Mechanistic Aspects of O2-Activation on Nickel(II) Tetrahydrosalen Complexes

Abstract

The activation of dioxygen in the coordination sphere of transition-metal centers is equally important for chemistry and biology and has been known for a long time for Fe-, Co- and Cu-containing active sites and the analogous model complexes.1 There is only very limited information on nickel(II) complexes interacting with dioxygen.2 Kimura et al. and others2 described nickel(II) complexes with macrocyclic N5 ligands, which are capable of both O2 addition and activation. H. Kanatomi3 reported that specifically substituted derivatives of tetrahydrosalen4 form nickel(II) complexes which, in aerated organic solution and in the presence of added hydroxide ions or pyridine, are subject to oxidative dehydrogenation to finally form the corresponding Schiff base complexes after prolonged heating. In an attempt to model the co-enzyme F430, Berkessel et al.5 prepared a nickel(II) complex of the Ni(ONNOS) type, in which the nickel is bound to a dihydrosalen ligand with a pendant thioether group. This complex obviously is subject to intramolecular oxidation by dioxygen and subsequent degradation. Very recently, we reported several O2 active nickel(II) complexes with the open-chain N2O2 ligand tetrahydrosalen,4 carrying specific substitution patterns on the benzene rings of the ligand.6 The activation of dioxygen by these latter complexes takes place under mild conditions (ambient temperature) and does not necessitate the addition of bases. The reaction with O2 is strongly solvent dependent, leads to oxidative dehydrogenation of one C-N bond under the given conditions (formation of the corresponding dihydrosalen nickel(II) complex) and produces O=PPh3 in the presence of PPh3.6