The interaction of aliphatic amines with transition-metal complexes of aromatic di-imines. Part 1. Kinetics and thermodynamics of reaction of the tris(5-nitro-1,10-phenanthroline)nickel(II) cation with aliphatic amines in dimethylformamide

Abstract

Addition of aliphatic amines to the complex [Ni(nphen)₃][ClO₄]₂(nphen = 5-nitro-1,10-phenanthroline) in dipolar aprotic solvents gives deep red solutions. A spectrophotometric investigation of these processes leading to the formation of 1:2 adducts of complex: amine, most likely ‘Meisenheimer’ complexes, has been carried out in dimethylformamide. Both equilibrium and rate constants for adduct formation between [Ni(nphen)₃][ClO₄]₂ and piperidine, n-propylamine, or benzylamine have been calculated by low-temperature stopped-flow techniques. The reactions are exothermic; the equilibrium data at –50 °C show that co-ordination of the ligand to nickel(II) largely increases the stability of the adduct between nphen and piperidine. Kinetic data are in agreement with a stepwise mechanism in which nucleophilic attack of the amine at the ligand is followed by proton transfer from the species formed to a second amine molecule. Proton transfer is either fast or rate determining depending on the nature of the amine.