Reaction of tri-(3-aminopropyl)amine with protons and some bivalent transition-metal ions: ΔH°, ΔS°, and visible spectra

Abstract

The four stepwise heats of protonation of the ‘tripod-like’ ligand tri-(3-aminopropyl)amine (tpt) have been determined calorimetrically at 25° in 0·1M-potassium chloride. From these results it appears that in the four stages both primary and tertiary amino-groups are protonated. The heats of formation of the complex ions [Ni tpt]²⁺, [Cu tpt]²⁺, [Cu Htpt]³⁺, [Cu tpt(OH)]⁺, and [Zn tpt]²⁺ have also been measured under the same experimental conditions. Through a combination of the values of the enthalpy changes with the corresponding free-energy changes, reported elsewhere, the entropies of protonation and complex formation have been obtained. A comparison between the heats of formation of [Cu Htpt]³⁺ and [Cu tpt]²⁺ shows that in the latter complex all four nitrogen atoms of the ligand are co-ordinated to the central atom. The thermodynamic functions and the electronic spectra seem to indicate a configuration that is five-co-ordinate for [Cu tpt]²⁺, [Cu tpt(OH)]⁺, and [Co tpt]²⁺, octahedral for [Ni tpt]²⁺ and [Cu Htpt]³⁺, and tetrahedral for [Zn tpt]²⁺. The low heats of formation of the copper(II) and nickel(II) complexes may be due both to the strain within the six-membered chelate ring and to the hindrance between the chelate rings.