Pyridine modifications regulate the electronics and reactivity of Fe-pyridinophane complexes

Area of Science:

• Coordination Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• 12-membered pyridinophanes are versatile ligands used as biological mimics, chelators, and catalytic precursors.
• Tuning pyridinophane reactivity is crucial for controlling metal complex applications.
• Structure-activity relationship studies in transition metal catalysis are complicated by simultaneous structural and electronic changes.

Purpose of the Study:

• To investigate the impact of 4-substitution on the pyridine ring of 12-membered tetra-aza pyridinophanes.
• To establish a regulatory handle on the electronic properties of the metal center.
• To correlate these electronic changes with the catalytic C-C coupling activity of iron complexes.

Main Methods:

• Synthesis of a series of 12-membered tetra-aza pyridinophanes with varying 4-substituents.
• Preparation and characterization of corresponding iron complexes.
• Evaluation of catalytic C-C coupling activity.
• Determination of iron redox potentials and metal binding constants.

Main Results:

• 4-substitution on the pyridine ring effectively modulated the electronic properties of the iron center.
• Catalytic C-C coupling yields (32-58%) showed a direct correlation with iron redox potentials (ΔE1/2 = 152 mV) and metal binding constants (Δlogβ = 3.45).
• Complex geometries remained virtually identical across the series, isolating electronic effects.

Conclusions:

• Pyridine substitution in pyridinophanes provides an independent regulatory handle on redox potential and metal binding constants.
• These modulated properties directly influence the catalytic C-C coupling reactivity of the iron complexes.
• This work offers a definitive method for tuning catalyst performance by controlling electronic properties separate from coordination geometry.