogens results in formation of BX
(X ) F, Cl, Br, I). See: (c) Hassanzadeh, P.; Andrews, L. J. Phys. Chem.
1993, 97, 4910.
mode
[cm-1]
I
rel
I
rel
1
0
2
2
1
1
8
6 [ B]
2662
2647
1084
1073
728
0.21
1.00
0.16
0.73
0.53
2743.0
2727.9
1105.0
1094.2
733.8
0.24
1.00
0.10
0.39
0.47
1
1
6 [ B]
1
0
7 [ B]
1
1
(6) The singlet-triplet energy splitting in the BH molecule is 10 410 cm
-1
7 [ B]
1
(
29.8 kcal mol- ): Brazier, C. R. J. Mol. Spectrosc. 1996, 177, 90. For
early computational investigations on the singlet-triplet energy splitting
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Schleyer, P. v. R. Chem. Phys. Lett. 1986, 129, 279. (b) Schleyer, P. v.
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computations, this band corresponds to the ν(C-B) stretching
vibration.The features due to 1b are rapidly bleached by longer
wavelength irradiation (λ ) 350-450 nm) with concomitant growth
(
7) For a very recent review, see: Braunschweig, H. AdV. Organomet. Chem.
2004, 51, 163.
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Force Office of Scientific Research, U.S. Clearinghouse Fed. Sci. Technol.
Inform., 1970.
14
of a further set of signals. Two signals of this set (2647, 1073
-
1
cm ) can already be detected after the initial 254 nm irradiation.
These bands are typical of a ν(B-H) stretching vibration and a
(9) Mennekes, T.; Paetzold, P. Z. Anorg. Allg. Chem. 1995, 621, 1175.
(10) (a) Travers, M. J.; Eldenburg, E. L.; Gilbert, J. V. J. Phys. Chem. A 1999,
ring breathing mode of a benzoborirene.15 Hence, this set is assigned
103, 9661. (b) Travers, M. J.; Gilbert, J. V. J. Phys. Chem. A 2000, 104,
to 4, and this assignment is also supported by comparison with
3
780.
-
1
computations (Table 3).
The ground state of C2V symmetric 1b is a singlet state ( A
(11) An unknown photolabile intermediate with an absorption at 2050 cm
1
is formed initially, but also quantitatively bleached by the time 6 is
1
),
decomposed completely.
but due to the two vacant orbitals on boron, two different triplet
(12) The
H. F. J. Organomet. Chem. 2006, accepted for publication.
1
A and
1
A states are 15 kcal mol-1 higher in energy. See: Bettinger,
2
1
3
3
1 2
states, B and B , need to be very similar in energy. This is
-1
(
13) Upon longer irradiation, another signal (|D/hc| ) 0.870 cm and |E/hc|
confirmed by CCSD(T)/cc-pVTZ computations, which give sin-
glet-triplet energy splittings of 36.1 and 36.9 kcal mol-1, respec-
-1
)
0.0007 cm ) can be detected (see Supporting Information). While its
origin is unclear, we note that the C
6
H
5
BNN isomer also has a triplet
), and B3LYP/6-31G* computations show a larger
3
ground state ( A
2
6 5
tively. According to eq 1, the phenyl substituent (R ) C H )
delocalization of spin density from the terminal N than in 7, consistent
with a smaller D value.
-
1
stabilizes the singlet state (∆E
S
(1) ) -2.7 kcal mol ) and
3
-1
(14) Additional bands form during irradiation, but their origin is not clear
beyond doubt at this time.
1 T
destabilizes the B state (∆E (1) ) +2.6 kcal mol ), resulting in
an increased singlet-triplet gap compared to BH (calcd 30.8, exp
(15) Bettinger, H. F. Chem. Commun. 2005, 2756.
-
1 6a
2
9.8 kcal mol ).
(
16) Azidoboranes are good precursors to iminoboranes. See: Paetzold, P. AdV.
Inorg. Chem. 1987, 31, 123.
BH + RBH f RB + BH3
(1)
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