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 ( L 1 ) and 2,9-bis(phenyldiazo)-1,10-phenanthroline ( L 2 ) are reported. The ligand L 1 is a tridentate pincer having NNN donor atoms, whereas L 2 is tetradentate having two azo-N donors and two N-donor atoms from the 1,10-phenanthroline moiety. Reaction of FeCl 2 with L 1 or L 2 produced the pentacoordinated mixed-ligand Fe(II) complexes Fe L 1 Cl 2 ( 1 ) and Fe L 2 Cl 2 ( 2 ), respectively. Homoleptic octahedral Fe(II) complexes, mer -[Fe( L 1 ) 2 ](ClO 4 ) 2 [ 3 ](ClO 4 ) 2 and mer -[Fe( L 2 ) 2 ](ClO 4 ) 2 [ 4 ](ClO 4 ) 2, have been synthesized from the reaction of hydrated Fe(ClO 4 ) 2 and L 1 or L 2 . The ligand L 2, 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.