N-(aryl)picolinamide complexes of ruthenium: usual coordination and strategic cyclometalation

Abstract

Reaction of five N-(4-R-phenyl)picolinamides (R = OCH₃, CH₃, H, Cl, and NO₂) with [Ru(PPh₃)₂(CO)₂Cl₂] in refluxing 2-methoxyethanol in the presence of a base (NEt₃) affords two geometrical isomers of a group of complexes (1-R and 2-R), each of which contains an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphanes, a carbonyl, and a hydride. Similar reaction of N-(naphthyl)picolinamide with [Ru(PPh₃)₂(CO)₂Cl₂] affords an organometallic complex, 3, in which the amide ligand is coordinated to the metal center, by C-H activation of the naphthyl ring at the 8-position, as a dianionic tridentate N,N,C donor along with two triphenylphosphanes and one carbonyl. Structures of the 1-OCH₃, 2-CH₃, and 3 complexes have been determined by X-ray crystallography. In all the complexes the two triphenylphosphanes are trans. In the 1-R complexes the hydride is trans to the pyridine nitrogen and in the 2-R complexes it is trans to the amide-nitrogen. All the complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ru^II-Ru^III oxidation within 0.71-0.93 V versus SCE. An oxidation and a reduction of the coordinated amide ligand are also observed within 1.29-1.69 V versus SCE and -1.02 to -1.21 V versus SCE respectively.