While N-heterocyclic carbenes (NHCs) have been well-studied ligands for transition metal complexes, their group 15 analogues, which instead incorporate a phosphorus atom in the heterocycle, remain relatively unexplored in comparison. In contrast to NHCs, N-heterocyclic phosphenium cations (NHP+s) are weak σ-donors and strong π-acceptors, leading to different reactivity, properties, and potential applications. Similar to nitrosyl ligands, NHPs are thought to interconvert between a phosphenium (NHP+) and a phosphido (NHP-) through a two-electron process, which could yield unique properties and coordination in comparison to NHCs. Incorporation of an NHP unit into the center of a rigid chelating pincer ligand and its coordination to a manganese center has yielded a (PPClP)Mn(CO)2Br complex, which can further be treated with two equivalents of a reducing agent to afford a coordinatively unsaturated manganese complex (PPP)Mn(CO)2. Structural characterization of this compound reveals a fluxional geometric process about the manganese center that varies in the solution and solid state. In the solution phase, (PPP)Mn(CO)2 adopts a more trigonal bipyramidal geometry and is analogous to pincer-ligated manganese dicarbonyl complexes used for hydrogenation and dehydrogenative coupling catalysis in the literature. Progress toward reactivity of (PPP)Mn(CO)2 and the nature of the metal-phosphorus bonding in the NHP+/NHP- moiety will be discussed.

Manganese carbonyl complexes incorporated into an N-heterocyclic phosphenium/phosphido pincer ligand framework

ACS Spring 2022

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