Facile regiospecific aromatic hydroxylation in palladium azopyridines and structural characterization of phenolato product

Abstract

The reaction of Na₂PdCl₄ with 2-(arylazo)pyridines (A) in ethanol affords yellow complexes of composition [PdACl₂] in which the PdCl₂ fragment has acis configuration [v(PdCl): 350, 365 cm^-1]. Upon treating [PdACl₂] with dilute sodium hydroxide in air the pendent aryl group is selectively hydroxylated at theortho position, affording the phenolato complex [PdBCl] in high yields [B^- is deprotonated 2-(2'-hydroxyarylazo)pyridine]. A possible reaction pathway is proposed by analogy with the hydroxylation of certain organic compounds by OH^-/O₂. The crystal and molecular structure of one [PdBCl] complex is reported. In the highly planar complex, the PdN(azo) length is significantly shorter than the PdN(pyridine) length. A single PdCl stretch at 365 cm^-1 characterizes [PdBCl] which, unliked [PdACl₂], has a structured intense absorption in the visible region near 670 nm.