Photoactivated formation of hydrogen spin adduct from phenylsilane. A new example of inverted spin trapping?

Abstract

Photodissociation of phenylsilane was studied by EPR spectroscopy using N-benzylidene-tert-butylamine N-oxide (PBN) and bis(benzene)chromium (BBC) as spin trapping systems. Under controlled UV irradiation and in the presence of dioxygen, hydrogen spin adducts were detected. Conditions for the formation and stability of the adducts have been studied. A mechanism involving "inverted spin trapping" was propounded that implies photoactivation of the spin traps followed by the formation of the superoxide radical ion O 2 Ϫ and the abstraction of H Ϫ from phenylsilane.

Spin trapping by bis(benzene)chromium: A density functional study

“…The results are summarised in Table 3 and compared to the available experimental values. 5,50 The most remarkable features of the data collected in this last table is that for all the theoretical methods hydrogen and chromium spin populations are in good agreement with experimental results.…”

“…2 This proposed mechanism is supported by two experimental features: (i) the direct observation of the endo position of the isobutyronitril (R ¼ IBN) spin adduct by angle-selected ENDOR spectroscopy in frozen solution, 2 and (ii) the observation in liquid solution at room temperature of a dynamic interannular radical transfer between the two equivalent structures B and C (see Scheme 1), independently of the nature of the radical adduct. 2,4 A more detailed study of the hydrogen addition (R ¼ H) revealed that the mechanism is more complex than proposed in Scheme 1, and necessitates traces of oxygen and a low level of UV irradiation: 5 BBC !…”

“…This inverted spin trapping mechanism implies the photoactivation of the spin trap BBC (step (i)) followed by the formation of the superoxide radical ion O 2 À by the electron transfer from the activated BBC* to the molecular oxygen (step (ii)) and the abstraction of H À from phenylsilane (step (iii)), a molecule that was not known to release such species. The key steps in such a mechanism are the generation of the BBC + radical cation and of the (Z 6 -C 6 H 6 )(Z 5 -C 6 H 7 )Cr (BBC-H in reaction (1)) species. While the first radical has been well characterized experimentally, there are some doubts on the identification of BBC-H .…”

“…While the first radical has been well characterized experimentally, there are some doubts on the identification of BBC-H . 2,5 Actually, ENDOR spectroscopy is able to supply the magnitudes of hyperfine interaction parameters pertaining to hydrogen nuclei but, as the more classical EPR technique, does not provide any information about the structure of the short-lived spin adducts (see Scheme 1). Moreover, even if ENDOR results suggest that the hydrogen adduct species (Z 6 -C 6 H 6 )(Z 5 -C 6 H 7 )Cr (BBCH ) was obtained, the attacking hydrogen being in the endo position, they failed to reveal any distinctive feature of the trapped hydrogen.…”

“…Moreover, even if ENDOR results suggest that the hydrogen adduct species (Z 6 -C 6 H 6 )(Z 5 -C 6 H 7 )Cr (BBCH ) was obtained, the attacking hydrogen being in the endo position, they failed to reveal any distinctive feature of the trapped hydrogen. 5 Owing to the high resolution of ENDOR spectroscopy and to the short distance between the unpaired electron spin on the chromium Scheme 1 Model for the formation of spin adducts.…”