Iron Coordination Chemistry of N2Py2 Ligands Substituted by Carboxylic Moieties and Their Impact on Alkene Oxidation Catalysis

A biomimetic approach based on Rieske dioxygenase mimics has been undertaken, which uses the tetradentate N2Py2 ligand platform that contains two pyridine moieties linked to a 1,2-diaminoethane or a trans-1,2-diaminocyclohexane backbone. Here we report the impact of the incorporation of carboxylic functionalities with the N2Py2 ligand on the catalytic efficiency of its FeII complexes during epoxidation with H2O2 as the oxidant. Five complexes have been characterized in the solid state and solution. The X-ray structure of a ferrous complex with a ligand that contains two carboxylic acid moieties shows an unexpected N4O3-type iron coordination sphere. Moreover, the two carboxylic groups of the ligand remain protonated and bound by its sp(2) oxygen atom. Two substitutions of the ligand with carboxylic moieties was found to be deleterious to the reactivity of the complex during alkene oxidation, whereas monosubstitution led to a slight change in the reactivity. Moreover, the catalyst built from the optically active trans-1,2-diaminocyclohexane backbone, L-3,catalyzes the asymmetric epoxidation of trans-2-heptene with up to 17% yield. The addition of acetic acid leads to better selectivity, enantioselectivity (38%), and yield of the epoxidation of nonaromatic alkenes.