Communication
Al5Br7⋅5 THF—The First Saltlike Aluminum Subhalide
Christoph Klemp,
Gregor Stößer,
Ingo Krossing Dr.,
Hansgeorg Schnöckel Prof. Dr.,
Christoph Klemp
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorGregor Stößer
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorIngo Krossing Dr.
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorHansgeorg Schnöckel Prof. Dr.
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorChristoph Klemp,
Gregor Stößer,
Ingo Krossing Dr.,
Hansgeorg Schnöckel Prof. Dr.,
Christoph Klemp
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorGregor Stößer
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorIngo Krossing Dr.
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorHansgeorg Schnöckel Prof. Dr.
Institut für Anorganische Chemie der Universität Karlsruhe (TH) Engesserstrasse, Gebäude 30.45 76128 Karlsruhe (Germany) Fax: (+49) 721-608-4854
Search for more papers by this authorFirst published: 13 October 2000
Citations: 38
Abstract
Multiple insertion of AlBr in bonds of AlBr3 leads to Al5Br7⋅5 THF—a new type of low-valent aluminum halide. The actual, ionic structure of the subhalide [Al5Br6⋅6 THF]+[Al5Br8⋅4 THF]− (the anion is shown schematically) appears plausible on the basis of density functional theory calculations. This and similar subhalides can play the role of potential intermediates in the electrolytic deposition of metals.
References
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- 9 X-ray structure analyses of 1 and 2: The data were collected on a STOE-IPDS diffractometer with MoKα radiation (λ=0.71073) and graphite monochromator. The structures were solved by direct methods and refined anisotropically against F 2 for all observed reflections. Hydrogen atoms were refined in calculated positions according to a riding model. Programs used: SHELXS and SHELX(T)L (G. M. Sheldrick, Universität Göttingen), Resview, Diamond. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-143867 (1) and CCDC-143868 (2). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: [email protected]). 1: Colorless needle, 0.3×0.1×0.1 mm3, measurement temperature 180 K, orthorhombic, space group Pca21 (no. 29), a=30.060(3), b=9.0121(6), c=16.0246(11) Å, V=4341.1(6) Å3, Z=4, ρcalcd=1.644 Mg m−3, μMo=2.938 mm−1, θmin=2.26°, θmax=20.81°, reflections: 16 026 measured, 4302 independent, 3400 observed, 350 parameters, numerical absorption correction, Rint=0.0544, R1=0.0428, wR2=0.1089, GOF=1.046, 1 restraint, max./min. residual electron density 0.501/−0.462 e Å−3. The THF molecules in 1 are slightly disordered. This disorder was incorporated in the stucture refeinement by increased anisotropic temperature factors and H atoms (at calculated positions). Four toluene molecules (likewise disordered) per Al2I4 unit are housed in the crystal, which evidently contribute to the low melting point (ca. 0 °C). 2: Colorless platelets, 0.3×0.2×0.05 mm3, measurement temperature 200 K, triclinic, space group P1¯ (no. 2), a=10.980(4), b=19.075(5), c=21.080(7) Å, α=109.88(4), β=102.07(4), γ=103.97(2)°, V=3818(2) Å3, Z=2, ρcalcd=1.835 Mg m−3, μMo=7.496 mm−1, θmin=1.93°, θmax=24.01°, reflections: 23 476 measured, 11 262 independent, 6457 observed, 964 parameters, numerical absorption correction, Rint=0.0947, R1=0.0546, wR2=0.1366, GOF=0.879, 100 restraints (for THF in the disordered portion), max./min. residual electron density 0.884/−1.049 e Å−3 (localized around Br atoms). Several crystals of 2 from different solutions were investigated by X-ray crystallography. The structure refinement repeatedly revealed a disorder, in particular, of the ligand-bearing Al atoms, in the molecules 2 a and 2 b, which was described by two completely isomeric molecules for 2 a and for 2 b, whereby the one isomer (83 %) can be converted into in the other isomer (17 %) by reflection at the respective central atom (two local inversion centers at Al10 and Al50 in Figure 1). The four ligand-bearing Al atoms of the S4-symmetric Al frameworks are thus the only atoms that are not inversion-symmetric, in contrast to the ligand shell (bromide and THF), which, however, determines the packing in the crystal. For a 1:1 disorder and only one type of molecule (thus only one inversion center), this problem could have been solved through the choice of an appropriate space group, as an analogous problem has shown.[8] Here the detailed description using two complete molecules (with R1=0.0546 for a reflection/parameter ratio of 6.7) was favored over a description using only disordered Al atoms (with R1≈0.09 for a reflection/parameter ratio of 10). The superposition of these disorders can falsify bond lengths; thus, especially the distances to the central Al atoms (in particular in 2 b) should not be considered and quoted for an exact discussion.
- 10 The bond lengths in 1 lie in the expected range and are similar to those of Al2I4⋅2 OEt2, which were compared with calculated distances in ref. [7a].
- 11 All quantum-chemical calculations were carried out with the RIDFT module (B-P86/SVP functional) of the TURBOMOLE program[26] with SV(P) basis set on the following systems: [Al5Br6⋅6 THF]+ in C1, [Al5Br8⋅4 THF]− in S4, Al5Br7⋅5 THF in C1 (and analogously for Ga), Al2Br4⋅2 H2O in C2h, Al22Cl20⋅12 H2O in Ci.
- 12 The population analysis[27] of 2 a shows a partial charge of −0.76 for the central Al0 atom and +0.12 for the surrounding AlII atoms. In 2 b these values are −0.81 for Al0, +0.07 for AlII, and +0.19 for AlI atoms. The partial charge of the Br atom bound to AlI (−0.17) is less negative than those for the Br atoms bound to AlII. For these the value is −0.20 and for Br52 as well as Br54 (longer Al−Br bond) −0.24.
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- 18 An overview of other follow-up products of such insertion reactions with subsequent AlX3 cleavage revealed compound Si(AlCl2⋅OEt2)4 which is isoelectronic to 2 a. In the presence of AlCl and AlR this finally leads to a remarkable SiAl14R6 cluster.[28]
- 19 An analogous case in the third and fourth periods of Group 15 is PCl5, which exists in the ionic components PCl4+ and PCl6−, whereas AsCl5 is unstable in the ionic form.
- 20 Calculated reaction energies and free reaction enthalpies do not differ in this reaction, because no entropy is produced. The solvation enthalpy liberated in the reaction in solution is almost identical for the ionic Al and Ga compounds and can here amount to maximal 185 kJ mol−1 (dielectric constant εr→∞) according to the Born equation.[29] For decreasing polarity it sinks for a THF solution (εr=7.52) to 160 kJ mol−1, for diethyl ether (εr=4.27) to 142 kJ mol−1, and for toluene (εr=2.2) to 101 kJ mol−1. The solvation enthalpy of the uncharged starting materials is neglected in this approximation. Whether the halide transfer favored for 2 already occurs in solution or first in connection with the crystallization, has yet to be verified.
- 21
The organometallic contact ion pair Cp
Al5I6 is to date the only example for Al−Al bonds in a cationic component (Cp
Al3I2+).[30]
- 22
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- 25 As a low-valent halide, 2 provides a potential reaction channel in the electrolytic deposition of Al, by which initially formed AlX species can immediately react further with AlX3 present in excess to give Al2X4. Further follow-up reactions such as our observed formation of 2, however, presumably proceed only for large local AlX concentrations. The formation of low-valent aluminum compounds in the anodic oxidation of aluminum under high current denisty is already known.[31]
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- 30
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