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}†

Abstract

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 Ni^II 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 ¹E_½ = –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 [Ni^IL]⁺ species.