Tetrazarsoles - A new class of binary arsenic-nitrogen heterocycles

Article abstract of DOI:10.1002/anie.200704367

Croup 15 galore: A new five-membered ring containing only nitrogen and arsenic was synthesized by a GaCl₃-assisted [3+2] cycloaddition. The novel tetrazarsole was stabilized as the GaCl₃ adduct (see structure; Cl green, Ga purple, N blue, As red, C gray). (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200704367

Angewandte
Chemie
DOI: 10.1002/anie.200704367
Arsenic–Nitrogen Compounds
Tetrazarsoles—A New Class of Binary Arsenic–Nitrogen
Heterocycles**
Axel Schulz* and Alexander Villinger
Triazadiphosphole[1] was recently prepared from (Me3Si)2N
À
N(SiMe3)PCl2[2] and GaCl3 under mild conditions in a formal
GaCl3-assisted [3+2] cycloaddition. The second example of a
binary azaphosphole, the tetrazaphosphole,[3] was formed in
=
the reaction of Me3SiN3 with Mes*N PCl (Mes* = 2,4,6-
tBu3C6H2), again catalyzed by GaCl3. Both reactions only
succeed when GaCl3 is added, because GaCl3 triggers the
elimination of Me3SiCl and kinetically stabilizes the hetero-
cycle at the end of the reaction sequence by adduct formation
(yields for both reactions are greater than 95%).
To our knowledge, binary five-membered arsenic–nitro-
gen heterocycles composed of only these two elements are not
yet known, and hence we tried to apply the GaCl3-assisted
[3+2] cycloaddition to the arsenic species.[4a] In analogy to the
azaphosphole synthesis, two routes seemed promising: 1) The
Scheme 1. Synthetic routes to tetrazarsoles.
À
reaction of (Me3Si)2N N(Me3Si)AsCl2 with GaCl3, which,
À
however, led to (ClSiMe2)2N N(SiMe3)AsMe2, after an
A dimer/monomer ratio of 1:1.7 was found at 298 K,
corresponding to a free enthalpy DG8 = À2.6 kJmolÀ1.
If one equivalent of Me3SiN3 (relative to the monomer)
and subsequently one equivalent of GaCl3 are added at
À408C to 1 and 2 in equilibrium, the red color of the
monomer immediately disappears, and one new colorless
species is formed, according to 1H and 13C NMR studies.
Indeed, the end of the reaction (complete formation of the
new species) can be titrated utilizing the distinct color
change.[6a,c]
The same species is obtained in the reaction of Me3SiN3
and Mes*N(SiMe3)AsCl2 in CH2Cl2 at À408C when a solution
of GaCl3 in CH2Cl2 is added (Scheme 1). In this case, the
reactive species, the iminoarsane 2, is generated in situ by
GaCl3-assisted elimination of Me3SiCl. The workup for both
procedures is the same. The reaction mixture is allowed to
warm to ambient temperature over a period of one hour. The
solution is then concentrated in vacuo to incipient crystal-
lization and stored in a freezer at À258C. After crystallization
and drying in vacuo, 3 remains as colorless crystals (yield: 92–
98%).[6a,c] Pure, dry 3 is stable at temperatures up to 190–
1958C (decomposition), is neither heat nor shock sensitive,
and decomposes very slowly at ambient temperatures in the
solid state and in solvents, releasing N2 gas. Colorless crystals
of 3 rapidly turn yellow when traces of water or oxygen are
present.
intriguing intrinsic Cl/methyl exchange.[4b] 2) Utilization of
“disguised” kinetically stabilized iminoarsanes that give high
yields of the desired tetrazarsole (Scheme 1). Herein, we
provide an interesting extension of the GaCl3-assisted formal
[3+2] cycloaddition to the chemistry of arsenic, which led to
the isolation of the first binary arsenic–nitrogen five-mem-
bered heterocycle, a tetrazarsole.
We started with an investigation of the equilibrium
between cyclic diarsadiazane (1) and its monomer, the
iminoarsane (2; Scheme 1), by means of temperature-depen-
dent 1H NMR spectroscopy. The dimer was first described by
Burford et al., and the authors speculated on its reversible
dissociation.[5] The white crystalline dimer 1 readily dissolves
in CH2Cl2, but the initially colorless solution slowly turns red,
the color of the monomeric species. To derive thermochem-
ical data for the dimer–monomer equilibrium, 1H NMR
spectra were collected between 233 and 298 K, yielding a
value of 2.2 kJmolÀ1 for the enthalpy of monomerization.[6a]
[*] Prof. Dr. A. Schulz, A. Villinger
Universität Rostock
Institut für Chemie
Albert-Einstein-Strasse 3a, 18059 Rostock (Germany)
and
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock, (Germany)
Fax: (+49)381-498-6382
X-ray structure analysis of 3 confirms the formation of the
desired tetrazarsole derivative (Scheme 1). This compound
crystallizes as colorless needles in the monoclinic space group
P21/c with four molecules per unit cell.[6b] As depicted in
Figure 1, the AsN4 ring is planar, like in the pentazoles,[7]
triazadiphospholes,[1] and tetrazaphospholes (5),[3] and the
structural features of 3 and 5 are very similar. The AsN4 ring is
E-mail: axel.schulz@uni-rostock.de
Homepage: http://www.chemie.uni-rostock.de/schulz
[**] Financal support by the Deutsche Forschungsgemeinschaft (SCHU
1170/4-1) is gratefully acknowledged. We are indebted to Dr. D.
Michalik for the NMR study.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
À
À
slightly distorted, with two longer N N bonds (d(N1 N2) =
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À
The Ga N bond length of 1.964(2) Š is in the typical
range for GaCl3 adducts (1.990(4) in 5). The N-As-N angle of
82.8(1)8 (88.2(2)8 in 5) is very small compared to the As-N-N
(114–1168) and N-N-N angles (113–1148). In the crystal, all
AsN4 rings are parallel to each other; the same holds true for
the Mes* groups, which, however, are almost perpendicular to
À
À
the AsN4 rings (Figure 3). The short As N and N N bonds,
Figure 1. ORTEPdrawing of the crystal structure of 3. Thermal
ellipsoids are shown with 50% probability at 173 K (hydrogen atoms
are omitted for clarity). Selected bond lengths [Š] and angles [8]:
As–N4 1.784(2), As–N1 1.805(2), N1–N2 1.349(3), N1–C1 1.458(4),
N2–N3 1.286(3), N3–N4 1.366(3), N4–Ga 1.964(2), Ga–Cl3 2.1463(9),
Ga–Cl1 2.1499(9), Ga–Cl2 2.1503(9); N4-As-N1 82.8(1), N2-N1-C1
117.3(2), N2-N1-As 115.2(2), C1-N1-As 127.4(2), N3-N2-N1 113.2(2),
N2-N3-N4 114.0(2), N3-N4-As 114.8(2), N3-N4-Ga 117.4(2), As-N4-Ga
127.7(1), N4-Ga-Cl3 105.15(7), N4-Ga-Cl1 101.48(7), Cl3-Ga-Cl1
114.61(4), N4-Ga-Cl2 106.33(7), Cl3-Ga-Cl2 113.70(4), Cl1-Ga-Cl2
113.97(4).[6b]
À
1.349(2) and d(N3 N4) = 1.366(3); 5: 1.355(5) and
À
À
1.374(5) Š) and one very short N N bond (d(N2 N3) =
À
1.286(3); 5: 1.286(5) Š). These N N bonds of 1.28–1.38 Š
are substantially shorter than the sum of the covalent radii
(dcov(N N) = 1.48 and dcov(N N) = 1.20), which indicates
partial double-bond character for all the N N bonds, with the
[8]
À
=
À
Figure 3. View along the c axis of 3. Color code as in Figure 2.
À
N2 N3 bond having a bond order close to two. Although both
3 and 5 seem predisposed to the release of molecular nitrogen,
the arsenic analogue is stable at ambient temperatures, and no
significant release of molecular nitrogen was observed, in
contrast to 5.[3] Obviously, the “naked” AsN4 ring is better
kinetically protected in the sandwich between the large aryl
and GaCl3 units (Figure 2).
together with the planarity, indicate the presence of a strongly
delocalized 6p-electron system, which is supported by molec-
ular orbital (MO) and natural bond orbital (NBO) calcula-
tions.[6e,13,14] As expected, according to NBO analysis the s
À
Partial double-bond character can also be assumed for the
and p As N bonds within the AsN4 ring of 3 are strongly
À
À
two As N bonds (d(As N1) = 1.784(2) and 1.805(2) Š) of
polarized (s: 20/80 and p: 22/78%) with partial charges of
+ 1.21 at arsenic and À0.53 and À0.84e at N1 and N4,
respectively, while the bonds between the adjacent nitrogen
atoms of the ring are almost ideally covalent (q(N2) =
q(N3) = 0.00). We calculate a charge of + 0.12e at the
exocyclic carbon atom (C1) and + 1.44e at the gallium
center. The computed charge transfer in this Lewis base–acid
adduct (3) is 0.14e (5: 0.15e).
À
=
the AsN4 ring (dcov(N As) = 1.91 and dcov(N As) = 1.71 Š;
1.82–1.84 Š in cations of 1,3,2-diazarsolenes[8,9]). In contrast
to the well-known iminophosphanes,[10] little is known about
[5,9,11]
=
the corresponding compounds containing As N bonds.
=
The first fully characterized compound with an As N bond
À
(d(As N) = 1.714(7) and 1.745(7) Š) was N,N’-bis(2,4,6-tri-
tert-butylphenyl)amino-iminoarsane, prepared by Lappert
et al. in 1986.[12]
Finally, we wanted to answer the question of what
happens when GaCl3 is added to 4 without the 1,3-dipole
Me3SiN3 (Scheme 2).[6a–c] It can be assumed that in the first
reaction step, Me3SiCl is also eliminated upon adding GaCl3,
but the very reactive iminoarsane (formally a dipolarophile)
cannot be quenched with Me3SiN3, and hence it slowly attacks
the tert-butyl group of the Mes* moiety, finally yielding the
GaCl3 diadduct 6 (Scheme 2). The formation of 6 was
unequivocally confirmed by X-ray crystallography and
NMR spectroscopy.[6a–c] Compound 6 crystallizes as colorless
blocks in the monoclinic space group C2/c with four
molecules per unit cell.[6b] The molecular structure of 6
(Figure 4) shows it to be a formal dimer, with two nonplanar
Figure 2. Space-filling representation of 3 (left: AsN4 ring in the plane
of the page, right: turned by 908); Cl green, Ga purple, N blue, As red,
C gray.
À
five-membered rings connected by a long As As bond
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C 39.17, H 5.56, N 9.97. 1H NMR (258C, CD2Cl2, 300 MHz): d = 1.04
(s, 18H, o-tBu), 1.38 (s, 9H, p-tBu), 7.66 ppm (s, 2H, CH);
13C{1H} NMR (258C, CD2Cl2, 75 MHz): d = 31.5 (C8, (CH3)3), 33.5
(C6, (CH3)3), 35.9 (C7, C(CH3)3), 37.6 (C5, C(CH3)3), 125.1 (C3),
132.7 (C2), 146.8 (d, C1), 154.5 ppm (C4) (see Figure S1[6] in the
Supporting Information). Raman (75 mW, 258C, 3000 scans): n˜ =
3111 (1), 2969 (9), 2936 (7), 2911 (10), 2786 (2), 2755 (2), 2713 (1),
1596 (2), 1466 (1), 1442 (1), 1285 (1), 1244 (2), 1198 (1), 1178 (1), 1146
(1), 1007 (1), 924 (1), 821 (2), 772 (1), 594 (1), 574 (1), 523 (2), 451 (1),
404 (1), 358 cmÀ1 (2). IR (ATR, 258C, 32 scans): n˜ = 2962 (s), 2867
(m), 1596 (m), 1568 (w), 1474 (m), 1421 (m), 1395 (m), 1363 (s), 1271
(m), 1235 (m), 1215 (m), 1198 (w), 1128 (w), 1030 (m), 1015 (s), 963
(m), 927 (w), 880 (m), 771 (m), 712 cmÀ1 (s). MS (EI, m/z, > 5%,): 43
(12) [C3H7]+, 57 (20) [C4H9]+, 69 (20), 91 (20) [NAsH2]+, 112 (14), 230
[Mes*HÀMe]+, 246 (100) [Mes*H]+, 261 (17) [Mes*NH2]+.
Scheme 2. Reaction of 4 with GaCl3 without Me3SiN3.
Received: September 21, 2007
Published online: December 3, 2007
Keywords: arsenic · azarsoles · cycloaddition ·
.
nitrogen heterocycles · structure elucidation
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Figure 4. ORTEPdrawing of the crystal structure of 6. Thermal
ellipsoids are shown with 50% probability at 173 K (hydrogen atoms
are omitted for clarity). Selected bond lengths [Š] and angles [8]: As–C
1.947(3), As–N 1.999(2), As–As 2.4174(7), N–C 1.451(3), N–Ga
2.018(2); C-As-N 95.4(1), C-As-As 89.92(7), N-As-As 86.90(6), C-N-As
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13C, 1H-HETCOR, 29Si{1H} NMR); b) crystal data; c) full
description of the experimental data, technique, and details of
3 and 4, including improved syntheses of all starting materials;
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1) partial charges, 2) hybridization effects, and 3) polarization.
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(2.4174(7); for comparison: 2.2560(5) in base-stabilized
amidodiarsenes[15] and 2.433(2) Š in Me2As AsMe2[16]).
À
In summary, we have reported a simple synthetic route to
the first tetrazarsole, which was isolated as a GaCl3-stabilized
adduct in high yield (greater than 90%). Compound 3 was
fully characterized and has an electronic structure that is
related to those of aromatic hydrocarbons that have 4n + 2
p electrons and therefore formally obey the Hückel rule. The
tetrazarsole can formally be regarded as the [3+2] cyclo-
+
À
=
addition product of [Mes*N As] and N3 ions. This cyclo-
addition only occurs if Me3SiCl elimination is induced by the
Lewis acid GaCl3, releasing the dipolarophile. Without a 1,3-
dipolar molecule, GaCl3 still triggers the Me3SiCl elimination,
but the very reactive iminoarsane is not quenched, and in an
intermolecular process a cyclic diarsane is formed, while one
tert-butyl group is removed.
Experimental Section
[12] P. B. Hitchcock, M. F. Lappert, A. K. Rai, H. D. Williams, J.
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[13] Single-point calculations were computed with a tight conversion
criterion using the program package Gaussian 98. For computa-
General information is included in the Supporting Information.[6]
3: M.p. 190–1958C (dec.). Elemental analysis (%) calcd
C18H29AsCl3GaN4 (552.44 gmolÀ1): C 39.13, H 5.29, N 10.14; found:
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Communications
tional details, see the Supporting Information; M. J. Frisch, et al.
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