Electrochemical and X-ray studies of nickel(II) Schiff base complexes derived from salicylaldehyde. Structural effects of bridge substituents on the stabilisation of the+3 oxidation state

The oxidative chemistry of three Ni(II) complexes with Schiff base ligands derived from salicylaldehyde and diamines with different steric demands, N, N'-2-methylpropane-2,3-diyl-bis(salicylideneiminate)nickel(II) (1), N,N'-1, 2-cyclohexyl-1,2-dyil-bis(salicylideneiminate)nickel(II) (2) and N,N'-2,3-d imethylbutane-2,3-diyl-bis(salicylideneiminate)nickel(II) (3), was studied by cyclic voltammetry and chronoamperometry in N,N'-dimethylformamide and ( CH3)(2)SO. The electrogenerated species were characterised by EPR spectrosc opy. All three complexes exhibited metal-centred oxidised processes and the oxidised products were low-spin six-coordinate Ni(III) species (d(z2) grou nd state) with two solvent molecules axially coordinate. Addition of pyridi ne resulted in the replacement of solvent molecules with no changes in the ground state. The crystal structures of compounds 1 and 3 were determined f rom single crystal X-ray diffraction data, and the crystal packing for any of the complexes did not show any systematic parallel orientation of any pa rt of the molecules. X-ray structural data for the Ni(II) complexes provide d a rationale for the E-1/2 values obtained in the oxidation processes and for the relative energy of the low-lying excited duplets of the electrogene rated Ni(III) species. (C) 2000 Elsevier Science Ltd All rights reserved.