Dialkylaluminium-, -gallium-, and -indium-based poly-lewis acids with a 1,8-diethynylanthracene backbone

Article abstract of DOI:10.1002/chem.201001618

Potential host systems based on a rigid 1,8-diethynylanthracendiyl backbone were synthesised by treatment of 1,8-diethynylanthracene with the Group 13 trialkyls AlMe₃, GaMe₃, InMe₃, AlEt₃ and GaEt₃. The resulting products were characterised by IR and multinuclear NMR spectroscopy, elemental analyses and determination of their crystal structures by X-ray diffraction. The compounds are dimeric in the solid state and comprise two M₂C₂ heterocycles. Depending on the steric demand of the alkyl substituents at the metal atom, different types of binding modes were observed, which can be classified to lie between the ideals of side-on coordination with almost linear primary M-C≡C units and the 3c-2e coordination with symmetrically bridging alkynyl units in M-C-M bonds. As a solution in THF the dimers are broken into monomers and some are found to undergo ligand scrambling reactions. Moving four-ward: Four Lewis acidic metal atoms held together by rigid hydrocarbon frameworks can be synthesised from 1,8-diethynylanthracene with metal alkyls MR₃ (M=Al, Ga, In; R=Me, Et) under alkane elimination (see figure). The arrangement of four close-lying metal functions is broken up in donor solvents like THF.

Full text of DOI:10.1002/chem.201001618

DOI: 10.1002/chem.201001618
Dialkylaluminium-, -Gallium-, and -Indium-Based Poly-Lewis Acids with a
1,8-Diethynylanthracene Backbone
Jasmin Chmiel, Beate Neumann, Hans-Georg Stammler, and Norbert W. Mitzel*[a]
Dedicated to Professor Hans H. Karsch on the occasion of his 65th birthday
Abstract: Potential host systems based
on a rigid 1,8-diethynylanthracendiyl
backbone were synthesised by treat-
ment of 1,8-diethynylanthracene with
the Group 13 trialkyls AlMe3, GaMe3,
InMe3, AlEt3 and GaEt3. The resulting
products were characterised by IR and
multinuclear NMR spectroscopy, ele-
mental analyses and determination of
their crystal structures by X-ray diffrac-
tion. The compounds are dimeric in the
solid state and comprise two M2C2 het-
erocycles. Depending on the steric
demand of the alkyl substituents at the
metal atom, different types of binding
modes were observed, which can be
classified to lie between the ideals of
side-on coordination with almost linear
ꢁ ꢀ
primary M C C units and the 3c–2e
coordination with symmetrically bridg-
ing alkynyl units in M-C-M bonds. As a
solution in THF the dimers are broken
into monomers and some are found to
undergo ligand scrambling reactions.
Keywords: alkynes
· aluminum ·
anthracene · gallium · Lewis acids
Introduction
or diaryl compounds of Group 13 metals have been structur-
ally characterised so far.[5] This situation has changed during
the last few years and new results in alkynyl aluminium and
gallium chemistry now provide more detailed insights into
the structures and bonding situations.[6a–d] Uhl et al. figured
out that the metallation of terminal alkynes leads to alkynyl
compounds with two different binding modes of the bridging
alkynido groups. The motifs depend on the steric demand of
the substituents terminally attached to the metal atoms.[6b]
All the quoted examples are dimeric and the carbanionic
carbon atoms of the alkynido groups assume bridging posi-
tions. Smaller substituents (like methyl) result in a side-on-
The field of s-alkynyl complexes is well documented in or-
ganometallic chemistry, especially with respect to synthetic
use such as stereoselective epoxide-opening reactions.[1] In
1956, Wilke and Mꢀller reported the first synthesis of an al-
ꢀ
kynyl aluminium compound (Et2AlC CEt) by salt elimina-
[2]
ꢀ
tion from diethylaluminium chloride and NaC CEt. This
initiated the preparation of several other alkynyl aluminium
compounds through metallation of CH-acidic terminal al-
kynes with dialkylaluminium hydride and the formation of
elemental hydrogen or with trialkyl- or triarylaluminium
under evolution of the corresponding gaseous or liquid hy-
drocarbon.[3] Nevertheless, until 2003 only a few alkynyl de-
rivatives of Group 13 elements were described in the litera-
ture.[4] In particular, the bonding situation between the
metal centres and alkynyl groups remained poorly under-
stood due to the fact that only a few derivatives of dialkyl
ꢁ ꢀ
type coordination with linear Al C C groups and an inter-
action with a second aluminium atom through a p-orbital lo-
calised at the a-carbon atom (type A, Scheme 1). More
bulky substituents like tert-butyl lead to structures in which
ꢀ
the C C triple bonds are oriented perpendicularly to the
M···M axis of the central M2C2 heterocycle (type B,
Scheme 1). It has to be noted that the energy differences be-
tween these two idealised borderline cases are very small
and thus the structures are expected to be dependent on var-
ious factors including the size of all substituents in the mole-
cules and intermolecular forces in the case of solid-state in-
vestigations (crystal structures). Intermediate cases are
known;[7] in particular, a gas-phase structure determination
[a] J. Chmiel, B. Neumann, Dr. H.-G. Stammler, Prof. Dr. N. W. Mitzel
Fakultꢁt fꢀr Chemie
Lehrstuhl fꢀr Anorganische Chemie und Strukturchemie
Universitꢁt Bielefeld, Universitꢁtsstraße 25
33615 Bielefeld (Germany)
Fax : (+49)521-1066-026
E-mail: mitzel@uni-bielefeld.de
ꢁ ꢀ
of (Me2Al C CH)2 has demonstrated a preference for the
C2h symmetric form (A) over the D2h symmetric case (B) in
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FULL PAPER
With respect to new insights into the field of host–guest
systems of Group 13 elements aluminium, gallium and
indium, with a tailor-made distribution of acceptor sites by a
rigid backbone, we set out to functionalise the 1,8-
anthracenediACTHUNTGRNEUNGethynyl (1) backbone with Lewis acidic func-
tionalities based on these elements. As for the above-men-
tioned boron compound, the new host molecules promise to
be more highly associated towards a Lewis base than analo-
gous complexes of simply monodentate chelating Lewis
acid.[10] In this way we aim to contribute to the rapidly grow-
ing field of poly-Lewis acid chemistry.[11]
Results and Discussion
Scheme 1. Two different aggregation motifs found for dimeric s-alkynyl
complexes of the earth metals.
Synthesis of 1,8-diethynylanthracene (1): The synthesis of
1,8-diethynylanthracene (1) was performed by modified lit-
erature procedures: 1,8-dichloroanthraquinone was reduced
to 1,8-dichloroanthracene[12] with zinc powder in aqueous
ammonia, and the formed dihalide was coupled with a
Grignard reagent (prepared by the reaction of trimethylsilyl-
acetylene with ethynylmagnesium bromide) in the presence
of a nickel catalyst.[13] Suitable reaction conditions had to be
determined to prevent the formation of a monosubstituted
byproduct and the homo-coupled 1,4-bis(trimethylsilyl)-1,3-
the absence of pronounced substituent or intermolecular in-
teractions.[8]
There is still a distinct paucity of di- or oligometallated al-
kynyl frameworks with Group 13 elements, and in many of
the few known cases the structural characterisation is miss-
ing. One of the earliest examples with non-metallic boron is
ꢁ
ꢁ
ꢁ
butadiyne (TMS CꢀC CꢀC TMS). In our hands, pro-
longed reaction times of the Kumada coupling to 48 h reflux
were found to be advantageous, which is due to the fact that
the substitution of the second chlorine functionalities takes
place more slowly than that of the first. The homo-coupled
byproduct was removed by sublimation (408C, 1 mbar). The
last reaction step was a cleavage of the trimethylsilyl pro-
tecting groups by use of potassium carbonate (Scheme 3).
Reactions of
1 with trialkylaluminium, -gallium and
-indium: Attempts to prepare a dilithiated 1,8-diethynylan-
thracene and to transmetallate this with dialkylmetal halides
failed or did not turn out to afford suitable yields of doubly
Scheme 2. Schematic representation of the functional units 1,8-bis(cate-
cholboranylethynyl)anthracene.[7]
1,8-bis(catecholboranylethynyl)-
anthracene[9] (Scheme 2). The
anthracenediethynyl unit incor-
porates the two rigid spacers
(anthracene and acetylene) and
is therefore a promising rigid
framework with potential use in
the field of host–guest chemis-
try. Both Lewis acidic boron
functions are in close proximity
to each other and offer a 5 ꢃ
bridging distance with minimal
steric interference, which is ad-
vantageous in the synthesis of
complexation agents for Lewis
basic guests of suitable size.
Scheme 3. General procedures for the syntheses of 1,8-bis(dialkylmetalethynyl)anthracenes.
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N. W. Mitzel et al.
metallated products. The successful dimetallation was ach-
ieved by treatment of 1,8-diethynylanthracene (1) with the
metal trialkyls under evolution of the corresponding alkane
(methane or ethane) in good yields (Scheme 3).[14] The prod-
ucts obtained in this way are bright yellow solids. They are
virtually insoluble in non-polar solvents like toluene or
hexane. Initiating the reactions requires temperatures of 50
to 758C. Catalytic amounts of auxiliary base (triethylamine)
were employed in the reactions with aluminium trialkyls.
The trialkylamine enhances the reactivity by decomplexa-
tion of AlR3 and provides a reversible binding donor, which
is important for later complexation of a Lewis base.[15]
tons bonded to C-9 (the position between the ethynyl
groups). Note that such redistribution reactions have already
been established for dimethyl(phenylethynyl)aluminium,
-gallium or -indium. Their NMR spectra in [D8]THF indicate
such an equilibrium according to Equation (1).[14a]
2 ðPhCCÞAlMe2 ꢂ THF ! Me3Al ꢂ THF þ ðPhCCÞ2AlMe ꢂ THF
ð1Þ
The spectrum of 3 shows the same features as the spectrum
of 2. A double set of signals is detected for the protons of
the anthracene and two sets of signals for the ethyl substitu-
ents bonded to aluminium (two triplets at d=1.15 and
0.99 ppm and two quartets at d=0.02 and ꢁ0.12 ppm).
The spectra of compounds 4, 5 and 6 do not indicate the
occurrence of similar redistribution reactions as described
above, as they show less signals for methyl or ethyl resonan-
ces in their 1H NMR spectra. The proton signals of their
ethyl or methyl substituents are found to be strongly shifted
towards higher field (d=ꢁ0.21 ppm (4), 0.52 and 1.24 ppm
(5), ꢁ0.23 ppm (6)), but only one set of signals is detected
for each compound. The resonances of the protons attached
to the anthracene backbone have similar shifts independent
of the kind of metal atom and substituents attached to it. In
this respect, the aluminium compounds 2 and 3 are thus
clearly distinct in their behaviour from the gallium and
indium compounds 4, 5 and 6.
IR and NMR spectroscopic studies: For all compounds, the
triple-bond stretching frequencies measured in the solid
state are close to 2054 cmꢁ1. This value is in accordance with
literature-known triple-bond stretching frequencies of di-
meric Me2PhCCAl,[14a] Me2PhCCGa[14a] and Me2PhCCIn[14a]
in benzene.
Due to the fact that the dimeric compounds 2–6 are in-
soluble in toluene or other hydrocarbon solvents, NMR
spectroscopy experiments were carried out in [D8]THF. The
use of a donor solvent induces a disruption of the dimeric
structure (see below for the solid-state structures). The
1H NMR spectrum of 2 in [D8]THF shows three singlets at
d=ꢁ0.67, ꢁ0.70 and ꢁ0.96 ppm. These are characteristic for
aluminium-bonded methyl groups and comply with the sig-
nals of the related compounds Me3Al (d=ꢁ0.96 ppm),
(PhCC)AlMe2 (d=ꢁ0.83 ppm) and (PhCC)2AlMe (d=
ꢁ0.78 ppm) in [D8]THF.[14a] This might either be due to the
simultaneous presence of monomers and dimers (see the
crystal structures below) or due to possible redistribution re-
actions of ethynyl and methyl functionalities as depicted in
Scheme 4. In addition, the number of signals and their rela-
tive intensities of the signals match the constitutions of the
postulated products.
Bonding situations and molecular structures
1,8-Diethynylanthracene (1): For comparison with the metal-
lated compounds, we determined the crystal structure of 1,8-
diethynylanthracene (1; Figure 1). Single crystals suitable
for X-ray diffraction were obtained by slow evaporation of
dichloromethane. It crystallises
in the monoclinic space group
P21/n. Three molecules are
found in the asymmetric unit.
Important bond lengths and
angles are listed in Table 1.
Comparison of corresponding
parameter values for the three
independent molecules show
impressively how structural pa-
rameter values of relatively
rigid molecules may scatter for
the same molecule. The relia-
bility of the lengths and angles
in terms of a determination of
Scheme 4.
values independent of the sur-
rounding is at best 0.01 ꢃ and
Further proof for the redistribution stems from the occur-
rence of two sets of signals for the protons of the anthracene
backbone in the range between d=9.69 and 7.34 ppm with
the characteristic singlets at d=9.69 and 9.56 ppm for pro-
18 in this case, which corresponds to about three estimated
standard deviations of the individual parameters. Neverthe-
less, within this range one is able to recognise structural
trends.
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Poly-Lewis Acids
FULL PAPER
1,8-Bis(dialkylmetalethynyl)anthracenes (2–6): Single crys-
tals of compounds 2 to 6 were obtained by slowly cooling
the reaction mixtures in toluene from around 50–708C to
ambient temperature overnight. The molecular structures
are shown in Figures 2 to 7. Due to structural similarities,
compounds 2, 4 and 6 (R=Me) and 3 and 5 (R=Et) are de-
scribed together. Abnormalities or significant differences
Figure 1. Molecular structure of 1,8-diethynylanthracene (1).
Table 1. Selected bond lengths [ꢃ] and angles [8] of 1.
Molecule A
Molecule B[a]
Molecule C[a]
ꢁ
C1 C15
1.433(4)
1.431(4)
1.180(4)
1.188(4)
120.1(2)
119.7(2)
120.2(2)
120.5(2)
119.8(2)
119.7(2)
179.4(3)
179.0(3)
1.436(3)
1.438(3)
1.174(3)
1.171(3)
121.2(2)
119.9(2)
118.8(2)
120.1(2)
120.0(2)
119.9(2)
178.0(3)
178.7(3)
1.429(4)
1.433(4)
1.190(4)
1.185(4)
120.7(3)
120.1(3)
119.2(2)
120.8(2)
120.1(3)
119.1(2)
179.3(3)
178.1(3)
ꢁ
C5 C17
ꢁ
C15 C16
ꢁ
C17 C18
C14-C1-C15
C2-C1-C14
C2-C1-C15
C6-C5-C17
C4-C5-C6
C4-C5-C17
C1-C15-C16
C5-C17-C18
[a] The labelling scheme of molecule A is used. The other parameter
values are the corresponding data from the other two molecules.
Figure 2. Wire models of compounds 2–6 in a view along the planes of
the anthracene rings and on top of the M2C2 rings to illustrate the struc-
tural trends to the two binding modes shown in a general scheme in
Scheme 5.
Due to the fact that all three structures are similar, only
one of them is described in detail. The anthracene skeleton
is planar with both alkyne substituents in plane. Only in
molecule C is one alkyne group slightly displaced to one
side of the anthracene plane by 1.48.
The atoms C1 and C5 are trigonal-planar-coordinated
with the three surrounding angles being close to 1208. The
alkynyl groups are bonded to the anthracene skeleton with
2
ꢁ
ꢁ
expectedly short bonds (sp sp bond); the bond C1 C15
ꢁ
measures 1.433(4) ꢃ and C5 C17 measures 1.431(4) ꢃ.
These values are in the range observed for the correspond-
ing ones in the free molecules of phenylacetylene (rg =
1.400(3) ꢃ by gas electron diffraction[16] and rs =1.448 ꢃ by
microwave spectroscopy[17]).
The alkynyl groups are almost linear (angles in molecule
A: C1-C15-C16 179.4(3)8 and C5-C17-C18 179.0(3)8), and
ꢁ ꢀ
the smallest of the six observed C C C angles is 178.0(3)8,
that is, is the largest deviation is 28. The lengths of the
ꢁ
ꢁ
bonds C15 C16 and C17 C18 are 1.180(4) and 1.188(4) ꢃ,
respectively. This is only slightly shorter than the standard
length of a C C triple bond of 1.20 ꢃ[18] and almost identical
ꢀ
to the corresponding ones in gaseous phenylacetylene (rg =
1.205(5) ꢃ by gas electron diffraction[16] and rs =1.208 ꢃ by
microwave spectroscopy[17]). The bond lengths for the an-
thracene skeleton in 1 (1.36–1.45 ꢃ) fall in the same range
as for unsubstituted anthracene (1.37 to 1.43 ꢃ)[19] and gas-
eous 1.392(6)–1437(4) ꢃ[20]).
Figure 3. Molecular structure of dimeric 1,8-bis(dimethylalanylethyn-
AHCTUNGTREGyNNNU l)anthracene (2).
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N. W. Mitzel et al.
Figure 6. Molecular structure of dimeric 1,8-bis(diethylgallanylethyn-
Figure 4. Molecular structure of dimeric 1,8-bis(diethylalanylethynyl)an-
AHCTUNGTREGyNNNU l)anthracene (5).
thracene (3).
Figure 7. Molecular structure of dimeric 1,8-bis(dimethylindanylethyn-
AHCTUNGTREGyNNNU l)anthracene (6).
Figure 5. Molecular structure of dimeric 1,8-bis(dimethylgallanylethyn-
ACHTUNGTRENNUNGyl)anthracene (4).
All compounds (2–6) crystallise in the triclinic system in
¯
will be mentioned separately. Figure 2 shows side views of
compounds 2–6 that illustrate the two different types of
binding modes. A general and ideal schematic representa-
tion of a side-on coordination (type A) and a 3c–2e bonding
(type B) with important structural features is displayed in
Scheme 5. Table 2 lists the corresponding bond lengths and
angles of all compounds (2–6) for facilitated comparison
and for a description of the bonding situations of methyl-
and ethyl-substituted compounds. Additional structural pa-
rameter values of compounds 2, 4 and 6 and 3 and 5 are
compiled in Tables 3 and 4, respectively.
the space group P1. They form dimers linked by two M2C2
heterocycles. The carbanionic alkynyl carbon atoms thereby
act as bridging units. The structural parameters listed in
Table 2 indicate that 2–6 crystallise in aggregation modes,
which differ from both the ideal side-on coordination and
from geometries that representing 3c–2e bonding. The side-
on coordination mode was earlier found for di(m-phenyl-
AHCTUNGTREGeNNUN thynyl)bis(dimethylaluminium) or di(m-phenylethynyl)bis-
(dimethylgallium).[6b] Close to ideal 3c–2e bonding was re-
ported for di(m-phenylethynyl)bis(di-tert-butylaluminium)
and di(m-phenylethynyl)bis(di-tert-butylgallium).[6b] Which of
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Poly-Lewis Acids
FULL PAPER
Table 3. Selected bond lengths [ꢃ] and angles [8] of 2, 4 and 6.
2
[a] (M=Al)
4[a] (M=Ga)
6[a] (M=In)
ꢁ
M1 C16
2.032(2)
2.129(2)
1.959(2)
1.954(2)
2.040(2)
2.141(1)
1.948(2)
1.951(2)
1.219(2)
1.217(2)
2.879(1)
87.2(1)
87.3(1)
90.7(1)
90.2(1)
117.9(1)
119.0(1)
2.029(2)
2.203(3)
1.966(2)
1.952(2)
2.047(3)
2.237(2)
1.956(3)
1.960(3)
1.209(3)
1.209(3)
2.951(1)
87.4(1)
87.9(1)
90.4(1)
89.0(1)
118.4(2)
119.4(2)
2.224(2)
2.430(2)
2.156(3)
2.150(2)
2.246(3)
2.437(2)
2.144(3)
2.146(3)
1.219(3)
1.218(3)
3.298(1)
90.0(1)
89.6(1)
87.8(1)
87.2(1)
118.3(2)
119.9(2)
ꢁ
M1 C20’
ꢁ
M1 C17
ꢁ
M1 C18
ꢁ
M2’ C20’
ꢁ
M2’ C16
ꢁ
M2’ C21’
ꢁ
M2’ C22’
ꢁ
C16 C15
ꢁ
C20’ C19’
ꢁ
M1 M2’
M1-C16-M2’
M1-C20’-M2’
C16-M1-C20’
C16-M2’-C20’
C14-C1-C15
C6’-C5’-C19’
[a] Methyl as substituent.
Table 4. Selected bond lengths [ꢃ] and angles [8] of 3 and 5.
3[a] (M=Al)
5[a] (M=Ga)
ꢁ
Scheme 5. General schematic representation of important structural fea-
M1 C16
2.044(2)
2.103(1)
1.970(2)
1.963(2)
2.053(1)
2.119(2)
1.957(1)
1.965(1)
1.219(2)
1.217(2)
2.811(1)
84.9(1)
85.1(1)
93.0(1)
92.3(1)
153.8(1)
130.4(1)
120.3(1)
144.0(1)
118.0(1)
118.4(1)
2.056(2)
2.162(2)
1.984(1)
1.974(2)
2.072(1)
2.193(1)
1.962(2)
1.974(2)
1.218(2)
1.217(2)
2.885(2)
84.4(1)
84.8(1)
93.4(1)
92.1(1)
153.9(1)
130.6(1)
120.6(1)
144.0(1)
118.4(1)
118.6(1)
ꢀ
tures of the central M
G
ꢁ
M1 C22’
modes. The letters describe selected bond lengths ($) and angles (\)
that emanate from side-on coordination (type A) and 3c–2e bonding
(type B).
ꢁ
M1 C17
ꢁ
M1 C19
ꢁ
M2’ C22’
ꢁ
M2’ C16
ꢁ
M2’ C23’
ꢁ
Table 2. Selected bond lengths [ꢃ] and angles [8] of 2–6 (see Scheme 5
for an understanding of which bonds/angles the letters represent).
M2’ C25’
ꢁ
C16 C15
ꢁ
C22’ C21’
2[a]
3[b]
4[a]
5[b]
6[a]
ꢁ
M1 M2’
(M=Al)
(M=Al)
(M=Ga)
(M=Ga)
(M=In)
M1-C16-M2’
M1-C22’-M2’
C16-M1-C22’
C16-M2’-C22’
M1-C16-C15
M1-C22’-C21’
M2’-C16-C15
M2’-C22’-C21’
C14-C1-C15
C6’-C5’-C21’
A
1.219(2)
1.219(2)
1.217(2)
2.044(2)
2.053(1)
2.119(2)
2.103(2)
176.3(1)
177.2(1)
153.8(1)
144.0(1)
84.9(1)
1.209(3)
1.209(3)
2.029(2)
2.047(3)
2.237(3)
2.203(2)
175.6(2)
177.7(2)
160.7(2)
157.2(2)
87.4(1)
1.218(2)
1.217(2)
2.056(2)
2.072(1)
2.193(2)
2.162(2)
176.2(2)
177.0(2)
153.9(1)
144.0(1)
84.4(1)
1.219(3)
1.218(3)
2.224(2)
2.246(3)
2.437(2)
2.430(2)
174.7(2)
178.2(2)
161.4(2)
159.4(2)
90.0(1)
A’ 1.217(2)
2.032(2)
B’ 2.040(2)
2.141(1)
C’ 2.129(2)
176.4(2)
D’ 176.6(2)
162.2(1)
E’ 159.8(1)
87.2(1)
F’ 87.3(1)
90.7(1)
G’ 90.2(1)
B
C
D
E
[a] Ethyl as substituent.
F
85.1(1)
93.0(1)
92.3(1)
87.9(1)
90.4(1)
89.0(1)
84.8(1)
93.4(1)
92.1(1)
89.6(1)
87.8(1)
87.2(1)
G
ꢀ
atom of the C C group in the other unit. In the case of the
earlier reported di(m-phenylethynyl)bis(dimethylaluminium)
[a] Methyl as substituent. [b] Ethyl as substituent.
or di(m-phenylethynyl)bis(dimethylgallium),[6] the M C C
ꢁ ꢀ
groups are almost linear (173.78 and 173.58) and have rela-
ꢁ
ꢀ
these modes is adopted depends primarily on the steric
demand of the alkyl substituents.
tively short M C bonds (1.994 and 2.001 ꢃ). The C C bond
lengths are close to the standard triple bond length of
ꢁ
As already outlined in the Introduction, in aggregation
type A the metal atoms can be described as s-bonded to
one alkynyl group of one 1,8-anthracenediethynyl unit
(stronger bond) and side-on coordinated to the alkynyl
group of the other 1,8-anthracenediethynyl unit (weaker sec-
ondary interaction between the metal atom and the p-elec-
1.20 ꢃ. The remaining M C distances of the asymmetric
ꢁ
bridge perpendicular to the M CꢀC groups are strongly
elongated to 2.224 and 2.378 ꢃ.
Compared with these results, the new compounds with
metal-bound methyl substituents, 2, 4 and 6, contain angles
M C C far away from the ideal 1808 by 17.88 and 20.28 for
2, by 19.38 and 22.88 for 4 and by 18.68 and 20.68 for 6. The
M1 C16 bonds (2.032(2) (2), 2.029(2) (4) and 2.224(2) ꢃ
(6)) are somewhat shorter than the M2’ C20’ bonds
ꢁ ꢀ
ꢀ
trons of the C C bond). It may be seen as consisting of two
ꢁ
distinct monomer units joined by long secondary bonds be-
ꢁ
tween the metal atom of one monomer unit and the a-C
Chem. Eur. J. 2010, 16, 11906 – 11914
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11911

N. W. Mitzel et al.
ꢀ
ꢀ
ꢀ
(2.040(2) (2), 2.047(3) (4) and 2.246(3) ꢃ (6)), but altogether
longer than for the monodentate alkynides[6b] described
above. Compared to this, the metal-to-bridging-carbon dis-
C C triple bond lengths C16 C15 and C22’ C21’ of 1.217 to
1.219 ꢃ are somewhat larger than the accepted standard
value of 1.20 ꢃ.
ꢁ
ꢁ
tances of M1 C20’ and M2’ C16 are elongated by 0.107 and
0.089 ꢃ for 2, by 0.208 and 0.156 ꢃ for 4 and by 0.213 and
0.184 ꢃ for 6. This trend is approximately proportional to
the widening of the M-C-M angles and to the compression
of the C-M-C angles. The larger the radii of the metal
atoms, the more widened are the angles M2’-C16-M1 and
M2’-C20’-M1, which have nearly the same value for the
three individual compounds (87.28 and 87.38 for 2, 87.48 and
The non-bonded M···M distances are 2.811(1) ꢃ for the
Al···Al (3) and 2.951(1) ꢃ for the Ga···Ga cases (5). This is
comparable to the values in structures 2 and 4, respectively.
It is possible that these results indicate an attractive interac-
ꢀ
tion between the p-bonding electrons of the C C bond and
the metal atoms (side-on coordination). The facts outlined
here illustrate that the metal–ethyl-substituted compounds
adopt a kind of intermediate bonding situation between the
side-on coordination and the 3c–2e bonding motif. This in-
terpretation is also supported by the fact that the length
ꢁ
87.98 for 4 and 90.08 and 89.68 for 6). In addition, the M1
M2’ distances are found to be elongated with increasing
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
metal atom size (2.879(1) ꢃ for Al1 Al2’, 2.951(1) ꢃ for
M1 C22’ and M2 C16’ exceed that of M1 C16 and M2
C22’ by 0.05 and 0.08 ꢃ in the case of 3 and 0.09 and 0.14 ꢃ
in the case of 5.
ꢁ
ꢁ
ꢀ
Ga1 Ga2’ in 4 and 3.298(1) ꢃ for In1 In2’ in 6). The C C
bond lengths are in the range of 1.219 to 1.209 ꢃ and thus
ꢀ
nearly equal to the usual acetylenic bond length and the C
ꢁ ꢀ ꢁ
C bond in CH3 C C CH3 determined by gas-phase elec-
tron diffraction to 1.206(1) ꢃ.[16b] This can be rationalised by
the donation of p electrons into empty orbitals of the metal
atoms. The four-membered rings formed by the metal atoms
and the two bridging carbon atoms are not planar.
Conclusion
Alkane elimination is a process efficient enough to be em-
ployed to prepare dimetallated alkynyl complexes of
Group 13 elements, that is, to affect both subsequent metal-
lation steps in reasonable to high yields. The new poly-
Lewis acids with a 1,8-diethynylanthracenediyl backbone
form dimers with different types of molecular structures. In
the solid state, tetranuclear compounds with a relatively
fixed arrangement of the four metal atoms are present.
Their structural motifs depend in a subtle way on the steric
demand of the alkyl groups attached to the Group 13 metal
atoms. A motif closer to side-on coordination was found for
the dimers 2, 4 and 6 with small methyl substituents, where-
as with the only slightly larger ethyl substituents the struc-
tural motif tends more toward a 3c–2e bonding situation
(compounds 3 and 5). In a solution of the donor solvent
THF, the dimers are broken into solvated monomers that
are bidentate Lewis acids; some of these undergo ligand-
scrambling reactions.
A representative structure for a 3c–2e bonding is the di-
meric trimethylaluminium molecule with the bridging
methyl group interacting equally with both Al atoms and an
Al···Al distance of 2.619 ꢃ. Two other examples on the basis
of three-centre two-electron bonds are di(m-phenylethynyl)-
bis(di-tert-butylaluminium) and di(m-phenylethynyl)bis(di-
tert-butylgallium).[6b] The C C triple bonds in these com-
ꢀ
plexes are oriented perpendicularly to the M···M axes of the
central M2C2 heterocycles. For these established compounds,
ꢁ
all four M C distances of the central rings fall over a rela-
tively narrow range between 2.063 and 2.097 ꢃ for the alu-
minium compound and between 2.110 and 2.141 ꢃ for the
ꢁ ꢀ
gallium compound. The M C C angles are 136.48 and
135.68 for the gallium and 129.78 and 143.08 for the alumini-
ꢀ
um derivative. The C C triple bond lengths are close to the
ꢀ
standard C C bond length (1.20 ꢃ). The M···M distances
are shorter (2.864 ꢃ for Al···Al and 2.951 ꢃ for Ga···Ga)
than the ones in the side-on coordination type.
Compared to this, the new compounds with metal–ethyl
substituents, 3 and 5, with their slightly smaller ethyl sub-
stituents do not feature a pure 3c–2e bonding motif. The
Experimental Section
ꢁ
General: 1,8-Diethynylanthracene (1) was synthesised by modified litera-
ture-known procedures.[12,13] Its purification was performed by column
chromatography on silica gel 60 (0.04–0.063 mm mesh) with pentane as
the mobile phase to separate the monosubstituted byproduct in the first
fraction. All metallation reactions were carried out under an anhydrous,
inert atmosphere of nitrogen or argon using standard Schlenk and glove-
box techniques in dried solvents (THF and toluene were dried over po-
tassium and sodium and were freshly condensed before being used for
the reactions). [D8]THF was dried over Na/K alloy and the auxiliary base
triethylamine was dried over CaH2 and degassed. Trimethylaluminium,
triethylaluminium, trimethylgallium, triethylgallium and trimethylindium
were commercially available. NMR spectroscopic measurements were re-
corded using a Bruker DRX 500 and a Bruker Avance 600 at room tem-
perature; the chemical shifts (d) were measured in ppm with respect to
the solvent ([D8]THF, 1H: d=1.73 and 3.58 ppm, 13C: d=25.5 and
67.7 ppm). Elemental analyses were performed using a Leco CHNS 932
instrument.
four-membered M2C2 rings are not planar and the M C
ꢁ
ꢁ
bond lengths are unequal. The distances M1 C16 and M2’
C22’ are 2.044(2) and 2.053(1) ꢃ for 3 and 2.056(2) and
ꢁ
ꢁ
2.072(1) ꢃ for 5. M1 C22’ and M2’ C16 are 2.103(2) and
2.119(2) ꢃ for 3 and 2.430(2) and 2.437(2) ꢃ for 5. In addi-
tion, the CꢀC bonds are not perpendicular to the M···M
ꢁ ꢀ
axes and the M C C angles are: M1-C16-C15 153.8(1)8,
M2’-C16-C15 120.3(1)8, M1-C22’-C21’ 130.4(1)8 and M2’-
C22’-C21’ 144.0(1)8 for 3. The corresponding angles for
5 are M1-C16-C15 153.9(1)8, (M2’-C16-C15
120.6(1)8, M1-C22’-C21’ 130.6(1)8 and 144.0(1)8 M2’-C22’-
C21’.
compound
It is worth mentioning the deformation caused by a tilt of
the ethynyl groups towards one metal atom of the rings. The
11912
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 11906 – 11914

Poly-Lewis Acids
FULL PAPER
Table 5. Crystallographic data for 1–6.
1
2
3
4
5
6
formula
Mr
crystal
C18H10
226.26
yellow
plates
monoclinic
P21/n
13.343(2)
11.545(1)
23.693(3)
90
94.42(1)
90
3638.8(7)
12
1.239
0.070
C44H40Al4
676.68
yellow
C52H56Al4
788.89
yellow
C44H40Ga4
847.64
yellow
C52H56Ga4
959.85
yellow
C44H40In4
1028.04
yellow
fragment
triclinic
fragment
triclinic
fragment
triclinic
fragment
triclinic
fragment
triclinic
crystal system
space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
¯
¯
¯
¯
¯
P1
P1
P1
P1
P1
8.664(1)
10.731(1)
12.418(1)
65.26(1)
81.69(1)
66.36(1)
960.2(1)
1
9.901(1)
10.457(1)
12.904(1)
67.07(1)
82.10(1)
64.17(1)
1106.5(1)
1
8.714(1)
10.756(1)
12.411(1)
65.86(1)
81.89(2)
66.73(1)
974.9(1)
1
9.913(1)
10.394(1)
12.880(1)
66.53(1)
81.62(1)
64.62(1)
1099.2(1)
1
7.350(1)
12.221(1)
12.383(1)
61.43(1)
88.00(1)
88.75(1)
970.9(2)
1
a [8]
b [8]
g [8]
V [ꢃ3]
Z
1calcd [gcmꢁ3
]
1.170
0.151
30
1.184
0.140
27.47
1.444
2.763
30
1.450
2.459
30
1.758
2.374
27.50
m [mmꢁ1
2qmax [8]
]
25
reflns collected
independent reflns
Rint
28692
6402
0.088
21780
5572
0.034
34117
5059
0.033
35102
5665
0.038
52593
6398
0.038
25891
4456
0.037
observed reflns
3421
4488
4254
4419
5603
3895
1min/max [eꢃꢁ3
]
ꢁ0.20/0.18
6402/0/487
0.0508
0.0976
0.1204
0.1232
0.968
ꢁ0.29/0.31
5572/0/221
0.0409
0.1102
0.0538
0.1186
1.027
ꢁ0.23/0.29
5059/0/263
0.0361
0.0980
0.0449
0.1030
1.058
ꢁ0.95/0.56
5665/0/221
0.0334
0.0876
0.0471
0.0958
0.965
ꢁ0.60/0.54
6398/0/268
0.0242
0.0614
0.0298
0.0637
1.058
ꢁ0.65/0.44
4456/0/221
0.0218
0.0459
0.0289
0.0485
1.039
data/restraints/params
R1 (I>2s(I))
wR2 (I>2s(I))
R1 (all data)
wR2 (all data)
GOF
F
(000)
1416
356
420
428
492
500
Crystallographic structure determinations: Single crystals of 2–6 were
prepared inside a glovebox by means of a suspension in a Paratone-N/
paraffin oil mixture, fixed on a glass fibre and transferred onto the goni-
ometer of the diffractometer. The data were collected with MoKa radia-
tion (l=0.71073 ꢃ). The structures were solved by direct methods and
refined by full-matrix least-squares cycle programs SHELXS-97 and
SHELXL-97.[21] Crystallographic data are provided in Table 5. CCDC-
779257 (1), CCDC-779258 (2), CCDC-779259 (3), CCDC-779260 (4),
CCDC-779261 (5) and CCDC-779263 (6) contain the supplementary crys-
tallographic 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.
8.8 Hz, 2H; H-2/H-7), 7.62 (d, 3JH,H =6.8 Hz, 2H; H-4/H-5), 7.36 (dd,
3JH,H =6.9, 8.4 Hz, 2H; H-3/H-6), ꢁ0.71 ppm (s, 12H; (CH3)4); 13C NMR
ꢀ ꢁ
ꢀ
(125 MHz, [D8]THF): d=133.0–125.3 (C-aryl), 105.3 (C C Al), 83.0 (C
C Al), ꢁ8.9 ppm (CH3); 27Al NMR (130 MHz, [D8]THF): d=182 ppm;
ꢁ
IR (KBr plates): n˜ =3053–2820 (s) (aryl-H), 2060 (vs) (CꢀC), 1938–
1672, 1604–1425 cmꢁ1 (w) (C=C); elemental analysis calcd (%) for
C44H40Al4: C 78.37, H 5.96; found C 78.37, H 5.96.
1,8-Bis(diethylalanylethynyl)anthracene (3): Triethylamine (10% based
on amount of trimethylaluminium) was added to a solution of 1 and trie-
thylaluminium in toluene. The reaction started at RT. For completion of
the reaction the mixture was heated to 728C for 3 h. Yield: 170 mg
(81%) (corresponding to a mixture of product and by-product (3:1)).
1H NMR (500 MHz, [D8]THF, 258C): d=9.56 (s, 1H; H-9), 8.42 (s, 1H;
H-10), 7.91 (d, 3JH,H =8.5 Hz, 2H; H-2/H-7), 7.65 (d, 3JH,H =6.8 Hz, 2H;
H-4/H-5), 7.37 (dd, 3JH,H =7.0 Hz, 2H; H-3/H-6), 1,15 (t, 3JH,H =8.1 Hz,
12H; (CH3)4), 0.02 ppm (q, 3JH,H =8.1 Hz, 8H; (CH2)4); 13C NMR
General procedure for the synthesis of alkynides 2–6: The 1,8-diethynyl-
anthracene (1) was dissolved in a small amount of toluene and an excess
amount of the metal trialkyl (3 equiv) was slowly added. The reactions
were initiated by heating the mixtures several times (to temperatures de-
pending on the different metals and substituents) until gas evolution and
the formation of a bright yellow precipitate was observed. The products
were characterised without further purification. The numbering scheme
for NMR spectroscopic assignments (Scheme 6) is based on IUPAC
guidelines. Only the lower number de-
ꢀ
(125 MHz, [D8]THF): d=132.9–125.2 (C-aryl), 106.4 (C C), 10.1 (CH3),
0.8 ppm (CH2) (one signal missing due to overlapping or broadening);
27Al NMR (130 MHz, [D8]THF): d=178 ppm; IR (KBr plates): n˜ =2933–
ꢁ1
ꢀ
2789 (s) (aryl-H), 2054 (vs) (C C), 1936–1658, 1604–1450 cm (w) (C=
C); elemental analysis calcd (%) for C52H56Al4: C 79.17, H 7.15; found C
78.88, H 8.01 (an additional value of 0.341% N indicates the presence of
small amounts of triethylamine).
scriptor is used for equal protons or
carbon atoms.
1,8-Bis(dimethylalanylethynyl)anthra-
cene (2): Triethylamine (10% based
on amount of trimethylaluminium)
was added to a solution of 1 and tri-
methylaluminium in toluene. The reac-
tion started after 6 h after heating to
728C. Yield: 108 mg (78%) (corre-
sponding to a mixture of product and
1,8-Bis(dimethylgallanylethynyl)anthracene (4): The product was ob-
tained by heating the reaction mixture to 508C for 5 h. Yield: 123 mg
(73%). 1H NMR (500 MHz, [D8]THF, 258C): d=9.59 (s, 1H; H-9), 8.42
3
(s, 1H; H-10), 7.92 (d, 3JH,H =8.5 Hz, 2H; H-2/H-7), 7.62 (d, JH,H
=
6.9 Hz, 2H; H-4/H-5), 7.38 (dd, 3JH,H =7.1, 7.0 Hz, 2H; H-3/H-6),
ꢁ0.21 ppm (s, 12H; (CH3)3); 13C NMR (125 MHz, [D8]THF): d=132.8–
ꢀ ꢁ
ꢀ ꢁ
125.1 (C-aryl), 114.2 (C C Ga), 104.9 (C C Ga), ꢁ5.3 ppm (CH3); IR
byproduct (3:1)). 1H NMR (600 MHz,
ꢀ
(KBr plates): n˜ =3051–2895 (s) (aryl-H), 2065 (vs) (C C), 1930–1757,
Scheme 6. Numbering scheme
for NMR spectroscopic assign-
ments.
1604–1442 cmꢁ1 (w) (C=C); elemental analysis calcd (%) for C44H40Ga4:
[D8]THF, 258C): d=9.56 (s, 1H; H-9),
3
8.42 (s, 1H; H-10), 7.91 (d, JH,H
=
C 62.34, H 4.76; found C 62.38, H 4.91.
Chem. Eur. J. 2010, 16, 11906 – 11914
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11913

N. W. Mitzel et al.
1,8-Bis(diethylgallanylethynyl)anthracene (5): The product was obtained
by heating the reaction mixture to 708C for 3 h. Yield: 366 mg (89%).
1H NMR (600 MHz, [D8]THF, 258C): d=9.59 (s, 1H; H-9), 8.42 (s, 1H;
[7] C. Elschenbroich, S. Grimme, S. Haddadpour, M. Matar, M. Nowot-
ny, W. Uhl, A. Vogelpohl, Z. Anorg. Allg. Chem. 2009, 635, 2027–
2033.
[8] A. Almenningen, L. Fernholt, A. Haaland, J. Organomet. Chem.
1978, 155, 245–257.
H-10), 7.91 (d, 3JH,H =8.5 Hz, 2H; H-2/H-7), 7.63 (d, 3JH,H =6.7 Hz, 2H;
3
H-4/H-5), 7.37 (dd, 3JH,H =7.1, 7.0 Hz, 2H; H-3/H-6), 1.24 (t, JH,H
=
3
8.0 Hz, 12H; (CH3)4), 0.52 ppm (q, JH,H =8.1 Hz, 8H; (CH2)4); 13C NMR
[9] H. E. Katz, J. Org. Chem. 1989, 54, 2179–2183.
ꢀ ꢁ
(125 MHz, [D8]THF): d=132.6–124.9 (C-aryl), 112.5 (C C Ga), 106.1
[10] A. Cottone, M. J. Scott, Organometallics 2002, 21, 3610–3627.
[11] Examples of poly-Lewis acids: a) F. P. Gabbaꢅ, Angew. Chem. 2003,
115, 2318–2321; Angew. Chem. Int. Ed. 2003, 42, 2218–2221; b) F. P.
Gabbaꢅ , A. Schier, J. Riede, D. Schichl, Organometallics 1996, 15,
4119–4121; c) F. P. Gabbaꢅ , A. Schier, J. Riede, J. Chem. Soc.
Chem. Commun. 1996, 1121–1122; d) C. L. Dorsey, P. Jewula, T. W.
Hudnall, J. D. Hoefelmeyer, T. J. Taylor, N. R. Honesty, C.-W. Chiu,
M. Schulte, F. P. Gabbaꢅ, Dalton Trans. 2008, 4442–4450; e) M. H.
Lee, F. P. Gabbaꢅ, Inorg. Chem. 2007, 46, 8132–8138; f) K. Ju-
rkschat, H. G. Kuivila, S. Liu, and J. A. Zubieta, Organometallics
1989, 8, 2755–2759; g) M. Newcomb, J. H. Horner, M. T. Blanda, J.
Am. Chem. Soc. 1987, 109, 7878–7879; h) M. Schulte, G. Gabriele,
M. Schꢀrmann, K. Jurkschat, A. Duthie, D. Dakternieks, Organome-
tallics 2003, 22, 328–336; i) M. Schulte, M. Schꢀrmann, K. Jurkschat,
Chem. Eur. J. 2001, 7, 347–355; j) K. Tamao, T. Hayashi, Y. Ito, M.
Shiro, J. Am. Chem. Soc. 1990, 112, 2422–2424; k) K. Tamao, T.
Hayashi, Y. Ito, M. Shiro, Organometallics 1992, 11, 2099–2114;
l) D. Brondani, F. H. Carrꢄ, R. J. P. Corriu, J. J. E. Moreau, M. Wong
Chi Man, Angew. Chem. 1996, 108, 349–352; Angew. Chem. Int. Ed.
Engl. 1996, 35, 324–326; m) W. Uhl, F. Hannemann, W. Saak, R.
Wartchow, Eur. J. Inorg. Chem. 1998, 921–926; n) W. Uhl, F. Hanne-
mann, J. Organomet. Chem. 1999, 579, 18–23; o) W. Uhl, M. Matar,
J. Organomet. Chem. 2002, 664, 110–115; p) W. Uhl, F. Breher, S.
Haddadpour, Organometallics 2005, 24, 2210–2213; q) W. Uhl, S.
Haddadpour, M. Matar, Organometallics 2006, 25, 159–163; r) X.
Yang, C. B. Knobler, M. F. Hawthorne, Angew. Chem. 1991, 103,
1519–1521; Angew. Chem. Int. Ed. Engl. 1991, 30, 1507–1508; s) X.
Yang, C. B. Knobler, M. F. Hawthorne, J. Am. Chem. Soc. 1992, 114,
380–382.
ꢀ ꢁ
(C C Ga), 11.3 (CH3), 5.1 ppm (CH2) (one signal not observed due to
overlapping or broadening); IR (KBr plates): n˜ =2943–2812 (s) (aryl-H),
2054 (vs) (C C), 1930–1753, 1604–1415 cmꢁ1 (w) (C=C); elemental analy-
ꢀ
sis calcd (%) for C52H56Ga4: C 65.07, H 5.88; found C 64.25, H 5.69.
1,8-Bis(dimethylindanylethynyl)anthracene (6): Product 6 was obtained
by heating the reaction mixture to 558C for 2 h. Yield: 179 mg (87%).
1H NMR (600 MHz, [D8]THF, 258C): d=9.63 (s, 1H; H-9), 8.40 (s, 1H;
H-10), 7.88 (d, 3JH,H =8.4 Hz, 2H; H-2/H-7), 7.61 (d, 3JH,H =6.7 Hz, 2H;
3
H-4/H-6), 7.36 (dd, JH,H =7.5, 7.6 Hz, 2H; H-3/H-5), ꢁ0.23 ppm (s, 12H;
(CH3)4); 13C NMR (150 MHz, [D8]THF): d=132.9–125.8 (C-aryl), 121.9
ꢀ ꢁ
ꢀ ꢁ
(C C In), 107.4 (C C In), ꢁ7.1 ppm (CH3); IR (KBr plates): n˜ =3049–
ꢁ1
ꢀ
2874 (s) (aryl-H), 2054 (vs) (C C), 1925–1672, 1604–1425 cm (w) (C=
C); elemental analysis calcd (%) for C44H40In4: C 51.40, H 3.92; found C
52.64, H 4.40.
Acknowledgements
We are grateful to the NRW Graduate School of Chemistry (GSC–MS)
and the Deutsche Forschungsgemeinschaft for financial support. We
thank Dr. Andreas Mix and Peter Mester for NMR spectroscopic meas-
urements and Brigitte Michel for performing the elemental analysis
measurements.
[1] a) M. Sasaki, K. Tanino, M. Miyashita, J. Org. Chem. 2001, 66,
5388–5394; b) N. Powell, S. D. Rychnovsky, Tetrahedron Lett. 1998,
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Received: June 8, 2010
Published online: September 9, 2010
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