Pseudo-One-Coordinate Tetrylenium Salts Bearing a Bulky Carbazolyl Substituent

Article abstract of DOI:10.1002/chem.201806346

Syntheses of a bulky carbazole-based substituent, the parent 1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butyl-carbazole (R?H) and a series of chlorotetrylenes, RECl (E=Ge, Sn, Pb), are described. Detailed analysis of the properties of the carbazole-based substituent revealed that it features flexible high bulkiness and electronic non-innocence, which may make it suitable for many applications in both main-group and transition-metal chemistry. To further showcase the employability of the novel substituent, the chlorotetrylenes were subjected to halide abstraction reactions, affording the corresponding tetrylenium salts [RE][Al(OC₄F₉)₄] (E=Ge, Sn, Pb) as strongly coloured compounds.

Full text of DOI:10.1002/chem.201806346

A Journal of
Accepted Article
Title: Pseudo-one-coordinate Tetrylenium Salts Bearing a Bulky
Carbazolyl Substituent
Authors: Alexander Hinz
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Pseudo-one-coordinate Tetrylenium Salts Bearing a Bulky
Carbazolyl Substituent
Alexander Hinz*[a]
Abstract: A new bulky carbazole-based substituent was devised. The
synthesis of the parent 1,8-bis(3,5-ditertbutylphenyl)-3,6-di-tertbutyl-
carbazole (R–H) and a series of chlorotetrylenes, RECl (E = Ge, Sn,
Pb), is described. Detailed analysis of the properties of this substituent
revealed that it features flexible high bulkiness and electronic non-
innocence which may make it suitable for many applications in both
main group and transition metal chemistry. To further showcase the
employability of the novel substituent, the chlorotetrylenes were
subjected to halide abstraction reactions, affording the corresponding
affording five- and six-membered heterocyclic tetrylenium cations
reported by the groups of Dias,[12] Müller (B),[13] Driess (C, Si),[14]
Power (C, Ge)[15] and Fulton (C, Sn, Pb).[16] Synthetic strategies
to generate these cationic species involve halide abstraction,[
methanide abstraction with boranes or backbone protonation with
strong acids. The respective products are reactive which
17]
manifests for instance in the decomposition of [BPh
4
]– counterions
with B–C bond cleavage.
4 9 4
tetrylenium salts [RE][Al(OC F ) ] (E = Ge, Sn, Pb) as strongly
coloured compounds.
Owing to their high reactivity, low-coordinate main group species
have become of general interest, but they remain challenging
synthetic targets due to their sensitivity.[1] Such low-coordinate
species have to be kinetically stabilised by bulky substituents.
However, established monodentate monoanionic substituents
such as terphenyls, supermesityl or aryl(silyl)amides are redox
inactive. To enable control of both electronic flexibility and the
steric bulk of the organic framework,[2] we envisaged carbazolyl
substituents to be suitable candidates. While carbazoles find
freqent application in optoelectronics,[3,4] they are a class of
substituents which has not yet been investigated in depth for the
stabilisation of main group species, even though there are a few
reports on bulky carbazolyl compounds of the main group
elements, such as (1,8-diphenyl-3,6-dimethyl-carbazolyl)-
Scheme 1. Selected cationic tetrylenes (Dip = 2,6-diisopropylphenyl, Tip =
2,4,6-triisopropylphenyl, tol = toluene, NHC = N-heterocyclic carbene).
dimethylalane,[5]
potassium
(1,8-mesityl-3,6-dimethyl-
carbazolide), (1,8-mesityl-3,6-dimethyl-carbazolyl)-dichlorogal-
[
6]
lane
and 1,8-dimesityl-3,6-di-tertbutyl-carbazolyl-magnesium
iodide.[ These carbazolyl compounds feature tert-butyl, phenyl,
mesityl or triisopropylphenyl groups in 1 and 8 position.[5–7]
7]
A different approach to the synthesis of dicoordinate group 14
cations was chosen by the groups of Aldridge and Wesemann,
who employed NHCs as secondary neutral donor to stabilise their
The chemistry of cationic low coordinate group 14 was pioneered
by Jutzi who reported on the complete series of [Cp*E] (E = Si,
Ge, Sn, Pb; A, Scheme 1).[8–10] To date, several other derivatives
are known, including various substitution patterns on the Cp
moiety.[11] Formally, there is a single substituent on the group 14
element, but the bonding equally involves all C atoms of the Cp
ring.[9,11]
Another method in the generation of low-coordinate group 14
cations has been pursued by utilising bidentate N-donor
substituents such as aminotroponiminates or ß-diketiminates,
+
+
[18,19]
+
respective [R–Ge] (D) and [R–Sn] cations (E).
The former
cation was studied with emphasis on the reactivity of the cation
towards small molecules while the latter was investigated with
focus on its formation in the dehydrogenation of stannanes. For
lead, Power has shown that a donor as weak as toluene is
+
[20]
sufficient to stabilise [TerPb] (F). Following this line of thought,
Jones utilised his versatile aryl(silyl)amido substituents to
incorporate the stabilisation of germylenium and stannylenium
cations, where one of the flanking phenyl groups of the Ar* moiety
is coordinated to the group 14 element (G).[ Both examples
were corroborated by reactivity studies with pyridine-derived
bases.
21]
[
a]
Dr. Alexander Hinz
We have developed a carbazole-based bulky substituent and now
demonstrate its application in the synthesis of a series of salts
Institute of Inorganic Chemistry (AOC)
Karlsruhe Institute of Technology (KIT)
Engesserstr. 15, Gebäude 30.45, 76131 Karlsruhe
E-mail: alexander.hinz@kit.edu
incorporating
pseudo-mono-coordinate
germylenium,
stannylenium and plumbylenium cations.
Supporting information for this article is given via a link at the end of
the document.
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Our past experience with bulky substituents led us to favor 3,5-di-
tertbutyl-phenyl groups as they generally facilitate high solubility
of the compounds as well as steric shielding and electron-rich
arenes for Menshutkin interactions, which could help to stabilise
low-coordinate species. The carbazole framework was chosen as
it enforces a favorable geometry of the substituent with the arenes
in a suitable position to interact with the metal atoms.
While the chlorogermylene RGeCl is highly soluble in organic
solvents, the heavier homologues RSnCl and RPbCl are
suprisingly poorly soluble, particularly in aliphatics such as
hexane. Crystalline products could be obtained in all three cases,
but only for RGeCl the X-ray diffraction experiment furnished a
good data set (Figure 1), while for RSnCl and RPbCl the data
were only sufficient to confirm connectivity (see SI, Figures S11
and S14).
t
Scheme 2. Synthesis of carbazole 4 (R–H). Conditions: a) BuCl, ZnCl
2
, MeNO
2
;
2 2 3 4
b) Br , HOAc; c) Pd catalyst, ArB(OH) , K PO , dioxane/water.
Scheme 3. Synthesis of the tetrylenium ions [7E]+ (E = Ge, Sn, Pb; wca =
[
6 5 4 4 9 4
B(C F ) ] or [Al(OC F ) ]).
Figure 1. Molecular structures of R–K (5, a) and R–GeCl (6Ge, b).
From these, halide abstraction envisaged to give
impression of the capabilities of this class of substituents. The use
of the [B(C
]– anion had great success in Cp chemistry of
group 14. Consequently, we initially attempted halide abstraction
][25] in toluene,
from the tetrylenes RECl with [H(SiEt ][B(C
as this seemed to be the most straightforward and convenient
method because no filtrations are necessary. All reactions
proceed readily and produced strongly coloured solutions (red,
magenta and blue, respectively). Unfortunately it was not possible
a first
6 5 4
F )
The synthesis of 1,8-bis(3,5-ditertbutylphenyl)-3,6-di-tertbutyl-
carbazole (R–H, Scheme 2) could be achieved by the modification
of literature procedures. Starting from carbazole (1), Friedel-
Crafts alkylation afforded 3,6-di-tertbutyl-carbazole (2),[22] which
subsequently was brominated to yield 1,8-dibromo-3,5-di-
3
)
2
6 5 4
F )
[
23]
tertbutyl-carbazole (3). Compound 3 was then subjected to a
Suzuki-type cross-coupling reaction with 3,5-ditertbutylphenyl
boronic acid using a highly active catalyst developed by the group
of Buchwald.[24] Overall, this reaction can be readily carried out
and affords R–H (4) in good yields on multigram scale (three steps,
to crystallise the putatitive products, [R–E][B(C
[7E][B(C ]). Therefore, we eventually turned our attention to
employing perfluorinated alkoxyaluminates as counterions. Since
]– counterion
there are no stable silylium salts with [Al(OC
known,[26,27] the method had to be changed and the more
conventional methathesis of Li[Al(OC with RECl in
6 5 4
F ) ]
(
6 5 4
F )
4 9 4
F )
67%).
The carbazole R–H (1) was then deprotonated with benzyl
potassium in hexane, toluene or THF. The potassium carbazolide
thus obtained is strongly luminescent and could be crystallised
from hexane as a monomeric entity without any donor solvent
present (R–K, 5, Figure 1). The potassium carbazolide is a
versatile starting material for other metal compounds, as
4 9 4
F ) ]
dichloromethane (while the reaction proceeds in fluorobenzene
as well, from that solvent no crystals could be obtained). For all
three halide abstractions, the reactions proceeded readily and
colour changes were observed, but for R–SnCl and R–PbCl the
reaction mixtures were gently heated to 40 °C to ensure
completeness of the reaction. After filtration, concentration and
layering with pentane, large crystals of the desired salts [R–
metathesis reactions with metal halides, i.e. GeCl
and PbCl (Scheme 3), afforded the corresponding
chlorogermylene (R–GeCl, 6Ge, light yellow), chlorostannylene
R–SnCl, 6Sn, yellow) and chloroplumbylene (R–PbCl, 6Pb,
orange) in yields >80%.
2 2
·dioxane, SnCl
2
E][Al(OC
4
F
9
)
4
] ([7E][Al(OC
4
+
F
9
)
4
]) were obtained. The compounds,
(
+
in particular bearing [RSn] and [RPb] , are highly sensitive and
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10.1002/chem.201806346
COMMUNICATION
decompose readily upon exposure to trace moisture or air. In low
yields, several interesting decomposition products could be
the bond critical point, the electron density and the laplacian of
the electron density assume values (averaged, [RGe] ϱ 0.0257,
2
2
2
crystallised, such as [RH][Al(OC
4
F
9
)
4
] ([8][Al(OC
4
9
F )
4
], vide infra).
∇ 0.0483; [RSn] ϱ 0.0217, ∇ 0.0485; [RPb] ϱ 0.0211, ∇ 0.0560)
The molecular structures (Figure 2) reveal the expected
shortening of the R–Ge bonds upon halide abstraction from
considerably higher than in weak interactions (benzene stacking
2
2
ϱ 0.0073, ∇ 0.0049, acetylene dimer ϱ 0.0067, ∇ 0.0047), but in
the same order of magnitude as hydrogen bonds (H O dimer ϱ
1
.9373(14) Å [comp. 1.958 Å] to 1.848(2)/1.847(2) Å [1.861 Å] in
2
+
[28]
2
2
[32]
6
Ge and [7Ge] , respectively. The same tendency was found
0.0219, ∇ 0.0396, HOAc dimer ϱ 0.0425, ∇ 0.0335).
for the heavier congeners [R–E]+, where the N–Sn
NMR spectroscopy is a possibility to gain further insight into the
electronic structure of the tetrylenium cations. For tin, the halide
abstraction reaction from 6E to [7E] causes a change in the 119Sn
NMR resonance from +46.2 (1/2 = 115 Hz) to –278.2 (1/2 = 245
Hz) ppm, which is more shielded than both Jones’
2.071(4)/2.076(4) Å [comp. 2.094 Å] and N–Pb 2.161(7)/2.182(7)
+
Å [comp. 2.203 Å] bond lengths are considerably shorter than the
computed values for the chlorotetrylenes, 2.182 and 2.295 Å,
respectively.[29] The Wiberg bond indices of the N–E bond (6Ge
0
.4891, 6Sn 0.3810, 6Pb 0.3562) increase upon chloride
[ArN(SiMe
3
)Sn]+ (G, 46 ppm, 1/2 = 195 Hz) and Wesemann’s
abstraction ([7Ge]+ 0.6262, [7Sn]+ 0.4869, [7Pb]+ 0.4614), but
nevertheless are small which indicates a strong electrostatic
component of the interaction. Further NBO analysis reveals that
the additional charge is largely delocalised over the substituent,
as the partial charge on the tetrel increases only by 0.15-0.18 e
cations (E, 880/920 ppm), as well as cyclic examples with [1+1]
coordination by Fulton (C, 197 ppm)[ or Inoue (355 ppm).
[19]
16]
[33]
However, cationic compounds reported by the groups of Jambor
and Jurkschat comprising neutral two- or three-dentate N-donor
ligands on [SnCl] moieties exhibit similarly upfield-shifted 119Sn
+
+
[34–36]
upon chloride abstraction (6Ge 1.02, [7Ge] 1.20, 6Sn 1.19,
NMR resonances in the range of –330.4 to –435 ppm.
+
+
[
7Sn] 1.35, 6Pb 1.23, [7Pb] 1.38 e).
A similar observation can be made for the corresponding lead
species, where 6Pb and [7Pb]+ give relatively high-field 207Pb
NMR resonances at +2398 (1/2 = 280 Hz) to +2027 (1/2 = 590
+
Hz), respectively, while Power’s cation [TerPb(tol)] was observed
at 8974 ppm and at 4764 ppm with two pyridine donors,
[
TerPb(py)
2
]+. The high-field resonance in both 119Sn and 207Pb
NMR spectra highlights the electronic shielding provided by the
carbazolyl substituent, arising from donation from both the
nitrogen atom and the two arenes in close proximity.
Figure 2. Molecular structures of the cations of [7Ge][Al(OC
7Sn][Al(OC ] (b) and [7Pb][Al(OC ] (c, d).
4 9 4
F ) ] (a),
[
4
F )
9 4
4 9 4
F )
Figure 3. Top: HOMO (a) and LUMO (b) of [R–Sn]+ (isovalue 0.04). Bottom:
Steric maps of R–H (a), R–GeCl (b) and [R–Ge]+ (c), viewed along the N–H or
N–Ge axis, respectively.
The geometry of the carbazolyl substituent enables Menshutkin-
type interactions (interactions between main group elements and
arenes, observed first by Smith et al. but systematically studied
by Menshutkin in the beginning of the 20th century).[30,31] In 6Ge
only one arene is interacting with the Ge atom, while the other
arene is bent away, revealing the influence of the Ge lone pair.
All three tetrylenium salts are strongly coloured compounds which
is unusual as both the examples reported by Power and Jones
were observed as orange or yellow compounds.[
20,21]
For [7E] the
+
+
absorption maximum shifts from 472 (red-orange, [7Ge] ) to 517
+
+
(
magenta, [7Sn] ) and 616 nm (blue, [7Pb] ), respectively. The
+
Contrasting this, in all cations [7E] both arenes are essentially
colour of the compounds is caused by a high lying, carbazole-
based HOMO, which causes relatively small HOMO–LUMO gaps
with a transition into the LUMO which is dominated by a tetrel-
based p orbital (Figure 3). The high lying HOMO of the carbazole
coplanar and donating equally towards the tetrel center. This is
also indicated by QTAIM as there is a bonding path between the
ipso-C atoms of the arene subsituents and the heavy tetrels. At
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COMMUNICATION
substituent also serves as an explanation for the sensitivity of the
tetrylenium cations ([7E] ). Especially for the heavier elements of
the series, decomposition upon contact with air is rapid and leads
4 9 4 4 9 4
to the formation of [RH][Al(OC F ) ] ([8][Al(OC F ) ]).
Facility (KNMF), a Helmholtz Research Infrastructure at Karlsruhe
Institute of Technology (KIT) and Prof. Dieter Fenske is gratefully
acknowledged for help with XRD. Prof. Ingo Krossing and MSc
Arthur Martens are acknowledged for valuable advice. I thank
MSc Wolfram Feuerstein for discussions on catalysis and his gift
of catalysts. I acknowledge support by the state of Baden-
Württemberg through bwHPC and the German Research
Foundation (DFG) through grant no. INST 40/467-1 FUGG
+
The oxidation of R–H (4) could also be carried out by treatment
with suitable oxidants such as nitrosyl salts without structural
change in the backbone which emphasises the redox non-
innocence of the carbazolyl substituent based on a reversible
oxidation wave in cyclic voltammetry experiments observed at a
potential of +0.69 V vs Fc/Fc+ in dichloromethane. The N–H
stretch vibration shifts from 3467 to 3371 cm–1 upon oxidation
which is consistent with the removal of bonding electron density
from the carbazole scaffold and compares well to the computed
(
JUSTUS cluster).
Keywords: Steric hindrance • Cations • Germanium • Tin • Lead
[
1]
P. P. Power, Nature 2010, 463, 171–177.
–
1
+
energies of 3489 and 3385 cm for R–H and [R–H] , respectively
[2]
[3]
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[
28]
(
scaling factor 0.95). The EPR spectrum of [8][Al(OC
4 9 4
F ) ] at
[
[
[
4]
5]
6]
295 K shows a resonance centered at g = 2.00505 as expected
for an organic radical, that displays hyperfine splitting to a pseudo-
quartet due to large coupling constants with one nitrogen and one
hydrogen atom (Aiso(N) = 35.36, Aiso(N) = 40.19 MHz, see Figures
S29, S30 and Table S1).[37]
To evaluate the steric bulk provided by the carbazole R, buried
volume[38,39] steric maps[40] and cone angles can be considered.[41]
95–101.
N. D. Coombs, A. Stasch, A. Cowley, A. L. Thompson, S. Aldridge,
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[8]
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[
9]
P. Jutzi, F. Kohl, P. Hofmann, C. Krüger, Y. Tsay, Chem. Ber. 1980,
The comparison of the molecular properties of RH, RGeCl and
113, 757–769.
[
RGe]+ clearly shows that the bulk provided by R is strongly
[10]
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[
[
11]
12]
dependent on the metal centre. The buried volume (for a sphere
with r = 3.5 Å) for RH (d = 1.937 Å) amounts to 74.8%, but drops
to only 61.6% for RGeCl. With these values, the carbazolyl
substituent provides comparable bulk to Jones’ amides (62.4%
[13]
14]
[15]
16]
[17]
18]
3
945–3950.
M. Driess, S. Yao, M. Brym, C. Van Wüllen, Angew. Chem. Int. Ed.
006, 45, 6730–6733.
[
2
Ar*(Me
3
Si)N)[42] and modified variations thereof (up to 77.7%),[43]
M. Stender, A. D. Phillips, P. P. Power, Inorg. Chem. 2001, 40,
5314–5315.
M. J. Taylor, A. J. Saunders, M. P. Coles, J. R. Fulton,
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A. Rit, R. Tirfoin, S. Aldridge, Angew. Chem. Int. Ed. 2016, 55, 378–
382.
while the bulky TipTer only gives 48.9%. Even for the recently
[
Mes*
reported
TerSnCl, only a buried volume of 51.4% was
found.[
44,45]
+
Again, there is an increased value found for [RGe]
compared to RGeCl as halide abstraction enables stronger π
[
interactions. A similar picture can be derived from the cone angles
+
[19]
[20]
C. P. Sindlinger, F. S. W. Aicher, L. Wesemann, Inorg. Chem. 2017,
of RH (239.2°), RGeCl (220.1°) and [RGe] (251.4°). The steric
56, 548–560.
maps and solid state structures allow to identify two factors for the
high flexibility of the carbazolide: twisting and bending of the
arene substituents vs the carbazole scaffold. The former certainly
has a bigger impact on the variability of steric protection, as
dihedral angles between arene and carbazole vary strongly
between 43.6° and 88.7° (illustrated by the steric maps, Figure 3),
while the bending at the sp2 carbon center is limited, so that the
angle varies only between 114.7° and 120.3°.
S. Hino, M. Brynda, A. D. Phillips, P. P. Power, Angew. Chem. Int.
Ed. 2004, 43, 2655–2658.
[
21]
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[
[
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In conclusion, we have begun to explore the properties of a new
carbazole-based substituent for the preparation of low-coordinate
[25]
2
[
[
26]
27]
A. Martens, P. Weis, M. C. Krummer, M. Kreuzer, A. Meierhöfer, S.
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main group species, exemplified by the simple “naked” cations
+
[
RE] . The carbazolyl substituent is both sterically encumbering
with high flexibility and redox non-innocent. In the future, we seek
to apply this substituent in the stabilisation of hitherto unknown
multiple bond species.
[28]
4 9 4
All three salts [7E][Al(OC F ) ] crystallise with two ion pairs in the
asymmetric unit. Of these, for 7Sn and 7Pb, one is disordered and
only the major contribution is listed in the text.
[
29]
All computations were carried out with the Gaussian16 program
utilising the PBE1PBE level of theory with Def2SVP basis sets for all
elements and empiricial dispersion correction by Grimme’s D3
damping function.
Acknowledgements
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The Fonds der Chemischen Industrie is gratefully acknowledged
for a Liebig Fellowship. I would like to thank Prof. Frank Breher
and Prof. Peter Roesky for their generous support. This work was
partly carried out with the support of the Karlsruhe Nano Micro
[32]
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Chemistry - A European Journal
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COMMUNICATION
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Chemistry - A European Journal
10.1002/chem.201806346
COMMUNICATION
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COMMUNICATION
A novel sterically and electronically
flexible carbazole-based substituent
was utilised in the stabilisation of
pseudo-one-coordinate germanium,
tin and lead cations.
Alexander Hinz
Page No. – Page No.
Pseudo-one-coordinate Tetrylenium
Salts Bearing a Bulky Carbazolyl
Substituent
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