5- 4-Pyridyl porphyrin

Forshey PA, Kuwana T. 1983. Electrochemistry of oxygen reduction. 4. Oxygen to water conversion by iron(II)(tetrakis(N-methyl-4-pyridyl)porphyrin) via hydrogen peroxide. 

Fig. 1. Trivial closed dimers assembled from 4-pyridyl porphyrins.

To substitute the strongly bound axial CO ligand of the ruthenium or osmium center, it is necessary to employ more drastic conditions than simple stirring at room temperature. Imamura (11,20) used photolysis to synthesize porphyrin trimers on the basis of simultaneous coordination of two 4-pyridyl porphyrins to the same ruthenium porphyrin (12, Fig. 3). Some interesting photophysical behavior was observed for these systems. The trimers have an extra UV-Vis absorption band at about 450 nm which is ascribed to metal-ligand charge transfer (MLCT), a d7r(Ru(II))-7r (OEP) transition. This band shows a batho-chromic shift in more polar solvents, and decreased in intensity when... 

Fig. 2. Trivial closed trimers (a,b), tetramers (c), and pentamers (d) assembled on multi-dentate 4-pyridyl porphyrin ligands.

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... 

Relative reactivies of the species Zn2+, Zn(OH)+, Zn(OH) s, and Zn(OH)4 have been established for the reaction of zinc(II) with tetrad-methyl-4-pyridyl)porphyrins in basic solution (319). The rate constant for reaction of a typical zinc finger peptide with Zn2q has been estimated as 2.8 x 107 M-1 s 1, for dissociation of this complex 1.6 x 104 s 1 (282). 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

The water-soluble Fe porphyrin, 3Na+ [Fe(III)(TPPS)] -12H20 [H2TPPS4- = tetra-anionic form of meso-tetrakis(7r-sulfonatophenyl)porphine], has recently been shown to be an effective catalyst for the electroreduction of nitrite to ammonia [419]. The Fe meso-tetrakis(A -methyl-4-pyridyl) porphyrin and/or the Fe meso-tetrakis (jr -sulfophenyl) porphyrin complex shows a catalytic activity for the reduction of dioxygen in aqueous solutions, leading to hydrogen peroxide [420]. 

N,N -ethylenebis(salicylideneiminato) ion meso-tetra(2,4,6 -trimethyl- 3,5 -disulfonato)porphin 5,10,15,20 -tetrakis(N-methyl- 4 -pyridyl)porphyrin... 

Exclusive enone formation could be achieved by electrocatalytic oxygenation of 2-cyclopentene-l-acetic acid in the presence of a water-soluble iron(III) porphyrin (2-TMPyP)Fe [2-TMPyP =tetrakis(N-methyl-2-pyridyl)porphyrin]. Unfortunately, neither yields nor TONs are given [116]. 

A recent contribution to the chemoselectivity problem is the composite photocatalysis developed by Maldotti et al. [117]. Cyclohexene la was subjected to photo-initiated oxidation in Nation membranes containing Pd(II) porphyrin Pd(4-TMPyP) [4-TMPyP = weso-tetrakis(N-methyl-4-pyridyl)porphyrin] and Fe(III) porphyrin 11 to... 

Amino-l,2,4-benzotriazine 1,2-dioxide Thin layer chromatography Thymidine-5 -phosphate iV,iV,iV, iV -Telramclhylphcnylcncdiamine Tetranitromethane 3,5,4 -Trihydroxy-frans-stilbene 2-Amino-2-hydroxy-l, 3-propanediol Time-resolved light-scattering p-[meso-5-5,10,15,20-Tetra(pyridyl)porphyrin]tetra kis[ (ns-(bipyridine) chloride ruthenium(II)]... 

TRP = g [meso-5,10,15,20-tetra(pyridyl) porphyrin]tetrakis[bis(bipyridine)chloride ruthenium(ll)]... 

Fig. 37.11. Use of an NO microsensor for detection of the NO release from cultured endothelial cells. The sensor is a dual probe microsensor. The small sensor is a bare Pt UME used to position the sensor in the feedback mode. Onto the larger Pt electrode a polymer was deposited from an acrylic resin containing Ni(4-lV-tetramethyl) pyridyl porphyrin and served as amperometric NO sensor, (a) Schematic of the sensor, (b) optical microphotograph of the sensor surface, (c) Response of the NO sensor to the stimulation of the cells with bradykinin at different distances of the sensor to the surface of the cells. Reprinted with permission from Ref. [104], Copyright 2004, American Chemical Society.

To construct such a porphyrin-based positive allosteric system, we chose a cerium(iv) bis(porphyrinate) double decker compound 184-861 namely the tetrakis(4-pyridyl)porphyrin derivative 38.[871 This molecule satisfies the aforementioned requirements firstly, slow rotation of the two porphyrin planes with respect to one another should be possible at room temperature,1881 in analogy to similar cerium(iv) bis(diarylporphyrin) and bis(tetraarylporphyrin) complexes studied by Aida et al/861 Secondly, tilting of two porphyrin planes is more difficult than in 37, and thirdly, four pairs of 4-pyridyl groups are available as hydrogen bond acceptor sites for diols, hydroxycarboxylic acids, and dicarboxylic acids. Compound 39, which has only one pair of pyridyl groups, was used as a reference. 

Ng et al. first reported the axial ligation of zinc(II) l,8,15,22-tetrakis(3-pentyloxy) phthalocyanine (1) with meso-pyridyl porphyrins 2 and 3 in chloroform, which form the corresponding edge-to-face dyad and pentad, respectively [25], As shown by UV-Vis spectroscopy, the ground-state tt-tt interactions between the perpendicularly disposed macrocycles in these arrays are insignificant. Upon mixing of phthalocyanine 1, zinc(II) meso-tetra(/Molyl)porphyrin, and 4,4/-bipyridine in chloroform, the formation of a face-to-face hetero-dyad was also inferred by fluorescence quenching experiments. 

Cationic metalla-assemblies have been prepared using the same dinuclear arene ruthenium clips and 5,15-bis(4-pyridyl)-10,20-diphenylporphyrin (bpp) or 5,10, 15-tris(4-pyridyl)-20-phenylporphyrin (tppp) instead of 5,10,15,20-tetra(4-pyridyl) porphyrin (tpp) (Fig. 15). The in vitro study showed that, despite having less ruthenium atoms per metalla-assemblies and a reduced overall charge as compared to the octanuclear arene ruthenium metalla-cubes, the cytotoxicity of these tetra-and hexanuclear metalla-assemblies was similar to those observed for the octanuclear metalla-cubes [55]. 

II 000 cm-1). Nevertheless, the photophysics of lanthanide porphyrinates is attractive because it could be of great help in medicine. For instance, hematoporphyrin derivatives are known to accumulate in malignant tumours and are used in medical diagnosis and photodynamic therapy of cancer. It is noteworthy that the Yb(III) complex with meso-tetra(3-pyridyl)porphyrin displays a substantial quantum yield (1.4%) when inserted into micelles formed by the non-ionic surfactant Triton X-100, a medium that can be considered as a model for biological tissues. 

Tetra(4-pyridyl) porphyrin (TPyP) metalloligands also were explored as potential MOF constituents. It is of more than passing interest that TPyPs are often capable of self-association via bonding of the pyridyl nitrogens to the coordina-tively unsaturated central metal of another porphyrin molecule. Several research groups have made an active study of such stmctures (111-117). However, because these coordination polymers tend to be formed from a single molecular component, rather than having the metalloporphyrin bound to a secondary metal center or SBU, they will be omitted from further discussion here. 

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