Arsa-diazonium salts with an arsenic-nitrogen triple bond

Article abstract of DOI:10.1002/anie.201302725

Arsenic triply bound: Arsa-diazonium salts containing a cation with an arsenic-nitrogen triple bond ([R-N≡As]⁺), which is stable at room temperature, are isolated and fully characterized for the first time. Analogous to the diazonium salts, ars

Full text of DOI:10.1002/anie.201302725

Angewandte
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Communications
As–N Triple Bond
Arsenic triply bound: Arsa-diazonium
salts containing a cation with an arsenic–
nitrogen triple bond ([R-NꢀAs]+), which
is stable at room temperature, are iso-
lated and fully characterized for the first
time. Analogous to the diazonium salts,
arsa-diazonium salts can be used in [3+2]
dipolar cycloaddition reactions with
azides to prepare arsenic analogues
(RAsN4) of pentazoles (RN5).
M. Kuprat, A. Schulz,*
A. Villinger
&&&& — &&&&
Arsa-Diazonium Salts With an Arsenic–
Nitrogen Triple Bond
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Angewandte
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DOI: 10.1002/anie.201302725
As–N Triple Bond
Arsa-Diazonium Salts With an Arsenic–Nitrogen Triple Bond**
Marcus Kuprat, Axel Schulz,* and Alexander Villinger
Dedicated to Prof. Werner Uhl on the occasion of his 60th birthday
As early as 1858 Peter Griess[1] described the preparation of
+
ꢀ
a salt bearing a diazonium ion, [R-N N] , which soon led to
the discovery of compounds called azo dyes, compounds that
=
contain two aromatic fragments connected by a N N bond.
Diazonium salts, the important intermediates in the diazo-
tation/azo coupling reaction, triggered the rise of the syn-
thetic dyestuffs industry during the nineteenth century,[2] but
the impact and many-sided importance of this discovery in
terms of scientific, medical, ecological, and technical rele-
vance is still huge.[3] Besides structural simplicity, diazonium
ions exhibit high electrophilicity and thermodynamic insta-
bility with respect to loss of N2 because the presence of
a nitrogen–nitrogen triple bond makes the formation of
molecular dinitrogen favored. Triple bonds between nitrogen
Scheme 1. Synthesis of room-temperature stable arsa-diazonium salts
3.
and heavier main-group elements are still scarce,[4,5] only
[6–8]
ꢀ
ꢀ
a few examples of the type R-E N-R (E = In, Ga),
[R-N
+[11]
P]+,[9,10] and [S N]
could be isolated and structurally
ꢀ
characterized. Like the discovery of diazonium salts, the first
isolation of a salt containing a phospha-diazonium ion [Mes*-
+
ꢀ
N P] in 1988 by Niecke and co-workers represented a land-
mark in the systematic development of phosphorus chemis-
Scheme 2. In the presence of an azide species, a [3+2] cycloaddition
reaction occurs with 3 leading either to tetrazarsolium salt (5) or
tetrazarsole (4) stabilized as GaCl3 adduct.[24]
try.[9] In contrast to the isolobal phospha-alkynes,[12,13] it was
+
ꢀ
shown that the P atom in [Mes*-N P] has Lewis-acceptor
properties,[14–17] which was illustrated, for example, by the
formation of h6-arene complexes[18,19] or tetrazaphosphole in
the reaction with Me3Si-N3.[20]
+
À
ꢀ
affording room-temperature stable [Mes*-N As] [GaCl4]
(3[GaCl4], Mes* = 2,4,6-tri-tert-butylphenyl).
First evidence for the existence of an arsa-diazonium ion
(3) (also named arsenoazonium) was provided by high-
The yellowish crystalline dimer 1 is readily dissolved in
CH2Cl2 or toluene, with the initially yellowish solution slowly
turning red, the color of the monomeric species 2 at ambient
temperatures (Scheme 1).[24] 1H NMR spectroscopy data
revealed the existence of a monomer/dimer equilibrium.[22]
Addition of one equivalent of GaCl3 (with respect to the
concentration of the monomeric species) at À808C led to
a red solution of arsa-diazonium ions (3) as demonstrated by
1H, 13C, and 14N NMR spectroscopy studies (Figure 1). After
resolution mass spectrometry in 1994[21] indicating the pres-
+
ꢀ
ence of a [Me-N As] ion in the gas phase. The formation of
+
ꢀ
the [Mes*-N As] ion in CH2Cl2 solution was also assumed as
a transient species in the reaction of iminoarsane Mes*-N
ꢀ
As-Cl (2) with GaCl3 in the presence of Me3Si-N3 yielding the
tetrazarsole species 4 (Schemes 1 and 2).[22] We have now
studied the equilibrium between cyclic diarsadiazane 1[23] and
its monomer, the iminoarsane 2, exclusively in the presence of
the Lewis acid GaCl3 at low temperatures (À80 to À608C)
ꢀ
removing the solvent under reduced pressure, [Mes*-N
As]+[GaCl4]À (3[GaCl4]) was obtained as an orange crystal-
line solid in 90% yield, however, depending on the solvent, as
the CH2Cl2 (3[GaCl4]·CH2Cl2) or toluene solvate (3-
[GaCl4]·toluene, two different polymorphs, see Supporting
Information Table S1). A solvate-free salt is obtained when
two equivalents of GaCl3 are utilized affording the digallate
[*] M. Kuprat, Prof. Dr. A. Schulz, Dr. A. Villinger
Universitꢀt Rostock, Institut fꢁr Chemie
Albert-Einstein-Strasse 3a, 18059 Rostock (Germany)
and
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢀt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: axel.schulz@uni-rostock.de
+
À
[24]
ꢀ
salt [Mes*-N As] [Ga2Cl7] (3[Ga2Cl7]) in 85% yield.
X-ray diffraction data could be obtained for all three
species. The 14N NMR spectrum shows a broad singlet at d =
Homepage: http://www.chemie.uni-rostock.de/ac/schulz
+
ꢀ
À83 ppm, shifted downfield in comparison with [Mes*-N P]
[**] Financial support by the DFG is gratefully acknowledged (SCHU
1170/8-1).
+
ꢀ
(d = À124 ppm) and [Ph-N N] (d = À156 ppm, cf. d =
À71 ppm for solvated N2).[25] All the arsa-diazonium salts
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302725.
described herein are very difficult to isolate since they are
2
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À
with a very short N As (1.609 ꢀ) bond length in accord with
À
the experimentally observed data (cf. dexp(N As) = 1.591–
1.613 ꢀ). Natural bond orbital (NBO) analysis data[32] give
a large negative partial charge for the N atom (À0.86 e),
whereas the arsenic atom carries a large positive charge
(+ 1.36 e). The “Natural Resonance Theory” (NRT)[33] was
used to determine the major Lewis formulae and their
percentage weightings in the resonance scheme. The leading
resonance structures display a triple bond between the N and
As atom and a lone pair localized at the As center (Lewis
representations I and II in Figure 3). Interaction between the
Figure 1. Left: ORTEP drawing of the molecular structure of the [Mes*-
N As]+ cation in 3[Ga2Cl7] in the crystal. Thermal ellipsoids set at
À
As N p bond and the aromatic phenyl ring is described by
ꢀ
ꢀ
À
50% probability at 173 K. Right: Space-filling model of the [Mes*-N
Lewis formulae IVand V. The NRT bond order for the As N
As]+ ion. As N bond length [ꢂ]: 1.612(2) in 3[Ga2Cl7], 1.591(4) and
À
bond is 2.82 in accord with values expected for a triple bond
(cf. 2.85 in [Ph-N P] and 2.89 in [Ph-N N] ). The strong
+
+
1.613(3) in 3[GaCl4]·toluene, 1.611(3) in 3[GaCl4]·CH2Cl2.
ꢀ
ꢀ
extremely air and moisture sensitive. Even at low
temperatures slow decomposition cannot be
avoided. The orange color of 3 vanishes rapidly
when traces of H2O are present. Fast decom-
position starts at temperatures above 808C.
Single-crystal diffraction studies reveal an
almost linear arrangement of the C-N-As
moiety (angles between 175.4–177.88) with
À
a very short N As bond length between 1.591–
1.613 ꢀ (Figure 1) which nicely agrees with the
sum of the covalent radii for an arsenic–nitrogen
[26]
ꢀ
triple bond (ꢀrcov(N As) = 1.60 ꢀ).
In con-
À
trast, the As C triple bond with 1.657(7) ꢀ in
is considerably longer
which can be attributed to a smaller amount of
Figure 2. Left: Molecular structure of 3[Ga2Cl7]; right: Molecular structure of 3-
[GaCl4]·toluene (hydrogen atoms omitted for clarity). Selected interatomic separa-
tions [ꢂ] and angles [8]: 3[GaCl4]·toluene: As-N 1.613(3), N-C1 1.389(4), As···C20
3.351(4), As···C21 3.252(4), As···C22 3.163(6), As···C23 3.151(6), As···C24 3.208(5),
As···C25 3.307(4); As-N-C1 175.4(3). 3[Ga2Cl7]: As-N 1.612(2), N-C1 1.372(3), As···Cl1
3.1786(8), As···Cl7 3.3437(7), As···Cl5 3.4565(6), As···Cl5’ 3.3385(7); As-N-C1 177.8(2).
[27]
ꢀ
isolobal Mes*-C As
À
bond polarization. In the literature, As N bond
ꢀ
lengths between 1.707–1.714 ꢀ (cf. ꢀrcov(N
À
As) = 1.74 ꢀ and ꢀrcov(N As) = 1.92 ꢀ) are
reported for the only three known compounds
[28–30]
À
À
with an As N double bond.
All these structural param-
eters indicate the presence of a classical As N triple bond in
3. In these compounds the As N triple bond is kinetically
polarization of the As N bond towards nitrogen, which is
responsible for the large positive charge at the As center,
becomes clearly visible in the computed NBOs as well as in
the electron localization function (ELF). In agreement with
the structural and NRT/NBO data, the ELF at 0.82 exhibits
a torus in the proximity of the N atom which is a good
indicator for a classical (polarized) triple bond (Figure 4).[34,35]
The reactivity of the arsa-diazonium ion was tested in
[3+2] cycloadditions with Me3Si-N3 and Ph3C-N3.[24] Indeed,
like diazonium[36] and phospha-diazonium salts,[20] arsa-diazo-
nium ions form a five-membered heterocycle (Scheme 2),
À
À
protected by the Mes* substituent, however, not completely
+
ꢀ
as can be seen from the space-filling model of [Mes*-N As]
in Figure 1 (right). Thus significant van der Waals interactions
between 3 and the anion or solvent molecules are possible. A
closer inspection of the interionic interactions as well as to the
solvent molecules (3[GaCl4]·CH2Cl2 and 3[GaCl4]·toluene)
display a number of distances within the sum of the van der
Waals radii (ꢀrvdW(As···Cl) = 3.60 ꢀ, ꢀrvdW(As···C) =
3.55 ꢀ)[31] indicating stabilizing van der Waals interactions
(Figure 2). For all the species short contacts are found
between the As atom of the cation and Cl atoms of the
anion (d(As···Clanion) = 3.123–3.457 ꢀ). In addition all six
carbon atoms of the toluene molecule in 3[GaCl4]·toluene
coordinate to the As atom (d(C···As) = 3.163–3.351 ꢀ).
Analogous h6-toluene complexation was also reported for
the isotypical phosphorus compound.[18]
Quantum chemical methods[24] were applied to study the
+
ꢀ
model compound [Ph-N As] (which has a phenyl group in
place of the Mes* substituent). At the M06-2X/aug-cc-pvTZ
level of theory, [Ph-N As] is linear along the C-N-As unit
Figure 3. Lewis representations (>1%) of [Ph-N As]+ along with their
percentage weightings in the resonance scheme.
ꢀ
+
ꢀ
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N3) = 1.278(3) (cf. 4: 1.286(2) ꢀ). Although the release of
molecular nitrogen seems to be predisposed, the AsN4
heterocycle is stable at ambient temperatures and no
significant release of N2 is observed. Clearly, the “naked”
AsN4 ring is kinetically protected sandwiched between the
large aryl groups (Figure 5). Nevertheless, a clean trans-
formation of 5 into 4 along with the elimination of Ph3C-Cl
occurs in solution even at low temperatures within hours
(Scheme 2).
In summary, more than 150 years after the discovery of
a stable diazonium salt, we demonstrate that the heavier
arsenic analogue, the arsa-diazonium salt, can be isolated as
À
well. Arsa-diazonium ions feature a strongly polarized As N
triple bond and are highly reactive, strong electron-deficient
species. Like diazonium salts, they can be used to act as
dipolarophiles in [3+2] cycloadditions, for example, with
azide to form five-membered rings, such as neutral tetrazar-
soles or cationic tetrazarsolium ions.
Received: April 2, 2013
Published online: && &&, &&&&
Keywords: arsenic · bond analysis · heterocycles · nitrogen ·
.
triple bond
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À
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