Design of 9H-fluoren-9-imine based organic superbases stabilized by proton–π interactions

The emergence of neutral organic superbases has garnered significant interest owing to their distinctive reactivity and extensive applications in catalysis and molecular design. In this research, a range of 9H-fluoren-9-imine derivatives (1-12) were examined through quantum chemical calculations to clarify the influence of intramolecular proton-π interactions on improving gas-phase basicity and stabilizing the associated conjugate acids. Proton affinity (PA) and gas phase basicity (GB) data reveal that derivatives bearing electron-donating groups on suitably oriented aromatic rings exhibit significantly higher basicity, with compounds 4, 6, 7, 8, 9, and 10 exceeding the benchmark PA of 1,8-bis(N, N-dimethylamino)naphthalene (DMAN). E isomers also display higher PA and GB values than the corresponding Z isomers, attributed to their greater thermodynamic instability and the presence of proton-π interactions in their conjugate acids. Quantum theory of atoms in molecules (QTAIM) analysis shows that high-PA derivatives feature shorter H⁺-aryl distances, higher electron densities at the proton-π bond critical points, and larger interaction energies estimated via the Espinosa relationship, all correlating strongly with basicity values. This confirms the key role of proton-π interactions in stabilizing the conjugate acids. Furthermore, aromaticity indices of selected derivatives indicates that the formation of proton-π interactions induces a measurable loss of local aromaticity in the involved aryl ring. The most pronounced aromaticity reduction is observed in compound 4, reflecting π-density redistribution and stabilization upon protonation. The insights gained here provide a fundamental basis for the rational design of new neutral organic superbases by exploiting intramolecular proton-π interactions.