Vanadyl radical

The complex is normally described as the oxochromium(IV) [(H20)5Cr=0]2+ or chromyl cation, by analogy with the vanadyl cation [(H20)5Viv0]2+. It can also be regarded as a chromium(III) substituted hydroxyl radical (Fig. 1) and its H-atom abstracting properties are parallel to those of -OH (15). 

Vanadyl salts are salts of tetravalent vanadium, and contain the divalent [VO]- radical. Many vanadium compounds are known which appear to contain a [VO] group, but the vanadium is either trivalent or pentavalent. Throughout this book the term vanadyl is restricted to compounds of tetravalent vanadium, that is, to salts of the oxide V02. Hence, for example, the compound VOCI3, which contains pentavalent vanadium, is called vanadium oxytrichloride, and not by the more usual but less logical name vanadyl chloride. ... 

Vanadium pentoxide dissolves in acids, both organic and inorganic, to form vanadyl or unstable vanadic salts,7 and in alkalis to produce ortho-, pyro-, meta-, and poly-vanadates. The physico-chemical changes involved when vanadium pentoxide is heated with various basic oxides in the powder state have been investigated by Tammann.8 On being digested with liquid ammonia slow absorption of ammonia takes place the composition of the product has not been definitely established.9 The oxide also dissolves in alcohols to produce esters,10 and combines with methylamine and ethylamine to form compounds of the type 2(R.NHB).V205, where R represents the alkyl radical.11... 

In presence of triphenyl phosphite or tri(4-methyl 6-f-butylphenyl) phosphite, (6) also acts as an oxidation inhibitor for polyenes. The complexes formed have not been studied, but the effect is attributed to termination of radical chains and the complexes are more active than the phosphites alone.29 Vanadyl acetylacetonate is also employed as a hardener for unsaturated polyesters.30... 

Ramasarma, T. and H.N. Ravishankar. 2005. Formation of an oxo-radical of perox-ovanadate during reduction of diperoxovanadate with vanadyl sulfate or ferrous sulfate. Biochim. Biophys. Acta 1722 30-35. 

Keller, R.J., R.P. Sharma, T.A. Grover, and L.H. Piette. 1988. Vanadium and lipid peroxidation Evidence for involvement of vanadyl and hydroxyl radical. Biochim. Biophys. Acta 265 524-533. 

In a later article, complexes of Ni(II), Cu(II), Pd(II), and V02+ ions with the same tetra-substituted porphyrin were reported. Stepwise oxidation of these complexes gave products for which the authors proposed quinonoid, monoradical, and diradical structures. The most prolonged oxidations yielded the diradical products, which were isolated as dark purple crystals, relatively stable in air (40). The monoradical vanadyl complex was observed to be diamagnetic, suggesting antiferromagnetic coupling between the phenoxyl radical and unpaired electron on vanadium, whereas in the copper complex no such coupling was observed. More detailed studies of these systems seem warranted. 

Vanadyl salen complexes epoxidized cyclohexene presumably through intermediate hydroperoxides formed by radical chain autoxidation <1997105927, 1998TL5923>. Reactions using vanadium complexes have been carried out in liquid carbon dioxide <19990M4916>. 

The model of Schiott and Jorgensen [98] shows that the proximity of vanadium atoms in a dimer present on the (200) plane of (VOjaPaOy and ti -superoxo or r -peroxo species chemisorbed on coordinatively unsaturated vanadium site are required for the selective oxidation of hydrocarbons. In fact, such activated molecular oxygen species are responsible for the activity and selectivity of vanadium catalysts in homogeneous oxidations [99-101]. Such oxidations are one-electron processes and involve free radicals. Although Schiott and Jorgensen [98] did not provide the complete mechanism of n-butane oxidation to maleic anhydride on vanadyl dimers, their model indicated that the superoxo species and one-electron, free radical processes may be involved in such oxidation. 

The Franck-Condon analysis of the vibrational fine structure of the photoluminescence spectrum of the anchored vanadium oxide observed at 77 K indicates that the equilibrium V-0 bond distance of the vanadyl group is elongated in the charge-transfer excited state by 0.013 nm compared with the ground state value (725). UV irradiation of the anchored vanadium oxides at 280 K in the presence of CO led to the photoformation of CO2. Since the photoformation of CO2 from CO is accompanied by the removal of oxygen from the oxide (i.e.. the photoreduction of the oxide), such an elongation of the equilibrium nuclear distance of the V-0 bond in the excited state is closely associated with the facile photoformation of CO2 on the anchored vanadium oxides. In other words, the O hole trapped centers in the electron-hole pair state of the (V -0 ) complex exhibit a high reactivity similar to 0 anion radicals 66). 

Su YO, Czemuczewicz RS, Miller LA, Spiro TG (1988) Effects of solvents, axial ligation, and radical cation formation on the V 0 stretching raman frequency in vanadyl porphyrins implications for peroxidase intermediates. J Am Chem Soc 110 4150-4157... 

Petroleum asphaltenes exhibit two general types of signals when examined by electron spin resonance techniques. One is the 16-line, anisotropic, vanadyl (V=0+2) resonance of the solid state while the other arises from unpaired electrons which are present in the form of relatively stable free radicals. The crude and residuum asphaltenes were examined by ESR, and the relevant data are summarized in Table V. 

It is apparent from the chemical shifts (g-values), the hyperfine coupling constants (A-values), and the linewidths that the free radicals and vanadyl species are in very similar environments in both samples. It was not possible to obtain meaningful values for the absolute numbers of spins per gram for either species, but estimates of the relative concentrations obtained by measuring peak heights indicate that the vanadyl and free-radical concentrations do not differ significantly between the two asphaltenes. It thus appears that heat treatment of Cold Lake asphaltenes to 320°C does not alter the nature or abundance of paramagnetic centers. 

Alkoxy radicals for ring expansion can be generated from alcohols by oxidative methods such as hypohalite thermolysis/photolysis [19a] and lead tetraacetate oxidation [19b], or peroxide reduction [19c]. The recent development of the hyper-valent organoiodine reagent (diacetoxyiodo)benzene (DIB) provides another way for efficient generation of alkoxy radicals (Scheme 11) [19d]. Additional oxidative methods to prepare cyclopropyloxy radicals include reaction of tertiary cyclopropanols or their silyl ether derivatives with various reagents such as manganese(III) tris(pyridine-2-carboxylate) [Mn(pic)3] [20a], Fe(III) salts [20b], and vanadyl ace-tylacetate [20c] (Scheme 12). 

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