Preparation of NHC Stabilized Al(III)fluorides: Fluorination of [(SIMes)AlMe3] with SF4 or Me3SnF

Article abstract of DOI:10.1002/ejic.201900921

Various reaction routes have been developed to synthesize the N-heterocyclic carbene (NHC) stabilized Al(III) fluorides [(SIMes)Al(F)₃] (2) and [(SIMes)Al(F)Me₂] (3) through fluorination of [(SIMes)AlMe₃] (1) (SIMes: 1,3-bis(2,4,6-trimethylphenyl)-imidazolidin-2-ylidene) with fluorinating agents such as SF₄, SF₆ and Me₃SnF. The reactivity of 2 was investigated towards a F/Cl exchange reaction by treating it with Me₃SiCl.

Full text of DOI:10.1002/ejic.201900921

A Journal of
Accepted Article
Title: Preparation of NHC stabilized Al(III)fluorides: Fluorination of
[(SIMes)AlMe3] with SF4 or Me3SnF
Authors: Pooja Tomar, Thomas Braun, and Erhard Kemnitz
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10.1002/ejic.201900921
European Journal of Inorganic Chemistry
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Preparation of NHC stabilized Al(III)fluorides: Fluorination of
[(SIMes)AlMe3] with SF4 or Me3SnF
Pooja Tomar[a], Thomas Braun*,[a], and Erhard Kemnitz*,[a]
Abstract: Various reaction routes have been developed to synthesize
the N-heterocyclic carbene (NHC) stabilized Al(III) fluorides
[(SIMes)Al(F)3] (2) and [(SIMes)Al(F)Me2] (3) through fluorination of
their applications have been covered very extensively in reviews
by Organoaluminium fluorides such as
Roesky.[9]
dimethylaluminium fluoride (Me2AlF)4 were established to access
aluminum fluoride clusters by Roesky and coworkers.[10] Kemnitz
et al. also showed that reactions of Al(OiPr)3 with HF in pyridine
as a solvent yield polynuclear aluminum alkoxide fluoride
compounds.[11] In a recent report Crimmin and coworkers reported
on the activation of C−F bonds of fluoroalkenes by treating them
with an Al(I) complex to give organoaluminium fluorides such as
[{HC(CMeNAr)2}Al(F)2] (Ar = 2,6-iPr2C6H3).[12]
N-heterocyclic carbenes NHCs are widely used as ligands for
stabilizing a wide range of group 13 compounds resulting in
complexes bearing unique and interesting properties.[13] Lewis
pairs with Al-based Lewis acids have been prepared for instance
by treatment of AlMe3 with aromatic NHCs.[14] Here we present,
the development of reaction routes to access NHC stabilized
aluminium fluorides by fluorination of [(SIMes)AlMe3] (1) (SIMes:
[(SIMes)AlMe3]
(1)
(SIMes:
1,3-bis(2,4,6-trimethylphenyl)-
imidazolidin-2-ylidene) with fluorinating agents such as SF4, SF6 and
Me3SnF. The reactivity of 2 was investigated towards a F/Cl exchange
reaction by treating it with Me3SiCl.
Introduction
Aluminium trifluorides and derivatives have been used as
heterogeneous
Lewis
acid
catalysts
to
produce
chlorofluorocarbons and hydrofluorocarbons.[1] β-AlF3 was shown
to catalyze intermolecular Cl/F exchange reaction, for instance to
convert the greenhouse gas trifluoromethane CHF3) into CHCl3.[2]
Nanoscopic, amorphous aluminium fluorides such as high-
surface aluminium fluoride (HS-AlF3) and aluminum chlorofluoride
(ACF, AlClxF3-x, x=0.05-0.3) are known to exhibit a Lewis acidity
comparable to that of SbF5.[1b] In the presence of silanes or
germanes they are useful compounds to mediate
hydrodefluorination or dehydrofluorination reactions of fluorinated
alkanes.[3] Nanoscopic aluminium fluorides can be synthesized by
treating alumina or AlCl3 with chlorofluorocarbons (CFCs),
hydrochlorofluorocarbons (HCFCs), or with HF sources. Case
and Nyman also documented in 1962 the decomposition of SF6
at AlCl3 at 180 – 200 °C to give within 24 h AlF3 along with sulfur
chlorides.[4] SF4 was also used as fluorinating agent to obtain AlF3
by reaction with aluminium hydroxide [Al(OH)3] at 20°C .[5]
1,3-bis(2,4,6-trimethylphenyl)-imidazolidin-2-ylidene).
fluorinating agents are SF4, SF6 and Me3SnF.
Suitable
Results and Discussion
Treatment of complex 1 with SF4 in toluene yielded the aluminium
trifluorido complex [(SIMes)Al(F)3] (2). The reaction was
monitored by NMR spectroscopy and at -30 °C a complete
fluorination as well as the generation of considerable amount of
CH3SF3 were observed (Scheme 1). In the 19F NMR spectrum a
broad resonance at δ = -164.5 ppm was found for 2 (Figure1).
This resonance is consistent with data for four-fold coordinated
aluminium fluorides such as the complex [{HC(CMeNAr)2}Al(F)2]
(Ar=2,6-iPr2C6H3).[12] A signal at δ = 49.8 ppm in the 27Al NMR
spectrum appears also in a typical range for fourfold coordinated
aluminium halide species.[15] The 19F NMR spectrum shows two
signals for CH3SF3 at δ = -50.6 ppm and 60.9 ppm, which are in
accordance to the literature.[16]
There are rare examples reported for the synthesis of molecular
AlF3 complexes bearing neutral ligands. Notably, Reid et al.
documented a 1,4,7-triazacyclononane (tacn) stabilized AlF3
complex solvated with water molecules.[6] Kolis and coworkers
described the hydrothermal synthesis of AlF3(NH3)2 by treating
AlN and NH4F at 400°C in supercritical ammonia.[7] In 1957
Ziegler reported on the generation of the first organoaluminium
fluorides.[8] The organyl groups make the compounds soluble in
various organic solvents, and hence, reactivity in solution can be
investigated. The properties of organoaluminium fluorides and
[a]
P. Tomar, Prof. Dr. T. Braun, Prof. Dr. E. Kemnitz
Department of Chemistry
Humboldt-Universität zu Berlin
Brook-Taylor-Str. 2, 12489 Berlin (Germany)
E-mail: thomas.braun@chemie.hu-berlin.de
E-mail: erhard.kemnitz@chemie.hu-berlin.de
http://www2.hu-berlin.de/chemie/braun/
http://www.kemnitzlab.de/
Scheme 1. Formation of SIMes aluminium(III)fluoride 2.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201900921
European Journal of Inorganic Chemistry
FULL PAPER
sensitive and can react to furnish HF and the corresponding urea
derivative. Therefore, it can be speculated that when complex 1
was treated with SF6 at 70 °C some decomposition occurs to yield
SIMes, which then activates SF6 to produce 2,2-
difluoroimidazolidine and the respective 2-thiocarbene. The
former reacts with adventitious water to yield HF which fluorinates
1. Note, that on adding complex 1 to the 2,2-difluoroimidazolidine
the generation of complex 3 was observed with very low
conversion.
To develop a more reliable synthesis of the fluorido compounds 2
and 3, Me3SnF was employed as fluorination agent. Note that
Me3SnF has been widely used as fluorinating agent for
fluorination of transition and main group chlorides.[9, 20] Me3SnF
was also reported to convert dimethyl alane or aluminium
hydrides into aluminium fluorides as shown for the synthesis of
Figure 1. 19F NMR (282.4MHz, [D8]Toluene, -30 °C ) spectrum showing the
formation of complex 2 and CH3SF3. #SOF2 (impurity from SF4), +SF6, *CH3SOF
The reactivity of 1 towards SF6 has also been investigated. In
general SF6 activation has been considered to be challenging, but
several interesting procedures were developed in the past
years.[17] When the aluminium complex [(SIMes)AlMe3] (1) was
treated with SF6 and the reaction mixture was heated at 70°C for
15 h , monofluorination at aluminium took place to give a complex
in low yield (30 %), the data of which are consistent with
[(SIMes)Al(F)Me2] (3). The reaction did not proceed further, even
after prolonged reaction times. When complex 1 and SF6 were
treated together at 311 nm under UV light for 24 h, the formation
of 3 and subsequently of the aluminium trifluoride complex 2 was
observed. In the 19F NMR spectrum of 3 a broad signal was found
aminoalane
difluoride
[(2,6-iPr2C6H3)N(SiMe3)Al(F)2]
and
diamidoaluminium fluoride [(2,6-iPr2C6H3)N]2(CH2)3AlF(NMe3).[21]
Reaction of complex 1 with 1.5 equivalents of Me3SnF led to the
generation of 3 with full conversion of complex 1. Tetramethyltin
was also formed and identified in 1H NMR spectrum by a signal at
δ = 0.05 ppm, in 13C NMR spectrum at δ = -9.4 ppm and in 119Sn
NMR spectrum at δ = 0.00 ppm.[22] When three equivalents of
Me3SnF were added to complex 1 a mixture of complexes 2 and
3 was observed after 2h (ratio: 0.5 : 1). Monitoring the reaction by
NMR spectroscopy revealed that 3 converted into 2 when heated
at 80 °C for 4 h. However, treatment of complex 1 with an excess
of Me3SnF for 2h gave complex 2 after heating, as evidenced by
the resonance at -164.8 ppm in the 19F NMR spectrum (Scheme
3, Figure 2).
1
at δ = -169.9 ppm. The H NMR reveals a doublet at δ = -1.09
3
ppm with a coupling constant of JH-F = 3.2 Hz for the methyl
groups attached to the Al center. The coupling constant is in the
expected
range
of
3JH-F
values
reported
for
organofluoroaluminates.[18]
Along with complex 3 the evolution of methane was also observed
as indicated by 1H NMR spectroscopy. This suggests the
presence of HF in the reaction mixture, which might react with 1
to yield CH4. When the activation of SF6 was performed in the
presence of CsF, no reaction was observed, presumably due to
trapping of HF by the base, which further supports a fluorination
of 1 by HF. However, treatment of complex 3 with PVP · HF as an
HF source gave the protonated SIMes and [AlF2Me2]-[19], ,whereas
a reaction of 1 with PVP · HF also generates the protonated
carbene and [AlF4]- (Scheme 2). The same products were formed
when the reaction mixture of 1 and 3 were kept for 2 days at room
temperature.
Scheme 3. Fluorination of complex 1 with Me3SnF.
Scheme 2. Addition of HF to complex 3.
Note that it was reported in the literature that SIMes and SF6 can
react at 80 °C within 48 h to give a 2,2-difluoroimidazolidine
derivative in very low yield.[17d] The reaction can be promoted by
photolysis. 2,2-Difluoroimidazolidine C3H4N2(Mes)2F2 is water
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10.1002/ejic.201900921
European Journal of Inorganic Chemistry
FULL PAPER
observed at δ = -156.9 ppm in 19F{1H} NMR spectrum and at δ =
30.3 ppm in 29Si NMR spectrum.[28]
Scheme 5. Transformation of 2 into 4 with Me3SiCl.
Figure 2. 19F NMR spectra of the reaction of 1 with Me3SnF (3 equiv); (a) after
30 mins, (b) after 2 h, (c) after heating the mixture of complex 2 (-164.8 ppm)
and complex 3 (-169.9 ppm) at 80 °C for 4 h.
Conclusions
Herein we have explored different synthetic routes to access NHC
stabilized aluminium(III)fluorido complexes. [(SIMes)Al(F)3] (2)
was formed when [(SIMes)AlMe3] (1) was treated with SF4,
whereas a reaction with SF6 yielded [(SIMes)Al(F)Me2] (3).
Me3SnF was also employed for fluorination of complex 1 to
develop an alternative more reliable way to synthesize 2 and 3.
Complex 1 can be converted into the respective chlorido complex
4 via F/Cl exchange reaction by reaction with Me3SiCl.
For comparison, AlMe3 was also treated with Me3SnF and the
generation of AlMe2F was observed (Scheme 4). The latter exists
as a dynamic mixture of dimers, as described in the literature.[23]
The measured 1H{19F} NMR spectrum shows resonances at δ = -
0.43 ppm, -0.55 ppm, -0.63 ppm and -0.69 ppm, but not with equal
intensities as described in the literature.[24] It can be speculated
that binuclear Al compounds bearing only one fluorine atom
Al2Me5F were also present. However, the 19F NMR spectrum
reveals four resonances at δ = -143.6 ppm, -145.8 ppm, -148.9
ppm and -150.8 ppm. These values are in accordance with data
reported by Oliva et al.[25] Upon addition of SIMes to the reaction
mixture the generation of complexes 1 and 3 was observed
(Scheme 4), which supports the proposed structure of complex 3.
There is no hint for the generation of [(SIMes)Al(F)2Me].
Experimental Section
Instruments and Methods
All reactions were carried out in a glove box under argon
atmosphere or using standard Schlenk techniques under inert
conditions. The reaction with SF4 was performed on a stainless
steel vacuum line. Solvents were distilled using conventional
methods and degassed prior to use. Other reagents were bought
either from Sigma-Aldrich or abcr. The NMR spectra were
recorded at 25°C, if not stated otherwise, on a Bruker DPX 300,
a Bruker Avance III 300, or a Bruker Avance II 500 spectrometer.
The 1H NMR spectra were referenced to residual [D7]Toluene at
δ = 7.09 ppm or C6D5H at δ = 7.16. The 19F NMR spectra were
referenced to external CFCl3 at δ = 0.0 ppm. The 29Si NMR
spectrum was referenced externally to TMS at δ = 0.0 ppm.
27Al{1H} NMR spectra were referenced externally to AlCl3 in D2O
at δ = 0.00 ppm. 1,2-Difluorobenzene at δ = -138.1 ppm in the
19F NMR spectrum was used as an external standard for
quantification
1,3-Bis(2,4,6-trimethylphenyl)-imidazolidin-2-ylidene
(SIMes)
was synthesized by following the procedure described in the
literature.[29] The NHC stabilized aluminium complex 1 was
synthesised by treating saturated imidazolidine mesityl carbene
Scheme 4. Formation of complex 3 using Me3SnF as fluorinating reagent.
(SIMes:
1,3-bis(2,4,6-trimethylphenyl)-imidazolidin-2-ylidene)
with AlMe3 (2M in Toluene) as reported.[14] Trimethyltin fluoride
(Me3SnF) was synthesized by treating trimethyltin chloride
(Me3SnCl) with potassium fluoride (KF) in a water/ethanol
mixture.[30]
Caution! SF4 is a toxic and corrosive gas which easily hydrolyses
to give HF and exposure to these chemicals can cause serious
injuries.
It has been shown that metal fluorido complexes can be converted
into chlorido complexes using chlorosilanes.[26] Indeed, when
Me3SiCl was added to the SIMes stabilized aluminum(III)fluorido
complex [(SIMes)Al(F)3] (2) the chlorido complex [(SIMes)Al(Cl)3]
(4) was obtained along with Me3SiF (Scheme 5). This conversion
further confirms the identity of complex 2. A signal at δ = 104.7
ppm was found in the 27Al NMR spectrum, which is consistent with
data for 4, synthesized independently.[27] Signals for Me3SiF were
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10.1002/ejic.201900921
European Journal of Inorganic Chemistry
FULL PAPER
Formation of complex [(SIMes)Al(F)3] (2)
Method A: Fluorination with SF4:
We would like to acknowledge the CRC 1349 “Fluorine-Specific
Interactions” funded by the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation, project 387284271) as well
as the graduate school SALSA (School of Analytical Sciences
Adlershof) for financial support We thank the Solvay Fluor GmbH
for a gift of SF6.
In a PFA tube complex [(SIMes)Al(Me)3] (1) (37.8 mg , 0.10 mmol)
was dissolved in [D8]Toluene (0.3 mL). The solution was
degassed by freeze-thaw cycles followed by condensation of SF4
(1 equivalent) into it. The PFA tube was sealed and inserted into
a NMR tube. The reaction mixture was then brought to
-30 °C and analysed by NMR spectroscopy.
Conflicts of interest
Method B: Fluorination with Me3SnF:
In a Schlenk tube complex [(SIMes)Al(Me)3] (1) (37.8 mg , 0.10
mmol) was dissolved in toluene (5 mL) and an excess of Me3SnF
(6 equivalents) was added it. The solution was stirred for 2 h at
room temperature followed by filtration. The filtrate was
evaporated in vacuo to obtain complex 2. Yield: 68%
There are no conflicts to declare.
Keywords: N-heterocyclic carbenes • aluminium fluorides •
sulfur fluorides • tin fluorides
Spectroscopic data for complex 2: 1H NMR (300.1 MHz, C6D6): 
= 6.60 (s, 4H, m-ArH), 4.24 (s, 4H, CH2), 2.22 (s, 12H, o-CH3),
2.01 (s, 6H, p-CH3) ppm; 19F NMR (282.4 MHz, C6D6):  = -164.5
(bs, AlF) ppm; 27Al NMR (130.3 MHz, C6D6): 49.8 ppm.
Spectroscopic data for CH3SF3:[16] 1H NMR (300.1 MHz,
[D8]Toluene, -30 °C): 4.96 (bs, CH3SF3) ppm; 19F NMR (282.4
MHz, [D8]Toluene):  = 62.1 (dq, 2F, 2JFF = 73 Hz, 3JHF = 15.3 Hz,
CH3SF3), -50.6 (1F, tq, 2JFF = 73 Hz, 3JHF = 15.3 Hz, CH3SF3) ppm.
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Reactivity ofcomplex 2 towards Me3SiCl.
In a PFA tube complex [(SIMes)Al(F)3] (2) (39.2 mg , 0.10 mmol)
was dissolved in C6D6 (0.3 mL) and Me3SiCl (3 equivalents) was
added at room temperature. Formation of complex
[(SIMes)Al(Cl)3] (4) and trifluoromethyl silane (Me3SiF) was
observed after 1 h. The analytical data for complex 4 are in
accordance with those in the literature.[27]
Acknowledgments
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10.1002/ejic.201900921
European Journal of Inorganic Chemistry
FULL PAPER
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10.1002/ejic.201900921
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
Fluorination at Aluminium
P.Tomar, T. Braun*, E. Kemnitz*
Page No. – Page No.
Title: Preparation of NHC stabilized
Al(III)fluorides:
[(SIMes)AlMe3] with SF4 or Me3SnF
Fluorination
of
Efficient routes to access NHC stabilized Al(III)fluorides are reported. The
conversions include reactions of SF4 or Me3SnF at [(SIMes)AlMe3]. The generation
of the Al(III) fluoride [(SIMes)Al(F)3] was observed.
This article is protected by copyright. All rights reserved.