Pyridyne radical cations produced by photodissociation of Mg˙+(multifluoro-pyridine) complexes: A combined experimental and theoretical study

Abstract

Gas phase complexes Mg^˙+(2,6-difluoropyridine) (1) and Mg^˙+(pentafluoropyridine) (2) have been subjected to photodissociation in the spectral range of ∼230–440 nm. Except for the evaporative photofragment Mg^˙+, the primary photoproduct for 1 is C₅H₃N^˙+, which is associated with the rupture of two C–F bonds by the photoexcited Mg^˙+, forming very stable MgF₂. In contrast, the direct loss of MgF⁺ is more favorable for 2 due to fluorine substitution. Given enough energy, C₅H₃N^˙+ can undergo decomposition to form C₄H₂^˙+ and HCN. These results are very different from those for Mg^˙+(2-fluoropyridine), highlighting the significance of the additional F at C6 of 1 and 2. Density functional theory (DFT) calculations have been employed to examine the geometries and energetics of the complexes as well as relevant reaction mechanisms. All of the complexes feature the direct attachment of Mg^˙+ to the N atom. The key intermediate is found to be FMg⁺(C₅H_xF_4−xN) (x = 3 or 0), which can lead to the formation of MgF⁺ directly or MgF₂ through activation of another C–F bond adjacent to N, producing the pyridyne radical cations. However, hydrogen-transfer prior to the rupture of the second C–F bond followed by ring-opening of C₅H₃N^˙+ may result in the formation of chain forms of C₅H₃N^˙+. The influence of the fluorine substitution on the competition of the two routes have been demonstrated.