Mixed aza–thia crowns containing the 1,10-phenanthroline sub-unit. Substitution reactions in [NiL(MeCN)][BF4]2 {L = 2,5,8-trithia[9](2,9)-1,10-phenanthrolinophane}†

The substitution reactions of the co-ordinated acetonitrile molecule in [NiL(MeCN)][BF4]2 1 (L = 2,5,8-trithia[9](2,9)-1,10-phenanthrolinophane) with different anionic and neutral ligands L′ [Cl-, Br-, I-, CN-, SCN-, H2O, pyridine (py), aniline (an), 1,3-dimethyl-4-imidazoline-2-thione (etu) or 1,3-dimethyl-4-imidazoline-2-selone (eseu)] have been studied by using electronic spectroscopy. While the reaction with all the anionic ligands is quantitative, for the neutral ones an equilibrium takes place; the corresponding equilibrium constants have been determined in MeCN at 25°C. The complex cations [NiL(L′)](2 - n)+ (n = 0 for neutral and 1 for anionic ligands) have also been isolated in the solid state, mainly as BF4- salts and the compounds [NiL(H2O)][ClO4]2·H2O, [NiL(Cl)]Cl·H2O, [NiL(SCN)]BF4·MeNO2, [NiL(eseu)][BF4]2 and [NiL(py)][BF4]2 have been characterized by X-ray diffraction studies. In these complexes a distorted octahedral geometry is achieved at the NiII with five sites occupied by the macrocyclic ligand L and the sixth by the appropriate ligand L′. The electrochemistry of all the prepared compounds has been studied by cyclic voltammetry. In particular the reductive cyclic voltammetry of 1 in acetonitrile shows a quasi-reversible one-electron reduction wave near 1E1/2 = -1.0 V vs. Fc/Fc+. Electrochemical reduction by controlled-potential electrolysis at this potential in the presence of the axial ligand PMe3 and investigation of the reduced product by ESR spectroscopy confirm the reduction process to be metal based and to correspond to the formation of the [Ni1L]+ species.