Different Kinetic Behavior of Zinc(II), Cadmium(II), and Lead(II) Porphyrins in the Ligand-Substitution Reaction with Ethylenediaminetetraacetic Acid

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Abstract
Kinetics and mechanism of the ligand-substitution reaction of Zn-, Cd-, and Pb(tpps)4− (H2tpps4−=5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin) with ethylenediaminetetraacetic acid (EDTA) have been studied at 25°C and I=0.1 mol dm−3 (NaNO3). The rate equation for the decrease of M(tpps)4− is expressed as –d[M(tpps)4−]⁄dt=(k1[edta4−])+k2[Hedta3−]+k3k4[H]2(k−3+k4[H+])−1)[M(tpps)4−], where M=Zn2+, Cd2+, and Pb2+, and edta4− denotes the fully deprotonated form of EDTA, respectively. The values of k1 were found to be 0 for all of the metalloporphyrins studied, and k2=0 mol−1 dm3 s−1; k3k4(k−3)−1=8.7±0.2 mol−2 dm6 s−1 for Zn(tpps)4−, k2=4.1±0.2 mol−1 dm3 s−1; k3k4(k−3)−1=(8.3±0.4)×1012 mol−2 dm6 s−1 for Cd(tpps)4− and k2=2.0±0.1 mol−1 dm3 s−1; k3(5.4±0.1)×107 mol−1 dm3 s−1; k3k4(k−3)−1=2.0±0.1 mol2 dm6 s−1 for Pb(tpps)4−, respectively. Hedta3− directly attacks Cd- or Pb(tpps)4 to form its metal complex, whereas Hedta3− can not bind Zn(tpps)4−. Zn(tpps)4− dissociates to aquazinc(II) by proton attack, then the aquazinc(II) reacts with Hedta3. Therefore, the ligand-substitution reaction of Zn(tpps)4− with EDTA is so slow that the reaction can be negligible compared with those of Cd- and Pb(tpps)4− in an alkaline solution. The reaction mechanism of the ligand substitution reaction and the different reactivities of the metalloporphyrins in it are discussed in detail.