Nickel porphyrins determination

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

Factors that determine the site of the nickel porphyrin at which reduction occurs.)... 

Trevin, S., E. Bedioui, and J. Devynck (1996). Electrochemical and spectrophoto-metric study of the behaviour of electropolymerized nickel porphyrin films in the determination of nitric oxide. Talanta 43,303-311. 

Resonance Raman scattering provides a valuable method of determining the state of axial ligation in nickel-reconstituted heme proteins and Ni-porphyrin complexes. A pattern of shifts in the Raman coresize and oxidation-state marker lines can be used to monitor changes in axial coordination. The shifts in the core-size lines (e.g. indicate an expansion of the core from about 1.96 A for the 4-coordinate Ni porphyrin to 2.04 A for the 6-coordinate species,... 

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). 

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... 

One of the properties of fossil fuels of interest to refiners is metal content. Vanadium in particular is troublesome in refinery operations (2). In some crude oils it occurs in concentrations up to 1000 ppm (3). Of the other metals in petroleum, only nickel is known to be present in concentrations approaching that of vanadium. These two metals are com-plexed with porphyrins and other ligands, the exact nature of which is unknown (4), Crude oils that are high in sulfur are usually high in vanadium (5), and it is interesting to determine whether or not this criterion also applies to tar sands. 

In order to increase the solubility of porphyrin and phthalocyanine complexes, several structural modifications have been made, a, jS, y, 6-Tetra-(4-pyridyl)-porphin complexes of copper(II), nickel(II), and zinc(II) have been synthesized (35) and their ultraviolet spectra determined in chloroform and in acid solution. By utilizing sulfonic acid groups to increase solubility, complexes of 4,4, 4",4" -tetrasulfophthalocyanine complexes of many metals were prepared (94j 95). This chelating agent was found to have a ligand field strength comparable to cyanide (94y 95). 

Cobalt porphyrins and phthalocyanines, nickel(II) macrocyclic polyamines, and other metal centers have been reported to catalyze the electroreduction of nitrous oxide according to N20 + 2111 I 2e —-N2 + H20. Kinetic experiments suggest the presence of N20-metal adducts in the rate-determining step.123,124... 

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. 

Our knowledge of the stereochemistry of porphyrins and related tetrapyrrole macrocycles has expanded rapidly since the first reported x-ray structure determination in 1959 The structures of metallotetrapyrrole complexes are of interest because of the common occurrence of this type of macrocycle in biological systems. As is well known, foremost among these are the heme proteins (iron derivatives), the various photosynthetic pigments (magnesium complexes), the vitamin Bn coenzyme (cobalt corrinoids), and coenzyme F430 (nickel corphinoids) of the methanogenic bacteria. 

The typical electrodeposition process observed with nickel derivatives was first investigated by Milczarek using a Ni(II) tetrakis(3-methoxy-4-hydroxyphenyl) porphyrin as a monomer. The resulting conductive film was used for the electro-catalytic oxidation of methanol and other simple alcohols , nitric oxide, formaldehyde , or ascorbic acid and NADH. Later, [Ni(II)-69], [Ni(II)-70] and [Ni(II)-71] were shown to provide a large benefit in the electrochemical determination of nitric oxide in aqueous solution or for the electrocatalytic... 

Kazemi et al. [121] reported the electrochemical fabrication of a conducting polymer of 5,10,15,20-tetra(4-sulfophenyl) porphyrin-nickel as a nanostructured electrocatalyst for hydrazine oxidation. An electrode modified with this electrocatalyst provided a detection limit of 0.11 pmol and a sensitivity of 1 pA L pmoF for the determination of hydrazine, and showed good stability and reproducibility. 

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