Synthesis, structure and redox properties of some thiosemicarbazone complexes of rhodium

Abstract

Reaction of salicylaldehyde thiosemicarbazone (H₂L¹), 2-hydroxyacetophenone thiosemicarbazone (H₂L²) and 2-hydroxynaphthaldehyde thiosemicarbazone (H₂L³) (general abbreviation H₂L, where H₂ stands for the two dissociable protons, one phenolic proton and one hydrazinic proton) with [Rh(PPh₃)₃Cl] afforded a family of rhodium(III) complexes of the type [Rh(PPh₃)₂(L)Cl]. The crystal structure of [Rh(PPh₃)₂(L²)Cl] has been determined by X-ray diffraction. The thiosemicarbazone ligands are coordinated, via dissociation of the two protons, as dianionic tridentate O,N,S-donor ligands forming one six-membered and one five-membered chelate rings. The complexes are diamagnetic (low-spin d⁶, S = 0) and their ¹H NMR spectra are in excellent agreement with their compositions. All three [Rh(PPh₃)₂(L)Cl] complexes display intense absorptions in the visible and ultraviolet regions. They also show strong emission in the visible region at ambient temperature. Cyclic voltammetry on all the complexes shows two irreversible oxidations, the first one is observed within 0.77 to 0.85 V vs. SCE and the second one within 1.13 to 1.23 V vs. SCE, and one irreversible reduction within −1.05 to −1.14 V vs. SCE.