Cages and Clusters of Indium: Spherical Aromaticity?

Abstract

The cluster chemistry of main-group elements has developed fruitfully during the past decades [1, 2]. When focusing on the heavier Group 13 elements, aluminum and gallium clusters have been examined most extensively, showing both polyhedral [E n R n ] xÀ (n ¼ 4, 6, 8, 9, 12; x ¼ 0, 1, 2) and element-rich clusters [E n R m ] xÀ (n > m, for example n ¼ 5, 8, 9, 10, 22, 26, 77, 82) [3]. Cluster compounds of indium, despite the higher stability of low oxidation states, have been investigated only marginally. A variety of concepts has been introduced to explain the structures and stoichiometry of main-group cluster compounds, the most commonly known being the WadeWilliam-Rudolph rules [4-6]; for higher, conjugated clusters these rules were later modified by Jemmis [7,8]. Element-rich clusters [E n R m ] xÀ , especially of the heavier elements of Group 13, were classified as metalloids [2, 3].Cluster compounds such as the polyhedral boranates [B n H n ] 2À have been discussed as three-dimensional (3-D) aromatic systems [9][10][11]. According to Paul von Rague Schleyer, a diatropic ring current is the most typical property of an aromatic molecule [12]. For endohedral fullerenes, it was proved that chemical shifts for atoms in endohedral and exohedral positions behave comparable to shifts of atoms in and out of the ring plane of an aromatic hydrocarbons, respectively [13]. Aromaticity can be quantified by the calculated magnetic shielding constants at selected regions of a molecule, where no atoms reside. The resulting nucleus-independent chemical shifts (NICSs) are negative (diatropic) for aromatic molecules, positive (paratropic) for anti-aromatic molecules, and approximately zero for non-aromatic molecules [12].It has been demonstrated that the skeletal number of electrons, as well as the substituents of boron cluster compounds, influence the NICS values in the center of clusters [14]. Whilst B 8 Cl 8 and B 9 Cl 9 are well-known compounds [15], it was not possible to prepare B 8 H 8 and B 9 H 9 . According to NICS values in the cluster center, B 9 F 9 is aromatic, as well as [B 8 H 8 ] 2À [16, 17] and [B 9 H 9 ] 2À [18], a closo-cluster compound by the Wade rules. B 9 H 9 , in contrast, is paratropic, which means Modeling of Molecular Properties, First Edition. Edited by Peter Comba.