Nickel porphyrins, metalation reactions

Intrinsic reactivity patterns of the different porphyrins are reflected in their metal deposition profiles, which serve as fingerprints marking the reaction sequence. Vanadium profiles (Fig. 25) from pure VO-etiopor-phyrin in oil demetallation experiments are steeper with less deposit in the pellets center than the nickel profiles (Fig. 26) from the analogous experiment with pure nickel porphyrin. Pure compound intrinsic reactivity data revealed that vanadium was more reactive than nickel at most temperatures of interest. However, a reduction in VO-porphyrin diffusion by stronger adsorption interactions would also contribute to a steepening of the metal deposition profiles. Metal profiles have not been examined from demetallating model oils containing both Ni- and VO-porphyrins. 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Equations 1 to 3 show some of fixation reactions of carbon dioxide. Equations la and lb present coupling reactions of CO2 with diene, triene, and alkyne affording lactone and similar molecules [2], in a process catalyzed by low valent transition metal compounds such as nickel(O) and palladium(O) complexes. Another interesting CO2 fixation reaction is copolymerization of CO2 and epoxide yielding polycarbonate (equation 2). This reaction is catalyzed by aluminum porphyrin and zinc diphenoxide [3],... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

A spectrum of metal compound reactivities in petroleum could arise for several reasons. Nickel and vanadium exist in a diversity of chemical environments. These can be categorized into porphyrinic and non-porphyrinic species vanadyl and nonvanadyl or associated with large asphaltenic groups and small, isolated metal-containing molecules. Each can be characterized by unique intrinsic reactivity. Reaction inhibition which occurs between the asphaltenes and the nonasphaltenes, as well as between Ni and V species, can also contribute to reactivity distributions. The parallel reaction interpretation of the observed reaction order discrepancy is therefore compatible with the multicomponent nature of petroleum. Data obtained at low conversion could appear as first order and only at higher conversions would higher-order effects become obvious. The... 

Model compound studies have shown the importance of porphyrin macrocycle basicity, resulting from electron-withdrawing substituents and metal ligands, on the reducibility and susceptibility of the central metal to reaction. Similar insight into the differences in relative basicity of vanadium- and nickel-containing complexes found in petroleum may therefore be valuable in rationalizing the observed effects and predicting demetallation activity. 

Other metal salts, such as Cr3+, Rh3+ or Ru3+ derivatives, catalyze the cyclization of dihydrobilin to porphyrin [63] differently from copper, these metals afford the metal-free ligand and this seems to be a suitable route for the preparation of porphyrins with acid-labile substituents, which in the previous method suffer during the removal of the copper atom. On the other hand, dihydrobilin also cyclizes to porphyrin when heated in 1,2-dichlorobenzene or when treated with bases although in lower yields [3, 8, 10]. The presence of nickel or cobalt salts seems to be necessary to drive the cyclization reaction towards the formation of the octadehydrocorrin structure. 

Chlorophylls and Iron porphyrins are prevalent In plant and animal matter whereas only nickel (as Nl(II)) and vanadium (as oxovanadlum V(IV), V 0) metalloporphyrlns are found In petroleum. To determine a plausible reaction sequence for these conversions, we are studying hydrolysis and metallatlon reactions of metal complexes of pheophytlns (the demetallated ligands of chlorophylls) and of porphyrins. The pheophytlns and metal pheophytlnates, Including the chlorophylls and the most abundant natural porphyrins, are highly llpophyllc and have very low solubilities In aqueous... 

The problem of the origin of the metal-porphyrins is closely related to that of the origin of petroleum and is oie of the most basic and interesting questions of petroleum geochemistry. The most probable conclusion seems to be that the nickel and vanadium porphyrin complexes are formed by metal exchange reactions from animal and/or plant metabolic pigments such as hemoglobin and chlorophyll. 

Several different sequences of reactions may be postulated for the conversion of the magnesium complexes of pheophytlns (chlorophylls) and Iron complexes of protoporphyrin IX and related porphyrins (hemes and hemlns) Into the nickel and vanadium porphyrins found In petroleum. One possible reason for the Isolation of only the nickel and oxovanadlum metalloporphyrlns Is that only they were resistant to degradation. While studies of Hodgson do Indicate that complexatlon of vanadium and nickel do Impart added thermal stability to porphyrins (12). Berezin has found that complexatlon of other metal Ions such as cobalt and copper also Imparts added thermal stability (13. 14). In addition, Hodgson s study Indicates that relatively little thermal degradation of the metalloporphyrlns has taken place In most crude oils (which would lead to unbound vanadium and nickel), One would expect that if little degradation of these metalloporphyrlns has occurred, complete disappearance of other metalloporphyrlns by thermal degradation Is an unreasonable assumption. 

Four years after their initial report, the first alternative synthesis of homoporphyrins was reported by Callot and coworkers [14], In this work it was found that the reaction of zinc tetraphenylporphyrin 20 with disubstituted diazoalkanes (Scheme 6) gives, after workup, the corresponding free-base homoporphyrins, namely 21-25. Treatment of these expanded porphyrins with Ni(II) gives the corresponding nickel complexes, namely 26-30. Reactions of 21 with other metals also gives the expected metal complexes 31-34 [15-17]. 

Porphyrin complexes can be commonly produced in sparking mixtures. Thus, Hodgson and Baker showed that pyrrole and paraformaldehyde in the presence of copper(II), nickel(II) and vanadyl salts give rise to both free and metal complexes of porphyrins. Tlie most efiective ion for the template reaction is Ni", a result parallelling the observations outlined earlier on the preservation of porphyrin complexes in asphalts, sediments and rocks. It was pointed out that coloured species noticed in earlier experiments were also probably porphyrin coordination complexes. More recently it has been demonstrated that Ni(CN)/ increases the yield of porphyrins in such reactions, as does Fe(CN) . Since these ions were probably present in reasonable concentrations in the primitive oceans, their significance is obvious. 

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