A seven-membered aluminum sulfur allenyl heterocycle arising from the conversion of an aluminacyclopropene with CS2

Article abstract of DOI:10.1021/ja049462t

The reaction of an aluminacyclopropene LAl[η²-C₂(SiMe₃)₂] (1, L = HC(CMeNAr)₂, Ar = 2,6-iPr₂C₆H₃) with CS₂ in the temperature range from -78 °C to room temperature affords the first seven-membered aluminum sulfur-containing heterocyclic compound [LAl]₂(μ-S)[η²-SC(SiMe₃)=C=C(SiMe₃)] (2) bearing an allenyl group. The structural characterization of 2 and the analogous compound LAl[OC(O)C₂(SiMe₃)₂] (3) of the proposed intermediate A and the variable-temperature ¹H NMR kinetic study of this reaction may give a better understanding on this unusual conversion. Copyright

Full text of DOI:10.1021/ja049462t

Published on Web 07/29/2004
A Seven-Membered Aluminum Sulfur Allenyl Heterocycle Arising from the
Conversion of an Aluminacyclopropene with CS2
Hongping Zhu,† Jianfang Chai,† Qingjun Ma,† Vojtech Jancik,† Herbert W. Roesky,*,†
Hongjun Fan,‡ and Regine Herbst-Irmer†
Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 4, D-37077 Go¨ttingen, Germany, and
Labor fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Siegen, D-57068 Siegen, Germany
Received January 30, 2004; E-mail: hroesky@gwdg.de
There is a widespread interest in main group heterocyclic
compounds due to their applications in pharmaceutical, agrochemi-
cal, and material science.1 In recent years, the number of group 13
metal heterocycles has increased. Within the heterocycles, those
containing multiple bonds have shown interesting structural features
and properties. MnN2C2 (M ) Al, Ga, In, n ) 2; M ) Ga, n ) 1,
3)2,3 and MC3N2 (M ) Al, Ga)4 heterocycles with CdC or CdN
double bonds exhibit high electron density. The three-membered
AlC2 ring with a CdC bond shows a highly strained structure and
good reactivity.5 Nonetheless, such species are rare, and the
heteroatoms within the cycle are limited to C, N, and O elements
due to the lack of appropriate synthetic routes. Herein we report
on a seven-membered Al2C3S2 heterocycle bearing a CdCdC
allenyl group prepared from the reaction of an aluminacyclopro-
pene, LAl[η2-C2(SiMe3)2] (1, L ) HC(CMeNAr)2, Ar ) 2,6-iPr2-
C6H3)5b with carbon disulfide. Interestingly, the unusual formation
Figure 1. Molecular structure of 2 (without H atoms). Selected bond lengths
(Å) and angles (deg): Al(1)-S(1) 2.2298(11), Al(1)-C(56) 1.990(3),
of the allenyl group, to the best of our knowledge, is hitherto
unknown in organoaluminum chemistry.
Al(2)-S(1) 2.2188(11), Al(2)-S(2) 2.2270(11), S(2)-C(54) 1.809(3),
C(54)-C(55) 1.325(4), C(55)-C(56) 1.316(4); Al(1)-S(1)-Al(2) 122.57(4),
The addition of neat carbon disulfide to the solution of 1 in
toluene was carried out at -78 °C. In the course of warming the
solution to room temperature, the color changed from red-black to
green, to yellow green, and finally to yellow. Additional stirring
for 12 h resulted in the formation of 2.
C(54)-C(55)-C(56) 178.0(3).
Scheme 1. Proposed Mechanism for the Conversion of 1 with
CS2 to 2
The initial reaction of 1 with CS2 closely resembles that of 1
with CO2,5b resulting in the generation of the five-membered
heterocycle LAl[SC(S)C2(SiMe3)2] (A). Obviously, A further
interacts with a second molecule of 1 at the donor exocyclic S atom
in A and the acceptor Al center in 1 to give intermediate B. The
fact that use of either 1 equiv or an excess of CS2 led to isolation
of only product 2 indicates a stronger competitive reaction of A
with 1 than with CS2. B reacts further under elimination of one
molecule of Me3SiCCSiMe3 due to the weak Al-η2-Calkyne bonding
(the bond dissociation energy of Al-η2-C2 is only 95.8 kJ/mol and
6
therefore much less than that of Al-Cmethyl in AlMe3 (Daverage, 281.4
kJ/mol; see Experimental Section II.1). Free Me3SiCCSiMe3 was
1
detected in the H NMR kinetic study. An easy migration of one
Me3Si group7 and rearrangement of CdSexo and µ-S-C bonds led
to the unique formation of the thioallenyl functional group and the
final product 2. A proposed mechanism for the formation of 2 is
given in Scheme 1. A parallel investigation of 1 with S8 resulted
in the formation of a dimer [LAlS]2 (see Experimental Section I.3).
Evidently, the bulky â-diketiminato ligand with the steric and
electronic stabilizing properties acts as an optimal skeleton for the
aluminum center throughout the reaction process.
1), which is in sharp contrast to those of known dinuclear aluminum
sulfides or thiolates. Each aluminum atom is chelated by a
â-diketiminato ligand to form LAl moieties. Between the two LAl,
one S atom and one SC(SiMe3)CC(SiMe3) group each form a
bridge. The bond lengths of Al-Sbridge (2.2188(11), 2.2298(11) Å)
and Al-Sthioallenyl (2.2270(11) Å) are in the range (2.10-2.30 Å)
of dinuclear aluminum sulfides.9 However, the bond angle of
Al(1)-S(1)-Al(2) (122.57(4)°) is the widest among those of
dinuclear aluminum sulfides or thiolates (78.1-117.5°).9,10 The two
central Al atoms, the µ-S atom and the SCCC group constitute one
seven-membered Al2C3S2 ring, of which S(2), Al(2), S(1), Al(1),
and C(56) are arranged within a plane (mean deviation ∆ ) 0.0375
Å), while the C(56)C(55)C(54) group is located outside of this
Compound 2 was obtained as yellow crystals, which were
characterized by spectroscopy and by X-ray crystallography.8a The
structural analysis of 2 reveals a novel heterocyclic molecule (Figure
† Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
‡ Universita¨t Siegen.
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10194
J. AM. CHEM. SOC. 2004, 126, 10194-10195
10.1021/ja049462t CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
The variable-temperature 1H NMR kinetic study of this reaction
(Figure 3) reveals the gradual consumption of 1 (a) and formation
of 2 (b and c), including the byproduct Me3SiCCSiMe3 (d) in I. In
II, this process is accompanied by the occurrence and quick
disappearance of some resonances, indicating that this conversion
proceeds via the proposed intermediates A and B to completion.
Thus, an unusual conversion of 1 with CS2 to 2 has been shown,
where the search of sterically and energetically favored unsaturated
substrate CS2 is crucial. An investigation on such selenium- or
tellurium-containing heterocycles is in progress.
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft and the Go¨ttinger Akademie der Wissen-
schaften.
Figure 2. Molecular structure of 3 (without H atoms). Selected bond lengths
(Å) and angles (deg): Al(1)-O(1) 1.7696(12), Al(1)-C(3) 1.9891(17),
C(1)-O(1) 1.331(2), C(1)-O(2) 1.218(2), C(1)-C(2) 1.530(2), C(2)-C(3)
1.364(2); O(1)-C(1)-O(2) 121.78(15).
Supporting Information Available: Experimental Section, includ-
ing detailed synthetic procedures, analytical and spectral characterization
data, and DFT calculations (PDF), as well as CIF data for 2 and 3.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 3. Variable-temperature 1H NMR kinetic studies of the reaction of
1 with excess CS2 in [D8]toluene. (I) Resonances of SiMe3 (0.10-0.55 ppm),
which correlate with changes of the η2-C2(SiMe3)2 group, and (II) resonances
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(8) (a) Crystallographic data for 2‚0.55 toluene‚0.45 ether with Cu KR (λ )
1.54178 Å) radiation at 133(2) K: a ) 18.903(4), b ) 18.624(4), c )
20.125(4) Å, â ) 94.36(3)°, monoclinic, space group Cc, Z ) 4, R1
)
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may approach that of A (Figure 2). As expected, the CO2 was
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AlC3O planar ring with an exo CdO group (∆ ) 0.0199 Å). The
longer bond distances and weaker bond strength of CS2 than those
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of A with 1 and rearrangement of the CdSexo unit.
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2003-2004; Vol. 84, pp 9-19.
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