Tetrakis porphyrin cobalt complex

The dechlorination of chlorinated alkenes could also be performed by porphyrin cobalt complex such as 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin cobalt ((TCPP)-Co). This cobalt complex, structurally similar to vitamin B12, was found to have superior aqueous-phase dechlorination activity on chlorinated ethylenes relative to vitamin Bi2. Based on fully detailed parameters dependence, the authors suggest the catalytic cycle below.This methodology has been used to synthesize C-labeled air-DCE from TCE (Scheme 33). ... 

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... 

Chiral binaphthyl-bridged mevo-tetrakis(2-aminophenyl)porphyrin]cobalt or polymer-supported chiral (salen)Co complexes also catalyzed the cyclopropanation of styrenes albeit so far with lower diastereo- and enantioselectivities [363, 364],... 

The cobalt complex of tetrakis(mesityl)porphyrin, 57, is one of the most commonly utilized sterically hindered porphyrins because there are no atropo issues, with each side of the macrocycle having four ortho-substituents. 

Even though most of the work reported has involved planar cobalt complexes nonplanar macrocylics also catalyze the reaction. For example, nonplanar cobalt tetrakis-(4-sulfonatophenyl)/ -octabromoporphyrin catalyzed the reduction of O2 via two electrons to give peroxide . Vitamin B12, which is also nonplanar, and its structure resembles that of a porphyrin (see Figure 2.4.), catalyzes the reduction of O2 via two electrons to give peroxide at low potentials and via four electrons to give water at higher overpotentials... 

Pang, D.-W., B.-H. Deng, and Z.-L. Wang (1994). Electrocatalysis of metalloporphyrins—part 14 Electro-oxidation of hydrazine catalysed by water-soluble tetrakis(4-trimethylammoniumphenyl) porphyrin and its cobalt complex. 

The equilibrium constant for the addition of NCS ion to tetrakis-(3-iV-methyl-pyridyl)porphinecobalt(iii) is less (by a factor of ca. 2) than for addition to the less basic 4-A -methylpyridyl isomer, and the difference is reflected in a larger rate of dissociation of the Co—SCN bond in the case of the 3-7/-methylpyridyl complex. Two further studies of porphyrin-cobalt(iii) complexes have appeared. For the aquation of [Co(a,/S,y,6-tetra-4-A -methylpyridylporphine)(X)(H20)] + (X= NCS, Cl, Br, I, or py) a linear free-energy relationship holds between the rate... 

Pang DW, Deng BH, Wang ZL (1994) Electrocatalysis of metalloporphyrins-Part 14. Electro-oxidation of hydrazine catalyzed by water-soluble tetrakis (4-trimethylammonium-phenyl) porphyrin and its cobalt complex. Electrochim Acta 39 847-851... 

The authors have also reported on the comparison between the electrocatalytic activity of electropolymerized films of cobalt porphyrins and phthalocyanines (cobalt tetrakis(p-hydroxyphenyl)porphyrin cobalt tetrakis(o-aminophenyl)porphyrin and Co(II)-2), with emphasis on the effect of the film thickness on the electro-oxidation of 2-mercaptoethanol [52]. It was shown that, despite the porphyrins modified electrodes exhibit electrocatalytic activity, their activity strongly depends on the film thickness with a decrease of activity as the thickness increases, in contrast to the phthalocyanine-based electrodes. This difference in behavior was attributed to lower electronic conductivity of electropolymerized porphyrin films relative to phthalocyanine-based ones, and to the potential implication of the less reactive Co(III) form of the deposited porphyrin complexes. 

Although the cofacial diporphyrins represent a vibrant and innovative direction in dioxygen activation, simple porphyrins and their derivatives also remain an important research area. The dichlorophenyl-substituted porphyrin tdcpp [5,10,15,20-tetrakis(2,6-dichlorophenyl)-porphyrin] forms a complex with cobalt(II), [Co(tdcpp)], and catalyzes the oxidation of conjugated olefins to (after experimental workup) ketones in the presence of dioxygen and triethylsilane (80) a hydroperoxide intermediate has been isolated from these reactions (81). 

As with cobaloximes, substituents on the equatorial ligand have only a moderate effect on the value of Cc for the complexes in Table 3. The same is true for substituents on cobalt porphyrins, 1 and 45—51 (Table 4). For tetrakis(pentafluoroethylphenyl)-porphyrin—Co11 the substituent effect is not clear. The fluorinated porphyrin works moderately for the polymerization of MMA in supercritical C02 with chain-transfer constant Cc = 550 at 60 °C.126 Unfortunately, no data on the chain-transfer constant in bulk polymerization are available, so that it is not clear whether this reduced value of Cc is the result of solvent or the presence of a strong EWG such as pentafluorophenyl in the porphyrin macrocycle. Similar experiments with 9c (Table 2) led to Cc = 378 000, which is 20 times higher than in bulk MMA or in organic solvents.30 We may conclude at this point that additional experiments are required with different catalysts to allow us to make reliable conclusions. 

Third-order NLO properties have been described for liquid-crystalline phthalocyanine and porphyrin complexes. The values for the meso-genic cobalt(II), nickel(II), copper(II), zinc(ll) and vanadyl complexes of 5,10,15,20-tetrakis(4-w-pentadecylphenyl)porphyrin have been measured in benzene solution by the technique of degenerate four-wave mixing (DFWM) at 532 nm. The values varied between 1.5 and... 

Imaoka and Yamamoto " reported that porphyrins possessing four ionic substituents of (mCTO-tetrakis(Af-methyl-4-pyridiniumyl)porphyrinatoRu(II) (RuTMPyP) and (me.y(9-tetrakis(4-sulfonatophenyl)porphyrinato)cobalt(II) (CoTPPS) associate spontaneously to form a dinuclear complex. Formation of CoTPPS-RuTMPyP was confirmed by UV-vis titration and TOF-mass spectra. The dinuclear complex exhibits an acceleration of intrinsic FT when it is present in a Nation film deposited on glassy carbon. The CoTPPS-RuTMPyP-Nation system catalyzes the four-electron reduction of O2 with an efficiency of 95%. 

The multinulear complexes reported by Shi and Anson are another interesting example of specially designed metalloN4-macrocyclic complexes with exceptional activity for ORR. Cobalt (tetrakis(4-pyridyl)porphyrin), with four [Ru(NH3)5] and ([(NH3)50s]" (n) 2, 3) groups around the porphyrin periphery, generally termed as... 

The reaction of Ru(NH3)50H2 " with CoP(py)4 (cobalt meso-tetrakis(4-pjpidyDporphyrin) within Nafion coatings on graphite electrodes produces [CoP(pyRu(NH3)5)4]. This complex, immobilized witiiin the polyelectrolyte coating, acts as a catalyst for the four-electron reduction of O2 under conditions where mixtures of CoP(py)4 with Ru(NH3)e or Ru(NH3)5py do not. The results provide evidence for the intramolecular delivery of four electrons from the four coordinated Ru(NH3)5 groups to O2 molecules associated with the Co(n) center of the porphyrin. 

Anation of the water-soluble porphyrin complex [Co(P)(OH2)2l [H2P = 5,10,15,20-tetrakis-(A -methylpyridyl)porphine] is very fast for a substitution reaction at cobalt(ra). Rate data are also reported for the anation of the [Co(P)(NCS)-(0H2)] + and [Cb(P)(NCS)(OH)] + ions, and thiocyanate ion is shown to have a strong trans effect (> 10 ) upon the remaining co-ordinated water molecule. Omitting charges for convenience, rate data for the following reactions were as shown ... 

Miscellaneous Metal Ions.—Mention has been made already of a relatively slow formation reaction for the normally labile manganese(ii) ion. Incorporations of the labile bivalent metal ions zinc(ii), copper(ii), manganese(n), cobalt(ii), and nickel(n) into water-soluble porphyrin molecules such as tetrakis-(4-N-methyI-pyridyl)porphine, tetrasulphonated tetraphenylporphine, and uroporphine are also relatively slow reactions. However, by taking into account the porphyrin deformation which is necessary, an Id mechanism can be fitted to these reactions. The rates of formation and dissociation of nickel(ii), copper(n), cobalt(ii), and zinc(ii) complexes of the sterically hindered ligand 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetra-decane (Meicyclam) are also 10 —10 smaller than is normal for these metal ions. ... 

Organocobalt porphyrin complexes have also yielded useful mechanistic information. The reactions of (tetrakis(p-methoxyphenyl)porphyrinato)cobalt(II) ((TAP)Co(II)) with radicals derived from dialkylazo thermal initiators with acrylic monomers provide evidence for the intermediacy of Co(III)-H species in CCTP. Reaction of (TAP)Co (II) with tertiary alkyl radicals, for example, as derived from AIBN in the presence of monomers that form stable Co-alkyl complexes, such as methyl acrylate, results in quantitative formation of Co(III)-alkyl. Whereas with monomers leading to tertiary C-Co bonds, such as MMA, the Co (II) is very much a spectator as normal polymerization ensues. Thermodynamic and activation parameters have been measured for the dissociation of (TAP)Co(III)-C(CH3)2CN to Co(II) and organic radical in solution as a probe into CCTP mechanism by low-spin Co(II). ... 

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