Compounds containing λ3,σ2-Sb=C bonds: Synthesis and structural characterisation of the first stiba-enol, Mes*C(O)Sb=C(OH)Mes* (Mes* = C6H2But3-2,4,6) and a 2,3-distibabutadiene, {Mes(Me<su

Article abstract of DOI:10.1039/a902791b

The reactions of [Li{Sb(SiMe₃)₂}] with RCOCl, R = C₆H₂Bu^t₃-2,4,6 (Mes*) or C₆H₂Me₃-2,4,6 (Mes), afford mixtures of the 2,3-distibabutadienes, {R(Me₃

Full text of DOI:10.1039/a902791b

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Compounds containing ë3,ó2-Sb᎐C bonds: synthesis and structural
᎐
characterisation of the first stiba-enol, Mes*C(O)Sb᎐C(OH)Mes*
᎐
(Mes* ؍ C6H2But3-2,4,6) and a 2,3-distibabutadiene,
{Mes(Me SiO)C᎐Sb} (Mes ؍ C H Me -2,4,6)†
᎐
3
2
6
2
3
Cameron Jones,*a Jonathan W. Steedb and Ryan C. Thomasa
a Department of Chemistry, University of Wales, Cardiff, PO Box 912, Park Place, Cardiff,
UK CF1 3TB. E-mail: jonesca6@cardiff.ac.uk
b Department of Chemistry, King’s College London, Strand, London, UK WC2R 2LS
Received 8th April 1999, Accepted 20th April 1999
The reactions of [Li{Sb(SiMe3)2}] with RCOCl, R ؍
C6H2But3-2,4,6 (Mes*) or C6H2Me3-2,4,6 (Mes), afford
mixtures of the 2,3-distibabutadienes, {R(Me SiO)C᎐Sb} ,
᎐
2
and the 2-stiba-1,3-dionatolithium complexes3, [Li{OC(R)-
SbC(R)O}(DME)n], n ؍ 1 or 0.5, the latter of which (R ؍
Mes*) can be protonated to give the first stiba-enol,
Mes*C(O)Sb᎐C(OH)Mes*, which has been structurally
᎐
characterised.
Since the preparation of the first thermally stable phospha-
t
᎐
alkyne, P᎐CBu , in 1981 the field of low coordination
᎐
phosphorus chemistry has become well established.1 Not sur-
prisingly, the chemistry of compounds containing As–C
multiple bonds was slower to develop but is now relatively well
explored.2 By contrast, there is a paucity of knowledge of
analogous low coordination antimony compounds which prob-
ably results from their inherent thermal instability. In fact, to
date there is only one structurally characterised example of a
Fig. 1 Molecular structure of Mes*C(O)Sb᎐C(OH)Mes* 5. Selected
᎐
bond lengths (Å) and angles (Њ): Sb(1)–C(20) 2.078(3), Sb(1)–C(1)
2.192(3), O(1)–C(1) 1.248(4), O(2)–C(20) 1.319(4), O(2)–H(2) 0.96(5),
O(1) ؒ ؒ ؒ H(2) 1.61(5); C(20)–Sb(1)–C(1) 91.31(12), O(2)–H(2)–O(1)
172(5), O(1)–C(1)–C(2) 120.9(3), O(1)–C(1)–Sb(1) 121.2(2), C(2)–C(1)–
Sb(1) 118.0(2), O(2)–C(20)–C(21) 117.9(3), O(2)–C(20)–Sb(1) 124.7(2),
C(21)–C(20)–Sb(1) 117.2(2).
compound, {R(Me SiO)C᎐Sb} 1 [R = C H But -2,4,6 (Mes*)],
᎐
3
2
6
2
3
that contains largely localised Sb–C double bonds,3 though
related compounds have recently been implicated as reactive
intermediates in the formation of stibacycles.4 This remark-
ably stable compound was prepared in low yield from the
reaction of Mes*COCl with [Li{Sb(SiMe3)2}], a surprising
result considering that the analogous reaction of ButCOCl
with [Li{Sb(SiMe3)2}] affords a high yield of the delocalised 2-
stiba-1,3-dionatolithium complex, [{[Li{OC(But)SbC(But)O}-
(DME)0.5]2}∞],5 the coordination chemistry of which we are
currently investigating.6 Herein we report that a stibadionato-
lithium complex is, indeed, the major reaction product in the
preparation of 1 and that a similar product mixture is obtained
in the reaction of the less hindered acyl chloride MesCOCl
(Mes = C6H2Me3-2,4,6) with [Li{Sb(SiMe3)2}]. In addition, this
work has led to the synthesis and structural characterisation of
the first stiba-enol which, in the solid state, contains a rare
example of a localised Sb–C double bond.
R
Sb
Li
R
R
Me3SiO
C
i
+
Sb Sb
[Li{Sb(SiMe3)2}]
O
O
C
OSiMe3
(DME)n
R
1 R = Mes*
2 R = Mes
3 R = Mes*, n = 1
4 R = Mes, n = 0.5
ii
Sb
Mes*
Mes*
O
O
H
5
Scheme 1 Reagents and conditions: i, RCOCl, DME, 18 h; ii,
R = Mes*, HCl, Et2O, 0 ЊC, 2 h.
The product mixtures obtained from the treatment of [Li{Sb-
(SiMe3)2}] with 1 equivalent of either Mes*COCl or MesCOCl
were extracted with hexane to give the distibabutadienes, 1 and
2, in low yield (18% and 5% respectively) after concentration of
the extracts (Scheme 1). The hexane insoluble fractions of the
reaction mixtures were further extracted with diethyl ether
affording the 2-stiba-1,3-dionatolithium complexes, 3 and 4, in
moderate yields (38% and 45%) after recrystallisation. Treat-
ment of a diethyl ether solution of 3 with 1 equivalent of
anhydrous HCl, followed by recrystallisation from diethyl
ether gave red crystals of the light sensitive stiba-enol, 5, in high
yield (97%).
Compound 5 is stable in toluene solutions for only 15 min-
utes at room temperature. As a result, spectroscopic data (see
SUP 57542) for the compound were collected at 0 ЊC and are
consistent with it existing predominantly in the enol form in
this solvent (cf. its As analogue5). Evidence for this suggestion
1
comes from its H NMR spectrum which displays a low field
resonance at δ 18.48 in the region normally associated with
strongly hydrogen bonded alcoholic protons. The symmetry of
this spectrum also suggests that 5 possesses a fully delocalised
structure in solution in which the alcoholic proton is under-
going a rapid exchange between the two oxygen centres of the
molecule. In the solid state 5 is more thermally stable (decomp.
103 ЊC) and its crystal structure‡ (Fig. 1) confirms that it exists
in the enol form but with localised Sb(1)–C(20) and C(1)–O(1)
double bonds, the former of which compares well with those
in 1 [2.056(10) Å]3 and 2 [2.066(5) Å] (see below) but is con-
siderably shorter than normal Sb–C single bonds {e.g. 2.225
† Supplementary data available: synthetic and spectroscopic details for
compounds 2–5. For direct electronic access see http://www.rsc.org/
suppdata/dt/1999/1541/, otherwise available from BLDSC (No. SUP
57542, 4 pp.) or the RSC Library. See Instructions for Authors, 1999,
Issue 1 (http://www.rsc.org/dalton).
J. Chem. Soc., Dalton Trans., 1999, 1541–1542
1541

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DME molecule, as is the case for its As counterpart.9 These
differences in the degree of association between 3 and 4 would
be expected considering the bulk of the aryl substituent in 3. As
is the case for 6, the symmetry of the solution state 1H and 13
C
NMR spectra of 3 and 4 suggest that the ligand backbones of
these complexes are delocalised.
We are currently exploring the use of 2 and 5 as ligands in
inorganic synthesis and the utility of 3 and 4 as reagents for the
transfer of the 2-stiba-1,3-dionate fragments onto other metal
centres. We are also investigating the mechanisms of formation
of 1–4. The results of these investigations will form the basis of
forthcoming publications.
Acknowledgements
We gratefully acknowledge financial support from the EPSRC
(studentship for R. C. T.).
Notes and references
‡ Crystal data for 5: C38H59O2Sb, M = 669.60, orthorhombic, space
group Pcab, a = 11.4906(2), b = 20.0826(4), c = 31.2742(5) Å, V =
7216.9(2) Å3, Z = 8, Dc = 1.233 g cmϪ3, F(000) = 2832, µ = 7.94 cmϪ1
,
crystal 0.20 × 0.20 × 0.10 mm, radiation Mo-Kα (λ = 0.71070 Å), T =
100(2) K, 50378 reflections collected. For 2: C26H40O2Sb2Si2, M =
684.26, monoclinic, space group P21/c, a = 10.704(2), b = 14.043(3),
c = 10.889(2) Å, β = 109.57(3)Њ, V = 1542.2(5) Å3, Z = 2, Dc = 1.473 g
cmϪ3, F(000) = 1368, µ = 18.48 cmϪ1, crystal 0.20 × 0.20 × 0.10 mm,
radiation Mo-Kα (λ = 0.71070 Å), T = 100(2) K, 13007 reflections
collected. All crystallographic measurements were made using an
Enraf-Nonius Kappa-CCD diffractometer. Both structures were solved
by direct methods and refined on F 2 by full matrix least squares
(SHELX97)10 using all unique data. All non-hydrogen atoms are aniso-
tropic with H-atoms [except H(2) in 5] included in calculated positions
(riding model). Absorption corrections were carried out using Scale-
pack.11 Final R (on F) were 0.0426 (5) and 0.0331 (2) and wR (on F 2)
were 0.0838 (5) and 0.0940 (2) for I > 2σ(I). CCDC reference number
186/1431. See http://www.rsc.org/suppdata/dt/1999/1541/ for crystallo-
graphic files in .cif format.
Fig. 2 Molecular structure of {Mes(Me3SiO)C᎐Sb}2 2. Selected bond
᎐
lengths (Å) and angles (Њ): Sb(1)–C(10) 2.066(5), Sb(1)–Sb(1)Ј
2.8018(8), O(1)–C(10) 1.377(5), Si(1)–O(1) 1.697(3); C(10)–
Sb(1)–Sb(1)Ј 92.99(13), O(1)–C(10)–C(1) 110.6(4), O(1)–C(10)–Sb(1)
125.2(3), C(1)–C(10)–Sb(1) 124.3(3).
(average) in [But3SbؒFe(CO)4]7}. The acute nature of the
C–Sb–C angle in 5 [91.31(12)Њ] probably results from a signifi-
cant degree of s-character for the hetero-atom lone pair. This is
a common feature of other low coordinate Group 15 systems
(e.g. RE᎐ER, E = N, P, As, Sb, Bi) and has been found to be
᎐
augmented with increasing molecular weight of the Group 15
element.8 The alcoholic proton H(2) was located from differ-
ence maps and refined isotropically. It is bonded to O(2) and
appears to have a strong H-bonded interaction with O(1), the
angle O(1)–H(2)–O(2) being 172(5)Њ. As has been suggested for
13 the unusual stability of 5 can probably be attributed to a
combination of the steric protection afforded by its bulky aryl
substituents and the conjugated nature of the system.
1 K. B. Dillon, F. Mathey and J. F. Nixon, in Phosphorus: The Carbon
Copy, Wiley, Chichester, 1998, and refs. therein.
2 L. Weber, Chem. Ber., 1996, 129, 367 and refs. therein.
3 P. B. Hitchcock, C. Jones and J. F. Nixon, Angew. Chem., Int. Ed.
Engl., 1995, 34, 492.
4 P. C. Andrews, C. L. Raston, B. W. Skelton, V.-A. Tolhurst and
A. H. White, Chem. Commun., 1998, 575.
5 J. Durkin, D. E. Hibbs, P. B. Hitchcock, M. B. Hursthouse, C. Jones,
J. Jones, K. M. A. Malik, J. F. Nixon and G. Parry, J. Chem. Soc.,
Dalton Trans., 1996, 3277 and refs. therein.
6 S. J. Black, D. E. Hibbs, M. B. Hursthouse, C. Jones and J. W. Steed,
Chem. Commun., 1998, 2199.
7 A. L. Rheingold and M. E. Fountain, Acta Crystallogr., Sect. B,
1985, 41, 1162.
The distibabutadiene 2 (decomp. 105 ЊC) is not as thermally
stable as its more sterically protected counterpart 1 (decomp.
213 ЊC) but is nevertheless stable in air at ambient temperature
for days. Its molecular structure‡ (Fig. 2) is similar to that of
1 and shows it to exist in the trans- form with the atoms
C(10), Sb(1), Sb(1)Ј and C(10)Ј being necessarily co-planar. The
Sb–C bond length is close to those in 1 and 5 (see above) and
as with the C–Sb–C angle in 5 the sharp Sb–Sb–C angles in 2
[92.99(13), cf. 94.7(3)Њ in 13] can be explained by a high degree
of s-character for the Sb lone pairs.
The 2-stibadionato lithium complexes, 3 and 4, are consider-
ably more stable (3 decomp. 170, 4 decomp. 103 ЊC) than the
only other example of such a compound, [{[Li{OC(But)SbC-
(But)O}(DME)0.5]2}∞] 6 (decomp. 65 ЊC).5 No crystallographic
data were obtained for 4 but in the solid state it probably con-
sists of oxygen and lithium bridged dimeric units linked by non-
chelating, bridging DME molecules, as has been found for 6
and a number of related 2-arsa- and 2-phospha-dionatolithium
complexes.5 Compound 3 on the other hand is probably
monomeric in the solid state and has its Li centre chelated by a
8 N. Tokitoh, Y. Arai, T. Sasamori, R. Okazaki, S. Nagase, H. Uekusa
and Y. Ohashi, J. Am. Chem. Soc., 1998, 120, 433; N. Tokitoh,
Y. Arai, R. Okazaki and S. Nagase, Science, 1997, 277, 78; N. C.
Norman, Polyhedron, 1993, 12, 2431.
9 C. Jones and R. C. Thomas, unpublished work.
10 G. M. Sheldrick, SHELX97, University of Göttingen, 1997.
11 Z. Otwinowski and W. Minor, in Methods in Enzymology, ed. C. W.
Carter and R. M. Sweet, Academic Press, New York, 1996.
Communication 9/02791B
1542
J. Chem. Soc., Dalton Trans., 1999, 1541–1542