The influence of halogen substituents on the course of hydrogallation and hydroalumination reactions

Article abstract of DOI:10.1039/b913568e

Treatment of trimethylsilylethynylbenzene derivatives with HGaCl ₂ afforded products, [C₆H_6-x{C(H)C(SiMe ₃)GaCl₂}_x], in which by a very fast cis/trans-rearrangement the Ga and H atoms occupied opposite sides of the resulting CC double bonds. The stability of the cis-forms considerably increased upon application of 1,3-dibromo- and pentafluorophenylalkyne derivatives. Two pairs of cis/trans-isomers could be characterized by crystal structure determinations and allow the direct comparison of structural parameters. For the first time an equilibrium was detected between cis- and trans-forms in solution. Treatment of 1,4-di(tert-butylalkynyl)benzene with HAlR₂ (R = CMe₃, CH₂CMe₃) afforded cyclophane-type molecules by the release of AlR₃. Only the neopentyl derivative could be isolated and characterized by crystal structure determination. In contrast, the dibromo compound, 1,4-Br₂-2,5-(Me₃CCC) ₂C₆H₂, yielded the simple addition product, C₆H₂Br₂{C(AlR₂)C(H)CMe ₃}₂ (R = CMe₃). Condensation was hindered in this case by intramolecular Al-Br interactions. Surprisingly, the simple addition product was also isolated from the reaction of 1,4-(Me ₃CCC)₂C₆H₄ with the relatively small hydride HAlEt₂. Solid-state NMR spectra of the product revealed strong intermolecular Al-C interactions involving the negatively charged terminal vinylic carbon atoms, to give one-dimensional coordination polymers.

Full text of DOI:10.1039/b913568e

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PAPER
www.rsc.org/dalton | Dalton Transactions
The influence of halogen substituents on the course of hydrogallation and
hydroalumination reactions†
Werner Uhl,*a Michael Claesener,a Alexander Hepp,a Beate Jasper,a Andrej Vinogradov,a Leo van Wu¨llenb and
Thomas K.-J. Ko¨sterb
Received 8th July 2009, Accepted 17th September 2009
First published as an Advance Article on the web 16th October 2009
DOI: 10.1039/b913568e
Treatment of trimethylsilylethynylbenzene derivatives with HGaCl2 afforded products,
=
[C6H6-x{C(H) C(SiMe3)GaCl2}x], in which by a very fast cis/trans-rearrangement the Ga and H atoms
=
occupied opposite sides of the resulting C C double bonds. The stability of the cis-forms considerably
increased upon application of 1,3-dibromo- and pentafluorophenylalkyne derivatives. Two pairs of
cis/trans-isomers could be characterized by crystal structure determinations and allow the direct
comparison of structural parameters. For the first time an equilibrium was detected between cis- and
trans-forms in solution. Treatment of 1,4-di(tert-butylalkynyl)benzene with HAlR2 (R = CMe3,
CH2CMe3) afforded cyclophane-type molecules by the release of AlR3. Only the neopentyl derivative
could be isolated and characterized by crystal structure determination. In contrast, the dibromo
≡
compound, 1,4-Br2-2,5-(Me3CC C)2C6H2, yielded the simple addition product,
=
C6H2Br2{C(AlR2) C(H)CMe3}2 (R = CMe3). Condensation was hindered in this case by
intramolecular Al-Br interactions. Surprisingly, the simple addition product was also isolated from the
≡
reaction of 1,4-(Me3CC C)2C6H4 with the relatively small hydride HAlEt2. Solid-state NMR spectra of
the product revealed strong intermolecular Al–C interactions involving the negatively charged terminal
vinylic carbon atoms, to give one-dimensional coordination polymers.
trimethylsilyl group were attacked by the metal atoms. cis/trans
Isomerization with respect to the position of the hydrogen and
Introduction
Hydroalumination and hydrogallation reactions with oligoalkynes
represent facile methods for the generation of alkenylaluminium
or -gallium compounds which possess two or more Lewis-acidic
centers in a single molecule.1 Recent investigations of our group
showed that the reaction courses are relatively complicated and
depend strongly on the substituents of the alkynes and the
dialkylelement hydrides. Secondary condensation reactions by
the release of trialkylelement compounds were observed in the
=
aluminium or gallium atoms at the resulting C C double bonds
occurred. The trans-products are formed preferentially in the case
of sterically less shielded hydrides bearing substituents smaller
than tert-butyl groups.8-11 Intermolecular activation seems to be
required, and indeed quantum-chemical calculations have verified
=
a dramatically decreased rotational barrier about the C C bond
upon addition of a Lewis acid.11 However, the energetic difference
between both forms is small. The reversibility of the isomerisation
process was observed for only one case, which may be caused
by a minimization of intramolecular steric interactions.10 Model
considerations reveal that only the trans-forms of oligoalkenyl
derivatives are suitable to act as chelating Lewis acids.9 Hence, a
detailed knowledge of the factors determining the isomerisation
is crucial for the specific generation of oligo-acceptors. The
effectiveness of these acceptors has been impressively shown
by the isolation of carbocationic species in which the hydride
counterion is coordinated by two aluminium atoms.12 We report
here on the influence of halogen substituents attached to the aryl
groups of alkynylbenzenes on the course of hydroalumination and
hydrogallation reactions.
≡
cases of tert-butyl-phenylethynes, C6H6-x(C CCMe3)x (x = 2 or
≡
3) or dialkylelement alkynides, R2E–C C–Alkyl (E = Al, Ga). In
the latter cases carbaalane clusters2 or Ga6C4 heteroadamantane
cages3 were isolated, while cyclophane type molecules with two
and three Al or Ga atoms in the bridging positions resulted for
tert-butylethynes.4,5 Condensation reactions did not take place
with sterically highly shielded dialkylaluminium hydrides [R =
CH(SiMe3)2],6 with terminal alkynes7 and interestingly also with
dialkylgallium hydrides bearing small alkyl groups (R = Et,
nPr),5 for which the substituent exchange may be prevented by
strong intermolecular interactions between gallium and vinylic
carbon atoms. The simple addition products have also been
≡
isolated with trimethylsilylethynes, C6H6-x(C CSiMe3)x. In these
cases the b-carbon atoms of the triple bonds bonded to the
Results and discussion
aInstitut fu¨r Anorganische und Analytische Chemie, Universita¨t Mu¨nster,
D-48149, Mu¨nster, Germany. E-mail: uhlw@uni-muenster.de
Reactions of 1-bromo-2-trimethylsilylethynylbenzene with HGaCl2
bInstitut fu¨r Physikalische Chemie, Universita¨t Mu¨nster, D-48149, Mu¨nster,
Germany
13
Some reactions of HGaCl2 with alkynes have been reported
previously.14,15 They proceeded similarly to those of small
dialkylgallium hydrides and yielded quantitatively the products
† CCDC reference numbers 738510–738519. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b913568e
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of a formal trans addition. The products are highly functionalized
by chlorine atoms attached to gallium. They form dimeric species
by chlorine bridges, are effective Lewis acids by the electron
withdrawing properties of the chlorine atoms and can easily be
converted to the corresponding alkyl derivatives by treatment
with alkyllithium derivatives.14,16 The corresponding reaction with
≡
1-Br-2-(Me3SiC C)C6H4 followed standard conditions, eqn (1).
Stoichiometric quantities of the starting compounds in n-hexane
were heated under reflux for 12 h. Cooling of the concentrated
reaction mixture to +4 ◦C yielded the colourless addition product
(1) in 72% yield. In particular the very characteristic 3JSiH coupling
9-11
=
constant across the C C double bond (8.2 Hz) indicated the
cis arrangement of the hydrogen and silicon atoms and, hence,
the quantitative formation of the trans addition product. NMR
spectra of the reaction mixture and the pure product did not
give any evidence for the occurrence of the cis isomer. Crystal
structure determination (Fig. 1) revealed the formation of a
dimeric compound with a central Ga2Cl2 heterocycle. A terminal
=
chlorine atom and a C C double bond are further attached to
each gallium atom. The molecular symmetry approaches the point
Fig. 1 Molecular structure of 1. The thermal ellipsoids are drawn
at the 40% probability level. Hydrogen atoms with the exception of
the vinylic hydrogen atoms are omitted for clarity. Important bond
lengths (pm) and angles (◦): Ga(1)–C(1) 193.6(5), Ga(2)–C(2) 194.4(4),
Ga(1)–Cl(1) 214.3(2) Ga(1)–Cl(12) 234.6(1), Ga(1)–Cl(21) 236.4(1),
Ga(2)–Cl(2) 214.5(2), Ga(2)–Cl(12) 236.0(1), Ga(2)–Cl(21) 234.3(1),
C(1)–C(11) 131.9(7), C(2)–C(21) 133.1(7); C(1)–C(11)–C(12) 125.4(5),
C(11)–C(1)–Si(1) 121.9(4), C(11)–C(1)–Ga(1) 117.0(4), C(2)–C(21)–C(22)
124.7(4), C(21)–C(2)–Si(2) 122.0(4), C(21)–C(2)–Ga(2) 116.8(3).
group C2 with both terminal chlorine atoms and both alkenyl
groups on the same sides of the inner heterocycle. The ring is
folded across the Ga ◊ ◊ ◊ Ga axis by 26.1 . The C C double bonds
have the expected trans-arrangement of Ga and H atoms. The
◦
=
=
normals of the benzene ring and the plane C CGa(Si) enclose
angles of 55.5 and 54.0◦ which is slightly larger than in◦the
related compound not substituted by a bromine atom (48.0 ).15
The dimeric formula unit is preserved in benzene solution, as
shown by molar mass determination.
(1)
(2)
The bifunctional dialkyne 1,4-dibromo-2,5-bis(trimethyl-
silylethynyl)benzene behaved similarly, eqn (2). However, the
colourless product (2) was insoluble in hydrocarbon solvents,
which probably was caused by the formation of coordination poly-
mers via Ga–Cl–Ga bridges. NMR spectra in THF showed that
2 was directly formed in high purity. The formation of the trans-
product was indicated by the characteristic 3JSiH coupling constant
of 10.4 Hz. Extraction of the amorphous powder with diethyl ether
and cooling of the resulting solution afforded colourless crystals
of the monomeric etherate [2(OEt2)2] in which the gallium atoms
are coordinatively saturated by adduct formation, eqn (2). Crystal
structure determination (Fig. 2) gave a centrosymmetric molecule
=
with a trans arrangement of Ga and H atoms at the C C double
bonds. The structural parameters are similar to those of compound
=
1 with a tilt angle between the plane C CGa(Si) and the benzene
Owing to the insolubility of 2 the addition of further HGaCl2
ring of 56.2◦. These results are in accordance with those obtained
for bromine-free compounds.
molecules to both C C double bonds by formation of a
=
dialkanyl derivative did not succeed. Double hydrogallation was,
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Fig. 2 Molecular structure of 2(OEt2)2. The thermal ellipsoids are drawn
at the 40% probability level. Hydrogen atoms with the exception of the
vinylic hydrogen atoms and ethyl groups are omitted for clarity. Important
bond lengths (pm) and angles (◦): Ga(1)–C(1) 195.8(2), Ga(1)–Cl(1)
218.37(8), Ga(1)–Cl(2) 218.08(7), C(1)–C(2) 133.8(3); C(2)–C(1)–Si(1)
119.1(2), C(2)–C(1)–Ga(1) 122.0(2); symmetry equivalent atoms generated
by -x, -y + 2, -z + 1.
Fig. 3 Molecular structure of 3. The thermal ellipsoids are drawn
at the 40% probability level. Hydrogen atoms are omitted for clarity.
Important bond lengths (pm): Ga(1)–C(1) 196.0(2), Ga(2)–C(1) 196.6(2),
Ga(1)–Cl(11) 213.95(8), Ga(1)–Cl(12) 234.14(7), Ga(1)–Cl(12)¢ 232.66(7),
Ga(2)–Cl(21) 214.35(9), Ga(2)–Cl(22) 217.85(8), Ga(2)–Br(1) 257.34(5),
C(1)–C(2) 157.1(3); symmetry equivalent atoms generated by -x + 2,
-y + 1, -z.
however, achieved with the corresponding monoalkyne 1-Br-2-
≡
(Me3SiC C)C6H4, eqn (3). The product (3) was insoluble in
n-hexane and precipitated from the reaction mixture as a colour-
less amorphous solid. However, its solubility in benzene allowed
the NMR spectroscopic characterization in non-donor solvents.
The CH2 group gave a singlet in the 1H NMR spectrum (d 3.76).
The molecular structure (Fig. 3) comprises a dimeric formula unit
in which both molecular halves are bridged by a planar Ga2Cl2
heterocycle. This structure deviates from that of the corresponding
bromine-free compound [PhCH2C(SiMe3)(GaCl2)2]2 in which all
GaCl2 groups are involved in bridging to give a tricyclic molecular
core.14 The Ga atom of one GaCl2 group in 3 completes its
coordination sphere by an intramolecular interaction with the
bromine atom in ortho-position of the aryl group despite the
expected relatively low donor capability of those halogen atoms.
Similar structural motifs have been detected in only few cases,
most of which involved fluorine atoms attached to phenyl rings.17
The Ga–Br distances [257.34(5) pm] are relatively short. They may
be compared to the Ga–Br bond lengths of Ga–Br–Ga moieties
in dimeric organogallium bromides for which values of about
253 pm have been determined.18 The Ga–Br interaction is further
underlined by the fact that the atom Ga2 is located 35 pm above
the plane generated by the atoms Cl21, Cl22 and C1. Also the sum
of angles at Ga2 deviates from 360◦ (351.6◦).
Reactions of 1,3-dibromo-2-trimethylsilylethynylbenzene and
pentafluoro-trimethylsilylethynylbenzene with HGaCl2
A different reaction course was observed when the dibromo com-
pound 1,3-dibromo-2-trimethylsilylethynylbenzene was treated
with HGaCl2, eqn (4). First, only the cis-addition product
(cis-4) could be detected by 1H NMR spectroscopy. It was
identified by the characteristic coupling constant between H and
Si atoms (3JSiH = 19.3 Hz) which are in trans-positions at the
=
C C double bond. Colourless crystals of cis-4 were obtained by
rapid concentration and cooling of this solution. The resonances
of the trans-isomer (trans-4, silicon and hydrogen atoms in
cis-position, 3JSiH = 9.0 Hz) were observed only after about 20 min.
After about 12 h at room temperature a constant molar ratio
of 1 : 4 (cis-4 : trans-4) resulted independently of the solvent
(n-hexane or benzene). Single crystals of the trans-isomer were
isolated from these mixtures upon concentration and cooling.
An NMR spectrum of a freshly prepared solution of this solid
material gave only the resonances of the trans-isomer, the 1 : 4
ratio of both isomers was detected after about one day at
room temperature. A faster isomerisation took place at 70 ◦C
(6 h). Also at this temperature the final 1 : 4 ratio resulted
which remained constant even after prolonged heating for 7 d.
However, slow decomposition with the formation of unknown
secondary products occurred under these conditions. Molar mass
determination revealed dimeric formula units in benzene solution.
Surprisingly, dimeric species having both configurations in a single
molecule could not be detected by NMR spectroscopy. cis-4 and
trans-4 represent one of the very rare cases8a where the cis and
trans isomers of a hydroalumination or hydrogallation product
could both be characterized by crystal structure determinations
(Fig. 4 and 5). Both compounds form centrosymmetric dimers
with bridging Ga2Cl2 heterocycles but only cis-4 is located on a
crystallographic center of symmetry. In contrast to the molecular
structure of 1 the centrosymmetric forms of cis-4 and trans-4
reflect the standard situation for those dimeric compounds, e.g.
(MeGaCl2)2.19 The reason for the formation of the singular
structural motif of 1 with a cis arrangement of the substituents
(3)
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planes are almost perpendicular [angles between their normals
81.1◦ (cis-4) and 62.5/69.3◦ (trans-4)].
Fig. 4 Molecular structure of cis-4. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms with the exception of the vinylic
hydrogen atoms are omitted for clarity. Important bond lengths (pm)
and angles (◦): Ga(1)–C(1) 193.3(3), C(1)–C(2) 133.4(4); C(1)–C(2)–C(3)
125.9(3), C(2)–C(1)–Si(1) 128.6(2), C(2)–C(1)–Ga(1) 113.6(2); symmetry
equivalent atoms generated by -x + 1, -y + 1, -z.
(4)
≡
The pentafluoro compound F5C6C CSiMe3 behaved similarly
upon treatment with HGaCl2, eqn (4). A mixture of cis/trans
isomers (cis-5 : trans-5) was formed with a molar ratio of 1 : 5
and the preferred formation of the trans addition product. The
different configurations were clearly assigned by the characteristic
Fig. 5 Molecular structure of trans-4. The thermal ellipsoids are drawn
at the 40% probability level. Hydrogen atoms with the exception of
the vinylic hydrogen atoms are omitted for clarity. Important bond
lengths (pm) and angles (◦): Ga(1)–C(1) 193.7(2), C(1)–C(11) 134.1(3),
Ga(2)–C(2) 193.9(3), C(2)–C(21) 134.6(4); C(1)–C(11)–C(12) 129.1(2),
C(11)–C(1)–Si(1) 118.7(2), C(11)–C(1)–Ga(1) 119.0(2), C(2)–C(21)–C(22)
127.3(2), C(21)–C(2)–Si(2) 118.7(2), C(21)–C(2)–Ga(2) 118.6(2).
3
coupling constants JSiH of 19.3 (cis-5) and 9.0 Hz (trans-5).
Compared to the analogous halogen-free compounds (d 7.9) the
1
resonances of the vinylic protons in the H NMR spectra are
shifted to a higher field (d 7.27 and 7.12 of cis-5 and trans-5; d 7.48
and 7.25 of 4). Fractionated crystallization of 5 gave very small
quantities of the pure component cis-5 as an amorphous solid.
Dissolution gave the standard molar ratio of 1 : 5 (cis : trans)
after few hours in benzene at room temperature. A significant
temperature dependence of this ratio could not be determined.
Single crystals of 5 could not be obtained by recrystallization
from different non-donor solvents. Etherates were formed upon
treatment with diethyl ether. Interestingly, the molar ratio of
the cis- and trans-forms was not affected by the formation of
these adducts. A mixture of single crystals was obtained which
showed different crystal shapes and could be separated under
a microscope. The crystals of the cis compound [cis-5(OEt2)]
at the Ga2Cl2 ring is not understood, however, it is not influenced
by any significant intra- or intermolecular interaction. While the
bond lengths of cis-4 and trans-4 are almost indistinguishable, a
considerable difference was detected for the bond angles at the
=
C C double bonds. trans-4 shows a relatively close approach to
◦
◦
=
=
the ideal angles of 120 (average values: C C–Si 118.7 , C C–
Ga 118.8◦, Si–C–Ga 122.5◦), while significant deviations resulted
for cis-4 [128.6(2), 113.6(2) and 117.7(2)◦, respectively]. These
differences make an important contribution to the slight energetic
preference of the trans-products.7b,11 Caused by the steric influence
=
of both bromine atoms, the benzene rings and the C C(Ga)Si
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were finely-faceted with indistinct faces and edges, while trans-
5(OEt2) gave well formed rod-like crystals. Crystal structure
determinations revealed monomeric formula units in which the
Ga atoms are coordinatively saturated by coordination of an
ether molecule (Fig. 6 and 7). Once more the angles including
Reactions of di(tert-butylethynyl)benzenes with dialkylaluminium
hydrides
Trimethylsilylethynylbenzene derivatives described so far gave the
relatively persistent simple addition products upon hydroalumina-
tion and hydrogallation. These reactions are highly regioselective,
and the Al or Ga atoms of the products are attached exclusively to
those carbon atoms of the triple bonds which were bonded also to
the terminal trimethylsilyl substituents (the b-C atoms with respect
to the benzene ring). tert-Butylethynylbenzene derivatives behave
differently.4,5 For instance, 1,3,5-tri(tert-butylethynyl)benzene re-
acted with di(tert-butyl)aluminium hydride to form Al(CMe3)3
and a cyclophane type molecule with three bridging Al–CMe3
groups.4 The Al atoms are bonded to the carbon atoms in the
a-position to the benzene ring. The cis-isomers were isolated in
all cases, rearrangement was not observed at all. Similar reactions
were established for a broad variety of dialkylgallium hydrides.5
The reaction of the dialkyne 1,4-di(tert-butylethynyl)benzene
with di(tert-butyl)aluminium hydride proceeded in a similar way.
=
both carbon◦atoms of the C C double bonds approa◦ch the ideal
=
=
values of 120 for the trans isomer [Ga1–C C 123.8(1) , Si1–C C
117.7(1)◦, Si1–C1–Ga1 118.44(9)◦; cis-5(OEt2): 113.7(1), 127.4(1)
and 118.87(8) , respectively]. The plane C C(Ga)Si is almost
◦
=
perpendicular to the plane of the phenyl ring in both compounds
(77.2 and 62.4◦) indicating negligible mesomeric interactions. In
15
≡
≡
contrast to the alkynes Me3SiC CC6H5 and Me3SiC CC6H4Br,
double hydrogallation did not succeed with the dibromo- and
pentafluorophenylalkynes.
1
Tri(tert-butyl)aluminium was formed and detected by H NMR
spectroscopy, and the resonances characteristic of a condensation
product could be identified in the reaction mixture. However, we
were not able to purify the cage compound by crystallization from
different solvents or by sublimation in vacuum. Therefore we used
dineopentylaluminium hydride which gave a similar condensation
reaction, eqn(5). Acrystalline product (6)was isolated in 40%yield
which showed the resonance of the vinylic protons (d 6.19) in the
range characteristic of these addition products. Crystal structure
determination (Fig. 8) verified the formation of a cyclophane-
type molecule with two Al atoms in the bridging positions, a cis
Fig. 6 Molecular structure of cis-5(OEt2). The thermal ellipsoids are
drawn at the 40% probability level. Hydrogen atoms with the excep-
tion of the vinylic hydrogen atom and the ethyl groups of the ether
ligands are omitted for clarity. Important bond lengths (pm) and an-
gles (◦): Ga(1)–C(1) 195.0(2), Ga(1)–O(1) 198.0(1), C(1)–C(2) 133.6(2);
C(1)–C(2)–C(3) 125.9(1), C(2)–C(1)–Si(1) 127.4(1), C(2)–C(1)–Ga(1)
113.7(1).
=
arrangement of the Al and H atoms at the C C double bonds and
the attack of the metal atoms in a-position to the aromatic ring.
The distance between the coplanar benzene rings is 338 pm.
(5)
In order to get a solid product also with the tert-butyl derivative
we employed the analogous dibromo compound 1,4-dibromo-2,5-
di(tert-butylethynyl)benzene, but surprisingly a different reaction
course occurred, eqn (6). Tri(tert-butyl)aluminium could not be
detected in the reaction mixture by NMR spectroscopy even in
trace quantities. Instead the integration ratio of the solid product
(7) verified the formation of a persistent compound with intact
di(tert-butyl)aluminium groups. The vinylic hydrogen atoms gave
a singlet in the expected range of the 1H NMR spectrum (d 6.08).
Intact dialkylaluminium groups attached to the a-carbon atoms
Fig. 7 Molecular structure of trans-5(OEt2). The thermal ellipsoids
are drawn at the 40% probability level. Hydrogen atoms with the
exception of the vinylic hydrogen atom and the ethyl groups of the
ether ligands are omitted for clarity. Important bond lengths (pm) and
angles (◦): Ga(1)–C(1) 195.1(2), Ga(1)–O(1) 198.5(1), C(1)–C(2) 134.1(2);
C(1)–C(2)–C(3) 128.9(2), C(2)–C(1)–Si(1) 117.7(1), C(2)–C(1)–Ga(1)
123.8(1).
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Fig. 9 Molecular structure of 7. The thermal ellipsoids are drawn
at the 40% probability level. Hydrogen atoms with the exception of
the vinylic hydrogen atoms are omitted for clarity. Important bond
lengths (pm) and angles (◦): Al(1)–C(1) 199.3(2), Al(1)–Br(1) 270.27(8),
C(1)–C(2) 134.2(3), C(12)–Br(1) 191.7(2); C(2)–C(1)–C(11) 125.8(2),
C(2)–C(1)–Al(1) 120.5(2), C(1)–C(2)–C(3) 134.7(2); symmetry equivalent
atoms generated by -x, -y, -z + 1.
Fig. 8 Molecular structure of 6. The thermal ellipsoids are drawn at the
40% probability level. Hydrogen atoms with the exception of the vinylic
hydrogen atoms are omitted for clarity. Important bond lengths (pm) and
angles (◦): Al(1)–C(1) 196.2(3), Al(1)–C(5) 196.3(2), Al(1)–C(9) 197.8(2),
C(1)–C(11) 134.2(3), C(5)–C(51) 133.9(4); C(1)–Al(1)–C(5) 122.4(1),
Al(1)–C(1)–C(11) 114.8(1), Al(1)–C(1)–C(2) 122.2(2), Al(1)–C(5)–C(51)
116.5(2), Al(1)–C(5)–C(6) 119.2(2); symmetry equivalent atoms generated
by -x + 1, -y, -z + 1.
in the 1H NMR spectrum and from crystal structure determination
(Fig. 10). It has two alkenyl groups with the cis arrangement of
Al and H atoms and tetrahedrally coordinated aluminium atoms
in a-position to the aromatic ring. Stable addition products were
also isolated by hydrogallation of this dialkyne with the relatively
small diethyl- and di(n-propyl)gallium hydrides.5 In the solid state
they showed an intermolecular interaction between the Ga atoms
and the a-carbon atoms of the vinyl groups bearing partial
negative charges. Four-membered Ga2C2 heterocycles resulted
which, however, had two long Ga–C distances of about 255 pm
(compared to about 204 pm for the shorter two Ga–C bonds
in the ring) indicating relatively weak intermolecular bonding
interactions. Accordingly the gallium compounds were soluble in
=
of the C C double bonds and the cis arrangement of Al and H
atoms were verified by crystal structure determination (Fig. 9). The
most intriguing observation is a relatively short distance between
the Br atoms at the benzene ring and the Al atoms [270.27(8) pm].
Furthermore, the Al atoms are 19.5 pm above the plane generated
by the atoms C1, CT1 and CT2. The Al–Br distance is in the
upper range of compounds possessing Al–Br–Al bridges18 (see
compound 3 for comparison) and indicates a coordinative inter-
action. Obviously, this interaction is strong enough to block the
secondary reaction by substituent exchange and formation of the
cyclophane molecule which requires coordinatively unsaturated
metal atoms and the intermediate bridging of two Al atoms by
alkyl and alkenyl groups.
(6)
Interestingly, condensation did also not occur upon treatment of
1,4-di(tert-butylethynyl)benzene with HAlEt2. Instead the simple
addition product (8) was formed which precipitated from the
reaction mixture as a colourless solid and proved to be insoluble
in non-donor solvents such as n-hexane, benzene, toluene or fluo-
robenzenes. Its constitution was confirmed by elemental analysis
and mass spectrometry. 8 dissolved readily in the donor solvent
THF to yield an adduct [8(THF)2] with one THF molecule coor-
dinated to each Al atom, eqn (7). The constitution of 8(THF)2 was
derived from the characteristic integration ratio of all resonances
Fig. 10 Molecular structure of 8(THF)2. The thermal ellipsoids are
drawn at the 40% probability level. Hydrogen atoms with the exception
of the vinylic hydrogen atoms are omitted for clarity. Only one of both
independent molecules is shown. Important bond lengths (pm) and angles
(◦); parameters of the second molecule in square brackets: Al(1)–C(11)
199.3(1) [199.0(1)], Al(1)–O(1) 192.8(1) [193.5(1)], C(11)–C(12) 134.5(2)
[134.4(2)]; C(12)–C(11)–Al(1) 119.5(1) [117.9(1)], C(12)–C(11)–C(14)
124.8(1) [125.6(1)], C(11)–C(12)–C(13) 132.7(1) [134.3(1)]; symmetry
equivalent atoms generated by -x + 1, -y, -z + 1.
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hydrocarbons such as toluene or n-pentane. Owing to the larger
electronegativity difference between aluminium and carbon Al–C
bonds are more polar than Ga–C bonds. Hence, the intermolecular
interaction between the Al atoms and the negatively charged
C atoms is expectedly stronger than for the Ga–C couples leading
to stable polymeric aggregates for 8. The schematic drawing of
polymeric 8 in eqn (7) corresponds to the crystal structures of
the gallium compounds. Only donor solvents are able to break
these chains and to stabilize the monomeric formula units by
coordinative saturation of the Al atoms. A very slow reaction of
the dialkyne was observed with dimethylaluminium hydride. After
16 h in boiling n-hexane the majority of the alkyne was recovered.
The remaining resonances in the 1H NMR spectrum could not be
assigned unambiguously.
r = internuclear distance) may then be evaluated from a plot of
normalized difference intensities (S0 - S)/S0 as a function of the
dipolar evolution time t.
1
Fig. 11(a) shows the 13C{ H}-CPMAS NMR spectrum of 8.
The signals may be safely assigned with the help of the solution
NMR data. Specifically, the lines at 5.9 and 140.5 ppm represent
=
the aliphatic Al–CH2CH3 and vinylic Al–C C–CMe3 carbon
atoms. Assuming a formation of an Al2C2 four-membered ring,
the aliphatic carbon atoms are bound to only one aluminium
atom, whereas the bridging vinylic carbon atoms share bonds
with two metal atoms. Contrasted to this, both carbon atoms
are connected to only one aluminium atom if no four-membered
ring is present. Therefore, the dipolar coupling between 13C and
27Al should be significantly higher for the vinylic carbon atoms
than for the aliphatic ones in the first scenario. In Fig. 11(b)
the REAPDOR evolution curves for both 13C NMR signals (5.9
and 140.5 ppm) are compiled and compared to theoretical curves
(7)
The intermolecular interactions in pure compound 8 could
1
be verified by solid-state NMR investigations. The 13C{ H}-CP-
27
{ Al} REAPDOR experiment (CP = cross polarization) allows
a quantitative measurement of heteronuclear dipolar couplings
between carbon and aluminium nuclei.20 For the details of dipolar
NMR techniques, the reader is referred to the literature (e.g.,
refs. 20–22). In short, in REAPDOR (rotational echo adiabatic
passage double resonance) the heteronuclear dipolar couplings
(here between 13C and 27Al), which are averaged out under the
conditions of fast MAS are reintroduced by the action of a
27Al rf pulse of length 1/3 of the rotation period in the middle
of the pulse sequence. The 13C signal is observed employing a
rotor synchronized multipulse spin–echo without (intensity S0)
and with (intensity S) irradiation of the 27Al pulse. The dipolar
coupling constant D = (g Cg Alhm0)/(r34p) (g = gyromagnetic ratio,
1
Fig. 11 (a) 13C{ H}-CPMAS NMR spectrum of 8. The inserts highlight
the spectral regions that include the resonances of carbon atoms bound
to aluminium (5.9 and 140.5 ppm). (b) 13C{ H}-CP-{ Al} REAPDOR
1
27
evolution curves of 8. Dots represent data points for the NMR line at
=
140.5 ppm (Al–C CCMe3), the data points for the CH2CH3 carbon atoms
attached to Al (5.9 ppm) are denoted by diamonds. The curves are results
of simulations of a C–Al2 spin system (dashed line) and C–Al spin system
(dash-dotted line). More details are given in the text.
10556 | Dalton Trans., 2009, 10550–10562
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assuming a C–Al2 three spin system (C ◊ ◊ ◊ Al: 210 pm; Al–C–Al
angle: 78.1◦ (dashed line)) and to a C–Al two spin system assuming
an interatomic distance of 200 pm (dash-dotted line). These spin
geometries were taken from model compounds containing regular
Al2C2 four-membered rings with equivalent Al–C distances.7b
Fig. 11(b) demonstrates that the data points for the vinylic
C-atoms are in reasonably good agreement with the results of the
C–Al2 simulation. The solid-state NMRexperiment thus gives very
strong evidence for the formation of a linear polymer of compound
8. It should be noted that the other resonances present in the
CPMAS spectrum did not exhibit any significant REAPDOR
effect, which proves that the aforementioned positions are the only
carbon atoms with a significant dipolar coupling to aluminium.
their characteristic NMR data. They formed exclusively at the
beginning of the reactions. The rate of the rearrangement may
decrease because the transition state (Scheme 1) has a positively
charged carbon atom attached to the aromatic ring bearing
electron withdrawing halogen atoms. Furthermore, the partial
negative charge at the a-carbon atoms may be reduced which
is important for the intermolecular activation of the isomerisation
process. The same effect may prevent the double hydrogallation in
the case of the dibromo- and pentafluoro derivatives.
The rearrangement was incomplete for compounds 4 and
5, and even after long reaction times, prolonged heating or
the addition of ether (5) the cis-isomers could be detected in
considerable quantities. Hence, for the first time equilibrium
mixtures of the cis- and trans-addition products resulted. In
previous investigations7b,11 the energetic difference between both
isomers was calculated to be low, with a slight thermodynamic
preference of the trans-form by only 1–8 kcal mol-1 depending on
the respective element and substituent. This isomer is favoured by
an approximation of the bond angles of the ethylene group to the
ideal value of 120◦, an interaction of a C–H bond of the benzene
ring with coordinatively unsaturated Al or Ga atoms, and a slightly
better mesomeric interaction between the benzene rings and the
alkenyl groups. Despite the low energy difference the cis- or trans-
isomers were formed with very high selectivity in all so far reported
reactions.8-11 Only one example for the reversible formation of a
cis-isomer starting with the trans-compound has been observed.
This was probably caused by specific steric interactions.10 However,
also in that case only a single product could be detected. In the di-
or pentahalogenated species reported here a balanced situation
must exist in which both forms are energetically quite similar.
However, the structural parameters do not allow a conclusive
interpretation. The only systematic change has been observed for
the angles between the normals of the benzene planes and the
Discussion
As reported previously,15 reactions of HGaCl2 with PhC CSiMe3
≡
≡
and H4C6(C CSiMe3)2 gave selectively the trans-addition prod-
ucts. The cis-isomers, which represent the expected initiating
step upon the reasonable assumption of a concerted reaction
mechanism, could not be detected by NMR spectroscopy as
intermediates. However, these isomers could be isolated in rare
cases in which steric shielding prevented the rearrangement
process.8–11 This observation underlines the importance of an
intermolecular activation of the cis/trans-isomerisation. Indeed,
quantum-chemical calculations have revealed a dramatic decrease
=
of the activation barrier for the rotation about the C C double
bonds of the addition products when a Lewis-acid was allowed
to coordinate to the negatively charged carbon atom of the
ethylene group attached to aluminium or gallium.7b,11 cis/trans-
Isomerisation was postulated to proceed via four-membered
heterocycles (Scheme 1) which have a relatively high charge
separation in their ethylene groups and allow the isomerisation
by a dipolar mechanism.7,14,23 Such dimeric formula units have
=
planes C C(Ga)Si which steadily increase and approach values
of 90◦ in the cis-isomers.
7
≡
been isolated with terminal alkynes, e.g. HC CCMe3. The fast
Different reaction courses resulted also for hydroalumina-
tion and hydrogallation reactions with tert-butylethynylbenzene
derivatives. However, rearrangement to yield the trans-isomers was
not observed at all. Very bulky substituents such as CH(SiMe3)2
gave the simple addition products with intact dialkylelement
and quantitative formation of the trans-addition products was
also observed for the reaction of HGaCl2 with bromophenyl
derivatives which have one bromine atom in ortho-position to
≡
≡
the C C triple bonds, 1-Br-2-(Me3SiC C)C6H4 and 1,4-Br2-2,5-
≡
(Me3SiC C)2C6H2 (eqn (1) and (2)). In contrast, a relatively
=
≡
groups attached to those carbon atoms of the C C double bonds
slow rearrangement occurred when 1,3-Br2-2-Me3SiC CC6H3
or the pentafluoro derivative F5C6C CSiMe3 was treated with
HGaCl2 (eqn (4)). In both cases cis-isomers were detected by
which are in a-position to the benzene ring.6 Any secondary
reaction is clearly prevented by steric shielding. In contrast, two
(eqn (5)) or three alkenyl groups bonded to the central benzene ring
and medium sized alkyl groups attached to aluminium or gallium
such as tert-butyl or neopentyl give condensation by the release
of trialkylelement compounds and formation of cyclophane-type
molecules.4,5 These reactions obviously proceed by the approach
of two molecules to allow substituent exchange via bridging
alkyl and alkenyl groups. The relatively weak interaction between
aluminium and Br atoms attached to the central benzene ring
prevent this secondary reaction by an intramolecular coordinative
saturation of the metal atoms (7, eqn (6)). Interestingly conden-
sation also did not occur when 1,4-di(tert-butylethynyl)benzene
was treated with the small hydrides Et2GaH, nPr2GaH or Et2AlH
(eqn (7)) for which owing to the low steric shielding an exchange
process seems to be most favourable. Crystal structure deter-
minations of the gallium compounds5b,c and solid state NMR
spectra of the aluminium compound 8 (Fig. 11) have shown that
≡
Scheme 1
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-1
≡
persistent one-dimensional coordination polymers are formed by
interactions of the metal atoms exclusively with the negatively
charged ethenyl carbon atoms. Caused by the lower Lewis-acidity
of gallium they are less stable for the hydrogallation products
which readily dissolve in non-coordinating solvents. The larger
charge separation in the aluminium compounds and, hence, the
better acceptor capability of aluminium atoms creates relatively
persistent polymeric chains which dissociate only upon addition
of donor solvents. The preferred interaction of the metal atoms in
these compounds with both negatively charged ethenyl carbon
atoms prevents substituent exchange in these cases. Enhanced
steric shielding makes this interaction relatively unfavourable and
alkyl groups can occupy bridging positions in order to initiate
substituent exchange. These results give nice insight into the
different reaction courses of hydroalumination reactions and their
dependence on the respective substituents.
plates, cm ): 2243 m, 2208 m nC C, 1461 vs (phenyl), 1454 vs,
1375 s (paraffin), 1362 s, 1261 m dCH3, 1242 m, 1200 s, 1150 m,
1113 vw, 1061 s, 1030 m, 1015 m, 920 s, 883 s, 814 vw, 775 s dCH,
nCC, 721 m (paraffin), 635 vw, 581 s, 542 s, 500 s, 447 s. MS (EI,
70 eV): m/z (%) = 394 (46), 396 (100), 398 (52) [M+], 379 (6), 381
(13), 383 (6) [M+ - Me].
≡
Synthesis of 1,3-Br2-2-Me3SiC CC6H3
1-Iodo-2,6-dibromobenzene (5.17 g, 14.3 mmol) was dissolved in
250 mL of triethylamine. CuI (150 mg), PdCl2(PPh3)2 (200 mg) and
2.00 mL of trimethylsilylethyne (1.42 g, 14.5 mmol) were added.
The mixture was stirred at 70 ◦C for 20 h. After cooling to room
temperature the ammonium salt was separated by filtration. The
solid residue was washed with a small quantity of n-pentane. All
volatile components of the filtrate were removed in vacuum. The
remaining oily product was treated with n-pentane and purified
by column chromatography (Al2O3). The solvent was distilled
off under reduced pressure at room temperature. The remaining
viscous liquid was distilled in vacuum (83 ◦C, 8 ¥ 10-2 mbar). Yield:
3.66 g (77%).
Experimental
All procedures were carried out under purified argon.
n-Hexane, n-pentane, and cyclopentane were dried over
LiAlH4, diethyl ether and THF over Na/benzophenone. The
starting compounds Et2AlH,24 (tBuCH2)2AlH,25 tBu2AlH,26
Found: C, 39.7; H, 3.4; C11H12SiBr2 requires C, 39.8, H, 3.6%;
1H NMR (CDCl3, 400 MHz): d 7.52 (d, 3JHH = 8.0 Hz, 2 H, 4,6-H
of phenyl), 6.97 (t, 3JHH = 8.0 Hz, 1 H, 5-H of phenyl, 0.31 (s, 9 H,
SiMe3); 13C NMR (CDCl3, 100 MHz): d 131.2 (4,6-C of phenyl),
27
5b,6,28
Cl2GaH,13 Me3SiC CC6F5, 1,4-(Me3CC C)2C6H4
and
≡
≡
29
1-I-2,6-Br2C6H3 were obtained according to literature proce-
≡
dures. Commercially available 1-Br-2-(Me3SiC C)C6H4 (Sigma-
≡
129.7 (5-C of phenyl), 127.0 (CC C of phenyl), 126.5 (C–Br),
Aldich) was dried over molecular sieves. The assignment of the
NMR spectra in solution is based on HSQC, HMBC, ROESY and
DEPT135 data. Solid state NMR experiments were performed
on a BRUKER DSX 400 with resonance frequencies of 100.6,
104.3 and 400.1 MHz for 13C, 27Al and 1H. Since the combination
13C/27Al is not accessible with commercial probe hardware due to
the close resonance frequencies, a homebuilt setup was employed
which makes use of the principle of overcoupled resonators.21,30
105.3 (CC C), 101.7 (CC C), -0.2 (SiMe3); 29Si NMR (CDCl3,
≡
≡
79.5 MHz): d -16.3; IR (paraffin, CsBr plates, cm-1): 2164 vs nC≡
,
C
1930 w, 1854 w, 1541 vs (phenyl), 1427 vs, 1385 vs (paraffin), 1304
w, 1250 vs dCH3, 1225 vs, 1188 vs, 1148 s, 1112 vw, 1095 vw, 1065 s,
1049 w, 999 vw, 966 vw, 955 vw dCH, nCC, 852 vs, 765 vs, 729 vs
rCH3(Si), 717 vs (paraffin), 702 vs, 656 vs nSiC, 567 w, 547 vw,
507 vw, 488 vw; MS (EI, 70 eV): m/z (%) = 330 (44), 332 (91), 334
(46) [M+], 315 (52), 317 (100), 319 (57) [M+ - Me].
1
27
The 13C{ H}-CP-{ Al}-REAPDOR experiment was performed
with magic angle spinning at 10 kHz and employing rf field
strengths of 13C and 27Al of 41.6 and 46.3 kHz, respectively.
The REAPDOR data was then analysed utilizing the SIMP-
SON simulation package,31 using a value of 18.3 MHz for the
quadrupolar coupling constant CQ(27Al), as obtained from a
=
Synthesis of [1-Br-2-{Me3Si(Cl2Ga)C C(H)}C6H4]2 1
≡
The alkyne 1-Br-2-Me3SiC CC6H4 (1.90 mL, 2.26 g, 8.93 mmol)
was added to a solution of HGaCl2 (1.27 g, 8.93 mmol) in 100 mL
of n-hexane at room temperature. The mixture was stirred at
70 ◦C for 12 h. Small quantities of a colourless solid of unknown
composition were filtered off. The filtrate was concentrated and
cooled to +4 ◦C to obtain colourless crystals of compound 1. Yield:
2.54 g (72%), decomp. (argon, sealed capillary): above 105 ◦C.
Found: C, 33.2; H, 3.6; C22H28Si2Cl4Ga2Br2 requires C, 33.5,
1
27
13C{ H}-CP-{ Al} TRAPDOR experiment with variable 27Al
offset frequency.22
≡
Synthesis of 1,4-Br2-2,5-(Me3CC C)2C6H2
1,2,4,5-Tetrabromobenzene (10.2 g, 25.9 mmol), CuI (25 mg) and
PdCl2(PPh3)2 (200 mg) were treated with 300 mL of triethylamine.
3,3-Dimethylbut-1-yne (7.3 mL, 4.89 g, 59.6 mmol) was added
1
3
H, 3.6%; H NMR (C6D6, 400 MHz): d 7.82 (s, JHSi = 8.2 Hz,
=
1 H, Ga–C CH), 7.24 and 7.22 (m, 2 H, 3,6-H of phenyl), 6.87
◦
dropwise. The mixture was stirred at 75 C for 7 h and filtered.
(dt, 3JHH = 7.6 Hz, 4JHH = 0.8 Hz, 1 H, 4-H of phenyl), 6.65 (dt,
4
The solid residue was washed with n-pentane. All volatiles of the
filtrate were removed in vacuum. The product was purified by
column chromatography (Al2O3) with n-pentane as the solvent.
The product remained as a colourless solid after evaporation.
Yield: 3.50 g (34%); mp: 150 ◦C.
Found: C, 54.6; H, 5.2; C18H20Br2 requires C, 54.6, H, 5.1%; 1H
NMR (C6D6, 400 MHz): d 7.49 (s, 2 H, H of phenyl), 1.21 (s, 18
H, CMe3); 13C NMR (C6D6, 100 MHz): d 136.2 (C–H of phenyl),
3JHH = 8.0 Hz, JHH = 1.6 Hz, 1 H, 5-H of phenyl), 0.23 (s, 9
13
=
H, SiMe3); C NMR (C6D6, 100 MHz): d 156.8 [C C(Si)Ga],
=
150.5 [C C(Si)Ga], 141.9 (2-C of phenyl), 133.0 (6-C of phenyl,
130.6 (5-C of phenyl), 129.1 (3-C of phenyl), 128.6 (4-C of phenyl),
122.8 (CBr), -0.2 (SiMe3); 29Si NMR (C6D6, 79.5 MHz): d -0.9;
IR (paraffin, CsBr plates, cm-1): 1377 s (paraffin), 1311 w, 1277 w,
1248 s dCH3, 1207 w, 1163 vw, 1120 m, 1043 w, 1026 m, 984 w,
953 m dCH, nCC, 916 m, 883 m, 837 vs, 754 vs rCH3(Si), 717 s
(paraffin), 690 s, 664 m, 623 m nCSi, 592 m, 569 vw, 536 vw, 505 m,
416 vs nGaC, nGaCl.
≡
≡
124.2 and 126.9 (C–Br and CC C of phenyl), 106.0 (C CCMe3),
≡
77.7 (C CCMe3), 30.7 (CH3), 28.5 (CMe3); IR (paraffin, CsI
10558 | Dalton Trans., 2009, 10550–10562
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=
=
Synthesis of 1,4-Br2-2,5-{Me3Si(Cl2Ga)C C(H)}2C6H2 2 and its
Synthesis of [1,3-Br2-2-{Me3Si(Cl2Ga)C C(H)}C6H3]2 cis-4 and
ether adduct 2(OEt2)2
trans-4
1,4-Dibromo-2,5-bis(trimethylsilylethynyl)benzene (0.734 g,
1.71 mmol) was dissolved in 90 mL of n-hexane and added to
solid HGaCl2 (0.484 g, 3.41 mmol). The mixture was heated
under reflux for 14 h. A colourless solid of 2 precipitated. It
was filtered off and washed with 10 mL of n-hexane. NMR
spectra in [D8]THF showed that 2 was directly formed in a high
purity. Extraction of◦the solid with diethyl ether, filtration and
crystallization at +4 C afforded the etherate 2(Et2O)2, which is
soluble in hydrocarbons. Yield of 2: 0.985 g (81%), dec (argon,
sealed capillary): above 200 ◦C.
1,3-Dibromo-2-trimethylsilylethynylbenzene (1.315 g, 3.97 mmol)
was dissolved in 80 mL of n-hexane and added to solid HGaCl2
(0.562 g, 3.97 mmol). The mixture was heated under reflux for
14 h. Small quantities of a colourless solid precipitated which were
filtered off. All volatile components of the filtrate were removed
in vacuum. The residue was dissolved in toluene. Cooling of the
solution to +4 ◦C afforded colourless crystals of compound 4.
Solutions of 4 show a constant molar ratio of the cis- and trans-
isomers of 1 : 4. Yield: 1.184 g (63%).
Found: C, 27.7; H, 2.7; C22H26Si2Cl4Ga2Br4 requires C, 27.9,
H, 2.8%; MS (EI, 70 eV): m/z (%) = 472 (12), 474 (17), 476
(10) [1/2 M+ (based on the dimer)]; 457 (79), 459 (100), 461 (63)
[1/2 M+ - Me].
2. Found: C, 26.8; H, 3.0; C16H22Si2Cl4Ga2Br2 requires C, 27.0,
H, 3.1%; 1H NMR ([D8]THF, 400 MHz): d 7.83 (s, 2 H, phenyl),
7.72 (s, 3JSiH = 10.4 Hz, 2 H, Ga–C CH), 0.27 (s, 18 H, SiMe3);
=
cis-4. 1H NMR (C6D6, 400 MHz): d 7.48 (s, 3JSiH = 19.3 Hz,
13
=
=
C NMR ([D8]THF, 100 MHz): d 153.4 (C C–Ga), 153.1 (C C–
3
=
1 H, Ga–C CH), 7.04 (t, JHH = 8.0 Hz, 2 H, 4-H and 6-H of
=
Ga), 143.1 (C CC), 133.9 (H–C of phenyl), 122.5 (CBr), -0.2
3
(SiMe3); 29Si NMR ([D8]THF, 79.5 MHz): d 0.4; IR (paraffin,
CsBr plates, cm-1): 1564 m (phenyl, nC=C), 1460 vs (paraffin), 1406
vw dCH, 1377 s (paraffin), 1313 w, 1252 s dCH3, 1186 vw, 1155
vw, 1055 s nCC, dCH, 922 m, 889 s, 868 m, 835 vs, 754 s rCH3(Si),
723 w (paraffin), 692 w, 629 w nSiC, 530 w, 463 w, 419 m nGaC,
nGaCl.
phenyl), 6.27 (dt, JHH = 8.0 Hz and J = 0.8 Hz, 1 H, 5-H of
phenyl), 0.03 (s, 9 H, SiMe3); 13C NMR (C6D6, 100 MHz): d 156.2
=
=
=
(Ga–C C), 150.5 (Ga–C C), 141.6 (CC C), 131.4 (4-C and 6-C
of phenyl), 130.0 (5-C of phenyl), 123.1 (CBr), -0.1 (SiMe3); 29Si
NMR (C6D6, 79.5 MHz): d -4.4.
trans-4. 1H NMR (C6D6, 400 MHz): d 7.25 (s, 3JSiH = 9.0 Hz,
3
2(Et2O)2. 1H NMR (C6D6, 400 MHz): d 8.11 (s, 2 H, phenyl),
=
1 H, C CH), 7.09 (t, JHH = 8.0 Hz, 2 H, 4-H and 6-H of phenyl),
6.30 (dt, 3JHH = 8.0 Hz and J = 0.8 Hz, 1 H, 5-H of phenyl), 0.3
3
7.84 (s, 3JSiH = 10.3 Hz, 2 H, Ga-C CH), 3.48 (q, JHH = 7.2 Hz,
=
(s, 9 H, SiMe3); 13C NMR (C6D6, 100 MHz): d 154.7 (Ga–C C),
3
=
8 H, OCH2), 0.76 (t, JHH = 7.2 Hz, 12 H, OCH2CH3), 0.41 (s,
13
=
=
151.8 (Ga–C C), 139.4 (CC C), 132.2 (4-C and 6-C of phenyl),
130.5 (5-C of phenyl), 123.5 (CBr), -0.3 (SiMe3). 29Si NMR (C6D6,
79.5 MHz): d 0.2.
=
18 H, SiMe3); C NMR (C6D6, 100 MHz): d 152.9 (C C–Ga),
=
=
152.1 (C C–Ga), 142.8 (C CC), 133.4 (H-C of phenyl), 122.6
(CBr), 68.1 (OCH2), 13.7 (OCH2CH3), -0.2 (SiMe3); 29Si NMR
(C6D6, 79.5 MHz): d 1.2; IR (paraffin, CsBr plates, cm-1): 1375 vs
(paraffin), 1248 m dCH3, 1190 w, 1150 w, 1190 vw, 1055 w, 1041
w, 1020 w, 928 w nCC, dCH, 841 m rCH3(Si), 719 m (paraffin),
617 w, 513 w nGaC, nGaCl.
=
Synthesis of [Me3Si(Cl2Ga)C C(H)C6F5]2 (cis-5, trans-5) and
their etherates [cis-5(OEt2), trans-5(OEt2)]
Trimethylsilylethynyl-pentafluorobenzene (2.62 g, 9.91 mmol) was
dissolved in 80 mL of n-hexane and added to solid HGaCl2 (1.40 g,
9.91 mmol). The mixture was heated under reflux for 16 h and
filtered. The solvent of the filtrate was removed in vacuum. The
Synthesis of [1-Br-2-{Me3Si(Cl2Ga)2CCH2}C6H4]2 3
≡
1-Br-2-Me3SiC CC6H4 (0.67 mL, 0.795 g, 3.14 mmol) was
◦
residue was dissolved in 1,2-difluorobenzene. Cooling to -45 C
dissolved in 80 mL of n-hexane and added to HGaCl2 (0.889 g,
6.28 mmol). A colourless solid of 3 precipitated after a few minutes
at room temperature. The mixture was heated under reflux for 15 h.
The precipitate was filtered off, washed with 20 mL of n-hexane
and evacuated. Yield: 1.399 g (83%), mp (argon, sealed capillary):
208 ◦C.
yielded a colourless solid of 5 which upon dissolution in benzene
showed resonances in the 1H NMR spectrum characteristic of the
respective cis and trans isomers in a molar ratio of 1 : 5. Dissolution
in diethyl ether and cooling to -45 ◦C afforded colourless crystals
of the etherates cis-5(OEt2) and trans-5(Et2O). Yield of 5(OEt2)2:
2.76 g (58%).
Found: C, 24.7; H, 2.9; C22H30Si2Cl8Ga4Br2 requires C, 24.6, H,
2.8%; 1H NMR ([D8]THF, 400 MHz): d 7.91 (dd, 3JHH = 8.0 Hz,
1
=
cis-5. H NMR (C6D6, 400 MHz): d 7.27 (s, 1 H, C CH),
3
4JHH = 1.6 Hz, 1 H, 3-H of phenyl), 7.50 (dd, JHH = 8.0 Hz,
-0.05 (s, 9 H, SiMe3); 13C NMR (C6D6, 100 MHz): d 157.4 (Ga–
3
4JHH = 1.2 Hz, 1 H, 6-H of phenyl), 7.25 (dt, JHH = 7.6 Hz,
=
=
C
C), 143.7 (ortho-CF), 141.2 (para-CF), 140.6 (Ga–C C), 137.9
3
4JHH = 1.2 Hz, 1 H, 4-H of phenyl), 7.05 (dt, JHH = 7.2 Hz,
(meta-CF), 114.2 (ipso-C), -0.5 (SiMe3); 29Si NMR (C6D6, 79.5
MHz): d -4.1; 19F NMR (C6D6, 376.5 MHz): d -140 (ortho-CF),
-154 (para-CF), -162 (meta-CF).
4JHH = 1.6 Hz, 1 H, 5-H of phenyl), 3.76 (s, 2 H, CH2), 0.18 (s,
9 H, SiMe3); 13C NMR ([D8]THF, 100 MHz): d 142.9 (2-C of
phenyl), 133.6 (6-C of phenyl), 132.3 (3-C of phenyl), 128.9 (5-C
of phenyl), 128.1 (4 C of phenyl), 126.1 (CBr), 35.7 (CH2), 28.8
(GaCGa), 2.7 (SiMe3); 29Si NMR ([D8]THF, 79.5 MHz): d 2.7; IR
(paraffin, CsBr plates, cm-1): 1311 vw, 1286 w, 1259 s dCH3, 1202
w, 1120 w, 1013 s, 972 s, 949 s, 937 m nCC, dCH, 871 m, 839 s, 772
m, 754 s, 727 s rCH3(Si), 717 s (paraffin), 688 w, 667 s, 648 m, 623
w nSiC, 592 vw, 569 s, 536 vs, 488 vw, 467 s, 438 vs nGaC, nGaCl.
1
=
trans-5. H NMR (C6D6, 400 MHz): d 7.12 (s, 1 H, C CH),
0.17 (s, 9 H, SiMe3); 13C NMR (C6D6, 100 MHz): d 157.3 (Ga–
=
=
C
C), 144.4 (ortho-CF), 141.5 (para-CF), 139.4 (Ga–C C), 138.0
(meta-CF), 113.7 (ipso-C), -0.6 (SiMe3); 29Si NMR (C6D6, 79.5
MHz): d 1.0; 19F NMR (C6D6, 376.5 MHz): d -142 (ortho-CF),
-153 (para-CF), -162 (meta-CF).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 10550–10562 | 10559
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View Article Online
=
=
5(OEt2)2. Found: C, 37.5; H, 4.0; C15H20OF5SiCl2Ga requires
C, 37.6, H, 4.2%.
(C8), (C–Br), 37.4 (C CCMe3), 31.6 (C CCMe3), 30.6 (Me of
Al–CMe3), 18.4 (Al–CMe3); IR (CsI plates, paraffin, cm-1): 2095
=
vw, 1978 vw, 1932 vw, 1744 w, 1579 s nC C, phenyl, 1460 vs, 1361 s
cis-5(OEt2). 1H NMR (C6D6, 400 MHz): d 7.44 (s, 1 H,
C CH), 3.50 (q, 4 H, OCH2), 0.77 (t, 6 H, OCH2CH3), -0.03 (s, 9
(paraffin), 1225 m, 1196 s, 1081 w, 1052 s, 1031 s, 934 m, 888 s,
811 s, 741 w, 738 s nCC, dCH, 690 m, 638 s, 587 s, 556 m, 471 s,
437 w dCC, nAlC; MS (EI, 70 eV): m/z (%) = 621 (19), 623 (38),
=
H, SiMe3); 13C NMR (C6D6, 100 MHz): d 159.1 (Ga–C C), 143.7
(ortho-CF), 140.8 (para-CF), 139.8 (Ga–C C), 137.7 (meta-CF),
=
=
625 (22) [M+ - CMe3], 481 (49), 483 (100), 485 (52) [M+ - AltBu2 -
butene], 398 (50), 400 (97), 402 (51) [M - tBu2Al–C C(H)CMe3 -
115.9 (ipso-C), 67.6 (OCH2), 13.3 (OCH2CH3), -0.2 (SiMe3); 29Si
NMR (C6D6, 79.5 MHz): d -5.5; 19F NMR (C6D6, 376.5 MHz): d
-141 (ortho-CF), -156 (para-CF), -163 (meta-CF).
+
=
tBu].
trans-5(OEt2). 1H NMR (C6D6, 400 MHz): d 7.19 (s, 1 H,
C CH), 3.50 (q, 4 H, OCH2), 0.77 (t, 6 H, OCH2CH3), 0.27 (s, 9
=
Synthesis of [Me3CC(H) C(AlEt2)]2C6H4 8
=
H, SiMe3); 13C NMR (C6D6, 100 MHz): d 159.7 (Ga–C C), 144.4
(ortho-CF), 141.1 (para-CF), 139.4 (Ga–C C), 138.0 (meta-CF),
=
1,4-Bis(3,3-dimethylbutyn-1-yl)benzene (0.400 g, 1.68 mmol) was
dissolved in 40 mL of n-hexane. Diethylaluminium hydride
(0.36 mL, 0.289 g, 3.36 mmol) was added at room temperature.
The mixture was heated under reflux for 19 h. A colourless solid
started to precipitate after about 3 h. The solid was filtered off
and thoroughly evacuated. The amorphous powder is insoluble
in non-donor solvents. 1H NMR spectra in [D8]THF showed that
the pure compound 8 was formed. Yield: 0.468 g (68%), decomp.
(argon, sealed capillary): 189 ◦C.
=
115.8 (ipso-C), 67.6 (OCH2), 13.3 (OCH2CH3), -0.4 (SiMe3); 29Si
NMR (C6D6, 79.5 MHz): d 0.5; 19F NMR (C6D6, 376.5 MHz):
d -142 (ortho-CF), -156 (para-CF), -163 (meta-CF); MS (EI, 70
eV): m/z (%) = 404 (8), 406 (10), 408 (4) [M+ - Et2O]; 389 (35),
391 (45), 393 (18) [M+ - Et2O - Me]; 369 (15), 370 (7), 371 (14)
[M+ - Et2O - Cl].
Synthesis of the cyclophane 6
Found: C, 75.9; H, 10.6; C26H44Al2 requires C, 76.2, H, 10.8;
13
1
=
C{ H}-CPMAS NMR: d 143.8 (ipso-C), 141.5 (Al–C CCMe3),
1,4-Bis(3,3-dimethylbut-1-ynyl)benzene (0.190 g, 0.80 mmol) were
dissolved in 20 mL of n-pentane and treated with dineopentyl-
aluminium hydride (0.272 g, 1.60 mmol) dissolved in the same
solvent. The solution was stirred at room temperature for 16 h. The
solvent was removed in vacuum. The oily residue was thoroughly
evacuated to 10-3 Torr at 60 ◦C to remove trineopentylaluminium.
The remaining substance was dissolved in n-pentane and cooled to
-45 ◦C to obtain colourless crystals of compound 6. Yield: 0.108 g
(40%); mp (argon, sealed capillary): 139 ◦C.
Found: C, 81.2; H, 10.3; C46H70Al2 requires C, 81.6, H, 10.4%;
1H NMR (C6D6, 400 MHz): d 6.51 (s, 8 H, H of phenyl), 6.19 (s,
4 H, vinylic H), 1.31 (s, 18 H, CMe3 of neopentyl), 1.05 (s, 36 H,
vinylic CMe3), 0.72 (s, 4 H, Al–CH2); 13C NMR (C6D6, 100 MHz):
=
140.5 (Al–C CCMe3; CAl2 in the polymer), 131.8, 127.6, 124.3
=
(C–H of phenyl), 40.7 (C CCMe3), 31.5, 30.0 (CMe3), 11.5, 9.7,
1
(CH3 of ethyl), 5.9 (Al–CH2); H NMR ([D8]THF, 400 MHz): d
6.68 (s, 4 H, phenyl), 5.70 (s, 2 H, vinylic H), 0.99 (t, 3JHH = 8.13 Hz,
12 H, CH3 of ethyl), 0.90 (s, 18 H, CMe3), -0,17 (q, 3JHH = 8,13 Hz,
8 H, CH2 of ethyl); 13C NMR ([D8]THF, 100 MHz): d 155.0 (Al–
=
=
C
CCMe3), 148.6 (Al–C CCMe3), 144.9 (ipso-C), 126.0 (C–H
=
of phenyl), 37.5 (C CCMe3), 32.4 (CMe3), 10.0 (CH3 of ethyl),
-0.4 (CH2 of ethyl); IR (paraffin, CsI plates, cm-1): 1553 m, 1537 m
=
nC C, phenyl, 1456 vs (paraffin), 1408 m dCH3, 1375 s (paraffin),
1361 m, 1256 w dCH3, 1196 m, 1174 m, 1113 m, 1036 m, 1016 s,
965 w, 931 w, 914 w, 891 m, 872 m, 797 s nCC, dCH, 708 s, 629 s,
596 vs, 577 s, 556 s, 530 vs, 521 vs, 438 vs dCC, nAlC; MS (EI, 70
=
=
d 157.9 (Al–C CCMe3), 152.6 (Al–C CCMe3), 142.0 (ipso-C),
126.6 (C–H of phenyl), 37.7 (C CCMe3), 35.9 (Me of neopentyl),
+
+
=
eV): m/z (%) = 410 (3) [M ], 243 (24) [M - Et2Al–C C(H)CMe3].
=
=
31.9 (CMe3 of neopentyl), 31.6 (C CCMe3), 24.9 (Al–CH2); IR
(paraffin, CsI plates, cm-1) = 1454 vs, 1375 s (paraffin), 1362 m,
1269 s dCH3, 1227 w, 1200 w, 1161 w, 1096 s, 1020 vs, 918 m, 872
w, 800 vs, 737 m nCC, dCH, 675 w, 669 w, 627 m, 588 m, 544 m,
518 m, 459 s, 418 s nAlC, dCC.
=
Synthesis of [Me3CC(H) C{AlEt2(THF)}]2C6H4 8(THF)2
Compound 8 (0.145 g, 0.354 mmol) was treated with 5 mL
of THF. The mixture was stirred at room temperature for few
minutes and filtered. The solvent of the filtrate was removed in
vacuum. The solid residue was dissolved in n-pentane and cooled
to -30 ◦C. Colourless crystals formed after several days. Yield:
0.148 g (76%); mp (argon, sealed capillary): 57 ◦C.
=
Synthesis of 1,4-Br2-2,5-[Me3CC(H) C(AlCMe3)2]2C6H2 7
1,4-Dibromo-2,5-bis(3,3-dimethylbutyn-1-yl)benzene (0.364 g,
0.92 mmol) were dissolved in 20 mL of n-hexane, cooled to
0
Found: C, 73.5; H, 11.0; C34H60O2Al2 requires C, 73.6, H, 10.9%;
1H NMR (C6D6, 400 MHz): d 6.96 (s, 4 H, H of phenyl), 6.12 (s, 2
H, vinylic H), 3.46 (m, 8 H, OCH2 of THF), 1.46 (t, 3JHH = 8.13 Hz,
12 H, CH3 of ethyl), 1.18 (s, 18 H, CMe3), 1.10 (m, 8 H, THF), 0.28
◦C and treated with di(tert-butyl)aluminium hydride (0.263 g,
1.85 mmol) dissolved in 20 mL of n-hexane. The solution was
allowed to warm to room temperature and stirred for 20 h.
Filtration, concentration of the filtrate and cooling to -45 ◦C
afforded a colourless solid of compound 7 which was recrystallized
from n-hexane (20/-15 ◦C). Yield: 0.438 g (70%); mp (argon,
sealed capillary): 145 ◦C.
3
(q, JHH = 8.13 Hz, 8 H, Al–CH2); 13C NMR (C6D6, 100 MHz):
=
=
d 154.5 (Al–C CCMe3), 148.7 (Al–C CCMe3), 144,6 (ipso-C),
125.7 (C-H of phenyl), 71.0 (OCH2 of THF), 37.4 (C CCMe3),
=
32.3 (CMe3), 25.1 (OCH2CH2), 10.2 (CH3 of ethyl), -0.4 (Al–
Found: C, 58.7; H, 8.7; C34H58Br2Al2 requires C, 60.0, H, 8.6%;
1H NMR (C6D6, 400 MHz): d 7.46 (s, 2 H, phenyl), 6.08 (s, 2 H,
CH2); IR (paraffin, CsI plates, cm-1): 1908 m, 1649 w, 1601 s, 1582
=
vs, 1487 vs nC C, phenyl, 1471 vs (paraffin), 1410 s, 1391 s dCH3,
vinylic H), 1.16 (s, 36 H, Al–CMe3), 1.07 (s, 18 H, vinylic CMe3);
1356 vs (paraffin), 1321 w, 1298 w, 1250 s dCH3, 1229 s, 1200 s,
1182 s, 1098 s, 1036 s, 1016 vs, 989 s, 916 s, 885 s, 874 s, 854 s, 806 s,
770 m nCC, dCH, nCO, 725 s (paraffin), 692 m, 625 vs, 544 vs,
13
=
C NMR (C6D6, 100 MHz): d 154.3 (Al–C CCMe3), 147.5 (Al–
CCMe3), 144.6 (C-C CCMe3), 130.6 (C–H of phenyl), 124.5
=
=
C
10560 | Dalton Trans., 2009, 10550–10562
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©

View Article Online
517 s, 457 s, 434 s dCC, nAlC, nAlO; MS (EI, 70 eV): m/z (%) =
482 (4) [M+ - THF]; 425 (6) [M+ - THF - tBu].
Crystal structure determinations
Single crystals were obtained by different procedures (1: cooling
of the reaction mixture in n-hexane to +4 ◦C; 2(OEt2)2: treatment
of 2 with diethyl ether and cooling of the solution to +4 ◦C;
3: crystallization from toluene at +4 ◦C; cis-4: cooling of the
reaction mixture to +4 ◦C after 20◦min; trans-4: cooling of the
reaction mixture in n-hexane to +4◦ C after 14 h; cis- and trans-
5(OEt2): from diethyl ether at -45 C, different crystal forms as
described above; 6: cooling of the reaction mixture in n-pentane
to -30 ◦C; 7: crystallization from n-pentane at -15 ◦C; 8(THF)2:
crystallization from n-pentane at +2 ◦C). The crystallographic
data were collected with Bruker SMART APEX-II (Mo) and
Bruker SMART 6000 (Cu) diffractometers. The crystals were
coated with a perfluoropolyether, picked up with a glass fiber
and immediately mounted in the cooled nitrogen stream of the
diffractometer. The data collection method involved complete
w-scans in three different positions in j. Data reduction was
carried out using the program SAINT+.32 The structures were
solved by direct methods and refined with the program SHELXL-
9733 by a full-matrix least-squares method based on F2. Crystal
data, data collection parameters and structure refinement details
are given in Table 1.
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft for
generous financial support.
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