Mono- and Di-Mesoionic Carbene-Boranes: Synthesis, Structures and Utility as Reducing Agents

Article abstract of DOI:10.1002/ejic.202100273

Mesoionic carbenes (MIC) of the 1,2,3-triazol-5-ylidene type are currently popular ligands in organometallic chemistry. Their use in main group chemistry has been rather limited. In this contribution we present mono- and di-MIC-boranes with MICs based on triazolylidenes. The synthesis involves in-situ deprotonation of the corresponding triazolium salts and their reaction with boranes to form the desired compounds. Whereas this reaction route worked well for all triazolium salts derived from the 1,4-regioisomer of the triazoles, for the methlyene-bridged bi-triazolium salt derived from a 1,5-substiuted triazole, we observed the unexpected decomposition of the bi-triazolium and the formation of a triazole-borane with a new N?B bond. All compounds were characterized via multinuclear NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction. Furthermore, the MIC-boranes were used as reducing agents for the reduction of the C=O of aldehydes to the corresponding alcohols.

Full text of DOI:10.1002/ejic.202100273

European Journal of Inorganic Chemistry
Accepted Article
Title: Mono- and Di-Mesoionic Carbene-Boranes: Synthesis, Structures
and Utility as Reducing Agents
Authors: Felix Stein, Marius Kirsch, Julia Beerhues, Uta Albold, and
Biprajit Sarkar
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100273
Link to VoR: https://doi.org/10.1002/ejic.202100273
01/2020

10.1002/ejic.202100273
European Journal of Inorganic Chemistry
Mono- and Di-Mesoionic Carbene-Boranes: Synthesis, Structures and Utility as Reducing Agents
Felix Stein[a], Marius Kirsch[a], Julia Beerhues[a,b], Uta Albold[a] Biprajit Sarkar[a,b],*
[a] F. Stein, M. Kirsch, J. Beerhues, Dr. U. Albold, Prof. Dr. B. Sarkar
Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34-36, 14195, Berlin,
Germany
[b] J. Beerhues, Prof. Dr. B. Sarkar
Lehrstuhl für Anorganische Koordinationschemie, Institut für Anorganische Chemie, Universität
Stuttgart, Pfaffenwaldring 55, 70569, Stuttgart, Germany. Email: biprajit.sarkar@iac.uni-stuttgart.de
URL: https://www.iac.uni-stuttgart.de/en/research/aksarkar/
1
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
Abstract
Mesoionic carbenes (MIC) of the 1,2,3-triazol-5-ylidene type are currently popular ligands in
organometallic chemistry. Their use in main group chemistry has been rather limited. In this
contribution we present mono- and di-MIC-boranes with MICs based on triazolylidenes. The synthesis
involves in-situ deprotonation of the corresponding triazolium salts and their reaction with boranes to
form the desired compounds. Whereas this reaction route worked well for all triazolium salts derived
from the 1,4-regioisomer of the triazoles, for the methlyene-bridged bi-triazolium salt derived from a
1,5-substiuted triazole, we observed the unexpected decomposition of the bi-triazolium and the
formation of a triazole-borane with a new N-B bond. All compounds were characterized via
multinuclear NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction. Furthermore,
the MIC-boranes were used as reducing agents for the reduction of the C=O of aldehydes to the
corresponding alcohols.
2
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
Introduction
1,2,3-Triazol-5-ylidenes, which are examples of mesoionic carbenes (MICs), have been widely used as
ligands in transition metal organometallic chemistry in the last years.[1] While most of these metal
complexes were tested for their activity in homogeneous catalysis, recent works have also shown the
utility of such complexes in photochemistry,[2,3] in electrocatalysis,[3,4] in redox-switchable catalysis,[5]
in the construction of supramolecular assemblies[6] and as electro-active compounds.[7,8] In comparison
to transition metal chemistry, there are extremely few reports of the use of such MICs in combination
with main-group elements.[7,9,10,11] The better σ-donor ability of these MICs in comparison to their N-
heterocyclic carbene (NHC) counterparts as well as their tunable π-acceptor ability,[12] might also help
in generating unusual properties with main-group elements, a fact that has already been seen often in
transition metal chemistry. We have recently reported on triazoline-selones, and have used those
compound to gauge the π-acceptor strength of 1,2,3-triazolylidenes[7,12]. In this report we turn our
attention to the combination of 1,2,3-triazolylidenes with the element boron. More specifically, we
report on the reactions of three mono-1,2,3-triazol-5-ylidenes and one bi-1,2,3-triazol-5-ylidene, all of
which are derived from 1,4-disubstituted-1,2,3-triazoles, with BH3 (Figure 1). Furthermore, we also
report on the reaction of a methylene-bridged bi-1,2,3-triazol-4-ylidene, derived from a 1,5-
disubstituted-1,2,3-triazole, with BH3. The difference in the reactivities of these different isomers of
triazolylidenes will be discussed. Results from multi-nuclear NMR spectroscopy, mass spectrometry,
and single crystal X-ray diffraction are reported. Additionally, we show the utility of the MIC-borane
adducts in the reduction of the C=O group of aldehydes to the corresponding alcohols.
Figure 1: Structure of Mono- and Di-Mesoionic Carbene-Boranes.
Results and Discussion
Synthesis, Characterization and X-ray Crystallography
All the triazoles use in this work were reported earlier. These compounds were re-synthesized for the
current work by either using the copper catalyzed azide alkyne cycloaddition (CuAAC) reaction for the
1,4-substituted triazoles (T1-T4),[13–16] or a base catalyzed azide-alkyne cycloaddition for the 1,5-
substituted triazole (T5, see experimental section).[17] The mono triazolium salts (L1-L3, Scheme 1)
3
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
were synthesized by reaction of the corresponding triazolium with CH3I,[18] and the bitriazolium salt L4
by the reaction of the bitriazole with Meerwein salt.[19] For the methylene-bridged bitriazolium salt L5
a procedure, recently reported by us, which is based on the preactivation of diiodomethane with AgOTf
followed by a reaction with the 1,5-disubstituted triazole T5 was used (see experimental section).[20]
Method C/D
Method A
N3-R3
T1-T5
L1-L5
R1
N
R2
Method E
Method B
Dipp
Mes
Dipp
N
N
N
N
Ph
N
N
N
N
N
N
N
N
N
N
N
N
N
Ph
Ph
Ph
Dipp
Mes
Mes
T1[a]
T2[a]
T3[a]
T4[a]
T5[b]
, 90%
, 64%
,36%
, 25%
, 29%
(BF4)2
I
(OTf)2
Dipp
I
Dipp
I
Mes
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Ph
N
N
N
Ph
N
N
N
Ph
Ph
Ph
Dipp
, 64%
Mes
Mes
, 91%
Ph
L1[c]
L2[c]
L3[c]
L4[d]
L5[e]
, 77%
,72%
, 87%
Scheme 1: Synthetic route for the synthesis of L1-L5. Method A[a]: CuSO4·5H2O, sodium ascorbate, K2CO3, pyridine, water/t-
BuOH, r.t., 3d;[13-16] Method B[b]: tetramethylazanium hydroxide, DMSO, r.t., 12 h;[17] Method C[c]: CH3I, MeCN, 70°C, 2d;[18]
Method D[d]: Me3OBF4, DCM, r.t., 3d;[19] Method E[e]: Me2I2, AgOTf, hexane/toluene, refluxe, 12h.[20]
The mono MIC-borane adducts LB1-LB3 as well as the diborane aduct LB4 were all synthesized using the
same procedure (Scheme 2 and experimental section). The corresponding triazolium salt was
suspended in THF and cooled to 0 °C, whereupon a base was immediately added, and the mixture was
stirred for 30 minutes. Afterwards BH3·THF was slowly dropped to the reaction mixture, and the
solution was allowed to warm up to room temperature overnight. The crude product was purified over
a silica column (see experimental section) and the obtained compounds were characterized via 1H, 11B,
13C NMR spectroscopy, and mass spectrometry. The moderate yield for the mono MIC-borane adducts
and the low yield for the diborane adduct could be explained by the recovery of unreacted starting
material. Unfortunately, prolonging of the reaction time and using more equivalents of BH3·THF did
not increase the yield in all cases. The disappearance of the signal corresponding to the C-H proton of
1
the triazolium salt in the H NMR spectrum of the products was a first indication of their formation
(Figure S6, S9, S12, S18). In the 11B spectrum a quartet was observed for all the compounds (coupling
to three protons in BH3) between -34 and -37 ppm (Figure S8, S10, S14, S19). For LB4 only one signal
was observed in the 11B spectrum indicating the equivalence of the two boron centers in that
compound in the NMR timescale (Figure S16). The carbene-C of the compounds appeared in the 13C
NMR spectra between 130 and 140 ppm (Figure S7, S11, S13, S17, S20). In the ESI-MS measurements
a molecular peak corresponding to the Na+ adduct was observed for all the compounds.
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
Scheme 2: Synthesis of the Mesoionic Carbene-Boranes.
The reaction of L5 with BH3·THF under identical conditions as described above did not lead to the
desired product LB5. Instead, a decomposition product LB6, which is a N-coordinated triazole-borane
was isolated (Scheme 3). The nucleophilicity of the N3-atom of a 1,5-substituted-1,2,3-triazole is not
particularly high, and bonds between that N-atom and other groups were observed to be labile in
previous works.[11] This seems to be one of the reasons for the formation of the decomposition product
1
LB6. In the H NMR spectrum of this compound, a signal corresponding to triazole C-H proton is
observed at 8 ppm (Figure S18). In the 11B NMR spectrum LB6 displays a broad peak at –18 ppm
(Fig. S19). As the boron center is now coordinated to a N-atom (instead of a C-atom as for the other
compounds), the chemical shift in the 11B NMR spectrum is significantly different. Additionally, the
1
signal is broad, and the expected coupling to the three H nuclei is not resolved. This is likely due to
the quadrupolar moment of the N-atom to which the B-center is coordinated in this compound, which
is known to broaden lines and preclude the observance of coupling to nuclei in the surrounding, which
is otherwise expected. In the ESI-MS a molecular peak of the corresponding compound as the Na+
adduct was observed.
Scheme 3: Formation of LB6 by decomposition of LB5.
5
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
The BH3-adducts investigated in this work displayed only irreversible reduction or oxidation steps in
their cyclovotammograms (Figure S21). These results point to decomposition of the compounds in
response to electron transfer steps. Such an observation has been made earlier for other 1,2,3-triazol-
5-ylidene type MIC adducts.[21]
We were successful in growing single crystals of LB1, LB4 and LB6 that were suitable for X-ray diffraction
measurements. The three compounds crystallize in the monoclinic P21/c, orthorhombic Pccn and
orthorhombic P212121 crystal system and space group respectively (Table S2). The B-centers in LB1 and
LB4 are coordinated to the MIC-C atom of the triazolylidene units (Figure 2). The B-C bond lengths in
the two compounds are 1.600(4) and 1.607(3) Å, and are in the range of previously reported
compounds. As expected, these bond lengths point to a single bond, and likely a dative nature of the
C-B bond.[22] Furthermore, the N-N and C-N bond lengths within the triazolylidene rings are in the
expected range (Table 1), and point to a delocalized bonding situation in the heterocyclic rings. In LB4,
there is a local inversion center in the molecule, and the rings take up an anti-orientation with respect
to each other (Figure 2). The mesityl substituents in LB1 are twisted by 89.6 (1) and 70.2 (1) ° with
respect to the central triazolylidene ring. The two triazolylidene rings in LB4 are twisted by 55.10 (6)
with respect to each other. The phenyl substituents in LB4 are twisted by 49.92 (6) and 49.92 (5) ° with
respect to the triazolylidene ring.
Figure 2: ORTEP representation of LB1, LB6 and LB4: ellipsoids drawn at 50% probability. Solvent molecules and H atoms omitted (except
for the C-H of the triazole ring in LB6) for clarity.
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
Table 1. Selected Bond Length (Å) and bond angels (°) for LB1, LB4 and LB6.
LB1
LB4
LB6
C1-B1
1.600(4)
-
1.607(3)
-
-
N3-B1
N1-N2
N2-N3
N3-C1
C1-C2
1.594(4)
1.344(3)
1.320(3)
1.347(4)
1.371(4)
1.366(4)
105.2(2)
1.320(4)
1.341(3)
1.365(4)
1.394(4)
1.463(4)
102.8(2)
1.310(2)
1.333(2)
1.364(2)
1.387(3)
1.459(2)
103.8(2)
C2-N1
N1-N2-N3
The aforementioned decomposition of L5 and the concomitant formation of LB6 is also confirmed
through the molecular structure determined in the crystal (Figure 2). The B-N bond length in this
compound is 1.594(4) Å, and lies in the range of other B-N single bonds.[23] The two phenyl substituents
in LB6 are twisted by 30.1 (1)° and 71.93 (9) ° with respect to the central triazolylidene ring. To the best
of our knowledge this is the first example of a triazole-borane adduct in which the N3-atom of a 1,2,3-
triazole is coordinated to the B-atom of a borane.
Reduction of Carbonyl Functionalities
Our next step was to assess the potential of the synthesized MIC-borane adducts in the reduction of
electrophilic C-O double bonds, which is shown in Table 2. To ensure good comparability, only LB1 and
LB4 were tested. The protocol was selected from literature reports on similar reactions with either
NHC-boranes or with MIC-boranes. All reactions were carried out in DCM at room temperature. The
yield was calculated by NMR, using hexamethylbenzene as an internal standard. First, the reduction of
4-bromobenzaldehyde without the lewis acid was tested, and this led to the expected recovery of the
starting material. Also, in the reaction without the MIC-borane no reduction to the alcohol was
observed. Only the blank test just with BH3·THF gave a conversion of around 30%, which indicates the
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potential of boranes for the reduction of aldehydes, which was already shown previously.[24] On using
Sc(OTf)3 as the lewis acid in combination with the MIC-boranes presented here, high conversion to the
desired alcohol could be achieved with LB1 and low conversion with LB4. By changing the additive from
Sc(OTf)3 to silica gel, almost quantitative conversion after 2.5 hours could be observed for both MIC-
BH3 adducts. The comparison of the reaction time of LB1 and LB4 (150 min vs. 100 min) shows the
increased reduction potential of LB4 with the second BH3 unit. While it is not totally clear at this
moment as to why the activity of LB4 is higher than LB1, it is likely that the presence of two BH3 units
within the same molecule is beneficial for the observed reactivity. The conversion profile of the
aldehyde to the alcohol for both MIC-boranes is shown in figure 3.
Figure 3: Percentage conversion of benzaldehyde to benzyl alcohol in DCM at r.t. Yield was calculated by NMR, using
hexamethylbenzene as an internal standard. Left: LB1 as the catalyst; right: LB4 as the catalyst.
We next examined the influence of the substitution pattern of the phenyl ring of the substrate on the
reaction rate, using the more reactive LB4 as the reducing agent. By exchanging the p-Br with two Br in
the ortho position, the reaction time is decreased by 1.66 times (entry 9, Table 2), which proves, that
the additional steric hindrance in that substitution pattern does not hamper the reaction progress.
Interestingly, exchange of the Br substituent with higher or lower homologes of the halides, the
reaction progress after 180 minutes is decreased (entry 11-14). Especially, for the fluoro substituent
(entry 10, Table 2), the yield goes down to 50%. In keeping with this trend, the reaction with the
perfluorinated phenyl ring has the lowest yield with 35%. The enormous influence of the substitution
of the phenyl ring on the outcome of the reaction is also illustrated by the fact that simply replacing
the halide in the para position with a hydrogen does not deliver any conversion after 180 min (entry 14,
Table 2). Overall, the results of the reducing capacity of LB1 and LB4 are comparable with similar NHC[25]
-and MIC-borane[9] systems. Especially, the diborane adduct could show higher conversion in a shorter
time period, with a variety of different aldehydes.
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European Journal of Inorganic Chemistry
Table 2: Results of the reduction of different aldehydes with different MIC-BH3-adducts as the catalyst, promoted by a lewis acid (LA).
Entry
MIC-BH3
R1
Lewis Acid (LA)
Time
[min]
Yield[a] [%]
1
2
3
4
5
6
7
8
9
-
BH3·THF
LB1
p-Br
p-Br
p-Br
p-Br
p-Br
p-Br
p-Br
p-Br
o-Br2
SiO2
180
180
180
180
150
180
180
100
60
0
SiO2
35
0
-
LB1
20mol% Sc(OTf)3
71
99
0
LB1
SiO2
LB4
-
20mol% Sc(OTf)3
SiO2
LB4
12
99
99
LB4
LB4
SiO2
10
11
12
13
14
LB4
LB4
LB4
LB4
LB4
p-F
p-Cl
p-I
F5
SiO2
SiO2
SiO2
SiO2
SiO2
180
180
180
180
180
50
80
90
35
0
H
[a] Yield calculated by NMR using hexamethylbenzene as internal standard.
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European Journal of Inorganic Chemistry
Conclusions
In summary, we have presented here the reactions of several triazolium salts with BH3·THF in the
presence of a base. For mono-triazolium salts and the bi-triazolium salts derived from the 1,4-
substituted triazoles, the reactions led to the desired formation of the mono- and the bi-MIC-borane
adducts. This is the first example of a bi-MIC-bi-borane adduct based on MICs of the 1,2,3-triazol-5-
ylidene type. Additionally, while reacting a methylene-bridged bi-triazolium salt with BH3·THF under
identical conditions, we have observed the formation of a new N-borane coordinated 1,2,3-triazole
ring through decomposition of the bi-triazolium salt. To the best of our knowledge, this is the first
example of a N3-coordinated triazole-borane adduct. All the compounds were characterized via a
combination of multinuclear NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction
studies. Furthermore, the synthesized MIC-borane adducts were employed for reduction of halide
substituted benzaldehydes, and we observed that the reaction outcome is dependent on the
substitution pattern and on the reducing agent that was employed. We could also show that the
reaction rate of LB4 is 1.5 times faster than the monosubstituted LB1. Our results thus expand the main
group chemistry of both triazoles and triazolylidenes, and we have also displayed the utility of the MIC-
borane adducts in reduction reactions.
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European Journal of Inorganic Chemistry
Experimental Section
Unless otherwise noted, all reactions were carried out using standard Schlenk-line techniques under
an inert atmosphere of argon (Linde, HiQ Argon 5.0, purity≥99.999%). Commercially available
chemicals were used without further purification. T1-T3,[13–15] T4[16] and T5[17] were prepared according
to a literature known procedure. THF and diethyl ether were dried and distilled from
sodium/benzophenone. Other solvents were available from MBRAUN MB-SPS-800 solvent system. For
the synthesis part, all solvents were degassed by standard techniques prior to use. For NMR, CDCl3 was
passed through a small plug basic alumina.1H NMR and 13C{1H} spectra were recorded on JEOL ECS 400
spectrometer and JEOL ECZ 400R spectrometer at room temperature. Kinetic NMR spectra were
recorded in Fourier transform mode with a Bruker AVANCE 500 spectrometer at 298 K. Chemical shifts
are reported in ppm (relative to the TMS signal) with reference to the residual solvent peaks.[26]
Multiplets are reported as follows: singlet (s), doublet (d),triplet (t) quartet (q), quintet (quint), and
combinations thereof. Mass spectrometry was performed on an Agilent6210 ESI-TOF.
X-ray data were collected on a Bruker D8 Venture system at 100(2) K, using graphite-monochromated
MoKα radiation (λα= 0.71073 Å). The strategy for the data collection was evaluated by using the APEX3
software. The data were collected by ω+φscan techniques, and were scaled and reduced using Saint+
and SADABS software. The structures were solved by intrinsic phasing methods using SHELXT-2014/7.
The structure was refined by full matrix least-squares usingSHELXL-2014/7, refining on F2. Non-
hydrogen atoms were refined anisotropically.[27] The contribution of disordered solvent molecules to
the diffraction pattern was subtracted from the observed data by the “SQUEEZE” method as
implemented in PLATON.[28]
Cyclic voltammograms were recorded with a PAR VersaStat 4 potentiostat (Ametek) by working in
anhydrous and degassed MeCN (99.8% extra dry, Acros Organics) with 0.1 M NBu4PF6 (dried, > 99.0%,
electrochemical grade, Fluka) as electrolyte. Concentrations of the compounds were about 1x10-4 M.
A three-electrode setup was used with a glassy carbon working electrode, a coiled platinum wire as
counter electrode, and
a
coiled silver wire as
a
pseudo-reference electrode. The
ferrocene/ferrocenium couple was used as internal reference.
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European Journal of Inorganic Chemistry
General procedure for the synthesis of the triazolium salts: The desired triazol and a methylating
agent were dissolved in the solvent and stirred for 3 d. The reaction mixture was poured into diethyl
ether and dried in vacuo, affording essentially pure triazolium salt.
Compound [L1]: T1 (700 mg, 2.29 mmol, 1 eq.), methyliodide (1.4 ml, 22.9 mmol, 10 eq.) were
dissolved in MeCN (5 mL) and stirred at 70 °C. After purification by precipitation in diethylether (200
mL), the ligand was obtained as a white solid (929 mg, 2.08 mmol, 91%).1H NMR (400 MHz, CDCl3): δ =
9.39 (s, 1H, triazol-H), 7.09 (m, 2H, aryl-H), 7.07 (m, 2H, aryl-H), 4.26 (s, 3H, N- CH3), 2.39 (s, 3H, CH3),
2.37 (s, 3H, CH3), 2.21 (s, 6H, CH3), 2.20 (s, 6H, CH3) ppm.
Compound [L2]: T2 (175 mg, 0.505 mmol, 1 eq.), methyl iodide (0.3 ml, 5.05 mmol, 10 eq.) were
dissolved in MeCN (4 mL) and stirred at 70 °C. After purification by precipitation in diethylether
(200 mL), the ligand was obtained as a white solid (180 mg, 0.368 mmol, 72%).1H NMR (400 MHz,
CDCl3): δ = 9.16 (s, 1H, triazol-H), 7.65 (t, J = 7.9 Hz, 1H, aryl-H), 7.41(m, 2H, aryl-H), 7.09 (s, 2H, aryl-
H), 4.37 (s, 3H, N-CH3), 2.38 (s, 3H, CH3), 2.36 (hept, J = 7.0 Hz, 2H, CH), 2.23 (s, 6H, CH3), 1.27 (d,
J = 5 Hz, 12H, CH3) ppm.
Compound [L3]: T3 (784 mg, 2.02 mmol, 1 eq.) and methyl iodide (0.12 ml, 20.02 mmol, 10 eq.) were
dissolved in MeCN (4 mL) and stirred at 70 °C. After purification by precipitation in diethylether
1
(200 mL), the ligand was obtained as a white solid (704 mg, 1.30 mmol, 64%). H NMR (400 MHz,
CDCl3): δ = 9.29 (s, 1H, triazol-H), 7.64z (t, J = 7.6 Hz, 2H, aryl-H), 7.41(t, J = 7.4 Hz, 4H, aryl-H), 4.39 (s,
3H, N-CH3), 2.38 (hept, J = 2.4 Hz, 4H, CH), 1.27 (d, J = 5 Hz, 12H, CH3), 1.22 (d, J = 5 Hz, 12H, CH3) ppm.
Compound [L4]: T4 (268 mg, 0.93mmol, 1 eq.) and trimethyloxonium tetrafluoroborat (344 mg,
2.32 mmol, 2.5 eq.) were dissolved in DCM (12 ml) and stirred at room temperature. The reaction
mixture was poured into diethyl ether (300 mL) and dried in vacuo, affording essentially pure ligand as
a white solid (393 mg, 0.82 mmol, 87%).1H NMR (400 MHz, (CD3)2CO): δ = 10.11 (s, 2H, triazole-H), 8.02
(m, 4H, aryl-H), 7.75 (m, 6H, aryl-H), 4.5 (s, 6H, N-CH3) ppm.
Compound [L5]: Silver triflate (1.39 g, 5.5 mmol, 2.2 eq.) was suspended in hexane (10 mL). After
adding of freshly distilled diiodomethane (0.2 mL, 2.5 mmol, 1 eq.), the mixture was heated to 70 °C
and stirred for 4 h under exclusion of light. The corresponding suspension with the participating silver
iodide was filtered over a plug of Celite@ and washed with hexane (10 mL). The filtrate was added
dropwise to T5 (1.39 g, 6.25 mmol, 2.5 eq.) in toluene (15 mL). The resulting mixture was heated to
refluxed overnight. After cooling down to room temperature, the solution was diluted in diethyl ether
(500 mL). The resulting colourless precipitate was collected by filtration and washed with diethyl ether
(50–100 mL), to obtain the product as a colourless solid (1.45 g, 1.92 mmol, 77%).1H NMR (400 MHz,
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European Journal of Inorganic Chemistry
(CD3)2CO): δ = 9.69 (s, 2H, triazol-H), 8.09 (s, 2H, aryl-H), 7.81(m, 4H, aryl-H), 7.74 (m, 2H, aryl-H), 7.65
(m, 4H, aryl-H), 7.56 (m, 7H, aryl-H), 7.47 (m, 3H, aryl-H) ppm.
General procedure for the synthesis of MIC-BH3 adducts: The triazolium salt was suspended in THF
and cooled to 0 °C. After addition of the base and stirring for 30 min., BH3·THF [1 M] was dropped
slowly to the suspension and then stirred overnight by slowly warming up to room temperature. The
crude product was purified by column chromatography (SiO2, DCM/MeOH = 50:1).
Compound[LB1]: L1 (44.7 mg, 0.10 mmol, 1eq), KOtBu (13.5 mg, 0.12 mmol, 1.2 eq.), THF (4 mL),
BH3·THF (0.1 mL, 0.1 mmol, 1 eq.). After purification by column chromatography, the product was
obtained as a white solid (28.9 mg, 0.063 mmol, 63%). 1H NMR (400 MHz, CDCl3): δ = 7.01 (s, 2H, Aryl-
H), 7.00 (s, 2H, Aryl-H), 3.87 (s, 3H, N- CH3), 2.36 (s, 3H, CH3), 2.34 (s, 3H, CH3), 2.08 (s, 6H, CH3), 2.03
(s, 6H, CH3) ppm. 11B NMR (400 MHz, CDCl3): δ = -37 (q, 3H, BH3) ppm. 13C{1H} NMR (101 MHz, CDCl3):
δ = 142.3, 140.4, 140.3, 138.2, 134.6, 133.7, 129.1, 128.8, 122.7, 36.3, 21.4, 21.3, 19.9, 17.3 ppm. ESI-
MS: m/z: [M+Na]+ 356.2268 calc. 356.2267.
Compound[LB2]: L2 (51.1 mg, 0.10 mmol, 1eq), KOtBu (13.5 mg, 0.12 mmol, 1.2 eq.), THF (4 mL),
BH3·THF (0.1 mL, 0.1 mmol, 1 eq.). After purification by column chromatography, the product was
obtained as a white solid (19.0 mg, 0.05 mmol, 50%). 1H NMR (400 MHz, CDCl3): δ = 7.51 (t, J = 7.9 Hz,
1H, Aryl-H), 7.31 (d, J = 7.8 Hz, 2H, Aryl-H), 7.01 (s, 2H, Aryl-H ), 3.87 (s, 3H, N-CH3), 2.36 (sept, J = 6.9 Hz,
2H, CH), 2.35 (s, 3H, CH3), 2.09 (s, 6H, CH3), 1.29 (d, J = 6.9 Hz, 6H, CH3), 1.13 (d, J = 6.9 Hz, 6H, CH3) ppm.
11B NMR (400 MHz, CDCl3): δ = -36.5 (q, 3H, BH3) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 145.3, 142.3,
140.4, 138.3, 133.6, 130.9, 128.8, 124.7, 123.9, 122.7, 36.3, 28.1, 24.6, 22.9, 21.4, 19.8 ppm. ESI-MS:
m/z: [M+Na]+ 398.2721 calc. 398.2738.
Compound[LB3]: L3 (26.5 mg, 0.05 mmol, 1eq), KOtBu (6.75 mg, 0.06 mmol, 1.2 eq.), THF (3 mL),
BH3·THF (0.05 mL, 0.05 mmol, 1 eq.). After purification by column chromatography, the product was
obtained as a white solid (14.03 mg, 0.025 mmol, 51%).
1H NMR (400 MHz, CDCl3): δ = 7.51 (m, 2H, Aryl-H), 7.32 (m, 4H, Aryl-H), 3.86 (s, 3H, N- CH3), 2.47 (sept,
J = 6.7 Hz, 2H, CH), 2.37 (sept, J = 6.7 Hz, 2H, CH), 1.30 (d, J = 7.0 Hz, 6H, CH3), 1.26 (d, J = 7.0 Hz, 6H,
CH3), 1.19 (d, J = 7.0 Hz, 6H, CH3), 1.14 (d, J = 7.0 Hz, 6H, CH3) ppm. 11B NMR (400 MHz, CDCl3): δ = -36
(q, 3H, BH3) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 149.1, 145.2, 131.2, 131.1, 123.9, 123.5, 36.6,
31.7, 29.0, 24.5, 24.2, 23.7, 22.9 ppm. ESI-MS: m/z: [M+Na]+ 440.3182 calc. 440.3207.
Compound[LB4]: L4 (148.2 mg, 0.3 mmol, 1eq), KOtBu (80.8 mg, 0.72 mmol, 2.4 eq.), THF (10 mL),
BH3·THF (0.6 mL, 0.6 mmol, 2 eq.). After purification by column chromatography, the product was
13
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
obtained as a white solid (21.8 mg, 0.063 mmol, 21%). 1H NMR (400 MHz, CDCl3): δ = 7.98 (m, 4H, Aryl-
H), 7.11 (m, 6H, Aryl-H), 4.33 (s, 6H, N-CH3) ppm. 11B NMR (400 MHz, CDCl3): δ = -34.5 (q, 3H, BH3) ppm.
13C{1H} NMR (101 MHz, CDCl3): δ = 137.7, 130.8, 129.5, 125.3, 100.6, 39.5 ppm. ESI-MS: m/z: [M+Na]+
367.1982 calc. 367.1984.
Compound[LB5]: L5 (75.4 mg, 0.1 mmol, 1eq), KOtBu (27.0 mg, 0.22 mmol, 2.2 eq.), THF (5 mL), BH3·THF
(0.22 mL, 0.22 mmol, 2.2 eq.). After purification by column chromatography, the product could not be
isolated, according to 1H NMR spectroscopy, instead LB6 was isolated.
Compound[LB6]: LB6 was isolated during the attempted synthesis of LB5.
1H NMR (400 MHz, CDCl3): δ = 8.00 (s, 1H, triazol-H), 7.52 (m,1H, aryl-H), 7.46 (m, 3H, aryl-H), 7.37 (m,
4H, aryl-H), 7.19 (m, 2H, aryl-H) ppm. 11B NMR (400 MHz, CDCl3): δ = -17.7 (bs, 3H, BH3) ppm.
13C{1H} NMR (101 MHz, CDCl3): δ = 139.9, 135.1, 131.9, 130.9, 130.8, 129.9, 129.5, 128.9, 125.4, 124.2
ppm. ESI-MS: m/z: [M+Na]+ 274.0915 calc. 274.0912.
Acknowledgements
We would like to thank the Core Facility BioSupraMol, supported by the DFG, for assistance with NMR
und ESI-MS measurements.
Keywords: mesoionic carbene, triazole, triazolylidene, boranes, reduction
Accession codes: CCDC 2062463 (LB1), 2062459 (LB4), 2062461 (LB6) contain the supplementary
crystallographic data for this paper. These data are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service via
www.ccdc.cam.ac.uk/structures.
14
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10.1002/ejic.202100273
European Journal of Inorganic Chemistry
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TOC
A series of MIC-borane adducts are presented. In one case the unexpected formation of a triazole-
borane adduct through the decomposition of a bitriazolium salt was observed. The compounds were
used for the reduction of aldehydes to alcohols in the presence of Lewis acids.
18
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