Reductive electrochemical study of Ni(II) complexes with N 2O 2 Schiff base complexes and spectroscopic characterisation of the reduced species. Reactivity towards CO

Abstract

Reductive electrochemical properties of series of nickel(II) complexes with salen ligands, which have different diimine bridges and substituents in the aldehyde moiety have been studied in several solvents (CH 3CN, dmf and (CH 3) 2SO). In order to assess the relative importance of the Ni(I) and Ni(II) anion radical species, the reduced species have been characterised by combining EPR and UV–Vis spectroscopy. The results have shown that complexes with aliphatic diimine bridges are reduced to four-coordinate Ni(I) species with a B 1g (d xy ) 1 ground state, whereas those with aromatic diimine bridges are reduced to square–planar Ni(II) anion radical species that rapidly dimerise. None of the reduced species was found to bind pyridine, imidazole and triphenylphosphine, but in the presence of the stronger π-acceptor ligand CO, new Ni(I) species were formed that, and on the basis of EPR data, can be formulated as five-coordinate complexes with a B 1g (d xy ) 1 ground state, [NiL·CO] −. These new species are more stable than the parent complexes as confirmed by the more positive E 1/2 values as a consequence of the extensive π delocalisation M→CO.