Inverse sandwich compounds

FIGURE 13.34 An Inverse Sandwich Compound, KthfljCal/x-CgHj-I.B.S-Phj Ca(thf)j]. (Molecular structure drawing created from CIF data, with hydrogen atoms omitted for clarity.)... 

The 1 2 complexes of hexamethylbenzene with BiXs (X = Cl and Br) are inverse-sandwich compounds, with two bismuth atoms on each side of the arene plane. The supramolecular architecture is formed from tetranuclear tectons based upon eight-membered Bi4X4 quasi-rings, interconnected into a three-dimensional architecture [459]. 

A variation on the theme of metallocenes and related sandwich compounds is provided by the inverse sandwich in Figure 13.34, with calcium(I) ions on the outside and the cyclic pi ligand 1,3,5-triphenylbenzene in between. This compound was most efficiently prepared by reacting 1,3,5-triphenylbenzene with activated calcium in THF solvent using catalytic amounts of l-bromo-2,4,6-triphenylbenzene. Although the product of this reaction is highly sensitive to moisture and air and is pyrophoric, it represents a rare example of a +1 oxidation state among the alkaline earths. 

Interesting examples of self-assembly are provided by the arene complexes of antimony halides (so-called Menschutkin-type complexes). The addition compound between benzene and antimony trichloride, C6H6-2SbCl3, has been known for a long time [407, 408], but its crystal structure has been determined by X-ray diffraction only recently [409], It consists of SbCls molecules r-bonded to both faces of the benzene ring, to form an inverse sandwich moiety, 164. Further secondary Sb - bonds connect these tectons into a self-organized layer structure. 

Photo-irradiation of [Pd3 Ar(CH = CH)4Ar 2][BAr ]2 results in an unusual photochemical face inversion of the multihapto-coordinated p-olefin ligands on a metal-metal chain. This process originates from the unique electronic structure of the polyene sandwich compounds as well as from the 7i-stacking of the pTi-conjugated ligands (Scheme 4S)P... 

In a kind of inversion of this concept, hydrocarbons can also serve as platforms for polycluster systems the tri- and tetrametallic complexes shown in Chart 8 (A and B) were recently prepared in our laboratory and characterized via X-ray crystallography at the University of Heidelberg (59). Compounds of this type are prototypes for extended oligomeric and polymeric networks such as that proposed in Chart 8(C), that are expected to exhibit extensive electron-delocalization between the metal centers (60). Systems of this kind, together with alkynyl-and phenyl-linked polyhedra, and multidecker sandwich-based arrays, are current targets of our research (61). 

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