A structural and functional model for alkene dioxygenases

In this article, we report sterically-controlled iron sites based on non-chelating bulky imidazole ligands. Adding 6 equiv. of 1,2-dimethylimidazole (1,2-Me₂Im) to Fe(OTf)₂⋅2CH₃CN affords the first example of a 5-coordinate imidazole‑iron complex ([Fe(1,2-Me₂Im)₅](OTf)₂, 1). The structure is distorted square pyramidal (τ₅ = 0.41). When an ⁱPr group is substituted for the methyl group at the 2-position on the imidazole (2-ⁱPr-1-MeIm), the 14-electron complex ([Fe(2-ⁱPr-1-MeIm)₄](OTf)₂, 2) is obtained. This complex exhibits slightly distorted tetrahedral geometry (τ'₄ = 0.93) with four N-donors and serves as a 4-His iron structural model complex for carotenoid cleavage dioxygenases (CCD). The electronic structure of 1 and 2 were characterized by Mössbauer spectroscopy. Reactions of 1 and 2 with model olefin substrates (1-R-4-(1-methoxyprop-1-en-2-yl)benzene; R = Me or Br) in the presence of oxygen result in olefin cleavage yielding ketone and aldehyde products, although 2 yields more products than 1. Support for a proposed reaction mechanism for 2 is offered from Density Functional Theory (DFT) calculations.