Ligand Dynamics and Aerobic Allylic Oxidation with Bifunctional Ni(NHC) Complexes
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The synthesis of three new NiCl(allyl/cinnamyl)(NHC) complexes, containing a bifunctional NHC (N-heterocyclic carbene) ligand is described. Two different NHCs were utilized that contain a secondary amine (HNR; R=t-Bu or Mes) moiety on one of the N-imidazolylidene arms. Complexes with allyl (2 a) and cinnamyl (3 a and 3 b) were each crystallographically characterized and η3-allyl and κ1-C NHC ligand coordination modes were found in the solid-state. In the solution-state, each of the ligands shows highly fluxional behaviour. Halide substitution of the chloro ligand for iodo effectively slows the ligand dynamics and permits solution-state characterization that is in agreement with the solid-state structures. The complexes react with oxygen to ultimately oxidize the allyl or cinnamyl ligands to acrolein (AC) or cinnamaldehyde (CA) and phenyl vinyl ketone (PVK), respectively. The bifunctional nature of the NHC has limited influence on oxidation reactivity, but preliminary evidence suggests it plays a role in the stability of the nickel oxidation products.