Synthesis, structure and electrochemical properties of some oxime complexes of rhodium

Reaction of the oximes of salicyladehyde (H²L¹), 2-hydroxyacetophenone (H₂L²) and 2-hydroxynaphthaldehyde (H₂L³; general abbreviation H₂L, where H ₂ stands for the two dissociable protons, one phenolic proton and one oxime proton) with [Rh(PPh₃)₃Cl] afforded a family of rhodium(III) complexes of the type [Rh(PPh₃)₂(HL)(L)]. The crystal structure of [Rh(PPh₃)₂(HL²)(L ²)] has been determined by X-ray diffraction. One oxime ligand is coordinated via dissociation of only the phenolic proton, while the other oxime ligand is coordinated via dissociation of both the phenolic and oxime protons. Both the oxime ligands are coordinated as bidentate N,O-donors, forming six-membered chelate rings. The complexes are diamagnetic (low-spin d ⁶, S = 0) and their ¹H NMR spectra are in excellent agreement with their compositions. All three [Rh(PPh₃) ₂(HL)(L)] complexes display intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on all the complexes shows two oxidations; the first one is observed within the range 0.61 to 0.76 V vs. SCE and the second one within 1.20 to 1.32 V vs. SCE. There is also one irreversible reduction between -1.05 and -1.30 V vs. SCE.