Inverse sandwich

Another group of charged Gp derivatives needs to be mentioned, a group of inverse sandwich complexes of the type [CpM2]+. Here, a Gp unit is flanked by two alkali metals. Examples include the triple cation [(MeCp)Li2(TMEDA)2] 130,125 in addition to the [CpNa2(THF)6] 131126 and [ BuCpKAlS-crown-h)] 132127 cations. 

A highly unusual trinuclear Sm(n)/Sm(m) mixed valence inverse sandwich was obtained when Cp Sru[//-OSi(Ollu )3]3Sm was treated with either phenylacetylene or 4-Mc-2,6-Bu 2( V,I I2OII (=ArOFI) in toluene (Scheme 103). Most of the products were structurally characterized by X-ray analyses, and the reactions illustrated in Schemes 100 to 103 represent only part of the interesting chemistry discovered with this system.506... 

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.)... 

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. 

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]. 

Reaction of a digermyne with cyclooctatetraene has been reported to give a Ge(II) inverse sandwich, a result of complete Ge=Ge bond cleavage and formation of a 7T-bound cyclooctatetraene ring. " This isomerizes to the thermodynamic product, a tetracyclic diene-digermane, in which a single-bonded Ge-Ge moiety has inserted into a C=C bond of the cyclooctatetraene carbocycle (Scheme 26). 

CHART 11 Inverse sandwiches of anionic benzene rare earth complexes as a l,4-hexadiene-2,5-diyl (left) and as a benzene radical anion (rigjit) hgand reported by Lappert s group. 

FIGURE 1 Molecular structures of inverse sandwiches of dianionic biphenyl yttrium cranplexes reported by Fryzuk s group. Figures were redrawn cfier Fryzuk et al. (1997). 

CHART 12 Inverse sandwiches of dianionic biphenyl yttrium complexes reported by Fryzuk s group. 

CHART 13 Inverse sandwiches of dianionic naphthalene lanthanide complexes reported by Bochkarev s group. 

CHART 14 Inverse sandwiches of anionic naphthalene yttrium and lutetium complexes reported... 

This chapter summarized recent advances in the reduction chemistry of rare earth metals and described our own efforts in synthesizing inverse sandwiches of rare earth arene complexes using ferrocene-based diamide ligands. Unprecedented molecules were synthesized and their unusual electronic structures were studied. Highlights included the synthesis of the first scandium naphthalene complex and its reactivity toward P4 activation and the isolation and characterization of a 6-carbon, lOTi-electron aromatic system stabilized by coordination to rare earth metals. The reactivity of those complexes was also discussed. 

C—H Borylation of an Inverse Sandwich Uranium Arene Complex 66... 

The C—H bond activation of benzene leading to equimolar amounts of metal hydride and metal phenyl products resembles the oxidative addition mechanism by late transition metal complexes, in which a single, mixed metal hydride phenyl product forms. The intriguing reactivity and its generality for rare-earth metals prompted a detailed mechanistic study. Inspired by previous reports of inverse-sandwich rare-earth metal arene com-plexes and taking into account the capacity ofbenzene to accept elec-... 

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