Annulenes, dianions

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],... 

The nature of multiple bonding between germanium and the heavier chalcogens in the complexes (/74-Megtaa)GeE (E = Se, Te) is best described as an intermediate between the Ge+— E and Ge=E resonance structures. The preparation of these complexes involves the addition of the elemental chalcogen to (/74-Mestaa)Ge, which is synthesized by the metathesis of GeCl2(l,4-dioxane) and Li2[Mestaa] (Mestaa = octamethyldibenzotetraaza[14]annulene dianion). The molecular structures of both complexes are shown in Figures 5 and 610. 

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. 

The inner 16-membered macrocycle contains 18 7C electrons and is isoelec-tronic with the 16-annulene dianion. The bond lengths in this inner ring are all very close to the one in benzene, as shown in Figure 6.2.3. The UV/vis spectra of both compounds are as similar as one could have be predicted by the analogy (Fig. 6.2.4) (Gouterman, 1978). 

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.

Fig. 4 ACID surface mapped onto the localized, tub conformer of [8]annulene (left), and the planar, delocalized [8]annulene dianion (right). Discontinuous surfaces imply no delocalization at those corresponding points in space (with respect to the selected isosurface value here set at 0.05). View this art in color at www.dekker.com.)...

Annulene is a 4n -electron system, and thus, the (neutral) planar form would be expected to exhibit antiaromatic behavior. Indeed, Frost s circle analysis predicts an open-shell species (as with [4]annulene see Figs. 2 and 6). Extensive studies of cyclooctatetraene were performed, and it was established that the neutral state avoids the problem of antiaromaticity by adopting a tub conformation. Both the anion and dianion are known to exist in a planar conformation, the latter being aromatic. This fact is also predicted by simple analysis of the [8]annulene dianion by Frost s circle (see Fig. 6). It is predicted that the planar [8]annulene dianion is a closed-shell system. 

Annulene (Dianion) 6, outer protons 6, inner protons reference... 

The [12]annulene and (16]annulene dianions have (4n+2) v-electrons and are diatropic species, while the [18]annulene dianion, which has 20 iT-electrons, is a paratropic species. This reversal of character when two electrons are added to diatropic or paratropic compounds provides outstanding evidence for the different types of spectra associated with species having on the one hand (4n+2) TT-electrons, and on the other hand (4n) TT-electrons, spread over a ring. 

By analogy, a ring-metal interaction is symmetry permitted between the highest occupied MOs of two [8]annulene dianions and two lanthanide or actinide metal f-orbitals (1 = 2). An empirical model depicting this interaction is shown in Figure 2. In this example, the central metal ion shown, uranium(IV), has two electrons remaining in metal f-orbitals thus, the HOMO of uranocene is located on the metal. 

Since the synthesis of uranocene,2 there have been many other complexes synthesized involving a lanthanide or actinide element and one or more [8]annulene dianions. These complexes are generally unstable towards air (most are pyrophoric) and water all handling of these complexes must be carried out in an inert atmosphere. 

The first part of this paper reviews the synthesis and structures of the f-block metal [8]annulene dianion complexes. For the purpose of this paper, the [8]annulene dianion complexes of the actinides and lanthanides have been classified as bis- or mono-[8]annulene complexes and are further classified by oxidation state of the f-block metal (+3 and +4 for actinides, +2, +3, and +4 for lanthanides). 

The most utilized route for the synthesis of bis([8]annulene)-actinide(IV) complexes is the reaction of a tetrafiydrofuran (THF) solution of [8]annulene dianion with a stoichiometric amount of the actinide tetrachloride in THF. This method has been employed for the synthesis of bis([8]annulene)thorium(IV) (thorocene),a -protactinium(IV)... 

Bis([8]annulene)plutonium(IV), plutonocene, was synthesized by the reaction of di(tetraethylammonium)plutonium(IV) hexachloride suspended in THF with a stoichiometric amount of [8]annulene dianion in THF.A6... 

Treatment of 2 with two equivalents of potassium gives the corresponding [8]annulene dianion. 1,1, 3,3, 5,5, 7,7 -Octamethylthorocene,20 -uranocene,-neptunocenei, 21 and -protactinocene2i have all been synthesized by the addition of I,3,5,7-tetramethyl[8]annuIene dianion to the appropriate actinide salt. 

Attempts to make the octamethylplutonocene derivative by addition of tetramethyl[8]annulene dianion to the tetrachloride resulted in the reduction of Pu(IV) by the tetramethyl[8]annulene dianion to Pu(III).i When the borohydride complexes of neptunium and plutonium are used in place of the halide salts acceptable yields of the bis(tetra-roethyl[8]annulene) complexes were obtained.21... 

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 mechanism of the reaction to form the bis([8]annulene) complexes is not known. It is clear, however, that there are differences in the mechanism in the formation of uranocene compared to thorocene. When the reaction of two equivalents of n-butyl[8]annulene dianion with thorium tetrachloride was carried out in an NMR tube and monitored by iH-NMR, a resonance was observed which cannot be attributed to the bis([8]annulene) complex.22 Further experiments showed that this apparent intermediate in the reaction is the mono-ring complex, [8]annulenethorium(IV) dichloride (vide infra).22 The mechanism for the formation of bis([8]annulene)thoriuro evidently involves the stepwise substitution of [8]annulene dianion for two chloride ions. 

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