Nitroxyl complexes

The 0-0 bond cleavage of complex (IV) will produce two mole of nitroxyl complex,... 

Other enzymes that are not obviously related to the dioxygenases have at least superficially similar metal sites. The fatty acid desaturase of the endoplasmic reticuluum is a nonheme iron protein and requires both oxygen and reducing equivalents for activity (Strittmatter and Enoch, 1978). It is not known whether this enzyme forms a nitroxyl complex, but rat liver microsomes containing the enzyme form an S = nitroxyl adduct when treated with nitrite and dithionite. 

The above complexes were characterized in various tumor models it was shown that the DNA binding rate for these new complexes is comparable with that of cisplatinum but then-oxidizing effect is opposite the initial cisplatinum accelerates oxidation in model radical reaction the platinum-nitroxyl complexes, on the contrary, inhibit oxidation. From this point of view, it is easy to explain the reduction of the toxicity of the synthesized complexes. 

However, while there are several examples of HNO/NO complexes with mainly second- or third-row transition metals in the literatme (108,109), only few iron-nitroxyl complexes have been reported so far. In fact, stabilization of the heme Fe-HNO moiety has been achieved only for protein complexes Fe(globin) HNO (87,110) being the (Mb)HNO the most extensively characterized. Stabilization of the Fe(HNO) moiety in heme models, without the support of a protein environment, remains elusive, though a fairly stable Ru(Por)HNO complex, [Ru (TTP)(HNO)(l-MeIm)], was reported and the IR and H NMR spectra were recorded, providing the diagnostic signatures to clearly identify the complex as an HNO adduct, without a crystal... 

Significantly, voltammetry at fast scan rates (>1 s) under NO gas yielded anodic currents attributed to the re-oxidation of the Fe nitroxyl complex, which implied that a nitroxyl adduct had a measurable lifetime during the catalytic reaction. A similar voltammetric fingerprint was apparent after succes-... 

Lehnert N, Praneeth VKK, Paulat F (2006) Electronic structure of iron(II)-porphyrin nitroxyl complexes molecular mechanism of fungal nitric oxide reductase (P450nor). J Comput Chem 27 1338-1351... 

Electrophilic Attack on Metal-bound NO HNO (Nitroxyl) Complexes... 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Organic carbamates (RNHCOO-) commonly display monodentate coordination, as exemplified in the structurally characterized tetrahedral Co(bmc)2Cl2,438 (bmc = lV-(benzimidazoyl-2-yl)-O-methylcarbamate). An unusual route to a carbamato complex involves the reaction of Co2(CO)8 in the presence of a fourfold excess of the stable radical species tmpo, which yields the blue Co40(OOCNC9H18)6 cluster, presumably via a Co(CO)2(tmpo) intermediate, with the nitroxyl radical serving as oxidant.439... 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

The subj ect has been treated as part of a general discussion of N O-donors in a number of reviews [1-4]. This chapter will briefly introduce the general properties of these systems and the chief methods to synthesise them, after which it will concentrate on their NO-donor properties and NO-dependent biological activities. The term NO will be used here as a family name, embracing not only nitric oxide (NO ) but also its two redox forms, nitroxyl (HNO) and nitrosonium ion (NO+), which play important roles in the complex signalling system connected with NO [5]. The specific redox form involved in the NO-release will be indicated, if known, when necessary for the discussion. 

Later even more complexity was demonstrated (Makino et al., 1992) in the reaction between DMPO and Fe111 in water. The HO-DMPO" formed was transformed into a hydroxamic acid [24] which is a tautomer of 2-hydroxy-DMPO [25] in a Fenton system transfer of a hydroxyl (cf. p. 133) from the ligand-FeOOH complex to either of these species leads to additional epr-active nitroxyls [26] and [27] in reaction (72). 

Chloride dioxide (CIO2) forms red charge-transfer complexes with piperidine and imidazoline nitroxyl radicals that slowly transform into oxoammonium salts. ... 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

Scheme 8.15 compels attention to the following features of the resulting species Complexation of the cation-radical with parent neutral counterparts enhances mobility of an unpaired electron. Introduction of selenium (heavier and bulkier atom) increases the overlap among donor planes due to the better chalcogen-chalcogen intermolecular contacts. The presence of the paramagnetic nitroxyl moiety is decisive for magnetism of the product. 

Nitrosyidisulfonic acid, reaction mechanisms, 22 129, 130 Nitrous acid, 33 103 decomposition, rate constants, 22 157 as oxidizing agent, 22 133 reaction mechanisms, 22 143-156 electrophilic nitrosations, 22 144-152 with inorganic species, 22 148, 149 nitrite oxidation by metals, 22 152-154 oxidation by halogens, 22 154, 155 in solution, 22 143, 144 reduction by metals, 22 155, 156 Nitrous oxide reductase, 40 368 Nitroxyl, reaction mechanisms, 22 138 Nitrozation, pentaamminecobalt(III) complexes, 34 181... 

In recent years, much effort has been spent on developing both selective and environmentally friendly oxidation methods using either air or oxygen as the ultimate, oxidant. One of the most selective and efficient catalyst systems reported to date is based on the use of stable nitroxyl radicals as catalysts and transition metal salts as co-catalysts (15). The most commonly used co-catalysts are (NH4)2Ce(N03)6 (16), CuBr2-2,2 -bipiridine complex (17), RuCl2(PPh3)3 (18,19), Mn(N03)2-Co(N03)2 and Mn(N03)2-Cu(N03)2 (20). However, from an economic and environmental point of view, these oxidation methods suffer from one common drawback. They depend on substantial amounts of expensive and/or toxic transition metal complexes and some of them require the use of halogenated solvents like dichloromethane, which makes them unsuitable for industrial scale production. 

A Cu(II) complex containing two spin-labelled ligands was prepared with a rigid linkage between the terpyridine chelator and a nitroxyl ring.26 Due to the large anisotropy of the Cu(II) spectrum only a small fraction of the Cu(II) spins are excited by the pulses of a DEER experiment. Analysis of the echo modulation... 

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