Matrix-Assisted Laser Desorption/Ionization Studies on Transition Metal Complexes of Benzimidazole Thiosemicarbazones

The transition metal (M=Fe, Co, Ni, Cu, Zn, Cd and Hg) complexes of 2-acetylbenzimidazolethiosemicarbazone (L1) and 1-methyl 2-acetylbenzimidazolethiosemicarbazone (L2) were analyzed by matrix-assisted laser desorption/ionization (MALDI) using 2-cyano-4-hydroxycinnamic acid (HCCA), terthiophene (THP), methyl 5-(4-methoxy-1-naphthyl)-(2E,4E)-2,4-pentadienoate (MMNPD) and 2,7-dimethoxy naphthalene (DMN) as the matrices. All the MALDI spectra are clean without any contribution from the complex ions resulted by multiple proton addition/removal. All the complexes, except Cu, irrespective of the matrix used, show 1:2 complex ions wherein two ligands (neutral or deprotonated) complex with the metal ion depending on the nature and stable oxidation state of the central metal ion viz., [M + 2L – 2H]+ ion for Fe and Co complexes (+3 oxidation state) and [M + 2L – H]+ ion for Ni, Zn, Cd and Hg (+2 oxidation state). The Cu complex shows 1:1 complex ions corresponding to [2M + 2L – 2H]+ ions. When HCCA is used as a matrix, the complex ions due to ligand exchange by matrix are also found, and this process is relatively more if a neutral ligand is bound to the metal ion in the original complex ion. The type of complex ions found under MALDI experiments are similar to those found under electrospray ionization experiments. However, the complex ions due to reduction of Cu are found only in the MALDI analysis of Cu complexes.