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-MeIm) to Fe(OTf)⋅2CHCN affords the first example of a 5-coordinate imidazole‑iron complex (Fe(1,2-MeIm), 1). The structure is distorted square pyramidal (τ = 0.41). When an Pr group is substituted for the methyl group at the 2-position on the imidazole (2-Pr-1-MeIm), the 14-electron complex (Fe(2-Pr-1-MeIm), 2) is obtained. This complex exhibits slightly distorted tetrahedral geometry (τ' = 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.