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 comp lexes with various metal ions. In this work the thermokinetic investigation s of complex formation of tetraphenylporphyrins with copper(II) acetate wer e carried out in two anhydrous organic solvents: acetic acid and DMF at 25 degrees C. Measurements were performed in a heat conduction microcalorimete r [7] adapted for non-aqueous solvents. Total heat effects and thermokineti cs were determined. The results were discussed together with spectrophotome tric data. Taking into account the high stability of metalloporphyrin complexes, the h eat of their formation is rather small tin the case of acetic acid Delta Q= 32.7+/-1.2 kJ mol(-1), for DMF Delta Q= 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 energ y.