An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Abstract

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl (1a) along with the two hydrogenated complexes of 1a, ZnHLCl (1b), and ZnHLCl (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O and para-benzoquinone to HO and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

“…Azo/hydrazo couple for alcohol dehydrogenation. 174,175 The bis(iminopyridyl) ligand (PDI), the bis(oxazolinepyridyl) ligand (Pybox) and the 2,2':6',2''-terpyridine ligand (terpy) are among the most frequently utilized, privileged, potentially redox active pincer ligands. All three ligand classes consist of a central pyridyl unit decorated with imine, pyridyl or oxazoline groups in 2-position, respectively.…”

Cu(II) and Fe(III) Complexes Derived from N-Acetylpyrazine-2-Carbohydrazide as Efficient Catalysts Towards Neat Microwave Assisted Oxidation of Alcohols

“…The catalytic performance of 1 was also compared with some recent literature reports (Table 4) [50][51][52][53]. It is clear from Table 4 that complex 1 exhibits a good catalytic efficiency (a maximum yield of 91.3% was achieved in 1 h) in comparison to the reported ones.…”

A Redox‐Active Tetrazine‐Based Pincer Ligand for the Reduction of N‐Oxyanions Using a Redox‐Inert Metal

“…Dihydrotetrazines are especially reducing because the amines in such rings can be oxidized to aromatic imines, and thus analogs of Flavin and NADH [15] . Furthermore, zinc pincer complexes have recently shown promise for metal‐ligand cooperation to promote catalytic hydrogenation, [16] as well as an H 4 Zn pincer complex catalyzing the dehydrogenation of alcohols [17] . We have previously shown the ability of tetrazines to assist in the reduction of water by phosphines, where H 4 btzp is quantitatively generated [18] .…”