Synthesis and characterization of B-heterocyclic π-radical and its reactivity as a boryl radical

Article abstract of DOI:10.1021/ja3094372

The first isolation and full characterization of the stable, persistent diazaboracyclic neutral radical 3 is reported. Reduction of base-stabilized difluororoborane 2 provided radical 3 as a neutral molecule having a planar sp² boron atom attached to one fluorine and two nitrogen atoms. ESR spectroscopy and DFT calculations indicated that the unpaired electron is delocalized over the six-membered ring. Because of an electronic transition related to the singly occupied molecular orbital, 3 has a characteristic red color, as UV-vis spectroscopy showed an absorption maximum at 498 nm. Although DFT calculations suggested that radical 3 has relatively low spin density on the boron atom in comparison with the nitrogen and carbon atoms in the six-membered ring, 3 reacted as a base-stabilized boryl radical when treated with benzoquinone or benzoyl peroxide.

Full text of DOI:10.1021/ja3094372

Communication
pubs.acs.org/JACS
Synthesis and Characterization of B‑Heterocyclic π‑Radical and Its
Reactivity as a Boryl Radical
Yoshitaka Aramaki,† Hideki Omiya,† Makoto Yamashita,*,†,§ Koji Nakabayashi,‡ Shin-ichi Ohkoshi,*,‡
and Kyoko Nozaki*,†
†Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
‡Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
corresponding radical anions [R2B−BR2]•− possessing a one-
electron π bond.6 One-electron reduction of geometrically
ABSTRACT: The first isolation and full characterization
of the stable, persistent diazaboracyclic neutral radical 3 is
reported. Reduction of base-stabilized difluororoborane 2
provided radical 3 as a neutral molecule having a planar sp2
boron atom attached to one fluorine and two nitrogen
atoms. ESR spectroscopy and DFT calculations indicated
that the unpaired electron is delocalized over the six-
membered ring. Because of an electronic transition related
to the singly occupied molecular orbital, 3 has a
characteristic red color, as UV−vis spectroscopy showed
an absorption maximum at 498 nm. Although DFT
calculations suggested that radical 3 has relatively low spin
density on the boron atom in comparison with the
nitrogen and carbon atoms in the six-membered ring, 3
reacted as a base-stabilized boryl radical when treated with
benzoquinone or benzoyl peroxide.
strained 1,8-diborylnaphthalene resulted in the formation of a
similar type of radical anion in which the unpaired electron is σ-
delocalized between the two boron atoms.7 Recently, an
anionic borole radical was also studied by X-ray crystallography,
electron spin resonance (ESR) and UV−vis spectroscopy, cyclic
voltammetry (CV), and trapping with dibenzoyl peroxide.8
Examples of base-stabilized boron-centered radical species are
limited to the N-heterocyclic carbene (NHC)- and pyridine
•
derivative-stabilized boryl radicals NHC−BR2 (R = H, Mes)
•
and R′C5H4N−BH2 [R′ = NMe2, N(CH2)4, N(CH2CH2)2O]
9b
te and Curran,9a Gabbai, and Lalevee9c
́
̈
characterized by Laco
̂
and the singlet diradical [t-BuB•−P(i-Pr)2]2 reported by
Bertrand,10 which can be considered as a dimeric form of the
base-stabilized boryl radical t-BuB•−P(i-Pr)2. Thus, the
reactivity of neutral base-stabilized boryl radicals is still waiting
to be further explored.
To obtain a radical structure satisfying the above require-
ments, we became interested in the reduction of β-diiminate
boron difluoride 211 to give a neutral boron-centered radical.
While the reactions of base-stabilized aminodifluoroboranes
with Lewis bases to give six-π-electron borenium cations were
extensively studied,12 reduction of a difluoride to isolate a base-
stabilized boryl radical has never been explored.13 Here we
report the first isolation and full characterization of a stable,
persistent diazaboracyclic neutral radical, 3.14 Although radical
3 has very low spin density on the boron atom in the ground
state, it reacts as a base-stabilized boryl radical when treated
with benzoquinone (BQ) or benzoyl peroxide (BPO).
The precursor, base-stabilized difluoroborane 2, was
synthesized by deprotonation of β-diimine 1 with KH followed
by reaction with BF3·OEt2 (Scheme 1). The C2v-symmetric
ince the isolation of triphenylmethyl radical by Gomberg,1
Sa number of stable radicals have been isolated and
characterized.2 Among the second-row elements, most of the
stable radicals are oxygen-, nitrogen-, or carbon-centered
radicals having seven electrons around the central atom. In
contrast, there has been no report on the direct observation of
neutral boron-centered radicals in the literature.3 The electron
deficiency at the boron center of a neutral boron-centered
radical having five electrons (eq 1) would contribute to its
instability.
1
pattern of the signals in the H NMR spectrum of 2 and the
presence of only one singlet signal in the 19F NMR spectrum
indicated rapid interconversion between distorted structures or
a C2v-symmetric structure with a planar B-heterocycle. The 11B
NMR signal at relatively high field (δB 0.9) indicated the
presence of an sp3-hybridized boron center. Single-crystal X-ray
analysis of 2 revealed a planar structure of the B-heterocycle
with small dihedral angles in the six-membered ring (Figure 1).
One can expect that a neutral boron-centered radical could
be stabilized by the addition of an anionic ligand or a Lewis
base on the boron center to reduce the electron deficiency of
the boron atom (eq 2). In fact, triarylboron radical anions,
BAr3•−, have been well-studied with ESR spectroscopy4 and X-
ray crystallography.5 In related studies, one-electron reduction
of diborane(4) compounds was reported to form the
Received: September 26, 2012
Published: December 3, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
Scheme 1. Synthesis and Reduction of Base-Stabilized
Difluoroborane 2
Figure 2. ORTEP drawing of base-stabilized boryl radical 3. Thermal
ellipsoids are set at 50% probability. Selected bond distances (Å), bond
angles (deg), and dihedral angles (deg): B1−F1, 1.342(4); B1−N1,
1.416(4); B1−N2, 1.415(4); N1−C1, 1.440(3); C1−C2, 1.388(4);
C2−C3, 1.373(4); C3−N2, 1.438(3); F1−B1−N2, 118.5(3); F1−
B1−N1, 118.5(3); N2−B1−N1, 123.0(3); N2−B1−N1−C1, 8.2(4);
B1−N1−C1−C2, −15.5(4); N1−C1−C2−C3, 7.3(4); C1−C2−C3−
N2, 8.8(4); N1−B1−N2−C3, 8.0(4); C2−C3−N2−B1, −16.0(4).
seven π electrons delocalized over the six-membered hetero-
cycle was suggested for the open-shell compound 3. In fact, 3 is
ESR-active, and the EPR spectrum of its toluene solution at 180
K is shown in Figure 3. A simulated spectrum with hyperfine
Figure 1. ORTEP drawing of 2. Thermal ellipsoids are set at 50%
probability. Selected bond distances (Å), bond angles (deg), and
dihedral angles (deg): B1−N1, 1.564(3); B1−N2, 1.560(3); N1−C1,
1.332(2); C1−C2, 1.391(3); C2−C3, 1.396(3); C3−N2, 1.331(2);
F2−B1−F1, 108.73(16); F2−B1−N2, 109.52(15); F1−B1−N2,
109.45(15); F2−B1−N1, 109.17(15); F1−B1−N1, 109.28(15);
N2−B1−N1, 110.66(15); N2−B1−N1−C1, 16.6(2); B1−N1−C1−
C2, −5.9(3); N1−C1−C2−C3, −7.9(3); C1−C2−C3−N2, 8.0(3);
C2−C3−N2−B1, 5.6(3); N1−B1−N2−C3, −16.4(2).
The two B−N bond lengths [1.564(3) and 1.560(3) Å] are in
the range between typical B−N single bond (1.50 Å)15 and a
B−N dative bond (1.66 Å),16 as calculated for the complexes
H2N−BH2←NH3 and H3B·NH3. The B−N, N−C, and C−C
bond lengths are also comparable to those observed in the
previously reported β-diiminate-substituted borane (1.507−
1.550 Å).11b
Reduction of 2 with KC8 at −35 °C gave an NMR-silent red
crystalline solid, 3 (Scheme 1). X-ray crystallographic analysis
revealed that 3 is a neutral molecule having a planar sp2 boron
atom attached to one fluorine and two nitrogen atoms (Figure
2). The relatively short B1−F1 bond length of 1.342(4) Å,
which is shorter than the sum of the covalent radii (1.46 Å), is
close to that in triplet states of fluoroboraethene H2CBF
(1.356 Å),17 indicating an electrostatic interaction between the
boron and fluorine atoms.18 The two B−N bond lengths of
1.416(4) and 1.415(4) Å are similar to those calculated for
borazine (B3N3H6) (1.432 Å),19 indicating the sharing of π
electrons as observed for the other N−B−N systems.20 The
hybridization of the boron center was estimated to be sp2 as the
sum of the angles around the boron center was 360.0°; two
nitrogen atoms were somewhat pyramidalized (357.2 and
357.9°). In the boron-containing six-membered ring, the two
N−C bonds [N1−C1 = 1.440(3) Å and C3−N2 = 1.438(3) Å]
are longer than those in 2 while the C−C bonds [C1−C2 =
1.388(4) Å and C2−C3 = 1.373(4) Å] are similar to those in 2,
suggesting that the bonding between the nitrogen and carbon
atoms was changed by the reduction. On the basis of the NMR-
silent character and these structural features, a structure with
Figure 3. (top) Observed ESR spectrum of 3 in toluene solution at
180 K; (bottom) simulated ESR spectrum obtained using g = 2.002,
AH = 17.4 MHz, AB = 7.2 MHz, AN = 4.1 MHz, and AF = 3.5 MHz.
splittings corresponding to one hydrogen atom at the 4-
position (17.4 MHz, I = 1/2 for 1H), two nitrogen atoms at the
2- and 6-positions (4.1 MHz, I = 1 for 14N), one boron atom
(7.2 MHz, I = 3/2 for 11B), and one fluorine atom (3.5 MHz, I
1
= /2 for 19F) showed good agreement with the observed
spectrum. Similarly, the ESR spectrum of 3 was observed at
room temperature and could be reproduced using nearly
identical hyperfine splitting parameters [Figure S5 in the
Supporting Information (SI)].
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Journal of the American Chemical Society
Communication
To clarify the electronic character of 3, unrestricted density
functional theory (DFT) calculations were performed. The
UB3LYP/6-31G*-optimized structure of 3 reproduced the
experimentally obtained structure. The singly occupied
molecular orbital (SOMO) and highest occupied molecular
orbital (HOMO) for the optimized structure are illustrated in
Figure 4. The SOMO has antibonding character for the π bond
the X-ray crystallography, ESR, and DFT results, the following
three resonance structures are suggested to contribute the
properties of 3 (Figure 6): boron-centered radical 3a, allylic
radical 3b, and the ylide structure of nitrogen radical 3c. The
major contribution comes from structure 3b.
Figure 6. Possible resonance structures of 3.
Since 3 has a bright-red color, in contrast to its precursor 2,
the UV−vis absorption spectrum of 3 was recorded. It contains
two absorption maxima, one at 498 nm (broad, ε = 750 M−1
cm−1) and the other at 316 nm (sharp, ε = 4450 M−1 cm−1)
(Figure S2). Time-dependent DFT calculations indicated two
absorptions at 467 and 315 nm with oscillator strengths of
0.0193 and 0.0562, in good agreement with the experimentally
obtained spectrum. The absorption in the visible region at 467
nm could be mainly assigned to an excitation from the SOMO
to the LUMO+1 (78%) and the LUMO+4 (12%), where both
the LUMO+1 and LUMO+4 possess a significant MO
coefficient on the boron center.
Figure 4. UB3LYP/6-31G*-calculated MOs of radical 3: (upper left)
HOMO; (upper right) SOMO; (bottom left) LUMO+1; (bottom
right) LUMO+4.
over the N1−C1 and N2−C3 bonds, which is consistent with
the N−C bonds in 3 being longer than those in 2 longer
because of the half occupation of the antibonding orbital. Also,
the HOMO has π-bonding character of the B1−N1 and B1−
N2 bonds, in accord with the shortened B−N bonds found in
the crystallographic study. In the HOMO and SOMO of 3, the
C1−C2−C3 unit has allylic radical character, with the HOMO
having bonding character over three carbons and the SOMO
having a small MO coefficient for the central carbon.21
Calculation of the spin density in 3 (Figure 5) showed that it
Radical 3 was trapped the boron center by reactions with
oxygen-containing reagents (Scheme 2). A reaction of 3 with
Scheme 2. Reactions of 3 with (top) BQ and (bottom) BPO
BQ provided 1:2 adduct 4 having two newly formed B−O
bonds, as revealed by X-ray crystallographic analysis. Radical 3
was also trapped with 0.5 equiv of BPO to give 5. The
structures of 4 and 5 were confirmed by single-crystal X-ray
analysis (see the SI). The doublet signals at relatively high field
in the 11B NMR spectra of 4 and 5 (δB 1.4, 1JFB = 50 Hz for 4;
Figure 5. UB3LYP/6-31G*-calculated spin density of 3.
1
δB 1.7, JFB = 60 Hz for 5) indicated an sp3-hybridized boron
atom bound to a fluorine atom. Such reactivity of 3 is similar to
the previously reported B−O bond formation between anionic
borole radical and BPO.8a,23 Thus, the reactivity of 3 with
oxygen-containing reagents may be explained as follows: Steric
repulsion between the oxygen-containing reagent and the bulky
tBu and Dip substituents of 3 inhibits the direct reaction of 3 at
the C1, C3, N1, and N2 positions having an unpaired electron
in the resonance structures 3b and 3c.24 On the other hand,
more space is available around the B1 center. Therefore,
although 3 mostly exists as the allylic radical 3b, it is reactive
toward BQ and BPO as the boryl radical 3a.
is mainly localized on C1 and C3 (0.517), followed by C2
(−0.233), N1 and N2 (0.075), and a relatively small spin
density on B1 (−0.008). This distribution of spin density is
consistent with a small MO coefficient on the boron atom in
SOMO; in other words, allylic radical character is dominant in
3. In fact, the frontier orbitals and the spin density distribution
in radical 3 are close to those of allyl radical,22 with the HOMO
of 3 corresponding to the HOMO of allyl radical and the
SOMO of 3 to the SOMO of allyl radical. It is noteworthy that
the calculated LUMO+1 and LUMO+4 have a noticeable
orbital coefficient on the central boron atom (see below). From
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Journal of the American Chemical Society
Communication
P. Organometallics 2009, 28, 4252. (c) Lalevee
́
, J.; Blanchard, N.;
In conclusion, we synthesized and isolated B-heterocyclic
radical 3 with seven π electrons. Its ESR spectrum showed
delocalization of the unpaired electron over the six-membered
ring. While DFT calculations suggested that allylic radical
character (resonance structure 3b) is dominant in 3, radical 3
behaves as a boryl radical (resonance structure 3a) in its
reactions with BQ and BPO to form B−O bonds.
Tehfe, M.-A.; Chany, A.-C.; Fouassier, J.-P. Chem.Eur. J. 2010, 16,
12920.
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(14) We were told by private communication that the Cui group
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reduction of a β-diiminate-substituted difluoroborane.
(15) Leroy, G.; Sana, M.; Wilante, C. Theor. Chim. Acta 1993, 85,
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(16) Thorne, L. R.; Suenram, R. D.; Lovas, F. J. J. Chem. Phys. 1983,
78, 167.
(17) Deakyne, C. A.; Thomas, H. M.; Liebman, J. F. J. Fluorine Chem.
2009, 130, 836.
(18) (a) Robinson, E. A.; Heard, G. L.; Gillespie, R. J. J. Mol. Struct.
1999, 485−486, 305. (b) Robinson, E. A.; Johnson, S. A.; Tang, T.-H.;
Gillespie, R. J. Inorg. Chem. 1997, 36, 3022. (c) Brinck, T.; Murray, J.
S.; Politzer, P. Inorg. Chem. 1993, 32, 2622.
(19) Shen, W.; Li, M.; Li, Y.; Wang, S. Inorg. Chim. Acta 2007, 360,
619.
(20) Weber, L. Coord. Chem. Rev. 2008, 252, 1.
(21) Oliva, J. M.; Gerratt, J.; Cooper, D. L.; Karadakov, P. B.;
Raimondi, M. J. Chem. Phys. 1997, 106, 3663.
(22) Aquilante, F.; Jensen, K. P.; Roos, B. O. Chem. Phys. Lett. 2003,
380, 689.
(23) For the reaction of 3 with BQ, single electron transfer (SET)
from radical 3 to BQ to generate a borenium cation9b and quinone
radical anion might be proposed. However, this seems less likely
because 3 also reacts with BPO. In addition, no oxidation wave but
only irreversible reduction waves were detected for radical 3 by CV
(Figure S8).
(24) Although one may consider the possibile photoexcitation of
radical 3b to give a boron-centered radical, the reaction of 3 with BQ
proceeded under light-shielded conditions.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization of radical 3, and
crystallographic data (CIF) for 2, 3, 4, and 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
makoto@oec.chem.chuo-u.ac.jp; ohkoshi@chem.s.u-tokyo.ac.
jp; nozaki@chembio.t.u-tokyo.ac.jp
■
Present Address
§Department of Applied Chemistry, Faculty of Science and
Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku,
Tokyo 112-8551, Japan.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to Shuhei Kusumoto and Hiromi Oyama (U
Tokyo) for their technical help. This work was supported by
the Funding Program for Next Generation World-Leading
Researchers, Green Innovation, the Global COE Program
“Chemistry Innovation through Cooperation of Science and
Engineering” from the Japan Society for the Promotion of
Science (JSPS), and the Grant-in-Aid for Scientific Research on
Innovative Areas “Stimuli-Responsive Chemical Species”
(24109012) from MEXT, Japan. Y.A. is grateful to JSPS for a
Research Fellowship for Young Scientists.
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