Cation-Charge Delocalization in (Ammonio)amidyl Groups: Exceptionally Strong Neutral Nitrogen-Centered π-Electron Donors

Abstract

Neutral π-electron donors stronger than dialkylamino substituents enable control of the frontier-orbital energies and optical properties of π-conjugated molecules without altering their net charge. Here we report sp2-type (ammonio)amidyl (AA) groups in which the cationic moiety is extended to pyridinium or guanidinium frameworks. These groups were installed onto a 7-(N,N-dimethylsulfamoyl)benzofurazan scaffold by nucleophilic aromatic substitution using N-aminated salts. The resulting AA-substituted benzofurazan derivatives exhibited large bathochromic shifts, with the absorption maximum reaching 518 nm for the derivative bearing the acyclic guanidinium-derived donor. By cross-calibrating benzofurazan absorption against the p-nitrobenzene-based σ+DCM substituent scale, apparent σ+DCM values of –2.44, –2.57, and –2.72 were estimated for the pyridinium-, cyclic guanidinium-, and acyclic guanidinium-derived donors, respectively. These extrapolated donor-strength assignments were independently supported by empirical oxidation onset potentials and HOMO energies calculated by density functional theory. Charge analysis and X-ray crystallography indicate that cation-charge delocalization attenuates local electrostatic stabilization of the amidyl nitrogen, thereby enhancing π donation. The enhanced donation is accompanied by increased Brønsted basicity, revealing an intrinsic coupling between donor strength and protonation equilibrium. To the best of our knowledge, these cation-delocalized AA groups are the strongest sufficiently stable, neutral nitrogen-centered π-electron donors quantified to date.