C-H bond activation by hyperconjugation with Al-C bonds and by chelating coordination of the hydride ion

Article abstract of DOI:10.1021/ja073657u

On treating di(tert-butyl)butadiyne with dimethylaluminum hydride under different reaction conditions two unprecedented organoelement compounds, containing cationic carbon atoms stable in solution at room temperature, were obtained. A vinyl cation (2) in which the cationic carbon atom is part of a C=C double bond was produced from 3 equiv of the hydride, whereas a large excess of the hydride yielded an aliphatic carbocation (3) by complete hydroalumination of all C-C multiple bonds. Each compound is zwitterionic with the hydride counterion effectively coordinated in a chelating manner by two strongly Lewis acidic aluminum atoms. In agreement with quantum-chemical calculations the C-H bond activation and the stabilization of the cationic species are further supported by a strong hyperconjugation with Al-C single bonds. This considerably diminishes the effective positive charge at the respective cationic carbon atoms.

Full text of DOI:10.1021/ja073657u

Published on Web 08/18/2007
C-H Bond Activation by Hyperconjugation with Al-C Bonds
and by Chelating Coordination of the Hydride Ion
Werner Uhl,* Andrej Vinogradov, and Stefan Grimme
Contribution from the Institut f u¨ r Anorganische und Analytische Chemie and the Institut f u¨ r
Organische Chemie der UniVersit a¨ t M u¨ nster, Corrensstrasse 30, D-48149 M u¨ nster, Germany
Received May 22, 2007; E-mail: uhlw@uni-muenster.de
Abstract: On treating di(tert-butyl)butadiyne with dimethylaluminum hydride under different reaction
conditions two unprecedented organoelement compounds, containing cationic carbon atoms stable in
solution at room temperature, were obtained. A vinyl cation (2) in which the cationic carbon atom is part of
a CdC double bond was produced from 3 equiv of the hydride, whereas a large excess of the hydride
yielded an aliphatic carbocation (3) by complete hydroalumination of all C-C multiple bonds. Each compound
is zwitterionic with the hydride counterion effectively coordinated in a chelating manner by two strongly
Lewis acidic aluminum atoms. In agreement with quantum-chemical calculations the C-H bond activation
and the stabilization of the cationic species are further supported by a strong hyperconjugation with Al-C
single bonds. This considerably diminishes the effective positive charge at the respective cationic carbon
atoms.
Introduction
only be prevented, when a very bulky dialkylaluminum hydride
was employed or when trimethylsilylethynes bearing SiMe3
4
Hydroalumination is a very powerful method for the reduction
of unsaturated organic compounds containing homo- or hetero-
nuclear multiple bonds. In most cases the organoaluminum
groups attached to their triple bonds were treated with dialkyl-
5
1
aluminum hydrides. Similar hydrogallation reactions appear to
be more selective than hydroaluminations and were investigated
intermediates of these reactions were not isolated and character-
ized but immediately destroyed by hydrolysis, and the reduced
organic material was finally isolated. The knowledge of the
constitution of those intermediates is therefore quite limited,
and rather speculative suggestions on the structures and reaction
mechanisms can be found in former publications or in text
books. In some recent investigations we observed that the course
of hydroalumination reactions is much more interesting than
may be derived from that general knowledge, and several novel
and unexpected products could be isolated. On treating dialkyl-
aluminum alkynides in which the aluminum atoms are attached
to carbon atoms of CtC triple bonds, with the corresponding
dialkylaluminum hydrides for example, the new class of
carbaalanes were formed by the release of trialkylaluminum.
These compounds possess clusters formed by carbon and
aluminum atoms, e.g., (AlMe)8(CCH2C6H5)5H, and have a
6
in still greater detail.
In a recent paper we reported on the remarkable reaction of
di(tert-butyl)butadiyne, Me3CsCtCsCtCsCMe3, with di-
7
(
tert-butyl)aluminum hydride. Instead of condensation, a
singular persistent butadienyl cation resulted (1, Scheme 1).
Formally, 1 is produced by hydroalumination of both triple
bonds, heterolytic cleavage of one CsH bond, and a very
effective chelating coordination of the hydride anion by two
unsaturated aluminum atoms. Such butadienyl cations have not
been isolated before and were detected in superacidic media
8
only. Comparable silyl-substituted vinyl cations in which the
cationic carbon atom is part of a localized CdC double bond
9
were published only recently. Owing to the general importance
(4) Uhl, W.; Matar, M. Z. Anorg. Allg. Chem. 2005, 631, 1177.
(
5) (a) Uhl, W.; Breher, F.; Haddadpour, S. Organometallics 2005, 24, 2210.
(
b) Uhl, W.; Matar, M. J. Organomet. Chem. 2002, 664, 110.
2
delocalized bonding situation in their molecular cores. Similar
condensation reactions with the formation of cyclophane type
molecules were observed on treating benzene centered tert-butyl
ethynes with aluminum hydrides. Secondary reactions could
(6) (a) Uhl, W.; Breher, F.; Haddadpour, S. Organometallics 2005, 24, 2210.
(b) Uhl, W.; Haddadpour, S.; Matar, M. Organometallics 2006, 25, 159.
c) Uhl, W.; Claesener, M.; Haddadpour, S.; Jasper, B.; Hepp, A. Dalton
(
Trans. 2007, 4, 417. (d) Uhl, W.; Bock, H. R.; Breher, F.; Claesener, M.;
Haddadpour, S.; Jasper, B.; Hepp, A. Organometallics 2007, 26, 2363.
7) Uhl, W.; Grunenberg, J.; Hepp, A.; Matar, M.; Vinogradov, A. Angew.
Chem. 2006, 118, 4465; Angew. Chem., Int. Ed. 2006, 45, 4358.
3
(
(
1) (a) Winterfeldt, E. Synthesis 1975, 10, 617. (b) Marek, J.; Normant, J.-F.
Chem. ReV. 1996, 96, 3341. (c) Eisch, J. J. In ComprehensiVe Organic
Synthesis; Frost, B. M., Flemming, J., Eds.; Pergamon: Oxford, 1991; Vol.
(8) (a) Siehl, H.-U.; Mayr, H. J. Am. Chem. Soc. 1982, 104, 909. (b) Siehl,
H.-U.; Brixner, S. J. Phys. Org. Chem. 2004, 17, 1039. (c) Apeloig, Y.;
Stanger, A. J. Org, Chem. 1982, 47, 1462. (d) van Alem, K.; Lodder, G.;
Zuilhof, H. J. Phys. Chem. A 2002, 106, 10681. (e) van Alem, K.; Lodder,
G.; Zuilhof, H. J. Phys. Chem. A. 2000, 104, 2780. (f) Cunje, A.; Rodriquez,
C. F.; Lien, M. H.; Hopkinson, A. C. J. Org. Chem. 1996, 61, 5212.
(9) (a) M u¨ ller, T.; Juhasz, M.; Reed, C. A. Angew. Chem. 2004, 116, 1569;
Angew. Chem., Int. Ed. 2004, 43, 1546. (b) M u¨ ller, T.; Margraf, D.; Syha,
Y. J. Am. Chem. Soc. 2005, 127, 10852. (c) M u¨ ller, T.; Meyer, R.; Lennartz,
D.; Siehl, H.-U. Angew. Chem. 2000, 112, 3203; Angew. Chem., Int. Ed.
2000, 39, 3074. (d) Zakharkin, L. I.; Kobak, V. V. J. Organomet. Chem.
1984, 270, 229.
8
, p 773.
(
2) (a) Uhl, W.; Breher, F. Angew. Chem. 1999, 111, 1578; Angew. Chem.,
Int. Ed. 1999, 38, 1477. (b) Uhl, W.; Breher, F.; L u¨ tzen, A.; Saak, W.
Angew. Chem. 2000, 112, 414; Angew. Chem., Int. Ed. Engl. 2000, 39,
4
06. (c) Uhl, W.; Breher, F.; Grunenberg, J.; L u¨ tzen, A.; Saak, W.
Organometallics 2000, 19, 4536. (d) Uhl, W.; Breher, F.; Mbonimana, A.;
Gauss, J.; Haase, D; L u¨ tzen, A.; Saak, W. Eur. J. Inorg. Chem. 2001, 3059.
3) Uhl, W.; Hepp, A.; Matar, M.; Vinogradov, A. Eur. J. Inorg. Chem., in
press.
(
10.1021/ja073657u CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 11259-11264
9
11259

A R T I C L E S
Scheme 1
Uhl et al.
of those carbocationic species, more systematic investigations
on the applicability of that particular method for generating
further derivatives were conducted. The synthesis of two new
compounds possessing carbocations but having completely
different structures compared to that of 1 is reported here.
Results and Discussion
Figure 1. Molecular structure and numbering scheme of 2; hydrogen atoms
of methyl groups are omitted for clarity. Important bond lengths [pm] and
angles [deg]: Al1-C2 204.2(2), C1-C2 151.8(2), C2-C3 127.3(2), C3-
C4 135.5(2), C4-Al2 210.0(2), C4-Al2′ 204.5(2), Al-C(Me) 194.7 (av.),
Al1-C2-C3 105.5(1), C1-C2-C3 128.7(2), C2-C3-C4 170.3(2), C3-
C4-Al2 95.6(1), C3-C4-Al2′ 100.6(1); Al1′ generated by -x + 1, -y,
Synthesis and Molecular Structure of Compound 2. Di-
(
tert-butyl)butadiyne and dimethylaluminum hydride did not
react in solution at room temperature. Successful reactions
required the treatment of the diyne with an excess of the hydride
without any solvent and heating of the mixture to 100 °C until
a homogeneous, colorless liquid had formed (eq 1). After
-
z + 1. Structural parameters of the optimized structure of 2 [PBE-D/
TZVPP,SV(P)]: Al2-C4 205.7, Al2-C4′ 210.0, Al1-C2 205.5, C1-C2
50.2, C2-C3 128.6, C3-C4 135.6, C2-C3-C4 169.3.
1
protons. The unusual chemical shift of one carbon atom (C3,
1
3
see below) in the C NMR spectrum (δ ) 196.6) verified the
formation of a carbocation. The remaining carbon atoms of the
inner C4 moiety resonated at δ ) 42.3, 70.1, and 86.9. The
low-field resonance is in the characteristic range of carbocations
stabilized by hyperconjugation; see for instance a vinyl cation
stabilized by hyperconjugation with C-Si bonds (δ ) 202.7).9
Determination of the crystal structure revealed a centrosym-
metric dimeric formula unit (Figure 1, schematic drawing in
eq 1). Two tetracarbon skeletons are bridged by two common
aluminum atoms. An Al2C2 heterocycle having slightly different
AlsC bond lengths of 210.0 (Al2sC4) and 204.5 pm (Al2s
C4′) is formed. These distances are relatively long compared
to standard AlsC bond lengths (e.g., AlsMe 195 pm on
average) and may indicate some hyperconjugative stabilization
as discussed later on. C3 is the cationic carbon atom. It has
only two neighboring atoms (C2 and C4) in an almost ideally
linear surrounding (angle C2sC3sC4 170.3°). The CsC
distances in the organic backbone vary in accordance with the
respective bonding situation. The C1sC2 bond length (151.8
pm) indicates a CsC single bond resulting from the double
hydroalumination of one CtC triple bond. The bonds C2sC3
(127.3 pm) and C3sC4 (135.5 pm) are considerably shorter
than the values characteristic of CsC double or single bonds.
They indicate some stabilization of the unsaturated cationic
carbon atom by hyperconjugation with the AlsC bonds Al2s
C4 and Al1sC2 (204.2 pm). The hydrogen atom formally
abstracted from C3 is coordinated by both aluminum atoms Al1
and Al2 to form a 3c-2e bond. The result is therefore a ladder
type tricyclic molecular structure having two six-membered
Al2C3H rings connected by a four-membered Al2C2 ring. The
dimer is formed from a condensation reaction similar to those
described in the Introduction. The hypothetic intermediate
obtained by triple hydroalumination of the butadiyne with three
AlMe2 groups attached to the tetracarbon backbone (schematic
cooling to room temperature all volatiles were removed in
vacuum, and the residue was recrystallized from n-hexane to
yield colorless crystals of 2. Trimethylaluminum was unambigu-
ously identified by NMR spectroscopy as an essential byproduct
1
of the reaction. Compound 2 has a relatively complicated H
NMR spectrum with three different resonances of equal intensity
for the methyl groups attached to aluminum. Two chemically
different tert-butyl groups could be detected, and two doublets
indicated the occurrence of a CH2 group with diastereotopic
11260 J. AM. CHEM. SOC.
9
VOL. 129, NO. 36, 2007

C−
H Bond Activation by Hyperconjugation with Al
−C
A R T I C L E S
drawing in eq 1) eliminates one AlMe3 molecule per formula
unit to yield the inner Al2C2 ring.
Synthesis and Molecular Structure of Compound 3.
Complete hydroalumination of all multiple bonds was observed
upon treatment of di(tert-butyl)butadiyne with a large excess
of dimethylaluminum hydride without a solvent at 100 °C.
Compound 3 was isolated in 70% yield, and once again
trimethylaluminum was formed as a byproduct (eq 2). NMR
Figure 2. Molecular structure and numbering scheme of 3; hydrogen atoms
of methyl groups are omitted for clarity. Important bond lengths [pm] and
angles [deg]: C1-Al1 200.0(2), C1-Al2 207.3(2), C1-C2 142.6(2), C2-
C3 141.3(2), C3-C4 154.2(2), C3-Al3 197.2(2), C3-Al4 209.7(2), Al1-
C1-Al2 97.62(8), C2-C1-Al1 104.8(1), C2-C1-Al2 109.9(1), C1-C2-
C3 128.1(2), C2-C3-C4 116.2(1), C2-C3-Al3 111.8(1), C2-C3-Al4
1
00.1(1). Structural parameters of the optimized structure of 2 [PBE-D/
TZVPP,SV(P)]: C1-C2 142.2, C2-C3 141.6, C3-C4 153.3, C1-Al1
02.5, C1-Al2 210.0, C3-Al4 211.0, C3-Al3 199.2, C1-C2-C3 128.0.
2
bonds C1-C2 (142.6 pm) and C2-C3 (141.3 pm). They are
close to the average values between single and double bonds
but significantly longer than those detected for compounds 1
and 2 in which the cationic carbon atoms are part of a π-system.
The shortening in 3 compared to C-C single bonds may be
caused by a hyperconjugative stabilization of the cation and an
electron transfer from Al-C σ-bonds into the empty p-orbital
of that carbon atom. There is strong evidence for such an
interaction in the structural data. The carbon atoms neighboring
the cation are coordinated by two aluminum atoms each with
two strongly differing Al-C bond lengths (198.6 and 208.5 pm
on average). While the shorter bonds are in the normal range
of Al-C distances and are close to the average plane of the
atoms C1, C2, H2, and C3, the longer bonds are perpendicular
to that plane and are in an ideal position to interact with the
empty p-orbital at the cationic carbon atom. The bonds become
longer and weaker on electron donation, whereas the corre-
sponding C-C bonds become stronger and shorter. A similar
hyperconjugative stabilization with comparable C-C distances
in the molecular core was observed for the tert-butyl cation
which was isolated in the presence of a noncoordinating
spectroscopic characterization verified the formation of another
carbocationic species. The 1H NMR spectrum showed the
occurrence of six chemically different methyl groups attached
to aluminum. One resonance with an unusual chemical shift of
δ ) 9.28 was assigned to a hydrogen atom bonded to a
positively charged carbon atom, although it is considerably
shifted to a higher field compared to, e.g., the isopropyl cation
which is only available in superacidic media (δ ) 13.5). These
differences are probably caused by hyperconjugation and the
reduction of the positive charge localized at the cationic carbon
atom by transfer of electron density from Al-C bonds. A similar
effect may influence the 13C NMR shift of that particular carbon
atom (δ ) 217.5) to a higher field by about 100 ppm as
compared to the resonances of more naked cations in superacidic
solvents.10
The molecular structure of 3 (Figure 2) comprises a relatively
complicated cage with four heterocycles at its surface, four
chemically different aluminum atoms, and owing to the mo-
lecular symmetry six different methyl groups bonded to the
metal atoms. Three Al-H-Al three-center bonds occur, two
of which bridge an Al-Me group with an AlMe2 residue (Al1-
H2-Al4 and Al3-H3-Al2), while the remaining one is
between two Al-Me groups (Al1-H1-Al3). These particular
aluminum atoms (Al1 and Al3) are coordinated by two hydrogen
atoms and are therefore involved in two 3c-2e bonds while
the others have contact to only one bridging hydrogen atom.
Two carbon atoms of the former butadiyne moiety bond each
to two aluminum atoms (C1 and C3). C4 represents a methylene
group bearing two hydrogen atoms. The cationic carbon atom
C2 has one terminal hydrogen atom and an ideal trigonal planar
surrounding (sum of the angles 360°). The C-C bond lengths
correspond to a normal single bond for the couple C3-C4
10
-
11
counterion (Sb2F11 ).
The release of trimethylaluminum in the course of the
synthesis of 3 is required to provide the correct number of
bridging hydrogen atoms. Formally 3 is produced by the addition
of 2 equiv of dimethylaluminum hydride and 2 equiv of
methylaluminum dihydride to the starting dialkyne. Each hydride
molecule adds one Al-H bond to the π-bonds to give the
aliphatic organic backbone, and two hydride atoms remain for
the bridging of the metal atoms. The third bridge results from
the formal breaking of a C-H bond and the chelating coordina-
tion of the eliminated hydride anion by two unsaturated
aluminum atoms. It is however by no means clear whether the
hydrogen-methyl exchange occurs before the reaction starts in
a kind of a preequilibrium or whether it occurs after addition
(154.2 pm), while shorter ones are observed for the remaining
(
10) (a) Olah, G. A.; Donovan, D. J. J. Am. Chem. Soc. 1993, 115, 7240. (b)
Olah, G. A. Angew. Chem. 1995, 107, 1519; Angew. Chem., Int. Ed. 1995,
3
4, 1393.
(11) Hollenstein, S.; Laube, T. J. Am. Chem. Soc. 1993, 115, 7240.
J. AM. CHEM. SOC.
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VOL. 129, NO. 36, 2007 11261

A R T I C L E S
Uhl et al.
Figure 3. Localized MOs (multicenter-bonds) of 3 (rotated view, only C
and Al atoms are shown).
Figure 4. Optimized molecular structure of 4 [PBE-D/TZVPP,SV(P)].
Calculated bond lengths [pm] and angles [deg]: C1-C2 156.9, C2-C3
156.1, C1-Al1 193.9, C1-Al2 204.2, C3-Al3 203.5, C3-Al4 213.3, C1-
C2-C3 107.9.
from an intermediate organoaluminum derivative. To the best
of our knowledge there is no real evidence for the formation of
methylaluminum dihydride in liquid dimethylaluminum hydride
at all. Bulky substituents are required to isolate organo aluminum
or gallium dihydrides.12 The maximum hydrogen content with
definitely verify an interaction of the Al-C bonds with the
cationic center at C3. Further evidence results from the
1
5
calculated Wiberg bond orders between C2 and the metal
atoms (C2-Al1 0.147, C2-Al2 0.179, C2-Al3 0.152, C2-
Al4 0.214). The numbers indicate a bonding interaction between
C2 and all four aluminum centers which supports the description
of the bonding situation by multicenter bonds and the hyper-
conjugative diminution of the electron deficiency at the central
carbon atom. Orbital analyses of the butadienyl compound 1
and the vinyl compound 2 reveal very similar interactions. Thus,
the stabilization of the formal cationic centers by hyperconju-
gation with Al-C single bonds seems to be a common property
of all three compounds.
smaller substituents was achieved with the sesquihydrides
Me3C-EH2]2[(Me3C)2E-H]2 (E ) Al, Ga).13
[
Quantum-Chemical Calculations. We first optimized the
molecular structure of the vinyl compound 2 in Ci symmetry.
The most important bond lengths and angles observed experi-
mentally are reproduced quite accurately (data are included in
the legend of Figure 1). The NBO analysis indeed reveals a
partial cationic character of C3, bearing a charge of q ) +0.116.
The two directly bonded carbon atoms have a negative charge,
the magnitude of which correlates quite well to the number of
bonded aluminum atoms [q(C2) ) -0.505, q(C4) ) -0.920].
A substantial negative charge is further localized at the hydride
atom H1 [q(H1) ) -0.457]. For comparison we reinvestigated
Finally the stability of an alternative isomer (4) which has
the hydrogen atom H1 shifted to the carbocationic center with
an intact C-H bond was investigated in comparison with 3, in
which that particular hydride anion is coordinated in a chelating
manner by two aluminum atoms. We obtained a stable structure
7
the electronic structure of the butadienyl cation 1 (Scheme 1).
The cationic carbon atom has a positive charge similar to that
of 2 [q(C2) ) +0.130], and those carbon atoms which are
attached to AlR2 groups have around one-half electron charge
(
(
Figure 4) of 4 that, however, is significantly higher in energy
+27.9 kcal mol ) than that of 3.
-
1
[q(C3) ) -0.516 and q(C5) ) -0.440]. A similar value was
calculated for the hydrogen atom coordinated in a chelating
manner by two aluminum atoms [q(H1) ) -0.521].
Conclusion
The optimized structural parameters of compound 3 are added
to the legend of Figure 2. NBO analysis attributes almost one
electron charge to the carbon atoms that are coordinated by two
dialkylaluminum groups [q(C1) ) -0.979, q(C3) ) -0.959].
In this case, the formal cationic center has a relatively small
negative charge (q(C2) ) -0.089) which indicates the transfer
of electron density from the neighboring atoms. The hypercon-
jugative interaction is visible in the shape of the corresponding
two-electron-multi-center MOs that are found in a Pipek-Mezey
orbital localization procedure14 (Figure 3): The localized MOs
Three different types of carbocationic species were realized
in the hydroalumination products 1, 2, and 3. Schematic
representations are depicted in Scheme 2. They represent a
butadienyl cation (1), a vinyl cation (2), and an aliphatic
carbocation (3). A steadily increasing number of aluminum
atoms attached to the organic backbone from two in 1 to four
in 3 is representative for these different structural motifs. All
compounds are zwitterionic with an intramolecular compensa-
tion of the positive charge. Thus, the problem of applying strictly
noncoordinating counterions does not occur in these cases at
all. Furthermore, compounds with these particular structures
have the considerable advantage of being soluble in nonpolar
solvents such as n-hexane. Their thermal stability will enable
their application in secondary reactions and a thorough inves-
(
12) (a) Cowley, A. H.; Gabbai, F. P.; Isom, H. S.; Carrano, C. J.; Bond, M. R.
Angew. Chem. 1994, 106, 1354; Angew. Chem., Int. Ed. Engl. 1994, 33,
1
253. (b) Wehmschulte, R. J.; Ellison, J. F.; Ruhlandt-Senge, K.; Power,
P. P. Inorg. Chem. 1994, 33. 6300.
(13) Uhl, W.; Cuypers, L.; Graupner, R.; Molter, J.; Vester, A.; Neum u¨ ller, B.
Z. Anorg. Allg. Chem. 2001, 627, 607.
(14) Pipek, J.; Mezey, P. G. J. Chem. Phys. 1989, 90, 4916.
(15) Wiberg, K. B. Tetrahedron 1968, 24, 1083.
11262 J. AM. CHEM. SOC.
9
VOL. 129, NO. 36, 2007

C−
H Bond Activation by Hyperconjugation with Al
−C
A R T I C L E S
Scheme 2. Schematic Representation of the Different Cationic
Table 1. X-ray Data for 2 and 3
Systems Obtained by Hydroalumination
2
3
formula
fw
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C30H60Al4
264.35
triclinic
P1h
8.395(1)
9.446(1)
12.695(2)
91.388(3)
102.358(3)
114.027(2)
891.2(2)
1
0.985
0.146
30.08
153(2)
5116
C18H42Al4
366.44
triclinic
P1h
9.337(1)
11.450(1)
11.762(1)
78.416(2)
85.403(2)
89.980(2)
1227.7(2)
2
0.991
0.187
31.32
153(2)
7313
Z
-
3
Dcalcd, Mg m
-
1
µ, mm
θmax, deg
temp, K
no. indep. reflns
no. reflns observed
no. of params
3935
167
5387
223
tigation of their chemical properties. The successful generation
of these cations with the chelating coordination of the hydride
ion by two aluminum atoms favored over the formation of a
C-H bond depends on two important effects. Hyperconjugation
with Al-C bonds at neighboring carbon atoms activates the
corresponding C-H bonds and diminishes the positive charge
at the inner carbon atoms which remains after the heterolytic
C-H bond cleavage. This interpretation is strongly supported
by structural data and the results of quantum-chemical calcula-
tions. The second point of interest is the effective intramolecular
coordination of the hydride ion by two coordinatively unsatur-
R1 [I > 2σ(I)]
0.0612
0.1592
0.0575
0.1514
wR2 (all data)
νCdC; 1454 vs, 1386 s (Nujol); 1360 s, 1303 w, 1254 m δCH
s, 1157 m, 1040 s, 1026 s, 995 m, 928 w, 895 m, 872 s, 783 m, br.,
95 m, br. νCC, FCH , δAlH; 621 m, 571 m, 538 m, 509 w, 461 m,
451 s νAlC.
Synthesis of 3. Di(tert-butyl)butadiyne (0.202 g, 1.25 mmol) was
treated with an excess of dimethylaluminum hydride (0.650 g, 11.21
mmol) without a solvent. The mixture was stirred at 100 °C for 1.5 h.
After cooling to room temperature all volatiles were removed in
vacuum. The residue was dissolved in n-pentane and cooled to
-15 °C to yield colorless crystals of compound 3. Yield: 0.320 g (70%
based on the butadiyne). Mp (argon, closed capillary): 113 °C (dec).
3
; 1196
6
3
1
6
ated aluminum atoms which act as a chelating Lewis acid.
Experimental Section
All procedures were carried out under an atmosphere of purified
argon in dried solvents (n-hexane and n-pentane over LiAlH ).
Dimethylaluminum hydride was obtained according to literature
+
+
MS (EI, 70 eV): m/z 366 (6%) (M ); 351 (4%) (M - CH
3
); 309
, 400 MHz): δ 9.28 (1 H, s,
cationic-C-H), 3.58, 3.33 and 3.01 (each 1 H, br., AlHAl), 2.04 and
4
+
1
(
3 6 6
19%) (M - CMe ). H NMR (C D
1
7
procedure. Commercially available di(tert-butyl)butadiyne (2,2,7,7-
dimethylocta-3,5-diyne, Aldrich) was dried in vacuum prior to use. The
assignment of the NMR spectra is based on HMBC, HSQC, ROESY,
and DEPT135 data.
2
1
.93 (each 1 H, d, JH-H ) 13.1 Hz, CH
to C1; see Figure 2 for the numbering scheme), 0.80 (9 H, s, CMe
the neopentyl group), -0.12 (3 H, s, CH at Al2), -0.14 and -0.29
each 3 H, s, CH at Al4), -0.25 (3 H, s, CH at Al3), -0.32 and
0.43 (each 3 H, s, CH , 75.5 MHz): δ 217.5
at Al1). 13C NMR (C
cationic carbon atom C2), 84.7 (C1), 79.4 (C3), 50.1 (C4, CH ), 37.0
CMe at C1), 33.6 (CMe at C1), 33.6 (CMe at C4), 29.8 (CMe at
at Al4), -7.7 and -10.7 (CH at Al1), -7.8
at Al3). IR (paraffin, CsBr plates, cm ):
652 w, br., 1584 m, br. νAlH, νCdC; 1462 vs, 1377 s (Nujol); 1366
; 1198 m, 1112 s, 1073 m, 1043 vs, br., 940
m, 901 m, 874 m, 845 m, 820 w, 714 m, 685 w νCC, FCH , δAlH;
2
), 1.14 (9 H, s, CMe
3
attached
3
of
3
(
-
(
(
3
3
Synthesis of 2. Di(tert-butyl)butadiyne (0.200 g, 1.23 mmol) was
treated with a small excess of dimethylaluminum hydride (0.286 g,
3
6 6
D
2
4
1
.93 mmol) without a solvent. The mixture was stirred at 100 °C for
h. After cooling to room temperature all volatiles were removed in
3
3
3
3
C4), -4.5 and -5.7 (CH
3
3
vacuum. The residue was dissolved in n-hexane and cooled to -15 °C
to yield colorless crystals of compound 2. Yield: 0.084 g (26% based
on the butadiyne). Mp (argon, closed capillary): 149 °C (dec). MS
-
1
(CH
3
at Al2), -12.1 (CH
3
1
s, 1310 w, 1261 w δCH
3
+
+
(
(
EI, 20 eV): m/z 528 (26%) (M ); 513 (2%) (M - CH
3
); 471 (36%)
3
+
1
M
- CMe
3 6 6
). H NMR (C D , 400 MHz): δ 4.03 (2 H, br., AlHAl),
6
48 w, 638 w, 618 w, 515 m, 461 s νAlC.
Crystal Structure Determinations. Single crystals of compounds
and 3 were obtained by recrystallization from n-hexane and n-pentane,
respectively, at -15 °C. The crystallographic data were collected with
a Bruker APEX diffractometer with graphite-monochromated Mo KR
radiation. The crystals were coated with high viscosity mineral oil,
picked up with a glass fiber, and immediately mounted in the cooled
nitrogen stream of the diffractometer. The crystallographic data and
details of the final R values are provided in Table 1. All structures
2
2
CMe
CMe
.45 and 1.96 (each 2 H, d, JH-H ) 12.3 Hz, CH
2
), 1.24 (18 H, s,
ring), 0.92 (18 H, s,
of the neopentyl groups), -0.12 (6 H, s, CH attached to the
ring), -0.14 and -0.23 (each 6 H, s,
moieties). 13C NMR
chemically different methyl groups of the AlMe
, 75.5 MHz): δ 196.6 (cationic carbon atom C3, for numbering
see Figure 1), 86.9 (C2), 70.1 (C4), 42.3 (CH ), 34.2 (CMe attached
to the Al ring), 32.7 (CMe attached to the Al ring), 31.9 (CMe
of the neopentyl group), 29.6 (CMe of the neopentyl group), -3.4
CH attached to the Al ring), -8.2 and -8.6 (CH of the AlMe
groups). IR (paraffin, CsBr plates, cm ): 1856 s,br. νAlH; 1580 m
3
2 2
attached to the carbon atoms of the Al C
2
3
3
2 2
aluminum atoms of the Al C
2
6 6
(C D
2
3
2
C
2
3
2
C
2
3
3
1
8
were solved by direct methods using the SHELXTL PLUS program
and refined with the SHELXL-97 via full-matrix least-squares
calculations based on F . All non-hydrogen atoms were refined with
(
3
2
C
2
3
2
18
-
1
2
anisotropic displacement parameters; hydrogen atoms attached to carbon
were calculated at ideal positions and allowed to ride on the bonded
(
16) (a) Uhl, W.; Hannemann, F.; Saak, W.; Wartchow, R. Eur. J. Inorg. Chem.
1
998, 921. (b) Uhl, W.; Hannemann, F. J. Organomet. Chem. 1999, 579,
1
8. (c) Uhl, W.; Layh, M. Z. Anorg. Allg. Chem. 1994, 620, 856. (d) Uhl,
W.; Koch, M.; Heckel, M.; Hiller, W.; Karsch, H. H. Z. Anorg. Allg. Chem.
1
994, 620, 1427.
(18) SHELXTL-Plus, REL. 4.1; Siemens Analytical X-RAY Instruments Inc.:
Madison, WI, 1990. Sheldrick, G. M. SHELXL-97, Program for the
Refinement of Structures; Universit a¨ t G o¨ ttingen: Germany, 1997.
(17) Lehmkuhl, H.; Ziegler, K. In Methoden der Organischen Chemie (Houben-
Weyl), 4th ed.; 1970, Vol. XIII/4, p 58.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 36, 2007 11263

A R T I C L E S
Uhl et al.
atom with U ) 1.2Ueq(C). Hydrogen positions of the Al-H-Al bridges
were taken from difference Fourier maps; the atoms were refined
isotropically.
(single point energies). Natural population analyses26 were performed
with the NBO program.27
Acknowledgment. We are grateful to the Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen Industrie
for generous financial support.
Quantum-Chemical Calculations. Density functional theory (DFT)
calculations provide a reasonable balance of computational efficiency
19,20
21
and chemical accuracy.
We have used the PBE functional together
with an empirical correction term in our calculations to compensate
for the insufficient description of dispersion forces in the commonly
Supporting Information Available: X-ray crystallographic
files in CIF format for the compounds 2 and 3. Further details
of the crystal structure determinations are also available from
the Cambridge Crystallographic Data Center on quoting the
depository numbers CCDC-646838 (2) and -646839 (3). This
material is available free of charge via the Internet at
http://pubs.acs.org.
22
used functionals. The large number of spatially close-lying bulky alkyl
groups make this important for the present study. The AO basis for
geometry optimizations consisted of a split valence set with polarization
23
functions [SV(P)] on all atoms except Al, the carbon atoms comprising
the skeleton, and the hydride atoms for which a triple-ú basis (TZV)24
augmented with three sets of polarization functions (2df,2pd) on all
elements was employed (TZVPP).25 The final energies were obtained
after replacing the SV(P) basis by TZVP without further optimization
JA073657U
(
19) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional
Theory; Wiley-VCH: New York, 2001.
(24) Sch a¨ fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
(25) All basis sets can be obtained from the Turbomole server at ftp://
ftp.chemie.uni-karlsruhe.de/pub/.
(
20) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules;
Oxford University Press: Oxford, 1989.
(26) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83,
735.
(
21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865.
22) (a) Grimme, S. J. Comput. Chem. 2004, 25, 1463. (b) Grimme, S. J. Comput.
Chem. 2006, 27, 1787.
(
(27) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Weinhold, F. NBO 4.M; Theoretical Chemistry Institute, University of
Wisconsin: Madison, WI, 1999.
(
23) Sch a¨ fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
11264 J. AM. CHEM. SOC.
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