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•+)­BPh4–] as single crystals and (b) an ultra-electron-deficient
NDI [(1a2+)­2BF4–] having the lowest LUMO level recorded for an NDI, overwhelming
the formative tetracyanoquinodimethane (TCNQ) molecule.
Both 1a•+ and 1a2+ 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···O interaction,
hypervalency, and propeller-like shape of the phosphonium groups in 1a2+ and 1a•+, were
substantiated by crystallography and theoretical studies. Natural
bond orbital (NBO) calculations validated the P···O
contact to be an nO → σP–C* orbital interaction.
Spontaneous electron transfer reactions of 1a2+ even in nonpolar solvents, anion−π interactions of 1a2+ with the naphthalene core, and panchromism
of 1a•+ are the other emergent properties.
The high-yielding (∼90%) in situ synthesis of 1a•+ 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.