Chemistry of 2-(phenylazo)pyridine complexes of osmium: synthesis, characterization and reactivities

Abstract

Reaction of 2-(phenylazo)pyridine (pap) with [Os(PPh₃)₃Br₂] afforded a mixed ligand complex of the type [Os(PPh₃)₂(pap)Br₂]. The structure of this complex was determined by X-ray crystallography. The PPh₃ ligands are trans and the bromides are in cis positions. The pap ligand is coordinated to osmium as a bidentate N,N-donor forming a five-membered chelate ring. The complex is diamagnetic (low-spin d⁶, S=0) and in dichloromethane solution shows intense MLCT transitions in the visible region. The two bromides were replaced from the coordination sphere of [Os(PPh₃)₂(pap)Br₂] under mild conditions by a series of anionic ligands L (where L=quinolin-8-olate (q), picolinate (pic), oxalate (Hox), 1-nitroso-2-naphtholate (nn) and acetyl acetonate (acac)) to afford complexes of the type [Os(PPh₃)₂(pap)(L)]⁺, which were isolated and characterized as the perchlorate salt. The structure of the [Os(PPh₃)₂(pap)(acac)]ClO₄ complex was determined by X-ray crystallography. The PPh₃ ligands occupy trans positions and the acetylacetonate anion is coordinated to osmium as a bidentate O,O-donor forming a six-membered chelate ring. The [Os(PPh₃)₂(pap)(L)]⁺ complexes are diamagnetic and show multiple MLCT transitions in the visible region. [Os(PPh₃)₂(pap)Br₂] shows an osmium(II)-osmium(III) oxidation at 0.49 V versus SCE. The same oxidation is displayed by the [Os(PPh₃)₂(pap)(L)]⁺ complexes within 0.69 to 0.96 V versus SCE. Two successive one-electron reductions of the coordinated pap ligand are also observed in all the complexes below -0.90 V versus SCE.