Aromaticity DOI: 10.1002/anie.201105081 Li NMR Spectroscopy on Crystalline Li 12 Si 7 : Experimental Evidence for the Aromaticity of the Planar Cyclopentadienyl-Analogous Si 5 6À Rings** Alexander Kuhn,* Puravankara Sreeraj, Rainer Pçttgen, Hans-Dieter Wiemhçfer, Martin Wilkening,* and Paul Heitjans* Since the pioneering work of West et al. in 1981 [1] that questioned the well-known double-bond rule, chemists work- ing in the field of organometallics have been trying to synthesize silicon analogues of organic compounds containing p bonds. However, most of these compounds inherently lack thermodynamic stability, thus making their preparation a very challenging task. In these compounds, Si =Si p bonds have to be kinetically stabilized by large, sterically hindering groups. Very recently, Abersfelder et al. [2] reported the synthesis of a tricyclic isomer of hexasilabenzene containing a Si 6 ring. Theoretical calculations suggest a nonclassical “dismuta- tional” aromaticity of the nonplanar hexasilabenzene. [3] However, classical benzene-like aromaticity is characterized by planarity and equal bond lengths that lead to an optimum overlap of the p orbitals to result in a cyclic electron delocalization. This effect increases the corresponding dia- magnetic susceptibility. Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique that can be used to experimentally assess whether a structure should be regarded as aromatic or not. Aromatic compounds show characteristic and relatively large chemical shifts d iso , which are explained by a ring current induced by the magnetic field. [4] A probe nucleus residing inside the anisotropic cone above or below the aromatic ring will be subjected to a shielding effect (upfield shift) that causes the NMR signal to appear at lower d iso values. To the best of our knowledge, classical aromaticity characterized by the criteria mentioned above has not been reported to date for a structural unit solely composed of Si. Lee and Sekiguchi [5] previously reported two probably aromatic, positively charged complex ligands containing Si 3 and Si 4 rings. The Si 4 ligands are nearly planar and their formal charge agrees with the Hückel rule; [6] these features are a good argument for the aromaticity of these compounds. However, the ultimate proof of their (carbon-like) aromatic- ity is still missing. In this work, the crystalline Zintl phase Li 12 Si 7 was investigated by both 1D and 2D 6 Li as well as 7 Li high- resolution, that is, magic angle spinning (MAS) NMR spectroscopy. The crystal structure of the binary silicide contains semi-infinite 1D 1 1 LiSi 10=2 Â Ã sandwich complexes (Figure 1). [7] Remarkably, the corresponding Li MAS NMR spectrum exhibits a distinct signal that is shifted upfield to À17.2 ppm. As we will show here, there is strong evidence that this NMR signal belongs to the Li ions sandwiched between the cyclopentadienyl-analogous Si 5 6À rings in Li 12 Si 7 . The suspected aromaticity of the planar Si 5 6À units, see for example Ref. [7b,c], results in a pronounced shielding of these Figure 1. a) Crystal structure of the Zintl phase Li 12 Si 7 (Pnma). In the right-hand unit cell, all Li atoms apart from Li6 are removed. b) Semi- infinite 1D sandwich complexes of 1 1 LiSi 10=2 Â Ã in Li 12 Si 7 . The notation follows that of Nesper et al., see Ref. [7b]. [*] A. Kuhn, Dr. M. Wilkening, Prof. Dr. P. Heitjans Institute of Physical Chemistry and Electrochemistry Leibniz University Hannover Callinstrasse 3a, 30167 Hannover (Germany) E-mail: kuhn@pci.uni-hannover.de wilkening@pci.uni-hannover.de heitjans@pci.uni-hannover.de Dr. P. Sreeraj, Prof. Dr. R. Pçttgen, Prof. Dr. H.-D. Wiemhçfer Institute of Inorganic and Analytical Chemistry University of Münster Corrensstrasse 28–30, 48149 Münster (Germany) [**] Financial support by the DFG and the BMBF is gratefully acknowl- edged. A.K. acknowledges support by the Studienstiftung des deutschen Volkes e.V. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201105081. 12099 Angew. Chem. Int. Ed. 2011, 50, 12099 –12102  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim