Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions.

Area of Science:

• Organometallic Chemistry
• Catalysis
• Polymer Science

Background:

• Pyridine(diimine) iron complexes catalyze [2+2] cycloadditions of alkenes and dienes, yielding vinylcyclobutanes.
• These reactions also enable butadiene oligomerization to divinyl(oligocyclobutane), a recyclable polymer microstructure.

Purpose of the Study:

• To investigate the catalytic mechanisms of aryl-substituted pyridine(diimine) iron and ruthenium complexes in cycloaddition and oligomerization reactions.
• To elucidate structure-activity relationships for designing advanced catalysts and promoting polymer chemical recycling.

Main Methods:

• Systematic study of iron and ruthenium butadiene complexes.
• Structural and computational analyses of iron butadiene complexes.
• Labeling experiments and mechanistic studies.

Main Results:

• Catalyst rigidity promotes s-trans diene coordination, facilitating oxidative cyclization.
• Cyclobutane formation proceeds via a metallacyclic intermediate with reversible C(sp3)-C(sp3) reductive coupling.
• Iron complexes utilize a spin crossover mechanism (S=0 to S=1) for C(sp3)-C(sp3) reductive elimination under thermal conditions.
• Ruthenium complexes require blue light irradiation for similar reactivity, with a redox-innocent pyridine(diimine) ligand.

Conclusions:

• Structural features of pyridine(diimine) iron catalysts are crucial for diene coordination and subsequent cycloaddition.
• Iron catalysts enable thermally driven C(sp3)-C(sp3) bond formation via spin crossover.
• Ruthenium catalysts offer light-driven reactivity, highlighting distinct activation pathways.
• Understanding these mechanisms provides design principles for next-generation catalysts and chemical recycling of polymers.