Syntheses and spectral characterization of Cobalt(III) complexes of N(4)-phenylsemicarbazones

Abstract

An interesting series of five new cobalt complexes of 2-benzoylpyridine-N-4-phenyl semicarbazone(HL1) and 2-acetylpyridine-N-4-phenyl semicarbazone(HL2) have been synthesized and physico chemically characterized. Cobalt(III) and various tridentate ligands form mainly mixed bis(ligand) complexes. Cobalt(III) complexes of the type [CoL2]X (X = Cl, Br, SCN, N3, NO3 etc., L = anion from the enol form of the semicarbazone ligand of the type, HL) which were proposed to be low-spin, diamagnetic and octahedral have been reported. Ligand can exist in keto or enol form or an equilibrium mixture of the two since it has an amide NH-C(O) function. However, the IR spectrum of HL1 and HL2 indicates that in the solid state it remains in keto form. The IR spectra of complexes, however, do not show any intense absorption bands around 1698 cm−1 and 1683 cm−1, due to the carbonyl stretching of the semicarbazone moiety. This shows that in solution, it tautomerises to the enol form and coordinates to the metal in the enolate form. A medium band in the region 3380 cm−1 in the free ligands due to v(N–H) vibration disappears in the spectra of complexes providing a strong evidence for the ligand coordination around cobalt(III) ion in its deprotonated form. Both the semicarbazones and their cobalt(lII) complexes show aring ᴨ → ᴨ * band in the range 37000–39000 cm−1 and an n → ᴨ* band in the range 32000–34000 cm-l involving transitions within the semicarbazone moiety.