Exploring Ligand-Centered Hydride and H-Atom Transfer

The nickel(II) complex [ON(H)O]Ni(PPh3) ([ON(H)O](2-) = bis(3,5-di-tert-butyl-2-phenoxy)amine), bearing a protonated redox-active ligand, was examined for its ability to serve as a hydrogen atom (H-center dot) and hydride (H-) donor. Deprotonation of [ON(H)O]Ni(PPh3) afforded the square-planar anion {[ONOcat]Ni(PPh3)}(1-), whereas hydrogen atom transfer from [ON(H)O]Ni(PPh3) to TEMPO center dot in the presence of added PPh3 afforded five-coordinate [ONO]Ni(PPh3)(2) that has been structurally characterized. In solution, this five-coordinate complex exists in equilibrium with four-coordinate [ONO]Ni(PPh3), 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(PPh3) in the presence of PPh3 afforded the octahedral complex, [ONOq]Ni(OTf)(PPh3)(2), which was characterized as an S = 1, nickel(II) complex. Bond dissociation free energy (BDFE) and hydricity (Delta G(H-)degrees) measurements benchmark the thermodynamic propensity of this complex to participate in ligand-centered H-center dot and H- transfer reactions.

Automated summary

The nickel(II) complex bearing a protonated redox-active ligand was studied for its ability to serve as a hydrogen atom and hydride donor. The research showed that the complex exists in equilibrium with different coordination states in solution, which correlates with valence tautomerization between the ligand and the nickel center. Additionally, the complex's thermodynamic propensity to participate in ligand-centered H transfer reactions was characterized by bond dissociation free energy and hydricity measurements.