Functionalized alkynyl-chlorogermanes: Hydrometallation, Ge-Cl bond activation, Ge-H bond formation and chlorine-tert-butyl exchange via a transient germyl cation

Article abstract of DOI:10.1039/c5dt03918e

Treatment of alkynyl-arylchlorogermanes Aryl_nGe(Cl)(C≡C-^tBu)_3-n (n = 1, 2) with HM^tBu₂ (M = Al, Ga) yielded mixed Al or Ga alkenyl-alkynylchlorogermanes via hydrometallation reactions. Intramolecular

Full text of DOI:10.1039/c5dt03918e

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Functionalized Alkynylꢀchlorogermanes: Hydrometallation,
GeꢀCl Bond Activation, GeꢀH Bond Formation and Chlorineꢀ
†
tertꢀButyl Exchange via a Transient Germyl Cation
a
b
a
a
Christian Honacker, ZhengꢀWang Qu, Jens Tannert, Marcus Layh, Alexander
a
b
a
Hepp, Stefan Grimme* , and Werner Uhl*
Received
t
Treatment of alkynylꢀarylchlorogermanes Aryl
n
Ge(Cl)(C≡Cꢀ Bu)3ꢀn (n = 1, 2) with
t
HM Bu
2
(M = Al, Ga) yielded mixed Al or Ga alkenylꢀalkynylchlorogermanes via hyꢀ
drometallation reactions. Intramolecular interactions between the Lewisꢀbasic Cl atoms
and the Lewisꢀacidic Al or Ga atoms afforded MCGeCl heterocycles. The endocyclic
MꢀCl distances were significantly lengthened compared to the starting compounds and
t
indicated GeꢀCl bond activation. Dual hydrometallation succeeded only with HGa Bu
2
.
One Ga atom of the product was involved in a GaꢀCl bond, while the second one had an
a
Institut für Anorganische und Analytische Chemie der Universität Münster,
Corrensstraße 30, Dꢀ48149 Münster, Germany. Eꢀmail: uhlw@uniꢀmuenster.de
b
Mulliken Center of Theoretical Chemistry, Institut für Physikalische und Theoretische
Chemie, Universität Bonn, Beringstraße 4, 53115 Bonn, Germany. Eꢀmail: grimꢀ
me@thch.uniꢀbonn.de
†
Electronic supplementary information (ESI) available. CCDC 1428775–1428785. For
ESI, crystallographic data in CIF or results of quantum chemical calculations see DOI:
…
……….
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interaction to a CꢀH bond of a phenyl group. In two cases treatment of chlorogermanes
t
with two equivalents of HAl Bu resulted in hydroalumination of one alkynyl group and
2
formation of unprecedented GeꢀH functionalized germanes, ArylꢀGe(H)(C≡Cꢀ
t
t
t
Bu[C(Al Bu
2
)=C(H)ꢀ Bu] (Aryl = mesityl, triisopropylphenyl). The Al atoms of these
compounds interacted with the αꢀC atoms of the alkynyl groups. Ph(Cl)Ge(C≡Cꢀ
t
t
t
t
2
Bu)[C(Al Bu }=C(H)ꢀ Bu] reacted in an unusual Cl/ Bu exchange to yield the tertꢀ
t
t
t
t
butylgermane Ph( Bu)Ge(C≡Cꢀ Bu)[C{Al( Bu)(Cl)}=C(H)ꢀ Bu]. Quantum chemical
calculations suggested the formation of a germyl cation as a transient intermediate.
Introduction
Hydroalumination and hydrogallation of donor functionalized alkynylsilanes and –
germanes afford unique compounds, in which intramolecular interactions between the
Lewisꢀacidic and –basic constituents result in an activation of the bonds between the
Group 14 elements and the donor groups. These compounds are active Lewis pairs that
after cleavage of the Mꢀdonor bonds (M = Al, Ga) are able to activate or coordinate
1
molecules and show a reactivity comparable to that of frustrated Lewis pairs. Treatꢀ
t
ment of a pyrrolidylꢀtrialkynylsilane, for instance, with equimolar quantities of HAl Bu
2
t
or HGa Bu
2
yielded alkenylꢀdialkynylsilanes with the Al or Ga atoms geminal to silicon
2
(
2
A, Chart 1). Strong intramolecular MꢀNR interactions afforded fourꢀmembered Siꢀ
CAlN heterocycles with a significantly lengthened SiꢀN bond. Similar compounds were
3
obtained by hydrometallation of diethylaminoꢀethynylgermanes (B). The activation and
2

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weakening of the SiꢀN and GeꢀN bonds was confirmed by decreased Wiberg bond indiꢀ
3
ces. A germane reacted as an active Lewis pair which after cleavage of the endocyclic
GaꢀN bond activated the CꢀH bond of a terminal alkyne and initiated an alkynylꢀamine
3
exchange with the release of diethylamine. Phenyl isocyanate was inserted into an actiꢀ
vated GeꢀN bond at room temperature, while isothiocyanates required more drastic reacꢀ
3
tion conditions. Dual hydrometallation afforded dialuminium and digallium comꢀ
pounds which have been applied as chelating Lewis acids for the effective coordination
4
of donors. Addition of chloride anions to a diethylaminodialuminium germane resulted
5
in the unique elimination of an imine, MeꢀC(H)=NꢀEt, by formation of a GeꢀH bond.
The remaining GeAl
2
compound coordinated the chloride anion in a chelating manner
(
C). Quantumꢀchemical calculations suggested an unprecedented mechanism, in which
imine formation under unexpectedly mild conditions is favoured by the intermediate
5
generation of a germyl cation. Cl atoms as donor substituents have been applied in a
6
single case. Hydrometallation of a chloroꢀtriethynylsilane gave Si/M compounds which
had strong MꢀCl bonding interactions and lengthened SiꢀCl bonds. Dual hydrogallation
yielded the expected digalliumsilane in which one Ga atom was coordinated to the Cl
atom, while the other showed an interaction to the αꢀC atom of the remaining alkynyl
t
t
group. Dual hydroalumination led to a remarkable Cl/ Bu exchange with a Bu group
migrating from Al to Si and the Cl atom from Si to Al (formal 1,3ꢀdyotropic rearrangeꢀ
6
ment, D). According to quantum chemical calculations this exchange is initiated by an
interaction of the Cl atom with Al and proceeds via a silyl cation, a revised version of
the unprecedented mechanism is included in this article. These examples clearly demonꢀ
strate that hydrometallated and donor functionalized alkynylsilanes and ꢀgermanes form
a fascinating new class of compounds which enable secondary reactions as active Lewis
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pairs or show interesting intramolecular transformations. In this article we report on the
synthesis of the first chlorine functionalized alkynylgermanes and present some prelimꢀ
inary aspects of their reactivity.
Chart 1. Compounds obtained from functionalized silanes and germanes
Results and Discussion
Synthesis of chlorine functionalized alkynylgermanes
The arylꢀsubstituted alkynyl(chloro)germanes 1a to 1d were readily obtained in moderꢀ
t
n
ate (Ar = Mes, Trip) to high yields (Ar = Ph) by lithiation of HꢀC≡Cꢀ Bu with BuLi and
reaction of the inꢀsitu generated alkynide with Ar
n
GeCl4ꢀn (Scheme 1). The commercialꢀ
7
3
8
3
ly not available precursors MesGeCl and TripGeCl were synthesized on new routes
from Mes Mg or Trip Mg and an excess of GeCl . The alkynyl C atoms of 1a to 1d
2
2
4
13
showed C NMR resonances in the expected ranges of
δ = 75.7 to 80.6 (GeꢀC≡C) and
4

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1
15.9 to 120.1 (GeꢀC≡C). The differences between the shifts of both C atoms are larger
= 43.4) for the phenyl derivatives (1a, 1b) as compared to the sterically better
shielded mesityl or triisopropylphenyl compounds 1c and 1d (ꢀ = 35.9). Such differꢀ
ences have been correlated to the charge distribution in the alkynyl groups and the
(ꢀδ
δ
4a,9
chemical reactivity. Two absorptions of the C≡C stretching vibrations were observed
–1
in the IR spectra in narrow ranges at 2185 and 2155 cm .
+ nBuLi
t
t
HꢀC≡Cꢀ Bu
LiꢀC≡Cꢀ Bu
t
+
x LiC≡Cꢀ Bu
t
Ar GeCl
Ar (Cl)Ge(C≡Cꢀ Bu)
n
4ꢀn
n
3ꢀn
ꢀ
x LiCl
Ar = Ph, n = 2
Ar = Ph, n = 1
Ar = Mes, n = 1
Ar = Trip, n = 1
1a: Ar = Ph, n = 2
1b: Ar = Ph, n = 1
1c: Ar = Mes, n = 1
1d: Ar = Trip, n = 1
Scheme 1. Syntheses of chlorine functionalized alkynylgermanes [x = 3ꢀn; Mes = 2,4,6ꢀ
Me C H ; Trip = 2,4,6ꢀ(Me CH) C H ].
3
6
2
2
3
6
2
Figure 1. Molecular structure and atomic numbering scheme of compound 1a (comꢀ
pounds 1b and 1c are similar). Displacement ellipsoids are drawn at the 40% level. Hyꢀ
drogen atoms have been omitted for clarity.
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XꢀRay crystal structure determinations of 1a to 1c (Fig. 1) revealed the distorted tetꢀ
rahedral coordination of the Ge atoms, and C≡C triple bond lengths in the normal range
(119.4 pm on average). The GeꢀCl distances correspond to standard values of terminal
1
0
GeꢀCl groups [215.9(5) to 220.8(1) pm; Table 1]. The GeꢀC distances depend on the
hybridization of the C atoms. The shortest bonds were accordingly observed to the alꢀ
kynyl groups [188.1(2) to 189.3(4) Å vs. 192.1(1) to 195.0(4) Å].
Hydrometallation and activation of Ge-Cl bonds
Treatment of the alkynylgermanes 1a to 1d with one equivalent of dialkylaluminum or
t
–
gallium hydrides, HM Bu (M = Al, Ga), at ꢀ78 ºC and stirring for 3 to 16 h at room
2
temperature yielded the cisꢀhydrometallation products 2 (cis refers to the relative orienꢀ
11
tation of Al or Ga to H in the resulting alkenyl groups). Concentration and cooling of
the mixtures or recrystallization of the crude products from 1,2ꢀdifluorobenzene affordꢀ
ed colourless solids in yields of 55 (2c) to 91% (2e) (Scheme 2). The reactions were
highly regioselective with respect to the addition of the electropositive metal atoms to
the C atoms bound to Ge. These C atoms carry a relatively high negative partial charge
due to the electronegativity difference between Ge and the spꢀhybridized C atoms. The
Cl atoms coordinate with a lone pair of electrons to the metal atoms hereby forming
fourꢀmembered GeCMCl heterocycles (see below, Fig. 2). In the absence of Lewis basic
substituents such as Cl or NR
2
the alkynyl groups were also found to interact with the
6

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9
,12,13
Lewis acidic metal atoms.
ing vibrations C≡C in the IR spectrum to lower wave numbers (by about 50 cm ) and a
comparably large difference
The essentially identical values of νC≡C in the IR spectra (2180 and 2150 cm ) and the
This was typically associated with a shift of the stretchꢀ
–1
ν
13
ꢀ
δ between the C NMR signals of the ethynyl C atoms.
–1
1
3
almost unchanged
ꢀδ values (35 to 45 ppm) in the C NMR spectra of the free alkynes
1
and the hydrometallation products 2 confirm the absence of interactions between the
respective C and metal atoms in compounds 2b to 2e. There was no evidence for an
isomerization of the kinetically favored cisꢀhydrometallation products into the thermoꢀ
11
dynamically favored transꢀisomers. This isomerisation was previously found to proꢀ
ceed via an intermolecular activation step in which the alkenyl functionality adopts a
bridging position between two metal atoms. Compounds 2 show characteristic signals at
1
δ
= 6.7 to 7.1 for the vinylic H atoms in the H NMR and at around
δ
= 145 (GeꢀC=C)
13
and 165 (GeꢀC=C) for the vinylic C atoms in the C NMR spectra. The tertꢀbutyl
groups bound to Al or Ga are diastereotopic for 2b to 2e and give two different sets of
signals as a consequence of the chiral coordination of the Ge atom. Equilibration by fast
rotation about the MꢀC(vinyl) bond is prevented by the intramolecular MꢀCl interaction.
2
Scheme 2. Hydrometallation of alkynylgermanes; if not stated otherwise R = ꢀC≡Cꢀ
t
Bu.
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A representative example (2a) of the structures of the hydrometallation products 2 is
shown in Fig. 2, selected bond lengths of 2a to 2e are summarized in Table 1. Comꢀ
pounds 2b to 2e feature a chiral coordination sphere at the Ge atoms and crystallize as
1
racemic mixtures in centrosymmetric space groups (P /c). Strong intramolecular
1, P2
interactions of the Cl atoms bound to Ge with the Al or Ga atoms result in the formation
of unprecedented almost planar fourꢀmembered MCGeCl heterocycles (largest deviation
from the idealized plane through the four atoms is <8 pm for C; c.f. Table 1). This interꢀ
action leads to a significant increase of the GeꢀCl distances from about 218 pm in comꢀ
pounds 1 to 232.85(3) to 237.89(5) pm in the hydrometallation products 2 (the longest
distance is observed for the sterically most shielded triisopropylphenyl derivative 2d).
The shorter GeꢀCl distance of 227.34(3) pm in the Ga compound 2e reflects the relativeꢀ
ly weak acceptor strength of Ga compared to Al atoms. The associated AlꢀCl and GaꢀCl
distances of 241.50(6) (2a) and 243.58(7) (2d) pm and 263.01(4) pm (2e; M = Ga), reꢀ
spectively, are on the longer side of bond lengths observed for typical metalꢀhalogen
4,14
bridges. The increase in GeꢀCl bond lengths is associated with a distortion of the
tetrahedral coordination sphere of the Ge atom. Krossing et al. used the sum of angles at
2
a Si atom to the directly bonded C atoms (ΣanglesSiC ) as an indicator for the charge
15
separation between Si and F. The sum of angles was correlated with the bond classifiꢀ
cation following the sequence 329º (covalent) < 345 – 354º (significant ionic character)
<
360º (ionic). According to this classification compounds 2 all show a significant ionic
character of the activated GeꢀCl bonds (ΣanglesGeC = 353 to 356°). The heterocycles
2
may be described as irregular rectangles resembling a kite with endocyclic angles that
are obtuse at the C atom [103.35(7) (2a) to 106.19(6)º (2e)], almost ideally 90º at Ge
[
93.37(4)° in 2e], acute at Cl (ca. 80º) and Al (ca. 85º); a slightly smaller endocyclic
8

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angle [81.30(4)°] resulted at the Ga atom of 2e. GeꢀC distances increase from GeꢀC≡C
to GeꢀC=CH and GeꢀC(Ar) in intervals of about 4 and 1 pm. Compound 2a forms
loosely bonded dimers in the solid state via an interaction between a Cl atom of one
molecule and an oꢀH atom of a second molecule (Cl···H16 288 pm, Cl···C16 383 pm,
angle ClꢀHꢀC 175º).
Figure 2. Molecular structure and atomic numbering scheme of compound 2a (comꢀ
pounds 2b to 2e are similar). Displacement ellipsoids are drawn at the 40% level. Hyꢀ
drogen atoms (except H32, arbitrary radius) have been omitted for clarity.
It was shown in previous experiments that dual hydrometallation of Si or Ge centered
bisꢀ and trisalkynes R E(C≡CR’) and RE(C≡CꢀR’) (R = alkyl, aryl; E = Si, Ge) afꢀ
2
2
3
fords silanes and germanes that carry two Lewis acidic metal functionalities in a moleꢀ
–
4
cule and are able to coordinate Lewis bases such as Cl anions in a chelating manner. It
was therefore hoped that double hydrometallation of compounds 1 may lead to a further
weakening and activation or even a complete cleavage of the GeꢀCl bond.
t
Reaction of 1b with two equivalents of HAl Bu produced an inseparable mixture of
2
t
compounds. HGa Bu
2
in contrast afforded after prolonged stirring (48 h) the bishydroꢀ
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gallation product 3 in 74% yield (Scheme 3). The analogous reaction of 1d yielded an
unidentifiable mixture, while HꢀCl exchange was observed upon treatment of 1c with
t
1
two equivalents of HAl Bu
2
(see below, Scheme 4). The H NMR spectrum of the
Ga Ge compound 3 at room temperature was deceptively simple. It showed two broad
2
t
t
singlets of different Bu groups of the Ga Bu
2
substituents and only one signal for the
t
t
vinylic H atom and the Bu group of the C=CHꢀ Bu moieties. This is consistent with a
t
dynamic behaviour of 3 in solution, and each of the Ga Bu units may alternately coorꢀ
2
dinate in a fast exchange process to the Cl atom (c.f. solid state structure, Fig. 3). Variaꢀ
ble temperature NMR studies demonstrated that at 190 K the structure in solution reꢀ
t
sembles that in the solid state as indicated by the magnetic inequivalence of all six Bu
groups and the two vinylic H atoms as well as a broadening of the resonance of the oꢀH
atoms. This is consistent with one Ga atom being coordinated to the Cl atom and the
second interacting with an oꢀC atom of the phenyl group, which in turn results in a hinꢀ
dered rotation of the phenyl substituent about the GeꢀC bond. The energy for the exꢀ
≠
change process was estimated to
of 210 K.
ꢀ
G = 44 kJ/mol based on a coalescence temperature
tBu
Ge
H
Cl
t
+
2 HGa Bu
2
t
GatBu2
Ph(Cl)Ge(C Cꢀ Bu)
tBu Ga
2
2
1b
H
tBu
3
Scheme 3. Dual hydrogallation of 1b and formation of 3.
One half of the molecular structure of 3 (Fig. 3) is strongly related to that of the
monoꢀhydrometallation product 2e. A fourꢀmembered GeCGaCl heterocycle is formed
by an interaction of the Cl with one of the Ga atoms. The Ga2ꢀCl1 distance [265.08(8)
1
0

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14
pm, Table 1] is in the longer range of typical MꢀClꢀGa bridges, and the Ge(1)ꢀCl(1)
bond [229.60(8) pm] is significantly lengthened compared to the starting compound 1b
[
215.93(5) pm]. Hydrogallation of the second alkynyl group resulted in the formation of
a divinyl compound with two Ga atoms in a single molecule. The second Ga atom is not
bonded to the Cl atom (Ga1···Cl1 441 pm), instead there is a relatively short contact
(
Ga1···C32 281 pm; Ga1···H32 240 pm]) to an oꢀCꢀH bond of the phenyl group resultꢀ
ing in a pyramidalization of the Ga atom (Ga1···C ꢀplane 29 pm). Such weak alumiꢀ
numꢀ or galliumꢀphenyl interactions have also been observed in the related compound
3
t
t
3
2
Ph(Et N)Ge[C(Al Bu )=CHꢀ Bu] (Al···oꢀC distance 260 pm) which shows a fascinatꢀ
2
2
ing chemical behavior (see Introduction). The longer M···C distance in 3 reflects the
weaker Lewisꢀacidity of the Ga atoms.
Figure 3. Molecular structure and atomic numbering scheme of compound 3. Disꢀ
placement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H12 and H22,
arbitrary radius) have been omitted for clarity.
Generation of Ge-H bonds by chlorine-hydrogen transfer
1
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In attempts to obtain double hydroaluminated compounds analogous to 3 (Scheme 3)
we treated the dialkynylgermane 1c with two equivalents and the monohydroaluminated
t
species 2d with equimolar quantities of HAl Bu
2
(Scheme 4). However, instead of the
tꢀ
doubly reduced divinyl compounds we obtained GeꢀH functionalized germanes. ClAl
1
2
6
Bu
was identified as a byꢀproduct by its characteristic NMR data.
Scheme 4. Hydroalumination of compounds 1c and 2d and ClꢀH exchange.
1
H NMR spectra of the reaction mixtures after 90 min at room temperature showed
t
t
only the signals of ClAl Bu and the germanes 4. Removal of ClAl Bu by sublimation
2
2
in vacuum afforded residues which were recrystallized from 1,2ꢀdifluorobenzene to
yield 4a and 4b in almost quantitative yields. The GeꢀH bonds of compounds 4a and 4b
were identified by characteristic strong absorptions in the IR spectra (
ν
GeH = 2066 and
–1 17
1
2
058 cm ) and by resonances in the H NMR spectra at δ = 6.5. Two absorptions in
–1
the IR spectra with comparatively low wave numbers of about 2150 and 2107 cm for
–1
the stretching vibrations of the C≡C bonds (compare 2180 and 2150 cm for 2b to 2e)
and increased ꢀδ values of 51.1 and 59.0 ppm between the resonances of the ethynyl C
1
3
atoms in the C NMR spectra (35 to 45 ppm for 2b to 2e; see above) were indicative of
1
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an interaction between the αꢀC atom of the C≡C triple bond (bearing a relatively high
1
2,13
negative partial charge) and the Lewis acidic Al atom.
Scheme 4) by treatment with an aluminum hydride is well documented in the literature,
and LiAlH or related hydrides have been used for the generation of GeꢀH bonds from
The observed HꢀCl exchange
(
4
18
the respective halogen precursors. The selective hydroalumination of the first alkynyl
group verifies that the reduction of the C≡C triple bond is actually the preferred reacꢀ
tion. Steric shielding may hinder the second hydoalumination step and favor HꢀCl exꢀ
t
t
change. The different reactivity of HGa Bu
2 2
and HAl Bu may result from the higher
polarity of the AlꢀH compared to the GaꢀH bond.
The molecular structure of 4b (Fig. 4) revealed a distorted tetrahedral coordination
sphere for the Ge atom with four different substituents: an H atom, alkynyl, aryl and
alkenyl groups (sum of the angles CꢀGeꢀC = 327.4°; HꢀGeꢀC with C21 and C31: 348.1°;
Table 1). The most interesting feature is the intramolecular contact of the αꢀC atom of
the alkynyl group to the Al atom which has been predicted by spectroscopic observaꢀ
tions. The Al1···C11 distance is with 250.8(2) pm comparable to related monomeric
t
t
species such as Me
2
(PhC≡C)Ge[(Al Bu
2
)=C(H)ꢀPh] or Ph
2 2
(PhC≡C)Ge[(Al Bu )=C(H)ꢀ
5
,12,13
Ph] (240 to 255 pm).
These interactions result in an activation of the Geꢀ
C(alkynyl) bonds and selective rearrangement reactions with the formation of germacyꢀ
9
clobutenes in the melt. Caused by this Al···C interaction the GeꢀC(alkynyl) bond of 4b
[
197.0(2) pm] is lengthened by about 10 pm compared to compounds 2b to 2e which
have exclusively terminal alkynyl groups. Further indicative for this interaction are the
formation of an almost planar AlC Ge heterocycle (torsion angle Al1ꢀC31ꢀGe1ꢀC11 =
2
15.9°), an acute angle C11ꢀGe1ꢀC31 of 93.98(7)° and the large deviation of the Al atom
(
37.2 pm) from the plane of the directly bonded C atoms (C31, C41, C51). The C≡C
1
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bond length is almost unaffected [C11ꢀC12 120.9(3) pm; 119.4 pm on average in 2b to
e], and the GeꢀC≡C group deviates only slightly from linearity [172.4(2)°]. The GeꢀH
2
19
bond length [142(2) pm] and other molecular parameters are unexceptional.
Fig. 4. Molecular structure and atomic numbering scheme of compound 4. Displaceꢀ
ment ellipsoids are drawn at the 40% level. Hydrogen atoms (except H1 and H32, arbiꢀ
trary radius) have been omitted for clarity.
Intramolecular chlorine-tert-butyl exchange
Heating compound 2b for 2 d at 50 ºC resulted in its conversion to compound 5 by
t
transfer of the bridging Cl atom from Ge to Al (Scheme 5) and a simultaneous Bu miꢀ
6
gration from Al to Ge (1,3ꢀdyotropic rearrangement ). The related gallium compound 2e
showed under similar conditions no reactivity and was recovered almost quantitatively.
1
Compared to the H NMR spectrum of 2b there is a small shift for the relevant signals
t
t
t
t
of the Bu groups from δ = 1.36 and 1.44 (Al Bu)
2
to 1.38 and 1.39 (Al Bu, Ge Bu), reꢀ
spectively, and the C=CH signal is shifted from δ = 7.00 to 7.33. The most significant
1
4

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13
change is found in the C NMR spectrum for the C
β
atom of the alkynyl substituent
which is shifted from δ = 122.9 in 2b to δ = 140.7 in 5. With only a small change in the
chemical shift for the corresponding C
α
atom (
δ
= 78.0 vs. 84.1) the difference
ꢀδ
CC
increases from 44.9 to 56.6. Large differences
ꢀ
δCC are indicative of an interaction beꢀ
1
2,13
tween the Lewis acidic Al atom and the alkynyl group (see above).
The observed
value therefore suggests that in contrast to the solid state (see below) compound 5 may
be monomeric in solution with an Alꢀalkynyl interaction similar to 4a and 4b.
tBu
t
Al Bu
2
C
Cl
Ge
C
H
Ph
Cl
5
0 ºC; 2 d
2
Ph
2
Ge
Al
toluene
tBu
tBu
tBu
C
C
tBu
H
tBu
5
(solution)
2b
tBu
C
tBu
Ge
H
Cl
Cl
tBu
C
Ph
tBu
tBu
Al
tBu
Al
Ge
Ph
C
tBu
C
H
tBu
5
(solid state)
Scheme 5. Thermal rearrangement of compound 2b
The molecular structure of 5 in the solid state consists of a centrosymmetric dimer
t
(
Fig. 5) with a central Al Cl heterocycle. It verifies the remarkable Cl/ Bu exchange
2
2
t
with the Cl atom bonded to Al and a Bu group attached to Ge. The Ge atom has a chiral
coordination sphere and is bonded to four C atoms of four different substituents. The
14
AlꢀCl distances are in the normal range of typical AlꢀClꢀAl bridges, but they differ
considerably [227.41(7) and 236.80(6) pm] and reflect an unsymmetric bonding situaꢀ
tion. The distance between Al and the αꢀC atom of the alkynyl group (Al1···C11) is
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with 309 pm too long for a strong intramolecular Al···C≡C bonding interaction, but it is
19
shorter than the sum of the van der Waals radii (354 pm ). The molecular conformation
seems to support a weak interaction between these atoms. The dimeric molecules are
perfectly preorganized to facilitate monomerisation. The GeꢀC(alkynyl) and AlC(vinyl)
bonds are parallel to each other as evident from the small Al1ꢀC31ꢀGe1ꢀC11 torsion
angle (1.6°). The αꢀC atom of the alkynyl group approaches the Al atom from the oppoꢀ
site side of the longer endocyclic AlꢀCl bond (angle C11···Al1ꢀCl1’ 157.9°) which reꢀ
sults in a surprisingly large C31ꢀAl1ꢀC51 angle of 125.01(8). The coordination sphere
of the Al atom may be described by a distorted trigonal bipyramide with the atoms C31,
C51 and Cl1 in the equatorial plane (deviation of the Al atom from that plane 31 pm).
One axial position is occupied by Cl1’, the other by the alkynyl C atom C11 with a
weak Al⋅⋅⋅C interaction. Dissociation may be supported by gradual strengthening of the
Al···C interaction and concomitant weakening of the Al1ꢀCl1’ bond, and in solution a
situation similar to 4a and 4b with an intramolecular Alꢀalkynyl interaction may help to
stabilize the monomeric formula units. This may be interpreted in terms of a small prefꢀ
erence of the Al···Cl over the Al···C(alkynyl) interaction in the solid state.
t
6
2
A similar Cl/ Bu exchange has previously been observed for an Al Si compound. In
contrast to the rearrangement of the GeAl compound 2b (Scheme 5) it proceeded under
mild conditions (room temperature) and much faster (<2 h). A revised version of the
calculated mechanism is shown below. Chelating coordination of the Cl atom by both
Al atoms and the intermediate formation of a silyl cation are the key steps to facilitate
this reaction. A higher reaction temperature and a longer reaction time is required for
the formation of the monoaluminium compound 5. A conceivable reaction mechanism
is discussed below.
1
6

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Figure 5. Molecular structure and atomic numbering scheme of compound 5. Disꢀ
placement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H32, arbitrary
radius) have been omitted for clarity. Equivalent atoms are generated by ꢀx, ꢀy, ꢀz.
Quantum-chemical calculations on the rearrangement mechanisms
t
To understand the mechanism of the Cl/ Bu exchange of the GeAl compound 2b, stateꢀ
ofꢀtheꢀart dispersionꢀcorrected DFT calculations were performed at the PW6B95ꢀ
D3/def2ꢀQZVP//TPSSꢀD3/def2ꢀTZVP + COSMOꢀRS(toluene) level of theory (simply
PW6B95 later on). Scheme 6 summarizes the most feasible free energy path, while deꢀ
tailed energies and geometries of various intermediates and transition structures are inꢀ
cluded in the Supporting Information (SI).
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7

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t
Scheme 6. The PW6B95ꢀD3 predicted free energy path (in kcal/mol; R = Bu) for the
t
thermal Cl/ Bu exchange of the Ge/Al compound 2b.
As shown in Scheme 6, heterolytic cleavage of the GeꢀCl bond of 2b and rotation
about an AlꢀC bond leads to the zwitterionic intermediate a in an endergonic step (20.0
kcal/mol). The acyclic intermediate a contains an anionic alanate moiety with Cl bondꢀ
20
ed to Al and a nearly planar coordinated cationic Ge atom, the latter being stabilized
t
by loose Ge···H interactions (209.7 pm) with an Al bound Bu group. From the metaꢀ
t
stable intermediate a, a Bu shift from the alanate part to the cationic Ge atom requires a
free energy barrier of only 7.9 kcal/mol, leading to the less polar molecule b in an exerꢀ
gonic step [ꢀ9.3 kcal/mol]. Molecule b contains two neutral centers with Ge and Al
t
bonded to the respective Bu and Cl groups and the Lewisꢀacidic Al atom being stabiꢀ
t
lized by weak Al···H interactions (209.2 pm to the Geꢀ Bu group). Intramolecular rotaꢀ
tion about the GeꢀC(alkenyl) bond of the acyclic molecule b affords the final product 5
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8

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(monomeric in solution), which is further stabilized by 9.4 kcal/mol via an Al···alkynyl
interaction. The dimeric form (5) is formed via two stronger AlꢀClꢀAl bridges. It is
2
however 2.9 kcal/mol higher in free energy in solution than two monomeric formula
units of 5 mainly due to unfavorable entropy (T⋅ꢀS = 19.5 kcal/mol) and solvation efꢀ
t
fects (3.8 kcal/mol). The overall Cl/ Bu exchange reaction involving the zwitterionic
molecule a as key intermediate is thus exergonic by 18.7 kcal/mol with a moderate free
energy barrier of 27.9 kcal/mol which is consistent with the observed experimental conꢀ
ditions (heating for 2 d at 50 ºC).
t
A similar Cl/ Bu exchange has previously been reported for an Al
2
Si compound (laꢀ
beled E in Scheme 7). The reaction occurred under surprisingly mild conditions (below
6
2
h at room temperature) although a stronger SiꢀCl instead of a GeꢀCl bond is involved.
Based on early DFT calculations, a zwitterionic mechanism similar to that in Scheme 6
t
was proposed for the Cl/ Bu exchange between one Si and one (rather than two) Al atꢀ
om, which however encounters a unreasonably high potential barrier of 37.7 kcal/mol
6
that is almost insurmountable under ambient conditions. The predicted ~10 kcal/mol
t
higher barrier for the Cl/ Bu exchange involving a single Al atom is consistent with the
stronger SiꢀCl compared to a GeꢀCl bond. This strongly suggested that the additional
2
Lewisꢀacidic Al atom in the Al Si compound E could act in a cooperative way to reduce
t
the Cl/ Bu exchange activation barrier, as corroborated by the revised mechanism based
on new DFT calculations (Scheme 7).
As shown in Scheme 7, rotation about the SiꢀC bond over low free energy barrier led
to the cleavage of the Al···alkynyl interaction in compound E and the formation of
molecule F with the Cl atom coordinated by both Al atoms in a chelating manner. The
chelating coordination of the Cl atom helps to facilitate the heterolytic cleavage of the
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20
−
SiꢀCl bond, to generate a silyl cation and to stabilize the Cl anion within the key tranꢀ
t
sition structure TSFG. As a result, the free energy barrier required for the Bu shift from
Al to the cationic Si atom is significantly reduced from 39.2 kcal/mol (via TSFG’, see
SI) to 29.4 kcal/mol for the respective cases involving one or two Al atoms. After the
t
Cl/ Bu exchange, rotation about two SiꢀC bonds results in cleavage of the AlꢀClꢀAl
bridge with one of the Al atoms being stabilized by a newly formed Al···alkynyl interꢀ
6
action in the final products H and D. Consistent with experimental NMR data, our DFT
calculations suggest that the isomer D (Chart 1) is about 1.9 kcal/mol lower in free enꢀ
ergy (compared with 0.9 kcal/mol derived from the H : D ratio of 18 : 82 in the NMR
6
t
spectra ) with a very low interꢀconversion barrier to the isomer H. The overall Cl/ Bu
exchange with one Si and two Al atoms interacting in a cooperative way is exergonic by
16.8 kcal/mol over a moderate free energy barrier of 29.4 kcal/mol, which is comparaꢀ
t
ble with that calculated for the Cl/ Bu exchange of the Ge/Al compound 2b. The similar
barriers underscore the efficiency of the chelating coordination of the Cl atom in F
which facilitates SiꢀCl bond cleavage despite the relatively high SiꢀCl bond energy.
2
0

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t
Scheme 7. The TPSSꢀD3 predicted free energy path (in kcal/mol; R = Bu) for the rearꢀ
rangement of the chlorine functionalized Al Si compound E.
2
Conclusion
t
The new functionalized chlorogermanes ArylGe(Cl)(C≡Cꢀ Bu)
2
(1a to 1d; Aryl = Ph,
Mes, Trip) are valuable starting materials for a variety of secondary reactions, but this
article is focused on hydrometallation as an important aspect of their chemical behavꢀ
t
t
2 2
iour. Treatment with equimolar quantities of HAl Bu or HGa Bu afforded by reduction
of one C≡C bond selectively alkenylgermanes (2a to 2e) in which the Ge and Al/Ga
atoms (M) adopted geminal positions at the resulting vinyl groups. Intramolecular AlꢀCl
or GaꢀCl interactions resulted in the formation of MCGeCl heterocycles and compared
to the starting compounds significantly lengthened GeꢀCl bonds. A similar lengthening
of Si/GeꢀN bonds has been observed previously for corresponding amino functionalized
2
,3
silanes and germanes. It correlates to a decrease of the calculated Wiberg bond indices
3
and an increased reactivity with the facile insertion of heterocumulenes. A similar beꢀ
haviour is expected for the ClꢀGe compounds 2a to 2e reported in this article with the
formation of interesting halogenated species of suitable substrates.
Dual hydrometallation of the Cl functionalized compounds was successful only on
t
t
2 2
one occasion (3) with HGa Bu as the reductant. HAl Bu led to ClꢀH exchange and the
t
t
2
formation of GeꢀH functionalized compounds, ArylGe(H)(C≡Cꢀ Bu)[C(Al Bu )=C(H)ꢀ
t
Bu] (4a, 4b). Hydroalumination of the first triple bond seems to be favoured in these
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1

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reactions, while steric shielding may facilitate ClꢀH exchange. The germanes 4 show an
intramolecular interaction between the Lewis acidic Al and the αꢀC atoms of the intact
alkynyl groups which has been recognized as an essential step in bond activation and
9
cyclisation reactions. An interesting challenge for future investigations is the observaꢀ
tion of an interaction of the Al atom to the Ge bound H atom. However, the polarity of
the GeꢀH bond and the charge at H seem to be too low to favour such a unique structurꢀ
al motif in competition to any other donor. The presence of the Lewis acidic Al atom in
the same molecule may help to facilitate insertion reactions into the GeꢀH bond.
t
Thermally initiated rearrangement of 2b with Cl/ Bu exchange afforded the compound
t
t
t
t
Ph( Bu)Ge(C≡Cꢀ Bu)[C{Al( Bu)(Cl)}=C(H)ꢀ Bu] (5) in which the Cl atom is bonded to
t
Al and a Bu group is attached to Ge. This process formally corresponds to a 1,3ꢀ
6
,21
dyotropic rearrangement. Quantum chemical calculations corroborate a unique reacꢀ
tion mechanism in which a germyl cation is formed as a reactive intermediate. A revised
2
mechanism for a similar rearrangement with an Al Si compound confirmed the forꢀ
mation of a silyl cation and the activation of the SiꢀCl bond by the chelating coordinaꢀ
tion of the Cl atom by two Lewis acids. The higher SiꢀCl bond energy compared to Geꢀ
Cl is compensated by this specific interaction. 5 is dimeric in the solid state via AlꢀClꢀ
Al bridges. Results of spectroscopic investigations verify its dissociation to yield the
monomeric fragments in solution in which the remaining alkynyl group interacts with
the Al atom similar to the molecular structures of 4a and 4b.
These silanes and germanes functionalized by Al or Ga atoms and donor substituents
are accessible on facile routes and are highly promising materials as active Lewis pairs
3
in the coordination, activation and insertion of substrates, for intramolecular rearꢀ
5
9a
rangement reactions, for the formation of silaꢀ and germacyclobutenes or the generaꢀ
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2

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2
0
tion of silyl and germyl cations by a complete transfer of the Lewis base from Si or Ge
to the Lewis acidic centers. They will find wide application in future investigations.
Experimental Section
All procedures were carried out under an atmosphere of purified argon in dried solvents
(
nꢀpentane, cyclopentane, nꢀhexane with LiAlH ; Et O, THF, toluene with
4 2
Na/benzophenone; 1,2ꢀdifluorobenzene, pentafluorobenzene with molecular sieves).
Microanalyses were carried out by the microanalytical laboratory of the Westfälische
WilhelmsꢀUniversität Münster. NMR spectra were recorded in C
6 6 6
D or C D12 at ambiꢀ
1
ent probe temperature using the following Bruker instruments: Avance I ( H, 400.13;
1
3
1
13
C, 100.62 MHz) or Avance III ( H, 400.03; C, 100.59 MHz) and referenced internalꢀ
1
3
ly to residual solvent resonances (chemical shift data in δ). C NMR spectra were all
proton decoupled. IR spectra were recorded as paraffin mulls between CsI or KBr plates
7
8
on a Shimadzu Prestige 21 spectrometer. MesGeCl
3 3
and TripGeCl have been reported
in the literature previously, we synthesized these trichlorides on different routes and
t
22
t
22
isolated them in 68% yield (see synthesis of 1c and 1d). HAl Bu2 and HGa Bu2 were
t
obtained according to literature procedures. HꢀC≡Cꢀ Bu, GeCl
4 3 2 2
, PhGeCl , Ph GeCl and
n
BuLi were used as purchased. The assignment of NMR spectra is based on HMBC,
H,HꢀROESY, HSQC and DEPT135 data. Only the most intensive mass of each fragꢀ
ment is given in the documentation of the mass spectra; the complete isotopic patterns
agree with the calculated ones.
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t
2
Synthesis of Ph Ge(Cl)C≡Cꢀ Bu 1a
n
A solution of BuLi (5.90 mL, 9.44 mmol, 1.6 M in nꢀhexane) was added at room temꢀ
t
perature to a solution of HꢀC≡Cꢀ Bu (1.20 mL, 0.80 g, 9.76 mmol) in Et
O (50 mL).
2
After stirring for 16 h at room temperature the pale yellow mixture was added dropwise
at ꢀ78 °C to a solution of Ph GeCl (2.00 mL, 2.83 g, 9.51 mmol) in Et O (50 mL). The
mixture was slowly warmed to room temperature and stirred for 16 h. Distilled water
20 mL) was added and the organic layer was separated. The aqueous layer was extractꢀ
ed three times with Et O (3 x 20 mL). The combined organic layers were dried over
MgSO and filtered. All volatiles of the filtrate were removed under reduced pressure,
and the residue was recrystallized from nꢀhexane to afford 1a as a colorless solid. Yield:
2
2
2
(
2
4
3.15 g (97%). M.p. (argon, sealed capillary): 53 ºC. Microanalysis: found C 62.8, H
1
5.6%; [C18
H
19ClGe (343.39)] requires C 63.0, H 5.6%. H NMR (C
6
D
6
, 400 MHz, 300
t
K): δ = 1.11 (s, 18 H, Bu), 7.11 (m, overlap, 4H, mꢀH), 7.13 (m, overlap, 2H, pꢀH), 7.79
3
4
13
(
dd, JHH = 6.5 Hz, JHH = 3.0 Hz, 4H, oꢀH). C NMR (C
6
6
D , 101 MHz, 300 K): δ =
2
1
1
8.6 (CMe ), 30.6 (CMe ), 75.9 (GeC≡C), 120.1 (GeC≡C), 129.0 (mꢀC), 130.9 (pꢀC),
3 3
–1
33.6 (oꢀC), 135.3 (ipsoꢀC). IR (KBr plates, paraffin, cm ): 2181 vs, 2151 vs ν(C≡C);
962 w, 1892 w, 1817 w, 1771 w, 1653 vw, 1582 w (phenyl); 1447 vs, 1439 vs, 1371 vs
(
paraffin); 1337 w, 1306 m, 1250 vs δ(CH ); 1204 m, 1194 m, 1159 vw, 1094 s, 1069
3
w, 1026 w, 995 w, 968 vw, 922 m, 849 w ν(CC); 743 vs (paraffin); 694 vs (phenyl); 617
vw, 550 vw, 490 s, 459 vs, 390 vs ν(GeC), ν(GeCl). Mass spectrum (EI+, 20 eV, 298
+
+
t
+
K): m/z (%) = 344 (4) [M] , 309 (1) [M – Cl] , 287 (8) [M – Bu] , 263 (15) [M – C≡Cꢀ
t
+
Bu] .
2
4

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t
Synthesis of PhGe(Cl)(C≡Cꢀ Bu)
1b
2
Compound 1b was obtained in an analogous manner to compound 1a from two equivaꢀ
n
t
lents of BuLi (15.5 mL, 24.8 mmol, 1.6 M in nꢀhexane), two equivalents of HꢀC≡Cꢀ Bu
(
3.00 mL, 2.00 g, 24.4 mmol) and one equivalent of PhGeCl (2.00 mL, 3.17 g, 12.4
3
mmol). Yield: 3.97 g (92%). M.p.: 51 °C (Ar, sealed capillary). Microanalysis: found C
1
62.4, H 6.7%; [C18
H
23ClGe (347.43)] requires C 62.2, H 6.7%. H NMR (C
6
D
6
, 400
t
MHz, 300 K): δ = 1.04 (s, 18H, Bu), 7.10 (m, overlap, 1H, pꢀH), 7.12 (m, overlap, 2H,
3
4
13
mꢀH), 7.92 (dd, JHH = 7.6 Hz, JHH = 1.7 Hz, 2H, oꢀH). C NMR (C
K): δ = 28.5 (CMe ), 30.3 (CMe
), 75.7 (GeC≡C), 118.4 (GeC≡C), 129.0 (mꢀC), 131.2
pꢀC), 132.9 (oꢀPh), 135.4 (ipsoꢀC). IR (CsI plates, paraffin, cm ): 2190 vs, 2155 vs
6 6
D , 101 MHz, 300
3
3
–
1
(
ν(C≡C); 2059 vw, 1983 vw, 1966 w, 1910 vw, 1892 w, 1841 vw, 1825 w, 1774 w, 1661
w, 1657 sh, 1590 w (phenyl); 1462 vs, 1377 vs (paraffin); 1337 s, 1310 vs, 1253 vs
δ
3
(CH ); 1204 s, 1187 s, 1172 m, 1159 m, 1097 vs, 1068 w, 1027 s, 997 m, 985 w, 972
w, 923 vs, 890 m, 863 m, 854 m, 755 vs, 736 vs ν(CC); 723 vs (paraffin); 694 vs, 679 s
(
(
(
phenyl); 617 vw, 554 w, 499 vs, 488 vs, 462 vs νGeC, δ(CC), ν(GeCl). Mass spectrum
+
+
+
EI+, 20 eV, 298 K): m/z (%): 348 (6) [M] , 333 (6) [M – CH ] , 313 (4) [M – Cl] , 291
3
t
+
t
+
11) [M – Bu] , 267 (36) [M – C≡Cꢀ Bu] .
t
Synthesis of Mes(Cl)Ge(C≡Cꢀ Bu)
1c
2
A solution of MesBr (5.40 mL, 6.98 g, 35.1 mmol) in THF (40 mL) was added dropꢀ
wise at 50 ºC to Mg turnings (1.00 g, 41.1 mmol). After completion the mixture was
heated for 5 min under reflux conditions. The mixture was allowed to cool to room
temperature, 1,4ꢀdioxane (10.0 mL, 10.3 g, 117 mmol) was added. The resulting susꢀ
pension was heated under reflux for 5 min, cooled to room temperature and allowed to
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2
settle. The supernatant solution was decanted and the solid MgBr (dioxane) was washed
with THF (20 mL). The decanted solution can be used directly for further transforꢀ
mations (preferred method). Alternatively, the THF fractions were combined and the
23
solvent was removed in vacuo. The remaining solid (Mes
with Et O (10 mL) and then treated with THF (100 mL). The mixture was filtered, and
the filtrate was added dropwise at ꢀ78 ºC to a solution of GeCl (5.00 mL, 9.40 g, 43.8
2
Mg ) was washed three times
2
4
mmol) in THF (100 mL). The mixture was allowed to warm to room temperature,
stirred overnight and then heated under reflux for 5 min. The solvent was removed in
vacuo at room temperature, and the remaining residue was treated with nꢀpentane (3 x
20 mL). After filtration the filtate was concentrated in vacuo until the product started to
precipitate (~4 mL). The solvent was removed with a pipette and the obtained product
was dried in vacuo to yield MesGeCl as a pale yellow powder. Yield: 7.10 g (68%
3
1
based on MesBr; H NMR (C
6
D
6
, 400 MHz, 300 K): δ = 1.87 (s, 3H, pꢀMe), 2.40 (s,
1
3
6
6 6
H, oꢀMe), 6.41 (s, 2H, mꢀH). C NMR (C D , 101 MHz, 300 K): δ = 20.8 (pꢀMe),
23.8 (oꢀMe), 130.8 (mꢀC), 130.9 (ipsoꢀC), 142.7 (oꢀC), 143.0 (pꢀC)).
t
A solution of HꢀC≡Cꢀ Bu (1.20 mL, 0.80 g, 9.76 mmol) in Et
2
O (50 mL) was treated
n
with BuLi (6.10 mL, 9.76 mmol, 1.6 M in nꢀhexane) at room temperature and the mixꢀ
ture was stirred overnight. It was added dropwise at ꢀ78 ºC to a solution of MesGeCl
1.46 g, 4.90 mmol) in Et O (50 mL). The mixture was slowly warmed to room temperꢀ
ature and stirred for 16 h. Distilled water (20 mL) was added, and the organic layer was
separated. The aqueous layer was extracted three times with Et O (3 x 20 mL). The
3
(
2
2
combined organic layers were dried over MgSO and filtered. All volatiles of the filtrate
4
were removed under reduced pressure, and the residue was recrystallized from nꢀhexane
(
ꢀ30 °C) to yield 1c as a colorless solid. Yield: 1.20 g (63%). M.p.: 92 ºC. Microanalyꢀ
2
6

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1
sis: found C 64.7, H 7.5%; [C21
H
29ClGe (389.51)] requires C 64.8, H 7.5%. H NMR
(
C D , 400 MHz, 300 K): δ = 1.06 (s, 18H, C≡CꢀCMe ), 2.02 (s, 3H, pꢀMe), 2.84 (s,
6
6
3
1
3
6
2
1
H, oꢀMe), 6.67 (s, 2H, mꢀH). C NMR (C
6
D
6
, 101 MHz, 300 K): δ = 21.0 (pꢀMe),
4.0 (oꢀMe), 28.4 (CMe
3 3
), 30.1 (CMe ), 80.4 (GeC≡C), 116.9 (GeC≡C), 129.2 (ipsoꢀC),
–
1
30.1 (mꢀC), 141.0 (pꢀC), 144.0 (oꢀC). IR (KBr plates, paraffin, cm ): 2189 vs, 2154 vs
ν(C≡C); 2008 vw, 1946 vw, 1906 vw, 1879 vw, 1749 vw, 1717 vw, 1601 m, 1555 s
aromatic ring); 1470 vs, 1377 vs (paraffin); 1296 s, 1252 vs (CH ); 1202 s, 1103 w,
028 m, 924 s, 845 s, 750 vs ν(CC); 725 s (paraffin); 590 m, 542 s, 492 vs ν(GeC),
(
δ
3
1
+
δ
(CC), ν(GeCl). Mass spectrum (EI+, 20 eV, 323 K): m/z (%) = 390 (2) [M] , 375 (2)
+
t
+
[
M – Me] , 354 (19) [M – HCl], 308 (39) [M – HꢀC≡Cꢀ Bu] .
t
Synthesis of [2,4,6ꢀ(Me CH) C H ]Ge(Cl)(C≡Cꢀ Bu) 1d
2
3
6
2
2
A solution of 2,4,6ꢀ(Me
2
CH)
3
C
6
2
H Br (7.83 g, 7 mL, 27.7 mmol) in THF (50 mL) was
added dropwise to Mg turnings (1.00 g, 41.1 mmol). The mixture was heated under reꢀ
flux conditions for 2.5 h. 1,4ꢀdioxane (10.0 mL, 10.3 g, 117 mmol) was added at room
temperature. The resulting suspension was heated under reflux for 5 min and cooled to
room temperature. MgBr
to settle overnight. The supernatant solution was decanted and added dropwise at ꢀ78 ºC
to a solution of GeCl (7.52 g, 4.00 mL, 35.1 mmol) in THF (50 mL). The mixture was
2
(dioxane) precipitated as a colourless solid which was allowed
4
warmed to room temperature and heated under reflux conditions (5 min). The solvent
was removed in vacuo, and the residue was extracted with nꢀpentane (3 x 20 mL). The
combined extracts were concentrated until the product started to precipitate (~4 mL).
The remaining solvent was removed by a pipette, and the remaining solid was dried in
2 3 6 2 3
vacuo to yield 2,4,6ꢀ(Me CH) C H GeCl as a pale yellow solid. Yield: 8.40 g (79%
2
7

Dalton Transactions
Page 28 of 50
View Article Online
DOI: 10.1039/C5DT03918E
1
3
based on aryl bromide; H NMR (C
6
D
6
, 400 MHz, 300 K): δ = 1.15 (d, JHH = 7.0 Hz,
3
3
6
H, pꢀCHMe ), 1.25 (d, J = 6.6 Hz, 12H, oꢀCHMe ), 2.68 (hept, J = 7.0 Hz, 1H,
2
HH
2
HH
3
pꢀCHMe
2
2
), 3.94 (hept, JHH = 6.6 Hz, 2H, pꢀCHMe ), 7.11 (s, 2H, mꢀH)).
t
A solution of HꢀC≡Cꢀ Bu (1.80 g, 21.9 mmol) in Et
O (50 mL) was treated at room
2
n
temperature with a solution of BuLi (13.5 mL, 21.6 mmol, 1.6 M in nꢀhexane). The
mixture was stirred for 3 h and added at ꢀ78 ºC to a solution of [2,4,6ꢀ
(
Me CH) C H ]GeCl (4.13 g, 10.8 mmol) in Et O (50 mL). The mixture was stirred for
2 3 6 2 3 2
2
h at ꢀ78 ºC, allowed to warm to room temperature over a period of 16 h and treated
with distilled water (20 mL). The phases were separated, and the aqueous phase was
extracted two times with Et O (2 x 10 mL). The combined organic phases were dried
over MgSO and filtered. The solvent of the filtrate was removed in vacuo, and the solid
2
4
residue was recrystallized from nꢀhexane (ꢀ40 °C) to afford compound 1d as colourless
crystals. Yield: 3.75 g (73%). M.p.: 96 ºC. Microanalysis: found C 68.3, H 8.8%;
1
[
C
27
H
41ClGe (473.67)] requires C 68.5, H 8.7%. H NMR (C
6
D
6
, 400 MHz, 300 K): δ =
3
3
1
.08 (s, 18H, CMe
3
), 1.20 (d, JHH = 6.8 Hz, 6H, pꢀCHMe
2
), 1.39 (d, JHH = 6.7 Hz,
3
3
1
2H, oꢀCHMe
2
), 2.75 (hept, JHH = 6.8 Hz, 1H, pꢀCHMe
2
), 4.40 (hept, JHH = 6.7 Hz,
13
2
H, oꢀCHMe ), 7.17 (s, 2H, mꢀH). C NMR (C D , 101 MHz, 300 K): δ = 24.0 (pꢀ
2
6
6
CHMe
2
2 3 3 2
), 25.4 (oꢀCHMe ), 28.5 (CMe ), 30.4 (CMe ), 33.1 (oꢀCHMe ), 34.7 (pꢀ
CHMe
2
), 80.6 (GeC≡C), 115.9 (GeC≡C), 122.8 (mꢀC), 128.8 (ipsoꢀC), 152.4 (pꢀC),
–
1
1
1
δ
53.4 (oꢀC). IR (CsI plates, paraffin, cm ): 2187 s, 2155 s ν(C≡C); 1763 vw, 1690 vw,
599 s, 1558 m (aromatic ring); 1466 vs, 1379 vs (paraffin); 1319 s, 1304 sh, 1252 vs
(CH ); 1202 s, 1165 s, 1136 m, 1103 s, 1070 m, 1059 m, 1032 m, 1016 m, 957 m, 922
3
vs, 878 vs, 839 m, 752 vs ν(CC); 723 vs (paraffin); 648 w, 611 w, 569 vw, 554 w, 515
m, 486 vs ν(GeC), (CC), ν(GeCl). Mass spectrum (EI+, 20 eV, 298 K): m/z (%): 474
δ
2
8

Page 29 of 50
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03918E
+
+
+
t
+
(
1) [M] , 439 (5) [M – Cl] , 438 (5) [M – HCl] , 417 (8) [M – Bu] , 392 (10) [M – Hꢀ
t
+
t
+
t
C≡Cꢀ Bu] , 381 (7) [M – HCl – Bu] , 357 (22) [M – HCl – C≡Cꢀ Bu].
t
t
2 2
Synthesis of Ph Ge(Cl)[C(Al Bu )=C(H)ꢀ Bu] 2a
A solution of compound 1a (1.00 g, 2.91 mmol) in nꢀpentane (10 mL) was slowly added
t
at ꢀ78 ºC to a suspension of HAl Bu
2
(0.42 g, 2.96 mmol) in nꢀpentane (15 mL). The
mixture was allowed to warm to room temperature and stirred for 16 h. The solution
was concentrated and stored at ꢀ30 ºC to yield 2a as colourless crystals. Yield: 0.89 g
(63%). M.p.: 108 °C (Ar, sealed capillary). Microanalysis: found C 64.6, H 8.0%;
1
[
C
26
H
38AlClGe (485.61)] requires C 64.3, H 7.9%. H NMR (C
6
D
6
, 400 MHz, 300 K):
δ = 0.86 (s, 9H, C=CꢀCMe
3
), 1.35 (s, 18H, AlCMe
m, overlap, 4H, mꢀH), 7.12 (s overlap, 1H, C=CH), 7.76 (m, 4H, oꢀH). C NMR
, 101 MHz, 300 K): δ = 17.5 (br., AlCMe ), 29.2 (C=CHꢀCMe ), 30.7 (AlCMe
8.7 (C=CHꢀCMe ), 129.1 (mꢀC), 131.2 (pꢀC), 134.8 (oꢀC), 137.1 (ipsoꢀC), 140.7 (br.,
GeC=C), 167.6 (GeC=C). IR (KBr plates, paraffin, cm ): 1956 vw, 1884 vw, 1815 vw,
3
), 7.10 (m, overlap, 2H, pꢀH), 7.11
13
(
(
C
6
D
6
3
3
3
),
3
3
–1
1
1
1
6
767 vw, 1682 vw, 1653 vw, 1593 m, 1562 m (phenyl); 1437 vs, 1379 vs (paraffin);
337 sh, 1306 m, 1250 m (CH ); 1192 m, 1157 w, 1088 s, 1067 w, 1050 w, 1022 m,
001 m, 970 w, 937 m, 887 m, 847 w, 810 vs, 787 s ν(CC); 731 vs (paraffin; 696 vs,
75 m (phenyl); 581 s, 542 m, 501 m, 455 m, 412 s ν(AlC), ν(GeC), (CC), ν(GeCl).
δ
3
δ
t
+
Mass spectrum (EI+, 25 eV, 298 K): m/z (%): 429 (52) [M – Bu] .
t
t
t
Synthesis of Ph(Cl)Ge(C≡Cꢀ Bu)[C(Al Bu )=C(H)ꢀ Bu] 2b
2
A solution of compound 1b (2.47 g, 7.11 mmol) in nꢀpentane (10 mL) was slowly addꢀ
t
ed at ꢀ78 ºC to a suspension of HAl Bu
2
(1.01 g, 7.11 mmol) in nꢀpentane (40 mL). The
2
9