Exploring Ligand-Centered Hydride and H-Atom Transfer.

The nickel(II) complex [ON(H)O]Ni(PPh) ([ON(H)O] = bis(3,5-di--butyl-2-phenoxy)amine), bearing a protonated redox-active ligand, was examined for its ability to serve as a hydrogen atom (H) and hydride (H) donor. Deprotonation of [ON(H)O]Ni(PPh) afforded the square-planar anion {[ONO]Ni(PPh)}, whereas hydrogen atom transfer from [ON(H)O]Ni(PPh) to TEMPO in the presence of added PPh afforded five-coordinate [ONO]Ni(PPh) that has been structurally characterized. In solution, this five-coordinate complex exists in equilibrium with four-coordinate [ONO]Ni(PPh), and this ligand exchange equilibrium correlates with a valence tautomerization between the redox-active ligand and the nickel center. Abstraction of a hydride from [ON(H)O]Ni(PPh) in the presence of PPh afforded the octahedral complex, [ONO]Ni(OTf)(PPh), which was characterized as an = 1, nickel(II) complex. Bond dissociation free energy (BDFE) and hydricity (Δ°) measurements benchmark the thermodynamic propensity of this complex to participate in ligand-centered H and H transfer reactions.