Synthesis and reactions of π-conjugated iminoboranes stabilized by intramolecular imine groups

Article abstract of DOI:10.1021/om4008407

Reduction of the boron difluorides HC[(CBu^t)(NAr)] ₂BF₂ led to C-N bond cleavage to give the iminoborane [C(Bu^t)CHC(Bu^t)NAr]B=NAr (2, Ar = 2,6-Me₂C ₆H₃; 2′, Ar = 2,6-P

Full text of DOI:10.1021/om4008407

Communication
pubs.acs.org/Organometallics
Synthesis and Reactions of π‑Conjugated Iminoboranes Stabilized by
Intramolecular Imine Groups
Liang Xie, Jianying Zhang, Hongfan Hu, and Chunming Cui*
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Collaborative Innovation Center of Chemical Science and
Technology (Tianjin), Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: Reduction of the boron difluorides HC[(CBut)-
(NAr)]2BF2 led to C−N bond cleavage to give the iminoborane
[C(But)CHC(But)NAr]BNAr (2, Ar = 2,6-Me2C6H3; 2′, Ar =
2,6-Pri2C6H3) stabilized by an intramolecular imine group.
Reaction of 2 with NHC resulted in nucleophilic attack on the
α-carbon atom to give [C(But)(NHC)CHC(But)NAr]BNAr
(3). 2 reacted with CO2 to give [C(But)CHC(But)NAr]B(CO2)-
NAr (4).
ecent research has revealed that conjugated B−N
= 2,6-Pri2C6H3; L″, Ar = o-MeC6H4) with β-tert-butyl groups at
the ligand backbone have been prepared with the expectation of
the stabilization of the elusive borylenes. It is noted that the six-
membered ring boryl radical [L′BF]•, prepared by the partial
reduction of L′BF2, has been recently reported by Nozaki and
co-workers.6
R
heterocycles possess unique chemical and physical features
due to the nitrogen lone-pair interaction with the boron empty
p orbital.1 Azaboroles (Chart1, A) are an attractive class of π-
Chart 1
The three boron difluorides LBF2 (1), L′BF2 (1′), and L″BF2
(1″) were similarly prepared by the reaction of HC[(CBut)-
(NAr)]2Li with BF3·Et2O in diethyl ether. Reduction of 1 and
1′ (Scheme 1) with 2.5 equiv of Na/naphthalene in THF at low
Scheme 1. Reduction of the Three Difluorides LBF2, L′BF2,
and L″BF2
conjugated boracycles that have been employed as luminescent
materials and for the generation of azaborolyl anions (B) as the
analogues of ubiquitous cyclopentadienyl ligands.2 Although a
number of azaboroles and azaborolyl anions have been
reported, there are no examples of an azaborole framework
featuring an exocyclic BE (E = C, N, P) double bond (C)
that have been prepared. Herein, we report on the synthesis of
a type C boracycle featuring an exocyclic BN double bond.
The synthesis of stable borylenes is one of the focuses in
main-group chemistry.3 Very recently, the parent borylene
(BH) stabilized by two carbene ligands was reported by the
Bertrand group,3a while N-heterocyclic anionic boryls, iso-
electronic with NHCs, have been studied by Nozaki group.3b,c
Our approaches to this end have focused on the hitherto
unknown neutral N-heterocyclic borylenes supported by β-
diketiminato ligands. It has been shown that this family of
ligands is particularly effective for the stabilization of heavy
group 13 carbene analogues.4 We recently reported the
synthesis of oxoboranes and their heavy analogues stabilized
by bulky β-diketiminato ligands featuring β-methyl groups.5a,b
To avoid the C−H bond cleavage of the β-methyl group on the
ligand backbone as observed in these studies,5a,b three ligands
of the type HC[(CBut)(NAr)]2 (L, Ar = 2,6-Me2C6H3;5c L′, Ar
temperature yielded a blue solution, from which compounds 2
and 2′ were isolated as blue crystals in ca. 42 and 11% yields,
respectively.6 In contrast, reduction of 1″ under the same
conditions led to a complicated mixture, which cannot be
separated and identified definitely. 2 and 2′ have been
characterized by multiple NMR spectra, IR spectra, and
elemental analysis. The 11B NMR spectra of 2 and 2′ exhibit
a broad resonance at δ 23.5 and 23.7 ppm, respectively, which
are comparable to those reported for N-silyl-substituted
iminoboranes but are shifted significantly to low field relative
Received: August 20, 2013
Published: November 11, 2013
© 2013 American Chemical Society
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to those for N-aryl-substituted iminoboranes (δ 10.6−12.2
ppm).7,8 It has been shown that donor-supported iminoboranes
are not easily accessible, since linear iminoboranes are resistant
to nitrogen-based Lewis bases.8 The formation of 2 and 2′ may
be attributed to the unique electronic structure of the
conjugated BC3N framework. They apparently resulted from
the C−N bond cleavage during the reduction process. This
type of C−N bond cleavage arising from alkali-metal reduction
has been recently reported by several groups.9 Furthermore,
C−N bond cleavage has also been observed in the reaction of
the aluminum(I) species HC[(CBut)(NAr)]2Al with an
isocyanide and NHCs.9f,g
resulting five-membered ring radical anion have been optimized
at the 6-31+G(d) level and subsequently verified by frequency
analysis. The cleavage of the C−N bond to form the five-
membered radical anion via TS2 is predicted to be much more
favorable (69.90 kcal/mol; see Figure 2 for the optimized
The molecular formula of 2 is identical with that of borylene
LB, which is shown in Scheme 2. Thus, 2 is very likely formed
Figure 2. Optimized structures for the C−N bond cleavage of the
singly reduced borylene [LB]•− to form the five-membered ring radical
anion via the transition state TS2 at the UB3LYP/6-31+G(d) level.
Scheme 2. Predicted Transition States for the C−N
a
Cleavage
structures and relative potential energies) with a very small
activation barrier of 4.89 kcal/mol. We reasoned that the
resulting radical anion with a five-membered ring may
participate in the reduction process, such as single-electron
reduction of the LB intermediate, to yield 2. Unfortunately, we
were unable to isolate and detect the radical anion due to the
complicated reduction process that involves some very reactive
intermediates. We have previously observed that the univalent
aluminum species HC[(CBut)(NAr)]2Al underwent a similar
C−N bond cleavage in the presence of NHCs and
isocyanides.9f,g It is apparent that the increased electron density
on the central atom facilitates its nucleophilic attack at the β-
carbon atom of the ligand, leading to the C−N bond cleavage.
To investigate whether the radical species isolated by Nozaki
group6 could be an intermediate toward the borylene LB,
reduction of 1 with 1 equiv of Na/naphthalene was performed
at −78 °C. However, this still resulted in the formation of 2
along with unreacted 1, which was due to the homogeneous
nature of Na/naphthalene in THF. Moreover, further reduction
of the radical featuring 2,6-dimethylphenyl groups similarly
prepared according to Nozaki’s method6 was studied, with an
equivalent amount of Na/naphthalene, but no blue product was
obtained. In light of the above results, the reported radical
might not be generated in the pathway of the formation of the
borylene intermediate.
a
See Figures 1 and 2 for the optimized structures of these
intermediates and transition states and relative potential energies
corrected by zero-point energies.
through isomerization of the borylene intermediate generated
by reduction of 1 with 2 equiv of Na/naphthalene. However,
attempts to isolate and trap these intermediates were
unsuccessful. In order to have an insight into this trans-
formation, DFT calculations have been performed. Calculations
on the borylene LB at the UB3LYP/6-31G(d) level disclosed
the singlet ground state and the very small singlet and triplet
energy gap of 3.09 kcal/mol (see the optimized structures in
Figure 1). The cleavage of the C−N bond of the singlet state to
The structure of 2 has been confirmed by an X-ray single-
crystal analysis and is shown in Figure 3. 2 features a planar
BC3N core with an exocyclic B1−N2 double-bond length of
1.3396(19) Å, which is much shorter than a typical B−N bond
in the three-coordinate aminoboranes (ca. 1.40 Å), in which the
B−N bonds have been considered to possess π character to
some extent. The short B−N bond indicates a strong π-
bonding character. However, it is longer than those in the linear
iminoboranes (1.22−1.25 Å).7,8,10 The geometry of the three-
coordinate boron atom is essentially planar (the sum of the
angles is 360.0°) with B1−N1 and B1−C1 single-bond lengths
of 1.5690(19) and 1.596(2) Å, respectively. The relatively long
C2−C3 distance of 1.465(2) Å and short C1−C2 and N1−C3
distances of 1.3478(19) and 1.3109(17) Å indicate a small
degree of electron delocalization in the ring system. Both the X-
ray and NMR data indicate the existence of only one isomer (E
isomer) in the solid state and in solution.
Figure 1. Optimized structures for the C−N bond cleavage of the
borylene LB to form 2 via the transition state TS1 at the UB3LYP/6-
31G(d) level.
form 2 is predicted to be exothermic (52.83 kcal/mol) at the
UB3LYP/6-31G(d) level. The transition state (TS1 in Scheme
2) for the transformation was optimized and further verified by
frequency calculations. The activation barrier for the C−N
cleavage via this transition state is predicted to be very high
(36.42 kcal/mol; see Figure 1), indicating that this mechanism
may not be operative. It is reasonable to assume that LB could
be further reduced to form the radical anion [LB]•− as long as
Na/naphthalene is not completely consumed (Scheme 2).
Thus, the geometries of the radical anion, TS2, and the
The molecular structure of 2 predicted at the B3LYP/6-
31G(d) level is in good agreement with that determined by X-
ray analysis (Figure S2 in the Supporting Information). Natural
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Scheme 3. Reactions of 2 with NHC and CO2
analysis, and X-ray single-crystal analysis. The 11B NMR
spectrum of 3 displays a singlet at δ 24.1 ppm, which is shifted
only slightly to low field in comparison to that for 2, indicating
their similar coordination environments. The 13C NMR
spectrum exhibits 17 signals at low field and 12 at high field,
respectively, which is mainly attributed to the restricted
rotation of the two Ar groups.
Figure 3. Ortep drawing of 2 with 30% probability ellipsoids.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg): B1−N2 = 1.3396(19), B1−N1 = 1.5690(19),
B1−C1 = 1.596(2), N1−C3 = 1.3109(17), C1−C2 = 1.3478(19),
C2−C3 = 1.465(2); N1−B1−N2 = 117.65(12), N2−B1−C1 =
141.23(13), N1−B1−C1 = 101.14(11).
The molecular structure of 3 (Figure 4) reveals the three-
coordinate, planar boron atom (the sum of the angles is
bond order (NBO) analysis of 2 indicates that the B−N π bond
is formed by the boron p orbital with the nitrogen p orbital and
is strongly polarized toward the nitrogen atom (70.86%). In
addition, NBO analysis, according to second-order perturbation
theory, discloses some donor−acceptor interactions (stabiliza-
tion energies 4.54, 4.73, 6.38, 9.67, 10.58, 30.23 kcal/mol)
among the B−Nexo, C−C, and C−N π and π* orbitals (see
Table S2 in the Supporting Information), indicating some
degree of the π-conjugated system. The LUMO of 2 is mostly
contributed by the π* orbitals of the central N2−B1−C1−C2−
C3−N1 framework.
In order to elucidate why 2 only exists as the E isomer, DFT
calculations on the Z isomer (N1 atom and Ar ring on the N2
atom at the same side of the BN bond) at the B3LYP/6-
31G(d) level have been performed. The optimization led to the
isomerization to the E isomer, indicating that the Z isomer is
not stable. DFT calculations on the parent molecule
[(CH)3NH]BNH (the Ar and But groups in 2 are replaced
by hydrogen atoms) predicted that the E and Z isomers are
both energy minima, with the E isomer being only 2.45 kcal/
mol more stable than the Z isomer. Furthermore, calculations
on the model compound of 2 (two Ar groups in 2 are replaced
by two C6H5 groups) revealed that both isomers are stable and
the E isomer is more stable than the Z isomer by only 0.18
kcal/mol. Therefore, it can be concluded that the steric effects
of Ar groups lead to there being only one stable isomer of 2.
The UV−vis absorption spectrum of 2 displays a weak
absorption maximum at 668 nm (broad, ε = 147 mol−1 cm−1
dm3) in the visible region (Figure S4 in the Supporting
Information), which is comparable to those of reported blue
compounds.11 As the observed maximum at 668 nm indicates
absorption of red light, compound 2 exhibits the comple-
mentary blue color.
Figure 4. Ortep drawing of 3 with 30% probability ellipsoids.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg): B1−N2 = 1.353(2), B1−N1 = 1.504(2), B1−C1
= 1.683(2), C1−C2 = 1.527(2), C2−C3 = 1.338(2), N1−C3 =
1.4124(18), C1−C28 = 1.531(2); N1−B1−N2 = 129.38(13), C1−
B1−N1 = 104.31(12), C1−B1−N2 = 126.04(14).
359.7°). The B1−N2 bond length (1.353(2) Å) is only
marginally longer than that in 2, and the B1−C1 (1.683(2) Å)
and B1−N1 (1.504(2) Å) bond lengths are noticeably
lengthened and shortened, respectively, relative to those in 2.
The C2−C3 bond length of 1.338(2) Å indicates a CC
double bond, while the C1−C2 and N1−C3 bond lengths are
in the range for a single bond. Thus, 3 can be viewed as a
donor-stabilized amino(iminoborane), in which the C1 atom in
the ylidic C1−C28 alkene fragment serves as a donor rather
than the nitrogen atom as observed in 2.12
Treatment of 2 with CO2 at room temperature resulted in
intermolecular [2 + 2] cycloaddition, affording 4 as bright
yellow crystals in 72% yield (Scheme 3), which unexpectedly
regenerated 2 quantitatively upon heating to 180 °C under
1
vacuum (see Figure S1 in the Supporting Information for H
Interestingly, reaction of 2 with 1 equiv of 1,3,4,5-
tetramethylimidazol-2-ylidene at room temperature led to the
disappearance of the blue color of 2 and yielded compound 3 as
colorless crystals (Scheme 3) in good yield. In this case, the
NHC attacks the unsaturated carbon atom bonded to the
boron atom rather than the boron atom itself. 3 has been fully
characterized by standard spectroscopic methods, elemental
NMR information). It is noted that this kind of decomposition
behavior was distinct from that concerning the cycloaddition
product of CO2 with the iminoborane TMPBNBut (TMP =
2,2,6,6-tertramethylpiperidine), which led to the formation of
(TMPBO)2 and ButNCO.13 Compound 4 has been charac-
1
terized by H, 11B, and 13C NMR, elemental analysis, and EI
mass spectroscopy. The 11B resonance observed at δ 6.8 ppm
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corresponds to a four-coordinate boron atom, revealing no
multiple bonds connected to it. The molecular structure of 4
has been determined by X-ray single-crystal analysis, which is
shown in Figure 5. The newly formed B1−O1−C4−N2 four-
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving synthetic procedures
and characterization data for compounds 1−4, crystallographic
data for compounds 2−4, and DFT calculations on 2. This
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S
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.C.: cmcui@nankai.edu.cn.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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(12) Furstner, A.; Alcarazo, M.; Goddard, R.; Lehmann, C. W. Angew.
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China and the 973 program (Grant No. 2012CB821600) for
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