Isolation of a 1,2-dialuminacyclobutene

Article abstract of DOI:10.1002/anie.200504329

Aluminum siding: Reduction of Ar′All₂ (Ar′ = 2,6-Dipp₂C₆H₃, Dipp = 2,6-iPr₂C ₆H₃) in the presence of Me₃SiCCSiMe₃ afforded (Ar′Al)₂(CSiMe₃)₂ (see picture), a dialuminum-substituted cyclobutene analogue. The central four-membered Al₂C₂ ring adopts a folded geometry and is isoelectronic with aromatic cyclobutadiene dications. (Figure Presented)

Full text of DOI:10.1002/anie.200504329

Angewandte
Chemie
Four-Membered Rings
DOI: 10.1002/anie.200504329
Isolation of a 1,2-Dialuminacyclobutene**
Chunming Cui,* Xiaofei Li, Chunhua Wang,
Jianying Zhang, Jinpei Cheng, and Xiaoqing Zhu
Dedicated to Professor Herbert W. Roesky
In recent years, small-ring systems that incorporate heavier
main-group elements have drawn intense attention because of
their unique structural features and high reactivity.[1,2,3]
Among such rings, those which contain three-coordinate
Group 13 elements and olefinic-carbon units (cyclopropene
and cyclobutene analogues) are particularly attractive since
they are isoelectronic with the smallest members of the
Hückel aromatic series, that is, the cyclopropenium ion and
the cyclobutadiene dication, respectively. The carboranes
(CH)2(BH)n (n = 1,2) and their derivatives have been inten-
sively investigated both experimentally and theoretically.[2]
Thus, theoretical studies of (CH)2(BH)2 revealed that the
stable 1,3-diboretene isomer preferentially adopted a non-
planar, puckered geometry similar to that predicted for
(CH)24þ, which was subsequently verified experimentally.[2]
Accordingly, the synthesis and characterization of the heavier
congeners of the formally 2p MC2 and M2C2 rings (M = Al,
Ga, In, Tl) are of fundamental interest as a result of their
potential aromaticity. Surprisingly, such ring systems are rare
in the literature. The recently reported AlC2 rings of cyclo-
propene derivatives with bulky ligands are the only known
examples in this series, in which the aluminum atoms are all
four-coordinate.[3] Stable metallacyclobutene M2C2 rings
ꢀ
which contain M M bonds have remained unknown until
now. Herein, we report the synthesis and single-crystal X-ray
structure of the 1,2-dialuminacyclobutene (Ar’Al)2(CSiMe3)2
(2; A r’ = 2,6-Dipp2C6H3, Dipp = 2,6-iPr2C6H3).
The bulky terphenyl ligand Ar’, which was originally
prepared by Power and co-workers, has proved effective for
the stabilization of monovalent heavier Group 13 elements.[4]
However, the “dialuminene” Ar’AlAlAr’ has not been
isolated and was postulated as an intermediate in the
formation of a bicyclic Al2C6 ring through a [2+4] cyclo-
addition upon crystallization from toluene.[5] Our interest in
this system stems from the recent findings that aluminum
[*] Prof. Dr. C. Cui, X. Li, C. Wang, J. Zhang, Prof. Dr. J. Cheng,
Prof. Dr. X. Zhu
State Key Laboratory of Elemento-Organic Chemistry
Nankai University
Tianjin, 300071, P.R. China
Fax: (+86)22-2350-3461
E-mail: cmcui@nankai.edu.cn
[**] This work was supported by the National Natural Science
Foundation of China (20421202) and the NCET.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 2245 –2247
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2245

Communications
cyclopropene analogues could be generated by facile reduc-
tive coupling reactions of aluminum diiodide precursors in the
presence of alkynes. Thus, reduction of Ar’AlI2 (1)[5] with
two equivalents of KC8 in the presence of Me3SiCCSiMe3 in
toluene at room temperature afforded 2 (Scheme 1) as
Scheme 1. Synthesis of 2.
Figure 1. Thermal-ellipsoid drawing of 2 (30% probability). Hydrogen
ꢀ
atoms are not shown. Selected bond lengths [Š] and angles [8]: Al1
orange-red crystals in good yield, along with a small amount
of the known compound Ar’(I)AlAl(I)Ar’ (3).[5] The similar
reaction with only one equivalent of KC8 leads to the
formation of 3 as the major product, as indicated by
spectroscopic and single-crystal X-ray analyses. Compound
2 could also be obtained by the reduction of 3 in the presence
of the acetylene. The mechanism for the formation of 2 is not
clear at this stage. It is possible that K+[Me3SiCCSiMe3]Cꢀ
promotes the reduction process,[3a] which is followed by KI
elimination as proposed previously,[3a] since in the absence of
the acetylene, the reduction did not proceed under similar
conditions. Compound 2 is soluble in aromatic solvents and
decomposed in [D8]toluene at 1008C within 20 minutes to
form a complicated mixture, but is stable in the solid state at
this temperature for several hours. Compound 2 was fully
characterized by 1H, 13C, and 29Si NMR spectroscopy, UV/Vis
spectroscopy, and single-crystal X-ray analysis.[6]
ꢀ
ꢀ
ꢀ
Al2 2.4946(9), Al1 C64 2.006(2), Al2 C65 2.007(2), C64 C65 1.362(3),
ꢀ
ꢀ
Al1 C18 1.979 (2), Al2 C48 1.978(2); C18-Al1-Al2 141.11(7), C18-Al1-
C64 137.53, C64-Al1-Al2 73.11(7), C65-Al2-Al1 72.17(7), C48-Al2-Al1
143.16(6), C48-Al(2)-C65 137.97(9), C64-C65-Al2 105.48(15), C65-C64-
Al1 103.82(15).
predict geometrical features that are quite similar to those
found in the X-ray structure of 2, except that the aluminum
atoms adopt a planar geometry in 4. The 1,2-dialuminacyclo-
butene structure of 4 is 67.9 kJmolꢀ1 more stable than the
corresponding puckered 1,3 isomer. Similar calculations on
the parent molecule (HAl)2(CH)2 (5) also show that the
1,2 isomer is energetically more favorable than its puckered
1,3 isomer, while the former is almost planar (dihedral angle:
0.038). This finding is consistent with our experimental result
that the isomerization of 2 to the 1,3-dialumina Al2C2 isomer
was not observed in solution or in the solid state at high
temperature (100–1608C). In contrast, the experimental and
theoretical studies for diboretenes revealed that puckered
1,3-B2C2 skeletons are more stable than the corresponding
planar 1,2 isomers.[2] The folded Al2C2 framework of 2 and 4
versus the nearly planar geometry of 5 may be attributed to
the sterically demanding SiMe3 and Ar’ groups. Close H···H
contacts were observed in 2 between the two SiMe3 groups
(2.086, 2.255 Š) and between the two Ar’ groups (2.293,
2.596 Š) and may also lead to the geometric distortion of the
aluminum atoms in 2 from planarity.
The calculated HOMO and LUMO maps for the model
compound 4 are shown in the Supporting Information. Most
of the electron density in the HOMO is located at the orbitals
of the olefinic carbon atoms in the ring. The LUMO has a
significant contribution from the empty Al 3p orbitals. The
calculated energy gap between the HOMO–LUMO orbitals
is DE = 13.4 kJmolꢀ1. The small HOMO–LUMO gap indi-
cates a non-aromatic ring.[10] The calculations also revealed
that the p electrons are mostly localized in the C C moiety.
The structure of 2 also does not meet the classic criteria for
aromaticity (ring planarity, cyclic bond equalization). The
short Al Al bond length in 2 might be related to the favored
The 29Si NMR spectrum of 2 contains one singlet (d =
ꢀ13.2 ppm), which indicates the existence of the
Me3SiCCSiMe3 olefinic carbon atoms.[3a] The low-field reso-
nance (d = 235.0 ppm) in the 13C NMR spectrum also sup-
ports the presence of this structural unit. The 1H NMR
spectrum shows the 1:1 ratio of the Ar’ ligands to the SiMe3
groups. The structure of 2 was finally confirmed by a single-
crystal X-ray analysis.
Single crystals of 2 suitable for X-ray analysis were
obtained from toluene at room temperature. The most
prominent feature of the structure (Figure 1) is the four-
membered Al2C2 ring, which is folded (dihedral angles: 24.6,
25.08). The geometry around each carbon atom in the ring is
almost planar, but the Si1-C64-C65-Si2 unit is twisted (torsion
angle: 18.68). The three-coordinate aluminum atoms are
distorted from planarity (angular sum: 351.758 at Al1, 353.228
ꢀ
at Al2). The C64 C65 bond length (1.362(3) Š) is consistent
ꢀ
ꢀ
with a C C double bond, and the Al Cring bond lengths
ꢀ
(2.006(2), 2.007(2) Š) indicate single bonding. The Al1 Al2
=
bond length (2.4946(9) Š) is shorter by about 0.1 to 0.26 Š
than those for known divalent aluminum dimers[5,7] and only
slightly longer than that found in the anionic radical
Trip2AlAlTrip2Cꢀ (2.470(2) Š; Trip = 2,4,6-iPr3C6H2), in
ꢀ
[8]
ꢀ
which an Al Al p bond was proposed.
ring-atom geometries that are associated with the strained
ring.
The UV/Vis spectrum of 2 in n-hexane showed two
maxima at 381.0 and 297.0 nm, and the former maximum
Preliminary B3LYP/6-31G* calculations[9] were per-
formed on the model compound (PhAl)2(CSiMe3)2 (4), for
which the Ar’ ligand was replaced by a phenyl group, and
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Angew. Chem. Int. Ed. 2006, 45, 2245 –2247

Angewandte
Chemie
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vanished immediately when the solution was exposed to air.
This absorption might be a result of electronic transitions
within the Al2C2 ring.
[3] a) C. Cui, S. Köpke, R. Herbst-Irmer, H. W. Roesky, M.
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In summary, the reduction of Ar’AlI2 in the presence of
Me3SiCCSiMe3 afforded the 1,2-dialuminacyclobutene 2, the
first heavier Group 13 M2C2 ring system. Compound 2
features an unprecedented folded Al2C2 skeleton and a
ꢀ
short Al Al separation. The geometrical features of 2 and
the calculations on the model compound 4 do not support the
notion of aromaticity of the 2p Al2C2 ring system. The
calculations predict that the folded 1,2-dialuminacyclobutene
is energetically more favorable than its puckered 1,3 isomer.
Investigation of the reaction chemistry of 2 is currently in
progress.
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Experimental Section
[6] Crystallographic data for 2·toluene: MoKa radiation (l =
0.71073 Š) at 273(2) K, monoclinic, space group P21/c, a =
16.8314(9), b = 16.4648(9), c = 25.4086(13) Š, b = 98.8530(10)8,
Z = 4, R1 = 0.0498, wR2 = 0.1341 for 12276 reflections (I >
2s(I)); R1 = 0.0784, wR2 = 0.1551 for all data. CCDC-291899
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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[9] All calculations were performed with the Gaussian 98 package.
The geometries of the model compound 4 were fully optimized
by employing the B3LYP/6-31G* method, and the nature of the
stationary points was characterized by analytical-frequency
calculations. The natural-bond-orbital method was used to
perform the charge-density analysis; Gaussian 98 (Revi-
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2: Asolution of Ar ’AlI2 (1, 0.67 g, 1 mmol) and bis(trimethylsilyl)
acetylene (0.17 g, 1 mmol) in toluene (35 mL) was added to a
suspension of KC8, which was freshly prepared from graphite and
potassium (0.080 g, 2.02 mmol), in toluene (10 mL). The mixture was
stirred at room temperature for 50 h and filtered, and the red filtrate
was concentrated (ca. 15 mL). Storage at 58C overnight afforded a
small amount of yellow crystals of 3 (ca. 3%). The mixture was
filtered, and the filtrate was concentrated (ca. 5 mL) and stored at
ꢀ308C overnight to give orange-red crystals of 2 (0.32 g, 62.0%).
M.p. 1618C (decomp.); 1H NMR (400 MHz, C6D6): d = ꢀ0.002 (s,
18H, SiMe3), 0.58 (d, J = 6.40 Hz, 12H, CHMe2), 0.93 (d, J = 6.80 Hz,
12H, CHMe2), 1.15 (d, J = 6.40 Hz, 12H, CHMe2), 1.34 (d, J =
6.80 Hz, 12H, CHMe2), 2.86 (sept, J = 6.80 Hz, 8H, CHMe2), 7.02
(d, J = 7.20 Hz, 4H, ArH), 7.06 (brs, 4H, ArH), 7.18–7.23 ppm (m,
10H, ArH); 13C NMR (125.8 MHz, C6D6): d = 2.8 (SiMe3), 22.3
(CHMe2), 23.2 (CHMe2), 26.0 (CHMe2), 27.0 (CHMe2), 30.4
(CHMe2), 31.2 (CHMe2), 123.5 (m-Dipp), 127.0 (p-C6H3), 129.8 (m-
C6H3), 142.4 (p-Dipp) 145.6 (i-Dipp), 147.0 (o-Dipp), 149.1 (o-C6H3),
150.6 (i-C6H3), 235.0 ppm (CSiMe3); 27Al NMR (104.24 MHz,
[D8]toluene): could not be observed; 29Si NMR (79.48 MHz,
[D8]toluene): d = ꢀ13.2 ppm; UV/Vis (hexanes): lmax [nm] (e in
molꢀ1 Lcmꢀ1): 297 (8000), 381 (2100). Elemental analysis (%) calcd
for C68H94Al2Si2 (1021.57): C 79.95, H 9.27; found: C 80.30, H 9.04.
Received: December 6, 2005
Revised: January 6, 2006
Published online: March 6, 2006
Keywords: aluminum · aromaticity · cyclobutenes ·
.
four-membered rings · pi interactions
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www.angewandte.org
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