Kinetics and Mechanism of Copper(II), Zinc(II), and Cadmium(II) Incorporation into 5,10,15,20-Tetraphenylporphine and N-Methyl-5,10,15,20-tetraphenylporphine in N,N-Dimethylformamide

Abstract

Abstract The kinetics of reactions of 5,10,15,20-tetraphenylporphine (H2(tpp)) and N-methyl-5,10,15,20-tetraphenylporphine (H(metpp)) with CuII, ZnII, and CdII ions was studied spectrophotometrically in N,N-dimethylformamide. The rate of incorporation of metal ions (M2+ : Cu2+, Zn2+, Cd2+) into H(metpp) is first order with respect to both H(metpp) and metal ion: d[M(metpp)+]/dt=[k[H(metpp)][M2+] with k(25 °C)=11.5±0.5 mol−1 dm3 s−1 for Zn2+ and (5.8±0.8)×102 mol−1 dm3 s−1 for Cd2+. The rate of incorporation of copper and zinc into H2(tpp) is first order in H2(tpp) and the conditional first-order rate constants are expressed as follows: k0(M)=K[M2+](1+K[M2+])−1(k1+k2[M2+]) where for Cu2+ and Zn2+ K/mol−1 dm3, k1/s−1, and k2/mol−1dm3 s−1 at 25°C are (1.6±0.2)×104 and (7.2±1.5)×102, (1.2±0.1)×10−4 and (3.5±0.7)×10−5, and (5.2±0.2)×10−2 and (8.6±0.2)×10−4, respectively. Metal ion incorporation into H2(tpp) proceeds through the formation of intermediates prior the rate-limiting step. For the H(metpp) system, the methyl group on the pyrrole in H(metpp) prevents the formation of such an intermediate. On the other hand, the rate of cadmium incorporation into H2(tpp) is first order with respect to Cd2+ and H2(tpp) and its second order rate constant k is (6.5±1.0)×10−3 mol−1 dm3 s−1. The large cadmium(II) ion can not fit into the porphyrin nucleus and its reaction proceeds through a pathway involving a mononuclear activated complex.