Nickel complexes porphyrins

Porphyrin, octaethyl-, nickel complex cyclic voltammetry, 4, 399 <73JA5140)... 

Synthesis, characterization, and chemistry of core-modified porphyrins and their nickel complexes 97NJC691. 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Crude oil consists mainly of a mixture of paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of metals-containing heterocyclic compounds. The most abundant metals found in oils are those contained in porphyrin or porphyrin-like complexes (nickel, copper, iron, and vanadium). These... 

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). 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

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. 

Interestingly, structural investigations of copper and nickel complexes of dodecasubstituted porphyrins bearing aryl groups at the meso positions and propano rings at the pyrrole p positions reveal considerable differences in their... 

One of the articles describes F-430, a nickel complex with a cyclic ligand related to porphyrins. In strict anaerobes and in anaerobes able to withstand a low level of dioxygen, e.g. sulphur bacteria, both vitamin B12, a cobalt complex, and heme, an iron complex, are present. They too are related to porphyrins. It is then of interest to compare and contrast their functions. Vitamin B12 has been described in volume 20 (Hogenkamp HPC, Sando GN (1974) Struct Bond 20 23). In this volume the value of heme to anaerobes is stressed. Putting these three articles together we begin to see that early life had a requirement for trace elements in highly specified forms. 

Owen and Thomley (547) have reviewed covalency in transition metal ions and, in particular, in nickel complexes. The NMR contact shift method has been used by Eaton et al. (195) to determine spin densities on organic ligands of paramagnetic molecules. In particular (194), a series of nickel aminotroponeiminates have been studied whereby conjugative and hyperconjugative effects within the molecule may be monitored. Similar studies of contact shifts have been carried out on Co and Ni pyrromethenes and porphyrins (196) and on many transition metal acetylacetonates (193) by Eaton et al. 

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. 

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