The group of Zvonomir Maksic at the Ruder Boskovic Institute (Zagreb, Croatia) is interested in the design of neutral organic superbases and superacids, since they present advantages compared to their inorganic counterparts. They are reactive under mild chemical environments and so are useful in catalysis and green chemistry. For design purposes the Croatian group has developed a particular strategy named the "Aufbau Prinzip" of organic superbases. This principle has proved useful in tailoring compounds of highly pronounced basicity in silico, some of which were later synthesized in the laboratory. An analogous strategy can be used to construct neutral organic superacids. Briefly, already known compounds that can serve as backbones of new superacids are selected and decorated by suitable substituents at strategic positions. The Zagreb group has examined a number of electronegative substituents and found that the CN group possesses features that are a very good compromise between the strong ?- and ?-electron withdrawing power and modest steric requirements. Maksic and co-workers have shown in a number of papers that polycyanation enormously amplifies the acidity of planar or almost planar hydrocarbons.

In his latest NJC paper, Maksic with co-author Robert Vianello reveals the structure of the first three members of the Rees family of hydrocarbons. The parent compound is 7bH-cyclopenta[cd]indene (I) methylated at the central C(sp3) atom (Chart 1). It turns out that the distribution of the C-C bond distances around the molecular rim is almost uniform, implying that compound I represents a highly elusive [10]annulene molecule, albeit in a quasi-form due to the presence of the central sp3 carbon. Derivatives 9cH-cyclopenta[jk]fluorene (II) and fluroadene (III) are obtained by annelation of one and two benzene rings, respectively, thus forming molecular wing(s). These, however, do not exhibit the features of the higher [14] and [18] annulenes. Instead, they involve ?-electron structural patterns of lower order, which conform to Hückel´s aromatic 4n+2 rule and compete for the ?-electrons. It was already known earlier that molecular properties can be reduced to molecular fragments as a rule, not to mention molecules themselves in supramolecular systems. The present finding is, however, an intriguing case of molecular "philogenesis" at the electronic level. It is a sort of a "memory effect". The larger system has retained some features of the smaller subsystems, which have served as its building blocks.

According to Maksic, it is important to replace the methyl group at the central carbon by hydrogen, which becomes the highly acidic center. Hence, their second and even more important result is that polycyano derivatives of molecules I, II and III yield powerful superacids. It is generally accepted that bases with ?H(base) value over the threshold, set by DMAN (1,8-bis(dimethylamino)naphthalene) at 245 kcal/mol, are superbases. We found it fitting to define superacids as substances with ?H(acid) lower than 300 kcal/mol, which is found for HClO4 in the gas phase. Moreover, it is convenient to term all superbases with ?H(base) larger than 300 kcal/mol as hyperbases. By the same token, all superacids with ?H(acid) lower than 245 kcal/mol can be characterised as hyperacids. According to these criteria, two tautomers of the percyano derivatives of systems I and II would qualify as superacids, since they exhibit ?H(acid) values of 261.8 and 259.0 kcal/mol. On the other hand, the most stable polycyano derivative of III has ?H(acid) equal to 246 kcal/mol, meaning that it is the first neutral organic hyperacid predicted by calculations. It is very interesting to mention that the former two are NH acids, because their most stable tautomers contain the keteneimine moiety, whereas the latter is a CH acid. It follows that the atom hosting the proton in these acids are not very important. A decisive influence is exerted by anionic resonance triggered in the conjugate bases upon deprotonation, as our analysis conclusively shows. The aforementioned three acids are more acidic in DMSO than H2SO4 by 25, 19 and 24 orders of magnitude, respectively.

Maksic observes that it is difficult to foresee what future research on super- and hyperacids will reveal. One of the roles of computational chemistry is to suggest pathways to explore experimentally. One of the outcomes of these calculations is that the negative charge is very effectively delocalised over the conjugate base after deprotonation of a strong superacid. Hence, their nucleophilic character should be low. Such anions with low coordination ability are very useful in the lab, both in academic and industrial ones. "Many researchers believe that it would be easier to obtain deprotonated anions than very strong superacids. However, the success of C.A. Reed and coworkers in obtaining pentacyanocyclopentadiene, albeit in polymeric form, is encouraging." continues Maksic.

Although the experimental procedures for polycyanation reactions are known, it is plausible to assume that percyanation could be difficult. In such cases, it would be perhaps useful to combine CN groups with some other highly electronegative atoms or fragments, which would act as intrinsic catalysts for additional cyanations. This experimental research could be executed in collaboration with computational chemists, doing calculations carried out "on the fly".