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

Abstract

Reaction of thiosemicarbazones of salicylaldehyde and 2-hydroxyacetophenone (H₂L¹ and H₂L²) with [Ir(PPh₃)₃Cl] affords complexes of type [Ir(PPh₃)₂(L)(H)] (L = L¹ or L²) 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(L²)(H)] and [Ir(PPh₃)2(L²)Cl] complexes has been determined by X-ray crystallography. Crystal data for [Ir(PPh₃)₂(L²)(H)]: space group, P2_1/c; crystal system, monoclinic; a=12.110(2) Å, b=17.983(4) Å, c=18.437(4) Å, β=103.42(3)°, Z=4; R 1=0.0591, wR 2=0.1107. Crystal data for [Ir(PPh₃)₂(L²)Cl]: space group, P2_1/c; crystal system, monoclinic; a =17.9374(11) Å;, b=19.2570(10) Å c=24.9135(16)Å β=108.145(5) ° ,Z=4;R1=0.0463,wR₂=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.