Structural characterization of alachlor complexes with transition metal ions by electrospray ionization tandem mass spectrometry

Electrospray ionization mass (ESI-MS) spectrometry was used to investigate the nature of metal complexes of alachlor and their dissociations on activation. Ions of the first row transition metal series were employed to react with alachlor and the products were subjected to collision-induced dissociation (CID) for further structural characterization. The formation of diverse complex ions including doubly charged metal/alachlor complexes; 3L + M]²⁺ and 4L + M]²⁺ (L: alachlor and M: transition metal ions) were observed depending on the experimental conditions including the tube lens offset voltage (TLOV) and relative concentrations of alachlor and transition metal ions. It is clear that complexation with transition metal ions alters the reactive site of alachlor, promoting the loss of chlorine over the loss of CH₃OH that is the major reaction pathway in uncomplexed system. Direct elimination of chlorine from alachlor molecule was confirmed by the use of MnBr₂ instead of MnCl₂. These evidences clearly illustrate the catalytic activities of the metal ions through insertion mechanism. The function of transition metal ions in complexation was emphasized comparing the fragmentation patterns with those of protonated molecule. A change in the oxidation state of copper from + 2 to + 1 during the dissociation of metal complex was observed in company with elimination of radicals which is specific for the copper complex ions.