Synthesis and unprecedented oxidation of a cationic Sb-analogue of an Arduengo's carbene

Article abstract of DOI:10.1039/b409657f

2-Chloro-1,3,2-diazastibolenes react with Lewis acids either via Sb-Cl cleavage to yield stable Sb-analogues of an N-heterocyclic carbene, or via an unprecedented oxidative fragmentation to give a diazadiene-SbCl₃ complex.

Full text of DOI:10.1039/b409657f

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Synthesis and unprecedented oxidation of a cationic Sb-analogue of an
Arduengo’s carbene{
Dietrich Gudat,*a Timo Gans-Eichlerb and Martin Niegerb
a Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart, 70550 Stuttgart, Germany.
E-mail: gudat@iac.uni-stuttgart.de; Fax: 149 711 685 4241
b Institut fu¨r Anorganische Chemie, Universita¨t Bonn, Germany
Received (in Cambridge, UK) 25th June 2004, Accepted 5th August 2004
First published as an Advance Article on the web 22nd September 2004
13
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2-Chloro-1,3,2-diazastibolenes react with Lewis acids either
in 5 (2.692 A ). This effect is quite surprising in the light of the
increase in atomic radii from P to Sb but may be explained if one
assumes that the cation 3a is a stronger chloride acceptor – and
thus a stronger Lewis acid – than the homologous diazapho-
spholenium ion. The validity of this hypothesis was independently
underlined by the observed reaction of 3a[AlCl4] with 5 which
proceeded via quantitative transfer of a chloride to give 2a and the
appropriate diazaphospholenium cation.
Crystalline 3a[SbCl4] (Fig. 2) contains anions that form, as in
[NPr4][SbCl4],16 centrosymmetric chloride-bridged dimers with
distorted square-pyramidal coordination at the Sb-atoms, and
cations whose dicoordinate Sb-atoms display weak contacts (3.43–
via Sb–Cl cleavage to yield stable Sb-analogues of an
N-heterocyclic carbene, or via an unprecedented oxidative
fragmentation to give a diazadiene–SbCl3 complex.
Carbenium (I, E = C),1 nitrenium (II, E = N)2 and phosphenium
ions (II, E = P)3 have received persistent interest due to their unique
bonding situation and reactivity. The access to analogues with
heavier group 14 and 15 elements was long obstructed by a growing
tendency of species with progressively larger and softer Lewis-
acidic centres to form adducts with Lewis bases,4 and only over the
last decades did examples of genuine cations I with E ~ Si,5 Ge,6
Sn7 and II with E ~ As,8 Sb,9 Bi9 become known.
˚
3.94 A) to Cl-atoms of adjacent anions that are clearly longer than
9
…
˚
in cyclo-[Me2Si(NtBu)2Sb][AlCl4] (6, Sb Cl 3.03, 3.07 A ). The
five-membered rings in the cations are planar and their structural
features indistinguishable in both 3a[SO3CF3] and 3a[SbCl4]. The
Sb–N bonds are slightly longer than in 2a or 6 (Sb–N 1.994,
9
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1.998 A ), and the CLC bond is longer and the C–N bonds are
The stability of known type II cations relies commonly on
mesomeric interaction with p-donor substituents.3,8,9 A highly
efficient p-stabilisation is present in the diazaphospholenium and
diazaarsolenium ions III (E = P, As)10–13 that are isoelectronic
analogues of N-heterocyclic carbenes IV (E = C). Having recently
preparedthefirststablestannyleneIV(E=Sn),14wereportnowonthe
synthesis andproperties ofthe novel Sb-centeredcations III(E=Sb).
By analogy to the synthesis of the stannylenes IV,14 the
a-aminoaldimines 1a,b reacted with ClSb(NMe2)2 to afford the
1,3,2-diazastibolenes 2a,b; alternatively, 2a was accessible by
reacting 1a with SbCl3 in the presence of NEt3 as acid scavenger.
Both products were isolated in 40–60% yield as orange-red, air
and moisture sensitive solids and were characterised by analytical
and spectroscopic data and a single-crystal X-ray diffraction study
of 2a.{ §
shorter than in 2a. The equalisation of single and double bonds is
more pronounced than in the appropriate diazaarsolenium ion10
and all trends reproduce similar findings in the diazaphospholene
system,11,13 thus suggesting a higher degree of p-conjugation in the
five-membered ring than in 2a.
Whereas treatment of P-chloro-diazaphospholenes with SbCl5 is
Conversion of 2a,b into the salts 3a,b[X]{ was achieved by Lewis-
acid induced Sb–Cl cleavage, using AlCl3, GaCl3, SbCl3, or
Me3SiOSO2CF3 as chloride acceptor. The presence of contact ion
pairs insolutionandthesolidstatewasproven bythesimilarityof the
1Hand13C NMR spectra of salts with different anions, the detection
of a sharp 27Al NMR signal attributable to free AlCl42, and single-
crystal X-ray diffraction studies of 3a[SbCl4] and 3a[SO3CF3].§
Crystals of 2a consist of molecules (Fig. 1) that are connected by
Fig. 1 ORTEP style drawing of 2a with thermal ellipsoids at the 50%
˚
probability level; H-atoms omitted for clarity. Selected bond lengths [A]:
Cl1–Sb1 2.646(2), Sb1–N1 1.998(4), Sb1–N2 2.000(4), N1–C1 1.407(6),
N2–C2 1.391(6), C1–C2 1.346(6).
…
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intermolecular Sb Cl contacts (3.86 A) slightly shorter than the
˚
sum of van der Waals radii (4.00 A). The Sb–N distances in the
planar five-membered rings are similar to those in cyclo-
˚
15
[Me2Si(NtBu)2SbCl] (4, 1.995(5) A ). The CLC and C–N bonds
match those in the homologous diazaphospholene cyclo-
13
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[(CH)2(NtBu)2PCl] (5, CLC 1.349, C–N 1.391 A ) and range
between double and single bonds; all features together suggest
that the bonding in 2a is as in 513 characterised by sizeable p(C2N2)
A s*(E–X)-hyperconjugation. The Sb–Cl bond in 2a is by 7%
longer than in 4 (2.472(3) A ) but still shorter than the P–Cl bond
15
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{ Electronic supplementary information (ESI) available: experimental
procedures and characterisations of all compounds described in this paper
and theoretical data of [(7c)(SbCl3)2] and 5c[SbCl6]. See http://www.rsc.org/
suppdata/cc/b4/b409657f/
2
Scheme 1 R ~ tBu (1a–3a), Mes (1b–3b); X ~ AlCl42, GaCl42, SbCl4
,
SO3CF32; (i) 1 equiv. ClSb(NMe2)2 or SbCl3/2 NEt3; (ii) 1 equiv. AlCl3,
GaCl3, SbCl3, or Me3SiOSO2CF3.
2 4 3 4
C h e m . C o m m u n . , 2 0 0 4 , 2 4 3 4 – 2 4 3 5
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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from ethereal reaction mixtures but was as well converted into 7a
and SbCl3 when the reaction proceeded.
Oxidative fragmentation was as well observed upon air oxidation
of 3a[AlCl4], and upon treatment of 2a with PCl5 to yield 7a, SbCl3,
and PCl3. This last reaction was completed by ring metathesis
of PCl3 with a second molecule of 2a to give the salt
[(CH)2(NtBu)2P][SbCl4] as isolable final product. The observed
reactivity of 2a towards SbCl5 and PCl5 is in marked contrast to
that of diazaphospholenes which react under identical conditions
exclusively via chloride abstraction and formation of diazapho-
spholenium salts12 that are stable in dry oxygen.
In conclusion, it has been shown that the mesomeric stabilisation
in the molecular framework of III is sufficient to allow the first
synthesis of cationic Sb-analogues of N-heterocyclic imidazoyl
carbenes, and that these species are distinguished from their lighter
homologues by increased Lewis-acidity and a significantly higher
lability to undergo unprecedented oxidation reactions under
cheletropic ring fragmentation.
Fig. 2 ORTEP style drawing of 3a[SbCl4] with thermal ellipsoids at the 50%
˚
probability level; H-atoms omitted for clarity. Selected bond lengths [A]:
Sb1–N1 2.025(2), Sb1–N2 2.023(2), N1–C1 1.356(3), N2–C2 1.353(3), C1–
C2 1.364(4), Sb2–Cl1 2.699(1), Sb2–Cl1# 3.054(1), Sb2–Cl2 2.517(1), Sb2–
Cl3 2.421(1), Sb(2)–Cl(4) 2.362(1), Sb1–Cl1 3.433(1).
known as viable access to phosphenium hexachloroantimonates,12
the corresponding reaction of 2a afforded, quite surprisingly, not the
salt 3a[SbCl6] but proceeded via fragmentation of the heterocycle to
give SbCl3 and the diazadiene 7a. The products were isolated as a
crystalline adduct of composition [(7a)(SbCl3)2] that was was
identified by a single-crystal X-ray diffraction study, but dissociated
according to 1H NMR studies into its constituents in solution.
The crystal structure of [(7a)(SbCl3)2] (Fig. 3) features two
Notes and references
{ Selected spectral data: 2a: 1H NMR (C6D6): d ~ 6.6 (2H, CH), 1.25
(18H, tBu); MS (EI, 16 eV, 150 uC): m/e (%) ~ 324 (85) (M1), 289 (85)
(M1 2 Cl). 2b: 1H NMR (C6D6): d ~ 6.79 (4H, m-H), 6.38 (2H, CH), 2.29
(12H, o-Me), 2.13 (6H, p-Me); MS (EI): m/z (%) ~ 448 (20) (M1), 413 (16)
(M1 2 Cl), 277 (100) (M1 2 SbCl, Me). 3a[SbCl4]: 1H NMR (CD3CN):
d ~ 8.57 (2H, CH), 1.71 (18H, tBu). 3b[SO3CF3]: 1H NMR (CD3CN): d ~
8.41 (2H, CH), 7.17 (4H, m-H), 2.40 (12H, o-Me), 2.20 (6H, p-Me).
§ Crystal data at 123 K: 2a: C10H20ClN2Sb, FW ~ 325.5, orthorhombic,
˚
pyramidal SbCl3-units with bond lengths of 2.38–2.40 A. The Sb1-
atom is chelated by the diazadiene nitrogen atoms to form a planar
˚
five-membered SbN2C2-ring. The Sb–N bonds are by some 0.4 A
longer than in 2a or 3a and the C–C and C–N bond lengths
represent normal single and double bonds, as expected for a
diazadiene. Both Sb-atoms exhibit secondary contacts with
˚
˚
˚
Pbca, a ~ 11.5242(5) A, b ~ 10.9946(4) A, c ~ 21.0980(9) A, V ~
3
2673.2(2) A , Z ~ 8; 13862 reflxns. collected (2344 unique), R1 ~ 0.038
˚
(I w 2s(I)), wR2 ~ 0.091. 3a[SbCl4]: C10H20Cl4N2Sb2, FW ~ 553.6,
monoclinic, P21/n, a ~ 10.1078(1) A, b ~ 19.5200(2) A, c ~ 10.2340(1) A,
b ~ 114.408(1)u, V ~ 1838.8(1) A , Z ~ 4; 16986 reflxns. collected (3213
unique), R1 ~ 0.018 (I w 2s(I)), wR2 ~ 0.042. 3a[SO3CF3]?CH2Cl2,
C12H22Cl2F3N2O3SSb, FW ~ 524.0, monoclinic, P21/c, a ~ 11.0192(1) A,
b ~ 10.3184(1) A, c ~ 18.6936(2) A, b ~ 104.138(1)u, V ~ 2061.1(1) A ,
Z ~ 4; 32263 reflxns. collected (3603 unique), R1 ~ 0.023 (I w 2s(I)),
wR2 ~ 0.061. [(7a)(SbCl3)2]: C10H20Cl6N2Sb2, FW ~ 624.5, monoclinic,
˚
3
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˚
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distances between 3.04 and 3.57 A to Cl-atoms of adjacent
SbCl3-units. Neglecting one weak interaction each (Sb–Cl w
3.50 A, see thin lines in Fig. 3), both Sb-atoms attain a coordination
˚
˚
˚
number of six (Sb1: 5 1 1; Sb2: 3 1 3) and a distorted octahedral
coordination geometry. If one adopts this view, the adduct [(7a)
(SbCl3)2] is formally described as a valence isomer of an ion pair
3a[SbCl6], with conversion between both forms being feasible by
simple shifts of electrons and bridging chlorides between Sb-atoms.
Computational studies (at the b3lyp/6-31g* level) on the model
compounds [(7c)(SbCl3)2] and 3c[SbCl6] (see Scheme 1, R = Me)
revealed that both species are local minima on the energy
hypersurface but that [(7c)(SbCl3)2] is – in the gas phase – by
34.4 kcal mol21 more stable than the ion pair 3c[SbCl6].
3
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˚
˚
˚
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C2/c, a ~ 12.0617(1) A, b ~ 17.4255(2) A, c ~ 19.3913(2) A, b ~
˚
3
102.081(1)u, V ~ 3985.4(1) A , Z ~ 8; 34881 reflxns. collected (3512
unique), R1 ~ 0.018 (I w 2s(I)), wR2 ~ 0.041. CCDC reference numbers:
CCDC-242440 (2a), CCDC-242441 (3a[SbCl4]), CCDC-242443
(3a[SO3CF3]), CCDC-242442 ([(7a)(SbCl3)2]). See http://www.rsc.org/
suppdata/cc/b4/b409657f/ for crystallographic data in .cif or other
electronic format.
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SbCl5 is rationalised by assuming that the initial step involves
oxidative chlorination of 2a rather than Lewis-acid induced Sb–Cl
cleavage, to give a hypothetical intermediate 8a which then decays
via cheletropic fragmentation to the diazadiene 7a and SbCl3. This
proposition was supported by the trapping of SbCl3 by unreacted
2a to give 3a[SbCl4]. The salt precipitated as isolable side-product
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Fig. 3 ORTEP style drawing of [(7a)(SbCl3)2] with thermal ellipsoids at the
50% probability level; H-atoms omitted for clarity; Sb2a, Cl1a, Cl2a, and
Cl4b denote atoms of neighboring molecular units. Selected bond lengths
14 T. Gans-Eichler, D. Gudat and M. Nieger, Angew. Chem., Int. Ed., 2002,
41, 1888.
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[A]: Sb1–N1 2.460(2), Sb1–N2 2.418(2), N1–C1 1.278(3), C2–N2 1.277(3),
C1–C2 1.472(3), Sb1–Cl1 2.384(1), Sb1–Cl2 2.546(1), Sb1–Cl3 2.596(1),
Sb1–Cl4 3.363(1), Cl1–Sb2a 3.334(1), Sb2–Cl2 3.196(1), Sb2–Cl3 3.041(1),
Sb2–Cl4 2.406(1), Sb2–Cl5 2.412(1), Sb2–Cl6 2.380(1).
15 M. Veith, B. Bertsch and V. Huch, Z. Anorg. Allg. Chem., 1988, 559, 7.
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C h e m . C o m m u n . , 2 0 0 4 , 2 4 3 4 – 2 4 3 5
2 4 3 5