Ligand Protonation Triggers H2 Release from a Dinickel Dihydride Complex to Give a Doubly "T"-Shaped Dinickel(I) Metallodiradical

Abstract

The dinickel(II) dihydride complex (1^K ) of a pyrazolate-based compartmental ligand with β-diketiminato (nacnac) chelate arms (L^- ), providing two pincer-type {N₃ } binding pockets, has been reported to readily eliminate H₂ and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2^Na , 2^K ) of L^- are now synthesized via a direct reduction route. They feature two adjacent T-shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local $d_{x^2-y^2}$ type magnetic orbitals are oriented into the bimetallic cleft, enabling metal-metal cooperative 2 e^- substrate reductions as shown by the rapid reaction with H₂ or O₂ . X-ray crystallography reveals distinctly different positions of the K⁺ in 1^K and 2^K , suggesting a stabilizing interaction of K⁺ with the dihydride unit in 1^K . H₂ release from 1^K is triggered by peripheral γ-C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H₂ elimination from the bimetallic cleft.

Keywords: dinuclear complexes; ligand protonation; magnetic properties; metalloradicals; nickel(I).