Reactivity of organoelement subhalides of gallium and indium - Ga-Ga and In-In bonds bridged by carboxylato ligands

Article abstract of DOI:10.1002/ejic.200300640

The organoelement subhalides R₂Ga₂I₂ (5) and R₃In₃I₂ [6; R = C(SiMe₃) ₃] reacted with silver benzoate AgOOCC₆H₅ to yield the dicarboxylato dielem

Full text of DOI:10.1002/ejic.200300640

FULL PAPER
Reactivity of Organoelement Subhalides of Gallium and Indium —
Ga؊Ga and In؊In Bonds Bridged by Carboxylato Ligands
Werner Uhl*[a] and Abdelhakim El-Hamdan[a]
Keywords: Carboxylate ligands / Gallium / Indium / Subvalent compounds
The organoelement subhalides R2Ga2I2 (5) and R3In3I2 [6; R =
C(SiMe3)3] reacted with silver benzoate AgOOCC6H5 to
yield the dicarboxylato dielement compounds R2E2(µ-
OOCC6H5-O,OЈ)2 (7, E = Ga; 8, E = In). Both compounds pos-
one another. Owing to the small bite of the carboxylato group
very short E−E bond lengths of 240.5 pm (Ga−Ga) and 265.4
pm (In−In) were observed.
sess E−E single bonds bridged by the chelating carboxylato ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
ligands. The chelating groups are arranged perpendicular to
Germany, 2004)
Introduction
Careful oxidation of the tetrahedral tetraelement clusters
Ga4[C(SiMe3)3]4 (1)[1] and In4[C(SiMe3)3]4 (2)[2] (Scheme 1)
with halogens or organic halogen donors yielded the corre-
sponding subhalides still containing the elements gallium
and indium in low oxidation states. The transfer of only one
bromine molecule by the reaction of 2 with 1,2-dibromoe-
thane gave the tetraindium compound 3 (Scheme 1), in
which the overall tetrahedral arrangement of the In atoms
is retained.[3] One face of the tetrahedron is bridged by a
µ3-Br atom, while the second Br atom is located above an
edge of that particular face. Reaction with hexachloroe-
thane, or mixtures of AlBr3 and Br2, yielded the diin-
dium() compounds 4, which contain localized InϪIn sin-
gle bonds and form dimers through chlorine or bromine
bridges.[3,4] The corresponding digallium() compound 5
was obtained by the reaction of 1 with ICl in the presence
of AlI3.[5] In contrast to the diindium compounds 4 it re-
mains monomeric, even in the solid state. An average oxi-
dation state of ϩ1.66 was found for the central atoms of
the triindium compound 6, which possesses a chain of three
In atoms connected by InϪIn single bonds and was ob-
tained by the treatment of 2 with I2/AlI3.[6]
These subhalides should be suitable starting compounds
for the generation of secondary products by salt-elimination
reactions. In a first investigation, which is described here in
detail, we employed silver benzoate as a reagent. The driv-
ing force of silver halide formation should favor these reac-
tions, and the chelating carboxylato ligand should stabilize
any secondary product by coordinative saturation of the Ga
or In atoms. Dicarboxylatodigallium compounds contain-
ing a GaϪGa single bond should result from the use of 5.
Scheme 1
Similar products have previously been obtained in our
group by another route when we treated tetraalkyldigal-
lane(4) R2GaϪGaR2 [R ϭ CH(SiMe3)2] with carboxylic ac-
ids. Two equivalents of bis(trimethylsilyl)methane were re-
leased in the course of these reactions, and two alkyl sub-
stituents were replaced by the chelating acid residues.[7,8] In
contrast, similar diindium dicarboxylates are not accessible
by such a reaction, which instead resulted in the quantitat-
ive cleavage of the InϪIn bond by oxidation of the In
atoms.[9] Owing to the singular structure of the digallium
compounds, which is discussed in detail below, the synthesis
and characterization of the corresponding diindium deriva-
tives was of particular interest. Indium subhalides seemed
to be suitable starting compounds for their generation.
[a]
Fachbereich Chemie der Philipps Universität,
Hans-Meerwein-Straße, 35032 Marburg, Germany
Fax: (internat.) ϩ49-(0)6421-282-5653
Eur. J. Inorg. Chem. 2004, 969Ϫ972
DOI: 10.1002/ejic.200300640
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
969

W. Uhl, A. El-Hamdan
FULL PAPER
Compound 8 is less stable than its digallium analogue and
decomposes already at 158 °C with the formation of a red-
brown solid.
Results and Discussion
Syntheses of the Dicarboxylatodigallium and -diindium
Compounds 7 and 8
Molecular Structures
Treatment of the digallium() diiodide R2Ga2I2 (5) with a
suspension of silver benzoate in toluene at low temperature
yielded colorless crystals of 7, after recrystallization of the
crude product from cyclopentane, in an almost quantitative
yield. The integration ratio of the 1H NMR spectrum
showed one phenyl group per alkyl substituent, as expected
for the complete replacement of both iodine atoms by
benzoato groups. The resonance of the methyl protons ap-
pears at δ ϭ 0.52 ppm, which is in the characteristic range
of compounds containing a GaϪGa single bond. The mo-
lecular structure of 7, possessing an intact GaϪGa bond,
was verified by an X-ray crystal-structure determination
(see below) and is shown schematically in Equation (1).
Compound 7 is thermally rather stable, with a melting point
of 222 °C.
Compounds 7 and 8 (Figure 1 and 2, respectively) con-
tain a GaϪGa or an InϪIn single bond terminally coordi-
nated by two tris(trimethylsilyl)methyl groups and bridged
by two carboxylato ligands. The bridging of metal-metal
bonds by carboxylato groups is a quite common structural
motif in transition metal compounds,[10] however, it is
extraordinarily rare in main-group chemistry.[11] While
GaϪGa bonds bridged by carboxylato groups have been
reported by our group previously,[7,8] the bridging of an
InϪIn bond is observed for the first time in compound 8.
Owing to the relatively long InϪIn bonds compared to
GaϪGa distances, and the small bite of the chelating li-
gand, such a structure was totally unexpected. According
to quantum-chemical calculations the terminal coordi-
nation of carboxylato groups is very unfavorable because of
the rather high energy required for the deformation of the
inner OCO angle.[8] Because of this inflexibility, a terminal
arrangement, with the coordination of only one metal
atom, leads to a relatively short distance between the inner
carbon atom of the chelate and the Ga or In atoms, which
results in considerable steric and electrostatic repulsion.
(1)
We hoped to synthesize the corresponding diindium()
carboxylate 8 by the treatment of the dimeric subhalide
(R2In2Br2)2 (4) with silver benzoate under similar reaction
conditions. However, elemental indium precipitated, and
the course of the reaction was more complicated. Crystals
of the colorless product 8 could be obtained in very small
quantities only. Interestingly, the diindium dicarboxylate 8
was finally isolated in 74% yield when we treated the triind-
ium diiodide R3In3I2 (6) with silver benzoate at low tem-
perature [Equation (1)]. The silver iodide precipitate be-
came black even at low temperature and in the dark, which
may be caused by the formation of elemental indium as a
by-product. However, tris(trimethylsilyl)methane, a possible
Figure 1. Molecular structure and numbering scheme of compound
7; the thermal ellipsoids are drawn at the 40% probability level;
methyl groups are omitted for clarity; selected bond lengths (pm)
and angles (°): Ga1ϪGa1Ј 240.48(7), Ga1ϪC1 198.9(2), Ga1ϪO1
200.6(1), Ga1ϪO2Ј 202.9(1); C1ϪGa1ϪGa1Ј 156.38(6),
O1ϪGa1ϪO2Ј 91.90(6), O1ϪGa1ϪGa1Ј 88.95(4), O2ЈϪGa1Ϫ
Ga1Ј 85.95(4); Ga1Ј and O2Ј generated by Ϫx, y, Ϫz ϩ 3/2
The rigidity of the CO2 backbone results in almost indis-
second product of a disproportionation reaction, could not tinguishable CϪO bond lengths [127.1 pm (7) and 126.8 pm
be detected by NMR spectroscopy. Formation of the tetra- (8), on average] and OϪCϪO bond angles [124.7° (7) and
indium cluster 2 as another possible side product could not 124.9° (8)]. Thus, the bite of the carboxylato ligands [225.2
be verified by NMR spectroscopy or its characteristic violet pm (7) and 224.9 pm (av., 8)] is almost the same in both
color, either. The crude product of the reaction shows two compounds despite the very different covalent radii of gal-
further SiMe3 resonances at δ ϭ 0.45 and 0.41 ppm, which lium and indium. A strong strain must result, which causes
may result from secondary products of the dispro- short EϪE bond lengths. While that of the digallium com-
portionation but could not be assigned unambiguously. pound 7 (240.5 pm) is in the lower range of GaϪGa single-
970
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Reactivity of Organoelement Subhalides of Gallium and Indium
FULL PAPER
cedures. Commercially available silver benzoate (99%; Aldrich) was
employed without further purification.
Synthesis of R2Ga2(µ-O2CC6H5-O,OЈ)2 [7; R ؍ C(SiMe3)3]: A sus-
pension of silver benzoate (69 mg, 0.301 mmol) in 20 mL of toluene
was cooled to Ϫ90 °C and treated with a solution of the diiodide
5 (129 mg, 0.151 mmol) in 10 mL of toluene. The mixture was
stirred and slowly warmed to Ϫ65 °C. The precipitate was filtered
off at Ϫ70 °C, and subsequently the solvent was removed in vacuo
at room temperature. The yellowish solid residue was recrystallized
from cyclopentane, cooling from 20 to Ϫ15 °C. Yield: 124 mg
1
(97%), colorless crystals, m.p. (argon, sealed capillary): 222 °C. H
NMR (C6D6, 400 MHz, 298 K): δ ϭ 0.52 (s, 27 H, SiMe3), 6.99
(m, 3 H, phenyl), 8.30 (m, 2 H, phenyl) ppm. 13C NMR (C6D6,
100.6 MHz, 298 K): δ ϭ 5.0 (GaC), 5.5 (SiMe3), 128.7 and 130.8
(m- and p-C of phenyl), 131.4 and 133.7 (ipso- and p-C of phenyl),
177.6 (CO2) ppm. IR (CsBr, nujol): ν˜ ϭ 1597 w phenyl; 1526 m
νCO2; 1462 vs. paraffin; 1411 s δCH3; 1377 vs. paraffin; 1306 w,
1260 s δCH3; 1174 vw, 1126 m, 1096 s, 1025 s δCH, νCC; 858 vs,
805 s, 754 vw, 718 m ρCH3(Si); 699 w, 684 w νasSiC; 639 vw, 625
vw νsSiC; 513 w, 468 w, 373 vw νGaC, νGaO.
Figure 2. Molecular structure and numbering scheme of compound
8; the thermal ellipsoids are drawn at the 40% probability level;
methyl groups are omitted for clarity; only the ipso-carbon atoms
of the phenyl groups are shown; selected bond lengths (pm) and
angles (°) [the atoms In3, In4, C3, C4, O5, O6, O7 and O8 belong
to the second independent molecule not shown here]: In1ϪIn2
265.44(9), In3ϪIn4 265.38(8), In1ϪC1 217.2(2), In2ϪC2 216.7(2),
In3ϪC3 217.0(2), In4ϪC4 216.9(2), In1ϪO1 222.8(2), In1ϪO3
222.8(2), In2ϪO2 221.6(2), In2ϪO4 221.1(2), In3ϪO5 221.4(2),
In3ϪO7 222.8(2), In4ϪO6 222.4(2), In4ϪO8 221.3(2);
C1ϪIn1ϪIn2 158.74(6), C2ϪIn2ϪIn1 161.07(6), C3ϪIn3ϪIn4
160.43(6), C4ϪIn4ϪIn3 160.50(6), O1ϪIn1ϪO3 89.93(6),
O2ϪIn2ϪO4 90.77(7), O5ϪIn3ϪO7 90.38(7), O6ϪIn4ϪO8
90.79(7), O1ϪIn1ϪIn2 85.13(4), O3ϪIn1ϪIn2 83.58(4),
O2ϪIn2ϪIn1 83.99(4), O4ϪIn2ϪIn1 85.53(5), O5ϪIn3ϪIn4
85.15(4), O7ϪIn3ϪIn4 83.41(4), O6ϪIn4ϪIn3 84.01(4),
O8ϪIn4ϪIn3 85.63(5)
Synthesis of R2In2(µ-O2CC6H5-O,OЈ)2 [8; R ؍ C(SiMe3)3]: Silver
benzoate (50 mg, 0.218 mmol) was suspended in 20 mL of toluene
and cooled to Ϫ90 °C. The suspension was treated with a solution
of the triindium diiodide 6 (138 mg, 0.107 mmol) in 15 mL of tolu-
ene. A black precipitate was formed immediately, which was filtered
off at Ϫ70 °C. The solvent was removed in vacuo at room tempera-
ture. The colorless residue was recrystallized from diisopropyl ether,
cooling from 20 to 8 °C. Yield: 82 mg (74%; 8·OiPr2); colorless
crystals which include up to one molecule of diisopropyl ether per
1
formula unit of 8; m.p. (argon, sealed capillary): 158 °C (dec.). H
NMR (C6D6, 400 MHz, 298 K): δ ϭ 0.49 (s, 27 H, SiMe3), 1.06
3
3
(d, JH,H ϭ 6 Hz, 6 H, Me of iPr2O), 3.43 (sept, JH,H ϭ 6 Hz, 1
H, CH of iPr2O), 7.03 (m, 3 H, phenyl), 8.33 (m, 2 H, phenyl) ppm.
13C NMR (C6D6, 100.6 MHz, 298 K): δ ϭ 5.6 (SiMe3), 15.5 (InC),
23.0 (Me of iPr2O), 68.1 (CH of iPr2O), 128.5 and 130.6 (m- and
p-C of phenyl), 132.8 and 133.1 (ipso- and p-C of phenyl), 176.6
(CO2) ppm. IR (CsBr, paraffin): ν˜ ϭ 1592 w phenyl; 1528 m νCO2;
1460 vs. paraffin; 1377 vs. paraffin; 1306 vw, 1260 s δCH3; 1124 m,
1086 m, 1025 s δCH, νCC; 859 vs, 843 s, 804 m, 718 m ρCH3(Si);
699 w, 680 w νasSiC; 654 vw, 614 vw νsSiC; 515 w, 460 m νGaC,
νGaO.
bond lengths reported for digallium() halides
Ga2X4(L)2,[12] for instance, and is similar to that of the
starting subhalide 5 (240.1 pm), the bridging of the InϪIn
bond forces a very close InϪIn contact in 8 of 265.4 pm,
which, to the best of our knowledge, is the shortest value
reported so far.[13] The InϪIn single bonds in the starting
subhalides 4 and 6 (Scheme 1) are considerably longer (282
pm on average).[3,6] The longer EϪE bond in 8 compared
to 7 causes slightly more acute EϪEϪO angles of 84.6°
(versus 87.5° in 7) and larger EϪOϪC angles of 122.3°
(119.3° in 7). The carboxylato ligands of both compounds
are arranged almost ideally perpendicular to one another
(OϪEϪO 91.9° and 90.5°, respectively). In accordance with
the almost linear CϪEϪE groups (156.4° and 160.2°), the
particular bonding situation of these compounds is de-
scribed by sp-hybridized Ga or In atoms[8] so that both
EϪO bonds are mainly formed by two p-orbitals of each
central atom. The phenyl substituents in both compounds
are almost ideally co-planar with the carboxylato groups
(angles between the normals of the corresponding planes
2.1° in 7 and 1.4 to 6.9° in 8).
Crystal-Structure Determinations: Single crystals of compounds 7
and 8 were obtained by cooling solutions in cyclopentane (Ϫ15
°C, 7) or diisopropyl ether (8 °C, 8). Crystal data and structure-
refinement parameters are given in Table 1. The molecules of com-
pound 7 reside on a crystallographic twofold rotation axis. They
crystallize with two cyclopentane molecules per formula unit,
which can easily be removed on evacuation. A slight disorder was
observed for the C(SiMe3)3 group; owing to the small occupation
factor of 0.055, only the silicon atoms were refined on split posi-
tions. Compound 8 crystallizes with two independent molecules in
the asymmetric unit, which further comprises two diisopropyl ether
molecules. A disorder was observed for the C(SiMe3)3 group at C4,
all atoms were refined at split positions with occupation factors of
0.76 and 0.24. A minor disorder was further detected for the
C(SiMe3)3 group at C2; owing to the small occupation of the se-
cond position (0.07), only the silicon atoms of the minor compo-
nent were refined.
Experimental Section
General: All procedures were carried out under purified argon in
dried solvents (toluene and diisopropyl ether over Na/benzo-
phenone; cyclopentane over LiAlH4). The compounds R2Ga2I2
(5)[5] and R3In3I2 (6)[6] were synthesized according to literature pro-
CCDC-218887 (7) and -218888 (8) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
Eur. J. Inorg. Chem. 2004, 969Ϫ972
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W. Uhl, A. El-Hamdan
FULL PAPER
Table 1. Crystal data, data collection parameters, and structure refinement of compounds 7 and 8[a]
7
8
Formula
Crystal system
Space group
Z
Temperature [K]
dcalcd. [g/cm3]
a [pm]
C34H64Ga2O4Si6·2C5H10
C34H64In2O4Si6·OC6H14
Monoclinic
P21/c no. 14[14]
8
193(2)
1.295
2260.3(5)
2814.9(6)
1782.8(4)
110.31(3)
10638(4)
Monoclinic
C2/c no. 15[14]
4
193(2)
1.211
2224.6(2)
1452.01(10)
1774.3(2)
109.47(3)
b [pm]
c [pm]
β [°]
V [10Ϫ30 m3]
5403.6(9)
1.166
µ [mmϪ1
]
1.037
Crystal size [mm]
Diffractometer
Radiation
θ range [°]
Index ranges
0.4 ϫ 0.3 ϫ 0.3
STOE IPDS
Mo-Kα radiation; graphite monochromator
1.90 to 25.93
Ϫ27 Յ h Յ 27
Ϫ17 Յ k Յ 17
Ϫ21 Յ l Յ 21
5238 [R(int) ϭ 0.0495]
4040
0.4 ϫ 0.3 ϫ 0.12
1.42 to 26.24
Ϫ28 Յ h Յ 28
Ϫ34 Յ k Յ 32
Ϫ22 Յ l Յ 22
21356 [R(int) ϭ 0.0415]
16776
Independent reflections
Reflections I Ͼ 2σ(I)
Parameters
275
0.0307
0.0758
0.604/Ϫ0.480
1131
0.0272
0.0863
0.804/Ϫ0.783
R ϭ Σ||Fo| Ϫ |Fc||/Σ|Fo| [I Ͼ 2σ(I)]
wR2 ϭ {Σw(F2o Ϫ Fc2)2 /Σw(F2o)2}1/2 (all data)
Max./min. residual [1030 e/m3]
[a]
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[11]
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Received September 8, 2003
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