Kinetics of Metalloporphyrin Formation with Particular Reference to the Metal Ion Assisted Mechanism

A classification of kinetics of metalloporphyrin formation is presented according to the composition of the activated complex involved in the reaction. In the reaction of large metal ions, such as mercury(II) and cadmium(II) with porphyrins, these metal ions can not fit into the porphyrin nucleus and they are out—of-plane in the metalloporphyrin. In this case the reaction is relatively fast and it proceeds through a pathway involving a mononuclear activated complex. At higher metal ion concentration, a dinuclear product results . In the incorporation of metal ions of medium size, such as cobalt(II) and copper(II) , the first metal ion weakly bound to porphyrin deforms the porphyrin nucleus, thus making easy the attack from the back by another metal ion. This reaction thus involves a homodinuclear species as an activated complex. In the presence of large metal ions like mercury (II) , the mercury(II)—porphyrin is formed rapidly and it is more reactive than the free porphyrin. This is the reason why a small amount of mercury(II) or cadmium(II) acts as a catalyst in the formation of metalloporphyrins involving transition metal ions of medium size such as manganese(II) cobalt(II) and nickel (II). Aheterodinuclear activated complex is involved in this metal ion assisted formation of metallo— porphyrins. N-methyl-nec o-tetraphenylporphine is favorably deformed and it is more reactive than the non—N—methylated analogue. For N—methylated porphyrins the reaction proceeds through a pathway involving a mononuclear activated complex: no homodinuclear activated complex is observed and the formation rate is always proportional to the metal ion concentration.