Spectroscopic and electrochemical properties of di-2-pyridyl ketone thiosemicarbazone [dpktsc] in non-aqueous media

Spectroscopic measurements on non-aqueous solutions of [dpktsc] divulged high sensitivity of [dpktsc] to its surroundings. 1H NMR studies performed on d6-dmso and d6-acetone solutions of [dpktsc] disclosed inter-molecular hydrogen bond between the amine proton and the solvent oxygen atom. In non-protophilic solvent (CDCl3), intra-molecular hydrogen bond between the amide proton and a nitrogen atom of a pyridine ring was elucidated. Electronic absorption measurements done on non-aqueous solutions of [dpktsc] revealed two intra-ligand charge transfer transitions (ILCT) due to n → π* and π → π* of the thione followed by thione to pyridine charge transfer. In protophilic solvents, a shoulder appeared at ∼400 nm along with the ILCT transitions. Spectrophotometric titrations of [dpktsc] with NaBX4 (X = H or F) and thermo-optical measurements in dmf disclosed that deprotonation of [dpktsc] is not favorable. When stoichiometric amounts of [MCl2] (M = Zn, Cd or Hg) were added to dmf solutions of [dpktsc], facile conversion of [dpktsc] to its conjugate base [dpktsc-H]− and in situ formation of [MCl2(κ3-N,N,S-dpktsc-H]− was observed. Dmf solutions of [dpktsc] are able to detect and determine [MCl2] in concentrations as low as 1.00 × 10−12 M. Protophilic solutions (dmf) of [dpktsc] show high affinity to [ZnCl2], compared to [MCl2] (M = Cd or Hg). Electrochemical measurements done on dmf solutions of [dpktsc] in the presence and absence of [MCl2] show reductive decomposition of [dpktsc] in the absence of [MCl2] and in the presence of [MCl2] electrochemical signatures consistent with the in situ formation of [MCl2(κ3-N,N,S-dpktsc-H)]− were observed.