First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

Abstract

The use of 3d metals in de-/hydrogenation catalysis has emerged as a competitive field with respect to 'traditional' precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as conceptual starting point for rational catalyst design. This reviews aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Rational selection of co-catalysts for the deaminative hydrogenation of amides

“…The few homogenous transition metal catalysts reported for deaminative hydrogenation are all proposed to follow similar pathways (Scheme 1), with Noyori-type bifunctional mechanisms being prominent. 14,15 Given the importance of non-metal mediated hemiaminal cleavage in our computed mechanism, we hypothesized that the co-catalytic enhancements observed here with Fe N should be generalizable to other systems. Indeed, highly active ruthenium catalysts recently reported by Beller and Sanford 24-26 also exhibit substantial enhancement in activity upon co-catalytic addition of TBD or formanilide (…”

“…Subsequent hydrogenation of the aldehyde affords the corresponding alcohol (step 3). 13,14 Most well-dened catalysts for deaminative hydrogenation rely on a Noyori-type, 14,15 bifunctional pathway whereby a metalhydride and adjacent ligand based proton are delivered to the carbonyl C]O moiety (Scheme 1). Intriguingly, recent mechanistic studies indicate that while the Noyori-type catalyst structure is essential for facilitating the dihydrogen addition steps of the process (1 and 3, in Scheme 1), the proton transfer between the O-and N-ends of the hemiaminal (step 2), which triggers the cleavage of the C-N bond, does not involve necessarily the metal catalyst.…”

Biomimetic Hydrogenation Catalyzed by a Manganese Model of [Fe]‐Hydrogenase

“…While the activity and scope of 5 in hydrogenation are the best among model complexes of [Fe]‐hydrogenase, they are below those of the most active base metal catalysts for common organic substrates –. We considered that the strength of a biomimetic catalyst like 5 might be its propensity to catalyze biomimetic reactions uncommon to synthetic chemistry.…”

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…In numerous cases, the different PNP ligands shown in Scheme and Scheme do not simply act as spectators but participate in the reactions via de‐ and re‐protonation processes, ligand‐centered redox reactions or via irreversible chemical transformations at the ligand backbones. Among those ligand‐based transformations, reversible metal‐ligand cooperative reactivity patterns often play a key role in catalysis, in particular in catalytic (de)hydrogenations and in hydrogen‐borrowing catalysis . Given that such application‐oriented reactivities have been covered in several excellent review articles,, the focus of our discussion will be on the metal complexes themselves and their stoichiometric reactions.…”