Thermokinetic Studies of Porphyrin Complexation in Non-Aqueous Solutions by Heat Conduction Microcalorimetry

Porphyrins are known as unique ligands forming some of the most stable complexes with various metal ions. In this work the thermokinetic investigations of complex formation of tetraphenylporphyrins with copper(II) acetate were carried out in two anhydrous organic solvents: acetic acid and DMF at 25°C. Measurements were performed in a heat conduction microcalorimeter [7] adapted for non-aqueous solvents. Total heat effects and thermokinetics were determined. The results were discussed together with spectrophotometric data.Taking into account the high stability of metalloporphyrin complexes, the heat of their formation is rather small (in the case of acetic acid ΔQ=32.7±1.2 kJ mol−1, for DMF ΔQ=26.2±1.3 kJ mol−1). Such result can suggest that: 1) the stability of complexes is influenced strongly by an entropy factor, 2) the complexation involves breaking of the NH-bond in the porphyrin coordination centre, accompanied by high consumption of energy.