Ligand Field Sensitive Spin Acceleration in the Iron-Catalyzed [2+2] Cycloaddition of Unactivated Alkenes and Dienes

Redox-active pyridine(diimine) (PDI) iron catalysts promote the reversible [2+2] cycloaddition of alkenes and dienes to cyclobutane derivatives that have applications ranging from fuels to chemically recyclable polymers. Metallacycles were identified as key intermediates, and spin crossover from the singlet to the triplet surface has been calculated to facilitate the reductive coupling step responsible for the formation of the four-membered ring. In this work, a series of sterically and electronically differentiated PDI ligands was studied for the [2+2] cycloaddition of ethylene and butadiene to vinylcyclobutane. Kinetic studies revealed that the fastest and the slowest turnover were observed with equally electron-deficient supporting ligands that either feature phenyl-substituted imine carbon atoms (MeBPDI) or a pyrazine core (MePZDI). While the oxidative cyclization was comparatively slow for both catalysts, the rate of reductive coupling – determined by stoichiometric 13C2H4 labeling studies – correlated with the TOFs. Two-state DFT studies and the distinct electronic structures of related (iPrBPDI) and (iPrPZDI) iron methyl complexes revealed a significantly different ligand field strength due to either diminished ligand σ-donation (MeBPDI) or promoted metal π-backbonding (MePZDI). Spin acceleration, leading to fast reductive coupling and catalytic turnover, was promoted in the case of the weaker ligand field and depends on both the nature and the position of the electron withdrawing group. This study provides strong evidence for the role of two-state reactivity in C(sp3)–C(sp3) bond formation and insights on how ligand design either promotes or inhibits spin acceleration in earth abundant metal catalysis.