Distal hydrogen-bonding effects and cofacial bimetallic salen architectures for oxygen activation chemistry

Abstract

Two distinct structural scaffolds elaborated from Schiff-base macrocycles were designed to study the proton-coupled electron transfer chemistry of 0-0 bond forming and activation chemistry. The "Hangman" architecture is composed of hydrogen-bonding functionalities poised over a redox active manganese salophen or salen platform. The complexes proved to be proficient catalase mimics (disproportionation of hydrogen peroxide to water and oxygen). Detailed spectroscopic, computational, and structure-function relationship studies elucidated the key redox, steric, and secondary coordination sphere effects for optimal catalytic ability. The incorporation a chiral backbone into the macrocycle led to catalysts that perform enantioselective epoxidation of unfunctionalized olefins. A macrocycle with an amide, imine, and bisphenolic functionalities was also incorporated as the redox platform in the Hangman framework; the manganese complex also performed catalytic oxygen atom transfer to olefins. The second framework, dubbed "Pacman", is composed of two salen platforms linked cofacially by rigid pillars xanthene or dibenzofuran. A series of bimetallic complexes, including chromium, iron, manganese, cobalt, copper, and zinc were generated. Mossbauer spectroscopy was used in the characterization of the iron salen complexes, which were also examined for photolytic oxidation chemistry.