N, N′-Bis(aryl)pyridine-2,6-dicarboxamide complexes of ruthenium: Synthesis, structure and redox properties

Abstract

Reaction of five N, N′-bis(aryl)pyridine-2,6-dicarboxamides (H 2L-R, where H 2 denotes the two acidic protons and R (R = OCH 3, CH 3, H, Cl and NO 2) the para substituent in the aryl fragment) with [Ru(trpy)Cl 3](trpy = 2,2′,2″-terpyridine) in refluxing ethanol in the presence of a base (NEt 3) affords a group of complexes of the type [Ru II(trpy)(L-R)], each of which contains an amide ligand coordinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [Ru II(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru II(trpy)(L-R)] complexes shows a Ru(II)–Ru(III) oxidation within 0.16–0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94–1.33 V versus SCE and a reduction of coordinated terpyridine ligand within −1.10 to −1.15 V versus SCE. Constant potential coulometric oxidation of the [Ru II(trpy)(L-R)] complexes produces the corresponding [Ru III(trpy)(L-R)] + complexes, which have been isolated as the perchlorate salts. Structure of the [Ru III(trpy)(L-CH 3)]ClO 4 complex has been determined by X-ray crystallography. All the Ru(III) complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region. A weak ligand-field band has also been shown by all the [Ru III(trpy)(L-R)]ClO 4 complexes near 1600 nm.