Redox Noninnocent Azo-Aromatic Pincers and Their Iron Complexes. Isolation, Characterization, and Catalytic Alcohol Oxidation

Abstract

The new redox-noninnocent azoaromatic pincers 2-(arylazo)-1,10-phenanthroline (L¹) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L²) are reported. The ligand L¹ is a tridentate pincer having NNN donor atoms, whereas L² is tetradentate having two azo-N donors and two N-donor atoms from the 1,10-phenanthroline moiety. Reaction of FeCl₂ with L¹ or L² produced the pentacoordinated mixed-ligand Fe(II) complexes FeL¹Cl₂ (1) and FeL²Cl₂ (2), respectively. Homoleptic octahedral Fe(II) complexes, mer-[Fe(L¹)₂](ClO₄)₂ [3](ClO₄)₂ and mer-[Fe(L²)₂](ClO₄)₂ [4](ClO₄)₂, have been synthesized from the reaction of hydrated Fe(ClO₄)₂ and L¹ or L². The ligand L², although having four donor sites available for coordination, binds the iron center in a tridentate fashion with one uncoordinated pendant azo function. Molecular and electronic structures of the isolated complexes have been scrutinized thoroughly by various spectroscopic techniques, single-crystal X-ray crystallography, and density functional theory. Beyond mere characterization, complexes 1 and 2 were successfully used as catalysts for the aerobic oxidation of primary and secondary benzylic alcohols. A wide variety of substituted benzyl alcohols were found to be converted to the corresponding carbonyl compounds in high yields, catalyzed by complex 1. Several control reactions were carried out to understand the mechanism of this alcohol oxidation reactions.