Isolation and oxidative reactivity via ligand-based non-innocence of a redox series of cobalt complexes

Metal ligand cooperativity is an essential strategy for both synthetic and biological systems that incorporate transition metals. Inspired by the oxidative reactivity of heme enzymes in biological systems and the wide range of cobalt-porphyrins used in synthetic oxidative chemistry, we have developed a cobalt complex that utilizes a similarly non-innocent ligand scaffold with pyrrole donors with the additional functionality of storing a full equivalent of dihydrogen. This cobalt compound with the redox-active ligand ^tBu,TolDHP (^tBu,TolDHP = 2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) has a cobalt(III) center coupled to a ligand-based radical (for a S= ½ compound) and displays a wide range of five electrochemically accessible oxidation states. Some of these alternative oxidation states have been synthetically accessed, with some being ligand-based and others metal-based, and we have also notably isolated a reduced species with a ligand-based radical and a cobalt(II) metal center with an agnostic C-H interaction to the ligand. These compounds have begun to demonstrate potential in synthetic applications to functionalize amines and amides and form new bonds with C-H substrates. Preliminary studies indicate that this reactivity is enabled by reversible storage of hydrogen on the dihydrazonopyrrole ligand. This discovery demonstrates a new paradigm which could potentially enable synthesis to bypass the use of hazardous, pre-activated reagents and suggests that cooperativity between metals and ligands that can store hydrogen is a powerful strategy for catalysis that requires the addition or removal of hydrogen.