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

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Electrochemistry

Background:

• Porphyrin double-decker complexes are known for redox activity.
• High LUMO energies of porphyrin ligands limit access to multiply reduced states.

Purpose of the Study:

• To synthesize and characterize a lanthanum(III) β-tetracyanoporphyrin double-decker complex.
• To investigate its unprecedented four-electron reduction capacity.
• To establish a design strategy for accessing multiply reduced porphyrin complexes.

Main Methods:

• Synthesis and characterization of the lanthanum(III) complex.
• Electrochemical reduction and UV-Vis/NIR spectroelectrochemistry.
• Single-crystal X-ray diffraction and density functional theory (DFT) calculations.
• Variable-temperature Nuclear Magnetic Resonance (NMR) studies.

Main Results:

• Achieved a four-electron reduction capacity, a significant increase compared to analogs.
• Incorporation of cyano groups shifted reduction potentials by ~1.3 V.
• X-ray diffraction revealed a distorted monoanion structure.
• DFT calculations indicated significant diradical character in the trianion.
• Observed slow ligand rotation attributed to steric hindrance.

Conclusions:

• The β-tetracyanoporphyrin complex demonstrates enhanced reducibility due to electron-withdrawing cyano groups.
• Steric effects, not electronic, dominate the observed slow ligand rotation.
• This work provides a rational design for multiply reduced porphyrin double-deckers for molecular electronics and redox-switchable devices.