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 weaker σ-donors and stronger 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. Thus, a closer look at electronics and the nature of the Ni-P bond is required to best describe our complexes as either Ni0/NHP+ or NiII/NHP- complexes. Incorporation of an NHP unit into the center of a rigid chelating pincer ligand and coordination to a nickel center has yielded a (PPClP)Ni(PMe3) complex, which can be treated with a hydride source to afford a new dimeric nickel complex [(PPHP)NiPMe3]2 in which the chloride bound to the central phosphorus is abstracted and a new P-H bond is installed. Solid-state characterization of this compound reveals an unprecedented structural motif in NHP coordination as the dimerization occurs through a P2N2 core of the two ligands, resulting in a bridging mode that does not involve the two transition metal centers or the hydride fragment. Furthermore, an unusual μ2-H ligand bridging between the nickel center and the central phosphorus atom of the NHP unit is also observed in this structure. Progress toward probing the electronic nature of the P-H bond and exploration of the novel coordination of the NHP framework will be discussed.

Characterization of a unique Ni-P bridging hydride in pincer-ligated N-heterocyclic phosphenium/phosphido nickel complexes

ACS Spring 2022

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