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.