Reaction of a germylene, stannylene, or plumbylene with trimethylaluminum and trimethylgallium: Insertion into Al-C or Ga-C bonds, a reversible metal-carbon insertion equilibrium, and a new route to diplumbenes

Article abstract of DOI:10.1021/ic502824w

The reaction of the tetrylenes Ge(Ar^Me₆)₂, Sn(Ar^Me₆)₂, and Pb(Ar^Me₆)₂ [Ar^Me₆ = C₆H₃-2,6-(C₆H₂-2,4,6-(CH₃)₃)₂] with the group 13 metal alkyls trimethylaluminum and trimethylgallium afforded (Ar^Me₆)₂Ge(Me)AlMe₂ (1), (Ar^Me₆)₂Ge(Me)GaMe₂ (2), and (Ar^Me₆)₂Sn(Me)GaMe₂ (3) in good yields via insertion reaction routes. In contrast, the reaction of AlMe₃ with Sn(Ar^Me₆)₂ afforded the [1.1.1]propellane analogue Sn₂{Sn(Me)Ar^Me₆}₃ (5) in low yield, and the reaction of AlMe₃ or GaMe₃ with Pb(Ar^Me₆)₂ resulted in the formation of the diplumbene {Pb(Me)Ar^Me₆}₂ (6) and AlAr^Me₆Me₂ (7) or GaAr^Me₆Me₂ (8) via metathesis. The reaction of Sn(Ar^Me₆)₂ with gallium trialkyls was found to be reversible under ambient conditions and analyzed through the reaction of Sn(Ar^Me₆)₂ with GaEt₃ to form (Ar^Me₆)₂Sn(Et)GaEt₂ (4), which displayed a dissociation constant K_diss and ΔG_diss of 8.09(6) × 10^-3 and 11.8(9) kJ mol^-1 at 296 °C. The new compounds were characterized by X-ray crystallography, NMR (¹H, ¹³C, ¹¹⁹Sn, and ²⁰⁷Pb), IR, and UV-vis spectroscopies.

Full text of DOI:10.1021/ic502824w

Article
pubs.acs.org/IC
Reaction of a Germylene, Stannylene, or Plumbylene with
Trimethylaluminum and Trimethylgallium: Insertion into Al−C or
Ga−C Bonds, a Reversible Metal−Carbon Insertion Equilibrium, and a
New Route to Diplumbenes
Jeremy D. Erickson, James C. Fettinger, and Philip P. Power*
Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616, United States
S
* Supporting Information
Me
6
ABSTRACT: The reaction of the tetrylenes Ge(Ar )2,
Me
Me
Sn(Ar )2, and Pb(Ar )2 [ArMe = C6H3-2,6-(C6H2-2,4,6-
6
6
6
(CH3)3)2] with the group 13 metal alkyls trimethylaluminum
Me
6
and trimethylgallium afforded (Ar )2Ge(Me)AlMe2 (1),
Me
(Ar )2Ge(Me)GaMe2 (2), and (ArMe )2Sn(Me)GaMe2 (3)
6
6
in good yields via insertion reaction routes. In contrast, the
Me
6
reaction of AlMe3 with Sn(Ar )2 afforded the [1.1.1]-
Me
6
propellane analogue Sn2{Sn(Me)Ar }3 (5) in low yield, and
Me
6
the reaction of AlMe3 or GaMe3 with Pb(Ar )2 resulted in the
Me
6
formation of the diplumbene {Pb(Me)Ar }2 (6) and
Me
Me
Me
6
6
6
AlAr Me2 (7) or GaAr Me2 (8) via metathesis. The reaction of Sn(Ar )2 with gallium trialkyls was found to be reversible
Me
Me
6
6
under ambient conditions and analyzed through the reaction of Sn(Ar )2 with GaEt3 to form (Ar )2Sn(Et)GaEt2 (4), which
displayed a dissociation constant Kdiss and ΔGdiss of 8.09(6) × 10−3 and 11.8(9) kJ mol−1 at 296 °C. The new compounds were
characterized by X-ray crystallography, NMR (1H, 13C, 119Sn, and 207Pb), IR, and UV−vis spectroscopies.
metalylene behaves as a donor to a trivalent group 13 metal
compound. Furthermore, anionic N-heterocyclic gallium
carbene analogues have been used in the formation of Lewis
adducts by acting as both electron pair donors19,20 and electron
pair acceptors21 in reactions with digermenes and distannenes
to give products with Ge−Ga and Sn−Ga bonds. Adduct
formation between various stannates and trimethylgallium or
-indium has also been observed.22
INTRODUCTION
■
Heavy group 14 metalylenes (:ER2, E = Si, Ge, Sn, or Pb; R =
bulky monodentate ligands) are Si−Pb analogues of carbenes.
However, they possess a longer history as stable species owing
mainly to the greater stability of their nonbonded electron
pairs. The first example, Sn{CH(SiMe3)2}2, was synthesized by
Lappert and co-workers in 1973,1 and several hundred stable
metalylenes are now known. They have an extensive chemistry
and have been shown to undergo a wide variety of reactions,
some of which resemble those of carbenes.2−5 They have
played a central role in the development of modern main group
chemistry, and more recently, they have attracted interest
because of their reactivity toward small molecules such as H2
and for their catalytic activity.6,7 Nonetheless, their reactions
with other heavier main group organometallic derivatives such
as group 13 metal alkyls are poorly studied, and the number of
well-characterized species with bonds between the heavier
members of group 14 elements germanium, tin, and lead and
group 13 metals is relatively low. The majority of the latter have
been obtained via simple salt metathesis reactions of group 13
chlorides with lithium salts of organic group 14 molecules, and
a number of Ge−Al,8−11 Ge−Ga,12−16 and Sn−Ga17,18 bonded
species have been obtained by this route. There also exist group
13−14 Lewis acid−base adducts, in which the group 14
Herein we demonstrate that heavy group 14 metalylenes
react with group 13 metal alkyls MMe3 (M = Al or Ga) to form
(ArMe )2E(Me)(MMe2) via insertion into the M−C bond. This
6
approach represents a simple and new route to bond types
previously obtainable only through salt metathesis and Lewis
adduct formation. The products obtained include a germyl−
alane and germyl−gallane, as well as a stannyl−gallane. The
insertion of Sn(ArMe )2 into GaMe3 and GaEt3 was found to be
6
reversible at room temperature. In contrast, the reaction of the
plumbylene Pb(ArMe
) with AlMe3 or GaMe3 results in
2
6
exchange reactions and the formation of the diplumbene
{Pb(Me)ArMe
}
and (ArMe )AlMe2 or (ArMe )GaMe2. Un-
6
6
6
2
expectedly, a 1:1 mixture of the diarylstannylene Sn(ArMe
)
6
2
Received: November 25, 2014
© XXXX American Chemical Society
A
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Article
with trimethylaluminum gave no reaction. However, upon
treatment of Sn(ArMe )2 with 2 equiv of AlMe3 and heating to
6
85 °C, a reaction occurred to give a rare pentastanna-
[1.1.1]propellane Sn2{Sn(Me)ArMe }3 in low yield.
6
RESULTS AND DISCUSSION
■
The behavior of heavy group 14 metalylenes as Lewis acids or
bases or their ability to undergo oxidative addition/insertion
reactions is characteristic of their chemistry. Insertions into
numerous single bonds such as H−H,23−25 C−C,26 C−O,27
O−H,28,29 P−P,30 and N−H5,31 have been described in
previous publications. Recent investigations involving the
reactions5,24,27,32 of the metalylenes Ge(ArMe )2 and Sn(Ar
)
Me6
6
2
with hydrogen, ammonia, or hydrazine provide examples of
insertion reactions that may or may not be accompanied by aryl
elimination. In reactions with Bronsted acids, however, it was
shown that both germylenes and stannylenes insert into O−H
and F−H bonds.33 In contrast, insertion reactions into metal−
Figure 1. Thermal ellipsoid plot (30% probability) of 1. H atoms are
not shown. Selected distances and angles are given in Table 1.
carbon bonded species have not been well investigated.
Me
6
Synthesis. Treatment of Ge(Ar )2 with trimethylalumi-
num or trimethylgallium yielded the products 1 and 2 in high
yield (Scheme 1). The reaction probably occurs through the
Scheme 1. Reactions of Group 14 Metalylenes with Parent
Group 13 Alkyls To Form Products 1, 2, and 3
Figure 2. Thermal ellipsoid plot (30% probability) of 2. H atoms are
not shown. Selected distances and angles are given in Table 1.
Me
6
initial formation of the Lewis acid−base adduct (Ar )2Ge·
MMe3 (M = Al or Ga) with subsequent insertion of the
germanium atom into one of the M−C bonds of the group 13
alkyl. Materials containing group 13−14 bonds exist in the solid
state and have been used in such applications as (AlGeIn)N
type thin-film light-emitting diodes,34 semiconductors,35 and
gallium-doped germanium layer superconductors.36 In molec-
ular species, however, only a handful of stable compounds with
heavier group 13−14 element bonds are known, some of which
have been structurally characterized (see below, Table 2).
The synthesis of 1 and 2 represent the first synthesis of a
germyl−alane and germyl−gallane from germylenes. Com-
pounds 1 and 2 have a four-coordinate germanium and three-
coordinate aluminum or gallium atoms as shown by their X-ray
crystal structures in Figures 1 and 2 (see below). The bulky
terphenyl substituents of the germanium are sufficient to
stabilize the three coordination at the group 13 metal. Product
1 is the second structurally characterized germyl−alane of this
solution initially becomes colorless, it slowly acquired a slight
purple color similar to that of the stannylene precursor upon
standing. Crystallization from pentane resulted in colorless
crystals and a pale purple mother liquor. The crystals were
confirmed to be the insertion product 3 by X-ray crystallog-
raphy (see below, Figure 3). When the colorless crystals of 3
were dissolved in deuterated benzene for NMR spectroscopic
geometry; an earlier example was reported in 2009 when Noth
̈
and co-workers synthesized tmp2AlGe(Me2)SiMe3 (tmp =
2,2,6,6-tetramethylpiperidino) by reaction of LiGe(Me)2Si-
(tBu)3 with tmp2AlCl.11
Reaction of Sn(ArMe
) with AlMe3 did not produce a
2
6
product analogous to 1 but instead gave the pentastanna-
[1.1.1]-propellane Sn2{Sn(Me)ArMe }3, 5, in low yield. In
6
contrast, the reaction of Sn(ArMe )2 with GaMe3 produced 3,
6
Figure 3. Thermal ellipsoid plot (30% probability) of 3. H atoms are
the tin analogue of 2 (Scheme 1). However, while the reaction
not shown. Selected distances and angles are given in Table 1.
B
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As noted above, the treatment of Sn(ArMe )2 with AlMe3
under the same conditions as those employed for the synthesis
of 1−3 afforded no reaction. Heating the reactants to reflux in
toluene also produced no reaction. However, the addition of a
second equivalent of AlMe3 and continued heating to 85 °C
afforded a low yield of a rare example of a pentastanna-
[1.1.1]propellane, 5 (see below, Figure 4). Tin−propellane
6
analysis, the solution slowly assumed a purple color resembling
that of the diarylstannylene precursor.
Colorless crystals of 3 were dissolved in hexanes, and the
solution was dried under reduced pressure. Repetition of this
cycle resulted in a progressively darker purple color of the
remaining solid. Eventually a pure purple solid was obtained
and crystallized from pentane. These purple crystals were
confirmed by X-ray crystallography to be Sn(ArMe )2. In effect,
6
dissolving 3 results in its dissociation to GaMe3 and Sn(ArMe )2,
6
which produces the purple color. This process can be reversed
by the addition of excess GaMe3. When the resulting colorless
solution is evaporated the removal of excess GaMe3 and
subsequent dissolution again regenerates a purple hue. The
addition of further GaMe3 regenerates the original colorless
solution of 3. This cycle of addition and removal of GaMe3 was
repeated five times with no observable difference between
Me
6
cycles. Reactions of Sn(Ar )2 with the less volatile gallium
Me
6
trialkyl GaEt3 afforded (Ar )2Sn(Et)GaEt2, 4, for which
variable-temperature 1H NMR spectroscopy permitted a
determination of the reaction equilibrium parameters (see
below).
While there exist reversible valence equilibrium reactions
between heavy main group species, most of these involve
monomer−dimer equilibrium of homometallic bridged species
dimerization or the formation and breaking of homonuclear E−
E bonds such as that observed for heavier element olefin
analogues.37−40 An insertion equilibrium similar to those in
Scheme 2 has previously been observed between Ge(C6H3-2,6-
Figure 4. Thermal ellipsoid plot (30% probability) of 5. H atoms and
mesityl substituents are not shown. Selected distances and angles are
given in Table 1.
analogues were first synthesized by Sita and co-workers via the
thermolysis of the trimer Sn3R6 (R = 2,6-diethylphenyl)43,44
and are of interest for their singlet diradical character.45,46
Product 5 was observed to be both air and moisture stable.
Scheme 2. Dissociation of 3 or 4 Occurs at Room
Temperature in Toluene
The reactions of Pb(ArMe )2 with AlMe3 and GaMe3 afforded
6
no Pb−Al or Pb−Ga bonded products or indeed any simple
oxidative addition products analogous to those seen for 1, 2, 3,
and 4. Instead, both reactions yielded the diplumbene, 6, as
Me6
well as ArMe AlMe2, 7, or Ar GaMe2, 8, coproducts (Scheme
6
Mes)2 or Cl2Al(NSiMe3)2P and P4 but required UV radiation
3). These reactions thus represent a new synthetic route to
diplumbenes. The few diplumbenes currently known37−40 have
been synthesized by the reaction of PbX2 (X = Cl, Br) with 2
equiv of a lithium derivative of a bulky alkyl, aryl, or silyl group
or by reaction of a halogen-bridging plumbylene dimer with an
alkyl Grignard reagent, followed by dimerization. The products
6, 7, and 8 can be accounted for by assuming an initial insertion
into the M−C bond of the group 13 alkyl. However, due to
lead’s tendency to favor a 2+ oxidation state,47 there is likely an
equilibrium between the reactants and the insertion product 5.
The overall reaction is in effect a metathesis of methyl and
terphenyl groups between aluminum and lead. The resulting
to rerelease the P4 cage.41 Unfortunately, both UV−vis and H
1
NMR variable-temperature experiments were unsuccessful in
determining the equilibrium parameters of solutions of 3
between −78° and 85 °C. This is because the reaction mixture,
even in a sealed tube, could not be made to regenerate the
inserted product 3 once a dissociation equilibrium was
established. This can be attributed to the high volatility of
GaMe3 leading to an equilibrium between the solution and the
vapor phase. To remedy this problem, the less volatile
triethylgallium42 was tested and also found to undergo a
reversible reaction with Sn(ArMe )2. The equilibrium constant
6
K
diss and ΔGdiss values at 296 K for the dissociation reaction of
plumbylene (ArMe )PbMe has insufficient bulk to remain
6
4 (Scheme 3) were found to be 8.09(6) × 10−3 and 11.8(9) kJ/
mol, respectively, for an initial concentration of 0.030(3) M of
4 in toluene.
monomeric and dimerizes via Pb−Pb bonding, yielding the
diplumbene 6 (Scheme 3).
The first well-characterized diplumbene, {Pb(Si(SiMe3)3)R}2
(R=C6H2-2,4,6-(CF3)3), and later the diplumbene {Pb(2,4,6-
tBu3C6H2)(CH2C(CH3)2-3,5-tBu2C6H3)}2 were obtained by
Klinkhammer via a ligand migration process similar to that of
the reaction described for 6−8, lending further credence to the
proposed reaction pathway.40,48 Furthermore, it is known that
diplumbylenes exist in an equilibrium with their dissociated
Scheme 3. Novel Synthesis of a Diplumbene (6) and
Formation of Corresponding Aryldimethylaluminum (7)
and -Gallium (8) Compounds
monomers at room temperature.38,40,48 Compound 6 differs
i
4
from the related previously reported species {PbMe(ArPr )}2
i
4
(ArPr = C6H3-2,6-{C6H3-2,6-Pr 2}2)40 in that it has a less bulky
i
C
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Table 1. Selected Bond Distances (Angstroms) and Angles (degrees) for Compounds 1−3 and 5−8
1
2
3
5
Ge(1)−Al(1)
Ge(1)−C(1)
Al(1)−C(2)
Al(1)−C(3)
2.4851(5)
Ge(1)−Ga(1)
2.4443(3)
Sn(1)−Ga(1)
2.6228(2)
2.1553(18) Sn(1)−Sn(1A)
1.980(2) Sn(1)−C(1)
1.9639(19) Sn(1)−C(2)
Sn(1)−Sn(2)
2.8515(3)
3.4878(5)
2.172(4)
1.9776(15) Ge(1)−C(1)
1.9622(18) Ga(1)−C(2)
1.9528(18) Ga(1)−C(3)
1.9741(19) Sn(1)−C(1)
1.983(2)
1.969(2)
Ga(1)−C(2)
Ga(1)−C(3)
2.193(3)
C(1)−Ge(1)−Al(1) 102.66(5)
Ge(1)−Al(1)−C(2) 109.23(6)
Ge(1)−Al(1)−C(3) 129.05(6)
C(1)−Ge(1)−Ga(1) 102.04(6)
Ge(1)−Ga(1)−C(2) 111.00(7)
Ge(1)−Ga(1)−C(3) 131.65(5)
C(1)−Sn(1)−Ga(1) 107.67(5)
Sn(1)−Ga(1)−C(2) 113.01(7)
Sn(1)−Ga(1)−C(3) 126.67(6)
Sn(1)−Sn(2)−C(2)
Sn(2)−Sn(1)−Sn(2A)
Sn(1)−Sn(2)−Sn(1A)
C(1)−Sn(2)−C(2)
C(1)−Sn(2)−Sn(1)
8
120.95(6)
86.502(8)
75.406(10)
107.40(14)
114.84(8)
C(2)−Al(1)−C(3)
115.50(8)
C(2)−Ga(1)−C(3)
114.68(9)
C(2)−Ga(1)−C(3)
118.16(9)
6
7
Pb(1)−Pb(1A)
Pb(1)−C(1)
Pb(1)−C(2)
C(1)−Pb(1)− Pb(1A)
C(2)−Pb(1)− Pb(1A)
C(1)−Pb(1)−C(2)
3.2854(5)
2.272(5)
Al(1)−C(1)
Al(1)−C(2)
Al(1)−C(3)
C(1)−Al(1)− C(2)
C(1)−Al(1)− C(3)
C(2)−Al(1)− C(3)
1.9450(15)
Ga(1)−C(1)
Ga(1)−C(2)
Ga(1)−C(3)
C(1)−Ga(1)− C(2)
C(1)−Ga(1)− C(3)
C(2)−Ga(1)− C(3)
1.953(3)
1.969(2)
1.9450(15)
1.9698(19)
121.18(10)
119.41(5)
2.335(4)
1.970(2)
102.38(13)
127.70(9)
95.23(16)
122.52(11)
118.37(9)
119.08(10)
119.41(5)
Table 2. Selected Bond Distances (Angstroms) Between Aluminum or Gallium and Germanium or Tin in Molecules Related to
a
1−3
Ge−Al
Ge−Ga
1
2.4851(5)
2.449(4)
2.545(1)
2.515(1)
2.532(2)
2.526(2)
2
2.4443(3)
52
Ge(AlCl2·OEt2)4
tmp2AlGe(Me2)SiMe3
Ph3GeAlMe2(OEt2)11
11
16
Ga2(Ge(I)ArMe
)
3
2.4934(6)
6
Me3SiMes2GeGa{[N(Ph)CH}2}20
K{Mes2GeGa[N(Ph)CH]2)}20
(Giso)Ge−Ga{N(Ph)CH)2}19
2.431(1)
11
10
[(Ph3Ge)3AlH]Li(THF)3
2.4600(8)
[(Ph3Ge)3AlMe]Li(Et2O)5
2.5157(7)
12
Ga4[Ge(SiMe3)3]4
2.582(5); 2.468(6)
Sn−Al
Sn−Ga
53
Cl3Al(SnNBut)4
Yb{Sn(2-pyR)3AlMe3}2
2.78(1)
2.8317(10)
3
2.6227(3)
2.6228(7)
2.7196(11)
2.7186(6)
22
[Me2Sn{ClGa(DDP)}2]54
[{Li(THF)Sn(2-pyR)3}GaEt3]20
[K(tmeda)][Sn{CH(SiMe3)2Ga-{[N(Ph)CH]2}]21
a
tmp = 2,2,6,6-tetramethylpiperidino, Giso = [Pri2NC{NPh}2]−, tmeda = tetramethylethylenediamine, pyR = C5H3N-5-Me, DDP = 2-{(2,6-
diisopropylphenyl)amino}-4-{2,6-diisopropyl)imino}-2-pentene
terphenyl ligand as a result of replacement of diisopropylphenyl
groups attached to the central aryl ring of the terphenyl ligand
by mesityl groups. Yet the Pb−Pb bond in 6 is longer
contains a near-planar three-coordinated aluminum, in which
aluminum is substituted by two amido substituents and
expected to have a smaller effective ionic radius owing to the
more electronegative amido substituents. This should result in a
shorter Al−Ge bond. It is a possibility that the longer and less
electronegative SiMe3 substituent increases the steric pressure
and electron density at germanium such that a longer Ge−Al
bond is produced. The Ge−Ga and Sn−Ga bond lengths in 2
and 3 are within the range of known bond lengths for Ge−Ga
and Sn−Ga bonded molecular species displayed in Table 2. We
note that the Ge−Ga bond (2.4443(3) Å) in 2 is shorter than
the Ge−Al bond (2.4851(5) Å) in 1, and this is consistent with
the smaller covalent radius of gallium despite its greater atomic
number.51
The sum of the interligand angles at the aluminum atom in 1
is 353.8°. The metal is displaced 0.367 Å out of the C(2)−
C(3)−Ge(1) plane toward a flanking ring of a terphenyl ligand.
The distance from the aluminum atom to the plane of the
flanking aryl ring is 2.679 Å, suggesting weak coordination,
which may cause the aluminum atom’s displacement from
planar coordination. The weaker aryl interactions with gallium
in 2 and 3 are consistent with a more planar gallium
coordination. The sums of the angles at the gallium atom for
2 and 3 are 357.6° and 357.9°, the displacement from the
(3.2854(5) vs 3.1601(6) Å), possibly as a result of greater
i
4
attractive dispersion forces in {PbMe(ArPr )}2, similar to those
seen in {Pb(Si(SiMe3)3)R}2 and related species.50 At room
49
i
4
temperature {PbMe(ArPr )}2 dissociates into two monomeric
plumbylenes, as shown by a chemical shift of 8738 ppm in its
207Pb NMR spectrum. Similarly, although 6 is a dimer in the
crystalline state, in solution it is the monomer, as evidenced by
its similar 207Pb NMR chemical shift of 8248 ppm.
Structures. The crystal structures for 1, 2, and 3 are shown
in Figures 1, 2, and 3, respectively, selected bond lengths and
angles are given in Table 1, and selected structural parameters
for related compounds are given in Table 2. The structures of
1−3 are very similar in their overall configuration. While the
Ge−Al bond length in 1 (2.485(5) Å) is longer than the 2.41 Å
predicted by the sum of the covalent radii of aluminum and
germanium,51 it is shorter than most of the related compounds
listed in Table 2. Only Ge(AlCl2·Et2O)4, which has
tetrahedrally coordinated germanium and aluminum, contains
shorter Ge−Al bonds (4.449(4) Å).52 However, only one other
compound, i.e., tmp AlGe(Me )SiMe of Noth and co-workers,
̈
2
2
3
D
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C(2)−C(3)-[Ge(1) or Sn(1)] plane is 0.199 and 0.183 Å, and
the distance from the gallium to the flanking aryl ring is 2.829
and 2.854 Å, respectively.
In the crystal structure of 5 (Figure 4), the five tin atoms are
arranged in a trigonal bipyramidal fashion (Figure 4). The axial
tins have no organic substituent, whereas the equatorial tins
each carry a methyl and a terphenyl ligand. The Sn(1)−Sn(1A)
bridgehead distance is 3.4878(5) Å, which is too long for strong
bonding. The distance is longer than the corresponding
distance in two compounds given in Table 3: Sn5[C6H3-2,6-
Table 4. Comparison of Selected Bond Distances
(Angstroms) and Angles (degrees) for {PbMe(ArPr )}2 and 6
i
4
i
4
{PbMe(ArPr )}2
6
Pb(1)−Pb(1A)
Pb(1)−C(1)
Pb(1)−C(2)
C(1)−Pb(1)−C(2)
C(1)−Pb(1)−Pb(1A)
C(2)−Pb(1)−Pb(1A)
∑Pb(1)°
3.1601(6)
2.280(6)
2.318(6)
91.8(2)
3.2866(5)
2.273(6)
2.329(6)
95.3(2)
109.35(16)
121.51(14)
322.66
102.33(17)
127.71(13)
325.34
Table 3. Comparison of Related Sn−Sn Distances in Known
Pentastanna[1.1.1]propellanes
bridgehead−bridgehead
Sn(1)−Sn(2)
distance (Å)
distance (Å)
5
3.4878(5)
3.367(1)
3.42
2.8515(3)
2.852(1)
2.852(1)
43
Sn5[2C6H3-2,6-Et]6
Sn5(C6H3-2,6-{O-
55
iPr}2)6
44
Et2]6 synthesized by Sita and co-workers and Sn5(C6H3-2,6-
{O-iPr}2)6 synthesized by Drost and co-workers.55 The sum of
the covalent radii of two tin atoms is 2.78 Å,51 and in known
pentastannapropellanes single Sn−Sn bonds between equato-
rial and axial tins are in the range of 2.82−2.87 Å,43,44,55,55 and
the Sn(1)−Sn(2) distance of 1 (2.8515(3) Å) is within these
limits (Table 3).
Figure 6. Thermal ellipsoid plots (30% probability) of 7 (left) and 8
(right). H atoms are not shown. Selected distances and angles are
given in Table 1.
ppm. For 1 and 2, the two methyl groups are in different
environments, as are all mesityl methyl groups. This persists
even to a temperature of 55 °C, with no change in signal shape.
The sharpness of the signals suggests no intermolecular or
intramolecular methyl exchange takes place.
Only one signal is observed for the two group 13 metal
methyl substituents of 3 at room temperature. The rotational
barrier in this species is probably lower because it has the
longest group 13−14 bond length as well as the lowest degree
of steric crowding. At lower temperatures the signal broadens
and splits beginning at 15 °C, becoming fully resolved at −10
°C. The energy barrier for this phenomenon is 55(4) kJ mol−1
based on the coalescence temperature of 15 °C and final
Figure 5. Thermal ellipsoid plot (30% probability) of 6. H atoms are
not shown. Selected distances and angles are given in Table 1.
separation of the Ga−Me signals of 180 Hz at −10 °C.57
A
As noted above, the crystal structure of 6 (Figure 5) can be
i
4
compared to that of the diplumbene {PbMe(ArPr )}2 previously
lower temperature produces broadening of the three mesityl
methyl signals for 3 at 1.84, 1.93, and 2.17 ppm beginning at
−30 °C. At room temperature, when the mesityl methyl signals
are fully coalesced, there is a rapid exchange of aryl rings
coordinating to the gallium center. At lower temperatures, this
movement is slowed, leading to signal broadening. At a low
enough temperature, transformation into a multitude of signals
synthesized in this laboratory.40 A comparison of key structural
parameters of these two compounds is given in Table 4. The
i
4
Pb−Pb bond length in 6 is longer than that of {PbMe(ArPr )}2
despite the use of a less crowding terphenyl ligand. The shorter
i
4
Pb−Pb bond length in {PbMe(ArPr )}2 may be the result of
1
increased dispersion force attraction owing to the greater
as observed in the H spectra of 1 and 2 consistent with
number of flanking ring substituents.49,50
restricted rotation is expected, but our instrument lacked the
capacity to descend to low enough temperature to resolve these
signals.
Spectroscopy. As is apparent from Figures 1−3, the AlMe2
or GaMe2 moieties lie in cavities formed by flanking mesityl
rings from two different terphenyl ligands which inhibit
rotation around the Ge−Al, Ge−Ga, and Sn−Ga bond. In
1
The H NMR spectroscopy of 4 affords a similar pattern to
that of 3, and only one set of signals is observed for identical
functionalities of each ligand. A greater degree of signal
broadening is observed in 4, but the sharpness of the signals
1
the H NMR spectra of compounds 1, 2, and 3, the group 13
metal methyl substituent signals appear near or upfield of 0
E
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Inorganic Chemistry
Article
Figure 7. Variable-temperature 1H NMR of (ArMe )2Sn(Et)GaEt2 (4). Signals at 1.90 and 1.80 ppm correspond to C6H2−Me groups of the aryl rings
6
of 4. Quartet at 0.40 ppm is due to the CH2 group of GaEt3. Signal at 1.86 ppm is due to the C6H2−Me groups of Sn(ArMe )2. Remaining signals are
6
identified in the Experimental Section.
increases with increasing temperature, as also observed for 3. At
ppm in the 13C spectrum because of the electropositive
character of the lead atoms. The 207Pb signal is observed at
room temperature in solution, dissociation of 4 into Sn(ArMe
)
6
2
8248 ppm, which lies in a region similar to that of
and GaEt3 is observed. Upon increasing temperature (Figure
7), the observed signals for 4 lose intensity until fully
disappearing around ca. 360 K. This is accompanied by the
i
4
{PbMe(ArPr )}2 and Pb(CH2C6H4-4-Pr )ArPr 3, which display
207Pb signals at 8738 and 8550 ppm, respectively.40 Product 6
has a red color with a λmax absorption at 466 nm and a shoulder
i
i
appearance of signals for Sn(ArMe )2 and GaEt3. This process is
6
reversible, and upon cooling to room temperature, 4 is
regenerated. A van’t Hoff analysis of the variable-temperature
1H NMR spectra affords an enthalpy and entropy of association
of 52(4) kJ mol−1 and −134(11) J mol−1 K−1 as well as an
equilibrium constant of 8.09(6) × 10−3 and ΔGdiss of 11.8(9) kJ
mol−1 at 0.03 M concentration and 296 °C.
feature at 550 nm, which is similar to the red color of
i
{PbMe(ArPr )}2 which has a λmax at 468 nm.
4
1
The H NMR spectra of 7 and 8 are nearly identical, as
would be expected from their similar structure. The only
significant differences lie in the methyl group chemical shifts.
The aluminum methyl signals of 7 appear at −1.69 and −7.07
The 1H NMR spectrum of 5 provides no evidence of
paramagnetism. In addition, there is no evidence of an Sn−H
absorption in the IR spectrum.58 This combined with the
crystallographic data suggests that 5 exists as a singlet
diradical.45 Compound 5 was most soluble in deuterated
benzene and toluene but slowly reacted with these solvents
upon standing at room temperature to yield a black precipitate.
This limited the time in which samples could be analyzed,
making 13C and 119Sn NMR spectroscopy exceedingly difficult.
Attempts to obtain 13C NMR and 119Sn NMR spectra were
recorded in deuterated cyclohexane to avoid solvent reactivity,
but the solubility of 5 was so low that no signals were ever
observed. Product 5 is pale purple in color with a λmax
absorption at 560 nm.
1
ppm in their H and 13C NMR spectra, while the gallium
methyl signals of 8 appear at −0.26 and −0.08, ppm
respectively, consistent with the more electropositive character
of aluminum in comparison to gallium.60
EXPERIMENTAL SECTION
General Experimental Procedures. Standard Schlenk techniques
■
were used in the synthesis of all products under strictly anhydrous and
oxygen-free conditions. All solvents were dried over NaK. Ge(ArMe )2,
6
Me6
Sn(ArMe )2, and Pb(Ar )2 were synthesized according to published
6
methods.60 AlMe3 (1.4 M in heptane) and GaMe3 were used as
received. 1H and 13C NMR spectroscopies were carried out on a
Bruker 600 or 500 MHz spectrometer and referenced to residual
solvent peaks. 119Sn and 207Pb NMR spectroscopies were carried out
on a Bruker 500 MHz spectrometer and referenced to SnPh4 (−128.8
ppm) and PbMe4 (0 ppm). Melting points were measured in glass
capillary tubes sealed under argon using a Mel-Temp II apparatus.
Infrared spectroscopy was carried out on a Bruker Tensor 27 IR
spectrometer as a Nujol mull. UV−vis data were recorded on an Olis
14 UV/vis/NIR Spectrometer.
The diplumbene 6 obtained by the route described in
Scheme 3 could not be completely purified by recrystallization
from the reaction mixture to give NMR spectra that were
completely uncontaminated by 7 or 8. The signals from the
Me
6
reaction mixture of Pb(Ar )2 and GaMe3 were assigned by
(ArMe )2Ge(Me)AlMe2 (1). Ge(ArMe
)
(0.70 g, 1 mmol) was
6
6
recording a spectrum for 7 synthesized independently by the
2
Me
dissolved in toluene (40 mL) and cooled to ca. −78 °C. A 1.4 M
solution of AlMe3 in heptane (0.79 mL, 1.1 mmol) was diluted with
toluene (20 mL) and added dropwise. The mixture was stirred and
allowed to warm to room temperature overnight, which resulted in a
6
reaction of Me2GaCl with LiAr . This enabled the assignment
of the signals of 6 and 8 in a simple manner. The lead−methyl
1
signals are upfield at 0.19 ppm in the H spectrum and at 0.58
F
DOI: 10.1021/ic502824w
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
color change from purple to colorless. The solvents were removed
under reduced pressure, and the resulting white solid was recrystallized
purple solution. Toluene and excess GaEt3 were removed under
reduced pressure to afford 4 as a white solid. Crystallization from
pentane yielded colorless crystals from a pale purple solution. Yield:
0.3832 g, 91.9%. Mp: White solid gradually turns purple; melts 52−58
°C giving a purple liquid. 1H NMR (600 MHz, PhMe-d8, 25 °C, ppm):
0.54 (m, 4H, Ga{CH2CH3}2), 0.83 (t, JHH = 7.9 Hz, 3H, SnCH2CH3),
0.88 (t, JHH = 8.0 Hz, 2H, SnCH2CH3), 1.15 (t, JHH = 8.0 Hz, 6H,
Ga{CH2CH3}2), 1.81 (s, 12H, C6H2−Me), 1.90 (s, 12H, C6H2−Me),
2.18 (s, 12H, C6H2−Me), 6.68 (s, 4H, m-C6H3) 6.72 (s, 8H, m-C6H2),
7.09 (t, JHH = 7.5 Hz, 2H, p-C6H3). 119Sn NMR (186 MHz, C6D6, 25
°C, ppm): −146. IR: 2900 (br), 2700 (w), 1450 (s), 1370 (s), 1250
(w) 1145 (w), 795 (m), 710 (m).
from hexanes to give colorless crystals of 1. Yield: 0.481 g, 62%. Mp:
1
168 °C (decomposition to Ge(ArMe )2). H NMR (600 MHz, C6D6,
6
25 °C, ppm): −0.76 (s, 3H, Al−Me), −0.45 (s, 3H, Al−Me), −0.12 (s,
3H, Ge−Me), 1.48 (s, 3H, p-Me), 1.64 (s, 3H, p-Me), 1.76 (s, 3H, p-
Me), 1.84 (s, 3H, p-Me), 1.91 (s, 3H, o-Me), 1.92 (s, 3H, o-Me), 1.98
(s, 3H, o-Me), 1.99 (s, 3H, o-Me), 2.10 (s, 3H, o-Me), 2.15 (s, 3H, o-
Me), 2.23 (s, 3H, o-Me), 2.28 (s, 3H, o-Me), 6.59 (m, 2H, m-C6H2),
6.67 (m, 1H, m-C6H2), 6.75 (m, 7H), 6.80 (s, 1H, m-C6H2), 6.95 (d,
JHH = 7.5 Hz, 1H, p-C6H3), 7.05 (t, JHH = 7.5 Hz, 1H, p-C6H3), 7.09 (t,
JHH = 7.5 Hz, 1H, p-C6H3). 13C NMR (150 MHz, C6D6, 25 °C, ppm):
−3.82, −3.70 (Al−Me), 4.10 (Ge−Me), 20.69, 20.98, 21.56, 22.04,
22.46, 22.81, 22.88, 22.97, 23.66, 23.84 (p- and o-Me); 127.54, 128.29,
128.33, 128.52, 128.99, 129.14, 130.13, 130.25, 130.95, 131.06, 131.56,
132.25, 133.14, 134.78, 135.86, 135.91, 136.07, 136.12, 136.41, 136.56,
136.78, 138.41, 138.84, 139.20, 139.85, 142.12, 142.48, 143.88, 144.87,
145.47, 146.27, 147.35, 148.75, 150.83, 151.57 (Ar). IR: 2900 (br),
1600 (w), 1450 (s), 1370 (s), 1290 (w), 1250 (s), 1075 (br), 1010
Sn2{Sn(Me)ArMe
}
(5). Sn(ArMe
)
(0.746 g, 1 mmol) was
6
6
3
2
dissolved in toluene (40 mL), and to this was added an excess of
AlMe3 (1.4 M in heptane, 1.6 mL, 2.2 mmol) dissolved in 20 mL of
toluene dropwise at room temperature. The resulting solution was
heated to ca. 85 °C and stirred overnight, yielding a pale red/purple
solution. The solvents were removed under reduced pressure, and the
resulting dark solid was extracted into hexanes. Dark purple crystals of
5 were produced at −28 °C. Yield: 13 mg. Mp: gradual decomposition
above 250 °C, solid did not melt below 300 °C. IR: 2900 (br), 2700
(w), 1440 (s), 1365 (s), 1290 (w), 1250 (w), 1145 (w), 1075 (br),
1010 (br), 935 (w), 870 (w), 830 (w), 790 (s), 710 (s). UV−vis: λmax
(br), 840 (w), 790 (s), 710 (s), 550 (w, br), 350 (w), 275 (w).
Me6
(ArMe )2Ge(Me)GaMe2 (2). Ge(Ar )2 (0.70 g, 1 mmol), dissolved
6
in toluene (40 mL) and cooled to ca. −78 °C, was treated dropwise
with a solution of GaMe3 (0.115 g, 1 mmol) in toluene (20 mL). The
reaction mixture was stirred and allowed to warm to room temperature
overnight, which resulted in a color change from purple to colorless.
The solvents were removed under reduced pressure, and the resulting
white solid was recrystallized from pentane to give colorless crystals of
2. Yield: 0.588 g, 72%. Mp: 208 °C. 1H NMR (600 MHz, C6D6, 25 °C,
ppm): −0.30 (s, 3H, Ge−Me), −0.04 (s, 3H, Ga−Me), −0.01 (s, 3H,
Ga−Me), 1.46 (s, 3H, p-Me), 1.65 (s, 3H, p-Me), 1.71 (s, 3H, p-Me),
1.86 (s, 3H, p-Me), 1.90 (s, 3H, o-Me), 1.91 (s, 3H, o-Me), 1.97 (s,
3H, o-Me), 1.98 (s, 3H, o-Me), 2.04 (s, 3H, o-Me), 2.09 (s, 3H, o-Me),
2.24 (s, 3H, o-Me), 2.27 (s, 3H, o-Me), 6.54 (s, 1H, m-C6H2), 6.61 (d,
JHH = 7.5 Hz, 1H, m-C6H3), 6.65 (d, JHH = 7.4 Hz, 1H, m-C6H3), 6.67
(s, 1H, m-C6H2), 6.70 (s, 1H, m-C6H2), 6.72 (s, 1H, m-C6H2), 6.74
560 nm.
Me6
{Pb(Me)ArMe }2 (6). Method A. Pb(Ar )2 (0.996 g, 1.194 mmol)
6
was dissolved in toluene (40 mL) and cooled to ca. −78 °C. To this
solution, AlMe3 in toluene (15 mL) was added dropwise. The resulting
solution was stirred and allowed to warm to room temperature
overnight, which resulted in a color change from purple to deep red. A
thin dull metal mirror was present on the inside of the flask wall. The
solvents were removed under reduced pressure, and the resulting red
solid was recrystallized from pentane. This resulted in two different
crystal types which were separated by fractional crystallization to yield
the deep red diplumbene (6) and the colorless alane (7). Yield: 0.135
Me6
g of {Pb(Me)ArMe }2 + AlAr Me2, 12%.
6
Method B. Pb(ArMe
)
(1.050 g, 1.25 mmol) was dissolved in
6
(m, 3H), 6.78 (s, 1H, m-C6H2), 6.79 (s, 1H, m-C6H2), 6.91 (d, JHH
=
2
7.5 Hz, 1H, m-C6H3), 7.03 (t, JHH = 7.5 Hz, 1H, p-C6H3), 7.08 (t, JHH
= 7.5 Hz, 1H, o-C6H3). 13C NMR (150 MHz, C6D6, 25 °C, ppm):
2.80, 3.49, 3.98 (M−Me, M = Ge, Ga), 20.52, 20.75, 21.35, 21.84,
22.18, 22.42, 22.69, 22.75, 23.22, 23.52 (p- and o-Me); 128.13, 128.21,
128.87, 129.02, 129.06, 130.08, 130.12, 130.69, 130.78, 131.11, 131.14,
131.23, 134.79, 135.67, 135.80, 136.12, 136.42, 136.45 136.60, 137.27,
137.48, 139.27, 139.53, 141.82, 142.07, 143.36, 143.38, 144.51, 145.27,
147.71, 148.52, 150.83, 151.13 (Ar). IR: 2900 (br), 1600 (w), 1450 (s)
1360 (s), 1290 (w), 1250 (w), 835 (s), 790 (s), 710 (s), 540 (w).
toluene and cooled to ca. −78 °C, and GaMe3 (0.170 g, 1.5 mmol) in
toluene was added dropwise. The resulting solution was stirred and
allowed to warm to room temperature overnight, resulting in a color
change from purple to deep red. The solvents were removed under
reduced pressure, and the resulting red solid was recrystallized from
pentane. This resulted in two different crystal types which were
separated by fractional crystallization yielding the deep red plumbene
(6) and the colorless gallane (8). Yield: 0.999 g of {Pb(Me)ArMe }2 +
6
GaArMe Me2, 82%.
Mp: 190−195 °C. H NMR (500 MHz, C6D6, 25 °C, ppm): 0.19
6
1
(ArMe )2Sn(Me)GaMe2 (3). Sn(ArMe
)
(0.746 g, 1 mmol) was
6
6
2
dissolved in toluene (40 mL) and cooled to ca. −78 °C. To this
solution, GaMe3 (0.126 g, 1.1 mmol) in toluene (20 mL) was added
dropwise. The resulting mixture was stirred and allowed to warm to
room temperature overnight. This resulted in a color change from
purple to colorless at low temperature and then a further change to a
pale purple at room temperature. The solvents were removed under
reduced pressure, and the resulting white solid was recrystallized from
pentane, giving a slightly purple solution and colorless crystals of 3.
Yield: 0.469 g, 55%. Mp: Turns purple with increasing temperature;
(s, 6H, Pb−Me), 2.09 (s, 12H, p-Me), 2.27 (s, 24H, o-Me), 6.75 (s,
8H, m-C6H2), 7.40 (t, JHH = 7.5 Hz, 2H, p-C6H3), 7.51 (d, JHH = 7.5
Hz, 4H, m-C6H3). 13C NMR (126 MHz, C6D6, 25 °C, ppm): 0.58
(Pb−Me), 21.10 (p-Me), 21.34 (o-Me), 126.21, 129.03, 134.62,
136.05, 136.85, 137.56, 147.45. 207Pb NMR (105 MHz, C6D6, 25 °C,
ppm): 8428. IR: 2900 (br), 2700 (w), 1600 (w), 1450 (s), 1370 (s),
1255 (w), 1150 (w), 1015 (w), 840 (m), 790 (m), 715 (m). UV−vis:
λmax 466 nm.
Products 7 and 8, in addition to being obtained as a mixture with
product 6, were synthesized by alternative routes to acquire pure
products to facilitate spectroscopic characterization because they could
not be completely separated from the mixtures formed with 5. These
methods are given alongside the original isolation of a single-crystal
product.
1
melts 156−158 °C. H NMR (500 MHz, C6D6, 25 °C, ppm): −0.40
(t, 2JHSn 40.8 Hz, 3H, Sn−Me), −0.04 (s, 6H, Ga−Me), 1.83 (s, 12H,
o-Me), 1.92 (s, 12H, o-Me), 2.18 (s, 12H, p-Me), 6.72 (s, 4H, m-
C6H2), 6.73 (s, 4H, m-C6H2), 6.76 (d, JHH = 7.4 Hz, 4H, m-C6H3),
7.09 (t, JHH = 7.5 Hz, 2H, p-C6H3). 13C NMR (150 MHz, C6D6, 25
°C, ppm): −4.04 (Sn−Me), 5.12 (Ga−Me), 20.76 (p-Me), 22.20 (o-
Me), 22.74 (o-Me), 129.19, 129.55, 129.71, 129.80, 136.38, 136.51,
137.07, 143.55, 146.57, 150.84 (Ar). 119Sn NMR (186 MHz, C6D6, 25
°C, ppm): −170. IR: 2900 (br), 2700 (w), 1440 (s), 1360 (s), 1290
(w) 1145 (w), 940 (w), 875 (w), 835 (w), 790 (w), 710 (s).
AlArMe Me2 (7). From the product mixture for the synthesis of (Pb
6
ArMe Me)2, 6, via method A, AlMe2 ArMe , 7, was separated via
fractional crystallization until a single crystal of sufficient purity was
obtained for crystallographic study.
6
6
For spectroscopic studies, 7 was synthesized via an alternative route
(ArMe )2Sn(Et)GaEt2 (4). Sn(ArMe )2 (0.345 g, 0.462 mmol) was
dissolved in toluene (35 mL) and cooled to ca. −78 °C. To this
solution a 4-fold excess of GaEt3 (0.286 g, 1.82 mmol) in toluene (15
mL) was added dropwise. The resulting mixture was stirred and
allowed to warm to room temperature over 1 h, resulting in a pale
as follows. A solution of ArMe Li (0.350 g, 1.09 mmol) in toluene (30
6
6
6
mL) was added dropwise to Me2AlCl (1.1 mL, 1.0 M in heptane) at
ca. −78 °C. The resulting mixture was stirred and allowed to warm to
room temperature overnight. The solvents were removed under
reduced pressure, and the product was extracted with pentane.
G
DOI: 10.1021/ic502824w
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Inorganic Chemistry
Article
Crystallization at room temperature afforded 6 as a colorless crystalline
solid. Yield: 0.397 g, 98%.
along with the corresponding aryldimethyl−group 13 com-
pounds 7 and 8.
Mp: 123−126 °C. 1H NMR (500 MHz, C6D6, 25 °C, ppm): −0.69
(s, 6H, Al−Me), 2.11 (s, 6H, p-Me), 2.16 (s, 12H, o-Me), 6.84 (s, 4H,
m-C6H2), 7.03 (d, JHH = 7.6 Hz, 2H, m-C6H3), 7.34 (t, JHH = 7.5 Hz, p-
C6H3). 13C NMR (600 MHz, C6D6, 25 °C, ppm): −7.07 (Al−Me),
21.10 (o-Me), 21.13 (p-Me), 125.47, 129.10, 129.38, 136.38, 137.37,
141.34, 148.82 (Ar−H). IR: 2900 (br), 2710 (w), 1600 (m), 1550 (w),
1445 (s), 1355 (s), 1250 (m), 1170 (w), 1070 (m), 1010 (m), 840
(m), 795 (s), 720 (m), 680 (w), 640 (w), 550 (w), 350 (br).
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information files for 1−8, thermal ellipsoid
1
plot of the structure of 4, variable- temperature H NMR
spectra of 4, data used in van’t Hoff analysis of the equilibrium
of 4, infrared and NMR spectra for 1−8. This material is
available free of charge via the Internet at http://pubs.acs.org.
GaArMe Me2 (8). From the product mixture obtained by the
6
Me6
Method B synthesis of (PbArMe Me)2, GaMe2Ar was separated via
6
fractional crystallization until a single crystal of sufficient purity was
obtained for crystallographic study.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pppower@ucdavis.edu.
Notes
■
For spectroscopic studies, 8 was synthesized via an alternative route.
GaMe3 (0.300 g, 2.61 mmol) in pentane (10 mL) was cooled to ca.
−100 °C using an ethanol/liquid nitrogen bath. This solution was
treated dropwise with 0.5 equiv of GaCl3 (0.229 g, 1.3 mmol) in
pentane (20 mL). The resulting mixture was stirred and allowed to
warm up overnight. The mixture was again cooled to −78 °C and
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
treated dropwise with a slurry of ArMe Li (1.24 g, 3.9 mmol) in toluene
6
We are grateful to the U.S. Department of Energy, Office of
Basic Energy Sciences (FG02-07ER46475) for support of this
work.
(25 mL). The mixture was again stirred and allowed to warm to room
temperature overnight. The solvents were removed under reduced
pressure, and the product was extracted with pentane (25 mL).
Concentration to ca. 8 mL resulted in crystallization at room
temperature, affording a colorless crystalline solid. Yield: 1.270 g, 74%
Mp: 124−126 °C. 1H NMR (500 MHz, C6D6, 25 °C, ppm): −0.26
(s, 6H, Ga−Me), 2.11 (s, 6H, p-Me), 2.14 (s, 12H, o-Me), 6.81 (s, 4H,
m-C6H2), 7.03 (d, JHH = 7.5 Hz, 2H, m-C6H3), 7.32 (t, JHH = 7.5 Hz,
1H, p-C6H3). 13C NMR (500 MHz, C6D6, 25 °C, ppm): −0.08 (Ga−
Me), 21.01 (o-Me), 21.14 (p-Me), 125.97, 128.86, 128.89, 136.16,
137.16, 140.79, 147.04, 155.23 (Ar−H). IR: 2900 (br), 2720 (s), 1915
(w), 1850 (w), 1790 (w), 1760 (w), 1720 (w), 1695 (w), 1605 (s),
1550 (s), 1445 (br), 1365 (br), 1290 (w), 1250 (w), 1180 (s), 1150
(br), 1090 (w), 1080 (w), 1015 (br), 940 (br), 880 (w), 795 (s), 760
(s), 715 (s), 560 (s), 535 (s).
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■
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