Porphyrin metal incorporation

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

Figure 3.4 Mechanism of porphyrin metallation. (a) Out-of-plane saddle structure in which two pyrrole rings with unprotonated nitrogens (blue spheres) point upwards, while the other two, protonated (blue and white spheres) point downwards, (b) Steps in the mechanism for incorporation of the metal ion (red) into the porphyrin (pyrrole rings in green), described in the text. (From Al-Karadaghi et al., 2006. Copyright 2006, with permission from Elsevier.)...

Chiral porphyrin metal complex catalysts have also received much attention. In this situation, the flat, symmetrical porphyrin structure must be modified dramatically in order to incorporate dissymmetry. This has been achieved through strapping techniques. " Some examples are shown in Figure 11.7. 

The comparative photophysics of Pt(II) and Pt(IV) porphyrins [297] have been investigated. A correlation of X-ray crystallographic data and spectroscopic properties was done for a variety of octaethylporphyrin complexes of divalent metals, incorporating those of Ni, Pd, Pt, Cu, and Ag, giving useful UV/Vis and IR data for the synthetic chemist as well [298]. 

The enzymatic activity of AChE from electric eel is inhibited by monosulfonate tetraphenyl porphyrin (TPPSi) the structure of which can be seen in Figure 12.1 with Rj = SO3, R2 = no substituent group, and no metal incorporated [36]. A Lineweaver-Burk plot of enzymatic rates in the absence/presence of TPPSj determines the type of inhibition resulting from the presence of the porphyrin. The Lineweaver-Burk plot is the plot of the double-reciprocal form of the initial enzymatic rate versus the substrate concentration. Intersection of the lines generated in the absence and presence of inhibitor at the y-axis shows no change in maximal velocity but a change in the Michaelis constant K j, an indication of the substrate binding affinity, and indicates competitive inhibition by the porphyrin. Competitive inhibition involves competition of the inhibitor for occupation of the active site of the enzyme. 

Other systems of interest in order to study through-space electron transfer processes or through-bond electron transfer with different metals coimecting the macrocycle and the dumbbell were designed, hi these [2]rotaxanes, the Zn(II) bis-porphyrin dumbbell component remains the same as in Sect. 2.3, whereas the gold porphyrin is incorporated in the macrocycle. This affects the mutual arrangement of the chromophores and therefore their electronic communication. It also changes remarkably the dynamic of the system after removal of the metal template as will be described. 

With fully water-soluble porphyrins, the metal-incorporation rate has been found to be first order in free base, metal, and anion. However, it has been suggested that during incorporation of metals in poorly soluble porphyrins, which would correspond to the situation of polymeric porphyrins, intermediate ( sitting-atop complexes) can be formed. Incorporation kinetics were suggested to be second order in metal and first order in porphyrin. The number of monolayers of polymeric porphyrins occupied by Ni(II) increases with its initial concentration. At trace concentration levels, film thickness is not important because only the first few layers are occupied, and the analyte signal does not depend on film thickness. The concentration levels (10 -10 M) and preconcentration times (40-100 seconds) are not sufficient to exceed the film capacity of monolayer film to incorporate Ni(II). However, for high Ni(II) concentra-... 

Spiro-linked CMPs functionalized with metal phthalocyanine units show enhanced catalytic activity towards different reactions." The Co-phthalocyanine-incorporated CMP acts as a catalyst with improved activity for cyclohexene oxidation, hydroquinone oxidation and H2O2 decomposition, whereas the spiro-linked Fe-porphyrin network shows increased catalytic activity for hydroquinone oxidation. The spiro linkages in these networks open up a lot of free space around the catalytic sites to enhance the accessibility of substrates to reach more catalytic sites. More functionalization in this vray of conjugated networks by various metals improves the scope of these networks in heterogeneous catalysis. Oxidation of sulfides, reductive aminations and photocatalyzed aza-Heniy reactions are reactions effectively catalyzed by different metal-incorporated CMPs" (Figure 10.6). 

The Hupp group also used two kinds of porphyrin ligands, M -TCPP and F-M DPyP, as building blocks, to develop an extended family of porphyrinic MOFs that directly incorporate a series of metallopor-phyrins (Figure 12). The frameworks with two kinds of porphyrin metals formed by M -TCPP connecting with paddle wheel zinc nodes and pillared by dipyridyl F-M DPyP have large accessible channels and multiple active metal sites for catalytic application. 

MECHANISM OF METAL INCORPORATION INTO THE PORPHYRIN COMPLEX... 

The formation of the complex between the free base (PH2) and the metal ion has been extensively studied. It was found that the complicated metal incorporation mechanism can be best interpreted for the reaction of the dipositive metal ion and the neutral chelating ligand. However, this does not mean that the anionic porphyrin species (PH and P ) are not reactive. In fact, they are present in very small concentrations, so small that they can hardly be detected. On the other hand, the cationic porphyrin species are definitively unreactive. Earlier works, of some twenty years ago, preferred the mechanism of metal incorporation which assumes a pre-equilibrium between the free base (PH2) and the dissociated protons of pyrrole nitrogens, so that the metal cation would actually react with the P " dianion. Later works, however, indicate the existence of the so-called SAT (sitting-atop) complex intermediate, which was already foreseen in 1960 by Fleischer and Wang. It was later shown that the SAT metal-ion intermediate can deform porphyrin, thus... 

Dendrimer chemistry has taught us that these molecules create a nano-sized closed space that, presumably, is the origin of the specific physical properties of this class of materials. As the next stage of dendrimer chemistry, a macromolecule capable of creating such a space inside its molecule is proposed. To create the nano-sized space, porphyrin is considered to be the best candidate for the component molecules, because it has versatile properties associated with its expanded 7i-electron system and the incorporated metal. The resultant multi-detectable properties of porphyrin, that is, a number of its properties are detectable by many physical methods, may reveal the function of the nanometer-sized space. 

Fig. 3a,b. Template cyclization reactions of a crown ethers and b CPOs. The coordination bonds are illustrated by black arrows. In the crown ether synthesis, ethylene glycols coordinate toward the metal acting as the template (normal template reaction) however, the template coordinates to the incorporated metals of porphyrin In CPO synthesis (inverse-template reaction)... 

To select the metal to be incorporated into the substrate porphyrin unit, the following basic properties of metalloporphyrins should be considered. The stability constant of MgPor is too small to achieve the usual oligomeric reactions and purification by silica gel chromatography. The starting material (Ru3(CO)i2) for Ru (CO)Por is expensive and the yield of the corresponding metalation reaction is low. Furthermore, the removal of rutheniirm is difficult, and it is likewise difficult to remove the template from the obtained ruthenium CPOs. Therefore, ZnPor is frequently used as a substrate in this template reaction, because of the low prices of zinc sources (zinc acetate and/or zinc chloride), the high yield in the metalation reaction, the sufficient chemical stability of the ZnPor under con-... 

CPOs are best characterized by the following three features 1) axial coordination to the incorporated metals, 2) specific nano-sized space created by rigid porphyrin panels, and 3) specific (photo-induced) redox reactions associated with the porphyrin s rr-electron system. In this chapter, some examples are reviewed based on these properties. 

Attempts to realize enzymatic reactions have been reported over the past four decades in the context of host-guest chemistry, presently a well-established research field. In the field of CPOs, much attention has been paid to identical research objectives. The host-guest chemistry based on CPOs holds a special position, because specific selectivity and reactivity will be achieved using the coordination-bond-forming reactions between the substrate and the incorporated metals in the porphyrins, as well as the redox reaction associated with the porphyrin s rr-electron system. 

Sanders and his co-workers reported two enzymatic reactions using CPO 3 described above (Fig. 9). In the reactions, three zinc metals are incorporated into each porphyrin. The first reaction is the acylation reaction of pyridyl alcohol 18 using AT-acetylimidazole (19) [35] to afford 20. In the presence of 3, the reaction is accelerated 16 times compared to the reaction without 3. The replacement of 3 with 4 (M=Zn) gave no significant acceleration of the reaction therefore, the enzymatic reaction proceeds through a supramolecular... 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

It may seem curious to make specific mention of this matter, but in a biological context, it is crucial. The nuclei of porphyrins and related systems are hydrophobic it appears to be generally desirable that the photosensitizer has some degree of intermediate polarity (i.e., amphiphilic properties Section 9.22.3) and the incorporation of a metal, such as zinc(II), which can take on an axial ligand (e.g., H20 in aqueous media, or RNH2 in a biological fluid), or magnesium(II), which can take on two, is expected to enhance solubility in hydroxylic solvents, and shift the... 

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