Al4(C5Me4H)4: Structure, reactivity and bonding
Graphical abstract
Structure and Al–Al bond energy of the second substituted Al4Cp4 compound are discussed in comparison to and its high stability.
Introduction
The solid structures of Al(I) organyl-compounds are well known since 15 years. The first example which was characterized via X-ray crystallography was AlCp∗ (1) (Fig. 1), containing tetrameric units with a tetrahedral framework of four Al atoms in the centre [1], [2]. Every Al atom is η5 coordinated by a pentamethylcyclopentadienyl ring moiety. Up to now only five further tetrameric and tetrahedral Al(I) compounds were synthesized and structurally characterized [3], [4], [5], [6], [7], [8], [9], [10]. However, none of these compounds does contain only unsubstituted Cp-ligands as stabilizing units in the molecule. Thus also the existence of Al4Cp4 (2) was only proved in solution via 27Al NMR spectroscopy supported by quantumchemical calculations [11]. The reason for the lack of crystalline 2 is its higher tendency to disproportionate towards aluminium metal and AlCp3. This reaction starts in solution as well as in the solid state at temperatures already above −60 °C [12], [13]. In contrast to 2, (1) shows a higher stability: the decomposition starts slowly above ca. 140 °C. Furthermore, a temperature-dependent equilibrium in solution between compound 1 and its carbenanalogous AlCp∗ monomers has been observed experimentally. Because of the existence of AlCp∗ in solution at room temperature, reactions, e.g. with transition metal carbonyls were not unexpected [14], [15].
Now we were able to synthesize the compound Al4(C5Me4H)4 (3) and to characterize it by X-ray crystallography. Furthermore the bonding situation of 3 and the equilibrium between its monomeric and tetrameric moieties is described in detail on the basis of DFT calculations. These results are discussed in comparison with those of 1 and 2.
Section snippets
Experimental
All reactions were carried out under nitrogen atmosphere. The solvents were refluxed with sodium benzophenone and distilled under nitrogen prior to use. The compound Mg(C5Me4H)2 was prepared as described in literature [16]. All NMR spectra were recorded with a BRUKER ADVANCE 400. Chemical shifts are reported in δ units (ppm) referenced to C6D5H (7.16 ppm, 1H), C6H6 (128.8 ppm, 13C) and external [Al(H2O)6]3+ (0.5 M, 0 ppm, 27Al).
Results and discussion
Compound 3 is obtained via a metathesis reaction (Eq. (1)) of an AlBr solution (toluene/thf (3:1)) and Mg(C5Me4H)2 at −78 °C. The metastable AlBr solution is obtained by cocondensation of the AlBr molecules, generated at 900 °C, with a mixture of toluene/thf (3:1) at −196 °C and subsequent warming to −78 °C [12], [13]. Single crystals of 3 were isolated from heptane at −25 °C after extraction of the reaction products.
The chemical
Summary and outlook
Al4(C5Me4H)4, the second Al(I)-compound with a tetrahedral Al4 framework, which solely contains modified Cp ligands, was synthesized and characterized by X-ray crystallography. Furthermore, it could be shown via 27Al NMR investigations and thermodynamic considerations, based on DFT calculations, that in solution only tetrameric units exist and consequently no reactions as for – here carbene-like AlCp∗ molecules are present in solution – could be expected. Our investigations presented
Supplementary material
CCDC 634979 and 634980 contain the supplementary crystallographic data for 3 and 4. These data can be obtained free of charge via http://www.ccdc.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft, the Centre for Functional Nanostructures (CFN), and the Fonds der Chemischen Industrie for financial support.
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