The kinetics of zinc(II) incorporation into fifty-five free base porphyrins in DMF: structure-reactivity correlations

Abstract

The kinetics of zinc(II) incorporation into fifty-five free base porphyrins of varying charges and structures were investigated in dimethylformamide at 25 °C. The rate law is the same as found by Tanaka and co-workers ( Bull Chem. Soc. Jpn., 57 (1984) 204) for tetraphenylporphyrin. One interpretation is that Zn(II) forms a complex with the free base porphyrin H 2P and in a k 1 pathway, the coordinated Zn(II) enters the porphyrin, while in a k 2 route, a second zinc is needed to produce the final product. While little correlation is found between the rate constants and overall porphyrin peripheral charge, k 2 varies over a 26 000 fold range for uncharged species. The replacement of hydrogens by two bulky substituents in the ortho-phenyl positions of tetraphenyl type porphyrins leads to a 200–500 fold decrease in k 2, and such ortho groups might inhibit porphyrin ring deformation. Phenyl rings substituted in the para position by groups such as −NH 2,−OCH 3 and −OH produce more reactive porphyrins that if such substituents are present in the meso-phenyl positions, indicating electron donation to the porphyrin ring by resonance effects. This implies coplanarity between at least one phenyl ring and the porphyrin plane in a distorted metal-porphyrin activated complex. Mercury(II) markedly catalyzes the Zn(II)/ H 2P reactions through the very rapid formation of Hg(II)P, which is 10 4 times more reactive with Zn(II) than H 2P itself. The predeformed octabromo-tetramesitylporphyrin metallates ∼4 × 10 3 times faster than the tetramesitylporphyrin.