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

Abstract

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 H 2 , 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 H 2 . 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.

Hydride-Free Hydrogenation: Unraveling the Mechanism of Electrocatalytic Alkyne Semihydrogenation by Nickel–Bipyridine Complexes

“…It is interesting to mention here that the ECEC-type mechanism formally consists in the hydrogenation of a ligandin that case, an alkynewhich is well-known as a degradation pathway of molecular electrocatalysts − or sometimes as an initiation to access more active catalytic species. − In the present work, this phenomenon is desired and fully exploited as it is part of the catalytic cycle. The absence of substantial decomposition may be here due to the stabilization of the nickel catalyst by benzoate ligands.…”

Hydride-free Hydrogenation: Unraveling the Mechanism of Electrocatalytic Alkyne Semihydrogenation by Nickel-Bipyridine Complexes

“…It is interesting to note that this mechanism formally consists in the hydrogenation of a ligand -in that case an alkyne -which is well-known as degradation pathway of molecular electrocatalysts [45][46][47][48][49][50][51][52][53][54] or sometimes as an initiation to access more active catalytic species. [55][56][57] In the present work, this phenomenon is desired and fully exploited as it is part of the catalytic cycle.…”

Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons

“…43−57 We have specifically been investigating a dihydrazonopyrrole (DHP) ligand scaffold that can store a full equivalent of H 2 which enables catalytic thermal hydrogenations of alkenes and quinones. 32,58 Given this reactivity, we questioned whether H 2 could be replaced by acid and an electrode and also whether the metal−ligand cooperative hydrogenation reactivity and selectivity of this system would be altered under this electrochemical regime.…”