Kinetics of metal-ion complexation with N-methyltetraphenylporphyrin. Evidence concerning a general mechanism of porphyrin metalation

Abstract

The rate constants for complexation of A-methyltetraphenylporphyrin by Cu(II), Zn(II), Co(II), Mn(II), and Ni(II) perchlorate salts in , -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 (AH* in units of kcal/mol and AS* in eu) are AH* = 16.9 ± 1.8 and AS* = 9.7 ± 6.0 for Cu(II), AH* = 14.2 ± 0.7 and AS* = -6.7 ± 2.6 for Zn(II), AH* = 20.4 ± 1.1 and AS* = 8.6 ± 3.6 for Co(II), AH* = 21.6 ± 0.5 and AS* = 4.5 ± 1.6 for Mn(II), and AH* = 21.4 ± 2.0 and AS* = -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 A-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 A-methyl and planar porphyrins.

Conformational control of cofactors in nature – the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

“…203 The idea that macrocycle deformation was a key step in the mechanism of metal insertion in general was originally based on the observation that N-alkylporphyrins, which have a distorted macrocycle, 204 underwent metalation 3-5 orders of magnitude faster than non-methylated derivatives. 205,206 Soon after it was discovered that such compounds are potent inhibitors of ferrochelatase 207 and it was suggested that the sterically imposed non-planar conformation was similar to a reaction intermediate. This idea was subsequently supported by the generation of an efficient antibody metalation catalyst that had been raised against Nmethyl mesoporphyrin IX (N-MeMP, 49).…”

Dynamic Study of Metal-Ion Incorporation into Porphyrins Based on the Dynamic Characterization of Metal Ions and on Sitting-Atop Complex Formation

“…In this context, the kinetics of the metalloporphyrin formation have been extensively studied for many kinds of metal ions in a variety of solvents in order to clarify the metalation mechansim of porphyrins. [21][22][23][24][25][26][27][28][29][30] When the metal ion (M n+ ) is incorporated into the porphyrin (H2por) to form the metalloporphyrin (M(por) (n-2)+ ), two amine protons in H2por are dissociated from the two pyrrole groups as in Eq. (1).…”

Kinetics and mechanisms of the formation of a sitting-atop complex of N-alkylporphyrin with Al(III) in moist acetonitrile

“…Therefore, the 'dead-end' mechanism via a porphyrin monomer can be ruled out as a predominant step in the moist MeCN. ), where the deprotonation from porphyrin occurs [8,11,13]. The activation enthalpy for the formation of a sitting-atop complex of H 2 TPP with Cu(II) in acetonitrile is reported to be 56 kJ mol À1 by Funahashi and coworkers [18,19].…”

“…Therefore, N-alkylporphyrin is an important model for ferrochelatase, which catalyses the biological insertion of Fe(II) into protoporphyrin IX by ring distortion. The mechanism of metalation of N-substituted porphyrins (M = Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)) has been studied both in aqueous and nonaqueous media [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”