English abstract
Two series of potentially tridentate thiosemicarbazone ligands were produced from the condensation reactions of either 2-hydroxy-1-naphthaldehyde or variously substituted derivatives of salicylaldehyde with various forms of thiosemicarbazides. The ternary copper(II) complexes of the thiosemicarbazone ligands H2napdmtsc, Hänapetsc, Hunapptsc, H2(3,5-'Buz)-saldmtsc, H2(3,5-Buz)-saletsc, Hz(5,3-Br,OMe) salptsc and H2(5,3-Br, OMe)-saletsc were synthesised by the reaction of the appropriate ligand with an equivalent amount of copper(II) in the presence of a neutral bidentate ligand (phen, bipy or dpa) or an anionic monodentate ligand X (X = C1, Bror CH3C02). The former series of copper(II) complexes are five coordinate whereas the latter are four-coordinate. These complexes have been characterised by microanalyses, mass spectrometry, R-spectroscopy, single-crystal X-ray diffraction, ESR spectroscopy, electronic absorption spectroscopy and cyclic voltammetry.
The X-ray crystal structures of the complexes [Cu(napdmtsc) (phen)]2:2H2O. MOH, [Cu(napetsc)(bipy)]2 and [Cu(napetsc)(dpa)] have revealed a distorted square pyramidal coordination geometry about the copper(II) ion with the ligand ONS donor set arranged meridionally in the equatorial plane. The chelating bidentate heterocyclic ligands exhibit an axial-equatorial mode of bonding. The bite angles of phen and bipy are in the range 77.49(14)0–78.33(15)° whereas the bite angle of the more flexible dpa ligand is 87.70o. Evidently, the thiosemicarbazones have undergone both tautomerisation and deprotonation to form the dinegative thiolate forms that bring about electroneutrality of these complexes. The occurrence of these ligand transformations has been corroborated by vibrational spectroscopy. However, the crystal structure of the complex [Cu(H(3,5-Buz-saldmtsc)}Cl] shows a distorted square planar geometry at the metal centre whereby the thiosemicarbazone ligand coordinated in tridentate fashion but retained its thione form thus, facilitating the coordination of the chloro ligand. The formation of the square pyramidal and square planar complexes demonstrates the coordination versatilities of both copper and the thiosemicarbazones that often lead to unpredictable fascinating structural features.
The five-coordinate complexes are one-electron paramagnetic and exhibit axial X band ESR spectra in frozen EtOH MOH solutions at 77 K with gl > g1 > ge and A> Al indicating a dz2_y2 ground state. Similarly, the ESR spectra of the four-coordinate complexes show axial spectra typical of distorted square planar complexes. The optical spectra exhibit overlapping LMCT absorption bands in the range 408-434 nm associated with interactions of copper(II) with phenolato or naphtholato moieties in conjunction with the transference of negative charge form the thiolate sulfur to the central metal ion. The visible spectra of the five-coordinate complexes show ligand field absorptions in the range 567-575 nm pointing to structural retention in solution. In contrast, the d-d transitions of the four-coordinate complexes are represented by the absorption bands at 645 and 620 nm, respectively, consistent with a square planar geometry. The complexes are redox-active and, generally, exhibit quasi-reversible waves for the Cu"/Cu' couple in MeCN solution in the range 0.51–0.57 V versus the SHE. These metal-centred electrochemical processes are accompanied by ligand oxidations at higher potentials.
