Triple sandwich complex

Lanthanides form phthalocyanines PcMX, sandwich-like complexes of the type Pc2M, and triple-decker complexes of the type Pc3M2. 

Dihydro-l,3-diborolenes are accessible as pentaorgano derivatives " and serve as ideal precursors of the diborolyl radieal. The formation of the radical and its interaction with a metal center is formally described as a metal oxidation. The resulting 2,3-dihydro-1,3-diborolyI ligand is either a three-electron donor or an anionic four-electron donor. l,3,4,5-Tetraethyl-2-methyl-2,3-dihydro-l,3-diborole reacts with [T -CpNi(CO)2] or (T7 -Cp>2Ni in hot mesitylene to yield a sandwich complex and a triple-decked complex ... 

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. 

Figure 80 The structure of the triple-decker sandwich complex (CsPryBatCOTJBafCsPr s) 153.

Bocian, Lindsey and co-workers studied sandwich complex nanocapacitors comprised of porphyrin and phthalocyanine ligands separated by lanthanide metals [133]. A triple-decker sandwich of phthalocyanine-Eu-phthalocyanine-Eu-porphy-rin, with two phenylethynyl linker wires from the porphyrin, potentially has up to nine accessible oxidation states (—4 to +4). SAMs of monomers, dimers, trimers, and oligomers of this sandwich, anchored at one or both ends by thioacetyl groups, gave charge densities up to 10 10 mol cm-2, electron-transfer rates up to 105 electrons s-1, and charge-dissipation half-lives in the 10-50 s range. 

The cyclo-P3 unit can also behave as a three electron bridging ligand (45) to give a variety of homo and heterometallic triple-decker sandwich complexes which are treated in a recent review (100). 

A typical structure of these so called triple-decker sandwich complexes is that of the complex [(p3)Ni -(j)3-P3) Ni(p3)](BPh4)2-2.5Me2CO (73). In each complex the trihapto P3 (or trihapto As3) groups form a bridge between the two Ni(p3) residues.292 Each nickel atom is thus six-coordinated by three phosphorus atoms from the ligand p3 and by three phosphorus atoms from the cyclo-P3 (or arsenic atoms from the cyclo-As ), which acts as a three-electron-donor ligand. 

In the case of "simple sandwich complexes the X-ray diffraction studies have shown that the M-PCtriphos), M-P(cyclo) and P-(cyclo)-P[cyclo) distances reveal effects of both the increase in the principal quantum number and the "lantanide contraction". For "triple decker sandwich complexes the li-li distances increase whereas the P[cyclo)-P(cyclo) distances decrease with increasing number of valence electrons. 

Additional d electrons on the metals increase the number to be counted in the aromaticity formalism. As this can be applied by two metals (d6 - d8), the family of triple-decker sandwich complexes should consist of members having 30, 31, 32, 33, and 34 valence electrons. Analogous to the 17-valence-electron ferricenium cation, obtained by oxidation of ferrocene, 29-valence-electron species ( n,8/d5/7r8/d8/7r6) would be expected to be stable, and this has been confirmed by chemical and electrochemical studies (see Section III). 

Fig. 6. Triple-decker sandwich complexes having 30-34 valence electrons. (Compound 27 has 34 28 and 30, 30 and 29, 33 valence electrons.)...

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