The tetramethylcyclam (TMC) ligand has played an instrumental role in our understanding of iron-oxygen intermediates commonly proposed in the catalytic cycles of non-heme iron oxygenases that carry out a wide variety of remarkable oxidative transformations. Among all Fe(TMC)-oxygen intermediates known to date, it was the oxoiron(IV) species, ([FeIV(O)(TMC)(NCMe)]2+ (1), that surfaced first when Rohde et al. reported its generation and characterization in 2003. The crystal structure of 1 displays an anti orientation of the FeIV=O unit with respect to the four methyl groups in the TMC ligand. This binding mode is a unique feature of 1, as all other [Fe(TMC)(anion)] complexes reported to date show the anion bound to the iron center syn to the methyl groups. The corresponding syn oxoiron(IV) complex has been elusive. However, the crystal structure of the (TMC)FeIII–Osyn–Sc3+ complex reported in 2010 and obtained from the reaction of 1 ([FeIV(Oanti)(TMC)(NCMe)]2+) and Sc3+ was puzzling and suggested the possible existence of the syn isomer in the solution. Further, the implied formation of 1 from the reaction of [FeIII(h2-O2)(TMC)]+ (the side-on peroxo ligand is bound syn to the methyl groups) with Lewis acids, where one of the peroxo oxygen atoms ends up in 1, added further mystery to the puzzle. The work herein reports our effort to generate the syn isomer of ([FeIV(O)(TMC)(NCMe)]2+, along with its detailed characterization by a variety of spectroscopy techniques, and X-ray crystallography. How the syn isomer contributes to the resolution of the above-mentioned puzzles will be discussed.

Solving topological puzzles in tetramethylcyclam-supported oxoiron(IV) chemistry

253rd National Meeting (2017)

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