The relationship between ligand structures and their CoII and NiII complexes: Synthesis and characterization of novel dimeric CoII/CoIII complexes of bis(thiosemicarbazone)

Abstract

4,6-Diacetylresorcinol serves as a starting point for the generation of multidentate S/N/O or O/N/O symmetrical chelating agents by condensation with thiosemicarbazide or semicarbazide to yield the corresponding bis(thiosemicarbazone) H4L1 or bis(semicarbazone) H4L2, respectively. Reaction of H4L1 and H4L2 with M(NO3)2·6H2O (M = Co or Ni) afforded dimeric complexes for H4L1 and binuclear complexes for H4L2, revealing the tendency of S to form bridges. The dimeric cobalt complexes of H4L1 are very interesting in that they contain CoII/CoIII, side/side, low-spin octahedral coordinated CoIII-ions and high-spin square-planar coordinated CoII-ions. These complexes have the general formula [(H2L1)2Co2(H2O) (NO3)]·nEtOH. Arguments supporting these anomalous CoII/CoIII structures are based on a pronounced decrease in their magnetic moments, elemental and thermal analyses, visible and IR spectra, as well as their unreactivity towards organic bases such as 1,10-phenanthroline (phen), 2,2′-bipyridine (Bpy), N,N,N′,N′-tetramethylethylenediamine (Tmen) and 8-hydroxyquinoline (oxine, Ox). The dimeric octahedral NiII complex [(H2L1)2Ni2(H2O)4]·3H2O showed higher reactivity towards phen and Bpy and formed adducts; [(HL1)Ni2(B)(H2O)5] NO3 (B = phen or Bpy). In the presence of oxine, the dimeric brown paramagnetic octahedral complex [(H2L1)2Ni2(H2O)4]·3H2O was transformed to the dimeric brick-red diamagnetic square-planar complex [(H3L1)2Ni2](NO3)2. The latter showed dramatic behavior in its 1H NMR spectrum in DMSO-d 6, which was explained on the basis of H+-transfer. By contrast, the binuclear NiII–H4L2 complex (11) showed higher reactivity towards phen, Bpy and oxine. These reactions afforded mixed dimeric complexes having the molar ratio 2 : 2 : 1 (NiII : H4L2 : base). The binuclear CoII–H4L2 complex afforded an adduct with phen and trinuclear complexes with Bpy and oxine. All complexes were found to be unreactive towards Tmen. Structural characterization was achieved by elemental and thermal analyses, spectral data (electronic, IR, mass and 1H NMR spectra) and conductivity and magnetic susceptibility measurements.