Mechanism of the gas phase reactions of the C5H5Ni+ and C5H5Co+ ions with substituted pyridines. A combined experimental and theoretical study

Different products have been observed in the reactions of C5H5Co+ and C5H5Ni+ ions with halogen-substituted pyridines (XPy) that have been studied by ion trap mass spectrometry (ITMS) techniques. In particular, an addition product C5H5M(XPy)+ and a product ion C5H4M(Py)+ corresponding to a loss of a HX molecule (X = F, Cl, Br) have been detected. The relative yield of these products is determined by the nature of the metal and by the nature and position of the halogen on the pyridine ring. A computational study at the DFT level on model-systems formed by 2-fluoro and 2-bromopyridine reacting either with the C5H5Ni+ or the C5H5Co+ ion has been carried out. This study shows the existence of a general mechanistic pattern. The rate-determining step of this mechanism is the migration of the halogen from the pyridine ring to the metal. A final hydrogen abstraction step carried out by the halogen leads to the expulsion of a HX molecule. The existence of avoided crossings between surfaces of different multiplicities (ground and first excited state) allows the system to follow lower energy reaction pathways. The barrier determined for the reactions involving 2-bromopyridine is significantly lower than that found for 2-fluoropyridine. This is mainly due to the poor migrating/leaving character and low polarizability of fluorine compared to that of bromine.