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

Porphyrin double-decker complexes undergo multiple redox processes that significantly alter their structural and electronic properties, although most studies have focused on oxidation processes. Studies of reduction have remained limited, particularly for porphyrin-based systems in which accessing multiply-reduced states remains challenging. Here we report the synthesis and characterization of lanthanum(III) porphyrin double-decker complex [La(CNTPP)2]⁻ that exhibits unprecedented four-electron reduction capability, reaching the pentaanion state. Strategic incorporation of electron-withdrawing cyano groups at the β-positions shifts the reduction potentials by approximately 1.3 V toward more positive values compared to the tetraphenylporphyrin analogue, enabling reduction at –0.78, –1.00, –1.42, and –1.66 V (vs. Fc/Fc⁺) in DMSO. Single-crystal X-ray diffraction analysis revealed that the complexes adopt the monoanion form with an azimuthal rotation angle of 33° and pronounced dome-shaped distortion arising from steric interactions. Variable-temperature NMR studies showed unusually slow ligand rotation with an activation barrier of 17.8 kcal mol⁻¹, which computational analysis attributed primarily to steric rather than electronic effects. This work demonstrates a rational design strategy for accessing multiply-reduced porphyrin double-decker complexes and opens new possibilities for applications in molecular electronics and redox-switchable molecular devices.