Synthesis, structure and electrochemical properties of some thiosemicarbazone complexes of iridium

Reaction of thiosemicarbazones of salicylaldehyde and 2-hydroxyacetophenone (H2L1 and H2L2) with [Ir(PPh3)(3)Cl] affords complexes of type [Ir(PPh3)(2)(L)(H)] (L = L-1 or L-2) in ethanol. A similar reaction carried out in toluene affords the [Ir(PPh3)(2)(L)(H)] complexes along with complexes of type [Ir(PPh3)(2)(L)Cl], where a chloride is coordinated to iridium instead of the hydride. The structure of the [Ir(PPh3)(2)(L-2)(H)] and [Ir(PPh3)(2)(L-2)Cl] complexes has been determined by X-ray crystallography. Crystal data for [Ir(PPh3)(2)(L-2)(H)]: space group, P2(1)/c; crystal system, monoclinic; a = 12.110(2) angstrom, b = 17.983(4) angstrom, c = 18.437(4) angstrom, beta = 103.42(3)degrees, Z = 4; R-1 = 0.0591, wR(2) = 0.1107. Crystal data for [Ir(PPh3)(2)(L-2)Cl]: space group, P2(1)/c; crystal system, monoclinic; a = 17.9374(l 1) angstrom, b = 19.2570(10) angstrom, c = 24.9135(16) angstrom, beta = 108.145(5)degrees, Z = 4; R-1 = 0.0463, wR(2) = 0.0901. In all the complexes the thiosemicarbazones are coordinated to the metal center as dianionic tridentate O, N, S-donors and the two triphenylphosphines are trans. The complexes are diamagnetic (low-spin d(6), S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry on all the [Ir(PPh3)(2)(L)(H)] and [Ir(PPh3)(2)(L)Cl] complexes shows a quasi-reversible Ir(III)-Ir(IV) oxidation within 0.55-0.78 V vs. SCE followed by an irreversible oxidation of the thiosemicarbazone within 0.91-1.27 V vs. SCE. An irreversible reduction of the thiosemicarbazone is also observed within -1.10 to -1.23 V vs. SCE.