In Search of Ultrastrong Brønsted Neutral Organic Superacids: A DFT Study on Some Cyclopentadiene Derivatives

Abstract

An efficient but reasonably accurate B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) computational procedure showed that pentasubstituted cyclopentadienes such as (CN)5C5H, (NO2)5C5H, and (NC)5C5H containing strongly electron-withdrawing groups are neutral organic superacids of unprecedented strength. The boldface denotes the atom attached to the cyclopentadiene framework. All of them exhibit prototropic tautomerism by forming somewhat more stable structures with C=NH, NO2H, and N=CH exocyclic fragments, respectively. The acidity (DeltaH(acid)) of these is lower, but only to a rather small extent. The DeltaH(acid) enthalpies of these last three tautomers are estimated to be 271, 276, and 282 kcal mol(-1), respectively. Hence, the most stable tautomers of (CN)5C5H and (NC)5C5H represent a legitimate target for synthetic chemists. On the other hand, (NO2)5C5H is less suitable for practical applications, because of its high energy density. The origin of the highly pronounced acidity of these compounds was analyzed by using the recently developed triadic formula. It is found that very high Koopmans' ionization energy (IE)n(Koop) of conjugate bases exerts a decisive influence on acidity. It follows as a corollary that the overwhelming effect leading to very high acidity is due to the properties of the final state. An alternative picture is offered by homodesmotic reactions, wherein the cyclic systems are compared with their linear counterparts. It is found that the acidity of cyclopentadiene (CP) is a consequence of aromatic stabilization in the CP- anion. The extreme acidity of pentacyanocyclopentadiene (CN)5C5H is due to aromatization of the five-membered ring and a strong anionic resonance effect in the resultant conjugate base. The neutral organic superacids predicted by the present calculations may help to bridge the gap between existing very strong acids and bases.

Strong Acidity of Some Polycyclic Aromatic Compounds Annulated to a Cyclopentadiene Moiety and Their Cyano Derivatives – A Density Functional B3LYP Study

“…[49][50][51][52][53] This lends credence to the method employed here and puts all superacids examined so far on the same (theoretical) ladder. [13][14][15] It is widely accepted that rigorous ab initio and modern DFT methods predict acidity and basicity of molecules quite accurately in the gas phase. [4,5,54] It is much more difficult to describe these two features in solutions.…”

“…Namely, the hydrogen atom can be attached to the nitrogen atom, thus forming a ketene imine functionality by a hydrogen walk from the C(sp 3 ) center to the nitrogen atom of the CN group bonded to the same C(sp 3 ) carbon atom. This type of the hydrogen shift yields the most stable tautomer in all cases, [13][14][15] which makes an interesting rule governing the hierarchy in stability of the prototropic tautomers. Hence, it is not surprising that tautomer 1b CN is also more stable than 1a CN , albeit by only 4.0 kcal mol -1 .…”

“…[13][14][15] It should be noticed that the polycyano derivative of the most stable parent hydrocarbon tautomer 1a CN will be taken as a reference in this family of compounds and similarly in other families later on.…”

“…(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) both in the gas phase [9][10][11][12][13][14][15] and in solutions. [16,17] An interesting class of very strong acids is given by carboranes.…”

“…The first was based on the use of the electronic superacceptor substituents, [9,10] whereas the second utilized the concept of stabilization of the conjugate bases through very strong anionic resonance triggered by cyano substituents. [13][14][15] Both strategies have led to a large number of extremely powerful superacids pronounced acidity is identified as a very strong anionic resonance effect efficiently assisted by a large number of CN groups in a concerted manner. It is stressed that the polycyanated anions are very stable and have a highly dispersed negative charge, which should lead to a low nucleophilicity and weak coordinating properties, thus making them potentially very useful in academic research and technological applications.…”