Lanthanum β-Tetracyanoporphyrin Double-Decker Complexes: Four-Electron Reduction and Slow Ligand Rotation

Porphyrin double-decker complexes undergo multiple redox processes, but accessing multiply reduced states remains challenging owing to the inherently high LUMO energies of the porphyrin ligands. Here, we report the synthesis and characterization of a lanthanum(III) β-tetracyanoporphyrin double-decker complex with an unprecedented four-electron reduction capacity. Strategically incorporating electron-withdrawing cyano groups at the β-positions increases the reduction potentials by approximately 1.3 V compared to those of the tetraphenylporphyrin analog, enabling reduction at -0.78, -1.00, -1.42, and -1.66 V (vs. ferrocene/ferrocenium) in DMSO. Single-crystal x-ray diffraction reveals that the complex adopts the monoanion form with an azimuthal rotation angle of 33° and pronounced dome-shaped distortion caused by steric interactions between the meso-phenyl groups and β-cyano substituents. UV-Vis/NIR spectroelectrochemistry and chemical reduction using cobaltocene confirm the formation of multiply reduced species, whereas density functional theory calculations reveal that the trianion exhibits a significant diradical character. Variable-temperature NMR studies indicate unusually slow ligand rotation with a high activation barrier (ΔG^‡ = 17.8 kcal mol^{- 1} at 298 K), which computational analysis primarily attributes to steric rather than electronic effects. This study establishes a rational design strategy for use in accessing multiply reduced porphyrin double-decker complexes with potential for application in molecular electronics and redox-switchable devices.