A tale of two topological isomers: Uptuning [Fe IV (O)(Me 4 cyclam)] 2+ for olefin epoxidation.

TMC- anti and TMC- syn, the two topological isomers of [Fe IV (O)(TMC)(CH 3 CN)] 2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Me 4 cyclam), differ in the orientations of their Fe IV =O units relative to the four methyl groups of the TMC ligand framework. The Fe IV =O unit of TMC- anti points away from the four methyl groups, while that of TMC- syn is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC- syn reacts with HAT (hydrogen atom transfer) substrates at 1.3- to 3-fold faster rates than TMC- anti , but the reactivity difference increases dramatically in oxygen-atom transfer reactions. R 2 S substrates are oxidized into R 2 S=O products at rates 2-to-3 orders of magnitude faster by TMC- syn than TMC- anti . Even more remarkably, TMC- syn epoxidizes all the olefin substrates in this study, while TMC- anti reacts only with cis -cyclooctene but at a 100-fold slower rate. Comprehensive quantum chemical calculations have uncovered the key factors governing such reactivity differences found between these two topological isomers.