A simple synthesis for donor-stabilized Ga2I4 and Ga3I5 species and the X-ray crystal structure of Ga3I5·3PEt3

Article abstract of DOI:10.1039/a703776g

The reaction of 'GaI' synthesized by ultrasonic irradiation of gallium and diiodine in the presence of phosphanes leads to new gallium subhalides in which gallium-gallium bonds are involved.

Full text of DOI:10.1039/a703776g

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A simple synthesis for donor-stabilized Ga2I4 and Ga3I5 species and the X-ray
crystal structure of Ga3I5·3PEt3
Andreas Schnepf, Clemens Doriat, Evelyn Mo¨llhausen and Hansgeorg Schno¨ckel*
Institut fu¨r Anorganische Chemie, Universita¨t Karlsruhe, Engesserstraße, 76128 Karlsruhe, Germany
The reaction of ‘GaI’ synthesized by ultrasonic irradiation of
gallium and diiodine in the presence of phosphanes leads to
new gallium subhalides in which gallium–gallium bonds are
involved.
bond angle in the same direction [3, 101.83(5)°, 1 107.64(4)°; 2,
109.0(1)°], which demonstrates that a weaker donor bond is in
line with a more sp2-like geometry at the Ga atom.10
However, steric interaction may also influence the Ga–Ga
distance since Ga2I4·2PPh3 4, which has been synthesized by
ultrasonic treatment of ‘GaI’ with PPh3 exhibits a Ga–Ga
distance of 244.4(2) pm.
The second compound isolated from the yellow solution is
Ga3I5·3PEt3 5 and its molecular structure is shown in Fig. 2. It
is the first neutral compound containing a Ga3 unit (similar to
B3F511).
The molecular chemistry of the heavier low-valent group 13
elements has gained special interest during recent years.1 One
reason for this rapid development is the cryochemical prepara-
tion method in which the high-temperature molecules AlX and
GaX (X = Cl, Br, I) are trapped in suitable cooled solvents.2 By
this method donor-stabilized Al4I42 and Ga8I83 compounds can
be synthesized.
In 5 a GaI centre is the bridging entity for two GaII units with
Ga–Ga distances of 245.1(1) and 246.0(1) pm. As expected, the
Ga–I distances in the terminal GaI2 units [261.0(1) pm], are
shorter than for the central GaI [262.7(1) pm], a fact which is in
line with the larger size of GaI relative to GaII. The same
argument is valid for the corresponding Ga–P distances:
242.7(3) and 240.5(3) pm. More unexpected are the 31P NMR
shifts in comparison with that of uncoordinated PEt3 (d 219).
The shifts of the PEt3 groups at the GaII centers are observed at
d 224 and of PEt3 at the central GaI unit at d 28. Owing to the
more positively polarized P atom coordinated to GaI (d 28) and
to a more tetrahedral like geometry at phosphorus [sum of C–P–
C bond angles is 310.3(13)° cf. 320.5(7)° for terminally
coordinated PEt3 ligands] a stronger donor bonding from PEt3
to GaI than to GaII is concluded.
A much more convenient starting material for many low-
2
valent gallium species is Ga2Cl4, which consists of Ga+GaCl4
ions in the solid4 as well as in the molten state.5 By addition of
arenes a series of Ga+–arene compounds with interesting Ga–p
interactions has been prepared.6 On the other hand geniune
gallium(ii) species with Ga–Ga bonds are formed by addition of
special donor compounds. Ga2Cl4·2dioxane7 was the first
prominent example which was followed by the analogous
bromides and very recently by two examples of donor-stabilized
Ga2I4 compounds.3,8 Here we present a new route to low-valent
donor-stabilized, gallium iodides, based on a method described
by Green et al., in which pale green ‘GaI’ is formed from a
reaction of gallium with iodine in toluene under ultrasonic
conditions at ca. 35 °C;9 however, no definite species have, as
yet, been isolated.
The overall reaction pathway may be described by Scheme 1.
This scheme demonstrates the easy synthesis of donor-
On treating ‘GaI’ in toluene with PEt3 at 278 °C a yellow
solution was obtained which exhibited 31P NMR signals at d
224 and 28 (doublet, triplet, coupling constants of 35 Hz) and
a singlet at d 220, which provides evidence for new molecular
species.
The solid metallic residue was analyzed to be elemental
gallium. This means that the soluble compound cannot be a pure
GaI species since the amount of gallium and iodine was
originally 1:1.
I(1)
P(1)
When the yellow solution was concentrated two new species
could be isolated. The first was obtained as a white solid at
278 °C. The white powder can be recrystallized from toluene to
provide colourless crystals. X-Ray analysis indicates the
formation of (GaI2·PEt3)2 1, the third example of donor-
stabilized Ga2I4, Ga2I4·2AsEt3 28 and Ga2I4·2NEt3 33 having
been characterized very recently.
These iodides exhibit the expected structure with tetrahed-
rally coordinated gallium atoms and gallium–gallium bonds,
Fig. 1.
The gallium–gallium distances are nearly identical for 1
[243.6(2) pm] and 2 [242.8(2) pm] but are shorter than in 3
[249.8(7) pm]. These changes in bond length may be explained
with a decrease of donor strength in the series NEt3 ? PEt3 ?
AsEt3. This interpretation is based on an increase of the I–Ga–I
I(2)
Ga(1)
P(2)
I(3)
Ga(2)
Ga(3)
I(5)
C(15)
C(16)
P(3)
C(13)
I(4)
C(17)
C(18)
C(14)
Fig. 2 Perspective view of the molecular structure of Ga3I5(PEt3)3 5.
Selected bond lengths (pm) and angles (°): Ga(1)–Ga(2) 245.1(1), Ga(2)–
Ga(3) 246.0(1), Ga(1)–I(1) 260.1(1), Ga(2)–I(3) 262.7(1), Ga(3)–I(4)
260.8(1), Ga(1)–P(1) 240.4(3), Ga(2)–P(2) 242.7(3); Ga(1)–Ga(2)–Ga(3)
121.9(1), Ga(2)–Ga(1)–I(1) 117.76(5), Ga(2)–Ga(1)–I(2) 109.34(5),
I(1)–Ga(1)–I(2) 107.23(5), P(1)–Ga(1)–Ga(2) 121.85(8), P(2)–Ga(2)–I(3)
98.76(8).
I
D
I
Ga Ga
I
D
I
Fig. 1 Schematic presentation of GaI2 compounds (D = donor group)
Chem. Commun., 1997
2111

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(GaI2·PPh3)2 4: monoclinic, space group C2/c,
a = 14.271(3),
PPh3
PEt3
b = 15.964(3), c = 17.310(3) Å, b = 95.16(3)°, U = 3927.7(14) Å3, Z = 8,
Dc = 1.981 g cm23, m = 46.21 cm21, F(000) = 2200. The structure
analysis was based on 4141 reflections (2qmax 49.98°), observed 2187 [F >
4s(F)], 199 parameters. Final residuals: R = 0.038, Rw = 0.081.
(GaI2•PPh3)2
4
2Ga + I2
'Gal'
(GaI2•PEt3)2
+
Ga3I5(PEt3)3
1
5
Ga3I5(PEt3)3 5: orthorhombic, space group Pna21, a
= 13.961(3),
b = 12.550(3), c = 21.080(4) Å, U = 3693.3(13) Å3, Z = 4, Dc = 2.155
g cm23, m = 64.89 cm21, F(000) = 2224. The structure analysis was based
on 6255 reflections (2qmax 55.02°), observed 3728 [F > 4s(F)], 264
parameters. Absorption correction (min., max. transmission 0.0125,
0.0517). Final residuals: R = 0.036, Rw = 0.074. CCDC 182/610.
Scheme 1 Overall reaction pathway
stabilized low-valent gallium iodides, although it does not
explain the reaction pathway. However, the results presented
here are a strong encouragement to apply this simple synthetic
method, besides co-condensation techniques, in order to prepare
uncoordinated gallium subhalides.
1 W. Uhl, Angew. Chem., Int. Ed. Engl., 1993, 32, 1386.
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1996, 35, 129.
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Chem., in the press.
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10 Extensive ab initio calculations (DFT, SCF, MP2) have been performed
to obtain a better understanding of the bonding. However, these results
are not presented here, since they did not allow a deeper understanding
than the data provided by experiments described herein.
11 P. L. Timms, J. Am. Chem. Soc., 1967, 89, 1629.
Footnotes and References
* E-mail: hg@achpc9.chemie.uni-karlsruhe.de
† (GaI2·PPh3)2 4: colourless crystals, ca. 30% yield, 31P NMR: d 218.23
(s). (GaI2·PEt3)2 1: colourless crystals, 22% yield, 31P NMR: d 220.09 (s).
Ga3I5(PEt3)3 5: yellow crystals, 33% yield, 31P NMR: d 28.38 (t, J 35 Hz),
224.05 (d, J 35 Hz).
‡ Crystal data: STOE STADI IV diffractometer, Mo-Ka radiation, T = 200
K. All structures have been solved by direct methods and refined by full-
matrix least squares on F2. Programs used: SHELXL-93 and SHELXS.
(GaI2·PEt3)2 1: monoclinic, space group P21/n,
a = 10.034(2),
b = 11.857(2), c = 10.763(2) Å, b = 92.60(2)°, U = 1279.2(4) Å3, Z = 4,
Dc = 2.293 g cm23, m = 70.51 cm21, F(000) = 812. The structure analysis
was based on 4311 reflections (2qmax 49.98°), observed 2044 [F > 4s(F)],
94 parameters. Absorption correction (min., max. transmission 0.47, 0.73).
Final residuals: R = 0.068, Rw = 0.180.
Received in Basel, Switzerland, 30th May 1997; 7/03776G
2112
Chem. Commun., 1997