Extraordinary Stability of Naphthalenediimide Radical Ion and Its Ultra-Electron-Deficient Precursor: Strategic Role of the Phosphonium Group

Stabilization of radical ions and highly electron-deficient systems under ambient conditions is of great significance. A new design concept is presented that applies the multifaceted features of the phosphonium group to achieve isolation of (a) the first naphthalenediimide (NDI) radical ion [(1a(center dot+))BPh4-] as single crystals and (b) an ultra-electron-deficient NDI [(1a(2+))2BF(4)(-)] having the lowest LUMO level recorded for an NDI, overwhelming the formative tetracyanoquinodimethane (TCNQ) molecule. Both 1a(center dot+) and 1a(2+) exhibit unprecedented stability to normal workup procedures, chromatography, and anion metathesis in open air. To our knowledge, this is the first instance where radical ions stable toward chromatography have been obtained, which is a noteworthy development in the field of synthetic radical chemistry. The crucial components of thermodynamic and kinetic stabilization, namely, the nonbonded P center dot center dot center dot O interaction, hypervalency, and propeller-like shape of the phosphonium groups in 1a(2+) and 1a(center dot+), were substantiated by crystallography and theoretical studies. Natural bond orbital (NBO) calculations validated the P center dot center dot center dot O contact to be an n(O) -> sigma*(P-C) orbital interaction. Spontaneous electron transfer reactions of 1a(2+) even in nonpolar solvents, anion-pi interactions of 1a(2+) with the naphthalene core, and panchromism of 1a(center dot+) are the other emergent properties. The high-yielding (similar to 90%) in situ synthesis of 1a(center dot+) and the extraordinary stability fostered by the phosphonium group have the potential to turn hitherto unstable organic systems into a new genre of stable off-the-shelf systems.