The first evidence for a transient stibaallene ArSb = C = CR2

Article abstract of DOI:10.1016/j.jorganchem.2004.09.022

Dechlorofluorination of ArSb(F)-C(Cl)CR₂ (CR₂ = fluorenylidene, Ar = 2,4,6-tri-tert-butylphenyl) by tert-butyllithium afforded a 3,4-bis(fluorenylidene)-1,2-distibacyclobutane. The formation of the latter probably involves the transient stibaallene ArSbCCR₂ followed by a head-to-head dimerization via two SbC double bonds. Molecular orbital calculations at the ab initio and DFT levels support the head-to-head dimerization of ArSbCCR₂ with the formation of a 1,2- distibacyclobutane.

Full text of DOI:10.1016/j.jorganchem.2004.09.022

Journal of Organometallic Chemistry 690 (2005) 307–312
www.elsevier.com/locate/jorganchem
The first evidence for a transient stibaallene ArSb@C@CR2
a
a
a,
b
Lise Baiget , Henri Ranaivonjatovo , Jean Escudie *, Gabriela Cretiu Nemes ,
´
b
Ioan Silaghi-Dumitrescu , Luminita Silaghi-Dumitrescu
b
a
´ ´ ´ ´
Heterochimie Fondamentale et Appliquee, Universite Paul Sabatier, UMR 5069, 118 Route De Narbonne, 31062, Toulouse Cedex 04, France
b
Universitatea Babes-Bolyai Cluj-Napoca, RO 400084 Cluj-Napoca, Romania
Received 15 July 2004; accepted 13 September 2004
Abstract
Dechlorofluorination of ArSb(F)–C(Cl)@CR2 (CR2 = fluorenylidene, Ar = 2,4,6-tri-tert-butylphenyl) by tert-butyllithium
afforded a 3,4-bis(fluorenylidene)-1,2-distibacyclobutane. The formation of the latter probably involves the transient stibaallene
ArSb@C@CR2 followed by a head-to-head dimerization via two Sb@C double bonds. Molecular orbital calculations at the ab initio
and DFT levels support the head-to-head dimerization of ArSb@C@CR2 with the formation of a 1,2-distibacyclobutane.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Distibacyclobutane; Stibaallene; Stibapropene
1. Introduction
ate: antimony–carbon doubly bonded compounds have
a higher tendency to dimerize than their phosphorus
or arsenic analogues since the stabilization of such low
coordinate derivatives is more and more difficult going
down a column.
We present in this paper the first evidence for the for-
mation of the transient stibaallene ArSb@C@CR2.
A growing variety of compounds containing stable
double bonds between carbon and main group heavy
elements have been reported in the last years. More re-
cently, much has been accomplished in the synthesis of
stable heteroallenes E=C=E0 (E, E0 = C, Si, Ge, Sn, P,
As, O, S) [1]. Based on their unique geometric and elec-
tronic structures, compounds containing cumulated
double bonds to a group 14 or 15 element are particu-
larly interesting and important molecules. In the field
of –E15@C@C< compounds, ketenimines (E = N) have
been known for a long time, many phosphaallenes
(E = P) have been prepared [1a,1c,1d] since the synthesis
of the first one by both Appel and Yoshifuji [2], and one
arsaallene (E = As) [3] has been obtained and isolated as
an air and moisture stable derivative.
2. Results and discussion
As phospha- and arsaallenes ArE@C@CR2
(Ar = 2,4,6-tri-tert-butylphenyl, CR2 = fluorenylidene)
(E = As [3], P [4]) could be easily isolated, we chose to
substitute the antimony atom by the same well-known
stabilizing ‘‘supermesityl’’ group and to include the ter-
minal carbon atom in a fluorenylidene group which pro-
vides a large steric hindrance with three fused cycles.
Many phosphaallenes –P@C@C< [1a,1c,1d] and the
arsaallene ArAs@C@CR2 [3] have been prepared by
routes involving the initial formation of the P(or As)@C
double bond. Such pathways were successful owing to
the rather good stability of P@C [1c,5] and As@C [3,6]
By contrast, no stibaallene (E = Sb) could be until
now isolated or even postulated as a transient intermedi-
*
Corresponding author. Tel.: +33
5
61 558347; fax: +33
´
5
61558204.
E-mail address: escudie@chimie.ups-tlse.fr (J. Escudie).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.09.022

308
L. Baiget et al. / Journal of Organometallic Chemistry 690 (2005) 307–312
double bonds provided that the steric protection around
the double bonds was important enough. But it is signif-
icantly more difficult to form and isolate compounds
with Sb@C bonds [7]. Only few of them have been iso-
lated as crystalline products: the stibaenols
R(CO)Sb@C(OH)R (R = t-Bu, Mes, Ar) [8] and two
m/z 1087. Two main fragments were detected at 543
and 352, corresponding to ArSb@C@CR2 + H and
R2C@C@C@CR2, respectively, obtained by a [2 + 2]
decomposition of 3 according to routes (a) and (b)
(Scheme 2). By electronic impact, one of the major frag-
ments correspond to m/z 352. The head-to-head dimeric
structure of 3 indicated by mass spectrometry was con-
2,3-distibabutadienes
R0(Me3SiO)C@Sb–Sb@C(Me3-
1
SiO)R0 (R0 = Mes, Ar) [9]. The stibaalkene 2-Pyr(Me3-
Si)2C–Sb@C(SiMe3)2-Pyr (Pyr = C5H4N) [10] has been
obtained as an oil and other two have been physico-
chemically or chemically characterized [11]. Thus, for
the synthesis of the stibaallene ArSb@C@CR2, it
appeared better to create the Sb@C double bond in
the final step by a dechlorofluorination of a 2-chloro-
3-fluorostibapropene with tert-butyllithium. Such a
route was successfully used for the preparation of a
heteroallene with a heavy group 14 element, the germa-
allene Tbt(Tip)Ge@C@CR2 [12] (Tbt = 2,4,6-tris[bis(tri-
meth- ylsilyl)methyl]phenyl).
firmed by H and 13C NMR data very similar spectra
to those of the stibapropene 2, with the non equivalence
of carbon and proton atoms of the two six-membered
rings in both fluorenylidenes. This result rules out the
head-to-tail dimer, in which these two six-membered
rings should be equivalent, leading to two doublets
and two triplets for the protons of the CR2 group in
1H NMR, and only four signals for C1–8 in 13C NMR.
Whereas the head-to-tail dimer 4 could have resulted
from the coupling of two molecules of the intermediate
lithio-compound R2C@C(Li)–Sb(F)Ar, the head-to-
head dimer 3 can only be obtained from the transient sti-
baallene 1. This reaction represents the first evidence for
the formation of a stibaallenic compound. Moreover, to
the best of our knowledge, such a four-membered heter-
ocycle containing two adjacent antimony atoms has
never been previously reported.
Stibapropene 2 was obtained nearly quantitative yield
from the aryldifluorostibane ArSbF2 (prepared from tri-
fluoroantimony and ArLi), and the carbenoid
R2C@C(Cl)Li (obtained in situ from R2C@CCl2 [13])
(see Scheme 1).
Compound 2 was characterized by mass spectrometry
and particularly by NMR spectroscopy which displays
as expected non equivalent aromatic rings of the fluor-
enylidene group: four doublets were observed for
Similar transient allenes substituted by a fluorenylid-
ene group have been evidenced by their dimer: the tran-
sient chloroallene R2C@C@C(Ph)Cl has been
postulated to give the 1,2-bis(9-fluorenylidene)cyclobu-
tane 5 by a head-to-head dimerization as in the case of
1 [14] (see Scheme 3).
1
H1, 4, 5 and 8 in H NMR and eight signals for the aro-
matic carbons in the 13C NMR spectrum. The protons
of the o-tert-butyl groups appear in the form of a broad
singlet indicative of a slow rotation of the Ar group.
This is confirmed by the 13C NMR spectrum in which
the methyl groups of o-tert-butyl groups also appear
as a broad signal.
A
1H NMR spectrum similar to that of 3, with
unequivalent protons and carbons in both fluorenylid-
ene groups, was observed [14]. Some head-to-head
dimerizations by the Si@C double bond of silaallenes
Ar02Si@C@CðSiMe3Þ (Ar0@Ph [15a,15b], o-, m- and p-
2
Addition of tert-butyllithium to a THF solution of 2
at low temperature afforded 1,2-bis(2,4,6-tri-tert-butyl-
phenyl)-3,4-bis(9-fluorenylidene)-1,2-distibacyclobutane
3.
Tol [15c]) leading to 1,2-disilacyclobutanes with two
exocyclic C@C bonds have also been reported.
2.1. A molecular orbital insight into the dimerization of 1
Suitable single crystals for an X-ray analysis could
not be obtained for 3. However, its structure has been
unambiguously determined by other physicochemical
methods. In mass spectrometry by chemical ionization
(NH3), the protonated molecula peak was observed at
Molecular orbital calculations at the PM3 level have
been carried out on the transient stibaallene
ArSb@C@CR2 1 and its head-to-head 3 and head-to-tail
4 (cis and trans) dimers. The parent compound
HSb@C@CH2, 1H and its dimers 3H and 4H have been
considered also by ab initio and DFT treatments.
Fig. 1 shows the optimized structure of the lowest en-
ergy (trans-3) head-to-head dimer and Table 1 includes
the calculated enthalpies of formation of 1, 3 and 4.
Data of Table 1 shows that there is a copious stabili-
zation of the dimer against the monomeric stibaallene
and that the head-to-head dimer 3 is much more favored
than the head-to-tail dimer 4. In both cases, the trans
isomer is lower in energy.
Cl
Cl
Li
F
CR2
Cl
ArSbF2
n-BuLi
-90 ˚C
R2C
C
R2C
C
Sb
C
Ar
Cl
2
2
3
1
4
11
12
13
9C
, CR2 =
Ar =
10
8
5
6
7
More elaborate methods (ab initio and DFT) have
been applied to 1H, 3H and 4H in order to learn more
Scheme 1.

L. Baiget et al. / Journal of Organometallic Chemistry 690 (2005) 307–312
309
-90 ˚C to RT
-LiF
t-BuLi
-90 ˚C
ArSb
F
C
Li
CR2
ArSb
C
CR2
2
1
x 2
(b)
Sb
Sb
(a)
Sb
Sb
3
4
Scheme 2.
Table 1
PM3 calculated enthalpies (kcal/mol) of 1, 3 and 4
Ph
R2C
R2C
Cl
Ph
C
System
DHf
Dimerization
energy DHf(dimer)-2 ·
DHf(monomer)
cis–trans
relative energies
R2C =
Cl
1
121.349
164.223
159.395
193.967
185.447
Scheme 3.
cis-3
trans-3
cis-4
trans-4
ꢁ78.475
ꢁ83.303
ꢁ48.731
ꢁ57.251
+4.828
0.0
+8.520
0.0
The main results coming out of these calculations are
that the order of preference for the head-to-head dimeri-
zation is maintained and the HF method gives stabiliza-
tion energies in the same range as the PM3 method. The
DFT results are also in agreement with the preferred for-
mation of the head-to-head trans-3H isomer. Note that
similar finding has been reported from a HF study on
the phosphagermaallene H2Ge@C@PH [16] where the
Ge–Ge head-to-head [2 + 2] cycloaddition product was
significantly more stable than the head-to-tail dimer.
Geometry data on the trans isomers of 3H and 4H are
given in Scheme 4. While 4H is predicted to be planar,
3H is bent along the Sbꢀ ꢀ ꢀC diagonal. The two C@CH2
moieties are in the same plane in trans-4H and twisted
around the ring C–C bond in trans-3H by 42.58ꢁ
(RHF). Some of the conjugation between the two
C@CH2 is however maintained since the central CC
Fig. 1. The PM3 calculated structure of trans-3 (hydrogen atoms
omitted). Relevant geometry data for the Sb2C2 cycle: Sb–Sb =
˚
˚
˚
2.733 A, Sb–C = 2.175 A, C–C = 1.435 A, \SbCC = 102.40ꢁ,
\CSbC = 69.95ꢁ, fold angle (along the Sbꢀ ꢀ ꢀC diagonal of the Sb2C2
ring) = 42.25ꢁ.
˚
bond (1.487 A) is still shorter than a normal single bond
(as noticed also at the PM3 level within the real systems).
The inspection of the frontier orbitals of 1H (Scheme
5) reveals that HOMO (ꢁ8.000 eV) and LUMO (1.497
eV) are composed mainly from antimony 5p and the
about the dimerization of the stibaallene. Table 2 in-
cludes the calculated energies at these two levels of
theory.

310
L. Baiget et al. / Journal of Organometallic Chemistry 690 (2005) 307–312
Table 2
Total (a.u.) and dimerization energies (kcal/mol) of the investigated models
System
RHF/gen
Total
B3LYP/lanl2dz(dp)
Total
E(dimer) ꢁ 2 · E(monomer)
E(dimer) ꢁ 2 · E(monomer)
1H
ꢁ316.6286888
ꢁ633.3844506
ꢁ633.3897640
ꢁ633.3488112
ꢁ633.3483974
ꢁ83.3271427
ꢁ166.7586775
ꢁ166.7635318
ꢁ166.7334955
ꢁ166.7335072
cis-3H
trans-3H
cis-4H
trans-4H
ꢁ79.75
ꢁ83.08
ꢁ57.39
ꢁ57.12
ꢁ65.51
ꢁ68.56
ꢁ49.71
ꢁ49.71
2.182 Å
thaw’’ cycles. NMR spectra were recorded in CDCl3
on the following spectrometers: 1H, Bruker AC200
(200.13 MHz); 13C{1H}, Bruker AC200 (50.32 MHz)
(reference TMS); 19F, Bruker AC200 (188.30 MHz) (ref-
erence CF3COOH). Melting points were determined on
a Wild Leitz–Biomed apparatus. Mass spectra were ob-
tained on a Hewlett–Packard 5989A spectrometer by EI
at 70 eV and on a Nermag R10-10 spectrometer by CI.
HF ab initio and B3LYP hybrid DFT methods have
been used to fully optimize the model HSb@C@CH2 al-
lene 1H as well as the head-to-head and head-to-tail di-
mers of 1H. The basis sets used were cc-pVDZ-PP for Sb
[17] and 6-31G* on carbon and hydrogen for the HF cal-
culations and Lanl2dz with (d,p) polarization functions
[18] for antimony. Note that both of the basis functions
on antimony account for the relativistic effects.
1.320 Å
H2C
CH2
HSb
HSb
C
SbH
C
2.856 Å
C
C
1.323 Å
2.176 Å
1.487 Å
HSb
79.00o
CH2
CH2
70.89o
126.00o 106.66o
100.58o
trans-3H
trans-4H
Scheme 4.
Sb
C
C
The ab initio and DFT calculations have been per-
formed by using G98 suite of programs [19]. The basis
sets and pseudopotentials were retrieved from the Pacific
NorthwestÕs Basis Set Library [20]. Semiempiric PM3
calculations on the real systems have been performed
by using SpartanÕ02 from Wavefunction [21].
HOMO
LUMO
Scheme 5.
3.1. Synthesis of stibapropene 2
central carbon 2p orbitals. Since the relative contribu-
tions from antimony are higher, it is expected that the
primary interaction of two stibaallene fragments occurs
between these two atoms and, first the Sb–Sb bond is
formed.
Thus, both HOMO–LUMO interactions as well as
the energy data support the experimental observation
of the head-to-head dimers of 1. The driving force for
the head-to-head dimerization could be the formation
of a dienic structure. Our efforts are now directed to-
wards stabilization of stibaallenes using bulkier groups
in order to avoid the approach of two molecules to
dimerize.
To an orange solution of 9-dichloromethylenefluo-
rene (1.57 g, 6.36 mmol) in 15 ml of THF were added
dropwise, at ꢁ78 ꢁC, 4.2 ml (6.72 mmol) of n-BuLi
(1.6 M in hexane); the solution turned purple and was
stirred at this temperature for 15 min. The lithium com-
pound was then added to a solution of ArSbF2 (2.58 g,
6.36 mmol) in 15 ml of THF cooled at ꢁ78 ꢁC. The reac-
tion mixture turned green during stirring for 10 min,
then it was allowed to warm to room temperature and
turned yellow. Solvents were removed in vacuo and
the residue dissolved in 20 ml of pentane/dichlorometh-
ane (50/50). After removal of the lithium salts by filtra-
tion, the resulting solution was crystallized to afford 3.18
1
g (83%) of 2 as a yellow powder (m.p. 155–160 ꢁC). H
3. Experimental
NMR d: 1.35 (s, 9H, p-t-Bu); 1.42 (broad s, 18H, o-t-
3
Bu); 7.24 and 7.35 (2t, JHH = 7.6 Hz, 2 · 2H,
HC2, 3, 6, 7 CR2); 7.43 (s, 2H, arom H Ar); 7.68 and
All experiments were carried out in flame-dried glass-
ware under a nitrogen atmosphere using high-vacuum-
line techniques. Solvents were dried and freshly distilled
from sodium benzophenone ketyl and carefully deoxy-
genated on a vacuum line by several ‘‘freeze–pump–
7.70 (2d, 3JHH = 7.6 Hz, 2 · 1H, two of HC1, 4, 5, 8 CR2);
8.12 (d, JHH = 7.6 Hz, 1H, one of HC1, 4, 5, 8 CR2); 8.56
3
3
(d, JHH = 7.6 Hz, 1H, one of HC1, 4, 5, 8 CR2).

L. Baiget et al. / Journal of Organometallic Chemistry 690 (2005) 307–312
311
13C NMR d: 31.4 (p-C(CH3)3 Ar); 33.4 (broad s, o-
C(CH3)3); 34.8 and 39.8 (o- and p-C(CH3)3); 119.3 and
119.9 (C4 and C5 CR2); 124.1 (m-C Ar); 124.9 (d,
JCF = 2.8 Hz), 126.9, 127.7, 128.2 (d, JCF = 4.6 Hz),
128.7 and 129.3 (CH CR2); 138.1, 140.8, 142.8, 143.1,
145.3, 151.0, 156.4, 156.8 and 158.6 (o-, p- and ipso-C
Ar, C@C and C10–13 CR2). 19F NMR d: ꢁ103.1. MS
(EI, 70 eV, m/z, %): 598 (M, 1); 578 (M–F-1, 3); 385 (M-
R2C@C(Cl), 5); 365 (M-R2C@C(Cl)–F– , 17); 245 (Ar,
28); 57 (t-Bu, 100). Anal. Calc. for C32H37SbClF
(M = 597.85): C, 64.29; H, 6.24. Found: C, 64.50; H,
6.31%.
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3.2. Synthesis of distibacyclobutane 3
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´
(d) H. Ramdane, H. Ranaivonjatovo, J. Escudie, N. Knouzi,
1.20 ml (1.88 mmol) of t-BuLi (1.5 M in pentane) were
added dropwise to a solution of ArSb(F)C(Cl)=CR2
(1.02 g, 1.70 mmol) in a mixture THF/Et2O (25/5 ml)
cooled at ꢁ78 ꢁC. The reaction mixture turned blue,
was stirred for 10 min at this temperature and then al-
lowed to warm to room temperature. After 30 min of
stirring, solvents were removed under vacuum, 20 ml of
pentane were added and the lithium salts were removed
by filtration. Fractional crystallizations from pentane af-
forded 0.46 g (49%) of 3 as an orange powder (m.p. 170–
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´
H. Ramdane, J. Am. Chem. Soc. 122 (2000) 12880.
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1
173 ꢁC). H NMR d: 1.37 (s, 18H, p-t-Bu Ar); 1.47 (s,
36H, o-t-Bu Ar); 7.04 (t, JHH = 7.6 Hz, 2H, two of
HC2, 3, 6, 7 CR2); 7.18–7.35 (m, 6H, six of HC2, 3, 6, 7
3
(b) P.B. Hitchcock, C. Jones, J.F. Nixon, Angew. Chem., Int. Ed.
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3
CR2); 7.48 (s, 4H, arom H Ar); 7.64 (d, JHH = 7.6 Hz,
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3
4H, four of HC1, 4, 5, 8 CR2); 8.13 (d, JHH = 7.9 Hz,
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3
2H, two of HC1, 4, 5, 8 CR2); 8.64 (d, JHH = 7.9 Hz, 2H,
two of HC1, 4, 5, 8 CR2). 13C NMR d: 31.4 (p-C(CH3)3
Ar); 34.0 (o-C(CH3)3 Ar); 35.1 (p-C(CH3)3 Ar); 40.0 (o-
C(CH3)3 Ar); 119.2, 119.6, 119.7, 123.3, 125.7, 126.5,
127.5, 128.2 and 128.9 (m-C Ar and CH CR2); 138.2,
138.7, 139.8, 140.4, 140.7, 147.2, 150.0, 150.8 and 159.0
(o-, p- and ipso-C Ar, C@C and C10–13 CR2). MS (EI,
70 eV, m/z, %): 366 (ArSb, 6); 352 (R2C@C–C@CR2,
29); 245 (Ar, 49); 176 (C@CR2, 8); 57 (t-Bu, 100). (CI/
NH3, m/z, %): 1087 (M + H, 5); 859 (M-Ar + NH4, 4);
841 (M–Ar, 1); 577 (ArSb–C@CR2 + N2H7, 100); 560
(ArSb–C@CR2 + NH4, 9); 543 (ArSb–C@CR2 + H,
10); 367 (ArSb + H, 25); 280 (Ar + N2H7, 60); 246
(Ar + H, 62). Anal. Calc. for C64H74Sb2 (M = 1086.79):
C, 70.73; H, 6.86. Found: C, 70.50; H, 6.61%.
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