Steric effects in the aerobic oxidation of π-Allylnickel(II) complexes with N-heterocyclic carbenes

Article abstract of DOI:10.1021/ic0612451

π-Allylchloro(NHC)nickel(II) complexes were synthesized and their reactions with O₂ were studied. Ligand steric effects were found to determine the difference between rapid oxidation of the allyl group to produce bis-μ-hydroxonickel complexes and no observable reaction. The ability of the metal-NHC bond to rotate correlates with the ability of the complex to react with O₂. In the limiting cases, conformationally restricted complexes are stable to O₂ and complexes with rapid Ni-NHC bond rotation react rapidly with O₂. Complexes with intermediate conformational flexibility were found to exhibit lesser reactivity with O₂. On the basis of the observed inertness of complexes with saddle-shaped ligands to O₂, we propose the adoption of a nonplanar geometry upon reaction with O₂ to be required. The issue of conformational flexibility versus rigidity is expected to directly impact the catalytic behavior of metal-NHC complexes.