KINETICS OF METAL-ION COMPLEXATION WITH N-METHYLTETRAPHENYLPORPHYRIN - EVIDENCE CONCERNING A GENERAL MECHANISM OF PORPHYRIN METALATION

The rate constants for complexation of N-methyltetraphenylporphyrin by Cu(II), Zn(II), Co(II), Mn(II), and Ni(II) perchlorate salts in N, N-dimethylformamide were determined at several temperatures in order to investigate general features of porphyrin metalation reactions. All reactions obey a second-order rate law, first order in porphyrin and first order in metal ion. At 25 °C, the complexation rate constants (in M-1 s-1) are 289 ± 52 for Cu(II), 10.4 ± 0.8 for Zn(II), 0.68 ± 0.04 for Co(II), 0.010 ± 0.002 for Mn(II), and 0.0003 for Ni(II). Activation parameters for these reactions (ΔH‡ in units of kcal/mol and ΔS‡ in eu) are ΔH‡ = 16.9 ± 1.8 and ΔS‡ = 9.7 ± 6.0 for Cu(II), AH‡ = 14.2 ± 0.7 and ΔS‡ = -6.7 ± 2.6 for Zn(II), ΔH‡ = 20.4 ± 1.1 and ΔS‡ = 8.6 ± 3.6 for Co(II), ΔH‡ = 21.6 ± 0.5 and ΔS‡ = 4.5 ± 1.6 for Mn(II), and ΔH‡ = 21.4 ± 2.0 and ΔS‡ = -3.2 ± 5.8 for Ni(II). With the exception of the Mn(II) reaction, the order of the porphyrin metalation rate constants, Cu(II) > Zn(II) > Co(II) > Ni(II), coincides with the rates of solvent exchange of the metal ions. The Mn(II) complexation rate is slower than would be predicted from solvent-exchange rates. This anomaly has also been found for Mn(II) complexation of both tetra(p-sulfophenyl)porphyrin and tetrakis(4-A-methylpyridyl)porphyrin. The general trend supports a porphyrin metalation mechanism in which solvent dissociation from the metal ion is a rate-determining factor. The rates for complexation of N-methyltetraphenylporphyrin are faster than rates for planar porphyrins, indicating the importance of porphyrin deformation to the rate. The results support a dissociative interchange mechanism as a predominant path for both N-methyl and planar porphyrins. © 1979, American Chemical Society. All rights reserved.