Easy Synthesis of a Self-Assembled Imine-Based Iron(II) Complex Endowed with Crown-Ether Receptors

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C-H Oxidation

Precise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)-H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site-selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C-H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis-Menten kinetics and increased reaction rates. In addition, C-H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

Insight into the chemoselective aromatic vs. side-chain hydroxylation of alkylaromatics with H2O2 catalyzed by a non-heme imine-based iron complex

Side-chain/ring oxygenated product ratio increases upon decreasing the benzylic bond dissociation energy in the oxidation of alkylaromatics with H₂O₂ catalyzed by an imine-based iron complex.

Increasing the steric hindrance around the catalytic core of a self-assembled imine-based non-heme iron catalyst for C-H oxidation

Sterically hindered imine-based non-heme complexes 4 and 5 rapidly self-assemble in acetonitrile at 25 °C, when the corresponding building blocks are added in solution in the proper ratios. Such complexes are investigated as catalysts for the H2O2 oxidation of a series of substrates in order to ascertain the role and the importance of the ligand steric hindrance on the action of the catalytic core 1, previously shown to be an efficient catalyst for aliphatic and aromatic C-H bond oxidation. The study reveals a modest dependence of the output of the oxidation reactions on the presence of bulky substituents in the backbone of the catalyst, both in terms of activity and selectivity. This result supports a previously hypothesized catalytic mechanism, which is based on the hemi-lability of the metal complex. In the active form of the catalyst, one of the pyridine arms temporarily leaves the iron centre, freeing up a lot of room for the access of the substrate.