Arsenic-Mediated C?C Coupling of Cyanides Leading to Cyanido Arsazolide [AsC4N4]?

Article abstract of DOI:10.1002/chem.201703317

The chemistry of arsenic cyanides has been investigated and is found to be completely different to the chemistry of the heavier analogs antimony and bismuth as well as phosphorus. The reaction of As(CN)₃ with cyanide salts resulted in the formation of an unknown cyanido arsazolide heterocycle, which represents a structure isomer of the desired [As(CN)₄]^?. The structure, bonding, and formation of this unusual heterocycle is discussed featuring an arsenic mediated C?C coupling of cyanides. [AsC₄N₄]^? salts with different counterions such as [PPh₄]⁺, [PPN]⁺=[Ph₃P-N-PPh₃]⁺, Ag⁺, and [BMIm]⁺ are reported with [BMIm][AsC₄N₄] being a low-temperature ionic liquid (T_m=?62 °C).

Full text of DOI:10.1002/chem.201703317

DOI: 10.1002/chem.201703317
Communication
&
Heterocycles
Arsenic-Mediated CꢀC Coupling of Cyanides Leading to Cyanido
Arsazolide [AsC4N4]ꢀ
Sçren Arlt,[a] Jçrg Harloff,[a] Axel Schulz,*[a, b] Alrik Stoffers,[a] and Alexander Villinger[a]
Abstract: The chemistry of arsenic cyanides has been in-
vestigated and is found to be completely different to the
Scheme 2. Synthesis of salts containing [E(CN)3+n]
nꢀ ions (X=F, Cl; n=1–3;
chemistry of the heavier analogs antimony and bismuth
as well as phosphorus. The reaction of As(CN)3 with cya-
nide salts resulted in the formation of an unknown cyani-
do arsazolide heterocycle, which represents a structure
isomer of the desired [As(CN)4]ꢀ. The structure, bonding,
and formation of this unusual heterocycle is discussed fea-
turing an arsenic mediated CꢀC coupling of cyanides.
[AsC4N4]ꢀ salts with different counterions such as [PPh4]+,
[PPN]+ =[Ph3P-N-PPh3]+, Ag+, and [BMIm]+ are reported
with [BMIm][AsC4N4] being a low-temperature ionic liquid
(Tm =ꢀ628C).
[Cat]+ =[PPh4]+, [PPN]+ =[Ph3PꢀNꢀPPh3]+; solvent=excess Me3SiCN,
CH3CN, ionic liquids).
nꢀ
ed synthesis of cyanido arsenates of the type [As(CN)3+n
]
(n=1, 2, 3), yielding an unprecedented cyanido arsazolide het-
erocycle (Scheme 3, Figure 1right).
Whereas the isolation and full characterization of all heavy
pnictogen tricyanides, E(CN)3 (E=PꢀBi),[1–8] were reported, ter-
nary cyanidopnictate anions have been known so far only for
phosphorus,[9–12] antimony,[13] and bismuth[13] but not for arsen-
ic (Schemes 1 and 2). Herein, we communicate the unusual Cꢀ
C coupling of cyanides mediated by arsenic upon the attempt-
Scheme 3. Known 5-membered As/N-heterocycles. (Ter=2,6-bis(2,4,6-trime-
thylphenyl)phenyl, Dmp=2,6-dimethylphenyl, Mes*=2,4,6-tri-tert-butyl-
phenyl).
nꢀ
Two approaches to [E(CN)3+n
]
ions were reported so
far:[11,13] i) Starting from E(CN)3 in the reaction with weakly coor-
dinating cyanide salts of the type [Cat]CN ([Cat]+ =for exam-
ple, [PPh4]+, [PPN]+ =[Ph3PꢀNꢀPPh3]+) and ii) treatment of EX3
with [Cat]CN in the presence of molar amounts of Me3SiCN
(X=halogen; Scheme 2). Interestingly, while [P(CN)4]ꢀ was de-
scribed as a highly labile intermediate in the reaction of P(CN)3
with CNꢀ sources, quickly decomposing to give [NCꢀPꢀCN]ꢀ
and the unusual [P2C10N10]2ꢀ (Scheme 1),[11] the formation and
Scheme 1. Isolated heavy ternary cyanido pnictogen anions.
[a] S. Arlt, Dr. J. Harloff, Prof. Dr. A. Schulz, A. Stoffers, Dr. A. Villinger
Institut fꢀr Chemie, Universitꢁt Rostock
Albert-Einstein-Straße 3a, 18059 Rostock (Germany)
E-mail: axel.schulz@uni-rostock.de
Homepage: http://www.schulz.chemie.uni-rostock.de/
isolation of the heavy cyanido pnictogen anions of the type
nꢀ
[E(CN)3+n
]
(E=Sb, Bi) was only achieved when EX3 was treated
with cyanide salts and Me3SiCN (Schemes 1 and 2),[13] since
E(CN)3 (E=Sb, Bi) are highly polymeric species that are almost
insoluble in common organic solvents, so that no reaction was
observed even after several days of reaction time and under
reflux. So it was interesting to see how arsenic performs, either
[b] Prof. Dr. A. Schulz
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201703317.
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creases the solubility of the silver salt in CH3CN (Figure 2,
Scheme S3 and Table S5 in the Supporting Information). Of
special interest might be the 1-butyl-3-methylimidazolium salt
[BMIm][AsC4N4] that was afforded by treating Ag[AsC4N4] with
Figure 1. Left) ORTEP representation of [As(CN)3Cl2]2ꢀ (left, disorder not
shown) and right) [AsC4N4]ꢀ in the crystal. Thermal ellipsoids correspond to
50% probability at 173 K. [PPh4]+ ions are omitted for clarity. Selected bond
lengths [ꢁ] and angles [8]: [As(CN)3Cl2]2ꢀ: As1ꢀC1 2.032(2), As1ꢀC2 1.922(7),
As1ꢀC3 2.01(1), As1ꢀCl1 2.736(1), As1ꢀCl2 2.808(1), C1-As1-C2 87.4(1), C1-
As1-C3 85.4(2), C2-As1-C3 89.8(4); Cl1-As1-Cl2 108.1(4). [AsC4N4]ꢀ: As1ꢀN1
1.801(2), As1ꢀN2 1.803(2), N1ꢀC1 1.334(2), N2ꢀC2 1.340(2), C2ꢀC4 1.439(2);
N1-As1-N2 93.29(6), C1-N1-As1 106.7(1), N3-C3-C1 177.5(2), N4-C4-C2
179.4(2).
Figure 2. Left) Section of a chain in [Ag(PPh3)2][AsC4N4] and right) view along
the threefold rotational axis of the helix (Ph groups omitted for clarity).
nꢀ
similar to phosphorus with no stable [E(CN)3+n
]
salts or like
the heavier antimony and bismuth that do form stable salts
[BMIm]Cl (83% yield). While all other here described salts de-
compose (without melting) above 1408 ([PPh4]+: 1448C;
[PPN]+: 1518C; Ag+: 1598C; [Ag(PPh3)]+: 1838C), [BMIm]
[AsC4N4] melts as low as ꢀ628C, thus representing a low-tem-
perature ionic liquid due to a very good charge delocalization
(see below). All these [AsC4N4]ꢀ salts can be prepared in bulk,
are only slightly moisture and air sensitive, but long term-
stable under argon. These properties allow follow-up chemistry
and thus render these salts a valuable starting material.
Single-crystal X-ray studies of all salts containing [AsC4N4]ꢀ
revealed a planar 5-membered heterocycle with rather short
AsꢀN (between 1.77–1.84, cf. Srcov(As=N)=1.74 ꢁ)[14] and
CringꢀNring distances (1.31–1.37, Srcov(C=N)=1.27 ꢁ)[14] featuring
partial double bond character. The first compound with an As=
N double bond (d(As-N)=1.714(7) and 1.745(7) ꢁ) was N,N’-
bis(2,4,6-tri-tert-butylphenyl)amino-iminoarsane, prepared by
Lappert et al. in 1986.[15] While a few cationic diazarsenium[16–18]
(A[16] d(AsꢀN)=1.763, B[19] 1.803–1.814 ꢁ) and neutral tetraza-
arsole (d(AsꢀN)=1.784–1.805 ꢁ)[20] heterocycles are known
(Scheme 3, species A–C), anionic diazarsolides are hitherto un-
known. Only recently, neutral triazarsole heterocycles were iso-
lated either by insertion of isonitriles into arsatriazanediyls
nꢀ
bearing [E(CN)3+n
]
ions.
In a first series of experiments, AsCl3 was treated with differ-
ent cyanide sources such as [PPh4]CN or KCN in Me3SiCN or
mixtures with acetonitrile at ambient temperatures as well as
slightly elevated temperatures (Scheme 2, path ii). The only
product that could be isolated in moderate yields (36%) was
[PPh4]2[As(CN)3Cl2] (Figure 1, left), but in no case complete Clꢀ/
CNꢀ substitution was observed. The solid state structure of
[PPh4]2[As(CN)3Cl2] (Figure 1, left) contains well-separated cat-
ions and monomeric anions, which display a sterically active
lone pair and a monomeric square-based pyramidal (pseudo-
octahedral) structure that is strongly distorted (Cl1-As1-Cl2
108.1(4)8, C1-As1-C3 85.4(2)8).
To avoid the Clꢀ/CNꢀ substitution problem (see below) we
started from As(CN)3 (reaction i, Scheme 2) in a second series
of experiments. Regardless of the stoichiometry, cyanide
source, and solvent (e.g., CH3CN, Me3SiCN, or even ionic liquids
such as [BMIm][OTf], OTf=F3CSO2Oꢀ) we used, we were never
capable of isolating any cyanide arsenate of the type [As-
(CN)3+n]
nꢀ, but always a structure isomer of [As(CN)4]ꢀ, the 4,5-
dicyano-1,3,2-diazarsolide [AsC4N4]ꢀ (Figure 1, right) in rather
good yields (up to 70%). The best yields were obtained with
[PPN]+ as counterion and a slight excess of [PPN]CN in the re-
action with As(CN)3 (1:1.25) in acetonitrile at ambient tempera-
tures (Scheme 2, reaction i). The presence of the [AsC4N4]ꢀ het-
erocycle was unequivocally proven by single-crystal X-ray stud-
ies on different salts ([PPN]+, [PPh4]+, [Ag(PPh3)2]+) containing
this heterocycle (Figure 1 and Tables S1–S6 in the Supporting
Information).
[As(m-NTer)2N]
(Ter=2,6-bis(2,4,6-trimethylphenyl)phenyl,
D d(AsꢀN)=1.875 ꢁ)[21] and by making use of a [3+2] cycload-
dition reaction between an arsaalkyne and an organic azide
(species E d(AsꢀN)=1.839 ꢁ).[22] Arsolides of the type [R4C4As]ꢀ
(R=Ph, Et) were described by Westerhausen et al.[23,24] Neutral
group 16 analogues of [AsC4N4]ꢀ were obtained in the reaction
of 2,3-diaminomaleonitrile with ECl4 (E=Se, Te) in the presence
of organic bases.[25–27]
With the [PPN][AsC4N4] in hand, we were able to substitute
the weakly coordinating cation [PPN]+ by Ag+ in the reaction
with AgNO3 in CH3CN in 95% yield. The silver salt turned out
to be hardly soluble in most common solvents. Even in di-
methyl sulfoxide, the solubility was very low. Solvates of the
silver salt were obtained upon dissolving small amounts and
recrystallization in DMSO or adding PPh3, which significantly in-
Interestingly, while the solid state structures of [Cat][AsC4N4]
([Cat]=[PPN]=[PPh4]) contain well-separated cations and
monomeric anions, all silver salts display coordination poly-
mers, for example, the most prominent structural feature in
[Ag(PPh3)2][AsC4N4] represents a chain with a threefold rota-
tional axis (Figure 2) with a tetrahedrally coordinated Ag+ ion
connecting two adjacent [AsC4N4]ꢀ ions via the ring N atoms in
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addition to coordination to two PPh3 molecules. The exocyclic
CN groups of [AsC4N4]ꢀ ions remain uncoordinated.
Such NMR experiments with 75As (I=3/2) are not feasible due
to the quadrupole moment. The crucial step with the largest
activation barrier (TS1 22.8 kcalmolꢀ1) is the attack of one CNꢀ
ligand at the carbon atom of an adjacent CNꢀ ligand, leading
to the formation of dicyanidoarsenide [As(CN)2]ꢀ and cyanogen
(NCꢀCN). Note, the attack of free CNꢀ at one carbon atom of
[As(CN)4]ꢀ does not work as in this case always [As(CN)5]2ꢀ is
formed in a barrier-free process. Once cyanogen is formed, all
other barriers are smaller and the reaction proceeds rather fast
to [AsC4N4]ꢀ (5); free CNꢀ ions attack cyanogen thereby form-
ing [(NC)2C=N]ꢀ (TS2 12.8 kcalmolꢀ1). This reaction seems to
be the reason why a slight access of CNꢀ in the reaction mix-
ture increases the yields. Actually, a similar reaction is known
for the synthesis of diiminosuccinonitrile from HCN and cyano-
gen in the presence of Et3N. Here, the base deprotonates HCN
forming CNꢀ, which attacks cyanogen twice. However, protons
are needed to form diiminosuccinonitrile. If no protons are
available, for example, in the reaction of NCꢀCN with KCN, the
authors describe the formation of a bicyclic [C7N7]ꢀ ion. Since
in our reaction setup [As(CN)2]ꢀ is present, [(NC)2C=N]ꢀ reacts
Finally, DFT, NBO, and NRT computations at the pbe/aug-cc-
pvdz level (see the Supporting Information, DFT=density func-
tional theory, NBO=natural bond theory, NRT=natural reso-
nance theory) were carried out to shed light into the bonding
and formation of [AsC4N4]ꢀ. Solvent effects, which are essential
to stabilize charged ions, are incorporated using the integral
equation formalism version of the polarizable continuum
model (PCM, solvent=acetonitrile). The frontier orbitals display
the characteristic features of aromatics and a strong coupling
of the 6p-electron 5-membered ring with the two CN groups
leading to a formal 10p-electron-9-center bonding system
(Table S7 in the Supporting Information). The LUMO and
HOMO are strongly localized on the arsenic atom. In accord
with these MO considerations, NBO/NRT analyses show reso-
nance between Lewis representations exhibiting delocalization
of the 6p electrons (Scheme 4). In accord with this, NBO occu-
pation of p atomic orbitals perpendicular to the five-mem-
bered ring amounts to 5.8 e (As 1.05, Nring 1.28, Cring 1.09 e).
This situation resembles the situation in nonlinear resonance-
stabilized pseudohalides such as tetracyanopyrrolides.[28]
Finally, we want to answer the question how [AsC4N4]ꢀ is
formed from its structural isomer [As(CN)4]ꢀ (2), which can be
assumed to be formed in the first reaction step when As(CN)3
(1) is treated with CNꢀ (Scheme 5). The barrier-free formation
of [As(CN)4]ꢀ was computed to be slightly exergonic (ꢀ9.3 kcal
molꢀ1), while the addition of a second CNꢀ ion and formation
of [As(CN)5]2ꢀ (3) represents a slightly endergonic process
(+1.3 kcalmolꢀ1). The transient formation of [P(CN)4]ꢀ in the
analogous reaction was proven by 31P NMR experiments.[11]
in
a
barrier free SN2 type substitution reaction to
[(NC)As(NC)2C=N)]ꢀ (4) and one CNꢀ of [As(CN)2]ꢀ is eliminat-
ed.[29] Again free CNꢀ can now attack one carbon atom of the
[(NC)2C=N]ꢀ ligand in 4, affording transition state TS3, an open
chain NCCNꢀAs species with an activation barrier of 21.0 kcal
molꢀ1. Upon formation of the final [AsC4N4]ꢀ heterocycle (5),
the last remaining CNꢀ ion in 4 is released. Interestingly, we
could not find a local minimum for an open chain isomer of
[AsC4N4]ꢀ along this reaction path, which means that the
attack of the CNꢀ ion at the [(NC)2C=N]ꢀ ligand in 4 leads di-
rectly to ring formation and elimination of CNꢀ. The formation
Scheme 4. Most important Lewis representations (out of 189) and their computed resonance weight (delocalization into CN triple bond omitted for clarity).
Scheme 5. Potential energy diagram for the formation of [AsC4N4]ꢀ (5) starting from As(CN)3 using the pbe density functional with aug-cc-pvdz basis sets for
C and N, and a ECP10MDF core potential for arsenic (see the Supporting Information, a=barrier free process, PCM method applied).
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of heterocycle [AsC4N4]ꢀ (5) is energetically favored by 46.3 kcal
molꢀ1 compared to its isomer [As(CN)4]ꢀ (2). It should be men-
tioned that the attack of CNꢀ at the free [(NC)2C=N]ꢀ yielding
the [(NC)2(C=N)2]2ꢀ ion is an endergonic process with 27.6 kcal
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molꢀ1 and a rather large activation barrier of 27.9 kcalmolꢀ1
,
again indicating an arsenic mediated CꢀC coupling process. At
last, we have computed the activation barriers for the heavier
congeners [E(CN)4]ꢀ, which are significantly higher
(TS1(Sb) 32.3 kcalmolꢀ1, TS1(Bi) 34.0 kcalmolꢀ1), nicely explain-
ing why in these cases no isomerization to an heterocycle
[EC4N4]ꢀ was observed, but always the formation of [E(CN)5]2ꢀ
,
since here also the second CNꢀ ion is bound in an exergonic
process ([E(CN)4]ꢀ +CNꢀ![E(CN)5]2ꢀ; E=Sb ꢀ3.2, Bi ꢀ5.3, cf.
As+1.3 kcalmolꢀ1).
In summary, we have demonstrated a facile synthetic access
to the first cyanido arsazolide [AsC4N4]ꢀ that has been fully
characterized. [AsC4N4]ꢀ is formed in an unusual isomerization
process from its structural isomer [As(CN)4]ꢀ, which is formed
first as a transient species when As(CN)3 is treated with CNꢀ.
Then, two arsenic mediated CꢀC coupling steps occur, leading
finally to the formation of the [AsC4N4]ꢀ aromatic ring system,
which can be considered a resonance-stabilized pseudohalide.
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Experimental Section
Experimental and computational details can be found in the Sup-
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The authors declare no conflict of interest.
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Keywords: arsenic · CꢀC coupling · cyanides · isomerization ·
heterocycles
Manuscript received: July 18, 2017
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Version of record online: && &&, 0000
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COMMUNICATION
Go get some cyanide and…isomerize:
A novel [AsC4N4]ꢀ heterocycle is formed
in an unusual isomerization process
from its structural isomer [As(CN)4]ꢀ,
which is formed as transient species
when As(CN)3 is treated with CNꢀ (see
scheme). Two arsenic-mediated CꢀC
coupling steps are necessary for the for-
mation of the [AsC4N4]ꢀ aromatic ring
system.
&
Heterocycles
S. Arlt, J. Harloff, A. Schulz,* A. Stoffers,
A. Villinger
&& – &&
Arsenic-Mediated CꢀC Coupling of
Cyanides Leading to Cyanido
Arsazolide [AsC4N4]ꢀ
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