Thiosemicarbazones as co-ordinating agents. Solution chemistry and X-ray structure of pyridoxal thiosemicarbazone trihydrate and spectroscopic properties of its metal complexes

Abstract

The ligand behaviour of pyridoxal thiosemicarbazone (H₂L) was examined in a series of metal complexes of formula M(H₂L)X₂, M(HL)X, and ML (M = Co, Ni, Cu, or Zn; X = NO₃ or CH₃CO₂) which were characterized by analytical methods and i.r. spectroscopy. Changes in the vibrational absorptions of the ligand upon co-ordination are discussed. The X-ray structure of uncomplexed H₂L·3H₂O was determined. Crystals are monoclinic, space group P2₁/n, with a= 16.126(1), b= 7.267(1), c= 12.466(1)Å, and β= 108.60(1)°. The structure was solved by direct methods and refined by full-matrix least squares to a final R value of 0.058 for 1 262 observed reflections. The thiosemicarbazone moiety shows an E configuration about both the C(2)–N(3) and C(1)–N(2) bonds, and the phenolic oxygen is traps to N(3). The protonation constants of H₂L were determined by spectrophotometric methods at 25 °C and I= 0.5 mol dm^–3(KCl): log β₀₁₁= 11.111(22), log β₀₂₁= 19.048(25), and log β₀₃₁= 23.342(27). The spectrophotometric characteristics of the various species are reported; the experimental spectra are in good agreement with calculated ones.