Utilisation des pièges à radicaux en vue de mettre en évidence des intermédiaires dans la photolyse de complexes contenant une liaison Co^III—C

Abstract

Using the spin trapping technique with 5,5′-dimethyl 1-pyrroline N-oxide (DMPO), phenyl-N-tert-butyl nitrone (PBN), nitrosodurene (ND), and α-4-pyridyl 1-oxide N-tert-butl nitrone (4-POBN), or their mixtures, we have been able to detect two types of radicals, one is a hydrogen atom spin adduct and the other is the corresponding alkyl of the alkylcobaloximes, salens or cobalamines.By the use of selective deuteration and the preparation of the benzyl bis(diphenylglyoximato)-pyridinato cobaloxime, we have shown that the spin trapped hydrogen atom comes from the chelated hydrogen of the dimethylglyoximato anion of the equatorial ligand of CoIII complexes. Using a mixture of two spin traps gives rise to an esr spectrum containing, at the same time, the hydrogen atom and alkyl spin adducts.To explain such an efficient spin trapping reaction while the homolysis is in competition with a β-elimination process, it should be postulated that the photolysis of such compounds proceeds through a solvent cage environment. This hypothesis explains the strong influence of solvents on the nature of the free radicals trapped. Also the spin trap diffuses in the solvent cage, and is able to trap in a very efficient manner any free radicals occurring in the solvent cage. The excited CoIII complexes and the spin trap in the solvent cage behave like an exciplex, which could explain the spin trapping of the hydrogen atom.

Photolysis Primary Products of Alkylcobaloximes Controlled by the Cobalt−Carbon Bond Strength

“…The cleavage of the cobalt−carbon bond, which is a central issue in B 12 chemistry, has been studied in model compounds, and several reports have appeared in the literature concerning anaerobic photolysis in protic and aprotic solvents of alkylcobaloximes with nitrogen − and phosphorus donor bases. It has been demonstrated that the photolysis products of alkylcobaloximes with symmetric phosphorus donor bases are independent of the alkyl group and that at all temperatures irradiation gives rise to a cobalt(II) five-coordinate species and an organic radical, thus implying the homolytic cleavage of the Co−C bond …”

“…For alkylcobaloximes with nitrogen bases it has been reported that the products observed upon photolysis with visible light depend (a) on the ligand trans to the alkyl group, as it may induce deformations on the equatorial moiety that control the reactivity of the photolysis primary products, , and (b) on the alkyl group, as photolysis at 250 K of methyl- and benzylpyridinecobaloximes was claimed not to induce homolysis of the cobalt−carbon bond but reduction of the metal center. However, it must be pointed out that this conclusion was based on results obtained only for two compounds and that the analysis of the ESR spectra was controversial 6a.…”

“…Photolysis has been performed with visible and UV light sources, both in fluid (300 and 250 K) and frozen (77 K) solution. Electron spin resonance (ESR) spectroscopy was used to monitor the photolysis process, to identify the paramagnetic species formed, − and to get relevant information about the structure of the cobalt(II) complexes 1 Alkylcobaloximes, Compounds of General Formula [RCo(Hdioxime) 2 L] 1 , CH 3 Co(DH) 2 py 7 , C 6 H 5 CH 2 Co(DH) 2 (3,5-lut) 2 , CD 3 Co(DH) 2 py 8 , CH 3 Co(PH) 2 py 3 , CH 3 Co(DH) 2 (3-pic) 9 , CH 3 Co(DH) 2 ( 15 Npy) 4 , CH 3 Co(DH) 2 (3,5-lut) 10 , C 6 H 5 CH 2 Co(DH) 2 ( 15 Npy) 5 , C 6 H 5 CH 2 Co(DH) 2 py 11 , CH 3 Co(GH) 2 py 6 , C 6 H 5 CH 2 Co(DH) 2 (3-pic) …”

Bis(dimethylglyoximato)(methyl)(pyridine)cobalt(III)

“…38 The primary step of this reaction is the formation of the methyl radical, which has been quenched with spin traps such as nitrosodurene or phenyl t-butyl nitrone. 39 Methyl radicals add to the β-position of alkyl methacrylates. The persistent cobaloxime(II) radical which is present in solution traps the adduct radical and regenerates the unsaturation in the product via hydridocobaloxime formation.…”

Enhanced acyl radical formation in the Au nanoparticle-catalysed aldehydeoxidation

“…To explore the origin of the DMPO-H adduct, perdeuterated acetaldehyde was oxidised in the presence of base and PI-Au catalyst. A DMPO-D adduct (a N = 14.54, a H = 18.79, a D = 2.84 G) 22 was observed, thus suggesting that the hydrogen adduct is formed as a consequence of C-H bond cleavage (see ESIw). Observation of a DMPO-H adduct in a related Au-catalysed alcohol oxidation reaction was recently attributed to the intermediate formation of Au-H as a consequence of hydride transfer from the C-H bond to gold.…”