Bis-and tris(dimethylgallyl)benzenes: Synthesis, solid-state structures, and redistribution reactions

Article abstract of DOI:10.1021/om900048c

derivatives containing dimethylgallyl substituents in 1,3- (compounds 5 and 6), 1,4- (compound 9), and 1,3,5-position (compound 12) were prepared by reaction of the corresponding chloromercuri- obenzenes with an excess of trimethylgallium at higher temperatures. These compounds decompose in solution at room temperature and in the solid state upon mild heating with elimination of trimethylgallium to give oligomeric condensation products of unknown detailed composition. These condensation products can be transformed back into the starting compounds by treatment with an excess of trimethylgallium at higher temperatures. Highly air-sensitive crystals of 5, 6, 9, and 12 suitable for an X-ray analysis are obtained from trimethylgallium as solvent. The X-ray crystallographic studies revealed the presence of higher coordinate gallium atoms, which lead to the formation of strand- or sheet-like polymers. A trigonal- bipyramidal coordination sphere is observed for the gallium atoms in 9. A distorted tetrahedral coordination is found for the gallium atoms in 5, 6, and 12. The latter compounds possess asymmetric aryl-dimethylgallyl bridging units.

Full text of DOI:10.1021/om900048c

Organometallics 2009, 28, 2619–2624
2619
Bis- and Tris(dimethylgallyl)benzenes: Synthesis, Solid-State
Structures, and Redistribution Reactions
Peter Jutzi,* Joseph Izundu, Henning Sielemann, Beate Neumann, and
Hans-Georg Stammler
Fakulta¨t fu¨r Chemie, UniVersita¨t Bielefeld, UniVersita¨tstrasse 25, 33615 Bielefeld, Germany
ReceiVed January 21, 2009
Benzene derivatives containing dimethylgallyl substituents in 1,3- (compounds 5 and 6), 1,4- (compound
9), and 1,3,5-position (compound 12) were prepared by reaction of the corresponding chloromercuri-
obenzenes with an excess of trimethylgallium at higher temperatures. These compounds decompose in
solution at room temperature and in the solid state upon mild heating with elimination of trimethylgallium
to give oligomeric condensation products of unknown detailed composition. These condensation products
can be transformed back into the starting compounds by treatment with an excess of trimethylgallium at
higher temperatures. Highly air-sensitive crystals of 5, 6, 9, and 12 suitable for an X-ray analysis are
obtained from trimethylgallium as solvent. The X-ray crystallographic studies revealed the presence of
higher coordinate gallium atoms, which lead to the formation of strand- or sheet-like polymers. A trigonal-
bipyramidal coordination sphere is observed for the gallium atoms in 9. A distorted tetrahedral coordination
is found for the gallium atoms in 5, 6, and 12. The latter compounds possess asymmetric aryl-dimethylgallyl
bridging units.
trimethylstannylbenzenes by chloromercurio groups, which
subsequently are replaced by dimethylgallyl groups.3 Table 1
shows the compounds involved in the synthetic procedure.
1. Introduction
1,1′-Bis(dimethylgallyl)ferrocene undergoes reversible con-
densation reactions due to the easy cleavage and formation of
covalent Ga-C bonds.1 This phenomenon has been used
synthetically within the framework of the concept of “dynamic
covalent chemistry”.2 In this context, we have become interested
in checking the possibility of the Ga-C cleavage and formation
in comparable benzene derivatives. Multiple dimethylgallyl-
substituted benzene derivatives are thus far unknown. Surpris-
ingly, the first monodimethylgallyl-substituted benzenes were
described only recently. They show the expected substituent
redistribution reactions.3 Here we describe the first multiple
dimethylgallyl-substituted compounds with the synthesis of the
1,3-disubstituted derivatives 5 and 6, of the 1,4-disubstituted
derivative 9, and of the 1,3,5-trisubstituted derivative 12.
Furthermore, we report the pronounced instability of these
compounds with regard to redistribution reactions in solution
and in the solid state and their structures in the solid state.
Most of the trimethylstannyl and chloromercurio benzenes
are known from the literature (see Table 1). The novel tin
compound 2 was prepared by reaction of sodium trimethylstan-
nide with the corresponding dibromobenzene (see Experimental
Section). Rather drastic conditions had to be applied for the
last step in the reaction sequence, the introduction of the
dimethylgallyl groups, which leads to the novel dimethylgallyl-
substituted benzene derivatives 5, 6, 9, and 12. The respective
chloromercuriobenzenes have to be treated with an excess of
trimethylgallium (using the reagent as solvent) at 100-140 °C
in a pressure vessel. After cooling to room temperature and
removal of the liquid components (dimethylmercury, chlo-
ro(dimethyl)gallane, and the excess of trimethylgallium), the
respective dimethylgallyl-functionalized benzene derivative
remained as a solid residue, which was crystallized from
trimethylgallium as solvent. The crystalline compounds are
extremely sensitive toward air and moisture. The use of high-
boiling solvents such as p-xylene together with an excess of
the reagent trimethylgallium instead of the use of trimethylgal-
lium as solvent and reagent was not successful.
2. Results
Synthesis and Characterization. The strategy for the
synthesis of the dimethylgallyl-functionalized benzene deriva-
tives is described in eq 1. The reaction sequence starts with the
substitution of trimethylstannyl groups in the corresponding
* Corresponding author. Phone: +49-5211066163. E-mail: peter.jutzi@
unibielefeld.de.
Redistribution Reactions in Solution and in the Solid
State. We have also been unsuccessful in preparing solutions
of the novel organogallium compounds 5, 6, 9, and 12 in organic
solvents such as hexane, toluene, or diethyl ether. Concomitant
substituent redistribution reactions were observed, which finally
led to the formation of trimethylgallium and of solid condensa-
tion products of thus far unidentified composition and structure.
The degree of condensation can be judged from the amount of
isolated trimethylgallium and varies with the reaction conditions
(1) (a) Jutzi, P.; Lenze, N.; Neuman, B.; Stammler, H. G. Angew. Chem.
2001, 113, 1470. (b) Jutzi, P.; Lenze, N.; Neuman, B.; Stammler, H. G.
Organometallics 2002, 21, 3018. (c) Jutzi, P.; Lenze, N.; Neuman, B.;
Stammler, H. G. Organometallics 2003, 22, 2766. (d) Althof, A.; Eisner,
D.; Jutzi, P.; Lenze, N.; Neumann, B.; Schoeller, W. W.; Stammler, H. G.
Chem. Eur. 2006, 12, 5471.
(2) Rowan, S. T.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.;
Stoddart, J. F. Angew. Chem. 2002, 114, 938; Angew. Chem. Int. Ed. 2002,
41, 898. .
(3) Jutzi, P.; Izundu, J.; Neumann, B.; Stammler, H. G. Organometallics
2008, 27, 4565–4571.
10.1021/om900048c CCC: $40.75
2009 American Chemical Society
Publication on Web 03/30/2009

2620 Organometallics, Vol. 28, No. 8, 2009
Jutzi et al.
Table 1. Compounds 1-12
and from compound to compound. A comparable behavior is
described in the literature for monodimethylgallyl-substituted
benzene derivatives.3 In the redistribution reaction of compound
5 in benzene-d6/THF solution, a soluble condensation product
Solid-State Structures. The solid-state structures of the
compounds 5, 6, 9, and 12 were determined by single-crystal
X-ray diffraction analysis. Suitable crystals were prepared in a
glovebox to avoid contact with air or moisture. X-ray structural
parameters are listed in Table 2. Parts of the solid-state structures
are presented in Figures 2 (5), 3 (6), 4 (9), and 5-7 (12), and
selected bond lengths and bond angles are incorporated in the
figure captions. Further information is given in the Supporting
Information.
1
was tentatively assigned to the first condensation step by H
NMR spectroscopy, as described in the Experimental Section.
Worth mentioning, treatment of the isolated solid condensation
products with an excess of trimethylgallium in a pressure vessel
at 100-140 °C regenerated the crystalline starting compounds.
With halogenated solvents such as dichloromethane, decomposi-
tion reactions were observed. As a result, it was not possible to
investigate the physical properties of the novel compounds in
solution (for example determination of the molecular mass and
of the NMR parameters).
A similar behavior was observed in the solid state of the
novel compounds. On treating the samples under reduced
pressure (<10 mbar) or on heating under normal pressure,
decomposition under GaMe3 elimination took place. Thus,
attempted melting point determinations were unsuccessful.
The amount of GaMe3 formed depends on the respective
decomposition temperature. Thus, the exact composition and
structure of the formed solid products remains unknown. All
these products could be retransformed into the crystalline
starting compounds by thermal treatment with an excess of
GaMe3. As an example for the above-described processes,
the decomposition of compound 5 in the solid state and its
regeneration by treatment with trimethylgallium is described
in the Experimental Section.
The parameters defined in Figure 1 are used for the
description of the crystal structures. The dip angle (R) is the
angle enclosed at the respective Cipso atom by the vector
Cpara-Cipso and the vector Cipso-Ga. The plane angle (ꢀ)
arises from the orientation of the two vectors Cortho-Cortho
in the aryl part and Cmethyl-Cmethyl in the dimethylgallyl part
of the molecule and determines the deviation from planarity.
The angle sum Φ characterizes the deviation from planarity
(Φ ) 360°) at the respective gallium atoms. For comparison,
parameters of the monomeric unit of the compound dimeth-
ylgallylbenzene3 are given here: the angle R and the angle ꢀ
both are 0°, and an angle Φ of 360° is found at the three-
coordinate gallium atom. In the novel compounds containing
asymmetric electron-deficient aryl-GaMe2 bridging units, the
parameters d, d*, and γ define the asymmetric bridging. With
the parameters d and d*, the presence of short and long
Cipso-Ga distances is characterized; this situation leads to
neighbored benzene rings that deviate from coplanarity
(coplanarity is found in symmetric bridges3). With the
parameter γ, the angle of the Ga-Ga vector in the bridging
unit is defined in relation to that benzene ring plane
(represented by the Cortho-Cortho vector), which is positioned
on the side of the short Cipso-Ga bond (parameter d); the
angle γ is determined from the view in the direction of the
Cpara-Cipso vector. There will be described bonding situations
in which the Ga-Ga vector is not perpendicular to the plane
of the benzene ring, so that the angle γ deviates from 90°.
As a consequence of the described properties of the novel
compounds, it was not possible to obtain reliable elemental
analyses. The observed deviations from calculated CH values
may be due to partial oxidation of the crystals or to the presence
of traces of chemisorbed trimethylgallium. Treatment of the
compounds in vacuo (<10 mbar) for the elimination of volatiles
and washing procedures is not indicated. A comparable behavior
is already described for monodimethylgallyl-substituted benzene
derivatives.3
Compound 5 crystallizes as colorless needles in the point
group P21/c with one monomeric molecule in the asymmetric
unit. This monomer is connected to other monomers by
transformation via a glide-plane along the c-axis under
formation of a coordination polymer (see Figure 2). The
polymer forms strands along the crystallographic c-axis. The
coordination at the gallium atoms in 5 differs from that found
in the monodimethylgallyl-substituted benzene molecule,
where a sp2-hybridized gallium atom in a nearly planar
structure shows additional π-contacts to two neighboring
molecules.3 Instead, an asymmetric bridging situation is
observed, as portrayed in Figure 2 and characterized by the
parameters R, ꢀ, Φ, γ, d, and d* at each gallium atom. The
highly asymmetric bridging units are characterized by small
(4) (a) Rot, N.; Bickelhaupt, F. Organometallics 1997, 16, 5027–5031.
(b) Chopa, A. B.; Lockhart, M. T.; Dorn, V. B. Organometallics 2002, 21,
1425–1429. (c) Eisch, J. J.; Kotowicz, B. W. Eur. J. Inorg. Chem., 1998,
761-769.
(5) Deeter, G. A.; Moore, J. S. Macromolecules 1993, 26, 2535–2541.
(6) Malaiyandi, M.; Sawatzky, H.; Wright, G. F. Can. J. Chem., 1961,
39, 1827–1835.
(7) (a) Havelkova, M.; Dvorak, D.; Hocek, M. Tetrahedron 2002, 58,
7431–7435. (b) Kiam, W.; Tesmann, H.; Bock, H. Berichte 1980, 113, 3221–
3234. (c) Chopa, A. B.; Lockhart, M. T.; Silbestri, G. Organometallics 2000,
19, 2249–2250. (d) Chopa, A. B.; Lockhart, M. T.; Silbestri, G. Organo-
metallics 2001, 20, 3358–3360.
(8) Sawatzky, H.; Wright, G. F. Can. J. Chem. 1958, 36, 1555–1569.
(9) Rot, N.; De Kanter, F. J. J.; Bickelhaupt, F.; Smeets, W. J. J.; Spek,
A. L. J. Organomet. Chem. 2000, 593-594, 369–379.

Bis- and Tris(dimethylgallyl)benzenes
Organometallics, Vol. 28, No. 8, 2009 2621
Table 2. Crystallographic Data of 5, 6, 9, and 12
5
6
9
12
C12H21Ga3
empirical formula
fw, g/mol
C10H16Ga2
C14H24Ga2
C10H16Ga2
275.67
331.77
275.67
374.45
cryst syst
space group
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/n
monoclinic
C2/c
lattice params
a, Å
b, Å
7.943(2)
14.358(2)
9.8740(3)
97.682(8)
1116.0(3)
4
14.3120(10)
9.9480(7)
22.5040(14)
103.736(5)
3112.4(4)
8
5.5550(4)
15.4270(13)
6.5310(9)
101.849(12)
547.76(10)
2
17.6600(3)
10.4860(2)
15.9950(11)
104.5570(11)
2866.91(8)
8
c, Å
ꢀ, deg
V, Å3
Z
cryst size, mm-3
0.26 × 0.20 × 0.14
0.30 × 0.08 × 0.06
0.30 × 0.20 × 0.16
0.30 × 0.29 × 0.27
dcalc g/cm3
1.641
1.416
1.671
1.735
F(000)
552
4.776
0.3698/0.5545
multiscan
2.5 to 27.5
99.9%
22 550
2571
2293
113
1360
3.438
0.4253/0.8203
multiscan
3.3 to 25.0
83.7%
12 266
4577
3342
229
276
4.865
0.3231/0.5099
multiscan
2.6 to 30.0
99.9%
13 910
1597
1434
57
1488
5.567
0.2859/0.3148
multiscan
3.0 to 30.0
99.8%
38 635
4175
3839
142
µ(Mo K), mm-1
min/max transmn
abs corr
θ range, deg
completeness to θmax
reflns collected
reflns unique
no. obsd data [I > 2σ(I)]
no. of params refined
final R1a [I > 2σ(I)]/wR2b
final R1/wR2 (all data)
largest peak in final diff map, e /Å3
0.0330, 0.0852
0.0386, 0.0901
0.568
0.0517, 0.1265
0.0799, 0.1445
0.775
0.0200, 0.0488
0.0239, 0.0510
0.404
0.0220, 0.0529
0.0254, 0.0541
0.476
b
a R1 ) ||Fo| - |Fc||/∑|Fo|. wR2 ) {∑[w(Fo - Fc2)2]/∑[w(Fo )2]}0.5
.
2
2
Figure 1. Parameters (R, ꢀ, Φ, d, d*, and γ) defined to describe
the crystal structures.
Figure 3. Solid-state structure of 6, with the thermal ellipsoids given
at the 50% probability level. Bridging at Ga(4A) and Ga(2A) is
not shown; also hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and bond angles [deg]: Ga(1) environment:
Ga(1)-C(11) 1.952(7); d ) 1.995; d* ) 2.689; R ) +6.6; ꢀ )
16.2, Φ ) 356.4, γ ) 87.7; Ga(2) environment: Ga(2)-C(13)
1.961(5); d ) 2.000, d* ) 2.741, R ) -9.9, ꢀ ) 11.6, Φ ) 355.7,
γ ) 97.7; Ga(3) environment: Ga(3)-C(25) 1.969(6); d ) 2.010,
d* ) 2.737, R ) -7.6; ꢀ ) 11.7, Φ ) 355.9, γ ) 98.8; Ga(4)
environment: Ga(4)-C(27) 1.973(6); d ) 1.999, d* ) 2.761, R )
+3.2; ꢀ ) 10.9, Φ ) 356.9, γ ) 82.4.
Compound 6 crystallizes as colorless needles in the space
group P21/c with two molecules in the asymmetric unit and with
translation along the b-axis under formation of a strand-like
coordination polymer (see Figure 3). The overall bonding
situation is comparable to that observed for compound 5, as
concluded from the structural parameters defined in Figure 1.
The additional butyl group on the benzene ring does not have
a pronounced influence on the asymmetric bridging situation.
Compound 9 crystallizes as colorless needles in the space
group P21/n with one-half of the monomer in the asymmetric
unit and with an inversion center located in the monomer. A
strand-like polymer chain is formed by translation along the
a-axis. These strands are connected by weak contacts between
gallium atoms in one strand and methyl groups from dimeth-
Figure 2. Solid-state structure of 5 with the thermal ellipsoids given
at the 50% probability level. Bridging at Ga(1A) and Ga(2B) is
not shown; hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and bond angles [deg]: Ga(1) environment: Ga(1)-C(7)
1.961; d ) 1.991; d* ) 2.718; R ) +7.5; ꢀ ) 6.8, Φ ) 355.8; γ
) 98.4; Ga(2) environment: Ga(2)-C(9) 1.956; d ) 1.997, d* )
2.706, R ) -7.2, ꢀ ) 8.1, Φ ) 356.2, γ ) 84.2.
values for the angle R and by only a small deviation of the
angle Φ from 360°. As expected, the Ga-C(methyl) distances
are shorter than the Ga-C(aryl) distances, because the
Ga-C(aryl) bonds are part of the bridging units. The
structural changes in the benzene unit of 5 are discussed in
a later section.

2622 Organometallics, Vol. 28, No. 8, 2009
Jutzi et al.
Figure 5. Structure of the monomeric unit of 12 with the thermal
ellipsoids given at the 50% probability level. Bridging of Ga(1)
with Ga(2A), Ga(2) with Ga(1B), and Ga(3) with Ga(3C) is not
shown; also hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and bond angles [deg]: Ga(1) environment: Ga(1)-C(7)
1.966, d ) 2.103; d* ) 2.307; R ) +28.8; ꢀ ) 16.3, Φ ) 344.3,
γ ) 85.2; Ga(2) environment: Ga(2)-C(9) 1.970, d ) 2.105, d*
) 2.321, R ) +27.3, ꢀ ) 21.2, Φ ) 345.4, γ ) 91.5; Ga(3)
environment: Ga(3)-C(11) 1.964, d ) 2.007, d* ) 2.681, R )
-13.3; ꢀ ) 9.8, Φ ) 354.7, γ ) 95.0.
Figure 4. Solid-state structure of 9, with the thermal ellipsoids given
at the 50% probability level. Full coordination is shown only for
Ga(1); also hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and bond angles [deg]: Ga(1)-C(1) 1.9594, Ga(1)-C(3)
1.9777, R ) +2.5, ꢀ ) 11.1, Φ ) 359.7, Ga(1)-C(4B) 3.04,
Ga(1)-C(Ga(1D) 3.448.
ylgallyl units in another (see Figure 4). Interestingly, the
coordination behavior of 9 differs from that observed for 5 and
6 and corresponds partly to that observed for monodimethyl-
gallyl-substituted arenes3 and partly to that found in trimethylgal-
lium.10,11 The molecular unit possesses a nearly planar structure
(see values for R, ꢀ, and Φ) with sp2-hybridized gallium atoms.
An overall only weakly distorted trigonal-bipyramidal coordina-
tion geometry at the gallium atoms is observed, arising from a
further π-type coordination to a neighboring arene unit [see
Ga(1)-C(4B) distance in Figure 4] and a further weak contact
(based on polarization forces) to a methyl group from a
dimethylgallyl unit of another neighbor [see Ga(1)-C(Ga1D)
distance in Figure 4].
Figure 6. Sketch of the structure of the coordination polymer 12
viewed in the direction of (top) and perpendicular to (bottom) the
benzene-ring planes.
Compound 12 crystallizes as colorless sheet-like crystals
in the space group C2/c with one molecule in the asymmetric
unit. The molecular structure of 12 is portrayed in Figure 5
without the coordination and in Figure 1* in the Supporting
Information with coordination to other molecules. All three
GaMe2 groups are involved in the formation of asymmetric
bridging units by interaction with aryl-GaMe2 groups from
neighboring molecules. Based on these intermolecular interac-
tions, a coordination polymer is formed, which consists of
undulated sheets built by weakly connected strands of double-
bridged monomers. In more detail, the atoms Ga(1)Ga(2A) and
Ga(2)Ga(1B), which form less asymmetric bridging units
(greater values of R, smaller values of Φ), connect the benzene
rings within the strand structure. These strands are connected
via a more asymmetric interaction including the Ga(3)Ga(3C)
atoms (smaller value of R, greater value of Φ) to give the final
sheet structure. The formation of strands with the benzene rings
positioned in two planes is schematically described in Figure 6
by the view from two different perspectives. The sheet formation
is portrayed in Figure 7 by a view in the direction of the b-axis
from the unit cell.
Figure 7. Sketch of the structure of the coordination polymer 12
viewed in the direction of the b-axis of the unit cell; the dashed
lines represent nonbonding Ga-Ga distances in bridging units
between adjacent chains represented in Figure 6.
substituents on the bonding parameters of the C6 perimeter of
the corresponding benzene unit was investigated. In this context,
the measured C-C bond lengths and CCC bond angles in these
compounds are presented in Table 1* of the Supporting
Information. The CCC angles at the carbon atoms connected
with dimethylgallyl substituents are smaller than 120° (∼116°),
and the corresponding C-C bonds are somewhat longer. These
changes do not indicate a pronounced carbocation character of
carbon atoms in the C6 perimeter. Thus, the electron deficiency
is mainly located in the bonds to the dimethylgallyl units.
For a rudimentary understanding of the bonding in the
molecules 5, 6, 9, and 12, the influence of the GaMe2
(10) Mitzel, N. W.; Lustig, C.; Berger, R. J. F.; Runeberg, N. Angew.
Chem., 2002, 114, 2629–2632; Angew. Chem. Int. Ed. 2002, 41, 2519-
2522. .
(11) Boese, R.; Downs, A. J.; Greene, T. M.; Wall, A. W.; Morrison,
C. A.; Parsons, S. Organometallics 2003, 22, 2450–2457.

Bis- and Tris(dimethylgallyl)benzenes
Organometallics, Vol. 28, No. 8, 2009 2623
in relation to the plane of the benzene rings (see type III in
Figure 8 and parameter γ in Figure 1). The asymmetry
parameters are different for each of the bridging dimethylgallyl
units. It is evident from our investigations that dimethylgallyl-
substituted benzene derivatives find quite different ways to avoid
the low coordination at gallium, presumably for steric reasons.
Experimental Section
General Comments. All experiments were conducted under a
purified argon atmosphere using standard Schlenk techniques. All
solvents were commercially available, purified by conventional
means, distilled, and stored under argon prior to use. The NMR
spectra were recorded on a Bruker Avance DRX 500 spectrometer
at 300 K (1H NMR, 500.1 MHz; 13C NMR, 125.8 MHz; 119Sn NMR,
186.5 MHz; 199Hg NMR, 89.6 MHz). The chemical shift values
are reported in ppm. The Microanalytical Laboratories of the
University of Bielefeld and Beller & Matthies, Goettingen, per-
formed the CH elemental analyses. Crystallographic data were
collected with a BrukerNonius KappaCCD diffractometer with Mo
KR radiation (graphite monochromator, λ ) 0.71073 Å) at 100 K.
Crystallographic programs used for structure solution and refinement
were from SHELXS-97 and SHELXL-97. The structures were
solved by direct methods and were refined by using full-matrix
least-squares on F2 of all unique reflections with anisotropic thermal
parameters for all non-hydrogen atoms.
Starting Materials. The bromobenzenes were purchased from
Merck Chemicals and Fluka Chemicals. The trimethyltin and
chloromercurio derivatives were prepared according to the cited
literature procedures.
Caution! Trimethylgallium is a pyrophoric liquid and should
be handled with care. Dimethylmercury is poisonous to the central
nervous system and has a long latency period before the charac-
teristic symptoms appear. It is necessary to use a combination of
gloves as suggested by Blaney et al.12-15 Dimethylmercury is
destroyed by treating it with aqua regia. The resulting mercury
dichloride is precipitated as the oxide under basic conditions before
it was appropriately disposed.
1,3-Bis(trimethylstannyl)-5-n-butylbenzene (2). A THF solu-
tion of sodium trimethylstannide16 (13.46 g, 72.0 mmol) in a
Schlenk flask was cooled to 0 °C. 1,3-Dibromo-5-n-butylbenzene
(7.10 g, 24.31 mmol) in 10 mL of THF was added to the solution
dropwise within 30 min. A rusty-brown suspension appeared after
a while. The reaction mixture was stirred at 0 °C for 5 h, allowed
to warm to room temperature, and stirred for a further 12 h.
Thereafter, 20 mL of degasified distilled water was added, and the
organic layer was separated. The aqueous layer was extracted three
times, each with 30 mL of diethyl ether. The combined organic
layer was dried over sodium sulfate to give a colorless solution.
The solvents were removed at reduced pressure to give a colorless
liquid. Fractional distillation of the liquid at reduced pressure (0.01
mbar; 105 °C) yielded 2 (5.40 g, 10.94 mmol, 45%) as a colorless
Figure 8. Schematic presentation of the coordination modes in
dimethylgallyl-substituted benzene derivatives (benzene rings are
given as thicker black lines).
Discussion and Conclusion
In this paper, the syntheses of benzene derivatives containing
dimethylgallyl substituents in 1,3- (compounds 5 and 6), 1,4-
(compound 9), or 1,3,5-position (compound 12) are described.
These highly air-sensitive compounds are thermolabile in
solution and to a smaller extent even in the solid state. They
decompose with elimination of trimethylgallium to give oligo-
meric condensation products of unknown detailed composition.
These condensation products can be retransformed into the
starting compounds by treatment with an excess of trimethyl-
gallium at higher temperatures. Unfortunately, the observed
cleavage and linking processes do not lead to molecularly
defined condensation products (for example ring structures
containing benzene units and trigonal-planar coordinate gallium
atoms) as desirable for a synthetic method in the frame of the
concept of “dynamic covalent chemistry”.2 This might be due
to the fact that in the desired condensation products the gallium-
bridged benzene rings cannot form energetically favored planar
ring systems, which are further stabilized by intermolecular
π-contacts. The interference of ortho-hydrogen atoms of the
benzene units might prevent the formation of such structures.
Crystalline samples of the compounds 5, 6, 9, and 12 are
obtained from trimethylgallium as solvent. X-ray crystal struc-
ture investigations show that all compounds form coordination
polymers, whereby old and novel structural features for higher
coordinated organogallium compounds are observed. Figure 8
gives a schematic presentation of the different coordination
modes. In compound 9 a structural motif is observed that
resembles on one hand that found for monodimethylgallyl-
substituted benzenes (type I)3 and on the other hand that
observed for trimethylgallium.10,11 In more detail, a coordination
number of five in a trigonal-bipyramidal geometry is realized
at the gallium atoms by formation of a π-contact to a
neighboring aryl system and of a van der Waals contact to a
methyl group from a neighboring dimethylgallyl unit (see type
II). Another structural feature is found for the compounds 5, 6,
and 12. A coordination number of four at the gallium atoms is
realized by the formation of highly asymmetric electron-deficient
bridge bonds involving the ipso-carbon atoms of the benzene
and the gallium atoms from the dimethylgallyl units. The
asymmetry concerns on one hand the different Ga-C(ipso) bond
lengths and on the other hand the position of the Ga-Ga vector
1
2
liquid. H NMR (CDCl3) [δ/ppm]: 0.31 (s, 18H Sn(CH3)3, JSn-H
) 46/53 Hz), 0.97 (t, 3H, 3JHH ) 8 Hz, CH2CH3), 1.42 (m, CH2CH3,
3JHH ) 8 Hz), 1.63 (m, 2H, JHH ) 8 Hz, CH2CH2CH3), 2.60 (t,
3
3
3
2H, JHH ) 8 Hz, CH2CH2CH2CH3), 7.29 (s, 2H, C-4/6, JSn-H
)
47 Hz), 7.58 (s, 1H, C-2, JSn-H ) 41 Hz). 13C NMR (CDCl3)
[δ/ppm]: -9.5 (Sn(CH3)3, 1JSn-13C ) 331/346 Hz), 14.1 (CH2CH3),
22.7 (CH2CH3), 34.0 (CH2CH2CH3), 36.0 (CH2CH2CH2CH3), 140.4
3
(12) Nierenberg, D. W.; Nordgren, R. E.; Chang, M. B.; Siegler, R. W.;
Blayney, M. B.; Hochberg, F.; Toribara, T. Y.; Cernichiari, E.; Clarkson,
T. New Engl. J. Med. 1998, 338 (23), 1672.
(13) Blayney, M. B. Appl. Occup. EnViron. Hyg. 2001, 16 (2), 233.
(14) Florea, A.; Buesselberg, D. BioMetals 2006, 19 (4), 419. Lockwood,
A. H.; Landrigan, P. J. New Engl. J. Med. 1998, 339 (17), 1243.
(15) Blayney, M. B.; Winn, J. S.; Nierenberg, D. W. Chem. Eng. News
1997, (May 12), 7.
(16) Bickelhaupt, F. J. Organomet. Chem. 2000, 593-594, 369.

2624 Organometallics, Vol. 28, No. 8, 2009
Jutzi et al.
2
2
(C4/6, JSn-13C ) 32 Hz), 141.6 (C-2, JSn-13C ) 37 Hz),142.0 (C-
mixture was heated at 120 °C for 5 h to give a colorless solution,
which was cooled without stirring to 40 °C overnight. Colorless
crystals of 5 separated from the solution. The crystals were isolated
(1.85 g) and characterized by X-ray diffraction (space group
determination).
3
3
2
1/3, JSn-13C ) 23 Hz), 143.2 (C-5, JSn-13C ) 33 Hz) JSn-13C ) 33
Hz). 119Sn NMR (CDCl3) [δ/ppm]: -28.26 (s, 1JSn-13C ) 335 Hz).
Anal. Found: C, 41.35; H, 6.52. Calcd: C, 41.79; H, 6.57.
1,3-Bis(chloromercurio)-(5-n-butyl)benzene (4). To a solution
of mercury dichloride (5.79 g, 21.32 mmol) in 70 mL of THF was
added slowly a solution of 2 (4.90 g, 10.66 mmol) in 50 mL of
diethyl ether at room temperature. The resulting colorless suspension
was stirred for 15 h and finally refluxed for 1 h. All volatile
components were removed at reduced pressure, and the colorless,
amorphous solid residue was washed twice with 50 mL of n-hexane
to give 5 (6.20 g, 10.30 mmol) as a colorless, amorphous solid in
97% yield. 1H NMR (DMSO-d6) [δ/ppm]: 0.88 (t, 3H, (CH2)3CH3),
1.29 (sext, 2H, (CH2)2CH2CH3), 1.51 (quint, 2H, (CH2CH2CH2CH3),
2.45 (t, 2H, CH2CH2CH2CH3), 3.34 (s, DMSO-coordinate), 7.19
(s, 2H), 7.24 (s, 1H). 13C NMR, (DMSO-d6) [δ/ppm]: 13.8
(CH2CH2CH2CH3), 21.7 (CH2CH2CH2CH3), 33.1 (CH2CH2CH2-
CH3), 35.0 (CH2CH2CH2CH3), 136.4 (C-4/6), 141.7 (C-2), 142.1
(C-5), 151.4 (C-1/3). 199Hg NMR (DMSO-d6) [δ/ppm]: -1,139.2.
Anal. Found: Hg, 65.7; Cl, 11.2. Calcd: Hg, 66.38; Cl, 11.73.
1,3-Bis(dimethylgallyl)benzene (5). Trimethylgallium (15.69 g,
136.6 mmol) was added to 1,3-bis(chloromercurio)benzene (4) (3.23
g, 5.88 mmol) in a thick-walled Schlenk flask. The resulting gray
suspension was heated at 120 °C for 7 h in the closed flask to give
a colorless solution, which was cooled without stirring to 40 °C
overnight. Colorless crystals separated from the solution. The liquid
components were removed by syringe, and the remaining colorless
crystals were washed twice with hexane (10 mL) and dried in vacuo
(10 mbar) for 3 h to give 5 (1.5 g, 5.44 mmol) in 92% yield.17
Redistribution of 5 in Benzene-d6/THF solution. In an NMR
tube, 0.15 g of 5 (0.54 mmol) was dissolved in 3 mL of a benzene-
d6/THF (1:1) solution. The measured NMR data were tentatively
assigned to the formation of the redistribution products bis(3-
dimethylgallylphenyl · THF)methylgallium · THF (13) and trimethyl-
1,3-Bis(dimethylgallyl)-5-n-butylbenzene (6). Trimethylgallium
(5.39 g, 46.92 mmol) was added to 1,3-bis(chloromercurio)-5-n-
butylbenzene (4) (1.18 g, 1.96 mmol) in a thick-walled Schlenk
flask. The resulting colorless suspension was heated at 120 °C for
4 h in the closed flask to afford a colorless solution. All volatile
components were removed at reduced pressure (10 mbar). To the
remaining colorless amorphous solid was added trimethylgallium
(1.0 mL) to give a colorless suspension, which was again heated
at 70 °C for 2 h to a give a colorless solution. The solution was
maintained at 40 °C without stirring overnight to give 6 as colorless
crystals. The liquid components were removed by syringe, and the
remaining crystals were washed with cold hexane (5 mL) at -80
°C and dried in vacuo (10 mbar) for 3 h to give 6 (0.63 g, 1.8
mmol) in 92% yield.17
1,4-Bis(dimethylgallyl)benzene (9). Trimethylgallium (16.62 g,
144.7 mmol) was added to 8 (3.31 g, 6.03 mmol) in a thick-walled
Schlenk flask. The resulting colorless suspension was heated at 140
°C for 5 h in the closed flask to give a colorless solution. The
solution was maintained at 40 °C without stirring for 3 days.
Colorless crystals of 9 separated from the solution. The liquid
components were removed with syringe, and the remaining crystals
were washed with cold hexane (5 mL) and dried in vacuo (10 mbar)
for 3 h to give 9 (1.42 g, 5.15 mmol) in 86% yield.17
1,3,5-Tris(dimethylgallyl)benzene (12). Trimethylgallium (7.40
g, 64.4 mmol) was added to 1,3,5-tris(chloromercurio)benzene (11)
(1.19 g, 1.61 mmol) in a thick-walled Schlenk flask. The resulting
gray suspension was heated at 100 °C for 7 h in the closed flask to
give a colorless solution. The solution was maintained at 40 °C
without stirring overnight. Colorless crystals of 12 separated from
the solution. The liquid components were removed by syringe, and
the remaining crystals were washed twice with hexane (10 mL)
and dried at reduced pressure (10 mbar) for 3 h to give 12 (0.56 g,
1.5 mmol) in 93% yield.17
1
gallium · THF (14). H NMR (benzene-d6) [δ/ppm] 13: -0.06 (s,
12H, GaMe2), 0.14 (s, 3H, GaMe), 1.48 br (THF), 3.51 br (THF),
7.42 (m, 2H, aryl), 7.74 (m, 4H, aryl), 8.22 (s, 2H, aryl). 14: -0.28
(s, 9H, GaMe3), 1.48 br (THF), 3.51 br (THF). Evaporation of all
volatiles led to an insoluble solid residue.
Solid-State Decomposition of 5 and Its Regeneration. Two
Schlenk flasks, A and B, were connected by a bent ground-glass
joint. Flask A was charged with compound 5 (2.10 g, 7.61 mmol),
and the system was evacuated (0.1 mbar). Flask B was then cooled
with liquid nitrogen. Flask A was heated at 120 °C for a period of
30 min. Thereafter, an amorphous insoluble solid remained in flask
A. Compound 5 liberated 0.79 g (7.02 mmol) of trimethylgallium,
which was collected in flask B and characterized by 1H NMR
spectroscopy [(benzene-d6) δ ) -0.28 ppm (s, GaMe3)].
The amorphous solid (1.31 g) was combined in a thick-walled
Schlenk flask with trimethylgallium (15.00 g, 135.9 mmol). The
Acknowledgment. We thank Professor Lorberth, Univer-
sity of Marburg, Germany, for a gift of trimethylgallium.
The financial support of the University of Bielefeld, the
Deutsche Forschungsgemeinschaft, and the Fonds der Che-
mischen Industrie is gratefully acknowledged.
Supporting Information Available: A figure of the structure
of 12 (Figure 1*) and a table with structure parameters of the
benzene rings in 5, 6, 9, and 12 (Table 1*) are deposited. The
crystallographic data for compounds 5, 6, 9, and 12 are deposited,
including the intramolecular distances, bond angles, and the
calculated positions of hydrogen atoms. This information is
available free of charge via the Internet at http://pubs.acs.org.
(17) As explained in more detail in the chapter “Synthesis and
Characterization”, it was not possible to get reliable elemental analyses
(partial oxidation, chemisorbed trimethylgallium).
OM900048C