Cobalt porphyrin derivatives

Cobalt porphyrin derivatives were also reported129 to be active for electrochemical reduction of C02 to formic acid at an amalgamated Pt electrode. More recently, Becker et al have reported130 that Ag2+ and Pd2+ metalloporphyrins acted as homogeneous catalysts for C02 reduction in dry CH2C12 oxalic acid and H2 (its source was not clear) were produced, but no CO was detected. 

The second structural type found for organometallic cobalt porphyrins contains an organic fragment bridged between the cobalt and one pyrrolic nitrogen. Cobalt complexes of N-alkyl- or N-arylporphyrins arc well established (but will not be specifically addressed here). The bridged complexes are derivatives of these where the N-alkyl group also forms a cr-bond to cobalt. They are also related to the axially... 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

We have demonstrated a new class of effective, recoverable thermormorphic CCT catalysts capable of producing colorless methacrylate oligomers with narrow polydispersity and low molecular weight. For controlled radical polymerization of simple alkyl methacrylates, the use of multiple polyethylene tails of moderate molecular weight (700 Da) gave the best balance of color control and catalyst activity. Porphyrin-derived thermomorphic catalysts met the criteria of easy separation from product resin and low catalyst loss per batch, but were too expensive for commercial implementation. However, the polyethylene-supported cobalt phthalocyanine complex is more economically viable due to its greater ease of synthesis. 

Bied-Charreton (63) showed by XH NMR spectroscopy that a zinc porphyrin bearing a primary amino group attached via a flexible spacer to the meta position of a /neso-phenyl ring spontaneously dimerizes in solution via NH Zn coordination. The corresponding ortho derivative shows an equilibrium between the dimeric form and the intramolecu-larly coordinated monomer. ESR spectroscopy was used to demonstrate the same behavior in the corresponding cobalt porphyrin (64). 

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

Recently, the high inhibitory efficiency of metalloporphyrins has been shown in lipid peroxidation of rat brain homogenates [346]. It was found that manganese and cobalt porphyrins were very effective inhibitors of lipid peroxidation while iron and especially zinc porphyrins had very weak inhibitory activity, if any. For example, /50 values were equal to 21, 29, 212, 946 pmol 1 1 for CoTBAP, MnTBAP, FeTBAP, and ZnTBAP, respectively, where TBAP is 5,10,15,20-tetrakis [4-carboxyphenyl]porphyrin similar values were obtained for other porphyrin derivatives. 

The incorporation of vitamin B12 derivatives into plasticized poly(vinyl chloride) membranes has resulted in the development of several ion-selective electrodes (ISEs). The response of the electrodes has been related to principles of molecular recognition chemistry. In addition, ISEs have been prepared by electropolymerization of a cobalt porphyrin. These electrodes have selectivity properties that are controlled by both the intrinsic selectivity of the metalloporphyrin and the characteristics of the polymer film (e.g., pore size). 

In this paper, we report the development of ISEs that have been designed by using molecular recognition principles. Specific examples include the development of polymer membrane anion-selective electrodes based on hydrophobic vitamin B12 derivatives and a cobalt porphyrin. The selectivity patterns observed with these electrodes can be related to differences in the structure of the various ionophores, and to properties of the polymer film. 

In summary, it has been demonstrated that ISEs can be designed by employing molecular recognition principles. In particular, the feasibility of using hydrophobic vitamin B12 derivatives and electropolymerized porphyrin films in the development of polymer membrane anion-selective electrodes has been demonstrated. The studies indicated that the changes in the selectivity of these ISEs can be explained by the difference in structure of the ionophores. In addition, it was shown that by electropolymerization of a cobalt porphyrin, anion-selective electrodes can be prepared that have extended lifetimes compared with PVC-based ISEs, which use a similar compound as the ionophore. 

Noteworthy exanples are sensitizers such as Ru(bipy)j (29). porphyrins derivatives (30) and acridine dyes, e.g., proflavin (31). Among the electron relay compounds investigated it is worth mentioning the viologens (32) Eu , and their respective salicylate complexes ( ), Ru(bipy)j (3U) and cobalt complexes (35). Thus, at present a considerable choice of sensitizer/relay pairs is available that fulfill the photochemiical and thermodynamic requirements for water decomposition. 

In a similar study, James and coworkers reported weak sensitization towards Chinese hamster ovary (CHO) cells with cationic cobalt(IIl) porphyrin derivatives... 

The catalytic reduction of O2 by ferrocene (Fc) derivatives (one-electron reductants like cytochrome c) with a monomer cobalt porphyrin and cofacial dicobalt porphyrins was examined in the presence of perchloric acid (HCIO4) in PhCN (168). This finding allowed for the difference between the single and bimetallic systems to be clarified. 

Groups 9 and 10. Paramagnetic NMR for cobalt-substituted amicyanin shows that the methionine side-chain residue interacts less strongly with the metal in Paracoccus denitrificans amicyanin than in the P. versutus variant.1215 The i I NMR spectra of Co(P), where P = dodeca-substituted porphyrins, show (dxz,dyz)4(dxy)1 electron configurations.1216 The paramagnetic 111 NMR spectrum of the cobalt(II) derivative of spinach plastocyanin has been completely assigned.1217... 

The reactions of a-hydroxyalkyl radicals with Co P also took place via addition to the metal but, unlike the case of Fe P, the adducts were found to undergo heterolysis leading to reduction of the porphyrin to Co P. Another difference between the iron and cobalt porphyrins is in the stability of the methyl adduct CH3Co P was found to be stable even in the presence of O2. Related studies were carried out on the reactions of alkyl radicals with Co -tetrasulfophthalocyanine (tspc) and with Bi2r. Several cobalt-carbon adducts have been observed and in many cases they were found to decompose to yield the oxidized Co -complex. However, in the reaction of Co tspc with the radical derived from t-BuOH, the oxidized product was formed in neutral solutions but the reduced product, Co tspc, was formed in alkaline solutions. 

Porphyrins. Rollmann (21) has described the preparation of a variety of metalloporphyrins on a macroretlcular polystyrene copolymer with either divinylbenzene or ethylene glycol dimethacrylate. Polymer attachment could be made through an amine ester or ketone linkage. Cobalt(III) derivatives catalyze the oxidation of thiols in air. The polymers did show a marked tendency to age with resultant loss of activity. 

Buck, T., Bohlen, H., Wohrle, D., and Schulz-Ekloff, G. (1993) Influence of substituents and ligands of various cobalt(ll) porphyrin derivatives coordinatively bonded to silica on the oxidation of mercaptan. J. Mol. Catal., 80, 253-267. 

ESR spectroscopy is effective for determining such coordination structures for cobalt(II) and iron(III) porphyrins [25]. For example, the ESR parameters gj. - 2.3 and g// = 2.0 Both signals are split into hyperfine lines by the Co nucleus, and several of the parallel signals are further split into equal intensity super-hyperfine lines. The latter are derived from the nitrogen nucleus of the nitrogenous ligand at the fifth coordination site of a cobalt porphyrin. 

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