Chromium porphyrin complexes

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... 

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). 

Chromium complexes (continued) organohalide activation, 5, 381 with phosphine-based supporting ligands, 5, 365 porphyrin-ligated derivatives, 5, 364 pyridinebis(imine) ligand derivatives, 5, 361 salen-ligated complexes, 5, 379 surface chemistry on oxides, 12, 525 tetradentate [04] compounds, 5, 352 tetradentate thioether derivatives, 5, 365 thiolate-bridged, Fe- and Rh complexes, 5, 308 with Ti(IV), 4, 627... 

The infra-red spectrum of Cr(TPP)py(02) obtained by the oxygenation of a chroinium(II) porphyrin complex suggests the presence of an dioxygen species The photolysis of chromium complexes such as [Cr2(> -C5H5)2(CO)6] in the presence of O2 gives paramagnetic complexes which may be studied by The g-values lie... 

Oxidations. By using Phl=0 (in presence of KBr) as an oxidant, alcohols are oxidized to acids and ketones in water in excellent yields. When catalyzed by either poly(4-vinylpyridine)-supported sodium ruthenate or a (salen)chromium complex chemoselective oxidation of alcohols (e.g., allylic alcohols to alkenoic acids) occurs, which is contrary to the effect of (salen)manganese and (porphyrin)iron complexes (giving epoxy alcohols). ... 

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. 

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 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... 

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... 

Chromium(III) salen will form chromium(V) derivatives in much the same way as porphyrin complexes do. By reaction of iodosylbenzene with chromium(III)salen oxochromium(V) complexes are obtained (equation 74) which will effect oxygen atom transfer to phosphines and alkenes such as norbornene (equation 75) in stoichiometric and catalytic systems.1270 The... 

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