Tuning the Diiron Core Geometry in Carboxylate-Bridged Macrocyclic Model Complexes Affects Their Redox Properties and Supports Oxidation Chemistry

We introduce a novel platform to mimic the coordination environment of carboxylate-bridged diiron proteins by tethering a small, dangling internal carboxylate, (CH<sub>2</sub>)<sub><i>n</i></sub>COOH, to phenol-imine macrocyclic ligands (H<sub>3</sub>PIMICn). In the presence of an external bulky carboxylic acid (RCO<sub>2</sub>H), the ligands react with [Fe<sub>2</sub>(Mes)<sub>4</sub>] (Mes = 2,4,6-trimethylphenyl) to afford dinuclear [Fe<sub>2</sub>(PIMICn)­(RCO<sub>2</sub>)­(MeCN)] (<i>n</i> = 4–6) complexes. X-ray diffraction studies revealed structural similarities between these complexes and the reduced diiron active sites of proteins such as Class I ribonucleotide reductase (RNR) R2 and soluble methane monooxygenase hydroxylase. The number of CH<sub>2</sub> units of the internal carboxylate arm controls the diiron core geometry, affecting in turn the anodic peak potential of the complexes. As functional synthetic models, these complexes facilitate the oxidation of C–H bonds in the presence of peroxides and oxo transfer from O<sub>2</sub> to an internal phosphine moiety.