Indazaboles - Synthesis and molecular structure

Article abstract of DOI:10.1002/aoc.1627

The reaction of 1-trimethylsilyl-indazole with boranes affords indazaboles accompanied by elimination of trimethysilane. Thus, the two isomers of parent indazabole are formed in a 1 : 1 ratio using borane in THF (BH3/THF), characterized by NMR spectroscopy in solution (1H, 11B and ¹³C NMR). In contrast, the analogous reaction with 1, 2-bis(tetramethylene)diborane(6) proceeds to give a single isomerof the B-alkylated indazabole via symmetric ring cleavage of the diborane(6), as shown by NMR in solution and X-ray structural analysis in the solid state. The molecular structure is fluxional in solution. In the solid state, the central B2N4 ring adopts a distorted boat conformation. Calculated gas phase geometries of the parent indazaboles and of the B-alkylated indazabole were optimized by DFT methods at the B3LYP/6-311+G(d,p) level of theory. Copyright

Full text of DOI:10.1002/aoc.1627

Full Paper
Received: 17 December 2009
Revised: 7 January 2010
Accepted: 7 January 2010
Published online in Wiley Interscience: 17 February 2010
(www.interscience.com) DOI 10.1002/aoc.1627
Indazaboles – synthesis and molecular
structure
a∗
b
a
a
¨
Bernd Wrackmeyer , Khadija Shahid , Rhett Kempe , Christian Doring
and Wolfgang Miliusa
The reaction of 1-trimethylsilyl-indazole with boranes affords indazaboles accompanied by elimination of trimethysilane.
Thus, the two isomers of parent indazabole are formed in a 1 : 1 ratio using borane in THF (BH3/THF), characterized by NMR
spectroscopy in solution (1H, 11B and 13C NMR). In contrast, the analogous reaction with 1,2-bis(tetramethylene)diborane(6)
proceeds to give a single isomer of the B-alkylated indazabole via symmetric ring cleavage of the diborane(6), as shown by NMR
in solution and X-ray structural analysis in the solid state. The molecular structure is fluxional in solution. In the solid state, the
central B2N4 ring adopts a distorted boat conformation. Calculated gas phase geometries of the parent indazaboles and of the
c
B-alkylated indazabole were optimized by DFT methods at the B3LYP/6-311+G(d,p) level of theory. Copyright ꢀ 2010 John
Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: boranes; indazoles; NMR; X-ray
Introduction
6, which otherwise is well known for its exceptionally low
reactivity.[18]
The pioneering work of Trofimenko has opened up access to
numerous pyrazaboles[1,2] of type 1 as well as 1-pyrazolylborates
such as 2[1a,b] or more generally to poly(1-pyrazolyl)borates,
which have been successfully applied as versatile tripodal ligands
(‘scorpionates’) in numerous areas of coordination chemistry
of both main group elements and transition metals.[3–7] In a
similar way to 2, the tri(1-indazolyl)borates 3 have found useful
applications.[8,9] In the case of 3, an ‘abnormal’ stereochemistry is
usually observed, since in contrast to 2 (R3 = R5), boron prefers
the sterically more hindered position (Scheme 1).
Pyrazaboles are promising for applications in the field of liq-
uid crystals,[10] π-conjugated hybrid polymers,[11] and attractive
materials with respect to photophysical properties, e.g. in organic
electroluminescent devices.[12] On the other hand, the stabilizing
effect exerted by the great variety of polypyrazolyl ligands for tran-
sition metal complexes has been extensively exploited.[3–7] Some
attractive areas of research concern biomimetric applications, e.g.
with respect to enzyme activity,[13] magnetic properties[14] or NLO
behavior.[15]
However, to the best of our knowledge, indazole analogs of
1, the indazaboles have not been reported as yet, although
applications of such materials (vide supra) can be anticipated
considering the rich chemistry of indazoles. We became interested
in azaboles, when we studied Si–N cleavage reactions using
boranes in order to eliminate silane and form B–N bonds.[16] Thus,
we found that the reaction of 1-trimethylsilyl-pyrazole 4 with
borane in THF (BH3/THF) readily affords the parent pyrazabole 1a
via the borane adduct as an intermediate which could be detected
by NMR spectroscopy (Scheme 2).[17]
Results and Discussion
Parent Indazaboles
When the THF solution containing an equimolar mixture of 1-
trimethylsilyl-indazole 4 and BH3/THF was warmed from −78 ◦C
to room temperature, the 11B NMR spectra showed a rather broad
signal at δ11B = −18 (see also the data in No¨th et al.[17a]) for
the borane adduct 4-BH3. In the 1H coupled 11B NMR spectrum,
the expected quartet with 1J(11B,1 H) ≈ 100 Hz is barely resolved.
After heating this solution for 1 h at reflux, the 29Si NMR spectra
indicated that only traces of 4 or its borane adduct were left,
and trimethylsilane was the major silicon compound. Similarly,
1
according to the 11B{ H} NMR spectra the reaction was judged to
be complete, since there were mainly (>90%; one minor signal
for 4-BH3, and one signal at δ11B = +18 for the decomposition
productofBH3/THF)threesignalsintheapproximateratioof1 : 2 : 1
in the region typical of the parent pyrazabole 1a [δ11B = −8.6,
1J(11B,1H) = 110 Hz].[19] These signals (δ11B = −7.5, −9.5, −11.4)
appeared as broad triplets [1J(11B,1H) = 105 Hz] in the 1H-coupled
11B NMR spectrum. 1H and 13C NMR spectra reveal severely
overlapping signals in the indazole region. The 1H(BH2) NMR
signalsaretypicallybroad,andupon11Bdecoupling,three1H(BH2)
signals become visible, again in a ratio of 1 : 2 : 1. Therefore, we
∗
¨
Correspondence to: Bernd Wrackmeyer, Anorganische Chemie II, Universitat
Bayreuth, D-95440 Bayreuth, Germany. E-mail: b.wrack@uni-bayreuth.de
Here we report on the results of the reaction of borane
in THF (BH3/THF) with 1-trimethylsilyl-indazole 5. Furthermore,
the application of the method is shown by the unexpectedly
smooth reaction of 5 with the 1,2-bis(tetramethylene)diborane(6)
¨
a
Anorganische Chemie, Universitat Bayreuth, D-95440 Bayreuth, Germany
b
Riphah Institute of Pharmaceutical Sciences, Riphah University, Islamabad,
Pakistan
c
Appl. Organometal. Chem. 2010, 24, 398–401
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

Indazaboles – synthesis and molecular structure
R4
B
B
6
R5
R3
BX2
H
H
R4
2
N N
H
B
70 °C
- Me3SiH
R5
R3
2
N
N
N
N
N
B
B
N N
70 °C
N
M+
N
M+
X2B
R3
N
N
3
N
N
HB
HB
3
SiMe3
SiMe3
N
5
3
2
8
R5
R3,4,5 = H, alkyl aryl, etc.
X = H, alkyl, halogen, etc.
R4
1
Scheme 4. Cleavage of the Si–N bond in 1-trimethylsilyl-indazole 5 in the
reaction with 1,2-bis(tetramethylene)diborane(6) 6. The intermediacy of
the borane adduct is proposed.
Scheme 1. Pyrazaboles 1, tri(1-pyrazolyl)borates 2 and tri(1-indazolyl)
borates 3.
chlorinated hydrocarbons. In this case, the formation of only one
isomer was observed, most likely for steric reasons.
N
N
N
N
+ BH3-THF
20 - 25 °C
-Me3SiH
NMR spectra (1H, 11B, 13C and 15N NMR) of 8 in solution indicate
the presence of a single isomer with a symmetric structure. There
are only two 1H and two 13C NMR signals of the boracyclopentane
ring. For a rigid boat-conformation one would expect four signals
each. Similar to pyrazaboles, the molecular structure of 8 is most
likely fluxional in solution with respect to fast inversion of the
central B2N4 ring. The 1H and 13C NMR spectra do not change
significantlyforCD2Cl2 solutionsfrom23to−60 ◦C.Thesedynamic
properties of pyrazaboles have been elucidated by solid-state
NMR spectroscopy of two pyrazaboles, 1a[24a] and its tetra-B-ethyl
derivative.[24b]
N
N
1/2 H2B
BH2
BH3
N
N
SiMe3
SiMe3
4
1a
Scheme 2. Cleavage of the Si–N bond in 1-trimethylsilyl-pyrazole 4 in the
reaction with borane.
propose that the reaction (Scheme 3) gives the two isomers of the
parent indazaboles 7 and 7ꢂ as major products in the ratio of ∼1:1.
The calculated[20] optimized gas phase geometries of 7 and 7ꢂ
[B3LYP/6-311+G(d,p) level of theory] show the boat- or roof-like
The proposed molecular structure of 8 was confirmed by
single crystal X-ray structural analysis (Fig. 1). The X-ray diffraction
measurements were carried out for different crystals at 133(2) and
293(2) K, since temperature-dependent phase transitions have
been reported for the pyrazabole bearing four B-ethyl groups.[24b]
In the case of 8, essentially identical results were obtained at
133 and 293 K. Intermolecular interactions appear to be weak
or negligible, since π stacking of the heteroaromatic systems
is absent (see however, Cavero et al.[2b]). The central B2N4 ring
adopts the familiar boat-like structure. However, the four nitrogen
atoms deviate significantly from a planar arrangement (∼ 5 pm
with respect to the best plane), in contrast to the situation in
pyrazaboles. The indazole ring systems are planar within the
experimental error, and they are twisted against each other by
an angle of 19.5◦. All bond angles and bond lengths are in the
expected ranges, and in particular the N–N, N–C, N–B and B–C
distances are close to those found for pyrazaboles.[1c,d,2a,b,23,24b]
The gas phase structure of 8 was calculated and optimized[20]
[B3LYP/6-311+G(d,p) level of theory], and the structural parame-
ters correspond closely to the experimental solid-state structure,
including the twist angle between the indazole rings. The latter
suggests that this particular structural feature does not result
from crystal packing. The calculated 11B chemical shift is in good
agreement with the experimental value (δ11B: calcd +4.2, exp.
+3.1).
structures similar to those found for many pyrazaboles in the solid
◦
state[1c,2b] with a folding angle along the B B axis of ∼132 for
· · ·
both 7 and 7ꢂ. The two planes of the indazole ring systems are
not twisted against each other. The energy difference between
7 and 7ꢂ is calculated to be extremely small (<0.15 kcal mol−1, 7
being slightly more stable than 7ꢂ). These structures appear to be
reliable, sincevaluesandthetrendof11Bchemicalshiftscalculated
for these geometries[21] for 7 (δ11B: calcd −12.2; exp. −9.5) and
7ꢂ (δ11B: calcd −14.3, −10.0; exp. −11.4, −7.5) are rather close to
the experimental data. This has been verified by calculating the
optimized geometry and the 11B chemical shif◦t for pyrazabole 1a:
· · ·
folding angle along the B B axis: calcd 132.3 (see also Alvarado
et al.[22]), to be compared with 134.7◦, determined for crystalline
1a;[23] and δ11B: calcd −10.8 and exp. −8.6.[19]
B-Substituted Polycyclic Indazabole
In contrast with BH3 and many other tetraalkyldiboranes(6), 1,2-
bis(tetramethylene)diborane(6) 6 does not react readily with
olefins,[18c,d] and Lewis bases such as amines do not cleave the
ring.[18a] However, the reaction of 6 with 1-trimethylsilyl-indazole
5 proceeds slowly (10 h) in boiling hexane via symmetric ring
cleavage of 6 to give the desired indazabole 8 (Scheme 4) as
a colorless crystalline material, stable in air, moderately soluble
in hexane or toluene and well soluble at room temperature in
5
4
Conclusions
6
9
3
7
8
1
2
The reaction of N-trimethylsilyl-azoles with boranes appears to be
anusefulalternativetoformB–Nbonds.Conventionalprocedures,
using simply the azoles and boranes, are sometimes difficult to
control in a stoichiometric way and give rise to side reactions, e.
g. multiple substitution at boron or hydroboration. In the present
work, the parent indazaboles were obtained as a 1 : 1 mixture of
isomers and a B-alkylated polycyclic indazabole was obtained as a
single isomer, characterized in solution and in the solid state.
N
N
N
N
N
N
+ BH3-THF
- Me3SiH
H2B
BH2
+
H2B
BH2
N
N
N
N
SiMe3
5
7
7′
Scheme 3. Cleavage of the Si–N bond in 1-trimethylsilyl-indazole 5 in the
reaction with borane.
c
Appl. Organometal. Chem. 2010, 24, 398–401
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/aoc

B. Wrackmeyer et al.
0.03 for 1H, 0.1 ppm for 13C, 15N, 29Si and 0.3 for 15N of 1a
and 7 (broad signals as a result of unresolved 15N–11B spin–spin
coupling[27]) and 0.4 ppm for 11B. Elemental analyses (C/H): Vario
Elementar EL III; melting points (uncorrected): Bu¨chi 510 melting
point apparatus.
The quantum chemical calculations were performed using the
program package Gaussian 03, revision B.02.[20] Gas phase struc-
tures were optimized with DFT methods [B3LYP/6-311+G(d,p)
level of theory] and the NMR parameters were calculated (GIAO
method) as implemented in the Gaussian 03 program. Nuclear
magnetic shielding constants σ(11B) were converted to δ11B data
by δ11B = σ(11B) [B2H6] − σ(11B) + 18, with σ(11B) [B2H6] = 84.1,
δ
11B [B2H6] = 18.0 and δ11B [BF3 –OEt2] = 0.
Reactions of 1-Trimethylsilyl-pyrazole and 1-Trimethylsilyl-
indazole with BH3 –THF/THF
The solution of BH3 –THF/THF (50 ml, 1 M) was cooled to −78 ◦C
and 1 equiv. of the respective 1-trimethylsillyazole was added in
one portion, while the solution was vigorously stirred. The mixture
was allowed to reach ambient temperature within 1 h and, after
measuring 11B NMR spectra, it was heated at 65 ◦C for 1 h. The
reactions were found to be complete by measuring both 11B and
29Si NMR spectra. All volatile materials were removed invacuo, and
the pure pyrazabole 1a and the mixture of isomers of indazabole
7 and 7ꢂ were left as colorless solid materials. Attempts to separate
7 and 7ꢂ by fractional crystallization (hexane, toluene or ether)
have not been successful so far. 1a: 1H, 11B and 13C NMR data
agree with the reported data set;[19] 50.4 MHz 15N NMR (CD2Cl2):
Figure 1. Two views of the molecular structure of 8. Top: ORTEP plot (50%
probability level; hydrogen atoms are omitted for clarity); bottom: ball
and stick model, showing the mutual tilt of the indazole groups and the
distorted roof-like structure. Selected bond lengths (pm) and angles (deg.):
N1–N2 137.1(2), N1–B1 158.2(3), N2–B2 159.6(3), N3–N4 136.6(2), N3–B1
158.4(3), N4–B2 157.3(3), C5–N3133.7(3), C7–N4136.5(2), C5–C6 139.4(3),
C6–C7 140.6(3), C6–C11 141.8(3), C10–C11 136.9(3), C9–C10 141.4(3),
C8–C9 136.5(3), C7–C8 141.6(3), C4–B1 162.5(3), C1–B1 163.8(3), C1–C2
154.2(3); C2–C3 1.531(3); N2N1B1 123.3(2), N1N2B2 122.8(2), N4N3B1
123.8(2), N3N4B2 123.4(2), N1B1N3 105.7(2), N4B2N2 105.3(2), C4B1C1
103.5(2), C19B2C22 103.8(2).
δ
15N = −152.0 (referencing in Layton et al.[19] was not given
precisely). 7/7ꢂ: Elemental analysis (% found/calcd): C 64.1/64.8,
Experimental
H 5.3/5.4; 400 MHz H NMR in CD2Cl2: δ1H = 8.35, 8.22 (s, s 2H,
1
All preparations and the handling of samples were carried out
under inert atmosphere (Ar) and in carefully dried solvents,
and oven-dried glassware was used. Starting materials such as
pyrazole, indazole, (Me3Si)2NH, BH3/THF (1 M) and butadiene
were commercial samples and used as received. 1-Trimethylsilyl-
pyrazole 4 [in C6D6; 250 MHz 1H NMR: δ1H = 0.26 (s, 9H, SiMe3),
6.20 (t, 1H, H4), 7.39, 7.74 (d, d 2H. H3,5); 62.9 MHz 13C NMR:
H3), 7.86, 7.81 (d, d, 4H), 7.57, 7.16 (m, m, 4H), 4.16, 4.22, 4.27 (s,
s, s, 2H, 4H, 2H, BH2; 1H{11B}); 100.6 MHz 13C NMR in CD2Cl2: δ13C
ꢂ
ꢂ
ꢂ
= 135,45 (C3,3 ), 122.62, 122.63, 121.49, 121.54 (C4,4 ,5,5ꢂ), 128,89,
ꢂ
ꢂ
128.92 (C6,6ꢂ ), 112.95, 113.15 (C7,7 ), 142.90, 142.94 (C8,8 ), 121.82,
121.88 (C9,9 ); 128.1 MHz 11B NMR: δ11B. −9.5 (7) and −11.4, −7.5
(7ꢂ).
δ
13C = −0.8 (SiMe3), 143.3 (C3), 106.4 (C4), 133.8 (C5); 49.7 MHz
29Si NMR: δ29Si = 14.2; 50.4 MHz 15N NMR: δ15N = −159.5 (N1),
−62.0 (N2)] and 1-trimethylsilyl-indazole 5 [in C6D6; 250 MHz
1H NMR: δ1H = 0.42 (s, 9H, SiMe3), 8.17 (s, 1H), 7.55, 7.42,
7.17, 7.02 (all m, 4H,); 62.9 MHz 13C NMR: δ13C = −0.1 (SiMe3),
138.1 (C3), 121.3, 120.9 (C4,5), 126.2 (C6), 111.5 (C7), 145.7 (C8),
126.5 (C9); 49.7 MHz 29Si NMR: δ29Si = 14.4; 50.7 MHz 15N NMR:
Reaction of 1-Trimethylsilyl-indazole 5 with 1,2-Bis
(tetramethylene)diborane(6) 6
A solution of 5 (4.81.g, 25 mmol) in hexane (30 ml) was cooled to
−78 ◦C and the diborane 6 1.70 g, 12.5 mmol) was added in one
portion. The mixture was warmed to ambient temperature and
heated at reflux for 10 h, until 11B and 29Si NMR spectra indicated
thatthereactionwascomplete. Volatilematerialswereremovedin
a vacuum, and the indazabole 8 was left as a colorless solid which
was recrystallized from hexane (4.0 g, 87%, m.p. 121–123 ◦C).
Elemental analysis (% found/calcd): C 71.2/71.8, H 7.0/7.1; 400 MHz
1H NMR (CD2Cl2): δ1H = 1.94–2.09 (m, 8H, BCH2, CH2), 7.99 (s, 2H,
H3), 7.95 (d, 2H), 7.4, 7.18, 6.85 (m, m, m 6H); 100.6 MHz 13C
NMR (CD2Cl2): δ13C = 24.6 (broad, BCH2), 29.4 (CH2), 134.5 (C3),
122.1, 121.7 (C4,5), 128.7 (C6), 114.5 (C7), 143.3 (C8), 122.6 (C9);
128.1 MHz 11B NMR (CD2Cl2): δ11B = 3.1; 50.4 MHz 15N NMR
(CD2Cl2): δ15N = −169.5 (N1), −151.0 (N2).
δ
15N = −186.0 (N1), −51.0 (N2)] were prepared via the reaction of
the azoles with (Me3Si)2NH and purified by distillation (yield
>90%). 1,2-Bis(tetramethylene)diborane(6) 6 was prepared as
described[18a,25] and purified by distillation [in C6D6; 400 MHz 1H
NMR: δ1H = 0.74 (m, 8H, BCH2), 1.54 (m, 8H, CH2); 100.6 MHz
13C NMR: δ13C = 16.9 (broad, BCH2), 26.9 (CH2); 128.1 MHz
11B NMR: δ11B = 28.2]. 1H, 11B, 13C, 15N and 29Si NMR spectra
were measured in 5 mm o.d. tubes at 23 ◦C if not mentioned
otherwise, using Bruker ARX 250, DRX 500 and Varian Inova 400
NMR spectrometers, equipped with multinuclear probe heads and
variable temperature units. The refocused INEPT pulse sequence
with and without 1H decoupling[26] served for measuring 29Si and
15N NMR signals. Chemical shifts are given relative to Me4Si [δ1H
(C6D5H) = 7.15, (CHDCl2) = 5.33; δ13C (C6D6) = 128.0, (CD2Cl2) =
53.4; δ29Si = 0 for ꢀ(29Si) = 19.867184 MHz], external BF3 –OEt2
[δ11B = 0 for ꢀ(11B) = 32.083971 MHz] and neat MeNO2 [δ15N
= 0 with ꢀ(15N) = 10.136767 MHz]. Chemical shifts are given as
X-Ray Structural Analysis of 8
The X-ray crystal structural analyses of 8 were carried out
(i) for a single crystal (selected in perfluorinated oil[28] at room
temperature) at 133(2) K using a STOE IPDS II system equipped
with an Oxford Cryostream low-temperature unit, and (ii) with
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 398–401

Indazaboles – synthesis and molecular structure
another single crystal, sealed in a Lindemann capillary, at 293(2) K
using a STOE IPDS I system; wavelength: λ = 0.71073 Å. Formula
weight: 368.09. Crystal system: monoclinic. Space group: P2(1)/n.
Unit cell dimensions [133(2) K]: a = 10.2058(8) Å, b = 10.9443(9)
Å, c = 17.5422(16) Å, α = 90.00◦, β = 101.036(7)◦, γ = 90.00◦.
Volume V = 1923.1(3) Å3. Z = 4. Absorption coefficient
µ = 0.075. F(000): 784. Crystal size: 0.40 × 0.39 × 0.30 mm3.
Theta range for data collection: 1.36–26.04◦. Index ranges:
−12 ≤ h ≤ 12, −13 ≤ k ≤ 13, −21 < l ≤ 12. Reflections
collected: 3568. Independent reflections: 2511 [R(int) = 0.0881].
Completeness to θ = 25.7◦: 97%. Data/restraints/parameters:
3568/0/253. Goodness-of-fit on F2: 1.040. Final R indices [I >
2σ(I)] 2511: R1 = 0.0541, wR2 = 0.1089. R indices (all data):
R1 = 0.0881, wR2 = 0.1186. Largest difference peak and hole:
0.259 and −0.150 e Å−3. Structure solution and refinement were
accomplished using SIR97,[29] SHELXL-97[30] and WinGX.[31] The
data have been deposited at the Cambridge Crystallographic Data
Centre as supplementary publications nos CCDC 757878 [8, 293(2)
K] and 757879 [8, 133(2) K]. These data can be obtained free
of charge from the Cambridge Crystallographic Data Centre, via
www.ccdc.cam.ac.uk/data request/cif).
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Supporting information
Supporting information may be found in the online version of this
article.
Acknowledgments
We thank the Deutsche Forschungsgemeinschaft for support, and
K. S. thanks HEC (Pakistan) for a fellowship.
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