Annulenes dianion from

Alternatively, the porphyrin ring can be constructed starting from [16]annulene. In the first step, two electrons are added to form the corresponding [16]annulene dianion, which is transformed into porphyrin Cl") by adding bridges and heteroatoms. Unlike the structure derived from [18]annulene, the dianion-based model has a fourfold symmetry, and was considered suitable for the description of metal complexes [23] (see Sect. 2.3.2). In yet another approach [24], based on the so-called perimeter model, the porphyrin macrocycle is derived from the [20]annulene dication (I "). Both the [16]- and [20]annulene models were employed to describe electronic absorption spectra and magnetic circular dichroism of porphyrinoids [24, 25],... 

Figure 2.14. Frontier orbitals of benzene and [8)annulene dianion. The nodal planes ( ) are obtained from the polygons with 2k vertices inscribed into the perimeter, and the magnitude of the LCAO coefficients (indicated by the size of the circles) may be estimated from the location of the nodal planes.

The simplest polycycles stem from two triply-bridged points. These systems are exemplified by Trost et al. s 4,8-dihydrodibenzo[oi,g7z]pentalene (69) [87, 88], a precursor for a purturbed [12]annulene dianion, and Mislow et al. s double-bridged biphenyl derivatives (generally shown as 70) [89], where X is methylene, carbonyl, or various heteroatoms. Other longer bridged biphenyls include the triple-bridged cyclophanes 71 made by Hubert and Dale [90], their unsym-metrical relatives by Cram and Reeves [91], and the recent polyalkynyl cyclo-phane (72) made by Rubin et al. [92], a proposed fullerene precursor. 

Figure 6.2.4 The electron conjugation pathways of the inner macrocycle of magne-sium-octaethylporphyrin (ethyl groups not shown) and the 16-annulene dianion with sodium counterions. The electronic spectra are as similar as one could predict from the Hiickel model of aromaticity.

Other substituted bis([8]annulene)actinide(IV) complexes of uranium,3-13 thorium,20 neptunium and plutonium have been prepared by the reaction of the substituted [8]annulene dianion with the appropriate actinide(IV) salt. X-ray diffraction data suggest that the alkyl-substituted neptunium(lV) and plutonium(IV) complexes are isomorphous with the corresponding uranium(IV) complex but none of the structures of these complexes has yet been determined from single crystal x-ray diffraction. 

The latter method is preferable, since in this manner of addition of the reagents the lanthanide trichloride is never exposed to an excess of reducing [8]annulene dianion. Bis([8]annulene)lanthanate(III) and -yttriate(III) anions have also been reported. 7 similiarly. the bis([81annulene)lanthanate(III) anion complexes from... 

It is postulated from these equilibria that in the reaction of [8]annulene dianion with a lanthanide(III) chloride the [8)annulene-lanthanate(III) chloride is formed initially as a isolable intermediate of the reaction. The mono-ring intermediate then reacts with additional [8]annulene dianion to form the bis[8]annulene complexes. 

The bis([8]annulene)lanthanate(III) anions which are complexed by an [8]annulenelanthanate(III) cation are the only other class of complexes which contain two [8]annulene dianions in a sandwich arrangement about a lanthanide(III) ion. This class was first reported for the cerium(III) ion, from the reduction of a cerium(IV) alkoxide with triethylaluminum in the presence of cyclooctatetraene.3 ... 

In this complex, the bis([8]annulene)cerate(II) complex is formally a dianion. The cerium complex as its di(l,2-dimethoxyethane) adduct was characterized by elemental analysis and infrared spectrum. The structure of [K(C Hjq02)]2[Ce(CgHg)2] is not known, but the infrared spectrum of the cerium complex does contain bands (887 and 682 cm-i) which are consistent with the presence of an [8]annulene dianion.S7 Although there are reports of Ce(II) from the reduction of Ce(III) in other systems,58,59 the presence of the cerium(II) ion in... 

Aromatic [16]Annulene Dication and Dianion from Antiaromatic [16]Ammlene... 

Eight-membered rings with two O, S or N atoms or combinations of these heteroatoms in a 1,2- or 1,4-relationship and three double bonds possess conjugated tt-electron frameworks and can be designated as dihetera[8]annulenes (78AHC(23)55). These 1,2- or 1,4-diheterocins are isoelectronic with the cyclooctatetraene dianion, and if planar represent potentially aromatic IOtt-electron systems. Considerable interest has attached to the degree of aromaticity of these compounds from both theoretical and experimental standpoints. Most theoretical treatments have led to the conclusion that 1,2-diheterocins, because of... 

Fig. 13 Aromaticity analysis in free-base porphyrin (top), porphyrin dianion, and porphyrin complex (bottom) [37]. Bond lengths (in Angstroms) are averages obtained from X-ray structural data. Values in parentheses correspond to calculated geometries (B3LYP/6-31G, no substituents, M = Mg). HOMA indices were calculated from the average (calculated) bond lengths. The values inside the macrocyclic ring and outside the macrocycle correspond respectively to the inner cross ([16]annulene circuit) and the complete macrocycle. NICS values (GIAO-RHF/6-31 + G ) were calculated at the points indicated with dots...

In theoretical interpretations of their electronic [Gouterman (85), Simpson (160), Weiss (199)] and pmr spectra [Jackman (105)] porphyrins have often been treated as cyclic polyene hydrocarbons, and it has recently been demonstrated that the (16)-annulene n dianion (which has 18 n electrons, is planar and aromatic) does indeed perfectly reproduce the spectra of porphyrin species with D,m symmetry [Oth (137)]. The analogy between aromatic hydrocarbons and the porphyrins stands on solid experimental and theoretical ground, and redox potentials together with any coupling constants from esr experiments should yield equally straightforward correlations. Zeeman effect measurements also support this conclusion [Malley (126)]. 

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