Porphyrins metal chelation/metallation

Fig.1. Structures of porphyrin 1, chlorophyll 2, and phthalocyanine 3. In the presence of metal salts M"+X (M=metal, X=counter anion, n=oxidation state or number of counter anions), porphyrins produce chelate complexes. Some metal chelates of the porphyrins, such as ZnPor, form further coordination bonds with other ligands such as pyridines...

The arrangement of four nitrogen atoms in the center of the porphyrin ring enables porphyrins to chelate various metal ions. Protoporphyrin that contains iron is known as heme ferroheme refers specifically to the complex and ferri-heme to Fe. Ferriheme associated with a chloride counter ion is known as hemin, or hematin when the counter ion is hydroxide. 

The most stable manganese oxidation state in porphyrins is +3, and Mn111 complexes of ETIO-type porphyrins are the most prominent example of metalloporphyrins with a split Soret band. Instead of the usual intense band at 400 nm, these chelates possess two bands at 350 and 460 nm with an intensity ratio of 2 1. This change was related to a strong porphyrin-metal n-d(XZjyZ) interaction in addition to the usual a interaction between the four pyrrole nitrogens and the metal [Boucher (11)]. Theoretical calculations do indeed predict dxz,yg-n interactions because the metal d orbitals in the first transition series roughly match... 

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

Chemical transformations at the macroeyclic chromophorc of expanded porphyrins are still not known. The complexation behavior of expanded porphyrins is very different from that of nonexpanded porphinoid macrocycles. The coordination hole of the expanded porphyrins is often too big for the complexation of a single metal ion, so in fact two metal ions can be chelated. With some expanded porphyrins, anion binding is observable, a striking difference to the nonexpanded porphyrins. The complexation behavior and the host-guest chemistry of expanded porphyrins is a rapidly growing field of research. The work in this field has been reviewed. Ie f... 

The corrins and porphyrins are another important class of natural chelator molecules (Figure 2.4). They are thermodynamically very stable and have four nearly coplanar pyrrole rings, the nitrogen atoms of which can accommodate a number of different metal ions in different oxidation states like Fe2+ in haem, Mg2+ in chlorophyll and Co3+ in vitamin B12. 

Cationic polymerization of alkylene oxides generally produces low molecular weight polymers, although some work [26] seems to indicate that this difficulty can be overcome by the presence of an alcohol (Fig. 1.3). Higher molecular weight polyethylene oxides can be prepared by a coordinated nucleophilic mechanism that employs such catalysts as alkoxides, oxides, carbonates, and carboxylates, or chelates of alkaline earth metals (Fig. 1.4). An aluminum-porphyrin complex is claimed to generate immortal polymers from alkylene oxides that are totally free from termination reaction [27]. 

---