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

✍ Scribed by Robert Vianello; Joel F. Liebman; Zvonimir B. Maksić

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 244 KB
• Volume: 10
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200400337

✦ Synopsis

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)~5~C~5~H, (NO~2~)~5~C~5~H, and (NC)~5~C~5~H 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, NO~2~H, and NCH exocyclic fragments, respectively. The acidity (Δ__H__~acid~) of these is lower, but only to a rather small extent. The Δ__H__~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)~5~C~5~H and (NC)~5~C~5~H represent a legitimate target for synthetic chemists. On the other hand, (NO~2~)~5~C~5~H 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)${{{{\rm Koop}\hfill \atop n\hfill}}}$ 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)~5~C~5~H 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.