Rhodium assisted C-H activation of N-(2'-hydroxyphenyl)benzaldimines. Synthesis, structure and electrochemical properties of a group of organorhodium complexes

Abstract

Reaction of a group of N-(2'-hydroxyphenyl)benzaldimines, derived from 2-aminophenol and five para-substituted benzaldehydes (the para substituents are OCH₃, CH₃, H, Cl and NO₂), with [Rh(PPh₃)₃Cl] in refluxing toluene in the presence of a base (NEt₃) afforded a family of organometallic complexes of rhodium(III). The crystal structure of one complex has been determined by X-ray crystallography. In these complexes the benzaldimine ligands are coordinated to the metal center, via dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring in the imine fragment, as dianionic tridentate C,N,O-donors, and the two PPh₃ ligands are trans. The complexes are diamagnetic (low-spin d⁶, S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry shows a Rh(III)-Rh(IV) oxidation within 0.63-0.93 V vs SCE followed by an oxidation of the coordinated benzaldimine ligand. A reduction of the coordinated benzaldimine is also observed within -0.96 to -1.04 V vs SCE. Potential of the Rh(III)Rh(IV) oxidation is found to be sensitive to the nature of the para-substituent.