Pentalene dianion, aromaticity

The HSE values of 14 thus obtained (Table 5) constitute about 30—50% of the HSE value calculated, based on the similar type reaction 5, for the most aromatic 10 7r electron bicyclic structure of naphthalene 62.2 (MP2(full)/6-31G ) and 53.7 (B3LYP/6-31G ) kcal mol . As shown in Figure 7, compounds 14 and their isoelectronic analogues possess the closed-shell jr-electron system similar to that of the aromatic pentalene dianion. Table 5 contains also the values of A characterizing the enhanced stability of the bicyclic Czv structures 14 relative to their unstrained monocyclic isomers 14a. 

As outlined in CHEC-II <1996CHEC-II(8)79>, only a few ring systems 1-34 having a 1 On-electron aromatic structure isoelectronic with the pentalene dianion 58 have been described. Triazoles 59, having two adjacent... 

For the purposes of this review, aromatic azapentalenes may be broadly defined as heterocyclic analogs of pentalene (I)1 which are aromatic by virtue of a 10- -electron system compounds of this type will thus be iso- -electronic with the pentalene dianion (2).2... 

We will now consider heterocycles with 5 doublets (10 -electrons) of which at least two are p-doublets. Among monocyclic systems (i.e., analogs of the cyclooctatriene dianion25 and the cycloheptadiene trianion26) the compounds known are few and unstable (CKL Sections III,F and III,E,2). The present review is devoted to a study of a group of 10-7r-electron bicyclic systems possessing two p-doublets the aromatic azapentalenes. These compounds are isoelectronic with the pentalene dianion (14) (Scheme 2). 

Pentalene has eight n electrons and is antiaromatic. Pentalene dianion, however, has ten tc electrons and is a stable, aromatic ion. 

D.117 Interestingly, if two electrons are added to pentalene, a stable dianion (43) results.118 It can be concluded that an aromatic system of electrons will be spread over two rings only if 10 electrons (not 8 or 12) are available for aromaticity. 

The structure analysis of the dilithium pentalenediide 45 reveals a C2-sym-metric ion triplet with the two lithium cations located on opposite sides of the two different rings. The structural parameters were extremely well reproduced by ab initio calculations [35] (see Fig. 3). Both the experimental structural parameters and calculated magnetic susceptibility exaltation classify the 107r-elec-tron species 45 as an aromatic compound. Apparently, the lithium counterions in 45 do not exert any significant effect on the bond lenghts of the dianion 22. On the other hand, the antiaromatic pentalene 2 and its aromatic dication 22+ show the characteristic bond length alternation (Fig. 3) [35]. 

The fully unsaturated dicyclopenta[cd,g/z]pentalene 4 (Fig. 7) remains elusive. Several attempts at generating 4 have been made, but so far all available knowledge about the system rests on computational studies which predict the structure and properties of this highly strained 7r-system [35]. In contrast to computational results for pentalene 2 and acepentalene 3, calculations for 4 predict an aromatic singlet ground state with a delocalized 7r-system. While the dianion of 4 is calculated to also be aromatic, the dication 42+ should be antiaromatic [35]. 

Solution Examination of the ring system of pentalene indicates a total of 8 tt electrons, which does not meet Hiickel s requirement for aromaticity (8 4n + 2). If two electrons were added to pantelene, forming a dianion. 

The dianion is an aromatic system of 10 tt electrons, but pentalene has 4n tr electrons. 

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