Synthesis, structure and redox properties of some 2-(arylazo)phenolate complexes of rhodium(III) †

Reaction of 2-(arylazo)phenols 2-(4′-RC6H4NN)C6H3OH(Me-4) (H2ap-R, where H2 stands for the two dissociable protons and R for the substituent in the phenyl ring of the arylazo fragment) with [Rh(PPh3)3Cl] afforded a family of organometallic complexes of rhodium(III) of type [Rh(PPh3)2(ap-R)Cl]. The crystal structure of [Rh(PPh3)2(ap-NO2)Cl] has been determined. The 2-(arylazo)phenols are coordinated via dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring in the arylazo fragment, as dianionic tridentate C,N,O-donors forming two five-membered chelate rings. 1H and 13C NMR spectra of the complexes are in excellent agreement with their composition and stereochemistry. The complexes are diamagnetic (low-spin d6, S = 0) and show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry for all the complexes show a quasi-reversible oxidation within 0.65 to 1.10 V vs. SCE and an irreversible reduction within −1.34 to −1.50 V vs. SCE. The potential of the oxidation is found to be sensitive to the nature of the substituent R in the 2-(arylazo)phenolate ligand.