What Is the Preferred Structure of the Meisenheimer−Wheland Complex Between sym-Triaminobenzene and 4,6-Dinitrobenzofuroxan?

The geometries, energies, and electronic properties of possible configurations of Meisenheimer−Wheland (M−W) complexes of sym-triaminobenzenes and 4,6-dinitrobenzofuroxan (DNBF) were investigated theoretically by MP2 and a variety of DFT methods. The π−π complex is preferred thermodynamically by more than 15 kcal/mol over the σ-complexes for the unsubstituted species. However, the N-substituents of the 1,3,5-triaminobenzenes influence the relative stabilities of the alternative configurations significantly. The σ-syn configuration of the M−W complex of 1,3,5-tris(N-piperidyl)benzene and DNBF has the lowest energy, followed closely by the σ-anti and π−π forms. The small energy differences between different configurations are consistent with the dynamic interconversion of three homomeric structures observed experimentally by NMR. The ca. 1.63 Å C−C interring bond exchanges among three equivalent sites. Quantum theory of atoms in molecules (QTAIM) analysis provided insights into the nature of the intermonomer interactions. Charge transfer and σ bonding account for the stability and remarkably large binding energies of the M−W complexes.