Formation of N-heterocyclic, donor-stabilized borenium ions

Article abstract of DOI:10.1039/c1cc11789k

Cationic and zwitterionic boryl bromide species and a borenium-boryl bromide cation have been synthesised which represent new N-donor stabilised cationic boron compounds with β-diketiminate ligands. The unexpected borenium-boryl bromide results from a hea

Full text of DOI:10.1039/c1cc11789k

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COMMUNICATION
Formation of N-heterocyclic, donor-stabilized borenium ionsw
¨
Chika I. Someya, Shigeyoshi Inoue, Carsten Prasang, Elisabeth Irran and Matthias Driess*
Received 29th March 2011, Accepted 10th April 2011
DOI: 10.1039/c1cc11789k
Cationic and zwitterionic boryl bromide species and a borenium–
boryl bromide cation have been synthesised which represent new
N-donor stabilised cationic boron compounds with b-diketiminate
ligands. The unexpected borenium–boryl bromide results from a
head-to-tail dimerisation of the corresponding zwitterionic boryl
bromide accompanied by proton migration. The electronic
nature of these new species was studied by DFT calculations.
aromatic 6p-electron stabilised silylidenium ions through addition
of electrophiles at the terminal anionic methylene moiety of I.11a
The peculiar reactivity of I and II prompted us to synthesize a
boron analogue L0BX, which is expected to show a unique
reactivity. Herein we wish to report the successful synthesis and
structural characterisation of the N-heterocyclic boryl bromide
L0BBr
2 via the N-donor stabilised borenium ion 1
[LBBr]+[BBr4]ꢀ, and of the related species [LBBr]+[OSO2CF3]ꢀ
3 and the head-to-tail dimer [LB-L0BBr] + Brꢀ 4. Because
compound 2 exhibits a zwitterionic nature akin to I, it is
susceptible to protonation and dimerization, affording the
6p-electron, N-donor stabilised borenium ion in 3 and the
unexpected ylidic boron cation in 4, respectively.
Boryl cations (borenium ions) are very reactive and highly
electrophilic species that play a key role in boron chemistry.1
Since the first crystallographic characterization of diammoniate
of diborane by Parry and co-workers,2 the structural chemistry
of boryl cations has attracted much interest over many years
because of their related electronic features of analogous group
14 cations in terms of orbital occupancy, electron count, and
net charge.3 On the other hand, b-diketiminato ligands L are
valuable ligands for the donor–acceptor stabilisation of
various chemical elements in different coordination modes
and oxidation states.4 In particular, stable N-heterocyclic
carbene analogues, i.e. silicon(II),5 germanium(II),6 aluminium(I),7
gallium(I),8 indium(I)9 and thallium(I)10 species were stabilized
by using the mono-anionic, bidentate b-diketiminato ligand L
(Scheme 1). Furthermore, cationic boron species have also
been synthesized using a b-diketiminato ligand or BODIPY
The reaction of LLi with two molar equiv. of tribromoborane,
BBr3, in toluene at ꢀ78 1C yielded the N-donor supported
borenium salt [LBBr]+[BBr4]ꢀ 1 (Scheme 2). Parallel to our
efforts, very recently, Cui and co-workers have reported the
synthesis of L0BBr 2 using an alternative protocol, however,
they were not able to determine the molecular structure by
X-ray crystallography.12 The boryl bromide 2 can be prepared
by reductive deprotonation of borenium ion 1 with lithium
naphthalenide (Scheme 2).
Yellow crystals of 2 were obtained from a concentrated
pentane solution. As shown in Fig. 1, the X-ray diffraction
analysis revealed that the six-membered C3N2B ring is
essentially planar. The B–Br [1.932(8) A] and B–N [1.412(9) A
and 1.417(9) A] bond lengths of 2 are within reported ranges of
the related heteronuclear bonds.13 The endo- and exocyclic
C–C distances [C1–C2 1.401(9) A, C2–C3 1.417(9) A, C3–C4
1.379(9) A and C4–C5 1.444(9) A] revealed a 1,3-butadiene-like
derivatives (BODIPY
=
dipyrromethene boron).3a,f,g,m
Recently, we have reported the intriguing reactivities of
zwitterionic N-heterocyclic silylene I5 and germylene II6
with a modified, dianionic b-diketiminato L0 (Scheme 1).11
Accordingly, we have reported the synthesis and structure of
Scheme 1 Ligand L and L0, zwitterionic silylene I and germylene II.
Technische Universitat Berlin, Institute of Chemistry:
¨
Metalorganics and Inorganic Materials, Sekr. C2,
Strasse des 17. Juni 135, D-10623 Berlin, Germany.
E-mail: matthias.driess@tu-berlin.de; Fax: + 49 (0)30-314-29732;
Tel: + 49 (0)30-314-29731
w Electronic supplementary information (ESI) available: Experimental
details of syntheses and X-ray structure determination. See DOI:
10.1039/c1cc11789k
Scheme 2 Synthesis of 1 and 2. LiNp = LiC10H8.
Chem. Commun., 2011, 47, 6599–6601 6599
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Fig. 1 Molecular structure of 2. Thermal ellipsoids are drawn at 30%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths [A] and angles [1]: Br1–B1 1.932(8), B1–N1 1.412(9),
B1–N2 1.417(9), C1–C2 1.401(9), C2–C3 1.427(9), C3–C4 1.379(9),
C4–C5 1.444(9), C2–N1 1.411(8), C4–N2 1.413(8), N2–B1–N1
121.8(6), N2–B1–Br1 119.2(5), N1–B1–Br1 119.0(5).
Fig. 2 Molecular structure of 4. Thermal ellipsoids are drawn at 30%
probability level. Hydrogen atoms and the Br atom as a counter anion
are omitted for clarity. Selected bond lengths [A] and angles [1]:
B1–Br1 1.945(5), B1–N1 1.399(6), B1–N2 1.421(6), C1–C2 1.495(6),
C2–C3 1.348(6), C3–C4 1.430(6), C4–C5 1.349(5), B2–C5 1.538(6),
B2–N3 1.450(6), B2–N4 1.467(6), C33–C34 1.505(6), C32–C33
1.359(6), C31–C32 1.357(6), C30–C31 1.509(6), N1–B1–N2 121.5(4),
N1–B1–Br1 118.8(4), N2–B1–Br1 119.7(4), N3–B2–N4 113.7(4),
N3–B2–C5 123.5(4), N4–B2–C5 122.6(4).
structure, which is consistent with those of silylene I and
germylene II.5,6
Because of the iso(valence)electronic structure of compound
2 with silylene I and germylene II, we have investigated its
reactivity towards electrophiles. Indeed, reaction of boryl
bromide 2 with trifluoromethanesulfonic acid in dichloro-
methane at ambient temperature furnishes the corresponding
N-donor stabilised borenium salt [L0BBr]+[OSO2CF3]ꢀ
3
borenium subunit in 3. This is contrary to the situation of
compound 2 [NICS(1) = +1.3, NICS(0) = +2.4].
as a separated ion pair (Scheme 3). The composition and
constitution of 3 was proven by HRMS and NMR spectro-
scopy (1H, 13C, and 11B). The 1H, 13C, and 11B NMR chemical
shifts for the borenium moiety in 1 and 3 are practically almost
Surprisingly, boryl bromide 2 underwent a slow but clean
dimerization at 110 1C to give compound 4 in 89% yield
(Scheme 2). The structure of 4 was established by NMR
spectroscopy and a single crystal X-ray diffraction analysis
as shown in Fig. 2. The structure of 4 consists of two
six-membered BN2C3 rings, featuring nearly planar geometry.
Both BN2C3 rings are arranged almost perpendicular to each
other. The B–Br bond length in 4 [1.945(5) A] is similar to that
in 2 [1.932(8) A]. The alternating lengths of the endo- and
exocyclic C–C bonds of 4 [C1–C2 1.495(6), C2–C3 1.348(6),
C3–C4 1.430(6), C4–C5 1.349(5)] suggest little p-conjugation
as observed for compound 2. The B1–N1 and B1–N2 distances
[B1–N1 1.399(6) and B1–N2 1.421(6)] are shorter than those of
the B2–N3 and B2–N4 bonds [B2–N3 1.450(6) and B2–N4
1.467(6)], indicating a stronger p interaction of the nitrogen
lone pair with the cationic boron centre (B2).
ꢀ
identical and independent of the counter anions (BBr4 and
OSO2CF3ꢀ), indicating that 1 and 3 contain least coordinating
1
ions even in solution. The H NMR chemical shift of the g-H
proton of the C3N2B ring in 3 (d = 7.95 ppm) suggests the
presence of aromatic 6p-electron stabilisation. Furthermore,
the 11B NMR of boron cation 3 exhibits a singlet resonance
(d = 29.8 ppm) that is slightly downfield from that of the
precursor 2 (d = 27.2 ppm). Indeed, the calculated NICS
values for compound 3 [NICS(1) = ꢀ4.3, NICS(0) = ꢀ2.5]
revealed negative values, indicating aromatic character of the
Although the reaction mechanism is unclear, a possible
stepwise path for the dimerisation of 2 to 4 can be envisaged
(Scheme 4). We assume that the betaine-like resonance
structure of compound 2 promotes this reaction. The initial
Scheme 3 Synthesis of 3 and 4.
Scheme 4 Proposed mechanism for the formation of 4.
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6600 Chem. Commun., 2011, 47, 6599–6601
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step is a nucleophilic attack at the boron atom to give
intermediate 5a. The next step is the migration of a proton
to the methylene carbon atom of the backbone to give inter-
mediate 5b. The tetracoordinate boron centre of intermediate
5b is unfavourable and affords the ion separated isomer 4 with
a tricoordinate cationic boron centre and p donor stabilisation
by the adjacent nitrogen lone pairs.
Notes and references
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In the 11B NMR of 4, two broad signals are observed at
d 27.3 and 29.6 ppm, respectively. These chemical shifts
are in good agreement with the values of boryl bromide 2
(27.2 ppm) and borenium-like cation 3 (29.6 ppm), respectively.
The 1H NMR spectrum of 4 reveals two characteristic singlets
at 4.84 and 9.33 ppm, corresponding to the g-H atoms of the
BN2C3 rings (4.84 ppm for the boryl bromide moiety and
9.33 ppm for the cationic boron moiety), respectively. This
significant downfield shifted signal of the g-H on C32 (cationic
moiety) of compound 4 as compared to that on C3 atom
(boryl bromide moiety) can be explained by the aromatic
character of the cationic BN2C3 ring. Indeed, these trends
are also supported by NICS values [boryl bromide moiety:
NICS(1) = +0.2, NICS(0) = +1.4; boryl cation unit:
NICS(1) = ꢀ4.3, NICS(0) = ꢀ2.7 ppm]. Apparently, the
boryl bromide unit BN2C3 bears a negative net charge, while
the cationic BN2C3 ring has a positive one [NPA charges in
borylbromide unit: B1 +0.82, N (mean) ꢀ0.69, C (mean) 0.05
vs. boron cation unit: B2 +1.04, N ꢀ0.60, C (mean) +0.12].
Furthermore, the HOMO’s show the presence of p-orbital
interaction within the butadiene moiety of the BN2C3 ring in
the boryl bromide moiety (Fig. 3). On the other hand, the
LUMO is located on the BN2C3 ring of the cationic unit.
These computed results indicate that compound 4 consists of a
borenium-like subunit (electron acceptor) and a boryl bromide
subunit (electron donor).
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reduction with lithium naphthalenide furnishes the boryl
bromide 2. Reactivity studies of 2 provide evidence for its
zwitterionic character, resulting in the formation of the
cationic borenium-like salts 3 and 4. The compounds 1, 3
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Fig. 3 Frontier orbitals of compound 4. (a) KS-HOMO ꢀ0.181 eV,
right and (b) KS-LUMO ꢀ0.271 eV, left. Hydrogen atoms are omitted
for clarity.
13 As determined from a survey of the Cambridge Crystallographic
Database, March, 2009.
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