N,N'-Substituted thioureas and their metal complexes: syntheses, structures and electronic properties

The synthesis of six N,N'-substituted thiourea ligands (L1a-L3b) was achieved in two steps. A corresponding extensive series of Cu(I), Cu(II), Ni(II) and Zn(II) complexes (1-24) with varying formulations were synthesised from these ligands by the reaction of a 1 : 1 or a 1 : 2 mixture of Cu(II), Ni(II) and Zn(II) perchlorate or chloride salts. Complexes 1-24 have been comprehensively characterised by mass spectrometry, elemental analysis, UV-vis., IR, and ¹H and ¹³C{¹H} NMR spectroscopies where applicable. The X-ray crystal structures were obtained for eight examples: [(L1a)₂Cu]ClO₄ (1), [(L1c)₂Zn](ClO₄)₂ (4), [(L2a)₂Cu]ClO₄ (6), [(L2c)₂Ni](ClO₄)₂ (7), [(L1b)₂Cu](ClO₄) (15), [(L1b)CuCl] (16), [(L4)₂CuCl₂] (19) and [(L3b)CuClO₄] (21). These studies reveal that L1c and L2c represent ligands that have undergone cleavage during reaction with the metal salt; L4 represents an intramolecular rearrangement (via a Hugershoff reaction) of L2b; and in most cases Cu(II) is reduced to Cu(I) during the ligand reaction. The X-ray crystal structures also reveal that 1, 4, 6, 15 and 16 are monometallic species in the solid state; that Cu(I) in 1, 6, 15 and 16 and Zn(II) in 4 are arranged in a distorted tetrahedral geometry; that Cu(I) in 21 adopts a trigonal planar geometry; and that in 7 and 19 the Ni(II) and Cu(II) centres, respectively, possess square planar geometry. Preliminary studies on the biological activity (using the Malaria Sybr Green I Fluorescence assay) of the thiourea containing complexes suggests that the d¹⁰ complexes, and increased ligand stoichiometries, may afford higher potency.