Chromium porphyrin

In another example, a polymer-supported chromium porphyrin complex was supported on ArgoGel Cl and then employed for the ring-opening polymerization of 1,2-cyclohexene oxide and C02 [95], This complex showed higher activity than a C02-soluble equivalent, and the solid nature of the catalyst meant that recycling of the catalyst was much easier. 

Several other metal complexes have promising photodynamic activity and are currently under development (248). Metalloporphyrins inhibit the enzyme heme oxygenase for example, chromium porphyrin and mesoporphyrin are potent inhibitors of heme oxygenase both in vitro and in vivo (249, 250) and are being used for the treatment of the neonatal jaundice. 

Chromium porphyrins constitute a comparatively new and important class of compound. In order to avoid an artificial separation, complexes of various oxidation states starting with chromium(II) are dealt with in this section and some chromium(V) salen derivatives are included. Electronic, ESR and IR spectra are routinely recorded to characterize chromium porphyrin complexes and chromatography is extensively used in their purification for details the original references should be consulted. The chemistry of the porphyrins is covered in a recent series.1241... 

A gold(I)/tri(methoxyphenyl) phosphine complex incorporated into PVC membranes led to formulation of ISEs with fast (14 s) Nernstian response (10 2-10 6 M) to perchlorate, with excellent selectivity even over chlorate in both water and urine [28]. Chromium porphyrins in PVC gave ISEs with near-Nernstian response to salicylate (10 2-10 6 M) with UV spectra confirming binding of salicylate to the chromium atom [29]. 

Vallier HA, Rodgers PA, Stevenson DK. 1993. Inhibition of heme oxygenase after oral vs. intraperitoneal administration of chromium porphyrins. Life Sci 52 79-84. 

D. J. Liston, B. O. West, Oxochromium compounds. 2. Reaction of oxygen with chromium(ll) and chromium(lll) porphyrins and synthesis of a, u-oxo chromium porphyrin derivative, Inorg. Chem. 24 (1985) 1568. 

There is also debate about the nature of the intermediates involved in the second step in the catalytic cycle illustrated in Fig. 19.1, that is, the transfer of oxygen to the alkene to form the epoxide. No intermediates have been detected experimentally, but five different possibilities have been proposed in the literature for the alkene complexed to the oxidized porphyrin [11,25-29]. The five proposed intermediates are radical, cation, concerted, metallaoxetane, and pi-radical-cation species. The literature is rather complicated due to the lack of direct experimental observation, and it is not clear that conclusions from, say, iron and chromium porphyrins also apply to manganese porphyrins [28]. Arasasingham et al. claim unequivocal evidence for a radical intermediate being involved in the oxidation of alkenes by manganese porphyrins [28]. They also discuss a charge-transfer complex that is similar to the concerted intermediate. Recently, density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) calculations were applied to styrene epoxidation by Mn-porphyrins ... 

Their H NMR spectra are characterized by resonances due to the PFe in the positions found for the symmetrical PFe-O-FeP dimer, and a resonance at 38 ppm that is due to the pyrrole-H of the chromium porphyrin unit. The temperature dependence of the magnetic susceptibility of the mixed-metal dimer leads to its formulation as antiferromagnetically coupled Fe S = 5/2) and Cr S = 3/2), yielding S =... 

Studies on chromium porphyrins found that both Cr P and Cr" P react with alkyl radicals to form relatively stable RCr P and RCr P, more stable than analogous products from chromium cyclam or aqueous chromium ions. Both of these products are slowly oxidized by O, the former producing Cr" P and the latter forming a product oxidized at the porphyrin ring. 

D.M. Guldi, P. Neta, and P. Hambright, Reaction of Alkyl Radicals with Chromium Porphyrins, J. Chem. Soc. Farady Trans., 88 (1992) 2337. 

The earliest electrochemistry of chromium porphyrins involved Gr(III) derivatives with anionic axial ligands and/or coordinated nitrogeneous bases, but a number of studies have since been carried out with complexes having Cr(II), Cr(IV) or Cr(V) central metal ions. The latter series of compounds are exemplified by the oxo-Cr(IV), oxo-Gr(V), and nitrido-Cr(V) [143-147] derivatives. Cr(IV) /r-oxo dimers are also known [148,149]. 

Mang S, Cooper AI, Colclough ME, Chauhan N, Hohnes A (2000) Copolymerization of CO2 and 1,2-cyclohexene oxide using a C02-soluble chromium porphyrin catalyst. Macromolecules 33 303-308... 

More recently, a C02-soluble chromium(III)-porphyrin catalyst, (344), has been reported to initiate the copolymerization of C02 with CHO.980 Carbonate linkages of >90% are observed and Mw/Mn values are consistently low (1.1-1.4). Complex (344) is highly active, producing 3,900 g polymer/g [Cr] over 18 hours even higher activities have been achieved by supporting the catalyst on polymer beads.981... 

C.H. Brubaker, Michigan State University In the case of the cytochromes, it has been proposed that electron transfer from the iron porphyrin may involve the pi system of the porphyrin and even nearby aromatic rings. Do you think that a similar thing may happen in the case of the reaction between these copper(I) p lastocyanins and the chromium(III) You seem to favor the idea that the important factor is that the Cr(III) be at a site that is reasonably close to the copper center. 

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. 

Complexes of acetylacetone (acacH), benzoylacetone (bzacH) and dipivaloylmethane (dpmH) have been reported. The acetylacetonate [Cr(acac)2] has been prepared from chromium(II) acetate and acetylacetone.142,143 It can also be obtained by the addition of aqueous sodium acetylacetonate to an aqueous solution of chromium(II) chloride, but in any preparation the yellow solid must be filtered off and dried as rapidly as possible, otherwise the chromium(III) compound is obtained. Its magnetic moment is 4.99 BM at room temperature consistent with a high-spin d4 configuration. 142The powerful reducing ability of [Cr(acac)2] has been used to prepare iron(II) and chromium(II) complexes80 of porphyrins and related ligands. 

Chromium(II) porphyrins and phthalocyanines are dealt with in Sections 35.4.9.1 and 35.4.9.3 respectively. 

Photolysis of the azido complex CrN3(salen)-2H20 produces1110 the nitrido chromium(V) complex CrN(salen)-H20 and the reaction1111 of [Cr(salen)(H20)2]+ with iodosylbenzene gives the oxo chromium(V) derivative [CrO(salen)]2+ chromium(III) porphyrin complexes behave similarly (Section 35.4.9.1). 

The chromium(II) complex of mesoporphyrin IX dimethyl ester (276) can be prepared by metal insertion with an excess of chromium hexacarbonyl under nitrogen.1242 After removal of solvent and extinction with toluene, Crn(MPDME) is crystallized by the addition of n-pentane. Presumably the weakly acidic NH protons of the porphyrin oxidize Cr° to Cr11 and form H2. 

Evidence is growing that chromium is an essential trace element (Section 35.4.8.3). If mammalian diets are supplemented with simple 51Cr-labelled salts it is found that less than 1% of the dose is absorbed because at intestinal pHs (>6) insoluble Cr111 hydrolysis products are produced. Complexes which remain intact in the add conditions of the stomach as well as in more alkaline conditions are required. As Cr porphyrin complexes are stable in these respects, methods for synthesis of the complexes and incorporation of 51Cr have been surveyed.1248... 

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