Oxochromium complexes

The oxochromium complex shows a very similar electrochemistry it also undergoes two metal centered reversible one-electron redox processes and one reversible oxidation of the ligand. 

The oxidation of alkyl aryl sulfides to sulfoxides with oxochromium(V) complexes is first order in oxidant and in substrate. The better correlation of log k with a+ rather than a and the low magnitude of p+ value (-1.19) were interpreted as evidence for a rate-determining single-electron-transfer mechanism. This was further supported by good correlation in the plots of log k versus oxidation potential/ionization energy. 

Oximes, formation by nitrosation, 7, 6 Oxochromium(VI)-amine complexes, 53, 1 Oxo process, 56, 1... 

R. S. Czemuszewicz, V. Mody, A. Czader, M. Galezowski, and D. T. Gryko, Why the chromyl bond is stronger than the perchromy bond in high-valent oxochromium(IV, V) complexes of tris(pentafluor-ophenyl)corrole,/. Am. Chem. Soc., 131 (2009) 14214-14215. 

H. Fujii, T. Yoshimura, and H. Kamada, ESR studies of oxochromium(V) porphyrin complexes Electronic structure of the Crv=0 moiety, Inorg. Chem., 36 (1997) 1122-1127. 

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). 

One oxochromium(V) complex, CrO(TETMC), containing the trinegative anion of a corrole (279), has been characterized as the solid.1266 It is prepared (Table 102) simply by exposure to air of a solution presumably containing a Cr11 complex. Aerial oxidation of Cr (TPP) produces the oxochromium(IV) complex CrO(TPP) so the corrole ligand apparently facilitates autoxida-tion. The redox behaviour of CrO(TETMC) has been examined by cyclic voltametry.1267... 

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... 

Cr(III) compounds, like Cr(III) hydroxide deposited on montmoril-lonite,408 Cr(III) stearate,409 Cr(acac)3,409b Cr(III) on a perfluori-nated sulfonic resin (NAFK),410 chloro(tetraphenylporphyrinate) chromium(III) [(TPP)CrCl] (6)411 and (salen)oxochromium(III) complex (7),412,413... 

Table 17. Half wave potentials (in V vs SCE) of oxochromium(V) and oxomolybdenum(V) macrocyclic complexes in dichloromethane. According to Refs. [47, 48]...

Cr porphyrin complexes react with O2 to form stable oxochromium(IV) species. Por example, the red [(TPP)CrO] contains a five-coordinate Cr displaced out of the porphyrin plane toward the oxygen atom (Cr-O, 157.2pm). There are also a few Cr scorpionates featuring terminally bound 0x0 ligands. The most recent of these is paramagnetic [Tp Cr(pz H)(0)]BARP (Cr-O, 160.2pm), which abstracts weakly bound hydrogen atoms from hydrocarbons. ... 

Formally, pentavalent neutral metallocorroles have been prepared by Murakami and coworkers.The first of these was the oxomolybdenum(V) corrole derivative 2.179. ° This complex was prepared by heating free-base corrole 2.82 with molybdenum pentachloride in oxygen-free decalin (Scheme 2.1.56). Alternatively, molybdenum hexacarbonyl (Mo(CO)e) could be used as the metal source. In both cases, oxidation to the oxomolybdenum complex 2.179 was believed to occur during workup (involving chromatography on neutral alumina followed by recrystallization). In this way, complex 2.179 was isolated in c. 40% yield. Similar yields of the oxochromium(V) complex 2.180 could be achieved via the reaction of 2.82 with anhydrous chromium(II) chloride in DMF. Here too, spontaneous oxidation during workup was used to afford the formally pentavalent oxo-complex 2.180. 

Matsuda Y, Yamada S, Murakami Y (1980) Preparation and characterization of an oxochromium(V) complex with a macrocyclic N4-ligand oxo-(2,3,17,18-tetramethyl-7,8,12,13-tetraethylcorrolato)chromium(V). Inorg Chim Acta 44 309-311... 

The identification of the reaction intermediates formed between [CrCl(l)] and iodosylbenzene (PhIO) were also investigated by EPR [115, 116]. The first intermediate was characterised by the reported spin-Hamiltonian parameters of g = 1.970-1.974, Acc = 54 MHz, 4 = 4.5-5.6 MHz, whilst the second species produced the parameters of g = 1.976-1.980, Acr = 54 MHz, 21 = 5.6-6.4 MHz. Based on the CW-EPR and H-NMR investigation, the first intermediate was identified as a reactive mononuclear oxochromium(V) intermediate, labelled [CrVO(l)L] where L = CP or solvent molecule. The second intermediate was identified as an inactive mixed-valence binuclear [L(2)Cr OCr (2)L] complex. Bryliakov et al. [115, 116] thereby proposed that the [CrCl(l)]-catalysed epoxida-tion of alkenes proceeds in accordance with a modified oxygen rebound cycle . 

Recent Applications of Oxochromium-Amine Complexes as Oxidants in Organic Synthesis. A Review"... 

The mechanism of picolinic acid (PA)-catalysed oxidation of formic acid by chro-mium(VI) involves the formation of a Cr(VI)-PA chelate complex in a pre-equilibrium step followed by attack at the Cr(VI) centre by the substrate, leading to a ternary complex within which electron transfer occurs giving a Cr(IV)-PA complex and CO2. The Cr(IV)-PA complex reacts faster with the substrate, ultimately giving rise to a Cr(III)-PA complex. The reaction is catalysed by anionic surfactants and inhibited by cationic surfactants. The oxidation of alcohols by modified oxochromium(VI)-amine reagents has been reviewed. ... 

Codd R, Lay PA. 2003. Oxochromium(V) species formed with 2,3-dehydro-2-deoxy-N-acetylneuraminic or N-acetylneuraminic (sialic) acids an in vitro model system of oxochromium(V) species potentially stabilized in the respiratory tract upon inhalation of carcinogenic chromium(VI) compounds. Chem Res Toxicol 16(7) 881—892. Signorella S, Garcia S, Rizzotto M, Levina A, Lay PA, Sala LF. 2005. The EPR pattern of Cr complexes of D-ribose derivatives. Polyhedron 24(9) 1079-1085. 

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