Demetalation of metalloporphyrins

Table 1 Demetalation of metalloporphyrins under acidic conditions.52,58-60...

The porphyrin metallo complexes in crude oils, asphaltenes and other natural bitumens are chiefly those of vanadium and nickel although copper, iron and even uranium have been suggested. Recently in a Precambrian shale, porphins were found to chelate with iron, zinc and copper in addition to vanadium and nickel.The origin of these complexes is still uncertain, although several theories have been advanced. Some of these theories could be verified or possibly even disproved if the porphyrin type bound to each metal was known. Furthermore, since these heavy metals are harmful to both health and catalysts, a systematic study of demetallation of metalloporphyrins should prove useful. 

The demetallation of metalloporphyrins by acid is a reversible one and can be represented in a generalized form by the following equation ... 

H. Levanon and P. Neta, One-Electron Oxidation and Demetallation of Metalloporphyrins and Chlorphyll a in Dichloroethane Solutions as Sudied by Pulse Radiolysis, Chem. Phys. Lett., 70 (1980) 100. 

The qualitative stabihty constants of metalloporphyrins are summarized in Table 1. The metals classified in class I produce the most stable metalloporphyrins and the demetalation reaction does not proceed smoothly even under concentrated sulfuric acid condition. The incorporated metals classified in classes 11 and III are removed using mild acids such as hydrochloric acid. Calcium classified in class V is removed by (the pH of) water. The mixed cyclization reactions afford the heterometalated CPO, and the acid treatment of the CPO obtained produces the CPO containing 2HPor moiety. Further treatment of the metal salt classified in a class lower than that of the unremoved metal(s), which is classified in a class higher in Table 1, produces another heterometalated CPO. Representative examples are summarized in Fig. 5 [25]. The initial cyclization reaction is carried out by using Ru (CO)Por 6, the metal of which is classified in class 1, and ZnPor 5, clas-... 

Table 7 Stability Classes of Metalloporphyrins Toward Demetallation with Acids...

The most common reactions involving nucleophiles and porphyrin systems take place on the metalloporphyrin 77-cation radical (i.e. the one-electron oxidized species) rather than on the metalloporphyrin itself. One-electron oxidation can be accomplished electrochemi-cally (Section 3.07.2.4.6) or by using oxidants such as iodine, bromine, ammoniumyl salts, etc. Once formed, the 77-cation radicals (61) react with a variety of nucleophiles such as nitrite, pyridine, imidazole, cyanide, triphenylphosphine, thiocyanate, acetate, trifluoroace-tate and azide, to give the correspondingly substituted porphyrins (62) after simple acid catalyzed demetallation (79JA5953). The species produced by two-electron oxidations of metalloporphyrins (77-dications) are also potent electrophiles and react with nucleophiles to yield similar products. 

If a metal oxidation state change occurs under the demetallation conditions, the charge (Z) and the radius (yf) should be those of the actually demetallated species. As is obvious from the definition of SI, a metalloporphyrin with the metal ion of a high charge, a high electronegativity and a small radius is stable. The SI underestimates the stability of metalloporphyrins where there are strong metal-porphyrin n interactions. 

One-electron oxidation of metalloporphyrins in this solvent is complicated by the production of HCl which rapidly demetallates these complexes. However, addition of pyridine as a base prevents this and allows the oxidation products to be investigated [29]. Under these conditions, the redox processes are similar to those observed in CCI4 solution (see above). 

There are discrepancies in reaction kinetic order with respect to the total metal concentration. Demetallation kinetics have been described in the literature by using first-order, second-order or in between first and second order rate laws (2). Measurements of intrinsic kinetics of metal removal from model oil comprised of metalloporphyrins have been reported with values ranging from 0.1 to... 

MP here is metalloporphyrin and PH2 is free base porphyrin. In general the forward equation is demetallation and the reverse is metallation. The dissociation of metalloporphyrins is governed by chemical stability. 

Radical cations of water insoluble porphyrins were studied in chlorinated organic solvents. Pulse radiolysis in dichloroethane led to production of the radical cations of several metalloporphyrins, but an additional process was observed in the case of Mg" and Zn" complexes. It was found that this second process was due to demetallation of these complexes by the HCl formed in the radiolysis. 

The stability of a metalloporphyrin can be roughly estimated by the charge-to-radius ratio (Z/yj). Buchler et al 1 found that the stability index, SI = 100(ZAr/yj), where X is the Pauling electronegativity, predicts the stability of a metalloporphyrin to acid demetallation, and correlated the SI to the Falk-Phillips stability criteria (Table 2). 

Trivalent yttrium and lanthanide metals, except for promethium, have been complexed to octaethylporphyrin by heating at 210 °C in an imidazole melt.17 The complexes obtained as hydroxides, Mm(OEP)(OH), are unstable in acidic media. As the charge radius ratio rule predicts, the early lanthanide metalloporphyrins, MIU(OEP)(OH) (M = La, Ce, PR, Nd), are demetallated during purification, and the middle series (M = Sm, Eu, Gd, Tb, Dy) in 1 % acetic acid in methanol, while the last five (M = Ho, Er, Tm, Yb, Lu) survive in 2% acetic acid in methanol but are dissociated in dilute hydrochloric acid. The Mnl(OEP)(OH) appears to coordinate more than one equivalent of pyridine and piperidine, and dimerizes in noncoordinating solvents such as benzene and dichloromethane at 10 4 M concentration. The dimer is considered to be a di-p-hydroxo-bridged species, different from the p-oxo dimer, Scin(OEP) 20 (Scheme 6). 

Fig. 19. Nickel analogs of model metalloporphyrins used in demetallation studies.

Single-electron oxidation of the metalloporphyrins 9 (e.g. by halogens or electrochemically) leads to the formation of radical cations. These undergo addition with nucleophiles at a methine bridge and, after acid demetalation, yield monosubstituted porphyrins 10 [18] ... 

The pKa values for such radicals were determined from changes in absorption spectra and found to be in the mildly alkaline range. On the other hand, metalloporphyrin radical anions cannot protonate on the nitrogen unless this leads to demetallation (see below). They may protonate on a carbon atom, but such species have not been shown to exist in the microsecond time scale under ambient conditions. Therefore, the above disproportionation mechanism is likely to be driven by protonation of the dianion rather than protonation of the radical anion. 

Complexes of Ni" and Co", for example, do not undergo demetallation but complexes of Zn" and Mg" do so very readily. To prevent demetallation, either a base (pyridine) was added to neutralize the HCl, or the solvent was changed to CCl4, ° where HCl is not produced. By using dichloroethane/ pyridine solutions rate constants could be determined for the electron transfer from Co"TPP and from chlorophyll a to the radical cations of various metalloporphyrins. 

Hambright et al. (1988) have also studied the kinetics of displacement of the Gd " ion from the gadolinium(III) complex of TSPP by ethylenediaminetetraacetate (EDTA) giving Gd(EDTA) and H2(TSPP) as the main products. This represents the first example of metal removal from a metalloporphyrin by a chelating ligand. A mechanism has been proposed to account for the kinetic data. The water-soluble lanthanide complexes of TMPyP also undergo demetallation in the presence of EDTA (Haye and Hambright 1991). Similar to the acid solvolysis reactions, a linear relationship between log k and the ionic radius of the metal center can be established, and complexes with smaller metal center are more stable toward demetallation by EDTA. 

---