Nickel(ii) complexes with amide ligands: oxidative dehydrogenation of the amines in a tetradentate diamide–diamine ligand

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Four complexes were prepared by the reaction of Ni(II) with three pyrrolidine based diamide–diamine ligands. The two amine groups in the ligand N,N′-bis(S-prolyl)-1,2-ethanediamine (S,S-bprolenH2) were oxidatively dehydrogenated during the preparation of the Ni(II) complex in air. The reaction involved O2 to yield a complex of a tetradentate ligand with 1-pyrroline terminal groups. The Ni(II) complex with S,S-bprolenH2 was synthesised under oxygen-free conditions, and the Ni(II) complexes with the analogous ligands N,N′-bis(S-prolyl)-R,R-1,2-cyclohexanediamine (R,R-(S,S)-bprolchxnH2) and N,N′-bis(S-prolyl)-1,2-benzenediamine (S,S-bprolbenH2) were prepared in air. The complexes were characterised by X-ray crystallography, 1H and 13C NMR spectroscopy and IR spectroscopy. The Ni(III/II) reduction potentials of complexes with pyridyl and pyrrolidine based tetradentate diamide ligands were measured by cyclic voltammetry to assess the stability of the Ni(III) oxidation state, but did not show any correlation with the ease of ligand oxidation.