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

Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N–H/N–C Bond Homolysis Due to the Formation of Stable Ligand Radicals

“…Recent studies on azo-containing pincer ligands have shown that also alcohol dehydrogenation can be catalyzed via a pathway that involves a reduced azo moiety. 11 Similarly, studies on molecular electrocatalysts for the hydrogen evolution reaction (HER) have identified several systems in which the mechanism does not involve “traditional” metal hydride intermediates; 12 instead, the organic ligand is proposed to be involved as the locus of reduction, protonation, or both. Thus, the assembly of two protons and two electrons as required for H 2 production requires a delicate interplay between the reactivity of the metal center and that of the ligand.…”

Metal Complexes Containing Redox-Active Ligands in Oxidation of Hydrocarbons and Alcohols: A Review

“…Redox-active, or 'noninnocent', ligands have more energetically-accessible levels that allow redox reactions to change their charge state [11]. In recent decades, noninnocent (or redox) ligands have been intensively studied for their unusual chemical behavior [12][13][14][15][16][17][18][19][20][21][22][23]. "Introduced in the late sixties, noninnocent (or redox) ligands have been extensively studied for their unusual and intriguing chemical behavior.…”

“…Redox-active ligands range from the small archetypical NO +/•/− and O 2 0/•−/2− systems via the classical 1,4-dihetero-1,3-diene chelates (e.g., α-diimine, dithiolene, or o-quinone redox series) to π-conjugated macrocycles. [15,22,23]. Currently, catalytic systems based on transition metal complexes (Rh, Ru, Pt, Pd, etc.)…”

Structure of Electronically Reduced N-Donor Bidentate Ligands and Their Heteroleptic Four-Coordinate Zinc Complexes: A Survey of Density Functional Theory Results

“…Such studies are particularly important with respect to the reliability of the B3LYP functional, which has been applied in countless related systems. [18][19][20][21][22][35][36][37][38]41,[43][44][45][50][51][52][53][66][67][68][69][70][71] A further point deserves attention with respect to the use of mono-reference DFT methods to study these compounds. It is known that static correlation can influence the electronic structure of transition metal complexes having non-innocent ligands, 48,52 notably in the case of iron complexes.…”