Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

The use of supporting ligands that can store either protons
or
electrons has emerged as a powerful strategy in catalysis. While these
strategies are potent individually, natural systems mediate remarkable
transformations by combining the storage of both protons and electrons
in the secondary coordination sphere. As such, there has been recent
interest in using this strategy to enable fundamentally different
transformations. Furthermore, outsourcing H-atom or hydrogen storage
to ancillary ligands can also enable alternative mechanistic pathways
and thereby selectivity. Here, we describe the application of this
strategy to facilitate radical reactivity in Co-based hydrogenation
catalysis. Metalation of previously reported dihydrazonopyrrole ligands
with Co results in paramagnetic complexes, which are best described
as having Co­(II) oxidation states. These complexes catalytically hydrogenate
olefins with low catalyst loadings under mild conditions (1 atm H2, 23 °C). Mechanistic, spectroscopic, and computational
investigations indicate that this system goes through a radical hydrogen-atom
transfer (HAT) type pathway that is distinct from classic organometallic
mechanisms and is supported by the ability of the ligand to store
H2. These results show how ancillary ligands can facilitate
efficient catalysis, and furthermore how classic organometallic mechanisms
for catalysis can be altered by the secondary coordination sphere.